The actions of the beta-adrenoceptor blocking agents propranolol and metoprolol in the maximally exercised horse.

The effects of two beta-adrenoceptor antagonists, propranolol (0.2 mg/kg) and metoprolol (0.2 mg/kg) on some physiological and metabolic changes produced by maximal exercise in the horse were investigated. Both drugs reduced the elevation in heart rate seen immediately following exercise and reduced performance as was seen by the increased time taken to perform each gallop. The rise in plasma glucose, glycerol and lactate, and the fall in blood pH seen following exercise were attenuated by both drugs. However, a greater increase in plasma free fatty acids occurred. Exercise produced a nine- to 12-fold increase in plasma noradrenaline levels. Neither drug had any effect on resting levels of noradrenaline but after metoprolol the levels during exercise were increased.