Heterologous desensitization of the rat tail artery contraction and inositol phosphate accumulation after in vitro exposure to phenylephrine is mediated by decreased levels of Galphaq and Galphai.

Desensitization of alpha-1 adrenoceptor (alpha1AR)-mediated responses in aortic smooth muscle after exposure to catecholamines or alpha1AR agonists has been widely demonstrated. To determine whether exposure to an alpha1AR agonist results in desensitization of alpha1AR-mediated responses in a resistance artery, rat tail artery rings were exposed to 7.5 or 75 microM phenylephrine (PE) for 22 hr in vitro. Norepinephrine-stimulated contraction was significantly reduced in PE-exposed tail artery rings. Contractions mediated by the alpha2AR 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 alpha1ARs, measured by specific [125I]2-([beta-(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]GTPgammaS binding to G proteins was assessed in membranes obtained from vehicle (control) and PE-treated tail arteries. In control membranes alpha1AR stimulation increased [35S]GTPgammaS binding to Galphaq and Galphai proteins, whereas the alpha2AR agonist UK14,304 activated [35S]GTPgammaS binding to Galphai exclusively. Both PE- and UK14, 304-induced responses were reduced in membranes from tail arteries that were exposed to either 7.5 or 75 microM PE for 22 hr. Western blot analyses of G protein alpha and beta subunits demonstrated that Galphaq and Galphai protein levels were decreased in PE-exposed tail artery membranes. These data show that the reduced transmembrane signaling for the alpha1AR in tail artery after in vitro PE exposure is associated with decreases in Galphaq and Galphai protein levels. The reduction in these Galpha proteins also appears to mediate the loss of function of alpha2AR and perhaps of other G protein-coupled receptors.