412 Awardee Talk: New Insights into Mechanisms of Serotonin Receptor-Mediated Vasoactivity and Complex Interactions with Ergot Alkaloids in Bovine Vasculature

Abstract

Abstract Ergovaline (ERV), produced in toxic endophyte-infected tall fescue, causes potent vasoconstriction of bovine vasculature by interacting with serotonin (5-HT) receptors. Ergovaline acts as a partial agonist of 5-HT2A-mediated vasoconstriction. The duration of ERV exposure affects 5-HT-mediated vasoactivity of mesenteric blood vessels. If ERV and 5-HT exposure occur simultaneously, ERV can increase 5-HT-mediated vasoconstriction. If 5-HT exposure occurs after blood vessels have had prior ERV exposure, increasing levels of 5-HT can induce vasorelaxation of blood vessels. To better understand mechanisms of 5-HT-mediated vasorelaxation, lateral saphenous veins from steers (n = 5) were assessed for vasoactivity in response to increasing concentrations of selective 5-HT receptor agonists Isolated vessel segments were suspended in chambers of a multi-myograph containing 5 mL of continuously oxygenated Krebs-Henseleit buffer and equilibrated to 1 g tension for 90 min. Vessels were pre-contracted with 1×10-4 M phenylephrine and exposed to increasing concentrations of 5-HT or 5-HT receptor agonists that were selective for 5-HT1B, 5-HT2B, 5-HT4, and 5-HT7. Vasoactive response data were normalized as a percentage of the maximum contractile response induced by the phenylephrine pre-contraction. Increasing concentrations of 5-HT resulted in divergent vasoconstriction and vasorelaxation responses that were dose-dependent. Serotonin caused an 86.3% increase (P < 0.01) in relaxation activity at 1×10-7 M and 5-HT caused a 166% increase (P < 0.01) in contraction activity at 1x10-4 M. Increasing concentrations of the selective 5-HT1B agonist did not result in any vasoactive response. Increasing concentrations of agonists selective for 5-HT2B, 5-HT4, or 5-HT7 resulted in a 27.3%, 91.6%, or 43.9% (P < 0.01) increase in maximal vasorelaxation activity. Of these 5-HT receptor agonists, the selective 5-HT4 receptor agonist resulted in the lowest -log IC50 value (6.30) compared with 5-HT2B and 5-HT7 receptor agonists (4.21 and 4.66, respectively). To confirm the involvement of 5-HT4 in 5-HT-mediated vasorelaxation, blood vessels were exposed to either DMSO (control), a selective 5-HT2A antagonist (1×10-5M), or selective antagonists of 5-HT2A and 5-HT4 simultaneously for 5-min before the pre-contraction with phenylephrine and additions of 5-HT. Antagonism of the 5-HT2A receptor did not result in increased vasorelaxation activity compared with control. Antagonism of the 5-HT2A receptor and 5-HT4 receptor simultaneously inhibited vasorelaxation activity and increased (P < 0.01) the maximal contractile response occurring at 1×10-4 M by 47% compared with control. Approximately 84% of the maximal vasorelaxation activity occurring in response to 5-HT could be accounted for through 5-HT4. These data provide strong evidence that 5-HT4 is the primary receptor involved with 5-HT-mediated vasorelaxation in bovine lateral saphenous vein. More research is needed to determine if 5-HT4-mediated vasorelaxation is responsible for relaxation responses observed in blood vessels with prior ERV exposure. This will aid in the development of strategies for alleviating symptoms of sustained vasoconstriction that occur during fescue toxicosis in ruminants.