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(ALPHA-Q) AND G(ALPHA-I)

Desensitization of alpha-1 adrenoceptor (alpha(1)AR)-mediated response s in aortic smooth muscle after exposure to catecholamines or alpha(1) AR agonists has been widely demonstrated. To determine whether exposur e to an alpha(1)AR agonist results in desensitization of alpha(1)AR-me diated responses in a resistance artery, rat tail artery rings were ex posed to 7.5 or 75 mu M phenylephrine (PE) for 22 hr in vitro. Norepin ephrine-stimulated contraction was significantly reduced in PE-exposed tail artery rings. Contractions mediated by the alpha(2)AR agonists, clonidine and UK 14,304, and by serotonin were also reduced in PE-trea ted tail artery rings. However, the contractile responses to KCl and i onomycin remained unchanged. Norepinephrine-, PE-, endothelin- and ser otonin-stimulated inositol phosphate accumulations were reduced in PE- exposed tail artery rings, whereas KCl- and ionomycin-stimulated inosi tol phosphate accumulation remained unchanged. The density of membrane alpha(1)ARs, measured by specific eta-(4-hydroxyphenyl)ethyl]aminomet hyl}-1-etralone binding was not changed in PE-desensitized tail arteri es. Further studies were performed to examine if alterations in recept or/G protein interaction accompanies arterial desensitization. In thes e studies receptor-stimulated increases in [S-35]GTP gamma S binding t o G proteins was assessed in membranes obtained from vehicle (control) and PE-treated tail arteries. In control membranes alpha(1)AR stimula tion increased [S-35]GTP gamma S binding to G(alpha q) and G(alpha i) proteins, whereas the alpha(2)AR agonist UK14,304 activated [S-35]GTP gamma S binding to G(alpha i) exclusively. Both PE- and UK14,304-induc ed responses were reduced in membranes from tail arteries that were ex posed to either 7.5 or 75 mu M PE for 22 hr. Western blot analyses of G protein alpha and beta subunits demonstrated that G(alpha q) and G(a lpha i) protein levels were decreased in PE-exposed tail artery membra nes. These data show that the reduced transmembrane signaling for the alpha(1)AR in tail artery after in viiro PE exposure is associated wit h decreases in G(alpha q) and G(alpha i) protein levels. The reduction in these G(alpha) proteins also appears to mediate the loss of functi on of alpha(2)AR and perhaps of other G protein-coupled receptors.