Effect of Starvation, Refeeding and Hydrocortisone Administration on Turnover of Myofibrillar Proteins Estimated by Urinary Excretion of Nτ-Methylhistidine in the Rat

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Abstract
Quantitative changes in fractional catabolic and synthetic rates of the myosin-actin pool in rat muscle under starvation and refeeding, during growth or after treatment with hydrocortisone were studied by estimating urinary excretion of N.tau.-methylhistidine (3-methylhistidine; Me-His). Following deprivation of food, urinary Me-His output increased from 0.35 mg/day- 0.45 mg/day during the 1st 2 days in spite of a decreasing body Me-His pool. This high rate of Me-His excretion was maintained for the following 4 days of starvation and then decreased. When rats were refed a 20% casein diet after 10 days of starvation, Me-His excretion continued to decrease even after 3 days of refeeding. On the 5th-day of refeeding, it began to rise progressively. During starvation, fractional catabolic rate of myosin-actin was about 3.7%/day in comparison with 2.6%/day of fed rats. After refeeding, the fractional catabolic rate decreased rapidly to a minimum value of 1.7%/day on the 3rd day. After that, it reached to a value of 2.6%/day of fed rats. Fractional synthetic rate of myosin-actin dropped immediately after fasting and the low rate of about 0.4%/day was maintained during starvation period. Fractional synthetic rate recovered quickly after refeeding. Urinary output of N and creatinine rose quickly on the 1st day after administration of hydrocortisone and on the 2nd day it fell to their normal value. While Me-His excretion increased after injection of hydrocortisone up to 0.52 mg/day on the 2nd day, this high excretion rate remained until the following day. Administration of hydrocortisone to rats apparently enhances catabolism and reduces synthesis of myosin-actin. The effect of this hormone on myofibrillar protein catabolism appears to last longer than its effect on N metabolism in the whole body judged from urinary N output. Fractional rates of catabolism and synthesis of rat myosin-actin were 3.3%/day (half-life of 21 days) and 7.2%/day, respectively, at the growth stage of 129 g body wt. These rates were 2.3%/day (half-life of 30 days) and 2.8%/day, respectively, at the mature stage of 363 g body wt. The fractional synthetic rate changed far more dramatically than catabolic rate. Mass of muscle protein is apparently primarily regulated by the rate of synthesis, although the rate of catabolism should not be neglected.