Inhibition of myocardial L- and T-type Ca2+ currents by efonidipine: possible mechanism for its chronotropic effect.

Effects of efonidipine, a dihydropyridine phosphonate Ca2+ channel antagonist, on the guinea-pig heart were compared with those of nifedipine. In the sino-atrial node, 1 microM efonidipine produced increase in cycle length accompanied by prolongation of the phase 4 depolarization which was not prominent with 0.1 microM nifedipine. In ventricular myocytes, both efonidipine and nifedipine produced inhibition of the L-type Ca2+ current, nifedipine being tenfold more potent than efonidipine. Efonidipine also inhibited the T-type Ca2+ current at higher concentrations but nifedipine did not. Both Ca2+ channel antagonists had no or only a weak effect on K+ currents. In addition, 40 microM Ni2+, which selectively inhibited the T-type Ca2+ current, had no effect on myocardial Ca2+ transients and contractile force. In conclusion, efonidipine was shown to have inhibitory effects on both L- and T-type Ca2+ currents, which may contribute to its high negative chronotropic potency.