Effects of insulin, amino acids and fasting on myofibrillar protein degradation in perfused hindquarters of rats.

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Abstract
The rate of 3-methylhistidine (3-MH) release from rat perfused hindquarters was measured to investigate the effects of insulin, amino acids and fasting on myofibrillar protein degradation. Since release of 3-MH into the perfusate increased linearly and the pool of free 3-MH in the perfused muscle did not change significantly during 2 h of perfusion, it was concluded that 3-MH release reflected the rate of myofibrillar protein degradation. Tyrosine release in the presence of cycloheximide represented the degradation rate of total muscle protein. Insulin suppressed the net release of tyrosine in normal rats, but did not affect the rate of release of 3-MH and tyrosine in the presence of cycloheximide. 3-MH release was not influenced by perfusate amino acid concentrations at zero to 5 times the normal plasma levels. When rats were fasted for one and two days, 3-MH release increased 1.7 and 2.6 times, respectively, compared with the fed rats, which showed that the rate of degradation of myofibrillar protein in skeletal muscle rose just after the beginning of fasting.

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