Activation and inhibition of lipolysis in isolated fat cells by various inhibitors of cyclic AMP phosphodiesterase

Effect of short-term fasting on the lipolytic response to theophylline

We investigated the hypothesis that the increase in lipolysis that occurs in short-term (86-h) fasting is due to a decreased inhibitory influence of adenosine. In normal volunteers who fasted for 14 and 86 h, the response to adenosine receptor blockade was assessed by the infusion of theophylline at a rate sufficient to produce plasma concentrations (30 microM) that blocked adenosine receptors but that were well below the threshold for inhibition of phosphodiesterase. Lipolysis was assessed by determining the rate of appearance of glycerol using D-5-glycerol infusion. Fatty acid flux was also determined by means of [1-13C]palmitate infusion, and total fatty acid oxidation was determined by indirect calorimetry. There was a mild stimulatory effect of theophylline on lipolysis at 14 h. After the subjects fasted for 86 h, theophylline infusion caused a much greater increase in both lipolysis and fatty acid oxidation. These results suggest that the inhibitory effect of adenosine on lipolysis is increased during short-term fasting.

Purinergic stimulation of astroblast proliferation: guanosine and its nucleotides stimulate cell division in chick astroblasts

A highly active fraction that was mitogenic for astroblasts but which contained no amino acids was identified during the purification of peptides from chick embryo brains. This material was purified by ultracentrifugation, ultrafiltration through Diaflo PM-30 and YM-2 membranes and retention on Diaflo YC-05, followed by ion exchange chromatography and reversed phase high performance liquid chromatography (HPLC) on a C18 Deltapak column. On thin layer chromatography and HPLC the material co-chromatographed with authentic commercially-obtained GMP. Its ultraviolet absorption spectrum was also identical with that of GMP. 1H and 31P nuclear magnetic resonance spectra of the isolated material were identical with those of GMP. The close match between the fast atom bombardment (FAB) mass spectra of the unknown material and authentic GMP indicated that the unknown material was GMP of molecular weight 363 Da. Authentic, commercial GMP stimulated the growth of cultured chick astroblasts in the same dose-dependent manner as the material from chick embryo brains; maximal stimulation was at 50 microM. Guanosine, GDP, and GTP also stimulated cell proliferation. The nucleotides were equally as effective as guanosine. 5'-Guanylyl imidodiphosphate, guanosine 5'-O-(2-thiodiphosphate), and guanosine 5'-N-(3-thiotriphosphate), guanine nucleotides which are relatively resistant to enzymatic hydrolysis, were also mitogenic, indicating that the nucleotides do not need to be degraded to nucleosides to be active and that they probably act extracellularly. Guanine nucleosides and nucleotides promoted astroblast growth when other growth factors were removed from the culture medium. The mitogenic effects of guanosine and its nucleotides were inhibited in a dose-dependent fashion by micromolar concentrations of theophylline, a characteristic of phenomena mediated by purinergic receptors. Guanosine and its nucleotides are released in micromolar concentrations by hypoxic or dying cells. Under these circumstances these compounds may stimulate division of adjacent cells in vivo.

A reexamination of the effects induced by adenosine and its degradation products on rat fat cell lipolysis

The effects of adenosine and of some products of its metabolic degradation on lipolysis were studied in rat fat cells isolated from epididymal adipose tissue. Basal glycerol release was not affected by adenosine and by uric acid, but it was significantly increased by inosine (1-100 microM) and by hypoxanthine (10-100 microM). Adenosine was more effective than inosine in antagonizing the lipolytic response of fat cells to theophylline. Also hypoxanthine and uric acid exerted a very potent, noncompetitive antagonism towards theophylline. Norepinephrine-induced lipolysis was inhibited by adenosine, hypoxanthine and uric acid approximately to the same extent, while inosine was ineffective at this level. Adenosine deaminase (0.5 U/ml) increased basal as well as theophylline- and norepinephrine-induced lipolysis. Moreover, adenosine deaminase enhanced the lipolytic rate in cells incubated with low (0.1, 1 microM) and, to a lesser extent, with high (10, 100 microM) inosine concentrations. These results suggest that inosine is the adenosine metabolite that may accumulate in the incubation medium following fat cell treatment with adenosine deaminase, thus contributing to the stimulatory effects of this enzyme on lipolysis.

Xanthine derivatives as antagonists at A1 and A2 adenosine receptors

A variety of alkylxanthines has been comparatively examined as antagonists of A1 adenosine receptors in rat fat cells, rat and bovine cerebral cortex and of A2 adenosine receptors in human platelets. With few exceptions all xanthine derivatives with 7-position substituents such as diprophylline, proxyfylline, pentoxifylline and etofylline were less potent antagonists than xanthine itself which had Ki-values of 170 mumol/l (A1) and 93 mumol/l (A2). Theophylline, caffeine and 3-isobutyl-1-methylxanthine were more potent than xanthine but nearly equipotent antagonists at both receptor subtypes. 8-Phenyl substituents considerably increased the antagonist potency at A1 and A2 receptors. 1,3-Diethyl-8-phenylxanthine was the most potent A2 antagonist (Ki 0.2 mumol/l) in human platelets. At A1 receptors 1,3-dipropyl-8-(2-amino-4-chlorophenyl)xanthine (PACPX) was the most potent antagonist in all three tissues with Ki-values from 0.3 to 8.6 nmol/l. Several 8-phenylxanthine derivatives were remarkably selective antagonists at A1 receptors. 8-Phenyltheophylline was approximately 700 times more potent as antagonist at A1 receptors (bovine brain) than at A2 receptors (human platelets), and PACPX was even 1,600 times more potent as A1 adenosine receptor antagonist. These compounds offer a possibility for a subtype-selective blockade of adenosine receptors.

Methylxanthine and non-xanthine phosphodiesterase inhibitors. Their effects on adenosine uptake and the low Km cyclic AMP phosphodiesterase in intact rat adipocyte

The effects of methylxanthines and non-xanthine phosphodiesterase-inhibitors on the low Km cyclic AMP phosphodiesterase of intact rat adipocytes were studied. Methylxanthines and papaverine stimulated rather than inhibited the enzyme when intact adipocytes were incubated in their presence. The effect of papaverine was not abolished by adenosine deaminase and was enhanced by adenosine. On the other hand, the effect of xanthine inhibitors and adenosine do not enhance each other. The difference in behaviour of these inhibitors could not be explained by their effects on adenosine uptake at the concentrations studied. Both agents inhibited adenosine uptake when measured after 15 sec and 10 min, with methylisobutylxanthine (MIX) having a greater inhibitory effect than papaverine only if uptake was measured after 15 sec. Effects similar to that of adenosine with the inhibitors on phosphodiesterase were obtained with insulin, which has been shown to act through a similar or related mechanism to that of adenosine. This was not the case with lipolytic agents whose effects were not potentiated by either MIX or papaverine. Under certain conditions the degree of stimulation of the enzyme was in fact decreased. Thus lipolytic and antilipolytic agents probably stimulated phosphodiesterase through distinct mechanisms.

An improved assay of cyclic 3',5'-nucleotide phosphodiesterases with QAE-Sephadex columns

A method for the assay of cyclic nucleotide phosphodiesterases is described which employs chromatographical separation of cyclic nucleotides and nucleosides on QAE A-25 Sephadex columns and 5'-nucleotidase as an auxiliary enzyme. The assay allows quantitative recovery of adenosine, guanosine and their metabolites from the anion exchanger and thus is suitable for use in crude phosphodiesterase preparations containing purine catabolizing enzymes. In comparative studies, this method was found to be considerably more sensitive than previous reported methods because of lower assay blanks and higher recoveries of the nucleoside catabolites. The method is suitable for kinetic analysis of crude enzyme preparations from micromolar to millimolar substrate concentrations.

On the mechanism of relaxation of tracheal muscle by theophylline and other cyclic nucleotide phosphodiesterase inhibitors

The mechamism of action of theophylline was studied by investigating the relationship between relaxant effect and inhibition of cyclic nucleotide phosphodiesterase (PDE) and by studying interactions with adenosine actions. Guinea pig tracheal smooth muscle cyclic AMP PDE had two apparent KmS': 0.4 and 70 microM for cyclic AMP. Theophylline and papaverine competetively inhibited the low Km form. Hydrolysis of 2.0 microM cyclic AMP and cyclic GMP was inhibited by several drugs. Some agents (e.g. ZK 62 711, ICI 63,197, Ro 20--1724, dipyridamol) were considerably more potent as inhibitors of cyclic AMP than of cyclic GMP hydrolysis, while other agents (M & B 22.948 and dilazep) selectively inhibited cyclic GMP breakdown, and some (theophylline, papaverine, IBMX and SQ 20,006) showed little selectivity. There was a weak but significant correlation between inhibition of cyclic AMP phosphodiesterase and relaxation of tracheal smooth muscle in vitro. There was also a correlation between the ratio of IC25 cyclic AMP/IC25 cyclic GMP and the smooth muscle relaxation, indicating that inhibition of cyclic AMP rather than cyclic GMP hydrolysis determined relaxation. However, there was a marked tachyphylaxis to the relaxant effect of the cyclic AMP selective PDE-inhibitors, while the nonselective methylxanthines did not show tachyphylaxis. The effect of theophylline was antagonized by low concentrations of adenosine, which by itself caused a weak tracheal contraction. The effect of PDI-inhibitors can be partly explained by decreased cyclic AMP breakdown but other mechanisms, such as antagonism of endogenous adenosine, may contribute to the observed relaxant action.

Uptake and release of adenosine in isolated rat fat cells

Radioactively labelled adenosine and adenine were rapidly taken up by isolated rat fat cells, and incorporated into nucleotides, of which ATP dominated. The overall process had an apparent Km of 1--5 micrometers. During incubation, especially in the presence of lipolytic agents, there was a reduction in labelled ATP with a compensatory increase in ADP, AMP, cAMP and nucleosides. The build-up of adenosine during incubation was inhibited by theophylline, which inhibits 5'-nucleotidase. Radioactivity released from perifused fat cells consisted mainly of nucleoside material, of which adenosine predominated. Lipolytic stimulation caused no significant increase in nucleoside outflow from perifused cells, whereas oxygenation was capable of reducing this outflow. It is concluded that adenosine is formed by fat cells as a consequence of ATP breakdown. Stimulation of lipolysis during activation of the sympathetic nerves leads to reversible ATP breakdown and adenosine release. Adenosine might therefore act as a modulator of lipolysis in vivo under these conditions, even though it does not serve as a feed back regulator in the proper sense.

Antilipolytic effect of adenosine in isolated perifused fat cells

Adenosine markedly inhibits cyclic AMP accumulation in isolated fat cells, whereas inhibitory effects of adenosine on lipopolysis have been difficult to demonstrate. The present study has been performed on isolated "perifused" fat cells where continuous monitoring of the lipolytic rate is possible and where modulating substances, such as adenosine, are not allowed to accumulate. Adenosine deaminase was ineffective as a lipolytic agent in perifused fat cells, suggesting no important background activity of adenosine in this system. Micromolar concentrations of adenosine inhibited lipolysis induced by noradrenaline (0.3--1 micrometer) and theophylline (1 mM). Theophylline was an effective lipolytic agent also in perifused fat cells suggesting that antagonism of adenosine is not the major mode of action of this drug on fat cells.

Cyclic AMP-dependent and independent inhibition of lipolysis by adenosine and decreased pH

NA-stimulated lipolysis and cAMP formation in isolated rat fat cells is inhibited by acidosis. In the present report we have examined the quantitative relationship between lipolysis and cAMP formation at normal and reduced pH and the possible involvement of adenosine, an endogenous inhibitor of cAMP formation. Adenosine antagonized cAMP accumulation and to a considerably lower degree lipolysis, effects potentiated by acidosis. Theophylline, an antagonist of adenosine effects, stimulated lipolysis and cAMP-accumulation, and potentiated responses to NA. Adenosine deaminase (ADA) had theophylline-like effects. Acidosis inhibited lipolysis and cAMP accumulation induced by ADA and theophylline to a larger extent than those induced by NA. It is suggested that adenosine modulates fat cell cAMP production and may contribute to the antilipolytic effect of acidosis. There was a curvilinear relationship between cAMP elevation and glycerol production in fat cell suspensions, which was different at pH 7.4 and at pH 6.6. The amount of cAMP needed for half-maximal activation of lipolysis increased from 1.3 (pH 7.4) to 3.1 pMol X 10(-5) cells (pH 6.6). The maximal glycerol production was reduced from 1 300 to 900 nMol X 10(-5) cells. The antilipolytic effect of acidosis is apparently due partly to an inhibition of cAMP formation and partly to inhibition of subsequent step(s) in the activation sequence.

The absence of stimulation of lipolysis by papaverine, a strong inhibitor of phosphodiesterase

Quantitative studies of the action of theophylline and papaverine were performed in rat epididymal fat pads, both on the lipolytic effect and on the activity of phosphodiesterase, adenylate cyclase and protein kinase. Papaverine, a stronger inhibitor of phosphodiesterase than theophylline, did not produce lipolysis. The maximum lipolytic effect (glycerol release) of theophylline was much higher than that of epinephrine and nearly approached the effect exerted by dibutyryl cyclic AMP. While theophylline potentiated or was without any effect on lipolysis produced by epinephrine and dibutyryl cyclic AMP, papaverine at concentration 10- minus 3 M reduced the effect of both drugs as well as of theophylline by 90 per cent. These concentrations of papaverine also strongly inhibited the activity of adenylate cyclase. Neither papaverine nor theophylline prevented the activation of protein kinase by cyclic AMP. The data suggest that the lack of a lipolytic effect of papaverine migth be caused by a combination of its inhibitory effect on adenylate cyclase and direct inhibition of activation of triglyceride lipase.