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

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

Tocopherol mobilization during intensive exercise

This work shows that the level of plasma tocopherol (vitamin E) which has free radical scavenging properties rises significantly during intensive exercise. It is proposed that mobilization of tocopherol could help to prevent lipoperoxidation phenomena occurring in exercising skeletal muscle. A hypothetical mechanism relating to a lipolysis effect is discussed to explain this mobilization.     

Effect of beta-adrenergic blockade on lipid mobilization induced by fasting in dogs

To evaluate the contribution of catecholamines to the fasting-induced lipid mobilization prolonged or acute blockade of beta-adrenergic receptors with propranolol was applied in dogs during 72 hrs of food withdrawal. Propranolol given orally in a dose of 15 mg twice daily throughout the whole period of fasting failed to modify the increases in the plasma FFA and glycerol concentrations. The acute beta-adrenergic blockade due to i.v. injection of propranolol (0.5 mg/kg b.w.) caused marked decreases in the plasma glycerol concentration both in the dogs fasting for 24 h and 72 hrs, whereas the effects of propranolol on the plasma FFA concentration was found only in the early stage of fasting. Plasma catecholamine concentrations were enhanced significantly by the 72 hrs food withdrawal and neither prolonged nor acute propranolol administration modified significantly this effect. The fasting-induced decreases in the serum insulin concentration were more pronounced in dogs treated with propranolol. Results of this study indicate that catecholamines are involved in the control of lipolysis during short term starvation. However, under these conditions beta-adrenergic blockade did not impair FFA mobilization most probably due to an enhanced contribution of other hormones to the control of this process.     

Exercise-induced lipid mobilization in subcutaneous adipose tissue is mainly related to natriuretic peptides in overweight men

Involvement of sympathetic nervous system and natriuretic peptides in the control of exercise-induced lipid mobilization was compared in overweight and lean men. Lipid mobilization was determined using local microdialysis during exercise. Subjects performed 35-min exercise bouts at 60% of their maximal oxygen consumption under placebo or after oral tertatolol [a beta-adrenergic receptor (AR) antagonist]. Under placebo, exercise increased dialysate glycerol concentration (DGC) in both groups. Phentolamine (alpha-AR antagonist) potentiated exercise-induced lipolysis in overweight but not in lean subjects; the alpha(2)-antilipolytic effect was only functional in overweight men. After tertatolol administration, the DGC increased similarly during exercise no matter which was used probe in both groups. Compared with the control probe under placebo, lipolysis was reduced in lean but not in overweight men treated with the beta-AR blocker. Tertatolol reduced plasma nonesterified fatty acids and insulin concentration in both groups at rest. Under placebo or tertatolol, the exercise-induced changes in plasma nonesterified fatty acids, glycerol, and insulin concentrations were similar in both groups. Exercise promoted a higher increase in catecholamine and ANP plasma levels after tertatolol administration. In conclusion, the major finding of our study is that in overweight men, in addition to an increased alpha(2)-antilipolytic effect, the lipid mobilization in subcutaneous adipose tissue that persists during exercise under beta-blockade is not dependent on catecholamine action. On the basis of correlation findings, it seems to be related to a concomitant exercise-induced rise in plasma ANP when exercise is performed under tertatolol intake and a decrease in plasma insulin.     

Adrenergic control of lipolysis and metabolic responses in obesity

Adrenergic modulation of lipolysis was determined in obese and lean women. Epinephrine was infused alone, or in combination with propranolol, or with phentolamine. In both obese and lean subjects slight alpha- and prevalent beta-adrenergic lipolytic responsiveness was observed. alpha-adrenergic blockade by yohimbine potentiated lipolysis and exercise energy expenditure. Yohimbine application during the slimming treatment increased weight loss without side effects.     

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

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

High-fat diet elevates resting intramuscular triglyceride concentration and whole body lipolysis during exercise

This study determined the role of intramuscular triglyceride (IMTG) and adipose lipolysis in the elevated fat oxidation during exercise caused by a high-fat diet. In four separate trials, six endurance-trained cyclists exercised at 50% peak O2 consumption for 1 h after a two-day control diet (22% fat, CON) or an isocaloric high-fat diet (60% fat, HF) with or without the ingestion of acipimox, an adipose lipolysis inhibitor, before exercise. During exercise, HF elevated fat oxidation by 72% and whole body lipolysis [i.e., the appearance rate of glycerol in plasma (Ra glycerol)] by 79% compared with CON (P < 0.05), and this was associated with a 36% increase (P < 0.05) in preexercise IMTG concentration. Although acipimox lowered plasma free fatty acid (FFA) availability, HF still increased fat oxidation and Ra glycerol to the same magnitude above control as the increase caused by HF without acipimox (i.e., both increased fat oxidation 13-14 micromol.kg(-1).min(-1)). In conclusion, the marked increase in fat oxidation after a HF diet is associated with elevated IMTG concentration and whole body lipolysis and does not require increased adipose tissue lipolysis and plasma FFA concentration during exercise. This suggests that altered substrate storage in skeletal muscle is responsible for increased fat oxidation during exercise after 2 days of an HF diet.     

Metabolic and hormonal response to physical exercise during beta 1-selective and non-selective beta-blockade

The effects of a beta 1-selective (metoprolol, 150 mg per day) and a non-selective beta-blocking agent (propranolol, 120 mg per day) on metabolic and hormonal responses to physical exercise (a 30 min bicycle ergometer test) were investigated against placebo in seven healthy male volunteers with a double blind cross-over design. The blood glucose level remained unchanged during placebo, it tended to increase during metoprolol, whereas it decreased during propranolol. Both metoprolol and propranolol counteracted the exercise-induced increase in plasma free fatty acids and caused a slight decrease in muscle glycogenolysis. The increase in blood lactate concentration during exercise was not influenced by beta-blockade. The secretion of glucagon and cortisol was not modified significantly by beta-blockade, whereas the growth hormone response to exercise was promoted equally by both beta-blocking agents. It has been assumed previously that, during treatment with beta-blocking agents, diminished hepatic gluconeogenesis, caused by the lack of lactate or free fatty acids, may result in a decline in blood glucose levels. The present results indicate that an inhibition of beta 2-mediated hepatic glycogenolysis by propranolol may also influence blood glucose homeostasis during exercise.     

Exercise induces pentane production and neutrophil activation in humans. Effect of propranolol

The effect of beta-adrenergic receptor blockade on exercise-induced lipid peroxidation in man has been examined by measuring the production of pentane in expired air. For this purpose, five healthy male subjects were subjected to dynamic exercise of graded intensity on a cycle ergometer (10 min at 45%, 5 min at 60% and 75% maximal oxygen uptake 1 h after ingestion of either a placebo or 40-mg propranolol. At rest, mean pentane concentration [( pent]) with placebo was 4.13 pmol.l-1, SD 2.14. After exercise, this value significantly increased by 310% (17.1 pmol.l-1, SD 7.73, P less than 0.01). Oral administration of 40-mg propranolol significantly lowered the mean resting [pent] to 1.75 pmol.l-1, SD 0.77, P less than 0.05. After exercise, the increase of [pent] was much smaller (240%) and was less significant (P less than 0.2) than with the placebo. The mechanism of this inhibitory effect of propranolol remains to be elucidated. However, as indicated by the measurement of plasma myeloperoxidase concentration, it can be concluded that the antioxidant property of propranolol cannot be attributed to the inhibition of neutrophil activation, a possible source of free radicals during exercise.     

Hormone-sensitive lipase is necessary for normal mobilization of lipids during submaximal exercise

For the working muscle there are a number of fuels available for oxidative metabolism, including glycogen, glucose, and nonesterified fatty acids. Nonesterified fatty acids originate from lipolysis in white adipose tissue, hydrolysis of VLDL triglycerides, or hydrolysis of intramyocellular triglyceride stores. A key enzyme in the mobilization of fatty acids from intracellular lipid stores is hormone-sensitive lipase (HSL). The aim of the present study was to investigate the metabolic response of HSL-null mice challenged with exercise or fasting and to examine whether other lipases are able to fully compensate for the lack of HSL. The results showed that HSL-null mice have reduced capacity to perform aerobic exercise. The liver glycogen stores were more rapidly depleted in HSL-null mice during treadmill exercise, and HSL-null mice had reduced plasma concentrations of both glycerol and nonesterified fatty acids after exercise and fasting, respectively. The data support the hypothesis that in the absence of HSL, mice are not able to respond to an exercise challenge with increased mobilization of the lipid stores. Consequently, the impact of the lipid-sparing effect on liver glycogen is reduced in the HSL-null mice, resulting in faster depletion of this energy source, contributing to the decreased endurance during submaximal exercise.     

Lipolysis in human adipose tissue during exercise: comparison of microdialysis and a-v measurements.

Subcutaneous adipose tissue lipolysis was studied in vivo by Fick's arteriovenous (a-v) principle using either calculated (microdialysis) or directly measured (catheterization) adipose tissue venous glycerol concentration. We compared results during steady-state (rest and prolonged continuous exercise), as well as during non-steady-state (onset of exercise and early exercise) experimental settings. Fourteen healthy women [age: 74 +/- 1 (SE) yr] were studied at rest and during 60-min continuous bicycling at 60% of peak O(2) uptake. Calculated and measured subcutaneous abdominal adipose tissue venous glycerol concentrations increased substantially from rest to exercise but were similar both at rest and during later stages of exercise. In contrast, during the initial approximately 40 min of exercise, calculated glycerol concentration was significantly lower (approximately 40%) than measured adipose tissue venous glycerol concentration. Despite several methodological limitations inherent to both techniques, the results strongly suggest that microdialysis and catheterization provide similar estimates of subcutaneous adipose tissue lipolysis in steady-state experimental settings like rest and continuous prolonged exercise. However, during shorter periods of exercise (<40 min), the results from the two techniques may differ quantitatively in the studied subjects. Caution should, therefore, be taken when lipolysis is evaluated, based on results obtained by the two techniques under non-steady-state conditions.     

Physiological and pathophysiological features of the control of lipolysis and lipid mobilization by natriuretic peptides

We have demonstrated a potent and specific lipolytic effect of natriuretic peptides (NP) in human and primates' fat cells. The lipolytic effect of NP is mediated through intracellular production of cGMP and activation of the cGMP-dependent kinase 1alpha. Local infusion of atrial-NP (ANP), directly within the subcutaneous adipose tissue through a microdialysis catheter, increases lipolysis and stimulates blood flow through its vasodilating effect in lean healthy men. This effect is blunted in overweight men and can be recovered by endurance training. Intravenous infusion of physiological doses of ANP induces lipid mobilization. Higher concentrations of ANP that are encountered during heart failure also stimulate lipid oxidation. ANP activates lipolysis and free fatty acids release from adipose tissue during endurance exercise. This effect is paradoxically amplified when exercise is performed under beta-blockade treatment, because of an enhanced cardiac release of ANP. No gender differences in ANP-induced lipid mobilization during exercise have been found. Heart failure is associated with high circulating levels of NP that could participate to the progression toward cachexia. On contrary, a negative correlation between NP levels and body mass index is found in obese persons. The molecular basis of this inverse correlation is not yet demonstrated from a functional standpoint. Further studies are needed to clearly define the pathophysiological role of NP in obesity and heart failure.     

Sex differences in lipolysis-regulating mechanisms in overweight subjects: effect of exercise intensity

Objective: To explore sex differences in the regulation of lipolysis during exercise, the lipid‐mobilizing mechanisms in the subcutaneous adipose tissue (SCAT) of overweight men and women were studied using microdialysis. Research Methods and Procedures: Subjects matched for age, BMI, and physical fitness performed two 30‐minute exercise bouts in a randomized fashion: the first test at 30% and 50% of their individual maximal oxygen uptake (Vo _2max) and the second test at 30% and 70% of their Vo _2max. Results: In both groups, an exercise‐dependent increment in extracellular glycerol concentration (EGC) was observed. Whatever the intensity, phentolamine [α‐adrenergic receptor (AR) antagonist] added to a dialysis probe potentiated exercise‐induced lipolysis only in men. In a probe containing phentolamine plus propranolol (β‐AR antagonist), no changes in EGC occurred when compared with the control probe when exercise was performed at 30% and 50% Vo _2max. A significant reduction of EGC (when compared with the control probe) was observed in women at 70% Vo _2max. At each exercise power, the plasma non‐esterified fatty acid and glycerol concentrations were higher in women. Exercise‐induced increase in plasma catecholamine levels was lower in women compared with men. Plasma insulin decreased and atrial natriuretic peptide increased similarly in both groups. Discussion: Overweight women mobilize more lipids (assessed by glycerol) than men during exercise. α₂‐Anti‐lipolytic effect was functional in SCAT of men only. The major finding is that during low‐to‐moderate exercise periods (30% and 50% Vo _2max), lipid mobilization in SCAT relies less on catecholamine‐dependent stimulation of β‐ARs than on an increase in plasma atrial natriuretic peptide concentrations and the decrease in plasma insulin.     

Atrial natriuretic peptide contribution to lipid mobilization and utilization during head-down bed rest in humans

Head-down bed rest (HDBR) increases plasma levels of atrial natriuretic peptide (ANP) and decreases norepinephrine levels. We previously demonstrated that ANP promotes lipid mobilization and utilization, an effect independent of sympathetic nervous system activation, when infused into lean healthy men at pharmacological doses. The purpose of the present study was to demonstrate that a physiological increase in ANP contributes to lipid mobilization and oxidation in healthy young men. Eight men were positioned for 4 h in a sitting (control) or in a HDBR position. Indexes of lipid mobilization and hormonal changes were measured in plasma. Extracellular glycerol, an index of lipolysis, was determined in subcutaneous adipose tissue (SCAT) with a microdialysis technique. A twofold increase in plasma ANP concentration was observed after 60 min of HDBR, and a plateau was maintained thereafter. Plasma norepinephrine decreased by 30-40% during HDBR, while plasma insulin and glucose levels did not change. The level of plasma nonesterified fatty acids was higher during HDBR. SCAT lipolysis, as reflected by interstitial glycerol, as well as interstitial cGMP, the second messenger of the ANP pathway, increased during HDBR. This was associated with an increase in blood flow observed throughout HDBR. Significant changes in respiratory exchange ratio and percent use of lipid and carbohydrate were seen only after 3 h of HDBR. Thus the proportion of lipid oxidized increased by 40% after 3 h of HDBR. The rise in plasma ANP during HDBR was associated with increased lipolysis in SCAT and whole body lipid oxidation. In this physiological setting, independent of increasing catecholamines, our study suggests that ANP contributes to lipid mobilization and oxidation in healthy young men.     

Effect of phenylephrine on lipolysis in rat adipocytes: no evidence for an alpha-adrenergic mechanism

Phenylephrine, a strong alpha 1-adrenergic agonist, exerted a concentration dependent antilipolytic effect against isoproterenol-activated lipolysis in rat adipocytes with the effect decreasing as the isoproterenol concentration increased. The alpha-adrenergic antagonists phentolamine and phenoxybenzamine did not reverse phenylephrine's antilipolytic effect. Phenylephrine alone activated lipolysis at concentrations above 10(-5) M and at 5 X 10(-4) M the rate of lipolysis was increased 3.4-fold. Propranolol abolished this effect. In the presence of sub-maximum concentrations of dibutyryl cyclic-AMP (less than 10(-4) M), 10(-4) M phenylephrine increased the rate of lipolysis above that activated by dibutyryl cyclic-AMP alone. At maximum dibutyryl cyclic-AMP concentrations, or in the presence of propranolol, phenylephrine had no effect on dibutyryl cyclic-AMP-dependent lipolysis. There is no evidence to support an alpha 1-adrenergic mechanism for regulation of lipolysis in the rat adipocyte. All effects of the alpha-adrenergic agonist phenylephrine appear to be due to its weak beta-adrenergic activity.     

Lipolysis in rat adipose tissue in vitro and its alpha 2 adrenergic control

Although the alpha 2-adrenergic inhibitory control on lipolysis in several mammalian species, e.g. in dogs, hamsters, rabbits and also in man has been proved recently, in rats some peculiarities have been described in the alpha-adrenergic regulation of lipid mobilization. In the present study the effect of the specific alpha 2-adrenergic blocking agent--yohimbine (YOH)--on lipid mobilization in young (45-55 days old) and old (6 months) rats of the Wistar strain was followed and also its interaction with the beta-adrenergic blocking agent propranolol and the specific alpha 2-adrenergic agonist--clonidine. Furthermore, the effect of YOH on the isoprenaline (ISO) induced lipolysis was studied. The adipokinetic effect was estimated from the amount of released non-esterified fatty acids from the tissue preparation into the incubation medium. In all experimental series YOH produced a significant response, which was concentration-dependent (pD2 6.2) with the maximum effect corresponding to 40% of the highest adipokinetic action of ISO. Young rats were more sensitive than old animals. In both age groups YOH non-competitively decreased the highest effect of ISO to the level of its own lipid-mobilizing activity. In the presence of YOH, the ISO dose-response curve therefore completely disappears. The adipokinetic effect of YOH can be antagonized competitively by clonidine (CLO) (pA2 6.25 and 6.01 for concentrations of CLO 1 and 10 mumol.1(-1) respectively). The inhibitory effect of propranolol is about two orders lower. It is concluded that in rats the alpha 2-adrenergic control rather concerns basal lipolysis, whereas induced lipid mobilization can be inhibited by depression of the alpha 2-activity.     

Adrenaline is a critical mediator of acute exercise-induced AMP-activated protein kinase activation in adipocytes

Exercise increases AMPK (AMP-activated protein kinase) activity in human and rat adipocytes, but the underlying molecular mechanisms and functional consequences of this activation are not known. Since adrenaline (epinephrine) concentrations increase with exercise, in the present study we hypothesized that adrenaline activates AMPK in adipocytes. We show that a single bout of exercise increases AMPKalpha1 and alpha2 activities and ACC (acetyl-CoA carboxylase) Ser79 phosphorylation in rat adipocytes. Similarly to exercise, adrenaline treatment in vivo increased AMPK activities and ACC phosphorylation. Pre-treatment of rats with the beta-blocker propranolol fully blocked exercise-induced AMPK activation. Increased AMPK activity with exercise and adrenaline treatment in vivo was accompanied by an increased AMP/ATP ratio. Adrenaline incubation of isolated adipocytes also increased the AMP/ATP ratio and AMPK activities, an effect blocked by propranolol. Adrenaline incubation increased lipolysis in isolated adipocytes, and Compound C, an AMPK inhibitor, attenuated this effect. Finally, a potential role for AMPK in the decreased adiposity associated with chronic exercise was suggested by marked increases in AMPKalpha1 and alpha2 activities in adipocytes from rats trained for 6 weeks. In conclusion, both acute and chronic exercise are significant regulators of AMPK activity in rat adipocytes. Our findings suggest that adrenaline plays a critical role in exercise-stimulated AMPKalpha1 and alpha2 activities in adipocytes, and that AMPK can function in the regulation of lipolysis.     

Effects of epinephrine, corticotropin, and thyrotropin on lipolysis and glucose oxidation in rat adipose tissue

(Log dose)-response curves have been determined for lipolysis and for the conversion of glucose-(14)C to (14)CO(2) by adipose tissue from rats in the presence of epinephrine, corticotropin, and thyrotropin. The stimulatory effect of epinephrine on lipolysis was greater than that of corticotropin or thyrotropin. Lipolysis induced by epinephrine was inhibited by propranolol but only slightly by phenoxybenzamine, whereas lipolysis induced by corticotropin was inhibited by phenoxybenzamine to a much greater extent than by propranolol. Neither blocking drug had a pronounced effect on the response to thyrotropin. Epinephrine stimulated the oxidation of glucose-(14)C to CO(2) more than did either thyrotropin or corticotropin. Moreover, epinephrine stimulated the conversion of glucose-(14)C to CO(2) and fatty acids even when lipolysis was not increased. These studies indicate that epinephrine can affect glucose utilization independently of its effect on lipolysis.     

Lipolytic activity in adipose tissue of mice bearing Ehrlich ascites carcinoma.

The hormonal background of lipid mobilization in mice bearing Ehrlich ascites tumor (EAT) was investigated. Glycerol production rates were measured in adipose tissues from healthy and tumor-bearing (TB) animals being in early and late stages of tumor growth. The basal rate of lipolysis was enhanced significantly in the epididymal fat pads from mice with EAT. The catecholamine- and theophylline-stimulated activity was also higher in the tumorous animals, and the hormone-stimulated lipolysis was more effectively repressed by insulin and propranolol in adipose tissue from TB mice, compared to healthy ones. Susceptibility of adipose tissues to hormonal manipulations suggested that an imbalance in the concentration of lipolytic and antilipolytic hormones in the blood might promote the lipid depletion in the TB host organism. The low glucose and insulin concentrations and high catecholamine levels found in the sera of the tumorous animals seem to support this conception.     

Regulation of hormone stimulation of adipose tissue lipolysis by guanosine triphosphate

Guanosine triphosphate (GTP) enhanced the rate of mobilization of free fatty acids from isolated rat epididymal fat cells and potentiated the lipolytic response to norepinephrine, adrenocorticotropic hormone, glucagon, and theophylline. ITP, CTP, UTP, and TTP also increased basal and norepinephrine-stimulated lipolysis but to a lesser extent than GTP. ATP differed from the other nucleotides by inhibiting norepinephrine-stimulated lipolysis. The degree of phosphorylation of the guanine was important for activity since GTP was more active than GDP which, in turn, was more active than GMP in potentiating hormone-sensitized free fatty acid mobilization. Cyclic 3′, 5′-GMP, guanine, and guanosine were inactive in this regard. Activation of lipolysis by GTP occurred immediately upon addition of the nucleotide. The lipolytic response to GTP alone or in combination with norepinephrine or theophylline was exquisitely sensitive to inhibition by prostaglandin E₂. Nicotinic acid also inhibited the GTP response but to a lesser extent than prostaglandin E₂ and the β-blocker, propranolol, had no effect. Lipolytic concentrations of GTP in combination with norepinephrine increased intracellular levels of cAMP. By some as yet unknown mechanism GTP and GDP sensitized the adenylate cyclase of adipocytes to the actions of both agonists and antagonists.