Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) may play a key role in Ca2+-induced insulin secretion. We have previously reported that treatment of insulinoma MIN6 cells with secretagogues activated CaM kinase II and increased the phosphorylation of synapsin I, followed by insulin secretion. Here, we identified isoforms of CaM kinase II in MIN6 cells and rat islets. Immunoblot analysis suggested that the major isoforms of CaM kinase II were β′e and δ2 at the protein level in MIN6 cells. Only the β′e isoform was detected in rat islets by both RT-PCR and immunoblot analysis. We transiently overexpressed β′e and δ2 isoforms in MIN6 cells and confirmed that treatment of cells with tolbutamide and glucose activated the isoforms. Immunoblot analysis with an antibody against synapsin I phosphorylated by CaM kinase II demonstrated that treatment with tolbutamide and glucose rapidly increased phosphorylation of synapsin I and that phosphorylation was potentiated by overexpression of the isoforms. The secretagogue-induced insulin secretion was potentiated by overexpression of the isoforms. Our results further support our conclusion that activation of CaM kinase II and the concomitant phosphorylation of synapsin I contribute to insulin secretion from pancreatic β-cells.
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