Fluid-electrolyte shift and renin-aldosterone responses to exercise under hypoxia.

In order to describe fluid-electrolyte shift and endocrine response to exercise under moderate acute hypoxia, 8 healthy male subjects (24 +/- 3 years old) were evaluated at 40, 60, 80 and 100% VO2 max in normoxic (N) and hypoxic (H) conditions (14.5% O2). VO2 max decreased from 55.5 +/- 1.3 to 45.8 +/- 1.4 ml/kg X min in H condition. Plasma volume reductions with increasing relative workloads were similar in N (9.4%) and H (9.9%) conditions. The rise in plasma osmolality was in part related to blood lactate accumulation which occurred in both conditions. However, variations in plasma solute content and osmolality suggested that exercise under hypoxia results in a greater electrolyte loss from vascular space and in a greater K+ loss from working skeletal muscles. Increase in catecholamine concentrations were similar in normoxic and hypoxic conditions except for lower maximal norepinephrine concentration under hypoxia. Finally, although plasma renin activity increased with workload in both conditions, plasma aldosterone did not significantly change. This dissociation between renin and aldosterone suggest that aldosterone release during exercise might depend upon other factors. However, changes in plasma potassium concentration do not appear as an important stimulus for aldosterone secretion during exercise.