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

Strong exogenous electrical stimulation (ES) can induce changes in intracellular calcium ion concentration ([Ca2+]i). It remains to be elucidated, however, whether physiologically relevant ES (e.g., 1-2 V/cm) could alter [Ca2+]i. We have used fluorescence microscopy to quantify [Ca2+]i changes in response to direct current (dc) ES in human fetal osteoblasts. Increases in [Ca2+]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 microM. Treatment of cells with verapamil does not affect ES-induced [Ca2+]i increases, but inhibition of phospholipase C (PLC) does prevent such increases, which suggests that receptor-regulated release of Ca2+ from intracellular stores is likely to be involved. Treatment of cells with the stretch-activated cation channel (SACC) blocker Gd3+ partially inhibits ES-induced [Ca2+]i increases, as does chelation of intracellular Ca2+. These results are consistent with a model in which physiologically relevant ES does not activate voltage-gated Ca2+ channels (VGCCs) directly, but rather stimulates PLC-coupled cell surface receptors that induce [Ca2+]i increases by activating IP3-dependent intracellular processes. The Ca2+ influx that follows PLC activation is likely mediated by activation of mechanically operated SACCs.