The effect of secretin on the exocrine and endocrine pancreas. Part III. Exocrine and endocrine responses to synthetic secretin in the perfused isolated rat pancreas (author's transl)

Abstract

The effects of synthetic secretin on the rate of flow of pancreatic juice, the rate of output of amylase, and the rate of release of immunoreactive insulin (IRI) and immunoreactive glucagon (IRG) were simultaneously investigated in the isolated perfused rat pancrease. In this experimental system, factors other than the direct effect of hormonal stimulus on the pancreas, such as the autonomous nervous system or alteration of blood glucose concentrations, could be eliminated while simultaneously determining pancreatic exocrine and endocrine function.Pancreases from male Wistar rats, weighing 250 - 300 g and fed ad libitum, were isolated and perfused according to the technique of Kanno. The pancreas was perfused with a Krebs Ringer bicarbonate solution of 4.6% dextran-T 70, 0.25% bovine serum albumin and 50 mg/100 ml glucose, and gassed constantly with a 95% 02, 5% CO2 mixture to achieve pH 7.4. Perfusate was administered into the superior mesenteric and celiac artery via a non-recirculating open circuit. Flow rate was kept at about 2.0 ml/min with the aid of a micro-tubing pump. The total portal effluent was collected in chilled tubes at 60 sec intervals for the measurement of IRI and IRG concentrations.Synthetic secretin was applied by changing the medium reservoir. When the effect of secretin was studied, a 20-min basal period was followed by a 20-min stimulatory period and then a 20-min recovery period.To determine whether secretin might require high glucose levels for an insulinotropic action, an infusion of secretin at a concentration of 100 ng/ml was superimposed on 100 and 150 mg/100 ml glucose stimulation.For measurement of pancreatic exocrine secretions, a calibrated capillary tube was attached to the free end of the pancreatic cannula, which was inserted into the distal end of the common duct at a point shortly before its entrance into the duodenum and tied in place. The proximal end of the bile duct was ligated. Every 10 min the capillary tube was replaced and the flow rate of the pancreatic juice was measured. The sample of juice was diluted with a 6% bovine serum albumin solution, and amylase activity was determined by the chromogenic method with blue-dyed starch polymer. Amylase activity was expressed as Somogyi units/10 min.IRI was measured by polyethylene glycol radioimmunoassay. IRG was determined by radioimmunoassay with the talc absorption technique. Rat insulin and porcine glucagon were used as standards in IRI and IRG assays, respectively.No changes of IRI levels in the perfusate were obtained with concentrations of synthetic secretin ranging from 0.01 ng/ml to 2 μg/ml. Secretin had no influence on glucose-induced IRI release when superimposed on 100 or 150 mg/100 ml glucose stimulation. On the other hand, uniphasic IRG release was elicited by synthetic secretin in a dose-related fashion in the presence of 50 mg/100 ml glucose. However, secretin-induced IRG responses were completely abolished in the presence of 100 or 150 mg/100 ml glucose.Contrary to our observations, most of the previous experiments have shown the insulinotropic effect of secretin but have failed to demonstrate the glucagonotropic effect. Moreover, Santensanio et al. have revealed the suppressive effect of secretin on basal and alanine-stimulated IRG secretion in dogs. The reason for this discrepancy between their results and ours is not apparent. It seems that not only species differences but also the doses and the preparation of secretin used, and the levels of glucose concentration perfusing the pancreas have played a part in the contrasting results.A dose-response relationship was obtained between synthetic secretin and the peak rates of the flow and output of amylase. Maximal flow rate of the pancreatic juice and amylase output were induced with a dose of secretin at 1 μg/ml.