INHIBITION OF SERINE THREONINE PROTEIN PHOSPHATASES PROMOTES OPENING OF VOLTAGE-ACTIVATED L-TYPE CA2+ CHANNELS IN INSULIN-SECRETING CELLS/

The biological activity of many proteins, including voltage-sensitive ion channels, is controlled by their state of phosphorylation. Ca2+ in flux through voltage-activated L-type Ca2+ channels serves as the majo r stimulatory signal in insulin-secreting cells. We have now investiga ted the extent to which Ca2+ handling in clonal insulin-secreting RiNm 5F cells was affected by okadaic acid, an inhibitor of various serine/ threonine protein phosphatases. Whole-cell patch-clamp experiments sho wed that okadaic acid generated an increase in membrane current, sugge sting that it promotes Ca2+ influx through L-type voltage-gated Ca2+ c hannels probably by modifying their phosphorylation state. Okadaic aci d was found to provoke a transient rise in the cytoplasmic free Ca2+ c oncentration ([Ca2+](i)), but had no further effect on the K+-induced increase. The Ca2+ transient induced by okadaic acid was dependent on the presence of extracellular Ca2+ and was abolished by D600, a blocke r of voltage-activated L-type Ca2+ channels. Concomitant with the rise in [Ca2+](i), okadaic acid induced insulin secretion, a phenomenon th at was also dependent on extracellular Ca2+. It is proposed that hyper phosphorylation of voltage-activated L-type Ca2+ channels in insulin-s ecreting cells lowers the threshold potential for their activation.