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 mitoch ondria-rich (MR) cells of amphibian skin is to energize active uptake of Cl- via an apical Cl-/HCO3--exchanger. The activity of the H+ pump was studied by monitoring mucosal [H+]-profiles with a pH-sensitive mi croelectrode. With gluconate as mucosal anion, pH adjacent to the corn ified cell larger was 0.98 +/- 0.07 (mean +/- SEM) pH-units below that of the lightly buffered bulk solution (pH = 7.40). The average distan ce at which the pH-gradient is dissipated was 382 +/- 18 mu m, corresp onding to an estimated ''unstirred layer'' thickness of 329 +/- 29 mu m. Mucosal acidification was dependent on serosal pCO(2), and abolishe d after depression of cellular energy metabolism, confirming that muco sal acidification results from active transport of H+. The [H+] was pr actically similar adjacent to all cells and independent of whether the microelectrode tip was positioned near an MR-cell or a principal cell . To evaluate [Hf]-profiles created by a multitude of MR-cells, a math ematical model is proposed which assumes that the H+ distribution is g overned 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+] prof ile would be governed by so many sources that their individual contrib utions could not be experimentally resolved. The flux equation was int egrated to provide a general mathematical expression for an external s tanding [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 e xpression derived was used for estimating stationary proton-fluxes. Th e external [H+]-gradient depended on the mucosal anion such as to indi cate that base (HCO3-) is excreted in exchange not only for Cl-, but a lso for Br- and I-, indicating that the active fluxes of these anions can be attributed to mitochondria-rich cells.