Markers of de novo lipogenesis in adipose tissue: associations with small adipocytes and insulin sensitivity in humans

AIMS/HYPOTHESIS

Previous studies have shown relationships between fatty acid ratios in adipose tissue triacylglycerol (TG), adipocyte size and measures of insulin sensitivity. We hypothesised that variations in adipose tissue de novo lipogenesis (DNL) in relation to adiposity might explain some of these observations.

METHODS

In a cross-sectional study, subcutaneous abdominal adipose tissue biopsies from 59 people were examined in relation to fasting and post-glucose insulin sensitivity. Adipocyte size, TG fatty acid composition and mRNA expression of lipogenic genes were determined.

RESULTS

We found strong positive relationships between adipose tissue TG content of the fatty acids myristic acid (14:0) and stearic acid (18:0) with insulin sensitivity (HOMA model) (p < 0.01 for each), and inverse relationships with adipocyte size (p < 0.01, p < 0.05, respectively). Variation in 18:0 content was the determinant of the adipose tissue TG 18:1 n-9/18:0 ratio, which correlated negatively with insulin sensitivity (p < 0.01), as observed previously. Adipose tissue 18:0 content correlated positively with the mRNA expression of lipogenic genes (e.g. FASN, p < 0.01). Lipogenic gene expression (a composite measure derived from principal components analysis) was inversely correlated with adipocyte cell size (p < 0.001). There was no relationship between dietary saturated fatty acid intake and adipose tissue 18:0 content.

CONCLUSIONS/INTERPRETATION

Our data suggest a physiological mechanism whereby DNL is downregulated as adipocytes expand. Taken together with other data, they also suggest that hepatic and adipose tissue DNL are not regulated in parallel. We also confirm a strong relationship between small adipocytes and insulin sensitivity, which is independent of BMI.

Effects of prolonged fasting and sustained lipolysis on insulin secretion and insulin sensitivity in normal subjects

Normal beta-cells adjust their function to compensate for any decrease in insulin sensitivity. Our aim was to explore whether a prolonged fast would allow a study of the effects of changes in circulating free fatty acid (FFA) levels on insulin secretion and insulin sensitivity and whether any potential effects could be reversed by the antilipolytic agent acipimox. Fourteen (8 female, 6 male) healthy young adults (aged 22.8-26.9 yr) without a family history of diabetes and a body mass index of 22.6 +/- 3.2 kg/m(2) were studied on three occasions in random order. Growth hormone and FFA levels were regularly measured overnight (2200-0759), and subjects underwent an intravenous glucose tolerance test in the morning (0800-1100) on each visit. Treatment A was an overnight fast, treatment B was a 24-h fast with regular administrations of a placebo, and treatment C was a 24-h fast with regular ingestions of 250 mg of acipimox. The 24-h fast increased overnight FFA levels (as measured by the area under the curve) 2.8-fold [51.3 (45.6-56.9) vs. 18.4 (14.4-22.5) *10(4) micromol/l*min, P < 0.0001], and it led to decreases in insulin sensitivity [5.7 (3.6-8.9) vs. 2.6 (1.3-4.7) *10(-4) min(-1) per mU/l, P < 0.0001] and the acute insulin response [16.3 (10.9-21.6) vs. 12.7 (8.7-16.6) *10(2) pmol/l*min, P = 0.02], and therefore a reduction in the disposition index [93.1 (64.8-121.4) vs. 35.5 (21.6-49.4) *10(2) pmol/mU, P < 0.0001]. Administration of acipimox during the 24-h fast lowered FFA levels by an average of 20% (range: -62 to +49%; P = 0.03), resulting in a mean increase in the disposition index of 31% (P = 0.03). In conclusion, the 24-h fast was accompanied by substantial increases in fasting FFA levels and induced reductions in the acute glucose-simulated insulin response and insulin sensitivity. The use of acipimox during the prolonged fast increased the disposition index, suggesting a partial reversal of the effects of fasting on the acute insulin response and insulin sensitivity.

Adipose tissue fatty acid metabolism in insulin-resistant men

AIMS/HYPOTHESIS

Increased NEFA production and concentrations may underlie insulin resistance. We examined systemic and adipose tissue NEFA metabolism in insulin-resistant overweight men (BMI 25-35 kg/m2).

METHODS

In a cohort study we examined NEFA concentrations in men in the upper quartile of fasting insulin (n = 124) and in men with fasting insulin below the median (n = 159). In a metabolic study we examined NEFA metabolism in the fasting and postprandial states, in ten insulin-resistant men and ten controls.

RESULTS

In the cohort study, fasting NEFA concentrations were not significantly different between the two groups (median values: insulin-resistant men, 410 micromol/l; controls, 445 micromol/l). However, triacylglycerol concentrations differed markedly (1.84 vs 1.18 mmol/l respectively, p < 0.001). In the metabolic study, arterial NEFA concentrations again did not differ between groups, whereas triacylglycerol concentrations were significantly higher in insulin-resistant men. Systemic NEFA production and the release of NEFA from subcutaneous adipose tissue, expressed per unit of fat mass, were both reduced in insulin-resistant men compared with controls (fasting values by 32%, p = 0.02, and 44%, p = 0.04 respectively). 3-Hydroxybutyrate concentrations, an index of hepatic fat oxidation and ketogenesis, were lower (p = 0.03).

CONCLUSIONS/INTERPRETATION

Adipose tissue NEFA output is not increased (per unit weight of tissue) in insulin resistance. On the contrary, it appears to be suppressed by high fasting insulin concentrations. Alterations in triacylglycerol metabolism are more marked than those in NEFA metabolism and are indicative of altered metabolic partitioning of fatty acids (decreased oxidation, increased esterification) in the liver.

The in vivo effects of the Pro12Ala PPARgamma2 polymorphism on adipose tissue NEFA metabolism: the first use of the Oxford Biobank

AIMS/HYPOTHESIS

To investigate the phenotypic effects of common polymorphisms on adipose tissue metabolism and cardiovascular risk factors, we set out to establish a biobank with the unique feature of allowing a prospective recruit-by-genotype approach. The first use of this biobank investigates the effects of the peroxisome proliferator-activated receptor (PPAR) Pro12Ala polymorphism on integrative tissue-specific physiology. We hypothesised that Ala12 allele carriers demonstrate greater adipose tissue metabolic flexibility and insulin sensitivity.

MATERIALS AND METHODS

From a comprehensive population register, subjects were recruited into a biobank, which was genotyped for the Pro12Ala polymorphism. Twelve healthy male Ala12 carriers and 12 matched Pro12 homozygotes underwent detailed physiological phenotyping using stable isotope techniques, and measurements of blood flow and arteriovenous differences in adipose tissue and muscle in response to a mixed meal containing [1,1,1-(13)C]tripalmitin.

RESULTS

Of 6,148 invited subjects, 1,072 were suitable for inclusion in the biobank. Among Pro12 homozygotes, insulin sensitivity correlated with HDL-cholesterol concentrations, and inversely correlated with blood pressure, apolipoprotein B, triglyceride and total cholesterol concentrations. Ala12 carriers showed no such correlations. In the meal study, Ala12 carriers had lower plasma NEFA concentrations, higher adipose tissue and muscle blood flow, and greater insulin-mediated postprandial hormone-sensitive lipase suppression along with greater insulin sensitivity than Pro12 homozygotes.

CONCLUSIONS/INTERPRETATION

This study shows that a recruit-by-genotype approach is feasible and describes the biobank's first application, providing tissue-specific physiological findings consistent with the epidemiological observation that the PPAR Ala12 allele protects against the development of type 2 diabetes.

Changes in adiponectin receptor expression in muscle and adipose tissue of type 2 diabetic patients during rosiglitazone therapy

AIMS/HYPOTHESIS

Adiponectin is important in the regulation of insulin sensitivity in man. Its receptors, adipoR1 and R2, have recently been identified, but their expression in adipose tissue and their regulation in response to insulin sensitisation of diabetic patients have never been assessed. We therefore explored the regulation of adipoR1/R2 and adiponectin expression in adipose tissue and skeletal muscle, and of adiponectin plasma concentrations in response to insulin sensitisation by rosiglitazone.

METHODS

Patients with type 2 diabetes were studied in a double-blind, placebo-controlled crossover study, using in vivo arteriovenous techniques of measuring adipose tissue and muscle blood flow, combined with measurement of adipose tissue and skeletal muscle gene expression.

RESULTS

Rosiglitazone treatment increased adiponectin concentrations by 69%. Skeletal muscle adipoR1 expression was down-regulated from 109.0 (70.1-165.7) (median [interquartile range]) to 82.8 (63.6-89.3) relative units (p=0.04), but adipose tissue adipoR1 expression was up-regulated from 5.3 (4.4-9.4) to 11.2 (4.8-15.3) relative units (p=0.02) by rosiglitazone. In contrast to adipoR1 expression, adipoR2 expression was not altered by rosiglitazone in either of the tissues. The increase in adipose tissue adipoR1 expression with rosiglitazone was associated with increased postprandial triglyceride clearance (r=0.67, p=0.05), and increased fasting fatty acid output (r=0.78, p=0.01) measured in subcutaneous adipose tissue.

CONCLUSIONS/INTERPRETATION

AdipoR1 expression is up-regulated in adipose tissue but down-regulated in skeletal muscle by rosiglitazone. These data suggest that adipoR1 plays a role in mediating the effects of adiponectin in specific tissues in relation to insulin sensitisation.

The effects of rosiglitazone on fatty acid and triglyceride metabolism in type 2 diabetes

AIMS/HYPOTHESIS

We investigated the effects of rosiglitazone on NEFA and triglyceride metabolism in type 2 diabetes.

METHODS

In a double-blind, placebo-controlled, cross-over study of rosiglitazone in diet-treated type 2 diabetic subjects, we measured arteriovenous differences and tissue blood flow in forearm muscle and subcutaneous abdominal adipose tissue, used stable isotope techniques, and analysed gene expression. Responses to a mixed meal containing [1,1,1-(13)C]tripalmitin were assessed.

RESULTS

Rosiglitazone induced insulin sensitisation without altering fasting NEFA concentrations (-6.6%, p=0.16). Postprandial NEFA concentrations were lowered by rosiglitazone compared with placebo (-21%, p=0.04). Adipose tissue NEFA release was not decreased in the fasting state by rosiglitazone treatment (+24%, p=0.17) and was associated with an increased fasting hormone-sensitive lipase rate of action (+118%, p=0.01). Postprandial triglyceride concentrations were decreased by rosiglitazone treatment (-26%, p<0.01) despite unchanged fasting concentrations. Rosiglitazone did not change concentrations of triglyceride-rich lipoprotein remnants. Adipose tissue blood flow increased with rosiglitazone (+32%, p=0.03). Postprandial triglyceride [(13)C]palmitic acid concentrations were unchanged, whilst NEFA [(13)C]palmitic acid concentrations were decreased (p=0.04). In muscle, hexokinase II mRNA expression was increased by rosiglitazone (+166%, p=0.001) whilst the expression of genes involved in insulin signalling was unchanged. Adipose tissue expression of FABP4, LPL and FAT/CD36 was increased.

CONCLUSIONS/INTERPRETATION

Rosiglitazone decreases postprandial NEFA and triglyceride concentrations. This may represent decreased spillover of NEFAs from adipose tissue depots. Decreased delivery of NEFAs to the liver may lead to lowered postprandial triglyceride concentrations. Upregulation of hexokinase II expression in muscle may contribute to insulin sensitisation by rosiglitazone.

Plasma leptin is influenced by diet composition and exercise

OBJECTIVE

A low-fat, high-carbohydrate diet (</=30% of total energy intake as fat) in conjunction with moderate intensity physical activity is widely recommended for weight maintenance and reduction. The aim of this study was to assess the effect of adding daily exercise to a short-term high-carbohydrate diet on fasting and postprandial leptin levels.

SUBJECTS

Eight healthy, postmenopausal women aged 60+/-4 y (mean+/-s.d.) (body mass index, BMI: 26.4+/-2.3 kg m(-2); predicted maximal oxygen uptake: 29+/-2 ml kg(-1) min(-1)).

DESIGN

Plasma responses were studied after subjects consumed the same high-fat, mixed meal on three occasions: after 3 days on a low-carbohydrate diet (35, 50 and 15% energy from carbohydrate, fat and protein, respectively) (Low-CHO); after 3 days on an isoenergetic high-carbohydrate diet (corresponding values 70, 15 and 15%) (High-CHO); and after 3 days on the same high-carbohydrate diet with 60 min of brisk walking daily (High-CHO-Ex).

MEASUREMENTS

Fasting and postprandial plasma or serum concentrations of leptin, glucose and insulin.

RESULTS

Fasting leptin was significantly higher (P<0.05) after the High-CHO (18.4+/-2.6 ng ml(-1)) (mean+/-s.e.m.) than after both the Low-CHO and the High-CHO-Ex interventions, which did not differ significantly from each other (16.9+/-2.1 and 15.5+/-2.0 ng ml(-1), respectively; P=0.08). Overall (fasted and postprandial states), plasma leptin concentrations were significantly higher after the High-CHO than after the High-CHO-Ex intervention. There was a strong, positive, linear relation between postprandial insulin responses and postprandial leptin concentrations at 6 h. In addition, there was a strong, negative, linear relation between whole-body insulin sensitivity (based on postprandial responses of glucose and insulin) and postprandial leptin concentrations at 6 h.

CONCLUSION

Daily moderate intensity exercise, without concomitant changes in body fat mass, suppressed fasting and postprandial circulating leptin concentrations after consumption of a short-term high-carbohydrate diet. As shown in previous studies, insulin appears to be an important modulator of leptinaemia.

Effects of acute administration of doxazosin on fasting and postprandial haemodynamics and lipid metabolism in healthy subjects

The selective alpha1 -adrenoceptor antagonist doxazosin has apparently beneficial effects on insulin sensitivity and on plasma lipid concentrations. In order to understand these effects better, we investigated the acute effects of doxazosin on adipose tissue and forearm blood flow and on postprandial lipid metabolism in healthy subjects. Nine subjects were studied in a balanced, placebo-controlled design. Pulse rate, blood pressure, forearm and subcutaneous adipose tissue blood flow were measured before and for 6 h after a mixed meal, with concomitant measurements of blood metabolites and insulin. Doxazosin increased pulse rate (p = 0.02) and forearm blood flow (p < 0.01 in fasting state), and decreased vascular resistance in forearm (p < 0.05 for fasting values) and subcutaneous abdominal adipose tissue (p = 0.04). Fasting plasma non-esterified fatty acid concentrations were increased by 40 % (p < 0.05). No other metabolic effects were detected. The effects on adipose tissue vascular resistance and lipolysis (reflected in elevated non-esterified fatty acid concentrations) were unexpected, as these are usually considered to be mediated by the balance of alpha2 - and beta-adrenoceptor activity in humans. We conclude that alpha1 -adrenoceptor activity may be more important in regulation of human lipid metabolism than previously recognized.

Energy balance, metabolism, hydration, and performance during strenuous hill walking: the effect of age

We aimed to examine the effect of age on energy balance, metabolism, hydration, and performance during 10 days of strenuous hill walking. Seventeen male subjects were divided into two groups according to their age. The nine subjects in group 1 constituted the younger group (age 24 +/- 3 yr), whereas eight older subjects were in group 2 (age 56 +/- 3 yr). Both groups completed 10 consecutive days of high-intensity hill walking. Mean (range) daily walking distances and ascent were 21 km (10-35 km) and 1,160 m (800-2,540 m), respectively. Energy intake was calculated from weighed food intake, and energy expenditure was measured by the doubly labeled water method. Blood and urine were sampled on alternative days to determine any changes in metabolism and hydration during the 10 days. Subjects also completed a battery of tests that included muscular strength (handgrip), jump performance, cognitive processing time, and flexibility. The younger group remained hydrated, whereas the older group became progressively dehydrated, indicated by a near twofold increase in urine osmolality concentration on day 11. This increased urine osmolality in the older group was highly correlated with impairment in vertical-jump performance (r = -0.86; P < 0.05) and decreased cognitive processing time (r = 0.79; P < 0.05). Despite energy expenditure of approximately 21 MJ/day, body mass was well maintained in both groups. Both groups displayed a marked increase in fat mobilization, reflected in significantly lowered prewalk insulin concentrations and elevated postwalk glycerol and nonesterified fatty acid concentrations. Despite the dehydration and impaired performance in the older group, blood glucose concentrations were well maintained in both groups, probably mediated via the increased mobilization of fat.

Physiological and metabolic responses to a hill walk

The physiological and metabolic demands of hill walking have not been studied systematically in the field despite the potentially deleterious physiological consequences of activity sustained over an entire day. On separate occasions, 13 subjects completed a self-paced hill walk over 12 km, consisting of a range of gradients and terrain typical of a mountainous walk. During the hill walk, continuous measurements of rectal (T(re)) and skin (T(sk)) temperatures and of respiratory gas exchange were made to calculate the total energy expenditure. Blood samples, for the analysis of metabolites and hormones, were taken before breakfast and lunch and immediately after the hill walk. During the first 5 km of the walk (100- to 902-m elevation), T(re) increased (36.9 +/- 0.2 to 38.5 +/- 0.4 degrees C) with a subsequent decrease in mean T(sk) from this time point. T(re) decreased by approximately 1.0 degrees C during a 30-min stop for lunch, and it continued to decrease a further 0.5 degrees C after walking recommenced. The total energy intake from both breakfast and lunch [5.6 +/- 0.7 (SE) MJ] was lower than the energy expended [14.5 +/- 0.5 (SE) MJ; P < 0.001] during the 12-km hill walk. Despite the difference in energy intake and expenditure, blood glucose concentration was maintained. The major source of energy was an enhanced fat oxidation, probably from adipose tissue lipolysis reflected in high plasma nonesterified fatty acid concentrations. The major observations were the varying thermoregulatory responses and the negative energy balance incurred during the hill walk. It is concluded that recreational hill walking can constitute a significant metabolic and thermoregulatory strain on participants.

Monitoring adipose tissue blood flow in man: a comparison between the (133)xenon washout method and microdialysis

INTRODUCTION

Adipose tissue blood flow (ATBF) increases after meal intake and a failure to regulate ATBF in the postprandial period seems to be a feature of insulin resistance and obesity. ATBF can be measured quantitatively by the (133)Xe washout technique, but the microdialysis ethanol escape method has also been employed to detect relative changes in ATBF.

METHODS

We compared (133)Xe washout and the recovery of exogenous ethanol and endogenous urea by microdialysis in abdominal subcutaneous adipose tissue, after physiological stimulation of ATBF by ingestion of oral glucose (75 g) in eight healthy people (age 23-52 y, body mass index (BMI) 19.4-29.6 kg/m(2)).

RESULTS

The ATBF response was heterogeneous. In subjects responding vigorously to the stimulus as measured by (133)Xe washout, the microdialysis ethanol escape was increased (indicating an increase in ATBF). An increased recovery of urea was observed, also indicating an increase in ATBF. The recovery of both small molecules was delayed compared with increased blood flow and failed to return to baseline in response to a rapid decline in ATBF.

CONCLUSION

We conclude that the (133)Xe washout technique is more responsive to physiological change in ATBF than ethanol escape or urea recovery by microdialysis.

Exercise prevents the accumulation of triglyceride-rich lipoproteins and their remnants seen when changing to a high-carbohydrate diet

We tested the hypothesis that daily aerobic exercise opposes the fasting hypertriglyceridemia and exaggerated postprandial lipemia observed after substituting dietary fat with carbohydrate. Eight healthy postmenopausal women aged 51 to 66 years consumed the same high-fat mixed meal on 3 occasions: (1) after 3 days on a low-carbohydrate diet (35%, 50%, and 15% energy from carbohydrate, fat, and protein, respectively); (2) after 3 days on an isoenergetic high-carbohydrate diet (corresponding values 70%, 15%, and 15%); and (3) after 3 days on the same high-carbohydrate diet with 60 minutes of brisk walking daily. Plasma triglycerides were higher after the high-carbohydrate diet than after the low-carbohydrate diet: fasting, 1.58+/-0.19 versus 0.96+/-0.12 mmol/L, respectively; 6-hour postprandial area under concentration versus time curve, 13.74+/-1.57 versus 10.12+/-1.15 (mmol/L)xhour, respectively (both P<0.01). In the fasted and postprandial states, concentrations of apolipoproteins B-48 and B-100 in the triglyceride-rich lipoprotein fraction were significantly higher after the high-carbohydrate diet, as was the concentration of remnant-like lipoprotein particle cholesterol (a measure of lipoprotein remnants). These carbohydrate-induced increases in the number of circulating triglyceride-rich particles and their remnants were abolished when subjects had exercised daily during the high-carbohydrate diet.

Disturbances in the normal regulation of SREBP-sensitive genes in PPAR alpha-deficient mice

Peroxisome proliferator-activated receptor alpha (PPAR alpha)-null mice were used to investigate the nature of the relationship between the normal circadian rhythm of hepatic PPAR alpha expression and the expression of the lipogenic and cholesterogenic sterol regulatory element-binding protein (SREBP)-regulated genes, acetyl-CoA carboxylase, fatty acid synthase (FAS), and 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoAR). The expression of FAS and HMG-CoAR varied rhythmically over the diurnal cycle in the normal mice, with patterns that were the opposite of that of PPAR alpha. The diurnal variation of lipogenic and cholesterogenic gene expression was attenuated or abolished in the PPAR alpha-null mice. This resulted in decreased expression compared with normal mice, but only during the dark phase of the cycle, when food intake was high. The diurnal variation in hepatic fatty acid and cholesterol synthesis was also abolished in the PPAR alpha-null animals and the variations in the concentration of plasma triacylglycerol, nonesterified fatty acids, and cholesterol were all attenuated. The failure of HMG-CoAR expression to increase during the feeding period in the PPAR alpha-null mice was associated with a decrease in hepatic nonesterified cholesterol content and an increase in cholesteryl ester compared with normal mice. There was no defect in the downregulation of hepatic HMG-CoAR mRNA in response to dietary cholesterol in the PPAR alpha-null mice. Under these conditions, hepatic PPAR gamma expression increased in both the control and PPAR alpha-deficient mice. The results suggest that PPAR alpha-deficiency disturbs the normal circadian regulation of certain SREBP-sensitive genes in the liver, but does not affect their response to dietary cholesterol. -- Patel, D. D., B. L. Knight, D. Wiggins, S. M. Humphreys, and G. F. Gibbons. Disturbances in the normal regulation of SREBP-sensitive genes in PPAR alpha-deficient mice. J. Lipid Res. 2001. 42: 328--337.

Determinants of within-subject variation of fasting serum leptin concentrations in healthy subjects

The hormone leptin is considered to contribute to body weight regulation through modulation of feeding behavior and energy expenditure. The aim of the present study was 1) to assess the day-to-day within-subject variation (biovariability) of serum leptin concentrations in healthy subjects and 2) to investigate whether this variation is associated with food intake, exercise, anthropometric measurements or various metabolic covariates (insulin, C-peptide and glucagon, glucose, lactate, 3-hydroxybutyrate (3-OHB), triglycerides, non-esterified-fatty acids and glycerol). Serum leptin levels were taken daily on 12 consecutive days after an overnight fast in 12 healthy subjects with a mean (SD) age of 22.7 (1.5) yr. and a BMI of 22.8 (1.6) kg/m2. Food intake, exercise, anthropometric measurements and various metabolic covariates were also determined during this period. The overall mean of serum leptin concentration was 33.3 pmol/L with a within-subject SD range of 27-41 pmol/L and a between-subject SD range of 18-61 pmol/L. The within-subject variance of serum leptin as a proportion of total variance was 9.5%. Within-subject variation of serum leptin concentrations is small in relation to between-subject variation in healthy, normal weight subjects. This has implications for the power of interventional or prospective studies. In men, 6.7% of the variation in serum leptin concentration was associated with body weight measured on the same day (p= 0.037). In women, however, 66% of the variation was negatively associated with 3-OHB measured on both the same and the previous day (p=0.0003 and 0.002), and positively associated with triglyceride concentration measured on the previous day (p=0.0017) and insulin measured on the same day (p=0.0002). Within-subject associations in women could be due to phasic changes in unmeasured variables, possibly related to the menstrual cycle or might suggest that energy balance may exert a delayed influence on serum leptin levels, with plasma 3-OHB and triglycerides acting as markers for the state of the fat stores that regulate leptin secretion. The differences between the genders remain unexplained, however.

Selection of oral microbial adhesion antagonists using biotinylated Streptococcus sanguis and a human mixed oral microflora

A microtitre assay has been developed using hydroxyapatite-coated wells and Streptococcus sanguis NCTC 10904 at 10(7) cells per ml. A number of models representing toothpaste and mouthwash usage were adopted to detect the anti-adherent efficacy of a polyvinylmethylether maleic acid copolymer (PVM/MA), polyoxypropylene/polyoxyethylene block copolymer (PO/EO), two casein-derived peptides and selected silicones. The results not only confirmed the anti-adherence property of the selected components but also indicated possible molecular interactions leading to the observed performance. To account for the diversity of oral microbial cells in vivo, a further testing system was developed. This involved submerging a hydroxyapatite disc in a mixed culture of human salivary microbial cells, and exposing it to different treatments using the active component either in an aqueous dispersion or in a toothpaste. The effect of toothpastes containing PO/EO, dimethicone copoyol or PVM/MA was investigated over a 4-h incubation with microflora. These tests showed that in a toothpaste formulation the anti-adherent efficacy may be reduced when compared with an aqueous dispersion containing the same or nearly the same concentration of the active component.

Prior exercise and postprandial substrate extraction across the human leg

Prior exercise decreases postprandial plasma triacylglycerol (TG) concentrations, possibly through changes to skeletal muscle TG extraction. We measured postprandial substrate extraction across the leg in eight normolipidemic men aged 21-46 yr. On the afternoon preceding one trial, subjects ran for 2 h at 64 +/- 1% of maximal oxygen uptake (exercise); before the control trial, subjects had refrained from exercise. Samples of femoral arterial and venous blood were obtained, and leg blood flow was measured in the fasting state and for 6 h after a meal (1.2 g fat, 1.2 g carbohydrate/kg body mass). Prior exercise increased time averaged postprandial TG clearance across the leg (total TG: control, 0.079 +/- 0.014 ml.100 ml tissue(-1).min(-1) ; exercise, 0.158 +/- 0.023 ml.100 ml tissue(-1).min(-1), P <0.01), particularly in the chylomicron fraction, so that absolute TG uptake was maintained despite lower plasma TG concentrations (control, 1.53 +/- 0.13 mmol/l; exercise, 1.01 +/- 0.16 mmol/l, P < 0.001). Prior exercise increased postprandial leg blood flow and glucose uptake (both P < 0.05). Mechanisms other than increased leg TG uptake must account for the effect of prior exercise on postprandial lipemia.

Uptake of individual fatty acids into adipose tissue in relation to their presence in the diet

BACKGROUND

The fatty acid composition of adipose tissue triacylglycerol reflects, but is not identical to, the fatty acid composition of the habitual diet.

OBJECTIVE

We investigated whether the fatty acid composition of adipose tissue is explained by differences between fatty acids in early storage in adipose tissue after a meal.

DESIGN

Nine healthy men ate a meal containing several fatty acids. Blood samples were taken for 6 h after the meal from an arterialized hand vein and a vein draining the anterior abdominal subcutaneous adipose tissue.

RESULTS

Net storage of fatty acids in adipose tissue occurred between 1 and 4 h after the meal. In relation to the amount fed, storage of fatty acids differed (P < 0. 01) between classes (n-3 polyunsaturated < saturated < n-6 polyunsaturated < monounsaturated); oleic acid was stored in the greatest amounts. These differences agreed closely with published data, except for n-3 polyunsaturated fatty acids. The only individual metabolic step at which significant differences between fatty acids was shown was incorporation of fatty acids into chylomicron triacylglycerol. Differences between fatty acids in rate of extraction from chylomicron triacylglycerol and net uptake into adipose tissue in the postprandial period were significant (P < 0. 01), but not when expressed in relation to proportions in chylomicron triacylglycerol.

CONCLUSIONS

The characteristic fatty acid pattern of adipose tissue may predominantly reflect the early metabolic handling of different fatty acids. Adipose tissue uptake of n-3 polyunsaturated fatty acids is slow in relation to that of other fatty acids.

Ethanol with a mixed meal increases postprandial triacylglycerol but decreases postprandial non-esterified fatty acid concentrations

Plasma triacylglycerol concentrations increase after the acute ingestion of alcohol (specifically ethanol). However, the effect of ethanol when consumed with a mixed meal has not been well studied. The objective of the present study was to determine the perturbations of lipid metabolism that occur after ingestion of ethanol in combination with a mixed meal of specific fatty acid composition. Blood samples were taken from seven healthy male subjects before and after a mixed meal, with and without ethanol. The specific fatty acid composition of the test meal allowed the fatty acids to be traced into the plasma non-esterified fatty acid pool during the postprandial period. Statistical analysis by repeated measures ANOVA showed significant effects of ethanol. For example, postprandial lipaemia was enhanced after the ethanol test meal compared with the control (P < 0.05), mainly due to increases in triacylglycerol-rich lipoproteins in the flotation range Sf 60-400 (VLDL1) (P < 0.05); those in the range Sf 20-60 (VLDL2) and also Sf > 400 (chylomicrons) were not significantly affected. The later postprandial increase in plasma non-esterified fatty acid concentrations was reduced after the ingestion of ethanol (P < 0.001), but the proportions of palmitoleic acid (a marker of fatty acid content of the test meal) and of linoleic acid (a marker of endogenous lipolysis) were not affected. The results suggest a primary effect of ethanol on the stimulation of secretion of large VLDL particles, which then compete for clearance with chylomicrons by lipoprotein lipase. The results do not support an effect of ethanol on the release of non-esterified fatty acid into the plasma. The suppression of plasma non-esterified fatty acid concentrations during the postprandial period may contribute towards the beneficial effects of moderate ethanol consumption.

Suppression of the nocturnal rise in growth hormone reduces subsequent lipolysis in subcutaneous adipose tissue

BACKGROUND

The aim of this study was to examine the effect of the nocturnal rise in growth hormone (GH) concentration on lipolysis in adipose tissue the following morning.

METHODS

Eight healthy subjects were studied on two occasions (control vs. suppression of GH secretion) and six were studied on a third occasion (control vs. replacement of GH). Lipolysis in the whole body was assessed by measurement of systemic glycerol turnover. Lipid metabolism in the subcutaneous adipose tissue of the anterior abdominal wall was studied by measurement of arterio-venous differences.

RESULTS

Suppression of the nocturnal rise in GH did not affect systemic glycerol turnover. However, in subcutaneous abdominal adipose tissue it led to a significant reduction in the veno-arterial differences in nonesterified fatty acid (NEFA, P = 0.041) and glycerol (P = 0. 014) concentrations, reflecting a reduction in intracellular lipolysis (P = 0.011). Although arterialized plasma triacylglycerol (TG) concentrations were reduced in the absence of the nocturnal GH pulse, the extraction of TG in subcutaneous abdominal adipose tissue remained unchanged.

CONCLUSION

We conclude that the normal nocturnal rise in plasma GH concentration leads to site-specific regulation of lipolysis in adipose tissue on the following day, with preferential fat mobilization from central depots.

Co-ordination of hepatic and adipose tissue lipid metabolism after oral glucose

The integration of lipid metabolism in the splanchnic bed and in subcutaneous adipose tissue before and after ingestion of a 75 g glucose load was studied by Fick's principle in seven healthy subjects. Six additional subjects were studied during a hyperinsulinemic euglycemic clamp. Release of non-esterified fatty acids (NEFA) from adipose tissue and splanchnic NEFA extraction followed a similar time-course after oral glucose, and there was a highly significant relationship between adipose tissue NEFA release and splanchnic NEFA uptake. There was no immediate inhibition of splanchnic very low density lipoprotein (VLDL)-triacylglycerol (TAG) output when plasma insulin levels increased after glucose. Adipose tissue extraction of VLDL-TAG tended to vary in time in a manner similar to splanchnic VLDL-TAG output and the two were significantly related. The area-under-curves (AUC) for splanchnic extraction of NEFA was significantly lower than that for output of VLDL, implying depletion of hepatic TAG stores during the experiment. In the hyperinsulinemic clamp experiments, there was on average suppression of splanchnic VLDL-TAG output although between-person variability was marked. This suppression could be explained by a very low supply of NEFA during the clamp. We conclude that there is an integrated pattern of metabolism in splanchnic and adipose tissues in the postabsorptive and post-glucose states. Flux of NEFA from adipose tissue drives splanchnic NEFA uptake. Splanchnic VLDL-TAG secretion appears to be regulated by a number of factors and in turn controls TAG extraction in adipose tissue. Insulin does not seem to play a key role in the acute regulation of hepatic VLDL metabolism under these particular conditions in vivo.

The use of microdialysis to monitor rapid changes in glucose concentration

As a result of the Diabetes Control and Complications Trial, there is increased emphasis on the importance of blood glucose concentration self-monitoring for people with diabetes. The current methods for this are not ideal, and there are many other possible techniques currently under investigation. One of these techniques is microdialysis, which can be used to analyse subcutaneous interstitial glucose concentrations. A system with high recovery has recently been used to monitor glucose concentrations with sampling over one- or two-hour periods. We have investigated whether this system can be used to monitor rapid changes in blood glucose concentration in healthy volunteers with collection intervals of only ten minutes. The results show that microdialysis can be used to monitor rapidly changing blood glucose concentration, but in some subjects, dialysate glucose lagged behind the whole blood and plasma glucose concentrations to a degree that would be clinically significant. It would therefore be necessary to assess the system, comparing dialysate with plasma glucose concentrations in each individual, prior to use in a clinical setting.

Lipoprotein lipase transport in plasma: role of muscle and adipose tissues in regulation of plasma lipoprotein lipase concentrations

Lipoprotein lipase (LPL) is synthesized in tissues involved in fatty acid metabolism such as muscle and adipose tissue. LPL is also found in the circulation, but is mostly lipolytically inactive. The proportion of active circulating LPL increases after a fatty meal. We investigated the release of active and inactive LPL from adipose tissue and muscle in the fasting and postprandial states. Arteriovenous concentration gradients of LPL across adipose tissue and forearm muscle were measured in male subjects before and after a fat-rich meal (n = 7) and before and during infusion of a triacylglycerol emulsion (Intralipid) (n = 6). Plasma LPL activity rose after the meal and more so during Intralipid infusion. Plasma LPL mass (>95% inactive LPL) increased after the meal but decreased after Intralipid infusion. In the fasting state (n = 13) muscle efflux of LPL activity was 0.263 +/- 0.098 mU/min per 100 ml of muscle tissue whereas there was an influx of LPL activity to adipose tissue of 0.085 +/- 0.100 mU/min per 100 g of adipose tissue (P < 0. 02 muscle vs. adipose tissue). Similarly in the postprandial state only muscle released LPL activity. Both tissues released LPL mass. In the fasting state efflux was 17.8 +/- 8.8 ng/min per 100 ml muscle and 55.2 +/- 21.3 ng/min per 100 g of adipose tissue (P < 0. 05 muscle vs. adipose tissue). Release of LPL, either active or inactive, was not correlated with levels of non-esterified fatty acids or plasma triacylglycerol. In conclusion, there is a substantial release of LPL from adipose tissue and muscle, most of which is inactive. A small proportion of active LPL seems to be redistributed from muscle to adipose tissue.

Metabolic and performance responses during endurance exercise after high-fat and high-carbohydrate meals

We studied the effects of preexercise meal composition on metabolic and performance-related variables during endurance exercise. Eight well-trained cyclists (maximal oxygen uptake 65.0 to 83.5 ml . kg-1 . min-1) were studied on three occasions after an overnight fast. They were given isoenergetic meals containing carbohydrate (CHO), protein (P), and fat (F) in the following amounts (g/70 kg body wt): high-carbohydrate meal, 215 CHO, 26 P, 3 F; high-fat meal, 50 CHO, 14 P, 80 F. On the third occasion subjects were studied after an overnight fast. Four hours after consumption of the meal, subjects started exercise for 90 min at 70% of their maximal oxygen uptake, followed by a 10-km time trial. The high-carbohydrate meal compared with the high-fat meal resulted in significant decreases (P < 0.05) in blood glucose, plasma nonesterified fatty acids, plasma glycerol, plasma chylomicron-triacylglycerol, and plasma 3-hydroxybutyrate concentrations during exercise. This was accompanied by an increase in plasma insulin (P < 0.01 vs. no meal), plasma epinephrine, and plasma growth hormone concentrations (each P < 0.05 vs. either of the other conditions) during exercise. Despite these large differences in substrate and hormone concentrations in plasma, substrate oxidation during the 90-min exercise period was similar in the three trials, and there were no differences in performance on the time trial. These results suggest that, although the availability of fatty acids and other substrates in plasma can be markedly altered by dietary means, the pattern of substrate oxidation during endurance exercise is remarkably resistant to alteration.

Peripheral fat metabolism during infusion of an exogenous triacylglycerol emulsion

OBJECTIVE

To test the hypothesis that intravenous infusion of lipid would bring about changes in adipose tissue metabolism, which would tend to spare net fat mobilization, and to attempt to identify the mediators of such responses.

DESIGN

The triacylglycerol (TG) emulsion, Intralipid, was infused and metabolic changes in subcutaneous adipose tissue and forearm muscle were assessed by measurements of arterio-venous differences.

SUBJECTS

Six normal male subjects aged 21-37 y, with body mass index (BMI) 23.0-25.9 kg/m2.

RESULTS

Plasma TG and non-esterified fatty acid (NEFA) concentrations rose during infusion as expected. The rise in systemic plasma NEFA concentration occurred despite decreased NEFA release from adipose tissue. Intralipid infusion resulted in a suppression of intracellular lipolysis in adipose tissue, by mechanisms which are not clear. Plasma leptin concentrations, measured in a search for the regulator of lipolysis, showed consistent leptin release from adipose tissue which did not change significantly with time.

CONCLUSION

The suppression of intracellular lipolysis in adipose tissue during Intralipid infusion is a new observation and may reflect a novel mechanism for regulation of fat storage.

Acute effect of fructose on postprandial lipaemia in diabetic and non-diabetic subjects

We investigated whether the potentiation of postprandial lipaemia by fructose occurs in both non-diabetic subjects and those with non-insulin-dependent diabetes mellitus. Six non-diabetic and six diabetic subjects were studied on two occasions. They were given a meal containing 1 g fat/kg body weight with, on one occasion, 0.75 g fructose/kg body weight, on the other occasion 0.75 g starch/kg body weight. In both groups, plasma glucose and insulin concentrations rose more after starch than after fructose. At 1-2 h after the meal, plasma non-esterified fatty acid concentrations were suppressed more after fructose than after starch, but later they rose more after fructose than after starch. Plasma triacylglycerol concentrations rose more slowly after fructose, but were considerably higher than those after starch from 4-6 h after the meal. There were no differences in post-heparin plasma lipoprotein lipase (EC 3.1.1.34) activity at the end of the test. The potentiation of postprandial lipaemia by fructose was positively related to the fasting plasma insulin concentration, suggesting that insulin-resistant subjects are more prone to this effect. We conclude that the potentiation of postprandial lipaemia by fructose is seen in both diabetic and non-diabetic subjects. Our results suggest that alterations in the dynamics of plasma non-esterified fatty acids might underlie the effects of fructose on triacylglycerol metabolism.

Adipose tissue metabolism in the postprandial period: microdialysis and arteriovenous techniques compared

We investigated whether two different methods of studying metabolism in adipose tissue, microdialysis and the arteriovenous technique, produced comparable results during the postprandial period. Interstitial glycerol concentrations measured by microdialysis are usually used as an index of intracellular lipolysis, and it is not known whether they also reflect the intravascular action of lipoprotein lipase in the postprandial period. The two techniques were compared in 10 healthy subjects fed mixed meals. Interstitial glycerol concentrations reflected those measured in adipose tissue venous plasma. However, the calculation of the rate of glycerol release from adipose tissue using the microdialysis data differed systematically from that using arteriovenous difference measurement. The former method gave, on average, 40% lower values than the latter one. The difference is probably due to the assumptions that had to be made for the calculation of glycerol release. The two techniques have complementary places in the study of postprandial adipose tissue metabolism, with microdialysis reflecting intracellular hormone-sensitive lipase action rather than intravascular lipoprotein lipase.

Effects of physiological hypercortisolemia on the regulation of lipolysis in subcutaneous adipose tissue

Cortisol is known to increase whole body lipolysis, yet chronic hypercortisolemia results in increased fat mass. The main aim of the study was to explain these two apparently opposed observations by examining the acute effects of hypercortisolemia on lipolysis in subcutaneous adipose tissue and in the whole body. Six healthy subjects were studied on two occasions. On one occasion hydrocortisone sodium succinate was infused i.v. to induce hypercortisolemia (mean plasma cortisol concentrations, 1500 +/- 100 vs. 335 +/- 25 nmol/L; P < 0.001); on the other occasion (control study) no intervention was made. Lipolysis in the s.c. adipose tissue of the anterior abdominal wall was studied by measurement of arterio-venous differences, and lipolysis in the whole body was studied by constant infusion of [1,2,3-2H5]glycerol for measurement of the systemic glycerol appearance rate. Hypercortisolemia led to significantly increased arterialized plasma nonesterified fatty acid (NEFA; P < 0.01) and blood glycerol concentrations (P < 0.05), with an increase in systemic glycerol appearance (P < 0.05). However, in s.c. abdominal adipose tissue, hypercortisolemia decreased veno-arterialized differences for NEFA (P < 0.05) and reduced NEFA efflux (P < 0.05). This reduction was attributable to decreased intracellular lipolysis (P < 0.05), reflecting decreased hormone-sensitive lipase action in this adipose depot. Hypercortisolemia caused a reduction in arterialized plasma TAG concentrations (P < 0.05), but without a significant change in the local extraction of TAG (presumed to reflect the action of adipose tissue lipoprotein lipase). There was no significant difference in plasma insulin concentrations between the control and hypercortisolemia study. Site-specific regulation of the enzymes of intracellular lipolysis (hormone-sensitive lipase) and intravascular lipolysis (lipoprotein lipase) may explain the ability of acute cortisol treatment to increase systemic glycerol and NEFA appearance rates while chronically promoting net central fat deposition.

Clearance of lipoprotein remnant particles in adipose tissue and muscle in humans

A major proportion of triglycerides in plasma triglyceride-rich lipoproteins (TRL) are removed in peripheral tissues by lipoprotein lipase, and hypothetically a minor proportion can also be removed by whole-lipoprotein particle uptake. This second removal pathway has not previously been directly demonstrated in humans. Simultaneous blood samples were drawn from arterialized blood, a vein draining the subcutaneous abdominal adipose tissue, and a deep antecubital vein of the forearm to provide arterio-venous gradients from blood-draining adipose tissue and muscle in seven male subjects. The men were given a fat-rich mixed meal containing vitamin A and the triglyceride and retinyl palmitate (RP) concentrations were quantified in the plasma. Density gradient ultracentrifugation was used to isolate TRL fractions, in which triglycerides, RP, apoB-48, and apoB-100 were quantified. There was clearance of triglycerides in muscle and adipose tissue and, in addition, removal of RP. By analysis of the TRL subfractions, the RP removal was likely to be confined to the largest chylomicron remnant particles. For the Sf > 400 fraction, the area under curve (AUC) relative to arterial for triglycerides were 79% (66-91%) and 81% (72-89%) in adipose tissue and muscle venous outflow, respectively (each P < 0.02 versus arterial). The corresponding values for RP were 87% (73-101%) and 85% (69-100%), respectively, (each P < 0.05 versus arterial). In the Sf 60-400 fraction there was further uptake of triglycerides, but not of RP. We hypothesize that the periphery could be of importance for removal of the largest chylomicron remnants, as their size might partially exclude them penetrating the fenestrated hepatic sinusoidal endothelium to reach the hepatic chylomicron remnant receptors.

Metabolic responses to isoenergetic meals containing different proportions of carbohydrate and fat

The purpose of the present study was to investigate the interrelationship between carbohydrate and fat metabolism at rest after isoenergetic meals of varying proportions of carbohydrate and fat. Eight physically-active subjects (BMI 18.1-23.4 kg/m2) were studied at rest on three occasions after an overnight fast. In a balanced design they were given meals containing carbohydrate, protein and fat in the following amounts respectively (g/70 kg body weight): meal 1 121, 16, 48; meal 2 70, 16, 70; meal 3 50, 14, 80. All meals were isoenergetic, containing 4.0 MJ/70 kg body weight, and were of similar appearance. In addition, on a fourth occasion five of the eight subjects consumed meal 4 (g/70 kg body weight): carbohydrate 0, protein 0, fat 108. Blood samples were taken before eating the meal and at intervals following the meal to determine metabolic and hormonal responses. Energy expenditure and substrate oxidation were measured by indirect calorimetry and balance was calculated over the 5 h postprandial period. The incremental areas under the time curves for fat oxidation were greatest after meals 3 and 4 (P < 0.05), whereas incremental areas under the carbohydrate oxidation v. time curves were relatively reduced after these two meals (P < 0.05). This was accompanied by lesser suppression of plasma non-esterified fatty acid concentrations (P < 0.001) and reduced plasma insulin concentrations (P < 0.001) following these meals. Energy balance was almost identical after the three isoenergetic meals. In contrast, there was an inverse relationship between carbohydrate and fat balance following these meals, with carbohydrate balance decreasing as carbohydrate intake decreased and fat balance increasing as fat intake increased. We conclude that there is a close interrelationship between carbohydrate and fat metabolism following isoenergetic meals in resting subjects.

Effects of morning rise in cortisol concentration on regulation of lipolysis in subcutaneous adipose tissue

Cortisol has a well-defined circadian rhythm. The aim of the study was to examine the effect of the morning rise in cortisol concentration on lipolysis in adipose tissue. Ten healthy subjects were studied on two occasions, and six of these were studied on three occasions. During the first two occasions, either a control or cortisol suppression study was performed by using metyrapone, and on the third occasion exogenous cortisol replacement was given in addition to metyrapone. Lipolysis in the subcutaneous adipose tissue of the anterior abdominal wall was studied by measurement of arteriovenous differences. Reduction in the early morning rise in cortisol led to significantly decreased venoarterialized differences for nonesterified fatty acids (P < 0.05) and glycerol (P < 0.01), attributable in part to decreased hormone-sensitive lipase (EC 3.1.1.3) action (P < 0.05) in adipose tissue. At the same time the arterialized plasma triacylglycerol concentration increased (P < 0.005) with a significant reduction in the adipose lipoprotein lipase (EC 3.1.1.34) rate of action (P < 0.05). In the replacement study, values were identical to those of the control study, showing that metyrapone had no nonspecific effects on lipolysis. We conclude that the morning rise in plasma cortisol concentration plays an important role in the regulation of lipolysis in adipose tissue in normal healthy adults.

Subcutaneous abdominal adipose tissue blood flow: variation within and between subjects and relationship to obesity

1. We assessed the variation in subcutaneous abdominal adipose tissue blood flow within and between subjects and investigated whether it is correlated with body mass index. 2. We measured body mass index and subcutaneous abdominal adipose tissue blood flow in 38 fasting subjects on the same day and on different days and, in a subgroup of 16 subjects, after a mixed meal. 3. In 190 measurements in the fasted state, subcutaneous abdominal adipose tissue blood flow was significantly more variable between subjects than could be accounted for by the within-subject variation alone. Subcutaneous abdominal adipose tissue blood flow was also significantly more variable between days within subjects than could be accounted for by within-day variation alone. Fasting and post-prandial subcutaneous abdominal adipose tissue blood flow were negatively correlated with body mass index, as was the post-prandial rise in subcutaneous abdominal adipose tissue blood flow. Multiple regression analysis showed that fasting blood flow was not dependent on insulin concentration after allowing for body mass index. There was no correlation between post-prandial subcutaneous abdominal adipose tissue blood flow and insulin concentration. 4. Insulin does not appear to have a direct vasodilatory effect in subcutaneous adipose tissue. Obese subjects have lower fasting and post-prandial subcutaneous abdominal adipose tissue blood flow. This may be because of a blunted response to sympathetic stimulation, or it may be another aspect of the insulin-resistant state.

Effects of epinephrine infusion on adipose tissue: interactions between blood flow and lipid metabolism

Epinephrine has effects on both blood flow and metabolism in adipose tissue. To investigate how these effects might interact in vivo, epinephrine was infused into six healthy volunteers at a rate of 25 ng.kg-1.min-1. The rates of action of lipoprotein lipase and hormone-sensitive lipase in adipose tissue were calculated by measurement of arteriovenous differences across subcutaneous abdominal adipose tissue, and adipose tissue blood flow was measured. Epinephrine caused a significant rise in adipose tissue blood flow (P < 0.001), and the net efflux of nonesterified fatty acids (NEFA) from adipose tissue increased significantly (P < 0.05). Most of this efflux could be accounted for by hormone-sensitive lipase-derived NEFA efflux from cells (P < 0.05), but there was also a significant rise in the contribution of lipoprotein lipase-derived NEFA (P < 0.05). We conclude that adipose tissue blood flow plays an important role in the regulation of lipid metabolism, controlling substrate presentation for lipoprotein lipase and also preventing the local accumulation of fatty acids derived from both hormone-sensitive lipase and lipoprotein lipase.

Effects of adrenaline infusion on the interstitial environment of subcutaneous adipose tissue as studied by microdialysis

1. Adrenaline was infused at a rate of 25 ng min-1 kg-1 into seven healthy volunteers and its effects on adipose tissue were studied by microdialysis. 2. Adrenaline infusion led to a rapid rise in plasma adrenaline concentration from a basal value of 0.15 +/- 0.03 nmol/l to a peak value of 1.82 +/- 0.15 nmol/l within 30 min (analysis of variance: P < 0.001). Plasma noradrenaline levels increased from the basal value of 1.48 +/- 0.21 nmol/l to 1.78 +/- 0.20 nmol/l within 30 min of infusion (analysis of variance: P < 0.05). Once the infusion was stopped the plasma adrenaline and noradrenaline levels returned to basal values within 15 min. 3. The arterialized blood glycerol concentration increased from the basal value of 47 +/- 6 mumol/l to 101 +/- 8 mumol/l (analysis of variance: P < 0.001), and the interstitial glycerol concentration increased from 181 +/- 17 mumol/l to 350 +/- 47 mumol/l (analysis of variance: P < 0.001) within 30 min of adrenaline infusion. Thereafter the concentrations declined gradually towards basal values. When the infusion was discontinued the levels declined further, below the basal value. 4. Adrenaline infusion caused a significant increase in the plasma glucose and blood lactate concentration (analysis of variance: P < 0.001 and P < 0.001), but no such effect was observed in the interstitial concentrations. 5. The basal dialysate concentrations of adrenaline and noradrenaline were 43 +/- 37 pmol/l and 1170 +/- 430 pmol/l respectively. There were difficulties in estimating the absolute interstitial concentrations of adrenaline and noradrenaline which are discussed. The dialysate adrenaline concentration increased to a peak value of 112 +/- 48 pmol/l (analysis of variance: P < 0.05) within 30 min. The dialysate noradrenaline concentration increased to a peak value of 2830 +/- 1470 pmol/l, but the rise was not significant. 6. These results show that the interstitial concentrations of metabolites and hormones can be significantly different from those of blood and plasma and these differences reflect tissue specialization.

Net carbon flux across subcutaneous adipose tissue after a standard meal in normal-weight and insulin-resistant obese subjects

OBJECTIVE

To elucidate the alterations in subcutaneous adipose tissue metabolism in obesity which affect substrate storage in the fed state, by describing as completely as possible the 'metabolic balance sheet' for adipose tissue.

SUBJECTS

13 lean people (age: 24-64 years, BMI: 19.3-29.4 kg/m2) and 9 insulin-resistant obese subjects (age: 38-57 years, BMI: 32.4-56.0 kg/m2).

MEASUREMENTS

Net flux of eight major substrates and CO2 across subcutaneous adipose tissue in vivo before and after eating a standard mixed meal (3.1 MJ, 41% from fat).

RESULTS

The net flux of total carbon atoms in the 8 substrates and CO2 was negative (net export from the tissue) in both groups in the fasting state. After the meal it changed rapidly to a positive balance (net substrate storage) in the control group and remained positive until 5 h, whereas in the obese group it remained negative throughout. The overall carbon balance in adipose tissue reflected predominantly the movement of fatty acids in and out of the tissue.

CONCLUSIONS

The fact that fat storage failed to occur despite elevated plasma concentrations of triacylglycerol, glucose and insulin indicates an alteration in metabolic regulation at the cellular level which might be called insulin resistance. Insulin resistance of fat metabolism may be an adaptation which limits further fat deposition, but is maladaptive in terms of risk factors for atherosclerosis.

Release of individual fatty acids from human adipose tissue in vivo after an overnight fast

The objective of this study was to investigate the mobilization of individual fatty acids from human subcutaneous adipose tissue in vivo. Concentrations of individual non-esterified fatty acids were measured in arterialized plasma and in the venous drainage from subcutaneous abdominal adipose tissue in eight normal subjects, after an overnight fast, and for the subsequent 6 h. Whilst the veno-arterial concentration difference for non-esterified fatty acids increased over this period, the relative proportions of different fatty acids remained constant. There was a close relationship between veno-arterial difference and arterialized concentration for the different fatty acids. The arterialized concentration of stearic acid consistently lay above the regression line drawn for unsaturated fatty acids (P = 0.001), probably reflecting lower clearance of stearic acid. The release of individual fatty acids was compared with their prevalence in adipose tissue triacylglycerol in biopsies taken from six subjects. Relative release decreased with increasing chain length, and for a given chain length increased with increasing unsaturation, in agreement with animal studies (P < 0.001 for differences in relative mobilization according to these two factors). The results suggest that the systemic plasma concentration of individual non-esterified fatty acids is determined by their release from adipose tissue, but that the relationship between release and systemic concentration for stearic acid is different from that for other fatty acids. The results confirm, in humans, differences in relative mobilization found previously in animal studies.

Regulation of lipid metabolism in adipose tissue during early starvation

We studied changes in lipid metabolism in adipose tissue in 24 healthy adults during early starvation (14-20 h) by cannulating the venous drainage of the subcutaneous adipose tissue of the anterior abdominal wall. Net nonesterified fatty acid (NEFA) efflux from adipose tissue increased steadily from 1,790 +/- 300 to 2,360 +/- 290 nmol.100 g-1.min-1 (P = 0.03), due to increasing transcapillary efflux of NEFA (release from adipocytes; P < 0.01). The reesterification rate after an overnight fast was close to zero; thus, reduction in the rate of reesterification played no part in the increased transcapillary efflux of NEFA. One-quarter of the net efflux of NEFA after an overnight fast arose from the action of lipoprotein lipase (LPL), although this relative contribution decreased during the study (P < 0.02). The increased transcapillary efflux of NEFA reflected a significant increase in the rate of action of hormone-sensitive lipase (HSL; P = 0.03). There was a strong relationship between mean arterial NEFA concentration and net NEFA release from adipose tissue (P < 0.001), implying that the particular depot studied reflects the behavior of adipose tissue as a whole. Thus the increasing efflux of NEFA from adipose tissue observed during early starvation is due to an increased rate of action of HSL, which may in turn be regulated by a fall in the plasma insulin concentration.

Carbohydrate metabolism in insulin resistance: glucose uptake and lactate production by adipose and forearm tissues in vivo before and after a mixed meal

1. To examine whether insulin resistance in vivo is manifest equally in both muscle and adipose tissues, we measured arteriovenous glucose and lactate fluxes across forearm (muscle) and abdominal subcutaneous (adipose) tissue in nine obese, glucose-intolerant subjects and 13 non-obese subjects of similar age and sex. 2. Compared with non-obese subjects, the forearm of the obese subjects was resistant to insulin stimulation of glucose uptake after a mixed meal. In contrast, adipose tissue showed little evidence of insulin stimulation of glucose uptake, and adipose tissue in subjects in both normal and obese groups behaved very similarly (assessed per 100 g of tissue). 3. For lactate flux, adipose tissue behaved very similarly (per 100 g of tissue) in obese and non-obese subjects, and was a consistent lactate exporter. 4. We conclude that insulin resistance of glucose uptake observed in the forearm of obese subjects is not evident in adipose tissue. Adipose tissue glucose uptake in obese, insulin-resistant subjects is similar to that in lean control subjects, although it occurs at elevated circulating insulin and glucose concentrations.

Is the growth hormone concentration regulated by the morning rise in the plasma cortisol concentration?

Exogenous steroids affect plasma growth hormone (GH) concentrations and hypothalamic somatostatin levels either directly, by stimulating the anterior pituitary gland, or indirectly by partially inhibiting the response of the pituitary gland to stimuli. The effect of the circadian rhythm of cortisol on plasma GH concentrations is unknown. In normal subjects, highest plasma GH concentrations are recorded within the first two hours of onset of sleep. During the early morning the plasma GH concentration decreases and the plasma cortisol concentration increases. We investigated the effect of the early morning rise in plasma cortisol concentration on plasma GH concentrations. We studied 12 healthy volunteers on two occasions. On one occasion the early morning rise in plasma cortisol concentrations was reduced by administering metyrapone (750 mg, 4 hourly from 03.00), while on the other occasion no intervention was made. Plasma GH and cortisol concentrations were measured every 30 and 60 minutes respectively from 22.00 until 14.00. Metyrapone caused a significant reduction in the rise of plasma cortisol concentration, but the GH concentration was identical on both occasions. We conclude that the morning rise in cortisol concentration has no acute regulatory effect on the plasma GH concentration.

Human triacylglycerol-rich lipoprotein subfractions as substrates for lipoprotein lipase

In order to test the hypothesis that lipoprotein lipase (LPL) acts preferentially on larger lipoprotein particles, we determined the susceptibility of triacylglycerol-rich lipoprotein (TRL) subfractions to hydrolysis by LPL in vitro. Chylomicrons (Sf > 400), very low density lipoproteins (VLDL)1 (Sf 60-400) and VLDL2 (Sf 20-60) were isolated from six subjects with a range of plasma-triacylglycerol (TAG) concentrations following an overnight fast and for up to 6 h after the consumption of a mixed meal (41% fat). The percent of TRL-TAG hydrolysed by LPL in subfractions isolated following overnight fast was VLDL1 > VLDL2 (46.8 +/- 10.2 vs. 25.9 +/- 7.4%, P = 0.006) and 3 h after the meal it was chylomicrons > VLDL1 > VLDL2 (81.0 +/- 12.6 vs. 52.8 +/- 10.2 vs. 27.7 +/- 6.2%, chylomicrons vs. VLDL1 and VLDL1 vs. VLDL2, both P < or = 0.005). The percent of VLDL1-TAG hydrolysed increased both within and between subjects as VLDL1-TAG concentrations increased. This relationship could be explained by the positive correlation observed between VLDL1-TAG and VLDL1-TAG:apolipoprotein B. In conclusion, increasing the size and TAG content of a lipoprotein particle increases its susceptibility to hydrolysis by LPL.

Impaired postprandial clearance of triacylglycerol-rich lipoproteins in adipose tissue in obese subjects

Adipose tissue is an important site of clearance of circulating triacylglycerol (TAG), especially in the postprandial period. Postprandial lipemia is usually increased in obesity. We studied the extraction of TAG from plasma and TAG-rich lipoproteins (TRLs) in subcutaneous adipose tissue in 11 control and 8 obese subjects before and after a mixed meal. Clearance of plasma TAG and very low-density lipoprotein (VLDL)-TAG was decreased in the obese subjects after an overnight fast. After the meal, chylomicron-TAG extraction increased in the control group whereas VLDL-TAG clearance decreased; these changes were not seen in the obese group, in whom the VLDL particles appeared to be better able to compete with the chylomicrons for clearance by lipoprotein lipase. In the control subjects, removal of TAG from the TRL in the postprandial period was accompanied by a shift toward addition of cholesterol to the high-density lipoprotein (HDL) fraction; this was not observed in the obese subjects. We conclude that disturbed TRL-TAG clearance in adipose tissue is related both to the elevated plasma TAG concentrations and the depressed HDL-cholesterol concentrations typical of obesity.

Catecholamine interference with enzymatic determination of nonesterified fatty acids in two commercially available test kits

We present evidence that catecholamines, which are commonly used to stimulate lipolysis in adipose tissue in vitro, interfere with the enzymatic determination of non-esterified fatty acid (NEFA) in two commercially available kits. Measurement of a 100 mumol/L standard with the Wako "NEFA C" test kit was 60% inhibited by 100 mumol/L norepinephrine and was completely inhibited by 100 mumol/L isoproterenol or by 1 mmol/L norepinephrine or epinephrine. Measurement with the Boehringer Mannheim "Free Fatty acids, Half-micro test" was completely inhibited by 100 mumol/L norepinephrine and was also affected by concentrations as low as 0.1 mumol/L. We propose that this effect is due to the catecholamines interfering with a step common to the two kits, the generation of hydrogen peroxide and oxidation of a chromagen; furthermore, this interference appears to be stoichiometric. We also give details of an alternative in-house method, which does not depend on the generation of hydrogen peroxide and is not affected by catecholamines.

Catecholamine interference with enzymatic determination of nonesterified fatty acids in two commercially available test kits

We present evidence that catecholamines, which are commonly used to stimulate lipolysis in adipose tissue in vitro, interfere with the enzymatic determination of non-esterified fatty acid (NEFA) in two commercially available kits. Measurement of a 100 mumol/L standard with the Wako "NEFA C" test kit was 60% inhibited by 100 mumol/L norepinephrine and was completely inhibited by 100 mumol/L isoproterenol or by 1 mmol/L norepinephrine or epinephrine. Measurement with the Boehringer Mannheim "Free Fatty acids, Half-micro test" was completely inhibited by 100 mumol/L norepinephrine and was also affected by concentrations as low as 0.1 mumol/L. We propose that this effect is due to the catecholamines interfering with a step common to the two kits, the generation of hydrogen peroxide and oxidation of a chromagen; furthermore, this interference appears to be stoichiometric. We also give details of an alternative in-house method, which does not depend on the generation of hydrogen peroxide and is not affected by catecholamines.

Coordinated regulation of hormone-sensitive lipase and lipoprotein lipase in human adipose tissue in vivo: implications for the control of fat storage and fat mobilization

The enzymes lipoprotein lipase (LPL, EC 3.1.1.34) and hormone-sensitive lipase (HSL, EC 3.1.1.3) apparently catalyze opposing functions in white adipose tissue: the former is concerned with fat storage, the latter with fat mobilization. We have studied their regulation in vivo in normal subjects in the postabsorptive state and after eating meals of different compositions, by measurement of arteriovenous concentration differences for triacylglycerol, non-esterified fatty acids and glycerol across a subcutaneous adipose depot. The two enzymes are regulated in a broadly reciprocal manner: in the overnight-fasted state, HSL is more active, but after a meal HSL is suppressed whilst LPL is activated. The movement of fatty acids in and out of adipose tissue appears to be driven by concentration gradients generated by regulation of these two enzymes, and also by activation, in the postprandial period, of the process of fatty acid esterification. The results show some interesting and perhaps unexpected features of metabolic regulation. Of the fatty acids generated by the action of LPL on circulating TAG, a large proportion is released directly into the venous plasma: close to 100% in the overnight-fasted state, and 50% or more at the peak of LPL action after a meal, making what appear reasonable assumptions. We suggest that this apparent 'inefficiency' of fat storage reflects the energetic cost of maintaining precise control over such a fundamental process. Although LPL is usually thought of as the enzyme regulating fat deposition, in fact the fatty acids and glycerol it releases from circulating TAG represent a substantial proportion of those released from adipose tissue, especially in the postprandial state. In addition, although HSL is considered the enzyme responsible for fat mobilization, suppression of its activity is essential to normal regulation of fat deposition. Thus, fat storage and fat mobilization during normal daily life are controlled by coordinated regulation of a number of enzymatic processes in white adipose tissue.