Effects of calcium on mitochondrial NAD(P)H in paced rat ventricular myocytes

Regulation of cardiac cellular bioenergetics: mechanisms and consequences

The regulation of cardiac cellular bioenergetics is critical for maintaining normal cell function, yet the nature of this regulation is not fully understood. Different mechanisms have been proposed to explain how mitochondrial ATP production is regulated to match changing cellular energy demand while metabolite concentrations are maintained. We have developed an integrated mathematical model of cardiac cellular bioenergetics, electrophysiology, and mechanics to test whether stimulation of the dehydrogenase flux by Ca²⁺ or Pi, or stimulation of complex III by Pi can increase the rate of mitochondrial ATP production above that determined by substrate availability (ADP and Pi). Using the model, we show that, under physiological conditions the rate of mitochondrial ATP production can match varying demand through substrate availability alone; that ATP production rate is not limited by the supply of reducing equivalents in the form of NADH, as a result of Ca²⁺ or Pi activation of the dehydrogenases; and that ATP production rate is sensitive to feedback activation of complex III by Pi. We then investigate the mechanistic implications on cytosolic ion homeostasis and force production by simulating the concentrations of cytosolic Ca²⁺, Na⁺ and K⁺, and activity of the key ATPases, SERCA pump, Na⁺/K⁺ pump and actin-myosin ATPase, in response to increasing cellular energy demand. We find that feedback regulation of mitochondrial complex III by Pi improves the coupling between energy demand and mitochondrial ATP production and stabilizes cytosolic ADP and Pi concentrations. This subsequently leads to stabilized cytosolic ionic concentrations and consequentially reduced energetic cost from cellular ATPases.

Regulation of mitochondrial Ca2+ and its effects on energetics and redox balance in normal and failing heart

Ca(2+) has been well accepted as a signal that coordinates changes in cytosolic workload with mitochondrial energy metabolism in cardiomyocytes. During increased work, Ca(2+) is accumulated in mitochondria and stimulates ATP production to match energy supply and demand. The kinetics of mitochondrial Ca(2+) ([Ca(2+)](m)) uptake remains unclear, and we review the debate on this subject in this article. [Ca(2+)](m) has multiple targets in oxidative phosphorylation including the F1/FO ATPase, the adenine nucleotide translocase, and Ca(2+)-sensitive dehydrogenases (CaDH) of the tricarboxylic acid (TCA) cycle. The well established effect of [Ca(2+)](m) is to activate CaDHs of the TCA cycle to increase NADH production. Maintaining NADH level is not only critical to keep a high oxidative phosphorylation rate during increased cardiac work, but is also necessary for the reducing system of the cell to maintain its reactive oxygen species (ROS) -scavenging capacity. Further, we review recent data demonstrating the deleterious effects of elevated Na(+) in cardiac pathology by blunting [Ca(2+)](m) accumulation.

Mitochondrial calcium as a key regulator of mitochondrial ATP production in mammalian cells

Mitochondrial Ca(2+) transport was initially considered important only in buffering of cytosolic Ca(2+) by acting as a "sink" under conditions of Ca(2+) overload. The main regulator of ATP production was considered to be the relative concentrations of high energy phosphates. However, work by Denton and McCormack in the 1970s and 1980s showed that free intramitochondrial Ca(2+) ([Ca(2+)](m)) activated dehydrogenase enzymes in mitochondria, leading to increased NADH and hence ATP production. This leads them to propose a scheme, subsequently termed a "parallel activation model" whereby increases in energy demand, such as hormonal stimulation or increased workload in muscle, produced an increase in cytosolic [Ca(2+)] that was relayed by the mitochondrial Ca(2+) transporters into the matrix to give an increase in [Ca(2+)](m). This then stimulated energy production to meet the increased energy demand. With the development of methods for measuring [Ca(2+)](m) in living cells that proved [Ca(2+)](m) changed over a dynamic physiological range rather than simply soaking up excess cytosolic [Ca(2+)], this model has now gained widespread acceptance. However, work by ourselves and others using targeted probes to measure changes in both [Ca(2+)] and [ATP] in different cell compartments has revealed variations in the interrelationships between these two in different tissues, suggesting that metabolic regulation by Ca(2+) is finely tuned to the demands and function of the individual organ.

Enhancing mitochondrial Ca2+ uptake in myocytes from failing hearts restores energy supply and demand matching

Mitochondrial ATP production is continually adjusted to energy demand through coordinated increases in oxidative phosphorylation and NADH production mediated by mitochondrial Ca2+([Ca2+]m). Elevated cytosolic Na+ impairs [Ca2+]m accumulation during rapid pacing of myocytes, resulting in a decrease in NADH/NAD+ redox potential. Here, we determined 1) if accentuating [Ca2+]m accumulation prevents the impaired NADH response at high [Na+]i; 2) if [Ca2+]m handling and NADH/NAD+ balance during stimulation is impaired with heart failure (induced by aortic constriction); and 3) if inhibiting [Ca2+]m efflux improves NADH/NAD+ balance in heart failure. [Ca2+]m and NADH were recorded in cells at rest and during voltage clamp stimulation (4Hz) with either 5 or 15 mmol/L [Na+]i. Fast [Ca2+]m transients and a rise in diastolic [Ca2+]m were observed during electric stimulation. [Ca2+]m accumulation was [Na+]i-dependent; less [Ca2+]m accumulated in cells with 15 Na+ versus 5 mmol/L Na+ and NADH oxidation was evident at 15 mmol/L Na+, but not at 5 mmol/L Na+. Treatment with either the mitochondrial Na+/Ca2+ exchange inhibitor CGP-37157 (1 micromol/L) or raising cytosolic Pi (2 mmol/L) enhanced [Ca2+]m accumulation and prevented the NADH oxidation at 15 mmol/L [Na+]i. In heart failure myocytes, resting [Na+]i increased from 5.2+/-1.4 to 16.8+/-3.1mmol/L and net NADH oxidation was observed during pacing, whereas NADH was well matched in controls. Treatment with CGP-37157 or lowering [Na+]i prevented the impaired NADH response in heart failure. We conclude that high [Na+]i (at levels observed in heart failure) has detrimental effects on mitochondrial bioenergetics, and this impairment can be prevented by inhibiting the mitochondrial Na+/Ca2+ exchanger.

Excitation-contraction coupling and mitochondrial energetics

Cardiac excitation-contraction (EC) coupling consumes vast amounts of cellular energy, most of which is produced in mitochondria by oxidative phosphorylation. In order to adapt the constantly varying workload of the heart to energy supply, tight coupling mechanisms are essential to maintain cellular pools of ATP, phosphocreatine and NADH. To our current knowledge, the most important regulators of oxidative phosphorylation are ADP, Pi, and Ca2+. However, the kinetics of mitochondrial Ca2+-uptake during EC coupling are currently a matter of intense debate. Recent experimental findings suggest the existence of a mitochondrial Ca2+ microdomain in cardiac myocytes, justified by the close proximity of mitochondria to the sites of cellular Ca2+ release, i. e., the ryanodine receptors of the sarcoplasmic reticulum. Such a Ca2+ microdomain could explain seemingly controversial results on mitochondrial Ca2+ uptake kinetics in isolated mitochondria versus whole cardiac myocytes. Another important consideration is that rapid mitochondrial Ca2+ uptake facilitated by microdomains may shape cytosolic Ca2+ signals in cardiac myocytes and have an impact on energy supply and demand matching. Defects in EC coupling in chronic heart failure may adversely affect mitochondrial Ca2+ uptake and energetics, initiating a vicious cycle of contractile dysfunction and energy depletion. Future therapeutic approaches in the treatment of heart failure could be aimed at interrupting this vicious cycle.

Parallel activation of mitochondrial oxidative metabolism with increased cardiac energy expenditure is not dependent on fatty acid oxidation in pigs

Steady state concentrations of ATP and ADP in vivo are similar at low and high cardiac workloads; however, the mechanisms that regulate the activation of substrate metabolism and oxidative phosphorylation that supports this stability are poorly understood. We tested the hypotheses that (1) there is parallel activation of mitochondrial and cytosolic dehydrogenases in the transition from low to high workload, which increases NADH/NAD+ ratio in both compartments, and (2) this response does not require an increase in fatty acid oxidation (FAO). Anaesthetized pigs were subjected to either sham treatment, or an abrupt increase in cardiac workload for 5 min with dobutamine infusion and aortic constriction. Myocardial oxygen consumption and FAO were increased 3- and 2-fold, respectively, but ATP and ADP concentrations did not change. NADH-generating pathways were rapidly activated in both the cytosol and mitochondria, as seen in a 40% depletion in glycogen stores, a 3.6-fold activation of pyruvate dehydrogenase, and a 50% increase in tissue NADH/NAD+. Simulations from a multicompartmental computational model of cardiac energy metabolism predicted that parallel activation of glycolysis and mitochondrial metabolism results in an increase in the NADH/NAD+ ratio in both cytosol and mitochondria. FAO was blocked by 75% in a third group of pigs, and a similar increase in and the NAHD/NAD+ ratio was observed. In conclusion, in the transition to a high cardiac workload there is rapid parallel activation of substrate oxidation that results in an increase in the NADH/NAD+ ratio.

Mitochondria and neuronal activity

Mitochondria are central for various cellular processes that include ATP production, intracellular Ca(2+) signaling, and generation of reactive oxygen species. Neurons critically depend on mitochondrial function to establish membrane excitability and to execute the complex processes of neurotransmission and plasticity. While much information about mitochondrial properties is available from studies on isolated mitochondria and dissociated cell cultures, less is known about mitochondrial function in intact neurons in brain tissue. However, a detailed description of the interactions between mitochondrial function, energy metabolism, and neuronal activity is crucial for the understanding of the complex physiological behavior of neurons, as well as the pathophysiology of various neurological diseases. The combination of new fluorescence imaging techniques, electrophysiology, and brain slice preparations provides a powerful tool to study mitochondrial function during neuronal activity, with high spatiotemporal resolution. This review summarizes recent findings on mitochondrial Ca(2+) transport, mitochondrial membrane potential (DeltaPsi(m)), and energy metabolism during neuronal activity. We will first discuss interactions of these parameters for experimental stimulation conditions that can be related to the physiological range. We will then describe how mitochondrial and metabolic dysfunction develops during pathological neuronal activity, focusing on temporal lobe epilepsy and its experimental models. The aim is to illustrate that 1) the structure of the mitochondrial compartment is highly dynamic in neurons, 2) there is a fine-tuned coupling between neuronal activity and mitochondrial function, and 3) mitochondria are of central importance for the complex behavior of neurons.

ATP regulation in adult rat cardiomyocytes: time-resolved decoding of rapid mitochondrial calcium spiking imaged with targeted photoproteins

The mechanisms that enable the heart to rapidly increase ATP supply in line with increased demand have not been fully elucidated. Here we used an adenoviral system to express the photoproteins luciferase and aequorin, targeted to the mitochondria or cytosol of adult cardiomyocytes, to investigate the interrelationship between ATP and Ca(2+) in these compartments. In neither compartment were changes in free [ATP] observed upon increased workload (addition of isoproterenol) in myocytes that were already beating. However, when myocytes were stimulated to beat rapidly from rest, in the presence of isoproterenol, a significant but transient drop in mitochondrial [ATP] ([ATP](m)) occurred (on average to 10% of the initial signal). Corresponding changes in cytosolic [ATP] ([ATP](c)) were much smaller (<5%), indicating that [ATP](c) was effectively buffered in this compartment. Although mitochondrial [Ca(2+)] ([Ca(2+)](m)) is an important regulator of respiratory chain activity and ATP production in other cells, the kinetics of mitochondrial Ca(2+) transport are controversial. Parallel experiments in cells expressing mitochondrial aequorin showed that the drop in [ATP](m) occurred over the same time scale as average [Ca(2+)](m) was increasing. Conversely, in the absence or presence of isoproterenol, clear beat-to-beat peaks in [Ca(2+)](m) were observed at 0.9 or 1.3 mum, respectively, concentrations similar to those observed in the cytosol. These results suggest that mitochondrial Ca(2+) transients occur during the contractile cycle and are translated into a time-averaged increase in mitochondrial ATP production that keeps pace with increased cytosolic demand.

Calcium-mediated coupling between mitochondrial substrate dehydrogenation and cardiac workload in single guinea-pig ventricular myocytes

We measured mitochondrial NADH autofluorescence or Ca(2+) using Rhod-2, simultaneously with cell shortening in isolated guinea-pig ventricular myocytes. When both frequency and amplitude of twitch shortening (work intensity) were increased by raising stimulus frequency in incremental steps from 0.1 to 3.3 Hz, the steady level of NADH signal increased in a frequency-dependent manner. Mitochondrial Ca(2+) also increased with increasing work intensity. Applying Ru360, an inhibitor of mitochondrial Ca(2+) uniporter, largely attenuated the response of both NADH fluorescence and mitochondrial Ca(2+). The increase in mitochondrial Ca(2+) was slow with t(1/2)=~12 s and no obvious cyclic changes were observed in the NADH signal. When a step change from 0.1 to 3.3 Hz stimulation was applied, the NADH signal first decreased to 83% and then increased to 155% of the control level. Upon returning to 0.1 Hz, the NADH signal showed an overshoot before declining to the control level. The biphasic onset time course was well explained by the delayed Ca(2+) activation of the substrate dehydrogenation superimposed on the feedback control of the ATP synthesis, while the offset time course with a delayed deactivation of dehydrogenation. A computer simulation using an oxidative phosphorylation linked to the cardiac excitation contraction model well reconstructed the response of NADH. This model simulation predicts that the activation of substrate dehydrogenation provides ~23% of driving force of the ATP synthesis to meet the increased workload induced by the jump of stimulus from 0.1 to 3.3 Hz, and remaining ~77% is supplied by the feedback control.

NAD(P)H fluorescence imaging of mitochondrial metabolism in contracting Xenopus skeletal muscle fibers: effect of oxygen availability

The blue autofluorescence (351 nm excitation, 450 nm emission) of single skeletal muscle fibers from Xenopus was characterized to be originating from mitochondrial NAD(P)H on the basis of morphological and functional correlations. This fluorescence signal was used to estimate the oxygen availability to isolated single Xenopus muscle fibers during work level transitions by confocal microscopy. Fibers were stimulated to generate two contractile periods that were only different in the PO2 of the solution perfusing the single fibers (PO2 of 30 or 0-2 Torr; pH = 7.2). During contractions, mean cellular NAD(P)H increased significantly from rest in the low PO2 condition with the core (inner 10%) increasing to a greater extent than the periphery (outer 10%). After the cessation of work, NAD(P)H decreased in a manner consistent with oxygen tensions sufficient to oxidize the surplus NAD(P)H. In contrast, NAD(P)H decreased significantly with work in 30 Torr PO2. However, the rate of NAD(P)H oxidation was slower and significantly increased with the cessation of work in the core of the fiber compared with the peripheral region, consistent with a remaining limitation in oxygen availability. These results suggest that the blue autofluorescence signal in Xenopus skeletal muscle fibers is from mitochondrial NAD(P)H and that the rate of NAD(P)H oxidation within the cell is influenced by extracellular PO2 even at high extracellular PO2 during the contraction cycle. These results also demonstrate that although oxygen availability influences the rate of NAD(P)H oxidation, it does not prevent NAD(P)H from being oxidized through the process of oxidative phosphorylation at the onset of contractions.

Coupling of neuronal activity and mitochondrial metabolism as revealed by NAD(P)H fluorescence signals in organotypic hippocampal slice cultures of the rat

During physiological activity neurons may experience localised energy demands which require intracellular signals for stimulation of mitochondrial NADH generation and subsequent delivery of ATP. To elucidate these mechanisms, we applied microfluorimetric monitoring of cytoplasmic (Fluo-3) and mitochondrial (Rhod-2) calcium concentration ([Ca(2+)](c), [Ca(2+)](m)), as well as of mitochondrial oxidative metabolism (NAD(P)H), whilst simultaneously measuring changes in extracellular potassium concentration ([K(+)](o)), as an indicator of neuronal activity in hippocampal slice cultures. Changes in neuronal activity were induced by repetitive stimulation at different frequencies (5, 20, 100 Hz) and intensities. Stimulation parameters were chosen to elicit rises in [K(+)](o) of less than 3 mM which is comparable to physiologically occurring rises in [K(+)](o). The mitochondrial uncoupler carbonyl cyanide m-chlorophenyl hydrazone (CCCP) reduced stimulus-induced changes in Rhod-2 fluorescence by 79%, indicating that Rhod-2 signals were primarily of mitochondrial origin. Repetitive stimulation at 20 Hz applied to areas CA1 or CA3 resulted in moderate rises in [K(+)](o) which were associated with stimulus-dependent elevations in [Ca(2+)](c) and [Ca(2+)](m) of up to 15%. The same stimuli also elicited biphasic changes in NAD(P)H fluorescence characterised by an initial decline and a subsequent prolonged elevation of up to 10%. Variation of stimulus parameters revealed close correlations between rises in [K(+)](o), in [Ca(2+)](m) and changes in NAD(P)H fluorescence. To elucidate the role of intracellular Ca(2+) accumulation in induction of NAD(P)H fluorescence signals, the effect of application of Ca(2+)-free solution on these signals evoked by repetitive antidromic stimulation of the alveus during recordings in area CA1 was studied. Lowering extracellular Ca(2+) led to complete blockade of postsynaptic field potential components as well as of rises in [Ca(2+)](c) and [Ca(2+)](m). Amplitudes of NAD(P)H signals were reduced by 59%, though rises in [K(+)](o) were comparable in presence and absence of extracellular Ca(2+). The results suggest i) that mitochondrial oxidative metabolism is fine-tuned to graded physiological activity in neurons and ii) that rapid mitochondrial Ca(2+) signalling represents one of the main determinants for stimulation of oxidative metabolism under physiological conditions.

Old players in a new role: mitochondria-associated membranes, VDAC, and ryanodine receptors as contributors to calcium signal propagation from endoplasmic reticulum to the mitochondria

In many cell types, IP(3) and ryanodine receptor (IP(3)R/RyR)-mediated Ca(2+) mobilization from the sarcoendoplasmic reticulum (ER/SR) results in an elevation of mitochondrial matrix [Ca(2+)]. Although delivery of the released Ca(2+) to the mitochondria has been established as a fundamental signaling process, the molecular mechanism underlying mitochondrial Ca(2+) uptake remains a challenge for future studies. The Ca(2+) uptake can be divided into the following three steps: (1) Ca(2+) movement from the IP(3)R/RyR to the outer mitochondrial membrane (OMM); (2) Ca(2+) transport through the OMM; and (3) Ca(2+) transport through the inner mitochondrial membrane (IMM). Evidence has been presented that Ca(2+) delivery to the OMM is facilitated by a local coupling between closely apposed regions of the ER/SR and mitochondria. Recent studies of the dynamic changes in mitochondrial morphology and visualization of the subcellular pattern of the calcium signal provide important clues to the organization of the ER/SR-mitochondrial interface. Interestingly, key steps of phospholipid synthesis and transfer to the mitochondria have also been confined to subdomains of the ER tightly associated with the mitochondria, referred as mitochondria-associated membranes (MAMs). Through the OMM, the voltage-dependent anion channels (VDAC, porin) have been thought to permit free passage of ions and other small molecules. However, recent studies suggest that the VDAC may represent a regulated step in Ca(2+) transport from IP(3)R/RyR to the IMM. A novel proposal regarding the IMM Ca(2+) uptake site is a mitochondrial RyR that would mediate rapid Ca(2+) uptake by mitochondria in excitable cells. An overview of the progress in these directions is described in the present paper.

Cardiac energy metabolism homeostasis: role of cytosolic calcium

The heart is capable of dramatically altering its overall energy flux with minimal changes in the concentrations of metabolites that are associated with energy metabolism. This cardiac energy metabolism homeostasis is discussed with regard to the potential cytosolic control network responsible for controlling the major energy conversion pathway, oxidative phosphorylation in mitochondria. Several models for this cytosolic control network have been proposed, but a cytosolic Ca(2+) dependent parallel activation scheme for metabolism and work is consistent with most of the experimental results. That model proposes that cytosolic Ca(2+) regulates both the utilization of ATP by the work producing ATPases as well as the mitochondrial production of ATP. Recent studies have provided evidence supporting this role of cytosolic Ca(2+). These data include the demonstration that mitochondrial [Ca(2+)] can track cytosolic [Ca(2+)] and that the compartmentation of cytosolic [Ca(2+)] can facilitate this process. On the metabolic side, Ca(2+) has been shown to rapidly activate several steps in oxidative phosphorylation, including F(1)F(0)-ATPase ATP production as well as several dehydrogenases, which results in a homeostasis of mitochondrial metabolites similar to that observed in the cytosol. Numerous problems with the Ca(2+) parallel activation hypothesis remain including the lack of specific mechanisms of mitochondrial Ca(2+) transport and regulation of F(1)F(0)-ATPase, the time dependence of Ca(2+) activation of cytosolic ATPases as well as oxidative phosphorylation, and the role of cytosolic compartmentation. In addition, the lack of cytosolic or mitochondrial [Ca(2+)] measurements under in vivo conditions is problematic. Several lines of investigation to address these issues are suggested. A model of the cardiac energy metabolism control network is proposed that includes a Ca(2+) parallel activation component together with more classical elements including metabolite feedback and cytosolic compartmentation.

Simultaneous measurements of mitochondrial NADH and Ca(2+) during increased work in intact rat heart trabeculae

The main goal of this study is to investigate the role of mitochondrial [Ca(2+)], [Ca(2+)](m), in the possible up-regulation of the NADH production rate during increased workload. Such up-regulation is necessary to support increased flux through the electron transport chain and increased ATP synthesis rates. Intact cardiac trabeculae were loaded with Rhod-2(AM), and [Ca(2+)](m) and mitochondrial [NADH] ([NADH](m)) were simultaneously measured during increased pacing frequency. It was found that 53% of Rhod-2 was localized in mitochondria. Increased pacing frequency caused a fast, followed by a slow rise of the Rhod-2 signal, which could be attributed to an abrupt increase in resting cytosolic [Ca(2+)], and a more gradual rise of [Ca(2+)](m), respectively. When the pacing frequency was increased from 0.25 to 2 Hz, the slow Rhod-2 component and the NADH signal increased by 18 and 11%, respectively. Based on a new calibration method, the 18% increase of the Rhod-2 signal was calculated to correspond to a 43% increase of [Ca(2+)](m). There was also a close temporal relationship between the rise (time constant approximately 25 s) and fall (time constant approximately 65 s) of [Ca(2+)](m) and [NADH](m) when the pacing frequency was increased and decreased, respectively, suggesting that increased workload and [Ca(2+)](c) cause increased [Ca(2+)](m) and consequently up-regulation of the NADH production rate.

Altered NADH and improved function by anesthetic and ischemic preconditioning in guinea pig intact hearts

NADH increases during ischemia because O(2) shortage limits NADH oxidation at the electron transport chain. Ischemic (IPC) and anesthetic preconditioning (APC) attenuate cardiac reperfusion injury. We examined whether IPC and APC similarly alter NADH, i.e., mitochondrial metabolism. NADH fluorescence was measured at the left ventricular wall of 40 Langendorff-prepared guinea pig hearts. IPC was achieved by two 5-min periods of ischemia and APC by exposure to 0.5 or 1.3 mM sevoflurane for 15 min, each ending 30 min before 30 min of global ischemia. During ischemia, NADH initially increased in nonpreconditioned control hearts and then gradually declined below baseline levels. This increase in NADH was lower after APC but not after IPC. The subsequent decline was slower after IPC and APC. On reperfusion, NADH was less decreased after IPC or APC, mechanical and metabolic functions were improved, and infarct size was lower compared with controls. Our results indicate that IPC and APC cause distinctive changes in mitochondrial metabolism during ischemia and thus lead to improved function and tissue viability on reperfusion.

Hypoxia causes downregulation of protein and RNA synthesis in noncontracting Mammalian cardiomyocytes

The aim was to identify energy-consuming processes, other than contraction, downregulated during moderate hypoxia ( approximately 5 micromol/L, 0.5% O(2)) and severe hypoxia (<0.5 micromol/L, <0.05% O(2)) in isolated neonatal cardiomyocytes. The metabolic response of cardiomyocytes to moderate and severe hypoxia was assessed by measuring rates of energy consumption and energetic status of cells maintained under these conditions. We found that the rates of energy production were decreased during both forms of hypoxia. Decreased rates of energy production under moderate hypoxia were associated with reduced energy wastage through a downregulation of proton leak in the mitochondria. Cellular protein synthesis and RNA synthesis, major energy-consuming pathways, were downregulated only during severe hypoxia, when oxygen concentrations were low enough to induce energetic stress (quantitatively defined as being any situation in which phosphocreatine concentrations had fallen by > or = 40%). Our results suggest that energetic stress is the signal responsible for this downregulation.

Changes in [Na(+)](i), compartmental [Ca(2+)], and NADH with dysfunction after global ischemia in intact hearts

We measured the effects of global ischemia and reperfusion on intracellular Na(+), NADH, cytosolic and mitochondrial (subscript mito) Ca(2+), relaxation, metabolism, contractility, and Ca(2+) sensitivity in the intact heart. Langendorff-prepared guinea pig hearts were crystalloid perfused, and the left ventricular (LV) pressure (LVP), first derivative of LVP (LV dP/dt), coronary flow, and O(2) extraction and consumption were measured before, during, and after 30-min global ischemia and 60-min reperfusion. Ca(2+), Na(+), and NADH were measured by luminescence spectrophotometry at the LV free wall using indo 1 and sodium benzofuran isophthalate, respectively, after subtracting changes in tissue autofluorescence (NADH). Mitochondrial Ca(2+) was assessed by quenching cytosolic indo 1 with MnCl(2). Mechanical responses to changes in cytosolic-systolic (subscript sys), diastolic (subscript dia), and mitochondrial Ca(2+) were tested over a range of extracellular [Ca(2+)] before and after ischemia-reperfusion. Both [Ca(2+)](sys) and [Ca(2+)](dia) doubled at 1-min reperfusion but returned to preischemia values within 10 min, whereas [Ca(2+)](mito) was elevated over 60-min reperfusion. Reperfusion dissociated [Ca(2+)](dia) and [Ca(2+)](sys) from contractile function as LVP(sys-dia) and the rise in LV dP/dt (LV dP/dt(max)) were depressed by one-third and the fall in LV dP/dt (LV dP/dt(min)) was depressed by one-half at 30-min reperfusion, whereas LVP(dia) remained markedly elevated. [Ca(2+)](sys-dia) sensitivity at 100% LV dP/dt(max) was not altered after reperfusion, but [Ca(2+)](dia) at 100% LV dP/dt(min) and [Ca(2+)](mito) at 100% LV dP/dt(max) were markedly shifted right on reperfusion (ED(50) +36 and +125 nM [Ca(2+)], respectively) with no change in slope. NADH doubled during ischemia but returned to normal on initial reperfusion. The intracellular [Na(+)] ([Na(+)](i)) increased minimally during ischemia but doubled on reperfusion and remained elevated at 60-min reperfusion. Thus Na(+) and Ca(2+) temporally accumulate during initial reperfusion, and cytosolic Ca(2+) returns toward normal, whereas [Na(+)](i) and [Ca(2+)](mito) remain elevated on later reperfusion. Na(+) loading likely contributes to Ca(2+) overload and contractile dysfunction during reperfusion.

Hibernation in noncontracting mammalian cardiomyocytes

BACKGROUND

-The aim of the present study was to establish whether isolated neonatal mammalian cardiomyocytes were capable of downregulating energy-using processes other than contraction while maintaining metabolic stability when oxygen availability was reduced.

METHODS AND RESULTS

Metabolic response of cardiomyocytes was investigated under moderate (5 to 6 micromol/L) and severe (2 to 3 micromol/L) forms of hypoxia. Cells exposed to oxygen concentrations of 5 to 6 micromol/L exhibited rates of oxygen consumption, which were decreased to 64% of normoxic rates. Rates of cellular energy usage were decreased because this reduced rate of oxygen consumption was not associated with either decreased intracellular ATP and phosphocreatine concentrations or a compensatory switch to glycolysis. When cells were exposed to oxygen concentrations of 2 to 3 micromol/L, rates of oxygen consumption decreased to 9% of normoxic rates. This decreased rate of oxygen consumption was associated with energetic stress, because a significant switch to glycolysis occurred and intracellular phosphocreatine concentrations were decreased by 40%. Rates of cellular energy usage were further decreased as indicated by stable intracellular ATP concentrations.

CONCLUSIONS

-Our results suggest that isolated cardiomyocytes are capable of downregulating energy-consuming processes other than contraction when oxygen supply is decreased. Regions of myocardial tissue are also capable of downregulating metabolic activity during ischemia by shutting down contractile activity (myocardial hibernation). We suggest that metabolic downregulation associated with myocardial hibernation may not be exclusively due to reduced rates of contractile activity. Other energy-using processes (eg, protein synthesis, mRNA synthesis, ion channel activity, and proton leak) may also be shut down.

Mitochondrial NAD(P)H, ADP, oxidative phosphorylation, and contraction in isolated heart cells

To examine the relationship between mitochondrial NADH (NADH(m)) and cardiac work output, NADH(m) and the amplitude and frequency of the contractile response of electrically paced rat heart cells were measured at 25 degrees C. With 5.4 mM glucose plus 2 mM beta-hydroxybutyrate, NADH(m) was reversibly decreased by 23%, and the amplitude of contraction was reversibly decreased by 27% during 4-Hz pacing. With glucose plus 2 mM pyruvate or with 10 mM 2-deoxy-D-glucose, NADH(m) was maintained during rapid pacing, and the contractile amplitude remained high. Phosphocreatine levels decreased with 2-deoxy-D-glucose administration but not with rapid pacing. Respiration increased to meet the increased ATP demand at 30 degrees C. The data suggest that 1) when NADH(m) is decreased during rapid pacing with defined substrates, the amplitude of contraction is decreased; 2) the amplitude of contraction during electrical pacing does not change with rate of pacing when both the ATP and NADH(m) levels are continuously replenished; and 3) the replenishment of NADH(m) during pacing with physiological substrates may be rate-limited by substrate supply to mitochondrial dehydrogenases. During activation of mitochondrial dehydrogenases, or a significant increase in free ADP induced by 2-deoxy-D-glucose, this rate limitation is bypassed or overcome.

Mitochondrial calcium in heart cells: beat-to-beat oscillations or slow integration of cytosolic transients?

Mitochondria have been implicated in intracellular Ca2+ signaling in many cell types. The inner mitochondrial membrane contains Ca2+-transporting proteins, which catalyze Ca2+ uptake and extrusion. Intramitochondrial (matrix) Ca2+, in turn, regulates the activity of Krebs cycle dehydrogenases and, ultimately, the rate of ATP synthesis. In the myocardium, controversy remains whether the fast cytosolic Ca2+ transients underlying excitation-contraction coupling in beating cells are rapidly transmitted into the matrix compartment or slowly integrated by the mitochondrial Ca2+ transporters. This mini-review critically summarizes the recent experimental work in this field.

The coupling of NAD(P)+-producing reactions to a semiautomated bioluminescent reaction

Many cellular metabolites can be measured with high sensitivity using bioluminescent techniques. These metabolites are coupled to an appropriate enzyme to produce NAD(P)H, which can then be coupled to the bioluminescent reactions. The sensitivity of bioluminescence cannot be readily applied to methods in which cellular metabolites consume NAD(P)H because of the difficulty in measuring, with sufficient sensitivity, decreases in the concentration of NAD(P)H against a high background NAD(P)H concentration. We have overcome these technical difficulties by developing a bioluminescent reagent to measure the production of NAD(P)+. Assays for creatine/creatine phosphate, pyruvate, and succinate, as well as the kinetic measurement of lactate, are described for a range of biological material. The assays are highly sensitive, quantitative, and reproducible and show no sample-specific inhibition. The range of assays and the diverse biological material tested suggests that NAD(P)+ bioluminescence has a wide potential for application.

The Ca2+ transport mechanisms of mitochondria and Ca2+ uptake from physiological-type Ca2+ transients

Mitochondria contain a sophisticated system for transporting Ca2+. The existence of a uniporter and of both Na+-dependent and -independent efflux mechanisms has been known for years. Recently, a new mechanism, called the RaM, which seems adapted for sequestering Ca2+ from physiological transients or pulses has been discovered. The RaM shows a conductivity at the beginning of a Ca2+ pulse that is much higher than the conductivity of the uniporter. This conductivity decreases very rapidly following the increase in [Ca2+] outside the mitochondria. This decrease in the Ca2+ conductivity of the RaM is associated with binding of Ca2+ to an external regulatory site. When liver mitochondria are exposed to a sequence of pulses, uptake of labeled Ca2+ via the RaM appears additive between pulses. Ruthenium red inhibits the RaM in liver mitochondria but much larger amounts are required than for inhibition of the mitochondrial Ca2+ uniporter. Spermine, ATP and GTP increase Ca2+ uptake via the RaM. Maximum uptake via the RaM from a single Ca2+ pulse in the physiological range has been observed to be approximately 7 nmole/mg protein, suggesting that Ca2+ uptake via the RaM and uniporter from physiological pulses may be sufficient to activate the Ca2+-sensitive metabolic reactions in the mitochondrial matrix which increase the rate of ATP production. RaM-mediated Ca2+ uptake has also been observed in heart mitochondria. Evidence for Ca2+ uptake into the mitochondria in a variety of tissues described in the literature is reviewed for evidence of participation of the RaM in this uptake. Possible ways in which the differences in transport via the RaM and the uniporter may be used to differentiate between metabolic and apoptotic signaling are discussed.

Mitochondrial calcium in relaxed and tetanized myocardium

The elemental composition of rat cardiac muscle was determined with electron probe x-ray microanalysis (EPMA) of rapidly frozen papillary muscles and trabeculae incubated with ryanodine (1 microM) in either 1.2 or 10 mM [Ca2+]o-containing solutions, paced at 0.6 Hz or tetanized at 10 Hz. Total mitochondrial calcium increased significantly, by 4.2 mmol/kg dry weight during a 7 s tetanus, only in muscles tetanized in the presence of 10 mM [Ca2+]o when cytoplasmic Ca2+ is 1-4 microM (Backx, P. H., W.-D. Gao, M. D. Azan-Backx, and E. Marban. 1995. The relationship between contractile force and intracellular [Ca2+] in intact rat trabeculae. J. Gen. Physiol. 105:1-19). Comparison of total mitochondrial with free mitochondrial Ca2+ reported in the literature indicates that the total/free ratio is approximately 6000 at physiological or near-physiological levels of total mitochondrial calcium. Increases in free mitochondrial [Ca2+] consistent with regulation of mitochondrial enzymes should be associated with increases in total mitochondrial calcium detectable with EPMA. However, such increases in mitochondrial calcium occur only as the result of prolonged, unphysiological elevations of cytosolic [Ca2+].

A first evaluation of the natural high molecular weight polymeric Lumbricus terrestris hemoglobin as an oxygen carrier

Lumbricus terrestris hemoglobin (LtHb), an unusually stable Hb (MW approximately 4x10(6) Da) with respect to dissociation and oxidation, circulates extracellularly in the earthworm and at neutral pH exhibits oxygen affinity and cooperativity similar to that of human HbA. Results suggest that LtHb may serve as a model for a high molecular weight extracellular oxygen carrier. Mice and a rat model partially exchanged with LtHb showed no apparent behavioral and physical changes. 31P NMR spectroscopy of perfused guinea pig hearts, used to assess phosphocreatine levels as an indication of the ability of LtHb to serve as an oxygen carrier to the heart, demonstrated that LtHb provides oxygen to the tissue and maintains the energy metabolism significantly better than the control non-Hb perfusion media. One day after infusion, video enhanced microscopy imaging of the mice cremaster muscle vasculature reveals temporal adhesion of leukocytes to the endothelial walls with temporal infiltration of leukocytes to the surrounding tissue, correlated with dosage. Exchanged mice rechallenged with LtHb show no overt allergic response or death. Further evaluation of this natural extracellular Hb as a potential polymeric Hb blood substitute/perfusion agent is warranted.

Inhibition of mitochondrial calcium efflux by clonazepam in intact single rat cardiomyocytes and effects on NADH production

The aims of this study were to determine: (i) whether clonazepam and CGP37157, which inhibit the Na+/Ca2+ exchanger of isolated mitochondria, could inhibit mitochondrial Ca2+ efflux in intact cells; and (ii) whether any sustained increase in mitochondrial [Ca2+] ([Ca2+]m) could alter mitochondrial NADH levels. [Ca2+]m was measured in Indo-1/AM loaded rat ventricular myocytes where the cytosolic fluorescence signal was quenched by superfusion with Mn2+. NADH levels were determined from cell autofluorescence. Upon exposure of myocytes to 50 nM norepinephrine (NE) and a stimulation rate of 3 Hz, [Ca2+]m increased from 59 +/- 3 nM to a peak of 517 +/- 115 nM (n = 8) which recovered rapidly upon return to low stimulation rate (0.2 Hz) and washout of NE. In the presence of clonazepam, the peak increase in [Ca2+]m was 937 +/- 192 nM (n = 5) which remained elevated at 652 +/- 131 nM upon removal of the stimulus. CGP37157 in some cells did give the same inhibition of mitochondrial Ca2+ efflux as clonazepam, but the effect was inconsistent since not all cells were capable of following the stimulation rate in presence of this compound. NADH levels increased upon exposure to rapid stimulation in the presence of NE alone and recovered upon return to low stimulation rates, whereas in clonazepam treated cells the recovery of NADH was prevented. We conclude that clonazepam is an effective inhibitor of mitochondrial [Ca2+] efflux in intact cells and also maintains the increase in NADH levels which occurs upon rapid stimulation of cells.

The relationship between mitochondrial state, ATP hydrolysis, [Mg2+]i and [Ca2+]i studied in isolated rat cardiomyocytes

1. As ATP has a higher affinity for Mg2+ than ADP, the cytosolic magnesium concentration rises upon ATP hydrolysis. We have therefore used the Mg(2+)-sensitive fluorescent indicator Magnesium Green (MgG) to provide an index of changing ATP concentration in single rat cardiomyocytes in response to altered mitochondrial state. 2. In response to FCCP, [Mg2+]i rose towards a plateau coincident with the progression to rigor, which signals ATP depletion. Contamination of the MgG signal by changes in intracellular free Ca2+ concentration (the KD of MgG for Ca2+ is 4.7 microM) was excluded by simultaneous measurement of [Ca2+]i and [Mg2+]i in cells dual loaded with fura-2 and MgG. The response to FCCP was independent of external Mg2+, confirming an intracellular source for the rise in [Mg2+]i. 3. Simultaneous measurements of mitochondrial NAD(P)H autofluorescence and mitochondrial potential (delta psi m; .-1 fluorescence) and of autofluorescence and MgG allowed closer study of the relationship between [Mg2+]i and mitochondrial state. Oligomycin abolished the FCCP-induced rise in [Mg2+]i without altering the change in autofluorescence. Thus, the rise in [Mg2+]i in response to FCCP is consistent with the release of intracellular Mg2+ following ATP hydrolysis by the mitochondrial F1F0-ATPase. 4. The rise in [Mg2+]i was correlated with cell-attached recordings of ATP-sensitive K+ channel (KATP) activity. In response to FCCP, an increase in KATP channel activity was seen only as [Mg2+]i reached a plateau. In response to blockade of mitochondrial respiration and glycolysis with cyanide (CN-) and 2-deoxyglucose (DOG), [Mg2+]i rose more slowly but again KATP channel opening increased only when [Mg2+]i reached a plateau and the cells shortened. 5. Oligomycin decreased the rate of rise of [Mg2+]i delayed the onset of rigor and increased the rate of mitochondrial depolarization in response to CN-_DOG. Thus, with blockade of mitochondrial respiration delta psi m is maintained by the mitochondrial F1F0-ATPase at the expense of ATP reserves. 6. In response to CN-_DOG, the initial rise in [Mg2+]i was accompanied by a small rise in [Ca2+]i. After [Mg2+]i reached a plateau and rigor developed, [Ca2+]i rose progressively. On reperfusion, in hypercontracted cells, [Ca2+]i recovered before [Mg2+]i and [ca2+]i oscillations were sustained while [Mg2+]i decreased. Thus on reperfusion, full recovery of [ATP]i is slow, but the activation of contractile elements and the restoration of [Ca2+]i does not require the re-establishment of millimolar concentrations of ATP.