Control of sodium permeability of the outer barrier in toad skin

External Ni2 + and ENaC in A6 cells: Na+ current stimulation by competition at a binding site for amiloride and Na+

In cultured A6 monolayers from distal Xenopus kidney, external Ni2+ stimulated active Na+ uptake via the epithelial Na+ channel, ENaC. Transepithelial capacitance measurements ruled out exocytosis of ENaC-containing vesicles underlying the Ni2+ effect. Na+ current noise analysis was performed using the neutral Na(+) -channel blocker 6-chloro-3,5-diamino-pyrazine-2-carboxamide (CDPC) and amiloride. The analysis of CDPC-induced noise in terms of a three-state channel model revealed that Ni2+ elicits an increase in the number of open channels as well as in the spontaneous open probability. While Ni2+ had no influence on CDPC-blocker kinetics, the macroscopic and microscopic blocking kinetics of amiloride were affected. Ni2+ turned out to compete with amiloride for a putative binding site but not with CDPC. Moreover, external Na(+)--known to compete with amiloride and so producing the "self-inhibition" phenomenon--and Ni2+ exerted mutually exclusive analogous effects on amiloride kinetics. Na+ current kinetics revealed that Ni2+ prevents ENaC to be downregulated by self-inhibition. Co2+ behaved similarly to Ni2+, whereas Zn2+ did not. Attempts to disclose the chemical nature of the site reacting with Ni2+ suggested cysteine but not histidine as reaction partner.

Determination of transepithelial (mucosal and serosal) electrical potentials in toad skin. Action of chemical agents

1. Potential differences across the mucosal or outer, and the serosal or inner, membranes of the toad skin (M and S) were recorded separately. Total potential difference across the skin (T) and the short-circuit current (SCC) were recorded by means of the classical Ussing method. 2. The independent determination of the M and the S is of importance in the elucidation of the mechanism of action of agents which alter ion fluxes across the skin. 3. The percentage values of the M and the S obtained in toad skins during the summer were similar to the percentage values obtained by microelectrode impalement of cells. 4. Angiotensin II (AII) and antidiuretic hormone (ADH) increased T with a notable rise in M and a slight increase in S. These agents act mainly by increasing mucosal membrane permeability to Na+ since M is principally affected. 5. Amiloride and ouabain reversed M, decreased T and increased S above T. The reversal of M might be explained by the flow of a cation to the mucosal aspect or of an anion to the cell interior. 6. These results show that the effects of several agents on the toad skin potential may be analysed independently across the mucosal and serosal membranes and reflect the behaviour of the entire tissue rather than of a single cell.

Structural and functional response of the isolated toad skin to mucosal lithium

Structural and functional changes induced by long-term Li-exposure of the outer surface of toad skin was studied. Electron microscopy revealed that total Na by Li replacement in the outer compartment of short-circuited toad skin promotes a conspicuous cellular damage expressed as focal swollen cells with altered intercellular spaces and nuclear morphology. Short-circuit current (SCC) decreases by about 70% over the first 60 min after 115 mmol/l Li-exposure. An amiloride sensitive transepithelial Li transport remains intact over a further 150 min despite the epithelial damage, indicating that the pathways across the apical barrier are functioning. Increase of the paracellular permeability is detected by elevation of Na-efflux. Partial 50% or 10% Na by Li replacements induce minor structural alterations and are not sufficient to trigger appreciable Na-efflux and SCC alterations. Therefore, low Li concentrations are less effective than 115 mmol/l in promoting morphofunctional responses. Although ouabain and Li reduce the SCC, ouabain does not promote structural lesions, showing that Li-inhibition of Na transport by itself is not responsible for the observed morphological alterations. In the light of this study, Li utilization as a tool to investigate transepithelial Na transport requires careful judgement.

Sodium flux in the apical membrane of the toad skin: aspects of its regulation and the importance of the ionic strength of the outer solution upon the reversibility of amiloride inhibition

Injection of small pulses of concentrate solutions of salts or drugs into the outer bathing fluid led to sudden increases of its solute concentration. Vigorous stirring of the outer bathing solution was used to minimize the thickness of the unstirred layer adjacent to the outer skin surface. Pulses of 1 M NaCl injected into the outer compartment induced sharp increases of the SCC following a time course variable with the magnitude of the pulse and the particular condition of each skin. Comparison of the spontaneous decline of the SCC with the decline induced by a small dose of amiloride, where an increase in R was observed, indicates that the spontaneous decline cannot be explained simply as a reduction of the Na permeability of the apical membrane by self-inhibition of feedback inhibition of the apical membrane Na channels. Reduction of the driving force for Na movement into the epithelial cells must play an important role in the process. Reversibility of the amiloride inhibition of the SCC was highly dependent upon the ionic strength of the solution used to rinse and wash out the inhibitor from the outer skin surface. With H2O, the amiloride molecules washed out slowly as compared to NaCl or KCl solutions. Na or K have the same ability to dislodge the amiloride molecules from their binding sites. This effect is apparently of a purely electrostatic nature.

Lack of PCMB action upon the outer barrier sodium permeability in the absence of Na in toad skin

The stimulation of apical Na permeability by p-chloromercuribenzoate (PCMB) was evaluated by studying its effect on unidirectional 24Na flux (JNaeff) from inner to outer compartment in the isolated short-circuited toad skin. With Na present in the inner compartment, addition of PCMB to the outer skin surface led to an increase of JNaeff. However, after replacing all Na of the inner compartment by K or choline, the amiloride inhibitable Na permeability of the outer cell membrane increased, and PCMB was no longer effective. It is concluded that PCMB releases the blocking effect of intracellular Na on apical Na channels. If intracellular Na is drastically reduced virtually all Na channels are opened so that PCMB does no longer act.

Passive cation permeability of turtle colon: evidence for a negative interaction between intracellular sodium and apical sodium permeability

The role of intracellular sodium in the regulation of apical sodium permeability was investigated in an electrically "tight" epithelium, the turtle colon. In the presence of low mucosal sodium (3 mM) and serosal ouabain, an inhibitor of the basolateral sodium pump, the apical membrane retained a substantial amiloride-sensitive, sodium conductance and the basolateral membrane exhibited a barium-sensitive potassium conductance in parallel with a significant sodium (and lithium) conductance. In the presence of a high mucosal sodium concentration (114 mM), however, inhibition of active sodium absorption by ouabain led to a disappearance of the amiloride-sensitive, transepithelial conductance that was due, at least in part, to a virtual abolition of the apical sodium permeability. Two lines of evidence indicate that this permeability decrease was dependent upon an increase in intracellular sodium content. First, raising the mucosal sodium concentration from 3-114 mM in the presence of ouabain reversibly inhibited the amiloride-sensitive conductance. The time course of the decline in conductance paralleled the apparent intracellular accumulation of sodium in exchange for potassium, which was monitored as a transient deflection in the amiloride-sensitive, short-circuit current. Second, the inhibitory effect of mucosal sodium-addition was markedly attenuated by serosal barium, which prevented the accumulation of sodium by blocking the electrically coupled, basolateral potassium exit. These results support the notion of a "negative feedback" effect of intracellular sodium on the apical sodium permeability.

Intracellular electrolyte concentrations in the frog skin epithelium: effect of vasopressin and dependence on the Na concentration in the bathing media

The intracellular electrolyte concentrations of the frog skin epithelium have been determined in thin freeze-dried cryosections using the technique of electron microprobe analysis. Stimulation of the transepithelial Na transport by arginine vasopressin (AVP) resulted in a marked increase in the Na concentration and a reciprocal drop in the K concentration in all epithelial cell layers. The effects of AVP were cancelled by addition of amiloride. It is concluded from these results that the primary mechanism by which AVP stimulates transepithelial Na transport is an increase in the Na permeability of the apical membrane. However, also some evidence has been obtained for an additional stimulatory effect of AVP on the Na pump. In mitochondria-rich cells and in gland cells no significant concentration changes were detected, supporting the view that these cells do not share in transepithelial Na transport. Furthermore, the dependence of the intracellular electrolyte concentrations upon the Na concentration in the outer and inner bathing solution was evaluated. Both in control and AVP-stimulated skins the intracellular Na concentration showed saturation already at low external Na concentrations, indicating that the self-inhibition of transepithelial Na transport is due to a reduction of the permeability of the apical membrane. After lowering the Na concentration in the internal bath frequently a Na increase in the outermost and a drop in the deeper epithelial layers was observed. It is concluded that partial uncoupling of the transport syncytium occurs, which may explain the inhibition of the transepithelial Na transport and blunting of the AVP response under this condition.

Vanadate and ouabain: a comparative study in toad skin

In this study we compare the effects of two inhibitors of the Na,K-ATPase, ouabain and vanadate, upon transport properties of the isolated short-circuited toad skin: The main conclusions are: Both inhibitors induce a similar decline in short-circuit current (SCC). They differ regarding skin electrical resistance (R). Ouabain induces an increase in resistance that, after some delay, builds up slowly after its addition to the preparation, while vanadate causes a fast increase in resistance that remains constant for most of the experimental period. Vanadate, but not ouabain, promotes an unspecific increase in skin permeability characterized by a delayed and progressive rise of 42K (JK eff) and 14C sucrose (J suc eff) effluxes. Vanadate effect upon skin permeability, as measured by JK eff, is not affected by pre-treating the skin with DIDS, a stilbene derivative, indicating that anion-exchange is not an important step for the entrance of vanadate into the epithelial cells to trigger its effect. Vanadate effect upon JK eff is also not affected by previous ouabain inhibition of the Na,K-ATPase, showing that this effect is not mediated by the inhibition of this enzyme. Vanadate action in toad skin seems to occur at junctional structures opening paracellular routes. A possible mechanism for the effect of vanadate is discussed in terms of cytosolic Ca2+ balance, cytoskeleton and their interplay with the sealing of tight junctions.

Feedback inhibition of sodium uptake in K+-depolarized toad urinary bladders

Ouabain-blocked toad urinary bladders were maintained in Na+-free mucosal solutions, and a depolarizing solution of high K+ activity containing only 5 mM Na+ on the serosal side. Exposure to mucosal sodium (20 mM activity) evoked a transient amiloride-blockable inward current, which decayed to near zero within one hour. The apical sodium conductance increased in the initial phase of the current decay and decreased in the second phase. The conductance decrease required Ca2+ to be present on the serosal side and was more rapid when the mucosal Na+ activity was higher. At 20 mM mucosal Na+ and 3 mM serosal Ca2+ the initial (maximal) rate of inhibition amounted to 20% in 10 min. The conductance decrease could be accelerated by raising the serosal Ca2+ activity to 10 mM. The inhibition reversed on lowering the serosal Ca2+ to 3 microM and, in addition, the mucosal Na+ to zero. Exposure of the mucosal surface to the ionophore nystatin abolished the Ca2+ sensitivity of the transcellular conductance, showing that the Ca2+-sensitive conductance resides in the apical membrane. The data imply that in the K+-depolarized epithelia, cellular Ca2+, taken up from the serosal medium by means of a Na+-Ca2+ antiport, cause feedback inhibition by blockage of apical Na+ channels. However, the rate of inhibition is small, such that this regulatory mechanism will have little effect at 1 mM serosal Ca2+ and less than 20 mM cellular Na+.

Chemical stimulation of Na transport through amiloride-blockable channels of frog skin epithelium

The stimulation of apical Na permeability caused by a number of reagents effective from the outer side of the membrane was investigated by fluctuation analysis. In the epidermis of R. ridibunda, parachloromercuriphenyl sulfonate (PCMPS) and benzimidazolyl guanidine (BIG) increase the number (N0) of conducting Na channels by releasing channels from Na self-inhibition. As a consequence, the apparent macroscopic affinity for amiloride is increased. 5-dimethyl amiloride and trinitrobenzene sulfonate (TNBS) also cause reversible stimulation by increasing N0; here release from self-inhibition is less clear. With each of the four stimulators investigated, the Na channel current remained unaffected or was only marginally increased. In addition to its stimulatory effect, TNBS caused irreversible blockage of Na channels. Apart from their stimulatory effects, BIG and 5-dimethyl amiloride, both of which have a side-chain terminated with an amidino group, are high rate-blocking competitors of amiloride.

Cellular Li+ opens paracellular path in toad skin: amiloride blockable effect

The presence of Li in the solution bathing the outer surface of toad skin under short-circuit condition promotes an unspecific permeability increase characterized by a delayed and progressive increase in the effluxes of 24Na, 42K and 14C sucrose. The effect of Li upon sucrose permeability might indicate an increased permeability of the paracellular pathway. The Li effect is mediated by an intracellular action since blockade of Li entrance into the cell compartment by amiloride prevents the increase in Na, K and sucrose permeability. A possible mechanism of this effect is discussed in terms of a disturbance in the cellular Ca++ balance leading to an increase in cytosolic Ca++ concentration which perturbs the organization of the cytoskeleton and the interplay between cytoskeleton and tight junctions.

Modelling the control mechanism of the sodium channel in the apical membrane of tight epithelia

The unidirectional inward Na + flux, J Na IN , passing through the sodium channel in the apical membrane of some tight epithelia depends approximately linearly on [Na] OUT as expected, but is also known to depend sensitively and inversely upon [Na] IN while the ratio of the fluxes obeys the relation J Na IN / J Na OUT = [Na] OUT / [Na] IN In this paper it is shown that such behaviour may be simply modelled based on the assumption of only single occupancy of the channel. With use of linear flux equations the calculated flux ratio is as measured. The additional assumption of single occupancy of the channel results in the calculated values of J Na IN and J Na OUT both being multiplied by the same factor, 1— P , where P is the probability that the channel is occupied by an ion. The flux ratio thus remains unchanged but J Na IN and J Na OUT may now depend inversely upon [Na + ] IN or [Na + ] OUT or both, owing to the dependence of P upon these parameters. To illustrate these general conclusions, detailed calculations are made on a particular assymetrical voltage-controlled model ion channel, based upon an ordered water structure. The results obtained are shown to simulate those found experimentally for tight epithelia.