title: Rates of lysine catabolism are inversely related to rates of protein synthesis when measured concurrently in adult female rats induced to grow at different rates.
abstract: To test the effect of changes in the rate of protein synthesis on amino acid oxidation, both were studied concurrently in individual 200-g female Sprague-Dawley rats. In a growth trial (Experiment 1), recombinant bovine somatotropin (rbST) was injected subcutaneously (0, 2 or 12 mg/d) over 6 d (n = 4 rats per rbST level). Weight gain increased with rbST level (P < 0.01); 1.96 +/- 0.8, 4.24 +/- 0.8 and 8.67 +/- 0.8 g/d, respectively. After treatment with rbST (0 or 12 mg/d) for 4 d (Experiment 2), rats were injected via a tail vein catheter with valine (400 mmol, 4.07 mBq L-[3,4(n)-3H]valine) at 0, 4, 10, 13 or 16 h after the daily rbST injection and killed 20 min later. This flooding dose was 5 to 6 times, not 10 times, the free pool as hoped. Protein synthesis in rbST-treated rats increased 46% in muscle (P < 0.001) and 36% in liver (P < 0.01). The ks was unaltered with time after rbST injection (0-16 h, P > 0.05). When 600 mmol valine (4.4 mBq L-[3,4(n)-3H]valine) was used in Experiment 3, specific activity (SA) of free valine was constant over 20 min and was 94 +/- 4% of that injected. Finally, in Experiment 4, protein synthesis and amino acid oxidation rates measured in the same rat revealed a 35% increase (P < 0.01) in protein synthesis in hind leg muscle and a 29% increase in liver (P < 0.05) from rbST-injected (12 mg/d) rats (n = 6). Lysine oxidation was estimated by continuous (12 h) infusion of L-[1-14C]lysine via the opposite tail vein catheter. Expired CO2 was collected over 20-min intervals and SA at plateau was estimated by fitting an exponential model. Lysine oxidation was reduced (P < 0.05) by 44% in rbST-treated rats. The idea that an increase in protein synthesis results in decreased amino acid oxidation remains tenable.
authors.affiliation: Department of Nutritional Sciences, University of Wisconsin-Madison, Madison, WI, USA.
authors.forename: M J
authors.initials: MJ
authors.lastname: Gahl
authors.forename: N J
authors.initials: NJ
authors.lastname: Benevenga
authors.forename: T D
authors.initials: TD
authors.lastname: Crenshaw
mesh_terms: D000818:Animals; D001835:Body Weight / Q000187:drug effects; D001944:Breath Tests; D002245:Carbon Dioxide / Q000032:analysis; D002940:Circadian Rhythm; D005260:Female; D006128:Growth / Q000187:drug effects / Q000502:physiology*; D013006:Growth Hormone; D007700:Kinetics; D008099:Liver / Q000378:metabolism; D008239:Lysine / Q000378:metabolism*; D008715:Methionine / Q000378:metabolism; D009132:Muscles / Q000378:metabolism; D010084:Oxidation-Reduction; D014176:Protein Biosynthesis; D051381:Rats; D017207:Rats, Sprague-Dawley; D014633:Valine / Q000378:metabolism
chemical_list: D002245:Carbon Dioxide; D013006:Growth Hormone; D008715:Methionine; D014633:Valine; D008239:Lysine