Measurement of intracellular pH and pCa with a confocal microscope

Simultaneous analysis of intracellular pH and Ca²⁺ from cell populations

Although changes in both pH(in) and [Ca(2+)](i) have been observed in response to a variety of agonists, it is not clear whether these ionic events work independently or are coordinated to lead to a specific physiological response. One of the fundamental problems in studying these ionic events is that changes in pH(in) modify Ca(2+) regulatory mechanisms and changes in Ca(2+) may modify pH regulation. It is desirable to use a technique that allows concomitant monitoring of these two ions in cell populations with high time resolution. Furthermore, like many Ca(2+) binding proteins, all Ca(2+)-sensitive fluoroprobes are inherently sensitive to pH owing to competition of H(+) for the Ca(2+)-binding sites. This chapter describes experimental paradigms that provide optimum conditions for simultaneous measurement of pH from the fluorescence emission of snarf-1, and Ca(2+) using fura-2. The fluorescence spectra of these compounds are sufficiently different to allow simultaneous measurement of pH and Ca(2+) both in vitro and in vivo. Moreover, the ratio of the H(+)-sensitive wavelengths of snarf-1 is unaffected by Ca(2+), or the concomitant presence of fura-2 in cells. Although the fluorescence ratio of fura-2 is insensitive to the presence of snarf-1, it is affected by pH, as indicated above. We describe procedures to correct for this effect and to obtain calibration parameters for fura-2 and snarf-1 required to facilitate analysis of pH and Ca(2+) concentrations within cell populations.

New insights in the spatially resolved dynamic pH measurement in macroscopic large absorbent particles by confocal laser scanning microscopy

Both, experimental investigation of protein adsorption processes and mathematical models describing such processes indicate, that the pH in the absorbent particle might be the key factor for an improved understanding of these chromatographic processes. Thus, a technique aiming at the spatially resolved pH measurement in macroscopic large absorbent particles is presented. The first application of this method, being based on confocal laser scanning microscopy (CLSM), revealed an apparent dependence of the pH calibration curve on the scanning depth. By a model-based approach, factors distorting the measurement signal are identified: The wavelength-dependent light scattering and the re-absorption of emitted light. The resulting consequences for further development and application of CLSM based techniques to measure pH in macroscopic large absorbent particles are illustrated and discussed.

Fluorescence tracing of intracellular proteins

Developments in fluorescence microscopy and the availability of fluorescently labeled antibodies and probes for localization of molecules and organelles have made the microscope an indispensable tool with which one can map specific molecules to subcellular loci allowing deep insight into cell and organelle biology. Furthermore, confocal microscopy permits analysis of the three dimensional architecture of cells that could not be accomplished by conventional light microscopy. The goal of fluorescence protein tracing by microscopy is to visualize cellular constituents and general cytoarchitecture as close to native organization as possible. To achieve this, and to preserve cellular structure in the best possible manner, the specimen is usually fixed chemically. Here I review several standard fixation, permeabilization and labeling schemes followed by examples of several standard imaging techniques.

Confocal laser scanning microscopy of calcium dynamics in living cells

Confocal laser scanning microscopy (CLSM) is widely used to monitor intracellular calcium levels in living cells loaded with calcium-sensitive fluorophores. This review examines the basic advantages and limitations of CLSM in in vivo imaging analyses of calcium dynamics. The benefits of utilizing ratioed images and dextran-conjugated fluorophores are addressed, and practical aspects of handling confocal datasets are outlined. After considering some relatively new microscopical methods that can be used in conjunction with conventional CLSM, possible future applications of confocal techniques in analyses of intracellular calcium dynamics are discussed.

Growing pollen tubes possess a constitutive alkaline band in the clear zone and a growth-dependent acidic tip

Using both the proton selective vibrating electrode to probe the extracellular currents and ratiometric wide-field fluorescence microscopy with the indicator 2', 7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF)-dextran to image the intracellular pH, we have examined the distribution and activity of protons (H+) associated with pollen tube growth. The intracellular images reveal that lily pollen tubes possess a constitutive alkaline band at the base of the clear zone and an acidic domain at the extreme apex. The extracellular observations, in close agreement, show a proton influx at the extreme apex of the pollen tube and an efflux in the region that corresponds to the position of the alkaline band. The ability to detect the intracellular pH gradient is strongly dependent on the concentration of exogenous buffers in the cytoplasm. Thus, even the indicator dye, if introduced at levels estimated to be of 1.0 microM or greater, will dissipate the gradient, possibly through shuttle buffering. The apical acidic domain correlates closely with the process of growth, and thus may play a direct role, possibly in facilitating vesicle movement and exocytosis. The alkaline band correlates with the position of the reverse fountain streaming at the base of the clear zone, and may participate in the regulation of actin filament formation through the modulation of pH-sensitive actin binding proteins. These studies not only demonstrate that proton gradients exist, but that they may be intimately associated with polarized pollen tube growth.

Regulation of cytosol-nucleus pH gradients by K+/H+ exchange mechanism in the nuclear envelope of neonatal rat astrocytes

In order to study the subcellular heterogeneity of intracellular H+ concentration in reactive astrocytes, the pH in the nucleus and cytosol of cultured astrocytes was measured using a confocal laser scanning microscope (CLSM) and pH indicator dye, 5'(and 6')-carboxyseminaphthofluorescein (carboxy SNAFL-1). The change in intracellular pH was indexed by the fluorescence ratio (F535/F610) at an excitation wavelength of 514.5 nm. The in vitro fluorescence ratio increased as pH decreased. This ratio in the nucleus was significantly lower than that in the cytosol of astrocytes when perfused by HEPES-buffered Hanks' balanced salt solution (HHBSS) at pH 7.4. Acid stimulations of cells (pH 5.0) raised the fluorescence ratio in both nucleus and cytosol. However, the increase in the fluorescence ratio of the nucleus was less than that of cytosol. Treatment with a K+/H+ ionophore, nigericin (20 microM), reversibly nullified this cytosol-nucleus pH gradient. These findings suggest that a buffering mechanism(s) for maintaining of intracellular pH exists between the nucleus and cytosol, and a K+/H+ exchanger may act on the nuclear envelope to eventuate intranuclear pH maintenance in the living cells.

Calcium imaging: a primer for mycologists

This article aims to encourage more fungal biologists to consider the imaging of cytoplasmic Ca2+ fluxes. Compared to other organisms, for fungi there have been remarkably few attempts to characterize the role of Ca2+ fluxes in signal transduction and general cellular activities, even though other approaches indicate that fungal growth and development are highly dependent upon Ca2+. The methodologies for imaging Ca2+ fluxes continue to develop rapidly. These methodologies are explained here in a style that should be accessible to a newcomer to the field, hopefully forming a bridge to the more complex methodological literature.