A HIGH-PROTEIN DIET DOES NOT IMPROVE PROTEIN-SYNTHESIS IN THE NON-WEIGHTBEARING RAT TIBIALIS ANTERIOR MUSCLE

We recently demonstrated that a high protein intake partially prevente d the decrease in protein synthesis in the atrophied dark soleus muscl e of rats that were hindlimb suspended (HS) for 21 d. To study the pos sible role of protein intake in a muscle more representative of the wh ole musculature, we measured the effect of a high protein (HP) (30%) a nd a medium protein (MP) (15%) diet on protein synthesis in the pale f ast-twitch tibialis anterior muscle of HS rats, The HS animals were su spended by the tail for 21 d so that only their front legs were able t o rest on the floor. The fractional rate of protein synthesis (K-s) wa s determined in vivo using a flooding dose method. A significantly low er K-s (24-25%) was found in both HS-MP and HS-HP rats compared with t heir pair-fed control groups. Reduced K-s in HS-MP rats relative to th eir pair-fed controls resulted from a decrease in the translational ef ficiency (KRNA, 23%, P < 0.01), while the ratio of RNA to protein (Cs) was unaffected. In contrast, the decrease in KRNA was prevented in th e HS-HP animals compared with their pair-fed controls (P < 0.05). Hind limb 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 i n both CT and HS animals fed the HP diet compared with those fed the M P diet (P < 0.05). These data clearly establish that depressed protein synthesis contributes to altered protein accretion in fast-twitch mus cles during long-term hindlimb suspension. Although the HP diet preven ted 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 h ad any beneficial effect on the overall musculature during weightlessn ess in rats.