Abstract

The object of this study was to determine the molecular species of verapamil (Vp) which is responsible for the negative inotropic activity of the drug. Toward this end, we developed a model system which allowed us to determine the concentrations of protonated and neutral Vp and four congeners in solution and bound to an amphiphilic interface. We then determined the effect of Vp and these congeners on twitch tension in sheep trabecular muscle and correlated species abundance with biologic activity. The cumulative log dose-response curves of Vp, acyano-Vp and nor-Vp were sigmoidal. The concentrations of drug which were required to inhibit twitch tension by 50% (IC50) were 1000 +/- 240, 3350 +/- 620 and 7140 +/- 950 nM, respectively (mean +/- S.D.; P less than .01). Quaternary-Vp, an amphiphilic, nonionizable protonated derivative, produced some nonspecific inhibition which never reached 50%. Tetrahydroxy-Vp, a hydrophilic ionizable derivative, was inactive. Concentrations of the membrane-bound, neutral form of Vp, acyano-Vp and nor-Vp were comparable at their IC50 values (86 +/- 19, 104 +/- 19 and 121 +/- 16 nM, respectively; mean +/- S.D.; P, N.S.). In contrast, the concentrations of the membrane-bound, protonated form were significantly different and accounted for the "apparent" difference in the IC50 values. Based on these data, we conclude that: 1) ionizability and amphiphilicity are required for the negative inotropic effects of verapamil, acyano-Vp and nor-Vp; 2) twitch tension correlates best with the concentration of the membrane-bound neutral form of each drug; and 3) the potency differences between Vp, acyano-Vp and nor-Vp, as reflected by their IC50 values, are apparent, not real.