Sarcoplasmic reticulum responses to repeated sprints are affected by conditioning of horses

Sarcoplasmic reticulum (SR) responses to repeated sprints and to physical c onditioning were studied in 10 Quarter Horses. Exercise tests (four repeate d sprints on a treadmill) were conducted before and after 12 wk of sprint c onditioning. Muscle samples from the middle gluteal muscle were taken befor e and after each exercise test, and SR vesicles were isolated. Calcium upta ke was determined spectrophotometricalIy using antipyrylazo III, and Ca2+-A TPase activity was determined using an enzyme-linked optical assay. Conditi oning increased calcium uptake rate and Ca2+-ATPase activity by 14 and 38%, respectively, before exercise and by 25 and 26% after exercise. Exercise d ecreased calcium uptake rate and Ca2+-ATPase activity by 37 and 27%, respec tively, before conditioning and by 28 and 21% after conditioning. Decreases in calcium uptake and Ca2+-ATPase activity of SR have been associated with fatigue during exercise, and this association is strengthened by the moder ating effect of conditioning.