Abstract

Desensitization of alpha-1 adrenoceptor (α₁AR)-mediated responses in aortic smooth muscle after exposure to catecholamines or α₁AR agonists has been widely demonstrated. To determine whether exposure to an α₁AR agonist results in desensitization of α₁AR-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 α₂AR 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 α₁ARs, measured by specific [¹²⁵I]2-{[β-(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]GTPγS binding to G proteins was assessed in membranes obtained from vehicle (control) and PE-treated tail arteries. In control membranes α₁AR stimulation increased [³⁵S]GTPγS binding to G_αq and G_αi proteins, whereas the α₂AR agonist UK14,304 activated [³⁵S]GTPγS binding to G_αi 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 andbeta subunits demonstrated that G_αq and G_αi protein levels were decreased in PE-exposed tail artery membranes. These data show that the reduced transmembrane signaling for the α₁AR in tail artery after in vitro PE exposure is associated with decreases in G_αq and G_αi protein levels. The reduction in these G_α proteins also appears to mediate the loss of function of α₂AR and perhaps of other G protein-coupled receptors.