Measurement of the limiting equivalent conductivities and mobilities of the most prevalent ionic species of EGTA (EGTA2- and EGTA3-) for use in electrophysiological experiments

How to Properly Measure a Current-Voltage Relation?-Interpolation vs. Ramp Methods Applied to Studies of GABAA Receptors

The relation between current and voltage, I-V relation, is central to functional analysis of membrane ion channels. A commonly used method, since the introduction of the voltage-clamp technique, to establish the I-V relation depends on the interpolation of current amplitudes recorded at different steady voltages. By a theoretical computational approach as well as by experimental recordings from GABA_A-receptor mediated currents in mammalian central neurons, we here show that this interpolation method may give reversal potentials and conductances that do not reflect the properties of the channels studied under conditions when ion flux may give rise to concentration changes. Therefore, changes in ion concentrations may remain undetected and conclusions on changes in conductance, such as during desensitization, may be mistaken. In contrast, an alternative experimental approach, using rapid voltage ramps, enable I-V relations that much better reflect the properties of the studied ion channels.

An optimised 3 M KCl salt-bridge technique used to measure and validate theoretical liquid junction potential values in patch-clamping and electrophysiology

Accurate potential measurements in electrophysiological experiments require correction for liquid junction potentials (LJPs), and, in patch-clamping especially, these can often be ~5-10 mV or more. They can be either calculated, if ion mobilities are known, or measured directly. We describe an optimised system to directly measure LJPs with a patch-clamp amplifier, using as a reference electrode, a freshly-cut 3 M KCl-agar salt-bridge (in polyethylene tubing) with its tip cut off by at least 5 mm during solution changes to eliminate its solution-history-dependent effects. We quantify such history-dependent effects and complement this with a de-novo theoretical analysis of salt diffusion to and from the salt-bridge. Our analysis and experimental results validate the optimised methodology for measuring LJPs, and the use of the Henderson equation for accurately calculating them. The use of this equation is also assessed and generally validated in the light of rigorous Nernst-Planck-Poisson and other numerical simulations and analytical studies of LJPs over recent decades. Digitizing, recording and amplifying the measured potentials increases their accuracy. The measured potentials still need correction for small, well-defined calculable, shifts in LJPs at the 3 M KCl-agar reference. Using this technique, we have measured changes in LJPs for diluted solutions of NaCl, LiCl, KCl, CsCl and NaF, obtaining excellent agreement within ±0.1 mV of predicted values, calculated using ion activities. Our de novo LJP measurements of biionic combinations of the above undiluted salts, and NaI and NaF (with halide anions I⁻ and F⁻), generally also gave excellent agreement with predicted values.

Study of membrane potential in T lymphocytes subpopulations using flow cytometry

Background

Ion channels are involved in the control of membrane potential (ψ) in a variety of cells. The maintenance of ψ in human T lymphocytes is essential for T-cell activation and was suggested to depend mostly on the voltage-gated Kv1.3 channel. Blockage of Kv1.3 inhibits cytokine production and lymphocyte proliferation in vitro and suppresses immune response in vivo. T lymphocytes are a heterogeneous cell population and the expression of Kv1.3 varies among cell subsets. Oxonol diBA-C4-(3) was used to determine ψ by flow cytometry. The presence of distinct T cell subsets was evaluated by immunophenotyping techniques and the contribution of Kv1.3 channels for the maintenance of ψ was investigated using selective blockers.

Results

The distribution of ψ in T lymphocytes varied among blood donors and did not always follow a unimodal pattern. T lymphocytes were divided into CD3⁺/CD45RO^- and CD3⁺/CD45RO⁺ subsets, whose peak channel values of ψ were -58 ± 3.6 mV and -37 ± 4.1 mV, respectively. MgTX (specific inhibitor of Kv1.3 channels) had no significant effect in the ψ of CD3⁺/CD45RO^- subsets but depolarized CD3⁺/CD45RO⁺ cells to -27 ± 5.1 mV.

Conclusion

Combination of optical methods for determination of ψ by flow cytometry with immuophenotyping techniques opens new possibilities for the study of ion channels in the biology of heterogeneous cell populations such as T lymphocyte subsets.