Insulin-like Actions of Ribonucleic Acid, Adenylic Acid, and Adenosine

The involvement of purinergic signalling in obesity

Obesity is a growing worldwide health problem, with an alarming increasing prevalence in developed countries, caused by a dysregulation of energy balance. Currently, no wholly successful pharmacological treatments are available for obesity and related adverse consequences. In recent years, hints obtained from several experimental animal models support the notion that purinergic signalling, acting through ATP-gated ion channels (P2X), G protein-coupled receptors (P2Y) and adenosine receptors (P1), is involved in obesity, both at peripheral and central levels. This review has drawn together, for the first time, the evidence for a promising, much needed novel therapeutic purinergic signalling approach for the treatment of obesity with a 'proof of concept' that hopefully could lead to further investigations and clinical trials for the management of obesity.

Adenosine, adenosine receptors and their role in glucose homeostasis and lipid metabolism

Adenosine is an endogenous metabolite that is released from all tissues and cells including liver, pancreas, muscle and fat, particularly under stress, intense exercise, or during cell damage. The role of adenosine in glucose homeostasis has been attributed to its ability to regulate, through its membrane receptors, processes such as insulin secretion, glucose release and clearance, glycogenolysis, and glycogenesis. Additionally, adenosine and its multiple receptors have been connected to lipid metabolism by augmenting insulin-mediated inhibition of lipolysis, and the subsequent increase in free fatty acids and glycerol levels. Furthermore, adenosine was reported to control liver cholesterol synthesis, consequently affecting plasma levels of cholesterol and triglycerides, and the amount of fat tissue. Alterations in the balance of glucose and lipid homeostasis have implications in both cardiovascular disease and diabetes. The ability of different adenosine receptors to activate and inhibit the same signaling cascades has made it challenging to study the influence of adenosine, adenosine analogs and their receptors in health and disease. This review focuses on the role and significance of different adenosine receptors in mediating the effect of adenosine on glucose and lipid homeostasis. J. Cell. Physiol. © 2013 Wiley Periodicals, Inc.

Possible central adenosinergic modulation of ethanol-induced alterations in [14C]glucose utilization in mice

The possible role of brain adenosine in acute ethanol-induced alteration in glucose utilization in the whole brain, as well as in the specific brain areas (cerebellum and brain stem), was investigated. Mice were killed 20-min postethanol, and the fresh tissue slices (300 microns) of brain and/or specific brain areas were incubated for 100 min in a 5.5 mM glucose medium in Warburg flasks using [6-(14)C]glucose as a tracer. Trapped 14CO2 was counted to estimate glucose utilization. Ethanol (2 g/kg, i.p.) markedly increased the glucose utilization in whole brain and in both motor areas of brain. Theophylline (50 mg/kg, i.p.), an adenosine antagonist, significantly reduced ethanol-induced increase in glucose utilization in whole brain, as well as in brain areas. However, adenosine agonist N6-cyclohexyladenosine (CHA; 0.1 mg/kg, i.p.) on the contrary, significantly accentuated ethanol-induced increase in glucose utilization in these tissues that was nearly completely blocked by theophylline pretreatment. Theophylline alone did not produce any significant change in glucose utilization, whereas CHA alone (in vivo and in vitro) significantly increased glucose utilization, as well as ethanol-induced increase in glucose utilization in an additive manner. Relevant supportive data were obtained by experiments in which adenosine deaminase (ADA), p-sulfophenyltheophylline (8-SPT), and CHA were administered in vitro to the slice preparations. Both ADA and 8-SPT were effective in almost completely blocking the ethanol-induced increase in glucose utilization, whereas CHA further enhanced the ethanol-induced increase in glucose utilization in an additive manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional role of intravascular coronary endothelial adenosine receptors

The endothelium is relatively 'impermeable' to adenosine. In addition, infusion of adenosine deaminase and transient infusion of large size adenosine agonists (molecular weight 100 kD) which are confined to the intravascular space depress effects of endogenous adenosine and retain physiologic activity respectively. Accordingly, the concept that intravascular adenosine may exert some of its action on the capillary lumen was tested by coupling the agonists: N6-([aminoethylamino]carbonyl)methylphenyladenosine (ADAC) and N6-octylamine adenosine (NOA) to carboxylated latex microspheres (0.07 microns diameter); thus, insuring their intravascular confinement. Our results demonstrated that sustained infusion of these particles into isolated saline perfused guinea pigs hearts caused a decrease in coronary vascular resistance, ventricular contraction, spontaneous ventricular rhythm, inhibition of auricular ventricular transmission and glycolytic flux. These effects were reversible and specific since microspheres without purines had no effect and the adenosine antagonist sulphophenyltheophylline blocked these responses. Furthermore, the effects were not the result that during the passage of the sphere-agonist complex through the heart the covalent bond hydrolyzed, releasing free agonist. Our data indicate that selective activation of intravascular coronary purine receptors may cause the release of endothelial bioactive messengers that regulate the function and metabolism of vascular and cardiac cells.

Adenosine, the heart, and coronary circulation

Adenosine is known to regulate myocardial and coronary circulatory functions. Adenosine not only dilates coronary vessels, but attenuates beta-adrenergic receptor-mediated increases in myocardial contractility and depresses both sinoatrial and atrioventricular node activities. The effects of adenosine are mediated by two distinct receptors (i.e., A1 and A2 receptors). A1 adenosine receptors, located in atrial and ventricular myocardium and sinoatrial/atrioventricular nodes, are responsible for inhibition of adenylyl cyclase activity. A2 adenosine receptors, located in coronary endothelial and smooth muscle cells, are responsible for stimulation of this enzyme activity. During increased myocardial oxygen demand due to rapid pacing and exercise, although both coronary blood flow and adenosine concentrations in the myocardium and coronary efflux increased, there is no clear consensus explaining its cause and effect relation at present. However, ischemia/reperfusion-induced coronary hyperemia is believed to be mostly attributed to released adenosine, and it has been proven that adenosine attenuates the severity of ischemia due to its coronary vasodilatory action. The beneficial effects of adenosine during ischemia/reperfusion processes do not seem simple. This is because myocardial ischemia and reperfusion injury is caused by 1) activated leukocytes and platelets, 2) ATP depletion and calcium overload of myocardium, and 3) catecholamine release from the presynaptic nerves as well as 4) the impaired coronary circulation. Intriguingly adenosine attenuates all of these deleterious actions and thereby attenuates ischemia/reperfusion injury. Indeed, adenosine attenuates the severity of contractile dysfunction (myocardial stunning) and limits the infarct size. Thus, administration of adenosine or potentiators of adenosine production in the ischemic myocardium may be beneficial for the attenuation of ischemic and reperfusion injuries, although further clinical investigations are necessary.

Myocardial glucose utilization. Failure of adenosine to alter it and inhibition by the adenosine analogue N6-(L-2-phenylisopropyl)adenosine

The effects of adenosine and the nonmetabolizable adenosine analogue N6-(L-2-phenylisopropyl)adenosine (PIA) on glucose transport or metabolism were determined in purified myocardial sarcolemmal vesicles, isolated cardiocytes, and perfused hearts. Adenosine (100 microM) did not affect hexose transport in myocytes. Also, adenosine deaminase, added to metabolize adenosine to inosine, did not alter transport of hexose into myocytes regardless of whether or not insulin was present. In contrast, PIA effectively inhibited 3-O-methyl-D-glucose uptake in myocytes even during insulin stimulation. PIA inhibited D-glucose-specific transport in both rat and bovine cardiac sarcolemmal vesicles (Ki = 26 microM at [D-glucose] = 5 mM). However, insulin did not affect glucose transport in sarcolemmal vesicles, which implies that receptor-coupled processes probably are not intact in this preparation. Thus, inhibition of PIA may not be receptor mediated. Also, PIA inhibited binding of cytochalasin B to bovine cardiac sarcolemmal vesicles, which supports the idea that PIA inhibits glucose flux by binding to the glucose transporter. To determine if adenosine altered glucose metabolism rather than transport, we measured the rate of 3H2O production from metabolism of D-[2-3H]glucose in paced rat hearts ([D-glucose] = 5.5 mM, [pyruvate] = 0.2 mM) perfused with a range of PIA or adenosine concentrations with or without 0.01 microM insulin. Adenosine (0.01-100 microM) in the presence or absence of insulin increased coronary flow but did not change glycolytic rates. Similar results were obtained with PIA (no insulin) rather than adenosine in the perfusate. However, with glucose as the only exogenous substrate, 100 microM PIA inhibited glycolysis by approximately 50%.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of age and day time on the adenosine modulation of basal and insulin-stimulated glucose transport in rat adipocytes

1. The relationship between the activity of adenosine metabolizing enzymes 5'nucleotidase (5'N), adenosine kinase (A.K.) and adenosine deaminase (A.D.) with basal and insulin-stimulated glucose transport in isolated fat cells from young and old animals was studied at 08:00 and 16:00 hr. 2. In cells from young animals a larger insulin-stimulation of glucose transport was observed at 16:00 hr than at 08:00 hr. Also at 16:00 hr small changes in 5'N, A.K. and A.D. activities suggest a decrease in adenosine formation. 3. In the cells from old animals no effect of insulin was observed at any time, while a 3-5-fold increase in 5'N indicated a predominance of adenosine formation at both times studied. 4. An inverse relationship was observed in the changes of adenosine metabolism and insulin action.

Carbohydrate metabolism in intact golden hamsters infected with plerocercoids of Spirometra erinacei (Cestoda: Diphyllobothriidae)

This study was performed to investigate the mechanism involved in a decrease in the serum glucose of golden hamsters infected with plerocercoids of Spirometra erinacei. The concentration of glucagon, the activity of glucose-6-phosphatase in the liver, and the in vivo incorporation of 2-deoxy-D-[1,2-3H]glucose into various organs increased in plerocercoid-infected hamsters compared with controls. Furthermore, the serum from the plerocercoid-infected hamsters enhanced the in vitro incorporation of [U-14C]glucose into adipose tissues, compared with control serum. The serum levels of immunoreactive insulin and somatomedin associated with nonsuppressible insulin-like activity in experimental animals, however, were not significantly different from those in controls. Therefore, we conclude that the decrease in serum glucose associated with plerocercoid infection is not the result of a decrease in gluconeogenesis, but the result of an increased utilization of glucose in the peripheral tissues.

Distribution of adenosine metabolising enzymes between adipocyte and stromal-vascular cells of adipose tissue

Rat adipose tissue was digested with collagenase and separated into adipocytes and stromal-vascular cells. The adipocytes accounted for 40% of the total adipose tissue adenosine deaminase activity, 32% of 5'-nucleotidase activity and 87% of adenosine kinase activity. This distribution suggests that adipocyte are the major cell type involved in adenosine utilization in adipose tissue. Furthermore, it suggests that the high sensitivity of isolated adipocytes to adenosine is representative of their sensitivity of isolated adipocytes to adenosine is representative of their sensitivity in vivo.

The mechanism of the lipolytic action of theophylline in fat cells

The lipolytic action of theophylline was examined using both intact fat cells and a fat globule system. Theophylline had similar lipolytic actions in both systems. However theophylline did not activate hormone-sensitive lipase in the fat globule system as measured with added Ediol. Pretreatment of the fat globules with phospholipase C suppressed theophylline-induced lipolysis, but phospholipase D had no effect. A theophylline-sensitive system was reconstituted from endogenous fat and a lipase fraction. Inhibitors of theophylline-induced lipolysis such as quinine and propranolol inhibited theophylline binding to artificial lipid micelles. Purine nucleosides such as adenosine, inosine and guanosine inhibited theophylline-induced lipolysis in the fat globule system. These results suggest that theophylline has a lipolytic action similar to that of adrenaline. Both share a lipolytic mechanism additional to that involving the activation of hormone sensitive lipase through the cyclic-AMP dependent protein kinase. Phospholipids play an important role in this additional mechanism.

Seasonal variation in the effect of dietary RNA on criteria of energy homoeostasis in the rat

1. RNA was administered to rats as part of a meal while standardizing food intake and minimizing the effects of psychological stress and diurnal metabolic rhythms. It was demonstrated that circulating levels of glucose and free fatty acids (FFA) in the animals, which were deprived of food for 48 h, were responsive to orally administered caffeine. 2. Inclusion of RNA in the diet slightly but consistently reduced the normal postprandial hyperglycaemia. Its effect on plasma FFA was variable although statistically significant in some experiments. The differences between RNA-and control-fed animals were not attributable to differences in the rate of passage of digesta along the gastrointestinal tract. 3. Evidence was obtained that the variability in the FFA response was related to a seasonally-dependent change in the state of animals. The synchronizer ('Zeitgeber') responsible for this change was not identified and no satisfactory way of suppressing its effect was found. 4. The present findings, taken in conjunction with those of previous workers, suggest that there is a seasonal influence on the sympathetic nervous system manifesting itself as a variable susceptibility to arousal or excitation.

Myocardial glucose uptake and breakdown during adenosine-induced vasodilation

In isolated K+ (16.2 mM)-arrested cat hearts perfused at constant pressure adenosine infusions (0.8 mumoles - min-1 - 100 g-1 for 10 min) caused an increase in myocardial 14C-glucose uptake and release of 14CO2 + H14CO3- AND 14C-lactate simultaneously with a rise in coronary flow. The ratio of the release of 14CO2 + H14CO3- to that of 14C-lactate and the specific activity of lactate in the effuate were not altered. In K+ -arrested hearts perfused with constant volume neither glucose uptake nor glucose breakdown were influenced by 0.8 or 100 mumoles - min-1 - 100 g-1 adenosine with 0.1 - 5 mM glucose in the perfusion medium. It is concluded that adenosine does not affect directly the myocardial glucose carrier system, aerobic or anaerobic glucose breakdown or glycogenolysis, but enhances glucose uptake secondarily by increasing coronary flow. This interpretation is substantiated by the finding that mechanically produced increases in perfusion volume caused similar increases in myocardial glucose uptake as were observed with comparable adenosine-induced coronary flow increments.