Superiority of HPLC to Assay for Enzymes Regulating Adenine Nucleotide Pool Intermediates Metabolism: 5'-Nucleotidase, Adenylate Deaminase, Adenosine Deaminase, and Adenylosuccinate Lyase-A Simple and Rapid Determination of Adenosine

Myocardial protection in beating heart cardiac surgery: I: pre- or postconditioning with inhibition of es-ENT1 nucleoside transporter and adenosine deaminase attenuates post-MI reperfusion-mediated ventricular fibrillation and regional contractile dysfunction

OBJECTIVE

To determine the role of the p-nitrobenzylthioinosine-sensitive equilibrative nucleoside transporter 1 (es-ENT1) in postmyocardial infarction reperfusion injury-mediated ventricular fibrillation and regional dysfunction. We used erythro-9 (2-hydroxy-3-nonyl)-adenine and p-nitrobenzylthioinosine to inhibit both adenosine deamination and transport in a canine model of off pump acute myocardial infarction.

METHODS

Anesthetized adult dogs (n = 37), instrumented to monitor the percentage of systolic segmental shortening and wall thickening using sonomicrometry, underwent 90 minutes of left anterior descending coronary artery occlusion and 120 minutes of reperfusion. Myocardial coronary blood flow, adenosine triphosphate pool, infarct size, and the incident of ventricular fibrillation and cardioversion were also measured. The dogs received an intravenous infusion of the vehicle (control) or 100 μM of erythro-9 (2-hydroxy-3-nonyl)-adenine and 25 μM p-nitrobenzylthioinosine before ischemia (preconditioning group) or just before reperfusion (postconditioning group).

RESULTS

In the control group, adenosine triphosphate depletion was associated with the accumulation of more inosine than adenosine during ischemia and washed out during reperfusion. Myocardial adenosine and inosine were the major nucleosides in the pre- and postconditioning groups during ischemia and remained detectable during reperfusion. In both groups, recovery of systolic segmental shortening and wall thickening and a reduction in the incidence of ventricular fibrillation (P < .05 vs the control group) coincided with retention of myocardial nucleosides. The infarct size in the 3 groups was not significantly different, independent of myocardial blood flow during ischemia.

CONCLUSIONS

Preconditioning or postconditioning with erythro-9 (2-hydroxy-3-nonyl)-adenine/p-nitrobenzylthioinosine significantly reduced the incidence of ventricular fibrillation and cardioversion and attenuated regional contractile dysfunction mediated by postmyocardial infarction reperfusion injury. It is concluded that p-nitrobenzylthioinosine-sensitive equilibrative nucleoside transporter 1 played a major role in these events.

Protection of mitochondrial function and improvement in cognitive recovery in rats treated with hyperbaric oxygen following lateral fluid-percussion injury

OBJECT

Hyperbaric oxygen (HBO2) has been shown to improve outcome after severe traumatic brain injury, but its underlying mechanisms are unknown. Following lateral fluid-percussion injury (FPI), the authors tested the effects of HBO2 treatment as well as enhanced normobaric oxygenation on mitochondrial function, as measured by both cognitive recovery and cellular adenosine triphosphate (ATP) levels.

METHODS

Adult male Sprague-Dawley rats were subjected to moderate lateral FPI or sham injury and were allocated to one of four treatment groups: 1) FPI treated with 4 hours of normobaric 30% O2; 2) FPI treated with 4 hours of normobaric 100% O2; 3) FPI treated with 1 hour of HBO2 plus 3 hours of normobaric 100% O2; and 4) sham-injured treated with normobaric 30% O2. Cognitive outcome was assessed using the Morris water maze (MWM) on Days 11 to 15 after injury. Animals were then killed 21 days postinjury to assess hippocampal neuronal loss. Adenosine triphosphate was extracted from the neocortex and measured using high-performance liquid chromatography. The results showed that injured animals treated with HBO2 or normobaric 100% O2 alone had significantly higher levels of cerebral ATP as compared with animals treated using normobaric 30% O2 (p < or = 0.05). The injured animals treated with HBO2 had significant improvements in cognitive recovery, as characterized by a shorter latency in MWM performance (p < or = 0.05), and decreased neuronal loss in the CA2/3 and hilar regions as compared with those treated with 30% or 100% O2, (p < or = 0.05).

CONCLUSIONS

Both hyperbaric and normobaric hyperoxia increased cerebral ATP levels after lateral FPI. In addition, HBO2 treatment improved cognitive recovery and reduced hippocampal neuronal cell loss after brain injury in the rat.

Myocardial protection with monophosphoryl lipid-A against aortic cross clamping-induced global stunning

BACKGROUND

Monophosphoryl lipid-A (MLA) has a late window (24 hours) of cardioprotection against acute myocardial infarction. It is not known whether MLA, administered, 24 hours before surgery, attenuates intraoperative ventricular dysfunction "stunning" associated with aortic cross-clamping and reperfusion during elective cardiac surgery. We determined the dose-response relationship between MLA and ventricular function in a canine model of global myocardial stunning in the absence of necrosis. The role of expression of inducible heat shock protein 70 (HSP 70i) was also investigated.

METHODS

Mongrel dogs (n = 32) were intravenously injected with either a vehicle solution or 3, 5, 10, 35 ug/kg MLA. Twenty four hours later, dogs were anesthetized and instrumented, in situ, to monitor the left ventricular performance (the slope of regression between stroke-work and end diastolic length). Tissue samples were obtained to determine HSP70i using immunoblot analysis. After a period of equilibration on cardiopulmonary bypass, the aortic cross-clamp was applied at normothermia for 30 minutes followed by 60 minutes of reperfusion. ATP and catabolites were determined in transmural myocardial biopsies. Triphenyl-tetrazolium chloride (TTC) staining was used to determine myocardial necrosis.

RESULTS

MLA treatment did not alter myocardial contractility or ATP metabolism. Global ischemia resulted in about 50% depletion of ATP and remained depressed during reperfusion in all groups. MLA-treated hearts had improved functional recovery in a dose dependent-manner. Significant recovery was observed at the highest dose (35 ug/kg) compared to the control group. Immunoblot analysis demonstrated significant increase in HSP 70i in the MLA-treated hearts.

CONCLUSIONS

MLA exhibits a delayed (24 hours) window of protection against myocardial stunning associated with aortic cross-clamping. HSP70i expression may play a role in MLA-mediated cardioprotection.

Hyperdynamic circulation of arteriovenous fistula preconditions the heart and limits infarct size

BACKGROUND

Chronic arteriovenous fistulae (AVF) create sustained hyperdynamic circulation. It is not known whether hyperdynamic circulation alters myocardial sensitivity to ischemia and reperfusion injury. We tested the hypothesis that AVF activate molecular responses that increase tolerance to infarction in dogs.

METHODS

Twelve dogs were divided into two groups: 1) AVF group, where an AVF in the femoral region was done; and 2) sham-operated group (each n = 6). After 8 weeks, left ventricular performance was determined from stroke work/end-diastolic length relationship. Myocardial biopsy was obtained to determine heat-shock protein 70 and adenosine triphosphate (ATP) pool. Left anterior descending coronary artery was occluded for 90 minutes at 37 degrees C, followed by 4 hours of reperfusion. Coronary blood flow was determined using different colored microspheres.

RESULTS

The fistula group showed improvement of left ventricular performance (p = 0.03). The infarct size was significantly lower in the fistula group; it was 9.2+/-2.0% in the fistula group versus 28.4+/-5.2% in the sham group (p < 0.05). ATP depletion during ischemia was less in the fistula group (p = 0.02). Regional myocardial blood flow was significantly higher in the fistula group (p = 0.03).

CONCLUSIONS

Peripheral AVF improve the left ventricular performance, and decrease infarct size and ATP depletion. This protective effect is caused by the development of collaterals in the coronary circulation without expression of heat-shock protein 70.

Intermittent aortic crossclamping prevents cumulative adenosine triphosphate depletion, ventricular fibrillation, and dysfunction (stunning): is it preconditioning?

This study was designed to determine whether intermittent warm aortic crossclamping induces cumulative myocardial stunning or if the myocardium becomes preconditioned after the first episode of ischemia in canine models in vivo. The role of adenosine triphosphate catabolism and subsequent release of purines on reperfusion-mediated postischemic ventricular dysfunction and arrhythmias was assessed with the use of selective inhibitors of nucleoside transport, p-nitrobenzylthioinosine (NBMPR), and a specific adenosine deaminase inhibitor, erythro-9-[2-hydroxy-3-nonyl] adenine (EHNA). Thirty-two anesthetized dogs were instrumented to monitor left ventricular contractility, off bypass, by sonomicrometry. During cardiopulmonary bypass dogs were treated before ischemia with either saline solution (control group, n = 8) or EHNA (100 mumol/L) and NBMPR (25 mumol/L) (EHNA/NBMPR group, n = 8). Hearts were subjected to either 60 minutes of global ischemia and 120 minutes of reperfusion (n = 16) or 6 episodes of 10 minutes of global ischemia and 10 minutes of reperfusion, followed by 60 minutes of reperfusion (n = 16). Sixty minutes of sustained ischemia resulted in 80% loss of adenosine triphosphate and induced reperfusion-mediated ventricular fibrillation and severe left ventricular dysfunction in the control group. EHNA/NBMPR treatment augmented myocardial adenosine trapping during ischemia, attenuated ventricular fibrillation, and enhanced left ventricular functional recovery, despite similar depletion of adenosine triphosphate (80% loss). In the intermittent ischemia experiment, the first episode of 10 minutes of ischemia and reperfusion caused significant adenosine triphosphate depletion, ventricular fibrillation, and left ventricular stunning in both control and drug-treated groups. The prevalence of ventricular fibrillation was greater in the control group than in the drug-treated group after the first episode of ischemia (p < 0.05). Adenosine was the major nucleoside accumulated in the myocardium at the end of 10 minutes of ischemia in the EHNA/NBMPR-treated group (p < 0.05 versus control). Subsequent episodes of ischemia prevented ventricular fibrillation and did not cause cumulative left ventricular stunning in either group. Left ventricular function fully recovered in the EHNA/NBMPR-treated group after intermittent ischemia, but remained stunned in the control group. Unlike sustained ischemia, intermittent ischemia and reperfusion preserved myocardial adenosine triphosphate, limited purine release, and prevented ventricular fibrillation and cumulative stunning. These results suggest that intermittent ischemia and reperfusion augmented the endogenous protective mechanism or mechanisms of "preconditioning." Nucleoside trapping improved functional recovery after sustained or repetitive ischemia. It is concluded that adenosine triphosphate preservation or blockade of nucleoside transport may play an important role in the activation of endogenous myocardial protective mechanisms that "precondition" against subsequent ischemic stress.

Orotic acid improves left ventricular recovery four days after heterotopic transplantation

Orotic acid accelerates compensatory myocardial hypertrophy after regional ischemia and improves left ventricular function acutely after global ischemia. In this study, the effect of orotic acid on left ventricular function was investigated 4 days after global ischemia (75 minutes, 21 degrees C) using heterotopically transplanted rabbit hearts (n = 18). Experimental animals received daily 100-mg/kg doses of intraperitoneally administered orotic acid, starting 1 day before transplantation, and showed a threefold increase in the serum level of orotic acid by 4 days. After 1 hour of reperfusion, the developed pressure was equally depressed in both the control and experimental groups; however, 4 days later, the developed pressure in control animals was decreased by 3 +/- 3 mm Hg (versus the developed pressure measured at 1 hour) while the developed pressure in experimental animals was significantly increased by 25 +/- 8 mm Hg. Heterotopically transplanted hearts manifested diminished systolic function (stemming from ischemia and unloading) as well as decreased expression of adult myosin. Because orotic acid has been observed to produce an increase in protein synthesis in other models, we investigated whether this improvement in systolic function resulted from an orotic acid-mediated augmentation (or preservation) or normal adult myosin expression. Both orotic acid-treated and untreated hearts manifested decreased expression of the beta-myosin heavy chain protein and steady-state messenger RNA levels. Because function improved with decreased beta-myosin heavy chain expression, an alternate mechanism underlying orotic acid-mediated improvement in function is implicated. Nevertheless, orotic acid may be a therapeutic agent facilitating long-term recovery from global ischemia.

Separation between ischemic and reperfusion injury by site specific entrapment of endogenous adenosine and inosine using NBMPR and EHNA

BACKGROUND

Although myocardial ATP is essential for myocardial viability and ventricular function, it is a major source of free radical substrates for endothelial xanthine oxidase. Correlation between myocardial ATP and ventricular function has been hindered by the impact of ATP catabolites on ventricular function during reperfusion.

OBJECTIVES

This work results from four separate experiments assessing the role of nucleoside efflux in reperfusion mediated injury to determine the dual role of myocardial ATP in postischemic ventricular dysfunction. An adenosine deaminase inhibitor, erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), and an adenine nucleoside transport blocker, p-nitrobenzylthioinosine (NBMPR), were used to specifically inhibit adenosine deamination and block nucleoside release, respectively. This pharmacological intervention results in site-specific entrapment of intramyocardial adenosine and inosine, generated during ischemia, and blocks degradation to free radical substrates during reperfusion, thereby limiting the impact of reperfusion mediated injury.

METHODS

Forty-three anesthetized dogs were instrumented to monitor left ventricular performance from the slope of the relationship between stroke work and end-diastolic length (SW/EDL). Hearts were subjected to varying periods (30, 60, or 90 min) of global ischemia and 60 or 120 minutes of reperfusion. Two control groups for 30 and 60 minutes of ischemia (16 dogs) received only saline solution. Four treated groups (27 dogs) received saline containing 100 microM EHNA and 25 mM NBMPR prior to ischemia or only during reperfusion (n = 7). Myocardial biopsies were analyzed for ATP catabolites and NAD+.

RESULTS

Myocardial ATP and left ventricular function were severely depressed by 50% and 80% in the untreated controls, following 30 and 60 minutes of ischemia (37 degrees C), respectively. Ventricular dysfunction was inversely related to inosine levels in the myocardium at the end of the ischemic period. Administration of EHNA/NBMPR before ischemia or only during reperfusion resulted in significant accumulation of mainly adenosine or inosine, respectively. Entrapment of nucleosides was associated with complete recovery of ventricular function after 30 or 60 minutes of ischemia. Hearts subjected to 90 minutes of ischemia developed contracture.

CONCLUSIONS

Despite severely reduced ATP levels, ventricular function significantly recovered to preischemic values only in the EHNA/NBMPR-treated groups. Selective blockade of purine release during reperfusion is cardioprotective against post-ischemic reperfusion mediated injury. It is concluded that nucleoside transport plays an important role in regulation of endogenous adenosine and inosine affecting the degree of myocardial injury or protection from reperfusion mediated injury.

Preimplantation alteration of adenine nucleotides in cryopreserved heart valves

To assess the initial metabolic phase of cellular injury from cardiac valve processing, high-energy phosphate concentrations were analyzed in valve leaflets subsequent to critical processing steps. Using a porcine model, valves were processed in a manner identical to human homografts, with 58 randomly assigned to five groups representing distinct preparation phases. Group I (controls) sustained 40 minutes of warm ischemia concluded by liquid nitrogen immersion. Remaining groups similarly endured 40 minutes of ischemia, but were subsequently prepared according to stepwise design: II, warm ischemia + 24 hours of 4 degrees C ischemia; III, warm ischemia + 24 hours of 4 degrees C antibiotic disinfection; IV, warm ischemia + 24 hours at 4 degrees C (without antibiotics) + cryopreservation (-1 degrees C/min cryoprotected freezing); and V, warm ischemia+disinfection+cryopreservation. At each regimen's conclusion leaflet extracts were assayed by high-performance liquid chromatography for high-energy adenine nucleotides (adenosine triphosphate, adenosine diphosphate, adenosine monophosphate) and catabolites. A 47% and 86% decrease in cellular adenosine triphosphate level was observed in group III and group V leaflets, respectively. The level of total adenine nucleotides was maintained up to cryopreservation; thereafter a 74% decrease was noted. Catabolite analysis confirmed incomplete degradation of adenine nucleotides indicating cellular metabolic resilience throughout standard homograft preparation in valves previously exposed to 40 minutes of warm ischemia.

Decrease in cold tolerance of aged rats caused by the enhanced endogenous adenosine activity

During severe cold exposure, old rats (24-28 months) were less capable of maintaining their body temperature compared to young rats (3-6 months) due to lower rate of heat production. Single injection of adenosine deaminase (AD) (converts adenosine to inosine) significantly increased thermogenesis in both young and old rats. However, doubling the dose of AD was required for optimal thermogenic response in old rats. In contrast, the similar enhancements in both thermogenesis and cold tolerance were observed in both young and old rats receiving the same optimal doses of specific adenosine receptor antagonists. These results lead to the suggestion that the lower capability of aged rats to withstand cold exposure could be due to an increase in adenosine stimulation because of the decreased endogenous AD activity rather than an increase in adenosine receptor sensitivity. This notion is further supported by the finding that the AD activity in the neck muscle, a key site for shivering thermogenesis, was significantly lower in old rats as compared to their younger counterparts before and after cold exposure.

Adenosine levels in the postimplantation mouse uterus: quantitation by HPLC-fluorometric detection and spatiotemporal regulation by 5'-nucleotidase and adenosine deaminase

Extracellular adenosine has the potential to influence many aspects of target cell metabolism. The present study has determined the endogenous levels of adenosine in the pregnant mouse uterus and developing embryo-decidual unit with respect to the expression of two key enzymes of adenosine metabolism, 5'-nucleotidase (5'-NT; EC 3.1.3.5) and adenosine deaminase (ADA; EC 3.5.4.4). To measure adenosine levels, nucleoside extracts were etheno-derivatized and quantitated by high-performance liquid chromatography-fluorescence detection (0.03 pmol/mg protein sensitivity). Adenosine levels were determined to be 0.18 nmol/mg protein in the nonpregnant uterus; however, two statistically significant changes were identified in the pregnant uterus: (1) a periimplantation surge between day 3 (0.24 nmol/mg protein) and day 5 (0.59 nmol/mg protein) of gestation (plug day 0; implantation day 4); and (2) an early postimplantation decline between day 6 (0.54 nmol/mg protein) and day 7 (0.10 nmol/mg protein). The periimplantation adenosine surge coincided with uterine expression of 5'-NT, an enzyme which catalyzes the irreversible dephosphorylation of 5'-AMP to adenosine. 5'-NT expression was shown by Northern blot analysis to peak in the embryo-decidual unit on day 5 of gestation and then to decline through day 9; transcripts remained elevated in the placenta between day 9 and day 13 (the latest day examined in this study). By use of specific enzyme histochemistry, most 5'-NT activity was localized to the primary decidual zone on day 5. This expression subsequently declined during regression of the primary decidua; however, 5'-NT appeared on giant trophoblast (days 7-13) and the metrial gland (days 11-13). Other purine catabolic enzymes degrading AMP (adenylate deaminase) or generating adenosine (S-adenosylhomocysteine hydrolase) were not detected in the embryo-decidual unit suggesting that the net flux of utero-placental AMP catabolism proceeds with adenosine as an intermediate, this being the major pathway of adenosine formation. The sharp drop in adenosine levels between day 6 and day 7 coincided with a rise in the activity and mRNA expression of ADA, an enzyme which catalyzes the irreversible deamination of adenosine to inosine. ADA was previously localized to the secondary decidual zone (days 6-11), secondary giant cells (days 7-13), and spongiotrophoblasts (days 8-13) in the mouse (Knudsen et al., 1991). Results of developmental Northern blot analysis demonstrated a direct correlation of relative 5'-NT/ADA mRNA band intensity to adenosine content between day 4 and day 9 of gestation, suggesting that the local availability of adenosine in the antimesometrium is dependent upon the distribution of these enzymatic activities.(ABSTRACT TRUNCATED AT 400 WORDS)

Efficacy of a hydroxyl radical scavenger (VF 233) in preventing reperfusion injury in the isolated rabbit heart

We tested the hypothesis that 3,4,5,-trihydroxybenzamidoxime (VF 233), a demonstrated hydroxyl radical scavenger and an effective Fe3+ chelator, attenuates reperfusion injury and improves isovolumic left ventricular function. Eighteen isolated, perfused rabbit hearts with intracavitary balloons were subjected to normothermic, global ischemia until the initiation of ischemic contracture. Effects on the adenine nucleotide pool metabolites were determined by high-pressure liquid chromatography from right ventricular biopsy specimens before ischemia and at 15-minute intervals throughout reperfusion. In the experimental group (n = 9), a 5-mL bolus of 1 mol/L VF 233 was given immediately before reperfusion and followed by a continuous infusion (0.125 mumol/min). The control group (n = 9) received the vehicle solution at identical times. Rabbits treated with VF 233 had significant improvement in left ventricular function (expressed as percent return of left ventricular peak developed pressure) within 15 minutes of reperfusion (55.0 +/- 3.0 versus 66.2 +/- 4.1; p less than 0.05 by analysis of variance) after global ischemia and remained significantly improved throughout the reperfusion period. Myocardial adenine nucleotide pool intermediates were not significantly different between groups. These results demonstrate that administration of VF 233 significantly improves ventricular function but does not affect adenine nucleotide metabolism after ischemia and reperfusion.

High energy phosphate depletion in leaflet matrix cells during processing of cryopreserved cardiac valves

Preimplantation preparation of cardiac valves includes three major steps: (1) harvesting with accompanying ischemia (warm time from cessation of donor heart beat), (2) antibiotic disinfection, and (3) controlled-rate cryopreservation. To define the interdependent injury effects of these manipulations on leaflet matrix cells and specifically the potential for prolonged harvest-related ischemia to predispose greater injury by the subsequent steps, 96 semilunar valves were harvested from pigs in a manner analogous to human heart valve retrievals and randomly allocated to study groups as follows: 48 control valves were exposed to increasing harvested-related ischemic times, (2, 6, 12, 24 hr) and immersed in liquid nitrogen to arrest metabolic activity (i.e., prior to cryopreservation) and conclude the ischemia; another 48 were similarly harvested, subjected to identical ischemic times, then disinfected in 4 degrees C RPMI medium with standard antibiotics for 24 hr and dimethylsulfoxide cryopreserved at -1 degrees C/min to -170 degrees C (i.e., formal cryopreservation protocol). At thawing, each valve was extracted in 12% trichloroacetic acid and assayed by high performance liquid chromatography for components of the adenine nucleotide pool including ATP, lower energy nucleotides (total adenine nucleotides, [TAN] = [ATP] + [ADP] + [AMP]), adenosine, and the diffusible purines. Results are reported as nanomoles metabolite/milligram of leaflet cell protein (Lowry) and reflect a maintenance of total high energy phosphates in the control groups (5.41 +/- 0.29 nmole TAN at 2 hr; 8.34 +/- 0.67 nmole TAN at 24 hr), which fell significantly in all cryopreserved groups (1.27 +/- 0.33 nmole TAN at 2 hr; 0.34 +/- 0.22 nmole TAN at 24 hr).(ABSTRACT TRUNCATED AT 250 WORDS)

Embryonic versus adult myocardium: adenine nucleotide degradation during ischemia

Neonatal myocardium demonstrates better recovery from ischemia than does adult tissue. We tested the hypothesis that developmental differences in adenine nucleotide degradation might facilitate recovery by quantitating depletion of high-energy phosphates in nine-day-old embryonic (n = 9) and 15-month-old adult (n = 14) chicken hearts at 15-, 30-, 45-, and 60-minute intervals of normothermic ischemia in vitro. Nucleotides adenosine triphosphate, adenosine diphosphate, and adenosine monophosphate and nucleosides adenosine, inosine, hypoxanthine, and xanthine were determined by high-performance liquid chromatography. Several observations in metabolite degradative response to ischemia were noted. The embryonic myocardium maintained higher adenosine triphosphate and adenosine monophosphate levels over the course of the investigation than did mature myocardium. Moreover, the adult group showed an increase in diffusible nucleoside pool metabolites. Relative immaturity of enzymes responsible for nucleotide degradation may facilitate postischemic recovery by preserving nondiffusible high-energy phosphate precursors to participate in salvage resynthesis of adenosine triphosphate.

Myocardial adenine nucleotide metabolism in pediatric patients during hypothermic cardioplegic arrest and normothermic ischemia

Quantitative assessment of high-energy phosphate levels, including degradation or utilization during ischemia, has not previously been performed in infants and children. Animal experiments suggest that high-energy phosphate metabolism varies with maturation. To help answer these questions, 24 patients aged 2 months to 8 years underwent myocardial biopsy immediately after the institution of cardiopulmonary bypass (16 to 20 degrees C). Additional samples were obtained at 16 and 45 minutes after aortic cross-clamping and administration of cardioplegia (St. Thomas's solution) (in vivo ischemia). Seven patients also underwent major myocardial resection. Resected specimens were placed in a 37 degrees C bath and divided into equal-sized samples that were removed at ten-minute intervals (in vitro ischemia). All samples were immersed in liquid nitrogen and analyzed for adenine nucleotide pool metabolites using high-performance liquid chromatography. Levels of adenosine triphosphate were high before cross-clamping but diminished during the period of protected ischemia. Adenosine triphosphate loss was much more pronounced in patients less than 18 months old (p less than 0.05) and was associated with accumulation of adenosine monophosphate and inosine, a finding not seen in patients more than 18 months old (p less than 0.05). The same trends documented during in vivo ischemia were noted during in vitro ischemia. Immaturity of 5'-nucleotidase results in accumulation of adenosine monophosphate during ischemia. It is known that 5'-nucleotidase is present in neonatal myocardial cell membranes and absent from the cytosol.(ABSTRACT TRUNCATED AT 250 WORDS)