Ischemic areas in perfused rat hearts: measurement by NADH fluorescence photography

The mechanisms of action of mitochondrial targeting agents in cancer: inhibiting oxidative phosphorylation and inducing apoptosis

The tumor microenvironment affects the structure and metabolic function of mitochondria in tumor cells. This process involves changes in metabolic activity, an increase in the amount of reactive oxygen species (ROS) in tumor cells compared to normal cells, the production of more intracellular free radicals, and the activation of oxidative pathways. From a practical perspective, it is advantageous to develop drugs that target mitochondria for the treatment of malignant tumors. Such drugs can enhance the selectivity of treatments for specific cell groups, minimize toxic effects on normal tissues, and improve combinational treatments. Mitochondrial targeting agents typically rely on small molecule medications (such as synthetic small molecules agents, active ingredients of plants, mitochondrial inhibitors or autophagy inhibitors, and others), modified mitochondrial delivery system agents (such as lipophilic cation modification or combining other molecules to form targeted mitochondrial agents), and a few mitochondrial complex inhibitors. This article will review these compounds in three main areas: oxidative phosphorylation (OXPHOS), changes in ROS levels, and endogenous oxidative and apoptotic processes.

Don't go breakin' my heart: cardioprotective alterations to the mechanical and structural properties of reperfused myocardium during post-infarction inflammation

Myocardial infarctions (MIs) kickstart an intense inflammatory response resulting in extracellular matrix (ECM) degradation, wall thinning, and chamber dilation that leaves the heart susceptible to rupture. Reperfusion therapy is one of the most effective strategies for limiting adverse effects of MIs, but is a challenge to administer in a timely manner. Late reperfusion therapy (LRT; 3 + hours post-MI) does not limit infarct size, but does reduce incidences of post-MI rupture and improves long-term patient outcomes. Foundational studies employing LRT in the mid-twentieth century revealed beneficial reductions in infarct expansion, aneurysm formation, and left ventricle dysfunction. The mechanism by which LRT acts, however, is undefined. Structural analyses, relying largely on one-dimensional estimates of ECM composition, have found few differences in collagen content between LRT and permanently occluded animal models when using homogeneous samples from infarct cores. Uniaxial testing, on the other hand, revealed slight reductions in stiffness early in inflammation, followed soon after by an enhanced resistance to failure for cases of LRT. The use of one-dimensional estimates of ECM organization and gross mechanical function have resulted in a poor understanding of the infarct's spatially variable mechanical and structural anisotropy. To resolve these gaps in literature, future work employing full-field mechanical, structural, and cellular analyses is needed to better define the spatiotemporal post-MI alterations occurring during the inflammatory phase of healing and how they are impacted following reperfusion therapy. In turn, these studies may reveal how LRT affects the likelihood of rupture and inspire novel approaches to guide scar formation.

Ab Initio Studies of NMNH(2-) Conformers in Water-Methanol Solutions: Comparative Analysis of the Biexponential Fluorescence Signals for NMNH(2-) and NADH

Ab initio studies of the structure of reduced β-nicotinamide d-ribonucleotide (NMNH(2-)) conformations in water and methanol solutions have been carried out for clarifying the role of the phosphate groups in fluorescence parameters of the NMNH(2-) molecule and the reduced β-nicotinamide adenine dinucleotide (NADH) molecule. Relaxed potential energy surfaces as a function of the dihedral rotation angle of the amide group in the NMNH(2-) molecule were calculated in the ground electronic state and the first excited electronic state to better understand the effect of phosphate groups on the nonradiative decay rates in the nicotinamide chromophore groups. The differences in the weighting coefficients in the biexponential fluorescence signals for NMNH(2-) and NADH molecules in solution were explained. A strong hydrogen bonding between the amide hydrogen atom and the nearest oxygen O^- atom from the phosphate group was detected by ab initio calculations for the folded NMNH(2-) conformations in the ground electronic state at trans configurations of the nicotinamide ring. This hydrogen bonding turned out to be much weaker for the first excited electronic state. These calculated data show that, after optical excitation of the NMNH(2-) molecule, a rapid change in the geometry of the molecule is possible. The strong interaction of the phosphate group with the amide group in NMNH(2-) molecules in aqueous solution leads to the predominance of the folded NMNH(2-) conformations and trans configurations of the nicotinamide ring. This explains the reason for the dominance of one fluorescence decay time of NMNH(2-) in the aqueous solution. Based on these data, an important conclusion can be drawn that the contribution of the exponent with the short decay time τ ≈ 0.28 ns to the fluorescence signal of NMNH(2-), NADH, and NADPH molecules is related to the trans configuration of the nicotinamide ring.

Disposable electrocatalytic sensor for whole blood NADH monitoring

Monitoring nicotinamide adenine dinucleotide (NADH) is important because NADH is involved in cellular redox reactions and cellular energy production. Currently, few biosensors quantify NADH in whole blood. However, they still have limitations due to several defects, including poor repeatability, long analysis time, and their requirement of extra sample pretreatment. In this study, we developed electrocatalytic sensors using screen-printed electrodes with a redox-active monolayer 4′-mercapto-N-phenylquinone diamine formed by a self-assembled monolayer of a 4-aminothiophenol (4-ATP). We exhibited their behavior as electrocatalysts toward the oxidation of NADH in whole blood. Finally, the electrocatalytic sensors maintained stability and exhibited 3.5 µM limit of detection, with 0.0076 ± 0.0006 µM/µA sensitivity in a mouse’s whole blood. As proof of concept, a polyhexamethylene guanidine phosphate–treated mouse model was used to induce inflammatory and fibrotic responses, and NADH level was measured for 45 days. This work demonstrates the potential of electrocatalytic sensors to analyze NADH in whole blood and to be developed for extensive applications.

Simultaneous triple-parametric optical mapping of transmembrane potential, intracellular calcium and NADH for cardiac physiology assessment

Investigation of the complex relationships and dependencies of multiple cellular processes that govern cardiac physiology and pathophysiology requires simultaneous dynamic assessment of multiple parameters. In this study, we introduce triple-parametric optical mapping to simultaneously image metabolism, electrical excitation, and calcium signaling from the same field of view and demonstrate its application in the field of drug testing and cardiovascular research. We applied this metabolism-excitation-contraction coupling (MECC) methodology to test the effects of blebbistatin, 4-aminopyridine and verapamil on cardiac physiology. While blebbistatin and 4-aminopyridine alter multiple aspects of cardiac function suggesting off-target effects, the effects of verapamil were on-target and it altered only one of ten tested parameters. Triple-parametric optical mapping was also applied during ischemia and reperfusion; and we identified that metabolic changes precede the effects of ischemia on cardiac electrophysiology.

Mimicking Cellular Metabolism in Artificial Cells: Universal Molecule Transport across the Membrane through Vesicle Fusion

Mass transport across cell membranes is a primary process for cellular metabolism. For this purpose, electrostatically mediated membrane fusion is exploited to transport various small molecules including glucose-6-phosphate, isopropyl β-D-thiogalactoside, and macromolecules such as DNA plasmids from negatively charged large unilamellar vesicles (LUVs) to positively charged giant unilamellar vesicles (GUVs). After membrane fusion between these oppositely charged vesicles, molecules are transported into GUVs to trigger the NAD⁺ involved enzyme reaction, bacterial gene expression, and in vitro gene expression of green fluorescent protein from a DNA plasmid. The optimized charged lipid percentages are 10% for both positively charged GUVs and negatively charged LUVs to ensure the fusion process. The experimental results demonstrate a universal way for mass transport into the artificial cells through vesicle fusions, which paves a crucial step for the investigation of complicated cellular metabolism.

Non-Destructive Monitoring via Electrochemical NADH Detection in Murine Cells

Nicotinamide adenine dinucleotide (NADH) is an important cofactor involved in metabolic redox reactions in living cells. The detection of NADH in living animal cells is a challenge. We developed a one-step monitoring method for NADH via an electrocatalytic reaction that uses a surface-modified, screen-printed electrode (SPE) having a redox active monolayer 4'-mercapto-N-phenlyquinone diamine (NPQD) formed by a self-assembled monolayer (SAM) of an aromatic thiol, 4-aminothiophenol (4-ATP). This electrode has a limit of detection (LOD) of 0.49 μM and a sensitivity of 0.0076 ± 0.0006 μM/μA in cell culture media, which indicates that it retains its selectivity. The applicability of this NADH sensor was demonstrated for the first time by cell viability monitoring via NADH-sensing in cell culture supernatants.

Two-Photon Excited Fluorescence Dynamics in Enzyme-Bound NADH: the Heterogeneity of Fluorescence Decay Times and Anisotropic Relaxation

The dynamics of polarized fluorescence in NADH in alcohol dehydrogenase (ADH) in buffer solution has been studied using the TCSPC spectroscopy. A global fit procedure was used for determination of the fluorescence parameters from experiment. The interpretation of the results obtained was supported by ab initio calculations of the NADH structure. A theoretical model was developed describing the polarized fluorescence decay in ADH-NADH complexes that considered several interaction scenarios. A comparative analysis of the polarization-insensitive fluorescence decay using multiexponential fitting models has been carried out. As shown, the origin of a significant enhancement of the decay time in the ADH-NADH complex can be attributed to the decrease of nonradiative relaxation rates in the nicotinamide ring in the conditions of the apolar binding site environment. The existence of a single decay time in the ADH-NADH complex in comparison with two decay times observed in free NADH was attributed to a single NADH unfolded conformation in the ADH binding site. Comparison of the experimental data with the theoretical model suggested the existence of an anisotropic relaxation time of about 1 ns that is related with the rotation of fluorescence transition dipole moment due to the rearrangement of the excited state NADH nuclear configuration.

Luminescence lifetime imaging of three-dimensional biological objects

A major focus of current biological studies is to fill the knowledge gaps between cell, tissue and organism scales. To this end, a wide array of contemporary optical analytical tools enable multiparameter quantitative imaging of live and fixed cells, three-dimensional (3D) systems, tissues, organs and organisms in the context of their complex spatiotemporal biological and molecular features. In particular, the modalities of luminescence lifetime imaging, comprising fluorescence lifetime imaging (FLI) and phosphorescence lifetime imaging microscopy (PLIM), in synergy with Förster resonance energy transfer (FRET) assays, provide a wealth of information. On the application side, the luminescence lifetime of endogenous molecules inside cells and tissues, overexpressed fluorescent protein fusion biosensor constructs or probes delivered externally provide molecular insights at multiple scales into protein-protein interaction networks, cellular metabolism, dynamics of molecular oxygen and hypoxia, physiologically important ions, and other physical and physiological parameters. Luminescence lifetime imaging offers a unique window into the physiological and structural environment of cells and tissues, enabling a new level of functional and molecular analysis in addition to providing 3D spatially resolved and longitudinal measurements that can range from microscopic to macroscopic scale. We provide an overview of luminescence lifetime imaging and summarize key biological applications from cells and tissues to organisms.

Two-Photon Excited Fluorescence Dynamics in NADH in Water-Methanol Solutions: The Role of Conformation States

The dynamics of polarized fluorescence in reduced nicotinamide adenine dinucleotide (NADH) at 460 nm under two-photon excitation at 720 nm by femtosecond laser pulses in water-methanol solutions has been studied experimentally and theoretically as a function of methanol concentration. A number of fluorescence parameters have been determined from experiment by means of the global fit procedure and then compared with the results reported by other authors. A comprehensive analysis of experimental errors was made. Ab initio calculations of the structure of NADH in water and methanol and of β-nicotinamide mononucleotide (NMNH) in vacuum have been carried out for clarifying the role of decay time heterogeneity. The main results obtained are as follows. An explanation of the heterogeneity in the measured fluorescence decay times in NADH has been suggested based on the influence of the internal molecular electric field in the nicotinamide ring on nonradiative decay rates. We suggest that different charge distributions in the cis and trans configurations result in different internal electrostatic field distributions that lead to the decay time heterogeneity. A slight but noticeable rise of the fluorescence decay times τ₁ and τ₂ with methanol concentration was observed and treated as a minor effect of a nonradiative relaxation slowing due to the decrease in solution polarity. Relative concentrations of the folded and unfolded NADH conformations in solutions have been determined using a new method of analysis of the rotational diffusion time τ_r as a function of methanol concentration on the basis of the Stokes-Einstein-Debye equation. The analysis of the fluorescence anisotropy parameters obtained under linearly and circularly polarized excitation and the parameter Ω has been carried out and resulted in the determination of the two-photon excitation tensor components and suggested the existence of two excitation channels with comparable intensities. These were the longitudinal excitation channel dominated by the diagonal tensor component S_zz and the mixed excitation channel dominated by the off-diagonal tensor components |S_xz² + S_yz²|^1/2.

Open-Source Multiparametric Optocardiography

Since the 1970s fluorescence imaging has become a leading tool in the discovery of mechanisms of cardiac function and arrhythmias. Gradual improvements in fluorescent probes and multi-camera technology have increased the power of optical mapping and made a major impact on the field of cardiac electrophysiology. Tandem-lens optical mapping systems facilitated simultaneous recording of multiple parameters characterizing cardiac function. However, high cost and technological complexity restricted its proliferation to the wider biological community. We present here, an open-source solution for multiple-camera tandem-lens optical systems for multiparametric mapping of transmembrane potential, intracellular calcium dynamics and other parameters in intact mouse hearts and in rat heart slices. This 3D-printable hardware and Matlab-based RHYTHM 1.2 analysis software are distributed under an MIT open-source license. Rapid prototyping permits the development of inexpensive, customized systems with broad functionality, allowing wider application of this technology outside biomedical engineering laboratories.

Paradoxical arteriole constriction compromises cytosolic and mitochondrial oxygen delivery in the isolated saline-perfused heart

The isolated saline-perfused heart is used extensively to study cardiac physiology. Previous isolated heart studies have demonstrated lower tissue oxygenation compared with in vivo hearts based on myoglobin oxygenation and the mitochondrial redox state. These data, consistent with small anoxic regions, suggest that the homeostatic balance between work and oxygen delivery is impaired. We hypothesized that these anoxic regions are caused by inadequate local perfusion due to a paradoxical arteriole constriction generated by a disrupted vasoregulatory network. We tested this hypothesis by applying two exogenous vasodilatory agents, adenosine and cromakalim, to relax vascular tone in an isolated, saline-perfused, working rabbit heart. Oxygenation was monitored using differential optical transmission spectroscopy and full spectral fitting. Increases in coronary flow over control with adenosine (27 ± 4 ml/min) or cromakalim (44 ± 4 ml/min) were associated with proportional spectral changes indicative of myoglobin oxygenation and cytochrome oxidase (COX) oxidation, consistent with a decrease in tissue anoxia. Quantitatively, adenosine decreased deoxymyoglobin optical density (OD) across the wall by 0.053 ± 0.008 OD, whereas the reduced form of COX was decreased by 0.039 ± 0.005 OD. Cromakalim was more potent, decreasing deoxymyoglobin and reducing the level of COX by 0.070 ± 0.019 OD and 0.062 ± 0.019 OD, respectively. These effects were not species specific, as Langendorff-perfused mouse hearts treated with adenosine demonstrated similar changes. These data are consistent with paradoxical arteriole constriction as a major source of regional anoxia during saline heart perfusion. We suggest that the vasoregulatory network is disrupted by the washout of interstitial vasoactive metabolites in vitro. NEW & NOTEWORTHY Regional tissue anoxia is a common finding in the ubiquitous saline-perfused heart but is not found in vivo. Noninvasive optical techniques confirmed the presence of regional anoxia under control conditions and demonstrated that anoxia is diminished using exogenous vasodilators. These data are consistent with active arteriole constriction, occurring despite regional anoxia, generated by a disrupted vasoregulatory network. Washout of interstitial vasoactive metabolites may contribute to the disruption of normal vasoregulatory processes in vitro.

Responsive monitoring of mitochondrial redox states in heart muscle predicts impending cardiac arrest

Assessing the adequacy of oxygen delivery to tissues is vital, particularly in the fields of intensive care medicine and surgery. As oxygen delivery to a cell becomes deficient, changes in mitochondrial redox state precede changes in cellular function. We describe a technique for the continuous monitoring of the mitochondrial redox state on the epicardial surface using resonance Raman spectroscopy. We quantify the reduced fraction of specific electron transport chain cytochromes, a metric we name the resonance Raman reduced mitochondrial ratio (3RMR). As oxygen deficiency worsens, heme moieties within the electron transport chain become progressively more reduced, leading to an increase in 3RMR. Myocardial 3RMR increased from baseline values of 18.1 ± 5.9 to 44.0 ± 16.9% (P = 0.0039) after inferior vena cava occlusion in rodents (n = 8). To demonstrate the diagnostic power of this measurement, 3RMR was continuously measured in rodents (n = 31) ventilated with 5 to 8% inspired oxygen for 30 min. A 3RMR value exceeding 40% at 10 min predicted subsequent cardiac arrest with 95% sensitivity and 100% specificity [area under the curve (AUC), 0.98], outperforming all current measures, including contractility (AUC, 0.51) and ejection fraction (AUC, 0.39). 3RMR correlated with indices of intracellular redox state and energy production. This technique may permit the real-time identification of critical defects in organ-specific oxygen delivery.

Cardiac performance is limited by oxygen delivery to the mitochondria in the crystalloid-perfused working heart

The left ventricular working, crystalloid-perfused heart is used extensively to evaluate basic cardiac function, pathophysiology, and pharmacology. Crystalloid-perfused hearts may be limited by oxygen delivery, as adding oxygen carriers increases myoglobin oxygenation and improves myocardial function. However, whether decreased myoglobin oxygen saturation impacts oxidative phosphorylation (OxPhos) is unresolved, since myoglobin has a much lower affinity for oxygen than cytochrome c oxidase (COX). In the present study, a laboratory-based synthesis of an affordable perfluorocarbon (PFC) emulsion was developed to increase perfusate oxygen carrying capacity without impeding optical absorbance assessments. In left ventricular working hearts, along with conventional measurements of cardiac function and metabolic rate, myoglobin oxygenation and cytochrome redox state were monitored using a novel transmural illumination approach. Hearts were perfused with Krebs-Henseleit (KH) or KH supplemented with PFC, increasing perfusate oxygen carrying capacity by 3.6-fold. In KH-perfused hearts, myoglobin was deoxygenated, consistent with cytoplasmic hypoxia, and the mitochondrial cytochromes, including COX, exhibited a high reduction state, consistent with OxPhos hypoxia. PFC perfusate increased aortic output from 76 ± 6 to 142 ± 4 ml/min and increased oxygen consumption while also increasing myoglobin oxygenation and oxidizing the mitochondrial cytochromes. These results are consistent with limited delivery of oxygen to OxPhos resulting in an adapted lower cardiac performance with KH. Consistent with this, PFCs increased myocardial oxygenation, and cardiac work was higher over a wider range of perfusate Po₂. In summary, heart mitochondria are limited by oxygen delivery with KH; supplementation of KH with PFC reverses mitochondrial hypoxia and improves cardiac performance, creating a more physiological tissue oxygen delivery. NEW & NOTEWORTHY Optical absorbance spectroscopy of intrinsic chromophores reveals that the commonly used crystalloid-perfused working heart is oxygen limited for oxidative phosphorylation and associated cardiac work. Oxygen-carrying perfluorocarbons increase myocardial oxygen delivery and improve cardiac function, providing a more physiological mitochondrial redox state and emphasizing cardiac work is modulated by myocardial oxygen delivery.

Ventilation strategy has a major influence on remote ischaemic preconditioning in mice

Whether oxygen should be administered acutely during ST‐segment elevation myocardial infarction is debated. Despite this controversy, the possible influence of supplementary oxygen on animal models of ischaemia–reperfusion injury or cardioprotection is rarely considered. We used an in vivo mouse model of ischaemia and reperfusion to investigate the effect of ventilation with room air versus 100% oxygen. The coronary artery of anaesthetized mice was occluded for 40 min. followed by 2‐hrs reperfusion. Infarct size was measured by tetrazolium staining and expressed as a percentage of area at risk, determined using Evan's blue. Unexpectedly, infarct size in mice ventilated with 100% oxygen was significantly smaller than in those ventilated with room air (33 ± 5% versus 46 ± 3%; n = 6; P < 0.01). We tested a standard protocol of 3 × 5 min. cycles of remote ischaemic preconditioning (RIPC) and found this was unable to protect mice ventilated with 100% oxygen. RIPC protocols using 2.5‐ or 10‐min. occlusion were similarly ineffective in mice ventilated with oxygen. Similar disparate results were obtained with direct cardiac ischaemic preconditioning. In contrast, pharmacological protection using bradykinin administered at reperfusion was effective even in mice ventilated with 100% oxygen, reducing infarct size from 33 ± 5% to 21 ± 3% (n = 4–6; P < 0.01). Laser speckle contrast imaging of blood flow and direct pO₂ measurements were made in the hindlimb, but these measurements did not correlate with protection. In conclusion, ventilation protocol can have a major influence on infarct size and ischaemic preconditioning protocols in mice.

Enzyme-dependent fluorescence recovery of NADH after photobleaching to assess dehydrogenase activity of isolated perfused hearts

Reduction of NAD⁺ by dehydrogenase enzymes to form NADH is a key component of cellular metabolism. In cellular preparations and isolated mitochondria suspensions, enzyme-dependent fluorescence recovery after photobleaching (ED-FRAP) of NADH has been shown to be an effective approach for measuring the rate of NADH production to assess dehydrogenase enzyme activity. Our objective was to demonstrate how dehydrogenase activity could be assessed within the myocardium of perfused hearts using NADH ED-FRAP. This was accomplished using a combination of high intensity UV pulses to photobleach epicardial NADH. Replenishment of epicardial NADH fluorescence was then imaged using low intensity UV illumination. NADH ED-FRAP parameters were optimized to deliver 23.8 mJ of photobleaching light energy at a pulse width of 6 msec and a duty cycle of 50%. These parameters provided repeatable measurements of NADH production rate during multiple metabolic perturbations, including changes in perfusate temperature, electromechanical uncoupling, and acute ischemia/reperfusion injury. NADH production rate was significantly higher in every perturbation where the energy demand was either higher or uncompromised. We also found that NADH production rate remained significantly impaired after 10 min of reperfusion after global ischemia. Overall, our results indicate that myocardial NADH ED-FRAP is a useful optical non-destructive approach for assessing dehydrogenase activity.

Integration of Artificial Photosynthesis System for Enhanced Electronic Energy-Transfer Efficacy: A Case Study for Solar-Energy Driven Bioconversion of Carbon Dioxide to Methanol

Biocatalyzed artificial photosynthesis systems provide a promising strategy to store solar energy in a great variety of chemicals. However, the lack of direct interface between the light-capturing components and the oxidoreductase generally hinders the trafficking of the chemicals and photo-excited electrons into the active center of the redox biocatalysts. To address this problem, a completely integrated artificial photosynthesis system for enhanced electronic energy-transfer efficacy is reported by combining co-axial electrospinning/electrospray and layer-by-layer (LbL) self-assembly. The biocatalysis part including multiple oxidoreductases and coenzymes NAD(H) was in situ encapsulated inside the lumen polyelectrolyte-doped hollow nanofibers or microcapsules fabricated via co-axial electrospinning/electrospray; while the precise and spatial arrangement of the photocatalysis part, including electron mediator and photosensitizer for photo-regeneration of the coenzyme, was achieved by ion-exchange interaction-driven LbL self-assembly. The feasibility and advantages of this integrated artificial photosynthesis system is fully demonstrated by the catalyzed cascade reduction of CO2 to methanol by three dehydrogenases (formate, formaldehyde, and alcohol dehydrogenases), incorporating the photo-regeneration of NADH under visible-light irradiation. Compared to solution-based systems, the methanol yield increases from 35.6% to 90.6% using the integrated artificial photosynthesis. This work provides a novel platform for the efficient and sustained production of a broad range of chemicals and fuels from sunlight.

Myocardial Microcirculation and Mitochondrial Energetics in the Isolated Rat Heart

Normal functioning of myocardium requires adequate oxygenation, which in turn is dependent on an adequate microcirculation. NADH-fluorimetry enables a direct evaluation of the adequacy of tissue oxygenation while the measurement of quenching of Pd-porphyrine (PpIX) phosphorescence enables quantitative measurement of microvascular pO2. Combination of these two techniques provides information about the relation between microvascular oxygen content and parenchymal oxygen availability in Langendorff hearts. In normal myocardium there is heterogeneity at the microcirculatory level resulting in the existence of microcirculatory weak units, originating at the capillary level, which reoxygenate the slowest upon reoxygenation after an episode of ischemia. Sepsis and myocardial hypertrophia alter the pattern of oxygen transport whereby the microcirculation is disturbed at the arteriolar/arterial level. NADH fluorimetry also reveals a disturbance of mitochondrial oxygen availability in sepsis. Furthermore it is shown that these techniques can also be applied to various organs and tissues in vivo.

Technical advances in studying cardiac electrophysiology - Role of rabbit models

Cardiovascular research has made a major contribution to an unprecedented 10 year increase in life expectancy during the last 50 years: most of this increase due to a decline in mortality from heart disease and stroke. The majority of the basic cardiovascular science discoveries, which have led to this impressive extension of human life, came from investigations conducted in various small and large animal models, ranging from mouse to pig. The small animal models are currently popular because they are amenable to genetic engineering and are relatively inexpensive. The large animal models are favored at the translational stage of the investigation, as they are anatomically and physiologically more proximal to humans, and can be implanted with various devices; however, they are expensive and less amenable to genetic manipulations. With the advent of CRISPR genetic engineering technology and the advances in implantable bioelectronics, the large animal models will continue to advance. The rabbit model is particularly poised to become one of the most popular among the animal models that recapitulate human heart diseases. Here we review an array of the rabbit models of atrial and ventricular arrhythmias, as well as a range of the imaging and device technologies enabling these investigations.

Mitochondrial NAD(P)H In vivo: Identifying Natural Indicators of Oxidative Phosphorylation in the (31)P Magnetic Resonance Spectrum

Natural indicators provide intrinsic probes of metabolism, biogenesis and oxidative protection. Nicotinamide adenine dinucleotide metabolites (NAD(P)) are one class of indicators that have roles as co-factors in oxidative phosphorylation, glycolysis, and anti-oxidant protection, as well as signaling in the mitochondrial biogenesis pathway. These many roles are made possible by the distinct redox states (NAD(P)(+) and NAD(P)H), which are compartmentalized between cytosol and mitochondria. Here we provide evidence for detection of NAD(P)(+) and NAD(P)H in separate mitochondrial and cytosol pools in vivo in human tissue by phosphorus magnetic resonance spectroscopy ((31)P MRS). These NAD(P) pools are identified by chemical standards (NAD(+), NADP(+), and NADH) and by physiological tests. A unique resonance reflecting mitochondrial NAD(P)H is revealed by the changes elicited by elevation of mitochondrial oxidation. The decline of NAD(P)H with oxidation is matched by a stoichiometric rise in the NAD(P)(+) peak. This unique resonance also provides a measure of the improvement in mitochondrial oxidation that parallels the greater phosphorylation found after exercise training in these elderly subjects. The implication is that the dynamics of the mitochondrial NAD(P)H peak provides an intrinsic probe of the reversal of mitochondrial dysfunction in elderly muscle. Thus, non-invasive detection of NAD(P)(+) and NAD(P)H in cytosol vs. mitochondria yields natural indicators of redox compartmentalization and sensitive intrinsic probes of the improvement of mitochondrial function with an intervention in human tissues in vivo. These natural indicators hold the promise of providing mechanistic insight into metabolism and mitochondrial function in vivo in a range of tissues in health, disease and with treatment.

Oxygen demand of perfused heart preparations: how electromechanical function and inadequate oxygenation affect physiology and optical measurements

NEW FINDINGS

What is the topic of this review? This review discusses how the function and electrophysiology of isolated perfused hearts are affected by oxygenation and energy utilization. The impact of oxygenation on fluorescence measurements in perfused hearts is also discussed. What advances does it highlight? Recent studies have illuminated the inherent differences in electromechanical function, energy utilization rate and oxygen requirements between the primary types of excised heart preparations. A summary and analysis of how these variables affect experimental results are necessary to elevate the physiological relevance of these approaches in order to advance the field of whole-heart research. The ex vivo perfused heart recreates important aspects of in vivo conditions to provide insight into whole-organ function. In this review we discuss multiple types of ex vivo heart preparations, explain how closely each mimic in vivo function, and discuss how changes in electromechanical function and inadequate oxygenation of ex vivo perfused hearts may affect measurements of physiology. Hearts that perform physiological work have high oxygen demand and are likely to experience hypoxia when perfused with a crystalloid perfusate. Adequate myocardial oxygenation is critically important for obtaining physiologically relevant measurements, so when designing experiments the type of ex vivo preparation and the capacity of perfusate to deliver oxygen must be carefully considered. When workload is low, such as during interventions that inhibit contraction, oxygen demand is also low, which could dramatically alter a physiological response to experimental variables. Changes in oxygenation also alter the optical properties of cardiac tissue, an effect that may influence optical signals measured from both endogenous and exogenous fluorophores. Careful consideration of oxygen supply, working condition, and wavelengths used to acquire optical signals is critical for obtaining physiologically relevant measurements during ex vivo perfused heart studies.

An intact small animal model of myocardial ischemia-reperfusion: Characterization of metabolic changes by hyperpolarized 13C MR spectroscopy

Hyperpolarized carbon-13 magnetic resonance spectroscopy ((13)C MRS) enables the sensitive and noninvasive assessment of the metabolic changes occurring during myocardial ischemia-reperfusion. Ischemia-reperfusion models using hyperpolarized (13)C MRS are established in heart preparations ex vivo and in large animals in vivo, but an in vivo model in small animals would be advantageous to allow the study of reperfusion metabolism with neuroendocrine and inflammatory responses intact with the option to perform a greater number of experiments. A novel intact rat model of ischemia-reperfusion is presented that incorporates hyperpolarized (13)C MRS to characterize reperfusion metabolism. Typically, in an in vivo model, a tissue input function (TIF) is required to account for apparent changes in the metabolism of injected hyperpolarized [1-(13)C]pyruvate resulting from changes in perfusion. Whereas the measurement of a TIF by metabolic imaging is particularly challenging in small animals, the ratios of downstream metabolites can be used as an alternative. The ratio of [(13)C]bicarbonate:[1-(13)C]lactate (RatioBic/Lac) measured within 1-2 min after coronary release decreased vs. baseline in ischemic rats (n = 10, 15-min occlusion, controls: n = 10; P = 0.017 for interaction, 2-way ANOVA). The decrease in oxidative pyruvate metabolism [RatioBic/Lac(Ischemia)/RatioBic/Lac(Baseline)] modestly correlated with area at risk (r = 0.66; P = 0.002). Hyperpolarized (13)C MRS was also used to examine alanine production during ischemia, which is observed in ex vivo models, but no significant change was noted; metrics incorporating [1-(13)C]alanine did not substantially improve the discrimination of ischemic-reperfused myocardium from nonischemic myocardium. This intact rat model, which mimics the human situation of reperfused myocardial infarction, could be highly valuable for the testing of new drugs to treat reperfusion injury, thereby facilitating translational research.

Tethering of nicotinamide adenine dinucleotide inside hollow nanofibers for high-yield synthesis of methanol from carbon dioxide catalyzed by coencapsulated multienzymes

Enzymatic conversion of carbon dioxide (CO2) to fuel or chemicals is appealing, but is limited by lack of efficient technology for regeneration and reuse of expensive cofactors. Here we show that cationic polyelectrolyte-doped hollow nanofibers, which can be fabricated via a facile coaxial electrospinning technology, provide an ideal scaffold for assembly of cofactor and multienzymes capable of synthesizing methanol from CO2 through a cascade multistep reaction involving cofactor regeneration. Cofactor and four enzymes including formate, formaldehyde, alcohol, and glutamate dehydrogenases were in situ coencapsulated inside the lumen of hollow nanofibers by involving them in the core-phase solution for coaxial electrospinning, in which cationic polyelectrolyte was predissolved. The polyelectrolyte penetrating across the shell of the hollow nanofibers enabled efficient tethering and retention of cofactor inside the lumen via ion-exchange interactions between oppositely charged polyelectrolytes and cofactor. With carbonic anhydrase assembled on the outer surface of the hollow nanofibers for accelerating hydration of CO2, these five-enzymes-cofactor catalyst system exhibited high activity for methanol synthesis. Compared with methanol yield of only 36.17% using free enzymes and cofactor, the hollow nanofiber-supported system afforded a high value up to 103.2%, the highest reported value so far. It was believed that the linear polyelectrolytes acted as spacers to enhance the shuttling of cofactor between enzymes that were coencapsulated within near vicinity, thus improving the efficiency of the system. The immobilized system showed good stability in reusing. About 80% of its original productivity was retained after 10 reusing cycles, with a cofactor-based cumulative methanol yield reached 940.5%.

Mitochondrial reactive oxygen species (ROS) and ROS-induced ROS release

Byproducts of normal mitochondrial metabolism and homeostasis include the buildup of potentially damaging levels of reactive oxygen species (ROS), Ca(2+), etc., which must be normalized. Evidence suggests that brief mitochondrial permeability transition pore (mPTP) openings play an important physiological role maintaining healthy mitochondria homeostasis. Adaptive and maladaptive responses to redox stress may involve mitochondrial channels such as mPTP and inner membrane anion channel (IMAC). Their activation causes intra- and intermitochondrial redox-environment changes leading to ROS release. This regenerative cycle of mitochondrial ROS formation and release was named ROS-induced ROS release (RIRR). Brief, reversible mPTP opening-associated ROS release apparently constitutes an adaptive housekeeping function by the timely release from mitochondria of accumulated potentially toxic levels of ROS (and Ca(2+)). At higher ROS levels, longer mPTP openings may release a ROS burst leading to destruction of mitochondria, and if propagated from mitochondrion to mitochondrion, of the cell itself. The destructive function of RIRR may serve a physiological role by removal of unwanted cells or damaged mitochondria, or cause the pathological elimination of vital and essential mitochondria and cells. The adaptive release of sufficient ROS into the vicinity of mitochondria may also activate local pools of redox-sensitive enzymes involved in protective signaling pathways that limit ischemic damage to mitochondria and cells in that area. Maladaptive mPTP- or IMAC-related RIRR may also be playing a role in aging. Because the mechanism of mitochondrial RIRR highlights the central role of mitochondria-formed ROS, we discuss all of the known ROS-producing sites (shown in vitro) and their relevance to the mitochondrial ROS production in vivo.

3D IMAGING OF THE MITOCHONDRIAL REDOX STATE OF RAT HEARTS UNDER NORMAL AND FASTING CONDITIONS

The heart requires continuous ATP availability that is generated in the mitochondria. Although studies using the cell culture and perfused organ models have been carried out to investigate the biochemistry in the mitochondria in response to a change in substrate supply, mitochondrial bioenergetics of heart under normal feed or fasting conditions has not been studied at the tissue level with a sub-millimeter spatial resolution either in vivo or ex vivo. Oxidation of many food-derived metabolites to generate ATP in the mitochondria is realized through the NADH/NAD+ couple acting as a central electron carrier. We employed the Chance redox scanner - the low-temperature fluorescence scanner to image the three-dimensional (3D) spatial distribution of the mitochondrial redox states in heart tissues of rats under normal feeding or an overnight starvation for 14.5 h. Multiple consecutive sections of each heart were imaged to map three redox indices, i.e., NADH, oxidized flavoproteins (Fp, including flavin adenine dinucleotide (FAD)) and the redox ratio NADH/Fp. The imaging results revealed the micro-heterogeneity and the spatial distribution of these redox indices. The quantitative analysis showed that in the fasted hearts the standard deviation of both NADH and Fp, i.e., SD_NADH and SD_Fp, significantly decreased with a p value of 0.032 and 0.045, respectively, indicating that the hearts become relatively more homogeneous after fasting. The fasted hearts contained 28.6% less NADH (p = 0.038). No significant change in Fp was found (p = 0.4). The NADH/Fp ratio decreased with a marginal p value (0.076). The decreased NADH in the fasted hearts is consistent with the cardiac cells' reliance of fatty acids consumption for energy metabolism when glucose becomes scarce. The experimental observation of NADH decrease induced by dietary restriction in the heart at tissue level has not been reported to our best knowledge. The Chance redox scanner demonstrated the feasibility of 3D imaging of the mitochondrial redox state in the heart and provides a useful tool to study heart metabolism and function under normal, dietary-change and pathological conditions at tissue level.

NADH fluorescence imaging of isolated biventricular working rabbit hearts

Since its inception by Langendorff¹, the isolated perfused heart remains a prominent tool for studying cardiac physiology². However, it is not well-suited for studies of cardiac metabolism, which require the heart to perform work within the context of physiologic preload and afterload pressures. Neely introduced modifications to the Langendorff technique to establish appropriate left ventricular (LV) preload and afterload pressures³. The model is known as the isolated LV working heart model and has been used extensively to study LV performance and metabolism^4-6. This model, however, does not provide a properly loaded right ventricle (RV). Demmy et al. first reported a biventricular model as a modification of the LV working heart model^{7, 8}. They found that stroke volume, cardiac output, and pressure development improved in hearts converted from working LV mode to biventricular working mode⁸. A properly loaded RV also diminishes abnormal pressure gradients across the septum to improve septal function. Biventricular working hearts have been shown to maintain aortic output, pulmonary flow, mean aortic pressure, heart rate, and myocardial ATP levels for up to 3 hours⁸.

When studying the metabolic effects of myocardial injury, such as ischemia, it is often necessary to identify the location of the affected tissue. This can be done by imaging the fluorescence of NADH (the reduced form of nicotinamide adenine dinucleotide)^9-11, a coenzyme found in large quantities in the mitochondria. NADH fluorescence (fNADH) displays a near linearly inverse relationship with local oxygen concentration¹² and provides a measure of mitochondrial redox state¹³. fNADH imaging during hypoxic and ischemic conditions has been used as a dye-free method to identify hypoxic regions^{14, 15} and to monitor the progression of hypoxic conditions over time¹⁰.

The objective of the method is to monitor the mitochondrial redox state of biventricular working hearts during protocols that alter the rate of myocyte metabolism or induce hypoxia or create a combination of the two. Hearts from New Zealand white rabbits were connected to a biventricular working heart system (Hugo Sachs Elektronik) and perfused with modified Krebs-Henseleit solution¹⁶ at 37 °C. Aortic, LV, pulmonary artery, and left & right atrial pressures were recorded. Electrical activity was measured using a monophasic action potential electrode. To image fNADH, light from a mercury lamp was filtered (350±25 nm) and used to illuminate the epicardium. Emitted light was filtered (460±20 nm) and imaged using a CCD camera. Changes in the epicardial fNADH of biventricular working hearts during different pacing rates are presented. The combination of the heart model and fNADH imaging provides a new and valuable experimental tool for studying acute cardiac pathologies within the context of realistic physiological conditions.

Glucose suppression of glucagon secretion: metabolic and calcium responses from alpha-cells in intact mouse pancreatic islets

Glucagon is released from alpha-cells present in intact pancreatic islets at glucose concentrations below 4 mm, whereas higher glucose levels inhibit its secretion. The mechanisms underlying the suppression of alpha-cell secretory activity are poorly understood, but two general types of models have been proposed as follows: direct inhibition by glucose or paracrine inhibition from non-alpha-cells within the islet of Langerhans. To identify alpha-cells for analysis, we utilized transgenic mice expressing fluorescent proteins targeted specifically to these cells. Measurements of glucagon secretion from pure populations of flow-sorted alpha-cells show that contrary to its effect on intact islets, glucose does stimulate glucagon secretion from isolated alpha-cells. This observation argues against a direct inhibition of glucagon secretion by glucose and supports the paracrine inhibition model. Imaging of cellular metabolism by two-photon excitation of NAD(P)H autofluorescence indicates that glucose is metabolized in alpha-cells and that glucokinase is the likely rate-limiting step in this process. Imaging calcium dynamics of alpha-cells in intact islets reveals that inhibiting concentrations of glucose increase the intracellular calcium concentration and the frequency of alpha-cell calcium oscillations. Application of candidate paracrine inhibitors leads to reduced glucagon secretion but did not decrease the alpha-cell calcium activity. Taken together, the data suggest that suppression occurs downstream from alpha-cell calcium signaling, presumably at the level of vesicle trafficking or exocytotic machinery.

High resolution contrast ultrasound and NADH fluorescence imaging of myocardial perfusion in excised rat hearts

Simultaneous imaging of myocardial flow and hypoxia could be vital for identifying acute ischemic mechanisms that may trigger an arrhythmia. We have studied the distribution of flow and hypoxia in excised locally ischemic rat hearts using simultaneous contrast ultrasound imaging and beta-nicotinamide adenine dinucleotide (NADH) fluorescence imaging. Local ischemia was induced by controlling flow within a major coronary artery. Intra-myocardial flow was imaged using contrast high-resolution ultrasound (linear probe; 13-6 MHz). An ultrasound contrast agent (UCA) was used to highlight the ischemic border. We observed distinct borders between two perfusion beds. UCA images showed high contrast borders of flow. The progression of UCA through the tissue was clearly visible. Intramyocardial regions of flow overlap could be identified by superimposing images of UCA from two perfusion zones. Borders between hypoxic and normoxic tissue were clearly revealed by increased NADH fluorescence. Hypoxic borders were oriented along borders of flow. In summary, simultaneous ultrasound and NADH imaging of excised hearts from small animals provide high fidelity images for characterizing the distribution of flow and hypoxic tissue during acute localized ischemia.

Mitochondrial NADH fluorescence is enhanced by complex I binding

Mitochondrial NADH fluorescence has been a useful tool in evaluating mitochondrial energetics both in vitro and in vivo. Mitochondrial NADH fluorescence is enhanced several-fold in the matrix through extended fluorescence lifetimes (EFL). However, the actual binding sites responsible for NADH EFL are unknown. We tested the hypothesis that NADH binding to Complex I is a significant source of mitochondrial NADH fluorescence enhancement. To test this hypothesis, the effect of Complex I binding on NADH fluorescence efficiency was evaluated in purified protein, and in native gels of the entire porcine heart mitochondria proteome. To avoid the oxidation of NADH in these preparations, we conducted the binding experiments under anoxic conditions in a specially designed apparatus. Purified intact Complex I enhanced NADH fluorescence in native gels approximately 10-fold. However, no enhancement was detected in denatured individual Complex I subunit proteins. In the Clear and Ghost native gels of the entire mitochondrial proteome, NADH fluorescence enhancement was localized to regions where NADH oxidation occurred in the presence of oxygen. Inhibitor and mass spectroscopy studies revealed that the fluorescence enhancement was specific to Complex I proteins. No fluorescence enhancement was detected for MDH or other dehydrogenases in this assay system, at physiological mole fractions of the matrix proteins. These data suggest that NADH associated with Complex I significantly contributes to the overall mitochondrial NADH fluorescence signal and provides an explanation for the well established close correlation of mitochondrial NADH fluorescence and the metabolic state.

Effects of anoxia and aglycemia on cytosolic calcium regulation in rat sensory neurons

Nociceptive neurons play an important role in ischemia by sensing and transmitting information to the CNS and by secreting peptides and nitric oxide, which can have local effects. While these responses are probably primarily mediated by acid sensing channels, other events occurring in ischemia may also influence neuron function. In this study, we have investigated the effects of anoxia and anoxic aglycemia on Ca²⁺ regulation in sensory neurons from rat dorsal root ganglia. Anoxia increased [Ca²⁺]_i by evoking Ca²⁺ release from two distinct internal stores one sensitive to carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP) and one sensitive to caffeine, cyclopiazonic acid (CPA), and ryanodine [assumed to be the endoplasmic reticulum (ER)]. Anoxia also promoted progressive decline in ER Ca²⁺ content. Despite partially depolarizing mitochondria, anoxia had relatively little effect on mitochondrial Ca²⁺ uptake when neurons were depolarized but substantially delayed mitochondrial Ca²⁺ release and subsequent Ca²⁺ clearance from the cytosol on repolarization. Anoxia also reduced both sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) activity and Ca²⁺ extrusion [probably via plasma membrane Ca²⁺-ATPase (PMCA)]. Thus anoxia has multiple effects on [Ca²⁺]_i homeostasis in sensory neurons involving internal stores, mitochondrial buffering, and Ca²⁺ pumps. Under conditions of anoxic aglycemia, there was a biphasic and more profound elevation of [Ca²⁺]_i, which was associated with complete ER Ca²⁺ store emptying and progressive, and eventually complete, inhibition of Ca²⁺ clearance by PMCA and SERCA. These data clearly show that loss of oxygen, and exhaustion of glycolytic substrates, can profoundly affect many aspects of cell Ca²⁺ regulation, and this may play an important role in modulating neuronal responses to ischemia.

Optimizing normoxic conditions in liver devices using enhanced gel matrices

For in vitro liver replacement devices, such as packed bed bioreactors, to maintain the essential functions of the liver, they must at least successfully support hepatocytes, the parenchymal cell of the liver. In vivo, the liver is a major consumer of oxygen. Hence it is unsurprising that the limited transport distance of oxygen (O(2)) governs the dimensions of the cellular space of engineered devices. Because cellular space capacity directly affects the device's performance, O(2) transport is a critical issue in the scale up of bioreactor designs. In the current investigation, the microporosity of the extracellular matrix (ECM) has been modified to further improve O(2) transport in packed bed devices beyond that previously reported in the literature. These improvements to the O(2) enhancement technique enabled O(2) transport distances of 481.7 +/- 12.5 microm to be achieved under acellular conditions; and distances of 418.1 +/- 6.0 microm to be attained in the presence of 1 million hepatocytes. Both values are significantly greater than the 170 microm baseline attained when 10(6) hepatocytes are packed within normal non-enhanced ECM gels. The study's results also illustrate that the O(2) enhancement technique has the added benefit of preventing regions of severe hypoxia and hyperoxia from developing within the cellular space. As such, enhanced ECM gels enable packed hepatocytes to maintain better hepatocellular metabolic status than is possible with normal non-enhanced gels.

Characterization of in vivo tissue redox status, oxygenation, and formation of reactive oxygen species in postischemic myocardium

The current study aims to characterize the alterations of in vivo tissue redox status, oxygenation, formation of reactive oxygen species (ROS), and their effects on the postischemic heart. Mouse heart was subjected to 30 min LAD occlusion, followed by 60 min reperfusion. In vivo myocardial redox status and oxygenation were measured with electron paramagnetic resonance (EPR). In vivo tissue NAD(P)H and formation of ROS were monitored with fluorometry. Tissue glutathione/glutathione disulfide (GSH/GSSG) levels were detected with high-performance liquid chromatography (HPLC). These experiments demonstrated that tissue reduction rate of nitroxide was increased 100% during ischemia and decreased 33% after reperfusion compared to the nonischemic tissue. There was an overshoot of tissue oxygenation after reperfusion. Tissue NAD(P)H levels were increased during and after ischemia. There was a burst formation of ROS at the beginning of reperfusion. Tissue GSH/GSSG level showed a 48% increase during ischemia and 29% decrease after reperfusion. In conclusion, the hypoxia during ischemia limited mitochondrial respiration and caused a shift of tissue redox status to a more reduced state. ROS generated at the beginning of reperfusion caused a shift of redox status to a more oxidized state, which may contribute to the postischemic myocardial injury.

Temperature dependence for fluorescence of beta-NADH in glycerol/water solution and in trehalose/sucrose glass

Fluorescence imaging of cells and tissue can be used to evaluate beta-NADH redox and location. At low temperature, beta-NADH fluorescence intensity increases and therefore sensitivity of imaging increases. In this paper, the temperature dependence of fluorescence was evaluated for beta-NADH in glycerol/water solution and in trehalose/sucrose glass. The average fluorescence lifetime for NADH in glycerol/water is 0.66 ns, compared with 5.3 ns in trehalose/ sucrose at 20 degrees C. Emission spectra were recorded from 290 to 12 K. The fluorescence of beta-NADH in glycerol/water increases approximately 16 fold and the emission shifts about 35 nm to the blue as temperature decreases. Much smaller change is seen for fluorescence of beta-NADH in sugar glass. Below 77 K, the beta-NADH spectral features did not change significantly with temperature change, and so no increase in sensitivity is obtained by going to very low temperatures. It is suggested that the sensitivity of beta-NADH fluorescence is related to water relaxation around the excited state molecule. Differences in water in various tissues may contribute to beta-NADH fluorescence changes when cells are altered.

O2 delivery and redox state are determinants of compartment-specific reactive O2 species in myocardial reperfusion

Reperfusion of the ischemic myocardium leads to a burst of reactive O(2) species (ROS), which is a primary determinant of postischemic myocardial dysfunction. We tested the hypothesis that early O(2) delivery and the cellular redox state modulate the initial myocardial ROS production at reperfusion. Isolated buffer-perfused rat hearts were loaded with the fluorophores dihydrofluorescein or Amplex red to detect intracellular and extracellular ROS formation using surface fluorometry at the left ventricular wall. Hearts were made globally ischemic for 20 min and then reperfused with either 95% or 20% O(2)-saturated perfusate. The same protocol was repeated in hearts loaded with dihydrofluorescein and perfused with either 20 or 5 mM glucose-buffered solution to determine relative changes in NADH and FAD. Myocardial O(2) delivery during the first 5 min of reperfusion was 84.7 +/- 4.2 ml O(2)/min with 20% O(2)-saturated buffer and 354.4 +/- 22.8 ml O(2)/min with 95% O(2) (n = 8/group, P < 0.001). The fluorescein signal (intracellular ROS) was significantly increased in hearts reperfused with 95% O(2) compared with 20% O(2). However, the resorufin signal (extracellular ROS) was significantly increased with 20% O(2) compared with 95% O(2) during reperfusion. Perfusion of hearts with 20 mM glucose reduced the (.)NADH during ischemia (P < 0.001) and the (.)ROS at reperfusion (P < 0.001) compared with 5.5 mM-perfused glucose hearts. In conclusion, initial O(2) delivery to the ischemic myocardium modulates a compartment-specific ROS response at reperfusion such that high O(2) delivery promotes intracellular ROS and low O(2) delivery promotes extracellular ROS. The redox state that develops during ischemia appears to be an important precursor for reperfusion ROS production.

Regional cardiac tissue oxygenation as a function of blood flow and pO2: A near-infrared spectroscopic imaging study

Near-infrared spectroscopic imaging (NIRSI) is useful to assess cardiac tissue oxygenation in arrested and beating hearts, and it shows potential as an intraoperative gauge of the effectiveness of bypass grafting. The purpose of this study was to determine whether NIRSI can reliably differentiate among a range of cardiac oxygenation states, using ischemia and hypoxia models independently. An ischemia-reperfusion model was applied to isolated, beating, blood-perfused porcine hearts, in which the left anterior descending (LAD) artery was cannulated. LAD flow was decreased stepwise to approximately 50, 20, and 0% of normal flow and was completely restored between ischemic episodes. Upon completion of the ischemia-reperfusion protocol, the hearts were further subjected to periods of increasingly severe global hypoxia. Regional oxy- and deoxy-hemoglobin (myoglobin) levels were derived from spectroscopic images (650 to 1050 nm) acquired at each step. Oxygenation maps vividly highlighted the area at risk for all degrees of ischemia. Oxygenation values differed significantly for different LAD flow rates, regardless of whether intermediate reperfusion was applied, and oxygenation values during progressive hypoxia correlated well with blood oxygen saturation. These results suggest that NIRSI is well suited, not only to identify ischemic or hypoxic regions of cardiac tissue, but also to assess the severity of deoxygenation.

Flow-independent myocardial ischemia induced by endothelin-1: an NADH fluorescence analysis

The endothelin-1 (ET-1) is known to cause myocardial ischemia; however, whether this effect is entirely dependent on vasoconstriction is uncertain. The aim of this study was to characterize the myocardial ischemia after the intracoronary administration of endothelin-1, and compare it with that induced by coronary stenosis. In the left anterior descending coronary artery of 15 dogs, a mild inflow reduction (30%) was produced for 10 minutes using intracoronary endothelin-1 (46 +/- 33 pmol/min) or coronary stenosis. The hearts were rapidly cross-sectioned at short axial plane and freeze-clamped within 120 milliseconds using a specially developed device to visualize and quantify the area of ischemia (%IA) with NADH fluorescence photography. The %IA was larger in the endothelin-1 group than in the stenosis group (66 +/- 23 versus 18 +/- 18, P = 0.0005); furthermore, the ischemia was transmural in the ET-1 group, but limited to subendocardium in the stenosis group. ET-1 increased the coronary arterial resistance especially in subepicardial region and produced smaller ischemic foci in microcirculation. The mechanism of larger ischemia produced by ET-1 might depend on pro-ischemic effects on myocytes and vasoconstriction of the coronary microcirculation, predominantly in the subepicardium in vivo.

Spectroscopic imaging for detection of ischemic injury in rat kidneys by use of changes in intrinsic optical properties

It is currently impossible to consistently predict kidney graft viability and function before and after transplantation. We explored optical spectroscopy to assess the degree of ischemic damage in kidney tissue. Tunable UV laser excitation was used to record autofluorescence images, at different spectral ranges, of injured and contralateral control rat kidneys to reveal the excitation conditions that offered optimal contrast. Autofluorescence and near-infrared cross-polarized light-scattering imaging were both used to monitor changes in intensity and spectral characteristics, as a function of exposure time to ischemic injury. These two modalities provided different temporal behaviors, arguably arising from two different mechanisms providing direct correlation of intrinsic optical signatures to ischemic injury time.

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.

NADH enzyme-dependent fluorescence recovery after photobleaching (ED-FRAP): applications to enzyme and mitochondrial reaction kinetics, in vitro

NADH enzyme-dependent fluorescence recovery after photobleaching (ED-FRAP) was evaluated for studying enzyme kinetics in vitro and in isolated mitochondria. Mass, optical, and nuclear magnetic resonance spectroscopy data were consistent with the UV NADH photolysis reaction being NADH --> NAD* + H+ + e-. The overall net reaction was O2 + 2NADH + 2H+ --> 2NAD+ + 2H2O, or in the presence of other competing electron acceptors such as cytochrome c, NADH + 2Cyt(ox) --> NAD+ + H+ + 2Cyt(red). Solution pH could differentiate between these free-radical scavenging pathways. These net reactions represent the photooxidation of NADH to NAD+. Kinetic models and acquisition schemes were developed, varying [NADH] and [NAD] by altering NADH photolysis levels, for extracting kinetic parameters. UV irradiation levels used did not damage mitochondrial function or enzymatic activity. In mitochondria, [NADH] is a high affinity product inhibitor that significantly reduced the NADH regeneration rate. Matrix NADH regeneration only slightly exceeded the net rate of NADH consumption, suggesting that the NADH regeneration process is far from equilibrium. Evaluation of NADH regeneration in active mitochondria, in comparison to rotenone-treated preparations, revealed other regulatory elements in addition to matrix [NADH] and [NAD] that have yet to be fully characterized. These studies demonstrate that the rapid UV photolysis of NADH to NAD is an effective tool in evaluating the steady-state kinetic properties of enzyme systems. Initial data support the notion that the NADH regeneration process is far from equilibrium in mitochondria and is potentially controlled by NADH levels as well as several other matrix factors.

Myocardial contractile function during postischemic low-flow reperfusion: critical thresholds of NADH and O2 delivery

The degree of myocardial oxygen delivery (Do2) that is necessary to reestablish functional contractile activity after short-term global ischemia in heart is not known. To determine the relationship between Do2 and recovery of contractile and metabolic functions, we used tissue NADH fluorometric changes to characterize adequacy of reperfusion flow. Isolated perfused rat hearts were subjected to global ischemia and were reperfused at variable flow rates that ranged from 1 to 100% of baseline flow. Myocardial function and tissue NADH changes were continuously measured. NADH fluorescence rapidly increased and plateaued during ischemia. A strong inverse logarithmic correlation between NADH fluorescence and reperfusion Do2 was demonstrated (r = -0.952). Left ventricular function (rate-pressure product) was inversely related to NADH fluorescence at reperfusion flows from 25 to 100% of baseline (r = -0.922) but not at lower reperfusion flow levels. An apparent reperfusion threshold of 25% of baseline Do2 was necessary to resume contractile function. At very low reperfusion flows (1% of baseline), another threshold flow was identified at which NADH levels increased beyond that observed during global ischemia (3.4 +/- 3.0%, means +/- SE, n = 9), which suggests further reduction of the cellular redox state. This NADH increase at 1% of baseline reperfusion flow was blocked by removing glucose from the perfusate. NADH fluorescence is a sensitive indicator of myocardial cellular oxygen utilization over a wide range of reperfusion Do2 values. Although oxygen is utilized at very low flow rates, as indicated by changes in NADH, a critical threshold of approximately 25% of baseline Do2 is necessary to restore contractile function after short-term global ischemia.

Reagentless detection of microorganisms by intrinsic fluorescence

Quick and accurate detection of microbial contamination is accomplished by a unique combination of leading-edge technologies described in this and the accompanying paper. In this contribution, a hand-held prototype instrument is described which is capable of statistically sampling the environment for microbial contamination and determining cell viability. The technology is sensitive enough to detect very low levels ( approximately 20 cells/cm(2) or cm(3)) of microbes in seconds.

Commonly used numbers of microspheres affect cardiac vascular resistance

The main goal of the study was to examine how the microsphere technique affects the hemodynamics and mitochondrial energy status of the Langendorff-perfused rat heart. The hearts were perfused at a constant flow with Tyrode solution. NADH videofluorometry of the surface of the left ventricle was used to record the mitochondrial energy status as indication of regional ischemia. The effects of seven successive (separated by 10 min) injections of 0.1 ml of saline or (0.05% Tween 20; polysorbate 20, Sigma-Aldrich, St. Louis, MO, U.S.A.) or (0.05% Tween 20 + microspheres) were studied. The number of microspheres per injection were: #1 (2,500), #2 (5,000), #3 (10,000), #4 (20,000), #5 (40,000), #6 (40000), and #7 (80000). The anti-aggregation agent Tween always caused a biphasic response in perfusion pressure. Compared with the Tween effect, the injection of microspheres caused an initial change (mm Hg) in perfusion pressure of #1 (-10), #2 (NS), #3 (NS), #4 (+7.5), #5 (+12.3), #6 (+14.4), #7 (+18.3), and a delayed change (10 min after injection) of #1 (-22.2), #2 (-6.0), #3 (-4.1), #4 (-4.5), #5 (NS), #6 (NS), and #7 (+5.9). The microspheres caused a significant delayed increase in NADH only for injection #6 and #7. Similar results were found for different durations of the input function or when hearts were perfused at constant perfusion pressure. In hearts without flow reserve (10 microM adenosine), Tween injections were without effect, whereas three successive injections of 60,000 microspheres each only caused increases in perfusion pressure and NADH. The data demonstrate that in hearts with flow reserve present, even very low numbers of microspheres (2,000/g heart) cause large decreases in perfusion pressure without obvious signs of ischemia. When flow reserve was exhausted by either microsphere loading or adenosine addition, microspheres only caused increases in perfusion pressure and resulted in detectable ischemia (NADH). It is concluded that microspheres affect the vascular resistance of the heart and that these effects are flow reserve dependent.

Non-invasive glucose monitoring by NAD(P)H autofluorescence spectroscopy in fibroblasts and adipocytes: a model for skin glucose sensing

The aim of this study was to develop an in vitro cell-culture model of skin-component cells to test the hypothesis that glucose can be monitored non-invasively by measuring NAD(P)H-related fluorescence changes in tissues. 3T3-L1 fibroblasts and adipocytes were grown in culture, and the response to added glucose was assessed by changes in steady-state autofluorescence at 400-500 nm [excitation at 340 nm, an index of NAD(P)H]. We also studied glucose-related fluorescence changes in cells stained with the mitochondrial marker, rhodamine-123. Fibroblasts and adipocytes showed glucose-dependent increases in autofluorescence with both short- and long-term exposure. Spectral properties indicated that the fluorescence was due to NAD(P)H production. With 5-h exposure to glucose, the maximal response was at 10-15 mmol/L glucose. Cells stained with the fluorescent mitochondrial marker, rhodamine-123, showed an immediate and marked decrease in fluorescence when exposed to glucose. We conclude that glucose can be sensed non-invasively by cellular fluorescence changes in fibroblasts and adipocytes. This is a model for the further exploration of fluorescence-based non-invasive metabolic monitoring in human diabetes.

Coronary microcirculation: physiology and pharmacology

Coronary microvessels play a pivotal role in determining the supply of oxygen and nutrients to the myocardium by regulating the coronary flow conductance and substance transport. Direct approaches analyzing the coronary microvessels have provided a large body of knowledge concerning the physiological and pharmacological characteristics of the coronary circulation, as has the rapid accumulation of biochemical findings about the substances that mediate vascular functions. Myogenic and flow-induced intrinsic vascular controls that determine basal tone have been observed in coronary microvessels in vitro. Coronary microvascular responses during metabolic stimulation, autoregulation, and reactive hyperemia have been analyzed in vivo, and are known to be largely mediated by metabolic factors, although the involvement of other factors should also be taken into account. The importance of ATP-sensitive K(+) channels in the metabolic control has been increasingly recognized. Furthermore, many neurohumoral mediators significantly affect coronary microvascular control in endothelium-dependent and -independent manners. The striking size-dependent heterogeneity of microvascular responses to all of these intrinsic, metabolic, and neurohumoral factors is orchestrated for optimal perfusion of the myocardium by synergistic and competitive interactions. The regulation of coronary microvascular permeability is another important factor for the nutrient supply and for edema formation. Analyses of collateral microvessels and subendocardial microvessels are important for understanding the pathophysiology of ischemic hearts and hypertrophied hearts. Studies of the microvascular responses to drugs and of the impairment of coronary microvessels in diseased conditions provide useful information for treating microvascular dysfunctions. In this article, the endogenous regulatory system and pharmacological responses of the coronary circulation are reviewed from the microvascular point of view.

Laser induced fluorescence attenuation spectroscopy: detection of hypoxia

The development of a new laser-induced fluorescence (LIF) spectroscopy technique for the measurement of the attenuation spectrum of tissue is described. The technique, termed laser-induced fluorescence attenuation spectroscopy (LIFAS), has been applied to study the effects of hypoxia on the in vivo optical properties of renal and myocardial tissue in the 350-600-nm band. Excimer laser (Xe-Cl) is used to excite a small volume of the tissue (rabbit model, N = 20) and induce autofluorescence. The emitted LIF is monitored fiberoptically at two locations that are unevenly displaced about the fluorescing volume. The optical attenuation of the tissue is calculated from the dual LIF measurements by assuming an exponential decay of the fluorescence with distance. The results indicate that hypoxia modulates the attenuation spectrum leading to characteristic changes in its shape. Primarily, the spectral profile becomes more concave between 455 nm and 505 nm and two spectral peaks at about 540 and 580 nm disappear leaving in their place a single peak at about 555 nm. The attenuation spectra of normoxic and hypoxic tissue are used to train partial least squares multivariate model for spectral classification. The model detected acute renal and myocardial hypoxia with an accuracy greater than 90% (range: 90%-96%) and 74% (range: 74%-90%), respectively.

Mapping of capillary flow, cellular redox state, and resting membrane potential in hypoperfused rat myocardium

The influence on myocyte viability of ischemia-induced changes in capillary perfusion was studied in the hearts of anesthetized rats subjected to partial occlusion of the left coronary artery for 45 min. Timed plasma labeling was applied to determine perfusion patterns. Changes in the fluorescence of preloaded potential-sensitive dyes [tetramethylrhodamine methyl ester (TMRM) and bis-oxonol], of trypan blue, and of endogeneous NADH were utilized in characterizing myocyte viability in histological sections of the heart. Within the hypoperfused zone, localized areas appeared vascularly nonlabeled for periods of at least 10 min. Within these areas a reduction in TMRM fluorescence occurred in 82. 5% of the tissue, signaling a reduced resting membrane potential. In the same areas 37.7% of the myocytes revealed an NADH fluorescence lower than that regularly found in anoxic tissues. This correlated with an especially low level of TMRM, with increased fluorescence bis-oxonol and with an accumulation of trypan blue. In conclusion, in localized hypoperfusion-induced zones lacking capillary flow, an inhomogeneous pattern of reductions in myocyte viability develops, which appears to be relevant in ischemia-induced arrhythmias.