UNIVERSITE LIBRE DE BRUXELLES MEDICAL FACULTY

UNIVERSITE LIBRE DE BRUXELLES

MEDICAL FACULTY

Promoter: Philippe van de Borne, MD, PhD Co-Promoter: Robert Naeije, MD, PhD

CONTRIBUTION TO THE STUDY OF SYMPATHETIC DYSREGULATION IN PULMONARY ARTERIAL

HYPERTENSION AND AFTER HEART TRANSPLANTATION

Agnieszka Ciarka

Application for PhD academic degree (Docteur en Sciences Médicales)

UNIVERSITE LIBRE DE BRUXELLES

MEDICAL FACULTY

Promoter: Philippe van de Borne, MD, PhD Co-Promoter: Robert Naeije, MD, PhD

CONTRIBUTION TO THE STUDY OF SYMPATHETIC DYSREGULATION IN PULMONARY ARTERIAL

HYPERTENSION AND AFTER HEART TRANSPLANTATION

Agnieszka Ciarka

Application for PhD academic degree (Docteur en Sciences Médicales)

Acknowledgments

I would like first to express my deep and sincere gratitude to my supervisor Professor Philippe van de Borne, MD, PhD, Head of the Hypertension Clinic, in the Cardiology Department of Erasme Hospital at the Free University of Brussels. His broad knowledge, logical way of thinking and scientific ideas inspired all my research projects and his personal kindness and enthusiasm have been of great value. His help throughout my research work cannot be overestimated and his understanding, encouragement and personal guidance were key to the accomplishment of this thesis.

I am also deeply grateful to my supervisor Professor Robert Naeije, MD, PhD, Head of the Department of Physiology at the Free University of Brussels and Consultant in the Department of Cardiology at Erasme Hospital, for his assistance during my research and manuscript preparation, and for his detailed and constructive comments, encouragement and support. His ideas and suggestions played an important role in determining the perspective of my thesis.

I wish to thank Professor Jean Paul Degaute, Head of the Cardiology Department at Erasme Hospital, who gave me the opportunity to work in the Cardiology Department and who was an untiring source of help for any difficulties that needed to be overcome.

I am really grateful to Professor Paul Linkowski, Head of the Psychiatry Department at Erasme Hospital, for attracting me to Belgium and to the Free University of Brussels in particular. His advice has been invaluable since the very beginning of my research and clinical work.

I would also like to thank the official Jury Members for their detailed review, constructive criticism and advice on my preliminary thesis project, as well as for making the discussion after the private presentation of my thesis such an enriching and enjoyable experience.

I wish to also thank Dr Karen Pickett for her assistance in revising the English of my manuscripts, and Miss Amélia Laurent, for her precious secretarial work during my research period. Special thanks go to Mrs Francoise Pignet, Professor Philippe van de Borne’s secretary, and a personal friend, who passed away unexpectedly, leaving a painful empty space.

During my research period I collaborated with many colleagues, for whom I have great regard. I wish to extend my appreciation to all those who helped me with this work in the Cardiology Department at Erasme Hospital and in the Medical Faculty of the Free University of Brussels.

I owe a deep debt of gratitude to my parents, who lost much due to my research abroad and were always of tremendous support with everything I undertook. Without their patience, encouragement and understanding I would never have been able to complete this task.

Finally, I thank Raphaël, for being there and giving such an unimaginable support, help and encouragement.

Agnieszka Ciarka

Members of the Jury

President

P.A. Gevenois, MD, PhD, Free University of Brussels, ULB Secretary and Promoter

P. van de Borne, MD, PhD, Free University of Brussels, ULB Co-Promoter

R. Naeije, MD, PhD, Free University of Brussels, ULB Experts from the Medical Faculty, ULB:

J.M. Boeynaems, MD, PhD, Free University of Brussels, ULB S. Motte, MD, PhD, Free University of Brussels, ULB

A. de Troyer, MD, PhD, Free University of Brussels, ULB P. Van der Linden, MD, PhD, Free University of Brussels, ULB

External experts

R. Fagard, MD, PhD, University of Leuven, KUL

M. Guazzi, MD, PhD, University of Milano, Italy

A. INTRODUCTION

A.1. The sympathetic nervous system.

A.1.1. General considerations and historical perspective.

A.1.1.1. Historical perspective

A.1.1.2. Reflex regulation of the autonomic nervous system A.1.1.3. Central control of the autonomic nervous system

A.1.1.4. Sympathetic and parasympathetic components of the autonomic nervous system

A.1.1.5. Organisation of the sympathetic nervous system A.1.1.6. Functions of the sympathetic nervous system

A.1.1.7. Neurotransmitters of the sympathetic nervous system A.1.1.8. Neurotransmitter secretion at effectors organ synapse A.1.1.9. Adrenoreceptors

A.1.2. Control mechanisms

A.1.2.1. Aortic arch and carotid baroreceptors A.1.2.2. Low pressure baroreceptors

A.1.2.3. Chemoreceptors

A.1.2.4. Effects of exercise on sympathetic nervous system activation A.1.2.5. Effects of left ventricular dysfunction on sympathetic nervous

system activation

A.1.2.6. Effects of right ventricular dysfunction and heart transplantation on sympathetic nervous system activity A.2. Methodological considerations.

A.2.1. Assessment of sympathetic activity in humans A.2.2. Circulating catecholamines

A.2.3. Microneurography A.3. Ergospirometry

A.3.1. Several aspects of physiology of exercise A.3.2. Principles of exercise testing

A.3.3. Exercise ventilation

A.4. Assessment of chemoreceptor regulation in humans A.4.1. Peripheral chemoreceptor inhibition

A.4.2. Peripheral and central chemoreceptor activation A.5. Brief summary of still unresolved questions

A.5.1. Pulmonary arterial hypertension A.5.2. Heart transplantation

B. SYMPATHETIC CONTROL IN PULMONARY ARTERIAL HYPERTENSION B.1. Hypothesis tested

B.2. Study populations

B.2.1. Study investigating sympathetic activity in PAH patients

B.2.2. Study investigating the effects of atrial septostomy on MSNA in PAH patients

B.3. Material, methods and study protocols

B.3.1. Particular measurements in the study investigating sympathetic activity in PAH patients

B.3.2. Particular measurements in the study investigating effects of atrial septostomy on MSNA in PAH patients

B.4. Sympathetic nervous activity in PAH and effects of disease severity B.5. Effects of chemoreflex activation

B.6. Effects of atrial septostomy

C. SYMPATHETIC CONTROL AFTER HEART TRANSPLANTATION C.1. Hypothesis tested

C.2. Patient population C.3. Material and methods

C.4. Effects of chemoreflex activation on sympathetic activity and blood pressure

C.5. Effects of chemoreflex activation on exercise intolerance

D. DISCUSSION

D.1. Sympathetic nervous system activation in patients with pulmonary arterial hypertension

D.2. Effects of atrial septostomy on sympathetic nervous system activation D.3. Chemoreceptors in heart transplant recipients

D.3.1. Peripheral chemoreceptors deactivation

D.3.2. Peripheral and central chemoreceptors sensitivity E. CONCLUSIONS

F. REFERENCE LIST G. ANNEXES

G.1. Publications

G.1.1. Velez-Roa and Ciarka et al, Increased sympathetic nerve activity in pulmonary artery hypertension, Circulation. 2004 Sep 7;110(10):1308- 12.

G.1.2. Ciarka et al, Atrial septostomy decreases sympathetic overactivity in pulmonary arterial hypertension, Chest. 2007 Jun;131(6):1831-7.

G.1.3. Ciarka et al, Effects of peripheral chemoreceptors deactivation on sympathetic activity in heart transplant recipients. Hypertension. 2005 May;45(5):894-900.

G.1.4. Ciarka et al, Increased peripheral chemoreceptors sensitivity and exercise ventilation in heart transplant recipients. Circulation. 2006 Jan 17;113(2):252-7.

G.2. Annexe thesis title.

G.3. Brief summary in French of described research

A. INTRODUCTION

A.1. The sympathetic nervous system

A.1.1. General considerations and historical perspective.

A.1.1.1. Historical perspective

he nervous system is divided into the somatic nervous system (responsible for voluntary control of different organs, mainly muscles) and the autonomic nervous system, which regulates individual organ function and homeostasis. T

The autonomic nervous system innervates every organ, creating, as Galen (129-216) suggested, ‘sympathy’ between various parts of the body. The autonomic nervous system has as complex neural organization as the somatic nervous system, but it remains largely involuntary and automatic. According to Claude Bernard,

‘nature thought it provident to remove these important phenomena from the capriciousness of the ignorant will’.

The expression ‘autonomic nervous system’ was not known before 1898, when in a paper about superior cervical ganglion, the Cambridge physiologist J.N.Langley suggested substituting the word ‘autonomic’ for the word ‘visceral’. He wrote: ‘The word ‘autonomic’ implies a certain degree of independent action, but exercised under control of a higher power. The autonomic nervous system means the nervous system of the glands and involuntary muscles, it governs the ‘organic’

functions of the body.’ However, the term ‘autonomic’ did not gain immediate and overwhelming acceptance, and for some years other expressions, such as ‘vegetative’

or ‘involuntary’ were also used to refer to the autonomic nervous system.

Before the 17

century, knowledge about the autonomic nervous system was

almost entirely derived from descriptions of ape and pig dissections performed by

Galen, as well as from his observations of wounded gladiators. He described a

ganglionic, sympathetic chain, which he thought arose from the brain and described

also white rami communicantes. For over a thousand years, his opinions were rigidly

maintained by medical and religious authorities. It was not until the 17

and 18

centuries that anatomists performed further studies. In particular, Thomas Willis

described vagus or ‘wandering nerves’ and named the ganglionic nerves the

‘intercostal nerves’, whereas Jacobus Winslow described the ganglia as ‘the small brains’, and introduced the term ‘great sympathetic nerves’, replacing the term

‘intercostal nerves’. The French anatomist Xavier Bichat distinguished between the somatic and visceral functions of the nervous system; however, the elucidation of the functions of these components was patchy for some time. Willis sectioned the vagus nerve of a dog and observed that its heart fluttered. François Poerfour observed that cutting cervical sympathetic nerves affected pupil size, and Albert Regnard and Paul Loye observed that stimulating the vagus nerve of criminals decapitated by guillotine provoked the secretion of the gastric juice on the inner surface of the stomach, 45 minutes after execution.

In the 19

century, the use of microscopy enabled Robert Remak to discover unmyelinated sympathetic fibers, and physiological experiments revealed further functional aspects of the autonomic system.

Milestones in the investigation of the autonomic nervous system were set by Walter Gaskell (1847-1914), a Cambridge physiologist, who distinguished both morphologically and functionally two major nervous outflows: the thoracolumbar (sympathetic) and the craniosacral (parasympathetic). His close colleague, the above- mentioned J.N.Langley, and his students, Henry Dale and Thomas Elliot, conducted further studies on chemical neurotransmission, and performed experiments with atropine, pilocarpine and adrenaline. In 1913, Dale published a dissertation on the effects of adrenaline; he described vasoconstriction, the normally predominating effect, which accounted for the pressor response, as well as the contrary effect, vasodilation, which occurred when a lower dose was applied. He was, however, unable to explain these phenomena.

Later, shortly after the Second World War, two major discoveries about the sympathetic nervous system and its neurotransmitters were made. First, Ulf van Euler demonstrated that noradrenaline occurred naturally in extracts of sympathetic nerves;

thus the longstanding objection to noradrenaline being a substance in animal nerves was removed. Second, Raymond Ahlquist, defined two major groups of adrenoreceptors (alpha and beta), with a range of potencies for catecholamines.

Therefore, an adequate explanation was offered for some apparently paradoxical

experimental differences observed by Dale and others.

At the beginning of our century, the hypothesis of one neuron-one neurotransmitter was formed. This idea was gradually modified as new neurotransmitters were discovered, and, in 1976, Burnstock formed the hypothesis that nerve cells might release more than one neurotransmitter. Understanding the details of these combinations of transmitters, known as ‘chemical coding’, has led to detailed physiological and pharmacological investigation into the effects of disease at the autonomic neuroeffector junction. Thus, throughout the 20

century, basic scientific studies of the autonomic nervous system have continued to elucidate the mechanisms that underlay complex nervous system activity.

A.1.1.2. Reflex regulation of the autonomic nervous system

The peripheral autonomic nervous system, an efferent system, consists of neurons that lie outside central nervous system and which are responsible for visceral innervation. Autonomic nerves constitute all of the efferent fibres that leave the central nervous system, except for those that innervate skeletal muscles.

The autonomic nervous system is primarily involved in reflex regulation, and consists of an autonomic or somatic afferent limb, and an autonomic or somatic efferent limb. As a result, the sensory signals, which enter the autonomic ganglia, and then the spinal cord, brain stem or hypothalamus, can elicit appropriate reflex responses directly back to the visceral organs to control their activities.

There are some afferent autonomic fibres that are responsible for the mediation of visceral sensation and regulation of vasomotor and respiratory reflexes, for example from the baroreceptors and chemoreceptors in the carotid sinus and aortic arch. These afferent fibres are usually carried to the central nervous system by major autonomic nerves such as the vagus, splanchnic or pelvic, although afferent pain fibres from blood vessels may be carried by somatic nerves. Reflex responses to stimuli may involve either autonomic efferent fibres, which cause contraction of smooth muscle in certain organs and which influence the function of the heart and glands, or the somatic nervous fibres, which cause, for instance, coughing and vomiting.

A.1.1.3. Central control of the autonomic nervous system

The autonomic nervous system is mainly activated by centers located in the spinal cord, brain stem and hypothalamus. Autonomic control can be also influenced by impulses originating from the cerebral cortex, especially the limbic cortex, which can transmit impulses to the lower centers. The autonomic nervous system operates by means of reflexes, and the efferent signals are transmitted to the visceral organs by two main subdivisions of the autonomic system: the sympathetic nervous system and the parasympathetic nervous system.

A.1.1.4. Sympathetic and parasympathetic components of the autonomic nervous system

The autonomic nervous system is divided into two separate components, called the parasympathetic system and the sympathetic system, on the basis of anatomical and functional differences. Both of these systems consist of myelinated preganglionic fibres that make synaptic connections with unmyelinated postganglionic fibres, and these fibers innervate effector organs. The synapses usually occur in clusters called ganglia. Most organs are innervated by fibres from both divisions of the autonomic nervous system, and the influence is usually opposing (e.g., the vagus slows the heart, while the sympathetic nerves increase heart rate and myocardial contractility), although activation may be parallel (e.g., effects in the salivary glands).

A.1.1.5. Organisation of the sympathetic nervous system

The sympathetic system has efferent neurons in the brain and spinal cord. The

cell bodies of sympathetic preganglionic fibers are in the lateral horns of the spinal

segments Th1-L2. The myelinated preganglionic sympathetic neurons travel a short

distance in the mixed spinal nerve, and then branch off as white rami communicantes

and synapse in the ganglia of the sympathetic system. These ganglia are arranged in

two paravertebral chains that lay anterolateral to the vertebral bodies and extend from

the cervical region to the sacral region. They are called the sympathetic ganglionic

chains. The preganglionic fibers that enter the chain make a synapse with a

postganglionic fiber at the same dermatomal level, or at a higher or lower level, and

then the unmyelinated postganglionic sympathetic fibers return to the adjacent spinal

nerve via the grey rami and are conveyed to the effector organ. Some preganglionic sympathetic fibers do not synapse in the sympathetic chains, but terminate in separate cervical or abdominal ganglia, or travel in the greater splanchnic nerve and then directly synapse with chromaffin cells in the adrenal medulla.

A.1.1.6. Functions of the sympathetic nervous system

In contrast to the parasympathetic system, the sympathetic system enables the body to be prepared for fear, flight or fight. Sympathetic responses include an increase in heart rate, blood pressure and cardiac output, a diversion of blood flow from the skin and splanchnic vessels to those supplying skeletal muscle, increased pupil size, bronchiolar dilation, contraction of sphincters, as well as metabolic changes, such as mobilisation of fat and glycogen.

A.1.1.7. Neurotransmitters of the sympathetic nervous system

Acetylcholine is a neurotransmitter that acts via nicotinic receptors at the preganglionic synapse (not blocked by atropine), whereas noradrenaline is the principal transmitter for postganglionic sympathetic nerves. However, there are several areas of cholinergic transmission at the sympathetic nervous system endings at effectors organs. These exceptions include sudomotor nerves, putative vasodilator fibres to muscle, and the adrenal medulla, which is innervated by preganglionic cholinergic fibres and which itself secretes both adrenaline and noradrenaline.

A.1.1.8. Neurotransmitter secretion at effector organs synapse

Adrenaline and noradrenaline are synthesised from the essential amino acid phenylalanine in a series of steps, including the production of dopamine. The terminal branches of sympathetic postganglionic fibres have varicosities and swellings, giving them the appearance of a string of beads. These swellings form contacts with the effector organs, and they are also the site of synthesis and storage of noradrenaline.

Upon the arrival of a nerve impulse, noradrenaline is released from granules in the

presynaptic terminal into the synaptic cleft. The action of noradrenaline is terminated

by diffusion from the site of action and then re-uptake into the presynaptic nerve

ending, where it is either inactivated by the enzyme monoamine oxidase in mitochondria, or metabolised locally by the enzyme catechol-O-methyl-transferase.

A.1.1.9. Adrenoreceptors

The actions of noradrenaline and adrenaline are mediated by specific receptors. Pharmacological subdivision of these receptors into two groups (alpha and beta) was first suggested by Ahlquist in 1948

; these have been further subdivided on functional and anatomical grounds. α-adrenoreceptors may be either postsynaptic (α

) or presynaptic (α

), the latter when stimulated decrease the release of neurotransmitter. α

adrenoreceptors mediate effects such as vasoconstriction. α

receptors are also present at the postsynaptic level and cause vasoconstriction when stimulated with agonists. ß adrenoreceptors mediate vasodilatation (especially in the skeletal muscle), increase the rate and the force of the heart (with a tendency to arrhythmias), and cause bronchial relaxation. They are further subdivided into ß

receptors, which mediate chronotropic cardiac action of isoprenaline, and ß

receptors, which are responsible for most of the peripheral effects of ß-adrenergic stimulation.

However, further research now shows that the classification is not as simple as this. For instance, many organs have both ß

and ß

adrenoceptors (e.g., in the heart, there is one ß

-adrenoceptor to every three ß

-adrenoceptors). The receptors also exhibit differing responses to adrenaline and noradrenaline. Adrenaline and noradrenaline appear to have an equivalent effect on ß

adrenoceptors in the heart, whereas the ß

adrenoceptors in smooth muscle are more sensitive to circulating adrenaline than noradrenaline.

A.1.2. Control mechanisms

A.1.2.1. Aortic arch and carotid baroreceptors

A s described above, functioning of the sympathetic nervous system

is based on reflex, with afferent neurons, which conduct impulses

towards the central nervous system, and efferent neurons, which

conduct impulses to the effector organs.

The most important arterial reflexes for circulatory haemostasis under normal physiologic conditions are probably those originating from carotid baroceptors.

Discovery of the carotid sinus reflex was made by Hering in 1923 after he clearly demonstrated that the sensory fibers of the reflex are located in the carotid sinus and carotid bifurcation

. In his subsequent papers, he demonstrated that the afferent pathway resides in the branch of the glosspharyngeal nerve (the sinus nerve or Hering’s nerve), and that stimulation of the sinus (or of its nerve) provokes reflex cardiac slowing and hypotension

.

Basically, the rise in pressure stretches the baroreceptors and causes them to transmit a signal to the central nervous system. The ‘feedback’ signals are then sent back through the autonomic nervous system to the circulation, which reduces the arterial pressure to normal levels.

Anatomical features of carotid sinus baroreceptors

The carotid sinus is a segmental enlargement of the carotid internal artery at the site of its origin from the carotid common artery. In humans, the carotid sinus may include the carotid bifurcation, and also the proximal segment of the external carotid

. The carotid sinus wall contains much more collagen and elastin than do the adjacent segments of the common carotid and external carotid artery. The sensitive fibers are of the small, myelinated type and most of their nerve terminals are located in the adventitia of the sinus wall adjacent to the media

. The sensory innervation of the carotid sinus is carried in the nerve of Hering, which is a small branch of the glossopharyngeal nerve. Signals from the Hering nerve are transmitted to the tractus solitarius in the medullary area of the brain stem.

Mechanical aspects of carotid sinus stimulation

Baroreceptors (or pressoreceptors) are in fact stretch receptors and are stimulated by deformation of adventitial tissue in which they reside. Altering the transmural pressure across the wall of the sinus by a special air-tight cylinder placed around the neck is an effective method to elicit reflex changes

.

Deformation was originally achieved by the altering of the intraluminal

pressure, but the rate of pressure change (pulse pressure) is also important. The

baroreceptors respond much more rapidly to changing pressure than to a stationary pressure. For example, if the mean arterial pressure is equal to 150 mmHg, but instantaneously rises rapidly, the rate of impulse transmission can be as much as twice that of when the pressure is held stationary at 150 mmHg.

The threshold for stimulation of the carotid sinus receptors by static, nonpulsatile, intraluminal pressure is 60 mmHg in most animal species. Maximal reflex changes are achieved at sinus pressures of 175-200 mmHg

. In a pulsatile system, the threshold systolic blood pressure for the carotid sinus stimulation (in a dog) is 62 mmHg

. There is a linear relation between sinus nerve activity and mean arterial pressure over the range of 80-180 mmHg

.

Anatomical considerations of aortic baroreceptors

The aortic baroreceptor system may be considered to be analogous to that of the carotid sinuses. Nerve endings in the aortic wall are found in adventitia

, and in this respect they do not differ from carotid sinus baroreceptors endings. They act as stretch receptors, and there is no evidence that the barosensory portions of the aortic arch and its major branches differ morphologically or have different stress-strain characteristics from the rest of the aortic arch. Signals from the arch of the aorta are transmitted through the vagus nerves to the tractus solitorius of the medulla.

Comparison of aortic and carotid reflex systems

Electrical stimulation of the central end of the aortic nerve produces bradycardia and hypotension, a response that is identical in comparison with the stimulation of Hering’s nerve. In line with this observation are those experiments that demonstrated that maximal changes in blood pressure in response to carotid sinus stimulation do not differ quantitatively from those accompanying stimulation of aortic depressor nerve

.

Although the potential for reflex circulatory regulation by the two baroreceptors systems may not differ upon maximal excitation, there are nevertheless important differences in the level of excitation achieved by a given pressure stimulus.

As a consequence, the responses of aortic baroreceptors are similar to those of carotid

baroreceptors, except that they operate at pressure levels about 30 mmHg higher.

Experiments have shown that the threshold systolic blood pressure that is sufficient to elicit a change in afferent impulse activity in the aortic nerve is 95 mmHg

, whereas the threshold pressure of the sinus nerve is much lower, averaging 62 mmHg

. The curve between systolic blood pressure and aortic afferent nerve activity was displaced to the right, which indicates that over a broad range of systolic pressures values, the aortic receptors are less sensitive to pressure changes than are the carotid sinus receptors (Figure 1). Further studies that analysed the effects of pulsatile pressure required to elicit reflex responses in the aortic arch reflex confirmed a threshold of 100-110 mmHg

.

Figure 1

Blutdruck-characteristik curves for the carotid sinus (filled circles) and aortic arch (unfilled circles) baroreflexes in eleven dogs. Response in perfusion pressure is expressed as percentage of maximal change evoked by carotid distension (mean ± SE). The aortic arch curve was displaced to the right, and its maximal slope and height were less than those for the carotid sinus curve. (From Donald and Edis, J Physiol 215: 521, 1971).

At normal pressure levels, there is a minimal excitatory activity in the aortic

nerves. This was deduced from experiments on vagotomised animals, in which

selective cold blockade of the aortic nerves with an intact sinus nerve caused a trivial

increase in heart rate and arterial pressure

. When a selective blockade of sinus nerve was performed with intact aortic nerves, a marked rise in blood pressure and heart rate was produced

. When this was followed by aortic nerve blockage, a further increase in blood pressure and heart rate was observed

. Thus, at higher pressures, aortic baroreceptors exert a buffering action, whereas at lower pressures, their modulating influence on the heart and systemic vessels is minor.

Reflex initiated by baroreceptors

The baroreceptors respond very rapidly to changes in arterial blood pressure.

Specifically, the rate of impulse firing from baroreceptors increases during systole and decreases during diastole (Figure 2). As a result, cardiac sympathetic nerve activity is coupled with heart beats

, as was demonstrated in neurographic studies in animals and microneurographic studies in humans

. The impulse grouping is explained by afferent activity from baroreceptors, with an inhibitory influence on the sympathetic activity with each systole

.

Figure 2

Blood pressure above; impulses in the carotid sinus nerve below. Impulses occur during the rise in pressure early in each beat. (After Bronk and Stella, Am J Physiol 110: 708, 1935).

The reflex initiated by baroreceptors is as follows. The afferent impulses from

baroreceptors enter the tractus solitorius of the medulla, and the secondary signals

inhibit the vasoconstrictor center of the medulla and excite the vagal center. This

results in the vasodilatation of veins and arterioles via the peripheral circulatory

system and a decreased rate and strength of heart contraction. The net effect is a

decrease of arterial blood pressure, as a result of decrease of both peripheral resistance and cardiac output.

Typical reflex changes in arterial blood pressure occur in animal models when the common carotid artery is occluded. This reduces carotid artery pressure and, as a result, the baroreceptors become inactive and lose their inhibitory effect on the vasomotor center. The vasomotor center becomes more active than usual, which provokes a substantial rise in the blood pressure. Removal of the occlusion allows the pressure to fall immediately to a level slightly below normal, as a momentary overcompensation, and then to return to normal within another minute or so.

Resetting of the baroreceptors

Baroreceptors reset in one to two days to whatever pressure they are exposed.

This means that if the pressure rises to a higher level, initially an extreme number of impulses are transmitted from the baroreceptors. During the next few seconds, the rate of firing diminishes considerably, and then the rate diminishes slowly over the next one to two days; finally, the rate of firing reaches a normal level, despite the fact that the blood pressure is still elevated. Conversely, when the blood pressure drops suddenly, the baroreceptors initially transmit no impulses; gradually, over about a day, their firing returns to a normal level. Baroreceptors are therefore unimportant for the long-term regulation of blood pressure.

A.1.2.2. Low pressure baroreceptors

I t has been recognised for over 100 years that stimulation of the reflexogenic areas of the heart and the lungs leads to dramatic changes in the heart rate and blood pressure. Despite the diversity of responses to stimulation of various areas within the heart and the lungs, there is still tendency to

‘lump’ them together under the term ‘cardiopulmonary baroreceptors’ or ‘low

pressure baroreceptors’. The experiments that lead to the concept of low pressure

baroreceptors were designed to induce changes in central blood volume, by

application of subatmospheric pressure to the lower part of the body, which results in

the transfer of blood away from the heart and the lungs. Since this procedure provokes

reflex changes in the heart rate and vascular resistance, with little effect on the mean

arterial blood pressure, it has been assumed that its effect was mediated through ‘low pressure baroreceptors’.

Location of low pressure baroreceptors

Sensory fibers in the heart were first described in 1895 by Berkley. In subsequent studies, the receptors of the heart were characterised by histological and electrophysiological techniques, and two groups of afferent vagal endings were described. The first group is found in restricted regions of the atria, ventricles, and pulmonary artery and consists of complex uncapsulated terminals of myelinated fibers, from which signals are conducted by vagal afferents

. These receptors exhibit a pulsatile discharge pattern. The second group was defined by histological studies as a system of fine nerve endings termed “end-nets”. Electrophysiological studies have shown that these nerve endings are distributed not only in the thoracic aorta, pulmonary artery, atriovenous junctions, and atrial appendages, but also in the ventricular endocardium. These fibers innervate the right ventricle much less than the left ventricle. The fibers are unmyelinated, have sparse discharge, and can be stimulated by excessive distension and specific chemical substances, such as phenyl diguanide

.

Receptors that consist of complex unencapsulated endings are found in both atria, but they are most numerous at the junction of the superior and inferior vena cava and right atrium, as well as at the junctions of the pulmonary veins and left atrium

. The endings are present in the loose tissue of the endocardium, have no capsule and are stimulated by the deformation of the chamber to which they are attached. Two types of afferent activity have been described for atrial receptors. In type-A receptors, the activity may be generated by atrial contraction, whereas in the type-B receptors, the activity is generated by atrial distension

.

Information from receptors located in the heart travels towards cardiac nervous centers by vagal afferents; however, sympathetic afferents are also involved in the transmission of signals. Several studies have reported that spontaneous cardiac activity in sympathetic cardiac fibers can originate from all chambers of the heart

.

Reflex elicited by atrial distension

Mechanical distension of various cardiac sites elicits circulatory reflexes. For example, an increase in atrial pressure provokes an increase in heart rate, sometimes by as much as 75%. A small part of this heart rate increase is caused by the direct effect of the increased atrial volume that stretches the sinus node; however, this direct stretch can increase the heart rate by only about 15%. The remaining heart rate increase can be explained by a reflex called the Bainbridge reflex.

Historically, Bainbridge was the first to observe (in 1915) that giving large infusions of saline or blood to anesthetised dogs caused an increase in their heart rates. Bainbridge claimed that this effect was secondary to stimulation of receptors in the atrial wall caused by the increased atrial pressure

. However, he gave no direct evidence that right atrium stretching was the stimulus from which this reflex originates.

Later, numerous studies on atrial reflex regulation were reported by Linden and his co-investigators

. Based on the knowledge of location of the atrial receptors, their experiments were designed in such a way that only areas of atrio-venous wall containing atrial receptors with myelinated fibers were distended, with no changes in atrial pressure or blood flow through the atria caused directly by the distensions

. This was achieved by the use of small balloons that were distended in the atriocaval junction or in the pulmonary vein-atrium junction during the experiments

. The stretching of the left atrium at its junction with the pulmonary veins resulted in an increase in heart rate in the animal studies

. The afferent pathway of this reflex was located in the vagi nerves, and the efferent pathways were situated in the sympathetic nerves to the heart

. Using a similar technique, it was demonstrated that stretching the junction of the superior vena cava and the right atrium resulted in a reflex increase in heart rate

. The efferent pathway of the heart response of this reflex is located in cardiac sympathetic nerves. Thus, tachycardia is prevented by cutting the ansae subclaviae, which contain cardiac sympathetic innervation

.

Tonic inhibition of the vasomotor center by vagal afferents from the cardiopulmonary region and sympathetic activation secondary to stimulation of myelinated receptors

Interruption of vagal afferents from the cardiopulmonary region results in no

change in blood pressure when the arterial baroreceptors are functional. However,

when the influence of arterial baroreceptors is eliminated, the block of cervical vagal

traffic results in a rise in blood pressure, tachycardia and constriction of the blood vessels of the skeletal muscle, intestine, and kidney, in addition to constriction of the splanchnic capacitance vessels. There is no change in the tone of cutaneous veins. The vasoconstriction is not affected by atropine and is due to an increase in sympathetic activity that is secondary to the decreased inhibition of central neurons that control sympathetic outflow. A series of experiments reported that unmyelinated vagal afferents are responsible for this tonic inhibition of the vasomotor center

.

Role of cardiac receptors with myelinated vagal afferents

It appears that myelinated receptors located primarily at the junctions of the atria and the veins (pulmonary veins and vena cava) respond to changes in atrial volume. Upon their stimulation, the circulatory effects are confined mainly to changes in heart rate, due to the increase of sympathetic activity towards the sinoatrial node. It has been suggested that this reflex contributes to maintain a relatively constant heart volume by increasing heart rate in response to the increased inflow

.

Role of cardiac receptors with unmyelinated vagal afferents

Although the spontaneous discharge from these afferents is low, interruption of the afferent unmyelinated fibers results in a decrease of the inhibition of the vasomotor center. However, the effect of these afferents on the circulatory system varies with the degree of inhibition on the vasomotor center exerted by carotid and arterial baroreceptors

. Therefore, during haemorrhage, hypotension with its accompanying decrease of intracardiac filling results in a combined withdrawal of arterial and cardiopulmonary baroreceptors, and maximal vasoconstriction occurs in nearly all vascular beds

. The activation of baroreceptors is different in the case of hypotension secondary to heart failure, especially when cardiac cavities are distended.

In this situation, vasoconstriction is attenuated because cardiopulmonary baroreceptors offset the vasoconstriction due to a reduction of afferent traffic from arterial baroreceptors

.

Reflex responses from cardiopulmonary baroreceptors elicited by lower body

negative pressure

Graded levels of lower body negative pressure (LBNP) can be applied when a subject is enclosed in a negative pressure chamber to the level of the iliac crest.

During mild levels of LBNP, it is possible to maintain constant systemic arterial pressure, which allows studying the cardiovascular response to decreased cardiopulmonary receptor stimulation, secondary to decreased venous return, without the stimulation of arterial baroreceptors.

By application of LBNP ranging from -5 to -15 mmHg in normal subjects, an increase of muscle sympathetic nerve activity (MSNA) and peripheral vascular resistance are observed

. This MSNA increase is not accompanied by any changes in arterial blood pressure or heart rate. Application of higher LBNP (40 mmHg) results in a decrease in central venous pressure and systemic arterial blood pressure with reflex tachycardia

. However, if the systemic blood pressure fall is prevented by phenylephrine perfusion in the above experiment, the heart rate is normalised and MSNA returns to a level equivalent to that produced by mild LBNP

.

Thus, low pressure baroreceptors in humans contribute to cardiovascular control because decreased stimulation of these receptors results in increased sympathetic traffic towards peripheral blood vessels and increased peripheral vascular resistance

.

Low pressure baroreflex and sinoaortic baroreflex interact in such a way that a reduction in cardiopulmonary afferent input augments the gain of the sinoaortic baroreflex

.

A.1.2.3. Chemoreceptors

C hemoreflexes exert an important influence on breathing, cardiac

and vascular control. Chemoreflex physiology is complex, and the

exact molecular mechanisms by which chemoreceptors are

activated remain unclear. While the chemoreflex involves hyperventilation and a

circulatory pressure response, important feedback loops exist between these two arms

of the reflex. During chemoreflex activation by hypoxia or hypercapnia,

hyperventilation and consequent stretch of thoracic afferents act to inhibit the

chemoreflex-mediated sympathetic activation, and this attenuates the chemoreflex-

mediated cardiovascular responses.

Chemoreflexes are represented by peripheral chemoreceptors located in the carotid bodies in the internal carotid artery region as well as in the aorta, and by the central chemoreceptors located in the brainstem. Peripheral chemoreceptors respond primarily to hypoxic stimulation, whereas central chemoreceptors respond to hypercapnia.

Peripheral chemoreceptors, anatomy and neurotransmitters

Peripheral chemoreceptors monitor changes in arterial blood oxygen tension.

Conventionally, we can distinguish peripheral chemoreceptors in carotid and aortic bodies. Carotid bodies are ovoid forms located at the bifurcation of the common carotid artery into the internal and external carotid artery

, whereas aortic bodies, usually four, are located on the interior and exterior surfaces of the aortic arch, and near the origin of the subclavian artery.

Reflexes arising from the peripheral chemoreceptors are the main defensive mechanisms acting against hypoxia. There are two main components of this reflex:

respiratory and circulatory, and both of them are essential to adapt the organism towards decreased oxygen content in the breathing air.

The carotid bodies are characterised by the following anatomic features: rich vascularisation and rich innervation. Carotid and aortic bodies are composed of clusters of cells; in each cluster, there is an afferent arteriole that spreads into a net of small capillaries, surrounded by two types of cells. Type I (glomus) cells contain numerous dense-core vesicles and make extensive synaptic contacts with sensory axons

. The cluster of type I cells is surrounded by both thin type II sustanticular cells and an extensive capillary network

. Blood supply for carotid bodies originates from the carotid internal artery and the occipital artery. Since glomus cells are in direct contact with afferent nerves endings, it is thought that they represent an initial site of chemotransduction

.

Different hypotheses have been proposed to explain the mechanism of

chemotransduction at the cellular level. Two main theories are used to explain the

mechanism by which hypoxia triggers transmitter release from glomous cells

. The

first assumes that K+ channels have an oxygen sensor, and that hypoxia provokes an

inhibition of K+ current as well as the consequent depolarisation, increase in cytosolic

Ca2+ and release of neurotransmitter

. The second suggests that a heme protein is an

oxygen sensor and that a biochemical reaction associated with the redox state of the protein triggers transmitter release

.

Type I glomous cells contain many neurotransmitters that are classified into inhibitory and excitatory types, according to their action on the transmission of the hypoxic stimulus. The proposed neurotransmitter that mediates the sensory excitation by hypoxia is acetylcholine, but there are several other substances that play neuromodulatory roles (e.g., dopamine, substance P, ATP, neurokinin 1, noradrenalin, adenosine, serotonin, endothelin-1)

. A general rule is that substances that activate adenylate cyclase and increase cAMP content in the glomus cells also activate chemoreceptor activity, whereas substances that activate guanylate cyclase and increase cGMP content exert an inhibitory action on peripheral chemoreceptors.

Aortic and carotid chemoreceptors are sensitive to the decrease in arterial oxygen tension (PaO

)

. A plot of the relation between PaO

and the frequency of discharge in the afferent endings of the peripheral chemoreceptors has a hyperbolic shape, and the exposure of an experimental animal to graded hypoxia causes an exponential response from the carotid bodies

. While hypoxia activates peripheral chemoreceptors, breathing of pure oxygen depresses chemoreceptor activity. This suppression is complete at a PaO

of 200 mmHg. The contribution of tonic peripheral activation to the normoxic eupneic breathing can be estimated by the Dejours test, when the examined subject takes a few breaths of pure oxygen. This results in a complete deactivation of peripheral chemoreceptors due to the high PaO

and absence of carbon dioxide in the inspired gas. The Dejours test decreases ventilation by 20%

and reveals the contribution of tonic peripheral chemoreceptor activation to ventilation under resting conditions. The peripheral chemoreceptors are also activated by the arterial carbon dioxide tension (PaCO

) and the decrease in arterial pH (pHa)

. When hypoxic and hypercapnic stimuli are combined, they have a multiplicative effect on the stimulation of peripheral chemoreceptors.

Aortic chemoreceptors have several distinct characteristics in comparison with

carotid bodies chemoreceptors. First, aortic chemoreceptors are less sensitive to

hypoxia and PaCO

, whereas they are more sensitive to low perfusion than are carotid

bodies chemoreceptors. This is because blood flow autoregulation is less developed in

the arotic bodies than in the carotid bodies. In humans, it is the carotid

chemoreceptors, and not aortic chemoreceptors, that play a decisive role in the

regulation of respiration. Denervation of the carotid chemoreceptors, which was once

a treatment for refractory asthma, turned out to have hazardous effects on the physiology of respiration. Mainly, it provokes a decrease in the acute hypoxic ventilatory responsiveness

and modest resting hypoxaemia and hypercapnia.

Reflex respiratory response from peripheral chemoreceptors

The peripheral chemoreceptors mediate the only reflex by which hypoxia provokes an increase in ventilatory movements, and consequently an increase in ventilation in humans. The carotid chemoreceptors are situated at a strategic point, near the carotid arteries, which supply the blood towards the brain.

Afferent fibers from carotid chemoreceptors travel with Hering’s nerves to the glossopharyngeal nerves, sensory ganglions of the IX nerve and then to the dorsal respiratory area of the medulla. The afferent fibers form the aortic chemoreceptors travel with the vagi and also to the dorsal respiratory area in the medulla. Neurons of the dorsal respiratory group are located mainly within the nucleus of the tractus solitorius (NTS). The neurotransmitter at the first synapse in the NTS is glutamate

. Postsynaptic neurons in the NTS synapse with the bulbo-spinal respiratory neurons alpha

, which activate the diaphragm and the spinal inspiratory motoneurons

.

If a subject breathes air with decreased PaO

, activation of the chemoreceptors results in an increase in ventilation, and as a result, in a reduction of the arterial PaCO

and hydrogen ion concentration. These last two responses depress the respiratory center; therefore, the effect of activation of peripheral chemoreceptors by hypoxia is mostly counteracted, and the ventilatory response is attenuated

.

When the PaCO

remains constant during hypoxia, the changes in ventilation are related only to the rise in the respiratory drive that are secondary to the effects of low PaO

in the arterial blood on the peripheral chemoreceptors. The response of the chemoreceptors and ventilation to hypoxia is a curvilinear increase with respect to the falling PaO

. Since the PaO

remains greater than 100 mmHg, no changes in ventilatory drive occur. At pressures lower than 100 mmHg however, ventilation doubles, for instance when PaO

falls to 60 mmHg.

Tonic peripheral chemoreceptor activation is responsible for 20% of resting

ventilation under normoxic conditions. However, the relative contribution of

peripheral chemoreceptors to ventilation increases during physical exercise and

during generalised hypoxia, such as at high altitude.

Reflex circulatory response from peripheral chemoreceptors

The main characteristics of the circulatory reflex from the peripheral chemoreceptors are sympathetic nervous system activation and vasoconstriction.

Direct evidence for this increased sympathetic traffic towards muscle blood vessels in humans comes from microneurographic recordings

. Consequent to sympathetic activation, total peripheral vascular resistance increases

.

Heart rate responses to peripheral chemoreceptor stimulation can be increased, decreased or remain unchanged. The reason for these variable responses is that stimulation of carotid chemoreceptors generates a number of secondary mechanisms, among which the most important are those initiated by the concomitant increase in respiratory minute volume. When these ventilatory movements are blocked (asphyxia), for example during an apnea or closure of the respiratory tree

, peripheral chemoreceptor stimulation provokes bradycardia through the activation of parasympathetic (cholinergic) and vagal efferents towards the heart

. Under the same conditions, vasoconstriction occurs secondary to the activation of sympathetic efferent nerve fibers

. Thus, this primary carotid chemoreflex response to hypoxia is a rare example of concomitant activation of sympathetic and parasympathetic components of the autonomic nervous system. The responses of bradycardia and vasoconstriction represent the direct (primary) carotid chemoreflex cardiac and vascular effects.

The ‘primary’ carotid chemoreflex response can be modulated by concomitant

change in pulmonary ventilation. Specifically, if pulmonary hyperventilation is

present together with hypoxia, sympathetic reflex activation predominates, and an

increase in heart rate and cardiac output is noted (instead of the bradycardia observed

in the primary reflex response from arterial chemoreceptors)

. There are two different

mechanisms by which an increase in pulmonary ventilation suppresses (or even

reverses) these primary cardiac and vascular responses evoked by stimulation of the

carotid bodies. The first is an increase in central respiratory drive

and the second

is an increased activity of pulmonary stretch receptors driven by lung inflation

.

These two respiratory mechanisms are responsible for altering the excitability of the

cardiac vagal preganglionic motoneurons that are situated in the nucleus ambiguous

region.

In conclusion, activation of the peripheral chemoreflex in the absence of secondary respiratory mechanisms provokes a predominant vasoconstrictor response and bradycardia, whereas in the presence of hyperventilation, heart rate acceleration occurs

. The variable cardiac and vascular responses evoked by stimulation of the carotid bodies in animal and human experiments depend on a balance between these primary and secondary mechanisms

.

Central chemoreceptors

The major areas of the central nervous system that are involved in the control of breathing are located in three anatomical regions: the pneumotaxic center called the pontine respiratory group (the PRG, lies in the dorsal lateral pons), the dorsal respiratory group (the DRG, lies in the region of the nucleus of the solitary tract in the dorsal medulla) and the ventral respiratory group (the VRG), which is located in the ventral medulla.

The PRG, DRG and VRG are three main clusters of respiratory neurons.

However, other neurons may also play a major role in the control of breathing. One such cluster of neurons is the rostral ventrolateral medulla (RVLM), and it extends rostrally from the VRG, dipping towards the medullary surface lying ventral to the facial nucleus.

The RVLM is known to be a site of central chemoreception

; thus this area is called a chemosensitive area. Central chemoreceptors detect changes in brain interstitial fluid pH and therefore monitor PaCO

. Experiments that attempted to locate chemosensitive regions in the brain took advantage of the observation that perfusion of the brain ventricular system with acidic fluids provoked an increase in ventilation. This led to further investigation aimed to identify possible locations of central chemoreception near brain surfaces. Studies using the direct application of acidic artificial cerebrospinal fluid resulted in the identification of chemosensitive areas located on, or near to, the ventrolateral medulla

. It is now generally accepted that central chemoreceptors are located superficially at a rostral and caudal area of the ventral medulla

.

Lesion in the RVLM in anesthetised animals produces decreased inspiratory

output and the virtually complete elimination of the normal stimulation of respiratory

output with increased PaCO

. In the absence of anesthesia, the RVLM lesions

decrease respiratory output and decrease the ventilatory response to increased PaCO

, but they do not cause apnea

.

Neurons from regions of central chemoreception increase the respiratory output in response to an increase in PaCO

, but they do not play a crucial role in the generation of the respiratory rhythm, which is generated by the neurons of the pre- Boetzinger complex, in the rostral part of the VRG

.

How is pH detected by neurons in the chemosensitive regions? First, we must assume that CO

is rapidly hydrated to carbonic acid, which dissociates into a proton and a bicarbonate ion, so that an increase in PaCO

would result in a decrease in pH.

Different mechanisms of chemoreception have been proposed. First, the difference in the intrinsic capacity of a cell to regulate pH can play a role, and neurons that respond to hypercapnia with less effective cellular pH regulation have been proposed to represent central chemoreceptors

. It is also plausible that pH has an effect on the release, synthase or uptake of neurotransmitters, and two of them have been proposed for this role: acetylcholine

and glutamate

. These two hypotheses for the mechanism of central chemoreception may both be valid, and it is possible that different mechanisms function at different chemoreceptor locations.

Respiratory effect of central chemoreflex

Central chemoreceptors account for two-thirds of the steady state ventilatory response to an increase in arterial PaCO

. Thus, the remaining third of the ventilatory response to hypercapnia is due to carotid chemoreceptors

. Moreover, it was noted that when only central chemoreceptors are activated, and when peripheral chemoreceptors are present but maintained at normal gas values, the response to abrupt increase of PaCO

is delayed by about 11 seconds

. This delayed response suggests that, unlike the peripheral chemoreceptors, the central carbon dioxide chemoreceptor does not instantly equilibrate with arterial blood.

PaCO

(via pH) is the most important chemical determinant of ventilation

under normal air breathing conditions

. CO

-sensitive central and peripheral

chemoreceptors are thought to provide ongoing and rapid feedback to the brainstem

respiratory control system concerning the levels of CO

in the arterial blood and

alveolar gas exchange space. If CO

production is maintained at a stable level, and if

alveolar ventilation is decreased, this would result in an increase in PaCO

. Hence,

CO

-sensitive chemoreceptors provide information concerning the adequacy of alveolar ventilation relative to metabolism, and can provide excitatory and inhibitory afferent information to the respiratory control system, thereby representing a classic feed-back control loop.

An increase in PaCO

in the arterial blood provokes a directly proportional increase in lung ventilation starting from a PaCO

equal to 40 mmHg and up to values of 80 mmHg

. PaCO

is a very potent stimulus to the respiratory control system; it increases both the respiratory frequency and tidal volume, and therefore their product ventilation. This response is so strong that it cannot be altered by conscious attempts to prevent it. For every 1 mmHg of increase in PaCO

in arterial blood, minute ventilation increases from 2 to 3-4 L/minute

.

Chemoreceptors can bring about the appropriate changes in ventilation if changes in arterial pH result from metabolic acid-base disorders. Metabolic acidosis increases ventilation by stimulation of central and peripheral chemoreceptors, provokes a decrease in PaCO

and consequently elevates (corrects) arterial pH.

Opposite effects occur in metabolic alkalosis

. The ventilatory response to changes in arterial pH is slightly delayed because chemoreceptor sites in the brainstem are to some degree protected from rapid changes in arterial pH that arise independently of changes in PaCO

. These chemoreceptor sites must be located in areas with less effective blood-brain barrier function.

A.1.2.4. Effects of exercise on sympathetic nervous system activation.

hronic exercise training results in cardiovascular adaptations that are present at rest. The most important cardiovascular changes at rest, related to chronic exercise training, are decreased heart rate and decreased systemic blood pressure. The first effect is related to the increased cardiac vagal activity, whereas the latter is associated with decreased systemic vascular resistance. Numerous studies have investigated the hypothesis that cardiovascular changes at rest are related to changes in sympathetic nervous system activation. Initially such studies measured catecholamines concentrations; further on a technique employing noradrenaline spillover was subsequently used, and finally microneurography was employed.

C

Training induced changes of sympathetic activation at rest

Studies investigating the effects of exercise on resting muscle MSNA gave conflicting results. Longitudinal studies, with the recordings done before and after a period of exercise training reported increases

, decreases

or no change in MSNA

. The majority of these studies examined endurance training. In those studies that found no effects of exercise on MSNA, there were several drawbacks that need to be highlighted. First, one study that found no change in MSNA after 8 weeks of cycling at 75% peak oxygen uptake (VO2) produced only a 7% increase of peak VO2

. Second, a similar study also found no influence of exercise training on MSNA, the post-training peak VO2 was only 35 ml/kg/min

. The training periods were short in all the published longitudinal studies; it is thus impossible to exclude the possibility that further improvements would be possible over a longer training period.

In contrast to the above results, Grassi et al. found a significant decrease in MSNA after 10 weeks of running

, whereas Sinoway et al. reported that rhythmic handgrip exercise increased resting MSNA after only 4 weeks of training

.

Cross-sectional studies can be of some use to compare resting MSNA between trained and untrained subjects because they eliminate the effect of training duration, which is a limiting factor in the longitudinal studies. However, the majority of cross- sectional studies have reported either no differences in terms of sympathetic activity between trained and untrained subjects, or even an increased MSNA in older trained athletes compared with sedentary subjects

.

Effects of exercise on chemoreceptors sensitivity

Exercise training in professional athletes results in a reduction in peripheral chemosensitivity

. Sprinters and runners exhibit an abnormally low ventilatory response to isocapnic hypoxia in comparison with untrained subjects

. However, no difference was noted in the heart rate response to hypoxia between these two groups

.

Central chemoreceptor sensitivity is also modulated by exercise in endurance

training. Attenuation of central chemosensitivity was observed both in professional

athletes

, as well as in semi-professional athletes

. Longitudinal studies of subjects

trained for four years demonstrated a gradual decrease in the ventilatory response to

hypercapnia at rest

. Moreover, central chemoreceptor sensitivity was inversely correlated to peak VO2 in the trained group

.

A.1.2.5. Effects of left ventricular dysfunction on sympathetic nervous system activation

eart failure is defined as a state of fatigue, dyspnea (at first during exercise and at rest) and lower limb oedema, which originate from a limitation of the increase in cardiac output, despite an excessive use of the mechanism of Starling, which allows the increase of cardiac output, secondary to the increase of preload.

H

Congestive heart failure provokes compensatory adjustments to low cardiac output in order to maintain adequate blood pressure and blood flow. It is characterised by complex pathophysiology, varying clinical expression and the activation of several neuroendocrine systems. Neuroendocrine activation already occurs in clinically asymptomatic patients with left ventricular dysfunction

; in large multicenter studies, an increase of plasma concentrations of noradrenaline, atrial natriuretic factor, plasma arginin vasopressin and elevation of plasma renin activity have been noted

. Moreover, plasma noradrenaline concentration is related to the severity of heart failure

and is represents a prognostic factor of survival

.

MSNA is elevated in heart failure, and in patients with advanced heart failure the muscle sympathetic firing rate approaches the cardiac frequency

. Moreover, an increase in cardiac noradrenaline spillover was documented in heart failure patients before the increase of MSNA and plasma noradrenaline concentration

. It seems thus that the myocardium is exposed to increased levels of noradrenaline over longer periods of time than other organs. Taking into account the fact that increased cardiac sympathetic drive may impair ß-adrenergic receptor function, induce myocyte necrosis and apoptosis, and precipitate arrhythmias and sudden death

, it is no wonder that such adrenergic activation is ominous sign in heart failure

. It was demonstrated that cardiac noradrenaline spillover is the principal risk factor for mortality in patients awaiting transplantation

.

The mechanisms responsible for sympathetic overactivation in heart failure

are multiple. There is evidence that sympathetic overactivation in heart failure can be

attributed to blunting of both the arterial baroreflex and cardiopulmonary reflexes.

The heart rate response to phenylephrine infusion is depressed in heart failure, which proves that the cardiac vagus nerve fails to slow the heart rate response to an increase in blood pressure

. Animal studies demonstrated that the peak discharge from the baroreceptors is significantly depressed in heart failure and that this decreased baroreceptor sensitivity cannot be explained by structural changes in the carotid sinus nerve fibers or by a reduction in carotid sinus compliance

. Some research groups, however, argue against impaired arterial baroreceptor sensitivity in heart failure; they postulate that adrenergic activation in heart failure is secondary to decreased activation of baroreflex by mechanical stimuli, which are decreased secondary to low cardiac output.

Other experiments provide evidence of abnormal functioning of low pressure baroreceptors. In heart failure, a loss of the inhibitory effects of increased filling pressures on the sympathetic nervous system activity is observed, as demonstrated by attenuation of the MSNA response to stimuli that increase and decrease cardiac filling pressure without affecting systemic blood pressure

. Moreover, heart failure patients present with decreased sympathetic response to the upright tilt test

and exhibit a significantly smaller sympathetic increase in response to a pharmacologically-induced vasodilatation

.

There is also evidence that sympathetic overactivity is due not only to a loss of inhibitory stimulus towards sympathetic activation (as observed in the baroreceptor dysfunction), but also to direct sympathetic excitation. This sympathetic nervous system activation is related to increased peripheral and central chemoreceptor sensitivity, and increased activation of metaboreceptors. Both peripheral and central chemoreceptors have increased sensitivity in heart failure patients

.

Activation of the renin-angiotensin-aldosterone system in heart failure leads to salt and water absorption and an increase in plasma volume, adding further complexity to reflex autonomic nervous system adjustments. This central volume expansion could be supposed to inhibit sympathetic stimulation by activation of inhibiting afferent receptors, except that this inhibitory reflex appears to be suppressed in heart failure.

A.1.2.6. Effects of right ventricular dysfunction and heart transplantation on

sympathetic nervous system activity