EXPERIMENTAL NUTRITION
EFFECT OF SOMATOSTATIN ON EXOCRINE PANCREATIC FUNCTION Somatostatin can inhibit the stimulation of pancreatic bicarbonate and protein secretion by foodstuff in the gut releasing endogenous secretin and cholecystokinin. Stimulation by exogenous secretin is also blocked by somatostatin but that by exogenous cholecystokinin is not. Key Words: somatostatin, growth hormone-release inhibiting hormone (GH-RIH), pancreatic secretion, cholecystokinin, cyclic AMP, cyclic GMP, pancreatic fistula
New hormones often gather several names before the ensuing confusion results in one, not necessarily the most apt, being chosen as the official appelation. Somatostatin, otherwise known as growth hormone-release inhibiting hormone or GH-RIH for short, is no exception. Somatostatin does much more than inhibit the release of growth hormone from the adenohypophysis. When the current sometimes frenetic interest in this peptide is seen in historical perspective it may acquire a name different from any of those currently in use. At present somatostatin is known to inhibit the secretion of anterior pituitary1 and pancreatic hormones.* It is also of gastroenterological interest since it blocks the secretion of gastrin3 and suppresses gastric acid and pepsin secretion.4 The presence of cross reactivity to somatostatin antibody by extracts of stomach, duodenum and pancreas5 raises the possibility that in somatostatin one may be dealing with an inhibitory peptide of multiple sites of origin and a general rather than a specific biological role implied by its name. The gastrointestinal facet of somatostatin physiology stimulated Konturek and his colleagues6 to investigate the effect of somatostatin on exocrine pancreatic secretion provoked by endogenously secreted or exogenously administered secretin and cholecystokinin. The model was the conscious dog prepared with chronic gastric and pancreatic fistulae. Each fistula was cannulated and the tip 210 NUTRITION REVIEWS I VOL. 35, NO. 8, I AUGUST 1977
of the pancreatic cannula was sited in the duodenum approximately 15 cm distal to the pylorus. Each of the four dogs was given water but no food for 18 hours before the start of the experiments. In each experiment the dog was given a constant rate infusion of physiological saline at 60 ml per hour via a fine tube threaded through the duodenal cannula and via a peripheralvein. The infusions were started after confirmation that basal pancreatic secretion was less than 1.5 ml in each of two consecutive 15 minute samples. During the experiment pancreatic juice was collected continuously and divided into 15 minute samples which were analyzed for volume, bicarbonate and protein content. Each experiment was performed twice on each animal. The results were presented as the mean of eight observations. The authors overlooked the paired nature of their experimental results when subjecting them to statistical analysis. The first set of experiments involved the intravenous infusion of secretin at a range of doses from 0.5 to 4.0 LI per kilogram per hour. An infusion rate of 2.0 U produced near maximum bicarbonate secretion in control animals which was sustained from the end of the first hour until the experiment finished two hours later. Secretin had a slight but not significant stimulatory effect on protein secretion. When somatostatin was added to the intravenous infusion at the rate of 2.5 p g per kilogram per hour from 45 to 105 minutes, it caused a prompt inhibition of bicarbonate release which reached a nadir in the second half of the somatostatin infusion. As soon as this was discontinued bicarbonate release rose rapidly and a maxi-
mum secretion rate was resumed in 30 minutes. A constant somatostatin infusion was tested against secretin stimulation rates of 0.5, 1.0, 2.0 and 4.0 U per kilogram per hour. The degree of inhibition decreased as the secretin dose increased, the absolute amount of inhibition remaining relatively constant. Analysis of the responses in a manner analogous to those of Michaelis-Menten kinetics showed good agreement between the two infusion types, secretin alone or secretin plus somatostatin insofar as the calculated mean maximum response for bicarbonate secretion was 4.81 and 4.89 mEq per 15 minutes respectively. The mean dose of secretin required to produce a half maximum response was 1.92 U per kilogram per hour and of secretin plus somatostatin, 4.58 U secretin per kilogram per hour. The intraduodenal infusion of hydrochloric acid at a rate of 8 mEq per hour stimulated endogenous secretin release, and provoked bicarbonate secretion at a rate similar to that in response to the 20 U secretin infusion which was also blocked by intravenous somatostatin. The pancreatic protein response to duodenal acidification was twice that caused by exogenous secretin. This stimulation was also completely abolished by somatostatin. The synthetic octapeptide of cholecystokinin (OP-CCK) was infused intravenously at a rate of 0.5 p g per kilogram per hour which was known to cause near maximum stimulation of pancreatic enzyme secretion.’ In response to this signal the duodenal fluid protein content rose to a peak during the first hour which was approximately double resting levels and sustained for the remainder of the experiment. When a somatostatin infusion was added there was no inhibition of OP-CCK stimulation. Pancreatic bicarbonate secretion was not affected by OP-CCK with or without somatostatin. Stimulation of both pancreatic protein and bicarbonate secretion was achieved by the instillation of 300 ml of a liver extract into the stomach. The meal consisted of a 10 percent aqueous solution of liver concentrate powder adjusted to pH 5 with hydrochloric acid. The intragastric pH was also kept at the same level by titration while the meal remained in place for 210 minutes. The pancreatic response to intragastric liver was to secrete protein at a rate
similar to that in response to OP-CCK and a significant but small increase in bicarbonate secretion which was equivalent to 10 percent of the maximal response to exogenous secretin. Somatostatin was given intravenously for one hour from 45 to 115 minutes at dose rates of 1.25, 2.5 and 5.0 pg per kilogram and caused a dose related inhibition of both protein and bicarbonate secretion. When the dogs were given a liver meal there was a quantitatively greater response in both protein and bicarbonate release each of which was inhibited by somatostatin. Duodenalperfusionwith a mixture of L-tryptophan and L-phenylalanine at the rate of 12 mmole per hour or with sodium oleate at 8 mmole per hour each produced a large protein and small bicarbonate response from the pancreas. Somatostatin blocked both bicarbonate and protein responses. These experiments demonstrated that somatostatin inhibits exocrine pancreatic secretion stimulated in a wide variety of ways, but what is the mechanism of action? The infusion of two gastrointestinal hormones secretin and OP-CCK, as well as stimulation of endogenous secretin and OP-CCK release by intragastric and intraduodenal nutrient, helped to answer this question. For example somatostatin blocked the pancreatic response to endogenously secreted CCK but not to an exogenous OP-CCK infusion. This indicated that the action of somatostatin is on the CCK secreting cell and not the pancreas. In the case of secretin the situation is less clear. Somatostatin and secretin interact on the pancreas with the characteristics of competitive inhibition, suggesting but not proving that the two molecules might act at a single effector site. Since the effects of somatostatin on endogenous secretin release were measured by a pancreatic end point no statement can be made regarding a direct effect of somatostatin on secretin secretion. Other authors8 showed inhibition of secretin release by somatostatin but much greater somatostatin concentrations were used to achieve the effect. An interesting difference between secretin and CCK induced pancreatic secretion is that the former appears to be mediated by cyclic AMP and the latter by cyclic GMP.9 It is posNUTRITION REVIEWS / VOL. 35, NO. 8 / AUGUST 1977 211
sible that somatostatin may influence cyclic AMP but not GMP generation. This may explain some of the differences in its effect on the two patterns of stimulated pancreatic secretion. Experiments in which analogues of the cyclic nucleotides were perfused through the pancreas or celiac axis, and the effect of somatostatin on the nucleotide-stimulated pancreatic enzyme or gut hormone secretion were studied, might be helpful in unravelling this point. That somatostatin probably plays a part in the physiologicalcontrol of pancreatic exocrine secretion is indicated by the profound inhibition by somatostatin of protein and bicarbonate secretion in response to feeding or the intragastric and intraduodenal instillation of nutrient which causes both secretin and CCK release. This observation, coupled with the preliminary evidence of a somatostatin-like substance in the stomach and duodenum, suggests that our thoughts on the local hormonal control of exocrine pancreatic function should be extended to include an inhibitor which is somatostatin or a closely related peptide. In the course of time it will be interesting to see what definitive name it is given. 0
3.
4.
5.
6.
7.
8. 1. P. Brazeau, W. Vale, R. Burgus, N. Ling, M. Butcher, J. Rivier and R. Guillemin: Hypothalamic Polypeptide that Inhibits the Secretion of Immunoreactive Pituitary Growth Hormone. Science 179: 77-79, 1973 2. C.H. Mortimer, W.M.G. Tunbridge, D. Carr, L. Yeomans, T. Lind, D.H. Coy, S.R. Bloom, A.
212 NUTRITION REVIEWS / VOL. 35, NO. 8, /AUGUST 1977
9.
Kastin, C.N. Mallinson, G.M. Besser, A.V. Schally and R. Hall: Effects of Growth-Hormone Release-Inhibiting Hormone on Circulating Glucagon, Insulin, and Growth Hormone in Normal, Diabetic,Acromegalic and Hypopituitary Patients. Lancet I: 697-701, 1974 S.R. Bloom, C.H. Mortimer, M.O. Thorner, G.M. Besser, R. Hall, A. Gomez-Pan, V.M. Roy, R.C. G. Russell, D.H. Coy, A.J. Kastin and A.V. Schally: Inhibition of Gastrin and Gastric-Acid Secretion by Growth-Hormone Release Inhibiting Hormone. Lancet 11: 1 106-1109, 1974 A.A.J. Barros D’Sa, S.R. Bloom and J.H. Baron: Direct Inhibition of Gastric Acid by GrowthHormone Release-Inhibiting Hormone in Dogs. Lancet I: 886-887, 1975 T. Hokfeld, S. Efendic, C. Hellestrom, 0. Johansson, R. Luft and A. Arimura: Cellular Localization of Somatostatin in Endocrine-Like Cells and Neurons of the Rat with Special Reference to the A, Cells of the Pancreas Islets and to the Hypothalamus. Acta Endocrinol. (Suppl.) 80: 1-41, 1975 S.J. Konturek,J. Tasler, W. Obtulowicz, D.H. Coy and A.V. Schally. Effect of Growth-Hormone-Release Inhibiting Hormone on Hormones Stimulating Exocrine Pancreatic Secretion. J. CIin. Invest. 58: 11-16, 1976 H.T. Debas and M.I. Grossman: Pure Cholecystokinin: Pancreatic Protein and Bicarbonate Response. Digestion 9: 469-481, 1973 G. Boden, M.C. Sivitz, O.E. Owen, N. EssaKoumar and J.H. Landor: Somatostatin Suppresses Secretin and Pancreatic Exocrine and Secretion. Science 190: 163-164, 1975 P. Robbercht in Stimulus Secretion Coupling in the Gastrointestinal Tract. R.M. Case and H. Goebell, Editors, p. 203. Baltimore, Llniversity Park Press, 1976
EFFECT OF SOMATOSTATIN ON EXOCRINE PANCREATIC FUNCTION Somatostatin can inhibit the stimulation of pancreatic bicarbonate and protein secretion by foodstuff in the gut releasing endogenous secretin and cholecystokinin. Stimulation by exogenous secretin is also blocked by somatostatin but that by exogenous cholecystokinin is not. Key Words: somatostatin, growth hormone-release inhibiting hormone (GH-RIH), pancreatic secretion, cholecystokinin, cyclic AMP, cyclic GMP, pancreatic fistula
New hormones often gather several names before the ensuing confusion results in one, not necessarily the most apt, being chosen as the official appelation. Somatostatin, otherwise known as growth hormone-release inhibiting hormone or GH-RIH for short, is no exception. Somatostatin does much more than inhibit the release of growth hormone from the adenohypophysis. When the current sometimes frenetic interest in this peptide is seen in historical perspective it may acquire a name different from any of those currently in use. At present somatostatin is known to inhibit the secretion of anterior pituitary1 and pancreatic hormones.* It is also of gastroenterological interest since it blocks the secretion of gastrin3 and suppresses gastric acid and pepsin secretion.4 The presence of cross reactivity to somatostatin antibody by extracts of stomach, duodenum and pancreas5 raises the possibility that in somatostatin one may be dealing with an inhibitory peptide of multiple sites of origin and a general rather than a specific biological role implied by its name. The gastrointestinal facet of somatostatin physiology stimulated Konturek and his colleagues6 to investigate the effect of somatostatin on exocrine pancreatic secretion provoked by endogenously secreted or exogenously administered secretin and cholecystokinin. The model was the conscious dog prepared with chronic gastric and pancreatic fistulae. Each fistula was cannulated and the tip 210 NUTRITION REVIEWS I VOL. 35, NO. 8, I AUGUST 1977
of the pancreatic cannula was sited in the duodenum approximately 15 cm distal to the pylorus. Each of the four dogs was given water but no food for 18 hours before the start of the experiments. In each experiment the dog was given a constant rate infusion of physiological saline at 60 ml per hour via a fine tube threaded through the duodenal cannula and via a peripheralvein. The infusions were started after confirmation that basal pancreatic secretion was less than 1.5 ml in each of two consecutive 15 minute samples. During the experiment pancreatic juice was collected continuously and divided into 15 minute samples which were analyzed for volume, bicarbonate and protein content. Each experiment was performed twice on each animal. The results were presented as the mean of eight observations. The authors overlooked the paired nature of their experimental results when subjecting them to statistical analysis. The first set of experiments involved the intravenous infusion of secretin at a range of doses from 0.5 to 4.0 LI per kilogram per hour. An infusion rate of 2.0 U produced near maximum bicarbonate secretion in control animals which was sustained from the end of the first hour until the experiment finished two hours later. Secretin had a slight but not significant stimulatory effect on protein secretion. When somatostatin was added to the intravenous infusion at the rate of 2.5 p g per kilogram per hour from 45 to 105 minutes, it caused a prompt inhibition of bicarbonate release which reached a nadir in the second half of the somatostatin infusion. As soon as this was discontinued bicarbonate release rose rapidly and a maxi-
mum secretion rate was resumed in 30 minutes. A constant somatostatin infusion was tested against secretin stimulation rates of 0.5, 1.0, 2.0 and 4.0 U per kilogram per hour. The degree of inhibition decreased as the secretin dose increased, the absolute amount of inhibition remaining relatively constant. Analysis of the responses in a manner analogous to those of Michaelis-Menten kinetics showed good agreement between the two infusion types, secretin alone or secretin plus somatostatin insofar as the calculated mean maximum response for bicarbonate secretion was 4.81 and 4.89 mEq per 15 minutes respectively. The mean dose of secretin required to produce a half maximum response was 1.92 U per kilogram per hour and of secretin plus somatostatin, 4.58 U secretin per kilogram per hour. The intraduodenal infusion of hydrochloric acid at a rate of 8 mEq per hour stimulated endogenous secretin release, and provoked bicarbonate secretion at a rate similar to that in response to the 20 U secretin infusion which was also blocked by intravenous somatostatin. The pancreatic protein response to duodenal acidification was twice that caused by exogenous secretin. This stimulation was also completely abolished by somatostatin. The synthetic octapeptide of cholecystokinin (OP-CCK) was infused intravenously at a rate of 0.5 p g per kilogram per hour which was known to cause near maximum stimulation of pancreatic enzyme secretion.’ In response to this signal the duodenal fluid protein content rose to a peak during the first hour which was approximately double resting levels and sustained for the remainder of the experiment. When a somatostatin infusion was added there was no inhibition of OP-CCK stimulation. Pancreatic bicarbonate secretion was not affected by OP-CCK with or without somatostatin. Stimulation of both pancreatic protein and bicarbonate secretion was achieved by the instillation of 300 ml of a liver extract into the stomach. The meal consisted of a 10 percent aqueous solution of liver concentrate powder adjusted to pH 5 with hydrochloric acid. The intragastric pH was also kept at the same level by titration while the meal remained in place for 210 minutes. The pancreatic response to intragastric liver was to secrete protein at a rate
similar to that in response to OP-CCK and a significant but small increase in bicarbonate secretion which was equivalent to 10 percent of the maximal response to exogenous secretin. Somatostatin was given intravenously for one hour from 45 to 115 minutes at dose rates of 1.25, 2.5 and 5.0 pg per kilogram and caused a dose related inhibition of both protein and bicarbonate secretion. When the dogs were given a liver meal there was a quantitatively greater response in both protein and bicarbonate release each of which was inhibited by somatostatin. Duodenalperfusionwith a mixture of L-tryptophan and L-phenylalanine at the rate of 12 mmole per hour or with sodium oleate at 8 mmole per hour each produced a large protein and small bicarbonate response from the pancreas. Somatostatin blocked both bicarbonate and protein responses. These experiments demonstrated that somatostatin inhibits exocrine pancreatic secretion stimulated in a wide variety of ways, but what is the mechanism of action? The infusion of two gastrointestinal hormones secretin and OP-CCK, as well as stimulation of endogenous secretin and OP-CCK release by intragastric and intraduodenal nutrient, helped to answer this question. For example somatostatin blocked the pancreatic response to endogenously secreted CCK but not to an exogenous OP-CCK infusion. This indicated that the action of somatostatin is on the CCK secreting cell and not the pancreas. In the case of secretin the situation is less clear. Somatostatin and secretin interact on the pancreas with the characteristics of competitive inhibition, suggesting but not proving that the two molecules might act at a single effector site. Since the effects of somatostatin on endogenous secretin release were measured by a pancreatic end point no statement can be made regarding a direct effect of somatostatin on secretin secretion. Other authors8 showed inhibition of secretin release by somatostatin but much greater somatostatin concentrations were used to achieve the effect. An interesting difference between secretin and CCK induced pancreatic secretion is that the former appears to be mediated by cyclic AMP and the latter by cyclic GMP.9 It is posNUTRITION REVIEWS / VOL. 35, NO. 8 / AUGUST 1977 211
sible that somatostatin may influence cyclic AMP but not GMP generation. This may explain some of the differences in its effect on the two patterns of stimulated pancreatic secretion. Experiments in which analogues of the cyclic nucleotides were perfused through the pancreas or celiac axis, and the effect of somatostatin on the nucleotide-stimulated pancreatic enzyme or gut hormone secretion were studied, might be helpful in unravelling this point. That somatostatin probably plays a part in the physiologicalcontrol of pancreatic exocrine secretion is indicated by the profound inhibition by somatostatin of protein and bicarbonate secretion in response to feeding or the intragastric and intraduodenal instillation of nutrient which causes both secretin and CCK release. This observation, coupled with the preliminary evidence of a somatostatin-like substance in the stomach and duodenum, suggests that our thoughts on the local hormonal control of exocrine pancreatic function should be extended to include an inhibitor which is somatostatin or a closely related peptide. In the course of time it will be interesting to see what definitive name it is given. 0
3.
4.
5.
6.
7.
8. 1. P. Brazeau, W. Vale, R. Burgus, N. Ling, M. Butcher, J. Rivier and R. Guillemin: Hypothalamic Polypeptide that Inhibits the Secretion of Immunoreactive Pituitary Growth Hormone. Science 179: 77-79, 1973 2. C.H. Mortimer, W.M.G. Tunbridge, D. Carr, L. Yeomans, T. Lind, D.H. Coy, S.R. Bloom, A.
212 NUTRITION REVIEWS / VOL. 35, NO. 8, /AUGUST 1977
9.
Kastin, C.N. Mallinson, G.M. Besser, A.V. Schally and R. Hall: Effects of Growth-Hormone Release-Inhibiting Hormone on Circulating Glucagon, Insulin, and Growth Hormone in Normal, Diabetic,Acromegalic and Hypopituitary Patients. Lancet I: 697-701, 1974 S.R. Bloom, C.H. Mortimer, M.O. Thorner, G.M. Besser, R. Hall, A. Gomez-Pan, V.M. Roy, R.C. G. Russell, D.H. Coy, A.J. Kastin and A.V. Schally: Inhibition of Gastrin and Gastric-Acid Secretion by Growth-Hormone Release Inhibiting Hormone. Lancet 11: 1 106-1109, 1974 A.A.J. Barros D’Sa, S.R. Bloom and J.H. Baron: Direct Inhibition of Gastric Acid by GrowthHormone Release-Inhibiting Hormone in Dogs. Lancet I: 886-887, 1975 T. Hokfeld, S. Efendic, C. Hellestrom, 0. Johansson, R. Luft and A. Arimura: Cellular Localization of Somatostatin in Endocrine-Like Cells and Neurons of the Rat with Special Reference to the A, Cells of the Pancreas Islets and to the Hypothalamus. Acta Endocrinol. (Suppl.) 80: 1-41, 1975 S.J. Konturek,J. Tasler, W. Obtulowicz, D.H. Coy and A.V. Schally. Effect of Growth-Hormone-Release Inhibiting Hormone on Hormones Stimulating Exocrine Pancreatic Secretion. J. CIin. Invest. 58: 11-16, 1976 H.T. Debas and M.I. Grossman: Pure Cholecystokinin: Pancreatic Protein and Bicarbonate Response. Digestion 9: 469-481, 1973 G. Boden, M.C. Sivitz, O.E. Owen, N. EssaKoumar and J.H. Landor: Somatostatin Suppresses Secretin and Pancreatic Exocrine and Secretion. Science 190: 163-164, 1975 P. Robbercht in Stimulus Secretion Coupling in the Gastrointestinal Tract. R.M. Case and H. Goebell, Editors, p. 203. Baltimore, Llniversity Park Press, 1976
