Comparison of electrophysiological effects of calcium channel blockers on cardiac repolarization.

Abstract

Dihydropyridine (DHP) calcium channel blockers (CCBs) have been widely used to treat of several cardiovascular diseases. An excessive shortening of action potential duration (APD) due to the reduction of Ca2+ channel current (ICa) might increase the risk of arrhythmia. In this study we investigated the electrophysiological effects of nicardipine (NIC), isradipine (ISR), and amlodipine (AML) on the cardiac APD in rabbit Purkinje fibers, voltage-gated K+ channel currents (IKr, IKs) and voltage-gated Na+ channel current (INa). The concentration-dependent inhibition of Ca2+ channel currents (ICa) was examined in rat cardiomyocytes; these CCBs have similar potency on ICa channel blocking with IC50 (the half-maximum inhibiting concentration) values of 0.142, 0.229, and 0.227 nM on NIC, ISR, and AML, respectively. However, ISR shortened both APD50 and APD90 already at 1 µM whereas NIC and AML shortened APD50 but not APD90 up to 30 µM. According to ion channel studies, NIC and AML concentration-dependently inhibited IKr and IKs while ISR had only partial inhibitory effects (<50% at 30 µM). Inhibition of INa was similarly observed in the three CCBs. Since the IKr and IKs mainly contribute to cardiac repolarization, their inhibition by NIC and AML could compensate for the AP shortening effects due to the block of ICa.