Differential regulation of G-protein expression by vasoactive peptides

Vasoactive peptides such as angiotensin II (AII), atrial natriuretic peptide (ANP) and vasopressin play an important role in the regulation of blood pressure. We have recently shown an augmentation of Gi alpha levels in heart and aorta from genetic and experimentally-induced hypertensive rats, which may be attributed to the increased levels of vasoactive peptides. We have therefore investigated the effect of AII and ANP on the expression of G-proteins (Gi alpha and Gs alpha) in cultured vascular smooth muscle cells (VSMC) and their relationship with adenylyl cyclase activity. Exposure of VSMC with AII resulted in the augmentation of the levels of Gi alpha-2 and Gi alpha-3 proteins and Gi alpha-2 and Gi alpha-3 mRNA and not of Gs alpha as determined by immunoblotting and Northern blotting techniques respectively. However, the stimulatory effects of N-ethylcarboxamide adenosine (NECA) and isoproterenol on adenylyl cyclase was diminished by AII treatment, whereas the inhibitory effects of AII and C-ANP4-23 were completely attenuated. On the other hand, pretreatment of the cells with C-ANP4-23 resulted in the reduction of the levels of Gi alpha-2 and Gi alpha-3 and not of Gs alpha. The inhibitory responses of adenylyl cyclase to C-ANP4-23 and AII were also attenuated and the stimulatory effects of GTP gamma S and other agonists were significantly augmented. These data indicate that AII and ANP modulate the expression of Gia protein in a different manner. It may be suggested that the enhanced levels of Gi alpha protein observed in hypertension may be attributed to the augmented levels of AII and not to ANP.