Nitric oxide modulates Gi-protein expression and adenylyl cyclase signaling in vascular smooth muscle cells

We have previously shown that treatment of rats with the nitric oxide (NO) synthase inhibitor N6-nitro-L-arginine methyl ester for 4 weeks resulted in the augmentation of blood pressure and enhanced levels of Giα proteins. The present studies were undertaken to investigate if NO can modulate the expression of Gi proteins and associated adenylyl cyclase signaling. A10 vascular smooth muscle cells (VSMC) and primary cultured cells from aorta of Sprague-Dawley rats were used for these studies. The cells were treated with S-nitroso-N-acetylpenicillamine (SNAP) or sodium nitroprusside (SNP) for 24 h and the expression of Giα proteins was determined by immunobloting techniques. Adenylyl cyclase activity was determined by measuring [32P]cAMP formation for [α-32P]ATP. Treatment of cells with SNAP (100 μM) or SNP (0.5 mM) decreased the expression of Giα-2 and Giα-3 by about 25–40% without affecting the levels of Gsα proteins. The decreased expression of Giα proteins was reflected in decreased Gi functions (receptor-independent and -dependent) as demonstrated by decreased or attenuated forskolin-stimulated adenylyl cyclase activity by GTPγS and inhibition of adenylyl cyclase activity by angiotensin II and C-ANP4–23, a ring-deleted analog of atrial natriuretic peptide (ANP) that specifically interacts with natriuretic peptide receptor-C (NPR-C) in SNAP-treated cells. The SNAP-induced decreased expression of Giα-2 and Giα-3 proteins was not blocked by 1H[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one, an inhibitor of soluble guanylyl cyclase, or KT5823, an inhibitor of protein kinase G, but was restored toward control levels by uric acid, a scavenger of peroxynitrite and Mn(111)tetralis (benzoic acid porphyrin) MnTBAP, a peroxynitrite scavenger and a superoxide dismutase mimetic agent that inhibits the production of peroxynitrite, suggesting that NO-mediated decreased expression of Giα protein was cGMP-independent and may be attributed to increased levels of peroxynitrite. In addition, Gsα-mediated stimulation of adenylyl cyclase by GTPγS, isoproterenol, and forskolin was significantly augmented in SNAP-treated cells. These results indicate that NO decreased the expression of Giα protein and associated functions in VSMC by cGMP-independent mechanisms. From these studies, it can be suggested that NO-induced decreased levels of Gi proteins and resultant increased levels of cAMP may be an additional mechanism through which NO regulates blood pressure.