Physiologic electrical stimulation provokes intracellular calcium increase mediated by phospholipase C activation in human osteoblasts

Abstract

Strong exogenous electrical stimulation (ES) can induce changes in intracellular calcium ion concentration ([Ca 2+ ] i ). It remains to be elucidated, however, whether physiologically relevant ES (e.g., 1–2 V/cm) could alter [Ca 2+ ] i . We have used fluorescence microscopy to quantify [Ca 2+ ] i changes in response to direct current (dc) ES in human fetal osteoblasts. Increases in [Ca 2+ ] i in response to 2 V/cm ES show a noticeable (20‐min) time delay, followed by a 45‐fold rise from the baseline of 40 nM to 1.8 μM. Treatment of cells with verapamil does not affect ES‐ induced [Ca 2+ ] i increases, but inhibition of phospholipase C (PLC) does prevent such increases, which suggests that receptor‐regulated release of Ca 2+ from intracellular stores is likely to be involved. Treatment of cells with the stretch‐activated cation channel (SACC) blocker Gd 3+ partially inhibits ES‐induced [Ca 2+ ] i increases, as does chelation of intracellular Ca 2+ . These results are consistent with a model in which physiologically relevant ES does not activate voltage‐gated Ca 2+ channels (VGCCs) directly, but rather stimulates PLC‐coupled cell surface receptors that induce [Ca 2+ ] i increases by activating IP 3 ‐dependent intracellular processes. The Ca 2+ influx that follows PLC activation is likely mediated by activation of mechanically operated SACCs.