title: L-NAME inhibits Mg(2+)-induced rat aortic relaxation in the absence of endothelium.
abstract: 1 L-NG-nitro-arginine methyl ester (L-NAME; 100 microM), a nitric oxide synthase (NOS) inhibitor, reversed the relaxation induced by 3 microM acetylcholine (ACh) and 2-10 mM Mg2+ in endothelium-intact (+E) rat aortic rings precontracted with 1 microM phenylephrine (PE). In PE-precontracted endothelium-denuded (-E) rat aorta, 3 microM ACh did not, but Mg2+ caused relaxation which was reversed by L-NAME, but not by D-NAME. 2 The concentration response profiles of L-NAME in reversing the equipotent relaxation induced by 5 mM Mg2+ and 0.2 microM ACh were not significantly different. 3 L-NAME (100 microM) also reversed Mg(2+)-relaxation of -E aorta pre-contracted with 20 mM KCl or 10 microM prostaglandin F2alpha (PGF2alpha). L-NG-monomethyl-arginine (L-NMMA; 100 microM) was also effective in reversing the Mg(2+)-relaxation. 4 Addition of 0.2 mM Ni2+, like Mg2+, caused relaxation of PE-pre-contracted -E aorta, which was subsequently reversed by 100 microM L-NAME. 5 Reversal of the Mg(2+)-relaxation by 100 microM L-NAME in PE-precontracted -E aorta persisted following pre-incubation with 1 microM dexamethasone or 300 microM aminoguanidine (to inhibit the inducible form of NOS, iNOS). 6 Pretreatment of either +E or -E aortic rings with 100 microM L-NAME caused elevation of contractile responses to Ca2+ in the presence of 1 microM PE. 7 Our results suggest that L-NAME exerts a direct action on, as yet, unidentified vascular smooth muscle plasma membrane protein(s), thus affecting its reactivity to divalent cations leading to the reversal of relaxation. Such an effect of L-NAME is unrelated to the inhibition of endothelial NOS or the inducible NOS.
authors.affiliation: Department of Physiology, Faculty of Medicine, The University of Hong Kong, Hong Kong, China.
authors.forename: R
authors.initials: R
authors.lastname: Das
authors.forename: G M
authors.initials: GM
authors.lastname: Kravtsov
authors.forename: H J
authors.initials: HJ
authors.lastname: Ballard
authors.forename: C Y
authors.initials: CY
authors.lastname: Kwan
mesh_terms: D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D001013:Aorta, Thoracic / Q000187:drug effects*; D004730:Endothelium, Vascular / Q000166:cytology / Q000187:drug effects*; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D066298:In Vitro Techniques; D007537:Isometric Contraction / Q000187:drug effects; D008274:Magnesium / Q000037:antagonists & inhibitors* / Q000494:pharmacology*; D008297:Male; D009126:Muscle Relaxation / Q000187:drug effects; D009131:Muscle, Smooth, Vascular / Q000166:cytology / Q000187:drug effects*; D019331:NG-Nitroarginine Methyl Ester / Q000494:pharmacology*; D011189:Potassium Chloride / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D019323:omega-N-Methylarginine / Q000494:pharmacology
chemical_list: D004791:Enzyme Inhibitors; D019323:omega-N-Methylarginine; D011189:Potassium Chloride; D008274:Magnesium; D000109:Acetylcholine; D019331:NG-Nitroarginine Methyl Ester