L-NAME inhibits Mg2+-induced rat aortic relaxation in the absence of endothelium

1 L-N-G-nitro-arginine methyl ester (L-NAME; 100 mu M), a nitric oxide synt hase (NOS) inhibitor, reversed the relaxation induced by 3 mu M acetylcholi ne (ACh) and 2-10 mM Mg2+ in endothelium-intact (+E) rat aortic rings preco ntracted with 1 mu M phenylephrine (PE). In PE-precontracted endothelium-de nuded (-E) rat aorta, 3 mu M 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 mu M ACh were not significantly di fferent. 3 L-NAME (100 mu M) also reversed Mg2+-relaxation of -E aorta pre-contracte d with 20 mM KCI or 10 mu M prostaglandin F-2 alpha (PGF2 alpha). L-N-G-mon omethyl-arginine (L-NMMA; 100 mu M) was also effective in reversing the Mg2 +-relaxation. 4 Addition of 0.2 mM Ni2+, like Mg2+, caused relaxation of PE-pre-contracte d -E aorta, which was subsequently reversed by 100 mu M L-NAME. 5 Reversal of the Mg2+-relaxation by 100 mu M L-NAME in PE-precontracted -E aorta persisted following pre-incubation with 1 mu M dexamethasone or 300 mu M aminoguanidine (to inhibit the inducible form of NOS, iNOS). 6 Pretreatment of either +E or -E aortic rings with 100 mu M L-NAME caused elevation of contractile responses to Ca2+ in the presence of 1 mu M PE. 7 Our results suggest that L-NAME exerts a direct action on, as yet, uniden tified vascular smooth muscle plasma membrane protein(s), thus affecting it s reactivity to divalent cations leading to the reversal of relaxation. Suc h an effect of L-NAME is unrelated to the inhibition of endothelial NOS or the inducible NOS.