Preconditioning in rat hearts is independent of mitochondrial F1F0 ATPase inhibition

Intracellular ATP Concentration and Implication for Cellular Evolution

Crystalline lens and striated muscle exist at opposite ends of the metabolic spectrum. Lens is a metabolically quiescent tissue, whereas striated muscle is a mechanically dynamic tissue with high-energy requirements, yet both tissues contain millimolar levels of ATP (>2.3 mM), far exceeding their underlying metabolic needs. We explored intracellular concentrations of ATP across multiple cells, tissues, species, and domains to provide context for interpreting lens/striated muscle data. Our database revealed that high intracellular ATP concentrations are ubiquitous across diverse life forms including species existing from the Precambrian Era, suggesting an ancient highly conserved role for ATP, independent of its widely accepted view as primarily "metabolic currency". Our findings reinforce suggestions that the primordial function of ATP was non-metabolic in nature, serving instead to prevent protein aggregation.

The ATP Synthase Deficiency in Human Diseases

Human diseases range from gene-associated to gene-non-associated disorders, including age-related diseases, neurodegenerative, neuromuscular, cardiovascular, diabetic diseases, neurocognitive disorders and cancer. Mitochondria participate to the cascades of pathogenic events leading to the onset and progression of these diseases independently of their association to mutations of genes encoding mitochondrial protein. Under physiological conditions, the mitochondrial ATP synthase provides the most energy of the cell via the oxidative phosphorylation. Alterations of oxidative phosphorylation mainly affect the tissues characterized by a high-energy metabolism, such as nervous, cardiac and skeletal muscle tissues. In this review, we focus on human diseases caused by altered expressions of ATP synthase genes of both mitochondrial and nuclear origin. Moreover, we describe the contribution of ATP synthase to the pathophysiological mechanisms of other human diseases such as cardiovascular, neurodegenerative diseases or neurocognitive disorders.

The potential role of mitochondrial ATP synthase inhibitory factor 1 (IF1) in coronary heart disease: a literature review

Cardiovascular disease (CVD) is the leading cause of death worldwide, and so the search for innovative and accurate biomarkers for guiding prevention, diagnosis, and treatment is a valuable clinical and economic endeavor. Due to a recent findings that the serum concentration of mitochondrial ATP synthase inhibitory factor 1 (IF1) is an independent prognostic factor in patients with coronary heart disease (CHD), we reviewed the role of this protein in myocardial ischemic preconditioning, its correlation to plasma high density lipoprotein (HDL), the predictive potential in patients with CHD, and its interplay with angiogenesis. IF1 has been positively correlated with plasma HDL-cholesterol, and is independently negatively associated with all-cause and CV mortality in patients with CHD. However, this conclusion is prevalently based on limited data, and more research is needed to draw definitive conclusions. IF1 seems to play an additional role in increasing cell vulnerability in oncologic diseases but may also function as modest inhibitor of angiogenesis in physiological conditions. It has been also explored that IF1 may rather act as a modulator of other molecules more significantly involved in angiogenesis, especially apolipoprotein A1 on which the largest effect could be observed. In conclusion, more research is needed to characterize the role of IF1 in patients with CHD.

The role of hexokinase in cardioprotection - mechanism and potential for translation

Mitochondrial permeability transition pore (mPTP) opening plays a critical role in cardiac reperfusion injury and its prevention is cardioprotective. Tumour cell mitochondria usually have high levels of hexokinase isoform 2 (HK2) bound to their outer mitochondrial membranes (OMM) and HK2 binding to heart mitochondria has also been implicated in resistance to reperfusion injury. HK2 dissociates from heart mitochondria during ischaemia, and the extent of this correlates with the infarct size on reperfusion. Here we review the mechanisms and regulations of HK2 binding to mitochondria and how this inhibits mPTP opening and consequent reperfusion injury. Major determinants of HK2 dissociation are the elevated glucose‐6‐phosphate concentrations and decreased pH in ischaemia. These are modulated by the myriad of signalling pathways implicated in preconditioning protocols as a result of a decrease in pre‐ischaemic glycogen content. Loss of mitochondrial HK2 during ischaemia is associated with permeabilization of the OMM to cytochrome c, which leads to greater reactive oxygen species production and mPTP opening during reperfusion. Potential interactions between HK2 and OMM proteins associated with mitochondrial fission (e.g. Drp1) and apoptosis (B‐cell lymphoma 2 family members) in these processes are examined. Also considered is the role of HK2 binding in stabilizing contact sites between the OMM and the inner membrane. Breakage of these during ischaemia is proposed to facilitate cytochrome c loss during ischaemia while increasing mPTP opening and compromising cellular bioenergetics during reperfusion. We end by highlighting the many unanswered questions and discussing the potential of modulating mitochondrial HK2 binding as a pharmacological target.

Linked Articles

This article is part of a themed section on Conditioning the Heart – Pathways to Translation. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue‐8

Resistance exercise training modulates acute gene expression during human skeletal muscle hypertrophy

We sought to determine whether acute resistance exercise (RE)-induced gene expression is modified by RE training. We studied the expression patterns of a select group of genes following an acute bout of RE in naïve and hypertrophying muscle. Thirteen untrained subjects underwent supervised RE training for 12 wk of the nondominant arm and performed an acute bout of RE 1 wk after the last bout of the training program ( training+acute). The dominant arm was either unexercised ( control) or subjected to the same acute exercise bout as the trained arm ( acute RE). Following training, men (14.8 ± 2.8%; P < 0.05) and women (12.6 ± 2.4%; P < 0.05) underwent muscle hypertrophy with increases in dynamic strength in the trained arm (48.2 ± 5.4% and 72.1 ± 9.1%, respectively; P < 0.01). RE training resulted in attenuated anabolic signaling as reflected by a reduction in rpS6 phosphorylation following acute RE. Changes in mRNA levels of genes involved in hypertrophic growth, protein degradation, angiogenesis, and metabolism commonly expressed in both men and women was determined 4 h following acute RE. We show that RE training can modify acute RE-induced gene expression in a divergent and gene-specific manner even in genes belonging to the same ontology. Changes in gene expression following acute RE are multidimensional, and may not necessarily reflect the actual adaptive response taking place during the training process. Thus RE training can selectively modify the acute response to RE, thereby challenging the use of gene expression as a marker of exercise-induced adaptations.

Altered expression of mitochondrial electron transport chain proteins and improved myocardial energetic state during late ischemic preconditioning

Altered expression of mitochondrial electron transport proteins has been shown in early preconditioned myocardial tissue. We wished to determine whether these alterations persist in the Second Window of Protection (SWOP) and if so, whether a favorable energetic state is facilitated during subsequent ischemia. Fourteen pigs underwent a SWOP protocol with ten 2-minute balloon inflations in the LAD artery, each separated by 2 minutes reperfusion. Twenty-four hours later, mitochondria were isolated from SWOP and SHAM pig hearts and analyzed for uncoupling protein (UCP)-2 content by western blot analysis, proteomic changes by iTRAQ(®) and respiration by an oxygen electrode. In parallel in vivo studies, high-energy nucleotides were obtained by transmural biopsy from anesthetized SWOP and SHAM pigs at baseline and during sustained low-flow ischemia. Compared with SHAM mitochondria, ex vivo SWOP heart tissue demonstrated increased expression of UCP-2, Complex IV (cytochrome c oxidase) and Complex V (ATPase) proteins. In comparison with SHAM pigs during in vivo conditions, transmural energetics in SWOP hearts, as estimated by the free energy of ATP hydrolysis (ΔG(0)), were similar at baseline but had decreased by the end of low-flow ischemia (-57.0 ± 2.1 versus -51.1 ± 1.4 kJ/mol; P < 0.05). In conclusion, within isolated mitochondria from preconditioned SWOP hearts, UCP-2 is increased and in concert with enhanced Complex IV and V proteins, imparts a favorable energetic state during low-flow ischemia. These data support the notion that mitochondrial adaptations that may reduce oxidant damage do not reduce the overall efficiency of energetics during sustained oxygen deprivation.

Mitochondrial adenine nucleotide transport and cardioprotection

Mitochondria are highly metabolically active cell organelles that not only act as the powerhouse of the cell by supplying energy through ATP production, but also play a destructive role by initiating cell death pathways. Growing evidence recognizes that mitochondrial dysfunction is one of the major causes of cardiovascular disease. Under de-energized conditions, slowing of adenine nucleotide transport in and out of the mitochondria significantly attenuates myocardial ischemia-reperfusion injury. The purpose of this review is to elaborate on and update the mechanistic pathways which may explain how altered adenine nucleotide transport can influence cardiovascular function. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

What makes the mitochondria a killer? Can we condition them to be less destructive?

Cardioprotection, such as preconditioning and postconditioning, has been shown to result in a significant reduction in cell death. Many of the signaling pathways activated by cardioprotection have been elucidated, but there is still a lack of understanding of the mechanisms by which these signaling pathways reduce cell death. Mitochondria have been reported to be an important player in many types of apoptotic and necrotic cell death. If mitochondria play an important role in cell death, then it seems reasonable to consider that cardioprotective mechanisms might act, at least in part, by opposing mitochondrial cell death pathways. One of the major mechanisms of cell death in ischemia-reperfusion is suggested to be the opening of a large conductance pore in the inner mitochondrial membrane, known as the mitochondrial permeability transition pore. Inhibition of this mitochondrial pore appears to be one of the major mechanisms by which cardioprotection reduces cell death. Cardioprotection activates a number of signaling pathways that reduce the level of triggers (reactive oxygen species and calcium) or enhances inhibitors of the mitochondrial permeability transition pore at the start of reperfusion. This article is part of a Special Issue entitled: Mitochondria and Cardioprotection.

Mitochondrial and cell-surface F0F1ATPsynthase in innate and acquired cardioprotection

Mitochondria are central to heart function and dysfunction, and the pathways activated by different cardioprotective interventions mostly converge on mitochondria. In a context of perspectives in innate and acquired cardioprotection, we review some recent advances in F(0)F(1)ATPsynthase structure/function and regulation in cardiac cells. We focus on three topics regarding the mitochondrial F(0)F(1)ATPsynthase and the plasma membrane enzyme, i.e.: i) the crucial role of cardiac mitochondrial F(0)F(1)ATPsynthase regulation by the inhibitory protein IF(1) in heart preconditioning strategies; ii) the structure and function of mitochondrial F(0)F(1)ATPsynthase oligomers in mammalian myocardium as possible endogenous factors of mitochondria resistance to ischemic insult; iii) the external location and characterization of plasma membrane F(0)F(1) ATP synthase in search for possible actors of its regulation, such as IF(1) and calmodulin, at cell surface.

Pharmacological profile of the selective mitochondrial F1F0 ATP hydrolase inhibitor BMS-199264 in myocardial ischemia

The mitochondrial F1F0 ATP synthase is responsible for the majority of ATP production in mammals and does this through a rotary catalytic mechanism. Studies show that the F1F0 ATP synthase can switch to an ATP hydrolase, and this occurs under conditions seen during myocardial ischemia. This ATP hydrolysis causes wasting of ATP that does not produce work. The degree of ATP inefficiently hydrolyzed during ischemia may be as high as 50-90% of the total. A naturally occurring, reversible inhibitor (IF-1) of the hydrolase activity is in the mitochondria, and it has a pH optimum of 6.8. Based on studies with the nonselective (inhibit both synthase and hydrolase activity) inhibitors aurovertin B and oligomycin B reduce the rate of ATP depletion during ischemia, showing that IF-1 does not completely block hydrolase activity. Oligomycin and aurovertin cannot be used for treating myocardial ischemia as they will reduce ATP production in healthy tissue. We generated a focused structure-activity relationship, and several compounds were identified that selectively inhibited the F1F0 ATP hydrolase activity while having no effect on synthase function. One compound, BMS-199264 had no effect on F1F0 ATP synthase function in submitochondrial particles while inhibiting hydrolase function, unlike oligomycin that inhibits both. BMS-199264 selectively inhibited ATP decline during ischemia while not affecting ATP production in normoxic and reperfused hearts. BMS-191264 also reduced cardiac necrosis and enhanced the recovery of contractile function following reperfusion. These data also suggest that the reversal of the synthase and hydrolase activities is not merely a chemical reaction run in reverse.

Mechanisms underlying acute protection from cardiac ischemia-reperfusion injury

Mitochondria play an important role in cell death and cardioprotection. During ischemia, when ATP is progressively depleted, ion pumps cannot function resulting in a rise in calcium (Ca(2+)), which further accelerates ATP depletion. The rise in Ca(2+) during ischemia and reperfusion leads to mitochondrial Ca(2+) accumulation, particularly during reperfusion when oxygen is reintroduced. Reintroduction of oxygen allows generation of ATP; however, damage to the electron transport chain results in increased mitochondrial generation of reactive oxygen species (ROS). Mitochondrial Ca(2+) overload and increased ROS can result in opening of the mitochondrial permeability transition pore, which further compromises cellular energetics. The resultant low ATP and altered ion homeostasis result in rupture of the plasma membrane and cell death. Mitochondria have long been proposed as central players in cell death, since the mitochondria are central to synthesis of both ATP and ROS and since mitochondrial and cytosolic Ca(2+) overload are key components of cell death. Many cardioprotective mechanisms converge on the mitochondria to reduce cell death. Reducing Ca(2+) overload and reducing ROS have both been reported to reduce ischemic injury. Preconditioning activates a number of signaling pathways that reduce Ca(2+) overload and reduce activation of the mitochondrial permeability transition pore. The mitochondrial targets of cardioprotective signals are discussed in detail.

Local delivery of PKCepsilon-activating peptide mimics ischemic preconditioning in aged hearts through GSK-3beta but not F1-ATPase inactivation

In adult heart, selective PKCepsilon activation limits ischemia (I)-reperfusion (R) damage and mimics the protection associated with ischemic preconditioning. We sought to determine whether local delivery of PKCepsilon activator peptide psiepsilon-receptor for activated C-kinase (psiepsilon-RACK) is sufficient to produce a similarly protected phenotype in aged hearts. Langendorff-perfused hearts isolated from adult (5 mo; n = 9) and aged (24 mo; n = 9) male Fisher 344 rats were perfused with psiepsilon-RACK conjugated to Tat (500 nM) or Tat only (500 nM) for 10 min before global 31-min ischemia. Western blotting was used to measure mitochondrial targeting of PKCepsilon, PKCdelta, phospho (p)-GSK-3beta (Ser(9)) and GSK-3beta in hearts snap-frozen during I. Recovery of left ventricular developed pressure was significantly improved by psiepsilon-RACK (P < 0.01) and infarct size reduced in 24-mo rats vs. age-matched controls (60% vs. 34%; P < 0.01). Mitochondrial PKCepsilon levels were 30% greater during I with psiepsilon-RACK in aged vs. control rats (P < 0.01). Interestingly, mitochondrial GSK-3beta levels were threefold greater in aged vs. adult rats during I, and psiepsilon-RACK prevented this increase (P < 0.01). Mitochondrial p-GSK-3beta levels were also greater in aged rats after psiepsilon-RACK (P < 0.01), and subsequent inhibition of GSK-3beta with SB-216763 (3 muM) before I/R elicited protection similar to that of psiepsilon-RACK (n = 3/group). Mitochondrial proteomic analysis further identified group differences in the F(1)-ATPase beta-subunit, and coimmunoprecipitation studies revealed a novel interaction with PKCepsilon. F(1)-ATPase-PKCepsilon association was affected by psiepsilon-RACK in adult but not aged rats. Our results provide evidence, for the first time, for PKCepsilon-mediated protection in aged rat heart after I/R and suggest a central role for mitochondrial GSK-3beta but not F(1)-ATPase as a potential target of PKCepsilon to limit I/R damage with aging.

Potential role and mechanisms of subcellular remodeling in cardiac dysfunction due to ischemic heart disease

Several studies have revealed varying degrees of changes in sarcoplasmic reticular and myofibrillar activities, protein content, gene expression and intracellular Ca-handling during cardiac dysfunction due to ischemia-reperfusion (I/R); however, relatively little is known about the sarcolemmal and mitochondrial alterations, as well as their mechanisms in the I/R hearts. Because I/R is associated with oxidative stress and intracellular Ca-overload, it has been indicated that changes in subcellular activities, protein content and gene expression due to I/R are related to both oxidative stress and Ca-overload. Intracellular Ca-overload appears to induce changes in subcellular activities, protein contents and gene expression (subcellular remodeling) by activation of proteases and phospholipases, as well as by affecting the genetic apparatus, whereas oxidative stress is considered to cause oxidation of functional groups of different subcellular proteins in addition to modifying the genetic machinery. Ischemic preconditioning, which is known to depress the development of both intracellular Ca-overload and oxidative stress due to I/R, was observed to attenuate the I/R-induced subcellular remodeling and improve cardiac performance. It is suggested that a combination therapy with antioxidants and interventions, which reduce the development of intracellular Ca-overload, may improve cardiac function by preventing or attenuating the occurrence of subcellular remodeling due to ischemic heart disease. It is proposed that defects in the activities of subcellular organelles may serve as underlying mechanisms for I/R-induced cardiac dysfunction under acute conditions, whereas subcellular remodeling due to alterations in gene expression may explain the impaired cardiac performance under chronic conditions of I/R.

Biochemical dysfunction in heart mitochondria exposed to ischaemia and reperfusion

Heart tissue is remarkably sensitive to oxygen deprivation. Although heart cells, like those of most tissues, rapidly adapt to anoxic conditions, relatively short periods of ischaemia and subsequent reperfusion lead to extensive tissue death during cardiac infarction. Heart tissue is not readily regenerated, and permanent heart damage is the result. Although mitochondria maintain normal heart function by providing virtually all of the heart's ATP, they are also implicated in the development of ischaemic damage. While mitochondria do provide some mechanisms that protect against ischaemic damage (such as an endogenous inhibitor of the F1Fo-ATPase and antioxidant enzymes), they also possess a range of elements that exacerbate it, including ROS (reactive oxygen species) generators, the mitochondrial permeability transition pore, and their ability to release apoptotic factors. This review considers the process of ischaemic damage from a mitochondrial viewpoint. It considers ischaemic changes in the inner membrane complexes I-V, and how this might affect formation of ROS and high-energy phosphate production/degradation. We discuss the contribution of various mitochondrial cation channels to ionic imbalances which seem to be a major cause of reperfusion injury. The different roles of the H+, Ca2+ and the various K+ channel transporters are considered, particularly the K+(ATP) (ATP-dependent K+) channels. A possible role for the mitochondrial permeability transition pore in ischaemic damage is assessed. Finally, we summarize the metabolic and pharmacological interventions that have been used to alleviate the effects of ischaemic injury, highlighting the value of these or related interventions in possible therapeutics.

Excessive ATP hydrolysis in ischemic myocardium by mitochondrial F1F0-ATPase: effect of selective pharmacological inhibition of mitochondrial ATPase hydrolase activity

Mitochondrial F(1)F(0)-ATPase normally synthesizes ATP in the heart, but under ischemic conditions this enzyme paradoxically causes ATP hydrolysis. Nonselective inhibitors of this enzyme (aurovertin, oligomycin) inhibit ATP synthesis in normal tissue but also inhibit ATP hydrolysis in ischemic myocardium. We characterized the profile of aurovertin and oligomycin in ischemic and nonischemic rat myocardium and compared this with the profile of BMS-199264, which only inhibits F(1)F(0)-ATP hydrolase activity. In isolated rat hearts, aurovertin (1-10 microM) and oligomycin (10 microM), at concentrations inhibiting ATPase activity, reduced ATP concentration and contractile function in the nonischemic heart but significantly reduced the rate of ATP depletion during ischemia. They also inhibited recovery of reperfusion ATP and contractile function, consistent with nonselective F(1)F(0)-ATPase inhibitory activity, which suggests that upon reperfusion, the hydrolase activity switches back to ATP synthesis. BMS-199264 inhibits F(1)F(0) hydrolase activity in submitochondrial particles with no effect on ATP synthase activity. BMS-199264 (1-10 microM) conserved ATP in rat hearts during ischemia while having no effect on preischemic contractile function or ATP concentration. Reperfusion ATP levels were replenished faster and necrosis was reduced by BMS-199264. ATP hydrolase activity ex vivo was selectively inhibited by BMS-199264. Therefore, excessive ATP hydrolysis by F(1)F(0)-ATPase contributes to the decline in cardiac energy reserve during ischemia and selective inhibition of ATP hydrolase activity can protect ischemic myocardium.

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.

Protection afforded by ischemic preconditioning is not mediated by effects on cell-to-cell electrical coupling during myocardial ischemia-reperfusion

The end-effectors of ischemic preconditioning (IPC) are not well known. It has been recently shown that transgenic mice underexpressing the gap junction protein connexin43 (Cx43) cannot be preconditioned. Because gap junctions allow spreading of cell death during ischemia-reperfusion in different tissues, including myocardium, we hypothesized that the protection afforded by IPC is mediated by effects on gap junction-mediated intercellular communication. To test this hypothesis, we analyzed the effect of IPC (5 min ischemia-5 min reperfusion x 2) on the changes in electrical impedance (four electrode probe) and impulse propagation velocity (transmembrane action potential) induced by ischemia (60 min) and reperfusion (60 min) in isolated rat hearts. IPC (n = 8) reduced reperfusion-induced lactate dehydrogenase release by 65.8% with respect to control hearts (n = 9) (P = 0.04) but had no effect on the time of onset of rigor contracture (increase in diastolic tension), electrical uncoupling (sharp changes in tissue resistivity and phase angle in impedance recordings), or block of impulse propagation during ischemia. Normalization of electrical impedance during reperfusion was also unaffected by IPC. The lack of effect of IPC on ischemic rigor contracture and on changes in tissue impedance during ischemia-reperfusion were validated under in vivo conditions in pigs submitted to 48 min of coronary occlusion and 120 min of reperfusion. IPC (n = 12) reduced infarct size (triphenyltetrazolium) by 64.9% (P = 0.01) with respect to controls (n = 17). We conclude that the protection afforded by IPC is not mediated by effects on electrical coupling. This result is consistent with recent findings suggesting that Cx43 could have effects on cell survival independent on changes in cell-to-cell communication.

Opening of mitochondrial K+ channels increases ischemic ATP levels by preventing hydrolysis

Mitochondrial ATP-sensitive K+ channels (mitoK(ATP)) have been proposed to mediate protection against ischemic injury by increasing high-energy intermediate levels. This study was designed to verify if mitochondria are an important factor in the loss of cardiac ATP associated to ischemia, and determine the possible role of mitoK(ATP) in the control of ischemic ATP loss. Langendorff-perfused rat hearts subjected to ischemia were found to have significantly higher ATP contents when pretreated with oligomycin or atractyloside, indicating that mitochondrial ATP hydrolysis contributes toward ischemic ATP depletion. MitoK(ATP) opening induced by diazoxide promoted a similar protection against ATP loss. Diazoxide also inhibited ATP hydrolysis in isolated, nonrespiring mitochondria, an effect accompanied by a drop in the membrane potential and Ca2+ uptake. In hearts subjected to ischemia followed by reperfusion, myocardial injury was prevented by diazoxide, but not atractyloside or oligomycin, which, unlike diazoxide, decreased reperfusion ATP levels. Our results suggest that mitoK(ATP)-mediated protection occurs due to selective inhibition of mitochondrial ATP hydrolysis during ischemia, without affecting ATP synthesis after reperfusion.

Myocardial ischemic preconditioning and mitochondrial F1F0-ATPase activity

A short period of ischemia followed by reperfusion (ischemic preconditioning) is known to trigger mechanisms that contribute to the prevention of ATP depletion. In ischemic conditions, most of the ATP hydrolysis can be attributed to mitochondrial F1F0-ATPase (ATP synthase). The purpose of the present study was to examine the effect of myocardial ischemic preconditioning on the kinetics of ATP hydrolysis by F1F0-ATPase. Preconditioning was accomplished by three 3-min periods of global ischemia separated by 3 min of reperfusion. Steady state ATP hydrolysis rates in both control and preconditioned mitochondria were not significantly different. This suggests that a large influence of the enzyme on the preconditioning mechanism may be excluded. However, the time required by the reaction to reach the steady state rate was increased in the preconditioned group before sustained ischemia, and it was even more enhanced in the first 5 min of reperfusion (101 +/- 3.0 sec in preconditioned vs. 83.4 +/- 4.4 sec in controls, p < 0.05). These results suggest that this transient increase in activation time may contribute to the cardioprotection by slowing the ATP depletion in the very critical early phase of post-ischemic reperfusion.