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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, (4-bromo-3,6-dimethoxybenzocyclobuten-1-yl) methylamine HCl (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 µM 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 multimyograph 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 the 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 µM ERV had higher (P < 0.01) contractile responses at lower concentrations of TCB-2. Simultaneous ERV addition at 1 × 10-4 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 ERV simply occupy receptor binding sites competitively, or influence receptor internalization to cause the observed divergent responses.

Ergovaline (ERV), produced in toxic endophyte-infected tall fescue, causes potent vasoconstriction of bovine peripheral and visceral vasculature. Ergovaline acts as both an agonist and an antagonist in bovine gut blood vessels through serotonin (5-HT) receptors and it appears that the type of action could be influenced by the extent of ERV exposure. Because it was unclear how the duration of ERV exposure influences 5-HT-mediated vasoactivity, experiments were designed to evaluate how simultaneous or prior ERV exposure influenced 5-HT-mediated vasoactivity of mesenteric artery (MA) and vein (MV) segments from Holstein steers (N = 10). Vessels were incubated in Krebs–Henseleit buffer containing 0, 0.01, or 0.1 μM ERV for 24 h prior to the 5-HT dose-response or exposed to fixed concentrations of 0, 0.01, or 0.1 μM ERV simultaneously during the 5-HT dose-response. Vessels were suspended in chambers of a multimyograph containing Krebs–Henseleit buffer and equilibrated to 1 g tension for 90 min. Vessels were exposed to increasing concentrations of 5-HT (5 × 10−8 M to 1 × 10−4 M) every 15 min and contractile responses were normalized as a percentage of the maximum contractile response induced by 120 mM KCl reference addition. Two-way analysis of variance was used to separately analyze data for each vessel type and duration of exposure using the MIXED procedure of SAS. When 5-HT concentration increased from 5 × 10−8 to 1 × 10−6 M, simultaneous addition of 0.1 μM ERV increased (P < 0.01) the contractile response of MV compared with additions of 0 and 0.01 μM ERV. At 1 × 10−4 M 5-HT, the simultaneous presence of 0.01 and 0.1 μM ERV decreased (P < 0.01) the contractile response of both MA and MV compared with 0 μM ERV addition. As 5-HT concentrations increased, the contractile response increased (P < 0.01) in both MA and MV with no previous ERV exposure, but decreased in MA and MV with 24 h prior exposure to 0.01 and 0.1 μM ERV. These data demonstrate that the duration of ERV exposure influences 5-HT-mediated vasoconstriction and likely vasorelaxation in bovine mesenteric vasculature. If ERV and 5-HT exposure occur simultaneously, ERV can act as a partial agonist of 5-HT-mediated vasoconstriction. If 5-HT exposure occurs after blood vessels have had prior ERV exposure, it appears that 5-HT may induce vasorelaxation of blood vessels. More research is needed to identify cellular and molecular mechanisms involved with 5-HT-mediated vasoactivity. [ABSTRACT FROM AUTHOR]

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 precontraction 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. [ABSTRACT FROM AUTHOR]

Ergovaline (ERV), produced in toxic endophyte-infected tall fescue, causes potent vasoconstriction of bovine peripheral and visceral vasculature. Ergovaline acts as both an agonist and an antagonist in bovine gut blood vessels through serotonin (5-HT) receptors and it appears that the type of action could be influenced by the extent of ERV exposure. Because it was unclear how the duration of ERV exposure influences 5-HT-mediated vasoactivity, experiments were designed to evaluate how simultaneous or prior ERV exposure influenced 5-HT-mediated vasoactivity of mesenteric artery (MA) and vein (MV) segments from Holstein steers (n = 10). Isolated vessel segments were incubated in Krebs-Henseleit buffer containing 0, 0.01, or 0.1 mM ERV for 24-h befoe the 5-HT doseresponse or exposed to fixed concentrations of 0, 0.01, or 0.1 mM ERV simultaneously during the 5-HT dose-response. Vessels 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 exposed to increasing concentrations of 5-HT (5 × 10-8 M to 1 × 10-4 M) every 15 min and contractile responses were normalized as a percentage of the maximum contractile response induced by 120 mM KCl reference addition. Two-way analysis of variance was used to separately analyze data for each vessel type and duration of exposure using the MIXED procedure of SAS for the main effects of 5-HT and ERV concentration and the 5-HT × ERV concentration interaction. When 5-HT concentration increased from 5 ' 10-8 M to 1 ' 10-6 M, simultaneous addition of 0.1 mM ERV increased (P < 0.01) the contractile response of MV compared with additions of 0 mM ERV and 0.01 mM ERV. At 1 ' 10-4 M 5-HT, the simultaneous presence of 0.01 mM ERV and 0.1 mM ERV decreased (P < 0.01) the contractile response of both MA and MV compared with 0 mM ERV addition. As 5-HT concentrations increased, the contractile response increased (P < 0.01) in both MA and MV with no previous ERV exposure, but decreased in MA and MV with 24-h prior exposure to 0.01 mM ERV and 0.1 mM ERV. These data demonstrate that the duration of ERV exposure influences 5-HT-mediated vasoconstriction and likely vasorelaxation in bovine mesenteric vasculature. If ERV and 5-HT exposure occur simultaneously, ERV can act as a partial agonist of 5-HT-mediated vasoconstriction. If 5-HT exposure occurs after blood vessels have had prior ERV exposure, it appears that 5-HT may induce vasorelaxation of blood vessels. More research is needed to identify receptors and mechanisms involved with 5-HT-mediated vasoconstriction and vasorelaxation before and after exposure to ERV. This will aid in the development of strategies for alleviating symptoms of sustained vasoconstriction that occur during fescue toxicosis in ruminants. [ABSTRACT FROM AUTHOR]

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, (4-bromo-3,6-dimethoxybenzocyclobuten- 1-yl) methylamine HCl (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 μM 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 multimyograph 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 the 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 μM ERV had higher (P < 0.01) contractile responses at lower concentrations of TCB-2. Simultaneous ERV addition at 1 × 10−4 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 ERV simply occupy receptor binding sites competitively, or influence receptor internalization to cause the observed divergent responses. [ABSTRACT FROM AUTHOR]