Circulating lactate and FFA during exercise: effect of reduction in plasma volume following exposure to simulated microgravity.

We examined the relationship between changes in plasma volume (PV) following simulated weightlessness and the response of blood lactate (BL) and plasma free fatty acids (FFA) during exercise. Ten healthy men (35-49 yr) performed 15 min of submaximal upright cycle ergoemetry at a workrate of 57% of pre-bed rest (BR) maximal oxygen uptake before and after 10 d of continuous 6 degrees head-down BR. During exercise, oxygen uptake (VO2) was measured to assure equivalent energy expenditure for pre-BR and post-BR workrate. Antecubital venous blood samples were collected at rest and during the last 30 s of exercise and analyzed for concentrations of BL and FFA, and hematocrit. Resting PV was measured pre-BR and post-BR with Evans blue dye and total circulating BL and FFA were calculated as the product of PV and concentrations of BL and FFA, respectively. VO2 and FFA concentration during submaximal exercise were unchanged post-BR. During exercise, post-BR BL concentration of 2.9 mmol . L-1 was slightly greater (p less than 0.05) than pre-BR BL concentration of 2.5 mmol . L-1. However, as a result of a 17% reduction (p less than 0.05) in resting PV, total circulating BL during exercise was unchanged while total circulating FFA was reduced (p less than 0.05) post-BR. We conclude that the concentrations of BL and FFA during exercise following simulated weightlessness can be significantly altered by the reduction in PV.