Characterization of sodium channels in cultured human uterine smooth muscle cells.

Voltage-clamp studies using the whole-cell patch clamp technique were performed on single cells of cultured human uterine smooth muscle obtained from term pregnancies. The small size of the cells allowed time resolution of transient ionic currents as short as 3 msec. A large, voltage-activated inward current was identified as a sodium channel conductance by the following criteria: (1) Removal of sodium from the bath eliminated the current; (2) current was blocked by the sodium-channel-blocking agent tetrodotoxin; (3) current was observed in the absence of calcium. The sodium current was large (maximal inward current 7.2 microA/cm2) and of short duration (decayed within 10 msec). The onset of activation of this current was -40 mV, with peak inward current at -10 mV. Steady-state voltage inactivation of this channel demonstrated half-maximal inactivation at -67 mV, indicating that this channel is largely inactivated at normal resting potentials.