CHOLESTEROL ENRICHMENT INCREASES BASAL AND AGONIST-STIMULATED CALCIUM INFLUX IN RAT VASCULAR SMOOTH-MUSCLE CELLS

The effect of cholesterol enrichment on vascular smooth muscle cell (VSMC) calcium homeostasis was studied by evaluating calcium uptake, efflux, and intracellular content in cultured VSMC derived from the rat pulmonary artery. Incubation of VSMC with liposomes consisting of free cholesterol (FC) and phospholipid (2:1 molar ratio, 1 mg FC/ml medium) for 24 h resulted in a 69 +/- 19% increase (P < 0.01; n = 10) in FC which was associated with a 73 +/- 11 % increase (P < 0.005; n = 10) in intracellular calcium content as assessed by isotopic equilibrium with Ca-45(2+) and a 65 +/- 11 % increase (P < 0.024; n = 3) as assessed by atomic absorption spectroscopy. Cholesterol enrichment caused a marked increase in the unidirectional calcium uptake rate from 0.026 +/- 0.03 to 0.158 +/- 0.022 nmol calcium/s per mg protein (P < 0.01; n = 3), but had no effect on calcium efflux. Nifedipine (1-mu-M) reduced (P < 0.05; n = 6) the effect of cholesterol enrichment on unidirectional calcium uptake by 78 +/- 16%; and verapamil (10-mu-M), diltiazem (1-mu-M), and nifedipine (1-mu-M) each significantly inhibited the effect of cholesterol enrichment on intracellular calcium accumulation. Exposure of cholesterol-enriched VSMC to cholesterol-poor liposomes for 24 h returned both FC and calcium contents to control levels. Serum- and serotonin-stimulated calcium uptakes were potentiated 3.7- and 1.7-fold, respectively, in cholesterol-enriched VSMC, whereas endothelin, vasopressin, and thrombin-stimulated calcium uptakes were not affected. We conclude that VSMC FC content plays a role in regulating cellular calcium homeostasis, both under basal conditions and in response to selected agonists.