Calcium channels are ‘Unblocked’ within a few milliseconds after photoremoval of nifedipine

Abstract

The organic Ca2+ antagonists are potent inhibitors of Ca2+ influx in cardiac and smooth muscle and are widely used clinically in the treatment of various cardiovascular disorders. It appears that Ca2+ antagonist binding prevents the normal movement of ions through Ca2+ channels, perhaps via an open-channel blockade mechanism. Although this concept is generally accepted, questions do remain about the detailed relationship between binding and blockade in the case of the structurally diverse organic Ca2+ channel blockers; e.g., (1) do they bind preferentially to open, closed and/or inactivated channels; (2) are there multiple binding sites; (3) do they act at extracellular and/or intracellular sites; and (4) does blocking or unblocking depend on membrane potential or its history? The dihydropyridine Ca2+ antagonist, nifedipine, contains an o-nitrobenzyl moiety and is photolabile; irradiation yields a molecule devoid of channel blocking activity and the photoconversion reactions are complete within 100 microseconds. Taking advantage of these properties to study the mechanistic details of nifedipine blockade of Ca2+ channels, we examined the waveform of the slow inward Ca2+ current (Isi) in atrial fibers before and following flash-induced removal of nifedipine. After flashes, we find that nifedipine blockade is reversed within at most a few milliseconds and that the rate of Isi reactivation parallels the normal, voltage-dependent activation rate. Our results imply that nifedipine binds to and stabilizes resting, closed Ca2+ channels and are not in agreement with the recent conclusions of Morad and coworkers that photoconversion of nifedipine must be followed by membrane repolarization in order to effect recovery of Isi and tension.(ABSTRACT TRUNCATED AT 250 WORDS)