Dynamics and density of mitochondria-rich cells in toad skin epithelium

Mitochondria-rich cells in amphibian skin epithelium: Relationship of immuno- and peanut lectin labeling pattern and transport functions

Mitochondria-rich cells are an integral component of the epidermis of amphibian skin and play a functional role. Whereas the principal cell compartment of the epithelium is specialized almost exclusively for active uptake of sodium, the mitochondria-rich cells perform other diverse ion-transport functions, including transport of Cl(-), H(+), HCO(3)(-) and organic molecules. These transporting functions differ in different species. Antibodies, such as those directed against band 3, H(+)-ATPase, and also peanut lectin (PNA), bind specifically to the mitochondria-rich cells, but do so differently in various species. Examination of these immunolocalizations and lectin labeling in the skin of over 10 amphibian species, including both Anurans and Urodeles, illustrate species-specific differences. The binding pattern and the transport capabilities of the skin in the various species do not show a universal correlation, they appear to be species specific and do not permit construction of a general scheme common to all the species studied. The mitochondria-rich cells of heterocellular epithelia and their roles in ion transport remain a subject that requires further studies to elucidate their particular functions within the framework of the whole epithelium.

Nitric oxide decreases ammonium release in tadpoles of the clawed frog, Xenopus laevis, Daudin

In the present study, we quantified the physiological consequences of nitric oxide (NO) on ammonium release in tadpoles of Xenopus laevis. Tadpoles exposed to S-nitro-N-acetylpenicillamine (SNAP), an NO-donor, or L: -arginine, the substrate of NO synthase (NOS), showed a reversible decrease, whereas animals exposed to the NOS inhibitor Nomega-methyl-L: -arginine (L: -NMMA) exhibited an increase in ammonium release. Release of ammonium may be of physiological relevance during stress response of the animal. Handling of tadpoles as well as exposure to hyposmotic environments increased ammonium release. To localize NO synthesizing cells, we used diaminofluorescein-diacetate (DAF-2DA), an NO-sensitive fluorescent dye, and NADPH-diaphorase histochemistry, an indicator for NOS activity. We observed a fluorescence signal as well as NADPH-diaphorase activity in small, solitary cells in the epidermis. Similarly to NADPH-diaphorase histochemistry, silver nitrate staining and rhodamine labelling, markers for mitochondria-rich cells, showed a strong reaction in these cells. These observations indicate that NO (1) inhibits ammonium release, and (2) is endogenously synthesized in mitochondria-rich cells in Xenopus tadpoles. Based on our histochemical results, we speculate that gill epithelium and epidermis work in parallel to release ammonium as epidermal tissue contains mitochondria-rich and NADPH-diaphorase positive cells.

Proton pump-driven cutaneous chloride uptake in anuran amphibia

Krogh introduced the concept of active ion uptake across surface epithelia of freshwater animals, and proved independent transports of Na(+) and Cl(-) in anuran skin and fish gill. He suggested that the fluxes of Na(+) and Cl(-) involve exchanges with ions of similar charge. In the so-called Krogh model, Cl(-)/HCO(3)(-) and Na(+)/H(+) antiporters are located in the apical membrane of the osmoregulatory epithelium. More recent studies have shown that H(+) excretion in anuran skin is due to a V-ATPase in mitochondria-rich (MR) cells. The pump has been localized by immunostaining and H(+) fluxes estimated by pH-stat titration and mathematical modelling of pH-profiles in the unstirred layer on the external side of the epithelium. H(+) secretion is voltage-dependent, sensitive to carbonic-anhydrase inhibitors, and rheogenic with a charge/ion-flux ratio of unity. Cl(-) uptake from freshwater is saturating, voltage independent, and sensitive to DIDS and carbonic-anhydrase inhibitors. Depending on anuran species and probably on acid/base balance of the animal, apical exit of protons is coupled to an exchange of Cl(-) with base (HCO(3)(-)) either in the apical membrane (gamma-type of MR cell) or in the basolateral membrane (alpha-type MR cell). The gamma-cell model accounts for the rheogenic active uptake of Cl(-) observed in several anuran species. There is indirect evidence also for non-rheogenic active uptake accomplished by a beta-type MR cell with apical base secretion and basolateral proton pumping. Several studies have indicated that the transport modes of MR cells are regulated via ion- and acid/base balance of the animal, but the signalling mechanisms have not been investigated. Estimates of energy consumption by the H(+)-ATPase and the Na(+)/K(+)-ATPase indicate that the gamma-cell accomplishes uptake of NaCl in normal and diluted freshwater. Under common freshwater conditions with serosa-positive or zero V(t), the K(+) conductance of the basolateral membrane would have to maintain the inward driving force for Na(+) uptake across the apical membrane. With the K(+) equilibrium potential across the basolateral membrane estimated to -105 mV, this would apply to external Na(+) concentrations down to 40-120 micromol/l. NaCl uptake from concentrations down to 10 micromol/l, as observed by Krogh, presupposes that the H(+) pump hyperpolarizes the apical membrane, which would then have to be associated with serosa-negative V(t). In diluted freshwater, exchange of cellular HCO(3)(-) with external Cl(-) seems to be possible only if the proton pump has the additional function of keeping the external concentration of HCO(3)(-) low. Quantitative considerations also lead to the conclusion that with the above extreme demand, at physiological intracellular pH of 7.2, the influx of Cl(-) via the apical antiporter and the passive exit of Cl(-) via basolateral channels would be possible within a common range of intracellular Cl(-) concentrations.

Mitochondria-rich cells as experimental model in studies of epithelial chloride channels

The mitochondria-rich (mr) cell of amphibian skin epithelium is differentiated as a highly specialised pathway for passive transepithelial transport of chloride. The apical membrane of mr cells expresses several types of Cl(-) channels, of which the function of only two types has been studied in detail. (i) One type of channel is gated by voltage and external chloride concentration. This intriguing type of regulation leads to opening of channels only if [Cl(-)](o) is in the millimolar range and if the electrical potential is of a polarity that secures an inwardly directed net flux of this ion. Reversible voltage activations of the conductance proceed with long time constants, which depend on V in such a way that the rate of conductance activation increases when V is clamped at more negative values (serosal bath grounded). The gating seems to involve processes that are dependent on F-actin localised in the submembrane domain in the neck region of the flask-shaped mr cell. (ii) The other identified Cl(-) pathway of mr cells is mediated by small-conductance apical CFTR chloride channels as concluded from its activation via beta-adrenergic receptors, ion selectivity, genistein stimulation and inhibition by glibenclamide. bbCFTR has been cloned, and immunostaining has shown that the gene product is selectively expressed in mr cells. There is cross-talk between the two pathways in the sense that activation of the conductance of the mr cell by voltage clamping excludes activation via receptor occupation, and vice versa. The mechanism of this cross-talk is unknown.

Chloride conductance and mitochondria-rich cell density in isolated skin of Rana catesbeiana acclimated to various environments

The Cl- conductance in isolated skin of frogs (Rana catesbeiana) acclimated to 30 mM solutions of NaCl, Na2SO4, MgCl2 and distilled water (DW) was studied. Transepithelial potential difference (PDtrans), short-circuit current (ISC) and total conductance (Gt) were measured under conditions such that there was Cl- flux in the presence and absence of Na+ transport. The Cl- content of the mucosal solution was acutely replaced with SO42- or gluconate to evaluate the effect of removal of Cl- conductance on electrophysiological parameters. Mitochondria-rich cell density (DMRC) was also measured. Skins from frogs acclimated to NaCl and Na2SO4 showed the lowest and the highest D(MRC), respectively, but no difference could be found between the skins from frogs acclimated to DW and MgCl2 indicating that DMRC is not unconditionally dependent on environmental Cl- in this species. Frogs acclimated to NaCl showed marked differences when compared to the other groups: the highest Gt, probably represented by a higher paracellular conductance; the lowest transepithelial electrical potential difference which remained invariant after replacement of mucosal Cl- with SO42- or replacement of mucosal Cl- with gluconate and an inwardly oriented positive current in the absence of bilateral Na+.

Expression of cystic fibrosis transmembrane conductance regulator in the skin of the toad, Bufo bufo and possible role for Cl- transport across the heterocellular epithelium

Evidence is discussed that apical CFTR Cl- channels of mitochondria-rich (MR) cells of Bufo bufo skin conduct beta-adrenergic receptor-activated Cl- currents. Ussing chambers studies revealed the following selectivity sequence of the receptor activated conductance, Cl- > Br- > NO3- > I-. With ion selective microelectrode-techniques, it was shown that receptor-coupled Cl- channels are not located in principal cells. A small conductance (7-10 pS) CFTR-like Cl- channel is located in the apical plasma membrane of MR cells. Short life times of sealed patches prevented detailed study of its selectivity to other halide ions and its molecular regulation. With monoclonal hCFTR-antibodies, selective expression in MR cells of the targeted antigens could be demonstrated. A transcript of CFTR was amplified in the skin, and a bbCFTR cDNA clone was generated from toad skin mRNA that exhibits 89% amino acid identity with the human homologue. The frequency of obtaining channels in patch clamp studies was too low for accounting quantitatively for the macroscopic conductance. Since MR cells were isolated by trypsin, and a putative extracellular loop of the deduced bbCFTR protein contains a target peptide bond for trypsin, enzyme treatment may have destroyed apical CFTR molecules.

Chloride conductance across toad skin: effects of ionic acclimations and cyclic AMP and relationship to mitochondria-rich cell density

The anionic conductance across toad (Bufo viridis) skin was studied using the voltage-clamp technique following long-term (more than 10 days) acclimation to NaCl and KCl solutions. The non-specific baseline conductance was approximately 0.6 mS cm(−)(2) and was similar in skins from all acclimation conditions. The voltage-activated Cl(−) conductance (G(Cl)) was maximal in skins from distilled-water- and KCl-acclimated toads (>3 mS cm(−)(2)) and was greatly reduced following acclimation to NaCl solutions. Cyclic AMP (EC(50)=13 micromol l(−)(1)) and isobutylmethyl xanthine (IBMX) (EC(50)=69 micromol l(−)(1)) exerted different effects on the activated conductance. IBMX only sensitized the activated conductance, whereas cyclic AMP (CPTcAMP) at high concentrations induced an increase in anionic conductance that was insensitive to electrical potential. Furthermore, external Cl(−) was not required for the stimulatory effect of cyclic AMP, and the conductive pathway had low selectivity. The effects of the two agonists were reversible and depended on the acclimation conditions. Following electrical measurements, the skin of the toads was removed and stained with silver to measure mitochondria-rich cell density (D(mrc)). There was no correlation between D(mrc) and Cl(−) conductance in the present study.

Mitochondria-rich cells in anuran amphibia: chloride conductance and regional distribution over the body surface

The distribution and density (D(mrc)) of mitochondria-rich cells (MR cells) in skin epithelium, were determined over the whole body surface in nine species of anuran Amphibia that live in a variety of habitats. It was found that the more terrestrial species (beginning with Hyla arborea) have a higher density of MR cells in their pelvic region. In the skin of aquatic (Xenopus laevis) or fossorial (Pelobates syriacus) species, D(mrc) is evenly distributed over the whole body surface. In dorsal skin pieces of H. arborea that lack detectable MR cells, transepithelial voltage activation did not induce Cl(-) conductance as it did in ventral pieces. Skins from Bufo viridis and X. laevis, both have MR cells in their skin, differ markedly in their biophysical properties: a Cl(-) specific current conductance is predominant in the skin epithelium of B. viridis, and is absent in X. laevis. In the latter, anionic conductance is due to glandular secretion. The biophysical properties cannot therefore be related solely to the presence or density of MR cells. Mitochondria-rich cells are sites of Cl(-) conductance across the skin of those amphibians that show this property, but must have different function(s) in other species. It is suggested that the specific zonal distribution of MR cells in the species that were examined in this study could be due to ion exchange activity and water conservation in more terrestrial environments.