Short- and long-term effects of long-chain free fatty acids on pancreatic α- and β-cell function

We have investigated the in vitro effects of long-chain free fatty acids (FFAs) on mouse pancreatic α- and β-cells by a combination of electrophysiological recordings, intracellular Ca2+ ([Ca2+]i) microfluorimetry and hormone release measurements. Acute addition of the saturated FFA palmitate to intact islets doubled insulin as well as glucagon secretion in the presence of 1 or 15 mM glucose and the FFA remained capable of enhancing release of both hormones in islets depolarised with high K+ in the presence of the KATP channel opener diazoxide, indicating that alterations of ATP sensitive potassium (KATP) channel activity is not part of the effect. Contrary to the stimulatory effect on α- and β-cell secretion, palmitate inhibited somatostatin secretion from pancreatic δ-cells by ~50%, suggesting that paracrine regulation might be part of the FFA stimulation of insulin and glucagon secretion. The stimulatory effect of palmitate correlated with a 3.5- and 1.3-fold increase in [Ca2+]i in α- and β-cells respectively and whole-cell peak Ca2+ currents were increased by ~25% in both cell types. The effect on the Ca2+ current amplitude was confined to L-type Ca2+ channels and the augmentation was abolished by inclusion of the L-type Ca2+ channel inhibitor isradipine. High resolution capacitance measurements of exocytosis showed that palmitate increased the size of the readily releasable pool of granules 1.5- and >2.3-fold in α-cells and β-cells respectively. While palmitate stimulation in β-cells appeared to encompass a direct effect on the exocytotic machinery, the stimulatory effect in α-cells seemed principally to result from increased Ca2+ influx. When β-cells were instead cultured in the presence of palmitate and a high glucose concentration (15 mM) for 72 hours, β-cell function was impaired. This was evident as increased basal and reduced glucose-stimulated [Ca2+]i, a ~50% reduction of glucose-stimulated insulin secretion as well as a marked degranulation (-75%) and a ~50% decrease in total insulin content. Also, the resting whole-cell KATP channel conductance (measured in 1 mM glucose) was reduced by ~75%. Pre-exposure to the unsaturated FFA oleate caused essentially the same impairments. Surprisingly, β-cells pre-incubated with palmitate demonstrated enhanced depolarisation-evoked Ca2+ currents (~50%) and exocytotic responses, elicited by depolarisations, were augmented by 100%. Oleate pre-treated cells responded similar to control cells. We conclude that short-term exposure to FFAs improves pancreatic α- and β-cell function while long-term exposure has adverse effects on β-cell function. We attribute the latter effect to suppression of KATP channel activity, reduced insulin expression/degranulation and possibly Ca2+-dependent Ca2+ channel inactivation.