Palmitate increases L-type Ca2+ currents and the size of the readily releasable granule pool in mouse pancreatic...

We have investigated the in vitro effects of the saturated free fatty acid palmitate on mouse pancreatic beta-cells by a combination of electrophysiological recordings, intracellular Ca(2+) ([Ca(2+)](i)) microfluorimetry and insulin release measurements. Addition of palmitate (1 mm, bound to fatty acid-free albumin) to intact islets exposed to 15 mm glucose increased the [Ca(2+)](i) by approximately 30% and insulin secretion 2-fold. Palmitate remained capable of increasing [Ca(2+)](i) and insulin release in the presence of tolbutamide and in islets depolarized by high K(+) in combination with diazoxide, indicating that the stimulation occurs independently of closure of ATP-regulated K(+) channels (K(ATP) channels). Palmitate (0.5 mm) augmented exocytosis (measured as an increase in cell capacitance) in single beta-cells and increased the size of the readily releasable pool (RRP) of granules 2-fold. Whole-cell peak Ca(2+) currents rose by approximately 25% following addition of 0.5 mm palmitate, an effect that was abolished in the presence of 10 microm isradipine indicating that the free fatty acid specifically acts on L-type Ca(2+) channels. The actions of palmitate on exocytosis and Ca(2+) currents were not mimicked by intracellular application of palmitoyl-CoA. We conclude that palmitate increases insulin secretion by a K(ATP) channel-independent mechanism exerted at the level of exocytosis and that involves both augmentation of L-type Ca(2+) currents and an increased size of the RRP.