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

Manuscript received 2 September 1997. Initial reviews completed 18 November 1997. Revision accepted 16 May 1998.

Mark J. Gahl^{*, ,} , N. J. Benevenga^{*, ,} , and Thomas D. Crenshaw

Departments of ^* Nutritional Sciences and  Animal Sciences, University of Wisconsin-Madison, Madison, WI

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 k_s 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.

The Journal of Nutrition Vol. 128 No. 9 September 1998, pp. 1503-1511
Copyright ©1998 by the American Society for Nutritional Sciences

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