Identification of a hyperpolarization-activated inward current in uterine smooth muscle cells during pregnancy

1. Rat uterine smooth muscle possesses spontaneous contraction and automaticity. This study was exerted to examine one of the underlying currents that may be involved in the generation of spontaneous activity, the hyperpolarization-activated inward current. 2. Whole-cell patch clamp experiments were performed on freshly-isolated single longitudinal smooth muscle cells of pregnant rat uterus (18-day gestation). The holding potential was -30 mV, and long-duration (3 sec) hyperpolarizing pulses were applied to -40 to -120 mV. Experiments were performed at room temperature (22 degrees C). 3. A hyperpolarization-activated inward current (I(f)) was produced at the larger hyperpolarizing steps. Its current density (at -120 mV) was -1.03 +/- 0.31 pA/pF (n = 5). The average cell capacitance was 64.3 +/- 12.3 pF (n = 8). The threshold potential for activation of I(f) was about -70 mV, and the reversal potential was -18.6 +/- 2.1 mV (n = 4). In the presence of Cs+ (3 mM), the I(f) current (at -120 mV) was decreased by 76.5 +/- 2.1% (n = 5, P < 0.01). 4. These results indicate that the Cs(+)-sensitive hyperpolarization-activated inward current is present in the longitudinal muscle cells of pregnant rat uterus. This I(f) current may contribute to some extent to the electrogenesis of the spontaneous activity.