Bepridil and cetiedil vasodilators which inhibit Ca²⁺-dependent calmodulin interactions with erythrocyte membranes

Two new vascular smooth muscle relaxants, bepridil and cetiedil, were found to possess specific CaM-inhibitory properties which resembled those of trifluoperazine. Trifluoperazine, bepridil, and cetiedil inhibited Ca²⁺-dependent ¹²⁵I-CaM binding to erythrocyte membranes and CaM activation of membrane Ca²⁺-ATPase with IC₅₀ values of ~ 12, ~ 17, and ~ 40 μM, respectively. This does not appear to be the result of a nonspecific hydrophobic interaction since inhibition was not observed with micromolar concentrations of many other hydrophobic agents. The predominant inhibition of binding and Ca²⁺-ATPase activation was competitive with respect to CaM. Bepridil and cetiedil bind directly to CaM since these drugs displaced [³H]trifluoperazine from sites on CaM. Inhibition of Ca²⁺-ATPase and binding by the drugs was not due to interference with the catalytic activity of this enzyme since: (a) neither inhibition of CaM-independent basal Ca²⁺-ATPase activity nor inhibition of proteolytically-activated Ca²⁺-ATPase activities were produced by these agents, and (b) no drug-induced inhibition of CaM binding was detected when membranes were preincubated with these agents but washed prior to addition of ¹²⁵I-CaM. Thus, bepridil and cetiedil competitively inhibit Ca²⁺-dependent interactions of CaM with erythrocyte membranes, most likely by a direct interaction between these drugs and CaM. The principal clinical actions of these drugs may be explained by their interactions with CaM or CaM-related proteins leading to reduced activation of Ca²⁺-regulated enzymes in certain other tissues, such as myosin light chain kinase in vascular smooth muscle.