Noradrenaline-stimulated Lipolysis Research Articles

Effects of insulin on lipolysis and lipogenesis in adipocytes from genetically obese (ob/ob) mice.

A method for the preparation of isolated adipocytes from obese mice is described. Similar yields of adipocytes (50--60%), as judged by several criteria, are obtained from obese mice and lean controls. Few fat-globules and no free nuclei were observed in cell preparations, which are metabolically active, respond to hormonal control and appear to be representative of intact adipose tissue. Noradrenaline-stimulated lipolysis was inhibited by insulin, equally in adipocytes from lean and obese mice. Inhibition in obese cells required exogenous glucose, and the insulin dose--response curve was shifted to the right. Basal lipogenesis from glucose was higher in adipocytes from obese mice, and the stimulatory effect of insulin was greater in cells from obese mice compared with lean controls. A rightward shift in the insulin dose--response curve was again observed with cells from obese animals. This suggests that adipose tissue from obese mice is insulin-sensitive at the high blood insulin concentrations found in vivo. The resistance of obese mice to the hypoglycaemic effect of exogenous insulin and their impaired tolerance to glucose loading appear to be associated with an impaired insulin response by muscle rather than by adipose tissue.

Alterations in response of rat white adipocytes to insulin, noradrenaline, corticotropin and glucagon after adrenalectomy. Correction of these changes by adenosine deaminase

1. Adipocytes isolated from rats 6--9 days after adrenalectomy had significantly increased sensitivity to insulin action against noradrenaline-stimulated lipolysis. In the presence of adenosine deaminase there was no significant difference in insulin sensitivity between cells from adrenalectomized and sham-operated rats. 2. Adipocytes from adrenalectomized rats had decreased lipolytic responses to all concentrations of noradrenaline and glucagon tested and a decreased lipolytic response to low but not high concentrations of corticotropin. There was no difference in lipolytic response to theophylline after adrenalectomy. Adenosine deaminase corrected the differences in response to noradrenaline and glucagon resulting from adrenalectomy. 3. In the presence of adenosine deaminase rates of lipolysis, after stimulation by high concentrations of noradrenaline, glucagon, corticotropin or theophylline, were the same in cells from adrenalectomized or sham-operated rats. 4. These findings and previously reported effects of adenosine and adrenalectomy on adipocyte function are discussed. It is proposed that changes in adipocyte hormone responsiveness after adrenalectomy may result from changes in adenosine metabolism or release.

Inhibition of Noradrenaline‐Stimulated Lipolysis and Cyclic AMP Accumulation in Isolated Rat Adipocytes by Purified Phospholipase C and Theta‐Toxin from Clostridium Perfringens

Purified phospholipae C (phosphatidylcholine cholinephosphohydrolase, EC 3.1.4.3) and theta-toxin from Clostridium perfringens both inhibited noradrenaline-stimulated lipolysis and cyclic AMP accumulation in isolated rat adipocytes in a dose-dependent manner. The action of phospholipase C was gradual in onset, while the effect of theta-toxin was almost immediate. Phospholipase C, but not theta-toxin, hydrolyzed membrane phospholipids and inhibited adenylate cyclase (EC 4.6.1.1) in a crude membrane fraction from fat cells. The inhibitory effects of phospholipase C were associated with morphological alterations detectable by electron microscopy, whereas effects of theta-toxin were observed at a time when no clearcut morphological alterations could be observed. It is concluded that the two purified principles from C. perfringens, which are both present in commercial preparations of phospholipase C, antagonize noradrenaline-stimulated cyclic AMP accumulation and lipolysis. Although their exact mechanisms of action have not been elucidated, phospholipase C and theta-toxin have different modes of attack.

Ffects of quinterenol (1-(5-(8-hydroxyquinolyl))-2-isopropylaminoethanol) on the metabolism of adipose tissue

1. The partial β-adrenergic agonist and catecholamine analogue, quinterenol (1-(5-(8-hydroxyquinolyl)-2-isopropylaminoethanol) was examined for its effects on adipose tissue metabolism in vitro. The drug promoted glycerol release from isolated fat cells in a dose-dependent hyperbolic manner with a maximum effect at 10 −3 M. 2. Quinterenol promoted an enhancement of lipolysis stimulated by a serum fat-mobilizing factor (lipolysin) over a narrow dose range (10 −5 M to 10 −4 M) and produced a similar narrow-band enhancement (at the single dose of 10 −5 M) of noradrenaline-stimulated lipolysis. This synergism has been interpreted as phosphodiestcrase inhibitory activity. 3. Propranolol at 10 −3 M totally inhibited both noradrenaline (10 −4 M) and quinterenol (10 −3 M)-induced lipolysis; and had reduced, though still significant effects, when at 10 −6 M. It is suggested that quinlerenol promotes lipolysis via the cyclic AMP system in similar fashion to noradrenaline. 4. Quinterenol alone was not able to enhance the incorporation of [U- 14C]glucose into total fat cell lipid. However, quinterenol did further stimulate insulin-induced lipid synthesis in fat cells obtained from both fasted and led rats.

Adipocyte metabolism in endogenous hypertriglyceridemia

In order to define specific metabolic abnormalities of adipose tissue metabolism in endogenous hypertriglyceridemia (EH) patients with this condition were compared with normolipidemic controls matched for body fat and fat cell size. In vitro the enlarged fat cells of EH were found to have an increased basal and noradrenaline-stimulated lipolysis in comparison with cells of the same size from normolipidemic controls. The insulin inhibition of noradrenaline-stimulated lipolysis was blunted. Lipoprotein lipase activity in these cells was clearly depressed. Basal triglyceride synthesis from labeled glucose was low in relation to plasma insulin. The reduction of insulin tolerance in vivo suggested that the depression of plasma glycerol and free fatty acid concentration was small in EH, suggesting that the more detailed findings in vitro were of relevance for in vivo conditions. It was suggested that the hyperinsulinemia and decreased glucose tolerance of EH may well be responsible for some of the aberrations of adipocyte metabolism in EH. The decreased responsiveness of lipolysis to insulin and the low lipoprotein lipase activity are, however, findings not typical for enlarged fat cells exposed chronically to insulin and might be characteristic for the fat cells of EH. It seems of importance to further define the factor(s) responsible for these metabolic aberrations, because the abnormalities of the acipocyte metabolism in EH may well offer a possible explanation to the pathogenesis of that condition.

Effect of antimycin a on hormone-stimulated lipolysis in vitro

The effect of antimycin A on hormone-stimulated lipolysis in vitro was investigated using noradrenaline, cyclic 3',5'-AMP and theophylline. The titration curve of antimycin against noradrenaline-stimulated lipolysis was sigmoidal, like the behaviour of antimycin on the respiratory chain of mitochondria. Antimycin A was found to antagonize the lipolytic effect of both cyclic 3',5'-AMP and theophylline. The range of active concentrations against the different drugs, was very similar. These data confirm that hormone-induced lipolysis needs ATP, generated by oxidative phosphorylation, both before and after cyclic 3',5'-AMP synthesis.

Growth and lipolysis of rat adipose tissue: effect of age, body weight, and food intake

The purpose of the present work was to study age- and weight-controlled rats to determine which is the primary factor in reducing the lipolytic response of free fat cells and which has the greater effect on the ratio of fat cells to nonfat cells in adipose tissue. The method for estimating fat cell and nonfat cell numbers is based on the analysis of adipose tissue and fat cell DNA and lipid. In adequately fed rats, epididymal adipocyte hyperplasia is complete between 9 and 14 wk of age. Chronic underfeeding delays, but does not eliminate, normal fat cell hyperplasia and is accompanied by a net loss in the nonfat cell population. During 9-14 wk of age, rat epididymal adipose tissue enlarges mainly through adipocyte hypertrophy. Total fat cells from the epididymal adipose tissue of control rats represent only 20-23% of the total cell population. Chronic underfeeding increases the percentage of fat cells in the fat pad from 23 to 28%. Noradrenaline-stimulated lipolysis is proportional to fat cell numbers but is inhibited when fat cell lipid increases to over 80% of fat pad wet weight. Rat age is apparently not primarily responsible for the decreased noradrenaline-stimulated lipolysis in fat cells of 350-g rats in vitro.

The mobilization of free fatty acids in relation to adipose tissue triglyceride fatty acids in the rat

Three diets, consisting respectively of formulations high in oleic and stearic acid, linolenic acid, and lauric acid, were fed to rats until the adipose tissue TGFA largely reflected the dietary pattern of fatty acids. The composition of the serum FFA under basal conditions and following noradrenaline-stimulated lipolysis, were examined in relation to the respective adipose tissue TGFA. It was found in both in vivo and in vitro studies that lauric acid appeared to be less easily mobilized than longer chain acids. The in vitro studies indicated that this could not be explained either by positional preference of the shorter chain acids for the alpha-position of esterification or by increased reesterification of the shorter chain acids. The possibility remains that the difference is due to some specificity of tissue lipases for certain ester linkages.

Effect of vitamin D 2 on hormone-stimulated lipolysis in vitro

The parameters of the antagonism of vitamin D 2 to noradrenaline-stimulated lipolysis in vitro were studied by determining the time-effect and the concentration-effect curves, and the influence on these of the ion's composition of the medium. The antagonism is of a non-competitive type (p D 2′ = 3.99 ± 0.05) and it was potentiated by a reduced Ca ++ concentration in the incubation medium of adipose tissue. The possible site of action of vitamin D 2 on the process studied was investigated by using caffeine and dibutyryl cyclic 3′5′-AMP. Vitamin D 2 also antagonized the lipolysis stimulating effect of these drugs, suggesting that the site of action of the vitamin is located at the level of some biochemical step following the cyclic 3′5′-AMP synthesis. The mechanism of action of vitamin D 2 on lipolysis is discussed.