Buffered diffusion around a spherical proton pumping cell: a theoretical analysis

¹ Istituto di Biofisica

^* To whom correspondence should be addressed. E-mail: pusch{at}ge.ibf.cnr.it.

Submitted on April 23, 2007
Revised on June 5, 2007
Accepted on 23 August 2007

	Abstract

H⁺ ions are a substrate of many active and passive membrane transporters in all cells. Absolute proton fluxes are often quantified using intracellular pH sensitive microelectrodes or pH sensitive dyes. These measurements, however, rely on a priori estimates of the intracellular buffer capacity and on the assumption of diffusive equilibrium inside the cell. Here, assuming local equilibrium of protons with a single mobile buffer, we model the diffusion of H⁺ in the extracellular medium around an H⁺ pumping cell, in order to estimate the expected pH changes as a function of time, distance from the cell, extracellular buffer capacity, and the absolute proton flux across the membrane. In particular, using accurate numerical simulation, we gauge the range of validity of an explicit, analytical solution of the linearized, non-stationary diffusion equation. Our results provide a framework to quantify the absolute membrane proton flux, if spatio-temporal information about the extracellular pH change is available, e.g. using imaging of pH dependent fluorescent dyes.

Key Words: CLC protein, ClC-5, channel, fluorometry, ion sensitive microelectrodes, transporter