Acute changes in high energy phosphates, nucleotide derivatives, and contractile force in ischaemic and nonischaemic canine myocardium following coronary occlusion

Ferroptosis: Emerging Role in Diseases and Potential Implication of Bioactive Compounds

Ferroptosis is a form of cell death that is distinguished from other types of death for its peculiar characteristics of death regulated by iron accumulation, increase in ROS, and lipid peroxidation. In the past few years, experimental evidence has correlated ferroptosis with various pathological processes including neurodegenerative and cardiovascular diseases. Ferroptosis also is involved in several types of cancer because it has been shown to induce tumor cell death. In particular, the pharmacological induction of ferroptosis, contributing to the inhibition of the proliferative process, provides new ideas for the pharmacological treatment of cancer. Emerging evidence suggests that certain mechanisms including the Xc- system, GPx4, and iron chelators play a key role in the regulation of ferroptosis and can be used to block the progression of many diseases. This review summarizes current knowledge on the mechanism of ferroptosis and the latest advances in its multiple regulatory pathways, underlining ferroptosis' involvement in the diseases. Finally, we focused on several types of ferroptosis inducers and inhibitors, evaluating their impact on the cell death principal targets to provide new perspectives in the treatment of the diseases and a potential pharmacological development of new clinical therapies.

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.

Myocardial infarction in a canine model monitored by two-dimensional 31P chemical shift spectroscopic imaging

We have developed a closed chest animal model that allows noninvasive monitoring of cardiac high energy phosphate metabolism before, during, and for at least 3 weeks after a myocardial infarction. Ten beagles underwent 2 h of coronary occlusion followed by 3 weeks of reperfusion. Myocardial high energy phosphates from 12-ml voxels were noninvasively tracked using 31P two-dimensional chemical shift imaging. Gadolinium enhanced 1H MRI identified the zone at risk, and radioactive microspheres assessed regional blood flow and partition coefficients. Occlusion of the left anterior descending coronary artery produced infarcts that were 13.7+/-8.8% (mean+/-SD) of the left ventricular volume. Rapid changes in the phosphocreatine and inorganic phosphate levels were observed during occlusion, whereas adenosine triphosphate levels decreased more slowly. All metabolites recovered to base-line levels 2 weeks after occluder release. Multiple inorganic phosphate peaks in the infarct voxel spectra indicated that more than one metabolically compromised tissue zone developed during occlusion and reperfusion. Microsphere data indicating three distinct blood flow zones during ischemia and reperfusion (<0.3, 0.3-0.75, and >0.75 ml/min/g) supported the grouping of pH values into three distinct metabolic distributions.

Cardioprotective effects of NIP-121, a novel ATP-sensitive potassium channel opener, during ischemia and reperfusion in coronary perfused guinea pig myocardium

We investigated the effect of NIP-121, a novel ATP-sensitive K+ channel opener, on myocardial damage during ischemia/reperfusion. The action potential and contractile force of coronary-perfused guinea pig right ventricular walls were recorded. The preparations were subjected to 30-min no-flow ischemia with or without NIP-121 or glibenclamide, followed by 60-min reperfusion. In untreated tissues, decreases in action potential duration (APD) and contractile force and an increase in resting tension were observed during the no-flow period. On reperfusion, transient arrhythmias were observed and resting or contractile force returned to <50% of preischemic values. NIP-121, at 0.3 microM, a concentration showing only a slight negative inotropic effect, caused a faster decrease in APD and contractile force but abolished the increase in resting tension (RT) during the no-flow period. On reperfusion, no arrhythmia was observed in NIP-121-treated preparations, and contractile force recovered to approximately 80% of the preischemic value. Glibenclamide 1 microM attenuated the decrease in APD but affected neither the decrease in contractile force nor the increase in RT during the no-flow period. On reperfusion, the incidence of arrhythmia was increased in glibenclamide-treated preparations, and the recovery of basal tension and contractile force was inhibited: Contractile force recovered to only approximately 15% of the preischemic value. NIP-121 was also shown to attenuate the decrease in tissue ATP during ischemia and reperfusion. We demonstrated that NIP-121 may have protective effects against myocardial injury during ischemia and reperfusion. Activation of ATP-sensitive K+ current may be an adaptive mechanism for cardioprotection under compromised blood flow.

Inosine--a natural modulator of contractility and myocardial blood flow in the ischemic heart?

The energetic role of inosine (INO) remains controversial. The aim of the present study was first to test whether endogenous INO consumption/production correlates with regional myocardial contractile performance and second to test whether locally increased levels of INO influence contractility and blood flow in severely ischemic myocardium. Fentanyl-anesthetized dogs with implanted sonomicrometry crystals and independently perfused left anterior descending coronary arteries were studied. Two relatively load-independent indexes of regional myocardial contractility derived from left ventricular pressure-segment length loops were used: the regional stroke work-end-diastolic segment length relationship (Wr/L(ed)) and the end-systolic pressure-segment length relationship (Plv/L(es)). Very good correlations between myocardial contractile performance (as measured by the slope of the regional Wr/L(ed) relationship) and endogenous INO consumption/production under both nonischemic and ischemic conditions were found. Ischemia severely depressed contractility, significantly shifting rightward the Wr/L(ed) and Plv/L(es) relationships. INO infused into the left anterior descending bypass, in a concentration of 600 to 800 mumol/L, partially restored contractile performance as evidenced by a significant leftward displacement of both relationships. Wr, measured at a common maximum L(ed), increased significantly by 61 +/- 5%. Border-zone collateral flow (microspheres) increased by 35 +/- 7% within the endocardial segments and by 34 +/- 9% in the epicardial segments, but no increase in flow in the ischemic region was measureable. With the current emphasis on recanalization with thrombolytic therapy and considering the apparent safety of INO, this naturally occurring nucleoside might prove to be a useful adjunctive agent in the treatment of acute myocardial ischemia.

The contribution of nonreentrant mechanisms to malignant ventricular arrhythmias

Evidence obtained from experimental animals and man indicates that reentry is a major mechanism underlying arrhythmogenesis. However, focal or nonreentrant mechanisms also appear to be operative under a wide variety of pathophysiologic conditions. For example, results obtained using three-dimensional (3D) mapping from 232 simultaneous sites in the feline heart in vivo revealed that nonreentrant or focal mechanisms were prominent during both ischemia and reperfusion. During early ischemia, nonreentrant mechanisms were responsible for initiation of ventricular tachycardia (VT) in 25% of cases and, in cases where VT was initiated by reentry, it often could be maintained by a nonreentrant mechanism. During reperfusion of ischemic myocardium, nonreentrant mechanisms were responsible for initiation of VT in 75% of cases. Most importantly, the transition from VT to ventricular fibrillation in response to reperfusion was secondary to acceleration of a nonreentrant mechanism in either the subendocardium or subepicardium. Potential cellular mechanisms include: 1) sarcolemmal accumulation of amphiphiles such as long-chain acylcarnitines and lysophosphatidylcholine; 2) alpha- and beta-adrenergic mediated effects of catecholamines on the transient inward current (ITI) secondary to an increase in intracellular Ca2+; and 3) alpha-adrenergic receptor-induced decrease in IK mediated by activation of protein kinase C. Recent findings obtained using 3D intraoperative mapping in patients with refractory VT and a previous myocardial infarction also indicate that both reentrant and nonreentrant or focal mechanisms contribute. For example, in 13 selected patients, mapping was of a sufficient resolution to define the mechanisms of 10 runs of VT. Intraoperative mapping indicated that five runs of VT were initiated by intramural reentry, whereas five runs of VT were initiated by a focal or nonreentrant mechanism. The mechanisms underlying ventricular arrhythmias associated with ischemic cardiomyopathy have recently been delineated in dogs after multiple sequential intracoronary embolizations with microspheres (with a decrease in mean ejection fraction from 64% to 25%). Spontaneous VT initiated by focal mechanisms from the subendocardium in 82% and epicardium in 18%, with no evidence of macroreentry. Thus, in divergent pathophysiologic settings, nonreentrant mechanisms appear to contribute importantly to the genesis of lethal ventricular arrhythmias, suggesting that development of novel therapeutic approaches should be directed at inhibition of not only reentrant circuits, but also nonreentrant mechanisms, including triggered activity.

Effect of graded reductions of coronary pressure and flow on myocardial metabolism and performance: a model of "hibernating" myocardium

The term "hibernating" myocardium has been applied to chronic left ventricular dysfunction without angina or ischemic electrocardiographic changes in patients with coronary artery disease that is reversed by therapy that increases myocardial blood flow. To investigate the relation between coronary blood flow and ventricular function experimentally, graded reductions in coronary artery pressure were produced in isolated perfused rat hearts as contractile performance (peak systolic pressure and its first derivative [dP/dt]) and metabolic variables were measured using phosphorus-31 nuclear magnetic resonance (NMR) spectroscopy. As coronary pressure and flow were reduced, significant reductions in myocardial oxygen consumption and contractile performance were observed, which returned to control levels when coronary artery pressure and flow were restored to baseline values. Two phases of metabolic abnormality were observed. With modest reductions in coronary perfusion, proportionate reductions in myocardial oxygen consumption and contractile behavior were accompanied by a slight reduction in creatine phosphate but no significant lactate production. With greater reductions in coronary artery pressure and flow, creatine phosphate decreased more, adenosine triphosphate levels and myocardial pH decreased significantly and myocardial lactate production increased. The balanced reductions in myocardial contractility and oxygen consumption without metabolic abnormalities traditionally associated with "ischemia" observed in the first phase provides evidence in normal hearts for resetting of the myocardial contractile behavior and oxygen consumption in the presence of reduced coronary flow (that is, hibernating myocardium). The data suggest that reductions in adenosine diphosphate and the index of the reduced form of nicotinamide adenine dinucleotide (NADH) (lactate formation) do not explain the coupling between coronary artery pressure and flow and myocardial oxygen consumption as contractile performance decreases.

Effects of nicardipine, a calcium antagonist, on myocardial salvage and high energy phosphate stores in reperfused myocardial injury

The current study determined the effectiveness of nicardipine, a 1,4-dihydropyridine calcium antagonist, in preserving reperfused myocardium in a cat model of temporary coronary occlusion and ascertained if replenishment of myocardial phosphate stores during reperfusion as defined by phosphorus-31 nuclear magnetic resonance (NMR) spectroscopy was indicative of salvage. Twenty open chest, anesthetized cats were studied with use of a snare ligature around the proximal left anterior descending coronary artery, with a coil sutured to the epicardial surface overlying the distribution of the artery. Peak areas of phosphocreatine, inorganic phosphate and adenosine triphosphate (ATP) NMR signals were measured during 1 h of occlusion followed by 1.5 h of reperfusion. Infarct size and jeopardy area were determined in vitro by simultaneous infusion of phthalocyanine blue dye and triphenyltetrazolium chloride into the aorta and the left anterior descending coronary artery, respectively, after 5 h of myocardial reperfusion. Nicardipine-treated and control groups had similar jeopardy area values (41.2 +/- 1.6% versus 47.4 +/- 3.1% of the left ventricle), but infarct area was significantly reduced in the nicardipine-treated group (3.2 +/- 1.1% versus 24.9 +/- 7.5% of jeopardy area, p less than 0.01). High energy phosphate compounds remained markedly altered during reperfusion in both groups. No significant improvement in phosphocreatine or inorganic phosphate recovery was observed in animals pretreated with nicardipine despite an 87% reduction in infarct size. Myocardial ATP was greater during reperfusion in the nicardipine-treated compared with the control group (average over initial 90 min of reperfusion 58 +/- 6% versus 46 +/- 3% of baseline values, p less than 0.05), suggesting improved recovery of ATP. However, the measured levels of high energy phosphate compounds during reperfusion and their ratios did not correlate with infarct size and thus were not predictive of myocardial salvage.

Depletion of myocardial adenosine triphosphate during prolonged untreated ventricular fibrillation: effect on defibrillation success

We studied left ventricular endomyocardial adenosine triphosphate levels in 13 large mongrel dogs before and during ventricular fibrillation induced cardiac arrest to assess whether myocardial adenosine triphosphate content could predict successful cardiopulmonary resuscitation. Endomyocardial biopsies were performed during sinus rhythm (control), after 15 min of ventricular fibrillation or 10 min of ventricular fibrillation and 5 min of open chest cardiopulmonary resuscitation, after 20 min of ventricular fibrillation and 10 min of open chest cardiopulmonary resuscitation and after 40 min ventricular fibrillation and 15-20 min open chest cardiopulmonary resuscitation. Myocardial adenosine triphosphate was measured utilizing a bioluminescence method adapted for use with endomyocardial biopsies and normalized to protein content. Left ventricular endomyocardial adenosine triphosphate content fell significantly over time from a control level of 8.88 +/- 0.9 micrograms/mg protein to 5.73 +/- 0.5 micrograms/mg protein at 15 min of cardiac arrest, to 3.4 +/- 0.4 micrograms/mg protein after 30 min of cardiac arrest and to 1.98 +/- 0.3 micrograms/mg protein after 60 min of cardiac arrest (P less than 0.001). Adenosine triphosphate levels were significantly different between animals that received 10 min of ventricular fibrillation and successful open chest cardiopulmonary resuscitation and those that received 40 min of ventricular fibrillation and unsuccessful open chest cardiopulmonary resuscitation (4.35 +/- 0.48 vs. 2.11 +/- 0.43 micrograms/mg protein; P less than 0.025).(ABSTRACT TRUNCATED AT 250 WORDS)

Interstitial purine metabolites during regional myocardial ischemia

The purpose of this study was to determine the changes in cardiac interstitial fluid (ISF) purine metabolites during 90 min of regional myocardial ischemia. To collect ISF metabolites and measure local coronary blood flow (CBF), cardiac microdialysis probes were implanted into the left anterior descending artery (LAD) and left circumflex artery (LC) perfused myocardium of chloralose-urethane anesthetized dogs (n = 7). Regional ventricular wall thickness was measured in the LAD and LC perfused regions with sonomicrometric crystals, using systolic wall thickening (SWT) as an index of regional ventricular function. Regional myocardial ischemia, produced by occlusion of the LAD, resulted in a decrease in CBF (hydrogen clearance) from 77.3 +/- 12.4 to 10.9 +/- 4.4 ml/min/100 g (P less than 0.05), and systolic wall thinning (control SWT = 15.5 +/- 2.2%; ischemic SWT = -6.8 +/- 1.7%). ISF adenosine was transiently elevated in the ischemic region, obtaining a maximum sixfold increase after 15 min of ischemia. Inosine, hypoxanthine, and to a lesser extent xanthine, composed the majority of metabolites which accumulated in the ISF of the ischemic region, accounting for greater than 95% of the total purine metabolites in the ISF after 20 min of ischemia. Despite the marked increase in ISF inosine, hypoxanthine, and xanthine levels, ISF uric acid levels did not increase in the ischemic region. Although CBF and SWT did not change in the nonischemic LC perfused area, there were small transient increases (two- to fourfold) in ISF adenosine, inosine, and hypoxanthine levels. In summary, these data demonstrate that purine metabolites accumulate rapidly in the ISF during myocardial ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Haemodynamic effects of inosine. A new drug for failing human heart?

Haemodynamic effects of inosine were studied in intact dogs and subsequently in patients suffering from prolonged otherwise intractable cardiogenic shock. The nucleoside significantly improve myocardial performance in patients with an extremely low cardiac index by 63 +/- 29% as well as animals with no signs of haemodynamic deterioration, increasing cardiac output by 12.2 +/- 4.3%. These results suggest that inosine may become a promising inotropic agent. Further studies though are necessary to elucidate the mechanism of inosine action.

Kinetic evidence for a heart mitochondrial pore activated by Ca2+, inorganic phosphate and oxidative stress. A potential mechanism for mitochondrial dysfunction during cellular Ca2+ overload

Evidence that the Ca2+-induced permeabilization of mitochondria is attributable to a reversible Ca2+-activated pore [Al Nasser & Crompton (1986) Biochem. J. 239, 19-29] has been further investigated. Permeabilization is induced in a wholly synergistic manner by either Ca2+ plus phosphate or Ca2+ plus tert-butyl hydroperoxide. When permeabilization is complete, extramitochondrial [14C]sucrose equilibrates with the matrix space with a half-time of about 800 ms; [14C]mannitol equilibrates at least threefold faster. Permeabilization is essentially fully reversed on Ca2+ chelation with EGTA, when the half time for [14C]sucrose equilibration is increased 600-1400-fold (to 550-1150 s). A pulsed-flow [14C]solute-entrapment technique has been developed to measure the kinetics of EGTA-induced resealing. The technique incorporates a suitable choice of [14C]solute and an appropriate model for data analysis, and is competent to measure permeation state changes occurring in 100 ms. The data obtained are consistent with exponential resealing of mitochondria in which pores of any single mitochondria close with a high degree of synchrony. The rate of resealing is increased about eight-fold by ADP (half-time approximately 1 s; Km approximately 30 microM). CoA, Mg2+, AMP and also ATP, when account is taken of ADP arising by hydrolysis, are essentially ineffective. It is concluded that heart mitochondria do contain a pore whose permeation state is controlled over an approximate 1000-fold range by Ca2+ and other factors including phosphate, oxidative stress and ADP. The possible involvement of the pore in reoxygenation-induced injury in heart is discussed.

Correlation of contractile dysfunction with oxidative energy production and tissue high energy phosphate stores during partial coronary flow disruption in rabbit heart

The relationships between contractile function, myocardial oxygen consumption, and tissue high energy phosphate and lactate content were investigated during partial coronary flow disruption. The experimental preparation was an isolated, isovolumic retrograde blood-perfused rabbit heart. Both developed pressure (r = 0.94) and dp/dt (r = 0.95) exhibited strong linear correlations with myocardial oxygen consumption that were stable for up to 45 min after blood flow reduction. In contrast, tissue high energy phosphate content exhibited nonlinear relationships with both developed pressure and oxygen consumption such that systolic mechanical function and oxidative metabolism declined to 20 and 30% of control values, respectively, before significant abnormalities in myocardial high energy phosphate stores were observed. Similarly, developed pressure and oxygen consumption decreased to 36 and 48% of control, respectively, before abnormal tissue lactate content was detected. The results of this study indicate that: (a) mechanical function is closely related to the rate of oxidative energy production during partial coronary flow disruption, and (b) despite the development of significant contractile dysfunction, tissue high energy phosphate content remains at normal levels except under the most severely flow-deprived conditions. The preservation of tissue energy stores can be explained by the apparent coupling of contractile performance to oxidative energy production, which could function to maintain myocardial energy balance during partial coronary flow restriction.

Characterization of high-energy phosphate compounds during reperfusion of the irreversibly injured myocardium using 31P MRS

Phosphorus-31 magnetic resonance spectroscopy (MRS) was used to monitor regional changes in high-energy phosphorus compounds and intracellular pH during 60 min of acute regional ischemia (acute occlusion of left anterior descending artery) and reperfusion in open-chest cats using a 1.2-cm two-turn coil sutured to the myocardium. During the 60-min ischemic phase, phosphocreatine (PCr) intensity was reduced to 47 +/- 4.9% (mean +/- SE) of control (p less than 0.01) by 15 min postocclusion while adenosine triphosphate (ATP) intensity decreased more slowly with the decrease (66 +/- 5.6%) achieving significance (p less than 0.05) only at 60 min postocclusion. Inorganic phosphate (Pi) increased to a maximum of 397 +/- 42% of control (p less than 0.01) while the pH decreased progressively from 7.36 +/- 0.02 to 6.02 +/- 0.14 (p less than 0.01). After release of occlusion PCr intensity recovered to 86 +/- 12% of the initial control value at 15 min postreperfusion but showed a subsequent downward trend to 79 +/- 8.8%. The ATP did not recover but tended to decline further during reperfusion. The Pi intensity decreased to 260 +/- 38% of control while the pH increased to 7.01 +/- 0.23 by 15 min postreperfusion. Thus, the reperfused irreversibly injured myocardium is characterized by persistent depletion of PCr and ATP and elevation of Pi. Phosphorus-31 MRS provides a nondestructive method for characterizing the reperfused irreversibly damaged myocardium.

Methods for the metabolic quantification of regional myocardial ischemia

An adequate balance between oxygen supply and demand is a basic requirement for normal cardiac function. When oxygen supply does not meet the demand, progressive cellular damage occurs leading to cardiac dysfunction and, ultimately, tissue death. While traditionally "ischemia" has been defined as decreased oxygen supply secondary to a decrease in blood flow, and "hypoxia" as decreased oxygen supply secondary to a decrease in oxygen tension, this review defines ischemia in its broader sense, namely as a pathophysiologic state in which there is a lack of oxygen relative to the demand for it. In a large number of experimental studies involving the heart, there is need to promptly recognize the ischemic state, to monitor its course in vivo, and to quantify it. Because of cardiac autoregulatory mechanisms, research methods which attempt to quantify supply (e.g., measurement of myocardial blood flow) and/or demand (e.g., measurement of myocardial oxygen consumption) do not necessarily reflect the status of the balance between supply and demand. An imbalance between myocardial supply and demand is more likely to be reflected by metabolic fluxes and by the accumulation of products specific to the ischemic state. Thus, the purpose of this review is to summarize the various methods available to the cardiac surgical investigator today for the metabolic quantification of myocardial ischemia. Due to the complexity of the heart and its inherent regional differences, myocardial ischemic changes are frequently regional in nature. Thus, this review will address metabolic methods for the regional quantification of myocardial ischemia.

Effects of delay in administration of potassium cardioplegia to the isolated rat heart

Ischemic injury to the heart in the period between aortic cross-clamping and administration of cardioplegic solution was evaluated in the normothermic rat heart model. After isolation and control perfusion with oxygenated Krebs-Henseleit bicarbonate buffer, the hearts were given lactated Ringer's cardioplegic solution (30 mEq of K+ per liter) for 2 minutes at three different intervals following aortic clamping: no delay, 2-minute delay, and 5-minute delay. Thereafter, the hearts were left unperfused and the time to initiation of ischemic contracture was recorded. Adenosine triphosphate (ATP) and creatine phosphate levels were measured in all groups prior to and at the conclusion of cardioplegia administration. A 2-minute delay in the administration of cardioplegic solution resulted in significantly lower (p less than 0.001) ATP levels that were restored after 2 minutes of cardioplegia administration. Contracture times were not significantly altered. A 5-minute delay resulted in significantly shorter (p less than 0.001) contracture times and significantly lower (p less than 0.001) ATP levels that were not restored to preischemic levels by 2 minutes of cardioplegia administration. The fate of the myocardium may be insensitive to events that occur during the earliest moments of ischemia provided that rapid administration of oxygenated potassium cardioplegia follows the ischemic period and restores preischemic high-energy phosphate stores. However, there is a critical ischemic time during the initial interval before cardioplegia that is associated with an impaired ability of the myocardium to tolerate subsequent ischemia.

Pathobiology of acute myocardial ischemia: metabolic, functional and ultrastructural studies

Acute myocardial ischemia induced by coronary occlusion in dogs is most severe in the subendocardial region, whereas more collateral blood flow is often present in the subepicardial region. Initially, all ischemic myocytes are reversibly injured, but beginning at 15 to 20 minutes after the onset, and continuing for 3 to 6 hours, there is a wave front of cell death from the subendocardial region to the less ischemic subepicardial region, such that by 6 hours, the final transmural extent of the infarct is established. Thus, ischemic myocardium cannot be salvaged by reperfusion after greater than or equal to 6 hours of coronary occlusion in open-chest anesthetized dogs. In the severely ischemic subendocardial region, most of the creatine phosphate is lost within the first 3 minutes of ischemia in vivo, and adenosine triphosphate (ATP) is depleted to 35% of control by 15 minutes (when cellular injury is still reversible), and to less than 10% of control at 40 minutes (when injury is irreversible). Tissue ATP content and other indexes of subcellular damage have also been compared after different periods of ischemia using a model of total myocardial ischemia in vitro. As long as the ATP of the tissue was not depleted below 5 mumols/g dry weight, incubated slices of injured myocardium resynthesized high-energy phosphates and excluded inulin. However, lower tissue ATP was associated with depressed high-energy phosphate resynthesis and failure of cell volume regulation. Overt membrane damage, as measured by an increased inulin-diffusible space, was detected only after the tissue ATP decreased to less than 2.0 mumols/g of dry weight. Thus, marked ATP depletion is associated with the onset of structural and functional indexes of irreversible injury. However, whether irreversibility is caused by the marked ATP depletion or by other concomitant metabolic consequences of ischemia is not known. Myocardial ischemic cellular injury is reversible despite depletion of 70% of the control ATP. Nevertheless, when myocyte injury is reversible, there is slow repletion of adenine nucleotides. This slow metabolic recovery may explain the delayed recovery of contractile function observed after reperfusion of ischemic myocardium.

Role of adenosine in the maintenance of coronary vasodilation distal to a severe coronary artery stenosis. Observations in conscious domestic swine

The purpose of this study was to test the hypothesis that adenosine is required to maintain arteriolar vasodilation distal to a severe coronary stenosis. Eight closed-chest conscious pigs were prepared by placing a 7.5-mm long stenosis (82% lumenal diameter reduction) in the proximal left anterior descending coronary artery. Regional myocardial blood flow (microsphere technique) was measured at control 1, after 10 minutes of intracoronary infusion of adenosine deaminase (7-10 U/kg per min) distal to the stenosis, and 20-30 minutes after stopping adenosine deaminase infusion. Studies with 125I-labeled adenosine deaminase were conducted in six additional pigs to document the extent to which infused adenosine deaminase penetrated the interstitial space. 125I-labeled adenosine deaminase was infused for 10 minutes (10-11 U/kg per min) into the left anterior descending coronary artery. Calculated interstitial fluid concentrations of adenosine deaminase ranged between 71 and 272 U/ml and were at least one order of magnitude greater than that required to deaminate all the adenosine which would be released into the interstitium in response to 15-30 seconds of coronary occlusion. In the primary group of animals (n = 8), endocardial flow (ml/min per g) distal to stenosis at control 1 (1.15 +/- 0.33) was reduced vs. endocardial flow in the nonobstructed circumflex zone (1.59 +/- 0.38, P less than 0.05). Flows in epicardial layers were comparable at control 1 (distal zone = 1.40 +/- 0.36 vs. circumflex zone = 1.45 +/- 0.41). Distal zone endocardial and epicardial flows did not change vs. control 1 in response to infusion of adenosine deaminase. However, the distal: circumflex epicardial flow ratio declined vs. control 1 (0.98 +/- 0.14) during adenosine deaminase infusion (0.87 +/- 0.17, P less than 0.05). The distal:circumflex endocardial flow ratio during adenosine deaminase (0.72 +/- 0.20) was unchanged vs. control 1 (0.76 +/- 0.22) but was less than control 2 (0.80 +/- 0.18, P less than 0.05). Thus, destruction of all or most interstitial adenosine caused only slight relative reduction in regional myocardial blood flow distal to a severe coronary artery stenosis. Accordingly, adenosine contributes only modestly to maintenance of arteriolar vasodilation in this setting or else its absence is almost fully compensated for by another mechanism(s).

Effects of proton release from adenine nucleotide degradation during ischemic necrosis of myocardium in vitro

The effects of the progressive production of hydrogen ions and inorganic phosphate from adenine nucleotides on the lowering of pH in heart muscle has been investigated in dog myocardium maintained under anoxic conditions in vitro at 37 degrees C. Tissue samples were taken at 15-min intervals for up to 105 min and the following biochemical parameters determined: pH, lactate, Pi, ATP, ADP, AMP, G6P, and PC in selected instances. From these data the net proton changes during prolonged anoxia were calculated, assuming homogeneity of the tissue milieu. Net proton change was negative after 15 min and became increasingly more so throughout the remainder of the experimental period, indicating that adenine nucleotide catabolism in fact has a protective effect against fall in pH. When tissue pH falls to ca. 6.2 (45-60 min anoxia), proton production due to lactic acid is reduced by approximately 16% because of absorption of protons by phosphate and ammonia liberated from the nucleotides.

Inosine: a protective agent in an organ culture model of myocardial ischemia

Fetal mouse hearts in organ culture provide a model of ischemic-like injury in which the myocardial protective effect of pharmacological agents can be studied independent of blood flow. To investigate the potential protective effect of a diffusable purine under ischemic-like conditions, we used 4 mM inosine in fetal mouse heart organ cultures deprived of oxygen and oxidizable substrates for 1-10 hours. We studied hearts (n = 258) immediately after simulated ischemia (early) and after a 20-hour recovery period (late), by utilizing three indices of myocardial viability. Thallium-201 accumulation is an early marker of myocardial viability during injury, whereas the percentage of lactic dehydrogenase release from hearts to culture medium and the percentage of irreversibly injured myocytes assessed by planimetry of midventricular histological sections are late markers, used after recovery from injury. At 10 hours of injury, thallium-201 accumulation was 38% greater in inosine-supplied hearts, 3.50 +/- 0.16 vs. 2.54 +/- 0.08 (counts/min per mg wet weight)/(counts/min per microliter medium) (mean +/- SEM) (P less than 0.001). After recovery from 10 hours of injury, lactic dehydrogenase release was 29% less in inosine-supplied hearts, 35 +/- 3% vs. 49 +/- 3% (P less than 0.001). After recovery from 8 hours of injury, the percentage of histologically irreversibly injured tissue was 23% less in inosine-supplied hearts, 60 +/- 7% vs. 78 +/- 3% (P less than 0.05). These data indicate that inosine has a protective effect on fetal mouse myocardium during simulated ischemia and suggest that inosine deserves further evaluation.

Raised hypoxanthine, xanthine and uridine concentrations in meconium stained amniotic fluid and during labour

Amniotic fluid samples were obtained at induction of labour in 64 women; in 15 of these there was meconium staining of the amniotic fluid; the remainder showed no signs of fetal distress. Using high pressure liquid chromatography, compared to the samples from normal patients there were highly significantly raised levels of hypoxanthine, xanthine and uridine in the meconium stained samples; oxypurines in the meconium itself could not explain the difference. Where serial samples were obtained during labour by intrauterine catheter, a terminal rise in oxypurine levels was apparent. Where the proportion of oxypurine present as hypoxanthine exceeded one per cent in amniotic fluid at the time of induction, there was a significantly greater occurrence of late fetal heart rate decelerations in the ensuing labour. These findings are consistent with other evidence that when tissues become hypoxic the metabolic products of nucleotide breakdown escape from the cells and appear in extracellular fluid. Oxygen lack in the fetus probably causes loss of these compounds from the hypoxic kidneys to the urine so that they appear in amniotic fluid.

Release of nucleosides from canine and human hearts as an index of prior ischemia

During ischemia, myocardial adenosine triphosphate is degraded to adenosine, inosine and hypoxanthine. These nucleosides are released into coronary venous blood and may provide an index of ischemia; adenosine may also participate in the autoregulation of coronary flow. In dogs, the temporal relations between reactive hyperemic flow and nucleoside concentrations in regional venous blood were correlated after brief occlusions of a segmental coronary artery. Reactive hyperemia and adenosine release peaked together in 10 seconds, persisted for 10 to 30 seconds and then decreased in a pattern consistent with the hypothesis that they are related. During initial reflow after 45 seconds of ischemia, mean concentrations of adenosine, inosine and hypoxanthine increased, respectively, to 52, 67 and 114 nmol/100 ml plasma; after 5 minutes of ischemia, the respective levels increased to 58, 1,570 and 1,134 nmol and fell quickly. In nine patients there was a similar release of nucleosides into coronary sinus blood during reperfusion after 59 to 80 minutes of ischemic arrest during cardiac surgery. With initial reflow, adenosine, inosine and hypoxanthine levels reached 65, 655 and 917 nmol/100 ml of blood, respectively. Inosine and hypoxanthine concentrations remained high for 5 to 10 minutes after cardiac beating resumed, often when production of lactate had decreased. The results indicate that postischemic release of nucleosides reaches significant levels in man as well as animals, is parallel with the duration of ischemia, is temporary and may be a useful supplement to measurement of lactate as an index of prior myocardial ischemia.