The effect of lactate in canine subcutaneous adipose tissue in situ

Re-examination of the contribution of substrates to energy expenditure during high-intensity intermittent exercise in endurance athletes

BACKGROUND

It has been believed that the contribution of fat oxidation to total energy expenditure is becoming negligible at higher exercise intensities (about 85% VO_2max). The aim of the present study was to examine the changes in substrate oxidation during high-intensity interval exercise in young adult men.

METHODS

A total of 18 healthy well-trained (aged 19.60 ± 0.54 years, BMI = 22.19 ± 0.64 kg/m², n = 10) and untrained (aged 20.25 ± 0.41 years, BMI = 22.78 ± 0.38 kg/m², n = 8) young men volunteered to participate in this study. After an overnight fast, subjects were tested on a cycle ergometer and completed six 4-min bouts of cycling (at ∼80% VO_2max) with 2 min of rests between intervals. Energy expenditure and the substrate oxidation rate were measured during the experiment by using indirect calorimetry. The blood lactate concentration was collected immediately after each interval workout.

RESULTS

The fat oxidation rate during each workout was significantly different between the untrained and the athlete groups (p < 0.05), and the carbohydrate (CHO) oxidation rate during the experiment was similar between groups (p > 0.05). Moreover, lactate concentration significantly increased in the untrained group (p < 0.05), whereas it did not significantly change in the athlete group during the workouts (p > 0.05). Fat contribution to energy expenditure was significantly higher in the athlete group (∼25%) than in the untrained group (∼2%).

CONCLUSIONS

The present study indicates that 17 times more fat oxidation was measured in the athlete group compared to the untrained group. However, the athletes had the same CHO oxidation rate as the recreationally active subjects during high-intensity intermittent exercise. Higher fat oxidation rate despite the same CHO oxidation rate may be related to higher performance in the trained group.

Metabolic characteristics of human subcutaneous abdominal adipose tissue after overnight fast

Subcutaneous abdominal adipose tissue is one of the largest fat depots and contributes the major proportion of circulating nonesterified fatty acids (NEFA). Little is known about aspects of human adipose tissue metabolism in vivo other than lipolysis. Here we collated data from 331 experiments in 255 healthy volunteers over a 23-year period, in which subcutaneous abdominal adipose tissue metabolism was studied by measurements of arterio-venous differences after an overnight fast. NEFA and glycerol were released in a ratio of 2.7:1, different (P < 0.001) from the value of 3.0 that would indicate no fatty acid re-esterification. Fatty acid re-esterification was 10.2 ± 1.4%. Extraction of triacylglycerol (TG) (fractional extraction 5.7 ± 0.4%) indicated intravascular lipolysis by lipoprotein lipase, and this contributed 21 ± 3% of the glycerol released. Glucose uptake (fractional extraction 2.6 ± 0.3%) was partitioned around 20-25% for provision of glycerol 3-phosphate and 30% into lactate production. There was release of lactate and pyruvate, with extraction of the ketone bodies 3-hydroxybutyrate and acetoacetate, although these were small numerically compared with TG and glucose uptake. NEFA release (expressed per 100 g tissue) correlated inversely with measures of fat mass (e.g., with BMI, r(s) = -0.24, P < 0.001). We examined within-person variability. Systemic NEFA concentrations, NEFA release, fatty acid re-esterification, and adipose tissue blood flow were all more consistent within than between individuals. This picture of human adipose tissue metabolism in the fasted state should contribute to a greater understanding of adipose tissue physiology and pathophysiology.

New perspectives on nutritional interventions to augment lipid utilisation during exercise

The enhancement of fat oxidation during exercise is an aim for both recreational exercising individuals and endurance athletes. Nutritional status may explain a large part of the variation in maximal rates of fat oxidation during exercise. This review reveals novel insights into nutritional manipulation of substrate selection during exercise, explaining putative mechanisms of action and evaluating the current evidence. Lowering the glycaemic index of the pre-exercise meal can enhance lipid utilisation by up to 100 % through reduced insulin concentrations, although its application may be restricted to specific training sessions rather than competition. Chronic effects of dietary glycaemic index are less clear and warrant future study before firm recommendations can be made. A flurry of recent advances has overthrown the conventional view of l-carnitine supplementation, with skeletal muscle uptake possible under certain dietary conditions and providing a strategy to influence energy metabolism in an exercise intensity-dependent manner. Use of non-carbohydrate nutrients to stimulate muscle l-carnitine uptake may prove more beneficial for optimising lipid utilisation, but this requires more research. Studies investigating fish oil supplementation on fat oxidation during exercise are conflicting. In spite of some strong putative mechanisms, the only crossover trial showed no significant effect on lipid use during exercise. Ca may increase NEFA availability although it is not clear whether these effects occur. Ca and caffeine can increase NEFA availability under certain circumstances which could theoretically enhance fat oxidation, yet strong experimental evidence for this effect during exercise is lacking. Co-administration of nutrients to maximise their effectiveness needs further investigation.

International Union of Basic and Clinical Pharmacology. LXXXII: Nomenclature and Classification of Hydroxy-carboxylic Acid Receptors (GPR81, GPR109A, and GPR109B)

The G-protein-coupled receptors GPR81, GPR109A, and GPR109B share significant sequence homology and form a small group of receptors, each of which is encoded by clustered genes. In recent years, endogenous ligands for all three receptors have been described. These endogenous ligands have in common that they are hydroxy-carboxylic acid metabolites, and we therefore have proposed that this receptor family be named hydroxy-carboxylic acid (HCA) receptors. The HCA(1) receptor (GPR81) is activated by 2-hydroxy-propanoic acid (lactate), the HCA(2) receptor (GPR109A) is a receptor for the ketone body 3-hydroxy-butyric acid, and the HCA(3) receptor (GPR109B) is activated by the β-oxidation intermediate 3-hydroxy-octanoic acid. HCA(1) and HCA(2) receptors are found in most mammalian species, whereas the HCA(3) receptor is present only in higher primates. The three receptors have in common that they are expressed in adipocytes and are coupled to G(i)-type G-proteins mediating antilipolytic effects in fat cells. HCA(2) and HCA(3) receptors are also expressed in a variety of immune cells. HCA(2) is a receptor for the antidyslipidemic drug nicotinic acid (niacin) and related compounds, and there is an increasing number of synthetic ligands mainly targeted at HCA(2) and HCA(3) receptors. The aim of this article is to give an overview on the discovery and pharmacological characterization of HCAs, and to introduce an International Union of Basic and Clinical Pharmacology (IUPHAR)-recommended nomenclature. We will also discuss open questions regarding this receptor family as well as their physiological role and therapeutic potential.

Biological roles and therapeutic potential of hydroxy-carboxylic Acid receptors

In the recent past, deorphanization studies have described intermediates of energy metabolism to activate G protein-coupled receptors and to thereby regulate metabolic functions. GPR81, GPR109A, and GPR109B, formerly known as the nicotinic acid receptor family, are encoded by clustered genes and share a high degree of sequence homology. Recently, hydroxy-carboxylic acids were identified as endogenous ligands of GPR81, GPR109A, and GPR109B, and therefore these receptors have been placed into a novel receptor family of hydroxy-carboxylic acid (HCA) receptors. The HCA(1) receptor (GPR81) is activated by the glycolytic metabolite 2-hydroxy-propionic acid (lactate), the HCA(2) receptor is activated by the ketone body 3-hydroxy-butyric acid, and the HCA(3) receptor (GPR109B) is a receptor for the β-oxidation intermediate 3-hydroxy-octanoic acid. While HCA(1) and HCA(2) receptors are present in most mammalian species, the HCA(3) receptor is exclusively found in humans and higher primates. HCA receptors are expressed in adipose tissue and mediate anti-lipolytic effects in adipocytes through G(i)-type G protein-dependent inhibition of adenylyl cyclase. HCA(2) and HCA(3) inhibit lipolysis during conditions of increased β-oxidation such as prolonged fasting, whereas HCA(1) mediates the anti-lipolytic effects of insulin in the fed state. As HCA(2) is a receptor for the established anti-dyslipidemic drug nicotinic acid, HCA(1) and HCA(3) also represent promising drug targets and several synthetic ligands for HCA receptors have been developed. In this article, we will summarize the deorphanization and pharmacological characterization of HCA receptors. Moreover, we will discuss recent progress in elucidating the physiological and pathophysiological role to further evaluate the therapeutic potential of the HCA receptor family for the treatment of metabolic disease.

Fat as a fuel: emerging understanding of the adipose tissue-skeletal muscle axis

The early pioneers in the field of metabolism during exercise such as Lindhard and Krogh understood the importance of fat as a fuel for muscle contraction. But they could not have understood the details of the pathways involved, as neither the metabolic role of adipose tissue nor the transport role of non-esterified fatty acids (NEFA) in the plasma was clearly understood at the time. We now recognize that the onset of muscular contraction coincides with an increase in the delivery of NEFA from adipose tissue, probably coordinated by the sympatho-adrenal system. During light exercise, adipose tissue-derived NEFA make up the majority of the oxidative fuel used by muscle. As exercise is prolonged, the importance of NEFA increases. The onset of exercise is marked by an increased proportion of NEFAs entering beta-oxidation rather than re-esterification and recycling. At moderate intensities of exercise, other sources of fat, potentially plasma- and intramyocellular-triacylglycerol, supplement the supply of plasma NEFA. The delivery of NEFA is augmented by increased adipose tissue blood flow and by other stimuli such as atrial natriuretic peptide. Only during high-intensity exercise is there a failure of adipose tissue to deliver sufficient fatty acids for muscle (which is coupled with an inability of muscle to use them, even when fatty acids are supplied artificially). This limitation of adipose tissue NEFA delivery may reflect some feedback inhibition of lipolysis, perhaps via lactate, or possibly alpha-adrenergic inhibition of lipolysis at very high catecholamine concentrations.

An autocrine lactate loop mediates insulin-dependent inhibition of lipolysis through GPR81

Lactate is an important metabolic intermediate released by skeletal muscle and other organs including the adipose tissue, which converts glucose into lactate under the influence of insulin. Here we show that lactate activates the G protein-coupled receptor GPR81, which is expressed in adipocytes and mediates antilipolytic effects through G(i)-dependent inhibition of adenylyl cyclase. Using GPR81-deficient mice, we demonstrate that the receptor is not involved in the regulation of lipolysis during intensive exercise. However, insulin-induced inhibition of lipolysis and insulin-induced decrease in adipocyte cAMP levels were strongly reduced in mice lacking GPR81, although insulin-dependent release of lactate by adipocytes was comparable between wild-type and GPR81-deficient mice. Thus, lactate and its receptor GPR81 unexpectedly function in an autocrine and paracrine loop to mediate insulin-induced antilipolytic effects. These data show that lactate can directly modulate metabolic processes in a hormone-like manner, and they reveal a new mechanism underlying the antilipolytic effects of insulin.

Postburn trauma insulin resistance and fat metabolism

Hyperglycemia and insulin resistance have long been recognized in severe burn patients. More recently, it has been observed that controlling hyperglycemia, or alleviating insulin resistance, is associated with improved outcomes. This has led to a renewed interest in the etiology of insulin resistance in this population. The postinjury hyperglycemic response appears to be associated with multiple metabolic abnormalities, such as elevated basal energy expenditure, increased protein catabolism, and, notably, significant alterations in fat metabolism. The synergy of all of the responses is not understood, although many studies have been conducted. In this article we will review the present understanding of the relationship between fat metabolism and insulin resistance posttrauma, and discuss some of the recent discoveries and potential therapeutic measures. We propose that the insulin resistance is likely related to the development of "ectopic" fat stores, i.e., triglyceride (TG) storage in sites such as the liver and muscle cells. Deposition of TG in ectopic sites is due to an increase in free fatty acid delivery secondary to catecholamine-induced lipolysis, in conjunction with decreased beta-oxidation within muscle and decreased hepatic secretion of fats. The resultant increases in intracellular TG or related lipid products may in turn contribute to alterations in insulin signaling.

Determinants of fat oxidation during exercise in healthy men and women: a cross-sectional study

The aim of the present study was to establish fat oxidation rates over a range of exercise intensities in a large group of healthy men and women. It was hypothesised that exercise intensity is of primary importance to the regulation of fat oxidation and that gender, body composition, physical activity level, and training status are secondary and can explain part of the observed interindividual variation. For this purpose, 300 healthy men and women (157 men and 143 women) performed an incremental exercise test to exhaustion on a treadmill [adapted from a previous protocol (Achten J, Venables MC, and Jeukendrup AE. Metabolism 52: 747–752, 2003)]. Substrate oxidation was determined using indirect calorimetry. For each individual, maximal fat oxidation (MFO) and the intensity at which MFO occurred (Fatmax) were determined. On average, MFO was 7.8 ± 0.13 mg·kg fat-free mass (FFM)−1·min−1 and occurred at 48.3 ± 0.9% maximal oxygen uptake (V̇o2 max), equivalent to 61.5 ± 0.6% maximal heart rate. MFO (7.4 ± 0.2 vs. 8.3 ± 0.2 mg·kg·FFM−1·min−1; P < 0.01) and Fatmax (45 ± 1 vs. 52 ± 1% V̇o2 max; P < 0.01) were significantly lower in men compared with women. When corrected for FFM, MFO was predicted by physical activity (self-reported physical activity level), V̇o2 max, and gender ( R2 = 0.12) but not with fat mass. Men compared with women had lower rates of fat oxidation and an earlier shift to using carbohydrate as the dominant fuel. Physical activity, V̇o2 max, and gender explained only 12% of the interindividual variation in MFO during exercise, whereas body fatness was not a predictor. The interindividual variation in fat oxidation remains largely unexplained.

Lack of antilipolytic effect of lactate in subcutaneous abdominal adipose tissue during exercise

The purpose of our study was to evaluate the potential inhibition of adipose tissue mobilization by lactate. Eight male subjects (age, 26. 25 +/- 1.75 yr) in good physical condition (maximal oxygen uptake, 59.87 +/- 2.77 ml. kg-1. min-1; %body fat, 10.15 +/- 0.89%) participated in this study. For each subject, two microdialysis probes were inserted into abdominal subcutaneous tissue. Lactate (16 mM) was perfused via one of the probes while physiological saline only was perfused via the other, both at a flow rate of 2.5 microl/min. In both probes, ethanol was also perfused for adipose tissue blood flow estimation. Dialysates were collected every 10 min during rest (30 min), exercise at 50% maximal oxygen consumption (120 min), and recovery (30 min) for the measurement of glycerol concentration. During exercise, glycerol increased significantly in both probes. However, no differences in glycerol level and ethanol extraction were observed between the lactate and control probes. These findings suggest that lactate does not impair subcutaneous abdominal adipose tissue mobilization during exercise.

Modified hormonal effects on fat metabolism after severe head injury in children

Previous studies within our research group have indicated that the hormonal influences on whole body energy expenditure may be modified in severely head-injured children. The aim of this study was to examine plasma concentrations of nonesterified fatty acids (NEFA) and the hormonal and metabolic mediators which influence these to determine whether there is a similarly modified effect on fat metabolism. A total of 64 serial measurements were made in 21 fasting severely head-injured children aged 2-15 y (Glasgow Coma Score < or = 8) who were receiving neurointensive care. Circulating NEFA, ketone bodies, and lactate concentrations were analyzed using microenzymatic or electrochemical techniques. Plasma concentrations of adrenaline and insulin were measured using radioenzymatic and RIA techniques, respectively. Net fat oxidation rates were determined using indirect calorimetry. Plasma NEFA concentrations showed a significant positive relationship with both net fat oxidation rates (p = 0.02) and log ketone body concentrations (p = 0.008), indicating that NEFA concentrations were significantly related with utilization. When compared with reference values for normal resting adults, 59 (92%) adrenaline measurements were elevated, whereas only 8 (12%) NEFA values lay above the reference range. Surprisingly, between children, there was a significant negative relationship between NEFA and adrenaline concentrations, even after allowing for the effects of insulin and lactate (p = 0.015). Both plasma NEFA and adrenaline concentrations were significantly related with Glasgo Coma Score (p = 0.04, p = 0.007, respectively), the most severely injured children having the lowest NEFA and highest adrenaline concentrations. The mechanisms underlying these metabolic changes may be related to the severity of head injury and may involve changes in triglyceride/NEFA cycling and/or peripheral effects on adrenergic receptors. If children are to be treated effectively after trauma, it is important to discover the mechanism of these changes which must reflect a fundamental alteration in metabolism.

Time course of the early pituitary-adrenal and metabolic responses to accidental injury

We have made serial measurements of the plasma adrenocorticotrophic hormone (ACTH), cortisol, glucose, lactate, glycerol, nonesterified fatty acid (NEFA), and beta-hydroxybutyrate concentrations in 14 recently injured patients. We took 6 to 9 blood samples at about 20-minute intervals from each patient within the first 3 hours after accidental injury of moderate severity (Injury Severity Score of 9 to 13). Plasma ACTH varied greatly both with time and between patients; in the majority, it fell throughout the sampling period and in the others, the pattern varied. The changes in plasma cortisol were mostly appropriate to those in ACTH. Plasma glucose and glycerol did not change systematically with time. In contrast, plasma lactate fell, and plasma NEFA, beta-hydroxybutyrate, and the NEFA:glycerol ratio rose, throughout the sampling period. Analysis of variance for these data showed that time after injury made a contribution that was highly significant, although smaller than that arising from differences between patients.

Interstitial glucose and lactate balance in human skeletal muscle and adipose tissue studied by microdialysis

1. Microdialysis was used to gain insight into the substrate exchanges in the interstitial space of skeletal muscle and adipose tissue. Probes were inserted in the quadriceps femoris muscle and para-umbilical subcutaneous adipose tissue of thirteen subjects and microdialysis was performed at different flow rates (1-4 microliters min-1) and during changes in tissue blood flow. 2. When ethanol (5 mM) is included in the perfusion solution, the ethanol clearance from the probe is a measure of tissue blood flow. Blood flow changes induced by adenosine or vasopressin perfusion, by exercise or by circulatory occlusion resulted in ethanol clearance values of 69-139% of the basal level. The ethanol clearance was higher in skeletal muscle than in adipose tissue (32-62%, P < 0.001), a difference compatible with a higher blood flow in muscle tissue. 3. The fraction of the interstitial glucose concentration that was recovered with the microdialysis was similar in skeletal muscle (the absolute values being 1.70 +/- 0.14 mM at 1 microliter min-1 and 0.59 +/- 0.05 mM at 4 microliters min-1) and adipose tissue (1.89 +/- 0.20 mM at 1 microliter min-1; 0.54 +/- 0.05 mM at 4 microliters min-1) and correlated inversely with the tissue ethanol clearance, both in the basal state and during changes in tissue blood flow (muscle: r = -0.56 to -0.67; adipose tissue r = -0.72 to -0.95). Coefficients of variation were 6-8% (glucose) and 11-16% (lactate) and were similar during isometric exercise. The reproducibility of the technique (comparison of two contralateral probes; perfusion flow rate 4 microliters min-1) was 5.3-8.3% (ethanol) and 23.9-20.8% (glucose) in muscle (n = 6) and adipose tissue (n = 4) respectively. 4. The skeletal muscle dialysate lactate concentration (1 microliter min-1: 1.16 +/- 0.2 mM) was higher than in adipose tissue (0.76 +/- 0.08 mM, P < 0.05), where the absolute amount of lactate that could be removed from the tissue (at 4 microliters min-1) was only half of that in skeletal muscle (0.8 +/- 0.11 vs. 1.76 +/- 0.23 nmol min-1, P < 0.05). The dialysate lactate level was not affected in either tissue by large changes in the interstitial glucose concentration indicating that in neither tissue is blood glucose a significant source of lactate formation. 5. The blood flow effects on the dialysate glucose concentration are the likely consequence of probe glucose drainage artificially shifting the balance between the supply and consumption of interstitial glucose.(ABSTRACT TRUNCATED AT 400 WORDS)

Glycerol production in subcutaneous adipose tissue in lean and obese humans

To estimate the regional subcutaneous glycerol production rate in normal and obese humans, the venous arterialized plasma glycerol, interstitial glycerol in the subcutaneous adipose tissue together with adipose tissue blood flow (ATBF, ml/100 g.min) were measured in the postabsorptive state and for 2 h after ingestion of 100 g of oral glucose. Eight lean and eight obese men with normal oral glucose tolerance tests were investigated with the subcutaneous microdialysis technique and 133Xe clearance. In the postabsorptive state, the interstitial glycerol concentrations in lean and obese subjects were 170 +/- 21 vs. 282 +/- 28 microM (P less than 0.01) and 156 +/- 23 vs. 225 +/- 12 microM (P less than 0.05) in the abdominal and femoral subcutaneous adipose tissue, respectively. The corresponding arterial glycerol levels were 54 +/- 4 vs. 75 +/- 14 microM (NS). Abdominal ATBF was greater in lean subjects (3.2 +/- 0.6 vs. 1.6 +/- 0.3; P less than 0.05), whereas femoral ATBF was similar in both groups (2.7 +/- 0.4 vs. 2.4 +/- 0.7). Estimated mean local glycerol release (mumol/100 g.min) was similar in the lean and obese group (0.16 +/- 0.03 vs. 0.20 +/- 0.05 and 0.18 +/- 0.02 vs. 0.17 +/- 0.04) in the abdominal and femoral site, respectively. We conclude that glycerol production from the subcutaneous tissue is increased in obesity, irrespective of adipose tissue distribution. This enhancement is due to the increased adipose tissue mass.

Metabolic responses of forearm and adipose tissues to acute ethanol ingestion

Although excess ethanol consumption is often considered to lead to adiposity, the metabolic routes by which this might occur are not clear. We have investigated some metabolic consequences of acute ethanol ingestion by measuring arteriovenous differences across forearm muscle and subcutaneous adipose tissue for 6 hours after ingestion of 47.5 g ethanol, in seven normal subjects fasted overnight. The expected systemic effects of ethanol ingestion were observed: slight lowering of the plasma glucose concentration, depression of plasma nonesterified fatty acid (NEFA) concentrations, and elevation of the blood lactate/pyruvate and 3-hydroxybutyrate/acetoacetate ratios. There was a marked reduction in blood total ketone bodies in relation to plasma NEFA concentrations. However, the only major change observed in peripheral tissue metabolism was an increased uptake of acetate into forearm muscle, equivalent, in whole-body terms, to only 3% of the ethanol load. Adipose tissue appeared to show a reduced cytoplasmic state in that it exported an increased ratio of lactate to pyruvate after ethanol ingestion. However, this reduced state did not lead to increased fatty acid reesterification within adipose tissue. No mechanism was clearly identified whereby ethanol ingestion might lead to net deposition of triacylglycerol in adipose tissue.

Endotoxicosis modulates cytosolic free calcium and basal and ACTH-stimulated lipolysis in rat adipocytes

Lipolytic rates and intracellular Ca2+ concentration ([Ca2+]i) were determined under basal conditions and upon stimulation with adrenocorticotropic hormone (ACTH), norepinephrine (NE) and insulin (I), in adipocytes isolated from control and acutely endotoxin (ET)-treated rats (1 mg/100 g body weight, LD50 at 6 h). [Ca2+]i measurements were done using the fluorescent Ca2(+)-indicator Fura-2. NE and ACTH, but not I, produced a marked increase of [Ca2+]i in cells of both control and ET-treated rats. ET treatment elicited a significant increase in [Ca2+]i of resting cells, and enhanced the ACTH effect on this parameter. The changes in lipolytic activity correlated well with changes of [Ca2+]i induced by ACTH. The results indicate that ET-induced alterations in intracellular calcium homeostasis of adipocytes may contribute to the mediation of effects on fat mobilization during endotoxemia.

Effects of hypoxia on lipolysis in isolated rat myocardial cells

The effect of hypoxia on myocardial lipolysis (glycerol release) was investigated in freshly isolated, calcium-tolerant rat ventricular myocytes. Hypoxia was produced by gassing the incubation medium (Joklik-minimum essential medium, supplemented with 1.2 mM MgSO4, 1 mM DL-carnitine, 1.5 mM CaCl2 and 0.6 mM palmitate bound to 0.15 mM fatty acid free bovine serum albumin) with 95% N2-5% CO2. Control (normoxic) incubations were carried out under air-5% CO2 atmosphere. Basal glycerol release increased from 46.6 +/- 3.0 nmol/10(6) cells.30 min in normoxia to 64.5 +/- 4.3 nmol/10(6) cells.30 min in hypoxia (p less than 0.05). Addition of isoprenaline (10 microM) resulted in a significant (p less than 0.05) stimulation of the glycerol release both in normoxia and in hypoxia, but the enhancement above basal rates was apparently lower in hypoxia (8.7 +/- 2.5 nmol/10(6) cells.30 min) than in normoxia (12.2 +/- 2.7 nmol/10(6) cells.30 min). Furthermore, whereas the isoprenaline-induced rise in lipolysis both in normoxia and hypoxia was prevented by inclusion of propranolol (10 microM), propranolol did not affect the hypoxia-induced increase in lipolysis. Thus, the above findings suggest that myocardial lipolysis may be stimulated by local non-adrenergic mechanisms during hypoxia.

Blood hormone and metabolite levels during graded cycle ergometer exercise

To study the effect of the intensity of physical exercise on plasma hormone and metabolite levels, a group of 11 well-conditioned males participated in cycle ergometer exercise. The subjects pedalled at three different work loads, corresponding to 63% of VO2max (duration 10 min), 86% of VO2max (duration 10 min) and maximal work load (tolerated 5-7 min) The increases in blood adrenaline, noradrenaline, growth hormone, cyclic AMP, glycerol and lactate concentrations were remarkably similar and exponentially related to the work load. The concentrations of blood glucose, cortisol and glucagon increased only at maximal work load. Many of these changes in blood metabolite and hormone concentrations seem to be related to the increased sympathetic activity during graded exercise.

Alterations in carbohydrate and lipid metabolism following administration of endotoxin

Endotoxin-induced alterations in blood flow, carbohydrate and lipid metabolism in various organs of the body are outlined. Peripheral utilization of carbohydrates and lipids does not appear to be adversely affected by the administration of mild to moderate doses of endotoxin (in fact, glucose turnover is even elevated). On the other hand, the supply of free fatty acids from adipose tissue is diminished, at least in part, due to severe hemodynamic changes that occur in this tissue following endotoxin. Hepatic gluconeogenesis is elevated, but does not match the increased demand for glucose by the periphery. The experimental conditions discussed in this brief review elicit changes that are reversible. However, during more severe endotoxemia, the metabolic alterations lead to irreversibility.

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

In 6 dogs concentration differences for glycerol and FFA were measured between the aorta and the external pudendal vein, a vein which mainly drains subcutaneous adipose tissue in dogs, during prolonged exercise. It was found that the a-v differences increased about 2-fold for both glycerol and FFA, however great interindividual differences were found. In 4 dogs adipose tissue blood flow, glycerol and FFA a-v differences were measured simultaneously, and the mobilizations of glycerol and FFA as well as the re-esterification of FFA were calculated. After 2 h of exercise the values were in the range of 1-7 umol/(100 g.min) for FFA and glycerol mobilizations while the FFA re-esterification was in the range of 2-14 umol/(100 g.min). It was found that the FFA/albumin ratio in adipose venous blood, on average 3.6 was at a level at which the FFA mobilization has been shown to depend on the adipose tissue blood flow in isolated fat pads. In 11 dogs subcutaneous adipose tissue blood flow rose 2-fold during exercise from about 5-10 ml/(100 g.min). It is concluded that the subcutaneous adipose tissue blood flow response to exercise is equal in man and dog, that lipolysis, FFA mobilization and FFA re-esterification are increased in subcutaneous adipose tissue during exercise, and that the increase in blood flows is of importance for the enhanced FFA mobilization during exercise.

Hypothalamic regulation of lipid metabolism in the rat: effect of hypothalamic stimulation on lipolysis

In unanesthetized rats, electrical stimulation of the ventromedial hypothalamic nucleus (VMH) induced a marked increase in plasma concentration of glycerol, but did not increase the plasma free fatty acid (FFA) concentration, probably owing to a great elevation of plasma lactate which might inhibit the release of FFA from adipose tissue. In anesthetized rats, on stimulation of the VMH there was no remarkable increase in the plasma lactate, and the plasma glycerol and FFA concentrations were both elevated markedly. Electrical stimulation of the lateral hypothalamic nucleus (LH), on the other hand, had no significant effects on plasma glycerol and FFA levels. Bilateral adrenodemedullation did not prevent the lipolytic response to VMH stimulation, although it reduced slightly the increment of plasma glycerol and FFA. However, the lipolytic response was completely blocked by previous treatment of the animals with hexamethonium or propranolol, but not with phentolamine. These results suggest that sympathetic innervation of the adipose tissue is the dominant factor involved in VMH-induced lipolysis in the rat, while the role of the adrenal medulla is subdominant; the effect of VMH stimulation is mainly transmitted through the sympathetic nervous system to beta-adrenergic receptor of the adipose tissue.

Adipose tissue perfusion and fatty acid release in exercising rats

Arterial concentrations of free fatty acids (FFA), glycerol and lactate were measured in male Wistar rats before, during and after 30 minutes exercise on a treadmill. The arterial concentration of glycerol increased 120% and lactate 340% during exercise, but fell immediately upon its termination. The arterial concentration of FFA increased only 20% during the exercise, but was followed by a sharp rise in the recovery period. Maximum FFA concentration was found after 6 minutes of recovery, reaching 70% above the preexercise level, whereafter plasma FFA fell. Using radioactivity labelled microspheres it was found that the fraction of cardiac output reaching adipose tissue after 10 min of exercise was reduced by 30-90% in relation to the resting value. After 3 min recovery, however, the fractional distribution to the same tissue was in average 300% above the value measured 3 min before termination of exercise. It is concluded that the increase in plasma FFA observed during the early recovery period is caused by increased FFA release from adipose tissue, as a consequence of increased adipose tissue perfusion in the recovery period.

Influence of blood flow on fatty acid mobilization form lipolytically active adipose tissue

Subcutaneous, ingvinal adipose tissue from dogs was perfused with blood to which had been added isoprenaline and theophyllamine in order to stimulate lipolysis. The supply of free fatty acid (FFA) carrier to the tissue was varied either by variations in the rate of blood flow or by changes in the albumin concentration of the perfusing blood at constant flow rate. The net production of FFA from the tissue was found to depend on the supply of carrier over a range from 0.1 -- 12 mumoles of albumin x 100 g tissue-1 x min-1. The corresponding molar FFA/albumin ratios in adipose venous blood varied between 12.1 and 1.2. The changes in FFA production appeared to be due to varying degrees of reesterification rather than changes in the rate of lipolysis. The findings suggest that the increase in adipose tissue blood flow demonstrated during various lipolytic conditions is of physiological importance by facilitating the removal of FFA from adipose tissue. Equilibration experiments showed the FFA binding capacity of both dog serum and human plasma to be above than calculated from the association constants of purified human albumin, suggesting the binding of FFA to other plasma carriers than albumin.

Metabolic effects of blood flow restriction in adipose tissue

The metabolic effects of blood flow restriction were studied in isolated blood-perfused canine subcutaneous adipose tissue. Blood flow restriction (on the average to 20 per cent of control flow) was caused by either mechanical clamping of the arterial inflow or by i.a. injections of methoxamine or angiotensin. Glucose uptake in the adipose tissue was reduced during blood flow restriction. This was partially compensated for by a period of increased glucose uptake following restoration of flow. Blood flow restriction also caused an increase in the venous lactate/pyruvate ratio. The basal lipolytic rate was decreased during blood flow restriction. Lipolysis induced by brief (5 min) sympathetic nerve stimulation (4 Hz) was not inhibited by blood flow restriction as the total amount of glycerol released from the tissue was unaffected. The outflow rate was reduced during blood flow restriction, but glycerol trapped within the tissue was apparently not reutilized by the fat cells as it was released upon flow restroation. FFA outflow following nerve stimulation was, however, inhibited suggesting increased reutilization of FFA within the tissue. This increased reutilization may ultimately be caused by the observed change in red./ox.-balance and/or by the limited carrier capacity (albumin) available during blood flow restriction. Three main conclusions may be drawn from the present results. Firstly, plasma levels of glycerol and FFA do not necessarily reflect adipose tissue lipolysis at a given moment. Secondly, the decreased adipose tissue blood flow seems to be a major cause of the lowered FFA-levels during hemorrhage. Thirdly, in contrast to hemorrhage, even severe reduction of adipose tissue blood flow is insufficient to cause irreversible ischemic damage.

Vascular and metabolic responses to adrenergic stimulation in isolated canine subcutaneous adipose tissue at normal and reduced temperature

1. The circulatory and metabolic effects of temperature reduction were studied in autoperfused canine subcutaneous adipose tissue in situ. 2. Cooling the adipose tissue sufficiently to reduce venous effluent temperature by 5--6 degrees C decreased blood flow from an average of 6.4--4.1 ml. min-1 . 100g-1. 3. Vasoconstrictor responses to sympathetic nerve stimulation (4 HZ) and injected noradrenaline (5 n-mole) were potentiated by cooling while vasodilator components of the vascular responses, such as autoregulatory escape and post-stimulatory hyperaemia, were virtually abolished by this treatment. 4. Oxygen uptake was reduced by cooling without signs of tissue hypoxia. This reduced oxygen demand may partly cause the decrease in adipose tissue blood flow. 5. Cooling inhibited glycerol mobilization from the adipose tissue during sympathetic nerve stimulation. Post-stimulatory lipolysis was, however, not inhibited. In vitro studies with 'perifused' rat fat cells suggest that this may be due to impaired inactivation of the lipolytic process, rather than to changes in transmitter removal, following stimulation at low temperature. 6. Cooling inhibited the mobilization of fatty acids more than that of glycerol, suggesting increased re-esterification of fatty acids within the tissue at low temperature. 7. It is concluded that cooling increases the sensitivity to vasoconstrictor stimuli and that inhibition of metabolic vasodilator mechanisms play a role for this effect. The stimultaneous inhibition of activating and inactivating mechanisms could explain the unchanged vascular and lipolytic responses to brief stimuli. Some possible implications of the present findings for the physiology of adipose tissue during cooling are discussed.

Some metabolic consequences of the anaphylactic reaction in the rabbit

The administration of egg albumin to rabbits sensitized to this antigen caused marked increases in the arterial concentration of lactate, glucose and glycerol, but no change in the arterial FFA level. Antigen administration had no effect in non-sensitized rabbits. Phentolamine (3 mg/kg) or propranolol (1 mg/kg) did not significantly alter the responses to egg albumin in sensitized rabbits. Noradrenaline or sympathetic nerve stimulation decreased blood flow but caused no significant change in lipolysis in rabbit epigastric adipose tissue in situ. It is therefore questionable if catecholamines are the major cause of the observed metabolic consequences of the anaphylactic reaction in the rabbit. These metabolic events, i.e. increased lactate levels, lipolysis, and reesterification of fatty acids, are similar to those reported during hemorrhagic or endotoxin shock in dogs, in spite of specied-differences and the difference in the genesis of the shock.