CA2-CELLS( INFLUX DOES MORE THAN PROVIDE RELEASABLE CA2+ TO MAINTAIN REPETITIVE SPIKING IN HUMAN UMBILICAL VEIN ENDOTHELIAL)

We investigated why oscillations of intracellular Ca2+ concentrations ([Ca2+](i)) in endothelial cells challenged by submaximal histamine ru n down in Ca2+-free medium despite stores retaining most of their Ca2. One explanation is that only a small subpopulation of the Ca2+ store s oscillate and are completely emptied of Ca2+. To investigate if infl ux refills an empty store subpopulation, we differentiated between cat ions entering the cell and those released from internal stores by usin g extracellular Sr2+ as a Ca2+ surrogate; we distinguished between [Sr 2+](i) and [Ca2+](i) by using the larger effect of Sr2+ On fura 2 fluo rescence at 360 nm (F-360). Ca2+ was still available for release when oscillations had run down since oscillations promptly reappeared on ad dition of Sr-o(2+) and these were predominantly of Ca2+ (indicated by F-360 changes). Also, totally depleting Ca2+ stores inhibited Sr2+-ind uced oscillations, suggesting that Sr2+ entry leads to Ca2+ release. I n contrast, Ba-o(2+) was unable to stimulate oscillations. Finally, os cillations generated by photolytic release of inositol trisphosphate ( IP3) analogues were similarly sensitive to extracellular Ca2+ and Sr2. We conclude that stores (or a subpopulation) are not completely depl eted of Ca2+ when oscillations run down in Ca2+-free medium. Bivalent cation entry therefore maintains sensitivity to IP3, possibly by maint aining luminal bivalent cation levels.