Enhanced expression of Gi proteins in non-hypertrophic hearts from rats with hypertension-induced by L-NAME treatment

Objective The objective of the present studies is to investigate if the enh anced expression of Gs alpha protein and their mRNA observed in various mod els of hypertensive rats is due to the expressed hypertrophy or hypertensio n. Methods Hypertension, in Sprague-Dawley rats was induced by the oral admini stration of the arginine analog N-omega-nitro-L-arginine methyl ester (L-NA ME) in their drinking tap water for a period of 4 weeks. The control rats w ere given plain tap water only, The levels of inhibitory guanine nucleotide regulatory proteins (Gi alpha-2, Gi alpha-3), stimulatory guanine nucleoti de proteins (Gs alpha) and G beta proteins were determined by immunoblottin g, whereas the levels of Gi alpha-2, Gi alpha-3, Gs alpha and adenylyl cycl ase type V enzyme mRNA were determined by Northern-blotting techniques. Ade nylyl cyclase activity was determined by measuring [P-32]cAMP formation fro m [alpha(32)P]ATP. Results The systolic blood pressure was enhanced in L-NAME-treated rats com pared to control rats (190 +/- 9.2 mmHg versus 121 +/- 6.3 mmHg); however, heart-to-body-weight ratio was not different in two groups. The levels of G i alpha-2 and Gi alpha-3 proteins and their mRNA were significantly augment ed in hearts from L-NAME-treated rats, however, the levels of Gs alpha and G beta were unaltered. In addition, the effect of low concentrations of GTP gamma S on forskolin (FSK)-stimulated adenylyl cyclase activity (receptor- independent functions of Gi alpha) was significantly enhanced in L-NAME-tre ated rats. However, the inhibitions of adenylyl cyclase exerted by oxotremo rine, C-ANP(4-23) and angiotensin II (AII) (receptor-dependent function of Gi alpha) were completely attenuated in L-NAME- treated rats. On the other hand, cholera toxin stimulated GTP or GTP gamma S-sensitive adenylyl cyclas e activity (Gs alpha function) to similar extent in control and L-NAME-trea ted rats, suggesting that Gs alpha functions were not altered by L-NAME tre atment. However, the stimulatory effects of isoproterenol, glucagon, NaF on adenylyl cyclase were diminished in L-NAME-treated rats. In addition, FSK- stimulated enzyme activity was also diminished in L-NAME-treated rats witho ut any changes in the mRNA levels of type V enzyme. Conclusions These results suggest that L-NAME hypertensive rats that do not express cardiac hypertrophy exhibit enhanced expression of Gi alpha protei n and associated adenylyl cyclase activity. J Hypertens 2000, 18:1081-1090 (C) Lippincott Williams & Wilkins.