THE INFLUENCE OF NA CONCENTRATION ON NA TRANSPORT ACROSS FROG SKIN

Nutrition and health in amphibian husbandry

Amphibian biology is intricate, and there are many inter-related factors that need to be understood before establishing successful Conservation Breeding Programs (CBPs). Nutritional needs of amphibians are highly integrated with disease and their husbandry needs, and the diversity of developmental stages, natural habitats, and feeding strategies result in many different recommendations for proper care and feeding. This review identifies several areas where there is substantial room for improvement in maintaining healthy ex situ amphibian populations specifically in the areas of obtaining and utilizing natural history data for both amphibians and their dietary items, achieving more appropriate environmental parameters, understanding stress and hormone production, and promoting better physical and population health. Using a scientific or research framework to answer questions about disease, nutrition, husbandry, genetics, and endocrinology of ex situ amphibians will improve specialists' understanding of the needs of these species. In general, there is a lack of baseline data and comparative information for most basic aspects of amphibian biology as well as standardized laboratory approaches. Instituting a formalized research approach in multiple scientific disciplines will be beneficial not only to the management of current ex situ populations, but also in moving forward with future conservation and reintroduction projects. This overview of gaps in knowledge concerning ex situ amphibian care should serve as a foundation for much needed future research in these areas.

Sodium-glucose interactions in the goldfish intestine

1. Everted sacs of goldfish intestine transfer glucose and water to their serosal surfaces and the total transfer is greater in the anterior intestine than in the intestinal bulb or rectum.2. Transmural potentials, with the serosa positive to the mucosa, were recorded from all parts of the goldfish intestine but were highest in the anterior intestine and the rectum. In both these areas the total potential was dependent partly upon the presence of glucose.3. Reducing the concentration of sodium bathing the mucosa of the anterior intestine reduced the glucose-evoked potential in a non-linear way. The steady-state potentials, with or without glucose, first increased and later decreased as the sodium concentration was further reduced.4. Reducing the concentration of glucose bathing the mucosa from 27 to 5 mM slightly increased the glucose-evoked potential. Further reduction of the glucose concentration caused the glucose-evoked potential to fall.5. Phlorrhizin inhibited the glucose-evoked potential. The degree of inhibition was proportional to the log concentration of phlorrhizin over the range 2 x 10(-7)-6 x 10(-5)M. The steady-state potential with glucose present was lower than when glucose was absent at phlorrhizin concentrations, 6 x 10(-6)-6 x 10(-5)M.6. The glucose-evoked potential increased rapidly over the temperature range 5-15 degrees C and more slowly from 15 to 30 degrees C. The steady-state potentials also increased with temperature, the rate of increase being greater when glucose was present. Below 15 degrees C the glucose-independent potential was higher and above 15 degrees C lower than the steady-state potential recorded with glucose present.7. These findings are discussed in terms of sodium-glucose interaction taking place at the luminal side of the mucosal cell, on the outside of the mucosal cell membrane.

Active sodium transport by the colon of Bufo marinus: stimulation by aldosterone and antidiuretic hormone

1. The isolated colon of Bufo marinus transports sodium actively from the mucosal (lumen) to the serosal side, and this transport is expressed quantitatively by the short-circuit current.2. Upon dilution of sodium in Ringer solution on the mucosal side of the preparation, short-circuit current remained a fair expression of sodium transport from mucosa to serosa.3. In view of this, the relation between short-circuit current and dilution of sodium of the luminal side was examined. This relation was curvilinear, which suggests the intervention of a saturable step in the transfer of sodium from lumen to serosal surface of colon.4. The relation between short-circuit current on the one hand, and the amount of sodium drawn from the luminal side and recovered in the membrane (;active sodium transport pool') on the other hand, appeared (almost) linear instead. This is meant to indicate that the ;pump' operates far from capacity. Hence, the observed saturation of sodium transport, when concentration of sodium on the mucosal side was increased, probably occurs at the mucosal border of the preparation.5. After treatment with aldosterone, the ;active sodium transport pool' and short-circuit current increased to the same extent, from which it is inferred that the hormone merely allows sodium easier access to the ;pump' which would react in proportion. Consequently, no direct influence of aldosterone on the ;pump' proper need be postulated.6. Upon exposure of the colon to antidiuretic hormone, there were (modest) increases of short-circuit current and of osmotic water flow across the wall of the organ.

Control of tight junctional sealing: role of epidermal growth factor

Epithelia can adjust the permeability of their paracellular permeation route to physiological requirements, pathological conditions, and pharmacological challenges. This is reflected by a transepithelial electrical resistance (TER) ranging from a few tenth to several thousands Ω·cm2, depending on the degree of sealing of the tight junction (TJ). The present work is part of an effort to understand the causes and mechanisms underlying these adaptations. We observed that an extract of human urine (hDLU) increases TER in a concentration- and time-dependent manner and is more effective when added from the basolateral side of cultured monolayers of Madin-Darby canine kidney cells than from the apical one. We found that its main TER-increasing component is epidermal growth factor (hEGF), as depletion of this peptide with specific antibodies, or inhibition of its receptor with PD153035, abolishes its effect. Since the permeability of the TJ depends on the expression of several species of membrane proteins, chiefly claudins, we explored whether hDLU can affect five members of the claudin family, the three known members of the ZO family, and occludin. EGF present in hDLU decreases the content of claudins-1 and -2 as well as delocalizes them from the TJ and increases the content of claudin-4. As expected from the fact that the degree of sealing of the TJ must be a physiologically regulated parameter, besides of hEGF, we also found that hDLU appears to contain also other components that decrease TER, claudin-4 and -7, and that seem to act with different kinetics than the TER-increasing ones.

Cell Adhesion, Polarity, and Epithelia in the Dawn of Metazoans

Transporting epithelia posed formidable conundrums right from the moment that Du Bois Raymond discovered their asymmetric behavior, a century and a half ago. It took a century and a half to start unraveling the mechanisms of occluding junctions and polarity, but we now face another puzzle: lest its cells died in minutes, the first high metazoa (i.e., higher than a sponge) needed a transporting epithelium, but a transporting epithelium is an incredibly improbable combination of occluding junctions and cell polarity. How could these coincide in the same individual organism and within minutes? We review occluding junctions (tight and septate) as well as the polarized distribution of Na+-K+-ATPase both at the molecular and the cell level. Junctions and polarity depend on hosts of molecular species and cellular processes, which are briefly reviewed whenever they are suspected to have played a role in the dawn of epithelia and metazoan. We come to the conclusion that most of the molecules needed were already present in early protozoan and discuss a few plausible alternatives to solve the riddle described above.

INTRACELLULAR ELECTRICAL POTENTIALS IN FROG SKIN

The influence of changes in ionic composition of the bathing solutions on intracellular electrical potentials in frog skin has been examined. When the skin bathed in SO₄ Ringer's solution is penetrated with a microelectrode two approximately equal potential jumps were frequently observed and most experiments were carried out with the electrode located between these steps. Substitution of Cl for SO₄ in the bathing solutions caused a decrease in PD across both the "outer" and "inner" barriers. When the skin was short-circuited an average intracellular potential of -18 mv was found with both Cl and SO₄ Ringer's. With the skin in SO4 Ringer's, decrease in Na concentration of the outside solution caused a decrease in PD between the microelectrode and the outside solution which was approximately the same as the decrease in total skin PD. With SO₄ Ringer's, an increase in K concentration in the inside solution caused a marked decrease in total skin PD. However, only 50 per cent of this change occurred at the inner barrier, between the microelectrode and the inside solution. The remainder of the change occurred at the outer barrier. This observation does not appear to be consistent with the model of the skin proposed by Koefoed-Johnson and Ussing (Acta Physiol. Scand., 1958, 42, 298).

Interaction of calcium with plasma membrane of epithelial (MDCK) cells during junction formation

We have previously shown that upon transferring confluent monolayers of Madin-Darby canine kidney (MDCK) cells from low- to normal-Ca2+ medium, cytosolic Ca2+ increases and tight junctions (TJs) assemble and seal, but the increase in cytosolic Ca2+ does not seem to be necessary for junction formation. In the present work we establish that these are in fact two independent phenomena. We first measured unidirectional Ca2+ fluxes across the plasma membrane of MDCK cells to find suitable inhibitors and tested their effects on the ability of Ca2+ to seal the TJ. Likewise, we studied a variety of multivalent cations. We observed that 1) Ca2+ triggering of junction formation does not depend on its entering the cell, 2) cations like La3+ may impair the influx of Ca2+ without affecting the sealing of TJs, and 3) only Cd2+ is able to block both Ca2+ penetration and junction formation; however, 4) Cd2+ itself cannot trigger junction formation. We interpret that Ca2+ triggers junction formation by acting mainly on an extracellular membrane site and that this site has a higher Ca2+ selectivity than the mechanisms for Ca2+ translocation across the membrane.

Rescue of a wild-type MDCK cell by a ouabain-resistant mutant

When wild-type MDCK cells (W-MDCK) were cocultured in mixed monolayers with a ouabain-resistant mutant (R-MDCK), the wild-type cells were protected from the effect of ouabain up to concentrations as high as 100 microM. Rescue depended on the dose of ouabain and on the proportion of each cell type in the coculture. The survival of R-MDCK cells at 1 microM ouabain was not endangered by varying from 1:9 to 9:1 the proportion of W-MDCK cells to be rescued. Ouabain binding revealed two kinds of binding sites in R-MDCK cells, one with high and the other with low affinity. Only the high affinity site was present in W-MDCK cells. Electron probe analysis of individual cells revealed that rescued cells kept a high K and a low Na intracellular contents, similar to control cells. Histograms of intracellular K/Na in cocultured cells treated with ouabain were unimodal. Using microinjection of Lucifer yellow or electrophysiological techniques we estimated that at most 13% of the R-MDCK and W-MDCK cells may be connected at a given time through cell-to-cell junctions. Therefore permanent cell-to-cell communication did not seem to play a central role in the rescue. W-MDCK cells cocultured with R-MDCK cells and subsequently separated, were not rescued. Thus rescue did not seem to depend on the transfer from R-MDCK to W-MDCK cells of either ouabain-resistant Na-K pumps or of information to synthesize them. It is speculated that intercellular communications were sporadic events, so that all cells may become intermittently connected and rescued.

Exploration of apical sodium transport mechanisms in an epithelial model by network thermodynamic simulation of the effect of mucosal sodium depletion: I. Comparison of three different apical sodium permeability expressions

A model based on that of Koefoed-Johnsen & Ussing (1958) and elaborated by Hviid Larsen (1978) and Lew et al. (1979), is designed using network thermodynamic theory and used to simulate experiments performed on epithelia. Three different expressions for the apical sodium permeability are tested for their ability to reproduce the saturation of the short-circuit current with increasing mucosal sodium concentration. Using the parameters from the previous models, the sodium entry step is shown to be the rate limiting step. If the apical sodium permeability is constant, there is no saturation of the short-circuit current with increasing mucosal sodium. The saturation of the short-circuit current is simulated with versions of the model which include a variable apical sodium permeability. The phenomenological expressions used for the variable permeabilities are those proposed by Fuchs et al. (1977) and Civan & Bookman (1982). They describe the so-called feedback effect of the mucosal and intracellular sodium concentrations.

Effects of standard diuretics and RPH 2823 on transepithelial Na+ transport in isolated frog skin

Short-circuit current (SCC) techniques were used to monitor the effects of various diuretic agents on Na+ transport in isolated frog skin, a model for the late distal tubule and the collecting duct of the mammalian kidney. Acetazolamide, hydrochlorothiazide, torasemide, and ethacrynic acid did not affect sodium transport (as indicated by the SCC) or transepithelial electrical resistance when added either to the apical (outer) or to the inner (basolateral, corial) bathing solution of the tissue. However, Na+ transport was sensitive to amiloride, the triamterene derivate dimethylamino-hydroxypropoxytriamterene (RPH 2823), and to furosemide. Whereas apical amiloride, and RPH 2823 induced a dose-dependent decrease in SCC and increase in transepithelial electrical resistance, apical furosemide resulted in a dose-dependent increase in SCC and a decrease in electrical resistance. None of the three diuretic agents caused a significant change in SCC when applied to the inner bathing Ringer's solution. The small furosemide-induced decrease in resistance compared with the huge increase in SCC suggests that furosemide affects Cl- permeability as well as Na+ permeability. Evidence for this notion was achieved by the following findings: The decrease in resistance after furosemide was more pronounced in tissues bathed in Cl(-)-free solutions compared with Cl(-)-containing solutions. n contrast, SCC stimulation by apical furosemide is Cl(-)-ion independent, but strongly Na+-ion dependent. SCC stimulation by furosemide is amiloride-sensitive. With respect to the onset, locus, and reversibility of action, it seems reasonable to assume that amiloride, RPH 2823, and furosemide all influence transepithelial Na+ transport by interacting with the Na+ channel or a regulator site of it within the apical membrane. The stoichiometry of the amiloride (RPH 2823)-receptor site interaction revealed Hill-coefficient(s) of less than 1, indicating a negative cooperativity among the receptor sites. The interaction between Na+ ions and amiloride or RPH 2823 displayed mixed competitive-noncompetitive inhibition. Taken together, these results support the hypothesis that amiloride and Na+ as well as RPH 2823 and Na+ may act at different loci on the apical entry mechanism in Rana esculenta skin.

The amiloride-sensitive sodium channel

Net Na+ movement across the apical membrane of high-electrical resistance epithelia is driven by the electrochemical potential energy gradient. This entry pathway is rate limiting for transepithelial transport, occurs via a channel-type mechanism, and is specifically inhibited by the diuretic drug amiloride. This channel is selective for Na+, Li+, and H+, saturates with increasing extracellular Na+ concentration, and is not affected, at least in frog skin epithelium, by changes in apical membrane surface potential. There also appears to be multiple inhibitory regions associated with each Na+ channel. We discuss the possible implications of a voltage-dependent block by amiloride in terms of macroscopic inhibitory phenomena. We describe the use of cultured epithelial systems, in particular, the toad kidney-derived A6 cell line, and the preparation of apical plasma membrane vesicles to study the Na+ entry process. We discuss experiments in which single, amiloride-sensitive channel activity has been detected and summarize current experimental approaches directed at the biochemical identification of this ubiquitous Na+ transport system.

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.

Synthesis and characterization of methylbromoamiloride, a potential biochemical probe of epithelial Na+ channels

We report the synthesis of a radioactive, methylated analog of bromoamiloride which inhibits the amiloride-sensitive, epithelial Na+ channel reversibly and with high affinity. This synthesis was achieved by methylation of a nitrogen in the acylguanidinium moiety with tritiated methyliodide of high specific activity. This methylated bromoamiloride molecule (CH3BrA) was purified by both thin layer and high performance liquid chromatography. Proton nuclear magnetic resonance and mass spectroscopy techniques were used to determine the structure of this analog. This compound inhibited both short-circuit current of in vitro frog skin and 22Na+ influx into apical plasma membrane vesicles made from cultured toad kidney cells (line A6) with the same or lower apparent inhibitory dissociation constant as bromoamiloride. Irradiation with ultraviolet light rendered this inhibition irreversible in both A6 vesicles and frog skin. Preparation of radioactive CH3BrA yielded specific activities in excess of 1 Ci/mmol. We suggest that this compound will be useful in the isolation and purification of this ubiquitous Na+ channel.

A ouabain resistant epithelial cell that protects the wild type in co-cultures

MDCK cells (epithelioid of renal origin) can be cultured as monolayers that resemble natural epithelia. A mutant of these cells (R-MDCK) can grow in cultures exposed to high doses of ouabain (e.g. 100 microM) because one half of its population of pumps has a negligible affinity for this drug. It can also protect the wild type for at least 86 h in co-cultures exposed to ouabain. This article reviews several possible mechanisms of protection, and suggests that it is due to exchange of Na+ and K+ taking place through gap junctions. These connections though, do not seem to be continuous features, but the cells couple intermittently so that, in a given moment, only 28% of them communicate.

Sodium-selective micro-electrode study of apical permeability in frog skin: effects of sodium, amiloride and ouabain

The intracellular sodium ion activity (aiNa), apical membrane potential (psi ac) and apical sodium electrochemical driving force (delta mu Na) in Rana temporaria skin were measured using double-barrelled sodium-sensitive micro-electrodes, in the presence of various apical sodium activities (aoNa), amiloride, ouabain, and during voltage clamp of psi ac. The permeability and specific conductance of the apical cell membrane to sodium entry (PaNa and GaNa respectively) were calculated from the Goldman-Hodgkin-Katz equation and the Nernst-Planck (electrodiffusion) permeability equations respectively. The roles of aoNa and aiNa in the control of apical sodium entry were studied. PaNa increased linearly with log decrease in aoNa between 79 and 0.01 mM. Under short-circuit conditions, aiNa remained constant over the aoNa range of 10-79 mM, but decreased when aoNa was lower than 10 mM, due to a fall in delta mu Na and GaNa. Amiloride decreased PaNa, GaNa and aiNa, a result analogous to that observed in spontaneous low-transporting skins. Ouabain inhibited sodium transport and increased aiNa before any changes in PaNa occurred. The latter decreased only when aiNa rose above 15 mM. Increasing delta mu Na by hyperpolarizing voltage clamp of the apical cell membrane elicited a saturable increase in aiNa. The opposite effect was elicited by depolarizing psi ac. Electrodiffusion appears to be the sole mode of apical sodium entry.

Changes in paracellular and cellular ionic permeabilities of monolayers of MDCK cells infected with influenza or vesicular stomatitis viruses

MDCK cells (epithelioid line derived from the kidney of a normal dog) form monolayers which retain the properties of transporting epithelia. In these cells viruses bud asymmetrically: influenza from the apical, and vesicular stomatitis (VSV from the basolateral membrane (E. Rodríguez-Boulán and D.D. Sabatini, Proc. Natl. Acad. Sci. USA 75: 5071-5075, 1978; E. Rodríguez-Boulán and M. Pendergast, Cell 20: 45-54, 1980). In the present study, we analyzed whether these viruses affect specific ion-translocating mechanisms located in the plasma membrane. We studied the effect of infection on membrane and transepithelial conductance, passive and active unidirectional fluxes of Na+ and K+, intracellular potentials, cellular content of Na+ and K+, and formation of blisters which, in these preparations, are due to the vectorial transport of fluid. Two main observations are derived from these studies. First, infection with VSV caused an increase in transepithelial electrical conductance, due to the opening of tight junctions, 5 to 6 hr after the start of infection, coincident with the accumulation of envelope protein in the cell surface and with the rise in the curve of virus budding. Infection with influenza, on the other hand, increased the transepithelial conductance only late in the infection (12 to 14 hr) when virus production has already stopped. Second, viruses did affect membrane permeability. Yet, the changes observed may not be ascribed to a perturbation of the specific translocating mechanisms for Na+ and K+ which operate in the same region of the plasma membrane that the viruses use to penetrate and leave MDCK cells. The methods used in the present study are not suitable to decide whether the nonspecific changes in permeability elicited by the viruses occur over the whole cell membrane or are restricted to a given region.

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.

Intracellular ion activities in frog skin in relation to external sodium and effects of amiloride and/or ouabain

Intracellular activities of sodium, potassium and chloride ions, aiNa, aiK, and aiCl were measured with ion-selective single-, double- and triple-barrelled micro-electrodes in skin and isolated epithelia of Rana temporaria bathed on both sides with normal or modified physiological saline. Apical and basolateral membrane potentials, psi ac and psi cs and resistance Ra and Rb respectively were also measured and from the latter the fractional resistance of the apical membrane, F(Ra) and voltage divider ratio, delta psi ac/delta psi cs were measured as criteria of satisfactory membrane penetration by the micro-electrodes. Under control conditions, aiNa was 12.3 +/- 0.8 mM, aiK was 70.3 +/- 22 mM and aiCl was 20.3 +/- 1.6 mM with psi ac averaging -38.0 +/- 3.2 mV. When 10(-4) M-amiloride was added to the apical bathing fluid aiNa fell within 10 min to 1.18 +/- 0.1 mM and aiCl to 5.2 +/- 0.9 mM, while aiK increased to 86.2 +/- 3.8 mM as measured from the basolateral border of isolated epithelia. The sodium transport pool of the skin was measured from the fall in aiNa in the presence of amiloride and could be expressed as 33 X 10(-9) mol cm-2 of epithelium. The mean rate of fall of aiNa under these conditions corresponded to an efflux rate at the basolateral border of 30.1 X 10(-9) mol cm-2 min-1 (48 microA cm-2) giving a half-time for turnover of the sodium transport pool of 33 s. Reduction of sodium concentration in the apical fluid from the normal 79 mM-Na to 10, 1 and 0.1 mM caused aiNa to fall in stages to 2 mM. Because psi ac increased in negativity to -101 mV in the process, this driving force for passive sodium accumulation, more than offset the increased sodium gradient opposing sodium influx across the apical border.

Increased Na+ excretion by the skin of Rana pipiens after NaCl loading

In vivo the frog skin excretes sodium and the sodium excretion is increased in response to a NaCl load. The sodium excretion can be demonstrated in vitro, and the rate of excretion is greater in skin from NaCl-loaded animals than from control, non-loaded animals. Unidirectional 22Na flux experiments on paired frog skins, as well as 22Na and 24Na bidirectional flux experiments measured in vitro, confirm the above finding that net sodium excretion occurs in response to the NaCl load.

The in vivo electrical parameters of toad skin

Open-circuit voltage (PD) and short-circuit current (SCC) across toad skin were studied in in vivo conditions. An improved technique for fastening a lucite chamber on the abdominal region of the animal was developed. Saline bridges (230 mM NaCl in 4% agar solution) were placed subcutaneously to make the connections between the extracellular fluid and the half-cells. A clear relationship was observed between the electrical parameters and sodium transport by the skin, since PD and SCC were related to the sodium concentration of the bathing solution, and abolished by the presence of amiloride--a specific sodium transport inhibitor in epithelia. The initial control values of SCC in vivo were higher than those in vitro, which was attributed to hormonal stimulation. However, these high initial control values of SCC in vivo fell with time, reaching steady levels after a 2 hr period. Vasopressin failed to increase SCC in vivo when the external sodium concentration was 115 mM, being effective only when the sodium concentration was low (5 mM). On the other hand, in isolated preparations vasopressin significantly promoted an increase in both PD and SCC.

Control of sodium permeability of the outer barrier in toad skin

The 24Na efflux (JNaeff) (i.e., the rate of appearance of 24Na in the outer compartment) in the isolated short-circuited toad skin bathed by NaCl-Ringer's solution on both sides is composed of para- and transcellular components of almost equal magnitudes. This relies on the assumption that amiloride acts on the transcellular component only and could block it completely. Ouabain induces a large transient increase of the transcellular component. This increase, which starts within a few minutes after the addition of ouabain, is due to electrical depolarization of the outer barrier, rather than a consequence of blocking Na recirculation across the inner barrier. The subsequent decline of JNaeff, which takes place after the ouabain-induced JNaeff peak, is due to a progressive block of outer barrier Na channels with time, which can eventually be complete, depending on the duration of action of ouabain. As the external Na concentration was always kept high and constant in these experiments, the results indicate that a rise in cell Na concentration, and not in the outer bathing solution, is the signal that triggers the reduction of outer barrier Na permeability (PNao). Ouabain has no effect upon JNaeff with Na-free solution bathing the outer and NaCl-Ringer's solution the inner skin surface, showing the importance of Na penetration across the outer barrier, and not across the inner barrier due to its low Na permeability, in the process of closing the Na channels of this structure. Step changes from Na 115 mM to Na-free external solution, or vice-versa, may affect both the outer barrier electrical potential difference (PDo) and cell Na concentration (Na)c. Therefore, the behavior of JNaeff depends on which variable (if PDo or (Na)c regulated outer barrier Na permeability) is most affected by step changes in outer bathing solution Na concentration. Amiloride in the control condition blocks the transcellular component of JNaeff. However, in the condition of approximate short-circuiting of the outer barrier and high cellular Na concentration induced by long term effects of ouabain, when the Na channels of the outer barrier are already blocked by elevated cell Na concentration, amiloride may induce the opposite effect, increasing Na permeability of the outer barrier. With outer barrier Na channels completely blocked by high cell Na concentration, PCMB in the outer bathing medium induces a large increase of JNaeff, rendering these channels again amiloride sensitive. The results are consistent with the notion that Na efflux from cell compartment to the outer bathing solution goes through the amiloride-sensitive Na channels of the apical border of the superficial cell layer of toad skin, with an apparent Na permeability modulated by cell ionic environment, most probably the cell Na concentration.

Effect of ouabain, amiloride, and antidiuretic hormone on the sodium-transport pool in isolated epithelia from frog skin (Rana temporaria)

When tracer Na+ is added to the solution bathing the apical side of isolated epithelia the observed transepithelial tracer influx increases with time until a steady state is reached. The build-up of the tracer flux follows a single exponential course. The halftime for this build-up under control conditions was 0.92 +/- 0.06 min, and in the presence of ouabain 4.51 +/- 0.7 min. It is shown that the calculated Na+-transport pool is located in the cells. The Na+-transport pool under control conditions was 35.6 +/- 3.4 nmol/cm2, which corresponds to an intracellular Na+ concentration of 7.9 mM. Activation of the active Na+ transport by addition of antidiuretic hormone resulted in a highly significant increase in the Na+ transport pool, and inhibition of the transcellular Na+ transport with amiloride resulted in a decrease in the Na+-transport pool. Furthermore, the active Na+ transport increased along an S-shaped curve with increasing intracellular Na+ concentration (Na+-transport pool). The Na+ pump was found to be half saturated at an intracellular Na+ concentration of 12.5 mM.

Kinetic studies on the effects of ouabain on Na+ fluxes in frog skin

Among 48 pieces of paired frog skins of Rana pipiens in Ringer's solution, 10 pieces showed a strictly monotone decrease in the short circuit current (SCC) following ouabain treatment (10(-4) M). In 9 cases a transient attenuation, and in 27 cases a distinct wave in the ebb of the SCC, was seen. In 2 instances, two waves were seen. Associated with the not-monotone events was a transient rise in electrical skin conductance. The reasons for these mixed skin responses are unknown. One possible reason is considered here: Early during the ouabain action, some of the Na+ entering from the mucosal side is trapped in the skin by electroneutral processes, in keeping with the already known fact that ultimately cellular KCl is partly replaced by NaCl. Computer assisted model studies show how monotone, and not-monotone "transepithelial" net Na+ flux curves can be generated. Essential conditions for the generation of not-monotone Na+ flux curves are: 1. Presence of two distinct "cellular", active Na+ pools in the model. 2. Presence of a loop pathway in which a principal "transepithelial Na+ transport compartment", and a constituent "Na+/K+ maintenance compartment", are connected to each other and to the "extracellular" compartment. The model, then, predicts under which kinetic conditions monotone and not-monotone transepithelial Na+ flux curves will be seen.

The role of sodium-channel density in the natriferic response of the toad urinary bladder to an antidiuretic hormone

Urinary bladders of Bufo marinus were depolarized, by raising the serosal K concentration, to facilitate voltage-clamping of the apical membrane. Passive Na transport across the apical membrane was then studied with near-instantaneous current-voltage curves obtained before and after eliciting a natriferic response with oxytocin. Fitting with the constant-field equation showed that the natriferic effect is accounted for by an increase in the apical Na permeability. It is accompanied by a small increase in cellular Na activity. Furthermore, fluctuation analysis of the amiloride-induced shot-noise component of the short-circuit current indicated that the permeability increase is not due to increased Na translocation through those Na channels which were already conducting prior to hormonal stimulation. Rather, the natriferic effects is found to be based on an increase in the population of transporting channels. It appears that, in response to the hormone, Na channels are rapidly "recruited" from a pool of electrically silent channels.

Fluxes, junctions, and blisters in cultured monolayers of epithelioid cells (MDCK)

The formation of blisters is a transient phenomenon that led Dr. Leighton to describe its observation with time-lapse photography in the MDCK monolayer to a "gently boiling oatmeal." One may ask why is it transient and why most areas are momentarily not blistering. The observations discussed in this article indicate that (a) at the blister, junctions are really tight; (b) when transported fluid is allowed to escape through a permeable support, junctions are also tight, but (c) when the support is impermeable the junctions allow ouabain and peroxidase through. This suggests that, if the attachment of the monolayer is strong enough, the accumulation of fluid (Figure 7) bursts the junctions. If, on the contrary, junctions withstand the pressure, factor 3 of Figure 7 prevails over the others, and a blister is formed. In all the rest of the monolayer junctions seem to be open. This is in keeping with the observation by Rabito et al.,27 that if the monolayers are prepared on weakened supports, blisters are much bigger than under control conditions. It also allows the measurement of ionic fluxes and labeling of pumping sites at the basolateral membrane. As a corollary, one may say that some of the factors known to affect blistering 26 may very well act through modifications in the occluding junctions.

Oscillation of the electrical potential of the frog skin under the effect of Li+: theoretical formulation

A theoretical model of oscillation is proposed. It is based on the non-linearity introduced in the functioning of the active pump by the presence of lithium. Other plausible causes of oscillation are shown not to interfere in this case. The oscillation is of the local type. Synchronization between the local oscillators is not achieved by diffusional, but by electrical coupling. Numerical calculation shows that the model fits reasonably well to the experimental data.

Kinetics of ionic transport across frog skin: two concentration-dependent processes

Sodium and chloride influxes across the nonshort-circuited isolated skin of Rana esculenta were measured at widely varying external ionic concentrations. The curve describing sodium transport has two Michaelis-Menten components linked at an inflection point occurring at an external sodium concentration of about 7 meq. Chloride transport can also be represented by two saturating components. A possible explanation of these kinetics is discussed. At sodium concentrations lower than 4 meq it is possible to define a component of the sodium transport mechanism as having a high affinity for sodium and which is independent of the nature of the external anion. A high affinity for chloride of the chloride transport system functioning at low external concentrations is also found but is significantly different from that of sodium. These systems show the physiological characteristics of the countertransports (Na est(+)/H int(+); Cl ext(-)/HCO 3int(-)) functioning at low external concentrations. At external concentrations higher than 4 meq a low affinity transporting system in which chloride and sodium are linked superimpose on the high affinity compoents. The physiological significance of these results is discussed.

Cationic selectivity and competition at the sodium entry site in frog skin

The cation selectivity of the Na entry mechanism located in the outer membrane of the bullfrog (Rana catesbeiana) skin epithelium was studied. This selectivity was determined by measuring the short-circuit current when all of the external sodium was replaced by another cation and, also, by noting the relative degree of inhibition that the alkali metal cations produced on Na influx. The ability of the Group Ia cations to permeate the apical membrane was determined from the tracer uptake experiments. The results demonstrate that (a) only Li and Na are actively transported through the epithelium; (b) the alkali cations K, Rb, and Cs do not enter the epithelium through the apical border and, therefore, Na and Li are the only alkali cations translocated through this membrane; (c) these impermeable cations are competitive inhibitors of Na entry; (d) the cations NH4 and Tl exhibit more complex behavior but, under well-defined conditions, also inhibit Na entry; and (e) the selectivity of the cation binding site is in the sequence Li congruent to Na > Tl > NH4 congruent to K > Rb > Cs, which corresponds to a high field strength site with tetrahedral symmetry.

Re-evaluation of the 'contralateral' effects of sodium and potassium on sodium transport through toad skin

1. Changes in the concentration of Na in the outer bathing solution, [Na]o, or of K in the inner bathing solution, [K]i, alter the electrical responses of the isolated toad skin to changes in ionic concentrations in the contralateral solutions. The mechanism(s) of these apparently contralateral effects remain(s) unknown. 2. The phenomenon has been investigated here in the isolated abdominal skin of Xenopus laevis. Each skin was exposed to multiple levels of [Na]o and [K]i, of between 5 and 112 m-mole 1.(-1) The p.d. and short-circuit current (s.c.c.) responses were analysed both in terms of kinetics and in terms of changes in the equivalent electrical circuit of the Na transport mechanism. 3. Kinetic analysis revealed that the relationship between [Na]o and s.c.c., at any level of [K]i, followed Michaelis-Menten kinetics. Increasing levels of [K]i reduced the s.c.c. response to changes in [Na]o, conforming with the algebraic descriptions of 'slope-parabolic competitive inhibition'. High levels of [Na]o (of 60-112 m-mole 1.(-1)) occasionally reduced the s.c.c. in a manner reminiscent of 'substrate inhibition'; this effect was independent of the level of [K]i. At high [K]i and low [Na]o, s.c.c was again often less than that predicted by Michaelis-Menten kinetics. 4. In terms of the equivalent electrical circuit, increasing [Na]o produced a fall in Rseries; in the presence of 'substrate inhibition', however, Rseries rose on increasing [Na]o; in either case, ENa and Rsh remained unchanged. Increasing [K]i lowered both ENa and Rsh; Rseries fell with modest increments in [K]i, but increased at higher levels of [K]i. 5. These results can be interpreted without invoking unknown contralateral effects. Thus the changes in s.c.c., as induced by changes in [Na]o or [K]i, are consistent with homolaterally mediated effects on an enzymic mechanism of transepithelial Na transport; the changes in p.d., given the [K]i-dependent changes in Rsh, are similarly explicable.

Rheogenic sodium transport in a tight epithelium, the amphibian skin

1. Intracellular potentials from frog and toad skins were measured to identify rheogenic components of active Na transport across the basolateral membrane. Transcellular current flow and associated R . I-drops were blocked with amiloride or Na-free mucosal solution. 2. The potential difference across the basolateral membrane was found to be hyperpolarized by 18 . 5 +/- 1 . 6 mV above the steady-state value immediately after blockage of apical membrane Na conductance. The hyperpolarization disappeared within 15--25 min. 3. The final steady-state value of 93 . 1 +/- 2 . 5 mV was slightly less than reasonable estimates of the K equilibrium potential. 4. The hyperpolarization could not be observed 3--5 min after addition of ouabain (10(-4) M). 5. Both the magnitude and duration of the hyperpolarization correlate directly with the amount of Na accumulated in the intracellular space. 6. A fraction of the intracellular potential was missing when Na transport was re-established after long term blockage of apical membrane Na entry. It reappeared within 10--20 min. 7. It is suggested that the hyperpolarization is due to rheogenic Na transport across the basolateral membranes. This transport mechanism may contribute some 30--50% of the electrical gradient for passive Na entry across the mucosal membrane. 8. A coupling ratio between pumped fluxes of Na and K of about 2:1 is calculated from the data.

Decreased sensitivity to amiloride of amphibian epithelia treated with aldosterone. Further evidence for an apical hormonal effect

The reversible inhibition of transepithelial sodium transport achieved with amiloride (and triamterene) was evaluated in amphibian preparations stimulated with aldosterone so as to provide further information regarding a possible influence of this hormone on the apical border of target cells. When aldosterone secretion was enhanced by withdrawal of sodium from toad (Bufo marinus) habitat, sensitivity of abdominal skin to amiloride decreased; the same occurred in skin and bladder preparations incubated with aldosterone for several hours. Amiloride proved a less efficient blocker of sodium transport by toad skin exposed to vasopressin and to ouabain; both substances are capable or raising cell sodium content. It is therefore proposed that the decrease in sensitivity to amiloride of amphibian epithelial treated with aldosterone results from an increase in target cell sodium, itself due to a hormone-induced increas in sodium conductance at the apical cell border. Glucose, which enhanced markedly the rate of sodium transport in preparations treated with aldosterone for several hours, failed to decrease any further the response to amiloride; this is taken as an argument for an additional (? secondary) influence of aldosterone on the cell's metabolic machinery connected with the operation of the sodium 'pump'.

Uptake of [3H]benzamil at different sodium concentrations. Inferences regarding the regulation of sodium permeability

1. The effect of benzamil on short-circuit current in frog skin was measured at different external sodium concentrations. A linear relationship exists between the concentration of benzamil reducing short-circuit current by 50% and the external sodium concentration, indicative of some form of competitive antagonism between sodium and benzamil. 2. Uptake of [3H]benzamil into isolated frog skin epithelium and whole skin (0.95 cm2 pieces) was measured at different external sodium concentrations. With a sodium concentration of 111 mM in the external medium the uptake of [3H]benzamil is linear with concentration. Uptake amounted to 8.8 f-mole nM-1, a value similar to the linear component of the uptake measured at low (1.1 mM) sodium concentration. 3. Using a variety of other conditions the maximal number of specific binding sites for [3H]benzamil was calculated from displaceable binding and the fractional occupancy, the latter being derived from the inhibition of short-circuit current. This approach gave similar binding site densities to those reported previously at low sodium concentrations. 4. The reduction in specific [3H]benzamil uptake at high sodium may result from two mechanisms, competition of sodium with the ligand for an external binding site and a reduction in the site density as the intracellular sodium concentration increases. 5. It is concluded that the saturation of sodium transport which occurs at high sodium concentration is likely a consequence of the reduced availability of entry sites, rather than saturation of the uptake process.

Transient potassium fluxes in toad skin

Experiments were carried out in the isolated short-circuited skin of the toad Bufo marinus ictericus. 42K influx and efflux experiments were carried out with skins bathed on both sides by NaCl-Ringer's solution. Those fluxes showed very similar kinetics of equilibration with time and the results could be fitted by equations of a model of two intraepithelial compartments and the bathing solutions. In the steady state K influx is 3.99 +/- 0.36 nmol cm-2 hr-1 (n = 7) and efflux 3.62 +/- 0.38 nmol cm-2 hr-1 (n = 7) and are not statistically different, indicating that no net K flux is present across the epithelium. Different kinds of perturbations affecting the rates of 42K discharge into the bathing solutions were studied. Immediately after addition of amiloride (10(-4) M) to the outer solution, a sharp decline is observed in the rate of 42K discharge into the bathing solution, JK21, which falls from 3.62 +/- 0.38 nmol cm-2 hr-1 to 2.02 +/- 0.04 nmol cm-2 hr-1 (n = 7) 2 min after addition of the drug, followed by a partial recuperation with time. A complete Na by K substitution in the outer bathing solution induces a prompt and marked decline in JK21 which is similar to that induced by amiloride. Increase in the outer bathing solution Na concentration from zero Na concentration induces a nonlinear increase in JK21 and a linear relationship was observed between JK21 and short-circuit current in the range of 0 to 115 mM external Na concentration. The decline in JK21 induced by amiloride or by lowering external Na concentration was interpreted as being caused by electrical hyperpolarization of the external barrier of the epithelium induced by these procedures. Depolarization of the epithelial barriers by inner Na by K substitution in the short-circuited state (when the potential barriers are equal) drastically interfere with the rate of 42K discharge from the epithelium into the bathing solutions. Thus, transient increases are observed both in the rate of 42K discharge to the outer and to the inner bathing solutions upon depolarization of the barriers. These results indicate that at least the most important component of transepithelial K unidirectional fluxes goes through a transcellular route with a negligible paracellular component. Addition of ouabain (10(-3) M) to the inner bathing solution induces a transient rise in the rate of 42K discharge to the outer bathing solution with a peak on the order of 200% of the stationary value previous to the action of the inhibitor, followed by a return to new stationary values not statistically different from those observed previously to the effect of ouabain. The behavior of JK21 upon the effect of ouabain, as suggested by comparison with predictions from computer simulation, strongly supports the notion of a rheogenic Na pump in the inner barrier of the epithelium against the notion of a nonrheogenic 1:1 Na--K pump.

Comparison of the effects of increased intracellular calcium and antidiuretic hormone on active sodium transport in frog skin. A study with the calcium ionophore A23187

The addition of the Ca2+ ionophore A23187 (1 microM) to the inside solution of the frog skin resulted in an approx. 40% transient increase in the active influx of Na+ and ionic conductance, which decayed to an approx. 13% steady-state stimulation after 1--2 h. A23187 had no effect from the outside solution. A23187's stimulatory action is most likely the result of the ionophore's ability to increase intracellular Ca2+. This contention is supported by the following experimental results: (1) reintroduction of Ca2+ into a Ca2+-free inner solution stimulated Na+ transport only in the presence of A23187: (2) Mg2+ would not mimic these effects, and (3) EGTA in the inner solution would inhibit the A23187 response. The stimulation of active transport and ionic conductances elicited by A23187 were found to be very similar to those caused by antidiuretic hormone. Several lines of evidence suggest that A23187 may by-pass steps in the normal antidiuretic hormone stimulatory process: (1) A23187 and antidiuretic hormone are apparently non-additive; (2) A23187 acts three times faster than antidiuretic hormone; (3) A23187 stimulates antidiuretic hormone-insensitive frog skins, and (4) results from other laboratories indicate that A23187 does not increase cyclic AMP concentrations. It is speculated that an increase in free intracellular Ca2+ may be a step in the normal antidiuretic hormone stimulatory process. This increase in intracellular Ca2+ may in turn stimulate active sodium transport by increasing the Na+ permeability of the outer 'rate-limiting' membrane.

Temperature compensation of sodium transport and ATPase activity in frog skin

Na+ transport across frog skin, measured as short-circuit current (SCC) shows perfect temperature compensation in frogs acclimated to 6 degrees, 12 degrees, and 23 degrees C as SCC values observed at the acclimation temperatures are equal (about 13 muA/cm2). Reacclimation experiments show that this is not a starvation effect. While very little temperature compensation is seen in the activity of Na+, K+-ATPase in epidermal homogenates from frog skins, the activity of Mg2+-ATPase shows inverse compensation at assay temperatures from 4 degrees to 48 degrees C. This ATPase is apparently activated either by Mg2+ or by Ca2+ and it probably controls the passive permeability of epidermal cells. It is suggested that the inverse temperature compensation in the activity of this enzyme is the main mechanism by which the observed perfect temperature compensation of Na+ transport across frog skin occurs.

On the mechanism of the amiloride-sodium entry site interaction in anuran skin epithelia

The steady-state transport kinetics of the interaction between external sodium and the diuretic drug, amiloride, was studied in isolated anuran skin epithelia. We also investigated the effect of calcium on the amiloride-induced inhibition of short-circuit current (Isc) in these epithelial preparations. The major conclusions of this study are: (a) amiloride is a noncompetitive inhibitor of Na entry in bullfrog and grassfrog skin, but displays mixed inhibition in R. temporaria and the toad. A hypothesis which states that the interaction sites for amiloride and Na on the putative entry protein are spatially distinct in all of these species is proposed. (b) The stoichiometry of interaction between amiloride and the Na entry mechanism is not necessarily one-to-one. (c) The external Ca requirement for the inhibitory effect of amiloride is not absolute. Amiloride, at all concentrations, is equally effective in inhibiting Isc of bullfrog skin independently from the presence or absence of external Ca.

Compartmental analysis of Na efflux across ileum mucosal membrane

A new tracer analysis technique has been developed for efflux from a three compartment series system. With this technique, results are based on pools contributing to efflux and not on total tissue pools. The use of this technique is illustrated in a study of Na efflux across the luminal membrane of rabbit ileum. Na efflux was described by two exponentials with half-times of 1.78 min and 13.5 min. The fast component, with a flux of 68.0 mumoles/h cm2 and a calculated Na concentration of 166 mM, was identified as an extracellular compartment. The slow component, with a flux of 13.2 mumoles/h cm2 and Na concentration of 86 mM, was identified with efflux across the mucosal membrane of epithelial cells.

Chloride dependence of active sodium transport in frog skin: the role of intercellular spaces

1. In agreement with previous observations the replacement of Cl by a nonpenetrating anion in the solution bathing either the outside or both sides of the frog skin causes a fall in the short-circuit current. 2. When Cl is replaced by a non-penetrating anion in the solution bathing the outside of the frog skin the Isc is still a correct measure of the net Na transport. 3. Under the same conditions both active and shunt paths seem to be affected since there is a decrease in Isc, Na influx, amiloride-dependent conductance, and initial Na uptake across the external barrier, together with a decrease in Cl-backfluxes and amiloride-independent conductance. There is also a decrease in water permeability and a reduction in size of the intercellular spaces. 4. The removal of Cl does not appear to affect the entry step of Na but may have an effect on the shunt path. This in turn may change the active Na transport.

Studies on chloride permeability of the skin of Leptodactylus ocellatus: III. Na+ and Cl- effect on electrical phenomena

During their flux through the skin of the frog Leptodactylus ocellatus, Na+ and Cl- interact with each other. This interaction gives rise to electrical phenomena which are studied in the present paper. The skin is mounted in Na2SO4 Ringer's with 115 mM Na+ on the inside, and a variety of outer solutions. The osmolarity of all solutions is kept constant at 237.8 mosmol by adding sucrose. When the main anion used on the outside is SO=4 the electrical potential difference (deltapsi) rises steadily with the concentration of sodium (Na+)0 up to 87 mV, which is reached at about 20 mM. Thereafter deltapsi remains constant. When the main anion is Cl- it is observed that deltapsi rises steadily with (NaCl)0 with a slope similar to the curve obtained with SO=4 (37 mV per decade), but with a lower intercept attributed to an inward Cl pumping which is characteristic of this frog species. At 2--9 mM (NaCl)0 a Cl-specific channel is activated. Further increases of (NaCl)0 produce a decrease of deltapsi. The specificity of the activation of this site by monovalent cations and its use by monovalent anions is also studied.

Effects of pH, Ca, ADH, and theophylline on kinetics of Na entry in frog skin

The short circuit current as a function of Na concentration in both solutions was found to obey Michaelis-Menten kinetics under a variety of experimental conditions. Values of maximal transport rate (Im) and half-maximal Na concentrations (Kt) were determined from these experiments. Three type of results were obtained: 1) Im and Kt both decreased by approximately the same fraction when the pH of both solutions was reduced by increasing PCO2, 2) Im decreased and Kt increased when the external pH was decreased, and 3) Im increased with ADH and theophylline, decreased with external Ca, and Kt remained unchanged. Various criteria were utilized to determine that these were properties of the entry barrier for Na into the "transport pool." The results are explained in terms of a model that separates three different types of actions on the entry barrier: 1) competition of Na with other ions in the external solution for entry, 2) modulation of the number of sites available for Na translocation by changing the cytoplasmic pH, and 3) alterations in the rate of Na translocation caused by changes in the Na permeability or the electrochemical gradient across the entry barrier.

Evidence of active sodium transport in the visceral yolk sac of the rat in vitro

1. Na transport has been studied in the isolated rat visceral yolk sac from day 17.5 of gestation to term. 2. The transepithelial potential difference (p.d.) and the short circuit current (s.c.c.) in the isolated yolk sac were found to vary with gestational age, with peak values at day 19.5. The maximal p.d. and s.c.c. were 3.85 +/- 0.32 mV (the fetal side positive) and 19.5 +/- 5.3 microamperemeter cm-2 respectively. 3. Simultaneous determination of the two-way Na+ flux and the s.c.c. revealed a preferential active movement of Na in the maternal to fetal direction. The net flux was found to be 50% higher than the s.c.c. 4. Both the p.d. and the s.c.c. were found to be reduced by cooling and by the uncoupling agent 2,4-dinitrophenol. 5. The s.c.c. altered in a curvilinear fashion with the Na+ concentration in the bathing solution, with an apparent Km of about 20 mM-Na+. Removing Cl ions from the bathing solutions had no effect on the p.d. and s.c.c. 6. Addition of amiloride (10(-4) M) to either side of the visceral yolk sac had no effect on the s.c.c. but application of ouabain (10(-5) M) to the fetal side caused a profound fall in the s.c.c. 7. The possible physiological role of this active Na transport by the visceral yolk sac in the formation of amniotic fluid is discussed.