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Article

Comparison of Total Body Urea Production Potential with Total Body Carbamoyl Phosphate Synthetase (CPS-1) Activity in Newborn Piglets Infused with Alanine at 50% of Resting Energy Expenditure for 36 Hours¹

Julie A. Davis^*, Frank R^*,** and Norlin J. Benevenga^*,2

Departments of ^* Nutritional Sciences, Animal Sciences and ^** Pediatrics, University of Wisconsin-Madison, WI 53706.

²To whom correspondence should be addressed.

The calculated rate of urea production [U_p; mmol urea/(h · kg^0.75)], based on urinary urea-N (UUN) excretion and changes in total body urea-N, was compared with the calculated total body V_max of carbamoyl phosphate synthetase (CPS-1) of 24 neonatal piglets from four treatments as follows: 6 h baseline control (n = 8), 18 h of alanine intravenously (IV) at 50% of resting energy expenditure (REE; n = 4), 36 h of alanine IV at 50% of REE (n = 6), or 36 h of glucose IV at 50% of REE (n = 6). The following significant increases from baseline were seen in piglets infused with alanine for 36 h: 1) UUN excretion [10.6 ± 5.9 mg N/(h · kg^0.75) to 53.2 ± 11.1]; 2) BUN concentrations (9.1 ± 3.0 mmol urea N/L to 51.2 ± 7.0); 3) calculated urea production [0.34 ± 0.21 mmol urea/(h · kg^0.75) to 2.39 ± 0.53]; and 4) CPS-1 V_max [2.0 ± 0.81 mmol citrulline/(h · kg ^0.75) to 4.4 ± 1.5], (P < 0.05). With the exception of CPS-1 activity, significant decreases from baseline were seen in these values in piglets infused with glucose for 36 h (P < 0.05). Comparison of calculated urea production with calculated total body CPS-1 V_max at baseline, 18 or 36 h after the start of infusion of alanine or glucose revealed a positive relationship (slope = 0.263; P < 0.002). At all enzyme activities, infusion of alanine resulted in a significant increase in the rate of urea production compared with controls (P < 0.001). Total body CPS-1 activity varied from 1.8 to 5.8 times that of urea production, suggesting that CPS-1 did not limit urea production.

KEY WORDS: • piglets • neonate • urea production • alanine • carbamoyl phosphate synthetase (CPS-1)

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