Nitric oxide inactivates endothelium-derived contracting factor in the rat aorta.

Abstract

Acetylcholine evokes the simultaneous release of endothelium-derived relaxing and contracting factors in aortas from spontaneously hypertensive rats. Only relaxing factors are released in aortas from normotensive controls. Experiments were designed to determine whether inhibitors of endotheliumdependent relaxations modify endothelium-dependent contractions. Rings of thoracic aortas of normotensive and spontaneously hypertensive rats, with and without endothelium, were suspended in organ chambers for isometric tension recording. Oxyhemoglobin (a scavenger of endothelium-derived relaxing factor) and jV G -monomethyl L-arginine (an inhibitor of nitric oxide formation) augmented the contractions to acetylcholine. Methylene blue (an inhibitor of soluble guanylate cyclase) and superoxide dismutase (a scavenger of superoxide anions) did not modify these contractions. The contractions in the presence of oxyhemoglobin or JV G -monomethyl L-arginine, like those in untreated rings, were endothelium-dependent; they only occurred in aortas from spontaneously hypertensive rats and were abolished by indomethacin. The contractions to acetylcholine in the presence of oxyhemoglobin were not affected by superoxide dismutase or deferoxamine. These data suggest that endothelium-derived relaxing factor inhibits endothelium-dependent contractions to acetylcholine in the spontaneously hypertensive rat aorta, probably by chemical inactivation of the endothelium-derived contracting factor rather than by stimulation of guanylate cyclase or scavenging of oxygen-derived free radicals. A cetylcholine causes the simultaneous release of / \ endothelium-derived relaxing and contracting A. V . factors in aortas from spontaneously hypertensive rats (SHR).1 -2 Only relaxing factors are released from normotensive controls, Wistar-Kyoto (WKY) rats. -2 In rings contracted with norepinephrine, the concomitant release of endothelium-derived contracting factor from SHR aortas leads to reduced endothelium-dependent relaxations compared with aortas from WKY rats.1 In quiescent rings, acetylcholine causes endothelium-dependent contractions only in aortas from SHR but not from WKY rats.1 The nature of the contracting factors has not been identified so far. It appears to be an unstable product of cyclooxygenase (most likely endoperoxides) that stimulates thromboxane A 2 /prostaglandin H 2 receptors on vascular smooth muscle.1 -3 -5 The purpose of the present study was to determine whether endothelium-dependent contrac- tions to acetylcholine are modified by the simultaneous release of endothelium-derived relaxing factor. MethodsThe experiments were performed on the thoracic aorta from male, normotensive WKY rats and age-and weight-matched SHR (30-34 weeks old; weight, WKY 381 + 7 g, n=5, SHR 391+4 g, n = 16) (Harlan Sprague Dawley, Indianapolis, Ind.). Systolic blood pressure was determined by an indirect tail-cuff method in the unanesthetized animal before the experiment (WKY 122±8 mm Hg, SHR 202+4 mm Hg; p<0.05). The thoracic aorta was excised under anest...

Oxygen‐derived free radicals mediate endothelium‐dependent contractions in femoral arteries of rats with streptozotocin‐induced diabetes

“…To study endothelium-dependent contractions, all experiments were performed under quiescent conditions in the presence of L-NAME (N o -nitro-L-arginine methyl ester hydrochloride, 3 Â 10 À4 M; Auch-Schwelk et al, 1992;Yang et al, 2004a;Tang et al, 2005). The drugs were incubated in the organ chamber for 30 min before the administration of the calcium ionophore A23187 in a cumulative manner.…”

Acetylcholine and sodium nitroprusside cause long-term inhibition of EDCF-mediated contractions

“…The ability of nitric oxide to scavenge oxygenderived free radicals, which are essential for the production of EDCF also, could contribute to inhibition of EDCF formation (36). These inhibitory effects of nitric oxide justify the addition of nitric oxide synthase or guanylyl cyclase inhibitors to optimize endothelium-dependent contractions (1,19,22,36,38).Preliminary observations in the SHR aorta yielded unexpected results in that, if early in the experiments the presence of endothelium was tested using increasing concentrations of ACh (10 Ϫ10 to 10 Ϫ4 M, during contractions to phenylephrine), subsequent EDCF-mediated contractions were minimal (Fig. 1).…”

“…The ability of nitric oxide to scavenge oxygenderived free radicals, which are essential for the production of EDCF also, could contribute to inhibition of EDCF formation (36). These inhibitory effects of nitric oxide justify the addition of nitric oxide synthase or guanylyl cyclase inhibitors to optimize endothelium-dependent contractions (1,19,22,36,38).…”

“…To confirm this long-term inhibitory effect of ACh and to determine whether nitric oxide (NO) mediates the phenomenon, we suspended rings of spontaneously hypertensive rat (SHR) aortas in organ chambers for the recording of isometric force. The rings were incubated in the absence or presence of N -nitro-L-arginine methyl ester (L-NAME; inhibitor of NO synthases) or 1H- [1,2,4]oxadiazolo [4,3-␣]quinoxalin-1-one (ODQ; inhibitor of soluble guanylyl cyclase) before exposure to increasing concentrations of ACh or sodium nitroprusside (SNP) during contractions to phenylephrine. Thereafter, EDCF-mediated contractions to ACh or the calcium ionophore A-23187 were elicited.…”

“…Nitric oxide acutely inhibits EDCF-mediated responses, because these contractions can be augmented in the aorta of SHR when inhibitors of nitric oxide are added to the preparations (1,37). The same can be achieved by using inhibitors of soluble guanylyl cyclase, such as 1H-[1,2,4]-oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) (37).…”