Stimulation of insulin secretion by pancreozymin

The Origin and Understanding of the Incretin Concept

Gastrointestinal hormones that stimulate insulin secretion at physiological concentrations are incretins. This concept has recently attracted considerable attention in the wake of drugs developed from the gut hormone GLP-1 (glucagon-like peptide-1) for diabetes therapy. But the renewed enthusiasm has also restricted the concept to just two hormones, GLP-1 and GIP (glucose-dependent insulinotropic polypeptide). The purpose of the present overview is two-fold: First to tell that the incretin concept is far from new. It has a more than a century long history full of ups and downs. Second, that the incretin concept may now have become too narrow. Thus, it is likely that incretin comprises additional gastrointestinal hormones, which interact with GIP and GLP-1 during normal meals containing protein, fat and complex carbohydrates (and not just pure glucose). Such broader incretin concept may stimulate development of novel gut hormone-derived drugs.

Effects of CCK on pancreatic function and morphology

This report reviews the effects of CCK on the pancreas and in particular analyzes recent studies in which CCK antagonists were used to evaluate the physiological role of CCK in modulating pancreatic function and morphology. CCK is released from endocrine cells of the small intestine in response to a meal. In various animal species there are CCK receptors on pancreatic acinar cells with two sites; occupation of the high affinity site is thought to mediate pancreatic secretion and growth, whereas occupation of the low affinity site by high CCK concentrations is thought to be responsible for supramaximal inhibition of secretion and pancreatitis. Recently, CCK receptors were also found on postganglionic cholinergic neurons in the gastrointestinal tract. Administration of CCK agonists stimulates pancreatic secretion and growth. Although in some previous studies CCK was given at doses that mimic its postprandial increase in plasma, these studies did not prove that the actions of exogenous CCK were physiologically important. In addition, it was unclear if CCK primarily acts as a true hormone or as a neurotransmitter. The development of specific CCK receptor antagonists made it possible to better evaluate the physiological role of CCK. In humans, CCK-A antagonists like loxiglumide or L-364,718 at doses that completely inhibited the action of supraphysiological doses of exogenous CCK reduced meal-stimulated pancreatic enzyme secretion only by approximately 50%. On the other hand, atropine abolished the postprandial increase in pancreatic secretion and in addition markedly reduced the increase in pancreatic secretion due to infusion of "physiological" doses of CCK (i.e., CCK doses that mimic its postprandial increase in plasma). The increase in pancreatic bicarbonate secretion was only slightly reduced by CCK blockade. CCK antagonists failed to reduce the postprandial increase in plasma insulin, but markedly reduced the postprandial PP release. CCK-A antagonists caused slight hypotrophy and hypoplasia of the exocrine pancreas. However, even after 9 months of effective blockade of the CCK-A-receptor, mice had normal body weight and an almost normal pancreas. CCK antagonists were unable to alter short-term changes in pancreatic growth due to feeding and fasting. In some species, CCK agonists induced development of pancreatic nodules and increased the growth of malignant tumors. Studies about the effects of CCK antagonists on induction and growth of pancreatic tumors showed controversial results. In conclusion, CCK may act on the pancreas by three pathways: (1) At low doses it serves as a neurotransmitter by acting on cholinergic neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of cholecystokinin-receptor blockade on pancreatic and biliary function in healthy volunteers

This study used the specific cholecystokinin (CCK)-receptor antagonist loxiglumide to evaluate whether endogenous CCK, which is released after a meal, regulates pancreatic and biliary functions. Eight healthy volunteers were studied twice on separate days. The subjects received a continuous intraduodenal infusion of a 750-kcal liquid test meal for 2 hours either with or without IV infusion of 5 mg.kg-1.h-1 of loxiglumide. Loxiglumide at this dose abolishes the actions of CCK at various target organs including gallbladder and pancreas, when given at doses that mimic postprandial plasma concentrations of CCK. Loxiglumide markedly decreased the meal-stimulated outputs of amylase, trypsin, and chymotrypsin by 55%-70% of control values but only slightly decreased duodenal volume (25% inhibition of mean integrated secretion). Loxiglumide abolished gallbladder emptying induced by infusion of nutrients and even increased gallbladder volumes when compared with prior fasting values. Correspondingly, loxiglumide almost abolished the output of bilirubin after infusion of nutrients. However, loxiglumide failed to alter the increase in circulating concentrations of glucose, insulin, and C peptide after infusion of nutrients. The present results show that CCK is one of several factors that regulate pancreatic protein secretion after absorption of nutrients. However, CCK is probably not involved in regulation of pancreatic secretion of fluid. In contrast, gallbladder function is mainly regulated by CCK, both in terms of its emptying after intestinal absorption of nutrients and in terms of maintenance of its fasting volume. Cholecystokinin does not play a major physiological role as an insulinotropic factor.

Cholecystokinin in the entero-insular axis

Cholecystokinin (CCK) is a well-characterized gastrointestinal hormone which is released into the general circulation after meals. Targets for CCK include not only the gallbladder and the exocrine pancreas but also the endocrine pancreas. In this paper, we review the role of CCK from the perspective of the entero-insular axis, where CCK seems to function as one component of incretin and can raise insulin release synergistically with other incretin components. CCK also increases the sensitivity of B cells to subsequent glucose stimulation. At present, the role of CCK as incretin in disease states is uncertain. The pathophysiological role of CCK is likely to be revealed using CCK-specific radioimmunoassay and bioassay techniques and CCK receptor antagonists.

Secretory effects of cholecystokinins on the isolated perfused porcine pancreas

Three different molecular forms of cholecystokinin (CCK-39, -33, and -8) were used in concentrations from 10(-11) to 19(-8) mol/l to stimulate the endocrine and exocrine secretion from the isolated perfused porcine pancreas. During perfusion with a glucose concentration of 7.5 mmol/l CCK-39 in the highest concentration increased the insulin secretion slightly. No significant effect was observed at lower glucose concentrations (5.0 and 3.5 mmol/l). CCK-33 and -8 did not stimulate the secretion of insulin significantly, and neither of the cholecystokinins increased the secretion of glucagon and bicarbonate. All three molecular forms stimulated the secretion of fluid and protein in concentrations above 10(-11) mol/l in a dose-dependent manner and with equal potency. We conclude that the effect of these three cholecystokinins on the endocrine pancreas is without physiological significance, whereas all three are sufficiently potent to play a role in the control of pancreatic protein secretion.

Factors governing insulin and glucagon responses during normal meals

An experimental model is described which can be used to study substrate and hormone responses to normal meals administered in very near normal circumstances. After 500, 300 and 125 calorie meals, the relative proportion of fat or protein content did not influence the plasma glucose except for minor differences between the high protein-high fat meals. The insulin response to such meals was correlated positively with the increment in glucose but reduction of protein content below 8 g caused a signficant reduction in the increment in plasma insulin per unit increase in plasma glucose. Alterations in protein content above 8 g made no difference. Fat content of the meal did not significantly alter the insulin response. No evidence was obtained for a major component of insulin release attributable to either bulk or preabsorption phenomena such as sight or smell. It is concluded that a significant accentuation of the insulin response to meals is dependent on a minimum amount of protein and that this is probably mediated by one of the gastro-intestinal hormones. Glucagon release is dependent on protein and carbohydrate content of the meal and is independent of the fat content. There may also be an early stimulation of glucagon release, regardless of content, which may also be hormonally mediated.

The effects of gastrin, gastric inhibitory polypeptide, secretin, and the octapeptide of cholecystokinin upon immunoreactive somatostatin release by the perfused canine pancreas

The effects of gastrin, gastric inhibitory polypeptide, secretin, and the octapeptide of pancreozymin-cholecystokinin on immunoreactive somatostatin release were studied in the isolated perfused dog pancreas. Gastrin at a concentration of 65 ng/ml and the octapeptide of pancreozymin-cholecystokinin at a concentration of 25 ng/ml produced a prompt, but transient statistically significant, twofold rise in mean somatostatin concentration. Secretion at a concentration of 0.3 U/ml and gastric inhibitory polypeptide concentration of 58 ng/ml produced a prompt two- to threefold rise in mean somatostatin release, which persisted throughout the perfusion period. With all four polypeptides the pattern of the somatostatin response resembled that of insulin. It appears that pancreatic somatostatin release is stimulated by gastrointestinal hormones that influence the secretion of insulin and glucagon.

Relationship between changes of serum cholecystokinin-pancreozymin and serum insulin after different stimuli

The relationship between changes of serum immuno-reactive cholecystokinin-pancreozymin (CCK-PZ) and serum immuno-reactive insulin has been studied after various stimuli. The oral administration of 5 percent glucose or magnesium sulphate and the intra-duodenal administration of olive oil were all followed by a rise of serum CCK-PZ. The serum insulin rose after 5 percent glucose and also showed a small but insignificant rise with olive oil. There was, however, no change of serum insulin after the ingestion of magnesium sulphate suggesting that CCK-PZ in isolation does not stimulate insulin release.

The effects of caerulein on insulin secretion in anaesthetized dogs

1. Insulin concentration changes in pancreatico-duodenal venous plasma were studied in anaesthetized dogs injected with caerulein.2. Rises in insulin concentration were elicited by rapid intravenous injection of caerulein, as well as by intravenous infusion. Threshold doses were 10 ng/kg and 0.5-1 (ng/kg)/min respectively.3. At the highest dose used (500 ng/kg by rapid intravenous injection and (25 ng/kg)/min by intravenous infusion) the increase in immuno-reactive insulin release was approximately 7 to 9 times the base levels.4. Adrenalectomy potentiated the effects of intravenous infusion of caerulein.5. On a molar basis, caerulein was 2-3 times as active as pancreozymin.6. It is concluded that caerulein is a potent stimulant of pancreatic islets in the dog and that it may be considered as a model peptide, capable of being substituted for pancreozymin in any experiment.7. The mechanism of the insulin stimulating effect of caerulein is discussed. The possibility of a direct "beta-cytotropic" effect of the peptide is suggested.

The gastrointestinal stimulus to insulin release. I. Secretin

The gastrointestinal stimulus to the release of insulin has been investigated in man by the use of a radioimmunoassay for secretin. Serum secretin levels rose rapidly after the oral ingestion of glucose or protein and preceded the elevation of serum insulin. An intravenous infusion of highly purified secretin caused a release of insulin when the serum secretin levels were within the physiological range. Infusion of hydrochloric acid into the duodenum caused an elevation of serum secretin and serum insulin levels in normal subjects. A similar response of secretin and insulin was seen after intravenous infusion of pentagastrin even when the acid stimulus to the duodenum was prevented. The latter observation suggests that pentagastrin (and probably gastrin) releases secretin by a direct humoral effect which is later fortified by the arrival of gastric acid in the duodenum. These studies suggest that secretin participates in the augmentation of insulin release after oral stimuli, and that a rapid sequence of humoral events takes place, gastrin releasing secretin and secretin releasing insulin. Subsequently secretin release would be augmented by a local stimulus in the duodenum and insulin release by the rising level of blood glucose or amino acids. This humoral system, which could also involve other gastrointestinal hormones, would provide a mechanism for facilitating the release of insulin to coincide with the onset of metabolite absorption.

The role of aminogenic glucagon secretion in blood glucose homeostasis

Hyperaminoacidemia is a powerful stimulus of pancreatic glucagon secretion. These studies were designed to elucidate the role of aminogenic hyperglucagonemia in glucoregulation. Conscious dogs with previously implanted indwelling venous catheters were employed. The results support the view that a role of glucagon is to limit blood glucose decline during hyperaminoacidemia.First, a significant negative correlation between the area of glucagon increment during the 1st 20 min of a 10 amino acid infusion and the maximum fall in glucose concentration was observed. Second, when endogenous glucagon secretion was suppressed by means of a continuous glucose infusion, hyperaminoacidemia induced a maximal glucose decline which averaged 35 mg/100 ml, differing significantly from mean maximal fall of 3 mg/100 ml, which normally occurs in the presence of endogenous hyperglucagonemia. Third, when, during hyperglycemic suppression of endogenous glucagon secretion, 50 mmug of exogenous glucagon/min was infused via the mesenteric vein with the amino acids, the fall in glucose was reduced to an average of 5 mg/100 ml. Similarly when pancreozymin, administered during the combined infusion of glucose and amino acids, overcame glucose suppression of endogenous glucagon secretion, plasma glucose did not fall. Similar results were obtained when aminogenic hyperglucagonemia was prevented by other means. Hyperlipacidemia, induced by infusing a triglyceride emulsion and giving heparin injections, also suppressed aminogenic hyperglucagonemia in two of four experiments; in these two dogs glucose fell 15 and 11 mg/100 ml. In a final group of experiments, the canine pancreas was resected except for the uncinate process, which is virtually devoid of alpha-cells. In two dogs, in which this procedure resulted in zero portal venous glucagon levels, the administration of amino acids and/or pancreozymin resulted in a glucose decline of 14 and 16 mg/100 ml, despite the reduced beta-cell population resulting from the subtotal pancreotectomy. It thus appears that the secretion of pancreatic glucagon during hyperaminoacidemia in association with insulin secretion, serves to limit the decline of glucose concentration.

Effects of secretin, pancreozymin, or gastrin on the response of the endocrine pancreas to administration of glucose or arginine in man

Intravenous administration of porcine secretin or pancreozymin or synthetic human gastrin II resulted in raised increments in serum immunoreactive insulin during intravenous infusion of glucose in normal man. Enhancement of serum immunoreactive insulin by each hormone was associated with accelerated disposal of glucose. In response to prolonged intravenous infusion of arginine with pancreozymin there was a maintained rise in immunoreactive insulin and glucagon-like immunoreactivity in the blood. These effects of pancreozymin and arginine were not reproduced with secretin and arginine, and may have been due to the stimulation of glucagon secretion together with insulin by pancreozymin. Enteric infusion of hydrochloric acid, or stimulation of gastric acid secretion by betazole, presumed to cause release of endogenous secretin, led to enhancement of insulin secretion during intravenous infusion of glucose. Enteric infusion of arginine, presumed to cause release of endogenous pancreozymin, led to a rise in serum immunoreactive insulin not attributable to effects of circulating glucose and amino acids. It is concluded that secretin and pancreozymin released in response to physiological stimuli contribute to stimulation of the endocrine pancreas after ingestion of food.

Plasma insulin responses to oral and intravenous glucose: studies in normal and diabetic sujbjects

The plasma insulin responses of normal weight and obese, diabetic, and nondiabetic subjects to intravenous glucose was only 30-40% of that seen after oral glucose, indicating that alimentary mechanism(s) in addition to the arterial blood sugar concentration regulate insulin secretion. Observations made in subjects with diverted portal circulation indicate that the alimentary insulinogenic mechanism is located in the intestinal tract. The insulinogenic potency of the alimentary and glycemic stimuli expressed in terms of insulin secretion per gram of glucose were remarkably similar within each group of individuals. Between these groups, however, there were considerable differences. Obesity, with or without associated diabetes, was associated with a true hypersecretory responsiveness, whereas diabetes was characterized, with or without obesity, by a marked impairment in insulin secretion. The experimental design used in these studies permitted quantitation of the magnitude of the glycemic component of an oral glucose load. As a consequence of impaired insulin secretion, a greater than normal proportion of the oral glucose load escapes initial hepatic extraction in the maturity-onset diabetic and enters the peripheral circulation. Therefore, in the noninsulin-requiring maturity-onset diabetic, the glycemic insulinogenic stimulus for a given oral glucose load is significantly greater than in normal subjects and accounts for the excessive plasma insulin responses observed late in the course of an oral glucose tolerance test.