Effects of grazing different ergovaline concentrations on vasoactivity of bovine lateral saphenous vein

Authors: Klotz, James; AIKEN, GLEN; EGERT-MCLEAN, AMANDA - UNIVERSITY OF KENTUCKY; SCHRICK, F. NEAL - UNIVERSITY OF KENTUCKY; CHATTOPADHYAY, NABANITA - UNIVERSITY OF KENTUCKY; HARMON, DAVID - UNIVERSITY OF KENTUCKY

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/24/2018
Publication Date: 4/26/2018
Citation: Klotz, J.L., Aiken, G.E., Egert-McLean, A.M., Schrick, F., Chattopadhyay, N., Harmon, D.L. 2018. Effects of grazing different ergovaline concentrations on vasoactivity of bovine lateral saphenous vein. Journal of Animal Science. 6:3022–3030. doi: 10.1093/jas/sky163.
DOI: https://doi.org/10.1093/jas/sky163

Interpretive Summary: Cattle that graze tall fescue pastures are exposed to fungally produced ergot alkaloids or toxins in the grass that cause vasoconstriction and result in fescue toxicosis. One strategy to mitigate the effects of these toxins to chemically suppress tall fescue seedhead production, where concentrations of the toxins are the highest, thereby reducing the amount of toxins in a pasture available for consumption. The objective of this study was to evaluate the vasoactivity of serotonin receptors functionally involved in vasoconstriction in lateral saphenous veins biopsied from steers grazing non-toxic bermudagrass, standard endophyte-infected tall fescue untreated, and treated to suppress seedhead production. Chemical suppression of tall fescue seedhead production resulted in a lower toxin level that effected the activity of some serotonin receptor subtypes, but not other subtypes. This suggests that there is still some vascular adaptation in cattle to the toxins, even at lower levels. This work will be of value to researchers interested in the physiological effects of ergot alkaloids in grazing animals and others in the field as a contribution to further defining pasture threshold levels of the toxins associated with fescue toxicosis.

Technical Abstract: 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 (n = 5; 340 ±9 kg), or standard endophyte-infected tall fescue that was not treated (n = 5; 300 ±6; 0.56 ppm ergovaline) or was treated (n=5; 294 ±9 kg; 0.24 ppm ergovaline) with herbicide containing aminopyralid and metsulfuron-methyl. 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 (CP 93129), 5HT1D (L-694,247), and 5HT2A (TCB2) 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 5HT1B agonist in any of the pasture treatments. There were pasture × concentration interactions for contractile responses to 5HT2A agonist (P < 0.01) and EXT (P < 0.01). For both EXT and TCB2, veins from bermudagrass steers were more vasoactive to the higher concentrations of these compounds (P < 0.05) and there were no differences between veins collected from the unsuppressed or seedhead suppressed treatments. There was also a pasture × concentration interaction for the contractile responses to 5HT1D agonist (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 veins from the unsuppressed treatment (P < 0.05) and did not differ between veins from suppressed and bermudagrass treatments. Although lower levels of ergovaline in seedhead suppressed pastures did not alter vasoactivity of 5HT2A or 5HT1B receptors in the lateral saphenous vein, elevated vasoactivity of 5HT1D in veins from unsuppressed tall fescue pasture treatment suggests that lower ergovaline levels in seedhead suppressed pastures can influence the vascular effects of ergot alkaloids.