Impact of treadmill exercise on early apoptotic cells in mouse thymus and spleen

Lymphocyte apoptosis occurs in response to stressors such as thermal injury, trauma, sepsis, and surgery. This study evaluated the effect of a single bout of physical exercise stress on the induction of apoptosis in murine thymocytes and splenic lymphocytes. Female C57BL/6 mice, treadmill exercised at a submaximal intensity (35 m/min, 6% grade) for 90 min or serving as controls (walking on treadmill at 12 m/min, 6% grade, 5 min), were sacrificed 5 min or 120 min after completion of exercise. The percent of apoptotic, necrotic, and viable thymocytes and splenocytes were determined by flow cytometry using annexin V FITC and propidium iodide. There was a significantly higher percent of viable splenocytes in the mice sampled 120 min after cessation of exercise than treadmill control animals (p<0.05). In the thymus, there was a significantly lower percent of apoptotic (p<0.5) and a significantly higher percent of viable (p<0.05) cells in exercised mice sampled at 120 min after exercise relative to controls. Absolute numbers of thymocytes and splenocytes did not differ by exercise treatment condition. Plasma corticosterone levels were elevated immediately after exercise and were negatively correlated with the percent of viable lymphocytes in the spleen. During the time frame sampled, submaximal exercise is associated with a lower % of thymocytes expressing early markers of apoptosis, despite elevated plasma corticosterone levels. Retention of self-reactive, viable thymocytes which would normally be deleted or selective trafficking of apoptotic thymocytes out of the thymus may be involved in the exercise effect. Additional studies are necessary to identify the mechanisms for this shift in proportions of apoptotic and viable cells in lymphoid compartments with exercise.