L. H. Enevoldsen

Role of the sympathoadrenergic system in adipose tissue metabolism during exercise in humans

1 The relative roles of sympathetic nerve activity and circulating catecholamines for adipose tissue lipolysis during exercise are not known. 2 Seven paraplegic spinal cord injured (SCI, injury level T3‐T5) and seven healthy control subjects were studied by microdialysis and 133xenon washout in clavicular (Cl) and in umbilical (Um) (sympathetically decentralized in SCI) subcutaneous adipose tissue during 1 h of arm cycling exercise at ∼60 % of the peak rate of oxygen uptake. 3 During exercise, adipose tissue blood flow (ATBF) and interstitial glycerol, lactate and noradrenaline concentrations increased significantly in both groups. Plasma catecholamine levels increased significantly less with exercise in SCI than in healthy subjects. The exercise‐induced increase in interstitial glycerol concentration in subcutaneous adipose tissue was significantly lower in SCI compared with healthy subjects (SCI: 25 ± 12 % (Cl), 36 ± 20 % (Um); healthy: 60 ± 17 % (Cl), 147 ± 45 % (Um)) and the increase in ATBF was significantly lower (Cl) or similar (Um) in SCI compared with healthy subjects (SCI: 1.2 ± 0.3 ml (100 g)−1 min−1 (Cl), 1.0 ± 0.3 ml (100 g)−1 min−1 (Um); healthy: 2.8 ± 0.7 ml (100 g)−1 min−1 (Cl), 0.6 ± 0.3 ml (100 g)−1 min−1 (Um)). Accordingly, in both adipose tissues lipolysis increased less in SCI compared with healthy subjects, indicating that circulating catecholamines are important for the exercise‐induced increase in subcutaneous adipose tissue lipolysis. In SCI subjects, the exercise‐induced increase in subcutaneous adipose tissue lipolysis was not lower in decentralized than in sympathetically innervated adipose tissue. During exercise the interstitial noradrenaline and adrenaline concentrations were lower in SCI compared with healthy subjects (P < 0.05) and always lower than arterial plasma catecholamine concentrations (P < 0.05). 4 It is concluded that circulating catecholamines are important for the exercise‐induced increase in subcutaneous adipose tissue lipolysis while sympathetic nerve activity is not.

HORMONE-SENSITIVE LIPASE (HSL) EXPRESSION AND REGULATION IN SKELETAL MUSCLE

Because the enzymatic regulation of muscle triglyceride metabolism is poorly understood we explored the character and activation of neutral lipase in muscle. Western blotting of isolated rat muscle fibers demonstrated expression of hormone-sensitive lipase (HSL). In incubated soleus muscle epinephrine increased neutral lipase activity by beta-adrenergic mechanisms involving cyclic AMP-dependent protein kinase (PKA). The increase was paralleled by an increase in glycogen phosphorylase activity and could be abolished by antiserum against HSL. Electrical stimulation caused a transient increase in activity of both neutral lipase and glycogen phosphorylase. The increase in lipase activity during contractions was not influenced by sympathectomy or propranolol. Training diminished the epinephrine induced lipase activation in muscle but enhanced the activation as well as the overall concentration of lipase in adipose tissue. In agreement with the in vitro findings, in adrenalectomized patients an increase in muscle neutral lipase activity was found at the end of prolonged exercise only if epinephrine was infused. In accordance with feedforward regulation of substrate mobilization in exercise, our studies have shown that HSL is present in skeletal muscle cells and is stimulated in parallel with glycogen phosphorylase by both epinephrine and contractions. HSL adapts differently to training in muscle compared with adipose tissue.

The effect of exercise training on hormone-sensitive lipase in rat intra-abdominal adipose tissue and muscle

1 Adrenaline‐stimulated lipolysis in adipose tissue may increase with training. The rate‐limiting step in adipose tissue lipolysis is catalysed by the enzyme hormone‐sensitive lipase (HSL). We studied the effect of exercise training on the activity of the total and the activated form of HSL, referred to as HSL (DG) and HSL (TG), respectively, and on the concentration of HSL protein in retroperitoneal (RE) and mesenteric (ME) adipose tissue, and in the extensor digitorum longus (EDL) and soleus muscles in rats. 2 Rats (weighing 96 ± 1 g, mean ±s.e.m.) were either swim trained (T, 18 weeks, n= 12) or sedentary (S, n= 12). Then RE and ME adipose tissue and the EDL and soleus muscles were incubated for 20 min with 4.4 μm adrenaline. 3 HSL enzyme activities in adipose tissue were higher in T compared with S rats. Furthermore, in RE adipose tissue, training also doubled HSL protein concentration (P < 0.05). In ME adipose tissue, the HSL protein levels did not differ significantly between T and S rats. In muscle, HSL (TG) activity as well as HSL (TG)/HSL (DG) were lower in T rats, whereas HSL (DG) activity did not differ between groups. Furthermore, HSL protein concentration in muscle did not differ between T and S rats (P > 0.05). 4 In conclusion, training increased the amount of HSL and the sensitivity of HSL to stimulation by adrenaline in intra‐abdominal adipose tissue, the extent of the change differing between anatomical locations. In contrast, in skeletal muscle the amount of HSL was unchanged and its sensitivity to stimulation by adrenaline reduced after training.

The combined effects of exercise and food intake on adipose tissue and splanchnic metabolism

Seven young, healthy male subjects were each studied in two separate experiments. (1) Subjects exercised for 60 min at 55% of peak oxygen consumption in the fasted state ending 30 min before a meal (60% of energy as carbohydrate, and 20% of energy as lipid and protein each) comprising 25% of the total daily energy intake, and were then studied for another 150 min postprandially during rest (E→M). (2) One hour after a similar meal, subjects exercised for 60 min and were then studied for another 180 min postexercise during rest (M→E). Regional adipose tissue and splanchnic tissue metabolism were measured by Ficks Principle. Food intake before exercise reduced whole‐body lipid combustion during exercise to about 50% of the combustion rate found during exercise in the fasted state. The increase in subcutaneous, abdominal adipose tissue lipolysis during exercise was not influenced by preexercise food intake, while the fatty acid mobilization was increased by only 1.5‐fold during postprandial exercise compared to a fourfold increase during exercise in the fasted state. During exercise, catecholamine concentrations increased similarly in the fasted and the postprandial state, while the insulin concentration was twofold higher postprandially. These results indicate that the increase in catecholamine concentrations during exercise is a more important determinant of the adipose tissue lipolytic rate than the decrease in insulin concentration. Furthermore, food intake either 30 min after or 1 h before exercise prevents the postexercise increase in adipose tissue glycerol and fatty acid release which normally takes place in fasting subjects at least up to 2.5 h postprandially. Postprandial exercise led to a faster increase in postprandial lipaemia. This could not be accounted for by changes in the regional splanchnic tissue or adipose tissue triacylglycerol metabolism. Exercise was able to increase hepatic glucose production irrespective of food intake before exercise. It is concluded that exercise performed in the fasted state shortly before a meal leads to a more favourable lipid metabolism during and after exercise than exercise performed shortly after a meal.

Determination of adipose tissue blood flow with local 133Xe clearance. Evaluation of a new labelling technique.

Adipose tissue blood flow was measured in six healthy, non‐obese subjects with the xenon wash‐out technique after labelling of the tissue by either injection of 133Xe dissolved in isotonic sodium chloride (water depot) or injection of 133Xe in gas form (gas depot). The wash‐out rates were registered from four depots simultaneously. Two depots were placed above the umbilicus, and two depots were placed below the umbilicus in the abdominal, subcutaneous adipose tissue. A water depot and a gas depot were placed in the two positions, respectively. It was not possible to demonstrate any difference between the wash‐out rates registered from the two depot types, and it was also not possible to demonstrate any difference between the changes in wash‐out rates induced by an oral glucose load. Similarly, the tissue distribution of the water and the gas depots appeared to be similar as registered by a gamma camera. It is concluded that that the two tissue labelling modes give identical results. However, there are significant regional differences in the wash‐out rates of xenon from subcutaneous, abdominal adipose tissue, the wash‐out rates from infraumbilical depots being about 20% lower than from the supraumbilical depots.

Post-exercise abdominal, subcutaneous adipose tissue lipolysis in fasting subjects is inhibited by infusion of the somatostatin analogue octreotide.

To determine whether blockade of the exercise‐induced increase in growth hormone (GH) secretion may affect the regional lipolytic rate in the post‐exercise recovery period, the aim of the present experiments was to study the effect of infusion of the somatostatin analogue octreotide on the s.c., abdominal adipose tissue metabolism, before, during and after exercise in healthy, fasting, young male subjects. The adipose tissue net releases of fatty acids and glycerol were measured by arterio‐venous catherizations and simultaneous measurements of adipose tissue blood flow with the local Xe‐clearance method. Nine subjects were studied during 1‐h basal rest, and then during continuous octreotide infusion during 1‐h rest, 1‐h exercise at 50% of maximal oxygen consumption and 4‐h post‐exercise rest. A control study on seven subjects was performed under similar conditions but without octreotide infusion. The results show that octreotide infusion during rest increased lipolysis and fatty acid release from the abdominal, s.c. adipose tissue. The exercise‐induced increase in lipolysis and fatty acid release does not seem to be affected by octreotide when compared with the control study without octreotide infusion while the post‐exercise increase in lipolysis is inhibited by octreotide, suggesting that the exercise‐induced increase in GH secretion plays a role for the post‐exercise lipolysis in s.c., abdominal adipose tissue.

Lipid mobilization from human abdominal, subcutaneous adipose tissue is independent of sex during steady-state exercise.

The aim of the study was to elucidate whether there are sex differences of significant biological importance in the human abdominal, subcutaneous adipose tissue lipid metabolism when studied by Ficks Principle during rest and exercise in steady‐state conditions. The net mobilization of fatty acids and glycerol from the abdominal, subcutaneous adipose tissue was measured by arterio‐venous catheterizations and simultaneous measurements of adipose tissue blood flow with the local Xe‐clearance technique in 16 healthy, young normal weight men and women during rest, during 1 h of exercise at moderate intensity, and for another 60 min during post‐exercise recovery. The results show that there are not significant sex differences with respect to the steady‐state fatty acid and glycerol mobilizations neither during resting condition nor during exercise.

The effect of exercise on regional adipose tissue and splanchnic lipid metabolism in overweight Type 2 diabetic subjects

Aims/hypothesisTo test the hypothesis that adipose tissue lipolysis is enhanced in patients with Type 2 diabetes mellitus, we examined the effect of exercise on regional adipose tissue lipolysis and fatty acid mobilisation and measured the acute effects of exercise on the co-ordination of adipose tissue and splanchnic lipid metabolism.MethodsAbdominal, subcutaneous adipose tissue and splanchnic lipid metabolism were studied by conducting measurements of arterio-venous concentrations and regional blood flow in six overweight Type 2 diabetic subjects before, during and after exercise.ResultsExercise induced an increase in adipose tissue lipolysis and fatty acid release. However, the increase in adipose tissue blood flow was small, limiting fatty acid mobilisation from this tissue. Some of the fatty acids were released in excess in the post-exercise phase. The splanchnic fatty acid uptake was unchanged during the experiment but splanchnic ketogenesis increased in the post-exercise phase. The arterial glucose concentration decreased during exercise and continued to decrease afterwards, indicating an imbalance between splanchnic glucose production and whole-body glucose utilisation.Conclusions/interpretationRegional subcutaneous, abdominal adipose tissue lipolysis is no higher in patients with Type 2 diabetes than in young, healthy subjects. Exercise stimulates adipose tissue lipolysis, but due to an insufficient increase in blood flow, a high fraction of the fatty acids liberated by lipolysis cannot be released to the blood. Splanchnic glucose release is smaller than whole-body glucose utilisation during exercise and post-exercise recovery.

Postprandial triacylglycerol uptake in the legs is increased during exercise and post-exercise recovery

Six young, healthy male subjects were each studied in two experiments: (1) during resting conditions before and for 360 min after a meal (54% of energy as carbohydrate, 30% of energy as lipid, and 16% of energy as protein) comprising 25% of their total daily energy intake (M→R); and (2) while exercising on a cycle ergometer for 60 min at 50% of the peak oxygen consumption commencing 60 min after the meal (M→E) and then for another 240 min. Regional metabolism was measured by Ficks Principle in a leg and in the splanchnic tissue. The combination of food intake and exercise led to increased plasma triacylglycerol (TAG) uptake and clearance in the exercising legs immediately and for at least 4 h post‐exercise, while food intake per se did not change leg plasma TAG uptake or clearance for up to 6 h. It is hypothesized that the effect of exercise on leg plasma TAG metabolism is a result of capillary recruitment leading to exposure of the plasma lipoprotein particles to a larger amount of active LPL. In spite of the increased TAG uptake in the exercising legs the arterial plasma TAG concentration had a tendency to increase faster during exercise after a meal than during rest, but it also decreased faster implying that the total lipaemic response was the same whether exercise was performed or not. The amount of lipid taken up in the legs was higher than could be accounted for by whole body lipid oxidation during post‐exercise recovery, indicating accumulation of lipid in skeletal muscle in this period. Neither food intake alone nor the combination of food and exercise affected the splanchnic net balance of TAG. Finally, there is an additive effect of exercise and food intake on splanchnic net glucose balance.

Hormone-sensitive Lipase (HSL) Expression and Regulation By Epinephrine and Exercise in Skeletal Muscle

Triacylglycerol (TG) is stored in lipid droplets in the cytoplasm of skeletal muscle. The energy content of the TG depot is higher than the energy content of the muscle glycogen depot. The enzymatic regulation of intracellular TG hydrolysis in skeletal muscle has not been elucidated. Therefore, we investigated the expression and the regulation of hormone-sensitive lipase (HSL) in skeletal muscle. This enzyme is a neutral lipase and known as the rate-limiting enzyme of intracellular TG hydrolysis in adipose tissue. The total and the activated form of the neutral lipase are referred to as MOME and TO, respectively. In isolated rat skeletal muscle fibers, the presence of HSL was demonstrated by Western blotting. The expression of HSL was correlated to fiber type, being higher in oxidative than in glycolytic fibers. In incubated soleus and extensor digitorum longus (EDL) muscles stimulation with epinephrine or electrically induced contractions increased neutral lipase activity against triolein (TO), but not against a diacylglycerol analogue (MOME). Glycogen phosphorylase activity increased in parallel with TO activity. No measurable increase in muscle homogenate TO activity existed in the presence of an anti-HSL antibody. The effect of epinephrine could be blocked by propanolol and mimicked by incubation of a crude supernatant from control muscle with the catalytic subunit of cAMP-dependent protein kinase. The effect of contractions was transient as TO activity declined to basal levels after 10 min of electrical stimulation. Indicating involvement of protein kinase C the effect of contractions was abolished by Calphostin C. Okadaic acid doubled the contraction-mediated increase in TO activity, whereas the increase was reversed by phosphatase treatment. The effects of epinephrine and contractions were partially additive. In rats training increased epinephrine-stimulated TO activity and HSL concentration in adipose tissue but not in muscle. In humans, at the end of 60 min of exercise muscle, TO activity was increased in healthy, but not in adrenalectomized, subjects. In conclusion, HSL is present in skeletal muscle and can be activated by phosphorylation by both epinephrine and muscle contractions. In addition, HSL and glycogen phosphorylase are stimulated in parallel in muscle indicating simultaneous activation of triacylglycerol and glycogen breakdown.