Mediators regulating tryptamine-induced tone of rat insolated mesenteric arterial bed

M.A. Anwar, W.R. Ford and K.J. Broadley, Division of Pharmacology, Welsh School Of Pharmacy, Cardiff University, Cardiff, CF10 3FX, U.K.

Background: Trace amines (TAs) and related molecules have been implicated in several pathological disorders, including migraine, hypertension and myocardial infarction (Premont et al, 2001; Al-Motarreb et al, 2005). Recent discovery of trace amine receptors (TAR) has seen a resurgence of interest in their physio-pharmacological properties ( Borowsky et al, 2001). Currently, little is known about how trace amines affect vascular tone. We are not aware of any studies where the effects of TAs on mesenteric vasoreactivity have been investigated. The aim of this study was to investigate responses to tryptamine (Trp), a trace amine, in rat isolated mesenteric arterial bed.

Methods: Male Sprague-Dawley rats (250 – 350g body weight) were killed by concussion and cervical dislocation. The superior mesenteric artery was cannulated and the mesenteric arterial bed excised and placed in a perfusion chamber. The bed was perfused at a constant flow rate (4 mL min-1) with Krebs’ bicarbonate solution (pH 7.4), warmed to 37ºC and gassed (95% O2, 5% CO2). Perfusion pressure was monitored by means of a pressure transducer (Elcomatic EM 750) connected to computer data acquisition system (AD Instruments Powerlab Chart 5). Dose-response curves (DRCs) for Trp by bolus injection (100µL volume) were constructed in the absence and presence of nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME, 100 µM), non-specific potassium channel blocker tetraethylammonium (TEA, 1 mM) or cyclooxygenase (COX) inhibitor indomethacin (Indo, 10 µM). Each inhibitor was infused from 20 min before the second DRC. pD2 and EMax values were computed and expressed as means ± SEM, n indicating numbers of animals.

Results: Basal perfusion pressure did not vary among the experimental groups (20.1 ± 0.6 mmHg, n=23). Trp caused dose-related contractions of the mesenteric vascular bed (pD2 = 105.5 ± 17.5 nmol / 100 µL, EMax: 40.3 ± 3.6 mmHg; n = 19). There were no significant differences between the initial DRCs to Trp and subsequent DRCs to Trp following the administration of vehicle. Inhibition by L-NAME significantly elevated the maximum perfusion pressure (EMax): 34.9 ± 4.7 vs 90.8 ± 12.5 mmHg (P < 0.02, n = 4). Indo significantly attenuated (P < 0.05, n = 3) the responses to Trp at concentrations greater than 10 nmol / 100 µL. Moreover, there was tendency for an increase in maximum perfusion pressure by TEA (EMax: 49.5 ± 5.5 vs 64.3 ± 7.2 mmHg, P = 0.068, n = 5). However, there were no significant changes in pD2 values between control and the corresponding inhibitor group.

conclusion: This study has identified Trp-induced vasoconstriction which is partially mediated via cyclooxygenase products, and modulated by NO release. We suggest that in cardiovascular abnormalities such as diabetes and hypertension, involving malfunction of the endothelium, enhanced TAs activity may play a prominent part in regulation of blood flow.

Al-Motarreb A., et al. (2005) Br J Clin Pharmacol 59: 574-581.
Borowsky B., et al. (2001) Proc Natl Acad Sci USA 98: 8966-8971.
Premont R.T., et al. (2001) Proc Natl Acad Sci USA 98: 9474-9475.