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 AT1 receptor antagonist, has been shown to attenuate the L-NAME-induced 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 (Gi alpha-2 and Gi alpha-3) and stimulatory (Gs alpha) proteins and Gi alpha mRNA in hearts were determined by immunoblotting and Northern blotting, respectively. Adenylyl cyclase activity was determined by measuring [32P]cAMP formation from [32P]ATP.


RESULTS
Systolic blood pressure was enhanced in L-NAME-treated 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 Gi alpha-2 and Gi alpha-3 proteins and their mRNA, which was enhanced in L-NAME-treated rats, was reversed by losartan treatment. However, losartan alone did not alter the levels of Gs alpha or Gi alpha proteins. In addition, the stimulatory effects of guanosine 5'-gamma-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-NAME-treated 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 AT1 receptors in enhanced expression of Gi alpha proteins and increased blood pressure in L-NAME-induced hypertension.
Hashim
Shehla
S
Department of Physiology and Groupe de recherche sur le système nerveux autonome, Faculty of Medicine, University of Montreal, Montreal, Quebec, Canada.
Anand-Srivastava
Madhu B
MB
D000262:Adenylyl Cyclases / Q000187:drug effects / Q000378:metabolism; D000818:Animals; D000959:Antihypertensive Agents / Q000494:pharmacology*; D001794:Blood Pressure / Q000187:drug effects*; D005576:Colforsin / Q000494:pharmacology; D004195:Disease Models, Animal; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D051960:GTP-Binding Protein alpha Subunit, Gi2; D019206:GTP-Binding Protein alpha Subunits, Gi-Go / Q000187:drug effects* / Q000378:metabolism*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D006973:Hypertension / Q000378:metabolism*; D019808:Losartan / Q000494:pharmacology*; D008297:Male; D008955:Models, Cardiovascular; D019331:NG-Nitroarginine Methyl Ester / Q000494:pharmacology*; D011518:Proto-Oncogene Proteins / Q000187:drug effects* / Q000378:metabolism*; D012333:RNA, Messenger / Q000187:drug effects / Q000378:metabolism; D051381:Rats; D011918:Rats, Inbred SHR; D017207:Rats, Sprague-Dawley; D012969:Sodium Fluoride / Q000494:pharmacology; D013223:Statistics as Topic
D000959:Antihypertensive Agents; D004791:Enzyme Inhibitors; D011518:Proto-Oncogene Proteins; D012333:RNA, Messenger; D005576:Colforsin; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D012969:Sodium Fluoride; D051960:GTP-Binding Protein alpha Subunit, Gi2; D019206:GTP-Binding Protein alpha Subunits, Gi-Go; C498017:Gnai2 protein, rat; D000262:Adenylyl Cyclases; D019808:Losartan; D019331:NG-Nitroarginine Methyl Ester