[Effect of substrate supply on post-asphyctic restoration of adenine nucleotides in rabbit hearts in vivo]

Enhanced high energy phosphate recovery with ribose infusion after global myocardial ischemia in a canine model

High energy phosphate levels are depressed following global ischemia and require several days to completely recover. Short-term methods to enhance ATP recovery have included infusion of ATP precursors, inhibition of enzymes that catabolize AMP, and membrane transport stabilization. Several precursors have been used to augment adenine nucleotide synthesis including adenosine, inosine, adenine, and ribose. Because of the short-term nature of previous experiments, recovery had been incomplete and the effects in the intact animal unknown. The purpose of this study was to determine the effects of ribose infusion in a long-term model of global ischemia and attempt to identify the precursor which limits myocardial ATP regeneration in the intact animal. Global myocardial ischemia (20 min, 37 degrees C) was produced in dogs on cardiopulmonary bypass. With reperfusion either ribose (80 mM) in normal saline or normal saline alone was infused at 1 ml/min into the right atrium and the animals were followed for 24 hr. Ventricular biopsies were obtained through an indwelling ventricular cannula prior to ischemia, at the end of ischemia, and 4 and 24 hr postischemia and analyzed for adenine nucleotides and creatine phosphate levels. Radiolabeled microspheres were used to measure myocardial and renal blood flows and no significant difference was found between ribose-treated control groups. In both groups, myocardial ATP levels fell by at least 50% at the end of ischemia. No significant ATP recovery occurred after 24 hr in the control dogs, but in the ribose-treated animals, ATP levels rebounded to 85% of control by 24 hr.(ABSTRACT TRUNCATED AT 250 WORDS)

Dose effects of adenine on myocardial ATP content in the post-anoxic nonworking rat heart

1. Tissue ATP levels were measured in Langendorff perfused nonworking rat hearts subjected to 50 min anoxia prior to reperfusion with Krebs-Ringer bicarbonate (KRB) buffer alone or supplemented with 50 microM or 1 mM adenine for 60 min. 2. ATP content was restored to the normoxic range in hearts reperfused with 50 microM adenine in KRB (20.82 +/- SEM 1.90 mumol/g dry weight vs 24.95 +/- 0.83 in normoxic hearts, P = NS). 3. Reperfusion with oxygenated KRB alone or buffer with 1 mM adenine failed to improve ATP levels (17.23 +/- 0.91 mumol/g dry weight for buffer alone, 15.60 +/- 0.46 with 1 mM adenine and 13.45 +/- 0.93 for anoxic hearts not reperfused). 4. These findings indicate that adenine at 50 microM dosage can restore ATP concentrations to the normoxic range after 60 in of anoxia in the nonworking rat heart while raising the adenine dose to 1 mM inhibited the tissue ATP content.

Long term model for evaluation of myocardial metabolic recovery following global ischemia

Myocardial ATP levels remain depressed following significant periods of ischemia (Isc) despite reperfusion (Rpf). Neither the rate of in vivo ATP return following global Isc nor the factors which influence recovery have been defined. In order to determine the time course to complete the return of ATP levels and evaluate methods of enhancing recovery of ATP levels, we have devised a chronic canine model of global Isc. In this model serial ventricular biopsies can be taken in the awake animal over several days without reoperation which allows an investigation of the recovery of the myocardium following a uniform global insult to be performed. Recovery of ATP levels has been shown to depend, at least in part, on the availability of precursors and the activity of the ATP regenerating enzymes. Because complete recovery of ATP levels takes days, short term (hours) models have limitations. Previous attempts at enhancing ATP recovery following Isc have been only partially successful because either the degree of depression was not great or the period of observation was short, resulting in incomplete return. To identify the best precursor choice, we previously measured the activity of the AMP regenerating enzymes, adenosine kinase (AdK) (adenosine----AMP) and adenine phosphoribosyl transferase (APRT) (adenine----AMP). Because APRT activity was 20 fold higher than AdK with similar Km values for substrates, it appeared that adenine (A) is preferred to adenosine for AMP regeneration in the dog's myocardium. The formation of 5-phosphoribosyl 1-pyrophosphate (PRPP) may also be rate limiting and, therefore, the effect of ribose (R) on ATP recovery was also evaluated. Recovery of ATP levels was assessed in three groups: (1) normal saline (NS), (2) A (20 mM) in normal saline (A/NS) or (3) A with R (80 mM) in normal saline (A/R) were infused (1.0 ml/min) into the right atrium of dogs for 48 hours following Isc. In all groups, ATP levels fell to between 46-60% of pre-Isc levels during Isc. In the NS dogs, ATP levels continued to fall slightly to 46% pre-Isc levels during the first four hours of Rpf after Isc. By 24 hours no appreciable recovery had occurred and the measured ATP was only 51% of the pre-Isc value. Even by seven days, ATP had not returned fully, and by extrapolation, complete recovery required 9.9 +/- 1.4 days. Treated dogs showed, however, that ATP recovery could be significantly enhanced.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of adenosine and AICAR on ATP content and regional contractile function in reperfused canine myocardium

We investigated whether the postischemic acceleration of adenosine triphosphate (ATP) synthesis by means of precursor infusion is beneficial for the contractile function of reperfused myocardium. A coronary artery was occluded for 45 min in 21 dogs to produce a marked but reversible ischemia. During the following 3 hours of reperfusion either adenosine (n = 6) or AICAR (5-amino-imidazole-4-carboxamide-riboside) (n = 6) was infused intracoronarily by a small transfemoral catheter positioned in the LAD. ATP repletion by adenosine was nearly 50% of the deficit caused by the previous ischemia, the effect of AICAR on steady-state tissue ATP concentration was insignificant. Regional systolic function of these both groups was compared to that of a control group (n = 9) receiving only a saline infusion. We measured the regional function by subendocardially implanted ultrasound transducers using the transit time method. All three groups showed a reduction to about 25% of the initial segment shortening at the end of ischemia, followed by a quick recovery to half of the preocclusion segment shortening after reopening of the vessel. No further changes were observed in the control series during the 3 hours of reperfusion (50 +/- 10% SE segment shortening at the end). With adenosine infusion - in spite of the resulting considerable ATP elevation - no significant change of segmental contractile function occurred (44 +/- 5% SE segment shortening). Only the AICAR treated group differed from control. It produced a continuous deterioration during reflow resulting in a holosystolic bulging of -20% +/- 10% SE at the end of 3 hours of reperfusion. Our results show that there is no correlation between different ATP tissue levels achieved by adenosine infusion and systolic function in reperfused myocardium after regional reversible ischemia. We hypothesize that reperfusion dyskinesia is caused by a failure of energy utilisation rather than of energy supply.

Influence of ribose, adenosine, and "AICAR" on the rate of myocardial adenosine triphosphate synthesis during reperfusion after coronary artery occlusion in the dog

Recovery of adenosine triphosphate after myocardial ischemia is limited by the slow adenine nucleotide de novo synthesis and the availability of precursors of the nucleotide salvage pathways. We determined the adenine nucleotide de novo synthesis in the dog by infusion of [14C]glycine and the acceleration of adenine nucleotide built up by intracoronary infusion of ribose together with [14C]glycine or radiolabeled 5-amino-4-imidazolcarboxamide riboside or adenosine in the same animal model and with the same dosage of substrates (9 mmol) in postischemic and nonischemic myocardial tissue. After 45 minutes of occlusion of a side branch of the left coronary artery, the ischemic area was reperfused for 3 hours, and needle biopsies were taken for biochemical analysis. Adenine nucleotide de novo synthesis was found to be very slow (1.5 nmol/g wet weight per hour). The rate was doubled after ischemia. Adenine nucleotide synthesis was accelerated 5-fold by ribose, the basic substrate of the adenine nucleotide de novo synthesis, 9-fold by 5-amino-4-imidazolcarboxamide riboside, an intermediate of the adenine nucleotide de novo synthesis and 90-fold by adenosine, a substrate of the nucleotide salvage pathway. Therefore, only adenosine infusion resulted in a measurable increase of adenosine triphosphate levels after 3 hours of reperfusion, but over a longer time period, ribose or 5-amino-4-imidazol-carboxamide riboside also can be expected to replenish reduced myocardial adenosine triphosphate faster than adenine nucleotide de novo synthesis. Studies with radiolabeled 5-amino-4-imidazol-carboxamide riboside showed significant incorporation of radioactivity into 5-amino-4-imidazol-carboxamide ribose triphosphate which had also risen measurably during 5-amino-4-imidazol-carboxamide ribose infusion, and which is not normally found in heart muscle.

Adenosine-induced increase in myocardial adenine nucleotides without adenosine-induced systemic hypotension

In anaesthetized rabbits, the i.v. application of 1% adenosine (Ado) for 3 hours at a rate of 4 ml X h-1 X kg-1 body weight increased the myocardial tissue levels of adenine nucleotides (AN) above the normal values by 39%. This increase in ATP and the sum of AN is a metabolic effect of the continuous and high supply of Ado and does not result from the Ado-induced systemic hypotension: Neither a comparable hypotension and reduction of circulatory work induced by phentolamine nor a massive volume loading caused changes in the AN. The compensation of the Ado-induced hypotension by a simultaneous i.v. application of caffeine or xylometazoline did not interfere with the accumulation of AN. The increase in AN was less pronounced, if norepinephrine was infused to maintain normotension. The increase in AN occurred in left and right ventricular myocardium to a similar extent, although the pressure-volume-work of the left ventricle decreased, and that of the right ventricle increased during Ado-application.

Interrelation between salvage of purine nucleotides and protein synthesis in rat heart cells

The effect of transition from a respiring to a respiration-inhibited state on the rate of protein synthesis was investigated in glycolyzing, cultured rat heart cells. The rate was found to be significantly lower after blocking respiration, and it was further decreased by L-lactate. In contrast, pyruvate or phenazine methosulfate prevented the drop in the rate caused by lack of respiration. The changes in the respiratory state also affected the steady-state concentration of ATP, which varied in the same sense as the rate of protein synthesis. Pyruvate or phenazine methosulfate induced an increment in the concentration of ATP of respiration-inhibited cells. This increment could not be accounted for by more extensive phosphorylation of the available purine nucleotides, but required repletion of the pool by synthesis of purine nucleotides through the salvage pathway. Pyruvate and phenazine methosulfate were found to stimulate incorporation of labeled hypoxanthine into the purine nucleotide fraction in general, and into the nucleotide triphosphates in particular. Under similar incubation conditions an increase in the ATP/ADP ratio was also noted. The stimulatory effect of pyruvate on protein synthesis and on the cellular level of ATP was also observed in respiration-inhibited 3T6 cells and in human fibroblasts, but not in human fibroblasts deficient in the salvage enzyme, hypoxanthine-guanine-phosphoribosyltransferase. Based on the demonstrated influence of L-lactate, pyruvate, and phenazine methosulfate on the salvage synthesis of purine nucleotides [K. Ravid, P. Diamant, and Y. Avi-Dor, (1984) Arch. Biochem. Biophys. 229, 632-639] and on the present findings, the connection between protein synthesis and the salvage activity is discussed.

Metabolic intervention to affect myocardial recovery following ischemia

Myocardial recovery during reperfusion following ischemia is critical to patient survival in a broad spectrum of clinical settings. Myocardial functional recovery following ischemia correlates well with recovery of myocardial adenosine triphosphate (ATP). Adenosine triphosphate recovery is uniformly incomplete during reperfusion following moderate ischemic injury and is therefore subject to manipulation by metabolic intervention. By definition ATP recovery is limited either by (1) energy availability and application in the phosphorylation of adenosine monophosphate (AMP) to ATP or (2) availability of AMP for this conversion. Experimental data suggest that substrate energy and the mechanisms required for its application in the creation of high energy phosphate bonds (AMP conversion to ATP) are more than adequate during reperfusion following moderate ischemic injury. Adenosine monophosphate availability, however, is inadequate following ischemia due to loss of diffusable adenine nucleotide purine metabolites. These purine precursors are necessary to fuel adenine nucleotide salvage pathways. Metabolic interventions that enhance AMP recovery rather than those that improve substrate energy availability during reperfusion are therefore recommended. The mechanisms of various metabolic interventions are discussed in this framework along with the rationale for or against their clinical application.

Characterization of purine nucleotide metabolism in primary rat cardiomyocyte cultures

Primary rat cardiomyocyte cultures were utilized as a model for the study of purine nucleotide metabolism in the heart muscle, especially in connection with the mechanisms operating for the conservation of adenine nucleotides. The cultures exhibited capacity to produce purine nucleotides from nonpurine molecules (de novo synthesis), as well as from preformed purines (salvage synthesis). The conversion of adenosine to AMP, catalyzed by adenosine kinase, appears to be the most important physiological salvage pathway of adenine nucleotide synthesis in the cardiomyocytes. The study of the metabolic fate of IMP formed from [14C]formate or [14C]hypoxanthine and that of AMP formed from [14C]adenine or [14C]adenosine revealed that in the cardiomyocyte the main flow in the nucleotide interconversion pathways is from IMP to AMP, whereas the flux from AMP to IMP appeared to be markedly slower. Following synthesis from labeled precursors by either de novo or salvage pathways, most of the radioactivity in purine nucleotides accumulated in adenine nucleotides, and only a small proportion of it resided in IMP. The results suggest that the main pathway of AMP degradation in the cardiomyocyte proceeds through adenosine rather than through IMP. About 90% of the total radioactivity in purines effluxed from the cells during de novo synthesis from [14C]formate or following prelabeling of adenine nucleotides with [14C]adenine were found to reside in hypoxanthine. The activities in cell extracts of AMP 5'-nucleotidase and IMP 5'-nucleotidase, which catalyze nucleotide degradation, and of AMP deaminase, a key enzyme in the purine nucleotide cycle, were low. The nucleotidase activity resembles, and that of the AMP deaminase contrasts the respective enzyme activities in extracts of cultured skeletal-muscle myotubes. The results indicate that in the cardiomyocyte, in contrast to the myotube, the main mechanism operating for conservation of nucleotides is prompt phosphorylation of AMP, rather than operation of the purine nucleotide cycle. The primary cardiomyocyte cultures are a plausible model for the study of purine nucleotide metabolism in the heart muscle.

Management of acute myocardial infarction with natural physiological agents

A number of natural physiological agents deserve evaluation in the treatment of acute myocardial infarction. Prostacyclin and magnesium dilate large coronary arteries and could promote collateral circulation to ischemic regions, especially if used in conjunction with alpha-agonists to prevent a drop in coronary perfusion pressure. In addition, prostacyclin has anti-aggregatory and de-aggregatory effects on platelets and a stabilizing action on hypoxic tissue, while magnesium has anti-arrhythmic, potassium-retaining, and fibrinolytic effects, all of which could improve the outcome in acute MI. Adenosine or ribose infusion could be used to promote rapid repletion of adenine nucleotides in reperfused tissue, but unfortunately arteriolar vasodilation by adenosine might reduce collateral perfusion by "coronary steal". High-dose insulin has positive-inotropic (at minimal oxygen cost) and potent anti-arrhythmic actions that have not been adequately tested in previous clinical trials of "polarizing solutions". Carnitine infusion could improve the bioenergetics of ischemic myocardium by relieving inhibition of mitochondrial adenine nucleotide translocase.

Normalization of depressed heart function in rats by ribose

Severe constriction of the abdominal aorta and simultaneous injection of isoproterenol in rats induced depression in heart function and reductions in cardiac adenosine triphosphate and total adenine nucleotides. When ribose was continuously infused for 24 hours, biosynthesis of cardiac adenine nucleotides was stimulated to such an extent that the reductions in adenosine triphosphate and total adenine nucleotides were prevented and left ventricular hemodynamic parameters were normal. These results support the hypothesis that adenosine triphosphate is primarily responsible for depression in myocardial contractility and that ribose is cardioprotective through its pronounced effects on adenine nucleotide metabolism in heart muscle.

Nucleotide degradation and functional impairment during cardioplegia: amelioration by inosine

The degradation of adenine nucleotide levels and impairment of functional recovery associated with exposure to hypothermic (20 degrees C) cardioplegia was studied in 84 isolated working rat hearts. After a 1-hour control period, hearts were exposed to 1 hour of cardioplegia that consisted of increasingly longer periods of cardioplegic solution (CPS) infusion (30 seconds and 10, 30 and 60 minutes), followed by increasingly shorter periods of global ischemia (591/2 minutes and 50, 30 and 0 minutes). Hearts were then reperfused for 1 hour with control perfusate, during which recovery of cardiac output was monitored. Additional hearts were freeze-clamped at various points in the protocols to determine adenine nucleotide levels (ATP, ADP, AMP and their sum TAN). Exposure to increasingly longer periods of CPS perfusion resulted in proportionally greater degradation of nucleotides and poorer recovery of cardiac output. Addition of inosine to the recovery perfusate as well as the CPS further improved nucleotide levels and recovery of cardiac output. These results suggest that washout of nucleotide degradation products in the CPS or reperfusion prevents their salvage for nucleotide resynthesis and impairs functional recovery from cardioplegia.

Adenine nucleotide content and regional function during ischemia and reperfusion in canine ventricular myocardium

We have compared in open-chest dogs the evolution of adenine nucleotides and creatine phosphate to that of regional mechanical function during ischemia and reperfusion. Reperfusion restored normal value of creatine phosphate whereas adenine nucleotide level remained depressed. The percentage of systolic shortening, which after occlusion has a zero or negative value, increased to about 20% of control, after reperfusion similarly the pressure-length loop areas decreased to 14.9% of control during ischemia and increased to 39.3% of control during the early phase of the reperfusion period. No individual correlation was found between the ATP content and segmental function during ischemia or reperfusion, suggesting that a low level of ATP did not prevent some restoration of mechanical function. However, the ATP content measured at the end of the ischemic period was correlated with mechanical function measured 75 min after reperfusion, and thus ATP depletion could appear as an indicator of the severity of ischemic damage.

Reduction of the isoproterenol-induced alterations in cardiac adenine nucleotides and morphology by ribose

Continuous intravenous infusion of ribose (200 milligrams per kilogram per hour) for 24 hours induced a marked stimulation of cardiac adenine nucleotide biosynthesis in unanesthetized and unrestrained rats that had been treated with isoproterenol subcutaneously (25 milligrams per kilogram). The diminution of adenine nucleotides characteristic for the action of high doses of isoproterenol was entirely prevented, and the incidence of the isoproterenol-induced myocardial cell damage was significantly reduced when ribose was administered. These results support the view that depletion of adenine nucleotides is involved in the development of cardiac necrosis.

Systematic variations in the content of the purine nucleotides in the steady-state perfused rat heart. Evidence for the existence of controlled storage and release of adenine nucleotides

1. The contents of the major purine nucleotides in the isolated non-working perfused rat heart varied systematically during 80min of perfusion. In particular the amounts of ATP, ADP, GTP, cyclic AMP and cyclic GMP in the well-oxygenated myocardium showed changes ranging from 25 to 60% of the mean concentrations. The apparent periodicity was about 30min for some and about 60min for other nucleotides. 2. These data are in contrast with measurements of parameters reflecting heart performance, which remained constant over this period of perfusion. 3. The ATP/ADP ratio, the cyclic AMP content, the GTP content and the GTP/GDP ratio in the tissue bore a constant relationship to one another, and all showed the same temporal variation. 4. Increasing the energy demand on the heart by administration of bovine somatotropin (1mug/ml) tended to damp the variations, and generally lower the content of all the nucleotides. 5. The total extractable adenine nucleotide pool also showed systematic temporal variations of as much as 1.3mumol/g wet wt. of tissue within 10min. 6. These variations could not be accounted for as inter-conversion with adenosine, other purine nucleotides, nucleosides or purine-degradation products either in the tissue or in the perfusion medium. No evidence was found in this preparation of the purine nucleotide oscillations described by Lowenstein and his co-workers [see Tornheim & Lowenstein (1975) J. Biol. Chem.250, 6304-6314]. 7. Further, the pool size increases cannot be satisfactorily explained by either synthesis de novo or the breakdown of any purine macromolecular species in the cell. Thus it is suggested that an unsuspected substantial storage form of purine nucleotide may exist in heart.

Stimulation of myocardial adenine nucleotide biosynthesis by pentoses and pentitols

In rats, pentoses and pentitols, intravenously injected in a single dose of 100 mg/kg, induced a considerable enhancement of the available pool of 5-phosphoribosyl-1-pyrophosphate and of the rate of adenine nucleotide biosynthesis in the heart, but not in liver and kidney. De novo synthesis of adenine nucleotides not detectable in skeletal muscle of normal rats became measurable after application of ribose. The stimulatory effect of isoproterenol on myocardial adenine nucleotide biosynthesis could be further potentiated by ribose and xylitol, but not by glucose. The isoproterenol-induced decrease of cardiac adenine nucleotide concentrations could be almost completely prevented by repeated administrations of ribose. Thus, pentoses and pentitols in combination with beta-receptor stimulation markedly and quite specifically enhance adenine nucleotide biosynthesis in the rat heart. The results indicate that the increase in the available pool of 5-phosphoribosyl-1-pyrophosphate is an important factor for the enhancement of cardiac adenine nucleotide biosynthesis. Moreover, the availability of 5-phosphoribosyl-1-pyrophosphate and the rate of de novo synthesis of adenine nucleotides in the heart seem to be limited by the flow through the hexose monophosphate shunt.

Postbypass treatment

Ischemia induced by cross-clamping the aorta during open-heart operations initiates progressive metabolic derangement. If the duration of ischemia is short, these derangements are easily reversed by restoring the flow of blood containing oxygen and substrate. If ischemia is prolonged, treatment designed to ameliorate ischemic damage may be necessary. Three problems are discussed: (1) loss of adenine nucleotides, particularly adenosine triphosphate, (2) impairment of calcium sequestration, and (3) formation of microemboli in coronary vessels. The rationale for postbypass treatment is presented.