Lack of involvement of pertussis toxin-sensitive G-proteins in norepinephrine-induced vasoconstriction of rat aorta smooth muscle

Several studies have shown that stimulation of pertussis toxin (PTX)-sensit ive G-proteins amplified alpha -adrenoceptor alpha -AR) agonist-induced vas oconstriction in small muscular and resistance arteries. The aim of this st udy was to assess the potential involvement of PTX-sensitive G-proteins in norepinephrine (NE)-induced constriction in a large diameter artery, the ra t aorta. PTX (1 mug/mL, 2 hr; 3 mug/mL, 4 hr) did not modify concentration- response curves to NE in endothelium-denuded aortic rings. However, several lines of evidence suggested that aortic smooth muscle cells (SMC) had a PT X-sensitive G-protein pathway. [alpha-P-32]-ribosylation of G(i/o)-proteins by PTX (3 mug/mL, 4 hr) was demonstrated in situ in the intact aorta witho ut endothelium. alpha (i/o) subunits were identified in vitro by both immun oblotting and ADP-ribosylation experiments in rat aorta SMC membranes. The measurement of G(i/o)-specific GTPase activity evidenced an effective coupl ing between NE receptors and G(i/o)-proteins, as NE induced an increase in basal G(i/o)-specific GTPase activity (20.7 +/- 2.8 vs 7.2 +/- 2.2 pmol P-i /mg protein at 5 min; P < 0.05 vs basal). Co-immunoprecipitation revealed t he in vitro coupling between <alpha>(1D)-ARs and G(i)-protein in rat aorta SMC membranes. In conclusion, we identified a PTX-sensitive G(i/o)-protein pathway in rat endothelium-denuded aorta. We showed an effective coupling b etween NE receptors and G(i)-proteins via alpha (1D)-ARs. Since PTX has no effect on NE-induced vasoconstriction, the PTX-sensitive G(i)-protein pathw ay does not play a predominant role in NE-induced responses in rat aorta SM C in contrast to small diameter muscular and resistance arteries. (C) 2001 Elsevier Science Inc. All rights reserved.