The transient K+ current in rat ventricular myocytes: evaluation of its Ca2+ and Na+ dependence

1. The transient outward K+ current (Ito) was studied in enzymatically isolated rat ventricular myocytes using the whole-cell patch clamp technique. 2. At holding potentials between -100 and -60 mV, depolarizing pulses activated outward current which was composed of transient and maintained components. These components differed from each other in their activation voltage range as well as in their kinetics of inactivation. 3. The transient component, in turn, appeared to be composed of rapidly and slowly inactivating components. Subtraction of ICa from the total current, or nifedipine pre-treatment, eliminated the slowly inactivating component of Ito indicating that the time course of inactivation of Ito may be contaminated by ICa. 4. Reduction of the holding potential from -100 mV to less negative holding potentials reduced all components of Ito, such that at holding potentials of -40 mV, very little or no Ito could be measured. 5. Elevation of [Ca2+]o activated Ito at holding potentials of -40 mV, and substitution of external Ca2+ by Sr2+ suppressed Ito, consistent with findings from other preparations and in support of a Ca(2+)-activated component of Ito. 6. Elevations of [Ca2+]o, however, also shifted the steady-state activation and inactivation parameters of the transient K+ current, such that a greater proportion of Ito channels were activated at the less negative holding potentials. 7. The shifts in the activation and inactivation parameters of the transient outward current were not mimicked by equivalent changes in external Mg2+. 8. Modulators of Ca2+ release from the sarcoplasmic reticulum (SR) such as caffeine and ryanodine suppressed Ito regardless of whether the myocytes were dialysed with low or high concentrations of Ca2+ buffers (EGTA or BAPTA, 0.5-14 mM) or whether nifedipine was used to block ICa. 9. 4-Aminopyridine (4-AP) blocked Ito in a dose-dependent manner, completely suppressing it at 10 mM. Similarly, tedisamil, a new K+ channel blocker, completely and reversibly blocked Ito at 5-20 microM concentrations. 10. TTX (10 microM) or removal of external Na+ decreased Ito, consistent with the idea that a component of Ito was Na+ activated. Both interventions, however, also shifted the voltage dependence of the activation and inactivation of Ito to more negative potentials, such that at -100 mV neither intervention had a significant effect on Ito. Alterations in [Na+]i had no effect on Ito.