Heterogeneity of local myocardial flow and oxidative metabolism

The coronary capillary bed and its role in blood flow and oxygen delivery: A review

The assumption that the coronary capillary blood flow is exclusively regulated by precapillary vessels is not supported by recent data. Rather, the complex coronary capillary bed has unique structural and geometric characteristics that invalidate many assumptions regarding red blood cell (RBC) transport, for example, data based on a single capillary or that increases in flow are the result of capillary recruitment. It is now recognized that all coronary capillaries are open and that their variations in flow are due to structural differences, local O₂ demand and delivery, and variations in hematocrit. Recent data reveal that local mechanisms within the capillary bed regulate flow via signaling mechanisms involving RBC signaling and endothelial-associated pericytes that contract and relax in response to humoral and neural signaling. The discovery that pericytes respond to vasoactive signals (e.g., nitric oxide, phenylephrine, and adenosine) underscores the role of these cells in regulating capillary diameter and consequently RBC flux and oxygen delivery. RBCs also affect blood flow by sensing <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>P</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:msub> </mml:math> and releasing nitric oxide to facilitate relaxation of pericytes and a consequential capillary dilation. New data indicate that these signaling mechanisms allow control of blood flow in specific coronary capillaries according to their oxygen requirements. In conclusion, mechanisms in the coronary capillary bed facilitate RBC density and transit time, hematocrit, blood flow and O₂ delivery, factors that decrease capillary heterogeneity. These findings have important clinical implications for myocardial ischemia and infarction, as well as other vascular diseases.

Myocardial ischemia: lack of coronary blood flow, myocardial oxygen supply-demand imbalance, or what?

This opinionated article reviews current concepts of myocardial ischemia. Specifically, the historical background is briefly presented. Then, the prevailing paradigm of myocardial oxygen-supply-demand imbalance is criticized since demand is a virtual parameter that cannot be measured and data on measurements of myocardial blood flow and contractile function rather support matching between flow and function. Finally, a concept of myocardial ischemia that focusses on the reduction of coronary blood flow to below 8-10 µl/g per beat with consequences for myocardial electrical, metabolic, contractile and morphological features is advocated.

Coronary microcirculatory pathophysiology: can we afford it to remain a black box?

Coronary microvascular networks play the key role in determining blood flow distribution in the heart. Matching local blood supply to tissue metabolic demand entails continuous adaptation of coronary vessels via regulation of smooth muscle tone and structural dilated vessel diameter. The importance of coronary microcirculation for relevant pathological conditions including angina in patients with normal or near-normal coronary angiograms [microvascular angina (MVA)] and heart failure with preserved ejection fraction (HFpEF) is increasingly recognized. For MVA, clinical studies have shown a prevalence of up to 40% in patients with suspected coronary artery disease and a relevant impact on adverse cardiovascular events including cardiac death, stroke, and heart failure. Despite a continuously increasing number of corresponding clinical studies, the knowledge on pathophysiological cause–effect relations involving coronary microcirculation is, however, still very limited. A number of pathophysiological hypotheses for MVA and HFpEF have been suggested but are not established to a degree, which would allow definition of nosological entities, stratification of affected patients, or development of effective therapeutic strategies. This may be related to a steep decline in experimental (animal) pathophysiological studies in this area during the last 15 years. Since technology to experimentally investigate microvascular pathophysiology in the beating heart is increasingly, in principle, available, a concerted effort to build ‘coronary microcirculatory observatories’ to close this gap and to accelerate clinical progress in this area is suggested.

Regulation of Coronary Blood Flow

The heart is uniquely responsible for providing its own blood supply through the coronary circulation. Regulation of coronary blood flow is quite complex and, after over 100 years of dedicated research, is understood to be dictated through multiple mechanisms that include extravascular compressive forces (tissue pressure), coronary perfusion pressure, myogenic, local metabolic, endothelial as well as neural and hormonal influences. While each of these determinants can have profound influence over myocardial perfusion, largely through effects on end-effector ion channels, these mechanisms collectively modulate coronary vascular resistance and act to ensure that the myocardial requirements for oxygen and substrates are adequately provided by the coronary circulation. The purpose of this series of Comprehensive Physiology is to highlight current knowledge regarding the physiologic regulation of coronary blood flow, with emphasis on functional anatomy and the interplay between the physical and biological determinants of myocardial oxygen delivery. © 2017 American Physiological Society. Compr Physiol 7:321-382, 2017.

The effects of transit time heterogeneity on brain oxygenation during rest and functional activation

The interpretation of regional blood flow and blood oxygenation changes during functional activation has evolved from the concept of 'neurovascular coupling', and hence the regulation of arteriolar tone to meet metabolic demands. The efficacy of oxygen extraction was recently shown to depend on the heterogeneity of capillary flow patterns downstream. Existing compartment models of the relation between tissue metabolism, blood flow, and blood oxygenation, however, typically assume homogenous microvascular flow patterns. To take capillary flow heterogeneity into account, we modeled the effect of capillary transit time heterogeneity (CTH) on the 'oxygen conductance' used in compartment models. We show that the incorporation of realistic reductions in CTH during functional hyperemia improves model fits to dynamic blood flow and oxygenation changes acquired during functional activation in a literature animal study. Our results support earlier observations that oxygen diffusion properties seemingly change during various physiologic stimuli, and posit that this phenomenon is related to parallel changes in capillary flow patterns. Furthermore, our results suggest that CTH must be taken into account when inferring brain metabolism from changes in blood flow- or blood oxygenation-based signals .

The role of capillary transit time heterogeneity in myocardial oxygenation and ischemic heart disease

Ischemic heart disease (IHD) is characterized by an imbalance between oxygen supply and demand, most frequently caused by coronary artery disease (CAD) that reduces myocardial perfusion. In some patients, IHD is ascribed to microvascular dysfunction (MVD): microcirculatory disturbances that reduce myocardial perfusion at the level of myocardial pre-arterioles and arterioles. In a minority of cases, chest pain and reductions in myocardial flow reserve may even occur in patients without any other demonstrable systemic or cardiac disease. In this topical review, we address whether these findings might be caused by impaired myocardial oxygen extraction, caused by capillary flow disturbances further downstream. Myocardial blood flow (MBF) increases approximately linearly with oxygen utilization, but efficient oxygen extraction at high MBF values is known to depend on the parallel reduction of capillary transit time heterogeneity (CTH). Consequently, changes in capillary wall morphology or blood viscosity may impair myocardial oxygen extraction by preventing capillary flow homogenization. Indeed, a recent re-analysis of oxygen transport in tissue shows that elevated CTH can reduce tissue oxygenation by causing a functional shunt of oxygenated blood through the tissue. We review the combined effects of MBF, CTH, and tissue oxygen tension on myocardial oxygen supply. We show that as CTH increases, normal vasodilator responses must be attenuated in order to reduce the degree of functional shunting and improve blood-tissue oxygen concentration gradients to allow sufficient myocardial oxygenation. Theoretically, CTH can reach levels such that increased metabolic demands cannot be met, resulting in tissue hypoxia and angina in the absence of flow-limiting CAD or MVD. We discuss these predictions in the context of MVD, myocardial infarction, and reperfusion injury.

Mechanisms of metabolic coronary flow regulation

Coronary blood flow is tightly adjusted to the oxygen requirements of the myocardium. The underlying control mechanisms keep coronary venous pO(2) at a rather constant level around 20mm Hg under a variety of physiological conditions. Because coronary flow may increase more than 5-fold during exercise without any signs of under- or overperfusion, coronary flow must be controlled, at least in part, in a feed forward manner. Likely metabolic factors contributing to feed forward control are carbon dioxide and reactive oxygen species. Adaptation of coronary flow to exercise under physiological conditions involves in addition to metabolic control feed forward neuronal and endothelium-dependent control. Under pathological conditions, e.g. vessel stenosis or anemia, or specific environmental conditions, e.g. high altitude exposure, cardiac oxygenation may become critical, especially if oxygen demand is increased during physical exercise. Under such conditions the fall of coronary pO(2) may directly result in opening of oxygen sensitive potassium or closure of calcium channels. Furthermore the fall of pO(2) results in the production of vasoactive metabolites, e.g. adenosine, nitric oxide or prostaglandins, and in proton accumulation. All of these adaptations support a reduction of coronary vessel resistance. This article is part of a Special Issue entitled "Coronoray Blood Flow".

Stem cell-mediated neovascularization in heart repair

Accumulating clinical and experimental evidence indicates that stem cells from various sources are promising in the treatment of cardiac dysfunction. They may be incorporated into neovascular foci and thus contribute to postnatal physiological and pathological vasculogenesis and/or produce a variety of growth factors for angiogenesis and cytokines that home other stem cells from other organs for cardiac regeneration. This review focuses on the neovascularization of stem cells from different sources in cardiac repair, with emphasis on adult stem cells.

Origins of heterogeneity in tissue perfusion and metabolism

AIMS

In the heart and other tissues, perfusion and metabolic activity are heterogeneous and spatially correlated. The goal of this work is to investigate the causes of this behaviour. Theoretical simulations are used to examine the effects on flow distribution and oxygen levels in terminal vascular beds of inherent irregularity in network structure, considering structural adaptation of vessel diameters to haemodynamic and metabolic stimuli, and adaptation of oxygen demand to local oxygen availability.

METHODS AND RESULTS

A mathematical model based on experimentally observed microvascular network structures (rat mesentery and m. sartorius) is used to simulate blood flow, oxygen transport, and adaptation of vessel diameters and tissue oxygen demand. Inherent geometric heterogeneities of vascular networks cause heterogeneity of blood flow and oxygen levels that cannot be eliminated by increasing metabolic sensitivity of diameter adaptation. Adaptation of oxygen demand to differences in oxygen availability causes increased oxygen extraction, implying improved functional capacity, and establishes a correlation between local oxygen demand and flow rate, as observed experimentally. Such a correlation is not predicted if the heterogeneity of oxygen demand is assumed to be an intrinsic tissue property.

CONCLUSION

A central mechanism generating heterogeneous perfusion is the inevitable structural heterogeneity of terminal vascular beds, which cannot be fully compensated by structural adaptation of vessel diameters. Heterogeneity of metabolism may result from adaptation of tissue function to the heterogeneous oxygen availability. These findings are of interest for the understanding of tissue function, including the heart, and of results obtained by corresponding imaging approaches.

Pacing-induced dyssynchrony during early reperfusion reduces infarct size

OBJECTIVES

Considering the recent discovery of postconditioning, we investigated whether intermittent dyssynchrony immediately upon reperfusion induces cardioprotection as well.

BACKGROUND

Intermittent dyssynchrony, induced by ventricular pacing, preconditions myocardium.

METHODS

Isolated ejecting rabbit hearts were subjected to 30-min coronary occlusion and 2-h reperfusion. Control, left ventricular (LV) pacing preconditioning (LVPpreC) (3 x 5-min LV pacing), and LV pacing postconditioning (LVPpostC) (10 x 30-s LV pacing during early reperfusion) groups were studied. Mechanical effects of LV pacing were determined using local pressure-length loops (sonomicrometry), whereas effects on myocardial lactate release and coronary flow were assessed from coronary effluent and fluorescent microspheres, respectively. Anesthetized pigs underwent 60-min coronary occlusion and 3-h reperfusion in control and right ventricular (RV) pacing postconditioning groups (RVPpostC) (10 x 30-s RV pacing during early reperfusion). In all hearts, area at risk and infarct size were determined with blue dye and triphenyltetrazolium chloride staining, respectively.

RESULTS

Infarct size, normalized to area at risk, was 47.0 +/- 12.3% in control rabbit hearts, but significantly smaller in LVPpreC (17.8 +/- 6.4%) and LVPpostC hearts (17.9 +/- 4.4%). Left ventricular pacing significantly altered regional mechanical work, but did not affect coronary flow or lactate release. In pigs, infarct size was significantly smaller in RVPpostC (9.8 +/- 3.0%) than in control (20.6 +/- 2.2%) animals.

CONCLUSIONS

Intermittent dyssynchrony during early reperfusion reduces infarct size in 2 different animal models. Dyssynchrony-induced postconditioning cannot be attributed to graded reperfusion but may be induced by modulation of local myocardial workload. Dyssynchrony-induced postconditioning opens new possibilities for cardioprotection in the clinical setting.

Regional myocardial perfusion under exchange transfusion with liposomal hemoglobin: in vivo and in vitro studies using rat hearts

The purpose of this study was to test the hypothesis that exchange transfusion with liposomal hemoglobin (LH) reduces the microheterogeneity of regional myocardial flows while sustaining cardiac function. Neo Red Cell mixed with albumin was used as the LH solution, in which the LH volume fraction was 17∼18% and hemoglobin density was nearly two-thirds smaller than in rat blood. Regional myocardial flows in left ventricular free walls were measured by tracer digitalradiography (100-μm resolution) in anesthetized rats with or without 50% blood-LH exchange transfusion. Within-layer flow distributions showed lower heterogeneity with ( n = 8) than without ( n = 8) LH transfusion. No extravasation of hemoglobin was confirmed by 3,3-diaminobenzidin staining ( n = 2). Carotid flow increased by 68% due to LH transfusion, whereas arterial pressure and heart rate remained unchanged. On the other hand, cross-circulated rat hearts ( n = 7) were used to evaluate the effects of 50% blood-LH exchange on coronary flow and tone preservation under 300-beats/min pacing and 100-mmHg perfusion pressure. Blood-LH exchange caused a 71% increase of coronary flow and 10% decrease of percent flow increase during hyperemia after 30-s flow interruption. Myocardial O2 supply and consumption increased by 9% and 10%, respectively, whereas myocardial O2 extraction remained unchanged. The large increases of in vivo carotid flow and coronary flow in cross-circulated hearts due to LH coperfusion could be explained by the reduction of apparent flow viscosity. These results suggest that under LH coperfusion, the microheterogeneity of myocardial flows decreases with increased coronary flow while fairly preserving coronary tone and cardiac function.

Acute Changes in Muscle Blood Flow and Concomitant Muscle Damage after an Intramuscular Administration

PURPOSE

The intramuscular route (IM) is widely used but commonly induces injection site muscle damage. This study investigates the hemodynamic changes in an acute lesion induced by the IM administration of propylene glycol (PG) in rabbits.

METHODS

Control groups received 1, 2, or 3 ml of PG (IM). Others were pretreated with pancuronium, dantrolene, indomethacin, or SR140333 and then received 2 ml of PG. The muscle blood flow (MBF) was assessed using fluorescent microspheres before and at 15, 45, 60, 90 min, 3 and 6 h after IM administration. Different areas within the muscle damage were quantified.

RESULTS

Muscle contractions as well as a transient but major MBF increase were observed at the injection site. All treatments reduced hyperemia by up to 81% (dantrolene, 15 min) at 15, 45, and 90 min (p < 0.05). MBF had returned to basal values in all groups at 6 h. The central necrotic area was not modified, but peripheral damage (8.0 +/- 1.3 g) was reduced by dantrolene, indomethacin, and SR140333 (p < 0.05), but not by pancuronium.

CONCLUSIONS

Muscle contraction and hyperemia are not responsible for muscle damage at the injection site, which is the multifactorial phenomenon, involving intracellular calcium and inflammation.

Regional differences of myocardial infarct development and ischemic preconditioning

The spatial and temporal development of myocardial infarction depends on the area at risk (AAR), the severity and duration of blood flow reduction (energy supply) as well as on heart rate and regional wall function (energy demand). Both supply and demand can vary within the AAR of a given heart, potentially resulting in differences in infarct development. We therefore retrospectively analyzed infarct size (IS, %AAR, TTC) in 24 anesthetized pigs in vivo following 90 min hypoperfusion and 120 min reperfusion of the LAD coronary artery, which supplies parts of the LV septum (LVS) and anterior free wall (LVAFW). The total LAD perfusion territory averaged 49.8 +/- 14.2 (SD) g (49.2 +/- 8.4% of LV); 61.4 +/- 8.1% of the AAR was LVAFW. IS within the LVS was 25.3 +/- 15.1%, while IS within the LVAFW was 16.6 +/-10.1% (p<0.05). While ischemic blood flow (radiolabeled microspheres) did not differ between LVS (0.05 +/- 0.02 ml/min/g) and LVAFW (0.05 +/- 0.03 ml/min/g), perivascular connective tissue (56 +/- 9 vs. 38+/-7 microm(2), p < 0.05) and the capillary-to-myocyte distance (1.65 +/- 0.23 vs. 1.18 +/- 0.23 mm, p < 0.05) were larger in LVS than in LVAFW. Interestingly, IS in LVS (9.3 +/- 9.6%, n = 24) and LVAFW (9.2 +/- 9.1%) were reduced to the same absolute extent by ischemic preconditioning with one cycle of 10 min ischemia and 15 min reperfusion, suggesting that a similar regional difference exists also in the protection afforded by ischemic preconditioning. The mechanism(s) for that remain(s) to be established.

CONCLUSION

In pigs, regional differences in infarct development and protection from it exist in the LAD perfusion territory, which are independent of ischemic blood flow but apparently related to pre-existing structural differences.

Myocardial O2 consumption in porcine left ventricle is heterogeneously distributed in parallel to heterogeneous O2 delivery

Myocardial blood flow is unevenly distributed, but the cause of this heterogeneity is unknown. Heterogeneous blood flow may reflect heterogeneity of oxygen demand. The aim of the present study was to assess the relation between oxygen consumption and blood flow in small tissue regions in porcine left ventricle. In seven male, anesthetized, open-chest pigs, local oxygen consumption was quantitated by computational model analysis of the incorporation of 13C in glutamate via the tricarboxylic acid cycle during timed infusion of [13C]acetate into the left anterior descending coronary artery. Blood flow was measured with radioactive microspheres before and during acetate infusion. High-resolution nuclear magnetic resonance 13C spectra were obtained from extracts of tissue samples (159 mg mean dry wt) taken at the end of the acetate infusion. Mean regional myocardial blood flow was stable [5.0 ± 1.6 (SD) and 5.0 ± 1.4 ml·min−1·g dry wt−1 before and after 30 min of acetate infusion, respectively]. Mean left ventricular oxygen consumption measured with the NMR method was 18.6 ± 7.7 μmol·min−1·g dry wt−1 and correlated well ( r = 0.85, P = 0.02, n = 7) with oxygen consumption calculated from blood flow, hemoglobin, and blood gas measurements (mean 22.8 ± 4.7 μmol·min−1·g dry wt−1). Local blood flow and oxygen consumption were significantly correlated ( r = 0.63 for pooled normalized data, P < 0.0001, n = 60). We calculate that, in the heart at normal workload, the variance of left ventricular oxygen delivery at submilliliter resolution is explained for 43% by heterogeneity in oxygen demand.

Pattern differences between distributions of microregional myocardial flows in crystalloid- and blood-perfused rat hearts

Regional myocardial flow distributions in Langendorff rat hearts under Tyrode and blood perfusion were assessed by tracer digital radiography (100-μm resolution). Flow distributions during baseline and maximal hyperemia following a 60-s flow cessation were evaluated by the coefficient of variation of regional flows (CV; related to global flow heterogeneity) and the correlation between adjacent regional flows (CA; inversely related to local flow randomness). These values were obtained for the original images (642 pixels) and for coarse-grained images (322, 162, and 82 blocks of nearby pixels). At a given point in time during baseline, both CV and CA were higher in blood ( n = 7) than in Tyrode perfusion ( n = 7) over all pixel aggregates ( P < 0.05, two-way ANOVA). During the maximal hyperemia, CV and CA were still significantly higher in blood ( n = 7) than in Tyrode perfusion ( n = 7); however, these values decreased substantially in blood perfusion and the CV and CA differences became smaller than those at baseline accordingly. During basal blood perfusion, the 60-s average flow distribution ( n = 7) showed a smaller CV and CA than those at a given point in time ( P < 0.05, two-way ANOVA). Coronary flow reserve was significantly higher in blood than in Tyrode perfusion. In conclusion, the flow heterogeneity and the local flow similarity are both higher in blood than in Tyrode perfusion, probably due to the different degree of coronary tone preservation and the presence or absence of blood corpuscles. Under blood perfusion, temporal flow fluctuations over 60-s order are largely involved in shaping microregional flow distributions.

Flow heterogeneity following global no-flow ischemia in isolated rabbit heart

The purpose of this study was to evaluate flow heterogeneity and impaired reflow during reperfusion after 60-min global no-flow ischemia in the isolated rabbit heart. Radiolabeled microspheres were used to measure relative flow in small left ventricular (LV) segments in five ischemia + reperfused hearts and in five nonischemic controls. Relative flow heterogeneity was expressed as relative dispersion (RD) and computed as standard deviation/mean. In postischemic vs. preischemic hearts, RD was increased for the whole LV (0.92 +/- 0.41 vs. 0.37 +/- 0.07, P < 0.05) as well as the subendocardium (Endo) and subepicardium considered separately (1.28 +/- 0.74 vs. 0.30 +/- 0.09 and 0.69 +/- 0.22 vs. 0.38 +/- 0.08; P < 0.05 for both comparisons, respectively) during early reperfusion. During late reperfusion, the increased RD for the whole LV and Endo remained significant (0.70 +/- 0.22 vs. 0.37 +/- 0.07 and 1.06 +/- 0.55 vs. 0.30 +/- 0.09; P < 0.05 for both comparisons, respectively). In addition to the increase in postischemic flow heterogeneity, there were some regions demonstrating severely impaired reflow, indicating that regional ischemia can persist despite restoration of normal global flow. Also, the relationship between regional and global flow was altered by the increased postischemic flow heterogeneity, substantially reducing the significance of measured global LV reflow. These observations emphasize the need to quantify regional flow during reperfusion after sustained no-flow ischemia in the isolated rabbit heart.

Spatial heterogeneity in the heart: recent insights and open questions

Within the left ventricular myocardium and despite its rather homogeneous structure, local myocardial perfusion varies substantially. Areas of low and high local flow differ with regard to substrate uptake, energy turnover, and demand. This spatial heterogeneity is related to distinct differences in local protein expression, forming the basis of a novel homeostatic mechanism.

Low-O(2) affinity erythrocytes improve performance of ischemic myocardium

O(2) transport and O(2) diffusion interact in providing O(2) to tissue, but the extent to which diffusion may be critical in the heart is unclear. If O(2) diffusion limits mitochondrial oxygenation, a change in blood O(2) affinity at constant total O(2) transport should alter cardiac O(2) consumption (VO(2)) and function. To test this hypothesis, we perfused isolated isovolumically working rabbit hearts with erythrocytes at physiological blood-gas values and P(50) (PO(2) required to half-saturate hemoglobin) values at pH of 7.4 of 17 +/- 1 Torr (2,3-bisphosphoglycerate depletion) and 33 +/- 5 Torr (inositol hexaphosphate incorporation). When perfused at 40 and 20% of normal coronary flow, mean VO(2) decreased from the control value by 37 and 46% (P < 0.001), and function, expressed as cardiac work, decreased by 38 and 52%, respectively (P < 0.001). Perfusion at higher P(50) during low-flow ischemia improved VO(2) by 20% (P < 0.001) and function by 36% (P < 0.02). There was also modest improvement at basal flow (P < 0.02 and P < 0.002, respectively). The improvement in VO(2) and function due to the P(50) increase demonstrates the importance of O(2) diffusion in this cardiac ischemia model.

Evidence of separate pathways for lactate uptake and release by the perfused rat heart

The simultaneous release and uptake of lactate by the heart has been observed both in vivo and ex vivo; however, the pathways underlying these observations have not been satisfactorily explained. Consequently, the purpose of this study was to test the hypothesis that hearts release lactate from glycolysis while simultaneously taking up exogenous lactate. Therefore, we determined the effects of fatty acids and diabetes on the regulation of lactate uptake and release. Hearts from control and 1-wk diabetic animals were perfused with 5 mM glucose, 0.5 mM [3-(13)C]lactate, and 0, 0.1, 0.32, or 1.0 mM palmitate. Parameters measured include perfusate lactate concentrations, fractional enrichment, and coronary flow rates, which enabled the simultaneous, but independent, measurements of the rates of 1) uptake of exogenous [(13)C]lactate and 2) efflux of unlabeled lactate from metabolism of glucose. Although the rates of lactate uptake and efflux were both similarly inhibited by the addition of palmitate, (i.e., the ratio of lactate uptake to efflux remained constant), the ratio of lactate uptake to efflux was significantly higher in the controls compared with the diabetic group (1.00 +/- 0.14 vs. 0.50 +/- 0.07, P < 0.002). These data, combined with heterogeneous (13)C enrichment of tissue lactate, pyruvate, and alanine, suggest that glycolytically derived lactate production and oxidation of exogenous lactate operate as functionally separate metabolic pathways. These results are consistent with the concept of an intracellular lactate shuttle.