Myocardial adenosine cycling rates during normoxia and under conditions of stimulated purine release

Reduced adenosine release from the aged mammalian heart

Adenosine (ADO) released in the heart results in enhanced coronary blood flow and reduced catecholamine release and myocardial responsiveness to adrenergic stimulation (anti-adrenergic action). ADO release from the adrenergic-stimulated aged heart is less than that from the young adult heart. Because adrenergic signaling in the aged heart is impaired, this study was conducted to determine if reduced ADO release from the aged heart results from this reduced adrenergic responsiveness. Hearts of 3-4 months (young adult) and 21-22 months (aged) Fischer-344 rats were perfused with ADO deamination and re-phosphorylation inhibited. Coronary effluent ADO levels were determined. Cellular-free ADO levels with and without sodium acetate (NaAc)-induced mitochondrial AMP synthesis were assessed using formed S-adenosylhomocysteine (SAH) in L-homocysteine thiolactone (L-HC)-treated hearts. The activities of SAH-hydrolase were determined. Aged heart ADO release was 61% less than from young hearts. NaAc augmented young heart ADO release by 104%, while that of aged hearts remained unchanged. SAH synthesis was 51% and 56% lower in the aged heart in the absence and presence of NaAc, respectively, despite an 89% greater SAH hydrolase activity found in the aged hearts. Since synthesized AMP may be diverted to IMP and ultimately inosine by AMP deaminase, inosine release was determined. Aged heart inosine levels in the absence and presence of NaAc were 74% and 59% less than for the young hearts. It is concluded that a reduced mitochondrial AMP synthesis is in part responsible for the attenuation in ADO release from the adrenergic-stimulated aged heart.

CardioNet: a human metabolic network suited for the study of cardiomyocyte metabolism

Background

Availability of oxygen and nutrients in the coronary circulation is a crucial determinant of cardiac performance. Nutrient composition of coronary blood may significantly vary in specific physiological and pathological conditions, for example, administration of special diets, long-term starvation, physical exercise or diabetes. Quantitative analysis of cardiac metabolism from a systems biology perspective may help to a better understanding of the relationship between nutrient supply and efficiency of metabolic processes required for an adequate cardiac output.

Results

Here we present CardioNet, the first large-scale reconstruction of the metabolic network of the human cardiomyocyte comprising 1793 metabolic reactions, including 560 transport processes in six compartments. We use flux-balance analysis to demonstrate the capability of the network to accomplish a set of 368 metabolic functions required for maintaining the structural and functional integrity of the cell. Taking the maintenance of ATP, biosynthesis of ceramide, cardiolipin and further important phospholipids as examples, we analyse how a changed supply of glucose, lactate, fatty acids and ketone bodies may influence the efficiency of these essential processes.

Conclusions

CardioNet is a functionally validated metabolic network of the human cardiomyocyte that enables theorectical studies of cellular metabolic processes crucial for the accomplishment of an adequate cardiac output.

Pathways of adenine nucleotide metabolism: degradation and resynthesis of IMP in ageing chicken heart

The activities of enzymes involved in adenine nucleotide metabolism and the concentration of their metabolic products were studied in the hearts of chickens from birth to advanced age. In particular, in order to investigate the main mechanisms which contribute to ensure availability of adenine nucleotides during ageing of the heart, IMP concentration and the activities of enzymes involved in its turnover were studied. In newborn animals, AMP degradation, though limited in amount, was found to lead to the final products of purine metabolism. In fact, the activity of hypoxanthine phosphoribosyl-transferase (HPRT)-the salvage enzyme of IMP-was not detected. On the contrary, in young chickens, the low concentration of final products of purine metabolism, together with a remarkable activity of HPRT and a high concentration of IMP, indicates that metabolic flux converges on the salvage pathway. In adult chickens, an increase of purine catabolism was observed. This, together with an optimal concentration of endogenous adenine nucleotides, is indicative of a particularly high AMP metabolism. Finally, in chickens of advanced age, a reduced purine catabolism appeared to take place, thus contributing to the maintenance of the adenine nucleotide pool. In ageing heart, a major role of IMP turnover probably consists in the preservation of adenine nucleotides and in the recovery of high-energy phosphates.

Existence and role of substrate cycling between AMP and adenosine in isolated rabbit cardiomyocytes under control conditions and in ATP depletion

BACKGROUND

Adenosine, a physiological coronary vasodilator, has been proposed to regulate coronary circulation according to myocardial oxygen demand. In the present study, we investigated the mechanisms of adenosine formation and utilization in isolated rabbit cardiomyocytes and, in particular, the existence and the role of substrate cycling between AMP and adenosine in the regulation of its concentration.

METHODS AND RESULTS

Rabbit cardiomyocytes were isolated by collagenase perfusion and incubated in HEPES-buffered Krebs-Henseleit solution at 37 degrees C, pH 7.4, in control conditions and in ATP depletion achieved by inhibiting glycolysis with 5 mmol/L iodoacetate. Under control conditions, adenosine accumulated at a rate of 4 pmol.min-1.10(-6) cells. The 13-fold elevation of adenosine accumulation induced by iodotubercidin (ITu), an inhibitor of adenosine kinase, proves that adenosine is normally recycled into AMP. This recycling involves 95% of the adenosine formed. In ATP depletion, adenosine accumulated at the rate of 335 pmol.min-1.10(-6) cells and was no longer rephosphorylated after 20 minutes, as shown by the absence of effect of ITu after this time interval. Moreover, adenosine was deaminated, as indicated by the twofold increase of its accumulation induced by deoxycoformycin (dCF), an inhibitor of adenosine deaminase. Both in control conditions and in ATP depletion, adenosine-dialdehyde, an inhibitor of S-adenosylhomocysteine (SAH) hydrolase, had no significant effect on adenosine formation, indicating that the transmethylation pathway is not an important source of adenosine in rabbit cardiomyocytes.

CONCLUSIONS

The results indicate that recycling of adenosine into AMP is essential for the maintenance of low, nonvasodilatory concentrations of the nucleoside under control conditions and that interruption of recycling plays an important role in elevating adenosine during ATP depletion.

Rapid turnover of the AMP-adenosine metabolic cycle in the guinea pig heart

The intracellular flux rate through adenosine kinase (adenosine-->AMP) in the well-oxygenated heart was investigated, and the relation of the AMP-adenosine metabolic cycle (AMP<-->adenosine) to transmethylation (S-adenosylhomocysteine [SAH]-->adenosine) and coronary flow was determined. Adenosine kinase was blocked in isolated guinea pig hearts by infusion of iodotubercidin in the presence of the adenosine deaminase blocker erythro-9-(2-hydroxy-3-nonyl)adenine (5 mumol/L). Iodotubercidin (1 nmol/L to 4 mumol/L) caused graded increases in venous effluent concentrations of adenosine, from 8 +/- 3 to 145 +/- 32 nmol/L (mean +/- SEM, n = 3), and in coronary flow, which increased to maximal levels. Flow increases were completely abolished by adenosine deaminase (5 to 10 U/mL). Interstitial adenosine concentrations, estimated using a mathematical model, increased from 22 nmol/L during control conditions to 420 nmol/L during maximal vasodilation. The possibility that iodotubercidin caused increased venous adenosine by interfering with myocardial energy metabolism was ruled out in separate 31P nuclear magnetic resonance experiments. To estimate total normoxic myocardial production of adenosine (AMP-->adenosine<--SAH), the time course of coronary venous adenosine release was measured during maximal inhibition of adenosine kinase with 30 mumol/L iodotubercidin. Adenosine release increased more than 15-fold over baseline, reaching a new steady-state value of 3.4 +/- 0.3 nmol.min-1 x g-1 (n = 5) after 4 minutes. In parallel experiments, the relative roles of AMP hydrolysis and transmethylation (SAH hydrolysis) were determined, using adenosine dialdehyde (10 mumol/L) to block SAH hydrolase. In these experiments, adenosine release increased to similar levels of 3.4 +/- 0.5 nmol.min-1 x g-1 (n = 6) during inhibition of adenosine deaminase and adenosine kinase. It is concluded that (1) maximal increases in coronary flow are elicited by increases in interstitial adenosine concentration to approximately 400 nmol/L, (2) more than 90% of the adenosine produced in the heart is normally rephosphorylated to AMP without escaping into the venous effluent, (3) AMP hydrolysis is the dominant pathway for cardiac adenosine production under normoxic conditions, and (4) the high rate of adenosine salvage is due to rapid turnover of a metabolic cycle between AMP and adenosine. Rapid cycling may serve to amplify the relative importance of AMP hydrolysis over transmethylation in controlling cytosolic adenosine concentrations.

Hypoxia enhances isoproterenol-induced increase in heart interstitial adenosine, depressing beta-adrenergic contractile responses

Endogenous interstitial adenosine may protect the hypoxic heart by attenuating beta-adrenergic-induced contractile and metabolic responses, thereby reducing energy utilization. Constant-flow perfused rat hearts were used to study: 1) the effect of hypoxia on isoproterenol (ISO)-induced increase in interstitial adenosine, as estimated with epicardial surface transudates, and 2) the role of endogenous adenosine in hypoxic depression of ISO-induced contractile responses. ISO (1 nM for 10 minutes) in the normoxic heart increased transudate adenosine 114% from a pre-ISO normoxic value of 343 pmol/ml. ISO administered to the hypoxic heart increased transudate adenosine 357% from a pre-ISO hypoxic value of 797 pmol/ml. The absolute magnitude of the ISO-induced increase in transudate adenosine was 625% greater during hypoxia than during normoxia. This was associated with a reduction in the ISO-induced contractile response during hypoxia. In other experiments, with normoxia ISO (10 nM for 10 seconds) increased developed left ventricular pressure by 140 mm Hg, and the maximum rates of left ventricular pressure development and relaxation by 5,860 and 2,771 mm Hg/sec, respectively, above control values of 90 mm Hg, 2,250 mm Hg/sec, and 1,875 mm Hg/sec. Hypoxia reduced the three ISO-induced contractile responses by 50%, 56%, and 36%. However, 1,3-dipropyl-8-cyclopentylxanthine (5 x 10(-7) M), an adenosine A1-receptor antagonist, added to the hypoxic hearts resulted in only a 31%, 39%, and 9% reduction in the ISO-induced responses in developed left ventricular pressure and the maximum rates of left ventricular pressure development and relaxation, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Purinergic stimulation of cell division and differentiation: mechanisms and pharmacological implications

Extracellular purine nucleosides and nucleotides in micromolar concentrations stimulate proliferation of a variety of cell types in vitro and in vivo. As well they act synergistically with NGF to stimulate neurite outgrowth from PC12 cells. A variety of purine nucleosides and deoxyribonucleosides promote cell proliferation and increase intracellular cAMP. Their activities are inhibited by adenosine A2 receptor antagonists. Only adenosine interacts with the A2 receptor. We propose that the other nucleosides and deoxyribonucleosides inhibit extracellular adenosine deaminase, thereby increasing the extracellular concentration of adenosine. The nucleotides apparently act by stimulating P2y receptors coupled to inositol phosphate metabolism. We propose that the nucleosides and nucleotides act synergistically with other growth factors because each has distinct but complementary second messenger systems. If our hypotheses are correct, it should prove possible to modulate the growth and morphogenesis in several cell types using drugs that inhibit or stimulate adenosine A2 or purine P2y receptor agonists or the second messenger systems coupled to these receptors.

Activating effect of adenosine on rat erythrocyte glycolysis

1. Adenosine increases the adenine nucleotide pool in rat erythrocytes. Hence, we tested the effect of the nucleoside on the glycolytic pathway in red blood cells. 2. A 2.5-fold increase in the level of fructose-1,6-bisphosphate and a 34% augmentation in lactate pool were observed in rat erythrocytes, 30 min after adenosine treatment. 3. Under conditions preventing adenosine metabolism, 1 microM nucleoside addition to isolated erythrocytes induced an 89% increase in lactate production and an increase in glucose consumption. 4. Activation of red cell phosphofructokinase (PFK) is produced by addition of microM concentrations of adenosine. Our data suggest a role for adenosine in the glycolysis flux regulation through PFK activation.

Reperfusion-induced arrhythmias are not prevented by uric acid in the isolated rat heart

Oxygen-derived free radicals have been implicated in ventricular arrhythmogenesis during coronary reperfusion following an acute ischemic event. We have investigated the possibility that uric acid, a potentially important physiological antioxidant (inhibits lipid peroxidation and scavenges various radical species during oxidation to allantoin), or oxonic acid (inhibitor of uricase enzyme), are able to prevent reperfusion-induced ventricular dysrhythmias in isolated buffer-perfused rat hearts. Rat hearts (n = 12/group) underwent 15 minutes occlusion; arrhythmias were monitored during ischemia and for 10 minutes of reperfusion. There was no difference in the incidence of ventricular fibrillation or ventricular tachycardia in either uric acid or oxonic acid treated hearts compared to untreated controls. Mean duration of ventricular fibrillation appeared to be reduced in hearts treated with 10(-3) and 10(-4) M oxonic acid compared to controls but these data did not achieve a level of statistical significance. These results demonstrate that uric acid and oxonic acid failed to prevent reperfusion-mediated ventricular dysrhythmias in this experimental preparation. Although oxygen-derived free radicals may contribute to the initiation of either ischemia- or reperfusion-induced arrhythmogenesis, our findings provide little support for this hypothesis.

Uric acid as radical scavenger and antioxidant in the heart

Uric acid (UA) is released from the heart of many species, including man, and its site of formation has been shown to be the microvascular endothelium. Since UA reacts with oxygen radicals in vitro, experiments were conducted on guinea pig hearts perfused with Krebs-Henseleit buffer (KHB) to evaluate whether the formation of UA could afford protection from damage by radicals and oxidants. The following results were obtained: (1) Upon addition of the hydroxyl radical scavenger DMSO to the perfusate, the coronary rate of release of endogenous uric acid was increased relative to the precursor purines. (2) UA was degraded during passage through the coronary system and also in KHB in vitro after addition of substances generating hydroxyl radicals or hypochlorite. Superoxide (O2-) radicals did not seem to react directly with UA, though UA concentration-dependently quenched the chemiluminescence generated from luminol in the presence of O2- and OH radicals. (3) Coronary dilation by acetylcholine (Ach) and sub-microM concentrations of adenosine, induced by both via endothelial mechanisms, was attenuated after prolonged inhibition of endothelial UA formation by allopurinol. Furthermore, the effect of Ach but not of adenosine proved acutely sensitive to methylene blue and O2-, substances known to inactivate EDRF. This finding suggests involvement of EDRF in Ach-mediated, but not in adenosine-induced dilatation of the intact coronary system. Exogenously applied UA prevented the impairment of vascular responses to Ach and adenosine caused by allopurinol, and to Ach upon generation of O2-. (4) Hearts performed more pressure-volume work and exhibited greater functional stability when perfused with KHB supplemented with UA in a physiological concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenine nucleotide transport and adenosine production in isolated rat heart mitochondria during acetate metabolism

In view of its vasodilatory effect on the coronary circulation (probably mediated by adenosine) and its metabolic compartmentalization (intramitochondrial activation to form acetyl-CoA), the metabolic effects of acetate were studied in isolated rat heart mitochondria. Acetate caused conversion of adenylates to AMP and the formation of adenosine. Adenylate efflux was inhibited by carboxyatractyloside but not by N-ethylmaleimide. The intramitochondrial accumulation of AMP was enhanced by carboxyatractyloside during acetate metabolism and the formation of extramitochondrial adenosine inhibited. A carboxyatractyloside-sensitive unidirectional AMP influx with a Km of 50 microM and Vmax of 11 nmol/min per mg mitochondrial protein was also observed. The mitochondrial adenosine content was high and constant during the experiments. The steep apparent concentration gradient of adenosine indicates that most of the mitochondrial adenosine is tightly bound to protein. Adenosine formation was proportional to the extramitochondrial AMP concentration, showing that the 5'-nucleotidase activity of cardiac mitochondrial preparations is extramitochondrial in origin. The data suggest that the mitochondrial ATP/ADP carrier is capable of transporting AMP and that intramitochondrial AMP is recycled during acetate metabolism in the myocardium partially by means of the ATP/ADP translocator, leading to an increase in extramitochondrial AMP and adenosine formation.

AMP and IMP dephosphorylation by soluble high- and low-Km 5'-nucleotidases

Three distinct 5'-phosphomonoesterase activities were isolated from soluble fractions of human placenta, cultured human T and B lymphoblasts, and rat liver using 5'-AMP-sepharose 4B affinity chromatography. We define these activities as "low-Km" 5'-nucleotidase, "high-Km" 5'-nucleotidase, and nonspecific phosphatase. High-Km 5'-nucleotidase was eluted with 0.5 M NaCl, low-Km 5'-nucleotidase was eluted with 10 mM ADP, and nonspecific phosphatase was not retained on the column. We have found significant variability in the relative content of high- to low-Km activities in the tissues studied with the ratios ranging from 5.5 to 264. The properties were studied after further purification. The molecular mass of the low-Km enzymes ranged from 72.5 to 209 kDa, optimum pH ranged from 7.4 to 9.0, Km for AMP ranged from 7 to 15 microM, and Km for IMP ranged from 10 to 26 microM. The molecular mass of the high-Km enzymes ranged from 182 to 210 kDa, pH optimum was at 6.5, Km for AMP ranged from 3.0 to 9.4 mM, and the Km for IMP ranged from 0.3 to 0.5 mM. The data indicate that the soluble low- and high-Km 5'-nucleotidase coexist in the mammalian cells and tissues studied. These observations suggest a complex system for the regulation of nucleoside 5'-monophosphate dephosphorylation.

Acetate-induced changes in cardiac energy metabolism and hemodynamics in the rat

The hemodynamic and metabolic effects of acetate were studied in rats in vivo and in the isolated perfused heart. Hemodynamic parameters, myocardial phosphagens, inorganic phosphate, and adenosine were measured in vivo. Acetate uptake, coronary flow, O2 consumption, parameters of the cellular energy state, and hypoxanthine compounds and their washout were measured in heart perfusion experiments. Heart rate (HR), cardiac output, and the peak derivative of the left ventricular pressure rise (dP/dtmax) increased significantly during acetate infusion in vivo, but mean arterial pressure, systolic arterial pressure, and systemic vascular resistance decreased. Heart muscle ATP concentrations decreased after 7 min of acetate infusion. In vivo cardiac work load (HR.(peak left ventricular pressure] showed a positive correlation with tissue adenosine concentration and a negative correlation with phosphorylation potential. Acetate uptake in the perfused hearts was about 2.5 mumol/min per gram wet weight. Acetate perfusion increased O2 consumption and coronary flow concomitantly with a decrease in tissue ATP concentration. Tissue AMP and perfusate effluent adenosine concentration and adenosine output increased significantly, perfusate adenosine showing a non-linear positive correlation with coronary flow. The results demonstrate that acetate induces considerable changes in hemodynamics and metabolism in the heart.

Developmental differences in myocardial ATP metabolism

Little is known about postnatal changes in myocardial purine metabolism. We therefore studied how ATP catabolism was affected by hypothermia and ischaemia in neonatal and adult hearts. Hypothermia during ischaemia protected isolated adult and newborn hearts against ATP decline. Reperfusion after normothermic ischaemia resulted in higher ATP levels in newborn hearts with less release of ATP-catabolites. During normoxia adult hearts released mainly urate (80% of total purine release), while newborns released mainly hypoxanthine (64%). During early reperfusion adult and newborn hearts released mainly inosine (50-60%). The very low xanthine oxidase activity in the neonatal heart could be an important factor in the observed ATP preservation during reperfusion.

Coronary vasoactivity of acetate in dog and guinea-pig

This study compares the coronary vasoactivity of acetate in the blood-perfused heart of the open-chest dog and in the buffer-perfused guinea-pig heart. In the dog acetate is a weak but probably fully efficacious coronary agonist. Direct intracoronary infusions of isosmolar Na acetate caused dose-dependent coronary vasodilation and decreased transcoronary O2 extraction, resulting in an increase in cardiac O2 usage of up to 40%. Acetate raised coronary flow to at least 50% above control in 63 of 67 dogs but caused maximum coronary vasodilation (400% of control) in only 39 of the 67. The frequency distribution of the acetate EC-20 decreased monotonically from a mode at less than 1 mM over a range extending to greater than 6 mM, suggesting a single population of animals characterized by a rather wide range of sensitivity to acetate. Theophylline antagonized acetate vasodilation, in support of the idea that adenosine mediates the coronary effects of acetate. In the guinea-pig heart, acetate in concentrations up to 10 mM caused minimal increases in coronary flow that were completely accounted for by the small change in O2 usage that resulted from switching from glucose to acetate the main energy source. Acetate (10 mM) elicited a small release of adenosine and its degradation products.