Fast outward current controlling electrical activity in rat uterine smooth muscle during gestation.

Action potentials and ionic currents were measured on myometrial strips from 14-16 days and 20-22 days of pregnancy in rats by means of a double sucrose gap technique. 1. In mid-pregnancy, the patterns of outward current showed a distinct fast component. However, conditioning polarizations can induce or suppress the fast outward current in the uterus from different stages of gestation, suggesting a membrane potential dependence. 2. 4-aminopyridine (4-AP) specifically suppressed the fast outward current while the late outward current was more sensitive to tetraethylammonium ions TEA). This allowed a determination of activation and inactivation parameters of the fast outward current. The availability was maximal at - 80 mV and represented 20-25% at - 50 mV. The activation-voltage dependence of the fast outward current in mid-pregnancy was similar to that of the fast component of the outward current during parturition. 3. The apparent depression of the inward current during conditioning hyperpolarizations was suppressed by 4-AP, indicating that the fast outward current overlapped with the inward current. 4. It is suggested that two sets of pharmacologically distinct potassium channels contribute to the outward current in myometrium. Ovarian hormones could control the amount of available fast potassium channels and, by this way, modulate the resting membrane potential during pregnancy.