Vol. 283, Issue 2, 925-931, 1997

Heterologous Desensitization of the Rat Tail Artery Contraction and Inositol Phosphate Accumulation After in Vitro Exposure to Phenylephrine Is Mediated by Decreased Levels of G_q and G_i¹

Tammy M. Seasholtz, Hakan Gurdal², Hoau-Yan Wang, Guoping Cai, Mark D. Johnson and Eitan Friedman

Department of Pharmacology, MCP-Hahnemann School of Medicine, Allegheny University of the Health Sciences, Philadelphia, Pennsylvania

Desensitization of alpha-1 adrenoceptor (₁AR)-mediated responses in aortic smooth muscle after exposure to catecholamines or ₁AR agonists has been widely demonstrated. To determine whether exposure to an ₁AR agonist results in desensitization of ₁AR-mediated responses in a resistance artery, rat tail artery rings were exposed to 7.5 or 75 µM phenylephrine (PE) for 22 hr in vitro. Norepinephrine-stimulated contraction was significantly reduced in PE-exposed tail artery rings. Contractions mediated by the ₂AR agonists, clonidine and UK 14,304, and by serotonin were also reduced in PE-treated tail artery rings. However, the contractile responses to KCl and ionomycin remained unchanged. Norepinephrine-, PE-, endothelin- and serotonin-stimulated inositol phosphate accumulations were reduced in PE-exposed tail artery rings, whereas KCl- and ionomycin-stimulated inositol phosphate accumulation remained unchanged. The density of membrane ₁ARs, measured by specific [¹²⁵I]2-{[-(4-hydroxyphenyl)ethyl]aminomethyl}-1-etralone binding was not changed in PE-desensitized tail arteries. Further studies were performed to examine if alterations in receptor/G protein interaction accompanies arterial desensitization. In these studies receptor-stimulated increases in [³⁵S]GTPS binding to G proteins was assessed in membranes obtained from vehicle (control) and PE-treated tail arteries. In control membranes ₁AR stimulation increased [³⁵S]GTPS binding to G_q and G_i proteins, whereas the ₂AR agonist UK14,304 activated [³⁵S]GTPS binding to G_i exclusively. Both PE- and UK14,304-induced responses were reduced in membranes from tail arteries that were exposed to either 7.5 or 75 µM PE for 22 hr. Western blot analyses of G protein alpha and beta subunits demonstrated that G_q and G_i protein levels were decreased in PE-exposed tail artery membranes. These data show that the reduced transmembrane signaling for the ₁AR in tail artery after in vitro PE exposure is associated with decreases in G_q and G_i protein levels. The reduction in these G proteins also appears to mediate the loss of function of ₂AR and perhaps of other G protein-coupled receptors.