F0F1 ATP synthase activity is differently modulated by coronary reactive hyperemia before and after ischemic preconditioning in the goat

The amplitude of coronary reactive hyperemia (CRH), elicited by 15 s of ischemia, is reduced in hearts subjected to 5 min of ischemic preconditioning (IP). F0F1 ATP synthase activity and ATP concentration are also altered by IP. We hypothesized that F0F1 ATP synthase is differently modulated by the inhibitor protein IF(1) during CRH elicited before (CRHnp) and after (CRHprec) IP. Hemodynamic parameters were recorded in 10 anesthetized goats. Myocardial biopsies were obtained before IP (Cnp), during CRHnp, 4 and 6 min after the onset of CRHnp, after IP (Cprec), during CRHprec, and 4 min after CRHprec. F0F1 ATP synthase activity, ATP concentration, and ATP-to-ADP ratio (ATP/ADP) were determined. Compared with CRHnp, IP blunted CRHprec. F0F1 ATP synthase activity transiently increased during CRHnp, decreased 4 min after CRHnp, and returned to control 2 min later; it was lower after IP (Cprec) and did not change during and after CRHprec. All these changes in activity were modulated by IF1. During CRHnp, ATP concentration and ATP/ADP were reduced compared with Cnp and began to rise 6 min thereafter. During Cprec, both parameters were transiently reduced but increased during and after CRHprec. Hence, during CRHnp, F0F1 ATP synthase activity transiently increases and then decreases significantly. The short-lasting inhibition of the enzyme may explain why a few seconds of occlusion do not induce IP. After IP, F0F1 ATP synthase activity is blunted, and it is not affected by a subsequent 15 s of occlusion, which induces a blunted CRHprec. These results suggest that postischemic long-lasting inhibition of F0F1 ATP synthase activity may be a feature of the preconditioned heart. The increase in ATP concentration after preconditioning is in agreement with previous reports of reduced ATP hydrolysis by cytoplasmic ATPases.

Knockout of Kir6.2 negates ischemic preconditioning-induced protection of myocardial energetics

Although ischemic preconditioning induces bioenergetic tolerance and thereby remodels energy metabolism that is crucial for postischemic recovery of the heart, the molecular components associated with preservation of cellular energy production, transfer, and utilization are not fully understood. Here myocardial bioenergetic dynamics were assessed by (18)O-assisted (31)P-NMR spectroscopy in control or preconditioned hearts from wild-type (WT) or Kir6.2-knockout (Kir6.2-KO) mice that lack metabolism-sensing sarcolemmal ATP-sensitive K(+) (K(ATP)) channels. In WT vs. Kir6.2-KO hearts, preconditioning induced a significantly higher total ATP turnover (232 +/- 20 vs. 155 +/- 15 nmol x mg protein(-1) x min(-1)), ATP synthesis rate (58 +/- 3 vs. 46 +/- 3% (18)O labeling of gamma-ATP), and ATP consumption rate (51 +/- 4 vs. 31 +/- 4% (18)O labeling of P(i)) after ischemia-reperfusion. Moreover, preconditioning preserved cardiac creatine kinase-catalyzed phosphotransfer in WT (234 +/- 26 nmol x mg protein(-1) x min(-1)) but not Kir6.2-KO (133 +/- 18 nmol x mg protein(-1) x min(-1)) hearts. In contrast with WT hearts, preconditioning failed to preserve contractile recovery in Kir6.2-KO hearts, as tight coupling between postischemic performance and high-energy phosphoryl transfer was compromised in the K(ATP)-channel-deficient myocardium. Thus intact K(ATP) channels are integral in ischemic preconditioning-induced protection of cellular energetic dynamics and associated cardiac performance.

3-isobutyl-1-methylxanthine (IBMX) sensitizes cardiac myocytes to anoxia

Cardiac myocytes incubated with 3-isobutyl-1-methylxanthine (IBMX), a nonspecific cyclic nucleotide phosphodiesterase inhibitor, formed rigor complexes under anoxic conditions more readily than cells incubated with other phosphodiesterase inhibitors. Cardiac myocytes were incubated for 1 hr with either (a) no additions, (b) 150 microM zaprinast, or (c) 1 mM IBMX, and then were rendered anoxic for periods up to 60 min. Cells were >80% viable throughout the anoxic period; viability was unaffected by either drug. Rod count decreased more rapidly after the onset of anoxia in the IBMX-treated cells than in control or zaprinast-treated cells (11% rods vs. roughly 47% rods after 30 min of anoxia). IBMX-treated cell groups also formed more "contracted" myocytes (box-like rods) than their untreated or zaprinast-treated counterparts (50% contracted vs. roughly 27% contracted after 30 min of anoxia). While nucleotide degradation patterns were similar in all experimental groups, the ratio of ATP to ADP was lower in IBMX-treated cells than in control or zaprinast-treated cells. The L-type calcium channel was apparently not involved in this phenomenon; while cyclic AMP was elevated in the IBMX-incubated cells, verapamil did not protect IBMX-incubated cells from premature damage by anoxia. Incubation with 8-cyclopentyl-1,3-dipropylxanthine (CDPX), an A1 receptor antagonist, at concentrations up to 1 microM in place of 1mM IBMX did not reproduce the IBMX effect. We concluded that IBMX sensitizes cardiac myocytes to anoxia through a mechanism related to its effect on ATP/ADP, and unrelated to an elevation of intracellular calcium or preconditioning phenomena.

Lactate accumulation rather than ATP depletion predicts ischemic myocardial necrosis: implications for the development of lethal myocardial injury

In ischemia, the myocardial metabolic status determines the expansion of necrosis. Decreased ATP levels and increased lactate contents in ischemic myocardium undergoing lethal injury are known to be related to the expansion of irreversible damage. However, their individual contributions have not yet been firmly established. Using two differently effective protocols of ischemic preconditioning (IP short and IP long), ischemic cardioplegic arrest (CP) and their combination (IP+CP) to directly influence the metabolic status of porcine myocardium, graded preservations in ATP content and decreases in lactate accumulation during 45 min ischemia could be achieved (control: ATP, 0.15+/-0.03; lactate, 60.53+/-4.89 micromol/g wet weight; IP short, 0.33+/-0.10/27.42+/-3.90; IP long, 0.60+/-0.10/17.49+/-2.14; CP, 0.98+/-0.12/11.82+/-0.96; IP+CP, 2.24+/-0.28/10.88+/-0.89; all P<0.001 vs. control). At the same time, a graded reduction of myocardial necrosis was observed (90.0+/-3.1 vs. 31.7+/-4.55 vs. 5.05+/-2.1 vs. 0.0 [isolated patchy necroses] vs. none). Regression analysis revealed only a weak correlation of infarct size and ATP preservation (r=0.567). In fact, there was a biphasic relation: with ATP levels above 1 micromol/g wet weight, no infarction occurred. ATP levels below this threshold value were associated with steep increase in infarct size. However, even for this latter range, the regression coefficient remained low (r=0.654). Instead, over the entire range, there was a close, rectilinear correlation of infarct size and lactate accumulation (r=0.939). These data indicate that lactate accumulation rather than ATP depletion determines the development of lethal myocardial injury. However, the biphasic relation between ATP depletion and infarct size suggests the latter to play a permissive role, since above a threshold value of 1 micromol/g wet weight neither substantial lactate accumulation nor infarction was observed. Below this threshold, however, infarct size increased as lactate accumulated.

Opening of potassium channels protects mitochondrial function from calcium overload

Ischemic preconditioning (IPC) protects myocardium from ischemia reperfusion injury by activating mitochondrial K(ATP) channels. However, the mechanism underlying the protective effect of K(ATP) channel activation has not been elucidated. It has been suggested that activation of mitochondrial K(ATP) channels may prevent mitochondrial dysfunction associated with Ca(2+) overload during reperfusion. The purpose of this experiment was to study, in an isolated mitochondrial preparation, the effects of mitochondrial K(ATP) channel opening on mitochondrial function and to determine whether it protects mitochondria form Ca(2+) overload. Mitochondria (mito) were isolated from rat hearts by differential centrifugation (n = 5/group). Mito respiratory function was measured by polarography without (CONTROL) or with a potassium channel opener (PINACIDIL, 100 microM). Different Ca(2+) concentrations (0 to 5 x 10(-7) M) were used to simulate the effect of Ca(2+) overload; state 2, mito oxygen consumption with substrate only; state 3, oxygen consumption stimulated by ADP; state 4, oxygen consumption after cessation of ADP phosphorylation; respiratory control index (RCI: ratio of state 3 to state 4); rate of oxidative phosphorylation (ADP/Deltat); and ADP:O ratio were measured. PINACIDIL increased state 2 respiration and decreased RCI compared to CONTROL. Low Ca(2+) concentrations stimulated state 2 and state 4 respiration and decreased RCI and ADP:O ratios. High Ca(2+) concentrations increased state 2 and state 4 respiration and further decreased RCI, state 3, and ADP/Deltat. PINACIDIL improved state 3, ADP/Deltat, and RCI at high Ca(2+) concentrations compared to CONTROL. Pinacidil depolarized inner mitochondrial membrane, as evidenced by decreased RCI and increased state 2 at baseline. Depolarization may decrease Ca(2+) influx into mito, protecting mito from Ca(2+) overload, as evidenced by improved state 3 and RCI at high Ca(2+) concentrations. The myocardial protective effects resulting from activating K(ATP) channels either pharmacologically or by IPC may be the result of protecting mito from Ca(2+) overload.

Cardioprotection with kappa-opioid receptor stimulation is associated with a slowing of cross-bridge cycling

Opioid and alpha-adrenergic receptor activation protect the heart from ischemic damage. One possible intracellular mechanism to explain this is that an improvement in ATP availability contributes to cardioprotection. We tested this hypothesis by correlating postischemic left ventricular developed pressure (LVDP) and myofibrillar Ca(2+)-dependent actomyosin Mg(2+)-ATPase from isolated rat hearts treated with the kappa-opioid receptor agonist U-50488H (1 microM) or the alpha-adrenergic receptor agonist phenylephrine (10 microM) + propranolol (3 microM). Preischemic treatment with U-50488H or phenylephrine + propranolol improved postischemic LVDP recovery by 25-30% over control hearts. Ca(2+)-dependent actomyosin Mg(2+)-ATPase was found to be 20% lower in both U-50488H- and phenylephrine + propranolol-treated hearts compared with control hearts. The kappa-opioid receptor antagonist nor-binaltorphimine (1 microM) abolished the effects of U-50488H on postischemic LVDP and actomyosin Mg(2+)-ATPase activity. Reduced actomyosin ATP utilization was also suggested in single ventricular myocytes treated with either U-50488H or the protein kinase C activator, phorbol 12-myristate 13-acetate (PMA), because U-50488H and PMA lowered maximum velocity of unloaded shortening by 15-25% in myocytes. U-50488H and phenylephrine + propranolol treatment both resulted in increased phosphorylation of troponin I and C protein. These findings are consistent with the hypothesis that kappa-opioid and alpha-adrenergic receptors decrease actin-myosin cycling rate, leading to a conservation of ATP and cardioprotection during ischemia.

Preconditioning of swine heart with monophosphoryl lipid A improves myocardial preservation

BACKGROUND

Ischemic preconditioning has been proven to be a powerful tool for myocardial protection in the setting of ischemia and reperfusion. A new drug to provide pharmacologic preconditioning, monophosphoryl lipid A (MLA), was administered 24 hours before an acute coronary occlusion in pigs to determine the effect on pharmacologic preconditioning.

METHODS

Two studies were completed. In the first, swine were distributed into five groups: group I, control; group II,. aminoguanidine (AMG) (30 mg/kg), a selective inducible nitric oxide synthase (iNOS) blocker; group III, MLA (10 microg/kg); group IV, MLA (35 microg/kg); and group V, MLA and AMG (35 microg/kg and 30 mg/kg, respectively). Twenty-four hours after administration of the MLA, AMG, or both, regional left anterior descending coronary artery ischemia was induced for 15 minutes followed by one hour of global normothermic cardioplegic arrest and three hour reperfusion. Left ventricular function, tissue injury, and percentage of myocardial infarction were measured. Left ventricular myocardium in the left anterior descending coronary artery region was sampled for iNOS messenger RNA (mRNA) during ischemia and reperfusion. In the second study, pigs were sacrificed 0, 4, 6, 8, and 24 hrs after MLA/AMG administration for iNOS mRNA determination in nonischemic myocardium.

RESULTS

Use of MLA significantly improved postischemic ventricular function, and reduced creatinine kinase release and percentage of infarction. Monophosphoryl lipid A induced expression of iNOS mRNA in nonischemic myocardium within four hours of administration which returned to base line by 24 hours. Normothermic regional ischemia then induced expression of iNOS mRNA, which returned to base line during reperfusion. Aminoguanidine completely abolished both MLA-induced and ischemia-induced iNOS mRNA and blocked the beneficial effects of MLA.

CONCLUSIONS

Use of MLA can provide myocardial preservation through enhanced expression of iNOS mRNA.

Cardioplegia and ischemia in the canine heart evaluated by 31P magnetic resonance spectroscopy

BACKGROUND

Warm continuous blood cardioplegia provides excellent protection, but must be interrupted by ischemic intervals to aid visualization. We hypothesized that (1) as ischemia is prolonged, the reduced metabolic rate offered by cooling gives the advantage to hypothermic cardioplegia; and (2) prior cardioplegia mitigates the deleterious effects of normothermic ischemia.

METHODS

Isolated cross-perfused canine hearts underwent cardioplegic arrest followed by 45 minutes of global ischemia at 10 degrees C or 37 degrees C, or 45 minutes of normothermic ischemia without prior cardioplegia. Left ventricular function was measured at baseline and during 2 hours of recovery. Metabolism was continuously evaluated by phosphorus-31 magnetic resonance spectroscopy.

RESULTS

Adenosine triphosphate was 71% +/- 4%, 71% +/- 7%, and 38% +/- 5% of baseline at 30 minutes, and 71% +/- 4%, 48% +/- 5%, and 39% +/- 6% at 42 minutes of ischemia in the cold ischemia, warm ischemia, and normothermic ischemia without prior cardioplegia groups, respectively. Left ventricular systolic function, left ventricular relaxation, and high-energy phosphate levels recovered fully after cold cardioplegia and ischemia. Prior cardioplegia delayed the decline in intracellular pH during normothermic ischemia initially by 9 minutes, and better preserved left ventricular relaxation during recovery, but did not ameliorate the severe postischemic impairment of left ventricular systolic function, marked adenosine triphosphate depletion, and creatine phosphate increase. Left ventricular distensibility decreased in all groups.

CONCLUSIONS

When cardioplegia is followed by prolonged ischemia, better protection is provided by hypothermia than by normothermia. Prior cardioplegia confers little advantage on recovery after prolonged normothermic ischemia but delays initial ischemic metabolic deterioration, which would contribute to the safety of brief interruptions of warm cardioplegia.

The effects of ischemic preconditioning on resting coronary flow and reactive hyperemia: involvement of A1 adenosine receptors

During the myocardial protection induced by ischemic preconditioning a reduction in myocardial metabolism occurs due to activation of the A1 adenosine receptors. This study investigates whether preconditioning changes both resting coronary flow and the magnitude of coronary reactive hyperemia and whether A1 adenosine receptors are involved in the observed changes. Experiments were performed in 14 goats (30-50 kg body weight). After the animals were anesthetized with ketamine, an electromagnetic flow-probe was used to record blood flow in the left circumflex coronary artery. Distal to the probe, an occluder was placed to produce ischemic preconditioning and reactive hyperemia. Preconditioning was obtained with two periods of 2.5 min of coronary occlusion separated from each other by 5 min of reperfusion. Coronary reactive hyperemia was obtained with 15 s of occlusion of the artery before and after preconditioning. In a group of goats before preconditioning 0.2 mg kg(-1) of 8-cyclopentyl-dipropylxanthine (CPX), an A1 adenosine receptor blocker, were given intravenously. In all animals ischemic preconditioning did not alter resting coronary flow, but, in the absence of A1 adenosine receptor blockade, reduced the reactive hyperemic response. The total hyperemic flow and the excess/debt flow ratio were reduced by about 25% and 30% respectively. The A1 adenosine receptor blockade "per se" did not cause any change in the resting flow and in the parameters of the reactive hyperemia. Unlike what observed in the absence of blockade, after CPX ischemic preconditioning was unable to reduce total hyperemic flow and the excess/debt flow ratio. The results suggest that ischemic preconditioning reduces the coronary hyperemic response by decreasing the myocardial metabolism through the activation of the A1 adenosine receptors.

Preconditioning does not attenuate cardiac dysfunction after global ischaemia in the guinea-pig

Ischaemic preconditioning reduces infarct size, but the effects on cardiac function after global ischaemia are more controversial. Additionally, species differences may exist. The present study investigates the effects of preconditioning on cardiac performance in the globally ischaemic, Langendorff-perfused guinea-pig heart. Hearts were stabilized for 25 min, and divided into the following groups: (1) (n = 8) control perfusion for 16 min before 30-min global ischaemia and 30-min reperfusion, (2) (n = 7) two episodes of 3-min ischaemia and 5-min reperfusion before global ischaemia, (3) (n = 7) 5-min ischaemia and 10-min reperfusion before ischaemia, (4) (n = 8) control perfusion before 40-min ischaemia and 30-min reperfusion, (5) (n = 8) Preconditioning as group 2 before ischaemia as group 4, (6) (n = 9) Control perfusion before 50-min ischaemia and 30-min reperfusion, (7) (n = 10) Preconditioning as group 2 before ischaemia as group 6. A dose-dependent reduction of left ventricular systolic pressure, and increase of end-diastolic pressure was observed during reperfusion after 30-, 40- and 50-min ischaemia. Preconditioning did not influence these changes, nor did it attenuate the incidence of severe reperfusion arrhythmias or reduction of coronary flow. In conclusion, ischaemic preconditioning does not improve cardiac function during reperfusion of the globally ischaemic, isolated guinea-pig heart.

An essential role of NFkappaB in tyrosine kinase signaling of p38 MAP kinase regulation of myocardial adaptation to ischemia

We have recently demonstrated that myocardial adaptation to ischemia triggers a tyrosine kinase regulated signaling pathway leading to the translocation and activation of p38 MAP kinase and MAPKAP kinase 2. Since oxidative stress is developed during ischemic adaptation and since free radicals have recently been shown to function as an intracellular signaling agent leading to the activation of nuclear transcription factor, NFkappaB, we examined whether NFkappaB was involved in the ischemic adaptation process. Isolated perfused rat hearts were adapted to ischemic stress by repeated ischemia and reperfusion. Hearts were pretreated with genistein to block tyrosine kinase while SB 203580 was used to inhibit p38 MAP kinases. Ischemic adaptation was associated with the nuclear translocation and activation of NFkappaB which was significantly blocked by both genistein and SB 203580. The ischemically adapted hearts were more resistant to ischemic reperfusion injury as evidenced by better function recovery and less tissue injury during post-ischemic reperfusion. Ischemic adaptation developed oxidative stress which was reflected by increased malonaldehyde formation. A synthetic peptide containing a cell membrane-permeable motif and nuclear sequence, SN 50, which blocked nuclear translocation of NFkappaB during ischemic adaptation, significantly inhibited the beneficial effects of adaptation on functional recovery and tissue injury. In concert, SN 50 reduced the oxidative stress developed in the adapted myocardium. These results demonstrate that p38 MAP kinase might be upstream of NFkappaB which plays a role in ischemic preconditioning of heart.

The effects of intraoperative intravenous clonidine on fluid requirements, hemodynamic variables, and support during liver transplantation: a prospective, randomized study

In this prospective, nonblind study, we report the use of clonidine during orthotopic liver transplantation (OLT). Twenty adult patients in a stable medical condition were studied. General anesthesia consisted of isoflurane in air/oxygen and sufentanil. Patients in the clonidine group received a slow i.v. infusion (15 min) of 4 microg/kg clonidine during induction. The other patients were used as controls. I.v. fluid requirements were determined as follows: albumin (4% solution) was administered to maintain filling pressures to a pulmonary capillary wedge pressure (PCWP) of more than 12 mm Hg. Packed red blood cells were transfused to maintain a hemoglobin level of 8-9 g/dL. Circulatory stability was evaluated using: systolic and diastolic arterial blood pressure and heart rate recorded at 2-min intervals; and the vasopressor/inotropic support required to maintain adequate hemodynamic variables after reperfusion. Intraoperative albumin and packed red blood cell requirements were significantly reduced in patients in the clonidine group (1644 +/- 140 and 50 +/- 50 mL vs 2867 +/- 226 mL and 1350 +/- 443 mL; P < 0.05). Heart rate was significantly slower in patients of the clonidine group. There were no differences in systolic arterial blood pressure. After reperfusion, patients in the control group showed significantly lower diastolic arterial blood pressure, required more vasopressor/inotropic support, and were more acidotic than patients in the clonidine group. We conclude that the administration of 4 microg/kg clonidine during induction of OLT significantly reduced the intraoperative requirements of i.v. fluids and blood products without compromising circulatory stability. Improvement in immediate reperfusion-induced disturbances was observed.

IMPLICATIONS

The administration of 4 microg/kg clonidine during induction of liver transplantation significantly reduced the intraoperative requirements for i.v. fluids and blood products without compromising the circulatory stability. Improvement in immediate reperfusion-induced disturbances was also observed.

Role of kinins in the endothelial protective effect of ischaemic preconditioning

1. The aim of this study was to assess whether the protective effect of ischaemic preconditioning on endothelial function in coronary arteries of the rat involves kinins. 2. Isolated hearts of the rat were exposed to a 30-min low-flow ischaemia (flow rate of 1 ml min[-1]) followed by 20-min reperfusion, after which coronaries were precontracted with 0.1 microM U-46619, and the response to the endothelium-dependent vasodilator, 5-hydroxytryptamine (5-HT, 10 microM), compared to that of the endothelium-independent vasodilator, sodium nitroprusside (SNP, 3 microM). 3. In untreated hearts, ischaemia-reperfusion diminished selectively 5-HT-induced vasodilatation, compared with time-matched sham hearts. The vasodilatation to SNP was unaffected after ischaemia-reperfusion. Preconditioning (5 min of zero-flow ischaemia followed by 10 min reperfusion) in untreated hearts preserved the vasodilatation produced by 5-HT. 4. Blockade of B1 and B2 receptors with either 3 nM [Lys[0], Leu8, des-Arg9]-bradykinin (LLDBK) or 10 nM Hoe 140 (icatibant), respectively, (started 15 min before ischaemic preconditioning or a corresponding sham period and stopped just before the 20-min reperfusion period) had no effect on the vasodilatation produced by either 5-HT or SNP in sham hearts. Pretreatment with Hoe 140 did not block the protective effect of ischaemic preconditioning on the 5-HT vasodilatation. In contrast, LLDBK halved the protective effect of ischaemic preconditioning on endothelium-dependent vasodilatation. 5. Perfusion with either bradykinin or des-Arg9-bradykinin (1 nM) 30 min before and lasting throughout the ischaemia protected the endothelium. 6. In conclusion, ischaemic preconditioning affords protection to the endothelial function in coronary resistance arteries of the rat partly by activation of B1 receptors. Although exogenous BK perfusion can protect the endothelium, B2 receptors do not play an important role in this protection in the rat isolated heart.

Acute adenosine treatment is effective in augmentation of ischemic tolerance in muscle flaps in the pig

The objective of the present project was to investigate the efficacy and mechanism of acute (10-minute) adenosine treatment for augmentation of ischemic tolerance in muscle flaps in pigs. Varying doses of adenosine were infused into 28 latissimus dorsi muscle flaps through the axillary artery (0, 0.5, or 2.0 mg per flap) and 22 gracilis muscle flaps through the medial circumflex femoral artery (0, 10, or 20 mg per flap) over 10 minutes. Ten minutes after adenosine infusion, these muscle flaps were subjected to 4 hours of sustained warm global ischemia. In addition, one group of latissimus dorsi muscle flaps (n = 6) received a 10-minute intraarterial adenosine infusion (0.5 mg) at the beginning of reperfusion. Muscle biopsies (n = 4 or 5) for adenosine triphosphate (ATP) analysis were obtained before and after adenosine infusion and at the end of 4 hours of ischemia. The extent of muscle infarction was assessed at 48 hours of reperfusion by the tetrazolium dye staining technique. Muscle blood flow in latissimus dorsi muscle flaps was measured at the end of adenosine infusion (0 or 0.5 mg per flap, n = 8) by the radioactive microsphere (15-microns) technique. It was observed that adenosine, at all doses tested, significantly (p < 0.05) reduced the extent of muscle infarction in latissimus dorsi muscle flaps (control, 40.3 +/- 2.2 percent; 0.5 mg, 20.6 +/- 1.6 percent; 2.0 mg, 18.2 +/- 1 percent) and gracilis muscle flaps (control, 31.0 +/- 1.5 percent; 10 mg, 14.3 +/- 3 percent; 20 mg, 11.6 +/- 1.2 percent). Preischemic adenosine treatment (0.5 mg per flap) was associated with maintenance of a significantly (p < 0.05) higher muscle content of ATP in latissimus dorsi muscle flaps at the end of 4 hours of ischemia compared with saline-treated ischemic controls. Postischemic adenosine treatment did not protect latissimus dorsi muscle flaps against infarction. Furthermore, adenosine treatment did not have any significant effect on mean systemic arterial blood pressure or muscle blood flow in latissimus dorsi muscle flaps. It is concluded that acute (10-minute) preischemic adenosine treatment is effective in augmentation of ischemic tolerance in muscle flaps and that this protective effect of adenosine may be, at least in part, the result of slowing muscle ATP depletion during sustained ischemia. The possible mechanisms of this adenosine-induced energysparing effect are discussed.

Myocardial, neural and vascular aspects of ischemic preconditioning

Ischemic preconditioning can be obtained with brief coronary occlusions. It has been studied in different animal species including dogs, pigs, rabbits and rats. The suggested duration of the occlusions ranges from four periods of 5 min, separated from each other by 5 min of reperfusion, to one period of 2.5 min. In addition to the reduction of the size of a subsequent infarction, preconditioning is responsible for the attenuation of the ischemia-reperfusion injury. The protection has a short duration and does not exceed two hours. Myocardial, neural and endothelial factors are involved in preconditioning. The myocardial component includes an increased release of adenosine with activation of A1 adenosine receptors, the activation of a protein-kinase C and possibly of antioxidant enzymes. The neural component includes a reduction in the release of noradrenaline from the postganglionic sympathetic fibers and a reduced myocardial sensitivity to noradrenaline. The increased myocardial release of adenosine, together with the reduced adrenergic activity, is consistent with the reduction in myocardial metabolism which has been observed after preconditioning. The coronary vascular endothelium is concerned in an increased release of nitric oxide which seems to be responsible for a prevention of reperfusion arrhythmias. In addition to the protective effect exerted on the myocardium, ischemic preconditioning seems to be responsible for a change in the coronary responsiveness to short periods of occlusion followed by release. This change in responsiveness is mainly represented by a greater velocity of the increase in flow occurring in the coronary reactive hyperemia.

Bioenergetics, ischemic contracture and reperfusion injury

The mammalian heart is normally well oxygenated and anaerobic glycolysis is extremely rare except for the production of extra ATP during extreme exercise like a marathon race. Anaerobic glycolysis plays a role when there is a serious impairment in coronary blood flow such as during heart attack and open heart surgery. The control of glycolysis in ischemic myocardial tissue appears to be extremely complex. During aerobic glycolysis, phosphofructokinase is the most important regulatory enzyme that controls the energy requirements of the cell. Under anaerobic conditions, however, glyceraldehyde-3-phosphate dehydrogenase becomes the key enzyme because it responds promptly to any changes in the essential supply of co-factors for oxidation. The conversion of pyruvate to acetyl CoA (aerobic metabolism) involves a series of chain reactions primarily catalyzed by pyruvate dehydrogenase complex which is situated at the cross roads between both aerobic and anaerobic glycolysis. It is important to remember that substrate utilization is carefully controlled by substrate availability. During aerobic metabolism, control mechanisms using fatty acids, lactate and glucose as energy substrates regulate the rate of ATP production according to energy demand. This precise mechanism is upset during ischemia and post-ischemic reperfusion for reasons discussed in this review. The demand for ATP can no longer be met by its supply because of severely reduced anaerobic glycolysis and significantly inhibited beta-oxidation of fatty acids. The impairment of bioenergetics is discussed in the context of several diseases such as cardiomyopathy, heart failure, diabetes, arrhythmias, cardiac surgery, heart-lung transplantation, and also in aging and oxidative stress. The regulation of energy metabolism in preconditioned heart is also discussed. Finally, methods used to preserve energy in ischemic myocardium are summarized and quantitation of the high-energy phosphates is discussed. This review challenges scientists to discover drugs which will stimulate energy supply during myocardial ischemia.