Leucocytosis, thrombocytosis, and plasma osmolality during rest and exercise: an hypothesis

The mechanism for inducing leucocytosis (increase in white blood cells) and thrombocytosis (increase in platelets) during exercise is unclear. Because plasma osmolality (Osm) may influence T-cell proliferation, Osm and the number of leucocytes (WBC) and platelets in blood were measured periodically during a 90 min rest period, and were compared with those during upright sitting ergometer exercise in six untrained, healthy men who cycled for 70 min at 71% of their maximal oxygen uptake (VO2max). There were 6 experiments in which the subjects drank different fluid formulations (10 ml x kg(-1) of various ionic and osmotic concentrations intermittently during 60 min of the rest period and during the exercise period. Osmolality, and WBC and platelet counts increased significantly (p < 0.05) within the first 10 min of exercise, but the additional 60 min of exercise did not significantly change the leucocytosis or thrombocytosis. There were low but significant correlations between individual values of total WBC and total Osm during exercise (r0.001(2),284 = 0.39) and during rest plus exercise (r0.001(2),499 = 0.43). With combined data from the six experiments, mean Osm correlated highly and significantly with both mean WBC (r0.001(2),6 = 0.95, p < 0.001) and mean platelets (r0.001(2),6 = 0.94, p < 0.01) during the exercise phase. These data indicate that increases in leucocytes, thrombocytes, and osmolality occur primarily within the first 10 min of high-intensity exercise, but neither hypovolemia nor hyperthermia during exercise contributed to the leucocytosis, thrombocytosis, or hyperosmolality. The high correlations between plasma Osm and WBC or platelet counts suggest changes in osmolality may contribute to the mechanism of leucocytosis and thrombocytosis induced by exercise.