Effects of glucose and insulin on lipolysis rates in human fat cells of different sizes

Response to intra- and extracellular lipolytic agents and hormone-sensitive lipase translocation are impaired in adipocytes from rats adapted to a high-protein, carbohydrate-free diet

We showed previously that rats adapted to a high-protein (70%), carbohydrate-free (HP) diet have reduced lipolytic activity. To clarify the underlying biochemical mechanisms, several metabolic processes involved in adipose tissue lipolysis were investigated. The experiments were performed in rats adapted for 15 d to an HP or a balanced diet. In agreement with previous results, microdialysis experiments showed that the concentrations of adipose tissue interstitial and arterial plasma glycerol were lower in rats adapted to the HP diet. Under nonstimulated conditions, rates of lipolysis, estimated by glycerol release to the incubation medium, were reduced in adipocytes from HP rats. Under the same conditions, there was a small, but significant (17%) reduction in the activity of hormone sensitive lipase (HSL), with no change in the content of the enzyme. Upon stimulation with isoproterenol, the percentage of the enzyme in the adipocyte cytosol translocated to the fat droplet was 20-25%in HP rats and 40-50% in rats fed the balanced diet. Adipocytes from HP diet-adapted rats had a significantly reduced response (approximately 40%) to the lipolytic action of nonspecific (norepinephrine, epinephrine, isoproterenol) and specific (CL316,243, BRL37,344, dobutamine, clenbuterol) beta-adrenergic agonists. Adipocytes from HP rats also had a reduced lipolytic response to the intracellular agents, dibutyryl cAMP (44%), forskolin (46%), and isobutyl-methylxanthine (29%). The data suggest that the main mechanism responsible for the reduced basal and stimulated lipolysis in HP diet-adapted rats is an impairment in the intracellular process of lipolysis activation, with a deficient translocation of HSL to the fat droplet.

Body fat distribution and hyperinsulinemia as risk factors for diabetes and cardiovascular disease

Differences in body fat distribution between diabetics and nondiabetics have been recognized for several decades; diabetics have a more centralized or upper body fat pattern than nondiabetics. Recently, attention has focused on fat patterning and also on hyperinsulinemia as possible risk factors for cardiovascular disease, as well. The case for insulin as a cardiovascular risk factor is bolstered by theoretical considerations related to its possibly atherogenic effects on serum and arterial wall lipids. Empirical evidence for fat patterning and hyperinsulinemia as cardiovascular risk factors rests on six prospective epidemiologic studies, three on fat patterning and three on insulin. Although provocative, none of these studies can be regarded as definitive. In none was a dose-response effect demonstrated, and there are various inconsistencies within and across the studies. Moreover, in none of the studies were hyperinsulinemia and fat patterning evaluated simultaneously. This is of particular importance in view of the well-documented interrelationships between these two variables. For example, insulin resistance and hyperinsulinemia have been found to be greater in women with upper body obesity compared to women with lower body obesity of equivalent degree. Considerable progress has been made recently in understanding the mechanisms of the differential metabolic effects of these two types of obesity. The extent to which fat patterning and hyperinsulinemia are genetic or acquired has received relatively little attention. Further research on this question is warranted since elucidation of any environmental influences on these variables might suggest new clinical and public health control measures.

Lipolysis and the antilipolytic effect of insulin in adipocytes from rats adapted to a high-protein diet

Free fatty acid (FFA) mobilization during fasting was investigated in rats fed a high-protein, carbohydrate-free (HP) diet (70% casein, 8% fat, wt/wt) or a balanced diet (66% carbohydrate, 17% casein, 8% fat) for 30 to 40 days. In vivo, rats on the HP diet showed reduced rates of plasma FFA increase during fasting. Their blood sugar remained unchanged and was higher than that of control rats 24 hours after removal of food. In the fed state, serum insulin levels were smaller in HP-fed rats but did not differ significantly in the two experimental groups during fasting. In vitro, the rates of glycerol and FFA release by epididymal fat pads obtained from fasted rats were similar in rats consuming the HP diet. Fat cells isolated from rats on the HP diet also had reduced rates of basal lipolysis. Furthermore, they showed a significant increase in responsiveness to the lipolytic action of noradrenaline and an increase in both sensitivity and responsiveness to the inhibitory effect of insulin on noradrenaline-stimulated lipolysis. Adipocytes from HP-fed and control rats had mean diameters of 51 and 60 mu, respectively, and estimated average volumes of 90 and 142 pL. On the basis of existing data on the correlation between size and lipolytic activity of fat cells, the smaller size of the adipocytes from HP-fed rats might account for the lower rate of basal lipolysis but not for the increased response to the hormones. The increased sensitivity of fat cells to the antilipolytic action of insulin may have been an important factor in the reduced lipomobilization during fasting in rats under the high-protein regimen.

The effect of body fatness on energetic efficiency and fasting heat production in adult sheep

The effect of body fatness on the fasting heat production and energetic efficiency of adult sheep was studied. Energy balance and heat production were determined in adult wether sheep at three fatness levels given a diet of grass hay and maize at four feeding levels, including fasting, in open-circuit respiration chambers. The sheep weighted approximately 60 kg at the start and were fed over a period of 4-7 months to obtain one of three levels of body fatness: fat 90 kg, medium 70 kg and thin 55 kg. Neither the digestibility nor the metabolizability of the diet was affected by body fatness or feeding level. Average fasting heat production was 6.47, 7.00 and 8.20 MJ/d for the thin, medium and fat sheep respectively, but when expressed as a multiple of metabolic body size (kg W0.75) remained constant at 0.31 MJ/kg W0.75. At each level of feeding the fat sheep produced most heat, but when heat production was expressed as a multiple of fasting heat production or metabolic body size, the differences due to level of fatness disappeared. Heat increment was not affected by fatness and there was no suggestion that energy utilization was improved by loss of condition. It is concluded that white adipose tissue is highly active and contributes significantly to the maintenance energy requirements of adult sheep. Metabolic body size is a good unit of reference for comparison of energy metabolism in adult animals of the same species but varying in body size.

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.

Body weight: reduction by long-term glycerol treatment

Elevation of body glycerol concentration by multiple daily injections of glycerol was shown to lead to hypophagia and body weight loss followed by normal food intake and normal rate of body weight increase in rats. Termination of injections was followed by hyperphagia and an accelerated rate of growth. These findings suggest that the blood glycerol concentration plays an important role in the control of body weight and may be one signal by which the central nervous system monitors body lipid content.

Body composition and adipose tissue cellularity before and after jejuno-ileostomy in severely obese subjects

In severe obesity with adipose tissue hyperplasia it is difficult to achieve and sustain weight decrease by conventional therapy. Fifteen severely obese patients (mean body weight 135 kg) with hyperplastic obesity had a jejuno-ileostomy according to Payne to induce weight reduction by controlled malabsorption. Body composition, adipose tissue cellularity, and glucose tolerance were studied before and after the operation when the patients had reached a new stable weight (WS) a mean 21 months after surgery. Preoperative fat cell number was unchanged at follow-up in contrast to all other variables of body composition. Reductions in fat cell size were evenly distributed in different major subcutaneous regions. Body potassium was determined immediately after operation and at 6 months intervals until WS. The postoperative decrease stopped at 6 months, thus differing from the decrease in body weight. Significant positive correlations between the postoperative decrease in body weight or body fat, and preoperative body weight, body fat and body cell mass were analysed by multiple stepwise regression. Preoperative body weight and body fat were shown to predict postoperative weight loss at a leve of P less than 0.005. A positive correlation between body fat decrease and fat cell number could be explained hypothetically by an increased caloric demand in hyperplastic obesity.

Human adipose tissue in culture V. Studies on the metabolic effects of insulin

Specimens of human adipose tissue were cultured for one week with or without the addition of insulin. The basal as well as the noradenaline-stimulated lipolysis were enhanced in the explants cultured with insulin, showing that the long-term effect of the hormone is lipolytic. However, an acute antilipolytic effect of insulin could be demonstrated in these explants in the subsequent short-term incubations. The basal rate of glucose incorporation into the lipids was enhanced in the explants cultured with insulin. When insulin was added in the short-term incubations these explants did not further respond to the hormone while this was the case with the explants cultured without insulin. Thus, it seems that prolonged exposure to insulin leads to a diminished acute effect of the hormone on glucose metabolism. However, the same explants responded to the antilipolytic effect showing that insulin was able to bind itself to the membrane. The activities of hexokinase (HK), glucose-6-phosphage dehydrogenase (G6PDH), pyruvate kinase (PK) and lactate dehydrogenase (LDH) were increased in large fat cells both in freshly excised tissue and in cultured explants. However, the activity of phosphofructokinase (PFK) did not correlate with the cell size. The presence of insulin during the culture period enhanced the activities of G7PDH, PK, and LDH, while this was not found for HK or PFK. The data thus suggest that the metabolic capacity of human fat cells is enhanced by long-term exposure to insulin. Although enzyme induction could be shown for G6PDH, PK and LDH it seems unlikely that this is of importance for the increased rates of glucose metabolism in these explants since the rate-limiting enzymes, HK and PGK, were not increased. Most probably, then, this stimulating effect of insulin is exerted on the membrane and the rate of glucose transport.

Abnormal post-heparin lipolytic activity in obesity. A preliminary note

Published data have suggested that hypertriglyceridemia in obesity may result from the combination of hepatic overproduction and diminished removal of triglyceride-rich lipoproteins. Diminished catabolism might be expected if tissue lipoprotein lipase activity were decreased, a finding which has been reported in biopsies of adipose tissue from obese subjects. Abnormalities in heparin-released triglyceride lipase activity (PHLA) in obesity have not been reported, however. We have examined the possibility that methods for the measurement of PHLA might have failed to reveal such a defect because of the disproportionality between plasma volume and increasing body mass in obesity. Since it is usual to administer heparin on the basis of body weight, higher plasma heparin levels would be achieved in obese individuals. We performed standard PHLA assays in lean and obese volunteers. In the obese, heparin levels were consistently higher than in lean individuals although PHLA values were similar in both. Thus, PHLA in obesity appeared to be inappropriate for the heparin levels attained in plasma. Pharmacokinetic studies suggest that a decrease in PHLA available for release by heparin rather than heparin insensitivity underlies this phenomenon.

Metabolic factors in the control of energy stores

We have examined some of the factors involved in the control of food intake and integrated the data using methods of systems analysis from biomedical engineering. The central hypothesis is that energy stored in the body is a regulated variable. Alterations in the quantity of stored calories initiates changes designed to restore the store of calories to its original level. These responses are both short-term and long-term in nature. They involve integrating data on the quantities of protein, carbohydrate, and lipid stores in the body, probably through such feedback signals as amino acids, glucose, glycerol, and free fatty acids.