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EFFECTS OF DIET ON THE MOTILITY OF THE SMALL INTESTINE AND PLASMA INSULIN LEVELS IN SHEEP

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EFFECTS OF DIET ON THE MOTILITY OF THE

SMALL INTESTINE AND PLASMA INSULIN

LEVELS IN SHEEP

Lionel Bueno, T.E.C. Weekes, Y. Ruckebusch

To cite this version:

(2)

EFFECTS

OF

DIET

ON THE

MOTILITY

OF THE

SMALL

INTESTINE

AND PLASMA INSULIN LEVELS IN SHEEP

L.

BUENO

T.E.C. WEEKES

Y. RUCKEBUSCH*

*

Laboratoire de Physiologie-Pharmacadynamie, Ecole Nationale Vétérinaire,

31076 Toulouse Cedex, France

*

&dquo; Rowett Research

lnstitute,

Bucksburn, Aberdeen AB2 9SB, Scotiand

Résumé

INFLUENCE DE L’ALIMENTATION SUR LA MOTRICITE INTESTINALE ET L’INSULINEMIE CHEZ LE MOUTON. - L’influence de la nature des aliments sur la motricité

iéjuno-iléale

a

été

envisagée

sur

cinq

moutons en stabulation recevant au cours de

périodes

successives de 21

jours :

foin, herbe ou concentrés à base de céréales offerts ad lib. 8 h par

jour.

L’activité

électrique

de l’intestin

grêle

a été

enregistrée

de façon continue

pendant

toute la durée des essais à

partir

d’électrodes

implantées

chroniquement.

La teneur

plasmatique

en

glucose

et insuline a été évaluée à

partir

de

prélèvements sanguins

effectués avant et

pendant

les

périodes

d’essai.

Le nombre de

complexes myoélectriques

(14

± 3/24

h)

n’est pas modifié

significativement

(P 5

0,05)

par la nature de l’aliment ;

cependant,

la durée totale d’activité

rapide

est

supérieure

(13

et 18

%) lorsque

l’animal reçoit de l’herbe et des concentrés au lieu de foin

long.

La durée de l’activité

rapide

de la

période

d’accès aux aliments est accrue dans le cas des

concentrés ;

l’insulinémie, qui

n’est pas modifiée par

l’ingestion

de foin ou

d’herbe,

est, pour

les concentrés,

augmentée

de 36,8 °!.

En

définitive, quel

que soit le

régime

alimentaire,

les

complexes

myoélectriques

(MMC)

migrant

le

long

de l’intestin sont permanents chez le mouton. Une alimentation riche en éléments directement assimilables par

l’organisme

dans le cas où elle est distribuée sous la forme d’un seul repas

journalier

est

capable

de

désorganiser

de façon transitoire le

profil

moteur de l’intestin

grêle.

Ce

phénomène s’accompagne

d’une

hyperinsulinémie

ana-logue

à celle observée chez les

monogastriques.

Introduction

In adult

sheep

a

major

pattern of electrical activity of the small intestine is the

migrating

myoelectric complex (MMC) (Grivel

and

Ruckebusch,

1972).

This comprises a

phase

of

irregular spiking activity (ISA)

followed

by

a brief

period

of

regular spiking

(RSA)

which in turn is followed

by

a

period

of

quiescence

during

which

only

slow waves are

detected

(Ruckebusch

and Bueno,

1975). The

complexes

are

propagated along

the intestine

and recur at intervals of about 85 min

(Bueno,

(3)

the MMC

pattern

occurs

during fasting,

but it is

disrupted

by feeding ;

during feeding

and for up to 8 h

afterwards

irregular

spiking

is continuous

(Code

and Marlett,

1975)

and

is associated with an increased flow of

digesta

(Bueno

et al.,

1975).

The MMC

pattern

is not

disrupted by

a meal of

hay

in

sheep (Grivel

and

Ruckebusch,

1972)

or

ruminant calves

(Ruckebusch

and Bueno,

1973).

Recently,

Bueno and Ruckebusch

(1976)

have shown that

injections

of insulin or

insulin

secretagogues

into fasted

dogs

result

in a pattern of electrical

activity

similar to

that seen after

feeding.

Continuous

irregular

spiking

is also elicited in

sheep by

a

higher

dose of insulin or

by

an intravenous infusion

of volatile

fatty

acids into normal, but not into alloxan-induced diabetic

sheep (Bueno

and Ruckebusch,

1976).

Although

a

post-prandial

elevation in

plasma

immunoreactive insulin

(IRI)

levels has been described in

adult ruminants

(see

Bassett,

1975),

the

changes

are most consistent when cereals

are fed

(Lofgreen

and Warner, 1972 ;

Jenny

and

Polan, 1975).

The

present study

there-fore

investigated

the

relationship

between

plasma

insulin and the MMC in

sheep

fed on

either fresh cut grass,

hay,

or cereal

pellets.

Materials and methods

Animal and

surgical preparation

Five adult

Lacaune ewes

weighing

35 to

50

kg

were used. All animals had been

kept

at pasture and were

thoroughly

accustomed

to

handling.

Under

thiopental

sodium

(20

mg/

kg)

anaesthesia a 10 cm

long laparotomy

performed

on the

right

side

about

4 cm

posterior

to the

last

rib allowed access to

the

proximal jejunum.

Electrodes made of

insulated stainless-steel wires 120 pm in diameter and 1.5 m in

length (Grivel

and Ruckebusch,

1972)

were

implanted

on the

small intestine

(Ruckebusch,

1970)

in groups

of

three,

2 mm apart.

Groups

were

implanted

at 1 m intervals from 3 to 7 m from the ileo-caecal

junction.

The free ends of the elec-trodes were exteriorized on the

right

side.

Animals

were also

equipped

at surgery with

an

indwelling jugular

catheter.

Diets

Four animals received each of the

following

diets for successive

periods

of 20-22

days :

1)

fresh grass, cut

daily,

of medium

quality,

2) long hay

of

poor

quality,

3)

cereal

pellets (12.5 mm)

comprising

(g/kg

fresh

weight) ground

maize 400,

sharps

240, rolled

barley

184,

ground

lucerne 90, molasses

50,

urea 16, minerals

and

vitamins

20

(Magda pellets :

Les Fils d’Henri Vialars,

Toulouse)

together

with 0.1

kg/day

long hay.

The chemical

composition

of the diets is

given

in Table 1.

One animal failed to consume the cereal

pellets

and the electrical

recordings

from

another ewe were

disrupted

by

a

gastro-intestinal nematode infestation. Thus results

were

only obtained

from three of the

original

ewes for the

hay

diet and two for the cereal

pellets.

An additional animal

therefore

re-ceived

only

the cereal ration.

Experimental procedure

Sheep

were housed in metabolism cages

and fed ad libitum at 09.00 hours

(Zero

time)

each

day.

Uneaten food was removed at 17.00. Fresh water was available at all times.

Following

the recovery from surgery and

adaptation

to the fresh grass diet

(5-7

days),

the electrodes were

permanently

connected

to an e.e.g.

machine

(Reega

VIII, Alvar, 93107

Montreuil, RC

coupling,

time constant 0.3

sec).

Electrical

activity

from each site was

directly

recorded for several consecutive

hours on 2 or 3

days

each week. Summation

of the electrical

activity

from two groups of electrodes for each

subject

was

continuously

plotted

at 20 sec intervals

throughout

the

experiment, using

a double linear

integrator

circuit

(Latour, 1973)

connected to a

poten-tiometric recorder

(PM

8010,

Philips.

93002

Bobigny).

This

procedure

allowed an

(4)

dietary

period

were

separated

into

periods

of ISA,

RSA

and

quiescence.

Results were

discarded when an accurate

separation

of

MMC components was not

possible

through-out a

24

h

period (09.00-09.00 hours).

During

the final week of each

dietary

period jugular

blood

samples

were taken on

two non-consecutive

days

at 30 min intervals from 1 h before

feeding

until 9 h after

fee-ding,

with additional

samples

10 and 11 h

after

feeding

commenced.

Samples

(6 ml)

were

placed

in tubes

containing heparin.

Plasma was removed

by

centrifugation

at 4° and stored at -20° until

analysed.

Analytical

Plasma

glucose

concentrations were

me-sured

using

a Technicon

Auto-Analyser

(Technicon

Instruments

Corp.,

Tarrytown,

New

York)

and a modification of the method described

by

Hoffmann

(1937).

Plasma IRI

levels were estimated

by

a modification of the method of Bassett and Thorburn

(1971),

using

human insulin

(24.6

i.u./mg ;

Radio-chemical Centre, Amersham)

as standard

and

guinea pig

anti-bovine insulin serum

(Lot

GP3; Miles

Research Products,

Slough)

at a final dilution of 1:80.00. Antiserum and

samples

or standards were incubated at room

temperature

for 4 h

before

addition of 180 pg

’ 25

1-labelled insulin

(C.E.A&dquo;

91190 Gif sur

Yvette).

After further incubation for 20 h, free and

antibody-bound

hormone were se-parate

using

talc

(Bassett

and

Thorburn,

1971).

).

Calculations

Plasma IRI concentrations were converted

to

logarithms

before calculation of means

and statistical

computations (Bassett

and Thorburn,

1971).

Results

Electrical

spiking

activity

The ewes consumed

significantly

less

dry

matter when

fresh

cut grass was

given

than with the other two diets. The overall MMC

frequency

was, however, unaffected

by

diet

(Table

2)

and was not correlated with

dry

matter intake for any diet. The duration of the individual components of the MMC, ISA and RSA, were also unaffected

by

diet

when

values were

averaged

over 24 h

(Table 2).

The MMC recurred at intervals of 80-120 min

during

the

pre-feeding period (-8-0 h)

and

migrated along

the ileum at a mean rate of

178 ± 57 mm/min

(n

=

80), regardless

of

diet.

Eating

continued

intermittently

throughout

the

period

of access to food,

although

it

(5)
(6)

fresh grass or

hay

were

given, feeding

had

no effect on the

MMC pattern (Fig. 1)

or the

duration

of electrical

spiking

activity during

the

8 h

feeding period (Table 3). When

cereal

pellets

were

given,

electrical

spiking activity

was

significantly

increased

during

the

fee-ding period compared

to the

pre

-

feeding

and

post-feeding

periods (Table 3).

This

change

resulted from both shorter MMC

cycles, particularly

early

in the

feeding

period,

and

longer

periods

of ISA

(Fig. 1).

The duration of

spiking activity during

the

feeding

period

was similar for all diets,

but

in the

pre-feeding period

less

activity

was recorded when cereal

pellets

were

given

than

with grass

(P

<

0.01)

or

hay (P

<

0.001).

Spiking

activity

was also less in the

post-feeding

period

when the

sheep

received cereal

pellets

compared

to

hay (P

<

0.05).

Spiking

activity

during

the

feeding period

was not correlated with

dry

matter intake for

any diet.

Plasma

glucose

concentration

Plasma glucose

concentration before

fee-ding

was similar for all diets

(Fig. 2).

When food was offered

plasma

glucose

concentra-tions tended to rise ; levels were

significantly

higher

than the mean

pre-feeding

value at

0.5, 1.0 and 1:5 h when

hay

was

given

and at

0.5 and 1.0 h when fresh grass was

supplied

(P <

0.05 in all

cases).

Plasma insulin concentration

The

pre-feeding

plasma

IRI concentration

(7)
(8)

concentrations

displayed

considerable

varia-bility

during feeding (Fig. 2),

but differences from the mean

pre-feeding level

were not

significant

when the ewes received fresh grass or

hay

(Fig.

2).

When cereal

pellets

were

given,

IRI concentrations were

higher

at 2.5, 3.0, 4.0, 4.5, 5.0 and 5.5 h than before

feeding

(P

< 0.05 in all

cases)

and the mean

IRI concentration was also elevated

during

feeding (Table 4).

The net insulin response

to

feeding

was

significantly

greater

when the

sheep

received cereal

pellets

than for

hay

(Table

4).

Plasma IRI concentrations

during

the

feeding

period (IRI,

p.U/ml)

were

posi-tively

correlated

(P

<

0.001)

with

plasma

glucose

concentrations

(PG,

mmol/I)

when cereal

pellets

were

given.

The

relationship,

based on

eighty

observations, was :

log

lo

IRI = 0.902 ± 0.156 PG

(SE

of

regression

coefficient

0.033,

residual SD

0.207, R

2

0.225).

There was no

relationship

between IRI and PG when grass or

hay

was fed.

When

hay

was

given,

the mean

plasma

IRI

concentration

(IRI, ¡

L

U/ml)

was

linearly

rela-ted

(P

<

0.05)

to the

dry

matter intake

(DMI,

kg/day).

The

relationship,

based on six

observations, was :

log

io

IRI = 0.974 ± 0.359 DMI

(SE

of

regression

coefficient 0.112,

resi-dual SD 0.051,

R

2

0.70).

A similar trend was apparent when cereal

pellets

were

given,

but this was not

statistically

significant (0.1

> P >

0.05).

The net IRI response to

feeding

on cereal

pellets

was not related to either the duration of

spiking

activity

during

the

feeding period

or the difference in the duration of

spiking

between the

feeding

and

pre-feeding

periods.

Discussion

The MMC has been termed the

interdiges-tive

myoelectric complex

(Code

and

Marlett,

1975),

since in the

dog

it is

only

seen

during

fasting.

However, it is clear from our results that the MMC

pattern

is not

disrupted

in ruminants

by eating roughage

diets,

in

agree-ment with

previous

studies

using

adult

sheep

(Grivel

and

Ruckebusch, 1972)

and ruminant calves

(Ruckebusch

and

Bueno,

1973).

Even when the

sheep

received cereal

pellets,

changes

in electrical

activity

were

relatively

small, in

comparison

to the effects of a meal

in

dogs (Bueno

et at.,

1975 ;

Code and

Mar-lett,

1975).

The

relatively

poor food intake of the ewes

and the

long

period

of access to food may

have minimised the responses observed. Thus Ruckebusch and Bueno

(1976)

found

that the intestinal MMC

pattern

was

disrupted

for 6 h

postprandially

when

pigs

were fed

once

daily,

but it was

scarcely

modified at

all when the animals ate ad libitum,

consu-ming

small but

frequent

meals. However,

even at the

highest

levels of

dry

matter

intake achieved with the

roughage

diets,

0.8

kg/day

for grass and 1.2

kg/day

for

hay,

there was no indication of any diurnal

varia-tion in electrical

activity.

When

compared

to the other diets,

intes-tinal

spiking

activity

for the cereal

pellet

ration was decreased

during

the pre- and

post-feeding period.

This suggests that

fee-ding

relieved an inhibition of

spiking activity.

A

possible

inhibitory

factor is

enteroglucagon,

since a

patient

with a tumour

producing

this material

displayed

marked intestinal

hypo-motility.

The

postprandial

release of this hormone occurs later than the release of other

pancreatic

and intestinal hormones in both man and

pre-ruminant

calves

(Bloom

et

al., 1975).

This

glucagon-like

immuno-reactive material is very

heterogeneous

(Val-verde,

Dobbs and

Unger,

1975)

and it is not

known whether the fraction

affecting

intes-tinal

motility

is present in ruminants,

although plasma

levels of total intestinal

glucagon-like

immunoreactivity

are

higher

when cattle are fed on

barley

compared

to

hay.

The passage of

digesta along

the small

intestine of

sheep

mainly

occurs in

associa-tion with ISA,

particularly immediately

prior

to a

period

of RSA

(Bueno

et al.,

1975).

The increased

spiking

activity

and

changes

in

MMC

frequency. during

the

feeding

period

when the ewes received cereal

pellets

may

therefore indicate an increased flow of

di-gesta,

in

agreement

with the

findings

of

Thompson

and

Lamming

(1972).

These

au-thors

reported

that the flow of

digesta,

dry

matter and starch to the duodenum of

sheep

was

greatest

4-8 h after

feeding

a

daily

meal of

ground pelleted

maize and

chopped

or

ground barley

straw, but diurnal variations in

flow were much less when

long

straw was

fed.

The constancy of

plasma

IRI concentra-tions when fresh grass or

hay

was fed and the

hyperinsulinaemic

response to cereal

pellets

are in

agreement

with

previous

(9)

positively

related to the

proportions

of cereal

in the ration

(Lofgreen

and Warner, 1972 ;

Bhattacharya

and Alulu,

1975).

The

degree

of

hyperinsulinaemia

is however

generally

less than that found in

monogastric species

(Mutter

et al., 1970 ; Bueno and Ruckebusch,

1976).

Several factors may have contributed to the increased insulin secretion when cereal

pellets

were fed

(Bassett, 1975).

Although

the

quantities

of

propionate

and

butyrate

produced

in the rumen were

probably

grea-test when the

sheep

received cereal

pellets

(Bauman,

Davis and Bucholtz, 1971 ; Orskov,

Fraser and Gordon,

1974),

these metabolites

are

relatively

unimportant

regulators

of

in-sulin secretion

(Bassett,

1975; Weekes,

Bueno and Garcia-Villar,

1976).

A conside-rable

quantity

of the

ground

maize starch in the cereal

pellets probably

escaped

ruminal

digestion

(Orskov,

Fraser and

Kay,

1969).

The relation between

plasma

concentrations

of

glucose

and IRI

during

the

feeding period

when the

sheep

received cereal

pellets

sug-gests that

glucose

may

partially regulate

insulin release in

sheep

when considerable

intestinal starch

digestion

occurs. A strong

relationship

between

plasma

glucose

and IRI

concentrations was also

reported by Jenny,

Polan and

Thye (1974)

in blood

samples

taken 3-4 h after

feeding

a

high grain

ration to

lactating

dairy

cows. The mean intake of

crude

protein

when cereal

pellets

were fed

(0.15

kg/day)

was almost twice that achieved

on the other two diets and Bassett, Weston and

Hogan (1971)

found a close relation

bet-ween the amount of crude

protein

digested

in the small intestine and insulin levels in

sheep

receiving

a

variety

of diets. This

asso-ciation may have been mediated

by

the release of

gastrointestinal

hormones (Bassett,

1975).

The

identity

of the

gastrointestinal

hormone(s)

responsible

for insulin release is

uncertain

(see

Grossman,

1974),

but

gastric

inhibitory polypeptide

appears to be a

strong

candidate, at least in man

(Brown

et al.,

1975).

High

doses of exogenous insulin or

intra-venous infusions of volatile

fatty

acids pro-duce continuous

irregular spiking activity

in

the intestine of

sheep (Bueno

and

Rucke-busch,

1976).

The moderate

hyperinsuli-naemia

resulting

from

feeding

cereal

pellets

may therefore have removed the inhibition

of

spiking

activity

and mediated the

changes

in MMC

frequency during feeding.

However,

other

postprandial

responses may also have

contributed to the

changes

in electrical

spi-king

activity,

since there was no

quantitative

relationship

between the effects of

feeding

cereal

pellets

on

spiking

activity

and the net

insulin response to

feeding.

Since

vagotomy

reduces the duration of ISA in

sheep

(Bueno

and Ruckebusch,

1976),

a

vagal

reflex

me-chanism may be involved, similar to that

producing

a

rapid

hyperinsulinaemic

response

to

eating

in

sheep

(Bassett, 1975).

Gastrin

release has been

suggested

as a

partial

me-diator of the

postprandial changes

of

intes-tinal electrical

activity

in the

dog

(Weisbrodt

et al.,

1974)

and Bruce and Huber

(1973)

reported that

a hormone released

by

duo-denal modification stimulated intestinal

moti-lity

in

sheep.

This hormone was not

iden-tified, but

cholecystokinin

increases duodenal

motility

in cats

(Fara,

Rubinstein and

Son-nenschein,

1972).

Thus

gastrointestinal

hor-mones may both

participate

in the

hyper-insulinaemic response to

feeding sheep

on

cereal diets and,

together

with insulin, may

regulate

intestinal

myoelectric activity

when such diets are fed.

Summary

1. To determine the effects of diet on intestinal electrical

activity

and insulin secretion, five

sheep

were

given

diets of fresh cut grass,

hay,

and cereal

pellets

for successive

periods

of 20-22

days.

Food was available for 8 h

daily.

Continuous

electromyographical

recordings

were made from electrodes

placed

chronically

on the

jejuno-ileon.

Plasma

immunoreactive insulin

(IRI)

and

glucose

concentrations were measured before and

during

feeding.

2. The duration of electrical

spiking

activity

was increased

during

the

period

of

access

to

food when cereal

pellets

were fed, but not when the

sheep

received grass or

hay.

The

daily frequency

of the

migrating

myoelectric

complex

was unaffected

by

diet.

(10)

not

change during feeding

when fresh grass or

hay

were

given.

4. It is concluded that

postprandial changes

in intestinal electrical

activity

in the

sheep

depend

on the nature of the diet, but are less marked than in

monogastric

species.

Plasma IRI concentrations may

partially regulate

the pattern of electrical

activity.

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