Cardiac high-energy phosphates adapt faster than oxygen consumption to changes in heart rate

Orthophosphate increases the efficiency of slow muscle-myosin isoform in the presence of omecamtiv mecarbil

Omecamtiv mecarbil (OM) is a putative positive inotropic tool for treatment of systolic heart dysfunction, based on the finding that in vivo it increases the ejection fraction and in vitro it prolongs the actin-bond life time of the cardiac and slow-skeletal muscle isoforms of myosin. OM action in situ, however, is still poorly understood as the enhanced Ca²⁺-sensitivity of the myofilaments is at odds with the reduction of force and rate of force development observed at saturating Ca²⁺. Here we show, by combining fast sarcomere-level mechanics and ATPase measurements in single slow demembranated fibres from rabbit soleus, that the depressant effect of OM on the force per attached motor is reversed, without effect on the ATPase rate, by physiological concentrations of inorganic phosphate (Pi) (1-10 mM). This mechanism could underpin an energetically efficient reduction of systolic tension cost in OM-treated patients, whenever [Pi] increases with heart-beat frequency.

Omecamtiv mecarbil is a small molecule effector under clinical trial for the treatment of systolic heart failure. Here the authors define the molecular mechanisms of its inotropic action and find it can increase the efficiency of contraction in muscle fibres when the orthophosphate concentration rises with the beat frequency.

Reducing mitochondrial bound hexokinase II mediates transition from non-injurious into injurious ischemia/reperfusion of the intact heart

Ischemia/reperfusion (I/R) of the heart becomes injurious when duration of the ischemic insult exceeds a certain threshold (approximately ≥20 min). Mitochondrial bound hexokinase II (mtHKII) protects against I/R injury, with the amount of mtHKII correlating with injury. Here, we examine whether mtHKII can induce the transition from non-injurious to injurious I/R, by detaching HKII from mitochondria during a non-injurious I/R interval. Additionally, we examine possible underlying mechanisms (increased reactive oxygen species (ROS), increased oxygen consumption (MVO₂) and decreased cardiac energetics) associated with this transition. Langendorff perfused rat hearts were treated for 20 min with saline, TAT-only or 200 nM TAT-HKII, a peptide that translocates HKII from mitochondria. Then, hearts were exposed to non-injurious 15-min ischemia, followed by 30-min reperfusion. I/R injury was determined by necrosis (LDH release) and cardiac mechanical recovery. ROS were measured by DHE fluorescence. Changes in cardiac respiratory activity (cardiac MVO₂ and efficiency and mitochondrial oxygen tension (mitoPO₂) using protoporphyrin IX) and cardiac energetics (ATP, PCr, ∆G_ATP) were determined following peptide treatment. When exposed to 15-min ischemia, control hearts had no necrosis and 85% recovery of function. Conversely, TAT-HKII treatment resulted in significant LDH release and reduced cardiac recovery (25%), indicating injurious I/R. This was associated with increased ROS during ischemia and reperfusion. TAT-HKII treatment reduced MVO₂ and improved energetics (increased PCr) before ischemia, without affecting MVO₂/RPP ratio or mitoPO₂. In conclusion, a reduction in mtHKII turns non-injurious I/R into injurious I/R. Loss of mtHKII was associated with increased ROS during ischemia and reperfusion, but not with increased MVO₂ or decreased cardiac energetics before damage occurs.

Computer-aided analysis of biochemical mechanisms that increase metabolite and proton stability in the heart during severe hypoxia and generate post-ischemic PCr overshoot

During severe hypoxia in the heart, impaired supply of ATP by oxidative phosphorylation could lead to a great drop in ATP turnover and heart work. Anaerobic glycolysis enables unchanged ATP turnover to be maintained, but leads to huge changes in metabolite (PCr, ATP, ADP, P (i)) concentrations and to cytosol acidification. A computer model of heart energetics developed previously is used to analyze semi-quantitatively the effect of different processes/mechanisms that can partly counteract these effects. Down-regulation of ATP usage compromises cardiac output, but reduces changes in cytosolic pH and metabolite concentrations. AMP decomposition delays cytosol acidification but reduces metabolite homeostasis (concentration stability). An increase in the parallel activation of oxidative phosphorylation (OXPHOS) (a hypothetical mechanism involving direct activation of all OXPHOS complexes by a cytosolic factor, postulated to take place also during work increase) reduces cytosol acidification and elevates metabolite homeostasis. All these mechanisms can generate the post-ischemic PCr overshoot.

Dynamic changes of myocardial oxygen consumption at pacing increased heart rate - the first observation by the continuous measurement of systemic oxygen consumption

OBJECTIVES

To assess dynamic changes in myocardial oxygen consumption (myoVO(2)) during atrial pacing increased heart rate by continuous measurement of systemic oxygen consumption (sysVO(2)).

METHODS

Six mechanically ventilated pigs were atrially paced to increase heart rate from baseline 98 ± 9 to 120-140-160-180 bpm for 10 minutes at each stage, with 10 minute intervals without pacing between stages. sysVO(2) was continuously measured with a respiratory mass spectrometer. Left anterior descending coronary arterial flow, aorta and coronary sinus blood gases were measured to calculate index of whole heart myoVO(2).

RESULTS

sysVO(2) peaked at the initiation of pacing in the first two to three minutes, followed by a decrease and subsequent stabilization. As heart rate increased, sysVO(2) increased by 0.08 ± 0.06 ml/kg/min, 0.14 ± 0.05 ml/kg/min and 0.17 ± 0.10 ml/kg/min, representing a 1.2 ± 0.9%, 2.1 ± 0.7% and 3.0 ± 1.8% increase of sysVO(2) respectively; myoVO(2) increased by 0.16 ± 0.12 to 0.31 ± 0.14 to 0.36 ± 0.24 ml/100 g/min, representing a 11 ± 9%, 21 ± 9% and 26 ± 12% increase of myoVO(2), respectively. The absolute and relative increases in sysVO(2) were significantly correlated with the increases in myoVO(2).

CONCLUSIONS

On-line continuous sysVO(2) monitoring by respiratory mass spectrometry allows non-invasive assessments of dynamic changes in myoVO(2) in vivo. The mechanism for the peaked increase in sysVO(2) at the initiation of pacing remains to be explored.

Mitochondrial energetics, pH regulation, and ion dynamics: a computational-experimental approach

We developed a computational model of mitochondrial energetics that includes Ca(2+), proton, Na(+), and phosphate dynamics. The model accounts for distinct respiratory fluxes from substrates of complex I and complex II, pH effects on equilibrium constants and enzyme kinetics, and the acid-base equilibrium distributions of energy intermediaries. We experimentally determined NADH and ΔΨ(m) in guinea pig mitochondria during transitions from de-energized to energized, or during state 2/4 to state 3 respiration, or into hypoxia and uncoupling, and compared the results with those obtained in model simulations. The model quantitatively reproduces the experimentally observed magnitude of ΔΨ(m), the range of NADH levels, respiratory fluxes, and respiratory control ratio upon transitions elicited by sequential additions of substrate and ADP. Simulation results are also able to mimic the change in ΔΨ(m) upon addition of phosphate to state 4 mitochondria, leading to matrix acidification and ΔΨ(m) polarization. The steady-state behavior of the integrated mitochondrial model qualitatively simulates the dependence of respiration on the proton motive force, and the expected flux-force relationships existing between respiratory and ATP synthesis fluxes versus redox and phosphorylation potentials. This upgraded mitochondrial model provides what we believe are new opportunities for simulating mitochondrial physiological behavior during dysfunctional states involving changes in pH and ion dynamics.

Energetics of stress production in isolated cardiac trabeculae from the rat

The heat liberated upon stress production in isolated cardiac muscle provides insights into the complex thermodynamic processes underlying mechanical contraction. To that end, we simultaneously measured the heat and stress (force per cross-sectional area) production of cardiac trabeculae from rats using a flow-through micromechanocalorimeter. In a flowing stream of O(2)-equilibrated Tyrode solution (∼22°C), the stress and heat production of actively contracting trabeculae were varied by 1) altering stimulus frequency (0.2-4 Hz) at optimal muscle length (L(o)), 2) reducing muscle length below L(o) at 0.2 and 2 Hz, and 3) changing extracellular Ca(2+) concentrations ([Ca(2+)](o); 1 and 2 mM). Linear regression lines were adequate to fit the active heat-stress data. The active heat-stress relationships were independent of stimulus frequency and muscle length but were dependent on [Ca(2+)](o), having greater intercepts at 2 mM [Ca(2+)](o) than at 1 mM [Ca(2+)](o) (3.5 and 2.0 kJ·m(-3)·twitch(-1), respectively). The slopes among the heat-stress relationships did not differ. At the highest experimental stimulus frequency, pronounced elevation of diastolic Ca(2+) resulted in incomplete twitch relaxation. The resulting increase of diastolic stress, which occurred with negligible metabolic energy expenditure, subsequently diminished due to the time-dependent loss of myofilament Ca(2+)-sensitivity.

Adenine nucleotide-creatine-phosphate module in myocardial metabolic system explains fast phase of dynamic regulation of oxidative phosphorylation

Computational models of a large metabolic system can be assembled from modules that represent a biological function emerging from interaction of a small subset of molecules. A "skeleton model" is tested here for a module that regulates the first phase of dynamic adaptation of oxidative phosphorylation (OxPhos) to demand in heart muscle cells. The model contains only diffusion, mitochondrial outer membrane (MOM) permeation, and two isoforms of creatine kinase (CK), in cytosol and mitochondrial intermembrane space (IMS), respectively. The communication with two neighboring modules occurs via stimulation of mitochondrial ATP production by ADP and P(i) from the IMS and via time-varying cytosolic ATP hydrolysis during contraction. Assuming normal cytosolic diffusion and high MOM permeability for ADP, the response time of OxPhos (t(mito); generalized time constant) to steps in cardiac pacing rate is predicted to be 2.4 s. In contrast, with low MOM permeability, t(mito) is predicted to be 15 s. An optimized MOM permeability of 21 mum/s gives t(mito) = 3.7 s, in agreement with experiments on rabbit heart with blocked glycolytic ATP synthesis. The model correctly predicts a lower t(mito) if CK activity is reduced by 98%. Among others, the following predictions result from the model analysis: 1) CK activity buffers large ADP oscillations; 2) ATP production is pulsatile in beating heart, although it adapts slowly to demand with "time constant" approximately 14 heartbeats; 3) if the muscle isoform of CK is overexpressed, OxPhos reacts slower to changing workload; and 4) if mitochondrial CK is overexpressed, OxPhos reacts faster.

Regulation of oxidative phosphorylation through parallel activation

When the mechanical work intensity in muscle increases, the elevated ATP consumption rate must be matched by the rate of ATP production by oxidative phosphorylation in order to avoid a quick exhaustion of ATP. The traditional mechanism of the regulation of oxidative phosphorylation, namely the negative feedback involving [ADP] and [Pi] as regulatory signals, is not sufficient to account for various kinetic properties of the system in intact skeletal muscle and heart in vivo. Theoretical studies conducted using a dynamic computer model of oxidative phosphorylation developed previously strongly suggest the so-called each-step-activation (or parallel activation) mechanism, due to which all oxidative phosphorylation complexes are directly activated by some cytosolic factor/mechanism related to muscle contraction in parallel with the activation of ATP usage and substrate dehydrogenation by calcium ions. The present polemic article reviews and discusses the growing evidence supporting this mechanism and compares it with alternative mechanisms proposed in the literature. It is concluded that only the each-step-activation mechanism is able to explain the rich set of various experimental results used as a reference for estimating the validity and applicability of particular mechanisms.

Oxygen consumption and metabolite concentrations during transitions between different work intensities in heart

Steady-state metabolite (ADP, ATP, P(i), PCr, and NADH) concentrations usually differ little between different workloads with significantly different oxygen consumption rates in the heart. However, during transitions between steady states, metabolite concentrations may in some cases change transiently, exhibiting a significant overshoot or undershoot, whereas in other cases they approach near-exponentially new steady-state values. Oxygen consumption rate usually reaches the new steady-state value very quickly (within a few seconds). The present in silico studies, performed using a previously developed computer model of oxidative phosphorylation in the heart, demonstrate that such a behavior of the oxidative phosphorylation system can be reproduced only under the assumption that ATP usage, substrate dehydrogenation, and (particular steps of) oxidative phosphorylation are directly activated to a similar extend by some cytosolic factor/mechanism during transition from low work to high work (the so-called parallel-activation mechanism). Computer simulations show that some differences observed between different experimental systems can be explained by a slightly different balance of the activation of particular components of the system and/or by a delay in time of the activation/inactivation of substrate dehydrogenation and oxidative phosphorylation during low-to-high and high-to-low work transitions. Thus the presented theoretical approach offers a general idea that is able to unify, at least semiquantitatively, different experimental data available in the literature.

A computational model integrating electrophysiology, contraction, and mitochondrial bioenergetics in the ventricular myocyte

An intricate network of reactions is involved in matching energy supply with demand in the heart. This complexity arises because energy production both modulates and is modulated by the electrophysiological and contractile activity of the cardiac myocyte. Here, we present an integrated mathematical model of the cardiac cell that links excitation-contraction coupling with mitochondrial energy generation. The dynamics of the model are described by a system of 50 ordinary differential equations. The formulation explicitly incorporates cytoplasmic ATP-consuming processes associated with force generation and ion transport, as well as the creatine kinase reaction. Changes in the electrical and contractile activity of the myocyte are coupled to mitochondrial energetics through the ATP, Ca2+, and Na+ concentrations in the myoplasmic and mitochondrial matrix compartments. The pseudo steady-state relationship between force and oxygen consumption at various stimulus frequencies and external Ca2+ concentrations is reproduced in both model simulations and direct experiments in cardiac trabeculae under normoxic conditions, recapitulating the linearity between cardiac work and respiration in the heart. Importantly, the model can also reproduce the rapid time-dependent changes in mitochondrial NADH and Ca2+ in response to abrupt changes in workload. The steady-state and dynamic responses of the model were conferred by ADP-dependent stimulation of mitochondrial oxidative phosphorylation and Ca2+ -dependent regulation of Krebs cycle dehydrogenases, illustrating how the model can be used as a tool for investigating mechanisms underlying metabolic control in the heart.

Regulation of oxidative phosphorylation in intact mammalian heart in vivo

A dynamic computer model of oxidative phosphorylation in intact heart was developed by modifying the model of oxidative phosphorylation in intact skeletal muscle published previously. Next, this model was used for theoretical studies on the regulation of oxidative phosphorylation in intact heart in vivo during transition between different work intensities. It is shown that neither a direct activation of ATP usage alone nor a direct activation of both ATP usage and substrate dehydrogenation, including the calcium-activated tricarboxylate acid cycle dehydrogenases, can account for the constancy of [ADP], [PCr], [P(i)] and [NADH] during a significant increase in oxygen consumption and ATP turnover encountered in intact heart in vivo. Only a direct activation of all oxidative phosphorylation complexes in parallel with a stimulation of ATP usage and substrate dehydrogenation enabled to reproduce the experimental data concerning the constancy of metabolite concentrations. The molecular background of the differences between heart and skeletal muscle in the kinetic behaviour of the oxidative phosphorylation system is also discussed.

Glycolytic buffering affects cardiac bioenergetic signaling and contractile reserve similar to creatine kinase

Creatine kinase (CK) and glycolysis represent important energy-buffering processes in the cardiac myocyte. Although the role of compartmentalized CK in energy transfer has been investigated intensely, similar duties for intracellular glycolysis have not been demonstrated. By measuring the response time of mitochondrial oxygen consumption to dynamic workload jumps (tmito) in isolated rabbit hearts, we studied the effect of inhibiting energetic systems (CK and/or glycolysis) on transcytosolic signal transduction that couples cytosolic ATP hydrolysis to activation of oxidative phosphorylation. Tyrode-perfused hearts were exposed to 15 min of the following: 1) 0.4 mM iodoacetamide (IA; n = 6) to block CK (CK activity <3% vs. control), 2) 0.3 mM iodoacetic acid (IAA; n = 5) to inhibit glycolysis (GAPDH activity <3% vs. control), or 3) vehicle (control, n = 7) at 37 degrees C. Pretreatment tmito was similar across groups at 4.3 +/- 0.3 s (means +/- SE). No change in tmito was observed in control hearts; however, in IAA- and IA-treated hearts, tmito decreased by 15 +/- 3% and 40 +/- 5%, respectively (P < 0.05 vs. control), indicating quicker energy supply-demand signaling in the absence of ADP/ATP buffering by CK or glycolysis. The faster response times in IAA and IA groups were independent of the size of the workload jump, and the increase in myocardial oxygen consumption during workload steps was unaffected by CK or glycolysis blockade. Contractile function was compromised by IAA and IA treatment versus control, with contractile reserve (defined as increase in rate-pressure product during a standard heart rate jump) reduced to 80 +/- 8% and 80 +/- 10% of baseline, respectively (P < 0.05 vs. control), and significant elevations in end-diastolic pressure, suggesting raised ADP concentration. These results demonstrate that buffering of phosphate metabolites by glycolysis in the cytosol contributes appreciably to slower mitochondrial activation and may enhance contractile efficiency during increased cardiac workloads. Glycolysis may therefore play a role similar to CK in heart muscle.

Activation time of myocardial oxidative phosphorylation in creatine kinase and adenylate kinase knockout mice

Our goal was to determine whether mice genetically altered to lack either creatine kinase (M/MtCK(-/-)) or adenylate kinase (AK(-/-)) show altered properties in the dynamic regulation of myocardial oxygen consumption (MVO(2)). We measured contractile function, oxygen consumption, and the mean response time of oxygen consumption to a step increase in heart rate [i.e., mitochondrial response time (t(mito))] in isolated Langendorff-perfused hearts from wild-type (n = 6), M/MtCK(-/-) (n = 6), and AK(-/-) (n = 4) mice. Left ventricular developed pressure was higher in M/MtCK(-/-) hearts (88.2 +/- 6.8 mmHg) and lower in AK(-/-) hearts (46.7 +/- 9.4 mmHg) compared with wild-type hearts (60.7 +/- 10.1 mmHg) at the basal pacing rate. Developed pressure fell slightly when heart rate was increased in all three groups. Basal MVO(2) at 300 beats/min was 19.1 +/- 2.4, 19.4 +/- 1.5, and 16.3 +/- 1.9 micromol x min(-1) x g dry wt(-1) for M/MtCK(-/-), AK(-/-), and wild type, respectively, which increased to 25.5 +/- 3.7, 25.4 +/- 2.6, and 22.0 +/- 2.6 micromol. min(-1) x g(-1), when heart rate was increased to 400 beats/min. The t(mito) was significantly faster in M/MtCK(-/-) hearts: 3.0 +/- 0.3 versus 7.3 +/- 0.6 and 8.0 +/- 0.4 s for M/MtCK(-/-), AK(-/-), and wild-type hearts, respectively. Our results demonstrate that MVO(2) of M/MtCK(-/-) hearts adapts more quickly to an increase in heart rate and thereby support the hypothesis that creatine kinase acts as an energy buffer in the cytosol, which delays the energy-related signal between sites of ATP hydrolysis and mitochondria.

Theoretical studies on the regulation of oxidative phosphorylation in intact tissues

The theoretical studies on the regulation of oxidative phosphorylation that were performed with the aid of kinetic models of this process are overviewed. A definition of the regulation of the flux through a metabolic pathway is proposed and opposed to the control exerted by particular enzymes over this flux. Different kinetic models of oxidative phosphorylation proposed in the literature are presented, of which only the model proposed by myself and co-workers was extensively used in theoretical studies on the regulation and compensation in the oxidative phosphorylation system. These theoretical studies have led to the following conclusions: (1) in isolated mitochondria, an increase in the activity of an artificial ATP-using system stimulates mitochondria mainly via changes in [ADP], while changes in [ATP] and [P(i)] play only a minor role; (2) in non-excitable tissues (e.g. liver), hormones (acting via some cytosolic factor(s)) activate directly both ATP usage and at least some enzymes of the ATP-producing block; (3) in excitable tissues (e.g. skeletal muscle), neural signals stimulate (via some cytosolic factor(s)) in parallel all the steps of oxidative phosphorylation together with ATP usage and substrate dehydrogenation; (4) the decrease in the flux through cytochrome oxidase caused by a decrease in oxygen concentration is, at least partially, compensated by a decrease in Delta p and increase in the reduction level of cytochrome c. A theoretical prediction is formulated that there should exist and be observable a universal cytosolic factor/regulatory mechanism which directly activates (at least in excitable tissues) all complexes of oxidative phosphorylation during an increased energy demand.

Measurement of the activation time of oxidative phosphorylation in isolated mouse hearts

The purpose of this study was to develop a technique for determination of the dynamic regulation of oxidative myocardial metabolism in the mouse. The response time of myocardial oxygen consumption (MVO(2)) to a step in heart rate was determined in Langendorff-perfused mouse hearts. We examined the effect of glucose-only perfusate and glucose combined with 1, 3, or 6 mM pyruvate. Left ventricular systolic pressure (LVSP) decreased, yet the rate-pressure product (RPP) and MVO(2) increased with upward steps in heart rate. Pyruvate increased LVSP, RPP, and MVO(2) at the lower concentrations; however, when 6 mM pyruvate was added, LVSP and RPP became depressed while MVO(2) remained elevated. The mean response time of oxygen consumption to a step in heart rate from 270 to 350 beats/min was 9.8 s (n = 7) in the glucose-only perfused hearts. Perfusion with glucose plus 6 mM pyruvate decreased the response time to 5.3 s. These results are similar to those found in the rabbit heart and lay the groundwork for further examination of the dynamic regulation of oxidative myocardial metabolism in genetically altered mice. We concluded that the activation time of oxidative phosphorylation in the mouse is similar to that in larger species, despite the high mitochondrial content and natural heart rate of the mouse.

Dynamic adaptation of cardiac oxidative phosphorylation is not mediated by simple feedback control

The classic idea about regulation of cardiac oxidative phosphorylation (OxPhos) was that breakdown products of ATP (ADP and P(i)) diffuse freely to the mitochondria to stimulate OxPhos. On the basis of this metabolic feedback control system, the response time of OxPhos (t(mito)) is predicted to be inversely proportional to the mitochondrial aerobic capacity (MAC). We determined t(mito) during steps in heart rate in isolated perfused rabbit hearts (n = 16) before and after reducing MAC with nonsaturating doses of oligomycin. The reduction of MAC was quantified in mitochondria isolated from each perfused heart, dividing oligomycin-sensitive, ADP-stimulated state 3 respiration by oligomycin-insensitive uncoupled respiration. The t(mito) to heart rate steps from 60 to 70 and 80 beats/min was 5. 6 +/- 0.6 and 7.2 +/- 0.8 s (means +/- SE) and increased an estimated 34 and 40% for a 50% decrease in MAC (P < 0.05), respectively, which is much less than the 100% predicted by the feedback hypothesis. For steps to 100 or 120 beats/min, t(mito) was 8.3 +/- 0.5 and 11.2 +/- 0.6 s and was not reduced with decreases in MAC (P > 0.05). We conclude that immediate feedback control by quickly diffusing ADP and P(i) cannot explain the dynamic regulation of cardiac OxPhos. Because calcium entry into the mitochondria also cannot explain the first fast phase of OxPhos activation, we propose that delay of the energy-related signal in the cytoplasm dominates the response time of OxPhos.

Dynamics of tissue oxygenation in isolated rabbit heart as measured with near-infrared spectroscopy

We investigated the role of myoglobin (Mb) in supplying O2 to mitochondria during transitions in cardiac workload. Isovolumic rabbit hearts (n = 7) were perfused retrogradely with hemoglobin-free Tyrode solution at 37 degrees C. Coronary venous O2 tension was measured polarographically, and tissue oxygenation was measured with two-wavelength near-infrared spectroscopy (NIRS), both at a time resolution of approximately 2 s. During transitions to anoxia, 68 +/- 2% (SE) of the NIRS signal was due to Mb and the rest to cytochrome oxidase. For heart rate steps from 120 to 190 or 220 beats/min, the NIRS signal decreased significantly by 6.9 +/- 1.3 or 11.1 +/- 2.1% of the full scale, respectively, with response times of 11.0 +/- 0.8 or 9.1 +/- 0.5 s, respectively. The response time of end-capillary O2 concentration ([O2]), estimated from the venous [O2], was 8.6 +/- 0.8 s for 190 beats/min (P < 0.05 vs. NIRS time) or 8.5 +/- 0.9 s for 220 beats/min (P > 0.05). The mean response times of mitochondrial O2 consumption (VO2) were 3.7 +/- 0.7 and 3.6 +/- 0.6 s, respectively. The deoxygenation of oxymyoglobin (MbO2) accounted for only 12-13% of the total decrease in tissue O2, with the rest being physically dissolved O2. During 11% reductions in perfusion flow at 220 beats/min, Mb was 1.5 +/- 0.4% deoxygenated (P < 0.05), despite the high venous PO2 of 377 +/- 17 mmHg, indicating metabolism-perfusion mismatch. We conclude that the contribution of MbO2 to the increase of VO2 during heart rate steps in saline-perfused hearts was small and slow compared with that of physically dissolved O2.

Response time of myocardial oxygen consumption to cardiac work jumps at 28 degrees C varies with exogenous carbon substrate

In isolated rabbit heart perfused with hemoglobin-free Tyrode's solution at 28 degrees C the response time of myocardial oxidative phosphorylation to steps in heart rate, which is about 8 s, is decreased by about 2.5 s when lactate of pyruvate are given as exogenous carbon substrate. A hypothesis that may explain the decrease in response times is that glycolytic buffering in compartments near the energy consuming ATPases delays the transport of the energetic signal between sites of ATP consumption and the mitochondria. Pyruvate inhibits this glycolytic buffering, which would explain the faster response time. Whatever the mechanism may be, the type of exogenous substrate available to the heart has a major effect on the speed of response of oxidative phosphorylation to quick changes in cardiac workload.

CK inhibition accelerates transcytosolic energy signaling during rapid workload steps in isolated rabbit hearts

The effect of graded creatine kinase (CK) inhibition on the response time of mitochondrial O2 consumption to dynamic workload jumps (tmito) was studied in isolated rabbit hearts. Tyrode-perfused hearts (n = 7/group) were exposed to 15 min of 0, 0.1, 0.2, or 0.4 mM iodoacetamide (IA) (CK activity = 100, 14, 6, and 3%, respectively). Pretreatment tmito was similar across groups at 6.5 +/- 0.5 s (mean +/- SE). The increase observed over time in control hearts (33 +/- 8%) was progressively reversed to 16 +/- 6, -20 +/- 6 (P < 0.01 vs. control), and -46 +/- 6 (P < 0.01 vs. control) % in the 0.1, 0.2 and 0.4 mM IA groups, respectively. The faster response times occurred without reductions in mitochondrial oxidative capacity (assessed in vitro) or myocardial O2 consumption of the whole heart during workload steps. Isovolumic contractile function assessed as rate-pressure product (RPP) and contractile reserve (increase in RPP during heart rate steps) were significantly reduced by IA. We conclude that CK in the myofibrils and/or cytosol does not speed up transfer of the energy-related signal to the mitochondria but rather acts as an energetic buffer, effectively slowing the stimulus between myofibrils/ion pumps and oxidative phosphorylation. This argues against the existence of an obligatory creatine phosphate energy shuttle, because CK is effectively bypassed.

31P NMR studies of creatine kinase flux in M-creatine kinase-deficient mouse heart

Hearts of wild-type and cytosolic muscle creatine kinase (M-CK)-knockout mice were perfused with Krebs-Henseleit buffer containing 10 mM glucose and 5 mM pyruvate and studied during pacing at 400 and 600 beats/min and during K+ arrest. Phosphocreatine (PCr) and ATP concentrations in M-CK-deficient hearts were not significantly different from those in wild-type hearts. With the use of 31P NMR saturation transfer, the flux mediated predominantly by mitochondrial creatine kinase (Mi-CK) was clearly detected in M-CK-deficient hearts. Mi-CK flux was 4.8 +/- 0.6 and 4.5 +/- 0.6 mM/s during pacing at 400 and 600 beats/min, respectively, and was 3. 5 +/- 0.4 mM/s during cardiac arrest. In control hearts total CK flux was 7.8 +/- 1.1 and 6.6 +/- 1.3 mM/s during pacing at 400 and 600 beats/min, respectively, and decreased to 3.8 +/- 0.5 mM/s during arrest. It is suggested that the relative contribution of Mi-CK to the total NMR-measured CK flux in the wild-type heart is higher than that of the homodimeric M-CK isoform (MM-CK).

Undiminished mitochondrial function during stunning in rabbit heart at 28 degrees C

OBJECTIVE

To investigate effect of brief ischemia on mitochondrial function in intact myocardium, rather than in isolated mitochondria.

METHODS

The mitochondrial response was characterized by the mean response time (tmito) of cardiac mitochondrial O2 consumption to steps in heart rate. Isolated isovolumic rabbit hearts were perfused at 28 degrees C with a constant flow of Tyrode solution containing 11 mM glucose. O2 consumption and tmito were determined before ischemia and after 25 min of no-flow global ischemia during which hearts were either paced (I + P, n = 8) or unpaced (I - P, n = 8). A non-ischemic control group (N = 8) was also examined.

RESULTS

At 20 min reperfusion, developed left ventricular pressure (DLVP) after I + P was decreased to 47 +/- 3% (mean +/- s.e.m.; P < 0.05) of control DLVP without significant changes in venous creatine kinase efflux, indicating contractile stunning. In contrast complete contractile recovery was observed after I - P. Before ischemia, tmito was 11.2 +/- 0.6 and 14.9 +/- 0.7 s for heart rate steps from 60 to 70 and from 60 to 120 beats/min, respectively. The tmito was lower (P < 0.05) for the corresponding downward steps (10.5 +/- 0.6 and 12.4 +/- 0.6 s, respectively). An increase (P < 0.05) in tmito was observed in the course of the experiment for upward (1.2 +/- 0.3 s) and downward steps (1.4 +/- 0.3 s), but the change was similar after ischemia to that in time-matched controls (P > 0.05, both for I - P and I + P vs. control). Oxygen consumption, compared at fixed levels of the rate x pressure product, was unchanged after ischemia (P > 0.05, for both I - P and I + P vs. controls), suggesting undiminished efficiency of mitochondrial ATP production.

CONCLUSIONS

Twenty-five minutes ischemia does not affect mitochondrial function in rabbit hearts at 28 degrees C, even when contractile stunning resulted.