The effects of tumor on protein metabolism in F344 male rats in three different nutritional states (baseline, fasting, and refeeding) were studied. Nitrogen intake and output were measured and [15N]glycine was infused intravenously. Urine was analyzed for [15N]urea enrichment to measure whole body protein turnover, and 15N tissue enrichment was analyzed to determine tissue fractional synthesis rates (FSR). Urinary 3-methylhistidine was measured as an indicator of muscle catabolism. Nitrogen balance data were similar in control (C) and tumor-bearing (TB) animals. Whole-body protein turnover was increased in TB animals. Liver fractional synthesis rates were increased in TB animals. Muscle FSR were decreased in TB animals. Urinary 3-methylhistidine was increased in TB animals. Tumor FSR remained the same despite different nutritional states. These findings indicate that TB protein cachexia begins insidiously. Despite normal nitrogen balance, the TB animal has increased protein turnover and liver FSR. Tumor tissue protein synthesis continues undisturbed, while muscle protein synthesis is reduced and muscle catabolism is increased.
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