Effects of norepinephrine, theophylline and dibutyryl cyclic AMP on in vitro lipolysis of human adipose tissue in obesity

Metabolism of mono- and diacylglycerols in subcutaneous adipose tissue of obese and normal-weight subjects

Tissue monoacylglycerols (MG), diacylglycerols (DG), free fatty acids (FFA), and cyclic AMP (cAMP) and release of FFA and glycerol have been studied in vitro in subcutaneous adipose tissue of 6 obese and 7 normal-weight subjects. The tissue was incubated without or with 6 X 10(-5) mol/l of isoprenaline (ISNA). The DG level and the fat cell volume were strongly interrelated (r=+0.95, p less than 0.001). The concentration of DG was increased (p less than 0.05) in obesity. The changes in DG and MG were significantly interrelated (r=+0.65, p less than 0.05) during basal incubation. ISNA increased the DG concentration in a way that was correlated (r=+0.81, p less than 0.001) with the ISNA-induced glycerol release. This indicates that 1) the basal metabolic activities of MG and DG lipase are similar and 2) DG lipase is an important rate limiting factor in lipolysis. Without ISNA, tissue FFA and the release of FFA and glycerol were significantly increased in the obese patients. As a mean, MG and DG did not accumulate in the basal state in the two patient groups. The findings indicate that basal lipolysis was increased in obesity. This was probably due to increased basal metabolic activity of triacylglycerol lipase, since the basal cAMP levels were similar in the two patient groups. In the presence of ISNA, the production of FFA and the glycerol release were similar in both patient groups, as was the increase in tissue DG. Also the ISNA-induced maximal level of cAMP was similar in the two groups. With ISNA, a small increment of MG was observed in adipose tissue of the normal-weight subjects. Taking all metabolites into account, the rate of lipolysis as well as the activation of triacylglycerol lipase via cAMP in the presence of ISNA appeared to be unaltered in obesity. Separate experiments with 1-14C-glycerol provided further evidence for the existence of a MG pathway for the esterification of FFA.

Glycerol production in subcutaneous adipose tissue in lean and obese humans

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

Prostaglandin E2 action and binding in human adipocytes: effects of sex, age, and obesity

The antilipolytic effect of prostaglandin E2 (PGE2) was studied in subcutaneous human adipocytes. The influence of sex, age, and obesity on the PGE2 effect was investigated. The antilipolytic effects of PGE2 were related to the PGE2 binding data obtained in the same adipocytes. The maximal antilipolytic action of PGE2 was slightly reduced in adipocytes in males compared with females (maximal inhibition 86% v 97%, P greater than .05). The PGE2 binding was similar in adipocytes in females and males. The antilipolytic effect of PGE2 and the PGE2 binding was similar when young females were compared with older females. However, the antilipolytic effect of PGE2 was significantly reduced in obese compared with nonobese subjects. If lipolysis was only stimulated by adenosine deaminase, the sensitivity of PGE2 was reduced in obesity (IC50, 1.45 nmol/L v 0.47 nmol/L, P less than .01), but the maximal antilipolytic effect of PGE2 in the two groups was similar, with an inhibitory effect of 95% to 98%. If lipolysis was stimulated by both adenosine deaminase and theophylline (2 mmol/L), it was especially the maximal antilipolytic effect of PGE2 that was impaired in adipocytes from obese subjects (lipolysis was maximally inhibited by 61% v 92%, P less than .01). When the PGE2 binding was expressed in relation to adipocyte surface area, the total binding capacity (Bmax) was reduced in adipocytes in obese subjects from 26.5 to 17.9 fmol/100 cm2 (P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of exercise-training and detraining on fat cell lipolysis in men and women

The effects of training and detraining on adipose tissue lipolysis were studied in 19 healthy subjects (7 women and 12 men) who were submitted to a 20-week aerobic training program. Thereafter, subjects refrained from exercise for a period of 50 days. Suprailiac fat biopsies were performed before training, after training, and at the end of the detraining period. Mean fat cell diameter and epinephrine stimulated lipolysis (ESL) were assessed on collagenase isolated fat cells. Body density through underwater weighing and skinfolds at seven different sites were also obtained. Training significantly increased ESL (P less than 0.05) in men but not in women. However, ESL values in men returned to pretraining values after the exercise restriction period. No significant changes in women lipolysis were observed under any conditions. Changes in lipolysis were not correlated with changes in body fatness. However, a significant correlation was observed between the increase in ESL produced by training and the subsequent decrease caused by detraining (r = -0.53; P less than 0.05). The present results suggest that lipolysis in fat cells from the female subjects seems to be insensitive to changes in energy expenditure. Moreover, the present study demonstrates that there are high and low responders in adipocytes ESL to variations in habitual energy expenditure.

Regional differences in the control of lipolysis in human adipose tissue

Omental fat cells were 30% smaller than those in subcutaneous regions. In omental fat cells with a mean diameter of 95 mu, the basal cAMP concentration was 50% lower, but the basal rate of glycerol release was three times as rapid as in subcutaneous (epigastric) fat cells of identical size. Added at maximal effective concentration, noradrenaline increased the level of cAMP and the rate of glycerol release more markedly in the omental than in the subcutaneous adipocytes, whereas the response to isopropyl noradrenaline was similar. Before starvation the lipolytic effects of noradrenaline and isopropyl noradrenaline, respectively, were identical in the two regions of subcutaneous adipose tissue investigated (femoral and hypogastric). The findings were well related to the tissue levels of cAMP induced by the two agents. During starvation noradrenaline and isopropyl noradrenaline increased the cAMP level and the rate of lipolysis in fat cells obtained from the hypogastric region, whereas noradrenaline decreased these parameters in femoral adipocytes. Starvation was associated with a more prominent inhibitory effect of phenylephrine on basal and isopropyl-noradrenaline-induced lipolysis in femoral than in hypogastric adipose tissue. In conclusion, differences exist between different regions of adipose tissue in their lipolytic responsiveness to noradrenaline, which seems related to the balance between alpha- and beta-adrenergic receptor response.

Relationship between lipolysis, cyclic AMP, and fat-cell size in human adipose tissue during fasting and in diabetes mellitus

The in vitro relationship between fat-cell size, glycerol release, and peak concentration of cyclic AMP was investigated in human adipose tissue obtained from 25 obese nondiabetic patients before and after a 7-day fast and from 23 patients with untreated diabetes mellitus. In the obese nondiabetic patients there was a linear correlation between fat-cell size and cyclic AMP concentration, and fat-cell size and the rate of lipolysis. This was found both in nonfasting and fasting nondiabetic patients. However, in diabetes mellitus, there was only a relationship between cell size and cyclic AMP concentration. The alpha-adrenergic and beta-adrenergic activity in human adipose tissue was assessed by comparing the effect of isoprenaline and noradrenaline on the cyclic AMP concentration. The activity of both receptors was found to be increased in fasting obese patients and in diabetics. In both conditions the alpha-adrenergic response to catecholamines predominated in small fat cells, whereas in large ones the beta response predominated. The results suggest that during fasting and in diabetes mellitus there is a correlation between fat-cell size and the responsiveness of the adrenergic receptors. Thus, catecholamines may be involved in regulating the fat-cell volume. The view is expressed that the abnormal catecholamine-induced lipolysis is solely due to changes at the level of the adrenergic receptors during fasting, whereas in diabetes mellitus the sequentional activation of lipolysis is disturbed at deeper sites as well.

Relationship between body fat mass, carbohydrate tolerance and IRI response during glucose infusion in subjects with early diabetes

We have studied the interrelationship of total body fat mass, carbohydrate tolerance and IRI response in 17 non-obese and obese subjects, who were suspected of having early diabetes. We carried out an i.v. glucose infusion test consisting of a priming injection of 0.33 g/kg followed by constant glucose infusion of 12 mg/kg/min in all persons. Total body fat mass was estimated by the tritium dilution method. There was a positive correlation of body fat mass, fasting glucose concentration and blood glucose concentration at 150 min as well as a strong correlation between body fat mass and BG area 60--120 min as parameters of carbohydrate tolerance in all subjects, i.e. the degree of carbohyrate intolerance was directly related to the quantity of total body fat mass. A similar correlation was found when the non-obese and obese groups were analyzed separately. In neither group did total body fat mass correlate with parameters of IRI response. In obese subjects with pathological carbohydrate tolerance, however, a positive correlation of basal IRI concentration and total body fat mass was found. Furthermore, a close relation between basal IRI level and parameters of carbohydrate tolerance could be demonstrated in obese subjects. The present study failed to demonstrate any correlation of parameters of carbohydrate tolerance and glucose-induced IRI response in either group. Thus, the significant relationship between body fat mass and degree of carbohydrate intolerance indicates that body fat mass plays an important role in the disturbance of blood glucose homeostasis in early diabetes with and without obesity.

Primary insulin antagonism of glucose transport in muscle from the older-obese rat

Effects of insulin (1 mU/ml) on diaphragms removed from older-obese (70--110 days, 350--520 g) male Sprague-Dawley rats were compared to responses on muscle removed from younger-lean (27--36 days, 80--150 g) animals. Insulin antagonism on glucose transport (2DG uptake), glucose uptake, glycogen synthesis, glycolysis (lactate production), and glucose oxidation was demonstrated in tissue from the older-obese rats. Extracellular water spaces (measured with inulin-H3) were significantly decreased in these tissue. To determine if insulin antagonism of glucose transport could be secondary to inhibition of a rate-limiting reaction in the Embden-Meyerhof pathway with a subsequent negative feedback on transport, both tissue levels of glycolytic intermediates and oxidation of intracellular lipids were measured. No free intracellular glucose was found in diaphragms from either group of rats. Levels of G-6-P, F-6-P, F-1, 6-diP, PEP, and pyruvate were all lower in muscle from the older-obese animals. Incorporation of C14-FFA into tissue TG was slightly, but significantly, lower in this same tissue. Oxidation of intracellular TG and PL was similar in the two groups. In conclusion, diaphragms from older-obese rats manifest insulin antagonism of glucose transport that is probably responsible for the diminished hormonal effect on glucose uptake and the intracellular pathways of glycogen synthesis, glycolysis, and glucose oxidation. This inhibition of insulin action cannot be accounted for by changes in glycolytic intermediates causing a negative feedback on transport or enhanced lipid oxidation and therefore should be considered primary. The relative effects of age and obesity will need to be evaluated in future studies.

Influence of cell size on the effects of insulin and noradrenaline on human adipose tissue

In the present study dose-response relationships of the effects of noradrenaline and insulin on fat cells of different sizes were performed. Adipose cells larger than 100 mum were more responsive (expressed as absolute effects) to the lipolytic action of noradrenaline as well as to the antilipolytic effect of insulin. This suggests that in the larger cells the capacity, i.e. the sum of factors contributing to the ability to stimulate or inhibit the metabolic rates, was greater than in the smaller ones. In contrast the sensitivity to these agents, i.e. the readiness to respond, was not different between small and large cells. It is shown that the concentrations of insulin needed to obtain an antilipolytic effect is far below that needed to stimulate glucose incorporation. This discrepancy in insulin concentrations required may be due to binding of insulin to receptors with different affinity.

Metabolic studies in familial partial lipodystrophy of the lower trunk and extremities

A familial syndrome of partial lipodystrophy inherited as a dominant trait is reported. Subcutaneous fat loss was confined to the extremities and trunk. Diabetes mellitus, hyperlipidemia, hepatomegaly and renal disease were very prevalent in this family. Metabolic studies were performed on 3 members. In vivo tests suggested that the remaining fat tissue responded normally to stimulators and inhibitors of lipolysis. In vitro incubation of the dystrophic fat tissue of one patient suggested that the intracellular pathways of lipid and glucose metabolism were normal. The pattern of subcutaneous loss of adipose tissue observed in this family may be due to sympathetic nervous system overactivity of certain non-contiguous dermatomes.

Cell size and the antilipolytic effect of insulin in human subcutaneous adipose tissue

Lipolysis was studied in subcutaneous adipose tissue removed under local or general anaesthesia from subjects with marked difference in body weight and from obese subjects before and after intestinal shunt operations. The release of glycerol was measured in sections of adipose tissue incubated for two hours in bicarbonate buffer containing 4 per cent bovine albumin with no glucose added. The larger fat cells were more insulin sensitive than the smaller. A positive relationship was observed between the fat cell size and the antilipolytic effect of insulin (100 muU/ml). Insulin had no significant effect on the lipolytic effect of the catecholamines, when this is calculated as the increment in the glycerol release above the basal.