CONTRACTION OF RAT ISOLATED AORTA TO TRACE AMINES INDEPENDENT OF ADRENOCEPTOR STIMULATION

Martina Fehler, Emma Kidd, Kenneth Broadley
Welsh School of Pharmacy, Cardiff University, Cardiff, United Kingdom

Trace amines (TAs) including β-phenylethylamine (β-PEA), p-tyramine and octopamine (Borowsky et al., 2001; Bunzow et al., 2001) are regarded as indirect sympathomimetic amines (ISAs) exerting vasoconstriction via α1-adrenoceptors (α1-AR). However, they can additionally stimulate trace amine receptors (TARs), now called trace amine-associated receptors (TAARs) (Lindemann et al., 2005; Lewin, 2006). This study compared a range of trace amines in the rat aorta.

Sprague-Dawley rats (male, 250-300g) were killed by cervical dislocation and exsanguination. Rings of thoracic and abdominal aortae were removed and hung on hooks in Krebs-bicarbonate buffer (37°C) gassed with CO2/O2 (5%/95%). Endothelium was removed, confirmed by no relaxation to acetylcholine in U44169-precontracted rings. The upper hook was connected to an isometric transducer (Dynamometer UF1, 558.6mN sensitivity) and 14.7mN resting tension applied. Tension was recorded by computer (Power Lab, Chart 5, AD Instruments). Non-cumulative concentration-response curves (CRCs) or single dose responses were obtained, contractions expressed as % of KCl contraction (isotonic, 60mM) or as % of own maximum. Mean responses (±SEM) or geometric mean EC50 (KCl) were compared by paired or unpaired Student’s t-test or Analysis of Variance (ANOVA) followed by post hoc Bonferroni and Dunnett tests.

β-PEA, octopamine, tyramine and D-amphetamine caused concentration-related contractions independent of the endothelium. Prazosin (1μM, α1-AR antagonist), cocaine (10μM, uptake inhibitor), ICI-118,551 (1μM, β2-AR antagonist) and pargyline (10μM, MAO A and B inhibitor) (“inhibitors”) did not affect CRCs for β-PEA. The responses to 300μM in absence and presence of “inhibitors” were 138.2±23.1% (n=6) and 99.2±25.2% (n=4), respectively. The CRC for octopamine (-logEC50 absence, 6.01±0.99, n=4) was displaced to the right by “inhibitors” (-logEC50 presence, 4.5±0.39, n=4). However, D-amphetamine was not antagonised, the maximum responses at 100μM being 70.7±24.0 (n=5) and 29.2±38.7% (n=4) in the presence and absence of “inhibitors”, respectively. Contractions by p-tyramine were small both in the absence (18.0±17.9% at 300μM, n=3) and presence (4.8±4.8%, n=3) of “inhibitors”. Because of this weak activity of p-tyramine, it was examined as an antagonist of the other trace amines. In the presence of p-tyramine (1mM) and the “inhibitors”, the contractile responses to single doses (100μM) of β-PEA (n=4), octopamine (n=5) and D-amphetamine (n=4) were significantly inhibited (P<0.05) to 41.5±15.5, 15.1±5.1 and 4.4±13.4%, respectively of the pre-tyramine values. Persistent contractions in the rat aorta to β-PEA and D-amphetamine in the presence of “inhibitors” suggest mechanisms other than ISA and α-AR stimulation, possibly via TAARs. The weak contractile activity of p-tyramine and antagonism of other trace amines suggests that it is a weak partial agonist for other TAARs.

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