Proton Pump Activity of Mitochondria-rich Cells : The Interpretation of External Proton-concentration Gradients

We have hypothesized that a major role of the apical H(+)-pump in mitochondria-rich (MR) cells of amphibian skin is to energize active uptake of Cl(−) via an apical Cl(−)/HCO(3) (−)-exchanger. The activity of the H(+) pump was studied by monitoring mucosal [H(+)]-profiles with a pH-sensitive microelectrode. With gluconate as mucosal anion, pH adjacent to the cornified cell layer was 0.98 ± 0.07 (mean ± SEM) pH-units below that of the lightly buffered bulk solution (pH = 7.40). The average distance at which the pH-gradient is dissipated was 382 ± 18 μm, corresponding to an estimated “unstirred layer” thickness of 329 ± 29 μm. Mucosal acidification was dependent on serosal pCO(2), and abolished after depression of cellular energy metabolism, confirming that mucosal acidification results from active transport of H(+). The [H(+)] was practically similar adjacent to all cells and independent of whether the microelectrode tip was positioned near an MR-cell or a principal cell. To evaluate [H(+)]-profiles created by a multitude of MR-cells, a mathematical model is proposed which assumes that the H(+) distribution is governed by steady diffusion from a number of point sources defining a set of particular solutions to Laplace's equation. Model calculations predicted that with a physiological density of MR cells, the [H(+)] profile would be governed by so many sources that their individual contributions could not be experimentally resolved. The flux equation was integrated to provide a general mathematical expression for an external standing [H(+)]–gradient in the unstirred layer. This case was treated as free diffusion of protons and proton-loaded buffer molecules carrying away the protons extruded by the pump into the unstirred layer; the expression derived was used for estimating stationary proton-fluxes. The external [H(+)]-gradient depended on the mucosal anion such as to indicate that base (HCO(3) (−)) is excreted in exchange not only for Cl (−), but also for Br(−) and I(−), indicating that the active fluxes of these anions can be attributed to mitochondria-rich cells.