Nitric oxide synthase inhibition by N(omega)-nitro-L-arginine methyl ester modulates G-protein expression and adenylyl cyclase activity in rat heart.

Abstract

We have recently shown an enhanced expression of inhibitory guanine nucleotide regulatory proteins Gi alpha-2 and Gi alpha-3 and their respective mRNA in hearts from DOCA-salt hypertensive rats. However, it is not known whether these changes are due to the expressed hypertrophy or hypertension. The present studies were therefore undertaken to investigate this possibility. Hypertension in Sprague-Dawley rats was induced by the oral administration of the arginine analog N(omega)-nitro-L-arginine methyl ester (L-NAME) in their drinking tap water for a period of 4 weeks. The control rats were given plain tap water only. L-NAME-treated rats showed an enhanced blood pressure (190 +/- 9.23 mm Hg; n = 20) compared to control rats (121 +/- 6.3 mm Hg; n = 20). However, heart to body weight ratio was not different in the two groups. Guanosine 5'-o-(3-thiotriphosphate) (GTPgammaS) stimulated adenylyl cyclase activity in heart membranes from both groups, but the extent of stimulation was significantly decreased in L-NAME-treated rats. Similarly, stimulations exerted by isoproterenol, glucagon, NaF, and forskolin on adenylyl cyclase were also diminished in L-NAME-treated rats. On the other hand, the inhibitory effect of low concentrations of GTPgammaS on forskolin-stimulated enzyme activity was significantly enhanced. The extent of oxotremorine-mediated inhibition of adenylyl cyclase was unaltered in both control and L-NAME-induced hypertensive rats. The levels of Gi alpha-2 and Gi alpha-3, but not of stimulatory guanine nucleotide regulatory protein Gs alpha, as determined by immunoblotting, were significantly augmented in L-NAME-treated rats. Northern blot studies revealed a significant increase in Gi alpha-2 and Gi alpha-3 mRNA with no changes in Gs alpha mRNA. These results suggest that the altered expression of Gi alpha proteins and adenylyl cyclase activity in L-NAME-treated rats may be attributed to hypertension and not to hypertrophy.