Abstract
Objective—To determine the relationship between plasma β-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 (
O2max)
and to fatigue at 95% O2max. Plasma EN concentrations
were determined before exercise, after a 10-
minute warmup period, after 5, 10, 15, and 20 minutes
at 60% O2max or at the point of fatigue (95%
O2max), 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% O2max,
compared with those after 20 minutes of exercise at
60% O2max (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%
O2max, respectively).
Conclusions and Clinical Relevance—A critical
exercise threshold exists for EN concentration in
horses, which is 60% O2max 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.
(Am J Vet Res 2000;61:969–973)
