Figure 2.

Coexistence of I_Na and I_Ca in single rat uterine myocytes from different stages of pregnancy. Top trace shows voltage protocol applied to all cells shown. Holding potential −80 mV; step depolarized to 10 mV for 36 ms. All myocytes were bathed in solution containing (mM): 105 Na⁺, 3 Ca²⁺, 30 TEA⁺, 5 4-aminopyridine, and 5 Cs⁺. Pipette solution contained 120 Cs⁺, which, along with external TEA, 4-aminopyridine, and Cs⁺, was used to block outward K⁺ currents. Cell capacitances (pFs) are: 29.6 for nonpregnant (day 0) myocytes; 47.2 for day-5 myocytes; 90.8 for day-17 myocytes; 148.4 for day-21 myocytes, and 200 for postpartum myocytes. In all cells, fast and slower components are seen, which are identified as I_Na and I_Ca, respectively. Such currents were recorded in about half of nonpregnant and day-2–pregnant myocytes, and in >90% of late-pregnant myocytes. Note the variability of the currents in relative peak amplitudes and degree of overlap. At end of the depolarizing step, tail current is entirely I_Ca, because I_Na is fully inactivated by this time. Other details in text and Table III.