The oxygen-consumption rates and the activities of fumarase and beta-hydroxyacyl-CoA dehydrogenase were compared in mitochondria isolated from fetal- and neonatal-rat kidney. Whole-organ ATP, phosphocreatine and creatine contents were determined in parallel. Kidney mitochondrial respiratory rates in the presence of succinate, glutamate/malate and palmitoyl-L-carnitine increased between 21 days post coitum and 1 day post partum, together with activities of oxidative enzymes. However, this postnatal maturation of oxidative metabolism was not yet initiated in mitochondria isolated from kidney 1 h post partum. An increase in ATP and phosphocreatine was observed immediately after delivery; newborn-rat kidney ATP content then remained high, whereas phosphocreatine reserves decreased considerably between 6 h and 1 day post partum. It is concluded that the increase in high-energy phosphate compounds observed at birth is not initially related to an activation of oxidative phosphorylation, and probably involves a transient stimulation of anaerobic glycolysis, while a progressive mitochondrial maturation takes place in the rat kidney during the first day of newborn life.

The effects of cold acclimation on cellularity, lipoprotein lipase (LPL) activity and lipolysis were studied in white adipose tissue of rats fed a high fat diet. Male Osborne-Mendel rats (7 weeks old) were exposed at either 28 or 5 °C for 10 weeks. The rats were fed a semipurified diet (normal fat (NL): 5% lard, high fat (HL): 54% lard) for the last 9 weeks. Caloric intake with NL and HL diets were comparable and cold exposure led to the same increase with both diets. At 28 °C, HL diet initiated both hypertrophy and hyperplasia; however, at 5 ° C only hyperplasia was observed. Total LPL activity showed high stimulation both in 28 and 5 °C HL rats. In vitro lipolytic stimulation by norepinephrine was lowered at 5 °C and abolished at 28 °C in HL-fed rats. HL diet resulted in enhanced lipid deposition without an increase in caloric intake. Even in cold-adapted Osborne-Mendel rats a relative obesity could be produced by a HL diet.

The consequences of hyperlipidic feeding on interscapular brown adipose tissue (IBAT) activity were examined in the cold-acclimated rat. Male Osborne-Mendel rats (7 weeks old) were exposed for 10 weeks at either 28 or 5 °C. The rats were fed a semipurified diet (normal fat (NL): 5 % lard; high fat (HL): 54% lard) for the last 9 weeks. IBAT weight was greater in HL than in NL rats. This increase was entirely due to accumulation of neutral lipids. Among different IBAT components (proteins, DNA, phospholipids) no variations were observed in HL 28 °C rats. HL diet did not modify lipoprotein lipase and cytochrome oxidase activities, but an increase in purine nucleotide binding (taken as an index of thermogenic activity) was observed in HL 28 °C rats. Cold acclimation led to comparable stimulation in NL and HL rats. The calorigenic effect of norepinephrine in vivo was not modified by HL diet. Study of arteriovenous differences showed that IBAT fatty acid and glycerol release by norepinephrine was slightly increased in HL 28 °C rats. No effect of HL diet was observed at 5 ° C. These results indicate that HL feeding leads to a moderate increase in the IBAT thermogenic capacity of Osborne-Mendel rats. HL diet does not modify the normal development of nonshivering thermogenesis when rats are kept in a cold environment.