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 hours

The calculated rate of urea production [U-p; mmol urea/(h . kg(0.75))], bas ed 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 synthe tase (CPS-1) of 24 neonatal piglets from four treatments as follows: 6 h ba seline control (n = 8), 18 h of alanine intravenously (IV) at 50% of restin g 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 in creases 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 decreas es from baseline were seen in these values in piglets infused with glucose for 36 h (P < 0.05). Comparison of calculated urea production with calculat ed total body CPS-1 V-max at baseline, 18 or 36 h after the start of infusi on of alanine or glucose revealed a positive relationship (slope = 0.263; P < 0.002). At all enzyme activities, infusion of alanine resulted in a sign ificant 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 ure a production, suggesting that CPS-1 did not limit urea production.