Role of Insulin and Free Fatty Acids in the Regulation of ob Gene Expression and Plasma Leptin in Normal Rats

Objective: It is under debate whether free fatty acids (FFAs) play an independent role in the regulation of adipose cell functions. In this study, we evaluated whether leptin secretion induced by FFA is due directly to an increased FFA availability or whether it is mediated by insulin levels. Research Methods and Procedures: To test this hypothesis, we compared the effects of six different experimental designs, with different FFA and insulin levels, on plasma leptin: euglycemic clamp, euglycemic clamp + FFA infusion, FFA infusion alone, FFA + somatostatin infusion, somatostatin infusion alone, and saline infusion. Results: Our results showed that euglycemic clamp, FFA infusion, or both in combination induced a similar increment of circulating leptin (3.31 ± 0.30, 3.40 ± 0.90, and 3.35 ± 0.80 ng/mL, respectively). Moreover, the inhibition of FFA‐induced insulin increase by means of somatostatin infusion completely abolished the rise of leptin in response to FFA (1.05 ± 0.30 vs. 3.40 ± 0.90 ng/mL, p < 0.001). Discussion: In conclusion, our data showed that the effects of high FFA levels on plasma leptin were mediated by the rise of insulin concentration. These data confirm a major role for insulin in the regulation of leptin secretion from rat adipose tissue and support the hypothesis that leptin secretion is coupled to net triglyceride synthesis in adipose tissue.     

Relationships of Plasma Leptin Levels to Changes in Plasma Free Fatty Acids in Women Who Are Lean and Women Who Are Abdominally Obese

HENNES, MAGDA MI, ARNAVAZ DUA, DIANA L MAAS, GABRIELE E SONNENBERG, GLENN R KRAKOWER, AHMED H KISSEBAH. Relationships of plasma leptin levels to changes in plasma free fatty acids in women who are lean and women who are abdominally obese. Regulation of leptin production by the hormonal and metabolic milieu is poorly understood. Because abdominal obesity is commonly associated with elevated plasma free fatty acid (FFA) flux, we examined the effects of augmenting FFA on plasma leptin levels in women who were lean and of suppressing FFA in women with abdominal obesity. In study 1, nine subjects who were lean, after a 12-hour overnight fast, received either intravenous saline or Intralipid plus heparin to increase the plasma FFA concentration to approximately 1000 μmol/ L. After 3 hours of additional fasting, subjects underwent 3-hour hyperglycemic clamps. In study 2, seven subjects with abdominal obesity were evaluated by a similar protocol, but lipolysis and plasma FFA flux were instead maximally suppressed by acipimox. In the individuals who were lean, leptin levels were unchanged during clamping. Increasing plasma FFA reduced plasma leptin from 7.66 ± 0.66 to 7.05 ±0 0.66 (p=0.03), but 3 hours of hyperglycemia plus hyperinsulinemia had no additional effect on leptin levels (7.15 ± 0.71). Basal leptin levels, 4-fold higher in the subjects with obesity, were reduced from 34.6 ± 2.4 μg/L to 32.3 ± 1.1 μg/L (p=0.004) during the clamp period. When plasma FFA flux was suppressed, however, plasma leptin levels after clamped hyperglycemia/hyperinsulinemia were increased to 38.9 ± 1.2 μg/L (p=0.014 vs. time 0 and 0.001 vs. saline protocol). Changes in leptin concentrations are not correlated with changes in FFA. These results suggest that plasma FFA concentration does not regulate plasma leptin levels in basal, extended fasting, or hyperglycemic/hyperinsulinemic states.     

Metabolic Response to a Mixed Meal in Obese and Lean Women from Two South African Populations

Objective: Lower lipid and insulin levels are found during a glucose-tolerance test in obese black than obese white South African women. Therefore, β-cell function and lipid metabolism were compared in these populations during a mixed meal. Research Methods and Procedures: Blood concentrations of glucose, free fatty acids (FFAs), insulin, lipograms, and in vivo FFA oxidation were determined at fasting and for 7 hours after oral administration of a mixed emulsion containing glucose-casein-sucrose-lipid and [1-¹³C] palmitic acid in 8 lean black women (LBW), 10 obese black women (OBW), 9 lean white women (LWW), and 10 obese white women (OWW). Subcutaneous and visceral fat mass was assessed by computerized tomography. Results: Visceral fat area was higher in OWW (152.7 ± 17.0 cm²) than OBW (80.0 ± 6.7 cm²; p < 0.01). In OBW, 30-minute insulin levels were higher (604.3 ± 117.6 pM) than OWW (311.0 ± 42.9 pM; p < 0.05). Total triglyceride was higher in OWW (706.7 ± 96.0 mM × 7 hours) than OBW (465.7 ± 48.2 mM × 7 hours; p < 0.05) and correlated with visceral fat area (β = 0.38, p = 0.05). Palmitate oxidation was higher in lean than obese women in both ethnic groups and correlated negatively with fat mass (β = −0.58, p < 0.005). Discussion: The higher 30-minute insulin response in OBW may reflect a higher insulinotropic effect of FFAs or glucose. The elevated triglyceride level of OWW may be due to their higher visceral fat mass and possibly reduced clearance by adipose tissue.     

Alterations in fatty acid kinetics in obese adolescents with increased intrahepatic triglyceride content

Objective: It has been hypothesized that excessive fatty acid availability contributes to steatosis and the metabolic abnormalities associated with nonalcoholic fatty liver disease (NAFLD). The purpose of this study was to evaluate whether adipose tissue lipolytic activity and the rate of fatty acid release into plasma are increased in obese adolescents with NAFLD. Methods: Palmitate kinetics were determined in obese adolescents with normal (n = 9; BMI = 37 ± 2 kg/m²; intrahepatic triglyceride (IHTG) ≤5.5% of liver volume) and increased (n = 9; BMI = 36 ± 2 kg/m²; IHTG ≥ 10% of liver volume) IHTG content during the basal state (postabsorptive condition) and during physiological hyperinsulinemia (postprandial condition). Both groups were matched on body weight, BMI, percent body fat, age, sex, and Tanner stage. The hyperinsulinemic‐euglycemic clamp procedure, in conjunction with a deuterated palmitate tracer infusion, was used to determine free‐fatty acid (FFA) kinetics, and magnetic resonance spectroscopy was used to determine IHTG content. Results: The rate of whole‐body palmitate release into plasma was greater in subjects with NAFLD than those with normal IHTG content during basal conditions, (87 ± 7 vs. 127 ± 13 µmol/min; P < 0.01) and during physiological hyperinsulinemia, (24 ± 2 vs. 44 ± 8 µmol/min; P < 0.01). Discussion: These results demonstrate that adipose tissue lipolytic activity is increased in obese adolescents with NAFLD and results in an increase in the rate of fatty acid release into plasma throughout the day. This continual excess in fatty acid flux supports the hypothesis that adipose insulin resistance is involved in the pathogenesis of steatosis and contributes to the metabolic complications associated with NAFLD.     

Deconvolution as a novel approach to analyze moment-to-moment free fatty acid release

Objective: Recent studies have shown that free fatty acid (FFA) release is pulsatile and that this pattern is controlled by the sympathetic nervous system. It is, then, necessary to understand and characterize adipose tissue lipolysis to elucidate its effect on metabolism. In this study, we introduce deconvolution as a method to detect and quantify pulsatile FFA release. Research Methods and Procedures: Octanoate, a medium‐chain fatty acid, was infused in male mongrel dogs (n = 7) to mimic the pulsatile appearance of plasma FFAs. Deconvolution analysis was used to reconstruct the number and timing of infused octanoate pulses from plasma FFA concentrations. Results: Deconvolution analysis was able to reconstruct the exogenously infused pulses of octanoate used to mimic pulsatile appearance of FFAs (pulse frequency, 8 per hour; interpulse interval, 7 minutes). However, determination of pulse mass was less accurate (1.0 ± 0.0 vs. 0.54 ± 0.1 mM). The addition of varying levels of Gaussian noise to non‐oscillatory FFA time series did not lead to detection of extraneous FFA pulses. However, goodness of fit declined with increasing variability. Discussion: These results support the use of deconvolution as an accurate approach to determine the temporal sequence of endogenous FFA release.     

Muscle Type‐Dependent Responses to Insulin in Intramyocellular Triglyceride Turnover in Obese Rats

Objective: To understand the role of hyperinsulinemia in intramyocellular (imc) triglyceride (TG) accumulation and in regulating imcTG turnover. Research Methods and Procedures: imcTG was first prelabeled by continuous infusion of [U‐¹⁴C]glycerol (pulse), and then the rate of label loss from the prelabeled imcTG pool (turnover) in gastrocnemius, tibialis anterior, and soleus muscle of awake, high‐fat‐fed obese rats during the subsequent hyperinsulinemic‐euglycemic clamp experiments (chase) was determined. Results: Post‐absorptive basal fractional imcTG turnover rate in soleus was 0.010 ± 0.001/min, significantly lower than that in gastrocnemius (0.026 ± 0.002/min, p < 0.001) or tibialis anterior (0.030 ± 0.002/min, p < 0.0001), a pattern reciprocal to their imcTG pool size. Insulin infusion at 25 pmol/kg per minute resulted in pathophysiological hyperinsulinemia (5‐fold increase over the baseline value). This caused an increase in imcTG turnover by 3‐fold in soleus (0.029 ± 0.006/min, p = 0.002) but a decrease in gastrocnemius (0.012 ± 0.003/min, p = 0.001) and in tibialis anterior (0.0064 ± 0.001/min, p < 0.0001). Pathophysiological hyperinsulinemia suppressed hormone‐sensitive lipase activity in heart (p = 0.01) and mesenteric fat (p = 0.05) but not in skeletal muscle (p > 0.05). The pool size of imcTG was not affected by hyperinsulinemia. Discussion: The results demonstrated muscle‐type dependence in the response of imcTG turnover to hyperinsulinemia in the obesity model. The reciprocal insulin effects on imcTG turnover in oxidative vs. oxidative‐glycolytic muscle indicated a possibility that oxidative muscle contributes more to insulin resistance under hyperinsulinemia if imcTG‐fatty acid oxidation is a function of turnover. imcTG turnover does not seem to regulate imcTG pool size acutely.     

Higher Free Fatty Acid Uptake in Visceral Than in Abdominal Subcutaneous Fat Tissue in Men

Visceral adipose tissue has been shown to have high lipolytic activity. The aim of this study was to examine whether free fatty acid (FFA) uptake into visceral adipose tissue is enhanced compared to abdominal subcutaneous tissue in vivo. Abdominal adipose tissue FFA uptake was measured using positron emission tomography (PET) and [¹⁸F]‐labeled 6‐thia‐hepta‐decanoic acid ([¹⁸F]FTHA) and fat masses using magnetic resonance imaging (MRI) in 18 healthy young adult males. We found that FFA uptake was 30% higher in visceral compared to subcutaneous adipose tissue (0.0025 ± 0.0018 vs. 0.0020 ± 0.0016 µmol/g/min, P = 0.005). Visceral and subcutaneous adipose tissue FFA uptakes were strongly associated with each other (P < 0.001). When tissue FFA uptake per gram of fat was multiplied by the total tissue mass, total FFA uptake was almost 1.5 times higher in abdominal subcutaneous than in visceral adipose tissue. In conclusion, we observed enhanced FFA uptake in visceral compared to abdominal subcutaneous adipose tissue and, simultaneously, these metabolic rates were strongly associated with each other. The higher total tissue FFA uptake in subcutaneous than in visceral adipose tissue indicates that although visceral fat is active in extracting FFA, its overall contribution to systemic metabolism is limited in healthy lean males. Our results indicate that subcutaneous, rather than visceral fat storage plays a more direct role in systemic FFA availability. The recognized relationship between abdominal visceral fat mass and metabolic complications may be explained by direct effects of visceral fat on the liver.     

Metabolic and Hemodynamic Response of Adipose Tissue to Angiotensin II

Objective: Recent studies have revealed the presence of a local renin‐angiotensin system in adipose tissue. To examine the possible role of this system in adipose tissue, we performed microdialysis studies on the effect of angiotensin II (Ang II) on blood flow and metabolism in abdominal subcutaneous adipose tissue (aSAT) and femoral subcutaneous adipose tissue (fSAT) in young healthy men. Research Methods and Procedures: Using the microdialysis technique, two different protocols were run perfusion with Ringer's solution + 50 mM ethanol with the subsequent addition of 125, 250, and 500 μg/liter Ang II (n = 8) and Ringers's solution + 50 mM ethanol with the subsequent addition of isoproterenol (1 μM) alone and in combination with 500 μg/liter Ang II (n = 6). Dialysate concentrations of ethanol, glycerol, glucose, and lactate were measured for estimating blood flow (ethanol dilution technique), lipolysis, and glycolysis, respectively. Results: Perfusion with Ang II resulted in a dose‐dependent decrease in blood flow (fSAT > aSAT), lipolysis (fSAT > aSAT), and glucose uptake (fSAT = aSAT). Isoproterenol increased blood flow and lipolysis at both sites and those effects could be returned to baseline values by the addition of Ang II in aSAT but not fSAT. Discussion: In conclusion, our data indicate that in addition to its well‐known vasoconstricting effect, Ang II inhibits lipolysis in adipose tissue, whereby femoral fat depots seem to be more sensitive to this effect than abdominal depots.     

Aerobic exercise is necessary to improve glucose utilization with moderate weight loss in women

Objective: To determine the effects of weight loss (WL) alone and combined with aerobic exercise on visceral adipose tissue (VAT), intramuscular fat, insulin‐stimulated glucose uptake, and the rate of decline in free fatty acid (FFA) concentrations during hyperinsulinemia. Research Methods and Procedures: We studied 33 sedentary, obese (BMI = 32 ± 1 kg/m²) postmenopausal women who completed a 6‐month (three times per week) program of either WL alone (n = 16) or WL + aerobic exercise (AEX) (n = 17). Glucose utilization (M) was measured during a 3‐hour hyperinsulinemic‐euglycemic clamp (40 mU/m² per minute). M/I, the amount of glucose metabolized per unit of plasma insulin (I), was used as an index of insulin sensitivity. Results: Body weight, total fat mass, and percentage fat decreased similarly in both groups (p < 0.01). VAT, subcutaneous abdominal adipose tissue, mid‐thigh subcutaneous fat, and intramuscular fat decreased to a similar extent in both groups and between 14% and 27% after WL and WL+AEX (p < 0.05). WL alone did not change M or M/I; however, M and M/I increased 15% and 21% after WL+AEX (p < 0.05). Fasting concentrations and rate of decline of FFA did not change in either group. In stepwise regression models to determine the independent predictors of changes in M and M/I, the change in VAT was the single independent predictor of M (r² = 0.30) and M/I (r² = 0.33). Discussion: Intramuscular fat decreases similarly with 6 months of moderate WL alone or with aerobic exercise in postmenopausal women. In contrast, only WL combined with exercise results in increased glucose utilization and insulin sensitivity. These findings should be validated in a larger population.     

Epicardial Fat from Echocardiography: A New Method for Visceral Adipose Tissue Prediction

Objective: To validate transthoracic echocardiography as an easy and reliable imaging method for visceral adipose tissue (VAT) prediction. VAT is recognized as an important indicator of high cardiovascular and metabolic risk. Several methods are applied to estimate VAT, with different results. Research Methods and Procedures: We selected 60 healthy subjects (29 women, 31 men, 49.5 ± 16.2 years) with a wide range of body mass indexes. Each subject underwent transthoracic echocardiogram and magnetic resonance imaging (MRI) to measure epicardial fat thickness on the right ventricle. Measurements of epicardial adipose tissue thickness were obtained from the same echocardiographic and MRI views and points. MRI was also used to measure VAT cross‐sectional areas at the level of L4 to L5. Anthropometric indexes were also measured. Results: Subjects with predominant visceral fat accumulation showed higher epicardial adipose tissue thickness than subjects with predominant peripheral fat distribution: 9.97 ± 2.88 vs. 4.34 ± 1.98 (p = 0.005) and 7.19 ± 2.74 vs. 3.43 ± 1.64 (p = 0.004) in men and women, respectively. Simple linear regression analysis showed an excellent correlation between epicardial adipose tissue and waist circumference (r = 0.895, p = 0.01) and MRI abdominal VAT (r = 0.864, p = 0.01). Multiple regression analysis showed that epicardial adipose tissue thickness (r² = 0.442, p = 0.02) was the strongest independent variable correlated to MRI VAT. Bland test confirmed the good agreement between the two methods. Discussion: Epicardial adipose tissue showed a strong correlation with anthropometric and imaging measurements of VAT. Hence, transthoracic echocardiography could be an easy and reliable imaging method for VAT prediction.     

Visceral and Subcutaneous Adipose Tissue Diacylglycerol Acyltransferase Activity in Humans

Diacylglycerol acyltransferase (DGAT) could be a rate limiting step in triglyceride (TG) synthesis as it is the final step in this pathway. As such, between depot differences in DGAT activity could influence regional fat storage. DGAT activity and in vitro rates of direct free fatty acid (FFA) storage were measured in abdominal subcutaneous and omental adipose tissue samples from 12 nonobese (BMI <30 kg/m²) and 23 obese men and women (BMI >30 kg/m²) undergoing elective surgery. DGAT activity was greater in omental than in abdominal subcutaneous adipose tissue from nonobese patients (2.0 ± 0.9 vs. 0.9 ± 0.3 pmol/min/mg lipid, respectively, P = 0.003), but not from obese patients (1.4 ± 0.6 vs. 1.7 ± 0.7 pmol/min/mg lipid, respectively, P = 0.10). DGAT activity per unit adipose weight was negatively correlated with adipocyte size (P < 0.01) and positively correlated with direct FFA storage in omental (P < 0.001) but not in abdominal subcutaneous fat. Tissue DGAT activity varies as a function of adipocyte size, but this relationship differs between visceral and abdominal subcutaneous fat in obese and nonobese humans. Our results are consistent with the hypothesis that interindividual variations in DGAT activity may be an important regulatory step in visceral adipose tissue FFA uptake/storage.     

Fatty acid cycling in the fasting rat

Adipose tissue lipolysis and fatty acid reesterification by liver and adipose tissue were investigated in rats fasted for 15 h under basal and calorigenic conditions. The fatty acid flux initiated by adipose fat lipolysis in the fasted rat is mostly futile and is characterized by reesterification of 57% of lipolyzed free fatty acid (FFA) back into adipose triglycerides (TG). About two-thirds of FFA reesterification are carried out before FFA release into plasma, whereas the rest consists of plasma FFA extracted by adipose tissue. Thirty-six percent of the fasting lipolytic flux is accounted for by oxidation of plasma FFA, whereas only a minor fraction is channeled into hepatic very low density lipoprotein-triglycerides (VLDL-TG). Total body calorigenesis induced by thyroid hormone treatment and liver-specific calorigenesis induced by treatment with beta, beta'-tetramethylhexadecanedioic acid (Medica 16) are characterized by a 1.7- and 1.3-fold increase in FFA oxidation, respectively, maintained by a 1.5-fold increase in adipose fat lipolysis. Hepatic reesterification of plasma FFA into VLDL-TG is negligible under both calorigenic conditions. Hence, total body fatty acid metabolism is regulated by adipose tissue as both source and sink. The futile nature of fatty acid cycling allows for its fine tuning in response to metabolic demands.     

Acyl Coenzyme A Synthetase Regulation: Putative Role in Long‐Chain Acyl Coenzyme A Partitioning

Objective: Long‐chain acyl coenzyme A synthetase (ACSL) converts free fatty acids (FFAs) into their metabolizable long‐chain acyl coenzyme A (LC‐CoA) derivatives that are essential for FFA conversion to CO₂, triglycerides, or complex lipids. ACSL‐1 is highly expressed in adipose tissue with broad substrate specificity. We tested the hypothesis that ACSL localization, and resulting local generation of LC‐CoA, regulates FFA partitioning. Research Methods and Procedures: These studies used cell fractionation of rat adipocytes to measure ACSL activity and mass and compared cells from young, mature, fed, fasted, and diabetic rats. Functional studies included measurement of FFA oxidation, complex lipid synthesis, and LC‐CoA levels. Results: High ACSL specific activity was expressed in the mitochondria/nuclei (M/N), high‐density microsomes (HDM), low‐density microsomes (LDM), and plasma membrane (PM) fractions. We show here that, during fasting, total FFA oxidation increased, and, although total ACSL activity decreased, a greater percentage of activity (43 ± 1.5%) was associated with the M/N fraction than in the fed state (23 ± 0.3%). In the fed state, more ACSL activity (34 ± 0.5%) was associated with the HDM than in the fasted state (25 ± 0.9%), concurrent with increased triglyceride formation from FFA. Insulin increased LC‐CoA and ACSL activity associated with the PM. The changes in ACSL activity in response to insulin were associated with only minor changes in mass as determined by Western blotting. Discussion: It is hypothesized that ACSL plays an important role in targeting FFA to specific metabolic pathways or acylation sites in the cell, thus acting as an important control mechanism in fuel partitioning. Localization of ACSL at the PM may serve to decrease FFA efflux and trap FFA within the cell as LC‐CoA.     

Anti‐lipolytic Effects of Insulin in African American and White Prepubertal Boys

Objective: Relative to whites, African Americans have lower circulating triglycerides (TG) and greater highdensity lipoprotein cholesterol. The metabolic basis for this difference is not known. This study was conducted to test the hypothesis that insulin‐induced suppression of free fatty acids (FFA) results in lower serum TG in African American versus white prepubertal children. Research Methods and Procedures: Insulin, FFA, and TG were determined at baseline and during a frequently sampled, intravenous glucose tolerance test in eight African American and eight white prepubertal males pair‐matched for whole‐body insulin sensitivity. Results: Baseline TG was lower in African Americans (0.43 ± 0.10 vs. 0.79 ± 0.37 mM/L; mean ± SD; p < 0.01). African Americans had higher peak insulin (218 ± 102 vs. 100 ± 30 pM/L; mean ± SD; p < 0.01) and a greater acute insulin response (9282 ± 4272 vs. 4230 ± 1326 pM/L × 10 minutes; mean ± SD; p < 0.05). FFA and TG values determined at the FFA nadir were lower in African Americans (0.26 ± 0.02 vs. 0.30 ± 0.03 mEq/L; mean ± SD; p < 0.01 for FFA nadir and 0.49 ± 0.07 vs. 0.77 ± 0.33 mM/L; mean ± SD; p < 0.05 for TG). Among all subjects, FFA nadir was correlated with peak insulin (r = ?0.54; p < 0.05). After adjusting for FFA nadir, neither baseline nor postchallenge TG differed with ethnicity (p = 0.073 and 0.192, respectively). The ethnic difference in FFA nadir disappeared after adjusting for peak insulin (p = 0.073). Discussion: These data suggest that hyperinsulinemiainduced suppression of FFA among African Americans is a determinant of lower TG in this group.     

Regulation of Lipolysis and Lipoprotein Lipase after Weight Loss in Obese,Postmenopausal Women

Objective: To test the hypothesis that the greater β‐adrenoceptor (β‐AR)‐stimulated lipolysis and sensitivity (half‐maximal lipolytic response) in abdominal (ABD) adipocytes, greater gluteal (GLT) adipose tissue‐lipoprotein lipase (AT‐LPL) activity, and dyslipidemia associated with obesity in older women are modifiable by weight loss (WL) and are not due to menopause or aging. Research Methods and Procedures: The metabolic effects of 6 months of hypocaloric diet and low‐intensity walking WL program on the regional regulation of in vitro lipolysis and AT‐LPL activity in subcutaneous ABD and GLT adipocytes were measured in 34 obese (48.7 ± 0.7% body fat, mean ± SE) postmenopausal (59 ± 1 years) white women. Results: The lipolytic responsiveness to the β‐AR agonist isoproterenol and basal lipolysis in the presence of 1 U/mL adenosine deaminase‐uninhibited (lipolysis) were greater (p < 0.01) in ABD than GLT adipocytes before and after WL, but there were no regional differences in postreceptor (dibutyryl 3′, 5′‐cyclic adenosine monophosphate)‐stimulated lipolysis. β‐AR sensitivity was greater in ABD than GLT adipocytes before (p < 0.01) but not after WL. Regional AT‐LPL did not change after WL, but the change in the activity of ABD (but not GLT) AT‐LPL correlated with the baseline adenosine deaminase‐uninhibited lipolysis (r = 0.38, p = 0.03). There were no relationships between the declines in plasma triglyceride or increases in high‐density lipoprotein cholesterol associated with WL and the changes in regional fat cell metabolism. Discussion: Thus, despite improving lipoprotein lipid profiles in obese, postmenopausal women, WL does not affect the regulation of regional fat metabolism, and a greater tonic inhibition of basal lipolysis by endogenous adenosine may increase the activity of AT‐LPL after WL and predispose older women to develop ABD adiposity.     

Plasma Leptin Response to an Epinephrine Infusion in Lean and Obese Women

Objective: Because leptin production by adipose tissue is under hormonal control, we examined the impact of epinephrine administration on plasma leptin concentrations. Research Methods and Procedures: We measured plasma leptin, insulin, and free fatty acid (FFA) responses after a 60-minute epinephrine infusion (0.010 μg/kg fat free mass/min) followed by a 30-minute recovery period (no infusion) in a group of 11 lean (mean body mass index ± SD: 22.6 ± 1.1 kg/m²) and 15 obese (30.0 ± 1.3 kg/m²) premenopausal women. Leptin, insulin, and FFA levels were measured in plasma before (−15 and 0 minutes) and at every 30 minutes over the 90-minute period. Results: In both lean and obese individuals, plasma leptin was significantly reduced by epinephrine (p < 0.0001). Body fat mass was associated with fasting leptin levels (r = 0.64, p < 0.0005) as well as with the decrease in leptinemia (r = −0.51, p < 0.01) produced by epinephrine administration. Furthermore, we noted a large range of leptin response to epinephrine among our subjects, especially in obese women (from −12 to −570 ng/mL per 60 minutes). However, there was no association between postepinephrine leptin and FFA levels (r = −0.14, p = 0.55). Discussion: Results of this study indicate that leptin levels decrease after epinephrine administration in both lean and obese premenopausal women. However, the heterogeneity in the response of leptin to catecholamines suggests potential alterations of the leptin axis that may contribute to generate a positive energy balance and, thus, may favor weight gain in some obese individuals.     

Neuropeptides,Including Neuropeptide Y and Melanocortins,Mediate Lipolysis in Murine Adipocytes

Objective: To determine whether key appetite‐regulating neuropeptides such as melanin‐concentrating hormone (MCH), neuropeptide Y (NPY), and α‐melanocyte—stimulating hormone (α‐MSH), which are known to mediate energy balance through centrally mediated pathways, also have direct acute effects on the lipolytic activity of murine adipocytes. Research Methods and Procedures: Fully differentiated 3T3‐L1 adipocytes serum starved overnight in Dulbecco's modified Eagle medium containing 2% bovine serum albumin or freshly isolated mouse adipocytes were incubated for up to 2 hours in the absence and presence of 100 nM each of NPY, MCH, α‐MSH, the melanocortin receptor agonist MTII, or isoproterenol as a control. Free fatty acids secreted into the incubation medium were measured using a commercially available nonesterified fatty acid C test kit. Results: Treatment of 3T3‐L1 cells with 100 nM NPY decreased basal free fatty acid secretion (basal, 0.006 ± 0.001 vs. NPY, 0.001 ± 0.0003 nM at 90 minutes; p < 0.05), whereas both α‐MSH and MTII stimulated up to a 7‐fold increase in free fatty acid release (MTII, 0.238 ± 0.004 vs. basal, 0.024 ± 0.002 nM at 2 hours; p < 0.05; and α‐MSH, 0.22 ± 0.005 vs. basal, 0.04 ± 0.003 nM at 2 hours; p < 0.05). Treatment with 100 nM MCH had no effect on basal free fatty acid release or on α‐MSH—induced lipolysis during concurrent treatment. Conversely, concurrent treatment with 100 nM NPY dramatically inhibited (by ～90%) α‐MSH—induced lipolysis. Similar treatment of freshly isolated mouse adipocytes showed virtually identical results. Discussion: In addition to their centrally mediated actions, appetite‐regulating neuropeptides modulate adipose tissue mass through direct peripheral effects. Systemic administration of pharmacological agents altering the effects of these neuropeptides may form the basis of future obesity therapies. Thus, some of these agents will likely have direct effects on adipocytes that may serve to alter their therapeutic effectiveness.     

The effect of moderate alcohol consumption on fat distribution and adipocytokines

Objective: To investigate the effect of moderate alcohol consumption on fat distribution, adipose tissue secreted proteins (adiponectin and resistin), and insulin sensitivity in healthy middle‐aged men with abdominal obesity. Research Methods and Procedures: Thirty‐four healthy men between 35 and 70 years old, with increased waist circumference (≥94 cm), participated in a randomized, controlled cross‐over design trial. They drank 450 mL of red wine (40 grams of alcohol) or 450 mL of de‐alcoholized red wine daily during 4 weeks. At the end of each treatment period, fat distribution, adipose tissue proteins, and insulin sensitivity index (ISI) were measured. Results: Subcutaneous and abdominal fat contents and body weight did not change after 4 weeks of moderate alcohol consumption. Liver fat (quip index) was slightly higher after consumption of red wine (6.8 ± 0.1) as compared with de‐alcoholized red wine (6.5 ± 0.1) but not significantly different (p = 0.09). Plasma adiponectin concentration increased (p < 0.01) to 6.0 ± 0.1 μg/mL after 28 days of moderate alcohol consumption compared with de‐alcoholized red wine (5.5 ± 0.1 μg/mL). Serum resistin concentrations and ISI were not affected by alcohol consumption. Percentage changes in serum resistin correlated significantly with changes in ISI (r = ?0.69, p < 0.01), whereas this correlation was not present between changes in plasma adiponectin and ISI (r = 0.31, p = 0.22). Discussion: Moderate alcohol consumption for 4 weeks is not associated with differences in subcutaneous and abdominal fat contents or body weight. Thus, the 10% increase in adiponectin was not associated with a change in fat distribution or body weight change.