The binding of the calcium channel antagonist [3H]nitrendipine (3[H]Ntd) to purified bovine cardiac sarcolemmal vesicles was examined at different membrane potentials. The vesicles were loaded with 150 mM K and exposed to a range of external K concentrations to induce negative internal membrane potentials using N-methyl-D-glucamine or choline to substitute for K. The lipophilic cation [3H]tetraphenylphosphonium was used to quantitate the membrane potential, and the vesicles were capable of maintaining a relatively stable negative inside membrane potential for at least 15 min. Equilibrium binding studies of [3H]Ntd performed on vesicles maintained at a depolarized potential (0 mV) revealed 24.7 +/- 5.6% more high affinity [3H]Ntd-binding sites than were present on vesicles maintained at a hyperpolarized potential (approximately -40 to -50 mV) with N-methyl-D-glucamine substituting for K. There was no significant change in Kd values which were 0.398 +/- 0.069 and 0.388 +/- 0.032 nM, respectively. When choline was used to substitute for K, comparable results were obtained with 19.01 +/- 3.4% more high affinity [3H]Ntd-binding sites under depolarized conditions and no significant change in Kd (0.356 +/- 0.025 and 0.379 +/- 0.030 nM, respectively). Varying the external K concentration resulted in a graded change in [3H]Ntd binding over the same range of external K concentrations which resulted in the greatest change in membrane potential. These findings suggest that the observed effect was due to changes in membrane potential rather than changes in the composition of the incubation medium. Additional control experiments employing choline-loaded vesicles also support the hypothesis that the observed effect was primarily due to changes in membrane potential. The present results are compared with those from electrophysiological studies.
Animals Cattle