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

Arterial smooth muscle cells exhibit vasomotion, related to oscillations in intracellular Ca2+ concentration, but the origin and function of these has not yet been fully determined. We measured intracellular Ca2+ using conventional fluorescent methods in primary cultured, human umbilical cord artery smooth muscle cells (HUCASMC). Spontaneous oscillations in Ca2+ was found in only 1% of all cells but exogenous, micromolar concentrations of ATP could induce Ca2+ oscillations in 70% of cells with the most common pattern being one of regular amplitude and frequency with a return to basal levels between each peak. The P2Y agonist, UTP, but not the P2X agonist alpha beta-methylene ATP, could also induce Ca2+ oscillations. Once induced, these oscillations could not be blocked by G-protein, PLC, VGCC or TRP channel antagonists applied individually, but could be prevented when antagonists were applied together. In the presence of EGTA, micromolar concentrations of ATP induced an elevation in intracellular Ca2+ but did not induce Ca2+ oscillations. The oscillation frequency induced by ATP was affected by bath Ca2+ concentration. Taken together, these data suggest that external Ca2+ entry maintains the Ca2+ oscillation induced by activation of P2Y receptors. Once induced, multiple mechanisms are involved to maintain the oscillation and the oscillation frequency is determined by the speed of Ca2+ refilling. Chronic hypoxia enhanced the Ca2+ response and altered the oscillation frequency. We suggest that these oscillations may play a role in the maintenance of umbilical blood flow during situations in which GPCR are activated.