Sarcoplasmic reticulum responses to repeated sprints, conditioning and dietary lecithin in the horse

Sarcoplasmic reticulum (SR) releases and sequesters calcium during muscle contraction and relaxation. Calcium uptake rates have been shown to respond to high intensity exercise, and we propose this response may be affected by conditioning and dietary lecithin. Ten Quarter Horses performed repeated sprints on a high speed treadmill. Muscle samples were taken before and after exercise from the middle gluteal muscle, and SR vesicles were isolated. Horses were then conditioned for 12 weeks and the experiment was repeated. Calcium uptake was determined spectrophotometrically using antipyralazo IIl, and Ca²⁺- ATPase activity was determined using an enzyme-linked optical assay. Resting calcium uptake rates and Ca²⁺-ATPase activities were greater when the horses were conditioned than when unconditioned (p=.05, p=.004, respectively). After exercise calcium uptake was slower in horses unconditioned (p=.02) or conditioned (p=.002) and this fatigue response to exercise was reduced by conditioning (p=.01). After exercise Ca²⁺-ATPase activity was decreased (p=.007) to 71% of resting activity in unconditioned horses, and to 82% in conditioned horses (p=.001).

The same ten horses were divided into two groups and were fed either a conventional diet or a diet with 10% added dietary fat in the form of corn oil/soy lecithin for a 12 week period. The diets were switched and the study was continued for another 12 weeks. Horses were exercised 4 days a week on a high speed treadmill throughout the study. The repeated sprinting experiments, as in the first part of the study, were performed at the end of both 12 week periods. Resting calcium uptake rates tended to be higher (p=.09) when horses were fed the fat modified diet, but decreased to a greater extent with an acute bout of exercise (p=.058). Ca²⁺-ATPase activity was lower after exercise when horses consumed the lecithin (p=.02) and the decrease from resting values was greater (p=.05).

These results suggest that high intensity repeated sprinting fatigues SR function in horses, and that conditioning reduces the depression of SR function with exercise. Feeding a lecithin supplemented diet to conditioned horses increases the response of the SR to an acute bout of exercise.