ENDURANCE TRAINING AND HEAT ACCLIMATION: EFFECT ON BRIEF CYCLE-ERGOMETER EXERCISE IN A HOT AND COOL ENVIRONMENT

The first part of this study investigated the changes in blood volume and composition that occur as a result of endurance training and heat acclimation in human subjects. The influence of exercise intensity (20 or 60% VO(,2) max) and environmental temperature, (40(DEGREES)C d.b. 26(DEGREES)C w.b. or 10(DEGREES)C d.b., 6(DEGREES)C w.b.) on changes in the plasma content during cycle exercise were also examined.;The second part of this study examined the extra-renal role of antidiuretic hormone (ADH) and subfornical organ (SFO) involvement in fluid shifts in rats. The results were inconclusive in revealing the mechanism by which ADH brings about these fluid shifts and electrical stimulation of the SFO had no effect on the fluid dynamics of the rats. However, delivery of angiotensin II (AII) directly to the SFO did result in a significant hemodilution. Perhaps, with a direct delivery of AII to the SFO, enough SFO neurons were activated to stimulate ADH release from the posterior pituitary and cause the subsequent fluid influx into the vasculature.;The results indicated that while endurance training had no influence on resting blood volume and composition, heat acclimation resulted in a significant hypervolemia. This hypervolemia was best explained by an increased intravascular retention of protein and the corresponding increase in oncotic pressure. Further, the data showed that endurance training and heat acclimation attenuated the fluid loss during cycle exercise, when it was performed in a cold environment at a light exercise intensity. During the same exercise, training and heat acclimation resulted in a net gain in albumin intravascularly and a smaller loss of osmotically active particles from the vascular compartment. These events provided an increased oncotic and osmotic force respectively, for the retention of fluid intravascularly. Increases in plasma renin activity and aldosterone levels during the cycle exercise were not influenced by endurance training nor by heat acclimation.