Inhibition of the type 1 inositol 1,4,5-tris-phosphate-sensitive Ca²+ channel by calmodulin antagonists

This study describes the effects of a number of calmodulin antagonists on the cerebellar type 1 inositol 1,4,5-trisphosphate (InsP(3)) receptor. All the antagonists tested (trifluoperazine, fluphenazine, chlorpromazine and calmidazolium) inhibited the extent of InsP(3)-induced Ca2+ release (IICR) with similar IC50 values between 60 and 85 muM). They did not affect the efficacy of InsP(3) to release Ca2+, since the concentrations of InsP(3) required to cause half-maximal release was little affected in the presence of these agents. Tn addition, these agents did not affect InsP(3) binding to its receptor. Stopped-flow studies to determine the rate constants of IICR showed this process to be biphasic with a fast and slow component. All the calmodulin antagonists appeared to reduce the rate constants for Ca2+ release in a phase-specific manner, preferentially reducing the fast phase component. Chlorpromazine (75 muM) appeared to have the most potent effect on the fast phase rate constant, reducing it from 1.0 to 0.08 s(-1), while only reducing the rate constant for the slow phase about two fold (0.2-0.08 s(-1)). The fact that calmodulin itself inhibits both IICR and InsP(3) binding, while these calmodulin antagonists also reduce Ca2+ release and do not affect InsP(3) binding, suggests that the mechanism of action of these agents is unlikely to be due to the reversal of the modulatory action of calmodulin on this receptor. (C) 2001 Elsevier Science Inc. All rights reserved.