Abstract #264

# 264
Effect of grazing seedhead-suppressed tall fescue pasture on the vasoactivity of serotonin receptors.
James L. Klotz*1, Glen E. Aiken1, Amanda M. Egert2, David L. Harmon2, 1USDA-ARS, Forage-Animal Production Research Unit, Lexington, KY, 2Department of Animal and Food Sciences, University of Kentucky, Lexington, KY.

Previous research has demonstrated that exposure to ergot alkaloids reduces vasoactivity of serotonin (5HT) receptors. Chemical suppression of tall fescue seedhead production is a tool to reduce the level of exposure to ergot alkaloids by a grazing animal. Therefore, the objective was to evaluate contractility of lateral saphenous veins biopsied from mixed breed steers following a 87–101 d grazing period on 3-ha pastures of bermudagrass (BG; n = 5; 340 ± 9 kg), or standard endophyte-infected tall fescue that was not sprayed (UNSUP; n = 5; 300 ± 6; 0.56 ppm ergovaline) or sprayed (SUP; n = 5; 294 ± 9 kg; 0.24 ppm ergovaline) with herbicide containing aminopyralid and metsulfuron-methyl (Chaparral; Dow AgroSciences). To evaluate contractility, biopsied veins were mounted in a multimyograph and exposed to increasing concentrations of a tall fescue seed extract (EXT; ergovaline source); and 5HT1B (CP9), 5HT1D (L), and 5HT2A (TCB) agonists. All contractility data were normalized to a maximal response of 1 × 10−4 M norepinephrine and were analyzed as a split plot treatment design using SAS for effects of pasture treatment, agonist concentration, and the interaction. There was no contractile response to any concentration of CP9 in any of the pasture treatments. There were pasture × concentration interactions for contractile responses to TCB (P < 0.01) and EXT (P < 0.01). For both EXT and TCB, BG veins were more vasoactive to the higher concentrations (P < 0.05) and there were no differences between UNSUP and SUP veins. There was also a pasture × concentration interaction for the contractile responses to L (P < 0.01). However, these responses were not sigmoidal and reached a zenith at 5 × 10–7 and 1 × 10–6 M. At these concentrations, the response was greatest for UNSUP (P < 0.05) and did not differ between SUP and BG veins. Although lower levels of ergovaline in SUP pastures did not alter vasoactivity of 5HT2A or 5HT1B receptors in the lateral saphenous vein, elevated vasoactivity of 5HT1D in UNSUP veins suggests that lower ergovaline levels in SUP pastures reduces the vascular effects of ergot alkaloids at this receptor.

Key Words: ergot alkaloid, cattle, serotonin receptor