Voltage-dependent binding of dihydropyridine calcium channel blockers to guinea pig ventricular myocytes

The voltage dependence of the binding of dihydropyridine Ca channel blockers (+)-[3H]PN200-110 and [3H]nitrendipine to enzymatically isolated guinea pig ventricular myocytes was examined. The equilibrium saturation binding of (+)-[3H]PN200-110 could be well described by a simple 1:1 binding scheme under all conditions tested. The results demonstrated that the effect of depolarization induced by high extracellular K+ (50 mM) compared to normal K+ (4.8 mM) was an increase in the observed number of binding sites (Bmax) with no change in the measured affinity of binding (Kd). Similarly, depolarization induced by a combination of Na channel toxins in the presence of normal extracellular K+ caused an increase in the observed binding of (+)-[3H]PN200-110 comparable to that observed with high K+. [3H]Nitrendipine binding was likewise increased by depolarization in the presence of 50 mM K+ compared to 4.8 mM K+. Percoll density gradient centrifugation was found to enrich the cell preparation with viable myocytes and increased the measured voltage-dependent change in binding. In agreement with the predictions from previous studies (Kamp and Miller, 1987a), the magnitude of the observed change in Bmax was directly related to the fraction of cells which were viable in a given experiment. These results are discussed in comparison to the modulated receptor hypothesis proposed from electrophysiological studies.