Production of reactive oxygen species in neutrophils after repeated bouts of exercise in standardbred trotters

Six trained Standardbred trotters exercised on a racetrack on 2 days with a 3-day interval. On both exercise days the horses trotted three different e xercise bouts with increasing intensity with 60-min intervals. Exercise-ind uced stress was manifested as leucocytosis, an increase in the neutrophil: lymphocyte (N:L) ratio, and increased capacity to produce reactive oxygen s pecies in the peripheral blood as indicated by an increase in whole blood c hemiluminescence. The leucocytosis was mainly due to neutrophilia, which la sted for 6 h. Production of reactive oxygen species per single neutrophil s howed no significant change during a day of exercise, but was lower on the second exercise day. The cortisol concentrations and N:L ratio, used as ind icators of stress, behaved differently: Cortisol did not change significant ly after exercise, whereas the N:L ratio increased. These results suggest t hat in trained horses, the N:L ratio is a sensitive indicator of stress of short duration, and an attenuated N:L response can be taken as an indicator of adaptation to exercise stress.