Examining the Influence of Muscle Fiber Type on Protein Turnover Signaling in Growing Pigs

• Main Author: Seymour, Kacie Tinnesz
• Other Authors: Animal and Poultry Sciences
• Format: Others
• Published: Virginia Tech 2020
• Subjects: pig; muscle fiber type; skeletal muscle; protein synthesis; protein degradation
• Online Access: http://hdl.handle.net/10919/98592

Postnatal skeletal muscle growth occurs through myonuclear accretion and high protein turnover rate. While fiber type composition of the muscle could affect protein turnover rate, less is known about how fiber type influences the regulation of protein synthesis and degradation signaling pathways. Thus, the hypothesis of this work was that variation in fiber type composition will differentially affect the regulation of signaling pathways related to protein turnover in skeletal muscle hypertrophy in growing pigs. Downregulated protein synthesis signaling and reduced expression of type II MyHC isoforms have been reported in skeletal muscles of low birth weight (LBWT) neonatal pigs. Therefore, we sought to determine whether these changes are sustained until weaning and would explain the reduction in LBWT pig growth compared to their normal birth weight (NBWT) sibling at weaning. Another objective was to determine whether the regulation of protein turnover signaling pathways are correlated to fiber type differences in skeletal muscles. Our data suggest that the longissimus dorsi (LD, glycolytic) muscle of LBWT pigs experienced compensatory growth while the soleus (oxidative) remained proportionally smaller. Growth of the LD was accompanied by upregulation of translation initiation. Additionally, there was no difference in expression of MyHC isoforms between NBWT and LBWT pigs. These data suggest the rapid growth of the LD of LBWT pigs may be attributed to an upregulation of protein synthesis signaling and occurred only in glycolytic muscles. A caveat in LBWT pig model is that the reduction in type II MyHC at birth is not the only factor that could influence muscle growth, and that other factors may have confounded our results. This is why we aimed to use β-adrenergic agonist as a means to induce a shift fiber type in muscles to a more glycolytic phenotype. Our objective was to determine the influence of the β-adrenergic agonist Ractopamine (RAC) induced slow-to-fast fiber type transformation on the regulation of protein synthesis and degradation pathways. Although supplementation improved translational capacity, enhanced S6K1 phosphorylation, and reduced the abundance of calcium-dependent proteases, RAC feeding had no effect on body or muscle weights. These results suggest that a fiber type transformation without other physiological influences does not alter protein turnover signaling in favor of hypertrophy in growing pigs. === Master of Science === Skeletal muscles grow by increasing the amount of protein contained within them. The amount of protein deposited is determined by the net balance between the rates at which proteins are synthesized and degraded. However, not all skeletal muscles grow at the same rate. One factor that is thought to influence protein synthesis and degradation rates is the types of muscle fibers that are present within a muscle. These fibers can display a range of contractile and metabolic characteristics, from slow-twitch oxidative fibers to fast-twitch glycolytic fibers. In the presented studies, we sought to determine whether changes in fiber type composition result in difference to the signaling pathways the regulate protein synthesis and degradation, ultimately leading to differences in the muscle growth of young pigs. We have previously shown reduced activation of the protein synthesis pathway in the skeletal muscle of low birth weight (LBWT) newborn pigs. These pigs also had lower expression of glycolytic fibers. In experiment 1, we aimed to compare the signaling pathways regulating protein synthesis and degradation in LBWT and normal birth weight (NBWT) pigs at weaning. We also sought to determine if the regulation of these signaling pathways changed between muscles with differing fiber type compositions. The glycolytic longissimus dorsi (LD) muscle of LBWT pigs grew rapidly between birth and weaning whereas the highly oxidative soleus did not. In addition, the LD of LBWT pigs had greater protein synthesis signaling and similar expression of muscle fibers compared with NBWT pigs, suggesting the improvement in protein synthesis signaling of LBWT pigs between birth and weaning may be related to a shift in fiber type. In experiment 2, we used a compound called ractopamine hydrochloride (RAC) to promote a slow-to-fast fiber type switch in the muscle of young pigs. With this study, we sought to determine the effect of this fiber type transformation, without the influence of birth weight, on the regulation of protein synthesis and degradation pathways. Although RAC-fed pigs showed some minor changes that could improve protein synthesis and decrease protein degradation, RAC feeding had no observable effect on body weight or muscle growth. These results suggest that a fiber type transformation alone is not enough to promote muscle growth in growing pigs.