In cultured human myometrial smooth muscle cells, removal of external Na+ activates repetitive increases in intracellular Ca2+ ([Ca2+]i). The Ca2+ transients persist in isotonic K+ solutions which suggests that the activity does not arise from regenerative changes in membrane potential. In nominally Ca(2+)-free solution, the activity disappears after a few cycles suggesting the involvement of internal stores. On Na+ removal, a background influx of Ca2+ may be responsible for activating the cyclical release of Ca2+ from internal stores. The absence of any effect of caffeine on resting [Ca2+]i suggests that the classical cardiac type of Ca(2+)-induced-Ca(2+)-release mechanism is not operating in these cells.

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