EFFECTS OF METAL-CATIONS AND CALMODULIN ANTAGONISTS ON [H-3] NITRENDIPINE BINDING IN SMOOTH AND CARDIAC-MUSCLE

Abstract

It was previously reported that [3H]nitrendipine binding to a microsomal fraction from intestinal smooth muscle was dependent upon the presence of divalent metal cations. The effects of cations and calmodulin antagonists on [3H]nitrendipine binding in smooth and cardiac muscle have been studied further. Treatment of ileal and aortic smooth muscle and cardiac muscle with EDTA reduced specific [3H]nitrendipine binding by 70-95%. Microsomes from rabbit ventricle were more resistant to EDTA treatment than were those from ileal smooth muscle, but low concentrations of Ca2+ (< 10-5 M) produced half-maximal restoration of binding in both tissues. The ability of cations at a concentration of 10-3 M to restore binding to membranes from guinea-pig ileum was in the sequence, Ca2+ = Sr2+ > Mg2+ = Mn2+ = Co2+ > Ba2+ = Ni2+ > Zn2+ = Cd2+ > La3+ = Sm3+ = Tm3+. In contrast to the activation of calmodulin-dependent processes, the ability of these cations to restore [3H]nitrendipine binding did not correlate linearity with ionic radius. Calmodulin antagonists were found to inhibit [3H]nitrendipine binding with the order of potency: pimozide > < calmidazolium (R 24571) > trifluoperazine > chlorpromazine > promethazine > chlorpromazine sulfoxide; that correlates well with the potency of these drugs as inhibitors of calmodulin-dependent processes. Calmodulin antagonists bind apparently to a protein associated with the [3H]nitrendipine binding site that has a hydrophobic domain similar to that exposed on calmodulin by Ca2+, but this protein is not calmodulin itself.