title: Proton pump activity of mitochondria-rich cells. The interpretation of external proton-concentration gradients.
abstract: 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-/HCO3(-)-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 microns, corresponding to an estimated "unstirred layer" thickness of 329 +/- 29 microns. Mucosal acidification was dependent on serosal pCO2, 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 (HCO3-) 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.
authors.affiliation: Zoophysiological Laboratory, August Krogh Institute, University of Copenhagen, Denmark. ljjensen@aki.ku.dk
authors.forename: L J
authors.initials: LJ
authors.lastname: Jensen
authors.forename: J N
authors.initials: JN
authors.lastname: Sørensen
authors.forename: E H
authors.initials: EH
authors.lastname: Larsen
authors.forename: N J
authors.initials: NJ
authors.lastname: Willumsen
mesh_terms: D000818:Animals; D000838:Anions / Q000378:metabolism; D002021:Buffers; D002023:Bufo bufo / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D008928:Mitochondria / Q000648:ultrastructure*; D009092:Mucous Membrane / Q000378:metabolism; D009994:Osmolar Concentration; D017494:Proton Pumps; D011522:Protons; D012867:Skin / Q000378:metabolism* / Q000648:ultrastructure*; D013379:Substrate Specificity
chemical_list: D000838:Anions; D002021:Buffers; D017494:Proton Pumps; D011522:Protons