The α 1B/D-adrenoceptor knockout mouse permits isolation of the vascular α 1A-adrenoceptor and elucidates its relationship to the other subtypes

Abstract

Background and purpose: Mesenteric and carotid arteries from the α 1B/D-adrenoceptor knockout (α 1B/D-KO) were employed to isolate α 1A-adrenoceptor pharmacology and location and to reveal these features in the wild-type (WT) mouse. Experimental approach: Functional pharmacology by wire myography and receptor localization by confocal microscopy, using the fluorescent α 1-adrenoceptor ligand BODIPY FL-Prazosin (QAPB), on mesenteric (an 'α 1A-adrenoceptor' tissue) and carotid (an 'α 1D-adrenoceptor' tissue) arteries. Key results: α 1B/D-KO mesenteric arteries showed straightforward α 1A-adrenoceptor agonist/antagonist pharmacology. WT had complex pharmacology with α 1A- and α 1D-adrenoceptor components. α 1B/D-KO had a larger α 1A-adrenoceptor response suggesting compensatory up-regulation: no increase in fluorescent ligand binding suggests up-regulation of signalling. α 1B/D-KO carotid arteries had low efficacy α 1A-adrenoceptor responses. WT had complex pharmacology consistent with co-activation of all three subtypes. Fluorescent binding had straightforward α 1A-adrenoceptor characteristics in both arteries of α 1B/D-KO. Fluorescent binding varied between cells in relative intracellular and surface distribution. Total fluorescence was reduced in the α 1B/D-KO due to fewer smooth muscle cells showing fluorescent binding. WT binding was greater and sensitive to α 1A- and α 1D-adrenoceptor antagonists. Conclusions and implications: The straightforward pharmacology and fluorescent binding in the α 1B/D-KO was used to interpret the properties of the α 1A-adrenoceptor in the WT. Reduced total fluorescence in α 1B/D-KO arteries, despite a clear difference in the functionally dominant subtype, indicates that measurement of receptor protein is unlikely to correlate with function. Fewer cells bound QAPB in the α 1B/D-KO suggesting different cellular phenotypes of α 1A-adrenoceptor exist. The α 1B/D-KO provides robust assays for the α 1A-adrenoceptor and takes us closer to understanding multi-receptor subtype interactions. © 2009 The British Pharmacological Society.