Ca2+ influx does more than provide releasable Ca2+ to maintain repetitive spiking in human umbilical vein endothelial cells

We investigated why oscillations of intracellular Ca2+ concentrations ([Ca2+]i) in endothelial cells challenged by sub-maximal histamine run down in Ca(2+)-free medium despite stores retaining most of their Ca2+. One explantation is that only a small subpopulation of the Ca2+ stores oscillate and are completely emptied of Ca2+. To investigate if influx refills an empty store subpopulation, we differentiated between cations entering the cell and those released from internal stores by using extracellular Sr2+ as a Ca2+ surrogate; we distinguished between [Sr2+]i and [Ca2+]i by using the larger effect of Sr2+ on fura 2 fluorescence at 360 nm (F360). Ca2+ was still available for release when oscillations had run down since oscillations promptly reappeared on addition of Sr2+o and these were predominantly of Ca2+ (indicated by F360 changes). Also, totally depleting Ca2+ stores inhibited Sr(2+)-induced oscillations, suggesting that Sr2+ entry leads to Ca2+ release. In contrast, Ba2+o was unable to stimulate oscillations. Finally, oscillations generated by photolytic release of inositol trisphosphate (IP3) analogues were similarly sensitive to extracellular Ca2+ and Sr2+. We conclude that stores (or a sub-population) are not completely depleted of Ca2+ when oscillations run down in Ca(2+)-free medium. Bivalent cation entry therefore maintains sensitivity to IP3, possibly by maintaining luminal bivalent cation levels.