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A High Protein Diet Does Not Improve Protein Synthesis in the Nonweight-Bearing Rat Tibialis Anterior Muscle^1,2,

Centre de Recherche en Nutrition Humaine et Institut National de la Recherche Agronomique, Unité d'Etude du Métabolisme Azoté, 63122 Ceyrat, France ^* Centre d'Etudes et de Recherches de Médecine Aérospatiale, Laboratoire de Physiologie Métabolique, 91228 Brétigny-sur-Orge, France

We recently demonstrated that a high protein intake partially prevented the decrease in protein synthesis in the atrophied dark soleus muscle of rats that were hindlimb suspended (HS) for 21 d. To study the possible role of protein intake in a muscle more representative of the whole musculature, we measured the effect of a high protein (HP) (30%) and a medium protein (MP) (15%) diet on protein synthesis in the pale fast-twitch tibialis anterior muscle of HS rats. The HS animals were suspended by the tail for 21 d so that only their front legs were able to rest on the floor. The fractional rate of protein synthesis (K_s) was determined in vivo using a flooding dose method. A significantly lower K_s (24–25%) was found in both HS-MP and HS-HP rats compared with their pair-fed control groups. Reduced K_s in HS-MP rats relative to their pair-fed controls resulted from a decrease in the translational efficiency (KRNA, 23%, P < 0.01), while the ratio of RNA to protein (Cs) was unaffected. In contrast, the decrease in KRNA was prevented in the HS-HP animals compared with their pair-fed controls (P < 0.05). Hindlimb suspension did not alter fiber type distribution in the tibialis anterior muscle. However, a higher proportion of intermediate and Type I fibers with a concomitant decrease in Type II fibers was observed in both CT and HS animals fed the HP diet compared with those fed the MP diet (P < 0.05). These data clearly establish that depressed protein synthesis contributes to altered protein accretion in fast-twitch muscles during long-term hindlimb suspension. Although the HP diet prevented the decrease in translational efficiency in muscles from HS rats, it neither sustained protein synthesis nor prevented the reduction in muscle growth. Thus, it seems very unlikely that a high protein diet had any beneficial effect on the overall musculature during weightlessness in rats.

KEY WORDS: • protein intake • skeletal muscle • protein synthesis • simulated weightlessness • rats

¹ This study was supported by a grant from the French Ministry of Research and Technology (project Aliment 2000 88 G0518).

² The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

³ To whom reprint requests should be addressed.

Manuscript received 12 April 1995. Revision accepted 3 October 1995.