Liver arginase activity and plasma urea-nitrogen in steers fed diets containing different levels of protein.

Seventeen Charolais-Hereford crossbred steers averaging 220.0 kg in weight initially were randomly allotted to three treatments and fed diets containing (A) 9%, (B) 11%, or (C) 13% protein ad libitum in individual pens for 238 days. Liver samples were taken by aspiration biopsy on days 47 (sampling I) and 238 (sampling II) for arginase assay. Blood samples were taken by jugular puncture at these same times for plasma urea-nitrogen analysis. Mean hepatic arginase activities (mumole urea/mg protein/hr) of steers fed the three diets at sampling I were: (A) 224.8, (B) 327.8 and (C) 333.3. The activities of B and C were significantly (P less than .05) higher than A. Plasma urea-N levels at this same time averaged: (A) 4.44, (B) 8.55 and (C) 12.22 mg/100 ml, and were significantly different (P less than .01) from each other. Arginase activities at sampling II were: (A) 240.6, (B) 305.2 and (C) 353.0. A differed significantly (P less than .05) from C. Plasma urea-N at this time averaged: (A) 8.69, (B) 10.72 and (C) 15.00 mg/100 ml, and C was significantly (P less than 5.0) higher than A and B. These data suggest that in the bovine hepatic arginase activity levels increase with increases in dietary protein and that similar patterns in activity are maintained after feeding for an extended period of time. The increased arginase activity was accompanied by increased plasma urea-nitrogen.

Urea cycle enzyme activity in lambs fed different nitrogen levels at low dietary intakes.

Eleven Southdown male lambs averaging 19.8 kg were randomly allotted to two groups and fed diets containing 7.7% (low-N) or 15.8% (high-N) crude protein. All of the supplemental nitrogen in the high-N diet was supplied as urea. Intake of the low-N and high-N diets averaged 372.3 g and 340.5 g/day, respectively. Findings at the end of the thirty-day trial were: (1) mean body weights unchanged for the two groups; (2) plasma urea nitrogen three-fold higher in the high-N (19.07 mg/100 ml) than the low-N (6.57) animals; (3) similar hepatic activity levels of three urea cycle enzymes (ornithine transcarbamylase, argininosuccinase, arginase) in the two groups, and (4) similar liver weights and liver protein concentration. The absence of adaptive change in enzyme levels suggests the hypothesis that addition of non-protein nitrogen to maintenance diets may cause ammonia intoxication by exceeding the liver's reserve capacity for urea synthesis.