The α1B/D-adrenoceptor knockout mouse permits isolation of the vascular α1A-adrenoceptor and elucidates its relationship to the other subtypes

Background and purpose:  Mesenteric and carotid arteries from the α1B/D-adrenoceptor knockout (α1B/D-KO) were employed to isolate α1A-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 α1-adrenoceptor ligand BODIPY FL-Prazosin (QAPB), on mesenteric (an ‘α1A-adrenoceptor’ tissue) and carotid (an ‘α1D-adrenoceptor’ tissue) arteries. Key results:  α1B/D-KO mesenteric arteries showed straightforward α1A-adrenoceptor agonist/antagonist pharmacology. WT had complex pharmacology with α1A- and α1D-adrenoceptor components. α1B/D-KO had a larger α1A-adrenoceptor response suggesting compensatory up-regulation: no increase in fluorescent ligand binding suggests up-regulation of signalling. α1B/D-KO carotid arteries had low efficacy α1A-adrenoceptor responses. WT had complex pharmacology consistent with co-activation of all three subtypes. Fluorescent binding had straightforward α1A-adrenoceptor characteristics in both arteries of α1B/D-KO. Fluorescent binding varied between cells in relative intracellular and surface distribution. Total fluorescence was reduced in the α1B/D-KO due to fewer smooth muscle cells showing fluorescent binding. WT binding was greater and sensitive to α1A- and α1D-adrenoceptor antagonists. Conclusions and implications:  The straightforward pharmacology and fluorescent binding in the α1B/D-KO was used to interpret the properties of the α1A-adrenoceptor in the WT. Reduced total fluorescence in α1B/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 α1B/D-KO suggesting different cellular phenotypes of α1A-adrenoceptor exist. The α1B/D-KO provides robust assays for the α1A-adrenoceptor and takes us closer to understanding multi-receptor subtype interactions.