The rate of myofibrillar protein degradation was measured by 3-methylhistidine (3MH) release in moderately uremic and sham-operated control rats. The rats were studied in the fed state or after 24 and 48 hours of fasting. When fed, both uremic and control rats gained weight at the same rate. During 48 hours of fasting, the uremic animals lost more weight (17.1%) than the shams (13.2%). The ratio of gastrocnemius muscle protein per gram of wet weight was not significantly different under any conditions. When fed and 48-hour fasted animals are compared, urinary excretion of 3MH rose from 0.81 to 1.32 moles per 24 hr per 100 g of initial body wt in shams and from 0.90 to 1.30 moles per 24 hr per 100 g of initial body wt in the uremics. When the ratio of 3MH to creatinine excretion in the urine was compared, there was no change in 3MH excretion between 0 and 24 hours, but there was an increase between 24 and 48 hours. Analysis of serum and muscle samples from red and 48-hour fasted animals revealed that the concentrations of 3MH increased in both groups during fasting, with the uremic rats having a larger increase in the total body 3MH pool. Myofibrillar degradation rates calculated from the sum of urinary excretion plus the changes in body 3MH pool size revealed that with moderate fasting, degradation rates increased in both sham and uremic rats with a larger increase being seen in the uremic group. After 48 hours of fasting, the increased amount of 3MH released from muscle of uremic rats was comparable to their larger percentage weight loss.
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