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 nu mber of leucocytes (WBC) and platelets in blood were measured periodically during a 90 min rest period, and were compared with those during upright si tting ergometer exercise in six untrained, healthy men who cycled for 70 mi n at 71% of their maximal oxygen uptake ((V)over dotO(2max)). There were 6 experiments in which the subjects drank different fluid formulations (10 ml .kg(-1)) of various ionic and osmotic concentrations intermittently during 60 min of the rest period and during the exercise period. Osmolality, and W BC and platelet counts increased significantly (p < 0.05) within the first 10 min of exercise, but the additional 60 min of exercise did not significa ntly change the leucocytosis or thrombocytosis. There were low but signific ant correlations between individual values of total WBC and total Osm durin g exercise (r(0.001(2),284) = 0.39) and during rest plus exercise (r(0.001( 2),499) = 0.43). With combined data from the six experiments, mean Osm corr elated highly and significantly with both mean WBC (r(0.001(2),6) = 0.95, p < 0.001) and mean platelets (r(0.001(2),6) = 0.94, p < 0.01) during the ex ercise phase. These data indicate that increases in leucocytes, thrombocyte s, and osmolality occur primarily within the first 10 min of high-intensity exercise, but neither hypovolemia nor hyperthermia during exercise contrib uted to the leucocytosis, thrombocytosis, or hyperosmolality. The high corr elations between plasma Osm and WBC or platelet counts suggest changes in o smolality may contribute to the mechanism of leucocytosis and thrombocytosi s induced by exercise.