Properties of an early outward current in single cells of the mouse ventricle

Single ventricular myocytes of adult mice were prepared by enzymatic dissociation for voltage clamp experiments with the one suction pipette dialysis method. After blocking the Na current by 10(-4) mol/l TTX early outward currents (IEO) with incomplete inactivation could be elicited by clamping from -50 mV to test potentials (VT) positive to -30 mV. Interfering Ca currents were very small (less than 0.6 nA at VT = 0 mV). The approximation of IEO by the q4r-model showed a pronounced decrease in the time constant of activation (tau q) to more positive potentials. At 50 ms test pulses the time course of the incomplete inactivation could be described by two exponentials and a constant. The time constant of the fast exponential (tau r1) showed a slight decline towards more positive test potentials (8.1 +/- 1.0 ms at -10 mV; 5.8 +/- 1.2 ms at +50 mV, mean +/- SD, n = 5) whereas the time constant of the slow exponential (tau r2) was voltage independent (41.1 +/- 7.9 ms, mean +/- SD, n = 5). The contributions of the fast exponential and the pedestal increased towards positive test potentials. The Q10 value for the time constants of activation and fast inactivation was 2.36 +/- 0.19 and 2.51 +/- 0.09 (mean +/- SD, n = 3), respectively. After an initial delay the recovery of IEO at a recovery potential of -50 mV could be fitted monoexponentially with a time constant of 16.3 +/- 2.9 ms (mean +/- SD, n = 3). The time course of the onset of inactivation determined with the double pulse protocol was slower than the decay at the same potential, and could be described as sum of a fast (tau = 18.4 +/- 6.0 ms) and a slow (tau = 62.1 +/- 19.9ms, mean +/- SD, n = 3) exponential. IEO could be blocked completely by 1 mmol/l 4-aminopyridine at potentials up to +20 mV. Stronger depolarizations had an unblocking effect.