The calcium/calmodulin-dependent phosphodiesterase PDE1C down-regulates glucose-induced insulin secretion

To understand the role cAMP phosphodiesterases (PDEs) play in the regulation of insulin secretion, we analyzed cyclic nucleotide PDEs of a pancreatic beta-cell line and used family and isozyme-specific PDE inhibitors to identify the PDEs that counteract glucose-stimulated insulin secretion, We demonstrate the presence of soluble PDE1C, PDE4A and 4D, a cGMP-specific PDE, and of particulate PDES, activities in beta TC3 insulinoma cells. Selective inhibition of PDE1C, but not of PDE4, augmented glucose-stimulated insulin secretion in a dose-dependent fashion thus demonstrating that PDE1C is the major PDE counteracting glucose-dependent insulin secretion from beta TC3 cells. In pancreatic islets, inhibition of both PDE1C and PDE3 augmented glucose-dependent insulin secretion. The PDE1C of beta TC3 cells is a novel isozyme possessing a K-m, of 0.47 mu M for cAMP and 0.25 mu M for cGMP. The PDE1C isozyme of beta TC3 cells is sensitive to 8-methoxymethyl isobutylmethylxanthine and zaprinast (IC50 = 7.5 and 4.5 mu M, respectively) and resistant to vinpocetine (IC50 > 100 mu M) Increased responsiveness of PDE1C activity to calcium/calmodulin is evident upon exposure of cells to glucose. Enhanced cAMP degradation by PDE1C, due to increases in its responsiveness to calcium/calmodulin and in intracellular calcium, constitutes a glucose-dependent feedback mechanism for the control of insulin secretion.