Thermal and respiratory control in young rats with altered caloric intake during postnatal development.

We asked to what extent differences in caloric intake during the first postnatal weeks may modify thermal and respiratory control of 1-month old rats. Large-size (Large) and small-size (Small) rats were obtained by raising rats in, respectively, small (6 pups) and large (16 pups) litters. In Small, the rate of oxygen consumption (V(O(2))/kg) was less than in Large during the first 2-3 weeks, and higher thereafter, when the thermogenic needs to maintain body temperature (Tb) increased. At day 31, when body weight in Small was approximately 80% of Large, Small maintained Tb in the cold with higher V(O(2))/kg than Large. The total uncoupling protein of the brown adipose tissue was unchanged. Also pulmonary ventilation (VE/kg) was higher in Small, maintaining the proportionality with V(O(2)). Lung weight in Small was reduced in proportion to body weight, with higher protein-DNA ratio. The compliances of the respiratory system and lungs, normalized by body weight, and the hyperventilatory responses to hypoxia or hypercapnia, expressed as % increase in VE/V(O(2)), were similar in Small and Large. Differences between Small and Large were reduced or no longer present in a group of Small rats raised until their body weight was as in Large. We conclude that rather important developmental differences in caloric intake and metabolic level, in otherwise healthy rats, had no long-term carry over effects in the developmental processes of respiratory and thermal control, other than the effects strictly attributable to the alterations in body size. Copyright 2002 Elsevier Science B.V.