De novo synthesis of myocardial adenine nucleotides in the rat. Acceleration during recovery from oxygen deficiency

Maintaining energy provision in the heart: the creatine kinase system in ischaemia-reperfusion injury and chronic heart failure

The non-stop provision of chemical energy is of critical importance to normal cardiac function, requiring the rapid turnover of ATP to power both relaxation and contraction. Central to this is the creatine kinase (CK) phosphagen system, which buffers local ATP levels to optimise the energy available from ATP hydrolysis, to stimulate energy production via the mitochondria and to smooth out mismatches between energy supply and demand. In this review, we discuss the changes that occur in high-energy phosphate metabolism (i.e., in ATP and phosphocreatine) during ischaemia and reperfusion, which represents an acute crisis of energy provision. Evidence is presented from preclinical models that augmentation of the CK system can reduce ischaemia-reperfusion injury and improve functional recovery. Energetic impairment is also a hallmark of chronic heart failure, in particular, down-regulation of the CK system and loss of adenine nucleotides, which may contribute to pathophysiology by limiting ATP supply. Herein, we discuss the evidence for this hypothesis based on preclinical studies and in patients using magnetic resonance spectroscopy. We conclude that the correlative evidence linking impaired energetics to cardiac dysfunction is compelling; however, causal evidence from loss-of-function models remains equivocal. Nevertheless, proof-of-principle studies suggest that augmentation of CK activity is a therapeutic target to improve cardiac function and remodelling in the failing heart. Further work is necessary to translate these findings to the clinic, in particular, a better understanding of the mechanisms by which the CK system is regulated in disease.

Integrated Functions of Cardiac Energetics, Mechanics, and Purine Nucleotide Metabolism

Purine nucleotides play central roles in energy metabolism in the heart. Most fundamentally, the free energy of hydrolysis of the adenine nucleotide adenosine triphosphate (ATP) provides the thermodynamic driving force for numerous cellular processes including the actin-myosin crossbridge cycle. Perturbations to ATP supply and/or demand in the myocardium lead to changes in the homeostatic balance between purine nucleotide synthesis, degradation, and salvage, potentially affecting myocardial energetics and, consequently, myocardial mechanics. Indeed, both acute myocardial ischemia and decompensatory remodeling of the myocardium in heart failure are associated with depletion of myocardial adenine nucleotides and with impaired myocardial mechanical function. Yet there remain gaps in the understanding of mechanistic links between adenine nucleotide degradation and contractile dysfunction in heart disease. The scope of this article is to: (i) review current knowledge of the pathways of purine nucleotide depletion and salvage in acute ischemia and in chronic heart disease; (ii) review hypothesized mechanisms linking myocardial mechanics and energetics with myocardial adenine nucleotide regulation; and (iii) highlight potential targets for treating myocardial metabolic and mechanical dysfunction associated with these pathways. It is hypothesized that an imbalance in the degradation, salvage, and synthesis of adenine nucleotides leads to a net loss of adenine nucleotides in both acute ischemia and under chronic high-demand conditions associated with the development of heart failure. This reduction in adenine nucleotide levels results in reduced myocardial ATP and increased myocardial inorganic phosphate. Both of these changes have the potential to directly impact tension development and mechanical work at the cellular level. © 2024 American Physiological Society. Compr Physiol 14:5345-5369, 2024.

Mitochondrial-cell cycle cross-talk drives endoreplication in heart disease

[Figure: see text].

Xanthine oxidase inhibitor ameliorates postischemic renal injury in mice by promoting resynthesis of adenine nucleotides

Although oxidative stress plays central roles in postischemic renal injury, region-specific alterations in energy and redox metabolism caused by short-duration ischemia remain unknown. Imaging mass spectrometry enabled us to reveal spatial heterogeneity of energy and redox metabolites in the postischemic murine kidney. After 10-minute ischemia and 24-hour reperfusion (10mIR), in the cortex and outer stripes of the outer medulla, ATP substantially decreased, but not in the inner stripes of the outer medulla and inner medulla. 10mIR caused renal injury with elevation of fractional excretion of sodium, although histological damage by oxidative stress was limited. Ischemia-induced NADH elevation in the cortex indicated prolonged production of reactive oxygen species by xanthine oxidase (XOD). However, consumption of reduced glutathione after reperfusion suggested the amelioration of oxidative stress. An XOD inhibitor, febuxostat, which blocks the degradation pathway of adenine nucleotides, promoted ATP recovery and exerted renoprotective effects in the postischemic kidney. Because effects of febuxostat were canceled by silencing of the hypoxanthine phosphoribosyl transferase 1 gene in cultured tubular cells, mechanisms for the renoprotective effects appear to involve the purine salvage pathway, which uses hypoxanthine to resynthesize adenine nucleotides, including ATP. These findings suggest a novel therapeutic approach for acute ischemia/reperfusion renal injury with febuxostat through salvaging high-energy adenine nucleotides.

Ribose Supplementation Alone or with Elevated Creatine Does Not Preserve High Energy Nucleotides or Cardiac Function in the Failing Mouse Heart

BACKGROUND

Reduced levels of creatine and total adenine nucleotides (sum of ATP, ADP and AMP) are hallmarks of chronic heart failure and restoring these pools is predicted to be beneficial by maintaining the diseased heart in a more favourable energy state. Ribose supplementation is thought to support both salvage and re-synthesis of adenine nucleotides by bypassing the rate-limiting step. We therefore tested whether ribose would be beneficial in chronic heart failure in control mice and in mice with elevated myocardial creatine due to overexpression of the creatine transporter (CrT-OE).

METHODS AND RESULTS

FOUR GROUPS WERE STUDIED: sham; myocardial infarction (MI); MI+ribose; MI+CrT-OE+ribose. In a pilot study, ribose given in drinking water was bioavailable, resulting in a two-fold increase in myocardial ribose-5-phosphate levels. However, 8 weeks post-surgery, total adenine nucleotide (TAN) pool was decreased to a similar amount (8-14%) in all infarcted groups irrespective of the treatment received. All infarcted groups also presented with a similar and substantial degree of left ventricular (LV) dysfunction (3-fold reduction in ejection fraction) and LV hypertrophy (32-47% increased mass). Ejection fraction closely correlated with infarct size independently of treatment (r(2) = 0.63, p<0.0001), but did not correlate with myocardial creatine or TAN levels.

CONCLUSION

Elevating myocardial ribose and creatine levels failed to maintain TAN pool or improve post-infarction LV remodeling and function. This suggests that ribose is not rate-limiting for purine nucleotide biosynthesis in the chronically failing mouse heart and that alternative strategies to preserve TAN pool should be investigated.

Energy metabolism in the reversible and irreversible phases of severe myocardial ischemia

In summary, myocardial ischemia is associated with the progressive depletion of HEP and the adenine nucleotide pool. Anaerobic glycolysis is essential for energy production in the severely ischemic myocyte and accounts for 80% of the HEP utilized by severely or totally ischemic myocardium. However, the rate of anaerobic glycolysis is too slow to prevent the progressive depletion of ATP. Anaerobic glycolysis stops entirely prior to the complete utilization of glycogen. Without remaining HEP stores or HEP production from anaerobic glycolysis, HEP utilization no longer can occur. This point occurs in vivo after about 40 minutes of severe ischemia and coincides with the onset of cell death. Modest depletion of ATP due to brief periods of transient ischemia may not cause cell death, but is associated with partial depletion of the adenine nucleotide pool. The slow repletion of this pool may be responsible for prolonged depression of contractile function.

Proton magnetic resonance spectroscopy can detect creatine depletion associated with the progression of heart failure in cardiomyopathy

OBJECTIVES

This study noninvasively examined total creatine (CR) of the myocardium in dilated cardiomyopathy (DCM) or hypertrophic cardiomyopathy (HCM) using proton magnetic resonance spectroscopy ((1)H-MRS).

BACKGROUND

Abnormalities in CR metabolism in failing hearts have been reported. A biochemical study suggested that myocardial metabolic changes are very similar in DCM and HCM despite the different heart failure (HF) mechanisms.

METHODS

Using cardiac-gated (1)H-MRS with magnetic resonance image (MRI)-guided point-resolved spectroscopy (PRESS) localization, we quantitatively measured septal CR. Patients with either DCM (n = 11) or HCM (n = 7) and age-matched normal subjects (n = 14) were examined.

RESULTS

Myocardial CR was significantly lower in DCM patients (16.1 +/- 4.5 micromol/g wet weight [range 10.2 to 22.9], p < 0.05) than that in subjects with normal hearts (27.6 +/- 4.1 micromol/g [range 21.4 to 36.2]). Myocardial CR in HCM patients (22.6 +/- 8.1 micromol/g [range 12.2 to 34.5]) was significantly lower than that in subjects with normal hearts (p < 0.05) but was significantly higher than that in DCM patients (p < 0.05). In 18 patients with either DCM or HCM, myocardial CR correlated positively with left ventricular ejection fraction (LVEF) (y = 0.22x + 9.8, r = 0.73, p = 0.0006) but correlated negatively with plasma B-type natriuretic peptide (BNP) levels (y = -0.012x + 22.4, r = -0.54, p = 0.022).

CONCLUSIONS

This study showed that (1)H-MRS can noninvasively detect CR depletion associated with the severity of HF in cardiomyopathy.

Susceptibility to cardiac ischemia/reperfusion injury is modulated by chronic estrogen status

The purpose of this study was to test whether the susceptibility of the heart to ischemia/reperfusion injury is modulated by the chronic estrogen status, i.e., increased with estrogen deficiency and attenuated by pharmacologic estrogen supplementation. In addition, the study tested whether estrogen-dependent changes in mechanical function are associated with alterations of cardiac high-energy phosphate metabolism. Rats were ovariectomized, not ovariectomized, or ovariectomized and treated with subcutaneous estrogen pellets (1.5 mg/21 d) (n = 8-11 per group). Three weeks later, hearts were isolated and perfused isovolumically under constant perfusion pressure conditions. Hearts were subjected to 15 min of total global ischemia (37 degrees C) and 30 min of reperfusion. Simultaneous [31P] nuclear magnetic resonance spectra were recorded throughout this protocol to monitor changes in ATP, phosphocreatine, and inorganic phosphate content. Whereas preischemic values for heart rate, end-diastolic pressure, and coronary flow were not different among groups, left ventricular developed pressure was slightly but significantly decreased in the estrogen-treated group (p < 0.05). However, treated hearts showed improved recovery of left ventricular developed pressure on reperfusion (89 +/- 4% in control rats, 70 +/- 8% in ovariectomized hearts, and 114 +/- 9% of preischemic values in estrogen-treated rats). However, changes in ATP, phosphocreatine, and inorganic phosphate during ischemia were as previously described and were unaffected by chronic estrogen status. In conclusion, in the isolated buffer-perfused rat heart, estradiol treatment caused improved functional recovery after ischemia/reperfusion injury. This improvement, however, did not include preservation of high-energy phosphate metabolism. Other potential mechanisms include an anti-oxidant activity of 17beta-estradiol-and estrogen-induced alterations in glucose metabolism.

Tolerance of isolated rat hearts to low-flow ischemia and hypoxia of increasing duration: protective role of down-regulation and ATP during ischemia

We tested the hypothesis that down-regulated hearts, as observed during low-flow ischemia, adapt better to low O2 supply than non-down-regulated, or hypoxic, hearts. To address the link between down-regulation and endogenous ischemic protection, we compared myocardial tolerance to ischemia and hypoxia of increasing duration. To that end, we exposed buffer-perfused rat hearts to either low-flow ischemia or hypoxia (same O2 shortage) for 20, 40 or 60 min (n = 8/group), followed by reperfusion or reoxygenation (20 min, full O2 supply). At the end of the O2 shortage, the rate-pressure product was less in ischemic than hypoxic hearts (p < 0.0001). The recovery of the rate-pressure product after reperfusion or reoxygenation was not different for t = 20 min, but was better in ischemic than hypoxic hearts for t = 40 and 60 min (p < 0.02 and p < 0.0002, respectively). The end-diastolic pressure remained unchanged during low-flow ischemia (0.024 +/- 0.013 mmHg x min(-1)), but increased significantly during hypoxia (0.334 +/- 0.079 mmHg x min(-1)). We conclude that, while the duration of hypoxia progressively impaired the rate-pressure product and the end-diastolic pressure, hearts were insensitive of the duration of low-flow ischemia, thereby providing evidence that myocardial down-regulation protects hearts from injury. Excessive ATP catabolism during ischemia in non-down-regulated hearts impaired myocardial recovery regardless of vascular, blood-related and neuro-hormonal factors. These observations support the view that protection is mediated by the maintenance of the ATP pool.

Endothelial adenosine transporter characterization in perfused guinea pig hearts

Adenosine (Ado), a smooth muscle vasodilator and modulator of cardiac function, is taken up by many cell types via a saturable transporter, blockable by dipyridamole. To quantitate the influences of endothelial cells in governing the blood-tissue exchange of Ado and its concentration in the interstitial fluid, one must define the permeability-surface area products (PS) for Ado via passive transport through interendothelial gaps [PS(g)(Ado)] and across the endothelial cell luminal membrane (PS(ecl)) in their normal in vivo setting. With the use of the multiple-indicator dilution (MID) technique in Krebs-Ringer perfused, isolated guinea pig hearts (preserving endothelial myocyte geometry) and by separating Ado metabolites by HPLC, we found permeability-surface area products for an extracellular solute, sucrose, via passive transport through interendothelial gaps [PS(g)(Suc)] to be 1.9 +/- 0.6 ml. g(-1). min(-1) (n = 16 MID curves in 4 hearts) and took PS(g)(Ado) to be 1. 2 times PS(g)(Suc). MID curves were obtained with background nontracer Ado concentrations up to 800 micrometer, partially saturating the transporter and reducing its effective PS(ecl) for Ado. The estimated maximum value for PS(ecl) in the absence of background adenosine was 1.1 +/- 0.1 ml. g(-1). min(-1) [maximum rate of transporter conformational change to move the substrate from one side of the membrane to the other (maximal velocity; V(max)) times surface area of 125 +/- 11 nmol. g(-1). min(-1)], and the Michaelis-Menten constant (K(m)) was 114 +/- 12 microM, where +/- indicates 95% confidence limits. Physiologically, only high Ado release with hypoxia or ischemia will partially saturate the transporter.

Left ventricular myocardial adenosine triphosphate changes during reperfusion of ventricular fibrillation: the influence of flow and epinephrine

OBJECTIVE

To determine whether epinephrine in combination with high flow worsens left ventricular (LV) myocardial high-energy phosphate stores during reperfusion of ischemic ventricular fibrillation (VF).

DESIGN

Blinded, prospective block randomized, placebo controlled study.

SETTING

University medical center research laboratory.

SUBJECTS

A total of 22 mixed breed swine weighing 22.0+/-3.3 kg (SD).

INTERVENTIONS

Open-chest swine, anesthetized with alpha-chloralose, underwent 10 mins of nonperfused VF followed by reperfusion with cardiopulmonary bypass for 90 mins and then defibrillation. Animals were block randomized to four groups for reperfusion: Group 1 (n = 5), high flow (100 mL/kg/min) and epinephrine (2.5 microg/kg/min); Group 2 (n = 5), high flow and placebo; Group 3 (n = 6), low flow (30 mL/kg/min) and epinephrine; and Group 4 (n = 6), low flow and placebo.

MEASUREMENTS AND MAIN RESULTS

In vivo LV creatine phosphate (CP) and adenosine triphosphate (ATP) were determined using whole wall and spatially localized 31P NMR spectroscopy at 4.7 Tesla. During perfusion of the fibrillating myocardium, epinephrine significantly increased aortic pressure (p < .05) and improved defibrillation rates (p < .01). ATP levels during reperfusion were significantly decreased within all groups compared with baseline. There were no differences in ATP levels between groups. High flow, independent of epinephrine, was associated with increased preservation of ATP (p < .05), increased CP/ATP ratios (p < .02) in all layers of the LV wall, and decreased aortic and cardiac vein lactates (p < .001).

CONCLUSIONS

Epinephrine, in combination with flow higher than standard cardiopulmonary resuscitation flows, increased perfusion pressure and defibrillation rates, but did not significantly alter myocardial ATP during VF reperfusion in the in vivo heart Reperfusion flow, independent of epinephrine, is a critical determinant of myocardial ATP preservation.

Metabolic adaptation of endothelial cells to substrate deprivation

Endothelial cells are known to be metabolically rather robust. To study the mechanisms involved, porcine aortic endothelial cells (PAEC), cultured on microcarrier beads, were perfused with glucose (10 mM) or with substrate-free medium. Substrate-free perfusion for 2 h induced an almost complete loss of nucleoside triphosphates (31P-NMR) and decreased heat flux, a measure of total energy turnover, by >90% in parallel microcalorimetric measurements. Heat flux and nucleoside triphosphates recovered after addition of glucose. Because protein synthesis is a major energy consumer in PAEC, the rate of protein synthesis was measured ([14C]leucine incorporation). Reduction or blockade of energy supply resulted in a pronounced reduction in the rate of protein synthesis (up to 80% reduction). Intracellular triglyceride stores were decreased by approximately 60% after 2 h of substrate-free perfusion. Under basal perfusion conditions, PAEC released approximately 30 pmol purine. mg protein-1. min-1, i.e., 16% of the cellular ATP per hour, while ATP remained constant. Substrate deprivation increased the release of various purines and pyrimidines about threefold and also induced a twofold rise in purine de novo synthesis ([14C]formate). These results demonstrate that PAEC are capable of recovering from extended periods of substrate deprivation. They can do so by a massive downregulation of their energy expenditure, particularly protein synthesis, while at the same time using endogenous triglycerides as substrates and upregulating purine de novo synthesis to compensate for the loss of purines.

High-energy phosphates metabolism and recovery in reperfused ischaemic hearts

BACKGROUND

The aim of this study was to assess how coronary flow, oxygen supply and energy demand affect myocardial ATP, phosphocreatine and their metabolites during oxygen shortage and recovery.

METHODS

Isolated rat hearts were exposed for 20 min to either low-flow ischaemia or hypoxaemia at the same oxygen supply, followed by return to baseline conditions (20 min). Seventy-three hearts were divided into four groups: ischaemic or hypoxaemic, spontaneously beating or paced to increase energy demand.

RESULTS

During O2 shortage, myocardial performance was less in ischaemic, spontaneously beating hearts (SpIs), than in the other groups (14 +/- 1% of baseline vs. 25-48%). Consequently, the tissue levels of ATP, total adenylates and phosphocreatine were maintained in SpIs, in contrast to marked decreases in the other groups. Upon reflow, the recovery of performance and of myocardial ATP was 94 +/- 5% in SpIs (P = NS vs. baseline) compared with 64-85% (P < 0.05 vs. baseline) in the other groups. The degree of recovery was positively related to the ischaemic contents of ATP (P = 0.03) and adenylates (P = 0.001), but not to that of phosphocreatine (P = NS).

CONCLUSION

The maintenance of the ATP pool under low oxygen supply conditions is essential for good recovery. The most important factors that determine the ATP pool size are the energy demand, which increases the formation of diffusible ATP catabolites, and the coronary flow, which removes these catabolites, rather than the oxygen supply per se.

Regulation of cardiac AMP-specific 5'-nucleotidase during ischemia mediates ATP resynthesis on reflow

The ability to resynthesize ATP during recovery from ischemia is limited to the size of endogenous pool of adenine nucleotides. Cytosolic AMP-specific 5'-nucleotidase (5'-NT) plays a key role in ATP degradation and hence the capacity for ATP resynthesis. We have suggested (J. Clin. Invest. 93: 40-49, 1994) that intracellular acidosis [intracellular pH (pHi)] is a potent inhibitor of 5'-NT under in vivo conditions. To test this hypothesis further, we used the hyperthyroid rat heart because we could alter pHi during ischemia and determine the consequences of lower pHi on AMP accumulation (by chemical assay) and ATP resynthesis (by 31P nuclear magnetic resonance spectroscopy) during reperfusion. Global no-flow ischemia caused pHi to decrease from 7.1 under well-oxygenated control perfusion to 6.7. We found that decreasing pHi further from pH 6.7 to 6.4 leads to increased accumulation (30%) of AMP during ischemia and to a 2.5-fold increase in ATP resynthesis during reperfusion. Analysis of all known substrates, products, activators, and inhibitors of the 5'-NT suggests that 5'-NT is activated primarily by Mg2+ and ADP and is inhibited by H+. Thus these observations provide evidence for a salutary effect of intracellular acidosis on preserving the AMP pool due to inhibition of 5'-NT and suggest a novel role of H+ in protecting ischemic tissue.

Regulation of and intervention into the oxidative pentose phosphate pathway and adenine nucleotide metabolism in the heart

The capacity of the oxidative pentose pathway (PPP) in the heart is limited, since the activity of glucose-6-phosphate dehydrogenase (G-6-PD), the first and regulating enzyme of this pathway, is very low. Two mechanisms are involved in the regulation of this pathway. Under normal conditions, G-6-PD is inhibited by NADPH. This can be overcome in the isolated perfused rat heart by increasing the oxidized glutathione and by elevating the NADP+/NADPH ratio. Besides this rapid control mechanism, there is a long-term regulation which involves the synthesis of G-6-PD. The activity of G-6-PD was elevated in the rat heart during the development of cardiac hypertrophy due to constriction of the abdominal aorta and in the non-ischemic part of the rat heart subsequent to myocardial infarction. The catecholamines isoproterenol and norepinephrine stimulated the activity of myocardial G-6-PD in a time- and dose-dependent manner. The isoproterenol-induced stimulation was cAMP-dependent and due to increased new synthesis of enzyme protein. The G-6-PD mRNA was elevated by norepinephrine. As a consequence of the stimulation of the oxidative PPP, the available pool of 5-phosphoribosyl-1-pyrophosphate (PRPP) was expanded. PRPP is an important precursor substrate for purine and pyrimidine nucleotide synthesis. The limiting step in the oxidative PPP, the G-6-PD reaction, can be bypassed with ribose. This leads to an elevation of the cardiac PRPP pool. The decline in ATP that is induced in many pathophysiological conditions was attenuated or even entirely prevented by i.v. infusion of ribose. In two in vivo rat models, the overloaded and catecholamine-stimulated heart and the infarcted heart, the normalization of the cardiac adenine nucleotide pool by ribose was accompanied by an improvement of global heart function. Combination of ribose with adenine or inosine in isoproterenol-treated rats was more effective to restore completely the cardiac ATP level within a short period of time than either intervention alone.

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.

Myocardial effects of experimental acute brain death: evaluation by hemodynamic and biological studies

BACKGROUND

Because of problems concerning the functional quality of heart transplants, more and more interest has been focused on the physiologic changes occurring during brain death, one of the major possible contributing factors to the myocardial alterations.

METHODS

The aim of this study was to describe the link between acute experimental brain death and myocardial metabolism. This was achieved by in vivo 3-hour hemodynamic and biological (myocardial lactate production) studies and then in vitro 6-hour phosphorus-31 nuclear magnetic resonance spectroscopy. Two groups of pigs were involved in the study: group I (n = 10) as control and group II (n = 10) as brain-dead animals.

RESULTS

Within the first hour, we observed a strong increase in myocardial activity associated with the onset of myocardial lactate production, lasting 2 hours and corresponding to a myocardial anaerobic metabolism period. Despite the apparent normalization before excision of the hearts, phosphorus-31 nuclear magnetic resonance spectroscopy revealed a significant decrease in adenosine triphosphate levels in group II when compared with group I.

CONCLUSIONS

We conclude that, in our study, acute experimental brain death is associated with an early and transient period of myocardial anaerobic metabolism and adenosine triphosphate consumption. These myocardial consequences of brain death could partially explain some observations of heart graft dysfunction.

Acidosis during ischemia promotes adenosine triphosphate resynthesis in postischemic rat heart. In vivo regulation of 5'-nucleotidase

Capacity for ATP resynthesis during recovery from ischemia or hypoxia is limited to the size of the adenine nucleotide pool, which is determined in part by the activity of cytosolic 5'-nucleotidase (5'-NT): AMP-->adenosine plus inorganic phosphate (Pi). To define in vivo regulation of 5'-NT, we used the tools of 31P nuclear magnetic resonance (NMR), spectroscopy and chemical assay to measure the substrates (AMP), products (Pi, adenosine, and its catabolites), and inhibitors (Pi and H+) of 5'-NT in isolated perfused rat hearts exposed to hypoxia (where pH remains near 7) and no flow, global ischemia (where pH falls to 6.1). We estimated 5'-NT reaction velocity, assessed the relative contributions of Pi and H+ to enzyme inhibition, and defined the consequences of changes in 5'-NT activity on ATP resynthesis after hypoxia and ischemia. We conclude that (a) 5'-NT is activated during hypoxia and early ischemia but is inhibited during prolonged ischemia, (b) H+ (pH < 6.2) is a potent inhibitor of 5'-NT, and (c) differences in AMP accumulation are sufficient to explain the differences in the capacity for net ATP resynthesis in ischemic and hypoxic tissue. These observations have implications for our understanding of heterogeneity of ischemic injury and myocardial protection during ischemia.

Calcium and sodium control in hypoxic-reoxygenated cardiomyocytes

When oxygen-deprived cardiomyocytes become energy depleted, they accumulate Na+ and Ca2+ in the cytosol. Influx of Ca2+ via the Na+/Ca2+ exchange mechanism seems to contribute to the development of Ca2+ overload, but Ca2+ overload may eventually also occur when this route is blocked. Hypoxic-reoxygenated cardiomyocytes in a state of severe overload of Na+ and Ca2+ can rapidly re-establish a normal cation control when oxidative energy production is re-initiated. The recovery of cellular Ca2+ control may be divided into three stages: first, sequestration of large amounts of Ca2+ into the sarcoplasmic reticulum; second, oscillatory movement of Ca2+ from and back into the sarcoplasmic reticulum and gradual extrusion across the sarcolemma; third, re-establishment of constant low cytosolic Ca2+ concentrations. When the Na+/Ca2+ exchanger is inhibited, extrusion of Ca2+ from the cells' interior is impaired and oscillatory Ca2+ movements between cytosol and sarcoplasmic reticulum continue for long time. Thus, the functions of the sarcoplasmic reticulum and the Na+/Ca2+ exchanger are of crucial importance for the recovery of Ca2+ control in reoxygenated cardiomyocytes. In re-energized cardiomyocytes, a persistent elevation of the cytosolic Ca2+ concentration provokes maximal force development and consecutive mechanical cell injury ("oxygen paradox"). This injury can be prevented when the contractile machinery is inhibited during the initial phase of reoxygenation as long as necessary for the re-establishment of a normal cytosolic Ca2+ control.

Effect of lacidipine on ischaemic and reperfused isolated rabbit hearts

Lacidipine is a new developed dihydropyridine calcium-antagonist, showing a slow onset and long lasting-selective activity. To assess whether the administration of lacidipine protects the myocardium in a dose-dependent manner against ischaemia and reperfusion, isolated rabbit heart were infused with three different concentrations of lacidipine: 10(-10); 10(-9); 10(-8) M. Diastolic and developed pressures were monitored; coronary effluent was collected and assayed for CPK activity and for noradrenaline concentration; mitochondria were harvested and assayed for respiratory activity, ATP production and calcium content and tissue concentration of ATP, creatine phosphate (CP) and calcium were determined. Occurrence of oxidative stress during ischaemia and reperfusion was also monitored in terms of tissue content and release of reduced (GSH) and oxidized (GSSG) glutathione. Treatment with lacidipine at 10(-10) and 10(-9) M had no effects on the hearts when perfused under aerobic condition, whilst the higher dose reduced developed pressure of 36%. The ischaemic-induced deterioration of mitochondrial function was attenuated. On reperfusion treated hearts recovered better than the untreated hearts with respect to left ventricular performance, replenishment of ATP and CP stores and mitochondrial function. The reperfusion-induced tissue and mitochondrial calcium overload, release of CPK and of noradrenaline and oxidative stress were also significantly reduced. The effects of lacidipine were dose-dependent. The lower concentration (10(-10) M) failed to modify ischaemic and reperfusion damage. The dose of 10(-9) M was cardioprotective, but the best effect was found at 10(-8) M. It is concluded that lacidipine infusion provides a dose dependent protection of the heart against ischaemia and reperfusion. Because this protection occurred also at 10(-9) M, in the absence of negative inotropic effect during normoxia and of a coronary dilatory effect during ischaemia, it cannot be attributed to an energy sparing effect or to improvement of oxygen delivery. From our data we can envisage two other major mechanism: -1) membrane protection -2) reduction of oxygen toxicity. The ATP sparing effect occurring at 10(-8) M is likely to be responsable for the further protection.

Incorporation of [14C]hypoxanthine into cardiac adenine nucleotides: effect of aging and post-ischemic reperfusion

In order to investigate whether the 'hypoxanthine salvage' pathway of the cardiac muscle is modified with age, we aerobically perfused isolated hearts of 4-month- and 22-month-old male Wistar rats for 20 min with 0.18 microM [14C]hypoxanthine. A second group of hearts was subjected to a 30-min ischemic perfusion (95% reduction of the coronary flow), followed by 20 min of reperfusion. In this last 20 min, the perfusate contained the same concentration of [14C]hypoxanthine used under the aerobic condition. After 20 min of aerobic perfusion the myocardial levels of ATP were significantly lower (15%) in aged than young rat hearts, whilst no age-related differences were observed at the end of the reperfusion. In the young rats the incorporation of the isotope into ATP, ADP, and AMP was significantly higher (192%, 226%, and 300%, respectively), after 20 min of reperfusion with respect to the aerobic values. On the contrary, in the aged hearts, no significant change in the rate of [14C]-incorporation into ATP was observed during reperfusion, despite an increase of the [14C]-incorporation into ADP and AMP. Moreover, the content of each labeled adenine nucleotide was significantly higher in aged than young hearts at the end of the aerobic period, whereas the incorporation of the labeled hypoxanthine was not affected by age after 20 min of reperfusion. The release of uric acid into coronary effluents was greater (50%) in aged than young rats during the reperfusion period, but no age-dependent differences in the isotope incorporation into uric acid were observed. These data indicate that in the aged rat heart, perfused under aerobic conditions, there is an increased incorporation of hypoxanthine into ATP, although it does not further increase during postischemic reperfusion.

The nucleotide metabolism in lactate perfused hearts under ischaemic and reperfused conditions

It was examined whether lactate influences postischaemic hemodynamic recovery as a function of the duration of ischaemia and whether changes in high-energy phosphate metabolism under ischaemic and reperfused conditions could be held responsible for impairment of cardiac function. To this end, isolated working rat hearts were perfused with either glucose (11 mM), glucose (11 mM) plus lactate (5 mM) or glucose (11 mM) plus pyruvate (5 mM). The extent of ischaemic injury was varied by changing the intervals of ischaemia, i.e. 15, 30 and 45 min. Perfusion by lactate evoked marked depression of functional recovery after 30 min of ischaemia. Perfusion by pyruvate resulted in marked decline of cardiac function after 45 min of ischaemia, while in glucose perfused hearts hemodynamic performance was still recovered to some extent after 45 min of ischaemia. Hence, lactate accelerates postischaemic hemodynamic impairment compared to glucose and pyruvate. The marked decline in functional recovery of the lactate perfused hearts cannot be ascribed to the extent of degradation of high-energy phosphates during ischaemia as compared to glucose and pyruvate perfused hearts. Glycolytic ATP formation (evaluated by the rate of lactate production) can neither be responsible for loss of cardiac function in the lactate perfused hearts. Moreover, failure of reenergization during reperfusion, the amount of nucleosides and oxypurines lost or the level of high-energy phosphates at the end of reperfusion cannot explain lactate-induced impairment. Alternatively, the accumulation of endogenous lactate may have contributed to ischaemic damage in the lactate perfused hearts after 30 min of ischaemia as it was higher in the lactate than in the glucose or pyruvate perfused hearts. It cannot be excluded that possible beneficial effects of the elevated glycolytic ATP formation during 15 to 30 min of ischaemia in the lactate perfused hearts are counterbalanced by the detrimental effects of lactate accumulation.

Effects of ribose on exercise-induced ischaemia in stable coronary artery disease

There is no established treatment specifically aimed at protecting or restoring cardiac energy metabolism, which is greatly impaired by ischaemia. Even after reperfusion, myocardial content of ATP remains low for more than 72 h. Long-term post-ischaemic dysfunction and irreversibility of ischaemic damage have been associated with low ATP content. Evidence that the pentose sugar ribose stimulates ATP synthesis and improves cardiac function led us to test the possibility that ribose increases tolerance to myocardial ischaemia in patients with coronary artery disease (CAD). 20 men with documented severe CAD underwent two symptom-limited treadmill exercise tests on 2 consecutive days; we postulated that the ischaemia induced might bring about changes in ATP metabolism lasting for several days. Patients whose baseline tests showed reproducibility were randomly allocated 3 days of treatment with placebo or ribose 60 g daily in four doses by mouth. Exercise testing was repeated after treatment on day 5. At that time mean (95% confidence interval) treadmill walking time until 1 mm ST-segment depression was significantly greater in the ribose than in the placebo group (276 [220-331] vs 223 [188-259] s; p = 0.002). The groups did not differ significantly in time to moderate angina. In the ribose-treated group the changes from baseline to day 5 in both time to ST depression and time to moderate angina were significant (p less than 0.005), but these changes were not significant in the placebo group. In patients with CAD, administration of ribose by mouth for 3 days improved the heart's tolerance to ischaemia. The presumed effects on cardiac energy metabolism offer new possibilities for adjunctive medical treatment of myocardial ischaemia.

Effect of adenosine triphosphate on the postischemic left ventricular function of the immature myocardium

In this study, the effect of exogenous adenosine triphosphate (ATP) on the immature myocardium was evaluated. Isolated working neonatal rabbit hearts were perfused aerobically for 15 min with Krebs-Henseleit buffer (KHB) at 37 degrees C, and then arrested with St. Thomas solution (STS) in group 1 and STS containing 500 mumol/L of ATP in group 2 at 4 degrees to 6 degrees C and maintained at 10 degrees to 14 degrees C for 60 min. Hearts were reperfused with KHB aerobically at 37 degrees C for 15 min. Each heart served as its own control before and after arrest. Systolic function was significantly depressed in group 1 compared with group 2. There was a significant decrease in the peak left ventricular (LV) systolic pressure in group 1 (preischemia mean [PIM] 54 mm Hg to postischemia mean [PoIM] 42 mm Hg, Student's t test p = 0.007) than in group 2 (PIM 66 to PoIM 62 mm Hg, p = 0.5). The LV pulse pressure decreased in group 1 (PIM 72 to PoIM 54 mm Hg, p = 0.02) but not in group 2 (PIM 84 to PoIM 86 mm Hg, p = 0.9) and the rate of rise of LV pressure (dP/dT) in group 2 improved (PIM 5718 to PoIM 6926 mm Hg, p = 0.4) compared with group 1 (PIM 7021 to PoIM 4125 mm Hg, p = 0.008). The PoIM LV flow (LVF) was greater in group 2 than group 1 (LVF group 1 = 2.7 ml/min, group 2 = 4.5 ml/min). Diastolic pressures were not significantly different in the two groups. Our findings suggest that the incorporation of ATP in STS has a significant effect in improving postischemic LV systolic function in neonatal rabbit hearts.

Effects of norepinephrine on the oxidative pentose phosphate pathway in the rat heart

To examine whether stimulation of alpha-adrenergic receptors may affect the oxidative pentose phosphate pathway (PPP) in the rat heart, norepinephrine (NE) and the alpha-adrenergic agonist norfenephrine were used. NE was administered as a continuous intravenous infusion in awake rats for 3 days. It stimulated the activity of cardiac glucose-6-phosphate dehydrogenase (G-6-PD), the first and regulating enzyme of the oxidative PPP, in a dose-dependent manner. With the highest dose (0.2 mg.kg-1.hr-1), there was also a time-dependent enhancement. The increase observed after 48 hours was attenuated partially by the beta-receptor blocker metoprolol and the alpha-receptor blocker prazosin. It was entirely abolished when both drugs were administered. Carvedilol, a beta-adrenergic blocker and vasodilator with alpha 1-blocking activity (0.5 mg.kg-1.hr-1), prevented the NE-induced increase in cardiac G-6-PD activity, in functional parameters (heart rate, left ventricular systolic pressure, and left ventricular dP/dtmax), and in the heart weight/body weight ratio. The alpha-adrenergic stimulator norfenephrine increased myocardial G-6-PD activity; prazosin prevented this stimulation. NE and norfenephrine also elevated the available pool of cardiac 5-phosphoribosyl-1-pyrophosphate. G-6-PD activity was enhanced in cardiac myocytes freshly isolated from the left ventricle of rats that had received NE infusion for 3 days (12.3 +/- 1.4 units/g protein) compared with control rats (1.5 +/- 0.4 units/g protein). The activity of 6-phosphogluconate dehydrogenase, one of the enzymes in the oxidative PPP, was elevated only moderately from 12.7 +/- 0.7 to 19.1 +/- 1.4 units/g protein. Combined alpha- and beta-receptor blockade with carvedilol attenuated these effects.(ABSTRACT TRUNCATED AT 250 WORDS)

The oxidative pentose phosphate pathway in the heart: regulation, physiological significance, and clinical implications

The capacity of the oxidative pentose phosphate pathway (PPP) in the heart is small, since the activity of glucose-6-phosphate dehydrogenase (G-6-PD), the first and rate-limiting enzyme, is very low. Basically, two mechanisms are involved in the regulation of this pathway. Under normal conditions, G-6-PD is inhibited by NADPH. This can immediately be overcome in the isolated perfused rat heart by increasing the oxidized glutathione and by elevating the NADP+/NADPH ratio. Apart from this rapid control mechanism, there exists a long-term regulation which involves the synthesis of G-6-PD. All catecholamines that were administered stimulated the activity of myocardial G-6-PD in a time- and dose-dependent manner. This stimulation was due to increased new synthesis of enzyme protein, since the G-6-PDmRNA was specifically enhanced. As a consequence of the stimulation of the oxidative PPP, the available pool of 5-phosphoribosyl-1-pyrophosphate (PRPP) was elevated which serves as an important precursor substrate for purine and pyrimidine nucleotide synthesis. The limiting step in the oxidative PPP can be bypassed by ribose which leads to an elevation of the cardiac PRPP pool. The decline in the ATP that is induced in many pathophysiological conditions can be attenuated or even entirely prevented by i.v. infusion of ribose. In some experimental in vivo rat models such as in the overloaded and catecholamine-stimulated heart and in the non-ischemic region of the infarcted heart, the normalization of the metabolic situation was accompanied by an improvement of global heart function. Ribose application has been shown to be beneficial in several clinical disease states such as myoadenylate deaminase deficiency and McArdle's disease. Moreover, ribose facilitated thallium-201 redistribution and markedly improved the detection of reversible ischemic injury of the pig and human heart.

Tolerance of myocardium of aged animals to repeated oxygen deficiency

Hemodynamic and metabolic effects of three times 4 min of oxygen deficiency were investigated in 18-month-old rats in comparison to 4-month-old Wistar rats. Left-ventricular isovolumic-pressure-generating capacity and dp/dtmax during isovolumic conditions and hemodynamic indices during intact circulation were determined in open-chest rats. Additionally, high-energy phosphates were measured at the end of the experiments after 20 min of postasphyxial recovery. Older rats had a significantly reduced isovolumic left-ventricular pressure generating capacity (236 +/- 9 vs 269 +/- 5 mm Hg; p less than 0.05) and a low cardiac index (55 +/- 9 vs 117 +/- 8 ml x min-1 x kg-1). The effects of the oxygen deficiency were comparable in both groups. The isovolumic pressure generating capacity was reduced for 11% vs 14%, and dp/dtmax for 13% vs 13%. The myocardial ATP-content was also decreased for the same extent in both groups (0.6 vs 1.0 mumol/gww). Both hemodynamic and biochemical results indicate that aged myocardium does not have a reduced tolerance to repeated periods of oxygen deficiency.

Protective effects of adenosine in myocardial ischemia

Adenosine is released from the myocardium in response to a decrease in the oxygen supply/demand ratio, as is seen in myocardial ischemia; its protective role is manifested by coronary and collateral vessel vasodilation that increase oxygen supply and by multiple effects that act in concert to decrease myocardial oxygen demand (i.e., negative inotropism, chronotropism, and dromotropism). During periods of oxygen deprivation, adenosine enhances energy production via increased glycolytic flux and can act as a substrate for purine salvage to restore cellular energy charge during reperfusion. Adenosine limits the degree of vascular injury during ischemia and reperfusion by inhibition of oxygen radical release from activated neutrophils, thereby preventing endothelial cell damage, and by inhibition of platelet aggregation. These effects help to preserve endothelial cell function and microvascular perfusion. Long-term exposure to adenosine may also induce coronary angiogenesis.

Nucleotide precursors modify the effects of isoproterenol. Studies on heart function and cardiac adenine nucleotide content in intact rats

In closed-chest rats, isoproterenol (ISO, 25 mg/kg), 5 hours after subcutaneous administration, increased heart rate by 53%, left ventricular (LV) dP/dtmax by 80%, and cardiac output by 37%. LV systolic pressure (LVSP, -10%), mean arterial pressure (MAP, -12%), and total peripheral resistance (TPR, -36%) were diminished. In separate experiments, continuous intravenous infusion of adenine (50 mg/kg/hr) for 5 hours reduced heart rate (-11%), LVSP (-16%), MAP (-20%), TPR (-33%), and LV dP/dtmax (-20%). Cardiac output was increased (+20%). Inosine has been shown to have similar effects, except for a decline in cardiac output. Adenine (50 mg/kg/hr) attenuated the ISO-induced increase in heart rate and LV dP/dtmax and aggravated the decline in LVSP, MAP, and TPR. The increase in cardiac output was not changed. Inosine (200 mg/kg/hr) modified the ISO effects to a similar extent. Ribose (200 mg/kg/hr) added to the adenine infusion did not have functional effects. However, it aggravated the modifying influence of inosine on LVSP, LV dP/dtmax, and MAP. ISO reduced the cardiac ATP content (mumol/g) from a control value of 5.02 +/- 0.06 (n = 12) to 3.51 +/- 0.13 (n = 10). Adenine (3.56 +/- 0.21, n = 7) and ribose (3.64 +/- 0.11, n = 9) alone did not affect it, but inosine attenuated it (4.33 +/- 0.08, n = 8). Adenine and inosine in combination with ribose abolished the ISO-induced ATP decline (5.18 +/- 0.23, n = 7, and 4.76 +/- 0.10, n = 8, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Ribose infusion accelerates thallium redistribution with early imaging compared with late 24-hour imaging without ribose

To determine if early (4-h) thallium-201 imaging with ribose infusion would enhance detection of thallium redistribution better than late (24-h) imaging without ribose infusion, 15 patients with coronary artery disease underwent thallium stress tests by both methods within 2 weeks. All 15 patients had quantitative coronary angiography. After immediate postexercise planar imaging during the first of two exercise tests, patients were randomized to receive either intravenous ribose (3.3 mg/kg per min) or a control infusion of saline solution for 30 min. Images performed at 4 h for the ribose study were compared with those at 24 h for the saline control study. During the second test, exercise was carried to the same rate-pressure product and each patient received the opposite infusion. Four-hour postexercise images after ribose infusion identified 21 reversible defects not seen in the 24-h saline study. Three reversible defects were seen only in saline studies, but not with ribose at 4 h (p less than 0.01); 15 reversible defects were seen with both tests. When analyzed with respect to the 31 vascular territories supplied by a coronary artery with a greater than 50% stenosis, 8 territories had reversible defects present in the ribose but not the saline study and the saline study did not demonstrate reversible defects in territories that were seen in the ribose study (p less than 0.01). In 14 of these territories, reversible defects were seen with both tests. In 6 of 15 patients, additional vascular territories with reversible defects were identified after ribose infusion. It is concluded that ribose enhances the detection of thallium redistribution at 4 h compared with 24-h control images in patients with coronary artery disease and, therefore, substantially improves the identification of viable ischemic myocardium.

Adenine nucleotide synthesis in exercising and endurance-trained skeletal muscle

Strenuous exercise leads to increased efflux of purine nucleoside and base that should necessitate recovery of adenine nucleotides by either the de novo synthesis or salvage pathway. De novo synthesis of adenine nucleotide was measured in quiescent and contracting muscle of sedentary and exercise-trained rats using an isolated perfused hindquarter preparation. Synthesis rates were assessed by measuring the incorporation of [1-14C]glycine into adenine nucleotide in muscles of both resting and stimulated hindlimbs after 1 h of either low- or high-energy demand isometric contractions. In nonstimulated sedentary and trained muscles, rates of de novo synthesis were similar. The effect of muscle contractions on de novo synthesis varied among muscle fiber types. Contracting, nonfatigued fast-twitch muscle sections showed significant declines in de novo synthesis in both sedentary and trained groups. Rates in slow-twitch red fibers and fatigued fast-twitch white fiber sections were not different from rest. Supplementing the perfusate with 5 mM ribose caused de novo synthesis to rise three- to fourfold in each of the fiber sections. However, the response in synthesis rates due to exercise was similar with or without ribose supplementation. De novo synthesis does not increase during exercise but exhibits an unchanged or reduced rate depending on the expected energy balance within the cell. This would occur if the energy state of muscle exerts significant control over de novo synthesis of adenine nucleotide.

ATP precursor depletion and postischemic myocardial recovery

Although cardioplegia reduces myocardial metabolism during ischemia, adenosine triphosphate (ATP) depletion occurs, which may contribute to poor functional recovery after reperfusion. Augmenting myocardial adenosine during ischemia is successful in improving ATP repletion and myocardial recovery following ischemia. If adenosine is an important determinant of ischemic tolerance, then depletion or elimination of myocardial adenosine should lead to poor functional and metabolic recovery after ischemia. To test this hypothesis, isolated, perfused rabbit hearts were subjected to 120 min of 34 degrees C ischemia. Hearts received St. Thomas cardioplegia alone or cardioplegia containing 200 microM adenosine, or cardioplegia containing 15, 5, 2.5, or 0.025 micrograms/ml adenosine deaminase (ADA), which catalyzes the breakdown of adenosine to inosine, making adenosine unavailable as an ATP precursor. Functional recovery was determined and myocardial nucleotide levels were measured before, during, and after ischemia. Following ischemia and reperfusion, control hearts recovered to 51 +/- 3% of preischemic developed pressure (DP). There was significantly better recovery in adenosine-augmented hearts (68 +/- 7%), while ADA hearts had significantly worse recovery. Hearts treated with 0.025 microgram/ml ADA recovered to only 29 +/- 5% of DP and higher dose ADA hearts failed to demonstrate any recovery of systolic function. Furthermore, adenosine enhanced metabolic recovery, whereas ADA resulted in greatly depleted ATP and precursor reserves. Postischemic developed pressure closely paralleled the availability of myocardial adenosine, consistent with the hypothesis that myocardial adenosine levels at end ischemia and early reperfusion are important determinants of functional recovery after global ischemia.

Beta-adrenergic agonists stimulate the oxidative pentose phosphate pathway in the rat heart

The oxidative pentose phosphate pathway is poorly developed in the rat heart compared with other organs, since the activity of glucose-6-phosphate dehydrogenase (G-6-PDH), the first and rate-limiting enzyme of the oxidative pentose phosphate pathway, is low. As a consequence, the available pool of 5-phosphoribosyl-1-pyrophosphate and the rate of adenine nucleotide biosynthesis are limited. Isoproterenol, 24 hours after subcutaneous administration at 0.1, 1, and 25 mg/kg, stimulated the activity of G-6-PDH in whole hearts dose-dependently from 4.3 +/- 0.16 (control) to 6.6 +/- 0.35, 10.3 +/- 0.82, and 11.5 +/- 0.56 units/g protein, respectively. The activity of 6-phosphogluconate dehydrogenase, another of the enzymes in the oxidative pentose phosphate pathway, remained unchanged. G-6-PDH activity started to increase 12 hours after isoproterenol application, when the glycogenolytic and functional response was over, and reached a peak value between 24 and 48 hours. This stimulating effect was also demonstrated in cardiac myocytes that were isolated 28 hours after isoproterenol application. beta-receptor blockade with atenolol reduced the isoproterenol-induced increase in cardiac G-6-PDH activity by 90%. Cycloheximide, which inhibits translation, and actinomycin D, which interferes with transcription, attenuated it by 83% and 78%, respectively. These results indicate that cardiac beta-adrenergic receptors and enzyme protein synthesis are involved in this effect. Other beta-sympathomimetic agents such as dopamine, dobutamine, fenoterol, salbutamol, and terbutaline also stimulated myocardial G-6-PDH activity in a time- and dose-related manner. The calcium antagonist D 600 (gallopamil) reduced the isoproterenol-elicited stimulation by 65%, and verapamil blunted the fenoterol-induced increase by 50%. This suggests that Ca2+ ions also contribute to the stimulation of the cardiac oxidative pentose phosphate pathway.

Effect of ATP synthesis promoters on postischemic myocardial recovery

The use of cardioplegia during surgically induced ischemia greatly reduces myocardial metabolic requirements. However, adenosine triphosphate (ATP) depletion may occur, resulting in poor functional recovery after ischemia. This study investigated if augmentation of intracellular ATP could be achieved by delivering known ATP synthesis promoters (adenosine and/or phosphate) during cardioplegic arrest, and whether this could enhance myocardial functional and metabolic recovery following ischemia. Isolated, perfused rabbit hearts were subjected to 120 min of hypothermic (34 degrees C) cardioplegia-induced ischemia. Controls received St. Thomas cardioplegia (CTL); remaining hearts received cardioplegia containing 200 microM adenosine (ADO), or 25 microM phosphate (PO4), or both ADO and PO4. Following ischemia and reperfusion, recovery of developed pressure (%DP) and postischemic diastolic stiffness was significantly better in adenosine hearts when compared with control or PO4 hearts. To determine if ADO or PO4 minimized depletion of ATP during ischemia or accelerated synthesis of ATP in the postischemic period, nucleotide levels were obtained before, during, and after ischemia. During ischemia, ATP fell equally in all groups, indicating that ADO and PO4 did not alter ischemia-induced depletion of ATP. However, intracellular adenosine was augmented during ischemia in adenosine-treated hearts. Consequently, during reperfusion, ADO and ADO/PO4 hearts had significantly enhanced ATP levels, suggesting that augmenting myocardial adenosine accelerated synthesis of ATP postischemia. The addition of phosphate, a stimulus for ATP synthesis, did not augment postischemic ATP. In fact, the beneficial effect of adenosine may have been decreased when phosphate was added to adenosine. In conclusion, adenosine but not PO4 augments intracellular ATP by allowing better metabolic repletion following ischemia, thereby improving postischemic myocardial functional recovery.

Recovery of postischemic myocardial ATP levels and hexosemonophosphate shunt activity

Recovery of myocardial ATP levels after a period of ischemia is slow, requiring several days to return to normal. The biochemical limitation for ATP recovery appears to be the availability of the adenine nucleotide (AN) precursor, phosphoribosylpyrophosphate (PRPP), which is produced by the phosphorylation of ribose in the hexosemonophosphate shunt (HMP). In fact, ATP precursors, in particular ribose, have been shown to enhance the rate of postischemic ATP recovery. Infusion of exogenous ribose bypasses the rate-limiting steps in the HMP and speeds up adenine nucleotide (AN) biosynthesis. I propose another method for augmenting the rate of postischemic ATP recovery; increase the flux of substrate through the HMP. This would have the effect of making more PRPP available for AN biosynthesis. Effective physiologically and biologically tolerable means for enhancing HMP activity are presently available. These may be of significant utility in facilitating postischemic myocardial energy recovery.

Adenosine-induced increase in myocardial ATP: are there beneficial effects for the ischaemic myocardium?

The adenosine triphosphate (ATP) content of isolated Langendorff-perfused rat hearts may be increased by more than 40% above the normal value by a 2-h perfusion with adenosine (15 mumol/l). This metabolic manipulation was used to investigate the hypothetical relationship between total tissue ATP content and ischaemia-induced contractile failure, ischaemic contracture and post-ischaemic functional recovery. Adenosine perfused hearts were submitted to 20 min of normothermic ischaemia and reperfused for 45 min with or without adenosine. Control experiments were performed with adenosine-free preischaemic perfusion. In identically designed experiments the tissue-protective effect of diltiazem (0.5 mumol/l) was determined and compared with the experiments with adenosine. At the end of 120 min of preischaemic perfusion, the ATP content of the adenosine treated hearts was 34.3 +/- 1.8 mumol/g dry weight (control = 23.6 +/- 1.9 mumol/g, p less than 0.01). After a period of 20 min of normothermic ischaemia, the ATP content of the adenosine hearts decreased to 13.3 +/- .4 mumol/g, whereas ATP fell to 8.3 +/- 1.6 mumol/g in the control hearts. The creatine phosphate (CP) levels of adenosine hearts were significantly lower than those of the control group before ischaemia, but did not show major differences following ischaemia. During ischaemia, the contractile activity measured via an intraventricular balloon catheter, as well as ischaemic contracture did not differ between the adenosine and control hearts. The inclusion of diltiazem into the perfusate significantly delayed the onset of contracture. After 45 min of reperfusion, ATP contents of adenosine and control hearts reached similar values (8.4 +/- 2.3 and 8.3 +/- 2.9 mumol/g, respectively). Inclusion of adenosine (15 mumol/l in the reperfusion perfusate of the adenosine experiments prevented a further decrease, but did not increase tissue ATP content. CP values of all groups showed a partial recovery upon reperfusion, they did not differ significantly. Contractile recovery was equal in all experimental groups except for the diltiazem treated hearts, which showed during the first 10 min of reperfusion an improved mechanical performance. It is concluded that total tissue ATP is not necessarily a good indicator of functional capabilities under conditions of normothermic ischaemia and reperfusion in the isolated rat heart.

Triiodothyronine-induced changes in function, metabolism and weight of the rat heart: effects of alpha- and beta-adrenergic blockade

The involvement of alpha- and beta-adrenergic receptors in the triiodothyronine (T3)-induced hemodynamic and metabolic alterations and in the development of cardiac hypertrophy was analyzed in time-course studies. Female Sprague-Dawley rats received daily injections of T3 (200 micrograms/kg s.c.) and a continuous i.v. infusion of 0.9% NaCl or alpha- or beta-receptor blocking agents. NaCl-injected animals served as controls. Eighteen hours after T3 administration, heart rate and LV dP/dtmax were considerably elevated. Cardiac output (CO) was not significantly changed. These alterations were abolished by simultaneous infusion of the beta-adrenergic blocker metoprolol. After 48 hours, CO as well as the cardiac RNA concentration were markedly elevated. The rise in LV dP/dtmax and heart rate was similar to the 18-h-value and was prevented by metoprolol. However, metoprolol did not influence the increase in CO and RNA concentration. Likewise, after 72 hours, metoprolol antagonized the T3-induced increase in heart rate and LV dP/dtmax, but had no effect on the elevation of CO and RNA concentration, the enhancement of adenine nucleotide synthesis and cardiac hypertrophy. Like metoprolol, the alpha-adrenergic blocker prazosin did not influence the T3-evoked cardiac hypertrophy. Thus, the development of cardiac hypertrophy in this model can occur independently of alpha- and beta-adrenergic stimulation.

Enhanced myocardial protection with adenosine

This study was undertaken to investigate whether adenosine administered during cardioplegic arrest could enhance myocardial protection and improve recovery of function after ischemia. Isolated perfused rabbit hearts were subjected to 120 minutes of hypothermic (32 degrees C) multidose cardioplegia-induced ischemia. Control hearts (n = 23) received modified St. Thomas's cardioplegia, and the remaining hearts received cardioplegia with either 100 microM (n = 11), 200 microM (n = 11), or 400 microM (n = 11) adenosine. After ischemia and 45 minutes of reperfusion, left ventricular contractility was superior in all groups of adenosine-treated hearts compared with control hearts. Furthermore, there was a significant incremental increase in functional recovery with increasing dose of adenosine. Postischemic diastolic stiffness was significantly better in all adenosine groups compared with controls. No differences were noted in coronary flow or myocardial water content between adenosinetreated and control hearts. These data demonstrate that adenosine administered in these concentrations provides myocardial protection and improved recovery of both systolic and diastolic function after global ischemia, presumably metabolically by reducing depletion of adenosine triphosphate or enhancing repletion of adenosine triphosphate and enabling improved postischemic recovery.

Purification, immunoassay, and tissue distribution of rat C1-tetrahydrofolate synthase

C1-tetrahydrofolate synthase (C1-THF synthase), a eukaryotic trifunctional enzyme, catalyzes three sequential folate-mediated one-carbon interconversions. These three reactions supply the activated one-carbon units required in the metabolism of purines, thymidylate, and several amino acids. In order to study the regulation of C1-THF synthase expression in mammals, we have purified the enzyme to homogeneity from rat liver, raised polyclonal antisera to it in rabbits, and developed a sensitive solid-phase immunoassay for the enzyme. The enzyme was purified approximately 600-fold to a specific activity of 24.6 U/mg protein based on 10-formyl-THF synthetase activity. Western blot analysis indicated that the antisera is specific for one protein in crude liver extracts which comigrates with purified C1-THF synthase. Using the solid-phase immunoassay, as little as 200 pg of immunoreacting protein can be detected in tissue homogenates. Several rat tissues were examined for the three C1-THF synthase enzymatic activities and immunoreactive protein. The results indicated that the level of C1-THF synthase is regulated in a tissue-specific manner. Enzyme assays revealed that certain tissues differ by more than 100-fold in enzyme activity, with liver and kidney containing the highest levels, and lung and muscle the lowest. However, immunoassay of these same tissues indicated only a 10-fold difference in C1-THF synthase concentration. This apparent masking of enzyme activity was observed in all tissues, but to varying degrees. These results emphasize the advantages of an immunoassay in studying the regulation of C1-THF synthase.

De novo synthesis of adenine nucleotides in different skeletal muscle fiber types

Management of adenine nucleotide catabolism differs among skeletal muscle fiber types. This study evaluated whether there are corresponding differences in the rates of de novo synthesis of adenine nucleotide among fiber type sections of skeletal muscle using an isolated perfused rat hindquarter preparation. Label incorporation into adenine nucleotides from the [1-14C]glycine precursor was determined and used to calculate synthesis rates based on the intracellular glycine specific radioactivity. Results show that intracellular glycine is closely related to the direct precursor pool. Rates of de novo synthesis were highest in fast-twitch red muscle (57.0 +/- 4.0, 58.2 +/- 4.4 nmol.h-1.g-1; deep red gastrocnemius and vastus lateralis), relatively high in slow-twitch red muscle (47.0 +/- 3.1; soleus), and low in fast-twitch white muscle (26.1 +/- 2.0 and 21.6 +/- 2.3; superficial white gastrocnemius and vastus lateralis). Rates for four mixed muscles were intermediate, ranging between 32.3 and 37.3. Specific de novo synthesis rates exhibited a strong correlation (r = 0.986) with muscle section citrate synthase activity. Turnover rates (de novo synthesis rate/adenine nucleotide pool size) were highest in high oxidative muscle (0.82-1.06%/h), lowest in low oxidative muscle (0.30-0.35%/h), and intermediate in mixed muscle (0.44-0.55%/h). Our results demonstrate that differences in adenine nucleotide management among fiber types extends to the process of de novo adenine nucleotide synthesis.