Summary
Smooth muscle cells normally do not possess fast Na2+ channels, but inward current is carried through two types of Ca2+ channels: slow (L-type) Ca2+ channels and fast (T-type) Ca2+ channels. Using whole-cell voltage clamp of single smooth muscle cells isolated from the longitudinal layer of 18-day pregnant rat uterus, depolarizing pusles, applied from a holding potential of −90 mV, evoked two types of inward current, fast and slow [8]. The fast inward current decayed within 30 ms, depended on [Na]0, and was inhibited by TTX (K0.5 = 27 nM). The slow inward current decayed slowly, was dependent on [Ca]0, and was inhibited by nifedipine. These results suggest that the fast inward current is a fast Na2+ channel current, and that the slow inward current is a Ca2+ channel current was not evident. Thus, the ion channels which generate inward currents in pregnant rat uterine cells are TTX-sensitive fast Na+ channels and dihudropuridine-sensitive slow Ca2+ channels. The number of fast Na+ channels increased during gestation [9]. The averaged current density increased from 0 on day 5, to 0.19 on day 9, to 0.56 on day 14, to 0.90 on day 18, and to 0.86 pA/pF on day 21. This almost linear increase occurs because of an increase in the fraction of cells which possess fast Na2+ channels, and it suggested that the fast Na+ current may be involved in spread of excitation. The Ca2+ channel current density also was higher during the latter half of gestation. These results indicate that the fast Na+ channels and Ca2+ slow channels in myometrium become more numerous as term approaches, and may facilitate parturition. Isoproterenol (beta-agonist) did not affect either ICa(s) or INa(f), whereas Mg2+ (K0.5 of 12 mM) and nifedipine (K0.5 of 3.3 nM) depressed ICa(s). Oxytocin had no effect on INa(f) and actually depressed ICa(s) to a small extect. Therefore, the tocolytic action of beta-agonists cannot be explained by an inhibition of ICa(s), whereas that of Mg2+ can be so explained. The stimulating action of oxytocin on uterine contractions is not due to stimulation of ICa(s).
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Sperelakis, N., Inoue, Y. & Ohya, Y. Fast Na+ channels and slow Ca2+ current in smooth muscle from pregnant rat uterus. Mol Cell Biochem 114, 79–89 (1992). https://doi.org/10.1007/BF00240301
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DOI: https://doi.org/10.1007/BF00240301