Ca(2+)-induced loss of Ca2+/calmodulin-dependent protein kinase II activity in pancreatic beta-cells

Elevations in intracellular Ca2+ in electrically permeabilized islets of Langerhans produced rapid insulin secretory responses from beta-cells, but the Ca(2+)-induced secretion was not maintained and was irrespective of the pattern of administration of elevated Ca2+. Ca(2+)-insensitive beta-cells responded normally to activators of protein kinase C or cAMP-dependent kinase with increased insulin secretion. The loss of secretory responsiveness to Ca2+ was paralleled by a reduction in Ca(2+)-induced protein phosphorylation. This was caused by a reduction in Ca2+/calmodulin-dependent protein kinase II (CaMK II) activity in the desensitized cells, as assessed by measuring the phosphorylation of a CaMK II-specific exogenous substrate, autocamtide-2. The Ca(2+)-induced reductions in kinase activity and protein phosphorylation were not dependent on the activation of Ca(2+)-dependent protein kinases and were not caused by the activation of phosphoprotein phosphatases or of Ca(2+)-activated proteases. The concomitant reductions in CaMK II activity and Ca(2+)-induced insulin secretion suggest that the activation of CaMK II is required for normal insulin secretory responses to increased intracellular Ca2+ concentrations.