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Biomedical subjects

S M Humphreys

Publications and source records attributed to S M Humphreys.

At least 37 records · Page 2Linked to original sources

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.

3-Hydroxybutyric Acid↗

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.

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.

Adult↗

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.

3-Hydroxybutyric Acid↗

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.

Adult↗

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.

Adipose Tissue↗

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.

Adult↗

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.

Adipose Tissue↗

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.

Adipose Tissue↗

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.

Adipose Tissue↗

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.

Abdomen↗

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.

Adult↗

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.

Adipose Tissue↗

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.

Adipose Tissue↗

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.

Adipose Tissue↗

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.

Adipose Tissue↗

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.

Adipose Tissue↗