In the present study, we describe possible mechanisms by which hypercholesterolemia may contribute to the development of cardiovascular diseases. Treatment of rat aortic smooth muscle cells for 20 hours with cholesterol-rich liposomes (500 micro gram/mL cholesterol, 100 micro gram/mL low-density lipoprotein) resulted in a 76 +/- 12% increase in total cholesterol content. The effects of cholesterol enrichment were examined by determination of changes in cell membrane fluidity. Fluidity of the cholesterol-enriched cell membranes was decreased at all temperatures between 15 degrees C and 40 degrees C. Changes in membrane fluidity in whole cell membranes represented changes in fluidity of microsomal membranes isolated by Percoll gradient ultracentrifugation. The basal [Ca2+]iand the maximal platelet-derived growth factor (PDGF)-BB-induced [Ca2+]iwas elevated by 30% and 90% in cholesterol-enriched cells, respectively. In contrast, the resting pHiand the PDGF-BB-induced stimulation of the Na+/H+exchange were not affected in cholesterol-enriched cells. The effect of PDGF-BB on [(3) H]thymidine incorporation in cholesterol-enriched cells was elevated by 40% in comparison with untreated cells. Our findings show that cellular cholesterol may be involved in the development of vascular diseases via modulation of the PDGF-induced increase in [Ca2+]iand DNA synthesis in vascular smooth muscle cells. (Hypertension. 1997;29[part 2]:326-333.)