Sex-associated differences in cold-induced UCP1 synthesis in rodent brown adipose tissue

The effects of acute and chronic acclimation to cold on uncoupling protein 1 (UCP1) levels, as well as on GDP-binding to mitochondria, cytochrome c oxidase activity and mitochondrial protein concentration in brown adipose tissue (BAT) of intact male and female rats have been analyzed. Results reveal that females rats are more sensitive to cold because their threshold temperature for the thermogenic response is set at a higher value (around 22 degreesC) than that of males (around 18 degreesC), hence leading to differences in BAT UCP1 levels between the sexes at different environmental temperatures. In vitro experiments showed that steroid hormones, beta-estradiol, estrone and progesterone, can reduce norepinephrine-induced UCP1 synthesis in brown adipocytes differentiated in primary culture. Thus the different sex-associated response of cold-induced thermogenesis in rats does not appear to be explained by a direct action of sex steroids upon the adipocyte, implying that other factors in the thermogenic regulatory system must be involved.