The effect of four slow Ca2+channel blockers (felodipine, nifedipine, prenylamine and bepridil) that possess the ability to bind to calmodulin (CaM) and to inhibit myosin light chain kinase (MLCK) on CaM-regulated Ca2+pumping ATPase of cardiac sarcolemma (SL) and brain cyclic AMP phosphodiesterase (PDE) was studied. The ability of these drugs to inhibit Ca2+pumping ATPase correlated with their inhibitory effect on CaM-activated Ca2+-dependent PDE. Nifedipine was unable to inhibit markedly both enzymes. Prenylamine also was a weak inhibitor, which was unexpected because of its CaM binding potency. Felodipine (10-50 μM) and bepridil (50 μM) markedly reduced activities of SL Ca2+pumping ATPase and PDE. Striking differences were, however, demonstrated when Ca2+and CaM concentrations, respectively, were increased. Previously it was reported that inhibition of the SL Ca2+pumping ATPase by the CaM antagonist calmidazolium could be overcome by increasing Ca2+concentrations (J. M. J. Lamers and J. T. Stinis, Cell Calcium 4, 281-294, 1983). Felodipine (10-50 μM) in the present study, appeared to be equipotent with calmidazolium in reducing Ca2+pumping ATPase, but increasing Ca2+up to 12.2 μM could not counteract this effect. Felodipine (2-10 μM) also inhibited brain PDE noncompetitively with respect to CaM contrary to the competitive effectors calmidazolium and bepridil. On the other hand, bepridil (10-20 μM) decreased or increased Ca2+pumping ATPase activity depending on the Ca2+concentration (0.29 and 12.2 μM, respectively) used. These findings suggest at least two types of CaM antagonists, which can be discriminated on basis of their inhibition patterns of PDE and heart SL Ca2+pumping ATPase. © 1985.
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