Non-adrenergic Vasoconstriction of Guinea-pig Aorta by β-Phenylethylamine and Amphetamine

β-Phenylethylamine (PEA) and amphetamine increase blood pressure by an effect conventionally thought to be indirectly acting sympathomimetic through release of noradrenaline onto vascular α-adrenoceptors (1). However, they can also interact with trace amine-associated receptors (TAARs) and we have suggested that vasoconstrictor responses to PEA in rat and pig isolated blood vessels may be due to stimulation of TAARs (2,3). In this study, we examine vasoconstriction of guinea-pig aorta by PEA and its close analogue, amphetamine.

Aortic rings (0.5 cm) from male Dunkin-Hartley guinea-pigs were mounted onto fixed and mobile hangers and isometric contraction measured using a computerized Power Lab, Chart 5 data acquisition system (ADInstruments). The rings were immersed in Kreb’s solution gassed with 5% CO2 in O2 and maintained at 37±0.5oC, and 1.5 g resting tension applied. Cumulative concentration-response curves (CRC) for β-phenylethylamine hydrochloride (PEA), d-amphetamine sulphate or (-)-phenylephrine HCl were obtained and after washout (x2) to return to baseline tension a second curve was obtained in the presence of the α1-adrenoceptor antagonist prazosin (1µM) or the nitric oxide synthase inhibitor, L-NAME (100µM), or their vehicles. After the second CRC, KCl (60mM) was added to obtain the maximum contraction of the tissue. Contractions were measured by subtracting baseline tension preceding each CRC from plateau responses. Phenylephrine responses were expressed as a percentage of the first curve maximum. PEA and amphetamine responses were expressed as a percentage of the contraction to KCl. Mean (n≥4) responses were compared by Student’s paired t-test, P ≤ 0.05 indicating significant differences.

Phenylephrine caused concentration-related constriction of the aorta. The CRC was shifted to the right in the presence of prazosin, yielding a –log KD value of 7.47±0.09 calculated from the dose-ratio (DR) using -log KD=log[A]-log(DR-1). PEA also constricted the aorta, but prazosin had no significant effect on the CRC. The vehicle for prazosin, DMSO, significantly shifted the CRC to PEA upwards. Amphetamine also caused concentration-related constrictions of the aorta and these were enhanced by prazosin, the response to the maximum dose significantly increasing from 32±9.3 to 54.0±7.6%KCl. Thus, neither PEA nor amphetamine constrict guinea-pig aorta via α1-adrenoceptors. Potentiation of contractions was unlikely to be due the α1-adrenoceptor blocking action of prazosin, because α1-adrenoceptor stimulation would have to exert vasorelaxation. It is more likely that prazosin is inhibiting an opposing vasorelaxation by amphetamine leading to potentiated vasoconstriction. One possibility is that amphetamine releases nitric oxide (NO) and that prazosin inhibits nitric oxide synthase (NOS). The NOS inhibitor, L-NAME (100µM) significantly potentiated the contractions to PEA, from 51.1.3±2.8 to 103.1±8.0%KCl at the maximum dose and amphetamine from 42.0±1.0 to 69.0±2.5%KCl. The controls for PEA (65.7±6.0 and 74.8±1.4%KCl) and amphetamine (36.0±3.3 and 44.7±1.1%KCl) showed no significant differences. Endothelium-denuded aortae also showed significant potentiation of PEA by L-NAME from 60.8±6.9 to 86.9±2.3%KCl, indicating that the NO stimulation was independent of the endothelium.

PEA and amphetamine do not constrict guinea-pig aorta via α1-adrenoceptors but probably via TAARs. There is an underlying opposing vasodilator action through release of non-endothelium-derived NO.

1.Broadley KJ (2010) Pharmacol Ther 125;363-375.

2.Fehler M, Broadley KJ, Ford WR, Kidd EJ (2010) Naunyn-Schmiedeberg’s Arch Pharmacol 382,385-398.

3.Herbert, A.A., Kidd, E.J., Broadley, K.J. (2008) Br J Pharmacol 155;525-534.