Myofibrillar protein and collagen breakdown in mature male rats fed protein or energy deficient diets.

The urinary output of N tau-methylhistidine (3-methylhistidine: 3-Mehis) and total hydroxy proline (OH-pro) were evaluated to assess the effect of a low-protein diet (low-P) or a low-energy diet (low-E), given for 21 days, on the rates of myofibrillar protein and collagen breakdown in mature male rats. As compared to the control rats, the animals fed the deficient diets showed a significant reduction in the rate of growth and plasma insulin. No major changes were found in the weight of several muscles excised from the animals at the end of the experiment. Total OH-pro was significantly reduced in the low-P rats and 3-Mehis was unchanged when results were normalized to either muscle weight or creatinine output. In conclusion, collagen breakdown was found to be very sensitive to protein depletion in the mature rat, but myofibrillar protein breakdown was not notably affected by protein or calorie malnutrition.