α_1A/B‐Knockout mice explain the native α_1D‐adrenoceptor's role in vasoconstriction and show that its location is independent of the other α₁‐subtypes

Background and purpose:  Theoretically, three α₁‐adrenoceptor subtypes can interact at the signalling level to alter vascular contraction or at the molecular level to alter each other's cellular location. The α_1A/B‐adrenoceptor knockout mouse (α_1A/B‐KO) was used to study the isolated α_1D‐adrenoceptor to consider these potential interactions in native tissue.

Experimental approach:  Pharmacological analysis of carotid and mesenteric arteries employed wire myography and fluorescent ligand binding (α₁‐adrenoceptor ligand BODIPY FL‐prazosin, QAPB).

Key results:  α_1A/B‐KO carotid had clear α_1D‐adrenoceptor‐induced contractions. In WT carotid α_1D‐adrenoceptor dominated but all three α₁‐subtypes participated. α_1A/B‐KO mesenteric had α_1D‐adrenoceptor responses with high sensitivity and small maximum, explaining how α_1D‐adrenoceptor could determine agonist sensitivity in WT. In both arteries α_1A/B‐KO fluorescence levels were reduced but pharmacologically more consistent with ‘pure’α_1D‐adrenoceptors. α_1D‐Adrenoceptor binding in α_1A/B‐KO was observed on the cell surface and intracellularly and was present in a high proportion of smooth‐muscle cells in both strains, regardless of artery type.

Conclusions and implications:  ‘Pure’α_1D‐adrenoceptor pharmacology in α_1A/B‐KO provides a quantitative standard. Functionally, the α_1D‐ and α_1A‐adrenoceptors produce additive responses and do not significantly compensate for each other. α_1D‐Adrenoceptor contributes to sensitivity even in resistance arteries. In α_1A/B‐KO, the loss of α_1A‐ and α_1B‐adrenoceptors is reflected by a general decrease in fluorescence, but similar binding distribution to WT indicates that the α_1D‐adrenoceptor location in native smooth‐muscle cells is not influenced by other α₁‐adrenoceptors. Equivalent levels of receptors did not correspond to equivalent responses. In conclusion, α₁‐subtypes do not interact but provide independent alternative signals for vascular regulation.

DOI: 10.1111/j.1476-5381.2009.00462.x

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