Lipoprotein lipase and hormone-sensitive lipase activity and mRNA in rat adipose tissue during pregnancy. Am. J. Physiol. 266 (Endocrinol. Metab. 29): E930- E935, 1994.-To investigate the factors controlling maternal depot fat accumulation during early pregnancy and net decrease during late pregnancy, the activity and mRNA expression of adipose tissue lipoprotein lipase (LPL) and hormonesensitive lipase (HSL) were related to several other lipid metabolic parameters. Virgin control rats, pregnant rats (at days 12, 15, 19, and 21), and lactating rats (at days 5 and 10 postpartum) were studied. In adipose lumbar tissue of late pregnant rats, LPL activity decreased to about one-third that of the virgin control animals, with < 10% of initial LPL mRNA expressed as determined by Northern blots. HSL activity increased maximally 1.5-fold with a fourfold increase of HSL expression at days 12-15 of pregnancy and decreased to control levels after parturition. The HSL-to-LPL mRNA and activity ratios were enhanced from days 15 and 19 of pregnancy, respectively, and remained so even during lactation, mainly because of the marked lowering of the LPL values. This enhancement coincided with increments in plasma free fatty acids and glycerol levels indicating an increased depot fat breakdown. These results give no indication of an involvement of LPL and HSL gene expression changes in the accumulation of maternal depot during early pregnancy. In contrast, such changes could be responsible for the net breakdown of this fat depot during late gestation. Thus, during this physiological state, long-term (e.g., transcriptional) regulation of LPL and HSL gene expression could be an important mechanism for the control of adipose tissue mass breakdown during late gestation.