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 a1B/D-adrenoceptor knockout (a1B/D-KO) were employed to isolate a1A-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 a1-adrenoceptor ligand BODIPY FL-Prazosin (QAPB), on mesenteric (an 'a1A-adrenoceptor' tissue) and carotid (an 'a1D-adrenoceptor' tissue) arteries.Key results: a1B/D-KO mesenteric arteries showed straightforward a1A-adrenoceptor agonist/antagonist pharmacology. WT had complex pharmacology with a1A- and a1D-adrenoceptor components. a1B/D-KO had a larger a1A-adrenoceptor response suggesting compensatory up-regulation: no increase in fluorescent ligand binding suggests up-regulation of signalling. a1B/D-KO carotid arteries had low efficacy a1A-adrenoceptor responses. WT had complex pharmacology consistent with co-activation of all three subtypes. Fluorescent binding had straightforward a1A-adrenoceptor characteristics in both arteries of a1B/D-KO. Fluorescent binding varied between cells in relative intracellular and surface distribution. Total fluorescence was reduced in the a1B/D-KO due to fewer smooth muscle cells showing fluorescent binding. WT binding was greater and sensitive to a1A- and a1D-adrenoceptor antagonists.Conclusions and implications: The straightforward pharmacology and fluorescent binding in the a1B/D-KO was used to interpret the properties of the a1A-adrenoceptor in the WT. Reduced total fluorescence in a1B/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 a1B/D-KO suggesting different cellular phenotypes of a1A-adrenoceptor exist. The a1B/D-KO provides robust assays for the a1A-adrenoceptor and takes us closer to understanding multi-receptor subtype interactions