title: Frequency-dependent blockade of T-type Ca2+ current by efonidipine in cardiomyocytes.
abstract: Efonidipine is a dihydropyridine Ca2+ antagonist with inhibitory effects on both L-type and T-type Ca2+ channels and potent bradycardiac activity especially in patients with high heart rate. In the present study, we examined the frequency dependence of efonidipine action on the T-type Ca2+ channel in isolated guinea-pig ventricular myocytes. The potency of efonidipine to inhibit the T-type Ca2+ current was higher under higher stimulation frequencies. The IC50 values were 1.3 x 10(-8), 2.0 x 10(-6) and 6.3 x 10(-6) M under stimulation frequencies of 1, 0.2 and 0.05 Hz, respectively. The reduction of T-type Ca2+ current amplitude was not accompanied by change in the time course of current decay. Efonidipine (10 microM) inhibited T-type Ca2+ current elicited by depolarization from holding potentials ranging from -90 to -30 mV by about 30%; the voltage-dependence of steady-state inactivation was not changed by the drug. Efonidipine slowed the recovery from inactivation following an inactivating prepulse. In conclusion, efonidipine was shown to have frequency-dependent inhibitory effects on the T-type Ca2+ channel, which could be explained by slow dissociation of the drug from the inactivated state of the channel.
authors.affiliation: Department of Pharmacology, Toho University School of Pharmaceutical Sciences, Funabashi, Chiba, Japan.
authors.forename: H
authors.initials: H
authors.lastname: Masumiya
authors.forename: J
authors.initials: J
authors.lastname: Kase
authors.forename: Y
authors.initials: Y
authors.lastname: Tanaka
authors.forename: H
authors.initials: H
authors.lastname: Tanaka
authors.forename: K
authors.initials: K
authors.lastname: Shigenobu
mesh_terms: D000818:Animals; D000959:Antihypertensive Agents / Q000494:pharmacology*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D020747:Calcium Channels, T-Type / Q000187:drug effects* / Q000502:physiology; D004095:Dihydropyridines / Q000494:pharmacology*; D004558:Electric Stimulation; D006168:Guinea Pigs; D006352:Heart Ventricles / Q000166:cytology / Q000187:drug effects; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009206:Myocardium / Q000166:cytology*; D009596:Nitrophenols; D009943:Organophosphorus Compounds / Q000494:pharmacology*; D018408:Patch-Clamp Techniques; D016276:Ventricular Function
chemical_list: D000959:Antihypertensive Agents; D002121:Calcium Channel Blockers; D020747:Calcium Channels, T-Type; D004095:Dihydropyridines; D009596:Nitrophenols; D009943:Organophosphorus Compounds; C066425:efonidipine