Reduced alpha(2)-adrenergic sensitivity of subcutaneous abdominal adipocytes as a modulator of fasting and postprandial triglyceride levels in men

This study examined the postprandial lipemia of two groups of men displaying similar age, body weight, and regional fat distribution, but characterized by either low (n = 11) or high (n = 15) alpha(2)-adrenergic sensitivity of subcutaneous abdominal adipocytes. In addition to fat cell lipolysis, adipose tissue lipoprotein lipase (AT-LPL) as well as postheparin plasma LPL activities were measured in the fasting state. Fasting AT-LPL and PH-LPL activities were similar in both groups. Maximal adipose cell lipolysis induced by isoproterenol (beta-adrenergic agonist) as well as the beta-adrenergic sensitivity did not differ between both groups of men. The selective alpha(2)-adrenergic agonist UK-14304 promoted a similar antilipolytic response in subcutaneous abdominal adipocytes from both groups. However, the alpha(2)-adrenergic sensitivity, defined as the dose of UK-14304 that produced half-maximal inhibition of lipolysis (IC(50)), was significantly different between groups (P < 0.0001). Men with low versus high subcutaneous abdominal fat cell alpha(2)-adrenergic sensitivity showed higher fasting TG levels. In the whole group, a positive relationship was observed between log-transformed IC(50) UK-14304 values of subcutaneous adipocytes and fasting TG levels (r = 0.39, P < 0.05), suggesting that a low abdominal adipose cell alpha(2)-adrenergic sensitivity is associated with high TG levels. After the consumption of a high-fat meal, subjects with low subcutaneous abdominal adipose cell alpha(2)-adrenergic sensitivity showed higher TG levels in total, medium, and small triglyceride-rich lipoprotein (TRL) fractions at 0- to 6-h time points than men with high adipocyte alpha(2)-adrenergic sensitivity (P values ranging from 0.01 to 0.05). Stepwise regression analysis showed that the fasting TG concentration was the only variable retained as a significant predictor of the area under the curve of TG levels in total TRL fractions (73% of variance) among independent variables such as body weight, percent body fat, visceral and subcutaneous abdominal adipose tissue accumulation measured by CT, as well as subcutaneous abdominal fat cell alpha(2)-adrenoceptor sensitivity.Taken together, these results indicate that a reduced antilipolytic sensitivity of subcutaneous abdominal adipocytes to catecholamines may increase fasting TG levels, which in turn play a role in the etiology of an impaired postprandial TRL clearance in men.     

Depot- and gender-related differences in the lipolytic pathway of adipose tissue from severely obese patients.

The present study was performed to analyze in detail gender- and site-related alterations in the adrenergic signal transduction pathway of lipolysis in fat cells isolated from subcutaneous abdominal and visceral fat depots from severely obese patients. The study group consisted of 30 morbidly obese subjects (9 men and 21 women) aged 41.1+/-1.9 years, with a body mass index (BMI) of 54.7+/-1.7 kg/m2, who had undergone abdominal surgery. Protein levels of hormone-sensitive lipase (HSL) and adrenergic receptors (AR), as well as HSL activity and the lipolytic response to adrenergic agents were analyzed. Both fat depots had similar basal lipolysis, but the capacity of catecholamines to activate lipolysis was greater in visceral fat, both at AR and postreceptor levels. Basal lipolysis and lipolytic activity induced by dibutyryl cyclic AMP were higher in men than in women. However, the visceral depot of women showed a higher maximal stimulation by noradrenaline than that of men, in accordance with higher beta1- and beta3-AR protein levels. In conclusion, the main gender-related differences were located in the visceral depot, with women exhibiting a higher sensitivity to catecholamines associated with an increased provision of beta-AR, while men showed an enhanced lipolytic capacity at the postreceptor level.     

Impaired insulin action in subcutaneous adipocytes from women with visceral obesity

Visceral obesity is associated with resistance to the antilipolytic effect of insulin in vivo. We investigated whether subcutaneous abdominal and gluteal adipocytes from viscerally obese women exhibit insulin resistance in vitro. Subjects were obese black and white premenopausal nondiabetic women matched for visceral adipose tissue (VAT), total adiposity, and age. Independently of race and adipocyte size, increased VAT was associated with decreased sensitivity to insulin's antilipolytic effect in subcutaneous abdominal and gluteal adipocytes. Absolute lipolytic rates at physiologically relevant concentrations of insulin or the adenosine receptor agonist N(6)-(phenylisopropyl)adenosine were higher in subjects with the highest vs. lowest VAT area. Independently of cell size, abdominal adipocytes were less sensitive to the antilipolytic effect of insulin than gluteal adipocytes, which may partly explain increased nonesterified fatty acid fluxes in upper vs. lower body obese women. Moreover, increased VAT was associated with decreased responsiveness, but not decreased sensitivity, to insulin's stimulatory effect on glucose transport in abdominal adipocytes. These data suggest that insulin resistance of subcutaneous abdominal and, to a lesser extent, gluteal adipocytes may contribute to increased systemic lipolysis in both black and white viscerally obese women.     

Lipolysis in Intraabdorninal Adipose Tissues of Obese Women and Men

Intraabdominal fat in humans is located in two major depots, the omental and mesenteric. We compared basal and stimulated lipolysis in adipose tissue from these two depots and the subcutaneous abdominal depot of obese women and men. Omental fat cells of women are smaller and have lower rates of basal lipolysis than in men. Basal Iipolysis rates are significantly higher in subcutaneous than intraabdominal adipose tissues of both genders. In men, the incremental lipolytic response to norepinephrine is significantly greater in both intraabdominal fat depots than in the subcutaneous fat, while in women tlie response of tlie mesenteric is lower than tlie omental. In women, but not men, responsiveness to tlie beta agonist isoproterenol is also increased in omental tissue. Thus, in women, omental and mesenteric adipose tissues show distinctly different metabolic properties which may moderate the impact of intraabdominal obesity.     

Regional variation in adipose tissue lipolysis in lean and obese men.

Biopsies of adipose tissue were obtained from two subcutaneous regions (abdominal and femoral) in a sample of 54 men (32 obese and 22 lean subjects). Clonidine-induced antilipolysis in femoral adipose cells was similar in both groups, whereas subcutaneous abdominal adipocytes of obese individuals showed a higher alpha 2-adrenergic response than did subcutaneous abdominal adipose cells from lean subjects. In addition, epinephrine had a biphasic effect in subcutaneous abdominal adipocytes from obese individuals, as it induced antilipolysis at low concentrations, and a net lipolytic response at higher doses. In contrast, the physiological amine promoted lipolysis in subcutaneous abdominal adipose cells of lean subjects. Epinephrine- and clonidine-induced antilipolysis of subcutaneous abdominal adipocytes was positively associated with the level of subcutaneous abdominal fat measured by computed tomography (CT). Finally, men with a high alpha 2-adrenergic response of subcutaneous abdominal fat cells were fatter than those with a low alpha 2-adrenergic component. These results suggest that, in men with a wide range of body fatness, variations in the lipolytic response of subcutaneous abdominal adipose cells to epinephrine appear to involve changes in the functional balance between alpha 2- and beta-adrenoceptors.     

Expression of human alpha 2-adrenergic receptors in adipose tissue of beta 3-adrenergic receptor-deficient mice promotes diet-induced obesity

Catecholamines play an important role in controlling white adipose tissue function and development. beta- and alpha 2-adrenergic receptors (ARs) couple positively and negatively, respectively, to adenylyl cyclase and are co-expressed in human adipocytes. Previous studies have demonstrated increased adipocyte alpha 2/beta-AR balance in obesity, and it has been proposed that increased alpha 2-ARs in adipose tissue with or without decreased beta-ARs may contribute mechanistically to the development of increased fat mass. To critically test this hypothesis, adipocyte alpha 2/beta-AR balance was genetically manipulated in mice. Human alpha 2A-ARs were transgenically expressed in the adipose tissue of mice that were either homozygous (-/-) or heterozygous (+/-) for a disrupted beta 3-AR allele. Mice expressing alpha 2-ARs in fat, in the absence of beta 3-ARs (beta 3-AR -/- background), developed high fat diet-induced obesity. Strikingly, this effect was due entirely to adipocyte hyperplasia and required the presence of alpha2-ARs, the absence of beta 3-ARs, and a high fat diet. Of note, obese alpha 2-transgenic beta 3 -/- mice failed to develop insulin resistance, which may reflect the fact that expanded fat mass was due to adipocyte hyperplasia and not adipocyte hypertrophy. In summary, we have demonstrated that increased alpha 2/beta-AR balance in adipocytes promotes obesity by stimulating adipocyte hyperplasia. This study also demonstrates one way in which two genes (alpha 2 and beta 3-AR) and diet interact to influence fat mass.     

Effects of testosterone on fat cell lipolysis. Species differences and possible role in polycystic ovarian syndrome

Testosterone is a potent regulator of lipolysis by influencing catecholamine signal transduction in fat cells. Major species differences exist as regards the testosterone effect. In rodents testosterone increases beta-adrenergic receptor mediated signals to lipolysis at multiple steps in the lipolytic cascade. The sex hormone also increases alpha2-adrenoceptor antilipolytic signalling in hamster which unlike rat express this receptor in their fat cells. In humans the region of adipose tissue is critical. Visceral fat cell lipolysis is not responsive to testosterone but this sex hormone decreases catecholamine-induced lipolysis in subcutaneous fat cells due to inhibition of the expression of beta2-adrenoceptors and hormone sensitive lipase. In polycystic ovarian syndrome (PCOS), which is characterized as a hyperandrogenic state, the lipolytic effect of catecholamine is decreased in subcutaneous adipocytes due to low content of beta2-adrenoceptors and hormone sensitive lipase. It is possible that the increased testosterone levels are responsible for these abnormalities in catecholamine signal transduction in subcutaneous fat cells of PCOS women. However, in visceral fat cells of PCOS women catecholamine-induced lipolysis is enhanced which cannot be explained by testosterone.     

Adrenergic lipolysis in human fat cells: properties and physiological role of alpha-adrenergic receptors (author's transl)

Lipolytic activity of human isolated fat cells from different fat deposits was studied. The purpose of the present investigations was to determine the epinephrine responsiveness, with regard to alpha- and beta-adrenergic receptor site activity, of omental and subcutaneous adipocytes (abdominal or from the lateral part of the thigh). Adipocytes were obtained from normal subjects or from obese subjects on iso- or hypocaloric diets. The lipolytic effect of epinephrine varied according to the fat deposits, while the beta-lipolytic effect of isoproterenol was more stable (Fig. 1). We explored the possible involvement of adrenergic alpha-receptors, in order to explain these results. The potentiating action of phentolamine on epinephrine-induced lipolysis, and the antilipolytic effect of alpha-agonists on basal or theophylline--induced lipolysis, were found to be a good indication of alpha-adrenergic activity. The alpha-adrenergic antilipolytic effect was most prominent in adipose tissue from the lateral part of the thigh, and less noticeable in omental adipocytes. In conclusion, the inability of epinephrine to induce lipolysis, and the epinephrine-induced inhibition of lipolysis observed when the basal rate of FFA release was spontaneously increased in subcutaneous fat-cells of the thigh, could be explained by an increased alpha adrenergic responsiveness (Fig. 2). Moreover, various alpha-adrenergic agonists (phenylephrine, noradrenaline and adrenaline) showed a clear inhibiting effect on theophylline-stimulated adipocytes from the thigh. The pharmacological study of the antilipolytic effect of epinephrine on theophylline-induced lipolysis showed that the inhibition was linked to a specific stimulation of the alpha-receptors of the subcutaneous adipocytes (Fig. 4). From the different sets of experiments, it is shown that the modifications in the lipolytic effect of epinephrine on adipocytes of different areas could be explained by the occurrence of a variable alpha-adrenergic effect initiated by catecholamine. Furthermore, theophylline stimulation of lipolysis provides an accurate system to investigate the alpha-inhibiting effect of catecholamines. Our study was completed by the investigation of the lipolytic activity of subcutaneous fat cells from obese subjects submitted to a hypocaloric diet (800-1 000 Cal/day). An increased alpha-inhibitory effect of epinephrine was shown on the increased basal lipolytic activity observed in the fat cells of obese subjects on a hypocaloric diet (Fig. 5); a similar effect was observed when these adipocytes were stimulated by theophylline. To conclude, these investigations allow the alpha-adrenergic effect to be considered as a regulator mechanism of the in vitro lipolytic activity in human adipose tissue, since the antilipolytic effect is operative whenever the basal rate of lipolysis is increased (spontaneously, after caloric restriction, or with a lipolytic agent such as theophylline).     

Sex-dependent thermogenesis,differences in mitochondrial morphology and function,and adrenergic response in brown adipose tissue

Gender-related differences in brown adipose tissue (BAT) thermogenesis of 110-day-old rats were studied by determining the morphological and functional features of BAT. The adrenergic control was assessed by studying the levels of beta(3)- and alpha(2A)-adrenergic receptors (AR) and by determining the lipolytic response to norepinephrine (beta(1)-, beta(2)-, beta(3)-, and alpha(2)-AR agonist), isoprenaline (beta(1)-, beta(2)-, and beta(3)-AR agonist), and CGP12177A (selective partial beta(3)-AR agonist but beta(1)- and beta(2)-AR antagonist) together with post-receptor agents, forskolin and dibutyryl cyclic AMP. The female rats that had greater oxygen consumption showed higher UCP1 content, a higher multilocular arrangement, and both longer cristae and higher cristae dense mitochondria in BAT indicating heightened thermogenic capacity and activity; this picture is accompanied by a more sensitive beta(3)-AR to norepinephrine signal (EC(50) 10-fold lower for CGP12177A) and a lower expression of alpha(2A)-AR than male rats. Taken together, our results support the idea that the BAT hormonal environment could be involved in the control of different elements of lipolytic and thermogenic adrenergic pathways. Gender dimorphism is both at receptor (changing alpha(2A)-AR density and beta(3)-AR affinity) and post-receptor (modulating the links involved in the adrenergic signal transduction) levels. These changes in adrenergic control could be responsible, at least in part, both for the important mitochondrial recruitment differences and functional and morphological features of BAT in female rats under usual rodent housing temperatures.     

Polymorphism of the AHSG gene is associated with increased adipocyte beta2-adrenoceptor function

The alpha(2) Heremans-Schmid glycoprotein (AHSG) gene is implicated in the regulation of body fat and insulin sensitivity. The Met/Met genotype of the common single-nucleotide polymorphism (SNP), rs4917, in the AHSG gene has been shown to be associated with reduced plasma levels as well as lower body fat. Here, we studied the association of this variation with subcutaneous adipocyte lipolysis. Ninety-three obese and nonobese healthy men were genotyped for Thr230Met, and subcutaneous adipose tissue biopsies were analyzed for lipolysis characteristics. The Met/Met genotype was associated with a marked increase of 1.5 log units in the lipolytic sensitivity to the beta2-adrenoceptor agonist terbutaline (P=0.0008) as compared with the Thr/Thr and Thr/Met genotypes. This corresponds to an approximately 35-fold increase in beta2-adrenoceptor function. The genotype effect was independent of body mass index and waist circumference. In contrast, lipolytic sensitivity to both the beta1-adrenoceptor agonist dobutamine (P=0.25) and the alpha2A-adrenoceptor agonist clonidine (P=0.54) was unaffected by the Thr230Met variation. Moreover, no difference in either maximal stimulation or inhibition of lipolysis was found between genotypes. We conclude that a common variation (Thr230Met) in the AHSG gene is associated with a marked increase in beta2-adrenoceptor sensitivity in subcutaneous fat cells, which may be of importance in body weight regulation.     

Inhibition of epinephrine-induced lipolysis in isolated white adipocytes of aging rabbits by increased alpha-adrenergic responsiveness.

The aim of this study was to explain the unresponsiveness of rabbit perirenal adipose tissue to epinephrine. The in vitro lipolytic response to isoproterenol and to epinephrine alone or associated with alpha- or beta-adrenergic blocking agents, was studied in the adipocytes of rabbits of various ages. Epinephrine induces a large glycerol release in young rabbit adipocytes whereas an increase in the rate of lipolysis cannot be shown with adult rabbit fat cells. Moreover, an antilipolytic effect can be shown for low concentrations of epinephrine when the basal rate of lipolysis is high in older rabbit adipocytes. Isoproterenol (beta-adrenomimetic) always exerts a strong adipokinetic effect, thus revealing functional beta-receptor sites. The blockade of alpha-adreneoceptor sites by phentolamine, which has no effect on young rabbits, abolishes the antilipolytic effect and unmasks strong lipolytic effect of epinephrine on aged and normal rabbit adipocytes. The loss of beta-adrenergic responsiveness towards epinephrine in the aging rabbit is linked to the involvement of an increased alpha-adrenergic responsiveness. The stimulation of alpha receptor sites by epinephrine leads to a depressive effect on lipolysis (lack of adipokinetic effect or antilipolytic action).     

Adipocyte differentiation of multipotent cells established from human adipose tissue

In this study multipotent adipose-derived stem cells isolated from human adipose tissue (hMADS cells) were shown to differentiate into adipose cells in serum-free, chemically defined medium. During the differentiation process, hMADS cells exhibited a gene expression pattern similar to that described for rodent clonal preadipocytes and human primary preadipocytes. Differentiated cells displayed the key features of human adipocytes, i.e., expression of specific molecular markers, lipolytic response to agonists of beta-adrenoreceptors (beta2-AR agonist > beta1-AR agonist > beta3-AR agonist) and to the atrial natriuretic peptide, insulin-stimulated glucose transport, and secretion of leptin and adiponectin. hMADS cells were able to respond to drugs as inhibition of adipocyte differentiation was observed in the presence of prostaglandin F2alpha, tumour necrosis factor-alpha, and nordihydroguaiaretic acid, a natural polyhydroxyphenolic antioxidant. Thus, for the first time, human adipose cells with normal karyotype and indefinite life span have been established. They represent a novel and valuable tool for studies of fat tissue development and metabolism.     

Atypical beta-adrenergic receptor in 3T3-F442A adipocytes. Pharmacological and molecular relationship with the human beta 3-adrenergic receptor.

Expression of ligand binding properties for an atypical beta-adrenergic receptor (beta-AR) subtype was studied during the adipose differentiation of murine 3T3-F442A cells and compared with that of the human beta 3-AR expressed in Chinese hamster ovary cells stably transfected with the human beta 3-AR gene (CHO-beta 3 cells) Emorine, L. J., Marullo, S., Briend-Sutren, M. M., Patey, G., Tate, K., Delavier-Klutchko, C., and Strosberg, A. D. (1989) Science 245, 1118-1121). 3T3-F442A adipocytes exhibited high and low affinity binding sites for (-)-4-(3-t-butylamino-2-hydroxypropoxy) [5,7-3H]benzimidazole-2-one ((-)-[3H]CGP-12177) (KD = 1.2 and 38.3 nM) and (-)-[125I]iodocyanopindolol ([125I]CYP) (KD = 47 and 1,510 pM). The high affinity sites corresponded to the classical beta 1- and beta 2-AR subtypes whereas the KD values of the low affinity sites for the radioligands were similar to those measured in CHO-beta 3 cells (KD = 28 nM and 1,890 pM for (-)-[3H]CGP12177 and [125I]CYP, respectively). These low affinity sites were undetectable in preadipocytes but represented about 90% of total beta-ARs in adipocytes. The atypical beta-AR and the human beta 3-AR add similarly low affinities (Ki = 3-5 microM) for (+/-)-(2-(3-carbamoyl-4-hydroxyphenoxy)ethylamino-3)-(4-(1-methyl- 4- trifluormethyl-2-imidazolyl)-phenoxy)-2-propanol methane sulfonate (CGP20712A) or erythro-(+/-)-1-(7-methylindan-4-yloxy)-3-isopropylaminob utan-2-ol (ICI118551), highly selective beta 1- and beta 2-AR antagonists, respectively, in agreement with the poor inhibitory effect of the compounds on (-)-isoproterenol (IPR)-stimulated adenylate cyclase activity. Atypical beta-AR and beta 3-AR had an affinity about 10-50 times higher for sodium-4-(2-[2-hydroxy-2-(3-chlorophenyl)ethylamino]propyl)phenoxyace tate sesquihydrate (BRL37344) than the beta 1-AR subtype. This correlates with the potent lipolytic effect of BRL37344 in adipocytes. The rank order of potency of agonists in functional and binding studies was BRL37344 greater than IPR less than (-)-norepinephrine greater than (-)-epinephrine both in 3T3 adipocytes and CHO-beta 3 cells. As in CHO-beta 3 cells, the classical beta 1- and beta 2-antagonists CGP12177, oxprenolol, and pindolol were partial agonists in adipocytes. Although undetectable in preadipocytes, a major mRNA species of 2.3 kilobases (kb) and a minor one of 2.8 kb were observed in adipocytes by hybridization to a human beta 3-AR specific probe.(ABSTRACT TRUNCATED AT 400 WORDS)     

Hormone-sensitive lipase-independent adipocyte lipolysis during beta-adrenergic stimulation, fasting, and dietary fat loading

In white adipose tissue, lipolysis can occur by hormone-sensitive lipase (HSL)-dependent or HSL-independent pathways. To study HSL-independent lipolysis, we placed HSL-deficient mice in conditions of increased fatty acid flux: beta-adrenergic stimulation, fasting, and dietary fat loading. Intraperitoneal administration of the beta(3)-adrenergic agonist CL-316243 caused a greater increase in nonesterified fatty acid level in controls (0.33 +/- 0.05 mmol/l) than in HSL(-/-) mice (0.12 +/- 0.01 mmol/l, P < 0.01). Similarly, in isolated adipocytes, lipolytic response to CL-316243 was greatly reduced in HSL(-/-) mice compared with controls. Fasting for 相似文献   

Protein kinase C-dependent antilipolysis by insulin in rat adipocytes

Recently, we have shown that protein kinase C (PKC) activated by phorbol 12-myristate 13-acetate (PMA) attenuates the beta1-adrenergic receptor (beta1-AR)-mediated lipolysis in rat adipocytes. Stimulation of cells by insulin, angiotensin II, and alpha1-AR agonist is known to cause activation of PKC. In this study, we found that lipolysis induced by the beta1-AR agonist dobutamine is decreased and is no longer inhibited by PMA in adipocytes that have been treated with 20 nM insulin for 30 min followed by washing out insulin. Such effects on lipolysis were not found after pretreatment with angiotensin II and alpha1-AR agonists. The rate of lipolysis in the insulin-treated cells was normalized by the PKCalpha- and beta-specific inhibitor G? 6976 and PKCbeta-specific inhibitor LY 333531. In the insulin-treated cells, wortmannin increased lipolysis and recovered the lipolysis-attenuating effect of PMA. Western blot analysis revealed that insulin slightly increases membrane-bound PKCalpha, betaI, and delta, and wortmannin decreases PKCbetaI, betaII, and delta in the membrane fraction. These results indicate that stimulation of insulin receptor induces a sustained activation of PKC-dependent antilipolysis in rat adipocytes.     

Prolonged fasting on the lipolytic activity of isolated adipocytes of the rat epididymis

Male Wistar rats, 6-8 week old, were fasted for 72 hours. The in vitro lipolytic activity of epididymal adipocytes was measured in the presence of adrenalin (a alpha and beta adrenergic agonist), isoprenaline (a pure beta agonist), theophylline (a phosphodiesterase inhibitor) or UK 14304 (a alpha 2 adrenoceptor agonist) associated with adenosine deaminase. The basal lipolytic activity, expressed per 100 mg lipids, was higher in fasted adipocytes than in fed ones. Its stimulation by adrenalin or isoproterenol was decreased by fast. The effects of these drugs were more potentiated by theophylline in fasted adipocytes than in fed ones. The UK 14304 inhibition of adenosine deaminase-stimulated lipolysis was about 20% in fasted adipocytes and 50% in fed adipocytes. The in vitro resistance of fasted adipocytes to the lipolytic effect of adrenalin or isoproterenol may be related to the hypothyroid status of fasted rats.     

beta-Adrenergic stimulated lipolysis in pony adipocytes is exclusively via a beta2-subtype and is not affected by lactation

Catecholamines are important lipolytic agents in horses and ponies but the nature of the adrenergic receptor subtype distribution in their adipocytes is uncertain. A first objective was to identify the beta-adrenergic receptor subtype(s) present in adipocytes from horses and ponies. A second objective was to evaluate if the lipolytic responsiveness of isolated adipocytes to beta-adrenergic agonists is altered during lactation, a condition known to affect markedly maternal fat metabolism. Isoproterenol and salbutamol elicited strong lipolytic responses in adipocytes isolated from horse and pony subcutaneous adipose tissue. There were weak lipolytic responses to norepinephrine, dobutamine and BRL37344. The weak lipolytic response to NE compared to isoproterenol or salbutamol suggests an antilipolytic action from alpha2-adrenergic receptors. The relative order of potency for the beta-adrenergic agonists was isoproterenol>/=salbutamol>dobutamine=BRL37344. There was expression of beta2-adrenergic receptor mRNA in pony and horse adipose tissues, as estimated by relative RT-PCR, but no expression of mRNAs for beta1- or beta3-adrenergic receptors. Early lactation did not alter the lipolytic responses to beta-adrenergic agonists, nor the expression of beta2-adrenergic receptor mRNA. Thus, these results indicate a dominant if not exclusive presence of beta2-adrenergic receptors in pony and horse adipocytes that is not affected by lactation.     

Atrial natriuretic peptide stimulates lipid mobilization during repeated bouts of endurance exercise

Atrial natriuretic peptide (ANP) controls lipolysis in human adipocytes. Lipid mobilization is increased during repeated bouts of exercise, but the underlying mechanisms involved in this process have not yet been delineated. The relative involvement of catecholamine- and ANP-dependent pathways in the control of lipid mobilization during repeated bouts of exercise was thus investigated in subcutaneous adipose tissue (SCAT) by microdialysis. The study was performed in healthy males. Subjects performed two 45-min exercise bouts (E1 and E2) at 50% of their maximal oxygen uptake separated by a 60-min rest period. Extracellular glycerol concentration (EGC), reflecting SCAT lipolysis, was measured in a control probe perfused with Ringer solution and in two other probes perfused with either Ringer plus phentolamine (alpha(1/2)-AR antagonist) or Ringer plus both phentolamine and propranolol (beta-AR antagonist). Plasma epinephrine, plasma glycerol, and EGC were 1.7-, 1.6-, and 1.2-fold higher in E2 than in E1, respectively. Phentolamine potentiated exercise-induced EGC increase during E2 only. Propranolol reduced the lipolytic rate during both E1 and E2 compared with the probe with phentolamine. Plasma ANP concentration increased more during E2 than during E1 and was correlated with the increase in EGC in the probe containing phentolamine plus propranolol. The results suggest that ANP is involved in the control of lipolysis during exercise and that it contributes to stimulation of lipolysis during repeated bouts of exercise.     

In vitro and in vivo impairment of alpha2-adrenergic receptor-dependent antilipolysis by fatty acids in human adipose tissue.

The aim of the present study was to study the influence of fatty acids on the adrenergic control of lipolysis both in vitro and in vivo. Human subcutaneous adipose tissue explants were cultured for 48 h in the presence of 100 microM bromopalmitate (BrPal), and lipolysis was measured in isolated adipocytes. In control conditions, beta-AR-dependent activation of lipolysis by epinephrine was almost undetectable, and could be fully restored by pharmacological blockade of alpha2-AR-dependent antilipolysis. After BrPal treatment, epinephrine became fully lipolytic and was no longer influenced by alpha2-AR-blockade. Radioligand binding analysis revealed that BrPal treatment led to a significant reduction in the coupling of alpha2-AR to G proteins. In parallel, a chronic and significant increase in plasma fatty acids resulting from a 4-day high-fat diet (HFD) was accompanied by an impairment of the amplifying effect of the alpha2-AR antagonist phentolamine on exercise-induced lipolysis (measured in the subcutaneous adipose tissue with the use of a microdialysis probe) normally observed after a low-fat diet. In conclusion, in vitro and in vivo studies showed that fatty acids impair alpha2-AR-dependent antilipolysis.     

Sex- and depot-specific lipolysis regulation in human adipocytes: interplay between adrenergic stimulation and glucocorticoids

The purpose of this investigation was to explore interactions between adrenergic stimulation, glucocorticoids, and insulin on the lipolytic rate in isolated human adipocytes from subcutaneous and omental fat depots, and to address possible sex differences. Fat biopsies were obtained from 48 nondiabetic subjects undergoing elective abdominal surgery. Lipolysis rate was measured as glycerol release from isolated cells and proteins involved in lipolysis regulation were assessed by immunoblots. Fasting blood samples were obtained and metabolic and inflammatory variables were analyzed. In women, the rate of 8-bromo-cAMP- and isoprenaline-stimulated lipolysis was approximately 2- and 1.5-fold higher, respectively, in subcutaneous compared to omental adipocytes, whereas there was no difference between the two depots in men. Dexamethasone treatment increased the ability of 8-bromo-cAMP to stimulate lipolysis in the subcutaneous depot in women, but had no consistent effects in fat cells from men. Protein kinase A, Perilipin A, and hormone sensitive lipase content in adipocytes was not affected by adipose depot, sex, or glucocorticoid treatment. In conclusion, catecholamine and glucocorticoid regulation of lipolysis in isolated human adipocytes differs between adipose tissue depots and also between sexes. These findings may be of relevance for the interaction between endogenous stress hormones and adipose tissue function in visceral adiposity and the metabolic syndrome.