Effect of secretion on the exocrine and endocrine pancreas Part V: Pancreatic exocrine and endocrine responses to secretion in rats in vivo and in vitro (author's transl)

Abstract

Secretin is an established stimulator of exocrine pancreatic function and has been reported by some to also stimulate insulin release both in vivo and in vitro. Contrary to these observations with natural porcine secretin, synthetic porcine secretin has been revealed to have no influence on insulin release but does stimulate glucagon release from the isolated perfused rat pancreas. Unphysiologically large doses of synthetic porcine secretin has also been confirmed in in vivo experiments to be capable of stimulating not only glucagon but also insulin secretion from the rat pancreas. Thus, the discrepancies between our previous results with synthetic porcine secretin and those obtained with natural porcine secretin may be due to the differences of the preparations of secretin or to the species differences of the experimental animals, or of the experimental system itself.The present investigation was undertaken to determine the effect of natural porcine secretin on exocrine and endocrine secretions from the rat pancreas in vivo and in vitro.Pancreases from Wistar male rats were isolated and perfused according to the technique of Kanno. A Krebs Ringer bicarbonate solution containing 4.6% dextran T-70, 0.25% bovine serum albumin and 50 mg/100 ml glucose was used as a perfusion medium. Commercially available secretin (Secrepan®; Eisai Co., Ltd., Tokyo, Japan : supplied as lyophilized powder containing 30% pure natural porcine secretin and stabilizer), an inactive placebo of secretin containing cystaine and alanine as stabilizers for natural secretin, and 100% pure natural porcine secretin without stabilizer were used as stimulants of pancreatic exocrine and endocrine secretions.Overnight-fasted Wistar strain rats were used in the in vivo experiments. The rats were anesthetized with sodium pentobarbital by subcutaneous injection. The body temperature was held at 37°C by a heating pad. One of the following solutions was administered into the jugular vein by a single rapid injection : 1 and 50 U/kg natural porcine secretin (Se-crepan®), and a comparable dose of the placebo of secretin. Blood samples were collected from the portal vein at -10, 0, 2.5, 5, 10, 30 and 60 min.Immunoreactive insulin (IRI) was measured by polyethylene glycol radioimmunoassay. Immunoreactive glucagon (IRG) was determined by radioimmunoassay by means of dextran coated charcoal using antiserum 30 K. Rat insulin and porcine glucagon were used as standards in the IRI and IRG assays, respectively.The lowest secretin concentration causing a significant increase in pancreatic flow rate and amylase output from the isolated perfused rat pancreas was 1 mU/ml. The maximal pancreatic flow rate and amylase output required a secretin concentration of 1 U/ml. Higher concentrations of secretin, however, resulted in lower responses of both pancreatic juice flow and amylase output. Commercial secretin at concentrations of 1 mU/ml to 2 U/ml in the presence of 50 mg/100 ml glucose did not cause an increase in the IRI concentration in the portal effluent above the basal value. Secretin at the concentration of inducing the maximal pancreatic exocrine secretion with 50 mg/100 ml glucose (1 U/ml) was infused for 10 min in the presence of 100 and 150 mg/100 ml glucose to determine whether a threshold level of glucose was required for the insulinotropic action of secretin. Commercial secretin at a concentration of 1 U/ml elicited a significant short-termed increase in the IRI response to 100 and 150 mg/100 ml glucose, while 1 U/ml pure natural porcine secretin with-out stabilizer had no effect on IRI secretion, even when perfused with 100 or 150 mg/100 ml glucose.