Effects of Exercise on Density and Osmotic Fragility of Circulating Red Cells in Normal and Splenectomized Thoroughbred Horses

Changes in cell density and osmotic fragility of erythrocytes during exercise were determined in 3 control (Group C) and 3 splenectomized (Group S) horses that performed an incremental exercise test on a treadmill until the point of fatigue. Venous blood samples were drawn before the start of the treadmill program, and immediately after the warm-up and the incremental exercise test. After the incremental exercise in Group C, red cells in both groups were swelled by washing with PBS regardless of exercise. Warm-up did not change the density of fractionated red cells that were separated by the method of Murphy in both groups of horses. In contrast, incremental exercise significantly increased the density of cells in Group C but not in Group S. The osmotic fragility of fractionated red cells was significantly lower in Group S than in Group C horses regardless of exercise. Warm-up significantly decreased the fragility, but incremental exercise significantly increased it in all fractionated erythrocytes from both groups. There are two possible explanations for the above phenomena: 1) Erythrocytes which are released from the spleen by strong contraction during heavy exercise show a stronger tendency to regulatory volume decrease which shows osmotic sensitivity, but not during light exercise; and/or 2) The sensitivity of the regulatory volume decreases and membrane fragility versus exercise stress increases with the repeated accumulation of erythrocytes in the spleen.