Intracellular ion concentrations in the isolated frog skin epithelium: Evidence for different types of mitochondria-rich cells

Abstract

Intracellular ion concentrations were determined in split skins of Rana pipiens using the technique of electron microprobe analysis. Under control conditions, principal cells and mitochondria-rich cells (MR cells) had a similar intracellular ion composition, only the Cl concentration in MR cells was significantly lower. Inhibition of transepithelial Na transport by low concentrations of ouabain (2 x 10(-6) M, inner bath) resulted in a Na concentration increase of principal cells from 10.9 to 54.3 mmol/kg wet wt. The increase was completely abolished by simultaneous application of amiloride (10(-4) M, outer bath). Amiloride alone resulted in a significant decrease of the Na concentration to 6.1 mmol/kg. w.w. Among MR cells, two different groups of cells could be distinguished; cells that showed a Na increase after ouabain which was even larger than that in principal cells and cells that did not respond to ouabain. In about half of all ouabain-sensitive MR cells the Na increase could be prevented by amiloride. According to these results, a subpopulation of MR cells displays the transport characteristics expected for a transepithelial Na transport compartment, an apical amiloride-sensitive Na influx and a basal ouabain-inhibitable Na efflux. Given the small number of cells, however, it is unlikely that this subtype of MR cells contributes significantly to the overall rate of transepithelial Na transport.

Chloride currents of single mitochondria‐rich cells of toad skin epithelium.

“…For example, an efflux of protons is generated by an apical proton pump in MR cells (Page & Frazier, 1987;Ehrenfeld, Lacoste & Harvey, 1989;Harvey, 1992;Larsen, Willumsen & Christoffersen, 1992), and passive and active inward fluxes of Cl-have been localized to MR cells, with the active component energized by the apical proton pump (Vofute & Meyer, 1978;Larsen, 1991;. Finally, an apical amiloride-blockable Na+ influx mechanism and basolateral Na+-K+ pumps are present in MR cells, indicating that they also mediate active uptake of Na+ (Larsen, Ussing & Spring, 1987;Harvey, 1992;Rick, 1992).…”

“…For example, the capacities for Cl-uptake (Katz & Larsen, 1984;Willumsen & Larsen, 1986;Devuyst, Beaujean & Crabbe, 1991) and H+ secretion (Vanatta & Frazier, 1981;Page & Frazier, 1987;Ehrenfeld, Lacoste & Harvey, 1989) are increased by salt depleting and acid loading the animal, respectively, and are correlated with the density of MR cells in the skin. According to a recent study of Na+ pathways, frog epidermis may contain three different subpopulations of MR cells (Rick, 1992). There are studies in which no Cl- (Nagel & Dorge, 1990) and Na+ (Dorge, Nagel, Beck, Rick & Thurau, 1990) transport by MR cells was demonstrated.…”

Cytochemical localization of adenylate cyclase in the sodium-transporting epithelium isolated from frog skin

“…While adenylate cyclase was found on all membranes of the deeper cells comprising the different epithelial layers, the enzyme was conspicuously absent from the apical membranes of the outermost living granulosum cells. Microprobe analysis of the intracellular ion concentrations (Rick, 1992) and intracellular potential measurements of frog skin epithelium (Nagel, 1976) have demonstrated that all layers of the stratified epithelium participate in transepithelial Na + transport, forming a syncytial Na + transport compartment. Hence, our results are consistent with the classic two-step model of Koefoed-Johnsen and Ussing (1958) as illustrated in Fig.…”

“…The membranes of the mitochondria-rich cells also contain adenylate cyclase, indicative that these cells are also subjected to hormonal regulation. It has been suggested that these cells constitute a minor Na + transport pathway, parallel to the other, but not contributing substantially to the overall rate of Na + transport (Larsen, 1988;Rick, 1992).…”

Role of mitochondria‐rich cells in epithelial chloride uptake

“…Interestingly, this study dealt with a passive -not the activemechanism. However, by suggesting that the mitochondria-rich (MR) cells are specialized for epithelial Cl-uptake this study led other investigators to focus on this minority cell type (Foskett & Ussing, 1986;Katz & Sheffey, 1986;Willumsen & Larsen, 1986;Larsen, Ussing & Spring, 1987;Rick, 1992), and to perform studies of the proton pump of MR cells to investigate its role in active uptake of Na+ (Ehrenfeld, Lacoste & Harvey, 1989;Harvey, 1992) and Cl- (Larsen, Willumsen & Christoffersen, 1992), respectively.…”