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)-sensitive G-proteins amplified α-adrenoceptor (α-AR) agonist-induced vasoconstriction in small muscular and resistance arteries. The aim of this study was to assess the potential involvement of PTX-sensitive G-proteins in norepinephrine (NE)-induced constriction in a large diameter artery, the rat aorta. PTX (1 μg/mL, 2 hr; 3 μg/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 PTX-sensitive G-protein pathway. [α- 32P]ADP-ribosylation of G i/o-proteins by PTX (3 μg/mL, 4 hr) was demonstrated in situ in the intact aorta without endothelium. α i/o subunits were identified in vitro by both immunoblotting and ADP-ribosylation experiments in rat aorta SMC membranes. The measurement of G i/o-specific GTPase activity evidenced an effective coupling 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 the in vitro coupling between α 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 between NE receptors and G i-proteins via α 1D-ARs. Since PTX has no effect on NE-induced vasoconstriction, the PTX-sensitive G i-protein pathway does not play a predominant role in NE-induced responses in rat aorta SMC in contrast to small diameter muscular and resistance arteries.