Requirement of calmodulin-dependent protein kinase II in cyclic ADP-ribose-mediated intracellular Ca²⁺ mobilization

Cyclic ADP-ribose (cADPR) is generated in pancreatic islets by glucose stimulation, serving as a second messenger for Ca²⁺ mobilization from the endoplasmic reticulum for insulin secretion (Takasawa, S., Nata, K., Yonekura, H., and Okamoto, H. (1993) Science 259, 370-373). In the present study, we observed that the addition of calmodulin (CaM) to rat islet microsomes sensitized and activated the cADPR-mediated Ca²⁺ release. Inhibitors for CaM-dependent protein kinase II (CaM kinase II) completely abolished the glucose-induced insulin secretion as well as the cADPR-mediated and CaM-activated Ca²⁺ mobilization. Western blot analysis revealed that the microsomes contain the α isoform of CaM kinase II but do not contain CaM. When the active 30-kDa chymotryptic fragment of CaM kinase II was added to the microsomes, fully activated cADPR-mediated Ca²⁺ release was observed in the absence of CaM. These results along with available evidence strongly suggest that CaM kinase II is required to phosphorylate and activate the ryanodine-like receptor, a Ca²⁺ channel for cADPR as an endogenous activator, for the cADPR-mediated Ca²⁺ release.