In:
Canadian Journal of Physiology and Pharmacology, Canadian Science Publishing, Vol. 70, No. 4 ( 1992-04-01), p. 491-500
Abstract:
Smooth muscle cells normally do not possess fast Na + channels, but inward current is carried through two types of Ca 2+ channels: slow (L type) Ca 2+ channels and fast (T type) Ca 2+ channels. Whole-cell voltage clamp was done on single smooth muscle cells isolated from the longitudinal layer of the 18-day pregnant rat uterus. Depolarizing pulses, applied from a holding potential of −90 mV, evoked two types of inward current, fast and slow. The fast inward current decayed within 30 ms, depended on [Na] o , and was inhibited by tetrodotoxin (TTX) (K 0.5 = 27 nM). The slow inward current decayed slowly, was dependent on [Ca] o (or Ba 2+ ), and was inhibited by nifedipine. These results suggest that the fast inward current is a fast Na + channel current and that the slow inward current is a Ca 2+ slow channel current. A fast-inactivating Ca 2+ channel current was not evident. We conclude that the ion channels that generate inward currents in pregnant rat uterine cells are TTX-sensitive fast Na + channels and dihydropyridine-sensitive slow Ca 2+ channels. The number of fast Na + channels increased during gestation. 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 that possess fast Na + channels. The Ca 2+ channel current density was also higher during the latter half of gestation. These results indicate that the fast Na + channels and Ca 2+ slow channels in myometrium become more numerous as term approaches, and we suggest that the fast Na + current may be involved in spread of excitation. Isoproterenol (β-agonist) did not affect either I Ca(s) or I Na(f) , whereas Mg 2+ (K 0.5 = 12 mM) and nifedipine (K 0.5 = 3.3 nM) depressed I Ca(s) . Oxytocin had no effect on I Na(f) and actually depressed I Ca(s) to a small extent. Therefore, the tocolytic action of β-agonists cannot be explained by an inhibition of I Ca(s) , whereas that of Mg 2+ can be so explained. The stimulating action of oxytocin on uterine contractions cannot be explained by a stimulation of I Ca(s) .Key words: sodium current, fast sodium current, calcium currents, myometrial smooth muscle cells, pregnant uterine muscle.
Type of Medium:
Online Resource
ISSN:
0008-4212
,
1205-7541
Language:
English
Publisher:
Canadian Science Publishing
Publication Date:
1992
detail.hit.zdb_id:
2004356-9
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