AMP In Adipose Tissue Research Articles

Beta-adrenergic agents increase the phosphorylation of phosphofructokinase in isolated rat epididymal white adipose tissue.

Pieces of rat epididymal adipose tissue were incubated in medium containing [32P]phosphate for 2 h to achieve steady-state labelling of intracellular phosphoproteins and then with or without hormones for a further 15 min. Phosphofructokinase was rapidly isolated from the tissue by use of either Blue Dextran-Sepharose chromatography or immunoprecipitation with antisera raised against phosphofructokinase purified from rat interscapular brown adipose tissue. Similar extents of incorporation of 32P into phosphofructokinase were measured by both techniques. Exposure of the tissue to adrenaline or the beta-agonist isoprenaline increased phosphorylation by about 5-fold (to about 1.4 mol of phosphate/mol of enzyme tetramer). No change in phosphorylation was detected with the alpha-agonist phenylephrine, but exposure to insulin resulted in an approx. 2-fold increase. The increased phosphorylation observed with isoprenaline was found to be associated with a decrease in the apparent Ka for fructose 2,6-bisphosphate similar to that observed on phosphorylation of phosphofructokinase purified from rat epididymal white adipose tissue with the catalytic subunit of cyclic AMP-dependent protein kinase. These results support the view [Sale & Denton (1985) Biochem. J. 232, 897-904] that an increase in cyclic AMP in adipose tissue may result in an increase in glycolysis through the phosphorylation of phosphofructokinase by cyclic AMP-dependent protein kinase.

Peripheral resistance to the cellular action of insulin in obese diabetic subjects in vivo.

1. The concentrations of adenosine 3': 5'-cyclic monophosphate (cyclic AMP) in human adipose tissue were studied after intravenous insulin administration (0.3 unit/kg body weight) in obese diabetic and obese non-diabetic subjects. 2. The declines in plasma glucose, glycerol and free fatty acids in the two groups after insulin were similar if allowances were made for the fasting hyperglycaemia in the diabetic subjects. 3. Insulin in vivo lowered the concentrations of cyclic AMP in adipose tissue from non-diabetic subjects. 4. Insulin administration in vivo did not alter the tissue concentrations of cyclic AMP in adipose tissue of obese diabetic subjects. This lack of effect of insulin provides evidence for tissue resistance in vivo to the action of insulin in diabetes mellitus.

Catecholamine receptor sensitivity and the regulation of lipolysis in adult diabetes

The sensitivity of alpha- and beta-adrenergic receptors, and the antilipolytic actions of prostaglandin E1 or insulin on adipose tissue of obese diabetic and non-diabetic subjects have been studied. Accumulation of cyclic AMP in adipose tissue and release of glycerol in response to several catecholamines (adrenaline, noradrenaline and isoprenaline) in the presence or absence of an alpha-adrenergic blocker (phentolamine) have been used to assess catecholamine receptor sensitivity. No differences in beta-receptor activity were observed between diabetics and non-diabetics, either on glycerol release or accumulation of cyclic AMP; alpha-receptor activity was also similar, except for significantly less accumulation of cyclic AMP in diabetic tissue incubated with noradrenaline and phentolamine (p less than 0.01). The antilipolytic action of prostaglandin E1 (at concentrations of 30 fM to 30 pM) on lipolysis (stimulated submaximally with isoprenaline, 10(-7) M) was similar in diabetic and control groups. The antilipolytic action of insulin (from 10(-10) to 10(-6) M) on lipolysis was also similar between the groups. It is concluded that neither disorders of the catecholamine receptor nor of the antiliolytic actions of prostaglandin E1 or insulin are responsible for the abnormalities of fatty acid metabolism in adult diabetes.

Nicotinic acid reverses fasting ketosis by lowering the level of cyclic AMP

When nicotinic acid was administered intraperitoneally to fasted rats, it reversed ketosis, decreased concentrations of cyclic AMP in adipose tissue and liver, and partially suppressed lipolysis. Administration of dibutyryl cyclic AMP reinduced ketosis in fasted rats previously treated with nicotinic acid. The results that nicotinic acid reverses ketosis by lowering tissue levels of cyclic AMP with a consequent suppression of lipolysis and ketogenesis.

Effect of highly specific inhibitors of oxidative phosphorylation, dicyclohexylcarbodiimide, antimycin A and piericidin A, on cyclic AMP accumulation induced by noradrenaline and theophylline in rat adipose tissue

The effect of three inhibitors of oxidative phosphorylation (acting at different levels), dicyclohexylcarbodiimide, antimycin A and piericidin A, was tested on cyclic AMP accumulation and lipolysis in rat epididymal fat. Dicyclohexylcarbodiimide, antimycin and piericidin, showed a marked inhibitory effect on cyclic AMP accumulation induced by noradrenaline plus theophylline. The same drugs inhibited also the lipolytic response of adipose tissue to noradrenaline, theophylline and dibutyryl cyclic AMP. The effect on cyclic AMP was quantitatively similar to that on lipolysis in the case of antimycin and dicyclohexylcarbodiimide. In contrast, piericidin was less effective on the cyclic AMP level than on lipolysis. These data suggest that the synthesis of cyclic AMP in adipose tissue, is dependent on oxidative phosphorylation, but to a lesser degree than the whole final lipolytic process.

Effect of chlorophenoxyisobutyric acid (CPIB) on fat-mobilizing lipolysis and cyclic AMP levels in rat epididymal fat

High concentrations of chlorophenoxyisobutyric acid (CPIB) reduced basal glycerol release from rat epididymal fat pads in vitro and antagonized the lipolytic effects of noradrenaline. Furthermore, very high concentrations of CPIB significantly antagonized the effects or noradrenaline or ACTH on cyclic AMP accumulation by isolated rat adipocytes. These data are not incompatible with the hypothesis that a primary mechanism in the hypolipidemic action of CPIB is to lower the levels of cyclic AMP in adipose tissue, resulting in decreased hormone-sensitive lipase activity and/or increased lipoprotein lipase activity.

Clearing-factor lipase in adipose tissue. A possible role of adenosine 3',5'-(cyclic)-monophosphate in the regulation of its activity.

1. The rise in clearing-factor lipase activity that occurs when epididymal fat bodies from starved rats are incubated in appropriate media in vitro is inhibited in the presence of 6-N-2'-O-dibutyryl-3',5'-(cyclic)-AMP (1mm). 2. Inhibition occurs at a concentration of glucose in the incubation medium of 1.3mg./ml. or less, but not at a glucose concentration of 2.4mg./ml., unless caffeine (1mm), an inhibitor of 3',5'-(cyclic)-nucleotide phosphodiesterase, is also present. Caffeine (5mm) alone inhibits the rise in clearing-factor lipase activity at a glucose concentration of 2.4mg./ml. of medium. 3. The concentration of free fatty acids in the epididymal fat bodies normally falls during incubations in vitro as the rise in clearing-factor lipase activity occurs. In the presence of 1mm-6-N-2'-O-dibutyryl-3',5'-(cyclic)-AMP, however, either the tissue free fatty acid concentration is increased or it does not fall to the same extent. The concentration of glucose in the incubation medium is important in determining the direction and extent of the changes in tissue free fatty acid concentration that occur in the presence of 6-N-2'-O-dibutyryl-3',5'-(cyclic)-AMP. 4. Free fatty acid concentrations in epididymal fat bodies in vivo rise as the clearing-factor lipase activity of the tissue falls during starvation. 5. The possibility that the concentration of 3',5'-(cyclic)-AMP in adipose tissue may regulate clearing-factor lipase activity, and that the regulation may occur through effects of the nucleotide on tissue free fatty acid concentrations, is discussed.

Effects of growth hormone on the lipolytic response of adipose tissue to theophylline.

Adipose tissue from hypophysectomized rats has a reduced capacity to carry on lipolysis in response to theophylline and other lipolytic agents. Three hr after a single injection of growth hormone (100 μg), FFA and glycerol production in response to theophylline are increased, but basal lipolysis is unchanged. Preincubation of adipose tissue for 3 hr in the presence of 0.01–1.0 μg/ml of growth hormone significantly increased the lipolytic response to theophylline, again without affecting the basal rate of lipolysis. This action of growth hormone has a lag period of more than 1 hr, is blocked by insulin and augmented by dexamethasone, which has a similar, but less consistent effect. Actinomycin failed to block this response to growth hormone and cycloheximide significantly enhanced it. These data are thought to illustrate an effect of growth hormone on the level of 3′,5′-cyclic AMP in adipose tissue, and suggest a biochemical explanation for the “permissive” effect of growth hormone on lipolysis. (Endocrinology81: 1027, 1968)