Effect of an adequate-protein diet after a low-protein diet on protein catabolism in growing rats.

I . Metabolic alterations related to protein catabolism were studied in rats in transitional states induced by changing from a low-protein (LP) (so g caseinlkg) diet to a high-protein (HP) (250 g casein/kg) diet. 2. Twenty-four hours after the diet was changed, the rats showed a more rapid increase in live-weight gain than controls that had been fed on the HP diet throughout. On the 5th day after the diet change, their increase in body-weight had returned almost to the control rate. Food and therefore nitrogen intakes on the 1st and j t h days after the change in diet were the same as those of the controls. It seems likely therefore that the initial high rate of live-weight gain is an indication of a metabolic adaptation which occurred on the LP diet and which did not fully return to normal until the 5th day after the change of diet, 3 . N balance was higher 24 h after the change in diet than in the controls, owing to a reduction in total urinary N and in urea excretion, but when measured on the 5th day it was similar in both groups. 4. Carcass N determination showed that, after 7 d on the LP diet total-, trichloroacetic acid (TCA)-solubleand TCA-insoluble-N contents (in terms of mg/g rat) were all slightly lower than control values but they had almost returncd to normal 5 d after the diet change. There was a significant increase in the TCA-soluble-N content after 24 h on the HP diet to a value greater than the control value. j . Proteolysis was measured in vitro by incubation of liver slices and diaphragms under anaerobic conditions. Withliver slices it was significantlylower 24 hafterthe diet change than in control rats. On the j t h day it was significantly higher than 24 h after the diet change but had not quite reached the control level. In the diaphragm, proteolysis was also lower I d after the diet change, and had not increased a t all by the 5th day. 6 . Ureogenesis in the liver was reduced significantly 24 h after the diet change and it had almost returned to the control level on the 5th day. On the other hand, arginase (L-arginine amidinohydrolase ; EC 3 . 5 . 3 . I ) activity was significantly lower 24 h after the diet change and did return completely to the control level on the j t h day. 7. These results show that the initial increased N balance and reduccd N cxcrction were due to enzymic adaptation to the LP diet, the reduced N excretion being attributable to reduction in hepatic urea production.