During four days of fasting in rats skeletal muscle protein synthesis fell pro-gressively, whereas skeletal muscle protein breakdown was unchanged until the third and fourth days when it rose dramatically. In contrast, the synthetic rate of smooth muscle protein was unchanged during three days of fasting despite a loss of protein content, indicating an abrupt rise in protein breakdown in this tissue on the first day of fasting which was sustained thereafter. Urinary excretion of Nτ-methylhistidine was significantly increased throughout fasting. The concentration of free Nτ-methylhistidine in plasma and in muscle tissue was elevated throughout the period of fasting. This elevation was not caused by reduced renal clearance, but appears to have been mainly the result of increased breakdown of Nτ-methylhistidine-containing proteins in tissues other than skeletal muscle.

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