MEASUREMENTS OF INTRACELLULAR CA2+ IN CULTURED HUMAN MYOMETRIAL SMOOTH-MUSCLE CELLS BATHED IN LOW NA+ SOLUTIONS

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 Ca2+-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 Ca2+-induced-Ca2+-release mechanism is not operating in these cells.