RELATIONSHIP BETWEEN TISSUE MOBILIZATION AND STORAGE IN THE RAT

The amount of energy mobilized or stored as protein, expressed as a pr oportion of the total energy stored or mobilized (defined as the P rat io, Payne & Dugdale, 1977), was investigated in the young male (rapid growth) and adult female (slow growth) rat. Energy mobilization was in duced by a 3 d fast and the changes in body content of fat and lean ti ssues were used to estimate the fasting P ratio (P-fast). Tissue stora ge was subsequently effected by 17 d of refeeding and the correspondin g ratio (P-refed) was calculated from the amounts of lean and fat tiss ue regained. The same experimental protocol was used for measuring P-f ast and P-refed in weanling (30 d) and adult (130 d) rats. Weight-matc hed individuals were assigned to three groups. All animals in group 1 were killed at the beginning of the experiment. Animals in group 2 wer e fasted for 30 and then killed. Group 3 animals were first fasted for 3 d, then had free access to a stock diet for a further 17 d before b eing killed. During fasting, a close linear relationship was found bet ween weight change and body energy changes (r 0.985, and r 0.92, P < 0 .001 for weanlings and adult rats respectively) and between carcass N loss and urinary N loss (r 0.933, P < 0.001). These relationships were used to estimate the body energy and N content of each animal at the end of the initial fasting period. Hence, both P-fast and P-refed valu es could be calculated for all individuals at both ages. When P-fast a nd P-refed were calculated for adult rats, the mean values were simila r (0.138 (SE 0.002) and 0.130 (SE 0.006) respectively). Individually, the P-fast and P-refed values were correlated (r 0.54, P = 0.058). In weanling rats, the mean P-refed value was about 2.5 times the P-fast ( 0.421 (SE 0.0113) v. 0.156 (SE 0.004)). Despite the differences in P-f ast and P-refed, the individual values were again correlated (r 0.668, P < 0.05). These results support the suggestion made by Payne & Dugda le (1977), that particular P-ratio values are characteristic of indivi duals and describe the extent to which protein is mobilized or stored during fasting or refeeding. These observations are discussed in relat ion to the predictions made by the Payne-Dugdale model.