PHENYLALANINE METABOLISM IN SHEEP INFUSED WITH GLUCOSE PLUS INSULIN. II. EFFECTS ON IN VIVO AND IN VITRO PROTEIN SYNTHESIS AND RELATED ENERGY EXPENDITURES

Abstract

In vivo fractional rates of protein synthesis (FSR), based on both intracellular fluid (ICF) and arterial plasma specific radioactivity (SRA), were determined for the external intercostal muscle (EIC), gastrocnemius muscle, liver and kidneys of growing sheep during infusions of either saline or glucose (2 g h−1) plus insulin (1.2 U h−1; G+I). In vitro FSR and energy expenditures associated with protein synthesis (cycloheximide-sensitive respiration) and Na+, K+ transport (ouabain-sensitive respiration) were also determined in EIC muscle. In vivo FSR based on ICF SRA in muscle were not significantly different between G+I and S infused sheep (5.2 vs. 4.2% d−1 and 5.0 vs. 3.2% d−1 for EIC and gastrocnemius, respectively). In vivo FSR in the liver (54 vs. 61% d−1) and kidneys (38 vs. 55% d−1) were also not significantly different between G+I versus S infused sheep. Based on plasma SRA, FSR in all tissues were unaffected by treatments and were less (P < 0.05) than those calculated from ICF SRA. In vitro FSR and the energy expenditures associated with protein synthesis and Na+, K+ transport were not affected by G+I infusions. The average in vitro FSR in isolated EIC muscle (2.7% d−1) was 53% and 81% of the average in vivo FSR calculated from ICF and plasma SRA, respectively. Compared to data reported for nonruminants, these data suggest that rates of protein synthesis and energy expenditures associated with protein synthesis in ruminants are less influenced by insulin and glucose. Key words: Sheep, protein synthesis, insulin, glucose, Na+, K+ transport