Utilization of adenosine as a tool in studies on the regulation of liver glycogen biosynthesis

Recovery of the Cell Cycle Inhibition in CCl(4)-Induced Cirrhosis by the Adenosine Derivative IFC-305

Introduction. Cirrhosis is a chronic degenerative illness characterized by changes in normal liver architecture, failure of hepatic function, and impairment of proliferative activity. The aim of this study is to know how IFC-305 compound induces proliferation of the liver during reversion of cirrhosis. Methods. Once cirrhosis has been installed by CCl(4) treatment for 10 weeks in male Wistar rats, they were divided into four groups: two received saline and two received the compound; all were euthanized at 5 and 10 weeks of treatment. Liver homogenate, mitochondria, and nucleus were used to measure cyclins, CDKs, and cell cycle regulatory proteins PCNA, pRb, p53, E2F, p21, p27, HGF, liver ATP, and mitochondrial function. Results. Diminution and small changes were observed in the studied proteins in the cirrhotic animals without treatment. The IFC-305-treated rats showed a clear increase in most of the proteins studied mainly in PCNA and CDK6, and a marked increased in ATP and mitochondrial function. Discussion/Conclusion. IFC-305 induces a recovery of the cell cycle inhibition promoting recovery of DNA damage through the action of PCNA and p53. The increase in energy and preservation of mitochondrial function contribute to recovering the proliferative function.

Adenosine deaminase of cultured brain cells

Two types of adenosine deaminase (EC 3.5.4.4) were found in cultured cells of central-nervous-system origin. The predominant and more active enzyme was obtained in soluble form from the cytosol of mouse neuroblastoma (N-18), neonatal hamster astrocytes (NN), human oligodendroglioma (HOL) and human astrocytoma (Cox Clone). Particulate adenosine deaminase was probably associated with the plasma membrane. When radioactive adenosine was added to superfusates of monolayer cultures it was rapidly converted into inosine and hypoxanthine. The metabolic conversion required adenosine uptake by the cells, a probable transition through the intracellular ATP pool(s) and a rapid excretion into the superfusate of the catabolic products. We discuss the evidence that points to adenosine and its derivatives as neurohumoral modulators of central-nervous-system function.

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)

Circadian variations of adenosine and of its metabolism. Could adenosine be a molecular oscillator for circadian rhythms?

The present review describes the biological implications of the periodic changes of adenosine concentrations in different tissues of the rat. Adenosine is a purine molecule that could have been formed in the prebiotic chemical evolution and has been preserved. The rhythmicity of this molecule, as well as its metabolism and even the presence of specific receptors, suggests a regulatory role in eukaryotic cells and in multicellular organisms. Adenosine may be considered a chemical messenger and its action could take place at the level of the same cell (autocrine), the same tissue (paracrine), or on separate organs (endocrine). Exploration of the circadian variations of adenosine was planned considering the liver as an important tissue for purine formation, the blood as a vehicle among tissues, and the brain as the possible acceptor for hepatic adenosine or its metabolites. The rats used in these studies were adapted to a dark-light cycle of 12 h with an unrestrained feeding and drinking schedule. The metabolic control of adenosine concentration in the different tissues studied through the 24-h cycle is related to the activity of adenosine-metabolizing enzyme: 5'-nucleotidase adenosine deaminase, adenosine kinase, and S-adenosylhomocysteine hydrolase. Some possibilities of the factors modulating the activity of these enzymes are commented upon. The multiphysiological action of adenosine could be mediated by several actions: (i) by interaction with extracellular and intracellular receptors and (ii) through its metabolism modulating the methylation pathway, possibly inducing physiological lipoperoxidation, or participating in the energetic homeostasis of the cell. The physiological meaning of the circadian variations of adenosine and its metabolism was focused on: maintenance of the energetic homeostasis of the tissues, modulation of membrane structure and function, regulation of fasting and feeding metabolic pattern, and its participation in the sleep-wake cycle. From these considerations, we suggest that adenosine could be a molecular oscillator involved in the circadian pattern of biological activity in the rat.

Stimulation of rat liver glycogen synthesis by the adenosine kinase inhibitor 5-iodotubercidin

The adenosine kinase inhibitor 5-iodotubercidin (Itu) was found to have the following effects on glycogen metabolism in hepatocytes of fasted rats. (1) Itu strongly stimulated glycogen synthesis from different substrates (glucose, lactate plus pyruvate, dihydroxyacetone, glycerol and fructose). In cells incubated with these substrates, the well-known stimulating effect of amino acids and that of Itu was more than additive. (2) In parallel with the increase in glycogen deposition, there was an increase in synthase a and a decrease in phosphorylase a concentrations after administration of Itu. Synthase a was increased by Itu and amino acids in an additive manner, whereas the observed activation of phosphorylase after addition of amino acids was antagonized by Itu. (3) In contrast with amino acids, Itu increased neither the cell volume nor the aspartate and glutamate concentrations. (4) Itu enhanced the levels of cyclic AMP. The stimulation of glycogen deposition in the presence of Itu persisted when the cyclic AMP concentration was further increased by adenosine or 2-chloroadenosine. (5) Itu decreased the concentration of ATP, but its effects on glycogen synthesis, synthase a and phosphorylase a concentrations persisted when the ATP catabolism was prevented by adenosine. (6) The effect of Itu on glycogen synthesis was not the result of inhibition of adenosine kinase, since 5'-amino-5'-deoxyadenosine, another inhibitor of this enzyme, had no effect on glycogen deposition.

Effect of adenosine on the serum levels of glucose, insulin and glucagon in vivo

The in vivo effect of adenosine on the serum levels of glucose, insulin and glucagon in rats fasted for twenty four hours or after an oral glucose load were studied. Under fasting conditions adenosine produced an hyperglycaemia without change in the insulin or glucagon serum levels. After a glucose load adenosine induced a marked hyperglycaemia concomitant to a decrease in insulin serum levels and an increase in glucagon serum levels. Adenosine did not alter the relationship between insulin and glucagon. In vivo adenosine administration altered the secretion of hormones by the islets of Langerhans (increased the release of glucagon and decreased the secretion of insulin) but this was only clearly observable under stimulated conditions. Adenosine did not alter the regulatory mechanism(s) that modulate the relationship between insulin and glucagon.

Circadian variations of adenosine level in blood and liver and its possible physiological significance

The role of adenosine as a possible physiological modulator was explored by measuring its concentration in different tissues during a 24-hour period. Initially the circadian variations of adenosine and other purine compounds such as inosine, hypoxanthine, uric acid and adenine nucleotides were studied in the rat blood. A daily cyclic response was observed, with low levels of adenosine from 08.00 - 20.00 h, followed by an increase from this time on. Inosine and hypoxanthine levels were elevated during the day and low at night. The uric acid changes observed indicate that the decrease in purine catabolism coincides with a decrease in inosine and hypoxanthine levels and an increase in adenosine. The blood adenine nucleotides, energy charge and phosphorylation potential remained constant during the day and showed oscillatory changes during the night. Similar studies were made in the liver, a primary source of circulating purines. Liver adenosine was high during the night while inosine and hypoxanthine remained low along the 24 hours. The results suggest that liver purine metabolism might participate in the maintenance and renewal of the blood purine pool and in the energy state of erythrocytes in vivo.

The effects of NECA (adenosine-5'N-ethylcarboxamide) and of adenosine on glucagon and insulin release from the in situ isolated blood-perfused pancreas in anesthetized dogs

The effect of adenosine-5'-N-ethylcarboxamide, (NECA), a long-lasting adenosine derivative with pronounced vasoactivity was investigated on glucagon and insulin release from the in situ isolated blood perfused pancreas in the anesthetized dog: NECA (10(-9) to 10(-5) mol/l) led to a dose-dependent glucagon release. Insulin release was inhibited by NECA at low concentrations, but significantly increased at higher concentrations of the adenosine analogue. Similar effects were observed with infusion of adenosine at 10(-7) and 10(-6) mol/l. Aminophylline (10(-4) mol/l) produced a 10-fold attenuation of the actions of NECA. The preponderance of glucagon release at low concentrations of NECA and adenosine in contrast to that of insulin release at high concentrations may represent a local pancreatic regulatory mechanism of adenosine in glucose homeostasis.

In vivo effect of adenosine on adenine nucleotides and inorganic phosphate in rat blood

The effect of adenosine and the time response on adenine nucleotide and Pi levels in rat blood was investigated. An increase in adenine nucleotide with a concomitant decrease in Pi concentration 30, 60, and 90 minutes after the nucleoside administration were observed. Though the 100 mg/Kg dose showed the highest effect on nucleotide concentration, the maximal response on Pi content was achieved with the 50 mg/Kg dose. The results are discussed at the light of previous data obtained in hepatocytes, and using as indicators the energy charge and the phosphorylation potential.

Regulation of adenosine-sensitive adenylate cyclase from rat brain striatum

An adenosine-sensitive adenylate cyclase has been characterized from rat brain striatum. In whole homogenates as well as in particulate fractions, N6-phenylisopropyl adenosine (PIA), 2-chloroadenosine, and adenosine N'-oxide were equipotent in stimulating adenylate cyclase. Although GTP inhibited basal as well as PIA-stimulated activity of whole homogenates, the enzyme showed an absolute dependency on GTP for stimulation by PIA, dopamine, epinephrine, and norepinephrine in a particulate fraction derived from discontinuous sucrose gradient centrifugation. Adenosine exerts two effects on this adenylate cyclase, stimulation at low concentrations and inhibition at high concentrations, suggesting the presence of two adenosine binding sites. The stimulation of adenylate cyclase by PIA was dependent on the concentration of Mg2+. The degree of stimulation by PIA was greater at a low concentration of MG2+, which suggests that stimulation by PIA was accompanied by increasing the apparent affinity for Mg2+. Activation of adenylate cyclase by PIA was blocked by theophylline or 3-isobutyl-1-methylxanthine (IBMX). The pH optimum for basal or (PIA + GTP)-stimulated activities was broad, with a peak between 8.5 and 9.5. In the presence of GTP, stimulation by an optimal concentration of PIA was additive, with maximal stimulation by the catecholamines. Phospholipase A2 treatment at a concentration of 1 U/ml for 5 min completely abolished the stimulatory effect of dopamine, whereas PIA-stimulated activity remained unaltered. These data suggest that rat brain striatum either has a single adenylate cyclase, which is stimulted by catecholamines and adenosine by distinct mechanisms, or has different cyclase populations, stimulated by either adenosine or catecholamines.

Evidence that the severity of depletion of inorganic phosphate determines the severity of the disturbance of adenine nucleotide metabolism in the liver and renal cortex of the fructose-loaded rat

To test the hypothesis that in both the liver and renal cortex of the fructose-loaded rat, severity of depletion of inorganic phosphate (P(i)), and not the magnitude of accumulation of fructose-1-phosphate (F-1-P), determines the severity of the dose-dependent reduction of ATP, we intraperitoneally injected fed rats with fructose, 20 and 40 mumol/g, alone, and at the higher load, in combination with (a) sodium phosphate, 20 mumol/g, administered shortly beforehand or subsequently or, (b) adenosine, 2 mumol/g, administered beforehand. The following observations were made: (a) With fructose loading alone, at the higher load, both P(i) and total adenine nucleotides (TAN) were reduced by one third in the renal cortex and (as previously observed) by two thirds in the liver; and at either load, the reduction of ATP (and TAN) and the accumulation of F-1-P were less severe in the renal cortex than in the liver. (b) Prior phosphate loading largely prevented the reductions of ATP and TAN in the renal cortex and significantly attenuated them in the liver, yet doubled the renal cortical accumulation of F-1-P. (c) Adenosine loading substantially attenuated the reductions of ATP, TAN, and P(i) only in the renal cortex. (d) ATP varied directly with P(i) (P < 0.001, r = 0.98) in the domain of control and reduced values of P(i) taken from both liver and renal cortex. (e) As judged from tissue and plasma concentrations of fructose and glucose, and tissue concentrations of fructose-6-phosphate and glucose-6-phosphate, the rate at which the renal cortex and liver converted fructose to glucose was much lower at the higher fructose load. (f) Prior phosphate loading prevented this decrease in rate in the renal cortex and attenuated it in the liver; adenosine loading attenuated it only in the renal cortex. We conclude that in both the renal cortex of the fructose-loaded rat: (a) Depletion of P(i) is critical to the causation of the reductions in both ATP and TAN and, at the higher fructose load, of a decrease in the rate at which ATP is regenerated. (b) The severity of depletion of P(i) determines the severity of these disturbances. (c) By differentially mitigating the depletion of P(i), prior phosphate loading largely prevents these disturbances in the renal cortex, and attenuates them in the liver; and adenosine loading attenuates them only in the renal cortex. The findings provide some basis for the observation that in patients with hereditary fructose intolerance experimentally exposed to fructose, prior loading with sodium phosphate substantially attenuates the renal but not hepatic dysfunction.