Search

Trace amines including tyramine and β-phenylethylamine (β-PEA) increase blood pressure and cause vasoconstriction which is attributed to indirect sympathomimetic actions. However, there is evidence that they may also have non-sympathomimetic mechanisms. This study examined whether β-PEA causes vasoconstriction of rat aorta by a sympathomimetic action or through the recently described trace-amine-associated receptors (TAAR). Concentration-response curves (CRCs) for β-PEA were constructed either cumulatively or non-cumulatively in rat isolated aortic rings. TAAR-1 and TAAR-4 protein expression was determined in rat aorta by Western blotting and TAAR-1 mRNA by reverse transcriptase polymerase chain reaction (RT-PCR). β-PEA caused concentration-related constriction of rat aorta. The contractions were unaffected by endothelium removal or the nitric oxide synthase inhibitor, N(ω)-nitro-L-arginine methyl ester (L-NAME, 100 μM) or the cyclooxygenase inhibitor, indomethacin (10 μM). Non-cumulative CRCs showed greater contractions and sensitivity to β-PEA than cumulative. The α(1)-adrenoceptor antagonist, prazosin, failed to inhibit either curve. The β-adrenoceptor antagonist, propranolol, the adrenergic neuronal transport inhibitor, cocaine, and the monoamine oxidase inhibitor, pargyline, also failed to alter the CRC. In the combined presence of prazosin, cocaine, pargyline, and the selective β(2)-adrenoceptor antagonist, ICI-118,551, the trace amine contractile potency order was tryptamine > β-PEA > octopamine > D: -amphetamine > tyramine. Western blotting and RT-PCR revealed the presence of TAAR-1 in rat aorta, but TAAR-4 was poorly expressed. Vasoconstriction of rat aorta by β-PEA appears not to be an indirect sympathomimetic action. The presence of TAAR-1 suggests that vasoconstriction may be via these receptors; however, the potency order differed from that reported for transfected cells expressing rat TAAR-1.

The effects of ischaemia and reoxygenation on cardiac contractile function can be abrogated by ischaemic preconditioning (IPC). We tested whether beta-adrenoceptor agonists could mimic IPC and whether IPC was dependent on beta-adrenoceptor activation in rat-isolated cardiac tissues. Paced left atria and right ventricular strips were set-up in Krebs solution and isometric developed tension recorded. Ischaemia was simulated by replacing with hypoxic glucose-free Krebs solution for 30 min. IPC and isoprenaline (10(-7) M) preconditioning for 10 min were examined. Developed tension post-reoxygenation was expressed as a percentage of the pre-ischaemic baseline. Recovery at 15 min was significantly increased by IPC in atria (47 +/- 4.0% vs. 29.3 +/- 1.7%, p < 0.05) and ventricles (39.0 +/- 5.2% vs. 22.4 +/- 2.8%, p < 0.05). At 60 min, isoprenaline-treated atria recovery (75.8 +/- 16.6%) was significantly (p < 0.05) greater than controls (47.9 +/- 2.3%). Propranolol (10(-6) M) abolished both effects. Therefore, both IPC and beta-adrenoceptor agonist-induced improvement of contractile recovery was propranolol-sensitive and beta-adrenoceptor-mediated.

The P1 purinoceptor subtype mediating the negative inotropic responses of guinea-pig left atria and the negative chronotropic responses of beating right atria were characterized. Guinea-pig isolated paced left atria (2Hz, 5ms, threshold voltage+50%) and spontaneously beating right atria were set up in Krebs-bicarbonate solution and isometric tension and rate of contraction, respectively, were recorded. Concentration-response curves for the reduction of tension and rate, respectively, by adenosine receptor agonists, N6-cyclopentyladenosine (CPA), the R- and S- stereoisomers of N6-(2-phenylisopropyl) adenosine (R-PIA and S-PIA), 5'-(N-carboxamido) adenosine (NECA) and 2-p-((carboxyethyl)-phenethylamino)-5'-(N-carboxamido) adenosine (CGS21680) were obtained. The orders of potency on the left atria (CPA = NECA > R-PIA > S-PIA > CGS21680) and right atria (CPA = R-PIA > S-PIA > CGS21680) were consistent with the responses being mediated via A1 receptors. Antagonism of the responses to CPA or R-PIA by 8-cyclo-1,3-dimethylxanthine (CPT) was examined by a full Schild analysis. Concentration-response curves for CPA or R-PIA were obtained in the absence or presence of five or six concentrations (10(-7)-10(-5) or 3 x 10(-5)M) of CPT. The shift in the concentration-response by CPT was expressed as the concentration-ratio (CR) and plotted as -log(CR-1) against log molar concentration of CPT (Schild plot). pA2 values were calculated from the intercept on the concentration axis and by application of the equation; pA2 = log(antagonist concentration) -log (CR-1). The Schild plots had unity slopes indicating competitive antagonism and the pA2 values derived therefrom indicated that the responses were mediated via A1-receptor. Closer inspection of the Schild plots, however, showed that at the higher concentrations of CPT there was a limit to the displacement of the concentration-response curves of the left and right atria to CPA and of the left atria to R-PIA. There were also significant differences in the apparent pA2 values calculated from the equation, when different concentrations of antagonist were examined. These results indicated that at higher concentrations of agonist there may be a component of the response that is resistant to antagonism by CPT. Whether this is related to the proposal that cardiac responses are mediated via A3 receptors is discussed.