Inhibition of Glucagon and Insulin Secretion by Somatostatin in the Rat Pancreas Perfused in Situ D. G. JOHNSON, J. W. ENSINCK, D. KOERKER, J. PALMER, AND C. J. GOODNER Department of Medicine and Department Washington 98195
of Physiology, University of Washington,
tin did not affect basal or glucose-stimulated secretion of insulin from rat pancreatic islets isolated by the collagenase technique. Arginine-stimulated secretion of insulin was enhanced by somatostatin in isolated islets. These results demonstrate a direct effect of somatostatin on the pancreas to inhibit secretion of glucagon and insulin. The failure of somatostatin to inhibit insulin secretion in pancreatic islets may be due to alterations in the beta cells produced by the isolation procedure. It is also possible that the effect of somatostatin on insulin secretion may be mediated indirectly. (Endocrinology 96: 370, 1975)
ABSTRACT. Perfusion of growth hormone inhibitory factor (somatostatin) into rat pancreas inhibited secretion of glucagon and insulin into medium containing 5.5 mM glucose. A 15-min infusion of arginine (20 mM) greatly increased glucagon and insulin secretion. When perfused simultaneously with arginine, somatostatin (55 nM) abolished the increase in glucagon secretion. The acute phase of insulin secretion in response to arginine was attenuated by somatostatin, and subsequent secretion was decreased to control levels. Pretreatment for 5 min with somatostatin blocked even acute-phase insulin secretion in response to arginine. Somatosta-
R
ECENTLY a hypothalamic factor that inhibits secretion of growth hormone (somatostatin or SRIF) has been isolated and characterized by Guillemin and coworkers (1-3). Studies in our laboratory demonstrated inhibition of both glucagon and insulin secretion during in vivo administration of somatostatin in the baboon, dog, and rat (4,5). Somatostatin has also been shown to suppress insulin secretion in man (6). The decrease in secretion of glucagon and insulin began rapidly and preceded any perceptible change in the blood glucose concentration. This suggested that somatostatin might act directly on the pancreas. Alberti et al. (6) reported that somatostatin inhibits glucose-induced insulin secretion from the perfused dog pancreas. Therefore, we investigated the effect of somatostatin on rat pancreas perfused in situ to determine if the inhibition of hormonal secretion occurred directly in this organ. Rat pancreatic islets were also studied to see if the effects of somatostatin on secretion of pancreatic alpha and beta cells could be demonstrated in vitro. Received May 8, 1974.
Seattle,
Materials and Methods Wistar male rats weighing 350-400 g were used in all studies. The pancreas perfusion was performed by a modification of the method described by Penhos et al. (7). The stomach was not isolated in our preparation, but the splenic vessels were ligated at the hilum to decrease the admixture of blood sequestered in this organ. Synthetic interstitial fluid (8) containing 0.3% bovine serum albumin (Pentex) and 4% dextran (Pharmacia) was infused at a constant rate of 4-5 ml per min in each experiment. Perfusion pressure was monitored continuously and varied from 50-80 mm Hg. Splanchnic organs and perfusion medium were kept at 37 C. All solutions were oxygenated with 95% O 2 -5% CO2. The effluent was collected in iced glass test tubes representing 1 min intervals. A 1 ml aliquot was taken from each sample and treated with 50 ju.1 of 1.0M benzamidine to prevent destruction of glucagon. The samples were frozen (—20 C) until assayed for insulin and glucagon. Insulin was measured by radioimmunoassay (9) using rat insulin standard. Glucagon was determined by radioimmunoassay and compared to standard curves obtained with porcine glucagon (10). Rat pancreatic islets were isolated by the collagenase technique of Lacy and Kostianovsky (11) as modified by Johnson et al. (9). For each experiment 80-100 islets from one animal were
370
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371
SOMATOSTATIN INHIBITION OF GLUCAGON put in a Millipore filter chamber and perifused at a constant rate of 0.7 ml per min with synthetic interstitial fluid containing 0.3% bovine serum albumin (12). One min samples were collected and assayed for glucagon and insulin as described earlier.
100-
Materials. Arginine hydrochloride was obtained from Aldrich Co., Milwaukee, Wisconsin. Somatostatin was kindly given to us by Drs. Roger Guillemin and Jean Rivier.
Results
2 5-
INSULIN Basal glucagon arid insulin secretion. All ---GLUCAGON pancreases were perfused with medium containing 5.5 mM glucose (100 mg/100 ml) 10 0 5 for 30 min to achieve stable secretion rates before addition of any other agents. When MIN 55 nM somatostatin (100 ng/ml) was added FIG. 1. Effect of somatostatin (55 nM) on secretion of to the perfused medium, there was a rapid insulin and glucagon by perfused rat pancreas. Rat inhibition of both glucagon and insulin pancreases were perfused as described in Materials secretion (Fig. 1). In some preparations the and Methods with medium containing 5.5 mM glucose for 30 min. Somatostatin was then added to the decrease in rate of hormonal secretion was medium and infused for 10 min. The rate of hormonal evident in the first minute following addi- secretion is plotted as percent mean control on the tion of somatostatin. After 10 min the ordinate. Brackets indicate standard error of 3 preparaconcentration of glucagon and insulin pres- tions. ent in the effluent were at the lower limit After 8 min glucagon secretion began to of sensitivity of our assay methods. decline rapidly, reaching basal levels by Arginine-stimulated hormonal secretion. the end of the 15-min infusion of arginine. Addition of arginine (20 mM) to the perfu- Addition of somatostatin (50 nM) simulsion medium produced an immediate large taneously with the arginine completely increase in glucagon secretion (Fig. 2). blocked the increase in glucagon secretion.
4.4 -, FIG. 2. Effect of somatostatin (SRIF) on arginine-stimulated secretion of glucagon. Somatostatin (55 nM) was added simultaneously with arginine (20 mM) and both agents infused for 15 min compared to arginine alone in control preparations. Brackets indicate standard error of 4 preparations.
3.6-
I
2.82.0.2.4- 8 - 4
0
4
8
12
16
20
24
28
MIN
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JOHNSON ETAL.
372
Encln • 1975 Vol 96 • No 2
iArg in ine]
5.04.0-
CONTROL — SRIF N = 4
3.02.01.0-
-8
-4
8 MIN
12
16
Arginine stimulated insulin secretion with a large initial increase followed by a gradual decline to basal levels (Fig. 3). Insulin secretion became maximal 3 min after starting the arginine infusion. The peak secretory rate was 20 times greater than basal values. When somatostatin was added concurrently, the acute release of insulin was partially blocked. Peak insulin secretory rate in the presence of somatostatin was approximately 5 times basal levels. TABLE 1. Insulin secretion from perifused rat pancreatic islets ;t • min.
Addition
Control
Somatostatin
n
Glucose:
8.35 niM 16.70 mM
1.0 ± 0.1 4.2 ± 0.4
1.1 ±0.1 4.8 ± 0.4
16 16
Arginine: and glucose:
20 5.5
4.3 ± 0.5
6.5 ± 0.4*
29
mM mM
* P < 0.01 vs corresponding control. Islets were isolated and perifused as described in Materials and Methods. In the glucose studies the glucose concentration of the medium was maintained for 40 min before addition of somatostatin (55 nM). One min samples were taken every 5 min for 20 min. In the arginine studies the islets were preincubated for 30 min in medium containing 5.5 mM glucose. Arginine (20 IUM) with or without somatostatin was added and 1 min samples collected for 10 min. "n" refers to number of samples in each group. Values represent mean average ± standard error of insulin secretory rate during indicated time intervals.
20
24
FIG. 3. Effect of somatostatin (SRIF) on arginine-stimulated secretion of insulin. Somatostatin (55 nM) was added simultaneously with arginine (20 HIM) and both agents infused for 15 min compared to arginine alone in control preparations. Brackets indicate standard error of 4 preparations.
28
After 4 min of arginine infusion, insulin secretion was inhibited by somatostatin to levels that were insignificantly different from basal secretion. Pretreatment for 5 min with somatostatin abolished even the initial increase in insulin secretion in response to arginine. Pancreatic islet secretion. Secretion of insulin by perifused islets was increased by either glucose (16.7 mM) or arginine (20 mM) (Table 1). Glucagon secretion decreased gradually to very low levels during the first 30 min of perifusion and did not respond to either glucose or arginine. When somatostatin was added, there was no change in the rate of insulin secretion during basal conditions or in response to glucose. The increase in insulin secretion in response to arginine was actually greater in somatostatin-treated preparations. Discussion These studies indicate that somatostatin acts directly in the rat pancreas to block glucagon and insulin secretion. Alberti et al. (6) have reported recently that somatostatin inhibited insulin secretion in the isolated perfused dog pancreas in concentrations as low as 1 ng/ml. Due to lack of sensitive assay methods, it is not known
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SOMATOSTATIN INHIBITION OF GLUCAGON whether somatostatin is present in plasma or peripheral organs such as the pancreas. Nevertheless, it is possible that somatostatin may play an important role in glucose homeostasis through its effects on pancreatic endocrine secretion. Somatostatin did not completely suppress the acute release of insulin in response to arginine given simultaneously. Alberti et al. (6) found only partial inhibition by somatostatin of acute-phase insulin secretion in response to glucose in 5 normal human subjects. The lesser effect of somatostatin in blocking the acute phase of insulin secretion could occur by several mechanisms. Unlike glucagon secretion, which is completely blocked by somatostatin, pancreatic beta cells may respond faster to glucose or arginine stimulation or slower to somatostatin inhibition. This explanation is consistent with the finding that pretreatment for 5 min with 55 nM somatostatin totally blocked insulin secretion. It is also possible that somatostatin affects beta cell secretion of insulin indirectly. Glucagon stimulates insulin secretion (13,14), so inhibition of glucagon secretion by somatostatin might decrease the release of insulin by pancreatic beta cells. Somatostatin could affect the secretion of insulin indirectly by an action on other pancreatic islet cells or exocrine tissue. Secretin has been shown to stimulate insulin secretion by a process involving pancreatic exocrine tissues (15-17). Somatostatin could inhibit insulin secretion by an analogous antagonistic mechanism. The speed with which somatostatin acts both in vivo (4-6) and in perfused pancreas suggests that it blocks the secretory process itself rather than acting primarily by inhibiting further synthesis of hormone. The quick reversal of inhibition after discontinuing infusion of somatostatin (4-6, personal data) implies a rapid clearance or metabolism of hormone at its sites of action. It also suggests that somatostatin does not seriously damage structural com-
373
ponents or enzymes important for hormonal secretion. Somatostatin may affect pancreatic endocrine secretion by a mechanism similar to its inhibition of growth hormone secretion by pituitary cells. The explanation for the lack of effect of somatostatin on insulin secretion from isolated pancreatic islets is unclear. Glucagon is not secreted normally in response to known stimuli, such as arginine, in this preparation. If the effect of somatostatin were mediated through its inhibition of glucagon secretion, insulin secretion would not be affected by somatostatin in isolated islets. Similarly, if somatostatin inhibited insulin secretion by affecting pancreatic exocrine cells, it would not act in the isolated pancreatic islet. It is also possible that the collagenase method used to isolate pancreatic islets destroys the ability of pancreatic alpha and beta cells to respond to somatostatin. Glucagon and growth hormone are two of the most important hormones for increasing plasma glucose concentration. Although substances such as arginine can induce secretion of both glucagon and growth hormone, the physiologic factors that regulate secretion of these two hormones in response to hypoglycemia remain unknown. Somatostatin appears unique in its ability to suppress secretion of both glucagon and growth hormone. Infusion of somatostatin in the baboon causes hypoglycemia (4,5), presumably as a consequence of the inhibition of glucagon and growth hormone secretion. The decrease in insulin secretion, which would tend to increase the glucose concentration, is apparently insufficient to reverse the effects on glucagon and growth hormone. This suggests that in vivo somatostatin may act primarily to lower the blood glucose concentration. Even if future investigation does not support a significant role for somatostatin in regulating pancreatic endocrine secretion, study of the mechanism
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JOHNSON ETAL.
374
of somatostatin inhibition may give useful insight into the secretory processes of endocrine cells.
6.
Addendum After submission of this manuscript, Efendic et al. (FEBS Lett 42: 1969, 1974) have reported a lack of effect of somatostatin on acutely isolated pancreatic islets.
7. 8. 9. 10.
References 1. Brazeau, P., W. Vale, R. Burgus, N. Ling, M. Butcher, J. Rivier, and R. Guillemin, Science 179: 77, 1973. 2. Siler, T. M., G. Vandenberg, S. S. C. Yen, P. Brazeau, W. Vale, and R. Guillemin, / Clin Endocrinol Metab 37: 632, 1973. 3. Brazeau, P., J. Rivier, W. Vale, and R. Guillemin, Endocrinology 94: 184, 1974. 4. Ruch, W., D. J. Koerker, M. Carino, S. D. Johnson, B. R. Webster, J. W. Ensinck, C. J. Goodner, and C. C. Gale, In Raiti, S. (ed.), Advances in Human Growth Hormone Research, U.S. Government Printing Office, Washington, D.C., 1973, p. 271. 5. Koerker, D. J., W. Ruch, E. Chideckel, J. Palmer,
11. 12. 13. 14. 15. 16. 17.
Endo • 1975 Vo! 96 • No 2
C. J. Goodner, J. W. Ensinck, and C. C. Gale, Science 184: 482, 1974. Alberti, K. G. M. M., N. J. Christensen, S. E. Christensen, A. P. Hansen, J. Iversen, K. Lundbaek, K. Seyer-Hansen, and H. 0rskov, Lancet 2: 1299, 1973. Penhos, J., C. H. Wu, J. Basabe, N. Lopez, and F. Wolff, Diabetes 18: 733, 1969. Bretag, A. H., Life Sci 8: 319, 1969.' Johnson, D. G., W. Y. Fujimoto, and R. H. Williams, Diabetes 22: 658, 1973. Ensinck, J. W., C. Shepard, R. J. Dudl, and R. H. Williams,/ Clin Endocrinol Metab 35: 463, 1972. Lacy, P. E., and M. Kostianovsky, Diabetes 16: 35, 1967. , M. M. Walker, and C. J. Fink, Diabetes 21: 987, 1972. Samols, E., G. Marri, and V. Marks, Lancet 2: 415, 1965. Crockford, P. M., D. Porte, Jr., F. C. Wood, and R. H. Williams, Metabolism 15: 114, 1966. Hinz, M., N. Katsilambros, B. Schweitzer, S. Raptis, and E. F. Pfeiffer, Diabetologia 7: 1, 1971. Goberna, R., R. D. Fussganger, S. Raptis, M. Telib, and E. F. Pfeiffer, Diabetologia 7: 68, 1971. Raptis, S., R. M. Rau, K. E. Schroder, W. Hartman, J. D. Faulhaber, P. H. Clodi, and E. F. Pfeiffer, Diabetologia 7: 160, 1971.
The Endocrine Society. Downloaded from press.endocrine.org by [${individualUser.displayName}] on 29 August 2015. at 21:28 For personal use only. No other uses without permission. . All rights reserved.
of Physiology, University of Washington,
tin did not affect basal or glucose-stimulated secretion of insulin from rat pancreatic islets isolated by the collagenase technique. Arginine-stimulated secretion of insulin was enhanced by somatostatin in isolated islets. These results demonstrate a direct effect of somatostatin on the pancreas to inhibit secretion of glucagon and insulin. The failure of somatostatin to inhibit insulin secretion in pancreatic islets may be due to alterations in the beta cells produced by the isolation procedure. It is also possible that the effect of somatostatin on insulin secretion may be mediated indirectly. (Endocrinology 96: 370, 1975)
ABSTRACT. Perfusion of growth hormone inhibitory factor (somatostatin) into rat pancreas inhibited secretion of glucagon and insulin into medium containing 5.5 mM glucose. A 15-min infusion of arginine (20 mM) greatly increased glucagon and insulin secretion. When perfused simultaneously with arginine, somatostatin (55 nM) abolished the increase in glucagon secretion. The acute phase of insulin secretion in response to arginine was attenuated by somatostatin, and subsequent secretion was decreased to control levels. Pretreatment for 5 min with somatostatin blocked even acute-phase insulin secretion in response to arginine. Somatosta-
R
ECENTLY a hypothalamic factor that inhibits secretion of growth hormone (somatostatin or SRIF) has been isolated and characterized by Guillemin and coworkers (1-3). Studies in our laboratory demonstrated inhibition of both glucagon and insulin secretion during in vivo administration of somatostatin in the baboon, dog, and rat (4,5). Somatostatin has also been shown to suppress insulin secretion in man (6). The decrease in secretion of glucagon and insulin began rapidly and preceded any perceptible change in the blood glucose concentration. This suggested that somatostatin might act directly on the pancreas. Alberti et al. (6) reported that somatostatin inhibits glucose-induced insulin secretion from the perfused dog pancreas. Therefore, we investigated the effect of somatostatin on rat pancreas perfused in situ to determine if the inhibition of hormonal secretion occurred directly in this organ. Rat pancreatic islets were also studied to see if the effects of somatostatin on secretion of pancreatic alpha and beta cells could be demonstrated in vitro. Received May 8, 1974.
Seattle,
Materials and Methods Wistar male rats weighing 350-400 g were used in all studies. The pancreas perfusion was performed by a modification of the method described by Penhos et al. (7). The stomach was not isolated in our preparation, but the splenic vessels were ligated at the hilum to decrease the admixture of blood sequestered in this organ. Synthetic interstitial fluid (8) containing 0.3% bovine serum albumin (Pentex) and 4% dextran (Pharmacia) was infused at a constant rate of 4-5 ml per min in each experiment. Perfusion pressure was monitored continuously and varied from 50-80 mm Hg. Splanchnic organs and perfusion medium were kept at 37 C. All solutions were oxygenated with 95% O 2 -5% CO2. The effluent was collected in iced glass test tubes representing 1 min intervals. A 1 ml aliquot was taken from each sample and treated with 50 ju.1 of 1.0M benzamidine to prevent destruction of glucagon. The samples were frozen (—20 C) until assayed for insulin and glucagon. Insulin was measured by radioimmunoassay (9) using rat insulin standard. Glucagon was determined by radioimmunoassay and compared to standard curves obtained with porcine glucagon (10). Rat pancreatic islets were isolated by the collagenase technique of Lacy and Kostianovsky (11) as modified by Johnson et al. (9). For each experiment 80-100 islets from one animal were
370
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371
SOMATOSTATIN INHIBITION OF GLUCAGON put in a Millipore filter chamber and perifused at a constant rate of 0.7 ml per min with synthetic interstitial fluid containing 0.3% bovine serum albumin (12). One min samples were collected and assayed for glucagon and insulin as described earlier.
100-
Materials. Arginine hydrochloride was obtained from Aldrich Co., Milwaukee, Wisconsin. Somatostatin was kindly given to us by Drs. Roger Guillemin and Jean Rivier.
Results
2 5-
INSULIN Basal glucagon arid insulin secretion. All ---GLUCAGON pancreases were perfused with medium containing 5.5 mM glucose (100 mg/100 ml) 10 0 5 for 30 min to achieve stable secretion rates before addition of any other agents. When MIN 55 nM somatostatin (100 ng/ml) was added FIG. 1. Effect of somatostatin (55 nM) on secretion of to the perfused medium, there was a rapid insulin and glucagon by perfused rat pancreas. Rat inhibition of both glucagon and insulin pancreases were perfused as described in Materials secretion (Fig. 1). In some preparations the and Methods with medium containing 5.5 mM glucose for 30 min. Somatostatin was then added to the decrease in rate of hormonal secretion was medium and infused for 10 min. The rate of hormonal evident in the first minute following addi- secretion is plotted as percent mean control on the tion of somatostatin. After 10 min the ordinate. Brackets indicate standard error of 3 preparaconcentration of glucagon and insulin pres- tions. ent in the effluent were at the lower limit After 8 min glucagon secretion began to of sensitivity of our assay methods. decline rapidly, reaching basal levels by Arginine-stimulated hormonal secretion. the end of the 15-min infusion of arginine. Addition of arginine (20 mM) to the perfu- Addition of somatostatin (50 nM) simulsion medium produced an immediate large taneously with the arginine completely increase in glucagon secretion (Fig. 2). blocked the increase in glucagon secretion.
4.4 -, FIG. 2. Effect of somatostatin (SRIF) on arginine-stimulated secretion of glucagon. Somatostatin (55 nM) was added simultaneously with arginine (20 mM) and both agents infused for 15 min compared to arginine alone in control preparations. Brackets indicate standard error of 4 preparations.
3.6-
I
2.82.0.2.4- 8 - 4
0
4
8
12
16
20
24
28
MIN
The Endocrine Society. Downloaded from press.endocrine.org by [${individualUser.displayName}] on 29 August 2015. at 21:28 For personal use only. No other uses without permission. . All rights reserved.
JOHNSON ETAL.
372
Encln • 1975 Vol 96 • No 2
iArg in ine]
5.04.0-
CONTROL — SRIF N = 4
3.02.01.0-
-8
-4
8 MIN
12
16
Arginine stimulated insulin secretion with a large initial increase followed by a gradual decline to basal levels (Fig. 3). Insulin secretion became maximal 3 min after starting the arginine infusion. The peak secretory rate was 20 times greater than basal values. When somatostatin was added concurrently, the acute release of insulin was partially blocked. Peak insulin secretory rate in the presence of somatostatin was approximately 5 times basal levels. TABLE 1. Insulin secretion from perifused rat pancreatic islets ;t • min.
Addition
Control
Somatostatin
n
Glucose:
8.35 niM 16.70 mM
1.0 ± 0.1 4.2 ± 0.4
1.1 ±0.1 4.8 ± 0.4
16 16
Arginine: and glucose:
20 5.5
4.3 ± 0.5
6.5 ± 0.4*
29
mM mM
* P < 0.01 vs corresponding control. Islets were isolated and perifused as described in Materials and Methods. In the glucose studies the glucose concentration of the medium was maintained for 40 min before addition of somatostatin (55 nM). One min samples were taken every 5 min for 20 min. In the arginine studies the islets were preincubated for 30 min in medium containing 5.5 mM glucose. Arginine (20 IUM) with or without somatostatin was added and 1 min samples collected for 10 min. "n" refers to number of samples in each group. Values represent mean average ± standard error of insulin secretory rate during indicated time intervals.
20
24
FIG. 3. Effect of somatostatin (SRIF) on arginine-stimulated secretion of insulin. Somatostatin (55 nM) was added simultaneously with arginine (20 HIM) and both agents infused for 15 min compared to arginine alone in control preparations. Brackets indicate standard error of 4 preparations.
28
After 4 min of arginine infusion, insulin secretion was inhibited by somatostatin to levels that were insignificantly different from basal secretion. Pretreatment for 5 min with somatostatin abolished even the initial increase in insulin secretion in response to arginine. Pancreatic islet secretion. Secretion of insulin by perifused islets was increased by either glucose (16.7 mM) or arginine (20 mM) (Table 1). Glucagon secretion decreased gradually to very low levels during the first 30 min of perifusion and did not respond to either glucose or arginine. When somatostatin was added, there was no change in the rate of insulin secretion during basal conditions or in response to glucose. The increase in insulin secretion in response to arginine was actually greater in somatostatin-treated preparations. Discussion These studies indicate that somatostatin acts directly in the rat pancreas to block glucagon and insulin secretion. Alberti et al. (6) have reported recently that somatostatin inhibited insulin secretion in the isolated perfused dog pancreas in concentrations as low as 1 ng/ml. Due to lack of sensitive assay methods, it is not known
The Endocrine Society. Downloaded from press.endocrine.org by [${individualUser.displayName}] on 29 August 2015. at 21:28 For personal use only. No other uses without permission. . All rights reserved.
SOMATOSTATIN INHIBITION OF GLUCAGON whether somatostatin is present in plasma or peripheral organs such as the pancreas. Nevertheless, it is possible that somatostatin may play an important role in glucose homeostasis through its effects on pancreatic endocrine secretion. Somatostatin did not completely suppress the acute release of insulin in response to arginine given simultaneously. Alberti et al. (6) found only partial inhibition by somatostatin of acute-phase insulin secretion in response to glucose in 5 normal human subjects. The lesser effect of somatostatin in blocking the acute phase of insulin secretion could occur by several mechanisms. Unlike glucagon secretion, which is completely blocked by somatostatin, pancreatic beta cells may respond faster to glucose or arginine stimulation or slower to somatostatin inhibition. This explanation is consistent with the finding that pretreatment for 5 min with 55 nM somatostatin totally blocked insulin secretion. It is also possible that somatostatin affects beta cell secretion of insulin indirectly. Glucagon stimulates insulin secretion (13,14), so inhibition of glucagon secretion by somatostatin might decrease the release of insulin by pancreatic beta cells. Somatostatin could affect the secretion of insulin indirectly by an action on other pancreatic islet cells or exocrine tissue. Secretin has been shown to stimulate insulin secretion by a process involving pancreatic exocrine tissues (15-17). Somatostatin could inhibit insulin secretion by an analogous antagonistic mechanism. The speed with which somatostatin acts both in vivo (4-6) and in perfused pancreas suggests that it blocks the secretory process itself rather than acting primarily by inhibiting further synthesis of hormone. The quick reversal of inhibition after discontinuing infusion of somatostatin (4-6, personal data) implies a rapid clearance or metabolism of hormone at its sites of action. It also suggests that somatostatin does not seriously damage structural com-
373
ponents or enzymes important for hormonal secretion. Somatostatin may affect pancreatic endocrine secretion by a mechanism similar to its inhibition of growth hormone secretion by pituitary cells. The explanation for the lack of effect of somatostatin on insulin secretion from isolated pancreatic islets is unclear. Glucagon is not secreted normally in response to known stimuli, such as arginine, in this preparation. If the effect of somatostatin were mediated through its inhibition of glucagon secretion, insulin secretion would not be affected by somatostatin in isolated islets. Similarly, if somatostatin inhibited insulin secretion by affecting pancreatic exocrine cells, it would not act in the isolated pancreatic islet. It is also possible that the collagenase method used to isolate pancreatic islets destroys the ability of pancreatic alpha and beta cells to respond to somatostatin. Glucagon and growth hormone are two of the most important hormones for increasing plasma glucose concentration. Although substances such as arginine can induce secretion of both glucagon and growth hormone, the physiologic factors that regulate secretion of these two hormones in response to hypoglycemia remain unknown. Somatostatin appears unique in its ability to suppress secretion of both glucagon and growth hormone. Infusion of somatostatin in the baboon causes hypoglycemia (4,5), presumably as a consequence of the inhibition of glucagon and growth hormone secretion. The decrease in insulin secretion, which would tend to increase the glucose concentration, is apparently insufficient to reverse the effects on glucagon and growth hormone. This suggests that in vivo somatostatin may act primarily to lower the blood glucose concentration. Even if future investigation does not support a significant role for somatostatin in regulating pancreatic endocrine secretion, study of the mechanism
The Endocrine Society. Downloaded from press.endocrine.org by [${individualUser.displayName}] on 29 August 2015. at 21:28 For personal use only. No other uses without permission. . All rights reserved.
JOHNSON ETAL.
374
of somatostatin inhibition may give useful insight into the secretory processes of endocrine cells.
6.
Addendum After submission of this manuscript, Efendic et al. (FEBS Lett 42: 1969, 1974) have reported a lack of effect of somatostatin on acutely isolated pancreatic islets.
7. 8. 9. 10.
References 1. Brazeau, P., W. Vale, R. Burgus, N. Ling, M. Butcher, J. Rivier, and R. Guillemin, Science 179: 77, 1973. 2. Siler, T. M., G. Vandenberg, S. S. C. Yen, P. Brazeau, W. Vale, and R. Guillemin, / Clin Endocrinol Metab 37: 632, 1973. 3. Brazeau, P., J. Rivier, W. Vale, and R. Guillemin, Endocrinology 94: 184, 1974. 4. Ruch, W., D. J. Koerker, M. Carino, S. D. Johnson, B. R. Webster, J. W. Ensinck, C. J. Goodner, and C. C. Gale, In Raiti, S. (ed.), Advances in Human Growth Hormone Research, U.S. Government Printing Office, Washington, D.C., 1973, p. 271. 5. Koerker, D. J., W. Ruch, E. Chideckel, J. Palmer,
11. 12. 13. 14. 15. 16. 17.
Endo • 1975 Vo! 96 • No 2
C. J. Goodner, J. W. Ensinck, and C. C. Gale, Science 184: 482, 1974. Alberti, K. G. M. M., N. J. Christensen, S. E. Christensen, A. P. Hansen, J. Iversen, K. Lundbaek, K. Seyer-Hansen, and H. 0rskov, Lancet 2: 1299, 1973. Penhos, J., C. H. Wu, J. Basabe, N. Lopez, and F. Wolff, Diabetes 18: 733, 1969. Bretag, A. H., Life Sci 8: 319, 1969.' Johnson, D. G., W. Y. Fujimoto, and R. H. Williams, Diabetes 22: 658, 1973. Ensinck, J. W., C. Shepard, R. J. Dudl, and R. H. Williams,/ Clin Endocrinol Metab 35: 463, 1972. Lacy, P. E., and M. Kostianovsky, Diabetes 16: 35, 1967. , M. M. Walker, and C. J. Fink, Diabetes 21: 987, 1972. Samols, E., G. Marri, and V. Marks, Lancet 2: 415, 1965. Crockford, P. M., D. Porte, Jr., F. C. Wood, and R. H. Williams, Metabolism 15: 114, 1966. Hinz, M., N. Katsilambros, B. Schweitzer, S. Raptis, and E. F. Pfeiffer, Diabetologia 7: 1, 1971. Goberna, R., R. D. Fussganger, S. Raptis, M. Telib, and E. F. Pfeiffer, Diabetologia 7: 68, 1971. Raptis, S., R. M. Rau, K. E. Schroder, W. Hartman, J. D. Faulhaber, P. H. Clodi, and E. F. Pfeiffer, Diabetologia 7: 160, 1971.
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