BERGSTR~, J., P. F~~ RsT, L.-O. NORSE, AND E. VINNARS. Intracellular free amino acid concentration in human muscle tissue. J. Appl. Physiol. 36 (6): 693-697. 1974.-Twenty-one healthy subjects were studied after an overnight fast. Muscle tissue obtained by needle biopsy from m. quadriceps femoris was homogenized and precipitated with 4 y0 sulfosalicylic acid. The supernatant material was analyzed for 23-28 free amino acids by a modified Moore-Stein technique. Simultaneously obtained plasma samples were precipitated with 6% sulfosalicylic acid and in all other respects were treated in the same way. Extracellular water in the muscle was estimated using a modified chloride method. The extra cellular (EC) and intracellular (IC) concentration and the IC/EC gradient for each amino acid was calculated. The majority of the amino acids showed much higher concentration in intracellular water than in plasma. The concentration gradient was specially high for taurine, glutamic acid, and glutamine. Valine, leucine, isoleucine, phenylalanine, citrulline, and tyrosine had a concentration gradient below 2.0. The rest of the amino acids showed gradients between 5.0 and 10.0. The free amino acid pool in skeletal muscle tissue was calculated in a normal man weighing 70 kg to be 86.5 g without taurine and 121.5 g with taurine. Of the total pool of muscle free amino acids the eight essential amino acids represent only 8.4 y0, whereas glutamine, glutamic acid, and alanine constitute about 79%. free amino acid pools

HERBERT ET AL.(16) observed in animal experiments that the concentration of several free amino acids is considerably higher in the cells than in the extracellular fluid. They also found the largest pool of free amino acids in skeletal muscle tissue. Higher free amino acid concentrations in muscle tissue than in plasma were also found in man by Zachman et al.(26). However, major differences seem to exist between different species (12, 15, 16, 26). Evidently, it is not possible to draw any conclusions concerning the total pool of free amino acids in the body by only determining their concentration in plasma. By analysis of the intracellular free amino acid concentration in muscle tissue obtained from human subjects it should be possible to extend our knowledge of amino acid metabolism under different conditions in man. We have therefore developed methods by which intracellular free amino acids can be determined in muscle biopsy material. A description of the methods and results obtained in normal subjects are given in the present paper. This investigation