Skeletal muscle water and electrolytes following prolonged dehydrating exercise

We studied if dehydrating exercise would reduce muscle water (H2Omuscle) and affect muscle electrolyte concentrations. Vastus lateralis muscle biopsies were collected prior, immediately after, and 1 and 4h after prolonged dehydrating exercise (150min at 33 +/- 1 degrees C, 25%+/- 2% humidity) on nine endurance-trained cyclists (VO2max=54.4 +/- 1.05mL/kg/min). Plasma volume (PV) changes and fluid shifts between compartments (Cl- method) were measured. Exercise dehydrated subjects 4.7%+/- 0.3% of body mass by losing 2.75 +/- 0.15L of water and reducing PV 18.4%+/- 1% below pre-exercise values (P<0.05). Right after exercise H2Omuscle remained at pre-exercise values (i.e., 398 +/- 6mL/100g dw muscle(-1)) but declined 13%+/- 2% (342 +/- 12mL/100g dw muscle(-1); P<0.05) after 1h of supine rest. At that time, PV recovered toward pre-exercise levels. The Cl- method corroborated the shift of fluid between extracellular and intracellular compartments. After 4h of recovery, PV returned to pre-exercise values; however, H2Omuscle remained reduced at the same level. Muscle Na+ and K+ increased (P<0.05) in response to the H2Omuscle reductions. Our findings suggest that active skeletal muscle does not show a net loss of H2O during prolonged dehydrating exercise. However, during the first hour of recovery H2Omuscle decreases seemly to restore PV and thus cardiovascular stability.