Exogenous ATP raises cytoplasmic free calcium in fura‐2 loaded piglet aortic endothelial cells

Cultured piglet endothelial cells were grown to confluence on glass coverslips and loaded with the fluorescent Ca2+ indicator, fura-2. Using a dual-wavelength excitation fluorescence spectrophotometer it was found that ATP caused a rapid transient elevation in [Ca2+]i in the presence of extracellular calcium which decreased to a maintained elevated level. With no extracellular calcium ATP evoked a similar transient increase which returned to the basal level. Addition of 50 mM K+ had no effect on [Ca2+]i or on the effect of ATP on [Ca2+]i in the presence of extracellular Ca2+. The data suggest that ATP causes both discharge of calcium from an intracellular pool and influx across the plasma membrane although this is unlikely to be via a voltage-operated channel. ATP stimulated simultaneously the production of PGI2 to a similar extent in the presence or absence of extracellular calcium. Elevated [Ca2+]i may be an important activation pathway in the endothelial cell.