Succinate stimulates insulin secretion and proinsulin biosynthesis. We studied the effects of NADPH-modulating pathways on glucose- and succinate-stimulated insulin secretion and proinsulin biosynthesis in the rat and the insulin-resistant Psammomys obesus (P. obesus). Disruption of the anaplerotic pyruvate/malate shuttle by phenylacetic acid (PAA) inhibited glucose- and succinate-stimulated insulin secretion and succinate-stimulated proinsulin biosynthesis in both species. In contrast, PAA failed to inhibit glucose-stimulated proinsulin biosynthesis in P. obesus islets. Inhibition of the NADPH-consuming enzyme nitric oxide synthase (nNOS) with L-N^G-nitro-L-arginine methyl ester (L-NAME) or with N_G-monomethyl-L-arginine (L-NMMA) doubled succinate-stimulated insulin secretion in rat islets, suggesting that succinate and nNOS derived signals interact to regulate insulin secretion. In contrast, nNOS inhibition had no effect on succinate-stimulated proinsulin biosynthesis in both species. In P. obesus islets insulin secretion was not stimulated by succinate in the absence of glucose, whereas proinsulin biosynthesis was increased 5-fold. Conversely, under stimulating glucose levels succinate doubled insulin secretion, indicating glucose-dependence. Pyruvate ester and inhibition of nNOS partially mimicked the permissive effect of glucose on succinate-stimulated insulin secretion, suggesting that anaplerosis-derived signals render the -cells responsive to succinate.

We conclude that -cell anaplerosis via pyruvate carboxylase is important for glucose- and succinate-stimulated insulin secretion and for succinate-stimulated proinsulin biosynthesis. In P. obesus, pyruvate/malate shuttle dependent and independent pathways that regulate proinsulin biosynthesis coexist; the latter can maintain fuel stimulated biosynthetic activity when the succinate dependent pathway is inhibited. nNOS signaling is a negative regulator of insulin secretion, but not of proinsulin biosynthesis.