No abstract

1 The mechanism of the vasorelaxant effect of platelet activating factor (PAF) on rat thoracic aorta and the effect of aging on the PAF-induced relaxation were investigated. 2 PAF at concentrations causing relaxation induced marked increases in guanosine 3': 5'-cyclic monophosphate (cyclic GMP) production, but did not induce an increase in adenosine 3':5'-cyclic monophosphate (cyclic AMP). 3 Removal of the endothelium by mechanical rubbing, and treatment with the PAF antagonists CV-3988, CV-6209 and FR-900452, the nitric oxide biosynthesis inhibitor, N0-nitro L-arginine, the radical scavenger, haemoglobin, and the soluble guanylate cyqRlase inhibitor, methylene blue, inhibited PAF-induced relaxation and abolished or attenuated PAF-stimulated cyclic GMP production. 4 The relaxation was greatest in arteries from rats aged 4 weeks. With an increase in age, the response of the arteries to PAF was attenuated. Endothelium-dependent cyclic GMP production also decreased with increase in age of the rats. 6 These results suggest that PAF stimulates production of nitric oxide from L-arginine by acting on the PAF receptors in the endothelium, which in turn stimulates soluble guanylate cyclase in the smooth muscle cells, and so increases production of cyclic GMP, thus relaxing the arteries. Age-associated decrease in PAF-induced relaxation may result from a reduction of cyclic GMP formation.

To determine whether amrinone and its induced increase of cyclic AMP releases nitric oxide and enhances endothelium-dependent vascular relaxation, we studied nitric oxide production and vascular relaxation of rat thoracic aorta on treatment with amrinone and forskolin, an activator of adenylate cyclase. When 20 microM amrinone was applied to ring segments of aorta previously contracted with phenylephrine, relaxation was greater in segments with endothelium than in those without (% relaxation 94 +/- 4 vs 37 +/- 7%, P < 0.01). However, a higher concentration of amrinone (> 50 microM) induced the same degree of relaxation in ring segments with or without endothelium, probably due to its direct effect on vascular smooth muscle. The maximal relaxant concentration (100 microM) of amrinone induced an increase (2.5 fold) in cyclic GMP in ring segments. Forskolin also induced concentration-dependent relaxation, but removal of the endothelium attenuated the relaxation induced by forskolin about seven-fold. NG-nitro L-arginine reversed the relaxation induced by amrinone or forskolin in ring segments with, but not without, endothelium. Nitric oxide production in ring segments of aorta, following application of amrinone or forskolin, was detected by nitric oxide-selective electrode and electron paramagnetic resonance spin trapping methods. Pre-treatment with NG-nitro L-arginine or removal of the endothelium suppressed nitric oxide production by amrinone or forskolin. These data showed that amrinone enhances the release of nitric oxide from rat aortic endothelial cells, and induces endothelium-derived relaxing factor/nitric oxide-mediated vasodilation.

1 The effect of the Ca2+-ATPase inhibitor, cyclopiazonic acid (CPA), was studied on rat thoracic aortic ring preparations. 2 At concentrations above 0.3 JM, CPA induced relaxation in the arteries precontracted with phenylephrine. Removal of the endothelium abolished CPA-induced relaxation.3 The nitric oxide (NO) synthase inhibitor NG-nitro L-arginine (3-300 AM), the free radical scavenger haemoglobin (O.1-33 AM), the soluble guanylate cyclase inhibitor, LY83583 (0.1-10 JAM), each inhibited the endothelium-dependent relaxation to CPA. The potassium channel blocker, glibenclamide (1O AM) and cyclo-oxygenase inhibitor, indomethacin (100 JAM for 60 min and then washed out) did not alter the action of CPA. 4 The calmodulin inhibitors calmidazolium (3-10 IAM) and W-7 (100 AM) also abolished CPA-induced relaxation.5 CPA (10 AM) increased guanosine 3':5'-cyclic monophosphate (cyclic GMP) levels in arteries with an intact endothelium, without affecting adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels. 6 The inhibitors of NO synthesis and actions, the calmodulin inhibitor and removal of the endothelium abolished the CPA-stimulated increase in the levels of cyclic GMP. 7 In Ca2'-free solution, CPA failed to induce relaxation or to stimulate cyclic GMP production.Relaxation to nitroprusside was not affected under these conditions.

1 Endothelin-3 (ET-3) at concentrations below those which caused contraction (30 nM) elicited endothelium-dependent relaxation followed by rebound contraction in rat isolated thoracic aorta. 2 Endothelin-l also relaxed the rat aorta with a similar potency.3 The nitric oxide (NO) synthase inhibitor, NG-nitro L-arginine, the radical scavenger, haemoglobin and the soluble guanylate cyclase inhibitor, methylene blue, each inhibited the ET-3-induced relaxation. 4 The calmodulin inhibitor, calmidazolium, considerably attenuated the relaxation caused by without affecting that to nitroprusside. 5 Concentrations of ET-3 that were necessary to induce the relaxation also caused concentrationdependent elevation of guanosine 3':5'-cyclic monophosphate (cyclic GMP) levels. 6 NG-nitro L-arginine, haemoglobin, methylene blue, calmidazolium and removal of the endothelium completely abolished ET-3-stimulated cyclic GMP production. 7 These results suggest that ET-3 triggers NO formation possibly via ETB receptors on the endothelium to activate soluble guanylate cyclase, which in turn stimulates cyclic GMP production and smooth muscle relaxation. The enzyme contributing to the NO formation may be of the calcium/calmodulindependent, constitutive type.