ABSTRACT
The purpose of this study was to examine the impact of intensive training for competitive sports on natural killer (NK) cell lytic activity and subset distribution. Eight female college-level volleyball players undertook 1 mo of heavy preseason training. Volleyball drills were performed 5 h/day, 6 days/wk. Morning resting blood samples were collected before training (Pre), on the 10th day of training (During), 1 day before the end of training (End), and 1 wk after intensive training had ceased (Post). CD3(-)CD16(bright)CD56(dim) (CD56(dim) NK), CD3(-)CD16(dim/-)CD56(bright) NK (CD56(bright) NK), and CD3(+)CD16(-)CD56(dim) (CD56(dim) T) cells in peripheral blood were determined by flow cytometry. The circulating count of CD56(dim) NK cells (the predominant population, with a high cytotoxicity) did not change, nor did the counts for other leukocyte subsets. However, counts for CD56(bright) NK and CD56(dim) T cells (subsets with a lower cytotoxicity) increased significantly (P < 0.01) in response to the heavy training. Overall NK cell cytotoxicity decreased from Pre to End (P = 0.002), with a return to initial values at Post. Lytic units per NK cell followed a similar pattern (P = 0.008). Circulating levels of interleukin-6, interferon-gamma, and tumor necrosis factor-alpha remained unchanged. These results suggest that heavy training can decrease total NK cell cytotoxicity as well as lytic units per NK cell. Such effects may reflect in part an increase in the proportion of circulating NK cells with a low cytotoxicity.
