Beneficial effects of lidocaine and disopyramide on oxygen-deficiency-induced contractile failure and metabolic disturbance in isolated rabbit hearts

The purpose of the present study was to determine whether antiarrhythmic agents, lidocaine and disopyramide, which reveal a membrane stabilizing action, may exert a beneficial effect on posthypoxic recovery of cardiac function and metabolism. Rabbit hearts were perfused for 20 min under hypoxic conditions, followed by 45-min reoxygenation. Hypoxic insults induced cessation of cardiac contractile force, rise in resting tension, depletion of myocardial high-energy phosphates, accumulation of tissue calcium and release of creatine kinase and ATP metabolites such as adenosine, inosine and hypoxanthine. These alterations were not returned to the initial levels upon reoxygenation. Administration of either 69 microM lidocaine or 55 microM disopyramide after the onset of oxygen deficiency (between 8th and 20th min of the hypoxia) resulted in a significant suppression of hypoxia-induced rise in resting tension, tissue calcium accumulation and release of creatine kinase and ATP metabolites, whereas hypoxia-induced decline in cardiac contractile force and depletion of myocardial high-energy phosphates were not affected by the treatment. The latter two variables were improved markedly during 45-min reoxygenation when the heart had been treated with the agents. The improvement was accompanied by a suppression of the release of creatine kinase and ATP metabolites and the tissue calcium accumulation. The results suggest that lidocaine and disopyramide are beneficial for posthypoxic recovery of cardiac function and metabolism.     

Cardioprotective action of alpha-blocking agents, phentolamine and bunazosin, on hypoxic and reoxygenated myocardium

The present study was undertaken to determine whether alpha-blocking agents, phentolamine and bunazosin, may exert a cardioprotective effect on hypoxic and subsequently reoxygenated hearts. For this purpose, rabbit hearts were perfused for 20 min under hypoxic conditions, followed by a 45 min-reoxygenation. Agents were administered between the 8th and 20th min of hypoxic perfusion. Hypoxic perfusion for 8 min resulted in a decline of cardiac contractile force and myocardial high-energy phosphates and a loss of adenine nucleotide metabolites from the heart, whereas a rise in resting tension was not observed. Neither increase in perfusion pressure, release of creatine kinase from hearts nor increase in tissue calcium was observed. At 20 min-hypoxia, significant changes in resting tension and perfusion pressure of the heart and release of creatine kinase from the heart were observed. Cardiac contractile force after 45 min of reoxygenation was less than 10% of the initial value. Treatment with 83 microM phentolamine or 46 microM of bunazosin resulted in a significant suppression of hypoxia-induced increase in tissue calcium, release of creatine kinase and adenine nucleotide metabolites and rise in perfusion pressure and resting tension. Treatment with either phentolamine or bunazosin resulted in appreciable recovery of cardiac contractile force. Reoxygenation-induced release of creatine kinase was also suppressed significantly. Two possible mechanisms for the protective effect of this treatment are considered; 1) preservation of ATP metabolites which may be utilized as substrates for a salvage synthesis of ATP during reoxygenation and 2) prevention of a nonselective transmembrane flux of cellular constituents due to changes in cell membrane permeability.     

Beneficial effects of yohimbine on posthypoxic recovery of cardiac function and myocardial metabolism in isolated perfused rabbit hearts.

The present study was undertaken to elucidate the possible actions of yohimbine on cardiac function and metabolism in the hypoxic and subsequently reoxygenated myocardium. For this purpose, rabbit hearts were perfused for 20 min under hypoxic conditions, followed by 45 min reoxygenated perfusion, and their functional and metabolic alterations with and without yohimbine treatment were examined. Hypoxia induced cessation of cardiac contractile force, rise in resting tension and depletion of tissue high-energy phosphates, which were poorly recovered by subsequent reoxygenation. Hypoxia also induced release of creatine kinase and ATP metabolites from perfused hearts and increases in tissue calcium and sodium contents, which were further enhanced upon subsequent reoxygenation. When hypoxic hearts were treated with 3 to 30 microM yohimbine, several beneficial effects were observed in a concentration-dependent manner. This included enhancement of posthypoxic recovery of contractile function and suppression of the hypoxia- and reoxygenation-induced rise in resting tension. Hypoxia/reoxygenation-induced release of ATP metabolites was inhibited and restoration of myocardial high-energy phosphates enhanced. Inhibition of reoxygenation-induced rise in tissue calcium and sodium and creatine kinase release were also noted. The findings suggest that suppression of transmembrane flux of ions, substrates and enzymes during hypoxia/reoxygenation plays a role in the posthypoxic functional and metabolic recovery. Yohimbine (3-30 microM) significantly depressed the maximal stimulus frequency the left atria could follow. These results suggest a close relationship between depression in the maximal driving frequency of atria and enhancement of the posthypoxic contractile and metabolic recovery of perfused hearts.(ABSTRACT TRUNCATED AT 250 WORDS)     

Possible involvement of membrane-stabilizing action in beneficial effect of beta adrenoceptor blocking agents on hypoxic and posthypoxic myocardium

The present study was designed to elucidate a possible involvement of membrane-stabilizing action of beta blocking agents in posthypoxic recovery of cardiac contractile function and myocardial metabolism. Propranolol and acebutolol, which possess a membrane-stabilizing action, and atenolol and metoprolol, which lack this action, were used in the isolated, perfused rabbit heart. The membrane-stabilizing effects of these agents were assessed on the basis of the effects on the maximal driving frequency of the left atria. Reoxygenation of hearts for 45 min following 20-min hypoxia resulted in little recovery of cardiac contractile force, sustained rise in resting tension, insufficient recovery of myocardial high-energy phosphates, accumulation of the tissue calcium and sodium and marked release of creatine kinase and ATP metabolites from the hearts. Treatment of hypoxic hearts with either 100 microM propranolol, 200 microM acebutolol, 200 microM atenolol or 100 microM metoprolol was commenced when the contractile force declined to 30% of the initial level and terminated at 20-min hypoxia. Treatment with either propranolol or acebutolol produced a significant posthypoxic recovery of cardiac contractile force, resting tension and myocardial high-energy phosphates, and a profound suppression of the tissue calcium and sodium accumulation and the loss of ATP metabolites from perfused hearts. In contrast, neither atenolol nor metoprolol affected these changes induced by the hypoxic insult and the following reoxygenation. The results suggest that membrane-stabilizing action of beta blocking agents plays an important role in the protection against posthypoxic cardiac contractile dysfunction and metabolic disturbances.     

Role of an ATP-sensitive potassium channel opener, YM934, in mitochondrial energy production in ischemic/reperfused heart.

We examined a possible mechanism of action of an ATP-sensitive potassium (K(ATP)) channel opener, YM934, for the improvement of energy metabolism in hearts subjected to 35-min ischemia and 60-min reperfusion. The treatment with 30 nM YM934 for the final 15 min of preischemia enhanced postischemic recovery of left ventricular developed pressure, attenuated the postischemic rise in left ventricular end-diastolic pressure, and suppressed the release of creatine kinase and ATP metabolites during reperfusion. The treatment also restored myocardial ATP and creatine phosphate contents and attenuated the decrease in mitochondrial oxygen consumption rate during reperfusion. The higher mitochondrial function was also seen in YM934-treated hearts at the end of ischemia. In another set of experiments, myocardial skinned bundles were incubated for 30 min under hypoxic conditions in the presence and absence of YM934, and then mitochondrial oxygen consumption rate was determined. Hypoxia decreased the mitochondrial oxygen consumption rate of skinned bundles to approximately 40% of the prehypoxic value. In contrast, the treatment of skinned bundles with 30 nM YM934 preserved the mitochondrial oxygen consumption rate during hypoxia. The effect of YM934 on the hypoxic skinned bundles was abolished by combined treatment with either the K(ATP) channel blocker glyburide or the mitochondrial K(ATP) channel blocker 5-hydroxydecanoate in a concentration-dependent manner. The results suggest that YM934 is capable of attenuating ischemia/reperfusion injury of isolated perfused hearts due to preservation of mitochondrial function during ischemia, probably through opening of mitochondrial K(ATP) channels.     

Relation of myocardial oxygen consumption and function to high energy phosphate utilization during graded hypoxia and reoxygenation in sheep in vivo.

This study investigates the relation between myocardial oxygen consumption (MVO2), function, and high energy phosphates during severe hypoxia and reoxygenation in sheep in vivo. Graded hypoxia was performed in open-chested sheep to adjust PO2 to values where rapid depletion of energy stores occurred. Highly time-resolved 31P nuclear magnetic resonance spectroscopy enabled monitoring of myocardial phosphates throughout hypoxia and recovery with simultaneous MVO2 measurement. Sheep undergoing graded hypoxia (n = 5) with an arterial PO2 nadir of 13.4 +/- 0.5 mmHg, demonstrated maintained rates of oxygen consumption with large changes in coronary flow as phosphocreatine (PCr) decreased within 4 min to 40 +/- 7% of baseline. ATP utilization rate increased simultaneously 59 +/- 20%. Recovery was accompanied by marked increases in MVO2 from 2.0 +/- 0.5 to 7.2 +/- 1.9 mumol/g per min, while PCr recovery rate was 4.3 +/- 0.6 mumol/g per min. ATP decreased to 75 +/- 6% of baseline during severe hypoxia and did not recover. Sheep (n = 5) which underwent moderate hypoxia (PO2 maintained 25-35 mmHg for 10 min) did not demonstrate change in PCr or ATP. Functional and work assessment (n = 4) revealed that cardiac power increased during the graded hypoxia and was maintained through early reoxygenation. These studies show that (a) MVO2 does not decrease during oxygen deprivation in vivo despite marked and rapid decreases in high energy phosphates; (b) contractile function during hypoxia in vivo does not decrease during periods of PCr depletion and intracellular phosphate accumulation, and this may be related to marked increases in circulating catecholamines during global hypoxia. The measured creatine rephosphorylation rate is 34 +/- 11% of predicted (P < 0.01) calculated from reoxygenation parameters, which indicates that some mitochondrial respiratory uncoupling also occurs during the rephosphorylation period.     

Impairment of energy metabolism in intact residual myocardium of rat hearts with chronic myocardial infarction.

The purpose of this study was to test the hypothesis that energy metabolism is impaired in residual intact myocardium of chronically infarcted rat heart, contributing to contractile dysfunction. Myocardial infarction (MI) was induced in rats by coronary artery ligation. Hearts were isolated 8 wk later and buffer-perfused isovolumically. MI hearts showed reduced left ventricular developed pressure, but oxygen consumption was unchanged. High-energy phosphate contents were measured chemically and by 31P-NMR spectroscopy. In residual intact left ventricular tissue, ATP was unchanged after MI, while creatine phosphate was reduced by 31%. Total creatine kinase (CK) activity was reduced by 17%, the fetal CK isoenzymes BB and MB increased, while the "adult" mitochondrial CK isoenzyme activity decreased by 44%. Total creatine content decreased by 35%. Phosphoryl exchange between ATP and creatine phosphate, measured by 31P-NMR magnetization transfer, fell by 50% in MI hearts. Thus, energy reserve is substantially impaired in residual intact myocardium of chronically infarcted rats. Because phosphoryl exchange was still five times higher than ATP synthesis rates calculated from oxygen consumption, phosphoryl transfer via CK may not limit baseline contractile performance 2 mo after MI. In contrast, when MI hearts were subjected to acute stress (hypoxia), mechanical recovery during reoxygenation was impaired, suggesting that reduced energy reserve contributes to increased susceptibility of MI hearts to acute metabolic stress.     

Protection against hypoxic injury in isolated-perfused rat heart by ruthenium red.

Changes in intracellular calcium content and energy production during the period of hypoxia appear to be necessary for the development of cellular injury. Ruthenium red, a hexavalent dye which inhibits the active uptake of calcium by mitochondria, might improve a cell's energy status thereby minimizing hypoxic injury. Rat heart tissue was perfused retrogradely with Krebs-Henseleit medium containing 2.5 mM calcium and 10 mM glucose. The infusion of 0.1, 1.0 or 1.24, but not 0.01 microM, ruthenium red throughout 60 min of hypoxia and 30 min of reoxygenation decreased, in a dose-dependent manner, the release of lactate dehydrogenase normally seen at reoxygenation. When the infusion of 1.24 microM ruthenium red was begun after 45 min of hypoxia, lactate dehydrogenase release at reoxygenation after 60 min of hypoxia was decreased, but to a lesser extent than when this agent was present throughout hypoxia. Ruthenium red, 1.24 microM, had no significant effects on coronary flow or function in oxygenated heart tissue. When present throughout hypoxia and reoxygenation, 1.24 microM ruthenium red prevented the decrease in coronary flow normally seen and allowed recovery of heart rate, +dP/dT, -dP/dT and work (defined as the product of developed pressure and heart rate) to normal levels. Significant functional protection was not evident at reoxygenation when ruthenium red was infused after 45 min of hypoxia or in the absence of glucose. Cardiac ATP, creatine phosphate and energy charge were decreased after 60 min of hypoxia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Neutrophils delay functional recovery of the post-hypoxic heart of the rabbit

A postischemic contractile dysfunction termed myocardial stunning has been described in vivo and is attributed, in part, to the generation of oxygen-derived free radicals and the presence of neutrophils. An analogous contractile derangement occurs in the posthypoxic heart in vitro. This study determined the role of neutrophils in hypoxia/reoxygenation-induced cardiac dysfunction in the isolated buffer-perfused rabbit heart utilizing a recirculating system with or without neutrophils present. In control hearts perfused with a neutrophil-free buffer, reoxygenation after 20 min of hypoxia was associated with a slow recovery of contractility which returned to prehypoxic values by 30 to 45 min. Although perfusion with buffer-containing neutrophils did not affect the hypoxia-induced decrease in myocardial contractility, the recovery of contractile function during subsequent reoxygenation was significantly diminished (P less than .01 vs. control), remaining depressed by 30 to 35% at 45 min. The myocardial neutrophil content increased approximately 2-fold in response to hypoxia and reoxygenation, as assessed using 51Cr-labeled neutrophils. The deleterious effects of neutrophil perfusion on cardiac function could not be attributed to neutrophil-mediated plugging of coronary vessels or enhanced myocellular damage. These results support the concept that neutrophils contribute to the cardiac dysfunction described in this model.     

Cardioprotective profile of MET-88, an inhibitor of carnitine synthesis, and insulin during hypoxia in isolated perfused rat hearts

Summary— 3-(2,2,2-trimethylhydrazinium) propionate (MET-88) is an inhibitor of carnitine synthesis. This study was carried out to investigate whether or not reduction of carnitine content could attenuate hypoxic damage in isolated perfused rat hearts.
Rats were divided into four groups: 1) vehicle control; 2) pretreatment with MET-88 (MET-88); 3) application of insulin (500 μU/mL) in the perfusate (insulin); and 4) pretreatment with MET-88 and application of insulin (MET-88 + insulin). MET-88 (100 mg/kg) was orally administered once a day for 10 days until the day before the experiments. Hearts were initially perfused for a 10 min period under normoxia, followed by a 30 min period under hypoxia. Hearts were frozen at the end of hypoxia for the measurement of high-energy phosphates, carnitine derivatives, and glycolysis intermediates. In a separate series of untreated and MET-88 treated hearts, exogenous glucose and palmitate oxidation was measured.
MET-88 decreased the extent of the depression of cardiac contractility (+dP/dt), and aortic flow during the hypoxic state. Insulin also improved cardiac function, and co-treatment of MET-88 and insulin additionally improved cardiac function during hypoxia. MET-88 prevented the decrease of high-energy phosphate and the increase of long-chain acylcarnitine after 30 min of hypoxic perfusion. In addition, MET-88 increased the steady state of glucose oxidation in hypoxic perfused rat hearts. These results indicate that MET-88 has cardioprotective effects on contractile function and energy metabolism of isolated perfused rat hearts in a hypoxic condition. Preventing the accumulation of long-chain acylcarnitine may serve to protect hypoxic hearts.     

Induction of manganese superoxide dismutase in rat cardiac myocytes increases tolerance to hypoxia 24 hours after preconditioning.

Manganese superoxide dismutase (Mn-SOD) is induced in ischemic hearts 24 h after ischemic preconditioning, when tolerance to ischemia is acquired. We examined the relationship between Mn-SOD induction and the protective effect of preconditioning using cultured rat cardiac myocytes. Exposure of cardiac myocytes to brief hypoxia (1 h) decreased creatine kinase release induced by sustained hypoxia (3 h) that follows when the sustained hypoxia was applied 24 h after hypoxic preconditioning (57% of that in cells without preconditioning). The activity and content of Mn-SOD in cardiac myocytes were increased 24 h after hypoxic preconditioning (activity, 170%; content, 139% compared with cells without preconditioning) coincidentally with the acquisition of tolerance to hypoxia. Mn-SOD mRNA was also increased 20-40 min after preconditioning. Antisense oligodeoxyribonucleotides corresponding to the initiation site of Mn-SOD translation inhibited the increases in the Mn-SOD content and activity and abolished the expected decrease in creatine kinase release induced by sustained hypoxia after 24 h of hypoxic preconditioning. Sense oligodeoxyribonucleotides did not abolish either Mn-SOD induction or tolerance to hypoxia. These results suggest that the induction of Mn-SOD in myocytes by preconditioning plays a pivotal role in the acquisition of tolerance to ischemia at a later phase (24 h) of ischemic preconditioning.     

Hypoxia elicits liberation of anti-aggregatory substances from isolated rabbit hearts

Summary. The hypothesis was investigated that myocardial hypoxia stimulates the production of platelet anti-aggregatory substances in the heart. Rabbit hearts were perfused under normoxic or hypoxic conditions and the coronary and interstitial effluents from the hearts were separated. The occurrence of anti-aggregatory activity (AAA) in the interstitial effluent was detected in vitro from its capacity to inhibit ADP-induced platelet aggregation. The AAA in the effluent was deemed to be prostacyclin (PGI₂) if its release was abolished by administration of indomethacin (5× 10^-5 M) to the heart, and to be adenosine if it was abolished by incubation of the effluent with adenosine deaminase. During normoxic perfusion, only a minor efflux of AAA appeared from the heart; neither was the efflux appreciable during mild hypoxia (30 or 60% O₂). Severe hypoxia (venous pO₂ below 5 k Pa), on the other hand, was associated with a marked release of AAA. Incubation of hypoxic effluent with adenosine deaminase resulted in a small loss of activity, indicating that the major part of the AAA was not ascribable to adenosine. After indomethacin treatment, significant amounts of AAA still appeared in the effluent during hypoxia. However, unlike the case before indomethacin, this AAA was completely destroyed by adenosine deaminase. From these data, we conclude that myocardial hypoxia can mobilize either of two independent mechanisms for protection against platelet aggregation: an activation of the synthesis and release of prostacyclin, and a more complete breakdown of ATP, leading to an increased formation and efflux of adenosine.     

Decreased energetics in murine hearts bearing the R92Q mutation in cardiac troponin T

The thin filament protein cardiac troponin T (cTnT) is an important regulator of myofilament activation. Here we report a significant change in cardiac energetics in transgenic mice bearing the missense mutation R92Q within the tropomyosin-binding domain of cTnT, a mutation associated with a clinically severe form of familial hypertrophic cardiomyopathy. This functional domain of cTnT has recently been shown to be a crucial modulator of contractile function despite the fact that it does not directly interact with the ATP hydrolysis site in the myosin head. Simultaneous measurements of cardiac energetics using 31P NMR spectroscopy and contractile performance of the intact beating heart revealed both a decrease in the free energy of ATP hydrolysis available to support contractile work and a marked inability to increase contractile performance upon acute inotropic challenge in hearts from R92Q mice. These results show that alterations in thin filament protein structure and function can lead to significant defects in myocardial energetics and contractile reserve.     

Glucose,insulin and potassium (GIK) during reperfusion mediates improved myocardial bioenergetics

Previous studies suggest glucose, insulin and potassium (GIK) infusion during ischemia reduces infarct size and improves post-ischemic myocardial function in acute myocardial infarction and following surgical revascularization of the heart. The potential use of GIK when given only during reperfusion after a period of global ischemia, as might occur during cardiac arrest, is unclear. To test the hypothesis that GIK reperfusion improves post-ischemic myocardial bioenergetics and function, we utilized a perfused heart model. Hearts from Sprague-Dawley rats (350-450 g) were perfused at 85 mmHg with oxygenated Krebs-Henseleit bicarbonate containing 5.5 mM glucose and 0.2 mM octanoic acid. Following 20 min of global ischemia, hearts were reperfused for 30 min with original solution (control) or GIK in two different doses (10 or 20 mM glucose each with insulin 10 U/l and K(+) 7 meq/l). Hearts perfused with GIK solutions had significantly higher ATP, creatine phosphate, energy charge, and NADP(+) and lower AMP and inosine levels compared with control after 30 min of reperfusion. Hearts reperfused with GIK had significantly higher developed pressure and higher dP/dt than control reperfused hearts. Reperfusion with GIK improved post-ischemic recovery of both contractile function and the myocardial bioenergetic state. GIK may be a viable adjunctive reperfusion therapy following the global ischemia of cardiac arrest to improve post-resuscitation cardiac dysfunction.     

Myocardial contractility after brain injury

Cardiac contractile function in the acute period after brain injury (BI) was studied on 158 non-inbred male albino rats, by using the isolated isovolumetrically contracted heart according to the procedure described by Fallen et al. The injured rats showed a significantly depressed left ventricular myocardial contractility during the hypoxic test, followed by reoxygenation, a lower positive chronic inotropic effect, and an increased diastolic defect with an enhanced rate of cardiac stimulation rates. Brain injury increased the dependence of cardiac performance on the concentration of Ca2+ in the perfused solution. The findings suggest the decreased resistance of the hearts of the rats that had sustained BI to pathogenetic factors.     

Carnitine deprivation adversely affects cardiac performance in the lipopolysaccharide- and hypoxia/reoxygenation-stressed piglet heart

Sepsis and hypoxia are important stressors for the neonate. Newborn infants receiving total parenteral nutrition are routinely deprived of carnitine and develop low carnitine plasma and tissue levels. Because of its high metabolic rate and dependence on fatty acids for energy, the newborn heart may be particularly vulnerable to stress in the face of an inadequate carnitine supply. To investigate whether carnitine deprivation affects cardiac performance under stress, 23 neonatal piglets received parenteral nutrition for 2-3 weeks that was either carnitine free (CARN -) or supplemented (CARN +) with L-carnitine (400 mg/L). Bacterial endotoxin (lipopolysaccharide (LPS), 250 microg/kg intravenous bolus) or saline vehicle was administered to anesthetized piglets 3 h prior to study of isolated perfused hearts. Left ventricular systolic pressure (LVSP), left ventricular end diastolic pressure, and left ventricular developed pressure (LVDP) were measured in vitro under aerobic, hypoxic, and reoxygenation conditions in all animals. Plasma and tissue carnitine values were lower in CARN - than in CARN + piglets. In hearts from LPS-treated animals prior to hypoxia, there was no difference in ventricular compliance between CARN - and CARN + groups. LVSP and LVDP were lower in CARN - than CARN + hearts. During hypoxia, LVSP and LVDP fell, but left ventricular end diastolic pressure increased in hearts from both LPS- and saline- treated piglets. Reoxygenation led to poorer recovery in CARN - than CARN + hearts from LPS-treated animals, but not from saline controls. During hypoxia/reoxygenation, lactate efflux initially rose and then fell, while carnitine efflux increased continually. Acetyl- and medium-chain acylcarnitines were detected in the coronary effluent. Our findings suggest that carnitine deprivation diminishes heart carnitine concentrations and impairs cardiac recovery from combined endotoxic and hypoxic stress. Possible mechanisms include reduced acyl buffering and/or impaired transport of fatty acyl groups into mitochondria.     

Cardiac function, myocardial glutathione, and matrix metalloproteinase-2 levels in hypoxic newborn pigs reoxygenated by 21%, 50%, or 100% oxygen

After asphyxia, it is standard to resuscitate the newborn with 100% oxygen, which may create a hypoxia-reoxygenation process that may contribute to subsequent myocardial dysfunction. We examined the effects of graded reoxygenation on cardiac function, myocardial glutathione levels, and matrix metalloproteinase (MMP)-2 activity during recovery. Thirty-two piglets (1-3 days old, weighing 1.5-2.1 kg) were anesthetized and instrumented for continuous monitoring of cardiac index, and systemic and pulmonary arterial pressures. After 2 h of hypoxia, piglets were randomized to receive reoxygenation for 1 h with 21%, 50%, or 100% oxygen (n = 8 each), followed by 3 h at 21% oxygen. At 2 h of hypoxemia (PaO2 32-34 mmHg), the animals had hypotension, decreased cardiac index, and elevated pulmonary arterial pressure (P < 0.001 vs. controls). Upon reoxygenation, cardiac function recovered in all groups with higher cardiac index and lower systemic vascular resistance in the 21% group at 30 min of reoxygenation (P < 0.05 vs. controls). Pulmonary artery pressure normalized in an oxygen-dependent fashion (100% = 50% > 21%), despite an immediate recovery of pulmonary vascular resistance in all groups. The hypoxia-reoxygenated (21%-100%) hearts had similarly increased MMP-2 activity and decreased glutathione levels (P < 0.05, 100% vs. controls), which correlated significantly with cardiac index and stroke volume during reoxygenation, and similar features of early myocardial necrosis. In neonatal resuscitation, if used with caution because of a slower resolution of pulmonary hypertension, 21% reoxygenation results in similar cardiac function and early myocardial injury as 50% or 100%. The significance of higher oxidative stress with high oxygen concentration is unknown, at least in the acute recovery period.     

Peroxynitrite, a two-edged sword in post-ischemic myocardial injury-dichotomy of action in crystalloid- versus blood-perfused hearts

Peroxynitrite (ONOO(-)) is widely recognized as a mediator of NO. toxicity, but recent studies have indicated that this compound may also have physiologic activity and induces vascular relaxation as well as inhibition of platelet aggregation and neutrophil adhesion. The present experiment was designed to determine whether ONOO(-) may exert different effects on postischemic myocardial injury in a crystalloid perfusion environment versus a blood perfusion environment and, if it does, to clarify the mechanisms causing any differences. In Krebs-Henseleit buffer-perfused rabbit hearts, administration of ONOO(-) at the onset of reperfusion enhanced myocardial injury in a concentration-dependent fashion with a significant effective concentration of 30 microM. In contrast, in blood-perfused hearts, administration of ONOO(-) (1 to 30 microM) significantly attenuated postmyocardial injury as evidenced by improved cardiac function recovery, preserved endothelial function, decreased myocardial creatine kinase loss, and reduced necrotic size. The minimal and maximal protective concentrations were determined to be 1 and 3 microM, respectively. When a high concentration of ONOO(-) (i.e., 100 microM) was administered, a detrimental effect was observed. Administration of ONOO(-) decreased neutrophil accumulation in the ischemic-reperfused myocardial tissue in a concentration-dependent manner in blood-perfused hearts and inhibited neutrophil adhesion to cultured endothelial cells exposed to hypoxia/reoxygenation. Taken together, these results demonstrate that ONOO(-) may act as a "double-edged sword" in postischemic myocardial injury. This compound is directly toxic to the cardiac tissue at a relatively high concentration, but it can indirectly protect myocardial cells from neutrophil-induced injury at a much lower concentration.     

Release of Adenosine from Human Hearts during Angina Induced by Rapid Atrial Pacing

This study was designed to determine whether human hearts release adenosine, a possible regulator of coronary flow, during temporary myocardial ischemia and, if so, to examine the mechanisms involved. Release of adenosine from canine hearts had been reported during reactive hyperemia following brief coronary occlusion, and we initially confirmed this observation in six dogs hearts. Angina was then produced in 15 patients with anginal syndrome and severe coronary atherosclerosis by rapid atrial pacing during diagnostic studies. In 13 of these patients, adenosine appeared in coronary sinus blood, at a mean level of 40 nmol/100 ml blood (SE = +/-9). In 11 of these 13, adenosine was not detectable in control or recovery samples; when measured, there was concomitant production of lactate and minimal leakage of K(+), but no significant release of creatine phosphokinase, lactic acid dehydrogenase, creatine, or Na(+).THERE WAS NO DETECTABLE RELEASE OF ADENOSINE BY HEARTS DURING PACING OR EXERCISE IN THREE CONTROL GROUPS OF PATIENTS: nine with anginal syndrome and severe coronary atherosclerosis who did not develop angina or produce lactate during rapid pacing, five with normal coronaries and no myocardial disease, and three with normal coronaries but with left ventricular failure.The results indicate that human hearts release significant amounts of adenosine during severe regional myocardial ischemia and anaerobic metabolism. Adenosine release might provide a useful supplementary index of the early effects of ischemia on myocardial metabolism, and might influence regional coronary flow during or after angina pectoris.     

Hypoxia and glucose independently regulate the beta-adrenergic receptor-adenylate cyclase system in cardiac myocytes.

We explored the effects of two components of ischemia, hypoxia and glucose deprivation, on the beta-adrenergic receptor (beta AR)-adenylate cyclase system in a model of hypoxic injury in cultured neonatal rat ventricular myocytes. After 2 h of hypoxia in the presence of 5 mM glucose, cell surface beta AR density (3H-CGP-12177) decreased from 54.8 +/- 8.4 to 39 +/- 6.3 (SE) fmol/mg protein (n = 10, P less than 0.025), while cytosolic beta AR density (125I-iodocyanopindolol [ICYP]) increased by 74% (n = 5, P less than 0.05). Upon reexposure to oxygen cell surface beta AR density returned toward control levels. Cells exposed to hypoxia and reoxygenation without glucose exhibited similar alterations in beta AR density. In hypoxic cells incubated with 5 mM glucose, the addition of 1 microM (-)-norepinephrine (NE) increased cAMP generation from 29.3 +/- 10.6 to 54.2 +/- 16.1 pmol/35 mm plate (n = 5, P less than 0.025); upon reoxygenation cAMP levels remained elevated above control (n = 5, P less than 0.05). In contrast, NE-stimulated cAMP content in glucose-deprived hypoxic myocytes fell by 31% (n = 5, P less than 0.05) and did not return to control levels with reoxygenation. beta AR-agonist affinity assessed by (-)-isoproterenol displacement curves was unaltered after 2 h of hypoxia irrespective of glucose content. Addition of forskolin (100 microM) to glucose-supplemented hypoxic cells increased cAMP generation by 60% (n = 5; P less than 0.05), but in the absence of glucose this effect was not seen. In cells incubated in glucose-containing medium, the decline in intracellular ATP levels was attenuated after 2 h of hypoxia (21 vs. 40%, P less than 0.05). Similarly, glucose supplementation prevented LDH release in hypoxic myocytes. We conclude that (a) oxygen and glucose independently regulate beta AR density and agonist-stimulated cAMP accumulation; (b) hypoxia has no effect on beta AR-agonist or antagonist affinity; (c) 5 mM glucose attenuates the rate of decline in cellular ATP levels during both hypoxia and reoxygenation; and (d) glucose prevents hypoxia-induced LDH release, a marker of cell injury.