Inhibitory non-adrenergic non-cholinergic (NANC) nerves are thought to be important in the autonomic innervation of the gastrointestinal tract and other organ systems. The nature of their neurotransmitter is still debated. Speculation that nitric oxide (NO), formed from L-arginine in neuronal structures and other cells, could act as a neurotransmitter, is not yet supported by demonstration of its release upon nerve stimulation. Using a superfusion bioassay, we report the release of a vasorelaxant factor upon stimulation of the NANC nerves in the canine ileocolonic junction. Several pieces of evidence, including the selectivity of the bioassay tissues, chemical instability, inactivation by superoxide anion and haemoglobin, inhibition by NG-nitro-L-arginine (L-NNA) and potentiation by L-arginine all indicated that NO accounted for the biological activity of this transferable NANC factor.

We studied the effects of hypercholesterolemia on vascular responsiveness in different arteries isolated from rabbits: control groups of rabbits and groups receiving the atherogenic diet consisted of eight animals each. In the arteries, 16 weeks of cholesterol-rich (0.3%) diet evoked intimal lesions which were more pronounced than those noted after 8 weeks of hypercholesterolemia; the aortic arch was affected significantly more by the lesions than the abdominal aorta and the pulmonary artery. Segments of the arteries were mounted in organ chambers for isometric tension recording or for measurement of the endothelium-derived relaxant factor. Contractions caused by acetylcholine and prostaglandin F2 alpha were not altered by the hypercholesterolemia; those evoked by serotonin were moderately augmented only in the aortic arch of hypercholesterolemic rabbits. As the degree of intimal lesion formation increased, the contractions to norepinephrine and clonidine were progressively inhibited. The endothelium-independent relaxations to nitroglycerin were inhibited in only the most severely affected arteries; the endothelium-dependent relaxations to acetylcholine and adenosine triphosphate were progressively inhibited as the degree of fatty streak formation augmented. Thus, in the aortic arch, the relaxations to 3 X 10(-6) M acetylcholine, expressed as percent of the initial contraction, decreased from 86.7 +/- 3.3% in control tissues to 16.3 +/- 8.6% in the 16-week hypercholesterolemic vessels; in the abdominal aortas these relaxations averaged 93.5 +/- 2.2% in control vessels and 72.0 +/- 6.9% in the hypercholesterolemic tissues. The acetylcholine-induced release of endothelium-derived relaxant factor from the abdominal aorta was not significantly affected by the hypercholesterolemia. We conclude from these studies that in arteries obtained from hypercholesterolemic rabbits: the contractions caused by serotonergic mechanisms tend to be augmented, while those to alpha-adrenergic activation are decreased, the endothelium-independent relaxations are modified only in the more severely affected arteries, and the endothelium-dependent relaxations are progressively inhibited as the degree of fatty streak formation augments, probably because a step subsequent to the release of endothelium-derived relaxant factor is altered.

No abstract

1 The present study examined the responses of the rabbit carotid artery to five vasoconstrictors after neo-intima formation induced by perivascular collar treatment and evaluated the role of constitutive and inducible nitric oxide (NO) synthase and endothelial cells (ECs). 2 Ring segments of the rabbit carotid artery were mounted in organ chambers for isometric tension recording. Neo-intima-bearing vessels developed less force (Em,.) in response to KCl, the thromboxanemimetic U-46619 and 5-hydroxytryptamine (5-HT), but not to angiotensin I and II. 3 The collar-treatment increased the sensitivity to 5-HT, and decreased the sensitivity to angiotensin II. The sensitivity to U-46619 and angiotensin I remained unchanged. 4 Mechanical removal of ECs and inhibition of NO biosynthesis by N0-monomethyl-L-arginine (L-NMMA) and N0-nitro-L-arginine (L-NOARG) increased the sensitivity to 5-HT in sham and collartreated segments to the same extent. The effects of collar-treatment and endothelial removal or treatment with inhibitors of NO biosynthesis were additive. Inhibition of NO biosynthesis failed to augment sensitivity to 5-HT after endothelial denudation. L-NOARG increased the force development to KC1 in sham and collar-treated segments to the same extent. However, L-NMMA and L-NOARG failed to augment the contractile responses of neo-intima-bearing vessels to 5-HT and KCl after endothelial removal. 5 The responses to angiotensin I were not altered, either by the neo-intima or by endothelial removal. In arteries with a neo-intima the sensitivity to angiotensin II was decreased. Removal of the endothelium or incubation with L-NOARG counteracted this rightward shift and increased Em|. 6 Our results demonstrate that contractions to 5-HT, angiotensin II and KCl are modulated by NO in both sham and neo-intima-bearing vessels. Inhibition of NO biosynthesis and collar treatment resulted in additive effects on the EC50 values, suggesting that the 5-HT and angiotension (AT) receptors on the smooth muscle cells are also modified by the formation of a neo-intima. Furthermore, the reduced contractile responses of segments with a neo-intima are not due to NO formed by an inducible NO synthase in those vessels.

The purpose of this study was to determine whether the generation of a neointima, an early step in the development of atherosclerosis, affects endothelium-dependent or -independent vasodilation. The neointima was induced, within 7 days, by positioning a nonocclusive silicone collar around one carotid artery in rabbits. After 1, 2, 7, or 14 days segments were cut from the collar-surrounded region of this artery as well as from the sham-operated contralateral artery and were used for isometric tension recording or for bioassay of nitric oxide (NO). The acetylcholine-induced release of NO was significantly reduced at 7 days. The tension recordings suggested that this already occurred at the earliest stages of neointima formation. Neither the capacity of the endothelial cells to form NO in response to the calcium ionophore A23187 nor the capacity of the underlying smooth muscle cells to relax in response to sources of exogenous NO (3-morpholinosydnonimine and nitroglycerin) was affected by the neointima. Therefore, the impaired endothelium-dependent relaxations to acetylcholine are presumably due to a defect at the level of the endothelial muscarinic receptors. The presence of a fully developed neointima did not alter the responsiveness to isoproterenol and forskolin but enhanced prostacyclin-mediated responses (assessed by iloprost and 13-hydroxyoctadecadienoic acid). These results illustrate selective alterations of endothelial and smooth muscle cell function in intima generation before fatty streak formation.