Effects of selected combinations of tall fescue alkaloids on the vasoconstrictive capacity of fescue-naïve bovine lateral saphenous veins

¹ USDA-ARS, Forage-Animal Production Research Unit, Lexington, KY, USA
² Department of Plant and Soil Sciences, University of Kentucky, Lexington, USA

^* To whom correspondence should be addressed. E-mail: jim.strickland{at}ars.usda.gov.

	Abstract

Vasoconstriction is a response associated with consumption of toxic endophyte-infected tall fescue. It is not known if endophyte-produced alkaloids act alone or collectively in mediating the response. Therefore, the objective of this study was to examine vasoconstrictive potentials of selected ergot alkaloids, individually or in paired combinations, using bovine lateral saphenous veins biopsied from fescue-naïve cattle. Segments (2 to 3 cm) of vein were surgically biopsied from healthy cross-bred yearling heifers (n = 22; 330 ± 8 kg). Veins were trimmed of excess fat and connective tissue, sliced into 2 to 3 mm sections and suspended in a myograph chamber containing 5 mL of oxygenated Krebs-Henseleit buffer (95% O₂/5% CO₂; pH = 7.4; 37°C). Increasing doses of ergovaline, lysergic acid, and N-acetylloline individually or in combination, were evaluated. Contractile data were normalized as a percent of contractile response induced by a reference dose of norepinephrine (1x10^-4 M). Increasing concentrations of lysergic acid did not result in an appreciable contractile response until the addition of 1x10^-4 M lysergic acid. In contrast, the vascular response to increasing concentrations of ergovaline was apparent at 1x10^-8 M and increased to a maximum of 104.2 ± 6.0% with the addition of 1x10^-4 M ergovaline. The presence of N-acetylloline did not alter the onset or magnitude of vascular response to either lysergic acid or ergovaline. The presence of 1x10^-5 M lysergic acid with increasing concentrations of N-acetylloline and ergovaline generated an increased contractile response during the initial additions compared to the responses of N-acetylloline and ergovaline alone. In the presence of 1x10^-7 M ergovaline, the contractile response increased with increasing concentrations of N-acetylloline and lysergic acid. Neither N-acetylloline nor lysergic acid elicited a very intense contractile response individually (maximum contractile responses of 1.9 x 0.3% and 22.6 ± 4.1%, respectively), suggesting that this was the result of the repetitive addition of 1x10^-7 M ergovaline. These data indicate that ergovaline is a more potent vascular toxicant than lysergic acid and N-acetylloline. The contractile responses of the ergovaline and lysergic acid combinations appeared to differ from the individual dose-responses. These data support the possibility that an additive alkaloid exposure effect may exist and should be considered during evaluations of ergot alkaloids.

Key Words: cattle, ergovaline, fescue toxicosis, lysergic acid, N-acetylloline, vasoconstriction