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 β 2-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 β 1- or β 2-adrenoceptor agonists in the absence or presence of selective β 1- or β 2-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 β 2-adrenoceptor agonists. β 2-Adrenoceptor protein density was evaluated by Western blotting and β 2-adrenoceptor localization by immunohistochemistry. Epinephrine is absent in Pnmt-KO mice. The potency and the maximal effect of the β 2-adrenoceptor agonist terbutaline were lower in Pnmt-KO than in wild-type (WT) mice. The selective β 2-adrenoceptor antagonist ICI 118,551 [(±)- erythro -( S *, S *)-1-[2,3-(dihydro-7-methyl-1 H -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. β 2-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 β 2-adrenoceptor–mediated vasodilation and facilitation of norepinephrine release. In the absence of epinephrine, β 2-adrenoceptor protein density was decreased in aorta cell membranes, thus potentially hindering its functional activity.