Figure 3.

I_Na of uterine myocytes, all recorded with Cs⁺ electrodes. (A) Myocyte from 18-day pregnant uterus, 187.6 pF. Holding potential −80 mV; step depolarized to −40, and then to 30 mV in 10-mV increments. A, 1 shows myocyte response in solution containing (mM): 120 Na⁺, 1 Ca²⁺, 10 TEA, 5 4-aminopyridine, and 5 Cs⁺. The inward current is complex, with fast and slow components and tail current, similar to currents shown in Fig. 2. A, 2 was taken after 5 min in a similar solution in which 120 mM choline chloride replaced NaCl. The fast component disappeared, but the slow component as well as the tail current was unaffected. A, 3 was taken 4 min after returning to Na⁺ bath. (B) Myocyte from 13-day pregnant uterus, 71.2 pF. Holding potential, −80 mV; step depolarized to −40, and then to 30 mV in 10-mV increments. Bath solution as in A. First currents (not shown) were complex, similar to that in A, 1. Traces in B, 1–3 were taken with myocytes in similar bath solution, but containing an additional 2 μm nisoldipine. Slow component has been blocked. These currents are similar to those shown in C. B, 1 shows only the fast component, which could be fully blocked by 1 μM TTX (B, 2), in a reversible manner (B, 3). (C) Myocytes from 18-day pregnant uterus; 112 pF. Holding potential −80 mV; command to −40 mV, and then to 80 mV in 10-mV increments (similar to those in I-V plot in D). (C) Superimposed current traces at 10 mV. Trace 1 was taken in solution containing 105 mM Na⁺ and 3 mM Ca²⁺, with inward current showing both fast and slow components. Trace 2 was taken after 5 min in solution containing 105 mM Na⁺ and 0 Ca²⁺. Slow component has disappeared. The responses to ion substitutions (A and C) and to TTX and nisoldipine (B) indicate that the fast component is I_Na and the slow component is I_Ca. (D) I-V relation of I_Na of a different myocyte from 17-d pregnant uterus; 134 pF, in solution containing 120 mM Na⁺. Pipette solution contains 4 mM Na⁺. Actual currents are similar to those of myocytes shown in C. I_Na is first detected at ∼−30 mV; reaches a maximum at +10 mV. Reversal potential is 85 mV, in good agreement with expected Nernst potential of 85.7 mV.