The hormonal responses of lipoprotein lipase activity and lipolysis in adipose tissue differ depending on the stage of the estrous cycle in female rats

International Journal of Obesity (2002) 26, 610–617 ß 2002 Nature Publishing Group All rights reserved 0307–0565/02 $25.00 www.nature.com/ijo PAPER The hormonal responses of lipoprotein lipase activity and lipolysis in adipose tissue differ depending on the stage of the estrous cycle in female rats M Yamaguchi1{, S Katoh1, C Morimoto1, K Sakayama2, T Shiosaka3, H Masuno3* and H Okuda1{ 1 Second Department of Medical Biochemistry, School of Medicine, Ehime University, Shigenobu, Onsen-gun, Ehime, Japan; Department of Orthopedic Surgery, School of Medicine, Ehime University, Shigenobu, Onsen-gun, Ehime, Japan; and 3Department of Medical Laboratory Technology, Ehime College of Health Science, Takooda, Tobe-cho, Iyo-gun, Ehime, Japan 2 OBJECTIVE: This study was designed to elucidate whether there were differences in the hormonal responses of the parameters involving triacylglycerol (TG) deposition and mobilization in adipose tissue among the stages of the estrous cycle in female rats. MEASUREMENTS: Adipose tissue was obtained from the parametrial region in female rats at each stage of the estrous cycle. Lipoprotein lipase (LPL) activity in the extracts of acetone=ether powders of the tissues was measured as a parameter for TG deposition. Norepinephrine-stimulated lipolysis in isolated fat cells was measured as a parameter for TG mobilization. RESULTS: LPL activity changed periodically during the estrous cycle; the activity level was highest at diestrus, began to decrease at proestrus, reached a minimum at estrus, began to increase again at metestrus-1, and increased further at metestrus-2. At diestrus and proestrus, LPL activity was increased with an increase in plasma insulin levels, suggesting that plasma insulin was the predominant up-regulator of LPL. But at estrus, metestrus-1 and metestrus-2, LPL activity remained low even when plasma insulin levels were high, indicating that it was not up-regulated by plasma insulin. Norepinephrine-stimulated lipolysis in fat cells was high at estrus and metestrus-1 and low at diestrus. CONCLUSION: The hormonal responses of LPL activity and lipolysis in adipose tissue differed depending on the stage of the estrous cycle. International Journal of Obesity (2002) 26, 610 – 617. DOI: 10.1038=sj=ijo=0801987 Keywords: lipoprotein lipase; norepinephrine-stimulated lipolysis; estrous cycle; adipose tissue; female rats Introduction Adipose tissue consists of fat cells which store triacylglycerol (TG) as a fuel for the body. Approximately 80 – 90% of the wet weight of fat cells consists of TG. The TG content of fat cells depends on the balance between TG deposition and *Correspondence: H Masuno, Department of Medical Laboratory Technology, Ehime College of Health Science, Takooda, Tobe-cho, Iyogun, Ehime 791-2101, Japan. E-mail: { Current address: Department of Nursing, School of Allied Health Sciences, Faculty of Medicine, Osaka University, Yamadaoka, Suita-city, Osaka 565-0871, Japan. E-mail: { Current address: Department of Environment and Symbiotic Science, Prefectural University of Kumamoto, Tsukide, 3-1-100, Kumamoto city, Kumamoto 862-8502, Japan. Received 8 May 2001; revised 12 November 2001; accepted 18 December 2001 mobilization. Fat cells are metabolically active. The metabolic parameters in fat cells change in response to the nutritional and hormonal status. In the rat, the administration of estradiol has been reported to induce a depletion of the TG stores.1 – 4 Adipose tissue lipoprotein lipase (LPL) hydrolyzes TG in chylomicrons and very-low-density lipoproteins to free fatty acids and monoacylglycerols, which are transported to fat cells. These products are re-esterified and stored as TG in fat cells. Thus, adipose tissue LPL involves TG deposition in fat cells. In vivo administration of estradiol to rats has been known to decrease adipose tissue LPL activity.1 – 4 Several studies1 – 3 have shown that this decreased activity of adipose tissue LPL results in the estradiol-induced depletion of TG. TG mobilization also seems to be under the control of estradiol.4 – 7 Benoit et al4 reported that in vivo administration of estradiol to female rats potentiated the lipolytic action of epinephrine in fat cells. They suggested that the potentiation LPL activity and lipolysis during the estrous cycle M Yamaguchi et al 611 of epinephrine-stimulated lipolysis in fat cells by estradiol was likely to represent a major cause to the estradiol-induced depletion of TG. Ackerman et al5 reported that catechol estrogens potentiated in vitro the lipolytic action of epinephrine in rat fat cells. Adipose tissue LPL activity differs in different conditions, for example with the nutritional state2,3,8 – 12 and on induction of diabetes mellitus.2,3,12,13 Starvation caused a decrease in adipose tissue LPL activity.2,3,5 – 8 The changes in LPL activity that occur during the feeding – fasting – refeeding cycle are governed by the changes in plasma insulin levels that accompany such cycles.10,12 The induction of diabetes mellitus in rats by injection of either alloxan or streptozotocin caused a decrease in LPL activity and the administration of insulin to diabetic rats caused an increase in the activity.2,3,12,13 Thus, plasma insulin was the predominant upregulator of LPL activity. Almost all these studies have been carried out in male rats, because the activity level was almost constant in male rats, whereas it fluctuated widely in female rats. Female rats have an estrous cycle which recurs regularly in 4 – 5 day cycles. The cycle is divided into five stages and the length of each stage of the cycle is as follows; approximately 57 h for diestrus, approximately 12 h for proestrus, approximately 12 h for estrus, approximately 15 h for metestrus-1, and approximately 6 h for metestrus-2.14 The cycle is accompanied by the changes in plasma estradiol levels.15,16 There are very few detailed studies on the changes in the metabolic parameters involving TG deposition and mobilization in adipose tissue that accompany the estrous cycle. We describe here that periodic changes in adipose tissue LPL activity occurred during the estrous cycle. During our experiment, it became apparent that the response of adipose tissue LPL activity to plasma insulin differed depending on the stage of the estrous cycle. We also describe that there were differences in the rates of norepinephrine-stimulated lipolysis in isolated fat cells among the estrous cycle stages. Materials and methods Animal procedures Female Wistar – King strain rats (8 weeks old) were purchased from Clea Japan Inc. (Tokyo, Japan). They were allowed free access to a standard laboratory chow and tap water. The lights were turned on at 7 am and off at 7 pm. One month later, rats were divided into three groups. The first group, termed the fed (am) rats, was allowed free access to food and the rats were killed between 10 and 11 am under nembutal anesthesia. The second group, termed the fed (pm) rats, was al (...truncated)