Ionic currents in the uterine smooth muscle.

1. Short segments of isolated longitudinal myometrium from the pregnant rate uterus have been studied in a double sucrose-gap voltage-clamp arrangement. The clamped segment averaged 65 mum times 240 mum times 100 mum, has an average total capacitance of 0-14 muF, and may contain 50-200 individual myometrial cells. 2. A significant resistance exists in series with the membrane, and limits theprecision of the quantitative information. However, it is argued that some qualitative and some comparative information is useful. 3. In Krebs-bicarbonate solution, depolarizing steps produced initial transient inward currents followed by delayed outward currents. 4. When [Na+]o was reduced by 50%, the equilibrium potential Ea shifted by an average of -17-6 mV, the maximum inward current was reduced to 0-5, the time to peak of the early current was delayed by 1-1 msec, and the maximum chord conductances for the early(Ga) and late (GK) currents remained unchanged as compared with those in normal [Na+]o. 5. When [Ca2+] was reduced to 25% of normal, Ea shifted by an average of -20-3 mV, the maximum inward current was reduced to 0-5, the time to peak was delayed 3-1 msec, and Ga was significantly reduced, while GK was unaffected. 6. The early current, and its tail when repolarization was imposed, reversed direction from inward to outward when [Na+]o was reduced from 143 mM to zero, with [Ca2+]o remaining constant at 1-9 mM. 7. From the observations in 4, 5 and 6, it was concluded that Na+ is the main charge carrier for the early current, and that Ca2+ is important in regulating Ga. 8. The late current is outwards when [K+]o equals 5-9 mM, but inwards in some voltage range when [K+]o was elevated to 120 or 148 mM. K+ is the main charge carrier for the late current. 9. The equilibrium potential for the late current, EK, is about 15 mV more negative than the natural resting potential. 10. Prolonged holding of the preparations at voltages that differ significantly from the natural resting potential tends to shift EK in a way consistent with passive changes in [K+]i by the holding current. 11. The steady-state inactivation of the early current, h, is unusual. Inward current is macimum around the resting potential, and declines with both hyperpolarizing and depolarizing changes. Half-inactivation occurred with about 9 mV depolarization and 15 mV hyperpolarization. 12. The instantaneous current-voltage relations of both early and late currents are linear. The chord conductances Ga and GKare similar in form to those in other tissues.