Erythrocyte alterations endurance exercise in horses.

The erythrocytes of 14 conditioned horses participating in a 157-km endurance ride (requiring 14-21 h) were examined before the ride, immediately upon entering the 44-91-, and 130-km rest stops, and at the finish. At the first rest stop (44 km), the mean erythrocyte count increased 41% (P less than 0.001), the mean hematocrit (Hct) increased 30% (P less than 0.001) and the mean hemoglobin (Hb) increased 33% ( P less than 0.001). Although subsequent mean erythrocyte counts, Hct, and Hb values remained significantly elevated above controls, the values decreased 9-9% from the 4-km values later in the ride. These changes suggest a lost of red cells mass during the prolonged exercise. Spiculated red blood cells that increased markedly in number during exercise were also observed in these conditioned horses. The appearance of an increased number of spiculated red cells with exercise was associated with corresponding changes in red cell indices.