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 (Ito1) and/or Ca2+-activated transient outward Cl− current (ICa.Cl or Ito2) 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 Ito1 protein (Kv4.2 or Kv4.3) in pig ventricular myocytes. Results: A transient outward current (Ito) was activated upon depolarization voltage steps to between −10 and +60 mV from −50 mV in pig ventricular cells, and the Ito was resistant to 4-AP application, but sensitive to the inhibition by ryanodine (10 μmol/l) and the Ca2+ channel blockade, and the Cl− channel blocker 4,4′-diisothiocyanostilben-2,2′disulfonic acid (DIDS, 150 μmol/l). The current was diminished by external Cl− (Cl−o) replacement and showed a 'bell-shaped' I–V relationship at room temperature, typical of Ito2. No difference in Ito2 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/l), but abolished by replacing Cl−o and by application of 100 μmol/l DIDS, suggesting Ito2 contribution. Western immunoblotting analysis showed no evidence for the expression of 4-AP-sensitive Ito1 channel protein (Kv4.2 or Kv4.3) in pig ventricular cells. Conclusion: The results indicate that 4-AP-sensitive Ito1 is not expressed, and only Ca2+-activated Ito2 is present in pig cardiac cells, which contributes importantly to the phase 1 repolarization of ventricular APs in this species.