BAT thermogenic activity and capacity are reduced during lactation in ground squirrels

The effect of lactation on brown adipose tissue (BAT) thermogenic activity and capacity as well as on uncoupling protein (UCP) gene expression has been studied in the 13-lined ground squirrel. Lactating animals were studied after approximately 21 days of lactation. Parameters of thermogenesis were also studied in a group of postpregnant animals from which litters had been removed at 1-2 days postpartum. A group of animals that did not bear litters served as nonpregnant controls. BAT pad weights, protein concentration, and mitochondrial mass were all significantly decreased relative to nonpregnant controls and postpregnant animals. Specific binding of GDP and total GDP bound were significantly decreased in lactating animals relative to both nonpregnant and postpregnant animals. Scatchard analysis of GDP binding indicated that binding affinity (Kd) was unaffected by treatment. UCP concentrations were reduced by 26% during lactation, whereas total UCP content was reduced by 70%. A 3-wk period after pup removal was sufficient for UCP concentrations to return to nonpregnant levels, although total UCP content remained reduced. The changes in UCP concentrations were accompanied by parallel alterations of gene expression. UCP mRNA-to-beta-actin mRNA ratios during lactation were 30% of the levels found in nonpregnant controls and postpregnant animals. The results presented here clearly indicate that BAT activity and capacity are suppressed during lactation in the 13-lined ground squirrel.