Apoptosis of Muscle Cells Causes Weight Loss Prior to Impairment of DNA Synthesis in Tumor‐bearing Rabbits

The mechanism of weight loss induced by the growth of malignant tumors is still unknown. We investigated it by focusing on apoptosis of skeletal muscle. VX2‐tumor was implanted into rabbits and the apoptotic index (AI) of skeletal muscle was measured by in situ end‐labeling assay. Plasma of the tumor‐bearing rabbits was perfused repeatedly through non‐coated charcoal resin. The AI reached 54.6% early after tumor implantation, when weight loss amounted to an 18% decrease in lean body mass (LBM) without change in muscle DNA synthesis or urinary 3‐methylhistidine/ creatinine ratio (3‐MH/Cr). When the decrease of LBM reached 30%, DNA synthesis was decreased by 48% and 3‐MH/Cr was increased by 104%, whereas AI was only 4.7%. The plasma perfusion did not prevent apoptosis in muscle, but unproved LBM, DNA synthesis, and 3‐MH/Cr. There may be two mechanisms of muscle depletion during the tumor growth: apoptosis in the early stage and metabolic abnormalities in muscle in the late stage.