Effects of exercise intensity and duration on plasma beta-endorphin concentrations in horses

Objective--To determine the relationship between plasma beta-endorphin (EN) concentrations and exercise intensity and duration in horses. Animals--8 mares with a mean age of 6 years (range, 3 to 13 years) and mean body weight of 450 kg, Procedure--Horses were exercised for 20 minutes at 60% of maximal oxygen co nsumption ((V) over dot O(2)max) and to fatigue at 95% (V) over dot O-2 max . Plasma EN concentrations were determined before exercise, after a 10-minu te warmup period, after 5, 10, 15, and 20 minutes at 60% (V)over dot O-2 ma x or at the point of fatigue (95% (V) over dot O-2 max), and at regular int ervals after exercise. Glucose concentrations were determined at the same t imes EN concentrations were measured. Plasma lactate concentration was meas ured 5 minutes after exercise. Results--Maximum EN values were recorded 0 to 45 minutes after horses compl eted each test. Significant time and intensity effects on EN concentrations were detected. Concentrations were significantly higher following exercise at 95% (V) over dot O-2,max, compared with those after 20 minutes of exerc ise at 60% (V) over dot O(2)max (605.2 +/- 140.6 vs 312.3 +/- 53.1 pg/ml). Plasma EN concentration was not related to lactate concentration and was si gnificantly but weakly correlated with glucose concentration for exercise a t both intensities (r = 0.21 and 0.30 for 60 and 95% (V) over dot O(2)max, respectively). Conclusions and Clinical Relevance--A critical exercise threshold exists fo r EN concentration in horses, which is 60% (V) over dot O(2)max or less and is related to exercise intensity and duration. Even under conditions of co ntrolled exercise there may be considerable differences in EN concentration s between horses. This makes the value of comparing horses on the basis of their EN concentration questionable.