RESUMEN
Vascular relaxation by the organic (nitroglycerin) and inorganic (sodium nitroprusside) nitrovasodilators and the endothelium-dependent vasodilators (acetylcholine and histamine) has been associated with cyclic GMP accumulation. Tolerance to vasodilation by nitroglycerin commonly occurs following prolonged exposure to nitroglycerin. This study investigates the effects of in vivo nitroglycerin therapy on vascular relaxation and cyclic GMP accumulation induced by the nitrovasodilators and the endothelium-dependent vasodilators. Rats were injected with nitroglycerin or the propylene glycol diluent control for 4-7 days. Thoracic aortas from the nitroglycerin-treated rats were 750-fold less sensitive to the relaxant effects of nitroglycerin. In contrast, these aortas were only threefold less sensitive to the relaxant effects of sodium nitroprusside, while the maximum relaxation to acetylcholine and histamine was depressed by 50 and 41%, respectively. Desensitization to relaxation was associated with reduced cyclic GMP elevations to all the vasodilators. Relaxation to 8-bromo cyclic GMP, dibutyryl cyclic AMP, or diltiazem was unaffected by nitroglycerin therapy. Tolerance was also associated with an increased sensitivity to the contractile effects of low concentrations of norepinephrine. This increased sensitivity to norepinephrine was associated with a decrease in cyclic GMP levels. The present results suggest that: (1) desensitization to nitroglycerin, sodium nitroprusside, acetylcholine, and histamine by nitroglycerin therapy may be at the level of cyclic GMP accumulation; (2) cyclic GMP is the common mediator of relaxation induced by the nitro- and endothelium-dependent vasodilators; (3) the mechanisms involved in the activation of guanylate cyclase and relaxation by sodium nitroprusside, acetylcholine, and histamine are probably different than those of nitroglycerin; and (4) cyclic GMP may be acting as a physiological negative feedback signal in agonist-induced contraction.