Effect of calmodulin antagonists on calmodulin-induced biphasic modulation of Ca2+-induced Ca2+ release

1 Calmodulin (CaM) has a biphasic effect on Ca2+-induced Ca2+ release (CICR) from the sarcoplasmic reticulum (SR): potentiation and inhibition at low (pCa>6.0) and high (pCa 5) Ca2+ concentrations, respectively. To characterize the mode of action of CaM, we studied the effect of CaM antagonists on the CICR in skinned muscle fibres of the rabbit. Ca2+ release was measured by microfluorometry with Fura-2. 2 A CaM antagonist, trifluoperazine (TFP), potentiated the CICR in a dose-dependent manner (10-300 mu M) at pCa 6, where a simple reversal of the CaM effect would be inhibition of the CICR. Furthermore, 100 mu M TFP sensitized the CICR to Ca2+. A similar effect was produced by other CaM antagonists that were tested: chlorpromazine, W-7, mastoparan, and peptide fragment of CaM-binding residues of CaM-dependent protein kinase II. 3 The biphasic effect of CaM on the CICR was observed even in the presence of high concentrations of CaM antagonists or CaM-binding peptides. 4 From these results we suggest that CaM has a unique mode of action on the CICR which is quite different from the effect of CaM on known enzymes.