Previous reports have suggested that insulin may not regulate the breakdown of myofibrillar proteins in skeletal muscle. To further test the role of insulin, insulinopenia was produced by treating rats with streptozocin. After treatment, protein breakdown in skeletal muscle was evaluated with the isolated prefused rat hindquarter preparation. After the inhibition of protein synthesis with cycloheximide, total and myofibrillar protein breakdown were assessed by measuring the release of tyrosine and 3-methylhistidine, respectively, in the perfused hindquarters of diabetic and agematched control rats. Streptozocin-induced (65 mg/kg) diabetes (3- to 28-day duration) resulted in hyperglycemia, hypoinsulinemia, hyperphagia, increased plasma lipid levels, arrested body and muscle growth, and increased urea and 3-methylhistidine excretion. Despite this, protein breakdown in skeletal muscle diminished. The release of 3-methylhistidine by the perfused hindquarters of diabetic rats decreased, whereas the release of tyrosine remained unchanged, suggesting that the breakdown of myofibrillar proteins was affected specifically. 3-Methylhistidine (unbound) levels in skeletal muscle of unperfused diabetic rats as well as in skin decreased, whereas they increased twofold in the gastrointestinal tract. More severe diabetes (125 mg/kg streptozocin), which resulted in ketoacidosis, augmented protein breakdown in muscle; however, this response was due to a marked fall in food consumption (it was also evident when control rats were pair fed). These data reinforce previous conclusions that insulin does not play a major role in the regulation of myofibrillar protein breakdown in skeletal muscle. They also suggest that food consumption per se and/ or lipid availability may modulate myofibrillar proteolysis independent of insulin secretion. Lastly, increased 3-methylhistidine excretion in diabetic rats appeared to be of gastrointestinal origin. Because of this, caution must still be exercised in the use of 3-methylhistidine excretion as an index of myofibrillar proteolysis in skeletal muscle.

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