Modulation of vascular responses of guinea-pig aorta by non-endothelial nitric oxide: A minor role for the endothelium

Trace amine-induced vasoconstriction of human mammary artery and saphenous vein

Sympathomimetic amines, including β-phenylethylamine (PEA), constrict animal blood vessels but their mechanism of action is not now thought to be through α-adrenoceptors and release of noradrenaline but via trace amine-associated receptors (TAARs). This information is not available for human blood vessels. Functional studies were therefore performed on human arteries and veins to establish whether they constrict to PEA and whether any constrictions are adrenoceptor-mediated. Isolated internal mammary artery or saphenous vein rings were set up in Kreb's-bicarbonate solution at 37 ± 0.5 °C gassed with O2:CO2 (95:5) under class 2 containment. Isometric contractions were measured and cumulative concentration-response curves for PEA or the α-adrenoceptor agonist, phenylephrine were established. PEA showed concentration-related contractions. The maximum was significantly greater in arteries (1.53 ± 0.31 g, n = 9) than veins (0.55 ± 0.18 g, n = 10), but not when plotted as % of KCl contractions. PEA showed slowly developing contractions plateauing at 17,3 ± 3.7 min in mammary artery. The reference α-adrenoceptor agonist, phenylephrine, exhibited more rapid onset (peak 5.0 ± 1.2 min) but non-sustained contractions. In saphenous veins, PEA (62.8 ± 10.7%) and phenylephrine (61.4 ± 9.7%, n = 4) displayed the same maximum, but phenylephrine was more potent. The α1-adrenoceptor antagonist, prazosin (1 μM), blocked phenylephrine contractions of mammary arteries but not PEA contractions in either vessel. PEA causes substantial vasoconstriction of human saphenous vein and mammary artery, which explains its vasopressor actions. This response, however, was not mediated via α1-adrenoceptors, but likely due to TAARs. The classification of PEA as a sympathomimetic amine on human blood vessels is therefore no longer valid and requires revision.

Renal artery responses to trace amines: Multiple and differential mechanisms of action

PURPOSE

The rise in consumption of dietary supplements containing the trace amines p-tyramine, p-synephrine and p-octopamine has been associated with cardiovascular side effects. Since renal blood flow plays an important role in blood pressure regulation, this study investigated the mechanisms of action of these trace amines on isolated porcine renal arteries.

MAIN METHODS

Contractile responses to amines were investigated in noradrenaline-depleted rings of porcine main renal arteries in the absence and presence of the α₁-adrenoceptor antagonist, prazosin (1 μM), β-adrenoceptor antagonist, propranolol (1 μM), or the trace amine-associated receptor (TAAR-1) antagonist, EPPTB (RO-5212773; 100 nM or 100 μM).

KEY FINDINGS

All three amines induced constrictor responses of similar magnitude and potency. However, their mechanisms of action on the renal artery appeared to differ. Depleting endogenous noradrenaline stores significantly reduced maximum responses to tyramine and synephrine, but less for octopamine. When direct responses were examined after depleting tissues of noradrenaline, responses to synephrine and octopamine, but not tyramine, were reduced in the presence of prazosin(1 μM) and potentiated in the presence of propranolol (1 μM) or L-NNA (100 μM). Generally, vasoconstrictor responses remaining after noradrenaline-depletion and α-adrenoceptor blockade were not affected by the TAAR-1 antagonist EPPTB (0.1-100 μM), although responses to low concentration of synephrine and octopamine were enhanced by this antagonist.

SIGNIFICANCE

Tyramine appears to mediate constriction of the renal artery mainly via an indirect sympathomimetic mechanism, whereas synephrine and octopamine exert additional direct effects on α₁-adrenoceptors and possibly contractile TAAR (not TAAR-1). The two amines also activate simultaneous inhibitory responses via β-adrenoceptors, TAAR-1 and nitric oxide release.