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Gastrointestinal Hormones
Symposium on Changing Concepts of Disease
Castrointestinal Hormones Julian Katz, M.D.*
The gut is an endocrine organ. Hormones produced in the gastrointestinal tract have multiple effects and the interactions of these hormones match in complexity those which the endocrinologist has discovered in the pituitary, thyroid, and adrenal glands. In fact, gastroenterology may be considered the parent of endocrinology. The concept of hormonal control of physiologic function was first proposed and demonstrated in a laboratory of gastrointestinal physiology, and secretin was the first substance to be called a hormone. Gastrointestinal hormones have been isolated, characterized, and even synthesized. A great deal has been learned about the relationship between molecular structure and endocrine function. The ability to synthesize materials with biologic activities provides a greater understanding of how hormones act, and offers a possible new approach to therapy of gastrointestinal disorders. Since polypeptides are antigenic, sensitive radioimmunoassay techniques can be used to measure and define these hormones. Methods of bioassay for hormones which have similar and multiple actions are less satisfactory. The endocrine cells of the gastrointestinal tract can be grouped by electron microscopy; assignment of specific hormonal function is made by using immunochemical techniques to identify the cells which contain the various hormones. Although there surely are other, yet unidentified gastrointestinal hormones, the existence of certain hormones within the gut has been established. Gastrin, secretin, and cholecystokinin-pancreozymin (CCK) have been characterized and much is known about the effects of these three hormones on gastrointestinal function and on processes elsewhere in the body. In addition there is good evidence for the presence in the intestine of another substance which inhibits gastric acid secretion (enterogastrone), and evidence for material with glucagon-like activity (enteToglucagon). There are important interactions between these hormones, as well as interelationships with the autonomic nervous system. *Assistant Professor of Medicine, Division of Gastroenterology, Medical College of Pennsylvania; Consultant, Veterans Administration Hospital, Philadelphia, Pennsylvania
Medical Clinics of North America- Vo!. 57, No. 4, July 1973
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SECRETIN In 1902 Baylis and Starling observed the increased flow of pancreatic juice after acid was put into the lumen of the denervated jejunum. 1 The substance which stimulated the secretion of pancreatic fluid was termed secretin. The criteria which were proposed for classifying a substance as a hormone were that the material be active in a denervated preparation and that the physiologic effects be reproduced by the injection of the substance into the circulation. Secretin is present in high concentration in the upper portion of the small intestine, with low levels in the pylorus and more distal small bowel. The cells which appear to produce and store secretin have been identified in the duodenum; they have apical projections which extend into the lumen. 73 Secretin is a strongly basic single-chain polypeptide which consists of 27 amino acids. 38 , 44 There is a notable similarity between the molecular structure of secretin and glucagon -14 amino acid units occupy the same position in secretin and in the 20 amino acid glucagon (Fig. 1). The origin of these hormones from a single hormone during evolution has been suggested. 91 Secretin has been isolated and synthesized. 67 • 68 The alkaline nature of this hormone which is released from intestinal mucosa after the introduction of acid is intriguing. The major stimulus for the release of secretin is the presence of hydrochloric acid within the proximal small bowel,35, 90 Other hormonal and neural stimuli may play a role, but acidification of the duodenal mucosa with organic or mineral acids is the most potent stimulus to secretin release. 65 If the alkaline pancreatic juice is excluded from the duodenum, there is a marked increase in the pancreatic secretion after the stimulus of a meal. 18 Above pH 4.5, secretin is not released in amounts sufficient to stimulate pancreatic secretion. There is some evidence that the products of digestion of fat and protein stimulate secretin release. The effects noted, however, may actually be due to the release of CCK by fatty acids and the amino acids. Vagal stimulation plays, at best, a minor role in secretin release, but local anesthetics applied to the intestinal mucosa diminish the response of the pancreas to stimuli known to release secretin. 77 Secretin induces the secretion of water and electrolytes by the pancreas. The volume of secretion is related to the dose. 48 As the volume increases, so does the bicarbonate concentration; pancreatic secretion, under secretin stimulation, contains the highest concentration of bicarbonate in any body fluid. 37 Chloride concentration declines as the bicarbonate concentration increases. Secretin has little effect on enzyme Secretin:
His-Ser- Asp-Gly-Thr-Phe-Thr-Ser-Glu-Leu-Ser-Arg-Leu-Arg-Asp-Ser- Ala-Arg- LeuGln- Arg-Leu-Leu-Gln-Gly-Leu-Val-NH2
Glucagon:
His-Ser-Gln-Gly-Thr-Phe-Thr-Ser-Asp-Tyr-Ser-Lys-Tyr-Leu-Asp-Ser-Arg-Arg-AlaGln-Asp-Phe-Val-Gln-Try-Leu-Met-Asn-Thr Figure 1.
Amino acid sequences of secretin and glucagon.'
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secretion. The augmentation of enzyme output which has been noted with secretin may be due primarily to the passive washout of these enzymes by the fluid. The acinar cells in the pancreas do not discharge their granules after the administration of secretin. 40 Secretin has a variety of other actions on the gastrointestinal tract and even elsewhere in the body. It stimulates the liver to secrete bile with a high concentration of bicarbonate. BB Secretin, though, is probably not an important physiologic stimulus of hepatic secretion. The effect of CCK on gallbladder contraction is also enhanced by secretin. It has been suggested that the hormone secreted by the non-insulin-secreting islet cell tumor associated with watery diarrhea and hypokalemia is secretin. 54 There is, however, no good evidence that secretin is a very effective stimulus for intestinal secretion, although it does increase small bowel motility. Secretin has been thought to have effects on fluid and electrolyte secretion by the intestine, particularly the Brunner's glands. 79 But the significance of the action of secretin on electrolyte and water transport is uncertain, and the preparations of the hormone which were used may have been impure. The stimulant effect on cardiac output and splanchnic blood flow seems to be a minor one. 3B ,74 Gastric acid secretion which is stimulated by one gastrointestinal hormone, gastrin, is inhibited physiologically by secretin. 43 Secretin blocks the action of gastrin in inducing acid secretion, but does not act to prevent histamine or vagally stimulated secretion. Thus a hormone which is released by acidification of the duodenum inhibits further acid secretion. Gastric motility is probably reduced, seemingly in contrast to the action of gastrin, and gastric emptying is retarded by secretin. B7 Secretin produces a fall in serum immunoreactive gastrin, indicating perhaps that it suppresses the release of gastrin or even promotes its excretion or metabolism. 34 The inhibition of gastric acid secretion is potentiated by CCK, and the inhibition by these two duodenal hormones may be produced by different mechanisms. 3 Pepsinogen secretion by the stomach is increased by secretin,lO but a complex interaction between secretin, gastrin, CCK, and vagal stimuli is involved in the regulation of pepsinogen release. The interaction of hormones on the regulation of gastrointestinal function is well illustrated by the effects of gastrin and secretin on lower esophageal sphincter pressure. 16 Gastrin increases acid secretion but acts to increase sphincter competence and prevent reflux. Secretin is a sensitive inhibitor of this gastrin-mediated increase in pressure, and the inhibition is competitive. This action was shown to be physiologic since the doses of secretin used to achieve this effect were submaximal for the primary effect on the pancreas, and the effect on the esophageal sphincter was reproduced by the endogenous release of secretin. The effect of secretin is to return the sphincter strength to a basal level, at a time in the digestive process when acid secretion is reduced. There is similarity in the structures of secretin and glucagon, and these hormones have both similar and antagonistic effects. They both increase secretion of bile, inhibit gastric secretion, increase splanchnic blood flow, promote lipolysis, and are insulinotropic. Glucagon inhibits
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the secretin-stimulated output of fluid and bicarbonate by the pancreas. 22 These observations might indicate that there is competition for receptor sites by the structurally similar molecules.
GASTRIN Early in this century, Edkins demonstrated that the intravenous injection of an extract of antral mucosa stimulated gastric acid secretion in the cat.23 Gastrin was the name given to this material and it was the second known hormone. Subsequently the existence of such a hormone was doubted, and studies seemed to indicate that histamine was the secretogogue present in the antral extract. 75 Komarov, however, substantiated the presence of a protein hormone,47 and Gregory and fellow workers isolated the purified gastrin. 28 The polypeptide hormone not only has an important influence on gastric acid secretion, but also plays an important role in other gastrointestinal functions. 30 , 31, 76 The gastrin which was purified from hog antral mucosa contains 17 amino acids. Human antral gastrin has been isolated and it also is a heptadecapeptide (Fig. 2). It is remarkable that the terminal carboxyltetrapeptide (Fig. 3) can reproduce all the activities of which the whole molecule is capable. The function of the other 13 amino acids may be to enhance the activity of the attached tetrapeptide. Chemical manipulation using varied combinations of amino acids, based on the knowledge of the gastrin molecule has shown the relationship of molecular structure and function. 66 Function varies with dose, and there are also decided differences in the physiologic and pharmacologic effects of gastrin. It is fortunate that the biologically active portion ofthe gastrin molecule is also the anti genic determinant.59 Antibodies can be made and used for measuring the hormone.6o ,94 There are two components of immunoreactive gastrin; the major portion of plasma gastrin is more basic and has a molecular weight 3 112 times that of the heptadecapeptide. This "big gastrin" is biologically active, and the proportion of the larger molecule in the mucosa increases from the antrum distally.7 The antrum, the non-acid secreting portion of the stomach, is the abundant source of gastrin. The antral mucosal cell which produces and releases gastrin has been identified using immunochemical methods. Gastrin is contained on or in numerous cytoplasmic granules.61 Removal of the antrum substantially and significantly lowers the serum gastrin level. 64 Gastrin may be inactivated by enzymes in the small bowel mucosa,80 and extensive small bowel resection in man lS occasionally associated with excessive gastric acid secretion. Several factors are responsible for the release of gastrin from antral 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Glu . Gly . Pro· Try . Leu . Glu . Glu . Glu . Glu . Glu . Ala· Tyr . Gly . [Trp . Met· Asp· Phe . NH21 C-terminal tetrapeptide Figure 2.
Amino acid sequence of gastrin.
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15
14
Figure 3.
16
Carboxyl-terminal tetrapeptide of gastrin.
mucosa: an alkaline pH of the antral contents, mechanical distention of the antrum, acetylcholine, and certain food substances. An autoregulatory feedback mechanism is involved in gastrin release. An alkaline pH causes the release of gastrin from the antrum, and gastrin stimulates the parietal cell to secrete acid. Acid inhibits all mechanisms of gastrin release and this inhibition occurs without nervous intervention. The exact mechanism by which antral acidification results in suppression of gastrin release is not known. In patients with pernicious anemia, there is a constant alkaline pH because of achlorhydria and serum levels of gastrin are high. 63 Acidification of the antrum in these patients will lower the gastrin levels, indicating that the feedback mechanism is intact. Release of antral gastrin is a prime example of neuroendocrine interaction. Vagal stimulation causes the antral release of gastrin,51 and also has a direct stimulatory effect on the partietal cells.71 There is a significant rise in immunoreactive gastrin in normal individuals after vagal stimulation by insulin-induced hypoglycemia. 3:l. SI If the acid which is released is neutralized, then the rise is more marked. Acetylcholine induces release of gastrin, and this can be prevented by the application of local anesthetics, ganglionic blocking agents, and atropine. The systemic use of atropine, however, does not block the rise in serum gastrin which occurs after eating.89 Gastrin is released by antral distention, and this seems to involve an extravagal cholinergic mechanism present in plexuses within the antral wall. The ingestion of various foods elicits a rise in serum gastrin. 49 The peak occurs about 20 to 45 minutes after eating, but there is also a vagal component since the rise starts within 10 minutes after eating. Proteins and amino acids seem to be most potent in stimulating a rise in serum gastrin, while fats and carbohydrates are poor stimulants. Although alcohol has been thought to be highly effective in releasing gastrin,93 recent studies do not support this belief. Calcium increases gastric acid secretion and this too may be mediated by gastrin. 82 Gastrin has effects on the sphincters of the intestinal tract. The lower esophageal sphincter provides a barrier against reflux of the acid gastric contents. Gastrin acts to increase the gastroesophageal sphincter strength. 14 The effect of gastrin is more notable on the sphincter muscle than on adjacent esophageal muscle. 56 The role of antacids in treating
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esophageal reflux may be not only to neutralize acid, but also to strengthen the sphincter by inducing the release of endogenous antral gastrin. 15 Patients who have the Zollinger-Ellison syndrome with hypergastrinemia have an increased lower esophageal sphincter pressure. 41 Achalasia, a disease in which esophageal sphincter pressure is high and relaxation is insufficient, may represent a supersensitivity of the sphincter muscle to gastrin. 17 The ileocecal sphincter pressure, on the other hand, decreases after the administration of gastrin. 13 Perhaps the gastrocolic reflex involves the relaxation of the ileocecal valve after the release of gastrin from the antrum, with a flow of small bowel secretions into the colon. The pyloric sphincter is also affected by gastrin and other gastrointestinal hormones. 26 There seems to be an incompetency of the pyloric sphincter in gastric ulcer, and the reflux of bile salts which stimulate gastrin release (at least in dogs)2 may play a role in this disorder.8 This hormone which regulates and induces gastric acid secretion also has effects on other gastric functions and elsewhere in the digestive system. There is a weak stimulatory effect on pepsinogen secretion,57 and although gastrin increases gastric antral motility,41 gastric emptying might actually be delayed. 39 Gastrin stimulates the proliferation of cells in the acid-producing portion of the stomach. 92 In the pancreas, gastrin stimulates enzyme output, and weakly induces water and bicarbonate secretion. 29 Along with stimulating bile flow,96 there is a reduction in bile duct pressure. Gastrin may increase intestinal motility,4.78 but there is difficulty in differentiating physiologic from pharmacologic effects. There is inhibition by gastrin of the intestinal absorption of fluid, electrolytes and nutrients. 12 This inhibitory effect, however, occurs at a time in the digestive cycle prior to the emptying of food into the small bowel. Gastrin may even have an influence on the body metabolism of ingested substances. Gastrin immunoassay is the best diagnostic test for the ZollingerEllison syndrome. 62 The non-insulin-producing islet cell tumor secretes excessive amounts of gastrin, and these levels are sufficiently higher than those in peptic ulcer disease to permit diagnosis. Gastrin levels are also quite high when the negative feedback mechanism is interrupted, which is the case when the antrum is removed from the alimentary stream, as in the retained antrum after ulcer surgery, or in the achlorhydria of pernicious anemia. As expected, there have been numerous studies attempting to ascertain the role of gastrin in ulcer disease. Variable levels of gastrin are found in patients with duodenal ulcer. Berson and Yallow found that their ulcer patients had higher than normal values, and proposed that there was a degree of autonomy in the antral release of gastrin, with an incomplete shut-off after antral acidification. 6 Others found that the basal levels of gastrin in duodenal ulcer patients were either normal83 or in the low range ofnormal;52 the high acidity in the antrum was able to suppress antral release. Serum gastrin levels, though, may be raised to a higher degree in patients with duodenal ulcer after a protein meal,52 and vagal stimulation in these patients produces a higher rise in gastrin than in controls. 89 Patients with gastric ulcer have higher basal gastrin levels
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than controls. This is probably related to lowered antral acidity, and the relative hypergastrinemia does not seem to have an etiologic role. 50 The role of gastrin in peptic ulcer disease has not yet been ascertained. Gastric acid secretion, stimulated by gastrin, is inhibited by CCK. This inhibitory action has a structural basis since the carboxyl-terminal pentapeptide of gastrin (Fig. 3) is identical to that of CCK.9 CCK acts as a competitive inhibitor of gastrin by attaching to a receptor site and causing little stimulation of that site, but blocking the access of gastrin to the site. Caerulein, a decapeptide amide isolated from the frog's skin, also has the same carboxyl-terminal amino acid sequence and shows a similar competitive inhibition of gastrin-stimulated acid secretion. Selectively inhibitory peptide analogues which cause no acid secretion may be available in the future as antigastrin agents. 8i
CHOLECYSTOKININ-PANCREOZYMIN (CCK) Cholecystokinin and pancreozymin were thought to represent two different hormones, one stimulating gallbladder contraction, and the other evoking enzyme release from the pancreas. It now appears that there is just one substance, and that the gallbladder and pancreatic effects represent different biologic actions of the same chemical compound. 35 CCK is a polypeptide hormone containing 33 amino acids,44 and the hormone has been prepared in pure form from duodenal mucosa. The distribution of CCK in the gastrointestinal tract is similar to that of secretin. Since CCK is a polypeptide, a radioimmunoassay can be performed. The entry of the products of digestion into the small bowel is the major stimulus to the release of CCK. Polypeptides and amino acids, and digested and emulsified fats are believed to be the most potent substances. 58 Amino acids in physiologic concentrations evoke a sustained output of pancreatic enzymes and are a greater stimulus to CCK release than micellar fatty acids. 25 • 27 Acidification seems to be almost ineffective in man, although this is a most potent stimulant in the cat and dog. 5 Pure solutions of simple sugars show little effect. A nervous mechanism also is involved, since the application of local anesthetics to the small bowel mucosa seems to block release. The vagus nerve seems to have a tonic influence on pancreatic enzyme secretion, but atropine does not block the responses of the pancreas to parenterally injected CCK. There is an interrelation between nervous and hormonal influences on the pancreatic acinar cell, but after vagotomy the hormonal mechanism can regulate enzyme output. 36 Most studies have involved a bioassay and more information will be available when immunoassay techniques are utilized. CCK shares many of the physiologic properties of gastrin. CCK acts primarily to increase the enzyme output from the pancreas. Degranulation of the pancreatic acinar cells is the morphologic concomitant of the stimulatory action of CCK on pancreatic enzyme secretion. 40 Gastrin also increases the protein output from the pancreas. However, while CCK is a powerful stimulant to gallbladder contraction, gastrin is quite weak and probably has no role in the regulation of gallbladder evacuation. CCK also
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affects fluid tran~fer across the gallbladder mucosa. 69 CCK may, in addition to causing gallbladder contraction, stimulate bile flow and relax the sphincter of Oddi; the physiologic significance, however, is uncertain. CCK, which is released after the arrival of nutrients in the intestine, speeds up passage of the ingested food through the bowel by increasing small intestinal motility.
ENTEROGLUCAGON In the past, glucagon was thought to be a hormone found in the pancreas which functioned solely as a hyperglycemic agent.85 However, a substance with glucagon-like immunoreactivity and hyperglycemic properties has been found in the gastrointestinal tract, and is called enteroglucagon. Cells containing glucagon-like immunoreactivity have been found in the fundus of the stomach and in the jejunum. 72 It has been suggested that enteroglucagon differs from pancreatic glucagon in molecular size and some biologic properties, but there may actually be two components of gut glucagon,86.95 one quite similar to pancreatic glucagon and the other, which promotes insulin secretion, lipolysis and glycogenolysis, probably not identical to the pancreatic hormone. Glucose given by mouth is cleared more rapidly from the blood and elicits a greater insulin response than glucose given intravenously. Several of the hormones released from the intestinal mucosa are capable of stimulating insulin secretion. 19. 58 Secretin does contribute to the augmentation of insulin release, but it seems unlikely that it is the major mediator. CCK may also, under certain experimental conditions, release insulin; in addition, it stimulates pancreatic glucagon release. A direct stimulatory effect of glucagon on pancreatic insulin secretion, independent of its effect on blood glucose, has been demonstrated. Glucagon probably promotes insulin secretion by activating cyclic adenosine monophosphate. Immunoreactive glucagon in the blood rises after the ingestion of glucose,86 but it is not at all certain that glucagon is the alimentary substance responsible for insulin secretion. Surely there are some other factors involved. 84 The release of glucagon by the entry of sugar into the gut does have the dual and advantageous effect of suppressing glucose uptake by the liver and increasing insulin secretion, which promotes glucose assimilation. In postgastrectomy hypo glycemia, there may be excessive stimulation ofinsulin secretion because some hormones are released by the rapid introduction of carbohydrates into the small intestine. Some other properties of pancreatic glucagon may be shared with enteroglucagon. There is a choleretic effect which differs from that of secretin,21 and an inhibitory action on jejunal motility.45 Glucagon also inhibits pancreatic secretion; it is suggested that enteroglucagon regulates volume and electrolytes, while pancreatic glucagon (presumably stimulated by CCK) inhibits enzyme release. 2o Thus there is another negative feedback mechanism-with the arrival of food in the jejunum, pancreatic secretion is inhibited.
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ENTEROGASTRONE(S) The term "enterogastrone" was originally used to describe the humoral substance which is released from the small intestine after the arrival of fat, and which inhibits gastric acid secretion. 43 . 53 The introduction of acid and hypertonic solutions also inhibits acid secretion. 24 Secretin and CCK, which are released from duodenal mucosa, are inhibitors of gastrin-stimulated gastric secretion, and Johnson and Grossman questioned whether enterogastrone is a distinct hormone. They concluded that recent physiologic and chemical evidence indicates the existence of one or more enterogastrones, different from secretin and CCK. Such a gastric inhibitory substance has been isolated. The material has been partially purified and the probable amino acid composition revealed. 70 However, the polypeptide must be identified in the blood during a meal in order for it to be considered a physiologic regulator of gastric secretion. Theoretically, inhibitors of gastric secretion would have clinical value in the treatment of ulcer disease.
OTHER GASTROINTESTINAL HORMONES The list of chemically defined gastrointestinal hormones is certain to increase. Some have already been identified, but their physiologic importance has not been determined. Motilin is a polypeptide which increases motor activity in the stomach. l l It is released after alkalinization of the duodenum and is apparently distinct from the established gastrointestinal hormones. Contraction and relaxation of the villi of the small intestine is influenced by the action of an intestinal hormone called villikinin.46 Increased movements of the villi, which may contribute to absorption, are initiated by release of this hormone after the arrival of acid chyme in the duodenum. Gastric inhibitory substances,24 and intestinal polypeptides which enhance glucose-stimulated insulin release,t9.84 are also being studied. There is often difficulty when dealing with these materials in discerning pharmacologic from physiologic activities.
INTERACTIONS OF GASTROINTESTINAL HORMONES There is a remarkable interrelationship of these endocrine agents to exocrine and other gastrointestinal functions. Noting that, with very few exceptions, all three hormones act on the same target organs, Grossman 32 postulates that gastrin, secretin, and CCK all act on one receptor. This receptor would have two interacting sites, one with affinity for the chemically similar gastrin and CCK, and one with affinity for secretin. He also suggests that glucagon acts at this secretin receptor site. The simultaneous action of hormones leads to competitive or noncompetitive effects, depending on whether the hormone acts on the same or different
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receptor sites. There is augmentation if the agents give the same maximal response, and inhibition if one hormone has a lower efficacy of action. Examples of these forms of interaction can be found in the effects of hormones on gastric and pancreatic secretion. CCK is a competitive inhibitor of gastrin-stimulated acid secretion, whereas secretin seems to be a noncompetitive inhibitor of gastrin-stimulated acid secretion. Gastrin plus CCK gives competitive augmentation of pancreatic secretion, while gastrin or CCK plus secretin gives noncompetitive augmentation. Noncompetitive augmentation means that the maximal response attained with both hormones is greater than the maximal response that can be attained with either hormone separately. In order to fully explain the actions and interactions of hormones, more information is required. There is a need for accurate dose-response curves for single agents and for combinations, and for more quantitative data which will allow kinetic analysis. Further receptor sites may be classified and the role of mediators of hormonal action, such as cyclic AMP ,55 needs to be clarified. Radioimmunoassay techniques are increasingly being utilized and provide more qualitative information about the individual hormones; concepts of hormonal actions may need to be revised since much of our knowledge is based on bioassays of these hormones which have complex and overlapping effects. The effect of these hormones on blood flow to the target organs and the relationship of blood flow to secretion also require investigation. 42 Certainly more. hormones from the gastrointestinal tract will be characterized and these too will interact with other regulatory factors. REFERENCES 1. Bayliss, W. M., and Starling, E. H.: The mechanism of pancreatic secretion. J. PhysioL (London), 28:325-353, 1902. 2. Bedi, B. S., Debas, H. T., Gillespie, G., et al.: Effect of bile salts on antral gastrin release. Gastroenterology, 60:256-261, 1971. 3. Bedi, B. S., Debas, H. T., Wasunna, A. F. 0., et al.: Secretin and cholecystokininpancreozymin in the inhibition of gastric acid secretion. Gut, 12:968-972, 1971. 4. Bennett, A. J., Misiewicz, J. J., and Waller, S. L.: Analysis of the motor effects of gastrin and pentagastrin on the human alimentary tract in vitro. Gut, 8:470-474,1967. 5. Berry, H., and Flower, R J.: The assay of endogenous cholecystokinin and factors influencing its release in the cat and dog. Gastroenterology, 60:409-420, 1971. 6. Berson, S. A., and Yalow, R S.: Gastrin in duodenal ulcer. New Eng. J. Med., 284:445-446, 1971. 7. Berson, S. A., and Yalow, R S.: Nature of immunoreactive gastrin extracted from tissues of gastrointestinal tract. Gastroenterology, 60:215~222, 1971. 8. Black, R B., Roberts, G., and Rhodes, J.: The effect of healing on bile reflux in gastric ulcer. Gut, 12:552-558, 1971. 9. Brooks, A. M., Agosti, A., Bertaccini, G., et al.: Inhibition of gastric acid secretion in man by peptide analogues of cholecystokinin. New Eng. J. Med., 282:535-538, 1970. 10. Brooks, A. M., Isenberg, J., and Grossman, M. I.: The effect of secretin, glucagon, and duodenal acidification on pepsin secretion in man. Gastroenterology, 57: 159-162, 1969. 11. Brown, J. C., Cook, M. A., and Dryburgh,J. R: Motilin, a gastric motor-activity-stimulating polypeptide: final purification, amino acid composition, and c-terminal residues. Gastroenterology, 62:401-404, 1972. 12. Bynum, T. E., Jacobson, E. D., and Johnson, L. R: Gastrin inhibition of intestinal absorption in dogs. Gastroenterology, 61 :858-862, 1971. 13. Castell, D. 0., Cohen, S., and Hams, L. D.: Response of human ileocecal sphincter to gastrin. Amer. J. PhysioL, 219:712-715,1970. 14. Castell, D. 0., and Hams, L. D.: Hormonal control of gastroesophageal-sphincter strength. New Eng. J. Med., 282:886-889,1970.
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15. Castell, D. 0., and Levine, S. M.: Lower esophageal sphincter response to gastric alkalinization. Ann. Int. Med., 74:223-227, 1971. 16. Cohen, S., and Lipshutz, W.: Hormonal regulation of human lower esophageal sphincter competence: interaction of gastrin and secretin. J. Clin. Invest., 50:449-454, 1971. 17. Cohen, S., Lipshutz, W., and Hughes, W.: Role of gastrin supersensitivity in the pathogenesis of lower esophageal sphincter hypertension in achalasia. J. Clin. Invest., 50:12411247,1971. 18. Cooke, A. R., Nahrwold, D. L., and Grossman, M. I.: Diversion of pancreatic juice on gastric and pancreatic response to a meal stimulus. Amer. J. Physiol., 219:637-639, 1967. 19. Creutsfeldt, W., Feurle, G., and Ketterer, H.: Effect of gastrointestinal hormones on insulin and glucagon secretion. New Eng. J. Med., 282:1139-1141,1970. 20. Dyck, W. P.: Influences of intrajejunal glucose on pancreatic exocrine function in man. Gastroenterology, 60:861-869, 1971. 21. Dyck, W. P., and Janowitz, H. D.: Effect of glucagon on hepatic bile secretion in man. Gastroenterology, 60:400-404, 1971. 22. Dyck, W. P., Rudick, J., Hoexter, B., et al.: Influence of glucagon on pancreatic exocrine function. Gastroenterology, 56:531-537, 1969. 23. Edkins, J. S.: The chemical mechanism of gastric secretion. J. Physiol. (London), 34: 133144,1906. 24. Enterogastrone(s). Editorial. Lancet, 1 :1224-1225, 1971. 25. Ertam, A., Brooks, F. P., Ostrow, J. D., et al.: Effect of jejunalamino acidperfusion and exogenous cholecystokinin on the exocrine pancreatic and biliary secretion in man. Gastroenterology, 61 :686-692, 1971. 26. Fisher, R., and Cohen, S.: Pyloric sphincter dysfunction in patients with gastric ulcer. Clin. Res., 20:453, 1972. 27. Go, V. L. W., Hoffman, A. F., and Summerskill, W. H. J.: Pancreozymin bioassay in man based on pancreatic enzyme secretion: potency of specific amino acids and other digestive products. J. Clin. Invest., 49:1558-1564, 1970. 28. Gregory, R. A.: The isolation and chemistry of gastrin. Gastroenterology, 51 :953-960, 1966. 29. Gregory, R. A., and Tracy, H. J.: The constitution and properties of two gastrins extracted from hog antral mucosa. Gut, 5:103-114,1964. 30. Grossman. M. I., ed. Conference on Gastrin, September 27-30, 1964. Los Angeles, University of California Press, 1966. 31. Grossman, M. I.: Gastrin and its activities. Nature, 228: 1147-1150, 1970. 32. Grossman, M. I.: Gastrin, cholecystokinin, and secretin act on one receptor. Lancet, 1 :1088-1089, 1970. 33. Hansky, J., Korman, M. G., Cowley, D. J., et al.: Serum gastrinin duodenal ulcer. 11. Effect of insulin hypoglycemia. Gut, 12:959-962, 1971. 34. Hansky, J., Soveny, C., and Korman, M. G.: Effect of secretin on serum gastrin as measured by immunoassay. Gastroenterology, 61 :62-68, 1971. 35. Harper, A. A.: Hormonal control of pancreatic secretion. In Handbook of Physiology, Section 6: Alimentary tract. Volume 11. Secretion. Washington, D.C., American Physiological Society, 1967, p. 969-995. 36. Harper, A. A., Blair, E. L., and Scrathcherd, T.: The distribution and physiological properties of pancreozymin. In de Reuck, A. V. S., and Cameron, M. P. (eds.): The Exocrine Pancreas. Boston, Little, Brown and Company, pp. 168-182. 37. Harrison, L., and Jacobson, E. D.: Gastrointestinal hormones. J. Okla. Med. Assoc., 63:157-161,1970. 38. Hubel, K. A.: Secretin: a long progress note. Gastroenterology, 62:318-341,1972. 39. Hunt, J. N., and Ramsbottom, N.: Effect of gastrin 11 on gastric emptying and secretion during a test meal. Brit. Med. J., 4:386-387, 1967. 40. Ichikawa, A.: Fine structural changes in response to hormonal stimulation of the perfused canine pancreas. J. Cell. BioI., 24:369-385, 1965. 41. Isenberg, J., Csendes, A., and Walsh, J. H.: Resting and pentagastrin-stimulated gastroesophageal sphincter pressure in patients with Zollinger-Ellison syndrome. Gastroenterology, 61 :655-659, 1971. 42. Jacobson, E. D.: Secretion and blood flow in the gastrointestinal tract. In Handbook of Physiology, Section 6: Alimentary tract. Volume 11. Secretion. Washington, D.C., American Physiological Society, 1967, pp. 1043-1062. 43. J ohnson, L. R., and Grossman, M. I.: Intestinal hormones as inhibitors of gastric secretion. Gastroenterology, 60: 120-144, 1971. 44. Jorpes, J. E.: The isolation imd chemistry of secretin and cholecystokinin. Gastroenterology, 55:157-164, 1968. 45. Kock, N. C., Darle, N., and Dotevall, G.: Inhibition of intestinal motility in man by glucagon given intraportally. Gastroenterology, 53:88-92, 1967. 46. Kokas, E., and Johnston, C. L., Jr.: Influence of refined villikinin on motility of intestinal villi. Amer. J. Physiol., 208: 1196-1202, 1965.
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47. Komarov, S. A.: Studies on gastrin 11. Physiological properties of the specific gastric secretagogue of the pyloric mucous membrane. Rev. Canad. BioI., 1 :377-401, 1942. 48. Konturek, S. J.: Pancreatic dose-response curves to intravenous secretin in man. Gastroenterology, 54: 197-209, 1968. 49. Korman, M. G., Soveny, C., and Hansky, J.: Effect of food on serum gastrin evaluated by radioimmunoassay. Gut, 12:619-624, 1971. 50. Korman, M. G., Soveny, C., and Hansky, J.: Gastrin studies in gastric ulcer. Gut, 13:166169,1972. 51. Korman, M. G., Soveny, C., and Hansky, J.: Radioimmunoassay of gastrin: The response of serum gastrin to insulin hypoglycemia. Scand. J. Gastroenterol., 6:71-75,1971. 52. Korman, M. G., Soveny, C., and Hansky, J.: Serum gastrinin duodenal ulcer. I. Basal levels and effect of food and atropine. Gut, 12:899-902, 1971. 53. Kosaka, T., and Lim, R K A.: Demonstration of the humoral agent in fat inhibition of gastric secretion. Proc. Soc. Exper. BioI. Med., 27:890-891, 1930. 54. Kraft, A. R, Tompkins, R K, and Zollinger, R M.: Recognition and management of the diarrheal syndrome caused by nonbeta islet cell tumors of the pancreas. Amer. J. Surg., 119: 163-170, 1970. 55. Liddle, G. W., and Hardman, J. G.: Cyclic adenosine monophosphate as a mediator ofhormone action. New Eng. J. Med., 285:560-566, 1971. 56. Lipshutz, W., and Cohen, S.: Physiological determinants of lower esophageal sphincter function. Gastroenterology, 61: 16-24, 1971. 57. Makhlouf, G. M., McManus, J. P. A., and Card, W. 1.: Comparative effects of gastrin II and histamine on pepsin secretion in man. Gastroenterology, 52:787-791,1967. 58. Markes, V., and Samols, E.: Intestinal factors in insulin secretion regulation. Adv. Metab. Disord., 4:1-39,1970. 59. McGuigan, J. E.: Antibodies to gastrin. Fed. Proc., 27:1337-1340,1968. 60. McGuigan, J. E.: Studies of the immunochemical specificity of some antibodies to human gastrin. Gastroenterology, 56:429-438, 1969. 61. McGuigan, J. E., Greider, M. H., and Grawe, L.: Staining characteristics of the gastrin cell. Gastroenterology, 62:959-969, 1972. 62. McGuigan, J. E., and Trudeau, W. L.: Elevated serum levels of gastrinin Zollinger-Ellison syndrome. New Eng. J. Med., 278:1308-1312,1968. 63. McGuigan, J. E., and Trudeau, W. L.: Serum gastrin concentrations in pernicious anemia. New Eng. J. Med., 282:358-361, 1970. 64. McGuigan, J. E., and Trudeau, W. L.: Serum gastrin: effect of vagotomy with pyloroplasty or with antrectomy. New Eng. J. Med., 286:184-188,1972. 65. Meyer, J. H., Way, L. W., and Grossman, M. I.: Pancreatic bicarbonate response to various acids in the duodenum of the dog. Amer. J. Physiol., 219:964-970,1970. 66. Morley, J. S.: Structure-activity relationships. Fed. Proc., 27: 1314-1317, 1968. 67. Mutt, V., and Jorpes, J. E.: Contemporary developments in the biochemistry of the gastrointestinal hormones. Recent Progr. Hormone Res., 23:483-503, 1967. 68. Ondetti, M. A., Narayanan, V. L., von Saltza, M., et al.: The synthesis of secretin, J. Amer. Chem. Soc., 90:4711-4716,1968. 69. Onstad, G. R, Schoenfield, L. J., and Higgins, J. A.: Fluid transfer in the everted gall bladder. J. Clin. Invest., 46:606-614, 1967. 70. Pederson, R A., and Brown, J. C.: Inhibition of histamine-, pentagastrin-, and insulinstimulated canine gastric secretion by pure "gastric inhibitory polypeptide." Gastroenterology, 62:393-400, 1971. 71. Pevsner, L., and Grossman, M. 1.: The mechanism of vagal stimulation of gastric acid secretion. Gastroenterology, 28:493-499, 1955. 72. Polak, J. M., Bloom, S., Coulling, I., et al.: Immunofluorescent localization of enteroglucagon cells in the gastrointestinal tract of the dog. Gut, 12:311-318, 1971. 73. Polak, J. M., Bloom, S., Coulling, 1., et al.: Immunofluorescent localization of secretin in the canine duodenum. Gut, 12:605-610, 1971. 74. Ross, G.: Cardiovascular effects of secretin. Amer. J. Physiol., 218:1166-1170,1970. 75. Sacks, J., Ivy, A. C., Burgess, J. P., et al.: Histamine as the hormone for gastric secretion. Amer. J. Physiol., 101 :313-338, 1932. 76. Sanders, M. G., and Schimmel, E. M.: Gastrin. Amer. J. Med., 49:380-394. 1970. 77. Shapiro, H., and Woodward, E. R: Inhibition of the secretin mechanism by local anesthetics. Amer. Surg., 31:139-141,1965. 78. Smith, A. N., and Hogg, D.: Effect of gastrin II on the motility of the gastrointestinal tract. Lancet, 1 :403-404, 1966. 79. Stening, G. F., and Grossman, M. 1.: Hormonal control of Brunner's glands. Gastroenterology, 56:1047-1052, 1969. 80. Temperley, J. M., Stagg, B. H., and Wyllie, J. H.: Disappearance of gastrin and pentagastrin in the portal circulation. Gut, 12:372-376, 1971. 81. Treating like with like. Editorial. New Eng. J. Med., 282:565-566, 1970.
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82. Trudeau, W. L., and McGuigan, J. E.: Effects of calcium on serum gastrin levels in the Zollinger-Ellison syndrome. New Eng. J. Med., 281 :862-866, 1969. 83. Trudeau, W. L., and McGuigan, J. E.: Serum gastrin levels and rates of gastric hydrochloric acid secretion. New Eng. J. Med., 284:408-412, 1971. 84. Turner, D. S., and Marks, V.: Enhancement of glucose- stimulated insulin release by an intestinal polypeptide in rats. Lancet, 1: 1095, 1972. 85. Unger, R H.: Pancreatic glucagon in health and disease. Adv. Intern. Med., 17:265-288, 1971. 86. Unger, RH., Ohneda, A., Valverde, I., et al.: Characterization of the responses of circulating glucagon-like immunoreactivity to intraduodenal and intravenous administration of glucose. J. Clin. Invest., 47:48-65, 1968. 87. Vagne, M., and Andre, C.: The effect of secretin on gastric emptying in man. Gastroenterology. 60:421-424, 1971. 88. Waitman, A. M., Dyck, W. P., and Janowitz, H. D.: Effect of secretin and acetazolamide on the volume and electrolyte composition of hepatic bile in man. Gastroenterology, 56:286-294, 1969. 89. Walsh, J. H., Yalow, R S., and Berson, S. A.: The effect of atropine on plasma gastrin response to feeding. Gastroenterology, 60: 16-21, 1971. 90. Wang, C. C., and Grossman, M. I.: Physiological determination of release of secretin and pancreozymin from intestine of dogs with transplanted pancreas. Am. J. Physiol., 154:527-545,1951. 91. Weinstein, B.: On the relationship between glucagon and secretin. Experientia, 24:406408,1968. 92. Willems, G., Vansteenkiste, Y., and Limbosch, J. M.: Stimulating effect of gastrin on cell proliferation kinetics in canine fundic mucosa. Gastroenterology, 62:583-588, 1972. 93. Woodward, E. R, Robertson, C., Ruttenberg, H. D., and Schapiro, H.: Alcohol as a gastric secretory stimulant. Gastroenterology, 32:727-737, 1957. 94. Yalow, R S., and Berson, S. A.: Radioimmunoassay of gastrin. Gastroenterology, 58:1-14, 1970. 95. Young, G.: Hormonal control of pancreatic endocrine and exocrine secretion. Gu t, 13: 154161, 1972. 96. Zaterka, S., and Grossman, M. I.: The effect of gastrin and histamine on the secretion of bile. Gastroenterology, 50:500-505, 1966. Medical College of Pennsylvania Philadelphia, Pennsylvania 19129
Castrointestinal Hormones Julian Katz, M.D.*
The gut is an endocrine organ. Hormones produced in the gastrointestinal tract have multiple effects and the interactions of these hormones match in complexity those which the endocrinologist has discovered in the pituitary, thyroid, and adrenal glands. In fact, gastroenterology may be considered the parent of endocrinology. The concept of hormonal control of physiologic function was first proposed and demonstrated in a laboratory of gastrointestinal physiology, and secretin was the first substance to be called a hormone. Gastrointestinal hormones have been isolated, characterized, and even synthesized. A great deal has been learned about the relationship between molecular structure and endocrine function. The ability to synthesize materials with biologic activities provides a greater understanding of how hormones act, and offers a possible new approach to therapy of gastrointestinal disorders. Since polypeptides are antigenic, sensitive radioimmunoassay techniques can be used to measure and define these hormones. Methods of bioassay for hormones which have similar and multiple actions are less satisfactory. The endocrine cells of the gastrointestinal tract can be grouped by electron microscopy; assignment of specific hormonal function is made by using immunochemical techniques to identify the cells which contain the various hormones. Although there surely are other, yet unidentified gastrointestinal hormones, the existence of certain hormones within the gut has been established. Gastrin, secretin, and cholecystokinin-pancreozymin (CCK) have been characterized and much is known about the effects of these three hormones on gastrointestinal function and on processes elsewhere in the body. In addition there is good evidence for the presence in the intestine of another substance which inhibits gastric acid secretion (enterogastrone), and evidence for material with glucagon-like activity (enteToglucagon). There are important interactions between these hormones, as well as interelationships with the autonomic nervous system. *Assistant Professor of Medicine, Division of Gastroenterology, Medical College of Pennsylvania; Consultant, Veterans Administration Hospital, Philadelphia, Pennsylvania
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SECRETIN In 1902 Baylis and Starling observed the increased flow of pancreatic juice after acid was put into the lumen of the denervated jejunum. 1 The substance which stimulated the secretion of pancreatic fluid was termed secretin. The criteria which were proposed for classifying a substance as a hormone were that the material be active in a denervated preparation and that the physiologic effects be reproduced by the injection of the substance into the circulation. Secretin is present in high concentration in the upper portion of the small intestine, with low levels in the pylorus and more distal small bowel. The cells which appear to produce and store secretin have been identified in the duodenum; they have apical projections which extend into the lumen. 73 Secretin is a strongly basic single-chain polypeptide which consists of 27 amino acids. 38 , 44 There is a notable similarity between the molecular structure of secretin and glucagon -14 amino acid units occupy the same position in secretin and in the 20 amino acid glucagon (Fig. 1). The origin of these hormones from a single hormone during evolution has been suggested. 91 Secretin has been isolated and synthesized. 67 • 68 The alkaline nature of this hormone which is released from intestinal mucosa after the introduction of acid is intriguing. The major stimulus for the release of secretin is the presence of hydrochloric acid within the proximal small bowel,35, 90 Other hormonal and neural stimuli may play a role, but acidification of the duodenal mucosa with organic or mineral acids is the most potent stimulus to secretin release. 65 If the alkaline pancreatic juice is excluded from the duodenum, there is a marked increase in the pancreatic secretion after the stimulus of a meal. 18 Above pH 4.5, secretin is not released in amounts sufficient to stimulate pancreatic secretion. There is some evidence that the products of digestion of fat and protein stimulate secretin release. The effects noted, however, may actually be due to the release of CCK by fatty acids and the amino acids. Vagal stimulation plays, at best, a minor role in secretin release, but local anesthetics applied to the intestinal mucosa diminish the response of the pancreas to stimuli known to release secretin. 77 Secretin induces the secretion of water and electrolytes by the pancreas. The volume of secretion is related to the dose. 48 As the volume increases, so does the bicarbonate concentration; pancreatic secretion, under secretin stimulation, contains the highest concentration of bicarbonate in any body fluid. 37 Chloride concentration declines as the bicarbonate concentration increases. Secretin has little effect on enzyme Secretin:
His-Ser- Asp-Gly-Thr-Phe-Thr-Ser-Glu-Leu-Ser-Arg-Leu-Arg-Asp-Ser- Ala-Arg- LeuGln- Arg-Leu-Leu-Gln-Gly-Leu-Val-NH2
Glucagon:
His-Ser-Gln-Gly-Thr-Phe-Thr-Ser-Asp-Tyr-Ser-Lys-Tyr-Leu-Asp-Ser-Arg-Arg-AlaGln-Asp-Phe-Val-Gln-Try-Leu-Met-Asn-Thr Figure 1.
Amino acid sequences of secretin and glucagon.'
GASTROINTESTINAL HORMONES
895
secretion. The augmentation of enzyme output which has been noted with secretin may be due primarily to the passive washout of these enzymes by the fluid. The acinar cells in the pancreas do not discharge their granules after the administration of secretin. 40 Secretin has a variety of other actions on the gastrointestinal tract and even elsewhere in the body. It stimulates the liver to secrete bile with a high concentration of bicarbonate. BB Secretin, though, is probably not an important physiologic stimulus of hepatic secretion. The effect of CCK on gallbladder contraction is also enhanced by secretin. It has been suggested that the hormone secreted by the non-insulin-secreting islet cell tumor associated with watery diarrhea and hypokalemia is secretin. 54 There is, however, no good evidence that secretin is a very effective stimulus for intestinal secretion, although it does increase small bowel motility. Secretin has been thought to have effects on fluid and electrolyte secretion by the intestine, particularly the Brunner's glands. 79 But the significance of the action of secretin on electrolyte and water transport is uncertain, and the preparations of the hormone which were used may have been impure. The stimulant effect on cardiac output and splanchnic blood flow seems to be a minor one. 3B ,74 Gastric acid secretion which is stimulated by one gastrointestinal hormone, gastrin, is inhibited physiologically by secretin. 43 Secretin blocks the action of gastrin in inducing acid secretion, but does not act to prevent histamine or vagally stimulated secretion. Thus a hormone which is released by acidification of the duodenum inhibits further acid secretion. Gastric motility is probably reduced, seemingly in contrast to the action of gastrin, and gastric emptying is retarded by secretin. B7 Secretin produces a fall in serum immunoreactive gastrin, indicating perhaps that it suppresses the release of gastrin or even promotes its excretion or metabolism. 34 The inhibition of gastric acid secretion is potentiated by CCK, and the inhibition by these two duodenal hormones may be produced by different mechanisms. 3 Pepsinogen secretion by the stomach is increased by secretin,lO but a complex interaction between secretin, gastrin, CCK, and vagal stimuli is involved in the regulation of pepsinogen release. The interaction of hormones on the regulation of gastrointestinal function is well illustrated by the effects of gastrin and secretin on lower esophageal sphincter pressure. 16 Gastrin increases acid secretion but acts to increase sphincter competence and prevent reflux. Secretin is a sensitive inhibitor of this gastrin-mediated increase in pressure, and the inhibition is competitive. This action was shown to be physiologic since the doses of secretin used to achieve this effect were submaximal for the primary effect on the pancreas, and the effect on the esophageal sphincter was reproduced by the endogenous release of secretin. The effect of secretin is to return the sphincter strength to a basal level, at a time in the digestive process when acid secretion is reduced. There is similarity in the structures of secretin and glucagon, and these hormones have both similar and antagonistic effects. They both increase secretion of bile, inhibit gastric secretion, increase splanchnic blood flow, promote lipolysis, and are insulinotropic. Glucagon inhibits
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the secretin-stimulated output of fluid and bicarbonate by the pancreas. 22 These observations might indicate that there is competition for receptor sites by the structurally similar molecules.
GASTRIN Early in this century, Edkins demonstrated that the intravenous injection of an extract of antral mucosa stimulated gastric acid secretion in the cat.23 Gastrin was the name given to this material and it was the second known hormone. Subsequently the existence of such a hormone was doubted, and studies seemed to indicate that histamine was the secretogogue present in the antral extract. 75 Komarov, however, substantiated the presence of a protein hormone,47 and Gregory and fellow workers isolated the purified gastrin. 28 The polypeptide hormone not only has an important influence on gastric acid secretion, but also plays an important role in other gastrointestinal functions. 30 , 31, 76 The gastrin which was purified from hog antral mucosa contains 17 amino acids. Human antral gastrin has been isolated and it also is a heptadecapeptide (Fig. 2). It is remarkable that the terminal carboxyltetrapeptide (Fig. 3) can reproduce all the activities of which the whole molecule is capable. The function of the other 13 amino acids may be to enhance the activity of the attached tetrapeptide. Chemical manipulation using varied combinations of amino acids, based on the knowledge of the gastrin molecule has shown the relationship of molecular structure and function. 66 Function varies with dose, and there are also decided differences in the physiologic and pharmacologic effects of gastrin. It is fortunate that the biologically active portion ofthe gastrin molecule is also the anti genic determinant.59 Antibodies can be made and used for measuring the hormone.6o ,94 There are two components of immunoreactive gastrin; the major portion of plasma gastrin is more basic and has a molecular weight 3 112 times that of the heptadecapeptide. This "big gastrin" is biologically active, and the proportion of the larger molecule in the mucosa increases from the antrum distally.7 The antrum, the non-acid secreting portion of the stomach, is the abundant source of gastrin. The antral mucosal cell which produces and releases gastrin has been identified using immunochemical methods. Gastrin is contained on or in numerous cytoplasmic granules.61 Removal of the antrum substantially and significantly lowers the serum gastrin level. 64 Gastrin may be inactivated by enzymes in the small bowel mucosa,80 and extensive small bowel resection in man lS occasionally associated with excessive gastric acid secretion. Several factors are responsible for the release of gastrin from antral 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Glu . Gly . Pro· Try . Leu . Glu . Glu . Glu . Glu . Glu . Ala· Tyr . Gly . [Trp . Met· Asp· Phe . NH21 C-terminal tetrapeptide Figure 2.
Amino acid sequence of gastrin.
897
GASTROINTESTINAL HORMONES
15
14
Figure 3.
16
Carboxyl-terminal tetrapeptide of gastrin.
mucosa: an alkaline pH of the antral contents, mechanical distention of the antrum, acetylcholine, and certain food substances. An autoregulatory feedback mechanism is involved in gastrin release. An alkaline pH causes the release of gastrin from the antrum, and gastrin stimulates the parietal cell to secrete acid. Acid inhibits all mechanisms of gastrin release and this inhibition occurs without nervous intervention. The exact mechanism by which antral acidification results in suppression of gastrin release is not known. In patients with pernicious anemia, there is a constant alkaline pH because of achlorhydria and serum levels of gastrin are high. 63 Acidification of the antrum in these patients will lower the gastrin levels, indicating that the feedback mechanism is intact. Release of antral gastrin is a prime example of neuroendocrine interaction. Vagal stimulation causes the antral release of gastrin,51 and also has a direct stimulatory effect on the partietal cells.71 There is a significant rise in immunoreactive gastrin in normal individuals after vagal stimulation by insulin-induced hypoglycemia. 3:l. SI If the acid which is released is neutralized, then the rise is more marked. Acetylcholine induces release of gastrin, and this can be prevented by the application of local anesthetics, ganglionic blocking agents, and atropine. The systemic use of atropine, however, does not block the rise in serum gastrin which occurs after eating.89 Gastrin is released by antral distention, and this seems to involve an extravagal cholinergic mechanism present in plexuses within the antral wall. The ingestion of various foods elicits a rise in serum gastrin. 49 The peak occurs about 20 to 45 minutes after eating, but there is also a vagal component since the rise starts within 10 minutes after eating. Proteins and amino acids seem to be most potent in stimulating a rise in serum gastrin, while fats and carbohydrates are poor stimulants. Although alcohol has been thought to be highly effective in releasing gastrin,93 recent studies do not support this belief. Calcium increases gastric acid secretion and this too may be mediated by gastrin. 82 Gastrin has effects on the sphincters of the intestinal tract. The lower esophageal sphincter provides a barrier against reflux of the acid gastric contents. Gastrin acts to increase the gastroesophageal sphincter strength. 14 The effect of gastrin is more notable on the sphincter muscle than on adjacent esophageal muscle. 56 The role of antacids in treating
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esophageal reflux may be not only to neutralize acid, but also to strengthen the sphincter by inducing the release of endogenous antral gastrin. 15 Patients who have the Zollinger-Ellison syndrome with hypergastrinemia have an increased lower esophageal sphincter pressure. 41 Achalasia, a disease in which esophageal sphincter pressure is high and relaxation is insufficient, may represent a supersensitivity of the sphincter muscle to gastrin. 17 The ileocecal sphincter pressure, on the other hand, decreases after the administration of gastrin. 13 Perhaps the gastrocolic reflex involves the relaxation of the ileocecal valve after the release of gastrin from the antrum, with a flow of small bowel secretions into the colon. The pyloric sphincter is also affected by gastrin and other gastrointestinal hormones. 26 There seems to be an incompetency of the pyloric sphincter in gastric ulcer, and the reflux of bile salts which stimulate gastrin release (at least in dogs)2 may play a role in this disorder.8 This hormone which regulates and induces gastric acid secretion also has effects on other gastric functions and elsewhere in the digestive system. There is a weak stimulatory effect on pepsinogen secretion,57 and although gastrin increases gastric antral motility,41 gastric emptying might actually be delayed. 39 Gastrin stimulates the proliferation of cells in the acid-producing portion of the stomach. 92 In the pancreas, gastrin stimulates enzyme output, and weakly induces water and bicarbonate secretion. 29 Along with stimulating bile flow,96 there is a reduction in bile duct pressure. Gastrin may increase intestinal motility,4.78 but there is difficulty in differentiating physiologic from pharmacologic effects. There is inhibition by gastrin of the intestinal absorption of fluid, electrolytes and nutrients. 12 This inhibitory effect, however, occurs at a time in the digestive cycle prior to the emptying of food into the small bowel. Gastrin may even have an influence on the body metabolism of ingested substances. Gastrin immunoassay is the best diagnostic test for the ZollingerEllison syndrome. 62 The non-insulin-producing islet cell tumor secretes excessive amounts of gastrin, and these levels are sufficiently higher than those in peptic ulcer disease to permit diagnosis. Gastrin levels are also quite high when the negative feedback mechanism is interrupted, which is the case when the antrum is removed from the alimentary stream, as in the retained antrum after ulcer surgery, or in the achlorhydria of pernicious anemia. As expected, there have been numerous studies attempting to ascertain the role of gastrin in ulcer disease. Variable levels of gastrin are found in patients with duodenal ulcer. Berson and Yallow found that their ulcer patients had higher than normal values, and proposed that there was a degree of autonomy in the antral release of gastrin, with an incomplete shut-off after antral acidification. 6 Others found that the basal levels of gastrin in duodenal ulcer patients were either normal83 or in the low range ofnormal;52 the high acidity in the antrum was able to suppress antral release. Serum gastrin levels, though, may be raised to a higher degree in patients with duodenal ulcer after a protein meal,52 and vagal stimulation in these patients produces a higher rise in gastrin than in controls. 89 Patients with gastric ulcer have higher basal gastrin levels
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than controls. This is probably related to lowered antral acidity, and the relative hypergastrinemia does not seem to have an etiologic role. 50 The role of gastrin in peptic ulcer disease has not yet been ascertained. Gastric acid secretion, stimulated by gastrin, is inhibited by CCK. This inhibitory action has a structural basis since the carboxyl-terminal pentapeptide of gastrin (Fig. 3) is identical to that of CCK.9 CCK acts as a competitive inhibitor of gastrin by attaching to a receptor site and causing little stimulation of that site, but blocking the access of gastrin to the site. Caerulein, a decapeptide amide isolated from the frog's skin, also has the same carboxyl-terminal amino acid sequence and shows a similar competitive inhibition of gastrin-stimulated acid secretion. Selectively inhibitory peptide analogues which cause no acid secretion may be available in the future as antigastrin agents. 8i
CHOLECYSTOKININ-PANCREOZYMIN (CCK) Cholecystokinin and pancreozymin were thought to represent two different hormones, one stimulating gallbladder contraction, and the other evoking enzyme release from the pancreas. It now appears that there is just one substance, and that the gallbladder and pancreatic effects represent different biologic actions of the same chemical compound. 35 CCK is a polypeptide hormone containing 33 amino acids,44 and the hormone has been prepared in pure form from duodenal mucosa. The distribution of CCK in the gastrointestinal tract is similar to that of secretin. Since CCK is a polypeptide, a radioimmunoassay can be performed. The entry of the products of digestion into the small bowel is the major stimulus to the release of CCK. Polypeptides and amino acids, and digested and emulsified fats are believed to be the most potent substances. 58 Amino acids in physiologic concentrations evoke a sustained output of pancreatic enzymes and are a greater stimulus to CCK release than micellar fatty acids. 25 • 27 Acidification seems to be almost ineffective in man, although this is a most potent stimulant in the cat and dog. 5 Pure solutions of simple sugars show little effect. A nervous mechanism also is involved, since the application of local anesthetics to the small bowel mucosa seems to block release. The vagus nerve seems to have a tonic influence on pancreatic enzyme secretion, but atropine does not block the responses of the pancreas to parenterally injected CCK. There is an interrelation between nervous and hormonal influences on the pancreatic acinar cell, but after vagotomy the hormonal mechanism can regulate enzyme output. 36 Most studies have involved a bioassay and more information will be available when immunoassay techniques are utilized. CCK shares many of the physiologic properties of gastrin. CCK acts primarily to increase the enzyme output from the pancreas. Degranulation of the pancreatic acinar cells is the morphologic concomitant of the stimulatory action of CCK on pancreatic enzyme secretion. 40 Gastrin also increases the protein output from the pancreas. However, while CCK is a powerful stimulant to gallbladder contraction, gastrin is quite weak and probably has no role in the regulation of gallbladder evacuation. CCK also
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affects fluid tran~fer across the gallbladder mucosa. 69 CCK may, in addition to causing gallbladder contraction, stimulate bile flow and relax the sphincter of Oddi; the physiologic significance, however, is uncertain. CCK, which is released after the arrival of nutrients in the intestine, speeds up passage of the ingested food through the bowel by increasing small intestinal motility.
ENTEROGLUCAGON In the past, glucagon was thought to be a hormone found in the pancreas which functioned solely as a hyperglycemic agent.85 However, a substance with glucagon-like immunoreactivity and hyperglycemic properties has been found in the gastrointestinal tract, and is called enteroglucagon. Cells containing glucagon-like immunoreactivity have been found in the fundus of the stomach and in the jejunum. 72 It has been suggested that enteroglucagon differs from pancreatic glucagon in molecular size and some biologic properties, but there may actually be two components of gut glucagon,86.95 one quite similar to pancreatic glucagon and the other, which promotes insulin secretion, lipolysis and glycogenolysis, probably not identical to the pancreatic hormone. Glucose given by mouth is cleared more rapidly from the blood and elicits a greater insulin response than glucose given intravenously. Several of the hormones released from the intestinal mucosa are capable of stimulating insulin secretion. 19. 58 Secretin does contribute to the augmentation of insulin release, but it seems unlikely that it is the major mediator. CCK may also, under certain experimental conditions, release insulin; in addition, it stimulates pancreatic glucagon release. A direct stimulatory effect of glucagon on pancreatic insulin secretion, independent of its effect on blood glucose, has been demonstrated. Glucagon probably promotes insulin secretion by activating cyclic adenosine monophosphate. Immunoreactive glucagon in the blood rises after the ingestion of glucose,86 but it is not at all certain that glucagon is the alimentary substance responsible for insulin secretion. Surely there are some other factors involved. 84 The release of glucagon by the entry of sugar into the gut does have the dual and advantageous effect of suppressing glucose uptake by the liver and increasing insulin secretion, which promotes glucose assimilation. In postgastrectomy hypo glycemia, there may be excessive stimulation ofinsulin secretion because some hormones are released by the rapid introduction of carbohydrates into the small intestine. Some other properties of pancreatic glucagon may be shared with enteroglucagon. There is a choleretic effect which differs from that of secretin,21 and an inhibitory action on jejunal motility.45 Glucagon also inhibits pancreatic secretion; it is suggested that enteroglucagon regulates volume and electrolytes, while pancreatic glucagon (presumably stimulated by CCK) inhibits enzyme release. 2o Thus there is another negative feedback mechanism-with the arrival of food in the jejunum, pancreatic secretion is inhibited.
GASTROINTESTINAL HORMONES
901
ENTEROGASTRONE(S) The term "enterogastrone" was originally used to describe the humoral substance which is released from the small intestine after the arrival of fat, and which inhibits gastric acid secretion. 43 . 53 The introduction of acid and hypertonic solutions also inhibits acid secretion. 24 Secretin and CCK, which are released from duodenal mucosa, are inhibitors of gastrin-stimulated gastric secretion, and Johnson and Grossman questioned whether enterogastrone is a distinct hormone. They concluded that recent physiologic and chemical evidence indicates the existence of one or more enterogastrones, different from secretin and CCK. Such a gastric inhibitory substance has been isolated. The material has been partially purified and the probable amino acid composition revealed. 70 However, the polypeptide must be identified in the blood during a meal in order for it to be considered a physiologic regulator of gastric secretion. Theoretically, inhibitors of gastric secretion would have clinical value in the treatment of ulcer disease.
OTHER GASTROINTESTINAL HORMONES The list of chemically defined gastrointestinal hormones is certain to increase. Some have already been identified, but their physiologic importance has not been determined. Motilin is a polypeptide which increases motor activity in the stomach. l l It is released after alkalinization of the duodenum and is apparently distinct from the established gastrointestinal hormones. Contraction and relaxation of the villi of the small intestine is influenced by the action of an intestinal hormone called villikinin.46 Increased movements of the villi, which may contribute to absorption, are initiated by release of this hormone after the arrival of acid chyme in the duodenum. Gastric inhibitory substances,24 and intestinal polypeptides which enhance glucose-stimulated insulin release,t9.84 are also being studied. There is often difficulty when dealing with these materials in discerning pharmacologic from physiologic activities.
INTERACTIONS OF GASTROINTESTINAL HORMONES There is a remarkable interrelationship of these endocrine agents to exocrine and other gastrointestinal functions. Noting that, with very few exceptions, all three hormones act on the same target organs, Grossman 32 postulates that gastrin, secretin, and CCK all act on one receptor. This receptor would have two interacting sites, one with affinity for the chemically similar gastrin and CCK, and one with affinity for secretin. He also suggests that glucagon acts at this secretin receptor site. The simultaneous action of hormones leads to competitive or noncompetitive effects, depending on whether the hormone acts on the same or different
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receptor sites. There is augmentation if the agents give the same maximal response, and inhibition if one hormone has a lower efficacy of action. Examples of these forms of interaction can be found in the effects of hormones on gastric and pancreatic secretion. CCK is a competitive inhibitor of gastrin-stimulated acid secretion, whereas secretin seems to be a noncompetitive inhibitor of gastrin-stimulated acid secretion. Gastrin plus CCK gives competitive augmentation of pancreatic secretion, while gastrin or CCK plus secretin gives noncompetitive augmentation. Noncompetitive augmentation means that the maximal response attained with both hormones is greater than the maximal response that can be attained with either hormone separately. In order to fully explain the actions and interactions of hormones, more information is required. There is a need for accurate dose-response curves for single agents and for combinations, and for more quantitative data which will allow kinetic analysis. Further receptor sites may be classified and the role of mediators of hormonal action, such as cyclic AMP ,55 needs to be clarified. Radioimmunoassay techniques are increasingly being utilized and provide more qualitative information about the individual hormones; concepts of hormonal actions may need to be revised since much of our knowledge is based on bioassays of these hormones which have complex and overlapping effects. The effect of these hormones on blood flow to the target organs and the relationship of blood flow to secretion also require investigation. 42 Certainly more. hormones from the gastrointestinal tract will be characterized and these too will interact with other regulatory factors. REFERENCES 1. Bayliss, W. M., and Starling, E. H.: The mechanism of pancreatic secretion. J. PhysioL (London), 28:325-353, 1902. 2. Bedi, B. S., Debas, H. T., Gillespie, G., et al.: Effect of bile salts on antral gastrin release. Gastroenterology, 60:256-261, 1971. 3. Bedi, B. S., Debas, H. T., Wasunna, A. F. 0., et al.: Secretin and cholecystokininpancreozymin in the inhibition of gastric acid secretion. Gut, 12:968-972, 1971. 4. Bennett, A. J., Misiewicz, J. J., and Waller, S. L.: Analysis of the motor effects of gastrin and pentagastrin on the human alimentary tract in vitro. Gut, 8:470-474,1967. 5. Berry, H., and Flower, R J.: The assay of endogenous cholecystokinin and factors influencing its release in the cat and dog. Gastroenterology, 60:409-420, 1971. 6. Berson, S. A., and Yalow, R S.: Gastrin in duodenal ulcer. New Eng. J. Med., 284:445-446, 1971. 7. Berson, S. A., and Yalow, R S.: Nature of immunoreactive gastrin extracted from tissues of gastrointestinal tract. Gastroenterology, 60:215~222, 1971. 8. Black, R B., Roberts, G., and Rhodes, J.: The effect of healing on bile reflux in gastric ulcer. Gut, 12:552-558, 1971. 9. Brooks, A. M., Agosti, A., Bertaccini, G., et al.: Inhibition of gastric acid secretion in man by peptide analogues of cholecystokinin. New Eng. J. Med., 282:535-538, 1970. 10. Brooks, A. M., Isenberg, J., and Grossman, M. I.: The effect of secretin, glucagon, and duodenal acidification on pepsin secretion in man. Gastroenterology, 57: 159-162, 1969. 11. Brown, J. C., Cook, M. A., and Dryburgh,J. R: Motilin, a gastric motor-activity-stimulating polypeptide: final purification, amino acid composition, and c-terminal residues. Gastroenterology, 62:401-404, 1972. 12. Bynum, T. E., Jacobson, E. D., and Johnson, L. R: Gastrin inhibition of intestinal absorption in dogs. Gastroenterology, 61 :858-862, 1971. 13. Castell, D. 0., Cohen, S., and Hams, L. D.: Response of human ileocecal sphincter to gastrin. Amer. J. PhysioL, 219:712-715,1970. 14. Castell, D. 0., and Hams, L. D.: Hormonal control of gastroesophageal-sphincter strength. New Eng. J. Med., 282:886-889,1970.
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