Pfltigers Arch (1994) 426:351-353 Eliiiiihfin J0urnal of Physiology 9 Springer-Verlag 1994 Short communication Oxygen consumption of oestradiol-treated rats Alberto Fermindez, Maria Abelenda, Marla-Paz Nava, Marisa Puerta Departamento de Biologla Animal II (Fisiologla Animal), Facultad de Ciencias Bioldgicas, Universidad Complutense de Madrid, E-28040 Madrid, Spain Received October 6, 1993/Received after revision November 12, 1993/Accepted November 15, 1993 Abstract. Rectal temperature and oxygen consumption (~ro2) were monitored in female rats acclimated either to cold or to thermoneutrality and with and without chronic administration of oestradiol. The hormone is known to inactivate brown adipose tissue (BAT) and to reduce its response to noradrenaline (NA). The role of sympathetic control was studied by administering NA or the adrenergic blocker propranolol. Oestradiol treatment did not affect rectal temperature in the states of acclimation to thermoneutrality and to cold, nor did it change the hypothermic response of cold-exposed rats to temporary food deprivation. In the cold-acclimated rats, both controls and oestradiol-treated animals exhibited similar degrees of metabolic reduction after propranolol administration in the cold and similar degrees of metabolic activation by NA at thermoneutrality, Rats acclimated to thermoneutrality showed a larger metabolic response to NA when treated with oestradiol. The results suggest that oestradiol, while inactivating the BAT response to NA, activates the NA responsiveness of other metabolically active tissues in cold- induced thermogenesis. The observation of a greater oxidative capacity in the kidney and the rectus abdominis muscle of oestradiol-treated, cold-acclimated rats would be in line with this proposal. Key words: Rat - Brown adipose tissue - Noradrenaline responsiveness - Oxygen consumption - Cold acclimation - Body temperature - Oxidative capacity. Introduction Cold-acclimated rats maintain their body temperature in the cold mainly by non-shivering thermogenesis in brown adipose tissue (BAT). Brown adipocytes are activated by noradrenaline (NA) which is released from the sympathetic terminals innervating the BAT, with the degree of sympathetic activation corresponding to the degree of cold exposure. It has been shown by in vitro analysis that the Correspondence to: M. Puerta metabolic activity of BAT becomes inhibited [5] and responsiveness of brown adipocytes to NA decreases [6] when high levels of plasma oestradiol are maintained in cold-acclimated rats. This observation suggests differences in cold-induced metabolic responses between female rats with normal and experimentally elevated plasma levels of oestradiol. The present in vivo study was designed to compare thermoregulation and metabolic activity of oestradiol-treated female rats andnon-treated controlswhen acclimated to cold or to thermoneutrality, and to elucidate possible differences in the metabolic responses to NA and adrenergic blockade. Materials and methods Female Wistar rats, 215 +_1 g body weight, were acclimated to either thermoneutral (28 ~ or cold (6 ~ ambient temperatures. After 15 days of acclimation one Silastic capsule (Dow Coming, Midland, Michigan, USA), 5 mm long and with 3.2 outer and 1.5 inner diameter, either empty or filled with 17-13 oestradiol (about 4 rag; Sigma, St. Louis, USA) was subcutaneously implanted under ether anaesthesia. The rats remained at 28 ~ or 6 ~ for further 15 days. Throughout the experiment the animals were housed in individual cages with food and water ad libitum and kept on a reverse light:dark cycle of 12:12 h. The food provided was a commercial stock diet (Panlab, Barcelone, Spain) containing 66.7% carbohydrate, 19.3% protein, 3.4% fat, 4.9% cellulose and 5.7% minerals. To avoid the obligatory postprandial thermogenesis, food was removed 12-14 hours before oxygen consumption ('v'o z) measurements. Since food was withdrawn during the inactive phase of the daily cycle, severe fasting was avoided. Rectal temperature was measured twice with an electronic probe inserted 5 cm into the rectum: at the start of food withdrawal and in the food deprived state just before the start of the Vo2 measurements. ~/o2 of the cold-acclimated rats was measured at 6 ~ before and after the subcutaneous (s.c.) administration of 20 mg/kg propranolol (Sigma). A subgroup of cold-acclimated rats was transferred to 28~ and 2 hours later Voz was measured before and after the s.c. administration of 250 #g/kg NA (Sigma). ~ro 2 of rats acclimated to 28 ~ was measured in basal conditions and after s.c. application of 250 #g/kg NA. Vo2 was measured in the dark in a closed circuit system. Soda lime served as the CO 2 absorbent. The rats were allowed a 30 min period before closing the metabolic chamber. This time period was sufficient for the rats to come into a resting state. After closure of the chamber'Qo 2 was