Relationship of binding of a calcium channel blocker to inhibition of contraction in intact cultured embryonic chick ventricular cells.

To study the mechanism of action of calcium channel-blocking drugs in intact, functioning myocardial tissue under physiological conditions, we related the inhibition of contraction of spontaneously beating monolayers of cultured chick embryo ventricular cells to studies of binding of four calcium channel-blocking drugs to the same preparation under the same physiological conditions. Nitrendipine produced a concentration-dependent decrease in amplitude of contraction (IC50 = 23 nM) as determined by a computer-assisted optical-video system. Studies using [3H]nitrendipine and equilibrium-binding techniques revealed a high affinity binding site in ventricular homogenates (KD = 0.15 nM) and on intact cultured heart cells (KD = 0.26 nM, Bmax = 51 fmol/mg of protein). These dissociation constants are similar to those reported by other workers for ventricular homogenates, but 100-fold lower than the IC50 for the negative inotropic effect in intact cells. Computer analysis of displacement of [3H]nitrendipine by unlabeled nitrendipine yielded a slope factor of 0.76 and identified an additional low affinity binding site for nitrendipine with KD = 19 nM, in good agreement with the IC50 for inhibition of contraction. Verapamil partially interacted with nitrendipine binding, whereas diltiazem showed no interaction. We conclude that, in intact myocardial cells, there are two classes of binding sites for nitrendipine; interaction of dihydropyridines with the low affinity site appears to mediate their effects on contractile function.