Ca~(2+) influx does more than provide releasble Ca~(2+) to maintain repetitive spiking in human umbilical vein endothelical cells

摘要

We investigated why oscillations of intracelluiar Ca~(2+) concentrations ([Ca~(2+)],) in endothelial cells challenged by sub-maximal histamine run down in Ca~(2+)-free medium despite stores retaining most of their Ca~(2+). One explanation is that onlya small subpopulation of the Ca~(2+) stores oscillate and are completely emptied of Ca~(2+) To investigate if influx refills an empty stoie subpopulation, we differentiated between cations entering the cell and those released from internal stores by using extracellular Sr~(2+) as a Ca~(2+) surrogate; we distinguished between [Sr~(2+)], and [Ca~(2+), by using the larger effect of Sr~(2+) on fura 2 fluorescence at 360 nm (F_(360)). Ca~(2+) was still available for release when oscillations had tun down since oscillations promptly reappeared on addition of S~(2+) and these were predominantly of Ca~(2+) (indicated by F_(360) changes) Also, totaily depleting Ca~(2+) stores inhibited Sr~(2+)-induced oscillations, suggesting that Sr~(2+) entry leads to Ca~(2+) release. In contrast. Ba~(2+) was unable to stimulate oscillations Finally, oscillations generated by photoiytic release of inosito! trisphosphate (IP_3) analogues weie similarly sensitive to extracellular Ca~(2+) and Sr~(2+) We conclude that stores (or a sub-population) are not completely depleted of Ca~(2+) when oscillations run down in Ca~(2+)-fiee medium. Bivalent cation entry therefore maintains sensitivity to IP_3, possibly by maintaining luminal bivalent cation levels.