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© 2018 Bromocriptine is a dopamine receptor agonist known to cause hypotension and bradycardia in several species. Five experiments were conducted to compare possible perturbations on heart rate (HR) in horses after a brief (2 minutes) exercise bout when exposed to either short-term or long-term treatment with bromocriptine, cabergoline, or pergolide (all commonly used dopaminergic agonists in horses) or sulpiride, a dopaminergic antagonist. For all experiments, prolactin was measured as an indicator of drug efficacy. Experiments 1 and 4 were conducted as a replicated Latin square, whereas experiments 2, 3, and 5 were double split plot designs. Experiment 1 tested changes in HR, adrenocorticotropin (ACTH), and growth hormone (GH) concentrations when geldings were pretreated with 50 mg of bromocriptine 12 hours before exercise. Bromocriptine pretreatment reduced (P <.05) the exercise-induced rise in HR and the ACTH and GH responses (P <.05). Experiment 2 assessed the daily responses of HR to exercise after intramuscular administration of 5 mg of cabergoline in vegetable oil, which diminished the rise in HR because of exercise for the first 2 days of the 7-day experiment. In experiment 3, daily feeding of 2g of pergolide top dressed over sweet feed had no effect on HR in response to exercise. Similar results were seen in experiments 4 and 5, when horses were intravenously administered.01 mg/kg BW sulpiride in saline or intramuscularly administered 1g of sulpiride dissolved in vegetable oil. Taken together, bromocriptine and cabergoline, but not pergolide or sulpiride, dampened the cardiac sympathetic response to exercise, thus, lowering the HR.

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Journal of Equine Veterinary Science

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