The transient outward current of isolated bovine adrenal zona fasciculata cells: comparison between standard and perforated patch recording methods

Voltage-clamp experiments were performed on single bovine adrenal fasciculata cells in short-term primary culture using either standard (broken membrane) or perforated whole-cell patch clamp recording. The membrane current measured with the perforated method was dominated by a very stable transient outward current. By contrast, the transient outward current recorded using the standard method was unstable. The reversal potential of the transient outward current varied linearly with the logarithm of [K+]e with a slope of 47 mV per decade. The onset of activation was sigmoidal and was fitted with a power function where n = 4. Time constants ranged from 1 to 4 msec with a maximum at -25 mV. The steady-state activation curve spanned the voltage range -50 +80 mV without reaching a clear maximum. During a pulse, the current decayed in a biexponential manner. Time constants tau 1 and tau 2 were voltage-dependent and ranged from 50 to 200 msec respectively for a voltage step at +50 mV. The steady-state inactivation was dependent on the conditioning pulse duration. Using short conditioning pulses (1.2 sec), the curve which spanned the voltage range -40 to -20 mV, was 15 mV more positive than that obtained with longer conditioning pulses (60 sec). Time constants of this "very slow inactivation" process (tau vs) determined for voltage steps at -60 and -50 mV were 15 and 10 sec respectively. A "facilitation process" of the peak current was observed when the duration or the amplitude of conditioning pulses were increased in the voltage range -100 to -50 mV. Recovery from inactivation followed a biexponential time course which seemed a mixture of both inactivation processes. In some experimental conditions, isolated cells were able to produce overshooting action potentials. These results are discussed in relation with the membrane electrogenesis of this cell type.