Verapamil, diltiazem and nifedipine interactions with calmodulin stimulated (Ca2+ + Mg2+)-ATPase

The functional interactions of the three prototype Ca2+ antagonists, verapamil, diltiazem and nifedipine, were examined in relation to the calmodulin regulated plasma membrane Ca2+ pump ATPase. For this we used low ionic strength derived, calmodulin depleted, human red cell ghost membranes. Exogenously added calmodulin activated basal (Ca2+ + Mg2+)-ATPase in a concentration-dependent manner. Half-maximal activation by 6 nM calmodulin was antagonized by 10(-3) M verapamil and 10(-3) M diltiazem 25.1 and 12.1% respectively. The inhibition appeared to be specific for calmodulin activation since basal activity was not affected by these agents. Nifedipine had no effects on basal or calmodulin stimulated (Ca2+ + Mg2+)-ATPase activity. Unlike dihydropyridine modulation of verapamil and diltiazem binding at high affinity channel sites, nifedipine in this system did not alter the inhibitory responses of verapamil and diltiazem. The calmodulin directed antagonism of the two drugs was shown to be strictly additive over a full range of calmodulin concentrations and appeared to change predominantly the Vmax and, to a lesser degree, the affinity of calmodulin for the (Ca2+ + Mg2+)-ATPase. It is concluded that this model system provides evidence for additional functional discrepancies among the various classes of Ca2+ antagonists.