Introduction/purpose: The purpose of this study was to determine the affects of repeated bouts of exercise in the same day on circulating leukocyte number and NK cell activity (NKCA).
Methods: Ten males (18-25 yr of age) were chosen to complete four trials: two exercise bouts (TB), AM exercise (AM-EX), PM exercise (PM-EX), and control (CN) in a random, counter-balanced order. Exercise consisted of 1-h cycle ergometry, split into three, 20-min segments (5 min at 50% [V0](2peak) and 15 min at 70% [V0](2peak)), and was separated by a 4-h passive recovery. Blood samples were collected from a peripheral arm vein before, immediately, 2 h, and 24 h after the AM bout, and a second series was drawn for the PM bout. Leukocyte subpopulations were measured by manual counting using a hemacytometer and Wright-Giemsa-stained differential slides. NKCA was measured with a whole-blood 51Cr-release assay against K562 target cells. RESULTS Total leukocyte (13.2 +/- 1.2 x 109 L-1, P < 0.035) and neutrophil (7.6 +/- 0.7 x 109 L-1, P < 0.001) counts were significantly higher after two bouts of exercise than a single exercise bout completed in the morning (leukocyte: 7.8 +/- 0.7 x 109 L-1; neutrophil: 5.2 +/- 0.5 x 109 L-1) or afternoon (leukocyte: 9.9 +/- 0.8 x 109 L-1; neutrophil: 5.6 +/- 0.9 x 109 L-1). Lymphocyte counts were significantly greater (P < 0.001) after PM (4.2 +/- 0.5 x 109 L-1) than AM (3.7 +/- 0.4 x 109 L-1) exercise. NKCA was significantly greater (P < 0.001) after exercise in the afternoon (35.3 +/- 8.1%) than morning (27.2 +/- 5.9%).
Conclusion: PM exercise produced a larger increase in NKCA than AM exercise, possibly indicative of an interaction between exercise and a diurnal effect. These data suggest two bouts of endurance exercise in 1 d produce an "additive effect" for total leukocyte and neutrophil counts. and to a lesser degree lymphocyte counts, but did not appear to impact shifts in NKCA.