THE CALCIUM-INDEPENDENT TRANSIENT OUTWARD POTASSIUM CURRENT IN ISOLATED FERRET RIGHT VENTRICULAR MYOCYTES

Enzymatically isolated myocytes from ferret right ventricles (12-16 wk , male) were studied using the whole cell patch clamp technique. The m acroscopic properties of a transient outward K+ current I(to) were qua ntified. It. is selective for K+, with a P(Na)/P(K) of 0.082. Activati on of I(to) is a voltage-dependent process, with both activation and i nactivation being independent of Na+ or Ca2+ influx. Steady-state inac tivation is well described by a single Boltzmann relationship (V1/2 = -13.5 mV; k = 5.6 mV). Substantial inactivation can occur during a sub threshold depolarization without any measurable macroscopic current. B oth development of and recovery from inactivation are well described b y single exponential processes. Ensemble averages of single I(to) chan nel currents recorded in cell-attached patches reproduce macroscopic I (to) and indicate that inactivation is complete at depolarized potenti als. The overall inactivation/recovery time constant curve has a bell- shaped potential dependence that peaks between -10 and -20 mV, with ti me constants (22-degrees-C) ranging from 23 ms (-90 mV) to 304 ms (-10 mV). Steady-state activation displays a sigmoidal dependence on membr ane potential, with a net aggregate half-activation potential of +22.5 mV. Activation kinetics (0 to +70 mV, 22-degrees-C) are rapid, with I (to) peaking in approximately 5-15 ms at +50 mV. Experiments conducted at reduced temperatures (12-degrees-C) demonstrate that activation oc curs with a time delay. A nonlinear least-squares analysis indicates t hat three closed kinetic states are necessary and sufficient to model activation. Derived time constants of activation (22-degrees-C) ranged from 10 ms (+10 mV) to 2 ms (+70 mV). Within the framework of Hodgkin -Huxley formalism, I(to) gating can be described using an a3i formulat ion.