Losartan-induced attenuation of blood pressure in L-NAME hypertensive rats is associated with reversal of the enhanced expression of Gi alpha proteins

Objective We have previously reported that hearts from N[omega]-nitro-L-arginine methyl ester (L-NAME)-induced hypertensive rats exhibited an enhanced expression of Gi proteins. Since, losartan, an AT(1) receptor antagonist, has been shown to attenuate the L-NAME-incluced increase in blood pressure, we undertook the present studies to evaluate whether losartan-induced decreased blood pressure in this model of hypertension is associated with attenuation of enhanced expression of Gi proteins and adenylyl cyclase signalling. Methods L-NAME (70 mg/kg body weight) and losartan (10 mg/kg body weight), alone or in combination, were given orally to Sprague-Dawley rats for 4 weeks. The control rats received only plain tap water. The levels of inhibitory guanine nucleotide regulatory proteins (Gialpha-2 and Gialpha-3) and stimulatory (Gsalpha) proteins and Gialpha mRNA in hearts were determined by immunoblotting and Northern blotting, respectively. Adenylyl cyclase activity was determined by measuring [P-32]cAMP formation from [P-32]ATP. Results Systolic blood pressure was enhanced in L-NAMEtreated rats compared to control rats (164 +/- 5.2 versus 105 +/- 2 mmHg; n = 30), and was significantly attenuated by losartan treatment (164 +/- 5.2 mmHg versus 120 +/- 2.5 mmHg; n = 30). The expression of Gialpha-2 and Gialpha-3 proteins and their rnRNA, which was enhanced in LNAME-treated rats, was reversed by losartan treatment. However, losartan alone did not alter the levels of Gsalpha or Gialpha proteins. In addition, the stimulatory effects of guanosine 5'-y-thiotriphosphate (GTPgammaS), isoproterenol, 5'-N-ethylcarboxamideadenosine (NECA), glucagon, forskolin (FSK) and sodium fluoride (NaF) on adenylyl cyclase, which were diminished in L-NAME-treated rats, were reversed by losartan treatment. Furthermore, the inhibition of forskolin-stimulated enzyme activity by low concentrations of GTPgammaS (receptor-independent Gi functions), which was significantly enhanced in L-NAMEtreated rats, was attenuated by losartan treatment. In addition, losartan was able to reverse the attenuated receptor-mediated inhibitions of adenylyl cyclase by oxotremorine and angiotensin II towards control. Conclusions These results suggest the implication of AT, receptors in enhanced expression of Gialpha proteins and increased blood pressure in L-NAME-induced hypertension. (C) 2004 Lippincott Williams Wilkins.