The kinetics of flavoprotein and pyridine nucleotide oxidation in cardiac mitochondria in the presence of calcium

Rapid changes in NADH and flavin autofluorescence in rat cardiac trabeculae reveal large mitochondrial complex II reserve capacity

KEY POINTS

A photometry-based technique was developed to measure nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD) autofluorescence and contractile properties simultaneously in intact rat trabeculae at a high time resolution. This provides insight into the function of mitochondrial complex I and II. Maximal complex I and complex II activities were determined in saponin-permeabilized right ventricular tissue by respirometry. In trabeculae, complex II function was considerably smaller than the maximal complex II activity, suggesting large complex II reserve capacity. Up-down asymmetry in NADH and FAD kinetics suggests a complex interaction between mitochondrial and contractile function. These data show that simultaneous measurement of contractile properties and NADH and FAD kinetics in cardiac trabeculae provides a mean to study the differences in complex I and II function in intact preparations in health and disease.

ABSTRACT

The functional properties of cardiac mitochondria in intact preparations have been mainly studied by measurements of nicotinamide adenine dinucleotide (NADH) autofluorescence, which reflects mitochondrial complex I function. To assess complex II function, we extended this method by measuring flavin adenine dinucleotide (FAD)-related autofluorescence in electrically stimulated cardiac trabeculae isolated from the right ventricle from the rat at 27°C. NADH and FAD autofluorescence and tension responses were measured when stimulation frequency was increased from 0.5 Hz to 1, 2 or 3 Hz for 3 min, and thereafter decreased to 0.5 Hz. Maximal complex I and complex II activity in vitro were determined in saponin-permeabilized right ventricular tissue by respirometry. NADH responses upon an increase in stimulation frequency showed a rapid decline, followed by a slow recovery towards the initial level. FAD responses followed a similar time course, but in the opposite direction. The amplitudes of early rapid changes in the NADH and FAD concentration correlated well with the change in tension time integral per second (R(2)  = 0.833 and 0.660 for NADH and FAD, respectively), but with different slopes for the up and down transient. Maximal velocity of the increase in FAD concentration (16 ± 4 μm s(-1) ), measured upon an increase in stimulation frequency from 0.5 to 3 Hz was considerably smaller than that of the decrease in NADH (78 ± 13 μm s(-1) ). The respiration measurements indicated that the maximal velocity of NADH utilization (143 ± 14 μm s(-1) ) was 2 times smaller than that of FADH2 (291 ± 19 μm s(-1) ). This indicates that in cardiac mitochondria considerable complex II activity reserve is present.

Autofluorescence imaging of NADH and flavoproteins in the rat brain: insights from Monte Carlo simulations

There has been recently a renewed interest in using Autofluorescence imaging (AF) of NADH and flavoproteins (Fp) to map brain activity in cortical areas. The recording of these cellular signals provides complementary information to intrinsic optical imaging based on hemodynamic changes. However, which of NADH or Fp is the best candidate for AF functional imaging is not established, and the temporal profile of AF signals is not fully understood. To bring new theoretical insights into these questions, Monte Carlo simulations of AF signals were carried out in realistic models of the rat somatosensory cortex and olfactory bulb. We show that AF signals depend on the structural and physiological features of the brain area considered and are sensitive to changes in blood flow and volume induced by sensory activation. In addition, we demonstrate the feasibility of both NADHAF and Fp-AF in the olfactory bulb.

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.

Flavoprotein autofluorescence imaging in the cerebellar cortex in vivo

Autofluorescence optical imaging is rapidly becoming a widely used tool for mapping activity in the central nervous system function in vivo and investigating the coupling among neurons, glia, and metabolism. This paper provides a brief review of autofluorescence and of our recent work using flavoprotein imaging in the cerebellar cortex. Stimulation of the parallel fibers evokes an intrinsic fluorescence signal that is tightly coupled to neuronal activation and primarily generated postsynaptically. The signal originates from mitochondrial flavoproteins. The signal is biphasic, with the initial increase in fluorescence (light phase) resulting from the oxidation of flavoproteins and the subsequent decrease (dark phase) from the reduction of flavoproteins. The light phase is primarily neuronal, and the dark phase is primarily glial. Exploiting the spatial properties of molecular layer inhibition in the cerebellar cortex, we show that flavoprotein autofluorescence can monitor both excitatory and inhibitory activity in the cerebellar cortex. Furthermore, flavoprotein autofluorescence has revealed that molecular layer inhibition is organized into parasagittal domains that differentially modulate the spatial pattern of cerebellar cortical activity. The reduction in flavoprotein autofluorescence occurring in the inhibitory bands most likely reflects a decrease in intracellular Ca(2+) in the neurons inhibited by the molecular layer interneurons. Therefore, flavoprotein autofluorescence imaging is providing new insights into cerebellar cortical function and neurometabolic coupling.

Modeling mitochondrial function

The mitochondrion represents a unique opportunity to apply mathematical modeling to a complex biological system. Understanding mitochondrial function and control is important since this organelle is critical in energy metabolism as well as playing key roles in biochemical synthesis, redox control/signaling, and apoptosis. A mathematical model, or hypothesis, provides several useful insights including a rigorous test of the consensus view of the operation of a biological process as well as providing methods of testing and creating new hypotheses. The advantages of the mitochondrial system for applying a mathematical model include the relative simplicity and understanding of the matrix reactions, the ability to study the mitochondria as a independent contained organelle, and, most importantly, one can dynamically measure many of the internal reaction intermediates, on line. The developing ability to internally monitor events within the metabolic network, rather than just the inflow and outflow, is extremely useful in creating critical bounds on complex mathematical models using the individual reaction mechanisms available. However, many serious problems remain in creating a working model of mitochondrial function including the incomplete definition of metabolic pathways, the uncertainty of using in vitro enzyme kinetics, as well as regulatory data in the intact system and the unknown chemical activities of relevant molecules in the matrix. Despite these formidable limitations, the advantages of the mitochondrial system make it one of the best defined mammalian metabolic networks that can be used as a model system for understanding the application and use of mathematical models to study biological systems.

Cerebellar cortical molecular layer inhibition is organized in parasagittal zones

Molecular layer inhibitory interneurons generate on-beam and off-beam inhibition in the cerebellar cortex that is hypothesized to control the timing and/or spatial patterning of Purkinje cell discharge. On- and off-beam inhibition has been assumed to be spatially uniform and continuous within a folium. Using flavoprotein autofluorescence optical imaging in the mouse cerebellar cortex in vivo, this study demonstrates that the inhibition evoked by parallel fiber and peripheral stimulation results in parasagittal bands of decreases in fluorescence that correspond to zebrin II-positive bands. The parasagittal bands of decreased fluorescence are abolished by GABA(A) antagonists and reflect the activity of molecular layer interneurons on their targets. The same banding pattern was observed using Ca2+ imaging. The bands produce spatially specific decreases in the responses to peripheral input. Therefore, molecular layer inhibition is compartmentalized into zebrin II parasagittal domains that differentially modulate the spatial pattern of cerebellar cortical activity.

Mitochondrial function in vivo evaluated by NADH fluorescence: from animal models to human studies

Normal mitochondrial function is a critical factor in maintaining cellular homeostasis in various organs of the body. Due to the involvement of mitochondrial dysfunction in many pathological states, the real-time in vivo monitoring of the mitochondrial metabolic state is crucially important. This type of monitoring in animal models as well as in patients provides real-time data that can help interpret experimental results or optimize patient treatment. The goals of the present review are the following: 1) to provide an historical overview of NADH fluorescence monitoring and its physiological significance; 2) to present the solid scientific ground underlying NADH fluorescence measurements based on published materials; 3) to provide the reader with basic information on the methodologies used in the past and the current state of the art fluorometers; and 4) to clarify the various factors affecting monitored signals, including artifacts. The large numbers of publications by different groups testify to the valuable information gathered in various experimental conditions. The monitoring of NADH levels in the tissue provides the most important information on the metabolic state of the mitochondria in terms of energy production and intracellular oxygen levels. Although NADH signals are not calibrated in absolute units, their trend monitoring is important for the interpretation of physiological or pathological situations. To understand tissue function better, the multiparametric approach has been developed where NADH serves as the key parameter. The development of new light sources in UV and visible spectra has led to the development of small compact units applicable in clinical conditions for better diagnosis of patients.

Mitochondrial K(ATP) channels in cell survival and death

Since the discovery of the mitochondrial ATP-sensitive potassium channel (mitoK(ATP)) more than 13 years ago, it has been implicated in the processes of ischemic preconditioning (IPC), apoptosis and mitochondrial matrix swelling. Different approaches have been employed to characterize the pharmacological profile of the channel, and these studies strongly suggest that cellular protection well correlates with the opening of mitoK(ATP). However, there are many questions regarding mitoK(ATP) that remain to be answered. These include the very existence of mitoK(ATP) itself, its degree of importance in the process of IPC, its response to different pharmacological agents, and how its activation leads to the process of IPC and protection against cell death. Recent findings suggest that mitoK(ATP) may be a complex of multiple mitochondrial proteins, including some which have been suggested to be components of the mitochondrial permeability transition pore. However, the identity of the pore-forming unit of the channel and the details of the interactions between these proteins remain unclear. In this review, we attempt to highlight the recent advances in the physiological role of mitoK(ATP) and discuss the controversies and unanswered questions.

12-(9-Anthroyl)stearic acid, a fluorescent probe for the ubiquinone region of the mitochondrial membrane

1. 12-(9-Anthroyl)stearic acid can be incorporated into mitochondrial membranes. 2. The fluorescence properties of the membrane-bound probe are different from those of the free molecule. 3. The intensity of emission and fluorescence life-time of the probe is enhanced when, in the presence of substrate, the electron-transport chain is reduced. 4. This change in intensity has been demonstrated to be a result of collisional quenching by oxidised ubiquinone in the oxidised membrane but not when the respiratory chain is in the reduced state. 5. In pulsing anaerobic mitochondria with oxygen the rate of the fluorescence change is found to be slower than the rate of ubiquinone oxidation, suggesting that the probe detects a structural transition in the mitochondrial inner membrane. 6. This transition results in a constraint on ubiquinone motion in the reduced system. Model experiments, using lipid dispersions, have been carried out to test some of the interpretations.

Mechanisms for intracellular calcium regulation in heart. I. Stopped-flow measurements of Ca++ uptake by cardiac mitochondria

Initial velocities of energy-dependent Ca(++) uptake were measured by stopped-flow and dual-wavelength techniques in mitochondria isolated from hearts of rats, guinea pigs, squirrels, pigeons, and frogs. The rate of Ca(++) uptake by rat heart mitochondria was 0.05 nmol/mg/s at 5 microM Ca(++) and increased sigmoidally to 8 nmol/mg/s at 200 microM Ca(++). A Hill plot of the data yields a straight line with slope n of 2, indicating a cooperativity for Ca(++) transport in cardiac mitochondria. Comparable rates of Ca(++) uptake and sigmoidal plots were obtained with mitochondria from other mammalian hearts. On the other hand, the rates of Ca(++) uptake by frog heart mitochondria were higher at any Ca(++) concentrations. The half-maximal rate of Ca(++) transport was observed at 30, 60, 72, 87, 92 microM Ca(++) for cardiac mitochondria from frog, squirrel, pigeon, guinea pig, and rat, respectively. The sigmoidicity and the high apparent K(m) render mitochondrial Ca(++) uptake slow below 10 microM. At these concentrations the rate of Ca(++) uptake by cardiac mitochondria in vitro and the amount of mitochondria present in the heart are not consistent with the amount of Ca(++) to be sequestered in vivo during heart relaxation. Therefore, it appears that, at least in mammalian hearts, the energy-linked transport of Ca(++) by mitochondria is inadequate for regulating the beat-to-beat Ca(++) cycle. The results obtained and the proposed cooperativity for mitochondrial Ca(++) uptake are discussed in terms of physiological regulation of intracellular Ca(++) homeostasis in cardiac cells.