Evidence that epinephrine acts preferentially as an antilipolytic agent in abdominal human subcutaneous fat cells: assessment by analysis of beta and alpha 2 adrenoceptor properties

Comprehensive analysis of transcriptomic and metabolomic profiles uncovered the age-induced dynamic development pattern of subcutaneous fat in Ningxiang pig

Enhancing meat production and quality is the eternal theme for pig breeding industries. Fat deposition has always been the focus of research in practical production because it is closely linked to pig production efficiency and pork quality. In the current study, multi-omics techniques were performed to explore the modulatory mechanisms of backfat (BF) accumulation at three core developmental stages for Ningxiang pigs. Our results identified that 15 differentially expressed genes (DEGs) and 9 significantly changed metabolites (SCMs) contributed to the BF development via the cAMP signaling pathway, regulation of lipolysis in adipocytes, and biosynthesis of unsaturated fatty acids. Herein, we found a series of candidate genes such as adrenoceptor beta 1 (ADRB1), adenylate cyclase 5 (ADCY5), ATPase Na+/K+ transporting subunit beta 1 (ATP1B1), ATPase plasma membrane Ca2+ transporting 3 (ATP2B3), ATPase Na+/K+ transporting subunit alpha 2 (ATP1A2), perilipin 1 (PLIN1), patatin like phospholipase domain containing 3 (PNPLA3), ELOVL fatty acid elongase 5 (ELOVL5) and metabolites like epinephrine, cAMP, arachidonic acid, oleic acid, linoleic acid, and docosahexaenoic acid existed age-specificeffects and played important roles in lipolysis, fat accumulation, and fatty acid composition. Our findings provide a reference for molecular mechanisms in BF tissue development and the optimization of carcass quality.

Obstructive sleep apnoea increases lipolysis and deteriorates glucose homeostasis in patients with type 2 diabetes mellitus

Obstructive sleep apnoea (OSA) is associated with type 2 diabetes mellitus (T2DM). However, mechanisms mediating association between these two conditions remain unclear. This study investigated, whether the OSA-associated changes in adipose tissue lipolysis might contribute to impaired glucose homeostasis in patient with T2DM. Thirty-five matched subjects were recruited into three groups: T2DM + severe OSA (T2DM + OSA, n = 11), T2DM with mild/no OSA (T2DM, n = 10) and healthy controls (n = 14). Subcutaneous abdominal adipose tissue microdialysis assessed spontaneous, epinephrine- and isoprenaline-stimulated lipolysis. Glucose metabolism was assessed by intravenous glucose tolerance test. Spontaneous lipolysis was higher in the T2DM + OSA compared with the T2DM (60.34 ± 23.40 vs. 42.53 ± 10.16 μmol/L, p = 0.013), as well as epinephrine-stimulated lipolysis (236.84 ± 103.90 vs. 167.39 ± 52.17 µmol/L, p < 0.001). Isoprenaline-stimulated lipolysis was unaffected by the presence of OSA (p = 0.750). The α₂ anti-lipolytic effect was decreased in T2DM + OSA by 59% and 315% compared with T2DM and controls (p = 0.045 and p = 0.007, respectively). The severity of OSA (AHI) was positively associated with spontaneous (p = 0.037) and epinephrine-stimulated (p = 0.026) lipolysis. The α₂-adrenergic anti-lipolytic effect (p = 0.043) decreased with increasing AHI. Spontaneous lipolysis was positively associated with Insulin resistance (r = 0.50, p = 0.002). Epinephrine-stimulated lipolysis was negatively associated with the Disposition index (r =  - 0.34, p = 0.048). AHI was positively associated with Insulin resistance (p = 0.017) and negatively with the Disposition index (p = 0.038). Severe OSA in patients with T2DM increased adipose tissue lipolysis, probably due to inhibition of the α₂-adrenergic anti-lipolytic effect. We suggest that dysregulated lipolysis might contribute to OSA-associated impairments in insulin secretion and sensitivity.

Acetazolamide increases locomotion, exploratory behavior, and weight loss following social stress: A treatment for emotional eating?

Sympathomimetics are effective, centrally acting drugs that induce weight loss through their potent anorexic and locomotor properties. We reported that sympathomimetics antagonize catecholamine-dependent, alpha-2 adrenergic receptor-dependent signal transduction mediated by chloride/bicarbonate transport. We posit that other drugs that target cellular chloride/bicarbonate antiport would similarly demonstrate anorectic properties, induce locomotion, and diminish weight gain. Male and female inbred mice were housed in groups or stressed by prolonged social isolation. Mice consumed either normal chow or a high fat, high fructose corn syrup, (i.e. "Western") diet. To inhibit chloride/bicarbonate transport, acetazolamide (ACT, 3 mM) was added to the drinking water. Rodents underwent evaluations of exploratory locomotion and learning with the object recognition test. Mice consuming a "Western" diet gain more weight compared to mice given a normal diet. When placed on a "Western" diet, stressed mice gained weight more rapidly than unstressed. The body weight of mice fed a normal diet with ACT was significantly reduced compared to control mice not given ACT (weight, g ± SEM), 23.7 ± 0.8 v. 21.0 ± 0.5, p = 0.02. ACT did not reduce weight gain in animals chronically maintained on a "Western" diet. Compared to unstressed mice, living in social isolation reduced spontaneous exploratory locomotion time, an indicator of anxiety, in male mice (sec +SEM) from 22.8 ± 3.5 to 12.2 ± 2.1 (p < 0.001), and in female mice, from 47 ± 5.7 to 19.6 ± 2.3 (p < 0.001). ACT had no effect on exploration time in unstressed mice, but ACT completely restored the diminished exploratory locomotion time found in stressed mice compared to unstressed mice. The ratio of time spent exploring new objects compared to familiar items (discrimination ratio [DR]) was reduced following social isolation in males from 2.6 ± 0.5 to 1.2 ± 0.2 (p < 0.05) and in females from 3.8 ± 0.6 to 1.5 ± 0.2 (p < 0.01). ACT normalized the DR ratio of the stressed mice. Decreased food consumption and greater locomotor activity induced by ACT may contribute to acute weight loss; this effect is diminished when rodents were maintained on an unhealthful Western diet. Inhibition of chloride/bicarbonate transport through agents such as acetazolamide could offer a safe, new approach to achieving weight loss.

Identification and characterization of adipose surface epitopes

Adipose tissue is a central regulator of metabolism and an important pharmacological target to treat the metabolic consequences of obesity, such as insulin resistance and dyslipidemia. Among the various cellular compartments, the adipocyte cell surface is especially appealing as a drug target as it contains various proteins that when activated or inhibited promote adipocyte health, change its endocrine function and eventually maintain or restore whole-body insulin sensitivity. In addition, cell surface proteins are readily accessible by various drug classes. However, targeting individual cell surface proteins in adipocytes has been difficult due to important functions of these proteins outside adipose tissue, raising various safety concerns. Thus, one of the biggest challenges is the lack of adipose selective surface proteins and/or targeting reagents. Here, we discuss several receptor families with an important function in adipogenesis and mature adipocytes to highlight the complexity at the cell surface and illustrate the problems with identifying adipose selective proteins. We then discuss that, while no unique adipocyte surface protein might exist, how splicing, posttranslational modifications as well as protein/protein interactions can create enormous diversity at the cell surface that vastly expands the space of potentially unique epitopes and how these selective epitopes can be identified and targeted.

Cellulite: Is there a role for injectables?

BACKGROUND

Cellulite describes the cutaneous dimpling of the thighs, buttocks, and hips that is seen predominately in women. Current evidence suggests that structural differences in fat architecture between the sexes account for its appearance. Mesotherapy, a method of delivering medication locally with the use of numerous cutaneous injections, has recently become a popular method to purportedly treat the condition.

METHODS

An overview of cellulite and adipocyte physiology, with a literature review and appraisal of compounds commonly used in mesotherapy.

RESULTS

Experimental studies using individual mesotherapy ingredients for other conditions suggest a number of mechanisms, including lipolysis, disrupting connective tissue and augmenting circulation, which may theoretically improve cellulite. Peer-reviewed studies have not evaluated whether these effects translate clinically.

CONCLUSIONS

Until further studies are performed, patients considering mesotherapy for cellulite must be aware that the substances currently being injected to treat this cosmetically disturbing, but medically benign, condition have not been thoroughly evaluated for safety or efficacy.

Regulation of fat metabolism during resistance exercise in sedentary lean and obese men

The effect of acute resistance exercise (RE) on whole body energy expenditure (EE) and α2-adrenergic receptor (α2-AR) regulation of lipolysis in subcutaneous abdominal adipose tissue (SCAAT) was determined in sedentary lean (LN) and obese (OB) men. Lipolysis was monitored using microdialysis in 10 LN [body mass index (BMI) 20.9 ± 0.6] and 10 OB (BMI 36.2 ± 2.7) men before, during, and for 24 h after RE. EE was measured before and immediately after RE for 40 min. Changes in interstitial glycerol were measured in SCAAT with three microdialysis probes perfused with a control solution, phentolamine (α2-AR antagonist), or propranolol (β-AR antagonist). EE and fat oxidation (FOX) were significantly ( P < 0.001) elevated immediately post-RE compared with pre-RE in LN and OB subjects, with no differences between groups. RE-induced increases in SCAAT glycerol concentrations from rest to peak exercise were greater in LN than in OB men in the control (LN 142.1 ± 30.8 vs. OB 65.4 ± 14.2%, P = 0.03) and phentolamine probes (LN 187.2 ± 29.6 vs. OB 66.7 ± 11.0%, P = 0.002). Perfusion of propranolol had no effect on interstitial glycerol concentrations over the time course of the experiment in either group. Plasma insulin concentrations were significantly lower ( P = 0.002) and plasma growth hormone (GH) was significantly higher ( P = 0.03) in LN compared with OB men. The mechanism behind RE contributing to improved body composition may in part be due to enhanced SCAAT lipolysis and improved EE and FOX in response to RE in LN and OB men. The blunted SCAAT lipolytic response to RE in OB compared with LN men is unrelated to RE-induced catecholamine activation of the antilipolytic α2-ARs and may be due to depressed GH in OB subjects.

[Effects of puberty on glucose-lipid balance during exercise in the obese child]

The aim of the present study was to investigate effect of puberty on substrate oxidation rates using a graded exercise test to exhaustion. Two groups of obese adolescent males (34 prepubertal: body mass index (BMI) = 25,94 +/- 2,63; Z-score = 4,43 +/- 1,83; and 26 postpubertal: BMI = 31,14 +/- 4,88; Z-score = 5,264 +/- 1,76) performed an exercise test on a cycle ergometer. The test consisted in a series of graded exercises on a cycle ergometer. Stage duration was 3 min 30 s. Fat and carbohydrate rates were calculated during the last 30 s of each stage using stoichiometric equations, and this permitted us to calculate substrate oxidation according to exercise intensity. Lipid oxidation rates are significantly higher in the postpubertal group. When the fat oxidation rates are reported relative to fat free mass, fat oxidation rates are higher in the prepubertal group. Puberty decreases significantly the capacity of fat free mass to oxidize fat for a same level of exercise.

Differential regulation of atrial natriuretic peptide- and adrenergic receptor-dependent lipolytic pathways in human adipose tissue

The aim of the study was to investigate the regulation affecting the recently described atrial natriuretic peptide (ANP)-dependent lipolytic pathway in comparison with the adrenergic lipolytic cascade. We studied in vivo the effect of a euglycemic-hyperinsulinemic clamp on the changes occurring in the extracellular glycerol concentration (EGC) of subcutaneous adipose tissue (SCAT) during ANP or epinephrine perfusion in a microdialysis probe. Homologous desensitization and the incidence of hyperinsulinemia on the ANP- and catecholaminergic-dependent control of lipolysis were also investigated in vitro on fat cells from SCAT. When perfused in SCAT, epinephrine and ANP promoted an increase in EGC; the EGC increase was significantly lower during the clamp. The reduction of epinephrine-induced lipolysis was limited (18%) when phentolamine (an alpha(2)-adrenergic receptor [AR] antagonist) was perfused together with epinephrine. Unlike the effect of epinephrine, the response to ANP observed during the second perfusion was reduced by 32%. The increase in extracellular guanosine 3',5' -cyclic monophosphate concentration, which reflects ANP activity, was also reduced during the second perfusion. Desensitization of the lipolytic effects of ANP was observed in vitro after a 2-hour period of recovery, while the effects of alpha(2)-AR agonist or of epinephrine were unchanged. Insulin was without any effect on ANP-induced lipolysis and alpha(2)-AR-mediated antilipolysis, while it reduced beta-AR-induced lipolysis. The ANP-dependent lipolytic pathway undergoes desensitization in vitro and in situ. Insulin had no inhibitory effect on either ANP- or alpha(2)-AR-dependent pathways, while it counteracted the beta-AR pathway.

A thyroid hormone receptor alpha gene mutation (P398H) is associated with visceral adiposity and impaired catecholamine-stimulated lipolysis in mice

Thyroid hormone has profound effects on metabolic homeostasis, regulating both lipogenesis and lipolysis, primarily by modulating adrenergic activity. We generated mice with a point mutation in the thyroid hormone receptor alpha (TRalpha) gene producing a dominant-negative TRalpha mutant receptor with a proline to histidine substitution (P398H). The heterozygous P398H mutant mice had a 3.4-fold (p < 0.02) increase in serum thyrotropin (TSH) levels. Serum triiodothyronine (T3) and thyroxine (T4) concentrations were slightly elevated compared with wild-type mice. The P398H mice had a 4.4-fold increase in body fat (as a fraction of total body weight) (p < 0.001) and a 5-fold increase in serum leptin levels (p < 0.005) compared with wild-type mice. A 3-fold increase in serum fasting insulin levels (p < 0.002) and a 55% increase in fasting glucose levels (p < 0.01) were observed in P398H compared with wild-type mice. There was a marked reduction in norepinephrine-induced lipolysis, as reflected in reduced glycerol release from white adipose tissue isolated from P398H mice. Heart rate and cold-induced adaptive thermogenesis, mediated by thyroid hormone-catecholamine interaction, were also reduced in P398H mice. In conclusion, the TRalpha P398H mutation is associated with visceral adiposity and insulin resistance primarily due to a marked reduction in catecholamine-stimulated lipolysis. The observed phenotype in the TRalpha P398H mouse is likely due to interference with TRalpha action as well as influence on other metabolic signaling pathways. The physiologic significance of these findings will ultimately depend on understanding the full range of actions of this mutation.

Activation of alpha2-adrenergic receptors blunts epinephrine-induced lipolysis in subcutaneous adipose tissue during a hyperinsulinemic euglycemic clamp in men

The aim of this study was to investigate whether hyperinsulinemia modifies adrenergic control of lipolysis, with particular attention paid to the involvement of antilipolytic alpha2-adrenergic receptors (AR). Eight healthy male subjects (age: 23.9 +/- 0.9 yr; body mass index: 23.8 +/- 1.9) were investigated during a 6-h euglycemichyperinsulinemic clamp and in control conditions. Before and during the clamp, the effect of graded perfusions of isoproterenol (0.1 and 1 microM) or epinephrine (1 and 10 microM) on the extracellular glycerol concentration in subcutaneous abdominal adipose tissue was evaluated by using the microdialysis method. Both isoproterenol and epinephrine induced a dose-dependent increase in extracellular glycerol concentration when infused for 60 min through the microdialysis probes before and during hours 3 and 6 of the clamp. The catecholamine-induced increase was significantly lower during the clamp than before it, with the inhibition being more pronounced in hour 6 of the clamp. Isoproterenol (1 microM)-induced lipolysis was reduced by 28 and 44% during hours 3 and 6 of the clamp, respectively, whereas the reduction of epinephrine (100 microM)-induced lipolysis was significantly greater (by 63 and 70%, P < 0.01 and P < 0.04, respectively) during the same time intervals. When epinephrine was infused in combination with 100 microM phentolamine (a nonselective alpha-AR antagonist), the inhibition of epinephrine (10 microM)-induced lipolysis was only of 19 and 40% during hours 3 and 6 of the clamp, respectively. The results demonstrate that, in situ, insulin counteracts the epinephrine-induced lipolysis in adipose tissue. The effect involves 1) reduction of lipolysis stimulation mediated by the beta-adrenergic pathway and 2) the antilipolytic component of epinephrine action mediated by alpha2-ARs.

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.

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.

Activation of antilipolytic alpha(2)-adrenergic receptors by epinephrine during exercise in human adipose tissue

The involvement of the antilipolytic alpha(2)-adrenergic pathway and the specific role of epinephrine in the control of lipolysis during exercise in adipose tissue (AT) were investigated in healthy male subjects (age: 24.1 +/- 2.2 yr; body mass index: 23.0 +/- 1.6). An in vitro study carried out on isolated adipocytes showed that the weak lipolytic effect of epinephrine was potentiated after blockade of alpha(2)-adrenergic receptor (AR) by an alpha(2)-AR antagonist and reached that of isoproterenol, a beta-AR agonist. The effect of the nonselective alpha(2)-AR antagonist phentolamine on the response of the extracellular glycerol concentration (EGC) in AT during two successive bouts of aerobic exercise (50% maximum O(2) uptake, 60 min duration) was evaluated using the microdialysis method. The metabolic responses measured in perfused probes with Ringer solution were compared with those obtained in perfused probes with Ringer plus 0.1 mmol/l phentolamine. Plasma norepinephrine level was not different during the two exercise bouts, whereas that of epinephrine was 2.5-fold higher during the second exercise. EGC in AT was twofold higher in the second compared with the first exercise, and the same response pattern was found for plasma glycerol. The exercise-induced increase in EGC was higher in the probe perfused with phentolamine compared with the control probe in both bouts of exercise. However, the potentiating effect of phentolamine on EGC was significant during the second exercise bout but did not reach a significant level during the first. These results suggest that epinephrine is involved in the control of lipid mobilization through activation of antilipolytic alpha(2)-AR in human subcutaneous AT during exercise.

Regional differences in adrenoceptor binding and fat cell lipolysis in obese, postmenopausal women

In women there is an increase in visceral obesity, subcutaneous abdominal adipocyte lipolysis, and risk of cardiovascular disease (CVD) associated with weight gain after menopause. The mechanisms underlying this increase in adrenoreceptor (AR)-agonist catecholamine-stimulated lipolysis and abdominal obesity in postmenopausal women were studied in intact adipocytes isolated from the abdominal and gluteal subcutaneous fat depots in 19 obese (48% +/- 1% body fat, mean +/- SE) women with a mean +/- SE age of 58 +/- 1 years. The fat cell size and adipose tissue lipoprotein lipase (ATLPL) activity were similar in both sites. The maximal lipolytic responsiveness and sensitivity to isoproterenol were higher (P < .05) in abdominal compared with gluteal adipocytes, but maximal lipolytic response to a post-AR agent was similar. Abdominal adipocytes had a higher beta-AR ([3H]-CGP-12177) and alpha2-AR ([3H]-yohimbine) affinity than gluteal cells (P < .05), lower alpha2-AR density (P < .05), but similar beta-AR density as gluteal cells. Both abdominal and gluteal cell size correlated with alpha2-AR density (P < .01), but not with beta-AR density. Thus, a higher beta-AR affinity and lower alpha2-AR relative to beta-AR density may explain the higher in vitro catecholamine-mediated lipolysis in abdominal compared with gluteal adipocytes in obese, postmenopausal women.

Tissue-specific regulation of fat cell lipolysis by NPY in 6-OHDA-treated rats

The effects of neuropeptide Y (NPY), peptide YY (PYY), [Leu31, Pro34]NPY, and NPY (13-36) on adipocyte lipolysis have been studied in subcutaneous (inguinal) and visceral (parametrial) rat adipose tissues. A 48-h fasting period and chemical sympathectomy were used to evaluate the regulation of Y1 and Y2 pathways in rat adipocytes. NPY, PYY, and [Leu31, Pro34]NPY significantly inhibited fat cell lipolysis by about 25% in both tissues (p < or = 0.05). This inhibition was achieved mainly through the Y1 pathway. No significant response to NPY (13-36) was observed, suggesting a lack of involvement of the Y2 pathway in the antilipolytic effect of NPY and PYY. The 48-h fasting period led to the loss of the Y1 inhibitory effect previously observed in control rats. On the other hand, the chemical sympathectomy induced a 35% increase of fat cell lipolysis (p < or = 0.05). The latter involved the Y2 pathway as stimulated by NPY (13-36), and was observed in the parametrial tissue exclusively. These results suggest that: a) rat Y receptors reported to exhibit Gi responses can also express Gs-like responses, and b) visceral and subcutaneous adipose tissues exhibit specific regulation of fat cell lipolysis.

Fat cells

The adipocyte is a metabolically active cell that functions to store energy for times of energy deprivation or enhanced need. Obesity is characterized by increased lipid accumulation and turnover compared with the nonobese state. Both triglyceride synthesis and lipolysis are regulated metabolic processes in the adipocyte. Current research on the metabolic activities of the human adipocyte focus on plasma triglyceride hydrolysis and uptake of fatty acids by LPL, esterification of these fatty acids, and the subsequent triglyceride breakdown by hormone-sensitive lipase in response to stimulation of adrenergic receptors. These topics are discussed in relationship to the development of obesity.

Adrenergic receptors and fat cells: differential recruitment by physiological amines and homologous regulation

The control of fat cell lipolysis by the catecholamines involves at least four different adrenoceptor subtypes; three beta (beta 1-, beta 2-, and beta 3-ARs) and one alpha 2-adrenoceptor (alpha 2-AR). The physiological importance of the beta- and alpha 2A-ARs varies according to the species, the sex, the age, the anatomical location of fat deposits and the degree of obesity in humans and animals. The physiological amines operate through differential recruitment of these sites on the basis of their relative affinities. This point has been assessed by in vitro studies and has partly been confirmed in in vivo experiments using selected alpha/beta-AR antagonists and in situ microdialysis. The affinity of the beta 3-AR for catecholamines is less than that of the classical beta 1- and beta 2-ARs in the various species investigated. Conversely, it is the alpha 2-AR which exhibit the highest affinity for the physiological amines in all fat cells. The relative order of affinity of the various fat cell ARs for the physiological amines defined in binding studies and in vitro assays is alpha 2 > beta 1 > or = beta 2 > beta 3 for norepinephrine and alpha 2 > beta 2 > beta 1 > beta 3 for epinephrine. When considering differential beta-AR recruitment by catecholamines, it is the beta 1-AR which is always activated at the lowest norepinephrine levels, whatever the species, while the activation of the beta 3-AR requires higher norepinephrine levels. In addition to the differential recruitment, differential regulation by hormones could also occur for each fat cell AR subtype. The alpha 2-and beta 3-ARs are less prone to desensitization and down-regulation by comparison with the beta 1- and beta 2-AR.

Regional differences in adipose tissue lipolysis from lean and obese women: existence of postreceptor alterations

Lipolysis studies were performed on isolated adipose cells obtained from two subcutaneous regions (abdominal and femoral) in 26 premenopausal women (16 obese and 10 lean subjects). Because obese adipocytes from both sites were significantly larger than lean fat cells, glycerol release measured by an ultrasensitive bioluminescent method was corrected for variation in cell surface area. Epinephrine induced antilipolysis at low concentrations and a net lipolytic response at higher doses, regardless of the subjects' fatness and the anatomic location of fat. However, the catecholamine and the selective alpha 2-adrenergic agonist, UK-14304, promoted a greater maximal antilipolytic response in both femoral and subcutaneous abdominal adipose cells from obese individuals that from lean individuals. Epinephrine- and UK-14304-induced maximal antilipolysis of femoral adipocytes was also positively associated with indicators of total adiposity. On the other hand, the maximal lipolytic responses to postadrenoceptor agents such as dibutyryl adenosine 3',5'-cyclic monophosphate, forskolin, and theophylline were lower in both adipose regions in obese than in lean women. Femoral fat cell lipolysis in the presence of these agents was negatively correlated with body fatness indexes. These results suggest that, in women covering a wide range of adiposity, variations in the lipolytic response of femoral fat cells to epinephrine may involve changes in the functional balance between alpha 2- and beta-adrenoceptors and also alterations located at different postreceptor levels.

Insulin resistance in adipocytes of obese women: effects of body fat distribution and race

Upper-body obesity (UBO) in white women is associated with increased fatty acid turnover and resistance to the effects of insulin on systemic glucose metabolism. The present study determined whether the abilities of insulin to stimulate glucose transport and suppress lipolysis are impaired in adipocytes from white UBO (W-UBO) women. Because the clinical risks associated with UBO are attenuated in black women, the effects of race on adipocyte insulin sensitivity were assessed. Forty-two healthy, equally obese women were selected for study on the basis of race (black or white) and body fat distribution (UBO or lower-body obesity [LBO]). In white women, both abdominal and gluteal fat cells from the UBO versus LBO group were less responsive to the stimulatory effects of insulin on glucose uptake and less sensitive to the antilipolytic effects of insulin and the adenosine analog, phenylisopropyladenosine (PIA). In contrast, in black women, fat cells from UBO and LBO groups were equally sensitive to the stimulatory effects of insulin on glucose transport and the suppressive effects of insulin and PIA on lipolysis. These in vitro data correlate well with previous clinical findings that UBO in white women but not in black women is associated with insulin resistance and dyslipidemia. Thus, resistance to the antilipolytic effects of insulin and adenosine at the level of adipose tissue may increase systemic lipolysis and play a role in the development or maintenance of peripheral insulin resistance associated with UBO in white women, but not in black women.

DNA polymorphisms in the alpha 2- and beta 2-adrenoceptor genes and regional fat distribution in humans: association and linkage studies

The aim of this study was to investigate the relationships between DNA restriction fragment length polymorphisms (RFLP) in the alpha 2- and beta 2-adrenoceptor genes and body fat distribution in humans. Skinfold thickness measurements and genetic analyses (Southern blot) were performed on 280 individuals (142 parents and 138 offsprings) from the Québec Family Study. Using the association study design in unrelated adults, women but not men carrying the 6.3-kb allele of an alpha 2A-adrenoceptor/DraI RFLP had a significantly higher trunk to extremity skinfold ratio (= sum of subscapular+suprailiac+abdominal skinfolds/sum of biceps+triceps+medial calf skinfolds) compared to women without the allele (1.44 +/- 0.52 vs. 1.12 +/- 0.33; p < 0.005 after adjustment for age, p < 0.002 after adjustment for age and body mass index or for age and subcutaneous fat). Using the sib-pair linkage procedure, a significant inverse relationship was found between the proportion of alleles identical by descent shared by sibs at the alpha 2A RFLP marker locus and the squared differences of the trunk to extremity skinfold ratio (p = 0.02 after adjustment for age or for age and body mass index or for age and subcutaneous fat). For a beta 2-adrenoceptor/BanI RFLP, no significant association or linkage was found between fat distribution indicators and the marker. These results suggest that alpha 2A-adrenoceptor gene variability detected with DraI is associated with a relative subcutaneous fat pattern favoring accumulation of truncal-abdominal fat in women, and that the alpha 2A-adrenoceptor gene, or a locus in close proximity, may be linked to body fat distribution in humans independently of the overall level of fatness.

Heterogeneity in adipose tissue metabolism: causes, implications and management of regional adiposity

The observation that different patterns of adipose tissue distribution are associated with different metabolic abnormalities, has recently given new impetus to research in obesity. Due to several methodologic problems, however, many aspects of regional excess of adipose tissue are still poorly understood. Among them, the causes and the metabolic consequences of regional adiposity are particularly important. Heterogeneity in adipose tissue distribution may be determined by a combination of genetic and hormonal causes. Both factors may determine differences in metabolism of various adipose tissue compartments primarily by regulating LPL production, storage and release of triacylglycerols, and aromatization of androgens. Furthermore, changes in adipocyte sensitivity to hormones such as, sex steroids, glucocorticoids, insulin and adrenergic hormones may also regulate fat distribution in various adipose tissue compartments. The metabolic heterogeneity of adipose tissue from various compartments, particularly the differences between the "portal" and subcutaneous adipose tissues, may account for several metabolic abnormalities associated with "upper body adiposity". However, no direct evidence is available to confirm this hypothesis. Recent advances in the methodology to study adipose tissue distribution (mainly CT and MRI) may provide the necessary tools to evaluate the true impact of adiposity in various compartments on intermediary metabolism and to identify a "morbid" adipose tissue compartment. These observations may help in designing better therapeutic strategies targeted towards regional adiposity and its metabolic complications.

Functional involvement of α1and α2-adrenoceptors in86Rb efflux from liver slices and lipolysis in guinea-pig isolated adipocytes

1. The application of an alpha 1-adrenoceptor agonist, amidephrine, to guinea-pig liver slices increases glucose release and 86Rb efflux. Since prazosin was more potent than yohimbine in inhibiting both responses, alpha 1-adrenoceptors seem to be involved in the effects evoked by the agonist. 2. Clonidine (an alpha 2-adrenoceptor agonist) at doses unable to activate liver glycogenolysis increased 86Rb release and potentiated isoprenaline in promoting 86Rb efflux. Since yohimbine antagonized clonidine in promoting 86Rb efflux, alpha 2-adrenoceptors also seem to control plasmalemmal permeability to 86Rb. 3. The liver slice responses resulting from alpha 1- and alpha 2-adrenoceptor stimulation required extracellular calcium. Calcium absence or the administration of D-600 attenuated the effects of amidephrine on glucose release and 86Rb outflow and Ca2+ excess re-established both responses. D-600 and apamin blocked clonidine-induced 86Rb efflux, suggesting that alpha 2-adrenoceptor stimulation activates calcium dependent K+ channels. 4. alpha 2-adrenoceptors do not appear to mediate antilipolytic effects in guinea-pig fat cells.

Effect of a 7-day treatment with idazoxan and its 2-methoxy derivative RX 821002 [correction of RX 821001] on alpha 2-adrenoceptors and non-adrenoceptor idazoxan binding sites in rabbits

1. The present study investigates the influence of a 7-day treatment with 2 mg kg-1, s.c., twice daily of RX 821002 (an alpha 2-adrenoceptor antagonist which binds only to alpha 2-adrenoceptors) or idazoxan (alpha 2-antagonist which binds to alpha 2-adrenoceptors and also to non-adrenoceptor idazoxan binding sites: NAIBS) on alpha 2-adrenoceptor (labelled with [3H]-RX 821002) and NAIBS (labelled with [3H]-idazoxan) number in three tissues (adipocytes, colocytes and platelets) in the rabbit. 2. Acute administration of RX 821002 or idazoxan increased plasma non-esterified fatty acids (NEFA) and catecholamine levels with no change in plasma glucose levels. 3. The 7-day treatment with RX 821002 or idazoxan failed to influence food intake, total body weight or perirenal adipose tissue weight. 4. RX 821002 and idazoxan increased the number of [3H]-RX 821002 binding sites in adipose tissue with no change in colocytes or platelets. 5. RX 821002 and idazoxan failed to modify [3H]-idazoxan binding sites on adipocytes and colocytes. No significant [3H]-idazoxan binding was detected on rabbit platelets. 6. The results show that a 7-day treatment with alpha 2-antagonists induces an up-regulation in adipocyte alpha 2-adrenoceptors. In contrast, this phenomenon does not involve all the tissues since colocytes and platelets escape the effects of alpha 2-antagonists. The data suggest a differential regulation of alpha 2-adrenoceptors according to their location. 7. The fact that NAIBS did not vary suggests that alpha 2-adrenoceptors and NAIBS are two different entities. Finally, since RX 821002 and idazoxan exert similar effects after either acute or chronic treatment, it is suggested that NAIBS are not involved in the control of catecholamine release or in NEFA or glucose metabolism.

Regional differences in triglyceride breakdown in human adipose tissue: effects of catecholamines, insulin, and prostaglandin E2

Regional variation of adipose tissue triglyceride breakdown (lipolysis) has been suggested to play a role for the health consequences of some forms of obesity. Thus, in the present study we investigated the regulation of lipolysis in isolated adipocytes obtained from different fat depots in females. Intra-abdominal adipose tissue (omental) and subcutaneous abdominal adipose tissue were obtained from the same individuals undergoing abdominal surgery (n = 9); in addition, adipocytes from the subcutaneous gluteal region (n = 12) and from mammary adipose tissue (n = 5) were investigated. The lipolytic/antilipolytic properties of epinephrine (EPI), insulin, clonidine, and prostaglandin E2 (PGE2) were investigated. The most prominent observation was that EPI had none or only minor lipolytic effect in adipocytes from the subcutaneous regions, but significantly enhanced lipolysis by approximately 500% in omental adipocytes (P less than .001). In the presence of the alpha 2-adrenergic antagonist, yohimbine, EPI had similar stimulatory effects (fourfold to fivefold) in all fat depots. The antilipolytic compounds, insulin and clonidine, had greatly reduced antilipolytic properties in omental adipocytes as compared with subcutaneous adipocytes (P less than .01 and P less than .05, respectively). On the other hand, PGE2 had similar antilipolytic properties in adipocytes from the various depots. In conclusion, we found great regional variation in the regulation of lipolysis. Particularly, EPI was much more lipolytic in omental adipocytes than in subcutaneous adipocytes, mainly due to an enhanced functional alpha 2-receptor activity in subcutaneous adipocytes. These in vitro data suggest that free fatty acids (FFA) are more readily mobilized from omental adipose tissue than from subcutaneous adipose tissue.

A1 adenosine receptors in the human fat cell: tissue distribution and regulation of radioligand binding

Identification of adenosine A1 receptor binding sites was performed using the antagonist, [3H]1,3-dipropyl-8-cyclopentyl-xanthine ([3H]DPCPX), and the agonist [3H](-)-N6-R-phenylisopropyladenosine ([3H](-)-PIA) on human adipocyte membranes from different anatomical localizations (i.e. abdominal, femoral and omental fat deposits). Despite the strong antilipolytic effect initiated by various adenosine analogs, the human adipocytes were poorly equipped in adenosine A1 receptors whatever the anatomical location (Bmax less than or equal to 95 fmol/mg of protein for the antagonist [3H]DPCPX and less than or equal to 72 fmol/mg for the agonist [3H](-)-PIA). There was no marked difference between the three fat deposits in terms of maximal binding for both radioligands. Saturation and competition experiments showed that the proportion of receptors in the high-affinity state for the agonists was very high (70-91%), but only 33-44% of them were guanine nucleotide-sensitive. Moreover the guanine nucleotides were shown to enhance the specific binding of the antagonist [3H]DPCPX by decreasing its KD value. These binding properties are strongly different from those of another Gi-coupled receptor on the human fat cell, the alpha 2A-adrenoceptor, and indicates that the adenosine A1 receptor and the alpha 2-adrenoceptor could be differentially coupled with Gi proteins in the human fat cell.

Reserpine induces vascular alpha 2-adrenergic supersensitivity and platelet alpha 2-adrenoceptor up-regulation in dog

1. The aim of the present study was to investigate the influence of catecholamine levels on the regulation of alpha 2-adrenoceptor sensitivity in dogs. 2. Blood pressure and heart rate values at rest, plasma catecholamine levels, platelet and adipocyte alpha 2-adrenoceptors as well as the alpha 2-mediated cardiovascular responses to clonidine (10 micrograms kg-1 i.v., after alpha 1-, beta-adrenoceptor plus muscarinic blockade) or noradrenaline (0.5, 1, 2 and 4 micrograms kg-1 i.v. after alpha 1- and beta-adrenoceptor blockade) were measured before and after reserpine treatment (0.1 mg kg-1 day-1 s.c. over 15 days). 3. Reserpine induced a significant decrease in resting systolic and diastolic blood pressures (213 +/- 2/87 +/- 6 mmHg before vs 158 +/- 5/59 +/- 3 mmHg after treatment) as well as in heart rate (91 +/- 2 beats min-1 before vs 76 +/- 3 beats min-1 after treatment). 4. A 5 min tilt test performed under chloralose anesthesia, failed to modify blood pressure before treatment whereas it induced a significant fall in the same animals after the 15 day treatment. Plasma levels of noradrenaline significantly decreased (262 +/- 58 vs 66 +/- 31 pg ml-1) whereas plasma adrenaline levels were unchanged. 5. The alpha 2-mediated pressor responses to noradrenaline were significantly increased after reserpine. Clonidine induced a marked pressor effect (+72 and +45% in systolic and diastolic blood pressures respectively) after reserpine treatment. This effect was suppressed by administration of RX-821002, a new specific alpha 2-adrenoceptor antagonist. 6. Reserpine treatment significantly increased platelet alpha 2-adrenoceptor number (identified with [3H]- yohimbine or [3H]-RX821002) with no change in Kd values. alpha 2-Adrenoceptor number remained unchanged in adipocytes (identified with [3H]-RX821002). 7. These results show that a 15 day treatment with reserpine induces a vascular alpha 2-adrenergic supersensitivity and an up-regulation in platelet alpha 2-adrenoceptors. In contrast, this phenomenon does not involve all the tissues since adipocyte alpha 2-adrenoceptors escape the effect of reserpine. We suggest that the levels of plasma noradrenaline play an important role in the regulation of the platelet and vascular alpha 2-adrenoceptors. In contrast, adipocyte alpha 2-adrenoceptors are not affected by changes in plasma noradrenaline levels.

Identification of human platelet alpha 2-adrenoceptors with a new antagonist [3H]-RX821002, a 2-methoxy derivative of idazoxan

1. The binding of a new alpha 2-adrenoceptor antagonist, [3H]-RX821002 (2-(2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline), was investigated in human platelet membranes and compared with [3H]-yohimbine binding parameters. 2. Analysis of kinetic data revealed association and dissociation time courses consistent with a simple biomolecular reaction. Saturation isotherms showed that [3H]-RX821002 labelled a higher total number of alpha 2-binding sites (224 +/- 31 vs 168 +/- 24 fmol mg-1 protein) than [3H]-yohimbine and with higher affinity (Kd: 0.92 +/- 0.06 vs 1.51 +/- 0.08 nM). Moreover [3H]-RX821002 exhibited a lower percentage of nonspecific binding 3. The difference in total binding is due to a better labelling of the alpha 2-adrenoceptors in the low affinity state by [3H]-RX821002 since the labelled receptors number in high affinity state was identical with the two radioligands. 4. [3H]-RX821002 binding displayed a specificity similar to that obtained with [3H]-yohimbine. The potency of various compounds acting on adrenoceptors was: yohimbine greater than oxymetazoline greater than UK14304 greater than (-)-adrenaline greater than prazosin greater than or equal to (+)-adrenaline greater than isoprenaline. This order of potency is classical for an alpha 2A-adrenoceptor. 5. RX821002 is a more potent alpha 2-adrenoceptor antagonist than yohimbine on adrenaline-induced platelet aggregation. 6. These results indicate that [3H]-RX821002 is a suitable ligand for the identification of human platelet alpha 2-adrenoceptors.

Discrimination between alpha 2-adrenoceptors and [3H]idazoxan-labelled non-adrenergic sites in rabbit white fat cells

Previous results indicated that the rabbit could represent a suitable model for investigations on the functional role of alpha 2-adrenoceptors in fat cells, but the characterization of these receptors was not resolved yet. In the present report, imidazoline compounds were used to attempt a better definition of rabbit adipocyte alpha 2-receptivity by means of lipolysis and binding studies. Lipolysis data showed that UK14304 is a full alpha 2-adrenoceptor agonist promoting a strong antilipolysis in rabbit fat cells. Moreover, the methoxy derivative of idazoxan, RX821002, is a more potent antagonist of UK14304-induced antilipolysis than idazoxan or yohimbine. Whereas [3H]yohimbine failed to bind at rabbit adipocyte alpha 2-adrenoceptors, [3H]UK14304 and [3H]RX821002 are valuable tools to study this receptor. Analysis of binding data demonstrated that [3H]UK14304 labels the high-affinity-state receptor while [3H]RX821002 binds to the whole alpha 2-adrenergic population. Inhibition studies of [3H]RX821002 and [3H]UK14304 binding by various compounds confirmed the alpha 2-adrenergic nature of the sites labelled by both radioligands. The other alpha 2-adrenoceptor radioligand, [3H]idazoxan, labelled binding sites which are insensitive to catecholamines. Competition studies of [3H]idazoxan binding with imidazoline derivatives revealed structure-activity relationships different from those of alpha 2-adrenoceptors. The most striking observation is that substitutions in the 2-position of idazoxan markedly reduce the affinity for the non-adrenergic sites, whereas the alpha 2-potency is increased or unchanged.

Neuropeptide Y and peptide YY inhibit lipolysis in human and dog fat cells through a pertussis toxin-sensitive G protein

Neuropeptide Y (NPY) and peptide YY (PYY) are regulatory peptides that have considerable sequence homology with pancreatic polypeptide. Because (a) NPY has been shown to be colocalized with noradrenaline in peripheral as well as central catecholaminergic neurons, and (b) alpha 2-adrenergic receptors of adipocytes play a major role in the regulation of lipolysis, we investigated the effect of NPY and PYY on isolated fat cells. In human fat cells NPY and PYY promoted a dose-dependent inhibition of lipolysis elicited by 2 micrograms/ml adenosine deaminase (removal of adenosine) whatever the lipolytic index used (glycerol or nonesterified fatty acids). In dog fat cells NPY and PYY inhibited adenosine deaminase-, isoproterenol- and forskolin-induced lipolysis. In humans and dogs the effects of NPY or PYY were abolished by treatment of cells with Bordetella pertussis toxin, clearly indicating the involvement of a Gi protein in the antilipolytic effects. This study indicates that, in addition to alpha 2-adrenergic agonists, NPY and PYY are also involved in the regulation of lipolysis in human and dog adipose tissue as powerful antilipolytic agents. Further studies are needed to characterize the pharmacological nature of the receptor mediating the inhibitory effect of NPY and PYY in fat cells.

Imidazolinic radioligands for the identification of hamster adipocyte alpha 2-adrenoceptors

Imidazolinic radioligands ([3H]UK 14304, [3H]idazoxan and [3H]RX 821002) were used for the identification of alpha 2-adrenoceptors on hamster fat cell membranes since there are limitations to the use of [3H]yohimbine and [3H]clonidine, which suggest alpha 2-adrenoceptor heterogeneity. Biological assays (lipolysis measurements) were performed on isolated fat cells and binding studies were carried out on fat cell membranes. The imidazolinic derivative, UK 14304, was a full agonist as compared to clonidine. Idazoxan and RX 821002 (2-(2-methoxy-1,4-benzodioxan-2yl)-2-imidazoline), a recently developed alpha 2-antagonist, were more potent alpha 2-antagonists than yohimbine in this fat cell model. [3H]UK 14304 was the most suitable agent for the quantification of the 'high-affinity state' alpha 2-adrenoceptors in binding studies since it did not exhibit the sensitivity to the composition of the buffer shown by [3H]clonidine. Although it is a potent alpha 2-antagonist, [3H]idazoxan had major limitations for use in the identification of alpha 2-adrenoceptors in this cell model since it also bound to 'non-adrenaline displaceable' binding sites which were revealed when imidazolinic derivatives (phentolamine) were used instead of adrenaline to determine the non-specific binding. We demonstrated that [3H]RX 821002 was a more suitable radioligand than [3H]yohimbine for labelling hamster fat cell alpha 2-adrenoceptors (KD = 1.0 +/- 0.1 nM, Bmax = 776 +/- 60 fmol/mg protein). Moreover, since it exhibited low affinity for 'imidazoline-preferring sites', it represents a valuable ligand even in tissues possessing such binding sites. We suggest that [3H]RX 821002 can be used to identify alpha 2-adrenoceptors in various tissues when these sites cannot be labelled with [3H]yohimbine.

Lipomobilizing effects of procaterol and yohimbine in the conscious dog: comparison of endocrinological, metabolic and cardiovascular effects

1. Lipid mobilization during a hypocaloric diet may be enhanced by a pharmacological approach using beta 2-adrenoceptor agonists or alpha 2-adrenoceptor antagonists. Studies were undertaken in the dog, an animal model presenting fat cell antilipolytic alpha 2- and lipolytic beta-adrenoceptors, in order, firstly, to demonstrate the presence of beta 2 subtype adrenoceptors on adipocytes and, secondly, to compare the effects of procaterol (beta 2-adrenoceptor agonist) and of yohimbine (alpha 2-adrenoceptor antagonist) on metabolic, endocrinological and cardiovascular parameters. 2. Procaterol strongly stimulates lipolysis in dog adipocytes in vitro. The utilisation of selective beta 1- and beta 2-adrenoceptor antagonists (bisoprolol and ICI 118,551) in both lipolysis and binding studies (displacement of [3H]-dihydroalprenolol binding) demonstrated the presence of the two beta-adrenoceptor subtypes in dog fat cells. 3. Infusion of either yohimbine or procaterol (10 and 0.4 nmol min-1 kg-1, respectively for 30 min), provoked an equivalent increase in plasma non-esterified fatty acids (+100%). Procaterol, but not yohimbine, induced hyperglycaemia (+120%). Plasma insulin was weakly enhanced by yohimbine (+120%) as compared to the increase given by procaterol (+500%). 4. Both drugs stimulated sympathetic nervous system activity, as indicated by the increased plasma noradrenaline concentration, but only yohimbine increased the plasma adrenaline level. 5. Cardiovascular measurements indicated that procaterol strongly enhances heart rate and transiently decreases mean blood pressure. Yohimbine exhibits a weaker effect on heart rate and slightly increases mean blood pressure. 6. The present work clearly indicates that lipid mobilization is enhanced during fasting in the dog by selective beta 2-adrenoceptor stimulation or by alpha 2-adrenoceptor blockade. This enhanced lipolytic effect may result either from a direct action of the drugs on the adrenoceptors of fat cells or from an activation of the sympathetic nervous system. Procaterol suffers major limitations since it strongly increases heart rate, immunoreactive insulin and glycaemia. On the other hand, yohimbine induces only minor modifications both in cardiovascular and endocrinological parameters.

Microdialysis of intercellular adenosine concentration in subcutaneous tissue in humans

To determine the intercellular adenosine concentration the periumbilical subcutaneous interstitial fluid was investigated in five healthy subjects with two single dialysis fiber catheters perfused with isotonic saline at a rate of 2.5 mul/min. A newly developed in situ calibration technique (29) allows an estimation of the adenosine concentration in the perfusate equilibrating with the surrounding medium, i.e., the concentration in the interstitial fluid. The mean interstitial adenosine concentration was 128 +/- 26 nM (range 25-300 nM). The effect of these adenosine concentrations on adipose tissue metabolism was investigated in isolated human fat cells. The influence of 5 and 10 nM 2-chloroadenosine, which is equipotent to 150 and 300 nM adenosine, on the dose-response curve for the lipolytic effect of norepinephrine and the stimulatory effect of insulin on glucose transport was studied. 2-Chloroadenosine at 10 nM decreased basal lipolysis 53 +/- 8% and shifted the dose-response curve for norepinephrine approximately twofold to the right. At 5 nM, 2-chloroadenosine only slightly shifted the dose-response curve, whereas basal lipolysis was not significantly reduced. 2-Chloroadenosine at either concentration had no significant effect on basal or insulin-stimulated glucose transport. In conclusion, endogenous adenosine can reach concentrations in human subcutaneous tissue sufficient for an important modulating effect on lipolysis in vivo.

[3H]idazoxan binding at non-alpha 2-adrenoceptors in rabbit adipocyte membranes

The imidazoline ligand, [3H]idazoxan, labels a large population of high-affinity binding sites in rabbit fat cell membranes (Bmax = 1370 +/- 91 fmol/mg protein; KD = 1.6 +/- 0.6 nM) when imidazoline derivatives are used for definition of non-specific binding. [3H]Idazoxan sites are not alpha 2-adrenoceptors as assessed by competition studies which showed that epinephrine, norepinephrine and yohimbine do not inhibit [3H]idazoxan binding. Naphazoline, tramazoline and the Na+/H+ exchange inhibitor, amiloride, completely inhibited [3H]idazoxan binding. The Ki values were 9, 27 and 48 nM, respectively.

Alpha 2-antagonist compounds and lipid mobilization: evidence for a lipid mobilizing effect of oral yohimbine in healthy male volunteers

Investigations were carried out to analyse the interactions of alpha 2-antagonists (yohimbine, idazoxan, SK & F-86,466) with human fat cell alpha 2-adrenoceptors. All the alpha 2-antagonists enhanced the lipolytic potencies of epinephrine with an order of potency: yohimbine greater than idazoxan greater than SK & F-86,466; the same order was also found in 3H-yohimbine competition studies on human fat cell membranes. The most potent agent, yohimbine, was administered orally in humans to define the conditions of appearance and the time-course of a putative lipid-mobilizing action. Oral yohimbine administration (0.2 mg kg-1) elevated plasma glycerol and non-esterified fatty acids in fasting healthy subjects without significant action on heart rate or blood pressure during the time-course of the experiment. The lipid-mobilizing action of yohimbine was reinforced during physical exercise, completely suppressed after a meal and partially blocked by administration of propranolol (0.5 mg kg-1; 60 min before yohimbine). Plasma norepinephrine concentrations were increased (40-50%) after oral yohimbine administration. The rise in plasma catecholamine concentration elicited by yohimbine was not modified by propranolol treatment. The lipid-mobilizing effect of yohimbine could be attributable to: (i) the increase in synaptic norepinephrine with a resultant increment in lipolysis by beta-adrenergic agonism; (ii) a decrease in alpha 2-adrenoceptor stimulation of human fat cell alpha 2-adrenoceptors; (iii) a blockade of presynaptic alpha 2-adrenoceptors. The use of highly selective alpha 2-antagonists will allow investigations into alpha 2-adrenoceptors, which may represent a novel locus for pharmacological intervention in lipid-mobilization strategies.(ABSTRACT TRUNCATED AT 250 WORDS)

Influences of variation in total energy intake and dietary composition on regulation of fat cell lipolysis in ideal-weight subjects

Weight-maintaining fat-rich, "prudent," carbohydrate-rich, as well as energy-restricted diets (300 kcal/d) were fed in succession for 7 d to 12 healthy males of ideal body weight under metabolic ward conditions. At the end of each period isolated fat cells were prepared from subcutaneous abdominal adipose tissue and incubated in vitro in the absence or presence of adenosine deaminase, either alone or in combination with various lipolytic or antilipolytic hormones and agents. Variations in total energy intake and dietary composition had characteristic and specific effects on fat cell lipolysis in vitro. High carbohydrate and prudent diets resulted in low rates of nonstimulated glycerol release and impaired insulin action in the presence of adenosine deaminase (320 mU/ml). High-fat and energy restricted diets were characterized by high rates of nonstimulated glycerol release. Sensitivity of antilipolysis to insulin and prostaglandin E2 was 10 to 200 times lower respectively on energy-restricted than on fat-rich diets. The effects of alpha 2- and beta-adrenergic catecholamines and of N6-phenylisopropyladenosine were not affected by the preceding diets.

Heterogeneous distribution of beta and alpha-2 adrenoceptor binding sites in human fat cells from various fat deposits: functional consequences

Investigations have been carried out to explain the heterogeneity of response to catecholamines of human fat cells from various deposits. Adipocytes from two subcutaneous sites (abdominal and femoral) were studied concomitantly in women while omental fat cells were taken from another group of patients undergoing abdominal surgery. Alpha-2 and beta sites were identified in fat-cell membranes with [3H]yohimbine and [3H]dihydroalprenolol, respectively. Lipolytic responses were tested with isoproterenol (beta agonist), clonidine (alpha-2 agonist) and epinephrine. There are clear differences in the relative number of beta and alpha-2 sites according to the origin of the fat deposit; beta sites are less numerous than alpha-2 sites in subcutaneous fat cells of both regions (alpha-2:beta sites are in a ratio of 3 +/- 0.4:2 +/- 0.4). However, in membranes of omental fat cells, beta sites are at least as numerous as alpha-2 sites (ratio 0.9 +/- 0.2). Epinephrine always has a higher affinity for alpha-2 sites than for beta sites in the subcutaneous and omental deposits. In lipolysis studies, epinephrine, in the absence of adenosine in the incubation medium, initiated an anti-lipolytic effect in femoral fat cells and promoted inhibition of lipolysis at lower concentrations in abdominal subcutaneous fat cells, the effect being reversed at higher doses; epinephrine, however, was always lipolytic in omental adipocytes. There was no striking differences in the sensitivity to isoproterenol in the various deposits. Clonidine had a higher affinity for alpha-2 sites in femoral fat cells and was equipotent in the omental and abdominal ones. Thus, the differences in the lipolytic responses to epinephrine in adipocytes from different sites are linked to a variable alpha-2 inhibiting effect (and alpha-2 site number) rather than to a modified beta driven increase in lipolysis initiated by the physiological amine.

The antilipolytic effect of insulin in human adipocytes requires activation of the phosphodiesterase

Human fat cells were incubated with two different cAMP analogues, 8-bromocAMP and 6N-monobutyrylcAMP. The former analogue is an excellent substrate for the phosphodiesterase while the latter is resistant to hydrolysis. In the presence of adenosine deaminase, isoproterenol (10(-6)M) stimulated lipolysis 8-10 fold which was similar to the effect exerted by the cAMP analogues. Basal lipolysis and lipolysis activated by 6N-monobutyrylcAMP was not inhibited by insulin even at high concentrations, whereas the effect of 8-bromocAMP was virtually completely inhibited. This effect of insulin was completely prevented by the addition of IBMX. Thus, activation of phosphodiesterase by insulin is necessary to elicit the antilipolytic effect in human adipocytes.