Reduced responsiveness of hypercholesterolemic rabbit aortic smooth muscle cells to nitric oxide

The response to nitric oxide of intracellular free Ca2+ levels, measured by fura 2 fluorimetry, and cyclic GMP, measured by RIA, was evaluated on smooth muscle cells of the thoracic aorta in primary culture from normal and cholesterol-fed rabbits. Relaxation to acetylcholine and nitric oxide was also determined in isolated rings of aorta. After 10 weeks of high-cholesterol diet, the intact aorta relaxed less to both acetylcholine and nitric oxide. In cultured cells from hypercholesterolemic rabbits, intracellular Ca2+ oscillated, and the mean Ca2+ levels were approximately twofold greater than in normal aortic cells. Nitric oxide failed to affect basal Ca2+ in either cell type. The peak and sustained rise in intracellular Ca2+ induced by angiotensin II (10(-7) mol/L) were similar in the two cell types. However, nitric oxide (10(-10) to 10(-6) mol/L) decreased the sustained Ca2+ levels to a significantly smaller extent in cells from cholesterol-fed rabbits. In addition, in cells from hypercholesterolemic rabbits, nitric oxide added before angiotensin II inhibited to a smaller degree the transient increase in intracellular free Ca2+ caused by angiotensin II in the nominal absence of extracellular Ca2+, as well as the increase in Ca2+ associated with the addition of extracellular Ca2+. Measurements of fura 2 quenching caused by Mn2+ influx confirmed that nitric oxide inhibited the entry of extracellular divalent cations significantly less in cells from hypercholesterolemic rabbits. Basal levels of cyclic GMP were significantly less than normal, and nitric oxide increased levels of cyclic GMP to a significantly smaller degree in cells from cholesterol-fed rabbits. These data indicate a substantial resistance to nitric oxide action in aortic smooth muscle cells of cholesterol-fed rabbits. This observation is consistent with the notion that resistance of smooth muscle cells to nitric oxide contributes to abnormal endothelium-dependent vasodilation during hypercholesterolemia and can play a role in the pathogenesis of atherosclerosis.