Attenuated aortic vasodilation and sympathetic prejunctional facilitation in epinephrine-deficient mice: Selective impairment of β₂-adrenoceptor responses

It has been suggested that there is a link between epinephrine synthesis and the development of β₂-adrenoceptor-mediated effects, but it remains to be determined whether this development is triggered by epinephrine. The aim of this study was to characterize β-adrenoceptor-mediated relaxation and facilitation of norepinephrine release in the aorta of phenylethanolamine-N-methyltransferase-knockout (Pnmt-KO) mice. Catecholamines were quantified by reverse-phase high-performance liquid chromatography-electrochemical detection. Aortic rings were mounted in a myograph to determine concentration-response curves to selective β₁- or β₂-adrenoceptor agonists in the absence or presence of selective β₁- or β₂-adrenoceptor antagonists. Aortic rings were also preincubated with [³H]norepinephrine to measure tritium overflow elicited by electrical stimulation in the presence of increasing concentrations of nonselective β- or selective β₂-adrenoceptor agonists. β₂-Adrenoceptor protein density was evaluated by Western blotting and β₂-adrenoceptor localization by immunohistochemistry. Epinephrine is absent in Pnmt-KO mice. The potency and the maximal effect of the β₂-adrenoceptor agonist terbutaline were lower in Pnmt-KO than in wild-type (WT) mice. The selective β₂-adrenoceptor antagonist ICI 118,551 [(±)-erythro-(S∗,S∗)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol hydrochloride] antagonized the relaxation caused by terbutaline in WT but not in Pnmt-KO mice. Isoproterenol and terbutaline induced concentration-dependent increases in tritium overflow in WT mice only. β₂-Adrenoceptor protein density was decreased in membrane aorta homogenates of Pnmt-KO mice, and this finding was supported by immunofluorescence confocal microscopy. In conclusion, epinephrine is crucial for β₂-adrenoceptor-mediated vasodilation and facilitation of norepinephrine release. In the absence of epinephrine, β₂-adrenoceptor protein density was decreased in aorta cell membranes, thus potentially hindering its functional activity.