Ca(2+)-independent inhibition of inositol trisphosphate receptors by calmodulin: Redistribution of calmodulin as a possible means of regulating Ca(2+) mobilization

The interactions between calmodulin, inositol 1,4,5-trisphosphate (InsP(3)), and pure cerebellar InsP(3) receptors were characterized by using a scintillation proximity assay. In the absence of Ca(2+), (125)I-labeled calmodulin reversibly bound to multiple sites on InsP(3) receptors and Ca(2+) increased the binding by 190% ± 10%; the half-maximal effect occurred when the Ca(2+) concentration was 184 ± 14 nM. In the absence of Ca(2+), calmodulin caused a reversible, concentration-dependent (IC(50) = 3.1 ± 0.2 μM) inhibition of [(3)H]InsP(3) binding by decreasing the affinity of the receptor for InsP(3). This effect was similar at all Ca(2+) concentrations, indicating that the site through which calmodulin inhibits InsP(3) binding has similar affinities for calmodulin and Ca(2+)-calmodulin. Calmodulin (10 μM) inhibited the Ca(2+) release from cerebellar microsomes evoked by submaximal, but not by maximal, concentrations of InsP(3). Tonic inhibition of InsP(3) receptors by the high concentrations of calmodulin within cerebellar Purkinje cells may account for their relative insensitivity to InsP(3) and limit spontaneous activation of InsP(3) receptors in the dendritic spines. Inhibition of InsP(3) receptors by calmodulin at all cytosolic Ca(2+) concentrations, together with the known redistribution of neuronal calmodulin evoked by protein kinases and Ca(2+), suggests that calmodulin may also allow both feedback control of InsP(3) receptors and integration of inputs from other signaling pathways.