Effects of Female Sex Steroids in Thermogenic Tissues

Abstract

The great heat generating power of brown adipose tissue (BAT) has a profound influence in energy balance. Cold-acclimated animals become hyperphagic to sustain the high substrate combustion in brown fat depots without reducing body energy stores. On the other hand, the low activity of brown fat in obese animals contributes to the low energy expenditure and the development of obesity. Reproduction is one of the major energy challenges in the mammalian life cycle. On viewing the profound influence of BAT in energy balance, it seems not surprising that brown adipose tissue is suppressed during late pregnancy and lactation as a way to save substrates for growth of mother and litter (1,13). Female sex steroids, 17ß-oestradiol and progesterone seem to be involved in such disactivation. Brown adipocytes isolated from female rats with high plasma levels of sex steroids show a lower responsiveness to noradrenaline (NA) -as revealed by the lower increase in respiration rate after NA addition to the incubation medium-than that displayed by adipocytes isolated from untreated controls (10). The decreased responsiveness of brown adipocytes when plasma levels of oestradiol are high leads to a reduction of BAT thermogenic activity in cold-acclimated rats (11). This poses two additional questions, i.e., are oestradiol-treated females rats hypothermic? or are there alternative adjustments that compensate for the low BAT thermogenesis? On the contrary, progesterone impairment of responsiveness to NA does not decrease BAT thermogenesis (9). This raises another question, how progesterone treated animals maintain BAT thermogenesis despite the low responsiveness of brown adipocytes to NA? As in the case of oestradiol a compensatory adjustment was hypothesized.