Extra- and intracellular water spaces in muscles of man at rest and with dynamic exercise

A method was established to analyze the extracellular water space (H2Oe) in small muscle tissue samples as [3H]inulin distribution space. After initial experiments on rats, the method was applied on 13 men and 6 women. Muscles with different fiber compositions (soleus, S; vastus lateralis, (VL; gastrocnemius, G; triceps brachii, TB) were studied at rest. The total water content was the same for all muscles, 320 (313-330) ml/100 g dry wt. However, differences were demonstrated for H2Oe, with 26-34 ml/100 g dry wt in VL and 38-54 ml/100 g dry wt in S, (P less than 0.05); the values for G and TB were in between those for VL and S. The differences in H2Oe were not related to the fiber composition of the muscles. During 3 x 3 min of intense bicycle exercise demanding about 120% VO2 max (6 men), total water content increased in VL from 313 to 359 ml/100 g dry wt and H2Oe increased from 34 to 60 ml/100 g dry wt (P less than 0.05), In TB, which is relatively inactive during bicycle exercise, no such changes occurred. The calculated intracellular lactate concentration increased in VL from 5.7 to 30.6 mmol/l H2Oi. The extracellular lactate concentration amounted to 13.6 mmol/l H2Oe at the end of exercise. The concentration gradient for lactate of 2 from intra- to extracellular space favored a flux of water to the intracellular space. The relative large increase in H2Oe may then be caused by a hydrostatic rather than an osmotic factor.U