Summary Cumulative concentration response curves to 5‐hydroxytryptamine (5‐HT; 10‐10‐10‐4 mol/l) were constructed using isolated rings of equine digital, facial, tail and coronary arteries (endothelium intact). 5‐HT was 17.7 and 41 times more potent as a vasoconstrictor of digital arteries than facial and tail arteries respectively. Removal of the endothelium increased the vasoconstrictor potency of 5‐HT in the facial artery by 3.7‐fold (P<0.05) but did not alter the sensitivity of digital arteries to 5‐HT. Coronary arteries failed to contract to 5‐HT. Coronary arteries pre‐contracted with U44069 showed concentration dependent relaxation to 5‐HT, a response which was partially dependent on the presence of the endothelium. No vasorelaxant effects were found in the digital or facial arteries. The concentration of 5‐HT in platelet poor and platelet rich equine plasma was found to be 6.70 ± 1.1 times 10‐8 mol/l and 1.77 ± 0.36 times 10‐6 mol/l (mean ± s.e.) respectively by high performance liquid chromatography (HPLC). Plasma which contained no detectable platelets had a 5‐HT concentration of 1.12 ± 0.48 times 10‐8 mol/l. Isolated digital arteries constricted when exposed to dilutions of platelet poor and platelet depleted equine plasma. These plasma induced contractions were almost completely inhibited by 5‐HT receptor antagonists, ketanserin and methiothepin. The change in isometric tension in rings of equine digital artery in vitro was therefore used as a bioassay for plasma 5‐HT and the results obtained by this method showed an excellent correlation (r2= 97.2%, P<0.001) with the concentration estimated by HPLC. Circulating free concentrations of 5‐HT in normal horses may besufficient to constrict digital blood vessels partially in vivo but are well below the threshold for contraction of other peripheral blood vessels examined.
Equine Veterinary Journal – Wiley
Published: Mar 1, 1998
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