DIFFERENT PHYSIOLOGICAL TRAITS UNDERLYING INCREASED BODY-FAT OF FATTY(FA FA) AND HETEROZYGOUS (+/FA) RATS/

To find out whether the most characteristic physiological traits disti nguishing suckling-age fa/fa pups from lean littermates also differ be tween +/+ and +/fa littermates, we analyzed the body composition and c old defense of 7- and 16-day-old pups and the plasma concentrations of insulin, glucose, triglycerides, and free fatty acids in 16-day-old p ups. Zucker rat x Brown Norway hybrid pups were genotyped by using a m olecular marker within 0.5 cM of the fa gene. At both ages the +/fa pu ps had significantly more body fat than their +/+ Littermates. At 7 da ys this difference was as large as that between +/fa and fa/fa pups, b ut at 16 days it was only one-seventh of the fa/fa vs. +/fa difference . In contrast, there were no heterozygote differences for three parame ters that show crucial abnormalities in the fa/fa pups: thermoregulato ry thermogenesis and plasma concentrations of insulin and triglyceride s. The physiological mechanisms underlying the increased fat content o f +/fa pups thus differ from those known to fuel most of the excessive fat deposition of their fa/fa littermates.