Metabolic and secretory effects of methylamine in pancreatic islets

The metabolic and secretory effects of methylamine in rat pancreatic islets were investigated. Methylamine accumulated in islet cells, was incorporated into endogenous islet proteins, and inhibited the incorporation of [2,5-3H] histamine into either N,N-dimethylcasein or endogenous islet proteins. Methylamine (2 mM) did not affect the oxidation of glucose or endogenous nutrients or the intracellular pH in islet cells. Glucose did not affect the activity of transglutaminase in islet homogenates, the uptake of 14C-methylamine by intact islets or its incorporation into endogenous islet proteins. Methylamine inhibited insulin release evoked by glucose, other nutrient secretagogues, and non-nutrient insulinotropic agents such as L-arginine or gliclazide. The inhibitory effect of methylamine upon insulin release was diminished in the presence of cytochalasin B or at low extracellular pH. Methylamine retarded the conversion of proinsulin to insulin. Trimethylamine (0.7 mM) was more efficiently taken up by islet cells than methylamine (2.0 mM), and yet caused only a modest inhibition of insulin release. These findings suggest that methylamine interferes with a late step in the secretory sequence, possibly by inhibiting the access of secretory granules to their exocytotic site.