POTASSIUM CURRENTS IN FRESHLY DISPERSED MYOMETRIAL CELLS

K+ currents in freshly dispersed cells from rat myometrium at estrus were studied with the patch-clamp technique (whole cell). Three types of K+ currents were identified: 1) a fast-activating current (I(Kf)), 2) a slowly activating current (I(Ks)), and 3) a transient current (I(Kt)). I(Kf) had a half-activation voltage of 12 mV and a time constant of activation (tau-on) of approximately 3 ms at +50 mV. I(Ks) had a tau-on of approximately 9 ms at +50 mV and a half-activation potential of 33 mV. Both I(Kf) and I(Ks) were sustained and became potentiated by the entrance of Ca2+ from the patch pipette. These two Ca2+-activated K+ currents were inhibited by 100 nM external charybdotoxin and were blocked by external tetraethylammonium (TEA, 2-20 mM). The third current (I(Kt)) was transient, had a faster tau-on (almost-equal-to 1 ms), and a decay phase with a time constant of approximately 8 ms at +50 mV. This current had a half-activation potential of 22 mV. I(Kt) was not potentiated by intracellular Ca2+, was sensitive to 4-aminopyridine (1 mM), was insensitive to external charybdotoxin (100 nM) and TEA (2 mM), and spontaneously decreased with time.