Uptake of Br in mitochondria-rich and principal cells of toad skin epithelium

The route of passive chloride movement across amphibian skin: localization and regulatory mechanisms

Transepithelial Cl(-) conductance (G(Cl)) in amphibian skin can be activated in several species by serosa positive potentials. Mitochondria-rich cells (MRC) or tight junctions (TJ) between the epithelial cells are possible sites for this pathway. The properties and the techniques used to investigate this pathway are reviewed in the present paper. In situ techniques are preferable, since specific properties of the MRC are apparently not maintained in isolated cells. Volume measurements and electronprobe microanalysis of intracellular ions suggest the localization of voltage-activated G(Cl) to MRC. G(Cl) correlates poorly with the density of MRC. The vibrating voltage probe allows quantitative correlation of the local Cl(-) current through morphologically identified structures and the transepithelial Cl(-) current. Our analysis shows that 80% of the voltage-activated Cl(-) current is accounted for by current through MRC or their immediate vicinity. The activation patterns of this current and the inhibition by the alpha(1)-adrenergic agonist, epinephrine, conform to those of the transepithelial current. However, less than 20% of the MRC are active at a certain moment and the activity is spontaneously variable with time. The molecular nature of this pathway, physiological control mechanisms and their relation to the temporal activity of MRC remain to be studied.

Use of Rb(+) and Br(-) as tracers for investigating ion transport by X-ray microanalysis in the Malpighian tubules of the black field cricket Teleogryllus oceanicus

Substitution of Rb(+) for K(+) in the incubation saline for in vitro preparations of Malpighian tubules had little effect on tubule function. Secretion rates increased by 10% for whole tubules, 9% for distal segments and 10% for main segments. In the secreted fluids Rb(+) almost completely replaced K(+). Within the cells of the main segment of the tubules Rb replaced the majority of the intracellular K. Treatment by ouabain in Rb saline resulted in a considerable increase in intracellular Na and Cl concentrations but no change in Rb concentration. This suggests that Rb(+) did not enter the cell via Na K ATPase and that the latter was not directly involved in Rb(+) secretion and by inference K(+) secretion. Substitution of Br(-) for Cl(-) in the incubation saline resulted in a 30% reduction in secretion rate from the distal segments but only a 10% reduction for the main segment. Cl(-) was almost completely replaced by Br(-) in fluid from both main and distal segments. In cells of the main segment the intracellular concentration of Br(-) did not exceed 30mmol kg(-1) dry weight and the Cl(-) concentration was unchanged in the apical region of the cell and increased in the basal region. These data suggest that Br(-) was transported across the tubule epithelium by a paracellular route and that the basal cell membrane is relatively impermeable to Cl(-). By inference Cl(-) may also be transported by a paracellular route.

Effects of cyclic AMP and theophylline on chloride conductance across toad skin

1. The effects of the phosphodiesterase inhibitors theophylline and isobutylmethylxanthine (IBMX) on baseline and voltage-activated Cl- conductance (gCl) of toad skin were compared with those of the potent 2-chlorophenylthio analogue of cAMP (CPT-cAMP). 2. Using intact and split skins of Bufo viridis we confirmed that theophylline and IBMX raised the voltage-activated gCl with a pattern identical to that seen under control conditions. This effect was small or missing if gCl was already high in the control. 3. CPT-cAMP, in contrast, increased the Cl(-)-specific conductance by up to 6 mS cm-2 at short circuit. The characteristic time-dependent, slow activation of gCl by serosa-positive clamp potentials was completely lost under these conditions. 4. Coinciding with the loss of voltage activation of gCl the plateau value of the Lorentzian component of fluctuation in current at serosa-positive clamp potentials decreased by almost 50%. The corner frequencies were not notably different. 5. After CPT-cAMP, the sigmoidal voltage-conductance relation that is characteristic of control conditions or after theophylline disappeared; the patterns were variable and incompatible with voltage activation. 6. The voltage-activated gCl under control conditions and with theophylline was blocked by mucosal NO3-, I- or SCN-, the last two being almost equally effective. In the presence of CPT-cAMP, mucosal NO3- had minimal influence on tissue conductance, whereas the effects of I- and SCN- were essentially unchanged. Br- on the mucosal side could substitute for Cl- under all conditions. 7. The results suggest that protein phosphorylation by supramaximal concentrations of cAMP induces maximal conductance through anion-specific routes, while the voltage sensitivity of this pathway is lost. The effects of theophylline and IBMX on the voltage-activated Cl-conductance of toad skin cannot be explained solely by inhibition of the phosphodiesterase.

Cyclic AMP-and beta-agonist-activated chloride conductance of a toad skin epithelium

1. The control by intracellular cyclic AMP and beta-adrenergic stimulation of chloride conductance was studied in toad skin epithelium mounted in a chamber on the stage of an upright microscope. Impalement of identified principal cells from the serosal side with single-barrelled conventional or double-barrelled Cl(-)-sensitive microelectrodes was performed at x500 magnification. For blocking the active sodium current 50 microM-amiloride was present in the mucosal bath. 2. When clamped at transepithelial potential difference V = 0 mV, the preparations generated clamping currents of 0.9 +/- 1 microA/cm2 (mean +/- S.E.M.; number of observations n = 55). The intracellular potential of principal cells (Vb) was -96 +/- 2 mV with a fractional resistance of the basolateral membrane (fRb) of 0.016 +/- 0.003 (n = 54), and an intracellular Cl- activity of 40 +/- 2 mM (n = 24). 3. At V = 0 mV, serosal application of a cyclic AMP analogue, dibutyryl cyclic AMP (500 microM) or a beta-adrenergic agonist, isoprenaline (5 microM) resulted in a sixfold increase in transepithelial Cl- conductance identified by standard 36Cl- tracer technique. 4. The clamping current at V = 0 mV was unaffected by cyclic AMP (short-circuit current Isc = 0.1 +/- 0.3 microA/cm2, n = 16) indicating that subepidermal Cl(-)-secreting glands are not functioning in our preparations obtained by collagenase treatment. 5. Cyclic AMP- or isoprenaline-induced chloride conductance (Gcl) activation (V = 0 mV) was not reflected in membrane potential and intracellular Cl- activity in principal cells. Intracellular chloride activity was constant at approximately 40 mM at membrane potentials between -90 and -100 mV. Therefore, it can be concluded that the principal cells are not contributing to activated Cl- currents. 6. At V = -100 mV where the voltage-dependent chloride conductance of mitochondria-rich (MR) cells was already fully activated, GCl was unaffected by cyclic AMP or isoprenaline. The major effect of these treatments was a rightward displacement of the MR cell-generated GCl-V relationship along the V axis. 7. Our results indicate that the beta-adrenergically controlled cyclic AMP-mediated chloride conductance is localized to the mitochondria-rich cells.

Chloride-related current fluctuation in amphibian skin

Fluctuation in transepithelial current was analysed in skins of Rana esculenta and Bufo viridis. Perturbation of the tissues to serosa + 40 mV or + 80 mV activated a pathway for conductive Cl transport and led to the appearance of a Lorentzian component in the power density spectrum. The presence of Cl on the mucosal side was mandatory for the observation of Lorentzians. Corner frequencies, fc, ranged between 48 Hz and 102 Hz. The mean value was slightly larger at 40 mV than at 80 mV (88.3 +/- 4.4 versus 72.5 +/- 6.4 Hz). Plateau values, So, were proportional to the magnitude of the activated Cl current. Single-channel currents of 48.3 +/- 5.2 fA and 93.0 +/- 8.7 fA were calculated at 40 mV and 80 mV, respectively, with the assumption of equal open and closed probabilities of the channels. From these data, a voltage-dependent open-channel conductance of about 1.2 pS is obtained. Activation of Cl conductance was associated with an increase in channel density. Stimulation of Cl conductance by procaine did not affect So and fc despite considerable stimulation of transepithelial Cl current. This could indicate that non-gated channels participate in the voltage-sensitive Cl conductance. Mucosal application of inhibitors of Cl conductance (3',5-dichlorodiphenylaminocarboxy acid, MK-196) decreased So in a concentration-dependent manner, but had no effect on fc. The data could support the hypothesis that the voltage-sensitive transepithelial Cl transport is localized to a cellular compartment, which is most likely the mitochondria-rich cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of amiloride on electrolyte concentrations and rubidium uptake in principal and mitochondria-rich cells of frog skin

The role of mitochondria-rich cells (MR cells) in transepithelial Na transport was investigated by determining electrolyte concentrations and Rb uptake in individual cells of frog skin epithelium using electron microprobe analysis. Measurements were performed under control conditions and after blocking the transepithelial Na transport with amiloride. Under control conditions, Na and Cl concentrations of MR cells scattered much more than those of principal cells and ranged from a few up to more than 30 mmol/kg wet weight. Rb uptake from the basal side into individual MR cells also showed a large variation and was, on the average, much less pronounced than into the principal cells. In principal cells, amiloride reduced the Na concentration and Rb accumulation. In contrast, no effect was observed upon electrolyte concentration and Rb uptake of MR cells. Rb uptake was correlated to the Na concentration of MR cells both under control conditions and after amiloride. It is concluded that, in contrast to the principal cells, MR cells are not involved in amiloride-sensitive transepithelial Na transport and that their Na/K-pump activity is very low.

Analysis of anion conductance in frog skin

Electrophysiological characteristics of transepithelial Cl-specific conductance (gCl) and intracellular element concentrations were analyzed in frog skins before and during voltage perturbation to serosa +100 mV, both under control conditions and after mucosal application of procaine. Under control conditions, gCl was often minimal and almost insensitive to voltage perturbation. Procaine stimulated gCl in many cases considerably and further activation resulted then from voltage perturbation. Microelectrode determinations indicated that conductive pathways parallel to the principal cells account for the procaine-induced increase in gCl. The responses in gCl were not related to the density of mitochondria-rich (MR) cells. Electron microprobe analysis of intracellular electrolyte concentrations showed that procaine increased the Cl content of MR cells significantly. Gain of Cl was primarily due to uptake across the basolateral membrane, as indicated by the small accumulation of Br after unilateral mucosal application. Voltage perturbation to serosa +100 mV in the presence of Br on the mucosal side led in procaine-stimulated tissues to an increase of the ratio of Br/Cl content in the majority of MR cells. It was much less than predicted for conductive transcellular anion transport. Also, intracellular Cl concentrations of MR cells were far above those expected for a highly Cl-permeable basolateral membrane. The data, although indicating finite Cl/Br transport across MR cells, are incompatible with the idea that the voltage-activated conductive Cl transport occurs though these cells. Alternatively, we suggest passage across highly Cl-specific sites of a paracellular pathway.

Localization of transport compartments in turtle urinary bladder

To characterize different transport compartments in the urinary bladder epithelium of postabsorptive turtles, the electrolyte composition of individual cells was determined using electron microprobe analysis. After blocking the transepithelial Na transport, the short-circuit current decreased from positive to negative values (from 26.5 +/- 17.7 to -3.9 +/- 2.9 after ouabain and from 25.4 +/- 17.2 to -8.0 +/- 5.1 microA/cm2 after amiloride). Whereas under control conditions the Na and K concentrations were similar in all cell types and the same was true for Cl in most of the cells, some cells exhibited very low Cl concentrations. The epithelial cells were subdivided according to their electrolyte composition into ouabain-sensitive and ouabain-insensitive ones. In the ouabain-sensitive cells, which made up the majority of epithelial cells and showed a relatively high Cl concentration (about 36 mmol/kg wet weight), the Na concentration increased after ouabain by about 90 mmol/kg wet weight and the K concentration decreased by a similar amount. Since these alterations could largely be prevented when amiloride was applied before ouabain, it is suggested that the granular and basal cells form a syncytial Na transport compartment similar to that in other multilayered epithelia. The ouabain-insensitive cells, in which almost no alteration in Na and K concentrations was observed after ouabain, were subdivided into a Cl-rich (34.6 +/- 7.6 mmol/kg wet weight) and a Cl-poor (12.0 +/- 5.6 mmol/kg wet weight) population. Since in these cells no large mucin granules were detectable, they are regarded as carbonic anhydrase-rich cells involved in H and HCO3 transport.