title: Up-regulation of the levels of expression and function of a constitutively active mutant of the hamster alpha1B-adrenoceptor by ligands that act as inverse agonists.
abstract: The alpha1-adrenergic agonist phenylephrine stimulated phospholipase D (PLD) activity in Rat 1 fibroblasts transfected to express either the wild-type hamster alpha1B-adrenoceptor or a constitutively active mutant (CAM) form of this receptor. The EC50 for agonist stimulation of PLD activity was substantially lower at the CAM receptor than at the wild-type receptor as previously noted for phenylephrine stimulation of phosphoinositidase C activity. Sustained treatment of cells expressing the CAM alpha1B-adrenoceptor with phentolamine resulted in a marked up-regulation in levels of this receptor with half-maximal effects produced within 24 h and with an EC50 of approx. 40 nM. Such an up-regulation could be produced with a range of other ligands generally viewed as alpha1-adrenoceptor antagonists but equivalent treatment of cells expressing the wild-type alpha1B-adrenoceptor was unable to mimic these effects. After sustained treatment of the CAM alpha1B-adrenoceptor expressing cells with phentolamine, basal PLD activity was increased and phenylephrine was now able to stimulate PLD activity to greater levels than in vehicle-treated CAM alpha1B-adrenoceptor-expressing cells. The EC50 for phenylephrine stimulation of PLD activity was not altered, however, by phentolamine pretreatment and the associated up-regulation of the receptor. After phentolamine-induced up-regulation of basal PLD activity, a range of alpha1-antagonists were shown to possess the characteristics of inverse agonists of the CAM alpha1B-adrenoceptor as they were able to substantially decrease the elevated basal PLD activity.
authors.affiliation: Molecular Pharmacology Group, Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, U.K.
authors.forename: T W
authors.initials: TW
authors.lastname: Lee
authors.forename: S
authors.initials: S
authors.lastname: Cotecchia
authors.forename: G
authors.initials: G
authors.lastname: Milligan
mesh_terms: D058646:Adrenergic alpha-1 Receptor Agonists; D000316:Adrenergic alpha-Agonists / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D006224:Cricetinae; D018076:DNA, Complementary; D008024:Ligands; D016296:Mutagenesis; D010739:Phospholipase D / Q000378:metabolism; D011224:Prazosin / Q000378:metabolism / Q000494:pharmacology; D051381:Rats; D018340:Receptors, Adrenergic, alpha-1 / Q000235:genetics / Q000378:metabolism*; D015854:Up-Regulation
chemical_list: C547718:Adra1b protein, rat; D058646:Adrenergic alpha-1 Receptor Agonists; D000316:Adrenergic alpha-Agonists; D018076:DNA, Complementary; D008024:Ligands; D018340:Receptors, Adrenergic, alpha-1; D010739:Phospholipase D; D011224:Prazosin