The interaction between cAMP-dependent and cAMP-independent mechanisms in mediating the somatostatin inhibition of insulin secretion in isolated rat pancreatic islets

Abstract

To characterize the intracellular mechanisms by which somatostatin modulates the insulin secretion, studies were performed with isolated rat pancreatic islets at 12 mmol l-1 glucose. Somatostatin (0.1-1000 nmol l-1) inhibited the glucose-induced insulin secretion concentration-dependently. Increasing intracellular cAMP concentration either with dibutyryl-cAMP (1 mmol l-1) or by the adenylate cyclase activator forskolin (20 mumol l-1) partly reversed the inhibition by somatostatin (100 nmol l-1). Neither somatostatin (100 nmol l-1) nor dibutyryl-cAMP (1 mmol l-1 were able to affect the low insulin secretion observed in the absence of extracellular Ca2+. To study cAMP-independent mechanisms of somatostatin, the experiments were performed with and without dibutyryl-cAMP (1 mmol l-1) present. Both somatostatin (100 nmol l-1) and the Ca(2+)-channel blocker verapamil (25 mumol l-1) inhibited the insulin secretion both with and without dibutyryl-cAMP present. An additional inhibition of the insulin secretion was observed when somatostatin was combined with verapamil in the absence, but not in the presence of dibutyryl-cAMP. We conclude that somatostatin inhibits the glucose-induced insulin secretion both by cAMP-dependent mechanism which requires extracellular Ca2+, and by cAMP-independent/verapamil-sensitive Ca(2+)-channel-dependent mechanism.