This project will examine the hypothesis that insulin is critical for exercise-induced increases in rates of protein synthesis and skeletal muscle hypertrophy. Insulin Dependent Diabetes Mellitus causes an accelerated loss of skeletal muscle mass. This loss may be counteracted by resistance exercise which results in net protein accretion, however the role of insulin during such accretion has not been systematically studied.
Specific Aims : We will determine whether elevations in rates of protein synthesis after acute and chronic resistance exercise are due to insulin- dependent increases in peptide-chain initiation. Insulin regulates specific eukaryotic initiation factors in peptide-chain initiation (the first steps in the translation of mRNA into protein), and that regulation is more pronounced in type II skeletal muscle fibers. Insulin availability during and after exercise may be critical for increasing rates of protein synthesis after resistance exercise. We will then determine whether the ability of muscle to hypertrophy is compromised in rats rendered insulin- deficient and whether such a decrement is due to altered regulation of specific steps in peptide chain initiation. Experiments and Methods: Rats are conditioned to rise up on their hindlimbs and push a switch in response to a light cue (to avoid electric shock). Resistance is added using weight packs on a Velcro vest (weights over the scapula). Acute resistance exercise will be weight lifting sessions on 4 separate days. Chronic resistance exercise will last 3, 6 or 9 wks with progressively greater weights being added to the backpacks The effect of prevailing hypoinsulinemia on the ability to increase rates of protein synthesis after acute exercise and the ability of muscle to hypertrophy due to chronic exercise will be elucidated using 70 and 90% partial pancreatectomy of mature (250g) rats. Rates of protein synthesis will be measured by incorporation of 3H-Phenylalanine using either an in vivo tritiated phenylalanine flooding dose technique or in situ bilateral hindlimb perfusion 16 hr after acute or chronic exercise. Because the effects of insulin on peptide-chain initiation are more pronounced in fast-twitch fibers, we will compare and contrast the affects of graded insulinemia on gastrocnemius (predominantly Type II), soleus (predominantly Type I), and extensor digitorum longus (EDL, predominantly Type II, but not extensively recruited during the exercise). Changes in ribosomal aggregation and the ability to form the 435 preinitiation complex in individual muscles will be used as markers of alterations in peptide-chain initiation. Mechanisms of the proposed involvement of peptide chain initiation will be evaluated by measuring the expression of eukaryotic initiation factor 2B (eIF-2B) and eIF-4E, the activity of eIF- 2B and phosphorylation state of eIF-4E. Significance: The proposed studies will be the first to systematically assess a role for insulin in protein synthesis during periods of net protein accretion. We predict a critical and fiber-specific role for insulin in mediating exercise-induced hypertrophy.
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