Nitrendipine block of cardiac calcium channels: high-affinity binding to the inactivated state.

Abstract

Block of Ca2+ currents by the dihydropyridine drug nitrendipine was studied in single canine ventricular cells by using the whole-cell variant of the patch clamp technique. When cells were held at depolarized membrane potentials at which Ca2+ currents were .apprxeq. 70% inactivated, nitredipine blocked Ca2+ currents very potently, with half-block by subnanomolar concentrations. The concentration dependence of block had the form expected for 1:1 binding, with an apparent Kd of 0.36 nM. When cells were held at hyperpolarized potentials, nitrendipine blocked Ca2+ currents much less potently (Kd .apprxeq. 700 nM). The results can be explained if nitrendipine binds very tightly to the inactivated state of the Ca2+ channel and only weakly to the normal resting state. The Kd estimated for binding to the inactivated state is very similar to the dissociation constants previously found for high-affinity [3H]nitrendipine binding to membrane fragments from heart, smooth muscle, brain and other tissues; The concentration-dependent kinetics of binding to the inactivated state are similar to those reported for [3H]nitrendipine binding to membranes. These results make it seem very likely that the high-affinity [3H]nitrendipine binding site is an activated state of the Ca2+ channel.