Failure of endogenous stimulation of secretin and pancreozymin release to influence serum-insulin

Metabolic effects of secretin

Secretin (Sct), traditionally a gastrointestinal hormone backed by a century long research, is now beginning to be recognized also as a neuroactive peptide. Substantiation by recent evidence on the functional role of Sct in various regions of the brain, especially on its potential neurosecretion from the posterior pituitary, has revealed Sct's physiological actions in regulating water homeostasis. Recent advances in understanding the functional roles of central and peripheral Sct has been made possible by the development of Sct and Sct receptor (SctR) knockout animal models which have led to novel approaches in research on the physiology of this brain-gut peptide. While research on the role of Sct in appetite regulation and fatty acid metabolism has been initiated recently, its role in glucose homeostasis is unclear. This review focuses mainly on the metabolic role of Sct by discussing data from the last century and recent discoveries, with emphasis on the need for revisiting and elucidating the role of Sct in metabolism and energy homeostasis.

The effect of secretin on the glucose-induced insulin release from isolated pancreatic islets in mice

A simple and rapid method for isolation of pancreatic islets from mice using Percoll as a separation medium is described. Increasing concentrations of secretin from 10(-11) to 10(-6) M were without effect on the insulin release from the islets at 5 X 5 mM glucose, whereas significant increases were found at 12 mM glucose. Secretin did not elicit additional increases of the insulin release at 5 X 5 and 12 mM glucose with 5 mM theophylline. The mechanism by which secretin acts on the pancreatic islets is discussed. Based upon the above observations it is suggested that secretin apparently represents a modulator of the insulin release from the pancreatic islets.

Secretin N-terminal hexapeptide potentiates insulin release in mouse islets

Peptides representing the N-terminal part of secretin were synthesized and their effects tested on column-perifused isolated mouse pancreatic islets. Insulin release induced by D-glucose was potentiated by the two peptides His-Ser-Asp-Gly-Thr-Phe-OMe (S1-6) and Ser-Asp-Gly-Thr-Phe-OMe (S2-6). The consecutive smaller N-terminal peptides Asp-Gly-Thr-Phe-OMe (S3-6) and Gly-Thr-Phe-OMe (S4-6) had no effects while the dipeptide ester Thr-Phe-OMe (S5-6) also potentiated the release of insulin. The results suggest that the N-terminal part of secretin may be involved in the marked in vitro glucose-dependent insulin release induced by secretin.

Secretin and its C-terminal hexapeptide potentiates insulin release in mouse islets

Peptides representing the C-terminal end of secretin were synthetized and their effects tested along with secretin on column-perifused isolated mouse pancreatic islets. Insulin release induced by 10 mmol/l D-glucose was potentiated by secretin tested in a concentration range of 0.01-10 micrograms/ml; the maximal effect was obtained with 1 microgram/ml secretin. This effect was mimicked by 50-500 micrograms/ml NH2-Leu-Leu-Gln-Gly-Leu-Val-NH2, [S-(22-27)], which represents an amidated C-terminal sequence of the secretin molecule. The consecutive smaller secretin C-terminal peptides had either no effects [Val-NH2, S-(24-27)] or only marginally [S-(26-27), S-(23-27)] potentiating effects on insulin release in the presence of 10 mmol/l D-glucose. The effects of secretin and S-(22-27) were not influenced by 2 mmol/l glutamine. The intact hormone and the five synthetic peptides as well as Val-NH2 had no stimulatory effect on islet glutamate dehydrogenase activity. In fact, S-(23-27), S-(24-27), and S-(25-27) inhibited the islet glutamate dehydrogenase activity, the activation by which amino acids and amino acid derivatives are known to elicit a potentiation of insulin release. Our results suggest that the C-terminal part is important to the marked potentiation of glucose-induced insulin release in vitro by secretin.

Secretory effects of secretin on isolated perfused porcine pancreas

The effect of pure natural porcine secretin on endocrine and exocrine pancreatic secretion was studied in the totally isolated perfused porcine pancreas. The exocrine pancreatic responses to secretin correspond well with those obtained in the anesthetized pig. The lowest concentration of secretin observed to increase pancreatic secretion was 2.8 pmol/liter, whereas the maximum pancreatic responses were obtained at a secretin concentration of 92 pmol/liter. The infusion of secretin in concentrations ranging from 2.8 to 278 pmol/liter in the presence of a constant concentration of glucose (7.5, 5.0, or 3.5 mmol/liter) was without effect on the insulin and glucagon release. Infusion of secretin at a concentration of 834 pmol/liter in the presence of glucose at 7.5 mmol/liter provoked a significant (P less than 0.01) short-lived increase in insulin secretion. However, there was no effect on the glucagon secretion. The results of this study indicate that neither the augmented insulin response nor the suppression of glucagon elicited by oral glucose depend on secretin.

Effect of secretin on basal- and glucose-stimulated insulin secretion in man

Plasma immunoreactive secretin and insulin concentrations were measured in fasting normal humans after intraduodenal infusions of hydrochloric acid, isotonic or hypertonic glucose. The effect of intraduodenal acidification or intravenous bolus injections of secretin on plasma insulin concentrations during infusions of glucose was also examined. The intraduodenal glucose load did not cause an increase in plasma secretin concentrations. Secretin concentrations rose after acid both in the fasting state and during infusions of glucose. A concomitant rise in insulin levels was however only observed during infusions of glucose. Intravenous injection of secretin in a dose which mimicked the response to intraduodenal acidification was without effect on the glucose-stimulated insulin release, while a 30 times higher dose caused a highly significant augmentation of the insulin release. The insulin response pattern to this high dose of secretin differed completely from that observed after intraduodenal infusion of acid. It is concluded and confirmed that the stimulating effect of secretin on insulin secretion is pharmacological and that secretin plays no significant role in the entero-insular axis.

Hepatic-portal nutrient infustion: effect on feeding in intact and vagotomized rabbits

Hepatic-portal infusions of isotonic glucose did not influence food intake in free-feeding rabbits. In contrast, several other macronutrients and some of their metabolites, infused via the same route, cause anorexia followed by prolonged hypophagia. Its duration was generally decreased when the same infusions were performed during the nocturnal period. Vagotomy did not significantly alter the food intake responses to hepatic-portal infusions of either glucose or the other experimental substances. The results of related experiments have demonstrated that prolonged anorexia was not a consequence of the fast infusion rate or the generalized behavioral mallaise. However, when the infusions of most anorexigenic substances were followed by intraperitoneal administration of insulin, the duration of anorexia was shortened and food consumption was elevated. The results provided indirect evidence for the vagally mediated regulatory contribution of the previously studied neural and humoral intestinal mechanisms which are bypassed by infusing nutrient loads directly into the hepatic-portal circulation.

Secretin and insulin: response to intraduodenal acid

Immunoreactive secretin (IRS) and immunoreactive insulin (IRI) levels were measured in humans and dogs following the intraduodenal instillation of hydrochloric acid. IRS levels rose after acid in both instances, but a concomitant rise in peripheral IRI levels was not noted. Premedication of the humans with Scopolamine prevented a rise of IRS in the human subjects. It is concluded that the endovenous release of IRS alone does not result in increased IRI levels in peripheral blood and that IRS release may be under vagal control.

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.

Effects of intraduodenal administration of HCl and glucose on circulating immunoreactive secretin and insulin concentrations

A new radioimmunoassay for secretin was used to investigate (a) serum secretin responses to intraduodenally infused HCl and glucose, (b) the metabolic half-life and the volume of distribution of exogenous secretin and (c) the effect of endogenously released secretin on insulin secretion in 25 anesthetized dogs. Portal and femoral venous blood samples were taken simultaneously before, during, and after intraduodenal infusion of HCl (21 meq/30 min) and glucose (131 ml/30 min). Control experiments were performed with intraduodenal infusion of saline. Mean portal venous immunoreactive secretin concentration of six dogs rose from 313 muU/ml before to 1,060 muU/ml 10 min after initiation of the intestinal acidification (P < 0.005). Femoral venous immunoreactive secretin concentration rose from 220 muU/ml before to 567 muU/ml 15 min after intestinal acidification (P < 0.01). Secretin concentrations remained elevated during the remainder of the infusion. In the same six dogs mean portal venous immunoreactive insulin concentration rose from 38 muU/ml before to 62 muU/ml at the end of the infusion (P < 0.05). Peripheral immunoreactive insulin, glucose, and free fatty acid concentrations, however, did not change significantly. Pancreatic exocrine function was studied in four dogs. The rise in secretin concentration was followed promptly by a highly significant increase in exocrine pancreatic flow rate and bicarbonate secretion, indicating biological activity of the circulating immunoreactive secretin. The effect of intraduodenal infusion of glucose on immunoreactive secretin concentration was studied in 12 dogs. Glucose in concentrations ranging from 2.5% to 10% had no detectable influence on portal or peripheral secretin concentration. Infusion of 50% glucose caused a slight decline in secretin concentration. The metabolic clearance rate, half-life of disappearance, and volume of distribution of exogenous secretin was studied in three dogs by the constant infusion technic. The metabolic clearance rate was 730+/-34 ml/min, volume of distribution was 17.4+/-0.8% of body weight, and the half-life of disappearance was 2.8+/-0.1 min. It could be calculated that 1.38 U/kg-h(-1) of endogenous secretin was released into the peripheral circulation during the steady state period of the HCl infusion experiments. The data indicated that immunoreactive secretin was released rapidly after intestinal acidification, continued to be secreted throughout the duration of HCl infusion, and was promptly distributed in the extracellular compartment. Furthermore, they suggested that endogenously released secretin could stimulate insulin secretion. The HCl-mediated insulinogenic effect of immunoreactive secretin, however, was too weak to influence peripheral immunoreactive insulin, glucose, and free fatty acid concentrations. The failure of intraduodenal glucose to stimulate secretin release suggests that secretin is not the insulin-stimulatory factor released from the gastrointestinal tract in response to glucose.

Proteins and insulin release: a dual role of amino-acids and intestinal hormones

In two subjects concurrent infusion of amino-acids and the hormones secretin and pancreozymin provoked much higher plasma insulin levels than did administration of amino-acids or hormones individually. It is suggested that this may be a physiological phenomenon, augmenting the release of insulin from the pancreas after a meal containing protein.

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.

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.