Article
Reference
Effect of insulin on glucose- and arginine-stimulated somatostatin secretion from the isolated perfused rat pancreas
GERBER, Pietro, et al.
Abstract
The effects of exogenous insulin on somatostatin secretion from the isolated perfused rat pancreas have been investigated in the presence of 5.6 mM glucose and when somatostatin secretion was stimulated by either glucose (16.7 mM) or arginine (20 mM). Insulin (15 mU/ml) significantly and rapidly suppressed glucose- and arginine-stimulated somatostatin release.
However, at 5.6 mM glucose and, in the absence of other stimulators of somatostatin release, insulin had no effect on the somatostatin secretion rate.
GERBER, Pietro, et al . Effect of insulin on glucose- and arginine-stimulated somatostatin secretion from the isolated perfused rat pancreas. Endocrinology , 1981, vol. 109, no. 1, p.
279-83
DOI : 10.1210/endo-109-1-279 PMID : 6113132
Available at:
http://archive-ouverte.unige.ch/unige:40540
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Copyright© 1981 by The Endocrine Society Printed in U.S.A.
Effect of Insulin on Glucose- and Arginine-Stimulated Somatostatin Secretion from the Isolated Perfused Rat Pancreas*
PIETRO P. G. GERBER,t ELISABETH R. TRIMBLE, CLAES B. WOLLHEIM, AND
ALBERT E. RENOLD
Institut de Biochimie Clinique, Universite de Geneve, Sentier de la Roseraie, 1211 Geneve 4, Switzerland
ABSTRACT. The effects of exogenous insulin on somatostatin secretion from the isolated perfused rat pancreas have been investigated in the presence of 5.6 mM glucose and when so- matostatin secretion was stimulated by either glucose (16.7 mM) or arginine (20 mM). Insulin (15 mU/ml) significantly and rapidly
T
HE PANCREATIC hormones, insulin, glucagon, and somatostatin, appear to play a role in control- ling nutrient homeostasis (1). In addition to the effects of pancreatic hormones on target tissues, an intraislet action of the hormones is possible. Thus, glucagon is known to stimulate the release of insulin (2-4) and pan- creatic somatostatin (5, 6), somatostatin to inhibit both insulin (7-9) and glucagon (10-13) secretion, and insulin to suppress glucagon secretion (14, 15). In these studies, higher concentrations of the hormones than those found in peripheral blood were employed. This may be neces- sary for the assessment of paracrine effects, since the concentrations of the hormones to which the pancreatic endocrine cells are exposed are not precisely known, but they are certainly higher than the circulating levels. The increased plasma concentrations of glucagon and so- matostatin in dogs with experimental insulinopenic dia- betes may suggest an inhibitory action of insulin on the pancreatic a-cells (16) similar to that exerted on the a-cells (16, 17). However, all efforts to demonstrate a direct action of insulin on somatostatin secretion from mam- malian pancreas preparations have so far given negative results (6, 18-20). In the present study, we investigated the effect of insulin on somatostatin release from the isolated perfused rat pancreas under basal conditions and during stimulation with glucose or arginine.
Received May 6, 1980.
• This work was supported by Research Grants 3,744-0.76 SR and 3.487-0.79 SR from the Swiss National Science Foundation.
t To whom requests for reprints should be addressed.
279
suppressed glucose- and arginine-stimulated somatostatin release. However, at 5.6 mM glucose and, in the absence of other stimulators of somatostatin release, insulin had no effect on the somatostatin secretion rate. (Endocrinology 109: 279, 1981)
Materials and Methods
;
The isolated pancreas was prepared from overnight fasted male Wistar rats, weighing 200-250 g, by the method of Penhos et al. (21) with minor modifications. The perfusate was modified Krebs-Ringer bicarbonate buffer containing 1 mM Ca ++, 40 mg/
ml dextran, 2 mg/ml human serum albumin, and 400 kiU/ml aprotinin. The buffer was continuously gassed with a mixture of 95% 02 and 5% C02; the partial pressure of oxygen in the buffer entering the pancreas was 450 mm Hg, and the perfusion rate was 4 ml/min. Previous studies (22) have shown that steady basal concentrations of somatostatin are achieved in the effluent after the first 20 min of perfusion. Baseline samples were accordingly obtained 28, 29, and 30 min after starting the perfusion (designated -2, -1, and 0 min). The experimental variables were introduced at 0 min (i.e. 30 min after the perfu- sion had begun) according to protocols that are explained in the text. The somatostatin RIA used has been described in detail previously (22); its sensitivity was 15-20 pg/ml. Statistical analysis was made by Student's t test.
The following substances were used: Dextran T-40 (Phar- macia Fine Chemicals AB, Uppsala, Sweden); human serum albumin (Swiss Red Cross, Bern, Switzerland); aprotinin (Tra- sylol) kindly provided by Prof. G. L. Haberland (Bayer A. G., Wuppertal, West Germany); cyclic somatostatin and N-tyro- sylated somatostatin for standard and iodinated tracer, respec- tively (Serono, Freiburg, West Germany); somatostatin antise- rum, a gift from Dr. J. Ardill (Queen's University, Belfast, United Kingdom); and pork insulin (Actrapid; Novo Industri A/S, Copenhagen, Denmark).
Results
Throughout the experiments shown in Fig. 1, the glu- cose concentration was kept constant at 5.6 IllM. After ~0
280 GERBER ET AL. Endo • 1981
n = 6,
x
:t SEMGLUCOSE 5.6mM Insulin
SRIF (pg/min)
800
600
400
200
0
-10 0 10 20 30 40 50
minutes
FIG. 1. Effect of exogenous insulin (15 mU /ml) on somatostatin (SRIF) secretion from the isolated perfused rat pancreas in the presence of 5.6 mM glucose. The addition of insulin (0-30 min) was without effect on the SRIF secretion rate.
min of equilibration and the recording of three baseline somatostatin secretion rates (-2, -1, and 0 min), insulin was infused for 30 min (0-30 min) at a concentration of 15 mU/ml (0.63 ILg/ml). Sampling continued for 20 min after insulin was withdrawn (30-50 min). The somatosta- tin secretion rate did not vary significantly from the initial basal rate, either during the period of insulin infusion or subsequently. At this nonstimulatory glucose concentration, therefore, somatostatin secretion was not affected by insulin.
The next experiments were designed to evaluate fur- ther whether insulin affects somatostatin secretion by adding insulin after first inducing stimulation of somato- statin secretion by glucose or arginine. The basal perfu- sate contained 5.6 mM glucose (-30 to 0 min). Somato- statin secretion was then stimulated, either by increasing glucose to 16.7 mM or by adding 20 mM arginine (in the presence of 5.6 mM glucose) during the period from 0-40 min. After 20 min of stimulation with either glucose or arginine, insulin was infused into a side-arm to give a final concentration of 15 mU/ml insulin in the perfusate during the second half of the stimulatory period (20-40 min) without modifying the concentration of any other constituent. As shown in Fig. 2, 16.7 mM glucose caused
Vol109 • No I
n = 7,
x
± SEMGLUCOSE 5.6mM ARGININE 20 mM
SRIF
Insulin(pg/min) 600
500
400
300
200
100+---~---.----~----~--~
-10 0 10 20 30 40
minutes
FIG. 2. Effect of exogenous insulin (15 mU/ml) on glucose-stimulated somatostatin (SRIF) secretion from the isolated perfused pancreas.
The glucose concentration was 5.6 mM from -30 to 0 min and 16.7 mM from 0 min to the end of the experiment. Insulin was added from 20-40 min. SRIF secretion was significantly stimulated by 16.7 mM glucose (P < 0.02 at all time points, compared with the basal secretion rate).
The effect of insulin on stimulated SRIF release was significant at 22, 26, 28, and 40 min (P < 0.05) and at 23, 24, 27, and 30 min (P < 0.025).
a significant increase in the somatostatin secretion rate at all time points at which it was measured. The subse- quent addition of insulin resulted in an inhibition of this glucose-induced somatostatin release. At 8 of 12 time points during the 20 min of insulin infusion, the somato- statin secretion rate was significantly less than the mean somatostatin secretion rate for the previous 20 min when 16.7 mM glucose was infused in the absence of insulin.
Furthermore, the somatostatin secretion rate during in- sulin infusion with 16.7 mM glucose never significantly exceeded that during the initial baseline period when 5.6 mM glucose was infused alone.
Arginine (20 mM) added to 5.6 mM glucose resulted in an increase in somatostatin secretion (Fig. 3). The further
addition of insulin caused a rapid inhibition of somato- statin secretion, so that the secretion rate during insulin infusion was significantly less than the mean secretion rate during the previous 20 min of arginine infusion at all time points measured. In control experiments where 16.7 mM glucose or 20 mM arginine plus 5.6 mM glucose was infused for the period from 0-40 min (without the addi- tion of insulin at 20-40 min), the somatostatin secretion rate during the second 20-min period of stimulation (20- 40 min) was similar to that during the first 20-min period (0-20 min; Table 1). In addition, the results of all the experiments have been expressed as integrated incremen- tal somatostatin secretion for the period from 0-40 min (Table 2). In control experiments, when glucose was infused for 40 min without insulin, the integrated incre- mental somatostatin secretion from 0-20 min was similar to that for the subsequent 20-40 min (Table 2A). By contrast, in the experiments where insulin was added during the period from 20-40 min, the integrated incre- mental somatostatin secretion was markedly reduced compared to that during the 0-20 min period of the same experiment (P < 0.05, paired statistics) and compared to
G 5.6 mM
SRIF
(pg/min)
400
300
200
100
n
=
6,x
:t SEMGLUCOSE 16.7 mM Insulin
04---+----.---r----.---+----.
-10 0 10 20
minutes
30 40
so
FIG. 3. Effect of exogenous insulin (15 mU/ml) on arginine-stimulated somatostatin (SRIF) secretion from the isolated perfused pancreas.
Arginine stimulated SRIF secretion (P < 0.005 at 1 min and P < 0.05 at 3, 5, 6, 7, 10 and 20 min, compared with the basal secretion rate).
The effect of insulin on stimulated SRIF release was significant at all time points (P < 0.02). G, Glucose.
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282 GERBER ET AL. Endo • 1981 Vol109 • No I
TABLE 2. Integrated incremental somatostatin secretion during stim- ulation with 16.7 mM glucose or 20 mM arginine in the presence or absence of insulin during the second part of the stimulatory period (picograms per 20 min; mean ± SEM)
0-20 min p 20-40 min A. Glucose (16.7 mM) without 2293 ± 353 NS 3318 ± 454
insulin
p NS <0.001
B. Glucose (16.7 mM) with in- 2283 ± 479 <0.05 185 ± 661 sulin (20-40 min)
C. Arginine (20 mM) without 3708 ± 776 NS 2854 ± 497 insulin
p NS <0.005
D. Arginine (20 mM) with insu- 2064 ± 678 <0.01 -746 ± 413 lin (20-40 mM)
A + C, n = 4; B, n = 6; D, n = 7.
that during the 20- to 40-min period of the control experiments carried out in the absence of insulin (P <
0.001, unpaired statistics). Similarly, when insulin was introduced from 20-40 min in the arginine-stimulated pancreas, there was a marked reduction in the integrated incremental somatostatin secretion for this period com- pared with that during 0-20 min in the same experiment
(P < 0.01) or that during 20-40 min in control experi-
ments (P < 0.005; see Table 2, C and D).
Discussion
The results presented here demonstrate that somato- statin secretion stimulated by either glucose or arginine can be inhibited by an increase in the insulin concentra- tion surrounding the pancreatic A-cells. Basal somatosta- tin release (i.e. 5.6 mM glucose without added stimuli) was unaffected by insulin. This finding under basal con- ditions confirms previous work in the rat (18, 19) and dog (6).
With respect to the inhibition by insulin of glucose- stimulated somatostatin release, our results appear at variance with those of Hermansen et al. (20), who failed to observe any effect of 25 mU/ml insulin on somatosta- tin release. The reason for this difference is unclear; apart from the species difference, the composition of the per- fusate was also different (20). In the isolated perfused chicken pancreas, Honey and Weir (23) observed a stim- ulation of somatostatin secretion by 20 mU/ml insulin.
While this finding contrasts markedly with the results reported here, the interaction between hormones in the pancreas of birds may be different from that occurring in mammals. Indeed, in the duck, the infusion of somato- statin in vivo resulted in a stimulation of glucagon secre- tion rather than the expected inhibition (24).
The use of high concentrations of pancreatic hormones is necessary when investigating their possible paracrine effects. When insulin secretion was stimulated by 16.7 mM glucose, the insulin concentration measured in the
mixed effluent of the isolated perfused pancreas prepa- ration was 1 mU/ml (22). This concentration can be increased by a factor of 10 under maximal stimulatory conditions. It appears reasonable, therefore, to use a concentration of insulin large enough to permit the A- cells to recognize an increase.
Suppression of pancreatic somatostatin secretion by insulin under physiological conditions may explain the findings of increased somatostatin secretion in experi- mental insulinopenic diabetes in dogs (16) and rats (25).
In addition, insulin treatment of alloxan-diabetic dogs partially normalized the increased somatostatin and glu- cagon levels measured in peripheral blood (16). In acute in vivo experiments, insulin infused iv inhibited meal- stimulated pancreatic somatostatin secretion; it also in- hibited stimulated somatostatin release from the gastric antrum but not from the fundus (26). However, it is not possible to determine from in vivo studies whether in- sulin has a direct effect on the pancreatic A-cell. In in vitro experiments using the perfused rat stomach, 5.6 mM "' glucose, after a delay, caused a stimulation of somatosta- tin secretion, which could be prevented by simultaneous insulin infusion (27).
In conclusion, the results presented here suggest that insulin can exert a direct inhibitory influence on somato- statin secretion under certain conditions. It would seem that each of the three principal hormones of the pan- creatic islet, insulin, glucagon, and somatostatin, may affect the secretion of the other two within the pancreatic islet. The concentration of one of the hormones in the pancreatic effluent may thus depend on the secretory activity of the other hormone-secreting cells in the islets.
With regard to pancreatic polypeptide, knowledge is as yet insufficient to show whether it may be similarly involved in reciprocal paracrine regulation of pancreatic hormone secretion.
Acknowledgment
The authors acknowledge with gratitude the expert technical assist- ance of Jarmila Slesinger.
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