The stimulus-secretion coupling of glucose-induced insulin release. Metabolic and functional effects of NH4+ in rat islets.

Abstract

A B S T R AC T NH4+ caused a dose-related, rapid, and reversible inhibition of glucose-stimulated insulin release by isolated rat islets. It also inhibited glyceraldehyde-, Ba2+-, and sulfonylurea-stimulated insulin secretion. NH4 failed to affect glucose utilization and oxidation, glucose-stimulated proinsulin biosynthesis, the concentration of ATP, ADP, and AMP, and the intracellular pH. NH4+ also failed to affect the ability oftheophylline and cytochalasin B to augment glucose- induced insulin release. However, in the presence and absence of glucose, accumulation of NH4+ in islet cells was associated with a fall in the concentration ofNADH and NADPH and a concomitant alteration of "Rb+ and 45Ca2+ (or 133Ba2+) handling. These findings suggest that reduced pyridine nucleotides, generated by the metabolism of endogenous or exogenous nutrients, may modulate ionophoretic processes in the islet cells and by doing so, affect the net uptake of Ca2+ and subsequent release of insulin.