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

Abstract

1. L-N(G)-nitro-arginine methyl ester (L-NAME; 100 μM), a nitric oxide synthase (NOS) inhibitor, reversed the relaxation induced by 3 μM acetylcholine (ACh) and 2-10 mM Mg2+ in endothelium-intact (+E) rat aortic rings precontracted with 1 μM phenylephrine (PE). In PE-precontracted endothelium-denuded (-E) rat aorta, 3 μM 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 μM ACh were not significantly different. 3. L-NAME (100 μM) also reversed Mg2+-relaxation of -E aorta pre-contracted with 20 mM KCl or 10 μM prostaglandin F(2α) (PGF(2α)). L-N(G)-monomethyl-arginine (L-NMMA; 100 μM) was also effective in reversing the Mg2+-relaxation. 4. Addition of 0.2 mM Ni2+, like Mg2+, caused relaxation of PE-pre-contracted -E aorta, which was subsequently reversed by 100 μM L-NAME. 5. Reversal of the Mg2+-relaxation by 100 μM L-NAME in PE-precontracted -E aorta persisted following pre-incubation with 1 μM dexamethasone or 300 μM aminoguanidine (to inhibit the inducible form of NOS, iNOS). 6. Pretreatment of either +E or -E aortic rings with 100 μM L-NAME caused elevation of contractile responses to Ca2+ in the presence of 1 μM 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.