Contraction of rat isolated aorta to β -phenylethylamine not mediated by adrenoceptors

M. Fehler, E.J. Kidd and K.J. Broadley, Welsh School of Pharmacy, Cardiff University, King Edward VII Avenue, Cardiff, CF10 3XF.

Trace amines (TAs) including β-phenylethylamine (β-PEA), p-tyramine, octopamine and tryptamine (Borowsky et al., 2001; Bunzow et al., 2001) are usually regarded as indirectly acting sympathomimetic amines (ISAs) exerting vasoconstriction via α1-adrenoceptors ( α1 -AR). Additionally they stimulate trace amines receptors (TARs), now called trace amine-associated receptors (TAARs). Human and rat TAAR1 and human TAAR4 are mainly activated by β -PEA and tyramine and rat TAAR4 by β -PEA and tryptamine (Lindemann et al., 2005; Lewin, 2006). This study investigated the function of trace amine receptors in the rat aorta.

Thoracic aorta from Sprague-Dawley rats (male, 250-300g) were cut into rings, hung on two hooks and immersed in Krebs-bicarbonate buffer (37°C) gassed with CO2/O2 (5%/95%). In some experiments endothelium was removed by rolling on a cocktail stick; successful removal was shown by the absence of relaxation to acetylcholine in U44169-precontracted rings. The upper hook was connected to an isometric transducer (Dynamometer UF1, 57g sensitivity) and 1.5g resting tension applied. Tension was recorded by computer (Power Lab, Chart 5, AD Instruments). Cumulative or non-cumulative concentration-response curves (CRCs) were obtained and contractions expressed as % of contraction to KCl (60mM). Mean responses ( +SEM) or geometric mean EC50(KCl) were compared by ANOVA followed by Student’s unpaired t-test.

β-PEA caused concentration-related contractions independent of the endothelium. Lower and upper parts of the aorta showed no significant difference to β-PEA contraction. Non-cumulative CRCs showed greater maximum contractions (110% ± 15% at 100µM) than cumulative CRCs (58.7% ± 13.1% at 300µM). Prazosin (1μM, α1-AR antagonist) in the presence of cocaine (10μM, uptake inhibitor) and propranolol (1μM, β-AR antagonist) non-significantly decreased the cumulative CRC for β-PEA to 25.3% ± 10.5% compared to 33.0% ± 6.7% with cocaine (10μM) and propranolol (1μM) at 300µM. The response to a single β-PEA dose (30μM) was significantly reduced by prazosin (1μM) in presence of cocaine and propranolol (13.7% ± 25.2%) compared to cocaine and propranolol only (33.5% ± 19.4%). Cocaine did not significantly affect the maximum contraction to a single dose of β-PEA but shifted the CRC to the right, suggesting small ISA and α1 -AR effects by β-PEA. Noradrenaline was blocked by prazosin and amplified by cocaine. In the presence of ICI 118,551 (1µM, β2 -AR antagonist) and prazosin, β -PEA showed stronger contractions (54.2% ± 11%) than with prazosin and propranolol (34.7% ± 7.2% at 100µM). Further addition of a MAO A and B inhibitor, pargyline (PG, 10µM), further enhanced the maximum β-PEA response to 125.8% ± 23.9%, indicating metabolism of β-PEA via MAO. β-PEA contractions in the presence of prazosin, cocaine, propranolol or ICI 118,551 and PG, suggest mechanisms other than ISA and α-AR stimulation, possibly via TAARs.

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