Modulation of resistance artery tone by the trace amine β-phenylethylamine: dual indirect sympathomimetic and α1-adrenoceptor blocking actions

The trace amine β-phenylethylamine (PEA) is normally present in the body at low nanomolar concentrations but can reach micromolar levels following ingestion of drugs that inhibit monoamine oxidase and primary amine oxidase. In vivo, PEA elicits a robust pressor response but there is no consensus regarding the underlying mechanism, with both vasodilation and constriction reported in isolated blood vessels. Using functional and biochemical approaches we found that at low micromolar concentrations PEA (1-30 µM) enhanced nerve-evoked vasoconstriction in the perfused rat mesenteric bed but at a higher concentration (100 µM) significantly inhibited these responses. The α2-adrenoceptor antagonist rauwolscine (1 µM) also enhanced nerve-mediated vasoconstriction but in the presence of both rauwolscine (1 µM) and PEA (30 µM) together, nerve-evoked responses were initially potentiated and then showed time-dependent run-down. PEA (10 and 100 µM) significantly increased noradrenaline outflow from the mesenteric bed as determined by HPLC/ECD. In isolated endothelium-denuded arterial segments, PEA (1 µM-1 mM) caused concentration-dependent reversal of tone elicited by the α1-adrenoceptor agonists noradrenaline (EC50 51.69 ± 10.8 µM; n=5), methoxamine (EC50 68.21 ± 1.70 µM; n=5) and phenylephrine (EC50 67.74 ± 16.72 µM; n=5) but was ineffective against prostaglandin F2α- or U46619-induced tone. In rat brain homogenates, PEA displaced binding of both [3H]-prazosin (Ki ≈ 25 µM) and [3H]-rauwolscine (Ki ≈ 1.2 µM), ligands for α1- and α2-adrenoceptors, respectively. These data provide the first demonstration that dual indirect sympathomimetic and α1-adrenoceptor blocking actions underlie the vascular effects of PEA in resistance arteries.