Interaction of ergovaline with serotonin receptor 5-HT2A in bovine ruminal and mesenteric vasculature

Authors: TROTTA, RONALD - University Of Kentucky; HARMON, DAVID - University Of Kentucky; Klotz, James

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/18/2018
Publication Date: 8/24/2018
Citation: Trotta, R.J., Harmon, D.L., Klotz, J.L. 2018. Interaction of ergovaline with serotonin receptor 5-HT2A in bovine ruminal and mesenteric vasculature. Journal of Animal Science. 96:4912-4922. https://doi.org/10.1093/jas/sky346.
DOI: https://doi.org/10.1093/jas/sky346

Interpretive Summary: A fungal endophyte that exists within tall fescue grass. While the endophyte is beneficial to the plant by imparting tolerance to stressors, it also produces toxic ergot alkaloids that lead to fescue toxicosis in grazing livestock Ergot alkaloids have been shown to induce vasoconstriction in core and peripheral blood vessels. Structurally, ergot alkaloids are similar to neurotransmitters like serotonin and it has been found that the vasoconstrictive aspects of fescue toxicosis are mediated through the 5HT2A serotonin receptor. Using a compound selective to this receptor subtype, the objective of the study was characterize vasoactivity of blood vessels that were or were not exposed to ergot alkaloids previously. Findings demonstrated that previous exposure to ergovaline significantly diminished the contractile response to TCB-2 in all blood vessels and simultaneous exposure to ergovaline increased the contractile response at concentrations below the low concentration threshold of TCB-2 in all blood vessels. This work demonstrated that although initial exposure of a blood vessel to an ergot alkaloid like ergovaline stimulates the receptor, the toxin prevents any subsequent activity at that receptor essentially disrupting its normal biological function. This work will benefit future research and scientists seeking to understand how ergot alkaloids cause their effects in livestock and to eventually develop a treatment.

Technical Abstract: Ergot alkaloids from endophyte-infected (Epichloë coenophiala) tall fescue (Lolium arundinaceum) induce vasoconstriction. Previous work has shown that serotonin receptor subtype, 5HT2A, is present in bovine ruminal (R) and mesenteric (M) vasculature, plays a role in vasoconstriction, and could be influenced by ergot alkaloids. To determine the influence of ergot alkaloids on 5HT2A, the vasoactivity of an agonist selective for 5HT2A (TCB-2) was evaluated using bovine ruminal and mesenteric arteries and veins (RA, RV, MA, MV) that were exposed to ergovaline (ERV) prior to or during the TCB-2 additions. Ruminal and mesenteric blood vessel segments were collected, cleaned, and cut into 2- to 3-mm cross sections. Vessel segments were incubated in Krebs-Henseleit buffer containing 0, 0.01 or 1 uM ERV for 2 h prior to TCB-2 dose response or exposed to ERV concentrations simultaneously during TCB-2 dose response. For the dose response portion of the study, vessels were suspended in a multi-myograph containing 5 mL of continuously oxygenated Krebs-Henseleit buffer and equilibrated to 1 g tension for 90 min. Vessels were exposed to increasing concentrations of TCB-2 every 15 min and contractile response data were normalized as a percentage of the maximum contractile response induced by 120 mM KCl reference. Analysis of variance was evaluated separately for each vessel and each ERV exposure experiment using mixed models procedure of SAS for effects of TCB-2 and ERV concentrations. All blood vessels with previous ERV exposure had significantly lower contractile responses to TCB-2 (P < 0.01). All blood vessels with simultaneous exposure to 1 uM ERV had higher (P < 0.01) contractile responses at lower concentrations of TCB-2. Simultaneous ERV addition at 1x10-4 M TCB-2 did not affect contractility of RV, MA, MV (P > 0.05), but decreased contractility of RA (P < 0.01). These results indicate that ergopeptine alkaloid exposure influences contractility of bovine ruminal and mesenteric blood vessels through serotonin receptor subtype 5HT2A by acting as both an agonist and antagonist. Additional work is needed to determine if ergot alkaloids like ergovaline simply occupy receptor binding sites competitively, or influence receptor internalization to cause the observed divergent responses.