The effect of adrenergic suppression induced by guanabenz administration on exercising thoroughbred horses

Adrenergic activity accompanies intense exercise and mediates physiological and metabolic responses to exercise. Guanabenz, an antihypertensive drug marketed for human usage, depresses brain vasomotor and cardioaccelerator centres, blocks peripherally adrenergic neurons and is reportedly used as a calming agent in horses but little is known of its effects in the species.

To determine if guanabenz induces measurable signs of adrenergic suppression on fit Thoroughbred horses undergoing intense exercise.

In a random crossover design, 12 exercise conditioned Thoroughbred horses each received guanabenz (0.08 mg/kg bwt i.v.) and placebo at 3-week intervals. An incremental exercise test to exhaustion on a treadmill followed treatment by 1 h. Heart rate, oxygen consumption, carbon dioxide production, plasma lactate, catecholamines, adrenocorticotropic hormone (ACTH) and cortisol, and time to fatigue were monitored. Statistical analysis was performed using mixed-effects linear modelling.

Mean heart rate during the exercise period was lower in guanabenz-treated horses (P = 0.04). Mean concentrations of plasma cortisol (P = 0.02) and adrenaline (P = 0.03) were lower for guanabenz-treated horses during the exercise period. Mean run time was slightly but not significantly longer for guanabenz-treated horses, (P = 0.053). No significant effects of guanabenz administration were found for oxygen consumption, carbon dioxide production nor for plasma lactate, noradrenaline and ACTH concentrations.

Guanabenz administration induced signs of adrenergic suppression including plasma cortisol and adrenaline concentrations and heart rate and may enhance endurance, but did not eliminate increases in hormone concentrations induced by exercise. Clear determination of a positive performance effect of adrenaline, but not noradrenaline, suppression is needed before clinical significance can be determined.