Interactions of organic calcium channel antagonists with calcium channels in single frog atrial cells.

Abstract

Inhibition of whole-cell Ca currents in enzymatically dispersed frog atrial myocytes by D-600, diltiazem and nifedipine was studied using a single-micropipette voltage-clamp technique. The applicability of a modulated-receptor hypothesis similar to that proposed for local anesthetic interactions with Na channels was tested to account for the tonic and frequency-dependent interactions of these organic compounds with myocardial Ca channels. Data consistent with such a hypothesis included: prominent use-dependent block of iCa by D-600 and diltiazem, which were predominantly charged at physiological pH; iCa block by an externally applied, permanently charged dihydropyridine derivative was greatly attenuated; all 3 antagonists produced large negative shifts in the voltage dependence of iCa availability; block of iCa by these compounds was state-dependent; reactivation of iCa in the presence of all 3 antagonists was biexponential, which suggested that drug-free channels recovered with a normal time course and drug-bound channels recovered more slowly; and the kinetics of the drug-induced slow iCa recovery process may be determined largely by factors such as size and MW, in addition to lipid solubility of the compounds. Experiments in which the pH was modified, however, revealed some important differences for the interaction of organic Ca antagonists with myocardial Ca channels. Acidification, in addition to changing the proportion of charged and neutral antagonist in solution, selectively antagonized tonic inhibition of iCa by diltiazem and nifedipine, without changing the kinetics of the drug-induced slow iCa reactivation process. Two distinct receptor sites may be involved in block of iCa by some of these compounds: a proton-accessible site and a proton-inaccessible site.