Effects of metal cations and calmodulin antagonists on [³H]nitrendipine binding in smooth and cardiac muscle

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 (Bolger et al., J. Pharmacol. Exp. Ther. 225: 291-309, 1983). 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 to 95%. Microsomes from rabbit ventricle were more resistance to EDTA treatment than were those from ileal smooth muscle, but low concentrations of Ca⁺⁺ (<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, Ca⁺⁺ = Sr⁺⁺ > Mg⁺⁺ = Mn⁺⁺ = Co⁺⁺ > Ba⁺⁺ = Ni⁺⁺ > Zn⁺⁺ = Cd⁺⁺ > La⁺⁺⁺ = Sm⁺⁺⁺ = Tm⁺⁺⁺. In contrast to the activation of calmodulin-dependent processes, the ability of these cations to restore [³H]nitrendipine binding did not correlate linearly with ionic radius. However, 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 quite well with the potency of these drugs as inhibitors of calmodulin-dependent processes. The results suggest that calmodulin antagonists bind to a protein associated with the [³H]nitrendipine binding site that has a hydrophobic domain similar to that exposed on calmodulin by Ca⁺⁺, but that this protein is not calmodulin itself.