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

The research article explores the impact of two beta-adrenoceptor antagonists, propranolol and metoprolol, on the physiological and metabolic responses in horses after high-intensity exercise. It was found that both these drugs moderated heart rate increases and certain metabolic changes post-exercise, but negatively affected performance and led to higher fatty acid levels.

Research Context and Methodology

• The study aims to examine how propranolol and metoprolol, two beta-adrenoceptor blocking agents, impact the physiological and metabolic response during and after maximal exercise in horses.
• Both drugs were administered with the same dosage (0.2 mg/kg), and the performance of the horses was assessed through their ability to perform a gallop task. Various physiological and metabolic markers, including heart rate, plasma glucose, glycerol, and lactate levels, were tracked.

Effects on Heart Rate and Performance

• Both propranolol and metoprolol resulted in a decrease in the elevated heart rate usually noticed following strenuous exercise, likely due to their properties as beta-blockers that hinder adrenaline activity and thus reduce the heart rate.
• However, while the drugs reduced the exercise-induced escalation in heart rate, they also had an adverse impact on horses’ performance. This was evidenced by an increase in the time taken for the horses to complete each gallop after administration of either drug.

Effects on Metabolic Changes

• The study observed that both drugs led to smaller increases in plasma glucose, glycerol, and lactate following exercise, which are typically indicators of the body’s metabolic response to physical activity.
• A reduction in the blood pH levels, which generally drop post-exercise due to the production of lactic acid, was also mitigated by both medications. However, there was a larger increase in plasma free fatty acids, which suggests that fat metabolism was promoted in the presence of these beta-blockers.

Effects on Noradrenaline Levels

• The researchers noted that maximal exercise caused a significant increase (nine- to 12-fold) in plasma noradrenaline levels, a hormone typically released in response to stress or physical exertion.
• Neither propranolol nor metoprolol affected the resting levels of noradrenaline. However, during exercise, the levels of this hormone increased when metoprolol was administered, indicating that this specific beta-blocker might stimulate the release of noradrenaline during intense physical activity.