Calcium-activated transient outward chloride current and phase 1 repolarization of swine ventricular action potential

Objective: It is unknown whether 4-aminopyridine- (4-AP-) sensitive transient outward K+ current (I-to1) and/or Ca2+-activated transient outward Cl- current (I-Ca.Cl or I-to2) contribute(s) to phase 1 repolarization of pig ventricular action potential (AP). The purpose of the present study was to determine ionic contribution of the phase 1 repolarization of AP in pig ventricle. Methods: We used whole-cell patch techniques to record APs and membrane currents, and Western immunoblotting analysis to detect expression of I-to1 protein (Kv4.2 or Kv4.3) in pig ventricular myocytes. Results: A transient outward current (I-to) was activated upon depolarization voltage steps to between -10 and +60 mV from -50 mV in pig ventricular cells, and the I-to was resistant to 4-AP application, but sensitive to the inhibition by ryanodine (10 mumol/1) and the Ca2+ channel blockade, and the Cl channel blocker 4,4'-diisothiocyanostilben-2,2'disulfonic acid (DIDS, 150 mumol/1). The current was diminished by external Cl- (Cl-o(-)) replacement and showed a `bell-shaped' I-V relationship at room temperature, typical of I-to2. No difference in I-to2 was observed in the regional cells from epicardium, midmyocardium, and endocardium of left ventricle. APs showed significant phase 1 and `spike and dome' in pig ventricular myocytes. The phase 1 and `spike and dome' of APs were not affected by 4-AP (3 mmol/1), but abolished by replacing Cl. and by application of 100 mumol/1 DIDS, suggesting I-to2 contribution. Western immunoblotting analysis showed no evidence for the expression of 4-AP-sensitive I-to1 channel protein (Kv4.2 or Kv4.3) in pig ventricular cells. Conclusion: The results indicate that 4-AP-sensitive I-to1 is not expressed, and only Ca2+-activated I-to2 is present in pig cardiac cells, which contributes importantly to the phase 1 repolarization of ventricular APs in this species. (C) 2003 European Society of Cardiology. Published by Elsevier Science B.V. All rights reserved.