15-Lipoxygenase promotes resolution of inflammation in lymphedema by controlling Treg cell function through IFN-β

Lymphedema (LD) is characterized by the accumulation of interstitial fluid, lipids and inflammatory cell infiltrate in the limb. Here, we find that LD tissues from women who developed LD after breast cancer exhibit an inflamed gene expression profile. Lipidomic analysis reveals decrease in specialized pro-resolving mediators (SPM) generated by the 15-lipoxygenase (15-LO) in LD. In mice, the loss of SPM is associated with an increase in apoptotic regulatory T (Treg) cell number. In addition, the selective depletion of 15-LO in the lymphatic endothelium induces an aggravation of LD that can be rescued by Treg cell adoptive transfer or ALOX15-expressing lentivector injections. Mechanistically, exogenous injections of the pro-resolving cytokine IFN-β restores both 15-LO expression and Treg cell number in a mouse model of LD. These results provide evidence that lymphatic 15-LO may represent a therapeutic target for LD by serving as a mediator of Treg cell populations to resolve inflammation.

Semaphorin 3C is a novel adipokine linked to extracellular matrix composition

AIMS/HYPOTHESIS

Alterations in white adipose tissue (WAT) function, including changes in protein (adipokine) secretion and extracellular matrix (ECM) composition, promote an insulin-resistant state. We set out to identify novel adipokines regulated by body fat mass in human subcutaneous WAT with potential roles in adipose function.

METHODS

Adipose transcriptome data and secretome profiles from conditions with increased/decreased WAT mass were combined. WAT donors were predominantly women. In vitro effects were assessed using recombinant protein. Results were confirmed by quantitative PCR/ELISA, metabolic assays and immunochemistry in human WAT and adipocytes.

RESULTS

We identified a hitherto uncharacterised adipokine, semaphorin 3C (SEMA3C), the expression of which correlated significantly with body weight, insulin resistance (HOMA of insulin resistance [HOMAIR], and the rate constant for the insulin tolerance test [KITT]) and adipose tissue morphology (hypertrophy vs hyperplasia). SEMA3C was primarily found in mature adipocytes and had no direct effect on human adipocyte differentiation, lipolysis, glucose transport or the expression of β-oxidation genes. This could in part be explained by the significant downregulation of its cognate receptors during adipogenesis. In contrast, in pre-adipocytes, SEMA3C increased the production/secretion of several ECM components (fibronectin, elastin and collagen I) and matricellular factors (connective tissue growth factor, IL6 and transforming growth factor-β1). Furthermore, the expression of SEMA3C in human WAT correlated positively with the degree of fibrosis in WAT.

CONCLUSIONS/INTERPRETATION

SEMA3C is a novel adipokine regulated by weight changes. The correlation with WAT hypertrophy and fibrosis in vivo, as well as its effects on ECM production in human pre-adipocytes in vitro, together suggest that SEMA3C constitutes an adipocyte-derived paracrine signal that influences ECM composition and may play a pathophysiological role in human WAT.

Phosphodiesterase-5A and neutral endopeptidase activities in human adipocytes do not control atrial natriuretic peptide-mediated lipolysis

BACKGROUND AND PURPOSE

Atrial natriuretic peptide (ANP) stimulates lipolysis in human adipocyte through a cGMP signalling pathway, the regulation of which is poorly known. Since phosphodiesterases (PDE) and neutral endopeptidase (NEP) play a major role in the regulation of the biological effects of natriuretic peptides in the cardiovascular and renal systems, we investigated whether these mechanisms could regulate cGMP signalling and ANP-mediated lipolysis in human adipocytes.

EXPERIMENTAL APPROACH

The presence of cGMP-specific PDE and NEP in differentiated pre-adipocytes and in mature adipocytes was evaluated by real-time qPCR and Western blot. The effect of non-selective and selective inhibition of these enzymes on ANP-mediated cGMP signalling and lipolysis was determined in isolated mature adipocytes.

KEY RESULTS

PDE-5A was expressed in both pre-adipocytes and adipocytes. PDE-5A mRNA and protein levels decreased as pre-adipocytes differentiated (10 days). PDE-5A is rapidly activated in response to ANP stimulation and lowers intracellular cGMP levels. Its selective inhibition by sildenafil partly prevented the decline in cGMP levels. However, no changes in baseline- and ANP-mediated lipolysis were observed under PDE-5 blockade using various inhibitors. In addition, NEP mRNA and protein levels gradually increased during the time-course of pre-adipocyte differentiation. Thiorphan, a selective NEP inhibitor, completely abolished NEP activity in human adipocyte membranes but did not modify ANP-mediated lipolysis.

CONCLUSIONS AND IMPLICATIONS

Functional PDE-5A and NEP activities were present in human adipocytes, however these enzymes did not play a major role in the regulation of ANP-mediated lipolysis.

Atrial natriuretic peptide inhibits the production of adipokines and cytokines linked to inflammation and insulin resistance in human subcutaneous adipose tissue

AIMS/HYPOTHESIS

Increased adipose tissue secretion of adipokines and cytokines has been implicated in the chronic low-grade inflammation state and insulin resistance associated with obesity. We tested here whether the cardiovascular and metabolic hormone atrial natriuretic peptide (ANP) was able to modulate adipose tissue secretion of several adipokines (derived from adipocytes) and cytokines (derived from adipose tissue macrophages).

SUBJECTS AND METHODS

We used protein array to measure the secretion of adipokines and cytokines after a 24-h culture of human subcutaneous adipose tissue pieces treated or not with a physiological concentration of ANP. The effect of ANP on protein secretion was also directly studied on isolated adipocytes and macrophages. Gene expression was measured by real-time RT-quantitative PCR.

RESULTS

ANP decreased the secretion of the pro-inflammatory cytokines IL-6 and TNF-alpha, of several chemokines, and of the adipokines leptin and retinol-binding protein-4 (RBP-4). The secretion of the anti-inflammatory molecules IL-10 and adiponectin remained unaffected. The cytokines were mainly expressed in macrophages that expressed all components of the ANP-dependent signalling pathway. The adipokines, leptin, adiponectin and RBP-4 were specifically expressed in mature adipocytes. ANP directly inhibited the secretion of IL-6 and monocyte chemoattractant protein-1 by macrophages. The inhibitory effects of ANP on leptin and growth-related oncogene-alpha secretions were not seen under selective hormone-sensitive lipase inhibition.

CONCLUSIONS/INTERPRETATION

We suggest that ANP, either by direct action on adipocytes and macrophages or through activation of adipocyte hormone-sensitive lipase, inhibits the secretion of factors involved in inflammation and insulin resistance.

Effects of hypoxia on the expression of proangiogenic factors in differentiated 3T3-F442A adipocytes

OBJECTIVE

Adipocyte hypertrophy combined with hyperplasia, observed during the growth of adipose tissue in obesity, might promote the occurrence of hypoxic areas within the tissue. The aim of the present study is to assess the influence of hypoxia on the expression and secretion of adipocyte-derived proangiogenic factors.

DESIGN AND METHODS

Differentiated 3T3-F442A adipocytes were submitted either to ambient hypoxia (5% O(2)) or to chemically induced hypoxia by treatments with cobalt chloride or desferrioxamine. The activities of the matrix metalloproteinases 2 and 9 (MMP-2 and -9) were determined by gelatin zymography. The expression of vascular endothelial growth factor (VEGF), hypoxia inducible factor 1 alpha (HIF-1alpha), leptin, MMP-2 and -9 were studied by the use of Western blotting and RT-PCR analyses.

RESULTS

Low oxygen pressure exposure and hypoxia mimics treatments were associated with increased glucose consumption and release of lactate in differentiated 3T3-F442A adipocytes. They also led to an upregulation of the expression of leptin, VEGF and MMPs. An enhanced accumulation of HIF-1alpha protein was observed in the hypoxic adipocyte nuclei.

CONCLUSION

Hypoxia, in adipocytes, markedly enhances the expression of leptin, VEGF and MMPs and stimulates the HIF-1 pathway. The present data demonstrate that hypoxic adipocytes express more proangiogenic factors and suggest that hypoxia, if occurring in adipose tissue, might be a modulator of the angiogenic process.

Increased lipolysis in adipose tissue and lipid mobilization to natriuretic peptides during low-calorie diet in obese women

OBJECTIVE

We recently demonstrated that natriuretic peptides (NP) are involved in a pathway inducing lipolysis in human adipose tissue. Atrial NP (ANP) and brain NP (BNP) operate via a cGMP-dependent pathway which does not involve phosphodiesterase-3B inhibition or cAMP. The study was performed to evaluate the effect of ANP on lipid mobilization in obese women and secondly to examine the possible effect of a low-calorie diet (LCD) on the lipolytic response of subcutaneous abdominal fat cells to NP and on the lipid mobilization induced by ANP infusion (1 microg/m(2) min for 60 min).

SUBJECTS

Ten obese women from 40.5+/-3.4 y old were selected for this study. Their body weight was 96.4+/-5.7 kg and their BMI was 35.3+/-1.7 kg/m(2). They received a 2.5-2.9 MJ/day formula diet for 28 days.

DESIGN

Before and during the LCD, an adipose tissue biospy was performed for in vitro studies and, moreover, ANP was perfused i.v. to evaluate its lipid mobilizing action in toto and in situ in subcutaneous abdominal adipose tissue (SCAAT) using microdialysis.

RESULTS

The lipolytic effects of isoproterenol, ANP, BNP and bromo-cGMP (an analogue of cGMP) on fat cells increased by about 80-100% during LCD. The lipid mobilization during i.v. ANP infusion, assessed by plasma non-esterified fatty acids (NEFA) increase was enhanced during the LCD. However, during LCD, ANP infusion induced a biphasic effect on glycerol concentration in plasma and interstitial fluid of SCAAT; a significant increase was observed in glycerol levels during the first 30 min infusion period, followed by a steady decrease. The concentration of glycerol was lower during the post-infusion period than during the baseline period. This effect was stronger in obese subjects submitted to the LCD with a low-carbohydrate composition. Other plasma parameters were weakly increased (noradrenaline) or not modified (insulin, glucose) by ANP infusion and no difference was found before and during LCD treatment.

CONCLUSION

The present study shows that NP are powerful lipolytic agents in subcutaneous fat cells and that both isoproterenol- and NP-induced lipolysis increase during LCD, in obese women. These changes seem to be associated with an improvement of the lipolytic pathway at a post-receptor level. Moreover, i.v. administration of ANP induced a lipid mobilizing effect which was enhanced by a LCD in these objects.

Adipocyte produces matrix metalloproteinases 2 and 9: involvement in adipose differentiation

Adipocyte hypertrophy and hyperplasia together with angiogenesis contribute to the growth of the fat mass. Because changes in the extracellular matrix (ECM) components are often associated with such cellular remodeling, we studied the adipocyte expression of the matrix metalloproteinases (MMPs) 2 and 9, two key enzymes involved in the modulation of ECM. The present study provides the first evidence that human adipose tissue produces and secretes MMP-2 and -9 as shown by gelatin zymography analysis performed on media conditioned by human subcutaneous adipose tissue and human preadipocytes in primary cultures and by reverse transcriptase-polymerase chain reaction (RT-PCR) analysis on transcripts from mature human adipocytes. The further characterization performed on the murine 3T3F442A preadipocyte cell line demonstrates that MMP expression, assessed by RT-PCR and Western blot analysis, as well as activity, assessed by gelatin zymography analysis, increased during the adipocyte differentiation, whereas the expression of tissue inhibitor metalloproteinases 1 and 2 were abolished or not affected, respectively. Finally, preadipocyte treatment with MMP inhibitors such as batimastat and captopril, as well as neutralizing antibodies, markedly decreased adipocyte differentiation as demonstrated by the inhibition in the appearance of lipogenic (triglycerides) and lipolytic (glycerol release and hormone-sensitive lipase expression) markers. These data suggest that MMP-2 and -9 could be important key regulators of adipocyte differentiation. Thus, the adipocyte-derived MMPs might represent a new target for the inhibition of adipose tissue growth.

The lipid-mobilizing effect of atrial natriuretic peptide is unrelated to sympathetic nervous system activation or obesity in young men

We recently demonstrated that natriuretic peptides and especially the atrial natriuretic peptide (ANP) are powerful lipolytic agents on isolated human fat cells. To search for a possible influence of obesity on ANP responsiveness, we compared the lipolytic effects of human ANP (h-ANP) on isolated subcutaneous abdominal adipose tissue (SCAAT) fat cells from young healthy lean and obese men. The lipid-mobilizing effects of an intravenous infusion of h-ANP was studied, as well as various metabolic and cardiovascular parameters that were compared in the same subjects. h-ANP (50 ng/min/kg) was infused iv for 60 min. Microdialysis probes were inserted in SCAAT to measure modifications of the extracellular glycerol concentrations during h-ANP infusion. Spectral analysis of blood pressure and heart rate oscillations that were recorded using digital photoplethysmography were used to assess changes in autonomic nervous system activity. h-ANP induced a marked and similar increase in glycerol and nonesterified fatty acids, and a weak increase in insulin plasma levels in lean and obese men. Plasma norepinephrine concentrations rose similarly during h-ANP infusion in lean and obese men. The effects of h-ANP infusion on the autonomic nervous system were similar in both groups, with an increase in the spectral energy of the low-frequency band of systolic blood pressure variability and a decrease in the spectral energy of the high-frequency band of heart rate. In SCAAT, h-ANP infusion increased extracellular glycerol concentration and decreased blood flow similarly in both groups. The increase in extracellular glycerol observed during h-ANP infusion was not modified when 0.1 mM propranolol was added to the microdialysis probe perfusate to prevent beta-adrenoceptor activation. These data show that ANP is a potent lipolytic hormone independent of the activation of the sympathetic nervous system, and that obesity did not modify the lipid-mobilizing effect of ANP in young obese subjects.

Effects of the perfusion of beta-, beta2-, or beta3-adrenergic agonists or epinephrine on in situ adipose tissue lipolysis measured by microdialysis in underfed ewes

The effects of isoproterenol (ISO, a non-selective beta-agonist), terbutaline (TER, a selective beta2-agonist), CL316243 (CL, a selective beta3-agonist), and epinephrine (EPI, beta- and alpha2-agonist) on in situ lipolytic response of s.c. adipose tissue were investigated in vivo, using a microdialysis method to measure glycerol release, in 12 adult nonlactating and ovariectomized, underfed Lacaune ewes. All the adrenergic compounds were perfused for 120 min at 10(-6), 10(-5), and 10(-4) M. They had no lipolytic effect at 10(-6) M. Isoproterenol and EPI at 10(-5) and 10(-4) M enhanced, in the same way, maximal response and area under the concentration curve (AUC) of dialysate glycerol, thus suggesting that involvement of alpha2-adrenoceptors in the control of in situ lipolysis is of minor importance in underfed ewes. Terbutaline had only a slight lipolytic effect at 10(-5) M. This low effect could be due to a lower affinity of TER than of ISO for the beta2-adrenoceptors. The beta3-agonist, CL, had no lipolytic effect whatever the concentration perfused. Further studies are needed to prove the putative presence of beta3-adrenoceptors and their possible role in the ovine adipose tissue.

A single bout of exercise induces beta-adrenergic desensitization in human adipose tissue

This study was designed to assess whether physiological activation of the sympathetic nervous system induced by exercise changes adipose tissue responsiveness to catecholamines in humans. Lipid mobilization in abdominal subcutaneous adipose tissue was studied with the use of a microdialysis method in 11 nontrained men (age: 22. 3 +/- 1.5 yr; body mass index: 23.0 +/- 1.6). Adipose tissue adrenergic sensitivity was explored with norepinephrine, dobutamine (beta(1)-agonist), or terbutaline (beta(2)-agonist) perfused during 30 min through probes before and after 60-min exercise (50% of the maximal aerobic power). The increase in extracellular glycerol concentration during infusion was significantly lower after the exercise when compared with the increase observed before the exercise (P < 0.05, P < 0.02, and P < 0.01, respectively, for norepinephrine, dobutamine, and terbutaline). In a control experiment realized without exercise, no difference in norepinephrine-induced glycerol increase between the two infusions was observed. To assess the involvement of catecholamines in the blunted beta-adrenergic-induced lipolytic response after exercise, adipose tissue adrenergic sensitivity was explored with two 60-min infusions of norepinephrine or epinephrine separated by a 60-min interval. With both catecholamines, the increase in glycerol was significantly lower during the second infusion (P < 0.05). The findings suggest that aerobic exercise, which increased adrenergic activity, induces a desensitization in beta(1)- and beta(2)-adrenergic lipolytic pathways in human subcutaneous adipose tissue.

[Lipid mobilization and energy metabolism: impact of molecular and cellular approaches on the treatment of obesity]

There is strong evidence that reduced sympathetic nervous system (SNS) activity is involved in the etiology of obesity in several animal models of obesity. In humans the situation is more complex but humans with low SNS activity, reduced beta-adrenergic sensitivity, reduced lipid mobilizing efficacy of catecholamines have lowered energy expenditure and are at greater risk of obesity. The SNS with its effect on food intake, lipid mobilization and energy expenditure has a major potential as a target for novel pharmacotherapies in weight reducing strategies. Extended cellular and molecular knowledge about the nature, the distribution and the role of the adrenergic receptors (beta(1)-, beta(2)-, beta(3)-, alpha(2)- and alpha(1)-) existing in tissue effectors involved in the control of lipid mobilization (adipose tissue) and energy expenditure (brown adipose tissue, skeletal muscle) has opened new pathways for pharmacological strategies. In this manuscript, after a summary of current knowledge on the regulation of lipid mobilization and energy expenditure in humans, we briefly review the putative targets and the most recent attempts to develop agents acting at various adrenergic receptor types in SNS effectors or on SNS activity. These include major questions about putative utilization of beta(3)-agonists, alpha(2)-antagonists and beta-antagonists in pharmacotherapy and/or prevention of obesity in humans.

Recent developments on lipolysis regulation in humans and discovery of a new lipolytic pathway

In man, the major hormones controlling the lipolytic function are insulin (inhibition of lipolysis) and catecholamines (stimulation of lipolysis). Catecholamines are of major importance for the regulation of lipid mobilization in human adipose tissue and for the increase of non-esterified fatty acid supply to the working muscle. In vitro studies have shown that there are differences in the catecholaminergic control of fat cells from various fat deposits and a number of physiological and pathological alterations of catecholamine-induced lipolysis have been reported. Lipolytic resistance to catecholamines has been reported in subcutaneous adipose tissue, the major fat depot in obese subjects. Multiple alterations in catecholamine signal transduction pathways have been reported. In situ microdialysis allows a physiological exploration of adipose tissue biology. Recent data obtained on the catecholaminergic regulation of lipolysis and lipid mobilization, using microdialysis in humans, will be analysed. A potent lipolytic and lipomobilizing effect of atrial natriuretic peptide has recently been discovered; the mechanisms of action and physiological relevance will also be discussed.

Nitric oxide-dependent downregulation of adipocyte UCP-2 expression by tumor necrosis factor-alpha

Uncoupling protein-2 (UCP-2) is a mitochondrial protein expressed in adipocytes and has recently been involved in the control of energy dissipation. Because obesity is characterized by an imbalance between energy intake and expenditure and by an enhanced adipocyte-derived secretion of tumor necrosis factor-alpha (TNF-alpha), we asked whether TNF-alpha could directly influence UCP-2 expression in adipocytes. Experiments performed in differentiated 3T3F442A preadipocytes showed that TNF-alpha (10 ng/ml) induced a reduction of UCP-2 trancripts, assessed by Northern blot analysis. A significant decrease in UCP-2 expression (40%) was observed after 12 and 24 h of TNF-alpha stimulation of the cells. The characterization of the mechanisms responsible for the TNF-alpha effect on UCP-2 expression demonstrates an involvement of the TNF-alpha-induced inducible (i) nitric oxide synthase (NOS) expression. Cell treatment with the NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME; 1 mmol/l) significantly diminished the TNF-alpha-mediated sustained downregulation of UCP-2 expression, whereas cell treatment with a nitric oxide (NO) donor (10(-3) mol/l S-nitroso-L-glutathione) mimicked the TNF-alpha effect on UCP-2 expression. Moreover, Western blot analysis clearly showed that TNF-alpha alone induces the expression of iNOS after 12-24 h treatment of differentiated 3T3F442A cells. These experiments demonstrate that TNF-alpha directly downregulates UCP-2 expression via NO-dependent pathways that involve the induction of iNOS expression.

Activation of alpha(2)-adrenergic receptors impairs exercise-induced lipolysis in SCAT of obese subjects

With the use of the microdialysis method, exercise-induced lipolysis was investigated in subcutaneous adipose tissue (SCAT) in obese subjects and compared with lean ones, and the effect of blockade of alpha(2)-adrenergic receptors (ARs) on lipolysis during exercise was explored. Changes in extracellular glycerol concentrations and blood flow were measured in SCAT in a control microdialysis probe at rest and during 60-min exercise bouts (50% of heart rate reserve) and in a probe supplemented with the alpha(2)-AR antagonist phentolamine. At rest and during exercise, plasma norepinephrine and epinephrine concentrations were not different in obese compared with lean men. In the basal state, plasma and extracellular glycerol concentrations were higher, whereas blood flow was lower in SCAT of obese subjects. During exercise, the increase of plasma glycerol was higher in obese subjects (115 +/- 35 vs. 65 +/- 21 micromol/l). Oppositely, the exercise-induced increase in extracellular glycerol concentrations in SCAT was five- to sixfold lower in obese than in lean subjects (50 +/- 14 vs. 318 +/- 53 micromol/l). The exercise-induced increase in extracellular glycerol concentration was not significantly modified by phentolamine infusion in lean subjects but was strongly enhanced in the obese subjects and reached the concentrations found in lean sujects (297 +/- 46 micromol/l). These findings demonstrate that the physiological stimulation of SCAT adipocyte alpha(2)-ARs during exercice-induced sympathetic nervous system activation contributes to the blunted lipolysis noted in obese men.

Natriuretic peptides: a new lipolytic pathway in human adipocytes

Atrial natriuretic peptide (ANP) receptors have been described on rodent adipocytes and expression of their mRNA is found in human adipose tissue. However, no biological effects associated with the stimulation of these receptors have been reported in this tissue. A putative lipolytic effect of natriuretic peptides was investigated in human adipose tissue. On isolated fat cells, ANP and brain natriuretic peptide (BNP) stimulated lipolysis as much as isoproterenol, a nonselective beta-adrenergic receptor agonist, whereas C-type natriuretic peptide (CNP) had the lowest lipolytic effect. In situ microdialysis experiments confirmed the potent lipolytic effect of ANP in abdominal s.c. adipose tissue of healthy subjects. A high level of ANP binding sites was identified in human adipocytes. The potency order defined in lipolysis (ANP > BNP > CNP) and the ANP-induced cGMP production sustained the presence of type A natriuretic peptide receptor in human fat cells. Activation or inhibition of cGMP-inhibited phosphodiesterase (PDE-3B) (using insulin and OPC 3911, respectively) did not modify ANP-induced lipolysis whereas the isoproterenol effect was decreased or increased. Moreover, inhibition of adenylyl cyclase activity (using a mixture of alpha(2)-adrenergic and adenosine A1 agonists receptors) did not change ANP- but suppressed isoproterenol-induced lipolysis. The noninvolvement of the PDE-3B was finally confirmed by measuring its activity under ANP stimulation. Thus, we demonstrate that natriuretic peptides are a new pathway controlling human adipose tissue lipolysis operating via a cGMP-dependent pathway that does not involve PDE-3B inhibition and cAMP production.

Glucose-induced sympathetic activity and energy expenditure during acute alpha2-adrenergic antagonism in obese subjects

OBJECTIVE

To determine the effect of an alpha2-adrenoceptor antagonist, idazoxan, on the sympathetic nervous system and on energy expenditure responses after an oral glucose load, in obese patients. (idazoxan acts as an indirect sympathomimetic drug through blockade of presynaptic alpha2-adrenoceptors).

DESIGN

Double-blind randomized placebo-controlled cross-over study. Idazoxan (40 mg) or placebo were administered orally 90 min before a 100 g oral glucose load.

SUBJECTS

Twelve long-standing obese subjects (six men and six women, age range from 24 to 45 y, body mass index range from 30.2 to 41.3 kg/m2).

MEASUREMENTS

Energy expenditure was derived from oxygen consumption and carbon dioxide production according to indirect calorimetry. Plasma samples were obtained for plasma adrenaline and noradrenaline, glucose, non-esterified fatty acid (NEFA), glycerol and insulin determinations.

RESULTS

The plasma noradrenaline concentration response to the glucose load was significantly higher after idazoxan than after placebo administration. The time-course of glucose load-induced thermogenesis was not significantly different after administration of idazoxan nor placebo. Idazoxan administration did not modify the insulin, non-esterified fatty acids or glycerol concentration responses to the glucose load. Neither heart rate nor blood pressure values were modified by idazoxan when compared to placebo. However, idazoxan significantly improved glucose tolerance.

CONCLUSION

The alpha2-adrenergic antagonist idazoxan increases glucose-induced sympathetic activity but not energy expenditure in obese subjects. These data do not argue for the development of alpha2AR antagonist compounds as anti-obesity treatment.

Adipose tissue lipolysis is increased during a repeated bout of aerobic exercise

The goal of the study was to examine whether lipid mobilization from adipose tissue undergoes changes during repeated bouts of prolonged aerobic exercise. Microdialysis of the subcutaneous adipose tissue was used for the assessment of lipolysis; glycerol concentration was measured in the dialysate leaving the adipose tissue. Seven male subjects performed two repeated bouts of 60-min exercise at 50% of their maximal aerobic power, separated by a 60-min recovery period. The exercise-induced increases in extracellular glycerol concentrations in adipose tissue and in plasma glycerol concentrations were significantly higher during the second exercise bout compared with the first (P < 0.05). The responses of plasma nonesterified fatty acids and plasma epinephrine were higher during the second exercise bout, whereas the response of norepinephrine was unchanged and that of growth hormone lower. Plasma insulin levels were lower during the second exercise bout. The results suggest that adipose tissue lipolysis during aerobic exercise of moderate intensity is enhanced when an exercise bout is preceded by exercise of the same intensity and duration performed 1 h before. This response pattern is associated with an increase in the exercise-induced rise of epinephrine and with lower plasma insulin values during the repeated exercise bout.

Impaired atrial M(2)-cholinoceptor function in obesity-related hypertension

The aim of this study was to investigate the activity of the parasympathetic limb of the baroreflex arch in a canine model of obesity-related hypertension. Twelve male beagle dogs were randomized into 2 groups. Six dogs were fed with normal canine food and 6 were submitted to a 10-week high-fat diet (HFD). We have evaluated the consequences of HFD on heart rate (HR) and blood pressure (BP) circadian cycles and methylscopolamine dose-response curves. Binding of [(3)H]-AF-DX 384 and adenylyl cyclase activity were investigated to determine the density and functionality of M(2)-cholinoceptors on right atrial membranes from control and HFD dogs. HFD induced a significant increase in body weight (15+/-1 vs 12+/-1 kg), systolic BP (161+/-5 vs 145+/-4 mm Hg), diastolic BP (92+/-3 vs 79+/-2 mm Hg), and HR (96+/-4 vs 81+/-3 bpm). Circadian rhythms of HR and BP observed in the baseline period were abolished after 9 weeks of HFD. After propranolol (1 mg/kg) pretreatment, the dose of methylscopolamine able to induce 50% maximum tachycardia was significantly increased after 9 weeks of HFD (7.4+/-0.3 vs 4.7+/-0.1 microg/kg). In the control group, the experimental period failed to modify these parameters. The numbers of M(2)-cholinoceptors measured in right atrial membranes were significantly lower in HFD than in control groups (54+/-6 vs 27+/-6 fmol/mg protein). The ability of carbachol to inhibit isoproterenol-stimulated adenylyl cyclase activity was significantly lower in HFD than in control groups (IC(50)=47+/-12 vs 6.4+/-1.4 micromol/L). However, the basal activity of adenylyl cyclase was unchanged by HFD. HFD decreases M(2)-cholinoceptor number and function in cardiomyocytes. This could explain the abolition of circadian rhythm of HR and the changes in chronotropic effect brought about by methylscopolamine.

Increase in epinephrine-induced responsiveness during microgravity simulated by head-down bed rest in humans

The epinephrine (Epi)-induced effects on the sympathetic nervous system (SNS) and metabolic functions were studied in men before and during a decrease in SNS activity achieved through simulated microgravity. Epi was infused at 3 graded rates (0.01, 0.02, and 0. 03 microg. kg(-1). min(-1) for 40 min each) before and on the fifth day of head-down bed rest (HDBR). The effects of Epi on the SNS (assessed by plasma norepinephrine levels and spectral analysis of systolic blood pressure and heart rate variability), on plasma levels of glycerol, nonesterified fatty acids (NEFA), glucose and insulin, and on energy expenditure were evaluated. HDBR decreased urinary norepinephrine excretion (28.1 +/- 4.2 vs. 51.5 +/- 9.1 microg/24 h) and spectral variability of systolic blood pressure in the midfrequency range (16.3 +/- 1.9 vs. 24.5 +/- 0.9 normalized units). Epi increased norepinephrine plasma levels (P < 0.01) and spectral variability of systolic blood pressure (P < 0.009) during, but not before, HDBR. No modification of Epi-induced changes in heart rate and systolic and diastolic blood pressures were observed during HDBR. Epi increased plasma glucose, insulin, and NEFA levels before and during HDBR. During HDBR, the Epi-induced increase in plasma glycerol and lactate levels was more pronounced than before HDBR (P < 0.005 and P < 0.001, respectively). Epi-induced energy expenditure was higher during HDBR (P < 0.02). Our data suggest that the increased effects of Epi during simulated microgravity could be related to both the increased SNS response to Epi infusion and/or to the beta-adrenergic receptor sensitization of end organs, particularly in adipose tissue and skeletal muscle.