LINKAGE BETWEEN POSTABSORPTIVE AMINO-ACID RELEASE AND GLUTAMATE UPTAKE IN SKELETAL-MUSCLE TISSUE OF HEALTHY-YOUNG SUBJECTS, CANCER-PATIENTS, AND THE ELDERLY

Several diseases of varying etiology that are commonly associated with the loss of skeletal muscle mass were found to be associated with a d ecrease in muscular glutamate and glutathione levels and in glutamate uptake in the postabsorptive state. In view of the Na+ dependency and insulin responsiveness of glutamate transport we studied the postabsor ptive glutamate exchange in more detail. Our study demonstrates a link age between glutamate uptake and the export of other amino acids, sugg esting that protein catabolism and the resulting coexport of amino aci ds plus Na+ substitute for insulin as a driving force for the Na+ grad ient in the postabsorptive state. The regression function of the corre lation between relative glutamate exchange and cumulative amino acid e xchange in cancer patients was lower than that in non-tumor-bearing su bjects, suggesting that cancer patients must release more amino acids to achieve the same glutamate uptake. In addition, cancer patients had a lower average cumulative amino acid exchange rate than non-tumor-be aring subjects, suggesting that the abnormally low relative glutamate exchange capacity of cancer patients results mainly from inadequate po stabsorptive protein catabolism in the skeletal muscle tissue. Both ca ncer patients and non-tumor-bearing elderly subjects had higher arteri al glutamate levels and alanine release than young subjects, indicativ e of a substantial glycolytic activity in the skeletal muscle. However , elderly non-tumor-bearing subjects showed, in contrast to cancer pat ients, in the postabsorptive state a stronger cumulative amino acid re lease and postabsorptive glutamate uptake than healthy young subjects. These changes are discussed in view of the age-related loss of skelet al muscle mass.