Documentation Of Existence Of Endothelium-derived Contracting Factor(s) In The Aorta Of Spontaneously Hypertensive Rat

A change in endothelial function is a common phenomenon in patients with essential hypertension and in animals with hypertension. In the aorta of spontaneously hypertensive rat (SHR). the endothelium-dependent relaxations are impaired due to the occurrence of a concomitant endothelium-dependent contraction. Endothelium-dependent contractions can be explained by the endothelial production of diffusible substance(s), endothelium-derived contracting factor(s) (EDCF). In the aorta of SHR, endothelium-dependent contractions involve the activation of cyclooxygenase (COX) and thromboxane/prostaglandin H receptor (TP receptor). The nature of EDCF is still unknown. The mechanisms governing the formation and release of EDCF in hypertensive blood vessels as well as the delicate balance between relaxing and contracting mediators are important for the understanding of the physiological and pathological circumstances in the vasculature. The present thesis was designed to characterize EDCF in aorta of SHR.In the aorta of SHR, both receptor-dependent stimuli, acetylcholine, and receptor-independent stimuli, calcium ionophore A23187, evoked endothelium-dependent contractions. These contractions were potentiated by the presence of NG-nitro-L-arginine, an inhibitor of nitric oxide synthase. In the presence of NG-nitro-L-arginine, the contractions to acetylcholine and the calcium ionophore A23187 were abolished by a selective TP-receptor antagonist, S 18886, and a preferential inhibitor of cyclooxygenase-1, valeryl salicylate. These results suggest that the endothelium-dependent contractions require an increase in endothelialintracellular concentration of calcium.In the presence of NG-nitro-L-arginine, the endothelium-dependent contractions to acetylcholine and endothelium-independent contractions to oxygen-derived free radicals generated from xanthine plus xanthine oxidase were abolished by S 18886, and valeryl salicylate, but not by a preferential inhibitor of cyclooxygenase-2, NS-398. Allopurinol, deferoxamine and the combination of superoxide dismutase plus catalase inhibited the contractions to oxygen-derived free radicals but did not significantly affect those to acetylcholine. In contrast, diethyldithiocarbamic acid, an inhibitor of superoxide dismutase, or Tiron, a scavenger of superoxide anion, reduced endothelium-dependent contractions to acetylcholine in aortas from SHR. In aorta from SHR chronically treated with dimethylthiourea, endothelium-dependent contractions to acetylcholine were decreased, and reduced further by acute in vitro exposure to deferoxamine or the combination of superoxide dismutase plus catalase. These results suggest that in the SHR aorta acetylcholine-induced endothelium-dependent contractions involve endothelial superoxide anion production and the subsequent dismutation into hydroxyl radicals and/or hydrogen peroxide.The acetylcholine-induced endothelium-dependent contractions of SHR isolated aortic rings were significantly and similarly potentiated by two nitric oxide synthase inhibitors, NG-nitro-L-arginine and NG-nitro-L-arginine methyl ester, by two structurally different nitric oxide scavengers, hemoglobin and carboxyPTIO, by two inhibitors of guanylate cyclase, ODQ and NS2028, but to a lesser extent by methylene blue. The contraction of the isolated rat trachea in response to methacholine and thecontraction of the rat aorta in response to 8-isoprostane were significantly inhibited by methylene blue. Methylene blue may therefore interact both with the M3 receptor subtype, involved in the release of EDCF, and the TP receptor, involved in the action of EDCF. These negative effects of methylene blue are likely to counterbalance the potentiation of acetylcholine-induced contraction in the SHR aorta which are linked to guanylate cyclase inhibition. Therefore, in the SHR aorta, removing the cGMP-dependent endothelium-dependent inhibitory tone by either inhibiting nitric oxide synthase, or the guanylate cyclase or by scavenging nitric oxide produces the same potentiation of the endothelium-dep...