Subcutaneous adipose tissue blood flow and triacylglycerol-mobilization during prolonged exercise in dogs

Healthy versus Unhealthy Adipose Tissue Expansion: the Role of Exercise

Although the hallmark of obesity is the expansion of adipose tissue, not all adipose tissue expansion is the same. Expansion of healthy adipose tissue is accompanied by adequate capillary angiogenesis and mitochondria-centered metabolic integrity, whereas expansion of unhealthy adipose tissue is associated with capillary and mitochondrial derangement, resulting in deposition of immune cells (M1-stage macrophages) and excess production of pro-inflammatory cytokines. Accumulation of these dysfunctional adipose tissues has been linked to the development of obesity comorbidities, such as type 2 diabetes, hypertension, dyslipidemia, and cardiovascular disease, which are leading causes of human mortality and morbidity in modern society. Mechanistically, vascular rarefaction and mitochondrial incompetency (for example, low mitochondrial content, fragmented mitochondria, defective mitochondrial respiratory function, and excess production of mitochondrial reactive oxygen species) are frequently observed in adipose tissue of obese patients. Recent studies have demonstrated that exercise is a potent behavioral intervention for preventing and reducing obesity and other metabolic diseases. However, our understanding of potential cellular mechanisms of exercise, which promote healthy adipose tissue expansion, is at the beginning stage. In this review, we hypothesize that exercise can induce unique physiological stimuli that can alter angiogenesis and mitochondrial remodeling in adipose tissues and ultimately promote the development and progression of healthy adipogenesis. We summarize recent reports on how regular exercise can impose differential processes that lead to the formation of either healthy or unhealthy adipose tissue and discuss key knowledge gaps that warrant future research.

Loss of Adipocyte VEGF Impairs Endurance Exercise Capacity in Mice

PURPOSE

Reducing vascular endothelial growth factor (VEGF) in adipose tissue alters adipose vascularity and metabolic homeostasis. We hypothesized that this would also affect metabolic responses during exercise-induced stress and that adipocyte-specific VEGF-deficient (adipoVEGF-/-) mice would have impaired endurance capacity.

METHODS

Endurance exercise capacity in adipoVEGF-/- (n = 10) and littermate control (n = 11) mice was evaluated every 4 wk between 6 and 24 wk of age using a submaximal endurance run to exhaustion at 20 m·min(-1) at 10° incline. Maximal running speed, using incremental increases in speed at 30-s intervals, was tested at 25 and 37 wk of age.

RESULTS

White and brown adipose tissue capillarity were reduced by 40% in adipoVEGF-/-, and no difference in skeletal muscle capillarity was observed. Endurance run time to exhaustion was 30% lower in adipoVEGF-/- compared with that in controls at all time points (P < 0.001), but no difference in maximal running speed was observed between the groups. After exercise (1 h at 50% maximum running speed), adipoVEGF-/- mice displayed lower circulating insulin (P < 0.001), lower glycerol (P < 0.05), and tendency for lower blood glucose (P = 0.06) compared with controls. There was no evidence of altered oxidative damage or changes in carnitine palmitoyltransferase-1β expression in skeletal muscle of adipoVEGF-/- mice.

CONCLUSIONS

These data suggest that VEGF-mediated deficits in adipose tissue blunt the availability of lipid substrates during endurance exercise, which likely reduced endurance performance. Surprisingly, we also found an unchanged basal blood glucose despite lower circulating insulin in adipoVEGF-/- mice, suggesting that loss of adipocyte VEGF can blunt insulin release and/or increase basal insulin sensitivity.

Lactate and the GPR81 receptor in metabolic regulation: implications for adipose tissue function and fatty acid utilisation by muscle during exercise

Lactate is increasingly recognised to be more than a simple end product of anaerobic glycolysis. Skeletal muscle and white adipose tissue are considered to be the main sites of lactate production and release. Recent studies have demonstrated that there is a specific G-protein coupled receptor for lactate, GPR81, which is expressed primarily in adipose tissue, and also in muscle. Lactate inhibits lipolysis in adipose tissue by mediating, through GPR81, the anti-lipolytic action of insulin. A high proportion (50 % or more) of the glucose utilised by white adipose tissue is converted to lactate and lactate production by the tissue increases markedly in obesity; this is likely to reflect a switch towards anaerobic metabolism with the development of hypoxia in the tissue. During exercise, there is a shift in fuel utilisation by muscle from lipid to carbohydrate, but this does not appear to be a result of the inhibition of lipolysis in the main adipose tissue depots by muscle-derived lactate. It is suggested instead that a putative autocrine lactate loop in myocytes may regulate fuel utilisation by muscle during exercise, operating via a muscle GPR81 receptor. In addition to being an important substrate, lactate is a key signal in metabolic regulation.

Fasting decreases free fatty acid turnover in mice overexpressing skeletal muscle lipoprotein lipase

Skeletal muscle lipoprotein lipase (LPL) overexpression in mice results in whole-body insulin resistance and increased intramuscular triglyceride stores, but decreased plasma triglyceride concentration and unchanged plasma free fatty acid (FFA) concentration. The effects of skeletal muscle LPL overexpression and fasting duration on FFA kinetics are unknown. Transgenic mice with muscle-specific LPL overexpression (MCKhLPL) and control mice (Con) were studied at rest during a 50-minute constant infusion of [9,10- 3H]palmitate to determine FFA kinetics after both 4 and 16 hours of fasting. FFA concentration was not different between groups after the 4-hour (Con, 0.80 +/- 0.06 mmol/L; MCKhLPL, 0.83 +/- 0.07 mmol/L) and 16-hour (Con, 0.83 +/- 0.04 mmol/L; MCKhLPL, 0.80 +/- 0.07 mmol/L) fast. FFA turnover (Ra) was not significantly different between MCKhLPL and Con groups after the 4-hour fast (Con Ra = 2.52 +/- 0.36 micromol/min; MCKhLPL Ra = 2.37 +/- 0.27 micromol/min). However, FFA turnover was significantly decreased after the 16-hour fast in MCKhLPL mice vs controls (Con Ra = 2.89 +/- 0.52 micromol/min; MCKhLPL Ra = 1.64 +/- 0.17 micromol/min; P < .05). The significantly lower FFA Ra in MCKhLPL vs control mice was due to a decrease in MCKhLPL FFA turnover from the 4- to 16-hour fast, whereas FFA turnover was unchanged in controls. The changes in FFA appearance after the 16-hour fast in MCKhLPL mice are most likely explained by increased reliance by skeletal muscle on plasma triglyceride as a fuel. These data suggest increased skeletal muscle LPL expression decreases dependence on plasma FFA during prolonged fasting in mice.

Fat metabolism in exercise - with special reference to training and growth hormone administration

Despite abundance of fat, exclusive dependency on fat oxidation can only sustain a metabolic rate corresponding to 50-60% of VO(2max) in humans. This puzzling finding has been subject to intense research for many years. Lately, it has gained renewed interest as a consequence of increased obesity and physical inactivity imposed by Western lifestyle. Why are humans so poor at metabolizing fat? Can fat metabolism be manipulated by exercise, training, diet and hormones? And why is fat stored in specialized adipose tissue and not just as lipid droplets inside muscle cells? In the present review, human fat metabolism is discussed in relation to how human fat metabolism is designed. Limitations in this design are explored and examples of different designs for fat metabolism from animal physiology are included to illustrate these limitations. Various means of manipulating fat metabolism are discussed with special emphasis on exercise, training, growth hormone (GH) physiology and GH administration. It is concluded that fat stores, non-esterified fatty acids (NEFAs) availability and enzymes for fat oxidation can be increased substantially. However, it is almost impossible to increase fat oxidation during endurance exercise at higher intensities. It seems that, for some reason, the human being is far from optimally designed for fat oxidation during exercise. Acute GH administration has several unexpected effects on fat and carbohydrate metabolism during aerobic exercise, and future research in this area is likely to provide valuable information with respect to GH physiology and the regulation of fat and carbohydrate metabolism during aerobic exercise.

Subcutaneous abdominal adipose tissue lipolysis during exercise determined by arteriovenous measurements in older women

OBJECTIVES

To characterize the lipolytic response in the subcutaneous abdominal adipose tissue in older women to endurance exercise.

DESIGN

Cross-sectional exercise study.

SETTING

Exercise laboratory, Copenhagen, Denmark.

PARTICIPANTS

Seven healthy, older women (mean age +/- standard error = 75 +/- 2 years); weight: 67.8 +/- 4.9 kg; body fat: 40 +/- 3; maximal oxygen uptake (VO2max): 1.43 +/- 0.07 L.min 1).

MEASUREMENTS

Body composition (dual energy x-ray absorptiometry (DEXA)), maximal oxygen uptake (VO2max, maximal cycling test), lipolytic response to exercise (arterial and adipose tissue venous catheterization at rest and during 60 minutes of continuous cycling at a load corresponding to 60 of VO2max), adipose tissue blood flow (ATBF) (133Xenon (133Xe) washout), oxygen consumption and respiratory exchange ratio during exercise (indirect calorimetry), whole blood glycerol, plasma nonesterified fatty acids (NEFA), lactate, glucose, epinephrine, norepinephrine, insulin, serum growth hormone, and hematocrit.

RESULTS

Glycerol and NEFA mobilization rates increased by 250 and 180, respectively, from rest to exercise. This was achieved primarily by an increase in veno-arterial differences, because ATBF did not increase significantly. NEFA:glycerol mobilization ratio was about two at rest and remained at that level during exercise, indicating significant local reesterification in both conditions. After an initial decrease, arterial plasma NEFA concentration increased significantly, by 26, indicating that NEFA delivery exceeded muscle uptake.

CONCLUSIONS

Older women are capable of prompt and substantial increase in subcutaneous abdominal adipose tissue glycerol and NEFA mobilization rates in response to moderate acute endurance exercise. The lipolytic response matches skeletal muscle NEFA uptake, and decreased ability to mobilize fat during exercise is therefore not likely to cause increased fat mass with advancing age.

Post-exercise adipose tissue and skeletal muscle lipid metabolism in humans: the effects of exercise intensity

One purpose of the present experiments was to examine whether the relative workload or the absolute work performed is the major determinant of the lipid mobilization from adipose tissue during exercise. A second purpose was to determine the co-ordination of skeletal muscle and adipose tissue lipid metabolism during a 3 h post-exercise period. Six subjects were studied twice. In one experiment, they exercised for 90 min at 40% of maximal O2 consumption (VO2,max) and in the other experiment they exercised at 60% VO2,max for 60 min. For both experiments, catheters were inserted in an artery, a subcutaneous abdominal vein and a femoral vein. Adipose tissue metabolism and skeletal muscle (leg) metabolism were measured using Fick's principle. The results show that the lipolytic rate in adipose tissue during exercise was the same in each experiment. Post-exercise, there was a very fast decrease in lipolysis, but it began to increase about 1 h post-exercise and remained elevated for the following 2 h. The increase in post-exercise non-esterified fatty acid (NEFA) mobilization was greater after 60% exercise than after 40 % exercise. It is concluded that the lipolytic rate in abdominal subcutaneous adipose tissue during exercise is the same whether the relative workload is 40% or 60% of maximum. Post-exercise, there is a substantial lipid mobilization from adipose tissue and only a small fraction of this is taken up in the lower extremities. This leaves a substantial amount of NEFAs for either NEFA/TAG (triacylglycerol) recirculation post-exercise or immediate oxidation.

Effect of epinephrine on muscle glycogenolysis during exercise in trained men

To test the hypothesis that an elevation in circulating epinephrine increases intramuscular glycogen utilization, six endurance-trained men performed two 40-min cycling trials at 71 +/- 2% of peak oxygen uptake in 20-22 degrees C conditions. On the first occasion, subjects were infused with saline throughout exercise (Con). One week later, after determination of plasma epinephrine levels in Con, subjects performed the second trial (Epi) with an epinephrine infusion, which resulted in a twofold higher (P < 0.01) plasma epinephrine concentration in Epi compared with Con. Although oxygen uptake was not different when the two trials were compared, respiratory exchange ratio was higher throughout exercise in Epi compared with Con (0.93 +/- 0.01 vs. 0.89 +/- 0.01; P < 0.05). Muscle glycogen concentration was not different when the trials were compared preexercise, but the postexercise value was lower (P < 0.01) in Epi compared with Con. Thus net muscle glycogen utilization was greater during exercise with epinephrine infusion (224 +/- 37 vs. 303 +/- 30 mmol/kg for Con and Epi, respectively; P < 0.01). In addition, both muscle and plasma lactate and plasma glucose concentrations were higher (P < 0.05) in Epi compared with Con. These data indicate that intramuscular glycogen utilization, glycolysis, and carbohydrate oxidation are augmented by elevated epinephrine during submaximal exercise in trained men.

Rat adipose tissue amino acid metabolism in vivo as assessed by microdialysis and arteriovenous techniques

In fed, anesthetized rats, microdialysis demonstrated a net release of glycerol, glutamine, serine, tyrosine, and taurine and a net uptake of glutamate, aspartate, glycine, and arginine across the inguinal adipose depot. However, the results also indicated excessive proteolysis associated with implantation of the microdialysis probe, and a novel arteriovenous difference technique was developed. Arteriovenous difference across the inguinal fat pat demonstrated a net uptake of glucose and a net release of lactate and glycerol. Starvation (48 h) resulted in higher rates of glycerol and lactate release with lower rates of glucose uptake. A net uptake of triacylglycerol was seen in starved-refed animals. Net glutamine, tyrosine, and taurine release was seen in fed and starved animals, but in starved-refed animals taurine and serine were the only amino acids showing significant release. No significant net uptake or release of ammonia, pyruvate, or alanine was observed. These experiments confirm that adipose tissue is a site of glutamine synthesis and suggest that the principal substrates are derived from intracellular proteolysis. The results also demonstrate the viability of an arteriovenous difference technique for the study of adipose tissue in the rat.

Pathways for oxidative fuel provision to working muscles: ecological consequences of maximal supply limitations

The study of metabolic fuel provision and its regulation has reached an exciting stage where specific molecular events can be correlated with parameters of the organism's ecology. This paper examines substrate supply pathways from storage sites to locomotory muscle mitochondria and discusses ecological implications of the limits for maximal flux through these pathways. The relative importance of the different oxidative fuels is shown to depend on aerobic capacity. Very aerobic, endurance-adapted animals such as long distance migrants favor the use of lipids and intramuscular fuels over carbohydrates and circulatory fuels. The hypothesis of functional co-adaptation between oxygen and metabolic fuel supply systems allows us to predict that the capacity of several biochemical processes should be scaled with maximal oxygen consumption. Key enzymes, transmembrane transporter proteins, glucose precursor supply and soluble fatty acid transport proteins must all be geared to support higher maximal glucose and fatty acid fluxes in aerobic than in sedentary species.

Site specific effects of acute exercise on muscle and adipose tissue metabolism in sedentary female rats

The effects of endurance exercise on muscle, and adipose tissue metabolism were investigated. Female lean Zucker rats swam for two hours at high intensity. Three groups were examined: pre-exercise control (C), exercised (E) and 24 hours post-exercise (E-24). Exercise increased fat cell lipolysis in the inguinal depot (p less than 0.05) while no effect was detected in the parametrial depot. In contrast, parametrial pad lipoprotein lipase (LPL) activity decreased after exercise with 24 hours post-exercise values being reduced below E and C rats (p less than 0.05). Gastrocnemius LPL activity remained unchanged during exercise while heart LPL increased, E having higher values than C and E-24 (p less than 0.05). Gastrocnemius, but not heart, citrate synthase activity increased with exercise, with E-24 values increased compared to E and C (p less than 0.05). These results demonstrate that adipose tissue's response to exercise is site specific, and suggests a distinct physiological role for different adipose depots. Muscle LPL and citrate synthase activities were modified differently for gastrocnemius and heart, confirming the distinct metabolic response to exercise of these two muscles.

Circulatory changes associated with heat stroke: observations in an experimental animal model

1. Although heat stroke is a frequent cause of death in both humans and animals as a result of climatic or exercise-imposed stress, underlying mechanisms are understood poorly. In order to develop more effective strategies for prevention and treatment of this cause of death and suffering, controlled experiments were conducted on a small number of sheep to examine cardiovascular involvement in the thermoregulatory failure of heat stroke. 2. Sheep were studied in a hot environment at rest and then during exercise until collapse. 3. With exercise, mean arterial pressure (MAP) increased slightly, cardiac output (CO) increased markedly and total peripheral resistance (TPR) decreased slightly. As collapse was imminent, MAP increased but CO and TPR did not change significantly. On collapse, MAP and TPR increased markedly and CO decreased markedly. 4. Radioactive microsphere measurements demonstrated during exercise a redistribution of blood flow (BF) away from abdominal viscera and torso skin, to muscles involved in exercise, respiratory muscles, myocardium, fat, limb skin and nasobuccal tissues. With progressively increasing heat stress and exercise, BF increased in exercise muscles and decreased in limb skin and fat. As collapse was imminent, there were sharp increases in BF in exercise muscles, brain and spinal cord. On collapse, BF decreased markedly in exercise and respiratory muscles and fat. 5. It is concluded that collapse and ultimately heat stroke are not due primarily to cardiovascular 'failure' but, rather, to consequences of high body temperatures resulting from thermoregulatory failure attributable to demands for blood pressure regulation dominating requirements for body temperature regulation.

Exercise-induced increase in dog adipose tissue blood flow before and after denervation

Subcutaneous adipose tissue blood flow was examined during rest and exercise in the inguinal fat pads of four female dogs using the Xe wash-out technique. The experiments were performed before and after denervation of one of the pads. No difference between the resting flows in the two pads could be demonstrated either before or after denervation. The flow increased about two-fold on average from rest to exercise. This response was similar before and after denervation. It is concluded that direct sympathetic innervation is not involved in the regulation of adipose tissue blood flow during exercise.

Inhibition of fatty acid mobilization by arterial free fatty acid concentration

Subcutaneous, inguinal adipose tissue from dogs was perfused with blood in which the free fatty acid (FFA) concentration was varied corresponding to FFA/albumin molar ratios between 1 and 6. Otherwise the composition of the perfusate was kept constant. In order to stimulate lipolysis, isoprenaline and theophyllamine were added to the perfusate. A raise in arterial FFA/albumin molar ratio was without influence on lipolysis (as reflected in the release of glycerol), but reduced the FFA release indicating an increased re-esterification. At FFA/albumin ratios above 3 a marked increase in vascular resistance was seen. This increase was partly reversible within the time of a perfusion. When lipolysis is stimulated in the intact organism, the effects of increasing arterial FFA/albumin ratio on re-esterification and vascular resistance may serve as feedback mechanisms regulating FFA mobilization.

The response of adipose tissue blood flow to insulin-induced hypoglycemia in conscious dogs and rats

A comparative study, focusing on the modification of regional blood flow in adipose tissue during insulin-induced hypoglycemia, was performed on dogs and rats at room temperature (22 degrees C) and on rats at thermoneutrality (28-32 degrees C). Insulin dosages of 3 IU/kg in rats and 0.75 IU/kg in dogs were found to cause changes of comparable amplitude and kinetics in plasma glucose and catecholamine levels in both species. At thermoneutrality, hypoglycemia induced an increase in blood supply to adipose tissue in both species: in dogs, blood flow density was markedly increased from the periphery (+75% in subcutaneous region) to the deeper locations (+550% in perirenal region); in rats, the increase of fractional cardiac output was especially pronounced in brown adipose tissue. By contrast, in rats acclimated at room temperature, hypoglycemia induced a decrease of fractional cardiac output to white adipose tissue and even more markedly, to brown adipose tissue (-45%). These results strongly suggest that, in rats at ambient temperature below thermoneutrality, thermoregulatory heat production is shut off during hypoglycemia.