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This Article Full Text Full Text (PDF) All Versions of this Article: 295/1/E187    most recent 90253.2008v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Wilson, F. A. Articles by Davis, T. A. PubMed PubMed Citation Articles by Wilson, F. A. Articles by Davis, T. A.

Stimulation of muscle protein synthesis by somatotropin in pigs is independent of the somatotropin-induced increase in circulating insulin

United States Department of Agriculture/Agricultural Research Service Children's Nutrition Research Center, and Department of Pediatrics, Baylor College of Medicine, Houston, Texas

Submitted 25 February 2008 ; accepted in final form 2 May 2008

Chronic treatment of growing pigs with porcine somatotropin (pST) promotes protein synthesis and doubles postprandial levels of insulin, a hormone that stimulates translation initiation. This study aimed to determine whether the pST-induced increase in skeletal muscle protein synthesis was mediated through an insulin-induced stimulation of translation initiation. After 7–10 days of pST (150 µg·kg^–1·day^–1) or control saline treatment, pancreatic glucose-amino acid clamps were performed in overnight-fasted pigs to reproduce 1) fasted (5 µU/ml), 2) fed control (25 µU/ml), and 3) fed pST-treated (50 µU/ml) insulin levels while glucose and amino acids were maintained at baseline fasting levels. Fractional protein synthesis rates and indexes of translation initiation were examined in skeletal muscle. Effectiveness of pST treatment was confirmed by reduced urea nitrogen and elevated insulin-like growth factor I levels in plasma. Skeletal muscle protein synthesis was independently increased by both insulin and pST. Insulin increased the phosphorylation of protein kinase B and the downstream effectors of the mammalian target of rapamycin, ribosomal protein S6 kinase, and eukaryotic initiation factor (eIF)4E-binding protein-1 (4E-BP1). Furthermore, insulin reduced inactive 4E-BP1·eIF4E complex association and increased active eIF4E·eIF4G complex formation, indicating enhanced eIF4F complex assembly. However, pST treatment did not alter translation initiation factor activation. We conclude that the pST-induced stimulation of skeletal muscle protein synthesis in growing pigs is independent of the insulin-associated activation of translation initiation.

translation initiation; mammalian target of rapamycin; growth hormone; eukaryotic initiation factor 4G; protein kinase B