Role of G(i)-proteins in norepinephrine-mediated vasoconstriction in rat tail artery smooth muscle

We showed, in rat de-endothelialised tail artery, that pertussis toxin (PTX ) (1 mug/mL, 2 hr) attenuated norepinephrine (NE)-induced vasoconstriction without modifying intracellular calcium concentration [Ca2+](i) mobilisatio n. We suggested the existence of two NE-induced intracellular pathways: a f irst, which would be insensitive to PTX and lead to [Ca2+], mobilisation, a nd a second sensitive to PTX and involved in the [Ca2+], sensitivity of NE- induced contraction. The aim of this study was to demonstrate the existence of the second intracellular pathway. PTX-sensitive G(i/o)-proteins in rat tail artery SMC membrane were identified by immunoblot and ADP-ribosylation . [P-32]ADP-ribosylation of alpha (i/o)-subunits was demonstrated in situ b y perfusing rat de-endothelialised tail artery segments with PTX (1 mug/mL, 2 hr), which suggested that G(i/o)-protein inactivation was involved in th e reduction by PTX of the [Ca2+], sensitivity of NE-induced contraction. Co upling between G(i/o)-proteins and NE receptors was confirmed by the NE-ind uced increase in G(i/o)-specific GTPase activity (24.1 +/- 1.9 vs 8.8 +/- 0 .4 pmol P-i/mg protein at 5 min; P < 0.05 vs basal). [H-3]Prazosin-binding data showed the presence of a heterogeneous <alpha>(1)-AR population in rat tail artery smooth muscle cells. We demonstrated the in vitro coupling bet ween alpha (1A)-AR subtype and alpha (i)-subunits. In conclusion, we identi fied, in rat de-endothelialised tail artery, a PTX-sensitive G(i/o)-protein -modulated pathway that is coupled to NE receptors via alpha (1A)-AR. We su ggest that NE stimulates two alpha (1)-AR-mediated intracellular pathways: a first, which is mediated by a G(q)-protein and leads to [Ca2+](i) mobilis ation and contraction, and a second, which is mediated by a G(i)-protein an d is involved in the amplification of the [Ca2+](i), sensitivity of NE-indu ced tension. (C) 2001 Elsevier Science Inc. All rights reserved.