REDUCED RESPONSIVENESS OF HYPERCHOLESTEROLEMIC RABBIT AORTIC SMOOTH-MUSCLE CELLS TO NITRIC-OXIDE

The response to nitric oxide of intracellular free Ca2+ levels, measur ed by fura 2 fluorimetry, and cyclic GMP, measured by RIA, was evaluat ed on smooth muscle cells of the thoracic aorta in primary culture fro m 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 hypercholestero lemic rabbits, intracellular Ca2+ oscillated, and the mean Ca2+ levels were approximately twofold greater than in normal aortic cells. Nitri c 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 signifi cantly smaller extent in cells from cholesterol-fed rabbits. In additi on, in cells from hypercholesterolemic rabbits, nitric oxide added bef ore angiotensin II inhibited to a smaller degree the transient increas e in intracellular free Ca2+ caused by angiotensin II in the nominal a bsence of extracellular Ca2+, as well as the increase in Ca2+ associat ed with the addition of extracellular Ca2+. Measurements of fura 2 que nching caused by Mn2+ influx confirmed that nitric oxide inhibited the entry of extracellular divalent cations significantly less in cells f rom hypercholesterolemic rabbits. Basal levels of cyclic GMP were sign ificantly less than normal, and nitric oxide increased levels of cycli c 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 mu scle cells to nitric oxide contributes to abnormal endothelium-depende nt vasodilatation during hypercholesterolemia and can play a role in t he pathogenesis of atherosclerosis.