Figure 8.

Recovery from inactivation of I_Na. (A) Voltage protocol and representative currents from myocyte from 13-d pregnant uterus. 72 pF. In solution containing 1 mM Ca²⁺. V₁ and V₂ are both 20 ms, with an interval between two steps varying on 2-ms increments. Large initial current produced by V₁ is I_Na, but small I_Ca is still present, which is especially clear as a tail current. Contribution of I_Ca to V₂ current is negligible because of its much slower reactivation kinetics (see Fig. 9). (B) Averaged reactivation time course of I_Na. Abscissa is interval between voltage steps; ordinate is V₂ current as a fraction of V₁ current. Data are from eight myocytes, three from 20-d pregnant uteri, two from 19-d, and one each from 7-, 13-, and 21-d uteri (means; all SEM are smaller than symbol). Although not all myocytes contributed to the entire range of the curve, data points are well fitted by y = 1 − exp(−t/19.8) (solid line). Single exponential with time constant of 19.8 ms for myocytes from different stages of pregnancy suggests participation of one class of sodium channels throughout entire course of pregnancy.