Effects of isoprenaline and glucagon on insulin secretion from pancreatic islets

The effects of isoprenaline and glucagon on insulin secretion from pancreatic islets were investigated. In the presence of high concentrations of isoprenaline (10-50 mumol/l), glucose-induced (20 mmol/l) insulin secretion from isolated perifused mouse islets was inhibited. This inhibition was apparently mediated by alpha 2-adrenoceptors, as it was antagonized by rauwolscine. At low concentrations isoprenaline (0.1 or 1 mumol/l) did not affect glucose-induced (2.5; 10 or 20 mmol/l) insulin secretion from perifused mouse or rat islets, even if alpha 2-adrenoceptors were blocked by rauwolscine. A stimulatory effect of isoprenaline on insulin secretion was also not observed in the perfused rat pancreas. However, when incubated mouse islets were exposed to glucose (10 mmol/l), insulin secretion was further enhanced by isoprenaline (0.5 mumol/l). To elucidate the underlying mechanism, the effects of glucagon on insulin secretion were investigated, because glucagon is released from the pancreatic A-cells during stimulation with isoprenaline and is accumulated in the islets and the surrounding medium during incubations of pancreatic islets. Indeed, glucagon stimulated insulin secretion from perifused mouse islets in the presence of high glucose (10 or 15 mmol/l) concentrations but not of low glucose (5 mmol/l) concentrations. Thus it is concluded that direct beta-adrenergic stimulation of pancreatic B-cells does not occur in mouse or rat pancreatic islets. Augmentation of glucose-induced insulin secretion by isoprenaline observed in incubation systems can be explained as a result of stimulation by glucagon, which is released from pancreatic A-cells by isoprenaline.