Compensatory growth of adipose tissue after partial lipectomy: involvement of serum factors

The regulation of body weight/fat was studied by investigating mechanisms for compensatory adipose tissue growth after removal of bilateral epididymal fat pads from male adult Wistar rats. Food intake during the first 4 weeks and energy expenditure on Days 8-10 postsurgery were not different between lipectomized and sham operated rats. During Days 29-31 post surgery, a small (2.4%) but significant (P < 0.05) increase in heat production per metabolic body size was detected in lipectomized as compared with sham operated rats. The carcass composition of lipectomized and sham operated rats was not significantly different 16 weeks after surgery. The compensatory growth was fat pad-specific: mesenteric, retroperitoneal, and inguinal fat pads, but not perirenal fat pads, were heavier in lipectomized rats than in sham operated rats as early as 4 weeks postsurgery. Examination of fat cell size distribution in the compensating pads indicated a shift toward larger cells in retroperitoneal fat, but not in inguinal fat of lipectomized as compared with sham operated rats. Serum from lipectomized rats, but not media conditioned by exposure to retroperitoneal fat pads from lipectomized rats, stimulated proliferation of preadipocytes in vitro more than that from sham operated rats. Thus, compensatory adipose tissue growth after lipectomy may be mediated, in part, by blood-borne factors that are derived from tissues other than adipose tissue.     

Compensation for partial lipectomy in mice with genetic alterations of leptin and its receptor subtypes

One hypothesis for the regulation of total body fat suggests that leptin is a lipostatic feedback signal that acts at brain sites involved in regulation of energy balance. The importance of leptin in recovery from partial surgical lipectomy was tested by performing bilateral epididymal lipectomy or sham surgery on wild-type and leptin-deficient ob/ob mice. Eight weeks later, nonexcised pads of lipectomized mice were increased but total carcass fat was lower than in sham-operated ob/ob mice. In experiment 2, ob/ob mice, wild-type mice, and two db/db mutants, C57BL/6J db(Lepr)/db(Lepr) (BL/6J) mice possessing short-form and circulating leptin receptors and C57BL/6J db(3J)/db(3J) (BL/3J) mice expressing only circulating receptors, were lipectomized or sham operated. Sixteen weeks later, body mass and carcass lipid were not different between sham and lipectomized ob/ob mice, wild-type mice, or BL/6J db/db mice, whereas there was incomplete (decreased carcass fat) but suggestive recovery (increased retroperitoneal fat mass and cell number) in lipectomized BL/3J db/db mice. These data indicate that leptin is not required for the regulation of total body fat.     

Resistance to Aging-Associated Obesity in Capsaicin-Desensitized Rats One Year after Treatment

Previous studies demonstrated reduced weight of abdominal white adipose tissue depots and of carcass fat in capsaicin-desensitized (Cap-Des) rats up to 8 months after treatment. The objective of the present study was to find out whether aging-associated obesity and hyperplasia of retroperitoneal white adipose tissue was prevented in older (13.5 month old) Cap-Des rats, one year after treatment with Cap (done when they were 1.5 months old). The prevalence of obesity is known to increase in rats by this age. Abdominal white adipose tissue depots weighed less in old Cap-Des rats, both epididymal (9% less) and retroperitoneal (30% less). The number of mature white adipocytes was 28% less in the retroperitoneal depot but was not significantly different in the epididymal depot. Adipocyte size was not different. Carcass fat was less, both total and as percent of body weight. Food intake was normal for their reduced body size. The exponential increase in retroperitoneal white adipose tissue weight characteristic of aging rats that are becoming obese was virtually absent in Cap-Des rats. We conclude that lack of function of capsaicin-sensitive afferent autonomic nerves, known to be destroyed in Cap-Des rats, results in an alteration in energy balance conducive to leanness. We suggest that the attenuated age-associated increase in circulating CGRP (derived mainly from capsaicin-sensitive nerves) in the Cap-Des rat results in a lower degree of aging-associated insulin-resistance, hence in a lesser degree of obesity.     

Effects of glucose-containing peritoneal-dialysis solutions on rates of lipogenesis in vivo in the liver, brown and white adipose tissue of chronic uraemic rats.

Chronic uraemic rats had decreased food intake, and this was accompanied by decreased weight of the epididymal fat-pads and interscapular brown adipose tissue. Normal rats whose food intake was restricted to an amount similar to that of the uraemic rats showed similar decreases in weight of the adipose-tissue depots. In addition, the food-restricted rats had decreased liver weight compared with normal or uraemic rats. The basal rate of lipogenesis was decreased in liver and epididymal fat-pads of food-restricted and uraemic rats and in interscapular brown adipose tissue of uraemic rats. Administration of a low-glucose-containing (1.36%) peritoneal-dialysis solution slightly increased lipogenesis in liver of uraemic rats, but had no significant effect in epididymal fat-pads. For brown fat, the rate of lipogenesis was increased in normal, food-restricted and uraemic groups, but the values for the last group were 4-5-fold lower than for the food-restricted or control groups. A high-glucose-containing (3.86%) peritoneal-dialysis solution gave similar rates of lipogenesis in liver, epididymal fat-pads and brown fat of all three groups, but for brown fat moderately uraemic rats showed a considerably lower rate of lipogenesis than did mildly uraemic rats. The basal plasma insulin concentration was lower in the food-restricted (50%) and uraemic (70%) groups than in the control group. The low-glucose peritoneal-dialysis solution increased plasma insulin to control values in the food-restricted rats, but had no significant effect on plasma insulin in the uraemic rats, despite a significant increase in blood glucose in this group. It is concluded that there is an impairment of the lipogenic response to intraperitoneal glucose loads in interscapular brown adipose tissue of uraemic rats, and that this is not due to the accompanying decrease in food intake. The hypoinsulinaemia may be an important factor. The possible relevance of this finding to the obesity observed in some uraemic patients treated by peritoneal dialysis with glucose-containing solutions is discussed.     

A comparison of extrahepatic lipogenesis from a small glucose meal in obob and gold thioglucose obese mice fed low- or high-fat diets with or without the addition of Delta22-5beta-taurocholenic acid

Extrahepatic fatty acid synthesis from a 250 mg meal of [U-(14)C]-glucose was measured in epidymal fat pads and the remaining carcass of hyperglycemic obese (obob), gold thioglucose obese, and nonobese controls under conditions of maximum and minimum lipogenesis. Also assessed was the effect of Delta(22)-5beta-taurocholenic acid, previously shown to inhibit hepatic fatty acid synthesis. Both types of obese and nonobese mice were fed for 6 weeks glucose-based diets containing either 1% corn oil or 40% lard with or without the addition of 0.05% taurocholenic acid. In mice fed 1% corn oil, incorporation of labeled glucose into carcass fatty acids was 25% greater in nonobese than obese mice of either type of obesity. On this diet incorporation of labeled glucose into epididymal fatty acids was reduced 83% in hyperglycemic obese mice compared with nonobese littermates. The corresponding reduction in lipogenesis in gold thioglucose obese mice was only 23% compared with nonobese controls. Feeding 40% lard reduced incorporation of labeled glucose into epididymal and carcass fatty acid 67 to 95% compared with mice fed 1% corn oil in both types of obese and nonobese mice whether or not taurocholenic acid had been fed. Both types of obesity or feeding 40% lard reduced lipogenesis in fat pads to a greater extent than glucose uptake by the pads with the reductions additive. Feeding taurocholenic acid reduced pad weight 30% across strain and obesity status, increased uptake of labeled glucose into epididymal fat pads and increased the percentage of the labeled glucose in the pad recovered as fatty acid in both types of obese and nonobese mice when the diet was 1% corn oil. Similarities and differences between the two obesity models are discussed.     

Sympathetic denervation of one white fat depot changes norepinephrine content and turnover in intact white and brown fat depots

It is well-established that the sympathetic nervous system (SNS) regulates adipocyte metabolism and recently it has been reported that sensory afferents from white fat overlap anatomically with sympathetic efferents to white fat. The studies described here characterize the response of intact fat pads to selective sympathectomy (local 6-hydroxydopamine (6OHDA) injections) of inguinal (ING) or epididymal (EPI) fat in male NIH Swiss mice and provide in vivo evidence for communication between individual white and brown fat depots. The contralateral ING pad, both EPI pads, perirenal (PR), and mesenteric (MES) pads were significantly enlarged 4 weeks after denervating one ING pad, but only intrascapular brown adipose tissue (IBAT) increased when both ING pads were denervated. Denervation of one or both EPI pad had no effect on fat depot weights. In an additional experiment, norepinephrine turnover (NETO) was inhibited in ING, retroperitoneal (RP), MES, and IBAT 2 days after denervation of both EPI or of both ING pads. NE content was reduced to 10-30% of control values in all fat depots. There was no relation between early changes in NETO and fat pad weight 4 weeks after denervation, even though the reduction in NE content of intact fat pads was maintained. These data demonstrate that there is communication among individual fat pads, presumably through central integration of activity of sensory afferent and sympathetic efferent fibers, that changes sympathetic drive to white adipose tissue in a unified manner. In specific situations, removal of sympathetic efferents to one pad induces a compensatory enlargement of other intact depots.     

Relationships among serum triacylglycerol, fat pad weight, and lipolysis in iron-deficient rats

We studied the relationships among serum triacylglycerol (TG), fat pad weight, and lipolytic response to norepinephrine (NE) in iron-deficient rats. We used male Sprague-Dawley International Golden Standard rats. The rats were randomly divided into four groups: two iron-adequate groups for 1 week (1A) and 5 weeks (5A), and two iron-deficient groups for 1 week (1D) and 5 weeks (5D), based on the AIN-93G diet. Iron-deficient treatment caused a significant decrease in hemoglobin (Hb) and hematocrit (Hct) values and an increase in relative heart weight in 1D and 5D rats. Although serum TG was not affected by the 1-week iron-deficient treatment, it was significantly increased by 5-week iron-deficient treatment. The 1-week iron-deficient treatment significantly decreased the relative weight of the retroperitoneal fat pads, but not that of the epididymal fat pads. On the other hand, the 5-week iron-deficient treatment significantly decreased the relative weight of both fat pads; the degree of decrease was 41% and 32% for retroperitoneal and epididymal fat pads, respectively. Basal lipolysis significantly decreased in the epididymal adipocytes from 1D rats, whereas lipolytic response to NE markedly increased. No effect due to the 5-week treatment on basal lipolysis was observed in either retroperitoneal or epididymal adipocytes. In addition, lipolytic response to NE significantly increased in the retroperitoneal, but not the epididymal adipocytes. These results demonstrate that the effects of an iron-deficient diet on fat pad weight are different, depending on the duration of the treatment and the location of fat pads. In addition, iron deficiency-caused hypertriacylglycerolmia may be predominantly related to the increase in lipolysis in retroperitoneal rather than in epididymal adipocytes. The data further show that the increase in lipolysis of epididymal adipocytes occurs in the earlier stage prior to a severe iron-deficient state.     

Sympathetic denervation does not prevent a reduction in fat pad size of rats or mice treated with peripherally administered leptin

Leptin increases sympathetic nervous system (SNS) activity in brown adipose tissue and renal nerves. Experiments described here tested whether SNS innervation is required for peripheral, physiological concentrations of leptin to reduce body fat. In experiment 1, one epididymal (EPI) fat pad was sympathectomized by local injection of 6-hydroxydopamine (6OHDA) in C57BL/6 mice that were then infused for 13 days with PBS or 10 microg leptin/day from an intraperitoneal miniosmotic pump. Surprisingly, EPI denervation increased total body fat of PBS-infused mice but leptin decreased the size of both injected and noninjected EPI pads in 6OHDA mice. Experiment 2 was identical except for the use of male Sprague-Dawley rats that were infused with 50 microg leptin/day. Leptin had little effect on EPI weight or norepinephrine (NE) content, but denervation of one EPI pad decreased the effect of leptin on intact EPI, inguinal and retroperitoneal (RP) fat and increased the size of the mesenteric fat pad. Experiment 3 included groups in which either one EPI or one RP pad was denervated. RP denervation reduced RP NE content but did not prevent a leptin-induced reduction in fat pad mass. Therefore, the SNS is not required for low doses of leptin to reduce body fat. EPI denervation significantly increased adipocyte number in contralateral EPI and RP fat pads and this was prevented by leptin. These changes in intact pads of rats with one denervated fat pad imply communication between fat depots and suggest that both leptin and the SNS regulate the size of individual depots.     

Growth Hormone Treatment of Hypophysectomized Rats Increases Catecholamine-induced Lipolysis and the Number of β-Adrenergic Receptors in Adipocytes: No Differences in the Effects of Growth Hormone on Different Fat Depots

Growth hormone (GH) has a lipolytic effect in adipose tissue but this effect may differ in adipose tissue from various fat depots. This latter possibility was investigated in the present study, in which the effects of GH in vivo on catecholamine-induced lipolysis and the number of β-adrenergic receptors in isolated adipocytes from different fat depots of hypophysectomized rats were investigated. Female and male Sprague-Dawley rats were hypophysectomized or sham-operated at 45 days of age. One week after the operation, hormonal replacement therapy with L-thyroxine and hydrocortisone acetate was given. In addition, groups of rats were treated with GH (1.33 mg/kg per day, given as two daily subcutaneous injections). After 1 week of hormonal treatment, adipocytes were isolated from the parametrial, epididymal and inguinal fat pads, and glycerol release after catecholamine-stimulation and ¹²⁵I-cyanopindolol binding were measured. Hypophysectomy resulted in a marked decrease in the lipolytic response to catecholamines. GH treatment significantly increased catecholamine-induced lipolysis with similar effects in adipocytes from parametrial or epididymal and inguinal fat depots in both female and male rats. There were no differences between norepinephrine compared with isoproterenol-induced responses. ¹²⁵I-cyanopindolol binding was reduced after hypophysectomy and normalized by GH treatment, without differences between parametrial and inguinal adipose tissue regions. We conclude that the lipolytic effects of GH in the rat may partly be mediated by a stimulatory effect on β-adrenergic receptors in adipocytes. In addition, GH exerted similar effect on catecholamine-induced lipolysis and β-adrenergic receptors in adipocytes from parametrial, epididymal and inguinal fat depots.     

White adipose tissue sensory nerve denervation mimics lipectomy-induced compensatory increases in adiposity

The sensory innervation of white adipose tissue (WAT) is indicated by the labeling of sensory bipolar neurons in the dorsal root ganglion after retrograde dye placement into WAT. In addition, immunoreactivity (ir) for sensory-associated neuropeptides such as calcitonin gene-related peptide (CGRP) and substance P in WAT pads also supports the notion of WAT sensory innervation. The function of this sensory innervation is unknown but could involve conveying the degree of adiposity to the brain. In tests of total body fat regulation, partial surgical lipectomy triggers compensatory increases in the mass of nonexcised WAT, ultimately resulting in restoration of total body fat levels in Siberian hamsters and other animals. The signal that triggers this compensation is unknown but could involve disruption of WAT sensory innervation that accompanies lipectomy. Therefore, a local and selective sensory denervation was accomplished by microinjecting the sensory nerve neurotoxin capsaicin bilaterally into epididymal WAT (EWAT) of Siberian hamsters, whereas controls received vehicle injections. Additional hamsters had bilateral EWAT lipectomy (EWATx) or sham lipectomy. As seen previously, EWATx resulted in significantly increased retroperitoneal WAT (RWAT) and inguinal WAT (IWAT) masses. Capsaicin treatment significantly decreased CGRP- but not tyrosine hydroxylase-ir, attesting to the diminished and selective sensory innervation. Capsaicin-treated hamsters also had increased RWAT and, to a lesser degree, IWAT mass largely mimicking the WAT mass increases seen after lipectomy. Collectively, these data suggest the possibility that information related to peripheral lipid stores may be conveyed to the brain via the sensory innervation of WAT.     

Incorporation of glucose into lipid of perirenal and subcutaneous adipocytes of rats and sheep: influence of insulin

Adipocytes were prepared by collagenase digestion of perirenal and subcutaneous fat from rats and sheep and were incubated in vitro with various concentrations of glucose and insulin. The lipogenic rate of perirenal and subcutaneous adipocytes of rats showed a quadratic response to glucose concentration. The addition of 10 nM insulin increased lipogenesis, especially at low lipogenic rates. At constant glucose concentrations, insulin concentrations up to 50 nM stimulated lipogenesis a similar amount in adipocytes from both depots. The rate of lipogenesis increased relative to cell volume in perirenal adipocytes only. The lipogenic rate of perirenal and subcutaneous adipocytes of sheep showed a positive linear response to glucose concentration, but insulin did not affect the rate of lipogenesis in adipocytes from either depot. In both rats and sheep, the rate of lipogenesis was higher in the perirenal adipocytes. It was concluded that insulin is unlikely to be the agent responsible for the differential growth rates of subcutaneous and perirenal fat depots in rats or sheep.     

Factors influencing the utilization of ketone bodies by mouse adipose tissue

Factors influencing the utilization of ketone bodies by mouse adipose tissue in vitro were studied. Epididymal fat pads can oxidize DL-Beta-hydroxybutyrate-3-(14)C and acetoacetate-3-(14)C to (14)CO(2) as well as convert these compounds to fatty acid-(14)C. An increased output of (14)CO(2) from Beta-hydroxybutyrate-3-(14)C was noted in response to glucose plus insulin, succinate, oxaloacetate, L-asparate, and L-malate. Fatty acid synthesis from Beta-hydroxybutyrate was enhanced by glucose plus insulin, L-aspartate, L-malate, oxaloacetate, and citrate. Nicotinamide stimulated the oxidation of Beta-hydroxybutyrate but not of acetoacetate to CO(2), and did not affect fatty acid synthesis from either ketone body. Nicotinamide increased NAD(+) and NADP(+) levels in epididymal fat pads without affecting the concentration of NADH and NADPH. "Superlipogenesis" caused by fasting the mice for 48 hr and re-feeding them for 24 hr sharply enhanced CO(2) output and lipogenesis from Beta-hydroxybutyrate. The activities of glucose-6-phosphate dehydrogenase, 6-phosphogluconic dehydrogenase, NADP-malic dehydrogenase, and citrate cleavage enzyme from mouse adipose tissue were increased during "superlipogenesis." Free fatty acid release by epididymal fat pads in vitro was slightly increased by Beta-hydroxybutyrate. The relationship of ketone body metabolism and lipogenesis in adipose tissue is discussed.     

Effects of central or peripheral leptin administration on norepinephrine turnover in defined fat depots

Leptin preserves lean tissue but decreases adipose tissue by increasing lipolysis and/or inhibiting lipogenesis. The sympathetic nervous system (SNS) is a primary regulator of lipolysis, but it is not known if leptin increases norepinephrine turnover (NETO) in white adipose tissue. In this study, we examined the effect of leptin administered either as a chronic physiological dose (40 microg/day for 4 days from ip miniosmotic pumps) or as an acute injection in the third ventricle (1.5 microg injected two times daily for 2 days) on NETO and the size of brown and white fat depots in male Sprague Dawley rats. NETO was determined from the decline in tissue norepinephrine (NE) during 4 h following administration of the NE synthesis inhibitor alpha-methyl-para-tryrosine. The centrally injected leptin-treated animals demonstrated more dramatic reductions in food intake, body weight, and fat pad size and an increase in NETO compared with the peripherally infused animals. Neither route of leptin administration caused a uniform increase in NETO across all fat pads tested, and in both treatment conditions leptin decreased the size of certain fat pads independent of an increase in NETO. Similar discrepancies in white fat NETO were found for rats pair fed to leptin-treated animals. These results demonstrate that leptin acting either centrally or peripherally selectively increases sympathetic outflow to white fat depots and that a leptin-induced change in fat pad weight does not require an increase in NETO.     

Hypertrophy-driven adipocyte death overwhelms recruitment under prolonged weight gain

Fat pads dynamically regulate energy storage capacity under energy excess and deficit. This remodeling process is not completely understood, with controversies regarding differences between fat depots and plasticity of adipose cell number. We examined changes of mouse adipose cell-size distributions in epididymal, inguinal, retroperitoneal, and mesenteric fat under both weight gain and loss. With mathematical modeling, we specifically analyzed the recruitment, growth/shrinkage, and loss of adipose cells, including the size dependence of these processes. We found a qualitatively universal adipose tissue remodeling process in all four fat depots: 1), There is continuous recruitment of new cells under weight gain; 2), the growth and shrinkage of larger cells (diameter >50 μm) is proportional to cell surface area; and 3), cell loss occurs under prolonged weight gain, with larger cells more susceptible. The mathematical model gives a predictive integrative picture of adipose tissue remodeling in obesity.     

Resistin expression in different adipose tissue depots during rat development

Resistin is a hormonal factor synthesised by adipocytes that was first thought to be related with the resistance to insulin in obesity, but whose function is not yet completely established. Here we have studied the ontogenic pattern of resistin mRNA expression in different white adipose tissue depots (WAT) – epididymal, inguinal, mesenteric and retroperitoneal – and in brown adipose tissue (BAT), as well as the circulating resistin levels, in rats of different ages (from the suckling period to one year of age). Resistin mRNA was determined by Northern blotting, and serum levels by enzyme immunoassay. In WAT, resistin expression remains almost constant with age, except in early development, where there is a peak of expression in the epididymal and retroperitoneal depots, and a decrease in the inguinal one, while the expression remains constant for the mesenteric depot. Moreover, there is a site-specific difference regarding resistin expression: all the depots express characteristic levels of mRNA, especially at the age of 2 months, the moment when resistin mRNA levels are significantly higher in the epididymal and the retroperitoneal than in the inguinal and mesenteric WAT and than in the BAT. The transient increased resistin expression in the epididymal and the retroperitoneal WAT at a period of time in which there is a change in diet (from milk to chow) suggests a common nutritional regulation of the resistin gene. Circulating resistin levels increase with age probably reflecting the increase in the body fat content.     

Studies on lipogenesis in vivo. Lipogenesis during extended periods of re-feeding after starvation

1. Lipogenesis was studied in mice re-fed for up to 21 days after starvation. At appropriate times [U-(14)]glucose was given by stomach tube and incorporation of (14)C into various lipid fractions measured. 2. In mice starved for 48hr. and then re-fed for 4 days with a diet containing 1% of corn oil, incorporation of (14)C from [U-(14)C]glucose into liver fatty acids and cholesterol was respectively threefold and eightfold higher than in controls fed ad libitum. The percentages by weight of fatty acids and cholesterol in the liver also increased and reached peaks after 7 days. Both the radioactivity and weights of the fractions returned to control values after 10-14 days' re-feeding. These changes could be diminished by re-feeding the mice with a diet containing 20% of corn oil. Incorporation of (14)C from [U-(14)C]glucose into extrahepatic fatty acids (excluding those of the epididymal fat pads) was not elevated during re-feeding with a diet containing either 1% or 20% of corn oil. However, incorporation of (14)C from [U-(14)C]glucose into the fatty acids of the epididymal fat pads was increased in mice re-fed with either diet, as compared with non-starved controls. 3. Lipogenesis was also studied in mice alternately fed and starved. Mice given a diet containing 1% of corn oil for 6hr./day for 4 weeks lost weight initially and never attained the weight or carcass fat content of controls fed ad libitum. Incorporation of (14)C from dietary [U-(14)C]-glucose into the fatty acids of the epididymal fat pads was elevated threefold in the mice allowed limited access to food, although the incorporation into the remainder of the extrahepatic fatty acids was not different from that found for controls. Mice given a diet containing 20% of corn oil for 6hr./day adapted to the limited feeding regimen quicker and in 4 weeks did attain the weight and carcass fat content of controls. Incorporation of (14)C from [U-(14)C]glucose into the fatty acids of the epididymal fat pads and the remainder of the extrahepatic fatty acids was respectively fivefold and threefold higher than in controls fed ad libitum. 4. The elevation in liver lipogenesis during re-feeding was greatest on a diet containing 1% of corn oil, whereas in extrahepatic tissues the increase in lipogenesis was greater when the mice were re-fed or were allowed limited access to a diet containing 20% of corn oil. These results suggest that the causes of the increased rate of incorporation of (14)C from [U-(14)C]glucose into fatty acids during re-feeding may be different in liver from that in extrahepatic tissues.     

Effects of white adipose tissue grafts on total body fat and cellularity are dependent on graft type and location

Surgical removal of body fat (lipectomy) triggers compensatory increases in nonexcised white adipose tissue (WAT), thus restoring adiposity levels in many species, including Siberian hamsters. In Siberian hamsters, when their lipectomized WAT is transplanted to another site (autologous grafts, no net change in body fat), healthy grafts result, but the lipectomy-induced compensatory increases in nonexcised WAT masses are exaggerated, an effect that apparently occurs only when the grafts contact intact WAT. When WAT is added to nonlipectomized hamsters to increase body fat, native WAT pads do not decrease. Thus WAT addition or removal-replacement does not induce compensatory WAT responses consistent with total body fat regulation as does WAT subtraction. Therefore, we tested whether the exaggerated response to lipectomy occurring with autologous WAT transplantation is dependent on graft site placement and whether the donor graft source [inguinal or epididymal WAT (IWAT, EWAT), sibling vs. nonsibling] affected body fat responses to WAT additions in nonlipectomized hamsters. Lipectomized hamsters received subcutaneous autologous EWAT grafts placed remotely from other WAT (ventrum) or in contact with intact WAT (dorsum), whereas intact hamsters received EWAT or IWAT grafts from sibling or nonsibling donors. The exaggerated response to lipectomy only occurred when grafts were in contact with intact WAT. EWAT, but not IWAT, additions to nonlipectomized siblings or nonsiblings increased native IWAT and retroperitoneal WAT mass but not EWAT mass compared with controls. Collectively, WAT transplantation to either lipectomized or nonlipectomized hamsters increased body fat contingent on graft contact with intact or native WAT.     

5 alpha-reductase activity in rat adipose tissue

We measured the 5 alpha-reductase activity in isolated cell preparations of rat adipose tissue using the formation of [3H]dihydrotestosterone from [3H]testosterone as an endpoint. Stromal cells were prepared from the epididymal fat pad, perinephric fat, and subcutaneous fat of male rats and from perinephric fat of female rats. Adipocytes were prepared from the epididymal fat pad and perinephric fat of male rats. Stromal cells from the epididymal fat pad and perinephric fat contained greater 5 alpha-reductase activity than did the adipocytes from these depots. Stromal cells from the epididymal fat pad contained greater activity than those from perinephric and subcutaneous depots. Perinephric stromal cells from female rats were slightly more active than those from male rats. Estradiol (10(-8) M), when added to the medium, caused a 90% decrease in 5 alpha-reductase activity. Aromatase activity was minimal, several orders of magnitude less than 5 alpha-reductase activity in each tissue studied.     

Stimulation of lipogenesis in rat adipocytes by ATP, a ligand for P2-receptors

The activation of P2-receptors has a wide range of diverse effects in many tissues. Here we show that extracellular ATP stimulates lipogenesis in adipocytes derived from the epididymal fat pads of male Wistar rats. The lipogenic effect of ATP is not susceptible to treatment of adipocytes with adenosine deaminase or an adenosine receptor antagonist. Degradation of ATP in adipocyte suspension by ectonucleotidases is slow and remaining ATP concentrations are sufficient to activate P2-receptors. ATP does not affect basal or insulin stimulated glucose transport, or basal or isoproterenol stimulated lipolysis, respectively. The lipogenic effect of ATP is mimicked by the adenine compounds, ADP, AMP, and beta,gamma-methylene-ATP, but not by other nucleotides (UTP, UDP, CTP, GTP, ITP, and diadenosine tetraphosphate), indicating that extracellular nucleotides stimulate lipogenesis via a P2-receptor. ATP and its receptor may define a signalling system in adipocytes, which regulates fat stores independently from established hormones.     

Regulation of Visceral and Subcutaneous Adipocyte Lipolysis by Acute AICAR‐induced AMPK Activation

This study investigated the role of adenosine monophosphate–activated protein kinase (AMPK) in the regulation of lipolysis in visceral (VC) and subcutaneous (SC) rat adipocytes and the molecular mechanisms involved in this process. VC (epididymal and retroperitoneal) and SC (inguinal) adipocytes were isolated from male Wistar rats (160–180 g). Adipocytes were incubated either in the absence or in the presence of the AMPK agonist 5‐aminoimidazole‐4‐carboxamide‐1‐β‐d‐ribofuranoside (AICAR, 0–500 µmol/l). AMPK and acetyl‐CoA carboxylase (ACC) phosphorylation, basal and epinephrine‐stimulated (100 nmol/l) glycerol release, and hormone‐sensitive lipase (HSL) phosphorylation and activity were determined. AICAR‐induced (500 µmol/l) AMPK activation inhibited basal glycerol release by ～42, 41, and 44% in epididymal, retroperitoneal, and inguinal adipocytes, respectively. Epinephrine‐stimulated glycerol release was almost completely prevented by AICAR treatment in adipocytes from all fat depots. The AMPK inhibitor compound C (20 µmol/l) prevented AICAR‐induced phosphorylation of AMPK and significantly increased basal (～1.3‐, 1.4‐, and 1.7‐fold) and epinephrine‐stimulated (～1.3‐, 1.2‐, 1.4‐fold) glycerol release in epididymal, retroperitoneal, and inguinal adipocytes, respectively. AICAR increased phosphorylation of HSL_Ser565 and inhibited epinephrine‐induced phosphorylation of HSL_Ser563 and HSL_Ser660. This was also accompanied by a 73% reduction in epinephrine‐stimulated HSL activity. Compound C prevented the phosphorylation of HSL_Ser565 induced by AICAR and partially prevented the inhibitory effect of this drug on basal and epinephrine‐stimulated lipolysis in adipocytes in VC and SC fat depots. In summary, despite different fat depots eliciting distinct rates of lipolysis, acute AICAR‐induced AMPK activation suppressed HSL phosphorylation/activation and exerted similar antilipolytic effects on both VC and SC adipocytes.