Stimulation of glycogenolysis by adenine nucleotides in the perfused rat liver

A purinergic mechanism underlying metformin regulation of hyperglycemia

Metformin, created in 1922, has been the first-line therapy for treating type 2 diabetes mellitus for almost 70 years; however, its mechanism of action remains controversial, partly because most prior studies used supratherapeutic concentrations exceeding 1 mM despite therapeutical blood concentrations of metformin being less than 40 μM. Here we report metformin, at 10-30 μM, blocks high glucose-stimulated ATP secretion from hepatocytes mediating its antihyperglycemic action. Following glucose administration, mice demonstrate increased circulating ATP, which is prevented by metformin. Extracellular ATP through P2Y₂ receptors (P2Y₂R) suppresses PIP₃ production, compromising insulin-induced AKT activation while promoting hepatic glucose production. Furthermore, metformin-dependent improvements in glucose tolerance are abolished in P2Y₂R-null mice. Thus, removing the target of extracellular ATP, P2Y₂R, mimics the effects of metformin, revealing a new purinergic antidiabetic mechanism for metformin. Besides unraveling long-standing questions in purinergic control of glucose homeostasis, our findings provide new insights into the pleiotropic actions of metformin.

Neuromodulatory effect of the combination of metformin and vitamin D3 triggered by purinergic signaling in type 1 diabetes induced-rats

Several studies have indicated the vitamin D deficiency in the development of macro- and microvascular complications of diabetes mellitus (DM) including DM-related cognitive dysfunction. The purinergic system plays an important role in the modulation of a variety of mechanisms, including neuroinflammation, plasticity, and cell-cell communication. In addition, purines, their receptors, and enzymes can regulate the purinergic axis at different levels in type 1 DM (T1DM). This study evaluated the effects of vitamin D₃ alone or in combination with metformin in the behavioral performance of streptozotocin-induced T1DM rats. The effects of this combination on the metabolism of ATP and ADP were also studied by NTPDase (CD39), AMP by 5'-nucleotidase (CD73), and adenosine by adenosine deaminase (E-ADA) in the brain and peripheral lymphocytes of type 1 diabetic STZ-induced rats. The results showed that anxiety and memory loss from the DM condition reverted after 30 days of vitamin D₃ treatment. Furthermore, the DM state affected systemic enzymes, with no effect on the central enzymes hydrolyzing extracellular nucleotides and nucleosides. Vitamin D₃ treatment positively regulated ectonucleotidase (NTPDase and 5'-nucleotidase) activity, E-ADA, and the purinergic receptors as a mechanism to prevent oxidative damage in the cerebral cortex of T1DM rats. A neuroprotector effect of vitamin D₃ through adenosine signaling was also observed, by regulating A1 and A_2A receptors proteins levels. The present findings suggest that purinergic signaling through vitamin D₃ modulation may be a novel alternative strategy for T1DM treatment, and may compensate for the negative changes in the central nervous system.

Pathophysiological Role of Purinergic P2X Receptors in Digestive System Diseases

P2X receptors (P2XRs) are trimeric, non-selective cation channels activated by extracellular ATP and widely distributed in the digestive system. P2XRs have an important role in the physiological function of the digestive system, such as neurotransmission, ion transports, proliferation and apoptosis, muscle contraction, and relaxation. P2XRs can be involved in pain mechanisms both centrally and in the periphery and confirmed the association of P2XRs with visceral pain. In the periphery, ATP can be released as a result of tissue injury, visceral distension, or sympathetic activation and can excite nociceptive primary afferents by acting at homomeric P2X(3)R or heteromeric P2X(2/3)R. Thus, peripheral P2XRs, and homomeric P2X(3) and/or heteromeric P2X(2/3)R in particular, constitute attractive targets for analgesic drugs. Recently studies have shown that P2XRs have made significant advances in inflammation and cancer. P2X7R mediates NLRP3 inflammasome activation, cytokine and chemokine release, T lymphocyte survival and differentiation, transcription factor activation, and cell death. The P2X7R is a potent stimulant of inflammation and immunity and a promoter of cancer cell growth. This makes P2X7R an appealing target for anti-inflammatory and anti-cancer therapy. It is believed that with the further study of P2XRs and its subtypes, P2XRs and its specific antagonists will be expected to be widely used in the treatment of human digestive diseases in the future.

Receptor-specific Ca2+ oscillation patterns mediated by differential regulation of P2Y purinergic receptors in rat hepatocytes

Extracellular agonists linked to inositol-1,4,5-trisphosphate (IP₃) formation elicit cytosolic Ca²⁺ oscillations in many cell types, but despite a common signaling pathway, distinct agonist-specific Ca²⁺ spike patterns are observed. Using qPCR, we show that rat hepatocytes express multiple purinergic P2Y and P2X receptors (R). ADP acting through P2Y1R elicits narrow Ca²⁺ oscillations, whereas UTP acting through P2Y2R elicits broad Ca²⁺ oscillations, with composite patterns observed for ATP. P2XRs do not play a role at physiological agonist levels. The discrete Ca²⁺ signatures reflect differential effects of protein kinase C (PKC), which selectively modifies the falling phase of the Ca²⁺ spikes. Negative feedback by PKC limits the duration of P2Y1R-induced Ca²⁺ spikes in a manner that requires extracellular Ca²⁺. By contrast, P2Y2R is resistant to PKC negative feedback. Thus, the PKC leg of the bifurcated IP₃ signaling pathway shapes unique Ca²⁺ oscillation patterns that allows for distinct cellular responses to different agonists.

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Distinct stereotypic Ca²⁺ oscillations are elicited by P2Y1 and P2Y2 receptors

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P2X receptors do not contribute to the generation of Ca²⁺ oscillations

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Agonist-specific Ca²⁺ spike shapes reflect discrete modes of PKC negative feedback

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Bifurcation of IP₃/PKC signaling yields unique Ca²⁺ oscillation signatures

An Intermediary Role of Adenine Nucleotides on Free Fatty Acids-Induced Hyperglycemia in Obese Mice

Increased plasma free fatty acids (FFA) level plays a central role in the development of type 2 diabetes. Our previous studies have shown that plasma 5'-adenosine monophosphate (5'-AMP) elevates and acts as a potential upstream regulator of hyperglycemia in diabetic db/db mice. The relationship between FFA and plasma adenosine nucleotides in type 2 diabetes remains unclear. Here we found that plasma 5'-AMP level was also increased in diabetic mice induced by a high-fat diet and streptozotocin (HFD-STZ), as observed in diabetic db/db mice. The metabolites of adenosine nucleotides in plasma were increased in obese mice compared to lean mice. An acute oil gavage to lean mice increased both FFA and plasma purine metabolites, accompanying with glucose intolerance. 5'-AMP administration resulted in an increase in dose-dependent purine metabolites and different levels of glucose intolerance. FFA induced a release of adenine nucleotides from cultural human umbilical vein endothelial cells (HUVECs) prior to induction of their apoptosis. FFA also reduced red blood cells (RBCs) resistance to reactive oxygen species (ROS), leading to hemolysis, thereby increasing plasma nucleotides. Our results suggest that plasma adenine nucleotides play an intermediary role in FFA-induced glucose intolerance and hyperglycemia in obese mice.

Adenosine stimulates hepatic glycogenolysis via adrenal glands-liver crosstalk in mice

Adenosine signaling is involved in glucose metabolism in hepatocytes and myocytes in vitro. However, no information is available regarding the effect of adenosine on glucose metabolism in vivo. Thus, we examined how extracellular adenosine acts on glucose metabolism using mice. Subcutaneous injections of adenosine (10, 25, and 50 mg/kg bodyweight) dose-dependently increased blood glucose levels, with the peak occurring at 30 min post injection. At 30 min after adenosine injection (25 mg/kg bodyweight), glycogen content in the liver, but not the skeletal muscle, was significantly decreased. Hepatic glycogen depletion by fasting for 12 h suppressed the increase of blood glucose levels at 30 min after adenosine injection. These results suggest that adenosine increases blood glucose levels by stimulating hepatic glycogenolysis. To investigate the effect of adenosine on the adrenal gland, we studied the glycogenolysis signal in adrenalectomized (ADX) mice. Adenosine significantly increased the blood glucose levels in sham mice but not in the ADX mice. The decrease in hepatic glycogen content induced by adenosine in the sham mice was partially suppressed in the ADX mice. The level of plasma corticosterone, the main glucocorticoid in mice, was significantly increased in the sham mice by adenosine but its levels were low in ADX mice injected with either PBS or adenosine. These results suggest that adenosine promotes secretion of corticosterone from the adrenal glands, which causes hepatic glycogenolysis and subsequently the elevation of blood glucose levels. Our findings are useful for clarifying the physiological functions of adenosine in glucose metabolism in vivo.

Purinergic signalling in the liver in health and disease

Purinergic signalling is involved in both the physiology and pathophysiology of the liver. Hepatocytes, Kupffer cells, vascular endothelial cells and smooth muscle cells, stellate cells and cholangiocytes all express purinoceptor subtypes activated by adenosine, adenosine 5'-triphosphate, adenosine diphosphate, uridine 5'-triphosphate or UDP. Purinoceptors mediate bile secretion, glycogen and lipid metabolism and indirectly release of insulin. Mechanical stress results in release of ATP from hepatocytes and Kupffer cells and ATP is also released as a cotransmitter with noradrenaline from sympathetic nerves supplying the liver. Ecto-nucleotidases play important roles in the signalling process. Changes in purinergic signalling occur in vascular injury, inflammation, insulin resistance, hepatic fibrosis, cirrhosis, diabetes, hepatitis, liver regeneration following injury or transplantation and cancer. Purinergic therapeutic strategies for the treatment of these pathologies are being explored.

Purinergic effects of a hydroalcoholic Agaricus brasiliensis (A. blazei) extract on liver functions

The effects of a hydroalcoholic extract of Agaricus brasiliensis (A. blazei) on functional parameters in the perfused rat liver were examined with emphasis on its content of nucleotides and nucleosides. Several nucleosides and nucleotides were identified in the A. brasiliensis extract, which was active on several liver functions. A significant part of the effects is the result of the purinergic action of nucleosides and nucleotides: pressure increment, glycogenolysis stimulation, transient inhibition of oxygen consumption, and redox state changes. Other phenomena such as the stimulation of gluconeogenesis, ureogenesis, and oxygen consumption are more likely consequences of the metabolic transformation of substrates contained within the extract, especially amino acids. It seems apparent that consumption of A. brasiliensis represents not only the ingestion of metabolic precursors but also the ingestion of substances that, even at low concentrations, can exert important signaling functions in the liver as well as in the organism as a whole.

Modification of purinergic signaling in the hippocampus of streptozotocin-induced diabetic rats

Diabetic encephalopathy is a recognized complication of untreated diabetes resulting in a progressive cognitive impairment accompanied by modification of hippocampal function. The purinergic system is a promising novel target to control diabetic encephalopathy since it might simultaneously control hippocampal synaptic plasticity and glucose handling. We now tested whether streptozotocin-induced diabetes led to a modification of extracellular ATP homeostasis and density of membrane ATP (P2) receptors in the hippocampus, a brain structure involved in learning and memory. The extracellular levels of ATP, evaluated in the cerebrospinal fluid, were reduced by 60.4+/-17.0% in diabetic rats. Likewise, the evoked release of ATP as well as its extracellular catabolism was also decreased in hippocampal nerve terminals of diabetic rats by 52.8+/-10.9% and 38.7+/-6.5%, respectively. Western blot analysis showed that the density of several P2 receptors (P2X(3,5,7) and P2Y(2,6,11)) was decreased in hippocampal nerve terminals. This indicates that the synaptic ATP signaling is globally depressed in diabetic rats, which may contribute for diabetes-associated decrease of synaptic plasticity. In contrast, the density of P2 receptors (P2X(1,2,5,6,7) and P2Y(6) but not P2Y(2)) increased in whole hippocampal membranes, suggesting an adaptation of non-synaptic P2 receptors to sense decreased levels of extracellular ATP in diabetic rats, which might be aimed at preserving the non-synaptic purinergic signaling.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Non-synaptic transmission at autonomic neuroeffector junctions

Non-synaptic transmission is characteristic of autonomic neuroeffector junctions. The structure of the autonomic neuromuscular junction is described. The essential features are that: the terminal portions of autonomic nerve fibers are varicose and mobile, transmitters being released 'en passage' from varying distances from the effector cells; while there is no structural post-junctional specialization on effector cells, receptors for neurotransmitters accumulate on cell membranes at close junctions; muscle effectors are bundles rather than single smooth muscle cells, that are connected by gap junctions which allow electrotonic spread of activity between cells. A multiplicity of transmitters are utilized by autonomic nerves, and cotransmission occurs often involving synergistic actions of the cotransmitters, although pre- and post-junctional neuromodulation of neurotransmitter release also take place. It is suggested that autonomic neural control of immune, epithelial and endothelial cells also involves non-synaptic transmission.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Inhibition by extracellular ATP of organic anion transport in the perfused rat liver

The action of extracellular ATP on organic anion transport in the bivascularly perfused rat liver was investigated, using bromosulfophthalein as a model substance. Transport was measured by means of the multiple-indicator dilution technique. The action of portal 100 microM ATP presented the following characteristics: (a) inhibition of bromosulfophthalein single pass extraction; the inhibition degree decreased with increasing bromosulfophthalein doses; (b) diminution of the influx rate coefficients; (c) 86.7% decrease of the maximal activity of the saturable component for bromosulfophthalein transport, but 100% increase of the non-saturable component; (d) diminution of the bromosulfophthalein flow-limited distribution space; (e) no significant alteration of the rate coefficients for metabolic sequestration. The action of ATP on organic anion transport in the intact liver occurred at much lower concentrations (10x) than those previously reported for isolated hepatocytes. This reinforces the suggestion that inhibition of organic anion transport could be a physiologically relevant effect of extracellular ATP.

Changes in distribution spaces and cell permeability caused by ATP in the rat liver

AIMS/BACKGROUND

Cellular and extracellular volume changes caused by ATP were investigated in the liver as well as the possible formation of diffusion barriers, which could be responsible for some of its metabolic effects.

METHODS

The experimental system was the bivascularly perfused rat liver. [(14)C]Sucrose and [(3)H]water were simultaneously injected into either the portal vein or the hepatic artery. Mean transit times, distribution spaces, variances and linear superimpositions were calculated.

RESULTS

In the portal system, ATP reduced the transit time in the great vessels, had little or no effect on sinusoidal and cellular spaces, but impaired the flow-limited distribution of both [(14)C]sucrose and [(3)H]water. In the arterial bed ATP infused into either the portal vein or the hepatic artery produced vasodilation and increased the aqueous extra-sucrose space. These effects were inhibited by Nomega-nitro-L-arginine methyl ester infused into the hepatic artery.

CONCLUSIONS

Sucrose and extra-sucrose space changes caused in the arterial bed by portally infused ATP are most probably analogous to the transhepatic vasodilation effect already described for the rabbit liver. Impairment of flow-limited distribution of tracers in the sinusoidal bed indicates that ATP induces the formation of permeability barriers, which could be responsible for some of its metabolic effects.

Exogenous Mg-ATP induces a large inhibition of pyruvate kinase in intact rat hepatocytes

Mg-ATP infusion in vivo has been reported to be beneficial both to organ function and survival rate in various models of shock. Moreover, a large variety of metabolic effects has been shown to occur in several tissues due to purinergic receptor activation. In the present work we studied the effects of exogenous Mg-ATP in rat liver cells perifused with dihydroxyacetone to investigate simultaneously gluconeogenetic and glycolytic pathways. We found a significant effect on oxidative phosphorylation as characterized by a decrease in oxygen consumption rate and in the cellular ATP-to-ADP ratio associated with an increase in lactate-to-pyruvate ratio. In addition, exogenous Mg-ATP induced rapid and reversible inhibition of both gluconeogenesis and glycolysis. The main effect on gluconeogenesis was located at the level of the fructose cycle, whereas the decrease in glycolysis was due to a strong inhibition of pyruvate kinase. Although pyruvate kinase inhibition induced by exogenous Mg-ATP was allosteric when assessed in vitro after enzyme extraction, we found a large decrease in the apparent maximal velocity when kinetics were assessed in vivo in intact perifused hepatocytes. This newly described short-term regulation of pyruvate kinase occurs only in the intact cell and may open new potentials for the pharmacological regulation of pyruvate kinase in vivo.

Aggravation of chemically-induced injury in perfused rat liver by extracellular ATP

The effects of purinergic receptor agonists on acute liver damage and hemodynamics were studied using chemically-induced liver injury. Rat livers were perfused in situ 24 h after treatment with D-galactosamine (800 mg/kg, i.p.). In these livers, infusion of ATP (50 microM) into the portal vein caused a rapid increase in the leakage of LDH and AST from perfused liver in a dose dependent manner, accompanied with flow reduction. The similar but less effective responses were also observed by the infusion of ADP. Infusion of adenosine, a P1-receptor agonist, induced only minimal changes of liver damage and flow rate. The ATP-induced changes were almost completely suppressed by P2-receptor antagonist, suramin, but not affected by P1-receptor antagonist, 8-phenyltheophylline. Pretreatment of rats with gadolinium chloride, which depletes Kupffer cells, did not inhibit the potentiation of liver damage caused by ATP, whereas hemodynamic effects of ATP were significantly attenuated by gadolinium. These results indicate that extracellular ATP aggravates acute liver injury mediated by P2-type purinergic receptors.

Adenosine triphosphate: established and potential clinical applications

Adenosine 5'-triphosphate (ATP) is a purine nucleotide found in every cell of the human body. In addition to its well established role in cellular metabolism, extracellular ATP and its breakdown product adenosine, exert pronounced effects in a variety of biological processes including neurotransmission, muscle contraction, cardiac function, platelet function, vasodilatation and liver glycogen metabolism. These effects are mediated by both P1 and P2 receptors. A cascade of ectonucleotidases plays a role in the effective regulation of these processes and may also have a protective function by keeping extracellular ATP and adenosine levels within physiological limits. In recent years several clinical applications of ATP and adenosine have been reported. In anaesthesia, low dose adenosine reduced neuropathic pain, hyperalgesia and ischaemic pain to a similar degree as morphine or ketamine. Postoperative opioid use was reduced. During surgery, ATP and adenosine have been used to induce hypotension. In patients with haemorrhagic shock, increased survival was observed after ATP treatment. In cardiology, ATP has been shown to be a well tolerated and effective pulmonary vasodilator in patients with pulmonary hypertension. Bolus injections of ATP and adenosine are useful in the diagnosis and treatment of paroxysmal supraventricular tachycardias. Adenosine also allowed highly accurate diagnosis of coronary artery disease. In pulmonology, nucleotides in combination with a sodium channel blocker improved mucociliary clearance from the airways to near normal in patients with cystic fibrosis. In oncology, there are indications that ATP may inhibit weight loss and tumour growth in patients with advanced lung cancer. There are also indications of potentiating effects of cytostatics and protective effects against radiation tissue damage. Further controlled clinical trials are warranted to determine the full beneficial potential of ATP, adenosine and uridine 5'-triphosphate.

Regulation of cytosolic free calcium concentration by extracellular nucleotides in human hepatocytes

The effects of extracellular ATP and other nucleotides on the cytosolic free Ca2+ concentration ([Ca2+]i) have been studied in single primary human hepatocytes and in human Hep G2 and HuH-7 hepatoma cells. ATP, adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS), and UTP caused a concentration-dependent biphasic increase in [Ca2+]i with an initial peak followed by a small sustained plateau in most cells. In some cells, however, repetitive Ca2+ transients were observed. The rank order of potency was ATP >/= UTP > ATPgammaS, and complete cross-desensitization of the Ca2+ responses occurred between ATP and UTP. The initial transient peak in [Ca2+]i was resistant to extracellular Ca2+ depletion, which demonstrates mobilization of internal Ca2+ by inositol 1,4,5-trisphosphate whose formation was enhanced by ATP and UTP. In contrast, the sustained plateau phase required influx of external Ca2+. Ca2+ influx occurs most likely through a capacitative Ca2+ entry mechanism, which was shown to exist in these cells by experiments performed with thapsigargin. On the molecular level, specific mRNA coding for the human P2Y1, P2Y2, P2Y4, and P2Y6 receptors could be detected by RT-PCR in Hep G2 and HuH-7 cells. However, ADP and UDP, which are agonists for P2Y1 and P2Y6 receptors, respectively, caused no changes in [Ca2+]i, demonstrating that these receptors are not expressed at a functional level. Likewise, alpha,beta-methylene-ATP, beta,gamma-methylene-ATP, AMP, and adenosine were inactive in elevating [Ca2+]i, suggesting that the ATP-induced increase in [Ca2+]i was not caused by activation of P2X or P1 receptors. Thus, on the basis of the pharmacological profile of the nucleotide-induced Ca2+-responses, extracellular ATP and UTP increase [Ca2+]i by activating P2Y2 and possibly P2Y4 receptors coupled to the Ca2+-phosphatidylinositol signaling cascade in human hepatocytes. This suggests that extracellular nucleotides from various sources may contribute to the regulation of human liver cell functions.

Growth, development and differentiation: a functional food science approach

Few other aspects of food supply and metabolism are of greater biological importance than the feeding of mothers during pregnancy and lactation, and of their infants and young children. Nutritional factors during early development not only have short-term effects on growth, body composition and body functions but also exert long-term effects on health, disease and mortality risks in adulthood, as well as development of neural functions and behaviour, a phenomenon called 'metabolic programming'. The interaction of nutrients and gene expression may form the basis of many of these programming effects and needs to be investigated in more detail. The relation between availability of food ingredients and cell and tissue differentiation and its possible uses for promoting health and development requires further exploration. The course of pregnancy, childbirth and lactation as well as human milk composition and the short- and long-term outcome of the child are influenced by the intake of foods and particularly micronutrients, e.g. polyunsaturated fatty acids, Fe, Zn and I. Folic acid supplementation from before conception through the first weeks of pregnancy can markedly reduce the occurrence of severe embryonic malformations; other potential benefits of modulating nutrient supply on maternal and child health should be further evaluated. The evaluation of dietary effects on child growth requires epidemiological and field studies as well as evaluation of specific cell and tissue growth. Novel substrates, growth factors and conditionally essential nutrients (e.g. growth factors, amino acids, polyunsaturated fatty acids) may be potentially useful as ingredients in functional foods and need to be assessed carefully. Intestinal growth, maturation, and adaptation as well as long-term function may be influenced by food ingredients such as oligosaccharides, gangliosides, high-molecular-mass glycoproteins, bile salt-activated lipase, pre- and probiotics. There are indications for some beneficial effects of functional foods on the developing immune response, for example induced by antioxidant vitamins, trace elements, fatty acids, arginine, nucleotides, and altered antigen contents in infant foods. Peak bone mass at the end of adolescence can be increased by dietary means, which is expected to be of long-term importance for the prevention of osteoporosis at older ages. Future studies should be directed to the combined effects of Ca and other constituents of growing bone, such as P, Mg and Zn, as well as vitamins D and K, and the trace elements F and B. Pregnancy and the first postnatal months are critical time periods for the growth and development of the human nervous system, processes for which adequate substrate supplies are essential. Early diet seems to have long-term effects on sensory and cognitive abilities as well as behaviour. The potential beneficial effects of a balanced supply of nutrients such as I, Fe, Zn and polyunsaturated fatty acids should be further evaluated. Possible long-term effects of early exposure to tastes and flavours on later food choice preferences may have a major impact on public health and need to be further elucidated. The use of biotechnology and recombinant techniques may offer the opportunity to include various bioactive substances in special dietary products, such as human milk proteins, peptides, growth factors, which may have beneficial physiological effects, particularly in infancy and early childhood.

Hepatic heterogeneity in the response to ATP studied in the bivascularly perfused rat liver

The zonation of the purinergic action of ATP in the hepatic parenchyma was investigated in the bivascularly perfused rat liver by means of anterograde and retrograde perfusion. Livers from fed rats were used, and ATP was infused according to four different experimental protocols: (A) anterograde perfusion and ATP infusion via the portal vein; (B) anterograde perfusion and ATP via the hepatic artery; (C) retrograde perfusion and ATP via the hepatic vein; (D) retrograde perfusion and ATP via the hepatic artery. The following metabolic parameters were measured: glucose release, lactate production and oxygen consumption. The hemodynamic effects were evaluated by measuring the sinusoidal mean transit times by means of the indicator-dilution technique. ATP was infused during 20 min at four different rates (between 0.06-0.77 micromol min[-1] g liver[-1]; 20-200 microM) in each of the four experimental protocols. The results that were obtained allow several conclusions with respect to the localization of the effects of ATP along the hepatic acini: (1) In retrograde perfusion the sinusoidal mean transit times were approximately twice those observed in anterograde perfusion. ATP increased the sinusoidal mean transit times only in retrograde perfusion (protocols C and D). The effect was more pronounced with protocol D. These results allow the conclusion that the responsive vasoconstrictive elements are localized in a pre-sinusoidal region; (2) All hepatic cells, periportal as well as perivenous, were able to metabolize ATP, so that concentration gradients were generated with all experimental protocols. Extraction of ATP was more pronounced in retrograde perfusion, an observation that can be attributed, partly at least, to the longer sinusoidal transit times. In anterograde perfusion, the extraction of ATP was time-dependent, a phenomenon that cannot be satisfactorily explained with the available data; (3) ATP produced a transient initial inhibition of oxygen uptake when protocols A and B were employed. These protocols are the only ones in which the cells situated shortly after the intrasinusoidal confluence of the portal vein and the hepatic artery were effectively supplied with ATP. The decrease in oxygen consumption was more pronounced at low ATP infusions when protocol B was employed. These observations allow the conclusion that the former phenomenon is localized mainly in cells situated shortly after the intrasinusoidal confluence of the portal vein and hepatic artery. Oxygen consumption in all other cells, especially the proximal periportal ones, is increased by ATP; (4) In agreement with previous data found in the literature, glycogenolysis stimulation by ATP was more pronounced in the periportal region. The cells that respond more intensively are not the proximal periportal ones, but those situated in the region of the intrasinusoidal confluence of the portal vein and the hepatic artery.

Loss of glycogen during preconditioning is not a prerequisite for protection of the rabbit heart

Depletion of glycogen has been proposed as the mechanism of protection from ischemic preconditioning. The hypothesis was tested by seeing whether pharmacological manipulation of preconditioning causes parallel changes in cardiac glycogen content. Five groups of isolated rabbit hearts were studied. Group 1 experienced 30 min of ischemia only. Group 2 (PC) was preconditioned with 5 min of global ischemia followed by 10 min of reperfusion. Group 3 was preconditioned with 5 min exposure to 400 nM bradykinin followed by a 10 min washout period. Group 4 experienced exposure to 10 microM adenosine followed by a 10 min washout period, and the fifth group was also preconditioned with 5 min ischemia and 10 min reperfusion but 100 microM 8-(p-sulfophenyl)theophylline (SPT), which blocks adenosine receptors, was included in the buffer to block preconditioning's protection. Transmural biopsies were taken before treatment, just prior to the 30 min period of global ischemia, and after 30 min of global ischemia. Glycogen in the samples was digested with amyloglucosidase and the resulting glucose was assayed. Baseline glycogen averaged 17.3 +/- 0.6 mumol glucose/g wet weight. After preconditioning glycogen decreased to 13.3 +/- 1.3 mumol glucose/g wet weight (p < 0.005 vs. baseline). Glycogen was similarly depleted after pharmacological preconditioning with adenosine (14.0 +/- 1.0 mumol glucose/g wet weight, p < 0.05 vs. baseline) suggesting a correlation. However, when preconditioning was performed in the presence of SPT, which blocks protection, glycogen was also depleted by the same amount (13.3 +/- 0.7 mumol glucose/g wet weight, p = ns vs. PC). Bradykinin, which also mimics preconditioning, caused no depletion of glycogen (16.3 +/- 0.8 mumol glucose/g wet weight, p = ns vs. baseline). Because preconditioning with bradykinin did not deplete glycogen and because glycogen continued to be low when protection from preconditioning was blocked with SPT, we conclude that loss of glycogen per se does not cause the protection of preconditioning.

Partial characterization of a new nucleotide binding glycoprotein of hepatocyte plasma membrane

Hepatocyte plasma membranes contain a glycosylated 230-kDa Ca(2+) -dependent, Mg(2+)-stimulated ATPase (pgp230), which consists of two subunits, one of 120 kDa and the other of 110 kDa. pgp230 can be enriched by the use of affinity chromatography on Concanavalin A-Sepharose, wheat germ lectin-Sepharose, and 5'-AMP-Sepharose. It has a high-affinity Ca2+ binding site. In the presence of Ca2+, it forms a phosphorylated intermediate by autocatalytic transfer of the terminal phosphate residue from ATP. Maximal Ca(2+)-dependent autophosphorylation is observed at pH 5-6. Photoaffinity labeling using 8-azido-[alpha-32P]ATP or [y-32P]ATP confirms the presence of ATP binding sites. Incubation with [alpha-32P]ATP leads to a rapid but transient labeling of pgp230. Various nucleotides, nucleotide receptor agonists, or antagonists inhibit Ca(2+)-dependent phosphorylation by [y-32P]ATP. The concentrations of half-maximal inhibition range from 10(-7) M to 10(-3) M. The rank order of inhibitory potency is: ATP > alpha,beta-methylene-ATP > CTP = TTP > y-4-amino-phenyl-ATP = 2-methyl-thio-ATP > UTP = GTP > GDP = ADP = beta,y-methylene-ATP = beta, y-methylene-TTP = beta,y-methylene-GTP = adenosine-5'-O-2-thiodiphosphate = CMP = AMP > adenosine > cytidine > guanosine = suramin > Reactive blue 2 > iso-butyl-methyl-xanthine > thymidine > uridine. These data suggest a nucleotide binding capacity of this new hepatocyte membrane glycoprotein. Further investigations should be carried out to reveal its biological function.

The mechanism of action of hepatic sympathetic nerves on ketone-body output from perfused rat liver. The effect of the interaction of noradrenaline with ATP on the release of beta-hydroxybutyrate

The regulatory mechanism of ketone-body output by the hepatic sympathetic nerves was studied in rat liver perfused in situ. Enrichment of the perfusion medium with 1 mM octanoate increased the basal ketone-body output from the liver up to 1.5 mumol.min-1.g liver-1. Under these conditions, electrical stimulation of the hepatic nerves (20 V, 20 Hz, 2 ms) decreased the output of both acetoacetate and beta-hydroxybutyrate, and was accompanied by an accumulation of beta-hydroxybutyrate in the liver. The effects of nerve stimulation were inhibited by the alpha 1-antagonist bunazosin (10 microM). However, noradrenaline, a typical sympathetic neurotransmitter, at a concentration of 1 microM decreased the output of acetoacetate but did not affect beta-hydroxybutyrate output. Prostaglandin F2 alpha at a concentration of 10 microM produced an effect similar to treatment with noradrenaline, without a decrease in beta-hydroxybutyrate output. ATP at 50 microM, however, decreased the output of both acetoacetate and beta-hydroxybutyrate and increased the tissue concentration of beta-hydroxybutyrate, mimicking the effect of nerve stimulation. Moreover, in the presence of 0.2 microM ATP, a concentration that produced neither metabolic nor hemodynamic changes, noradrenaline (1 microM) was shown to decrease the beta-hydroxybutyrate output. These results indicate the possible involvement of ATP in the action of hepatic sympathetic nerves on beta-hydroxybutyrate output from the liver, presumably through its interaction with noradrenaline.

Adenosine inhibits protein synthesis in isolated rat hepatocytes. Evidence for a lack of involvement of intracellular calcium in the mechanism of inhibition

Extracellularly added adenosine and ATP are potent inhibitors of protein synthesis in liver cells. In this study, the possible involvement of Ca2+ in the mechanism of inhibition of protein synthesis by adenosine was investigated. Stimulation of freshly isolated hepatocytes with adenosine or ATP, at concentrations that impaired protein synthesis, induced an increase in the cytosolic free Ca2+ concentration ([Ca2+]i). However, there was no correlation between the increase in [Ca2+]i and inhibition of radiolabelled leucine incorporation into proteins. Thus, the stimulation of hepatocytes with the V1-receptor agonist, vasopressin, or with the nucleotide triphosphates, UTP and GTP, elicited changes in [Ca2+]i similar to those observed after ATP or adenosine addition, but did not affect protein synthesis. ATP produced near complete discharge of Ca2+ from the inositol 1,4,5-trisphosphate-sensitive Ca2+ pool in isolated hepatocytes, whereas adenosine only had a partial effect. Depletion of the hormone-sensitive Ca2+ pool by adenosine was transient. In contrast, prolonged depletion of internal Ca2+ by thapsigargin resulted in the inhibition of protein synthesis in hepatocytes. However, the inhibition of radiolabelled leucine incorporation into proteins by thapsigargin was further augmented by the additional presence of adenosine. These results show that the inhibition of protein synthesis by adenosine in isolated hepatocytes is not mediated by an increase in [Ca2+]i or depletion of internal pool(s) sensitive to inositol 1,4,5-trisphosphate or thapsigargin.

A new type of Ca(2+)-dependent, Mg(2+)-stimulated ATPase of rat liver plasma membrane

Incubation of a glycoprotein fraction obtained from rat liver plasma membrane which has been previously well characterized using [gamma-32P]ATP results in the phosphorylation of a 230-kDa glycoprotein (pgp230). It is composed of a 120-kDa subunit (pgp120) and a 110-kDa subunit (pgp110) linked by interchain disulfide bonds. Peptide maps of pgp120 and pgp110 suggest extensive similarity in their polypeptide chains. Glycan analysis reveals between four and six hybrid-type oligosaccharide chains for both phosphoproteins. Immunoblotting using monoclonal antibodies and endoglycosidase digestion exclude an identity of pgp120 or pgp110 with the hepatocyte plasma membrane glycoproteins dipeptidylpeptidase IV or the taurocholate transport protein, which co-purify and co-migrate in SDS/PAGE. Protein phosphorylation is Ca(2+)-dependent (K0.5(Ca2+) = 0.35 microM, in the absence of Mg2+). In the presence of Mg2+, the glycoprotein undergoes rapid cycles of phosphorylation and dephosphorylation, resulting in ATPase activity. Analysis of phosphorylated amino acids identifies phosphothreonine as the major one. Photoaffinity labeling with 8-azido-[alpha-32P]ATP demonstrates the presence of one or more ATP binding site(s). Preincubation of pgp230 with various purine or pyrimidine nucleotides (ATP, UTP, TTP, ADP, GDP, AMP, CMP) or known P2-purinoceptor agonists or antagonists (adenosine 5'-[alpha,beta-methylene]triphosphate, 2-methyl-thio-adenosine 5'-triphosphate, suramin) inhibits its phosphorylation by [gamma-32P]ATP. The biological function of pgp230 is unknown at present. Several findings of the present study are compatible with the idea that pgp230 may be involved in a P2-purinoceptor function of the hepatocyte. Following this concept, a mechanism is discussed where a cytosolically exposed high-affinity Ca(2+)-binding site of pgp230 would allow for receptor feedback control, via phosphorylation and dephosphorylation, by sensing changes in cytosolic Ca2+ concentration.

Dietary nucleotides affect hepatic growth and composition in the weanling mouse

The effect of dietary nucleotides upon hepatic growth and composition was examined in weanling mice. For 5 weeks, mice were fed either Purina Rat Chow, a nucleotide-free diet (NT-), a nucleotide-free diet supplemented with a mixture of five nucleotides (0.21% w/w), (NT+) or a nucleotide-free diet supplemented with adenosine 5'-monophosphate (0.0425% w/w) (NTA). Hepatic cholesterol and lipid phosphorous were significantly higher, whereas liver weight (expressed as a percentage of body weight), and glycogen were lower in animals fed NT- vs all other groups. NTA-fed animals presented a greater contrast to the NT- group than did animals fed the mixture of nucleotides. Liver fatty acid composition and distribution of phospholipid subclasses were not affected by dietary nucleotide supplementation. Dietary nucleotide supplementation in weanling mice affects hepatic growth and composition; adenosine 5'-monophosphate may play a unique role in these effects.

Signal transduction via P2-purinergic receptors for extracellular ATP and other nucleotides

Extracellular ATP, at micromolar concentrations, induces significant functional changes in a wide variety of cells and tissues. ATP can be released from the cytosol of damaged cells or from exocytotic vesicles and/or granules contained in many types of secretory cells. There are also efficient extracellular mechanisms for the rapid metabolism of released nucleotides by ecto-ATPases and 5'-nucleotidases. The diverse biological responses to ATP are mediated by a variety of cell surface receptors that are activated when ATP or other nucleotides are bound. The functionally identified nucleotide or P2-purinergic receptors include 1) ATP receptors that stimulate G protein-coupled effector enzymes and signaling cascades, including inositol phospholipid hydrolysis and the mobilization of intracellular Ca2+ stores; 2) ATP receptors that directly activate ligand-gated cation channels in the plasma membranes of many excitable cell types; 3) ATP receptors that, via the rapid induction of surface membrane channels and/or pores permeable to ions and endogenous metabolites, produce cytotoxic or activation responses in macrophages and other immune effector cells; and 4) ADP receptors that trigger rapid ion fluxes and aggregation responses in platelets. Current research in this area is directed toward the identification and structural characterization of these receptors by biochemical and molecular biological approaches.

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.

Activation of glycogenolysis by methotrexate. Influence of calcium and inhibitors of hormone action

The influence of Ca2+ and the possible action of hormone blockers on the activation of glycogenolysis by methotrexate were investigated. Methotrexate was inactive on glycogenolysis and oxygen uptake when the liver, depleted of intracellular Ca2+, was perfused with Ca(2+)-free medium. The action of methotrexate in calcium-depleted hepatocytes could be restored by the addition of extracellular Ca2+. When Ca2+ was absent in the extracellular medium, but the intracellular stores were not depleted, methotrexate produced transient and progressively attenuated increases in glycogenolysis and oxygen uptake. Like many agonists, methotrexate produced transient increases in Ca2+ efflux. The action of methotrexate was not blocked by the antagonists of norepinephrine, phenylephrine, isoproterenol, vasopressin and angiotensin II. It was concluded that methotrexate acts through a Ca(2+)-dependent mechanism, which is similar to that of the Ca(2+)-dependent agonists. This action, however, seems not to be receptor mediated.

Gluconeogenesis stimulated by extracellular ATP is triggered by the initial increase in the intracellular Ca2+ concentration of the periphery of hepatocytes

Extracellular ATP, ADP and GTP increased the intracellular free Ca2+ concentration ([Ca2+]i) in a suspension of isolated rat hepatocytes. The [Ca2+]i was determined by measuring fura-2 fluorescence, and its increase was biphasic. The initial transient rise was followed by a longer-lasting plateau. The peak of the early component preceded the plateau level of the second component. A time course of change in [Ca2+]i in single cells at 100 microM-ATP was very similar to that observed in the suspension system. Preincubation of hepatocytes with 40 mM-caffeine, 2 mM-oxalate or 60 microM-dantrolene sodium inhibited the P2 purinergic response. The plateau phase was not observed when measured in the presence of extracellular 100 microM-LaCl3 or in the absence of extracellular Ca2+. The distribution of [Ca2+]i in single hepatocytes was also determined by fluorescence image analysis. In the initial phase, the increase in [Ca2+]i is greater in the peripheral region than the central region of the cell. Degradation of extracellular ATP by ecto-ATPase in the hepatocyte suspension was measured; the amount of ATP degradation was less than 10-15% of the initial amount (100 microM) during the measurement of the intracellular [Ca2+]i in the cell suspension. Extracellular ATP stimulated glucose synthesis. The rate of glucose production also showed two components, the initial fast component within 1 min and the subsequent slower component. The rate of the initial fast component did not depend on the presence or absence of extracellular Ca2+, whereas the rate of the subsequent component depended on it. The present study shows that the initial transient rise in [Ca2+]i plays an important role in triggering the gluconeogenesis.

ATP diphosphohydrolase is responsible for ecto-ATPase and ecto-ADPase activities in bovine aorta endothelial and smooth muscle cells

An ATP diphosphohydrolase (EC 3.6.1.5) is an enzyme hydrolyzing pyrophosphate bonds in nucleoside di- and triphosphates with broad substrate specificity in the presence of divalent cations. The ATPase and ADPase activities in the enzyme purified to homogeneity from bovine aortic vessel wall were insensitive to oligomycin, ouabain, and various protease treatments, and sensitive to azide and Ap5A. Bovine aorta endothelial and smooth muscle cells were cultured separately to characterize the ectonucleotidase activities. The activities were dependent on the addition of divalent cations and had broad substrate specificity. The ecto-ATPase and -ADPase activities were insensitive to oligomycin, ouabain, and protease treatments, and sensitive to azide and Ap5A. No enzyme degrading only ADP was found in the aortic vessel wall. Moreover, antiserum raised against purified ATP diphosphohydrolase inhibited the ecto-ATPase and -ADPase activities. These results indicated that ecto-ATPase and ecto-ADPase are not separate enzymes but are expressed by one enzyme, ATP diphosphohydrolase.

Characterization of the biological effects of 2-methylthio-ATP on rat hepatocytes: clear-cut differences with ATP

1. In several tissues, 2-methylthio adenosine triphosphate (2MeSATP) is a very potent P2y-purine agonist. In rat hepatocytes, 2MeSATP half-maximally activated glycogen phosphorylase at 20 nM and was therefore about 25 times more effective than ATP (Ka 0.5-0.8 microM). This strong glycogenolytic potency of 2MeSATP suggests on its own the presence of P2Y-purinoceptors in liver. 2. Displacement of the radioligand ATP alpha[35S] from its receptor however occurred at much higher concentrations of 2MeSATP than was anticipated on the basis of its glycogenolytic potency. 3. The interaction of 2MeSATP with the receptor, characterized with ATP alpha[35S] as radioligand, cannot be considered as a pure competitive interaction. 4. 2MeSATP did not share the ability of ATP to counteract the effect of glucagon on the adenosine 3':5'-cyclic monophosphate levels. 5. 2MeSATP barely increased the levels of inositol trisphosphate (IP3). 6. The glycogenolytic effect of 2MeSATP was completely abolished by pretreatment of the hepatocytes with phorbol myristic acetate. 7. It is tentatively concluded that 2MeSATP and ATP are interacting with different P2 purinoceptors.

Biphasic rise caused by extracellular ATP in intracellular calcium concentration in bovine adrenocortical fasciculata cells

We examined the effect of extracellular ATP on intracellular calcium concentration ([Ca2+]i) in bovine adrenocortical fasciculata cells. The suspended cells and the monolayer cells on coverslips were loaded with fura2. We found that ATP caused a rapid transient rise [Ca2+]i in dose-dependent manner in the presence of extracellular Ca2+ and that the rise in [Ca2+]i caused by ATP was biphasic. We did not observe the second phase in the absence of extracellular Ca2+. These results suggest that the first rise in [Ca2+]i caused by ATP is due to Ca2+ release from intracellular Ca2+ stores and subsequent rise due to Ca2+ influx across the plasma membrane. Furthermore, we found that the first phase of the monolayer cells consisted of two peaks. So it is possible that Ca2+ release from intracellular Ca2+ stores caused by ATP is induced by two different manners in adrenocortical fasciculata cells. In addition, monolayer cells are more useful than suspended ones in the examination of Ca2+ mobilization in adrenocortical fasciculata cells.

Modulation of maximal glycogenolysis in perfused rat liver by adenosine and ATP

Rat livers perfused at constant flow via the portal vein with dibutyryl cyclic AMP produced glucose equivalents at a steady maximal rate (6 mumol/min per g of liver). Addition of adenosine (150 microM) caused a biphasic effect. (i) First, the glycogenolytic rate rose transiently, to a mean peak of 150% of control levels after 2 min. This glycogenolytic burst was reproduced by two P1-receptor agonists, but not by ATP, and was blocked by a P1-antagonist (8-phenyltheophylline), as well as by inhibitors of eicosanoid synthesis (indomethacin, ibuprofen or aspirin). It did not occur in phosphorylase-kinase-deficient livers. The adenosine-induced glycogenolytic burst coincided with moderate and transient changes in portal pressure (+6 cmH2O) and O2 consumption (-20%), but it could not be explained by an increase in cytosolic Pi, since the n.m.r. signal fell precipitously. (ii) Subsequently, the rate of glycogenolysis decreased to one-third of the preadenosine value, in spite of persistent maximal activation of phosphorylase. The decrease could be linked to the decline in cytosolic Pi: both changes were prevented by the adenosine kinase inhibitor 5-iodotubercidin, whereas they were not affected by ibuprofen or 8-phenyltheophylline, and were not reproduced by non-metabolized adenosine analogues. In comparison with adenosine, ATP caused a slower decrease of Pi and of glycogenolysis. The fate of the cytosolic Pi was unclear, especially with administered ATP, which did not increase the n.m.r.-detectable intracellular ATP.

Metabolic responses of isolated hepatocytes to adenosine; dependence on external calcium

The role of cyclic-adenosine monophosphate (cAMP) and calcium (Ca2+) in the metabolic responses to adenosine was studied in isolated hepatocytes from fed rats. In the presence of 1.2 mM Ca but not in the absence of Ca2+, adenosine stimulated ureagenesis without increasing cAMP. Adenosine inhibited the glucagon mediated increase in cAMP. Adenosine increased free cytoplasmic Ca2+ provided that cells were incubated in the presence of external Ca2+. In the absence of added Ca2+ adenosine did not stimulate ureagenesis or the movements of Ca2+. It is suggested that, in the liver cell, Ca2+ may be a second messenger for adenosine.

Overview. Purinergic mechanisms

An overview of the history of studies of the biological actions of extracellular ATP is presented. The basis of the subdivision of receptors for purines into P1-purinoceptors for adenosine and P2-purinoceptors for ATP and ADP are considered, as well as the recent proposal for subdivision of the ATP receptors into P2X-, P2Y-, P2Z-, and P2T- purinoceptor subtypes. These purinoceptor subtypes are discussed with respect to their transduction mechanisms, their distribution, and their physiological roles, including their roles in cotransmission and neuromodulation.

Stimulation of glucose production from glycogen by glucagon, noradrenaline and non-degradable adenosine analogues is counteracted by adenosine and ATP in cultured rat hepatocytes

The glycogenolytic potency of adenosine and ATP was studied in adult rat hepatocytes and compared with the action of glucagon and noradrenaline. In cells cultured for 48 h, adenosine and ATP as well as their analogues 2-chloroadenosine, phenylisopropyladenosine, N-ethylcarboxamidoadenosine and beta-gamma-methylene-substituted ATP (p[CH2]ppA) increased glycogen phosphorylase alpha to levels indistinguishable from those obtained by the addition of glucagon or noradrenaline. The P1 receptor antagonist 8-phenyltheophylline abolished the activation of phosphorylase by adenosine and by p[CH2]ppA, but not that by ATP. Protein kinase A was activated by p[CH2]ppA and ATP via their breakdown to adenosine. [14C]Glucose production from glycogen was stimulated only 3-fold by ATP and adenosine, compared with a 7-fold increase produced by the hormones. Stimulation of glucose production by glucagon or noradrenaline was almost completely abolished by ATP or adenosine, with half-maximal effects at around 10 microM. The non-degradable adenosine analogues were equipotent with glucagon with respect to stimulation of glucose production, and their action was also inhibited by adenosine. ATP and p[CH2]ppA, which were both degraded to adenosine, showed comparable metabolic effects, whereas the alpha, beta-methylene analogue was without biological action and also was not degraded to adenosine. In the presence of the adenosine transport inhibitor nitrobenzyl thioinosine (NBTI), adenosine exerted an increased glycogenolytic potency, reaching 80% of the maximal stimulation obtained by glucagon. The glucagon-antagonistic effect of adenosine could be completely abolished by NBTI, but was not affected by phenyltheophylline. It is concluded that, in the hepatocyte culture system, adenosine and ATP decrease the catalytic efficiency of phosphorylase alpha through signals arising from their uptake into the cell.

Stimulation of release of prostaglandin D2 and thromboxane B2 from perfused rat liver by extracellular adenosine

In isolated perfused rat liver, adenosine infusion (50 microM) led to increases in glucose output and portal pressure and a net K+ release of 3.7 +/- 0.21 mumol/g, which was followed by an equivalent net K+ uptake after cessation of the nucleoside infusion. These effects were accompanied by a transient stimulation of hepatic prostaglandin D2 and thromboxane B2 release. The Ca2+ release observed upon adenosine infusion (50 microM) was 23.5 +/- 5.2 nmol/g, i.e. 10-20% of the Ca2+ release observed with extracellular ATP (50 microM). Indomethacin (10 microM) prevented the adenosine-induced stimulation of glucose output and the increase in portal pressure by 79 and 63% respectively, and completely abolished the stimulation of prostaglandin D2 release. The thromboxane A2 receptor antagonist BM 13.177 (20 microM), the phospholipase A2 inhibitor 4-bromophenacyl bromide (20 microM) and the cyclo-oxygenase inhibitor ibuprofen (50 microM) also decreased the glycogenolytic and vasoconstrictive responses of the perfused rat liver upon adenosine infusion by 50-80%. When the indomethacin inhibition of adenosine-induced prostaglandin D2 release was titrated, a close correlation between prostaglandin D2 release and the metabolic and vascular responses to adenosine was observed. These findings suggest an important role for eicosanoids in mediating the nucleoside responses in the perfused rat liver. Since eicosanoids are known to be formed by non-parenchymal cells in rat liver [Decker (1985) Semin. Liver Dis. 5, 175-190], the present study gives further evidence for an important role of eicosanoids as signal molecules between the different liver cell populations.

Characterization of the purinoceptors present in rabbit and guinea pig liver

Micromolar concentrations of ATP induced a cAMP-independent glycogenolytic response in rabbit and guinea pig hepatocytes. With ATP alpha[35S] (adenosine 5'-[alpha-[35S]thio]triphosphate) as radioligand, we detected the presence of specific purinoceptors on hepatocytes and liver plasma membranes of both species. We determined a Kd value of 0.28 microM and a Bmax of 4.8 pmol/10(6) cells for rabbit hepatocytes and a Kd of 0.25 microM and a Bmax of 7.0 pmol/10(6) cells for guinea pig hepatocytes. The Kd values with purified plasma membranes from rabbit and guinea pig liver, were respectively 0.2 and 0.1 microM whereas the Bmax values were respectively 71 and 47 pmol/mg of protein. These purinoceptors belong to the P2Y-subclass as is evidenced by the high degree of similarity which exists between the binding affinities of several ATP analogues to either rabbit or guinea pig liver plasma membranes and rat liver plasma membranes, previously shown to possess P2Y-purinoceptors.

Hepatic circulation: potential for therapeutic intervention

In recent years, knowledge of the physiology and pharmacology of hepatic circulation has grown rapidly. Liver microcirculation has a unique design that allows very efficient exchange processes between plasma and liver cells, even when severe constraints are imposed upon the system, i.e. in stressful situations. Furthermore, it has been recognized recently that sinusoids and their associated cells can no longer be considered only as passive structures ensuring the dispersion of molecules in the liver, but represent a very sophisticated network that protects and regulates parenchymal cells through a variety of mediators. Finally, vascular abnormalities are a prominent feature of a number of liver pathological processes, including cirrhosis and liver cell necrosis whether induced by alcohol, ischemia, endotoxins, virus or chemicals. Although it is not clear whether vascular lesions can be the primary events that lead to hepatocyte injury, the main interest of these findings is that liver microcirculation could represent a potential target for drug action in these conditions.

Extracellular nucleotides stimulate receptor-mediated calcium mobilization and inositol phosphate production in human fibroblasts

In human skin fibroblasts, low concentrations of extracellular ATP stimulated 45Ca2+ efflux from a slow-turnover intracellular pool, accompanied by inositol phosphate generation. These effects of ATP were not due to a generalized increase in plasma-membrane permeability. The EC50 (concn. giving 50% stimulation) for ATP was dependent on Ca2+ and Mg2+ concentrations in a manner which indicates that a form of ATP uncomplexed with bivalent cations is the active species. The rank order of potency of nucleotides was: ATP = UTP greater than adenosine 5'-[gamma-thio]triphosphate greater than ITP greater than ADP greater than UDP greater than other nucleoside triphosphates. Adenosine 5'-[alpha beta-methylene]triphosphate, adenosine 5'-[beta gamma-methylene]triphosphate and 2-methylthio-ATP were inactive. Thus the nucleotide specificity of this receptor is different from that of previously characterized P2 purinoceptors. Nucleotide-stimulated 45Ca2+ mobilization and inositol phosphate production were markedly inhibited by phorbol ester, and partially inhibited by pertussis-toxin pretreatment. These findings suggest that the coupling of nucleotide receptor to phospholipase C is mediated both by a pertussis-toxin-sensitive G-protein and by a pertussis-toxin-insensitive mechanism.

Stimulation by insulin of glycolysis in cultured hepatocytes is attenuated by extracellular ATP and puromycin through purine-dependent inhibition of phosphofructokinase 2 activation

Activation of glycolysis by insulin in cultured rat hepatocytes is preceded by an activation of phosphofructokinase 2 (PFK 2) and subsequent rise of the fructose 2,6-bisphosphate [Fru(2,6)P2] level. Extracellular addition of ATP or puromycin prevented the hormonal effect on glycolysis. The mechanism through which the purines abolished glycolytic stimulation was investigated. 1. 50 microM ATP completely prevented the 3-5-fold insulin-dependent increase of glycolysis, irrespective of whether the cells initially possessed a low or a high Fru(2,6)P2 content. 50 microM puromycin prevented the stimulation of glycolysis by insulin only in cells whose initial Fru(2,6)P2 levels were low and had to be increased by insulin prior to the increase in glycolysis. It did not antagonize the action of insulin cells with initial high Fru(2,6)P2 content. 2. ATP exerted effects on its own; it decreased initially high Fru(2,6)P2 levels by 95% within 10 min and decreased the basal glycolytic rate by 60%. Half-maximal effects on the Fru(2,6)P2 level were obtained with about 25 microM ATP or 15 microM adenosine 5'[beta, gamma-methylene]triphosphate. ADP and adenosine-5-[gamma-thio]triphosphate were as effective as ATP, whereas 100 microM adenosine 5'[alpha, beta-methylene]triphosphate elicited no effect. Puromycin neither decreased high Fru(2,6)P2 levels nor inhibited basal glycolysis. 3. Extracellular ATP (100 microM) led to inhibition of the active form of PFK 2. Intracellular levels of Glc6P, citrate, ATP, ADP and AMP were increased by extracellular ATP, the phosphoenolpyruvate content was decreased, Fru6P and glycerol 3-phosphate levels stayed constant. Puromycin did not inhibit PFK 2. 4. Both puromycin and ATP prevented the insulin-dependent rise of the Fru(2,6)P2 level, they abolished the activation of PFK 2 by the hormone. Puromycin did not block the accumulation of Fru(2,6)P2 provoked by glucose addition; ATP also antagonized the glucose-dependent increase. 5. 100 microM ATP elevated the cAMP-dependent protein kinase activity ratio from 0.1 to 0.38 and increased the level of inositol trisphosphate by 16-fold within 5 min, whereas puromycin was without effect on either level. It is concluded that the two purines block the insulin effect on glycolysis by preventing the hormone increasing the Fru(2,6)P2 level. The mode of action, however, seems to be different: ATP antagonizes insulin action in that it leads to increased inhibition of PFK 2 whereas puromycin prevents the activation of PFK 2 by insulin.