Calcium oscillations induced by ATP in human umbilical cord smooth muscle cells

Abstract

Oscillations in smooth muscle calcium levels have been described, but the origin and function of these has not yet been fully determined. In cultured human umbilical cord smooth muscle cells (HUCSM), we found that ATP, at a micromolar dose that activates P2Y receptors, induced a steady calcium oscillation in previously quiescent cells and the calcium entry pathway was shown to be via TRP channel activation. These oscillations were quenched by higher, millimolar doses of ATP or membrane depolarization by high, bath K+. In addition, when external calcium was removed by EGTA, micromolar ATP could not induce oscillations. Surprisingly, if internal calcium stores were not pre depleted, we found that only a small proportion of the calcium increase following P2Y receptor activation, could be blocked by the P2 antagonist suramin, or the TRP channel inhibitor, 2APB, suggesting an alternative calcium entry pathway was activated under these conditions. However, if internal stores were pre depleted by EGTA or high K+, PLC or G-protein antagonists could almost fully reverse the calcium increase induced by ATP. After 60 hours incubation at 1–2% O2, the calcium oscillation frequency and amplitude were increased. Taken together, these data suggest that external calcium entry maintains the calcium oscillation induced by ATP but the status of the internal calcium store directs the precise calcium entry pathway utilised and that chronic hypoxia can modulate ion channel activity in these cells. This work was supported by The British Heart Foundation.