Attenuated Aortic Vasodilation and Sympathetic Prejunctional Facilitation in Epinephrine-Deficient Mice: Selective Impairment of b2-Adrenoceptor Responses

It has been suggested that there is a link between epinephrine synthesis and the development of b2-adrenoceptor–mediated effects, but it remains to be determined whether this development is triggered by epinephrine. The aim of this study was to characterize b-adrenoceptor–mediated relaxation and facilitation of norepinephrine release in the aorta of phenylethanolamine-Nmethyltransferase–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 b1or b2-adrenoceptor agonists in the absence or presence of selective b1or b2-adrenoceptor antagonists. Aortic rings were also preincubated with [H]norepinephrine to measure tritium overflow elicited by electrical stimulation in the presence of increasing concentrations of nonselectivebor selectiveb2-adrenoceptor agonists. b2-Adrenoceptor protein density was evaluated by Western blotting and b2-adrenoceptor localization by immunohistochemistry. Epinephrine is absent in Pnmt-KOmice. The potency and themaximal effect of the b2-adrenoceptor agonist terbutaline were lower in Pnmt-KO than in wild-type (WT) mice. The selective b2-adrenoceptor antagonist ICI 118,551 [(6)-erythro-(S*,S*)-1-[2,3-(dihydro-7methyl-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 WTmice only. b2-Adrenoceptor protein density was decreased in membrane aorta homogenates of Pnmt-KOmice, and this finding was supported by immunofluorescence confocal microscopy. In conclusion, epinephrine is crucial for b2-adrenoceptor–mediated vasodilation and facilitation of norepinephrine release. In the absence of epinephrine, b2-adrenoceptor protein density was decreased in aorta cell membranes, thus potentially hindering its functional activity.