Perinatal energy stores and excessive fat deposition in genetically obese (fa/fa) rats.

To find out whether perinatal differences in energy stores are essential for the phenotypic manifestation of the fa/fa genotype in 16-day-old rats, we investigated Zucker rats (13M) x Brown Norway (BN) hybrids. +/fa and fa/fa hybrids were identified by molecular polymorphisms approximately 1 cM from the fa locus. The fat content of all pups increased rapidly after birth, and differences between +/fa and fa/fa hybrid pups were significant in 7- and 16-day-old rats but not at 0, 1, and 4 days of age. For 16-day-old 13MBN hybrids as well as 13M Zucker rats, plots of carcass fat vs. body mass were closely fitted by parallel regression lines, with carcass fat similarily higher in fa/fa than +/fa pups of both strains. Fetal and neonatal plasma insulin concentration and hepatic glycogen in the 13MBN hybrid pups did not differ between +/fa and fa/fa litter-mates, nor did the hepatic and adipose tissue triglycerides in 24-h-old 13MBN hybrid pups. The manifest differences between fatty and lean 16-day-old pups thus do not depend on differences in perinatal energy stores.