Myosin, sarcoplasmic and mitochondrial protein synthesis rates in rats adapted to a high protein diet

Abstract

High protein diets have been shown to produce either no effect or a decrease in mixed muscle protein synthesis rates in rats and humans. However, no data regarding their effect on specific muscle protein fractions is available, despite their possible different metabolic regulation. In this context, we aimed at determining the fractional synthesis rates (FSR) of myosin, sarcoplasmic and mitochondrial protein fractions in gastrocnemius muscle of growing rats adapted to a high protein diet (HP, 49% of energy, n=16) for 14d or a normal protein diet (NP, 13% of energy, n=16). After the 14-d diets, rats were killed in the fasted or the fed state after a flooding dose of 13C-valine. NP and HP rats exhibited the same body weight, gastrocnemius mass, hydration and protein content. FSR and absolute synthesis rates of mixed muscle protein were neither significantly influenced by diet (NP or HP) nor by nutritional state (fasted or fed). There was also no effect of diet or nutritional state on the FSR of myosin fraction, sarcoplasmic or mitochondrial protein, which all ranged from 6 to 9%/d. Unlike splanchnic protein, which synthesis rates are greatly sensitive to an increased chronic protein intake, muscle protein fractions are not affected by dietary changes. This finding is in agreement with the lack of lean tissue anabolism usually accompanying high protein diets and suggest a very efficient system redistributing or oxidizing excess amino acids.