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⁺ influx through voltage-activated L-type Ca²⁺ channels serves as the major stimulatory signal in insulin-secreting cells. We have now investigated the extent to which Ca²⁺ handling in clonal insulin-secreting RiNm5F cells was affected by okadaic acid, an inhibitor of various serine/threonine protein phosphatases. Whole-cell patch-clamp experiments showed that okadaic acid generated an increase in membrane current, suggesting that it promotes Ca²⁺ influx through L-type voltage-gated Ca²⁺ channels probably by modifying their phosphorylation state. Okadaic acid was found to provoke a transient rise in the cytoplasmic free Ca²⁺ concentration ([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 blocker of voltage-activated L-type Ca²⁺ channels. Concomitant with the rise in [Ca²⁺](i), okadaic acid induced insulin secretion, a phenomenon that was also dependent on extracellular Ca²⁺. It is proposed that hyperphosphorylation of voltage-activated L-type Ca²⁺ channels in insulin-secreting cells lowers the threshold potential for their activation.