We have previously shown that hearts and aorta from SHR and other experimentally-induced hypertensive rats exhibit enhanced expression of Gia-protein that precede the development of hypertension suggesting that the enhanced levels of Gi proteins may be one of the contributing factors in the pathogenesis of hypertension. We have also shown that treatment of the rats with nitric oxide (NO) synthase inhibitor N⁶-nitro-L-arginine methylester (L-NAME) for 4 weeks resulted in the augmentation of blood pressure and enhanced levels of Gia proteins suggesting that decreased levels of NO may be responsible for the enhanced expression of Gia proteins. The present studies were undertaken to investigate this possibility. A10 vascular smooth muscle cells were used and treated with different concentrations of s-nitroso-N-acetylpenicillamine (SNAP) or 8-bromoguanosine-cyclic monophosphate (8-Br-cGMP) alone or in combination for 24 hrs and the expression of Gia proteins was determined by immunoblotting techniques. SNAP and 8-Br-cGMP decreased the expression of Gia-2 and Gia-3 proteins in a concentration and time-dependent manner. The maximal inhibition of about 40% was observed at 100mM SNAP or at 0.5mM 8-Br-cGMP and at 24 hrs of treatment. However, SNAP and 8-Br-cGMP together did not show any additive effect on the expression of Gia proteins. The decreased expression of Gia proteins was reflected in decreased Gi functions. Angiotensin II(AngII) and C-ANP_4-23 inhibited adenylyl cyclase activity by about 45% and 25%, respectively in control cells, however, this inhibition was significantly attenuated in cells treated with SNAP or 8-Br-cGMP or together. In addition, GTPgS, inhibited FSK-stimulated adenylyl cyclase activity in control cells which was significantly attenuated by SNAP, 8-Br-cGMP alone or in combination. On the other hand, the treatment of cells with SNAP or 8-Br-cGMP or in combination significantly enhanced the Gs-mediated stimulation of adenylyl cyclase activity by isoproterenol. The stimulation of adenylyl cyclase activity by FSK and NaF was also augmented in cells treated with SNAP, 8-Br-cGMP alone or together as compared to control cells. These results suggest that enhanced expression of Gia proteins observed in L-NAME hypertensive rats may be attributed to the decreased levels of NO.