Immunogold localization of adenosine 5'-monophosphate-specific cytosolic 5'-nucleotidase in dog heart

The decreased serum activity of cytosolic 5'-nucleotidase IA as a potential marker of breast cancer-associated muscle inflammation

Cytosolic 5'-nucleotidase IA (cN-IA) plays a central role in the regulation of the purine nucleotide pool in skeletal muscle, preferentially converting adenosine monophosphate to adenosine. cN-IA can act as an autoantigen in muscle diseases, including the paraneoplastic syndrome related to breast cancer (BC). As a result of myocyte damage, released cN-IA protein may trigger the production of anti-cN-IA antibodies (anti-NT5C1A). This work aimed to develop an effective method to measure cN-IA activity in the serum and analyze it in BC patients. Our study demonstrated that serum cN-IA activity was decreased in BC patients and we assumed it is due to the presence of specific autoantibodies. We found correlations between cN-IA activity and parameters of inflammatory muscle damage. Thus, cN-IA is worth further attention to clarify its usefulness as a biomarker of BC-associated polymyositis.

Cardiac purinergic signalling in health and disease

This review is a historical account about purinergic signalling in the heart, for readers to see how ideas and understanding have changed as new experimental results were published. Initially, the focus is on the nervous control of the heart by ATP as a cotransmitter in sympathetic, parasympathetic, and sensory nerves, as well as in intracardiac neurons. Control of the heart by centers in the brain and vagal cardiovascular reflexes involving purines are also discussed. The actions of adenine nucleotides and nucleosides on cardiomyocytes, atrioventricular and sinoatrial nodes, cardiac fibroblasts, and coronary blood vessels are described. Cardiac release and degradation of ATP are also described. Finally, the involvement of purinergic signalling and its therapeutic potential in cardiac pathophysiology is reviewed, including acute and chronic heart failure, ischemia, infarction, arrhythmias, cardiomyopathy, syncope, hypertrophy, coronary artery disease, angina, diabetic cardiomyopathy, as well as heart transplantation and coronary bypass grafts.

The 5'-nucleotidases as regulators of nucleotide and drug metabolism

The 5'-nucleotidases are a family of enzymes that catalyze the dephosphorylation of nucleoside monophosphates and regulate cellular nucleotide and nucleoside levels. While the nucleoside kinases responsible for the initial phosphorylation of salvaged nucleosides have been well studied, many of the catabolic nucleotidases have only recently been cloned and characterized. Aside from maintaining balanced ribo- and deoxyribonucleotide pools, substrate cycles that are formed with kinase and nucleotidase activities are also likely to regulate the activation of nucleoside analogues, a class of anticancer and antiviral agents that rely on the nucleoside kinases for phosphorylation to their active forms. Both clinical and in vitro studies suggest that an increase in nucleotidase activity can inhibit nucleoside analogue activation and result in drug resistance. The physiological role of the 5'-nucleotidases will be covered in this review, as will the evidence that these enzymes can mediate resistance to nucleoside analogues.

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.

Cardiac adenosine production in rat genetic models of low and high exercise capacity

We previously demonstrated that Copenhagen (COP) and DA inbred rat strains show a wide difference in a test for aerobic treadmill running that correlated positively with isolated cardiac function. The purpose of this study was to test adenosine production as a candidate intermediate phenotype that may explain part of the difference in running and cardiac performance in these genetic models for low and high aerobic capacity. Adenosine production was measured as the activity of soluble 5'-nucleotidase and membrane-bound ecto-5'-nucleotidase in the membrane pellet and supernatant fractions of left and right ventricular muscle and gracilis muscle taken from 10 DA and 10 COP rats. Ecto-5'-nucleotidase activity in the membrane pellet of hearts from both DA and COP accounted for the vast majority of the total tissue adenosine production (>90% in the left ventricle and >80% in the right ventricle). Ecto-5'-nucleotidase activity in the pellet fraction was significantly higher in the left (22.4%) and right (46.1%) ventricles of DA rats compared with COP rats, with no differences in total protein content. There were no significant differences between the strains for 5'-nucleotidase activity in the cardiac supernatant, the gracilis pellet, or the gracilis supernatant. These data support the hypothesis that an increase in cardiac adenosine production may contribute to the greater aerobic running capacity of the DA rats.

The effect of systemic hypoxia on interstitial and blood adenosine, AMP, ADP and ATP in dog skeletal muscle

1. We investigated the effect of moderate systemic hypoxia on the arterial, venous and interstitial concentration of adenosine and adenine nucleotides in the neurally and vascularly isolated, constant-flow perfused gracilis muscles of anaesthetized dogs. 2. Systemic hypoxia reduced arterial PO2 from 129 to 28 mmHg, venous PO2 from 63 to 23 mmHg, arterial pH from 7.43 to 7.36 and venous pH from 7.38 to 7.32. Neither arterial nor venous PCO2 were changed. Arterial perfusion pressure remained at 109 +/- 8 mmHg for the first 5 min of hypoxia, then increased to 131 +/- 11 mmHg by 9 min, and then decreased again throughout the rest of the hypoxic period. 3. Arterial adenosine (427 +/- 98 nM) did not change during hypoxia, but venous adenosine increased from 350 +/- 52 to 518 +/- 107 nM. Interstitial adenosine concentration did not increase (339 +/- 154 nM in normoxia and 262 +/- 97 nM in hypoxia). Neither arterial nor venous nor interstitial concentrations of adenine nucleotides changed significantly in hypoxia. 4. Interstitial adenosine, AMP, ADP and ATP increased from 194 +/- 40, 351 +/- 19, 52 +/- 7 and 113 +/- 36 to 764 +/- 140, 793 +/- 119, 403 +/- 67 and 574 +/- 122 nM, respectively, during 2 Hz muscle contractions. 5. Adenosine, AMP, ADP and ATP infused into the arterial blood did not elevate the interstitial concentration until the arterial concentration exceeded 10 microM. 6. We conclude that the increased adenosine in skeletal muscle during systemic hypoxia is formed by the vascular tissue or the blood cells, and that adenosine is formed intracellularly by these tissues. On the other hand, adenosine formation takes place extracellularly in the interstitial space during muscle contractions.

The mechanism of adenosine formation in cells. Cloning of cytosolic 5'-nucleotidase-I

Adenosine increases blood flow and decreases excitatory nerve firing. In the heart, it reduces rate and force of contraction and preconditions the heart against injury by prolonged ischemia. Based on indirect kinetic arguments, an AMP-selective cytosolic 5'-nucleotidase designated cN-I has been implicated in adenosine formation during ATP breakdown. The molecular identity of cN-I is unknown, although an IMP/GMP-selective cytosolic 5'-nucleotidase (cN-II) and an ecto-5'-nucleotidase (e-N) have been cloned. We utilized the high abundance of cN-I in pigeon heart to purify a 40-kDa subunit for partial protein sequencing and subsequent cDNA cloning. We obtained a full-length clone encoding a novel 40-kDa peptide, unrelated to cN-II or e-N, that was most abundant in heart, brain, and breast muscle. Immunolocalization in heart showed a striated cytoplasmic location, suggesting association with contractile elements. Transient expression in COS-7 cells, generated a 5'-nucleotidase that catalyzed adenosine formation from AMP, which was increased during ATP catabolism. In conclusion, the cloning and expression of cN-I provides definitive evidence of its ability to produce adenosine during ATP breakdown.

Magnesium activated adenosine formation in intact perfused heart: predominance of ecto 5'-nucleotidase during hypermagnesemia

Magnesium ion is an allosteric effector of 5'-nucleotidase and thus activates adenosine production from AMP. Two distinct 5'-nucleotidase systems, the membrane-bound ecto and the soluble cytosolic isoforms, exist in mammalian myocardium. The aim of this study was to delineate the contributions of the ecto vs. cytosolic isoforms to Mg2+-stimulated cardiac purine nucleoside formation and release. Isolated guinea pig hearts were retrogradely perfused at their physiological aortic pressure with Krebs-Henseleit bicarbonate buffer fortified with 10 mM glucose. AMP and the adenylate degradatives adenosine and inosine were measured in coronary venous effluent and in epicardial transudate, which was sampled to estimate concentrations of adenylate degradatives in the interstitium. When perfusate Mg2+ was increased from 0.6 to 6 mM, coronary vascular resistance and spontaneous heart rate fell, and steady-state coronary venous release of adenosine + inosine rose severalfold. Cytosolic free magnesium, as estimated by 31P-NMR after 15 min of perfusion with 6 mM Mg2+ or from chemically measured indicator metabolites after 30 min, rose 60 and 144% respectively (P < 0.05). Excess Mg2+ stimulated purine nucleoside release nearly threefold in coronary venous effluent and four- to sevenfold in epicardial transudate. 50 microM, alpha,beta-methylene adenosine 5'-diphosphate (AOPCP), a selective inhibitor of ecto 5'-nucleotidase, elevated interstitial AMP concentration tenfold, did not attenuate basal nucleoside release, but completely inhibited Mg2+-stimulated coronary venous purine nucleoside release and blunted Mg2+-stimulated interstitial purine nucleoside formation by 69%. During perfusion with exogenous 1 microM [8-14C]AMP, excess perfusate MgCl2 increased [14C]adenosine release by 63% in coronary effluent and 133% in epicardial transudate. AOPCP decreased baseline [14C]adenosine release in coronary effluent and epicardial transudate by 85-90%, caused equilibration of arterial and epicardial AMP, and attenuated MgCl2 activation of p[14C]adenosine formation by approx. 75%, in both the vascular and interstitial compartments. Intramyocytic concentrations of allosteric regulators of the cytosolic 5'-nucleotidases were evaluated in stop-frozen myocardium. Excess magnesium did not appreciably alter intracellular pH and ATP concentration, but lowered free cytosolic ADP and AMP concentrations by 50 and 70%, respectively. A simplified model of compartmentalized adenosine metabolism is proposed in which magnesium ion-activated cardiac purine release originates predominantly from the ecto 5'-nucleotidase; magnesium ion stimulation of metabolic flux through the cytosolic isoforms was constrained by concomitant reductions in intracellular AMP substrate and allosteric activator ADP. Magnesium ion-enhanced adenosine formation by 5'-nucleotidase could contribute to the known cardioprotective effects of this clinically used cation.