Effects of exercise intensity and duration on plasma β-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 consumption ((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-minute warmup period, after 5, 10, 15, and 20 minutes at 60% (V)over dot O-2 max or at the point of fatigue (95% (V) over dot O-2 max), and at regular intervals after exercise. Glucose concentrations were determined at the same times EN concentrations were measured. Plasma lactate concentration was measured 5 minutes after exercise. Results--Maximum EN values were recorded 0 to 45 minutes after horses completed 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 exercise 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 significantly but weakly correlated with glucose concentration for exercise at 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 for 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 controlled exercise there may be considerable differences in EN concentrations between horses. This makes the value of comparing horses on the basis of their EN concentration questionable.