Low-density lipoproteins stimulate internal free calcium and prostacyclin release in endothelial cells

We have studied the effects of low density lipoproteins (LDL) on the intracellular free calcium concentration [( Ca2+]i, measured with the fluorescent probe indo-1) and PGI2 release (measured with radioimmunoassay of 6-keto PGF1 alpha) in cultured endothelial cells from bovine aorta. Cells transferred to serum-free medium developed an increasing responsiveness to native LDL in terms of increases in [Ca2+]i. After 6 h, native LDL (20-160 micrograms/ml) concentration-dependently evoked immediate, but transient, elevations of [Ca2+]i (from 90 +/- 10 nmol/l to 530 +/- 60 nmol/l at 160 micrograms/ml LDL) and enhanced release of PGI2 at 160 micrograms/ml. Increases in [Ca2+]i were found also in the absence of extracellular Ca2+ indicating that native LDL induces mobilization of Ca2+ from internal stores. LDL oxidized by incubation with Cu2+ had no immediate effects on [Ca2+]i when applied at a concentration of 80 micrograms/ml, whereas 160 micrograms/ml, in the presence of 1 mmol/l extracellular Ca2+, elicited increases in [Ca2+]i and PGI2 release. We conclude that the stimulating actions of low concentrations of native LDL on endothelial cells are likely to be mediated by membrane receptors expressed after removal of serum.