Abstract
Changes in transmembrane ionic currents induced by OPC-8212 (3,4-dihydro-6-[4-(3,4-dimethoxybenzoyl)-1-piperazinyl]- 2(1H)-quinoline), a recently introduced positive inotropic agent which lengthens cardiac action potential duration, were examined using whole-cell voltage-clamp techniques in single rabbit, guinea pig and human ventricular myocytes. In rabbit, OPC-8212 (12 μmol/l) significantly increased membrane action potential duration measured at 90% of repolarization by an average of 88 ms (from 462 ± 25 to 550 ± 35 ms, n = 4; P < 0.05). In rabbit this increase in duration was not associated with significant changes in either the inward rectifier or transient outward K + currents. The magnitude of the secondary inward current evoked from a holding potential of −50 mV was significantly increased by 97 ± 8% (n = 6; P < 0.01) while a demonstrable delayed rectifier outward current could not be identified in the rabbit myocytes examined at room temperature. In guinea pig ventricular myocytes, where the delayed rectifier was large, 12 μmol/l OPC-8212 significantly depressed the current by 58 ± 10% (n = 6; P < 0.01). The effects of OPC-8212 in human ventricular myocytes obtained from the explanted heart of a single patient having an idiopathic cardiomyopathy most closely resembled those observed in isolated rabbit ventricular myocytes. Thus, in rabbit and a few human ventricular myocytes examined at room temperature, OPC-8212 appeared to lengthen cardiac membrane action potential duration primarily by increasing the amplitude of the secondary inward current believed to primarily represent current through L-type Ca 2+ channels. In guinea pig preparations, OPC-8212 also decreased the delayed rectifier outward K + current which also would account for an increase in action potential duration. OPC-8212 could not be demonstrated to affect Na + current inactivation in a manner similar to that produced by 1 mg/l veratrine, a recognized Na + channel agonist, which dramatically slowed this process.
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