PLATELETS FROM SPONTANEOUSLY HYPERTENSIVE RATS EXHIBIT DECREASED EXPRESSION OF INHIBITORY GUANINE-NUCLEOTIDE REGULATORY PROTEIN - RELATION WITH ADENYLYL-CYCLASE ACTIVITY

We have recently demonstrated an enhanced expression of inhibitory guanine nucleotide regulatory protein (G(i)) in the heart and aorta from spontaneously hypertensive rats (SHR) as compared with control Wistar-Kyoto (WKY) rats; this enhanced G(i) expression was associated with an increased inhibition of adenylyl cyclase by inhibitory hormones and decreased stimulation of adenylyl cyclase by stimulatory hormones. In the present studies, we have determined the levels of stimulatory and inhibitory guanine nucleotide regulatory proteins (G(s) and G(i), respectively) in platelets from SHR by cholera toxin- and pertussis toxin-catalyzed ADP-ribosylations, respectively, as well as by immunoblotting techniques using specific antibodies for G(s) and G(i). Cholera toxin catalyzed the ADP-ribosylation of a single protein of M(r) 45 000 in rat platelets from SHR and WKY rats, and the labeling of this band was not altered in SHR as compared with WKY rats. Pertussis toxin, on the other hand, catalyzed the ADP-ribosylation of a single protein band of M(r) 41 000 in platelets from SHR and WKY rats, and unlike the response in heart and aorta, the labeling of this band was significantly decreased in SHR as compared with WKY rats. Furthermore, immunoblotting experiments using AS/7 antibody, which is specific for G(ialpha-1) and G(ialpha-2), showed a decrease in G(ialpha-2) in platelets from SHR as compared with WKY rats. In addition, the inhibitory effects of angiotensin II and atrial natriuretic factor on adenylyl cyclase and cAMP levels were completely abolished in SHR platelets, whereas the stimulatory effects of GTP, N-ethylcarboxamide adenosine, prostaglandin E1, and forskolin on adenylyl cyclase and cAMP levels were enhanced. These results suggest that altered responsiveness of hormones to stimulate or inhibit adenylyl cyclase and cAMP levels in platelets from SHR may partly be attributed to the decreased expression of G(i) levels and not to the overexpression of G(s) protein.