Endoplasmic Reticulum Ca 2+ Depletion Unmasks a Caffeine-Induced Ca 2+ Influx in Human Aortic Endothelial Cells

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Abstract
Intracellular Ca2+ pools contribute to changes in cytosolic [Ca2+] ([Ca2+]i), which play an important role in endothelial cell signaling. Recently, endothelial ryanodine-sensitive Ca2+ stores were shown to regulate agonist-sensitive intracellular Ca2+ pools. Since caffeine binds the ryanodine Ca2+ release channel on the endoplasmic reticulum in a variety of cell types, we examined the effect of caffeine on [Ca2+]i in human aortic endothelial cell monolayers loaded with the fluorescent probe indo 1. Under baseline conditions, 10 mmol/L caffeine induced a small increase in [Ca2+]i from 86±10 to 115±17 nmol/L (mean±SEM); this effect was similar to that of 5 μmol/L ryanodine and was unaffected by buffer Ca2+ removal. After depletion of an intracellular Ca2+ store by the irreversible endoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin (1 μmol/L), ryanodine did not affect [Ca2+]i. In contrast, caffeine induced a large rapid increase in [Ca2+]i (176±19 to 338±35 nmol/L, P<.001) after thapsigargin exposure; this effect of caffeine was only observed when extracellular Ca2+ was present. A similar increase in [Ca2+]i was induced by caffeine after depletion of ryanodine- and histamine-sensitive Ca2+ stores or after pretreatment with the endoplasmic reticulum Ca2+-ATPase inhibitor cyclopiazonic acid (10 μmol/L). Thus, under baseline conditions the effect of caffeine on [Ca2+]i is similar to that of ryanodine and appears to be due to the release of an intracellular store. However, after depletion of an endoplasmic reticulum Ca2+ store, caffeine, but not ryanodine, stimulates Ca2+ influx, resulting in a large increase in [Ca2+]i. The data suggest that caffeine-induced Ca2+ influx is controlled by the status of Ca2+ loading of intracellular Ca2+ stores in human aortic endothelial cells.

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