Endocrine and respiratory responses to ergotamine in Brahman and Hereford steers.

Ergot alkaloids are considered causative agents of fescue toxicosis, a syndrome experienced by cattle consuming tall fescue (Festuca arundinacea) infected with the fungal endophyte Neotyphodium coenophialum. One sign of fescue toxicosis in cattle is severe hyperthermia. This study assessed hormonal responses to ergotamine in heat-sensitive and heat-tolerant cattle. Seven Hereford (heat-sensitive, Bos taurus) and 7 Brahman (heat-tolerant, Bos indicus) steers on a fescue-free diet received ergotamine tartrate iv. Blood was sampled every 15 min for 2 h before and 4 h after dosing for determination of circulating hormonal changes. Respiration rates were recorded hourly. Ambient temperature and relative humidity averaged 31C and 48%, respectively, during sampling. A breed x time interaction existed (p < 0.01) for plasma prolactin, LH, insulin, glucagon, cortisol, triiodothyronine and glucose concentrations. The breed x time interaction tended to affect (p = 0.14) growth hormone and influenced (p < 0.01) respiration rates. Ergotamine reduced (p < 0.01) plasma LH and increased (p < 0.01) growth hormone concentrations in Brahman. Both breeds responded to ergotamine with increased (p < 0.01) plasma cortisol, glucagon, and glucose and reduced (p < 0.01) insulin concentrations. The magnitude of response for cortisol, insulin, and glucose were greater for the Brahman. Ergotamine increased (p < 0.01) plasma triiodothyronine and respiration rates in Hereford, whereas these traits were unaltered in Brahman. Acute ergotamine exposure generally resulted in similar effects on Brahman and Hereford steers. The triiodothyronine and respiratory comparisons revealed modified responses in Brahman that suggest a potential benefit of using heat-tolerant genetics to reduce the adverse effects of fescue toxicosis in cattle.