Inter-organ unevenness and catch-up growth in rats

The recovery in body weight (catch-up growth) was studied in juvenile male and female rats either exposed to hypoxia during the first postnatal week (10% inspired O-2, Hypoxic) or with limited caloric intake (by growing in large litter size) during the first two postnatal weeks (LowCal). Compared to controls, the experimental rats had a severe stunting in body growth. The effects on the dry and wet weights of the internal organs (kidney, lungs, brain, heart, liver, thymus, skeleton) differed between Hypoxic and LowCal, but were similar between sexes. Catch-up growth during the following weeks was more apparent in females than in males, and accompanied by increased oxygen consumption. At 1 and, especially, 3 months, the organs had almost recovered their original weights, although differences in protein and DNA content (measured in brain, lungs, heart, brown adipose tissue) were still present in some cases. In general, organ recovery was more apparent in LowCal than in Hypoxic, and in females than in males. In the experimental groups the unevenness in weight among organs (coefficient of variation of the individual weights relative to control) at 10 days, 1 month and 3 months correlated with the specific growth rate of the first, second and third month. This finding supports the possibility that inter-organ unevenness may represent an additional factor controlling the rate of catch-up growth.