Abstract
1. 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_??_-methylhistidine (3-methyl-bistidine; Me-His).
2. Following deprivation of food, urinary Me-His output increased from 0.35mg/day to 0.45mg/day during first 2 day in spite of 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 con-tinued to decrease even after 3 days of refeeding. On the fifth 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 third day. After that, it reached to a value of 2.6%/day of fed rats. On the other hand, 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.
3. Urinary output of nitrogen and creatinine rose quickly on the first day after ad-ministration of hydrocortisone and on the second day it fell to their normal value. While Me-His excretion increased after injection of hydrocortisone up to 0.52mg/day on the second day and this high excretion rate remained until the following day. From these results, it was shown that administration of hydrocortisone to rats enhances catabolism and reduces synthesis of myosin-actin. The results also show that the effect of this hormone on myofibrillar protein catabolism appears to last longer than its effect on nitrogen metabolism in the whole body judged from urinary nitrogen output.
4. 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 129g body weight. These rates were 2.3%/day (half-life of 30 days) and 2.8%/day, respectively, at the mature stage of 363g body weight.
5. Under the dietary conditions in this experiment, fractional synthetic rate changed far more dramatically than catabolic rate. This suggests that mass of muscle protein is primarily regulated by the rate of synthesis, although the rate of catabolism should not be neglected.
