Investigation of intracellular signalling cascades mediating stimulatory effect of a Gymnema sylvestre extract on insulin secretion from isolated mouse and human islets of Langerhans

Aim Traditional plant-based remedies such as Gymnema sylvestre (GS) extracts have been used to treat diabetes mellitus for many centuries. We have shown previously that a novel GS extract, OSA®, has a direct effect on insulin secretion but its mode of action has not been studied in detail Thus this study investigated the possible underlying mechanism(s) by which OSA® exerts its action. Methods The effects of OSA® on [Ca2+]i and K+ conductances were assessed by Ca2+ microfluorimetry and electrophysiology in dispersed mouse islets and MIN6 β-cells, respectively. Isolated mouse (from 20 to 25 mice) and human (from 3 donors) islets, and MIN6 β-cells, were used to investigate whether the stimulatory effect of OSA® on insulin secretion was dependent on the presence of extracellular calcium and protein kinase activation. Results OSA ®-induced insulin secretion from mouse islets and MIN6 β-cells was inhibited by nifedipine, a voltage-gated Ca2+ channel blocker, and by the removal of extracellular Ca2+, respectively. OSA® did not affect the activities of KATP channels or voltage-dependent K+ channels in MIN6 β-cells but it caused an increase in intracellular Ca2+ ([Ca2+]i) concentrations in Fura-2-loaded mouse islet cells. The insulin secretagogue effect of OSA® was dependent, in part, on protein kinase activation since incubating mouse or human islets with staurosporine, a general protein kinase inhibitor, resulted in partial inhibition of OSA®-induced insulin secretion. Experiments using permeabilized, Ca2+-clamped MIN6 β-cells revealed a Ca2+-independent component action of OSA® at a late stage in the stimulus–response coupling pathway. OSA®-induced insulin secretion was unexpectedly associated with a decrease in intracellular cAMP levels. Conclusions These data indicate that the GS isolate OSA® stimulates insulin secretion from mouse and human islets in vitro, at least in part as a consequence of Ca2+ influx and protein kinase activation.