Assessment of Bovine Ruminal and Mesenteric Vascular Serotonin Receptor Populations

A previous study using a contractility bioassay determined that 5HT_2A receptors in bovine lateral saphenous veins play a role in ergot alkaloid-induced vascular contraction in steers grazing endophyte-infected (Epichloë coenophiala) tall fescue (Lolium arundinaceum). Ergot alkaloids have also been shown to be vasoactive in gut vasculature. To determine what 5HT receptors are involved in vasoconstriction of gut vasculature, contractility of ruminal and mesenteric arteries and veins collected from cattle were evaluated in the presence of agonists selective for 5HT_1D (L694-247), 5HT_2A (TCB-2), 5HT_2B (BW723C86) and 5HT₇ (LP44) receptors. Segments of vein and artery were collected from 18 mixed-breed and gender animals between 10 and 30 months of age (531 ± 29 kg). Vessels were sliced into 2- to 3-mm cross-sections, and suspended in a multi-myograph containing 5 mL of continuously oxygenated Krebs-Henseleit buffer (95% O₂/5% CO₂; pH = 7.4; 37°C). Vessels were equilibrated to 1.0 g for 90 min followed by the addition of 120 mM KCl. Tissues were exposed to increasing concentrations of 5HT agonists every 15 min. Contractile response data were normalized as a percentage of the maximum contractile response induced by KCl. Analysis of variance was evaluated using mixed models procedure of SAS for effects of agonist concentration for each vessel type. BW723C86, L694-247, and LP44 did not induce a contractile response for ruminal or mesenteric vasculature. However, when exposed to BW723C86 or L694-247, mesenteric veins relaxed (P<0.05). There was a trend for BW723C86 (P<0.05), L694-257 (P=0.06), and LP44 (P<0.05) to induce a contractile response for ruminal arteries, but at the lowest agonist concentration. This is an indication that the ruminal artery is still relaxing from exposure to KCl. Exposure of gut vasculature to TCB-2 induced vasoactivity. Onset was observed at addition of 5x10^-5 M TCB-2 for the mesenteric artery and vein and the ruminal vein (P<0.05). The mesenteric artery reached a maximal response when exposed to 5x10^-5 M TCB-2 and did not differ from the highest (1x10^-4 M) TCB-2 concentration addition (P>0.05). For the ruminal artery, a contractile response was observed at 5 x 10^-6 M TCB-2 (P<0.05). The findings of this study indicate that 5HT_1D and 5HT_2B are present in mesenteric veins and may play a role in vasorelaxation. Further, 5HT_2A is present in ruminal and mesenteric vasculature, plays a role in vasoconstriction of blood flow, and could be influenced by ergot alkaloid exposure as has been demonstrated in peripheral blood vessels.