Voltammetric measurement of oxygen in single neurons using platinized carbon ring electrodes

Carbon-Fiber Nanoelectrodes for Real-Time Discrimination of Vesicle Cargo in the Native Cellular Environment

Carbon-fiber microelectrodes have proven to be an indispensable tool for monitoring exocytosis events using amperometry. When positioned adjacent to a cell, a traditional microdisc electrode is well suited for quantification of discrete exocytotic release events. However, the size of the electrode does not allow for intracellular electrochemical measurements, and the amperometric approach cannot distinguish between the catecholamines that are released. In this work, carbon nanoelectrodes were developed to permit selective electrochemical sampling of nanoscale vesicles in the cell cytosol. Classical voltammetric techniques and electron microscopy were used to characterize the nanoelectrodes, which were ∼5 μm long and sharpened to a nanometer-scale tip that could be wholly inserted into individual neuroendocrine cells. The nanoelectrodes were coupled with fast-scan cyclic voltammetry to distinguish secretory granules containing epinephrine from other catecholamine-containing granules encountered in the native cellular environment. Both vesicle subtypes were encountered in most cells, despite prior demonstration of populations of chromaffin cells that preferentially release one of these catecholamines. There was substantial cell-to-cell variability in relative epinephrine content, and vesicles containing epinephrine generally stored more catecholamine than the other vesicles. The carbon nanoelectrode technology thus enabled analysis of picoliter-scale biological volumes, revealing key differences between chromaffin cells at the level of the dense-core granule.

Does a redox cycle provide a mechanism for setting the capacity of neuroglobin to protect cells from apoptosis?

We hypothesize that the various, previously reported, reactivities of neuroglobin with redox partners and oxygen provide for the establishment of a redox cycle within cells, such as neurons and retinal rod cells. Using native cell lysates, from cultured human cells of neuronal origin, we have estimated the rate of reduction of the oxidized form of neuroglobin in vivo. Furthermore we provide evidence that the cytosol of these cells contains factors (presumably enzymes) capable of employing either glutathione or NADH as re-reductants of ferric neuroglobin. Taken in conjunction with previous rate data, for the various redox reactions of neuroglobin, this information allows us to set up a computer model to estimate the steady state cellular level of the antiapoptotic ferrous form of neuroglobin. This model indicates that the steady state level of antiapoptotic neuroglobin is very sensitive to the cellular oxygen tension and moderately sensitive to the redox status of the cell. Further analysis indicates that such a system would be capable of significant modification, on the seconds time scale, following hypoxic transition, as is likely in stroke. We hypothesize that this mechanism might provide a moderately rapid mechanism for adjusting the antiapoptotic status of a cell, whilst the reaction of neuroglobin with mitochondrial cytochrome c provides a very rapid, but limited, capacity to intervene in the apoptotic pathway.

Monitoring changes of cellular metabolism and microviscosity in vitro based on time-resolved endogenous fluorescence and its anisotropy decay dynamics

Reduced nicotinamide adenine dinucleotide (NADH) is a well-known metabolic coenzyme and endogenous fluorophore. In this study, we develop a system that simultaneously measures time- and wavelength-resolved fluorescence to extract free and protein-bound NADH signals from total cellular fluorescence. We analyze temporal characteristics of NADH fluorescence in a mixture of NADH and lactate dehydrogenase (LDH) as well as in living cell samples. The results show that in both the NADH/LDH mixture and cell samples, a fraction of free NADH and protein-bound components can be identified. The extracted free and bound NADH signals are confirmed by time-resolved measurement of anisotropy decay of NADH fluorescence, based on the fact that free NADH is a small fluorescent molecule with much shorter rotational diffusion time than bound NADH. The ratio of free NADH signal to bound NADH signal is very different between normal and cancer cervical epithelial cells. In addition, the ratio changes significantly when the cell samples are treated with a mitochondrial inhibitor or uncoupler, demonstrating that the method is sensitive to monitor cellular metabolic activity. Finally, we demonstrate that the microviscosity for relatively small molecules such as NADH in cells could be extracted from wavelength- and time-resolved NADH fluorescence of living cell samples.

Electrochemical detection in a microfluidic device of oxidative stress generated by macrophage cells

The release of reactive oxygen species (ROS) or reactive nitrogen species (RNS), i.e., the initial phase of oxidative stress, by macrophage cells has been studied by electrochemistry within a microfluidic device. Macrophages were first cultured into a detection chamber containing the three electrodes system and were subsequently stimulated by the microinjection of a calcium ionophore (A23187). Their production of ROS and RNS was then measured by amperometry at the surface of a platinized microelectrode. The fabricated microfluidic device provides an accurate measurement of oxidative release kinetics with an excellent reproducibility. We believe that such a method is simple and versatile for a number of advanced applications based on the detection of biological processes of secretion by a few or even a single living cell.

Monitoring in real time with a microelectrode the release of reactive oxygen and nitrogen species by a single macrophage stimulated by its membrane mechanical depolarization

Macrophages are key cells of the immune system. During phagocytosis, the macrophage engulfs a foreign bacterium, virus, or particle into a vacuole, the phagosome, wherein oxidants are produced to neutralize and decompose the threatening element. These oxidants derive from in situ production of superoxide and nitric oxide by specific enzymes. However, the chemical nature and sequence of release of these compounds is far from being completely determined. The aim of the present work was to study the fundamental mechanism of oxidant release by macrophages at the level of a single cell, in real time and quantitatively. The tip of a microelectrode was positioned at a micrometric distance from a macrophage in a culture to measure oxidative-burst release by the cell when it was submitted to physical stimulation. The ensuing release of electroactive reactive oxygen and nitrogen species was detected by amperometry and the exact nature of the compounds was characterized through comparison with in vitro electrochemical oxidation of H2O2, ONOO-, NO*, and NO2(-) solutions. These results enabled the calculation of time variations of emission flux for each species and the reconstruction of the original flux of production of primary species, O2*- and NO*, by the macrophage.

Production of hydrogen peroxide in rat corpus cavernosum: An on-line study with microvoltammetric electrodes

Continuous measurements were realized in rat corpus cavernosum using microvoltammetric captor. After inhibition of endothelial NO-synthase by L-NMA vasodilatation was induced by intracavernous injection of acetylcholine. Blood flow of rat penis was monitored by laser Doppler. A new peak appeared in reduction. It was identified as hydrogen peroxide by its peak potential, by intracavernous injection of hydrogen peroxide or catalase. Intracavernous injection of various pharmacological agents permitted to demonstrate that its origin was due to endothelial NAD(P)H oxidases. DPI inhibited its synthesis; NADH and NADPH enhanced it. Intracavernous injection of diethyldithiocarbamic acid gave the disappearance of hydrogen peroxide peak and appearance of the superoxide peak. This preliminary study showed that when the L-arginine pathway was inhibited, the NAD(P)H oxidase pathway functioned in rat corpus cavernosum for vasodilatation.

Novel fluorescent oxygen indicator for intracellular oxygen measurements

Intracellular oxygen concentration is of primary importance in determining numerous physiological and pathological processes in biological systems. In this paper, we describe the application of the oxygen sensing indicator, ruthenium dibipyridine 4-(1"-pyrenyl)-2,2'-bipyridine chloride [Ru(bpy-pyr)(bpy)(2)], for molecular oxygen measurement in J774 murine macrophages. Ru(bpy-pyr)(bpy)(2) exhibits strong visible absorption, efficient fluorescence, long excited state lifetime, large Stokes shift, and high photo- and chemical stability. The fluorescence of Ru(bpy-pyr)(bpy)2 is efficiently quenched by molecular oxygen. It is 13 fold higher in a nitrogenated solution than in an oxygenated one. The dye passively permeates into cells and maintains its oxygen sensitivity for at least 5 h when the cells are stored in a phosphate buffered saline solution at pH 7.4. The oxygen sensitivity, photostability, and chemical stability of the indicator and the effect of hypoxia and hyperoxia on the intracellular oxygen level in single macrophages are discussed.

Development of a digital fluorescence sensing technique to monitor the response of macrophages to external hypoxia

Oxygen plays a very important role in living cells. The intracellular level of oxygen is under tight control, as even a small deviation from normal oxygen level affects major cellular metabolic processes and is likely to result in cellular damage or cell death. This paper describes the use of the oxygen sensitive fluorescent dye tris (1,10-phenanthroline) ruthenium chloride [Ru(phen)(3)] as an intracellular oxygen probe. Ru(phen)(3) exhibits high photostability, a relatively high excitation coefficient at 450 nm (18 000 M(-1) cm(-1)), high emission quantum yield ( approximately 0.5), and a large Stoke shift (peak emission at 604 nm). It is effectively quenched by molecular oxygen due to its long excited state lifetime of around 1 micros. The luminescence of Ru(phen)(3) decreases with increasing oxygen concentrations and the oxygen levels are determined using the Stern-Volmer equation. In our studies, J774 Murine Macrophages are loaded with Ru(phen)(3), which passively permeates into the cells. Fluorescence spectroscopy and digital fluorescence imaging microscopy are used to observe the cells and monitor their response to changing oxygen levels. The luminescence intensity of the cells decreases when exposed to hypoxia and recovers once normal oxygen conditions are restored. The analytical properties of the probe and its application in monitoring the cellular response to hypoxia are described.

Quantitative chemical analysis of single cells

A fundamental perspective can be achieved by targeting single cells for analysis with the goal of deconvoluting complex biological functions. However, single-cell studies have their own difficulties, such as minute volumes and sample amounts. Quantitative chemical analysis of single cells has emerged as a powerful new area in recent years due to several technological advancements. The development of microelectrodes has allowed the measurement of redox-active species as a function of cellular dynamics. This miniaturization trend is also evident in the separation sciences with the application of small column separations to single cells. Desorption ionization methods with mass spectrometric detection have shown single-cell capability owing to numerous technological developments. Finally, fluorescence imaging has also progressed to the point where single-cell dynamics can be probed by native fluorescence utilizing either single or multiple photon excitation. The results of these studies are reviewed with an emphasis on the quantitation of single-cell dynamics.

Oxygen-sensing microdialysis probe for in vivo use

Electrochemical conditions were optimized to allow the metal tube used for the shaft of commercial microdialysis (MD) probes to be coated with gold. In in vitro tests with phosphate-buffered Ringer's solution using double differential pulse amperometry (DDPA), the gold-coated shafts were capable of specifically measuring the reduction of oxygen and the oxidation of ascorbic acid in the presence of high concentrations of potentially interfering endogenous substances. By using fixed-potential amperometry (FPA), the gold-plated shaft also measured oxygen with minimal interference from high concentrations of potentially interfering endogenous substances. Concentric design MD probes were constructed that used a metal shaft (O.D = 0.4 mm), fused silica inlet and outlet tubes, and a 1.5 mm dialyzing membrane (O.D = 0.2 mm). A 0.5-0.7 mm gold collar was electroplated onto the metal shaft approximately 0.5 mm above the dialyzing membrane. The nongold outer surface of the MD probe was coated with an insulating polymer. In vivo tests demonstrated that DDPA was not suitable for use with this gold microdialyzing electrode (GMDE). However, brain oxygen levels were satisfactorily measured using FPA. In urethane-anesthetized rats, the reduction current to oxygen in the striatum was increased by brief (1 min) inhalation of O2 or CO2 and decreased by inhalation of N2. Transient application of noxious stimuli (foot pinch) increased cerebral O2, whereas bilateral carotid artery occlusion and death decreased striatal O2. The responses of the GMDE were indistinguishable from the reduction current simultaneously measured from a conventional carbon fiber electrode implanted adjacent to the gold-plated area of the MD shaft. Basal levels of striatal O(2) were 20 +/- 5 microM (n = 4) for the GMDE and 30 +/- 11 microM (n = 3) for the carbon fiber. The GMDE was robust and could be used for at least three animals. This technique can be used to provide information about the oxygen status of the tissue adjacent to the dialyzing membrane without the need for implantation of an additional electrode.

Microamperometric measurements of photosynthetic activity in a single algal protoplast

The effects of p-benzoquinone (BQ) on photosynthetic and respiratory electron transport in a single algal protoplast (radius, 100 microm) was investigated quantitatively by amperometric measurements using microelectrodes. Under light irradiation (25 kLx) in the presence of 1.00 mM BQ, a single protoplast consumed BQ by (2.9 +/- 0.2) x 10(-13) mol/s and generated p-hydroquinone (QH2) by (2.7 +/- 0.3) x 10(-13) mol/s, suggesting that BQ was quantitatively reduced to QH2 via the intracellular photosynthetic electron-transport chain. The generation of QH2 increased with light intensity and with concentration of BQ added to the outside solution but became saturated when the light intensity was above 15 kLx or the BQ concentration was higher than 0.75 mM. The addition of 3-(3, 4-dichlorophenyl)-1,1-dimethylurea, a photosynthetic electron-transport inhibitor, decreased the generation of QH2 upon light irradiation, suggesting that BQ accepts electrons from a site in the photosynthetic electron-transport chain after the photosystem II site. The presence of 1.00 mM BQ increased the generation of photosynthetic oxygen by approximately (2.6 +/- 1.0) x 10(-13) mol/s, which was approximately 1.5-2 times larger than that expected from the consumption of BQ. The electrons produced by the additional generation of oxygen is used to reduce intracellular species as well as to reduce BQ.

Permeation of redox species through a cell membrane of a single, living algal protoplast studied by microamperometry

Permeation of several redox species through a cell membrane of a single algal protoplast (radius 100 microns) was investigated by amperometry with a Pt microdisk electrode (disk radius, 6.5 microns) located near the membrane. The redox current observed at the microelectrode decreased as the microelectrode approached the cell membrane since the membrane acted as a barrier for diffusion of redox species from bulk to the microelectrode. Permeability coefficient (Pm) of the protoplast membrane was determined by the quantitative analysis of the variation of the redox current with microelectrode-membrane distance using digital simulation. The Pm values for Fe(CN)6(4-), Fe(CN)6(3-), Co(phen)3(2+), ferrocenyl methanol(FMA) and p-hydroquinone(QH2) were < or = 1.0 x 10(-4), < or = 1.0 x 10(-4), 1.0 x 10(-3), 5.0 x 10(-3) and 2.0 x 10(-2) cm/s, respectively. Using these Pm values, the concentration changes inside a model cell and chloroplast were theoretically calculated.

Neuroengineering in biological and biosynthetic systems

Recent advances in restoring neural function to biological systems and incorporating neural function into bioartificial devices have been made possible by developments in biology, materials science, engineering, and physics. Further progress is being achieved in relating cellular function to overall system behavior through the application of quantitative experimental and theoretical techniques.