Influence of exercise type, hydration, and heat on plasma volume shifts in men.

Four male Caucasians were studied during cycle ergometer exercise and stair stepping in a hot wet environment (32 degrees C db, 30 degrees C wb) after exertion was equated by matching heart rates during training. With each exercise, one session was conducted after 24 h of water deprivation, the other 50 min after ingestion of an amount of water equal to 1% body weight. Venous blood samples were obtained 24 h before each exercise and before and at 10-min intervals during each exercise. No changes in osmolality were found during stair stepping. A progressive osmoconcentration, however, occurred during cycling after dehydration and an initial osmoconcentration with little subsequent change accompanied ergometer exercise after hydration. This latter effect was due to a consistent osmodilution in all subjects, but occurring at different times during the session in each. All attained an osmolality of 290 mosmol/kg before dilution. Because this value is above the threshold of arginine vasopressin release, this hormone may have been responsible for the osmodilution. Therefore, the preexercise osmolality and the rate at which the threshold for vasopressin release is attained may determine whether osmodilution, osmoconcentration, or both occur during exercise.