Movement of sodium across the mucosal surface of the isolated toad bladder and its modification by vasopressin

Epithelial transport in The Journal of General Physiology

JGP hosts key papers that shaped the epithelial transport field.

Epithelia define the boundaries of the body and often transfer solutes and water from outside to inside (absorption) or from inside to outside (secretion). Those processes involve dual plasma membranes with different transport components that interact with each other. Understanding those functions has entailed breaking down the problem to analyze properties of individual membranes (apical vs. basolateral) and individual transport proteins. It also requires understanding of how those components interact and how they are regulated. This article outlines the modern history of this research as reflected by publications in The Journal of General Physiology.

Physiological and morphological effects of poly-L-lysine on the toad bladder

Studies were carried out on the morphological and physiological effects of the binding of poly-L-lysine (polylysine; mol wt≊120,000) to the apical surface membrane of the toad bladder epithelium. Paired hemibladders were mounted in chambers and exposed to polylysine concentrations of 2, 8, or 80 μg/ml in the mucosal medium for periods of up to 2 hr. Radioautographs prepared after addition of(3)H-polylysine showed that the polymer was localized to the apical surface of the epithelium and in dense subapical masses in lysed cells. No significant morphological changes were seen in the epithelium by light or electron microscopy at polymer concentrations of 2 and 8 μg/ml. Exposure to 80 μg/ml lysed many epithelial cells, i.e., converted them to slightly swollen ghosts with pycnotic nuclei and empty cytoplasm, except for remnants of mitochondria and vesicular fragments of the endoplasmic reticulum. All of the superficial epithelial cells were lysed in stretched hemibladders. The plasma membranes of the lysed cells were uniformly thickened, and their intercellular attachments remained intact. In contracted hemibladders, lysed and normal-appearing cells were interspersed, and the number of lysed cells in the epithelium was proportional to the duration of exposure to high concentrations of the polycation. In parallel experiments, the effects of varying concentrations of polylysine on active Na(+) transport and osmotic flow of water were measured with and without vasopressin, aldosterone, or amphotericin B in the media. At a concentration of 2 μg/ml of polylysine in the mucosal bathing solutions, no change in the basal rate of Na(+) transport was seen, and the response to vasopressin was unimpaired. At a concentration of 8 μg/ml, there was a significant but small fall in electrical potential difference (PD) and in short-circuit current (SCC) and no interference with the response to vasopressin. At a concentration of 80 μg/ml, there was a rapid curvilinear fall in SCC to 54±4% of the baseline value and in PD to 21±3% of the baseline value in a 2-hr period. Simultaneous unidirectional isotope flux studies with(22)Na and(24)Na showed a more than twofold increase in the serosal to mucosal flux but no discrepancy between net flux and SCC. Despite the inhibitory action of the polymer, the stimulatory response in Na(+) transport to vasopressin, aldosterone, and amphotericin B was relatively preserved in that the percentage increase in SCC was the same in the polymer-treated and control hemibladders. The polycation produced a small but significant increase in osmotic water flow, and striking and irreversible inhibition of the water-flow response to vasopressin.

Measurement of the composition of epithelial cells from the toad urinary bladder

Two methods are described by which epithelial cells from toad urinary bladders can be obtained for analysis of their intracellular water and electrolyte contents. In the first, a method similar to that described in 1968 by J. T. Gatzy and W. O. Berndt, sheets of epithelial cells are scraped from bladders after incubation in sodium Ringer's and collagenase (400 mg/liter). The scraped cells were incubated under various conditions and their composition subsequently determined. Oxygen consumption was also measured. In the second method, epithelial cells were scraped from hemibladders removed from chambers. These cells were then analyzed without further incubation. The morphology of epithelial cells obtained by each method is illustrated. Both methods yield similar results and evidence is provided that the derived intracellular values obtained truly reflect the composition of the epithelial cells.

The cellular specificity of the effect of vasopressin on toad urinary bladder

Phase and electron micrographs of toad bladders were obtained following dilution of bathing media in the presence and absence of vasopressin. Dilution of the mucosal medium alone resulted in no morphologic changes. Subsequent addition of vasopressin produced an increase in the cell volume of the granular cells, manifested by some or all of the following changes: increased area of granular cell profiles as observed in sections, rounding of the cell nucleus, displacement of the two components of the nuclear envelope, loss of nuclear heterochromatin, sacculation of the endoplasmic reticulum and the Golgi apparatus, and reduction in the electron density of the cell cytoplasm. No such morphologic changes were noted in the other cell types comprising the mucosal epithelium - the mitochondria-rich, the goblet, and the basal cells. On the other hand, dilution of the serosal bathing medium in the absence of vasopressin caused a marked increase in the cell volume of all these cell types. The results demonstrate that the action of vasopressin to enhance bulk water flow across toad bladder is exerted specifically on the apical surface of the granular cells. It is suggested that the hormonal effect on sodium transport may also be limited to the granular cells. The route of osmotic water flow and the possible role of the other mucosal epithelial cells is discussed.

Determination of the driving force of the Na(+) pump in toad bladder by means of vasopressin

Vasopressin stimulates Na(+) transport across toad bladder largely or entirely by decreasing the resistance to Na(+) entry into the transporting epithelial cells. Therefore, the hormone should induce proportional changes in short circuit current (I S ) and tissue conductance; the ratio of these changes should equal the driving force (E Na) of the Na(+) pump.Administration of vasopressin provided a rapid, reversible and reproducible technique for the measurement ofE Na. Values calculated forE Na ranged from 74 to 186 mV, in agreement with previously published estimates. The results were not dependent on the vasopressin concentration over a wide range of concentrations.Ouabain, an agent thought to inhibit specifically the Na(+) pump, decreased bothI S andE Na. On the other hand, amiloride, a diuretic thought to block specifically Na(+) entry, markedly reducedI S , without reducingE Na.It is concluded that vasopressin constitutes a probe for the rapid reproducible determination ofE Na under a wide variety of physiological conditions.

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.

The Role of Potassium in Active Transport of Sodium by the Toad Bladder

Studies were carried out on the isolated urinary bladder of the toad, Bufo marinus, in order to explain the dependence of active sodium transport on the presence of potassium, in the serosal medium. Attempts to obtain evidence for coupled sodium-potassium transport by the serosal pump were unsuccessful; no relation between sodium transport and uptake of K(42) from the serosal medium was demonstrable. Rather, the predominant effect of serosal potassium appeared to be operative at the mucosal permeability barrier, influencing the permeability of this surface to sodium. The mucosal effects of serosal potassium were correlated with effects on cellular cation content. When sodium Ringer's solution was used as serosal medium, removal of potassium resulted in significant decrease in tissue potassium content, commensurate increase in tissue sodium content, and marked depression of mucosal permeability and sodium transport. When choline replaced sodium in the serosal medium, removal of potassium resulted in only slight alterations of tissue electrolyte content, and effects on mucosal permeability and sodium transport were minimal.

Hans H. Ussing--scientific work: contemporary significance and perspectives

As a zoologist, Hans H. Ussing began his scientific career by studying the marine plankton fauna in East Greenland. This brought him in contact with August Krogh at the time George de Hevesy, Niels Bohr and Krogh planned the application of artificial radioactive isotopes for studying the dynamic state of the living organism. Following his studies of protein turnover of body tissues with deuterium-labeled amino acids, Ussing initiated a new era of studies of transport across epithelial membranes. Theoretical difficulties in the interpretation of tracer fluxes resulted in novel concepts such as exchange diffusion, unidirectional fluxes, flux-ratio equation, and solvent drag. Combining methods of biophysics with radioactive isotope technology, Ussing introduced and defined the phrases 'short-circuit current', 'active transport pathway' and 'shunt pathway', and with frog skin as experimental model, he unambiguously proved active transport of sodium ions. Conceived in his electric circuit analogue of frog skin, Ussing associated transepithelial ion fluxes with the hitherto puzzling 'bioelectric potentials'. The two-membrane hypothesis of frog skin initiated the study of epithelial transport at the cellular level and raised new questions about cellular mechanisms of actions of hormones and drugs. His theoretical treatment of osmotic water fluxes versus fluxes of deuterium labeled water resulted in the discovery of epithelial water channels. His discovery of paracellular transport in frog skin bridged studies of high and low resistance epithelia and generalized the description of epithelial transport. He devoted the last decade of his scientific life to solute-coupled water transport. He introduced the sodium recirculation theory of isotonic transport, and in an experimental study, he obtained the evidence for recirculation of sodium ions in toad small intestine. In penetrating analyses of essential aspects of epithelial membrane transport, Ussing provided insights of general applicability and powerful analytical methods for the study of intestine, kidney, respiratory epithelia, and exocrine glands-of equal importance to biology and medicine.

The effect of Ca and antidiuretic hormone on Na transport across frog skin. I. Examination of interrelationships between Ca and hormone

Ca added to the solution bathing the outside of isolated frog skin causes a decrease in net Na transport across the skin while antidiuretic hormone (ADH) causes an increase. Possible interrelations between the effects of these agents have been examined. The decrease in Na transport caused by Ca was the same before and after treatment of the skin with ADH and the increase in transport caused by ADH was unaffected by the presence of Ca. The relationship between Ca concentration and degree of inhibition of Na transport was not appreciably altered by ADH. These results indicate that Ca and ADH do not compete but act independently at two different sites and these sites appear to be located on the same barrier to Na movement in the skin. Further, Ca causes a decrease in Cl influx across the short-circuited skin but ADH has no effect on Cl movement, again suggesting that the actions of these agents are independent.

The electrical potential profile of the isolated toad bladder

The electrical potential profile of the isolated toad bladder was examined in the spontaneously active, chronically short-circuited, and intermittently short-circuited states by means of glass micropipettes. The position of the micropipette tip within the bladder was evaluated by measuring the D.C. resistance between the micropipette tip and the reference electrode on the serosal side of the bladder. In the spontaneously active state, with concentrations of sodium in the mucosal solution ranging from less than 1 to 114 meq per liter, the potential profile consisted in the majority of impalements of two steps, each positive to the mucosal solution. A minority of impalements showed more than two potential steps, each positive to the mucosal solution. In the short-circuited state, the interior of the bladder was found to be negative to the bathing solution by approximately 5 mv. The results are interpreted as showing a potential step at the two surfaces of the epithelial cell layer of the toad bladder. In the spontaneously active state the potential change at the mucosal boundary is of the wrong polarity to bring about net sodium entry; the small electrical driving force across the mucosal surface which is present in the short-circuited state may contribute to the net entry of sodium from mucosal solutions with low sodium concentration.

The effect of Ca and antidiuretic hormone on Na transport across frog skin. II. Sites and mechanisms of action

A method has been developed for determining unidirectional Na fluxes across the two faces of the transporting cells in the frog skin. The method has been used to investigate the location of the sites at which Ca and anti-diuretic hormone act to alter the rate of active Na transport across the skin. The results have indicated that the primary effect of both agents is on the Na permeability of the outward facing membrane of the cells. Ca decreases and the hormone increases permeability of this barrier. Neither agent appears to have a direct effect on the active transport system itself assuming that it is located at the inner membrane of the cells. The rate of active Na transport is altered as a result of changes in the size of the Na pool in the cells which occur because of changes in the rate of Na entry through the outer membrane. Thus, the results indicate that the Na permeability of the outer membrane plays an important role in controlling the rate of net active Na transport across the skin.

The electrical characteristics of active sodium transport in the toad bladder

The mechanism responsible for active sodium transport in the urinary bladder of the toad appears to be located at the serosal boundary of the epithelial cell layer of the bladder. Studies of the potential step observed at the serosal boundary in the open-circuited state were undertaken in an attempt to define the factors responsible for its production. Glass micropipettes were used to measure the serosal potential step in bladders exposed on the serosal side to solutions of high potassium or of high potassium and low chloride concentration. Observed potentials exceed the maximum values which would have been expected if the serosal potential step were a potassium or chloride diffusion potential. Measurements of net cation flux exclude the possibility of a diffusion potential at this border due to the passive movement of any anionic species. The observed independence of transbladder potential and short-circuit current from the pH of the serosal medium over a wide range of pH makes it unlikely that the observed serosal potential step is a hydrogen ion diffusion potential. We conclude that the active sodium transport mechanism in toad bladder is "electrogenic."

ION TRANSPORT IN ISOLATED RABBIT ILEUM. II. THE INTERACTION BETWEEN ACTIVE SODIUM AND ACTIVE SUGAR TRANSPORT

The addition of actively transported sugars to the solution bathing the mucosal surface of an in vitro preparation of distal rabbit ileum results in a rapid increase in the transmural potential difference, the short-circuit current, and the rate of active Na transport from mucosa to serosa. These effects are dependent upon the active transport of the sugar per se and are independent of the metabolic fate of the transported sugar. Furthermore, they are inhibited both by low concentrations of phlorizin in the mucosal solution and by low concentrations of ouabain in the serosal solution. The increase in the short-circuit current, ΔI_sc, requires the presence of Na in the perfusion medium and its magnitude is a linear function of the Na concentration. On the other hand, ΔI_sc is a saturable function of the mucosal sugar concentration which is consistent with Michaelis-Menten kinetics suggesting that the increase in active Na transport is stoichiometrically related to the rate of active sugar transport. An interpretation of these findings in terms of a hypothetical model for intestinal Na and sugar transport is presented.

SODIUM MOVEMENT ACROSS SINGLE PERFUSED PROXIMAL TUBULES OF RAT KIDNEYS

Using perfusion techniques in single proximal tubule segments of rat kidney, the relationship between net sodium movement and active transport of ions, as measured by the short-circuit method, has been studied. In addition, the role of the colloid-osmotic pressure gradient in proximal transtubular fluid and sodium movement has been considered. Furthermore, the limiting concentration gradient against which sodium movement can occur and the relationship between intratubular sodium concentration and fluid transfer have been investigated. Comparison of the short-circuit current with the reabsorptive movement of sodium ions indicates that this process is largely, perhaps exclusively, active in nature. No measurable contribution of the normally existing colloid-osmotic pressure gradient to transtubular water movement was detected. On the other hand, fluid movement across the proximal tubular epithelium is dependent upon the transtubular sodium gradient and is abolished when a mean concentration difference of 50 mEq/liter is exceeded.

THE EFFECT OF MUCOSAL AND SEROSAL SOLUTION CATIONS ON BIOELECTRIC PROPERTIES OF THE ISOLATED TOAD BLADDER

The spontaneous transtissue potential and the DC conductance of the isolated toad bladder were measured when the tissue was exposed to sulfate Ringer's solutions of modified ionic composition. Na(+) was replaced to varying extents by (C(2)H(5))(3)NH(+), (C(2)H(5))(4)N(+), Li(+), Cs(+), K(+), or Rb(+). Reversible and irreversible changes were observed. The reversible changes were consistent with equations derived from the Nernst-Planck diffusion equation, and gave the following functional description of the bladder: (a) the potential measurements were compatible with two membranes in series; (b) the mucosal surface was more permeable to Na(+) than to other monovalent cations; (c) the serosal surface was permeable to both K(+) and Na(+) but preferentially to K(+); (d) the rate of Na(+) diffusion across the mucosal membrane appeared to approach a maximum but two alternative interpretations are discussed; (e) the conductance data were consistent with the assumption of a constant concentration gradient for the penetrating ions within the membrane (Henderson's assumption) provided suitable hypotheses are made concerning the Na(+) distribution between the membrane surfaces and the bulk phases of the adjacent solutions; (f) the conductance and spontaneous potential data suggested that the mucosal membranes of a small fraction of the epithelial cells were more permeable than the mucosal membranes of the majority of these cells. The irreversible changes were almost entirely associated with cation substitution in the serosal solution. However, Li(+) produced an irreversible fall in voltage when added to either side of the tissue.

THE INFLUENCE OF NA CONCENTRATION ON NA TRANSPORT ACROSS FROG SKIN

The effects of changes in Na concentration of the bathing solutions on some transport and permeability properties of the isolated frog skin have been examined. Rate coefficients for unidirectional Na movements across the two major barriers in the skin have been estimated as functions of Na concentration. The results indicate that the "apparent Na permeability" of the outer barrier of the skin decreases markedly when Na concentration in the outer solution is increased from 7 to 115 mM. The observed saturation of rate of Na transport with increasing Na concentration can be ascribed, in part, to this permeability change rather than to saturation of the transport system itself. Unidirectional Cl flux across the short-circuited skin was not significantly altered by an increase in Na concentration from 30 to 115 mM suggesting that the changes in membrane properties are relatively specific for the Na ion. The results also suggest that the movement of Na across the outer membrane may not be due entirely to simple passive diffusion of free Na ions.

ELECTRON MICROSCOPY OF ABSORPTION OF TRACER MATERIALS BY TOAD URINARY BLADDER EPITHELIUM

The absorption of Thorotrast and saccharated iron oxide by the epithelium of the toad urinary bladder was studied by electron microscopy. Whether the toads were hydrated, dehydrated, or given Pitressin, no significant differences in transport of colloidal particles by epithelial cells were observed. This implies that these physiological factors had little effect on the transport of the tracer particles. Tracer particles were encountered in three types of epithelial cells which line the bladder lumen, but most frequently in the mitochondria-rich cells. Tracer materials were incorporated into the cytoplasm of epithelial cells after being adsorbed to the coating layer covering the luminal surface of the cells. In the intermediate stage (1 to 3 hours after introducing tracer) particles were present in small vesicles, tubules, and multivesicular bodies. In the later stages (up to 65 hours), the particles were more commonly seen to be densely packed within large membrane-bounded bodies which were often found near the Golgi region. These large bodies probably were formed by the fusion of small vesicles. Irrespective of the stages of absorption, no particles were found in the intercellular spaces or in the submucosa. Particles apparently did not penetrate the intercellular spaces of the epithelium beyond the level of the tight junction.

ELECTRICAL EXCITABILITY OF ISOLATED FROG SKIN AND TOAD BLADDER

When current of proper polarity and sufficient intensity is passed across isolated frog skin or toad bladder, an action potential of about 200 mv and 10 msec. duration with a sharp threshold and refractory period of several seconds' duration is elicited. Interruption of current during the action potential abolishes the response, and, as shown by appropriate bridge measurements, this occurs because the action potential results from resistance variations during the current flow. The ionic composition of the medium bathing the frog skin was varied, and it was found that the response is relatively insensitive to changes in the solution bathing the inner surface, but rapidly and reversibly affected by changes in the outer solution, particularly by replacement of sodium with potassium and by variations of calcium concentration. It was also observed that the resistance of the skin and action potential across it are reversibly altered by metabolic inhibitors and that these alterations occur independently of any changes in the intrinsic EMF of the system. From the finding that the action potential across frog skin and toad bladder results from a time-variant resistance, it is argued that this same phenomenon can be the basis of electrical excitability in general. This would attribute physical significance to the equivalent circuit commonly employed to represent the plasma membrane; i.e., the plasma membrane would be a mosaic structure of spatially separate permselective regions.

ION TRANSPORT IN ISOLATED RABBIT ILEUM. I. SHORT-CIRCUIT CURRENT AND NA FLUXES

The transmural potential difference, short-circuit current, and Na fluxes have been investigated in an in vitro preparation of isolated rabbit ileum. When the tissue is perfused with a physiological buffer, the serosal surface is electrically positive with respect to the mucosal surface and the initial potential difference in the presence of glucose averages 9 mv. Unidirectional and net Na fluxes have been determined under a variety of conditions, and in each instance, most if not all of the simultaneously measured short-circuit current could be attributed to the active transport of Na from mucosa to serosa. Active Na transport is dependent upon the presence of intact aerobic metabolic pathways and is inhibited by low concentrations of ouabain in the serosal medium. A method is described for determining whether a unidirectional ionic flux is the result of passive diffusion alone, in the presence of active transport of that ion in the opposite direction. Using this method we have demonstrated that the serosa-to-mucosa flux of Na may be attributed to passive diffusion with no evidence for the presence of carrier-mediated exchange diffusion or the influence of solvent-drag.

Activation of epithelial Na+ channels by protein kinase A requires actin filaments

We have recently demonstrated a novel role for "short" actin filaments, a distinct species of polymerized actin different from either monomeric (G-actin) or long actin filaments (F-actin), in the activation of epithelial Na+ channels. In the present study, the role of actin in the activation of apical Na+ channels by the adenosine 3',5'-cyclic monophosphate-dependent protein kinase A (PKA) was investigated by patch-clamp techniques in A6 epithelial cells. In excised inside-out patches, addition of deoxyribonuclease I, which prevents actin polymerization, inhibited Na+ channel activation mediated by PKA. Disruption of endogenous actin filament organization with cytochalasin D for at least 1 h prevented the PKA-mediated activation of Na+ channels but not activation following the addition of actin to the cytosolic side of the patch. To assess the role of PKA on actin filament organization, actin was used as a substrate for the specific phosphorylation by the PKA. Actin was phosphorylated by PKA with an equilibrium stoichiometry of 2:1 mol PO4-actin monomer. Actin was phosphorylated in its monomeric form, but only poorly once polymerized. Furthermore, phosphorylated actin reduced the rate of actin polymerization. Thus actin allowed to polymerize for at least 1 h in the presence of PKA and ATP to obtain phosphorylated actin filaments induced Na+ channel activity in excised inside-out patches, in contrast to actin polymerized either in the absence of PKA or in the presence of PKA plus a PKA inhibitor (nonphosphorylated actin filaments). This was also confirmed by using purified phosphorylated G-actin incubated in a polymerizing buffer for at least 1 h at 37 degrees C. These data suggest that the form of actin required for Na+ channel activation (i.e., "short" actin filaments) may be favored by the phosphorylation of G-actin and may thus mediate or facilitate the activation of Na+ channels by PKA.

Effects of PCMBS on the water and small solute permeabilities in frog urinary bladder

It has been reported that PCMBS (p-chloromercuribenzene sulfonate) blocks the water permeability of red cells and of the tubular kidney membranes. In this study we compare the effects of this mercurial compound on the permeability of water and other small solutes in the frog urinary bladder. We observed that: (i) 5 mM PCMBS applied at pH 5.0 to the mucosal side inhibited the net and unidirectional water fluxes induced by oxytocin without changing the delta Pf/delta Pd ratio. (ii) The oxytocin-induced urea and Na+ influxes were also inhibited by PCMBS. (iii) The unidirectional Cl- movement was first reduced and then increased during the course of PCMBS treatment. (iv) The short-circuit measured at low mucosal Na+ concentration (10 mM), diminished continuously, whereas the transepithelial resistance first increased and then diminished. (v) Mannitol, raffinose, alpha-methyl-glucose, antipyrine, caffeine and Rb+ movements were not changed significantly during the first 26 min of the water permeability inhibition.

IN CONCLUSION

(i) The ADH-sensitive water, urea and Na+ transport systems were inhibited by PCMBS, (ii) PCMBS did not induce a nonspecific and general effect on the permeability of the membrane during the development of the water permeability inhibition, and (iii) in terms of water channels, the inhibition of water transport with the maintenance of a high Pf/Pd ratio suggests that PCMBS closes the water channels in an all or none manner, reducing their operative number in the apical border of frog bladder.

Transient electrical phenomenon of the voltage-clamped toad urinary bladder

1. The effects of changes in voltage on the transepithelial current through the toad urinary bladder have been studied using Ussing chambers. 2. Step changes in voltage produced two transient currents of duration seconds and minutes respectively. 3. Amiloride, which was used to block all active transport, also eliminated the transient nature of the current responses, indicating that the phenomena were cellular in origin. In the presence of amiloride, amphotericin B regenerated the short-circuit current and the transient behaviour. 4. The effects of substituting gluconate for Cl- in the medium were examined. Similar transient responses were observed, indicating that they were not due to changes in a plasma membrane Cl- conductance. 5. The shape and magnitude of the first current transient changed with (i) changes in the mucosal Na+ concentration, (ii) the magnitude of the transepithelial voltage step, (iii) the addition of antidiuretic hormone, (iv) changes in the serosal K+ concentration, or (v) the addition of ouabain. 6. The second current transient was similarly affected by such challenges. 7. In some bladders the voltage step produced current oscillations similar to those obtained after the epithelium had been challenged with a serosal osmotic step (Gordon, 1988). 8. The results suggest that two major processes are initiated by a transepithelial voltage step. The first involves a change in the K+ conductance of the basolateral membrane and the second is associated with the alteration of cellular ion content and Na+ pump rate.

Ion selectivity of epithelial Na channels

Epithelial Na channels are apparently pore-forming membrane proteins which conduct Na much better than any other biologically abundant ion. The conductance to Na can be 100 to 1000 times higher than that to K. The only other ions that can readily get through this channel are protons and Li. Small organic cations cannot pass through the channel, and water may also be impermeant. The selectivity properties of epithelial Na channels appear to be determined by at least three factors: A high field-strength anionic site, most likely a carboxyl residue of glutamic or aspartic acid residues on the channel protein, probably accounts for the high conductance through these channels of Na and Li and to the low conductance of K, Rb and Cs. A restriction in the size of the pore at its narrowest point probably accounts for the low conductance of organic cations as well as the possible exclusion of water molecules. The outer mouth of the channel appears to be negatively charged and may control access to the region of highest selectivity and may serve as a preliminary selectivity filter, attracting cations over anions. These conclusions are illustrated by the cartoon of the channel in Fig. 3. This picture is obviously both fanciful and simplified, but its general points will hopefully be testable. It leaves open a number of important questions, including: does amiloride block the channel by binding within the outer mouth? what does the inner mouth of the channel look like, and does this part of the channel contribute to selectivity? and what, if any, are the interactions between the features of the channel that impart selectivity and those that control the regulation of the channel by hormonal and other factors?

Intracellular solute gradients during osmotic water flow: an electron-microprobe analysis

In an attempt to quantify possible intracellular water activity gradients during ADH-induced osmotic water flow, we employed energy dispersive X-ray microanalysis to thin, freeze-dried cryosections obtained from fresh, shock-frozen tissue of the toad urinary bladder. The sum of all detectable small ions (Na + K + Cl) in the cellular water space was taken as an index of the intracellular osmolarity. Presuming that all ions are osmotically active, they comprise about 90% of the cellular solutes. When the cells were exposed to dilute serosal medium, the reduction in the sum of the ions agreed well with the expected reduction in osmolarity. After inducing water flow by addition of ADH and dilution of the mucosal medium, all epithelial cells showed a fall in osmolarity. The change was more pronounced in granular cells than in basal or mitochondria-rich cells, consistent with the notion that granular cells represent the main transport pathway. Most significantly, intracellular osmolarity gradients, largely caused by an uneven distribution of K and Na, were detectable in granular cells. The gradients were not observed after ADH or mucosal dilution alone, or when the direction of transepithelial water flow was reversed. We conclude from these results that there is a significant cytoplasmic resistance to water flow which may lead to intracellular gradients of water activity. Concentration gradients of diffusible cations can be explained by a flow-induced Donnan-type distribution of fixed negative charges. With regard to transepithelial Na transport, the data suggest that ADH stimulates transport by increasing the Na permeability of the apical membranes of granular cells specifically.

The pathogenesis of cystic fibrosis: a proposal for calmodulin as the basic biochemical defect

In cystic fibrosis (CF) there are two major defects which lead to most clinical manifestations of the disease. These are the electrolyte sweat defect and the abnormality of mucous secretions. Both may be satisfactorily explained by increased intracellular Ca++. In Na+ reabsorbing cells, such as exocrine sweat glands, increased Ca++ inhibits transepithelial Na+ transport. In mucus-secreting cells, high Ca++ levels may lead to physiochemical changes in secreted mucus. Increased intracellular Ca++ levels are hypothesized to be caused either by a defective Ca++ efflux cellular mechanism, or by increased intracellular binding of Ca++.

Volume-regulatory responses of Amphiuma red cells in anisotonic media. The effect of amiloride

Amphiuma red cells were incubated for several hours in hypotonic or hypertonic media. They regulate their volume in both media by using ouabain-insensitive salt transport mechanisms. After initially enlarging osmotically, cells in hypotonic media return toward their original size by losing K, Cl, and H2O. During this volume-regulatory decrease (VRD) response, K loss results from a greater than 10-fold increase in K efflux. Cells in hypertonic media initially shrink osmotically, but then return toward their original volume by gaining Na, Cl, and H2O. The volume-regulatory increase (VRI) response involves a large (greater than 100-fold) increase in Na uptake that is entirely blocked by the diuretic amiloride (10(-3) M). Na transport in the VRI response shares many of the characteristics of amiloride-sensitive transport in epithelia: (a) amiloride inhibition is reversible; (b) removal of amiloride from cells pretreated with amiloride enhances Na uptake relative to untreated controls; (c) amiloride appears to act as a competitive inhibitor (Ki = 1-3 microM) of Na uptake; (d) Na uptake is a saturable function of external Na (Km approximately 29 mM); (e) Li can substitute for Na but K cannot. Anomalous Na/K pump behavior is observed in both the VRD and the VRI responses. In the VRD response, pump activity increases 3-fold despite a decrease in intracellular Na concentration, while in the VRI response, a 10-fold increase in pump activity is observed when only a doubling is predicted from increases in intracellular Na.

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.

Voltage-dependent block by amiloride and other monovalent cations of apical Na channels in the toad urinary bladder

Inhibition of the Na conductance of the apical membrane of the toad urinary bladder by amiloride, alkali cations and protons was voltage dependent. Bladders were bathed with a high K-sucrose serosal medium to reduce series basal-lateral resistance and potential difference. Transepithelial current-voltage relationships were measured over a voltage range of +/- 200 mV with a voltage ramp of frequency 0.5 to 1 Hz. Na channel I-V relationships were obtained by subtraction of currents measured in the presence of maximal doses of amiloride (10 to 20 microM). With submaximal doses of amiloride (0.05 to 0.5 microM), the degree of inhibition of the Na channel current (INa) increased as the mucosal potential was made more positive. The data can be reasonably well explained by assuming that amiloride blocks Na transport by binding to a site which senses approximately 12% of the transmembrane voltage difference. INa was reduced in a qualitatively similar voltage-dependent manner by mucosal K, Rb, Cs and Tl (approximately 100 mM) and by mucosal H (approximately 1 mM). Block by these cations cannot be explained in terms of interactions with a single membrane-voltage-sensing site; a model in which there are two or more blocking sites in series provides a better description of the data. On the other hand, amiloride block was reduced competitively by mucosal Na and K, suggesting that occupation of the channel by one cation excludes occupancy by the others. ADH and ouabain also reduce the apparent affinity of amiloride for its blocking site. Thus, intracellular Na may also compete with amiloride for occupancy of the channel.

Evidence for concerted effects of aldosterone on a target sodium-transporting epithelium

The sodium-transporting activity of toad skin is stimulated in vitro with aldosterone in the absence of energy-providing substrate; it can be stimulated further upon addition of glucose after prolonged (overnight) incubation. The magnifying effect exerted by glucose in these conditions could be blocked by inhibitors of ribonucleic acid and protein biosynthesis. In addition, exposure to cycloheximide prevented the increase in thermodynamic affinity resulting from aldosterone treatment. A synthetic 19-nor steroid, (RU 24411), dimethyl-2,2-hydroxy-21-nor-19-pregnene-4-dione-3,20, also stimulated sodium transport by toad skin incubated in the absence of glucose, but there was no magnifying effect of this substrate. Furthermore, there was no change in thermodynamic affinity with RU 24411. Therefore, the magnifying effect seen with glucose and the increase in thermodynamic affinity are not necessarily integral parts of the response of sodium-transporting epithelial to "mineralocorticoids."

Amiloride-sensitive trypsinization of apical sodium channels. Analysis of hormonal regulation of sodium transport in toad bladder

Incubation of the mucosal surface of the toad urinary bladder with trypsin (1 mg/ml) irreversibly decreased the short-circuit current to 50% of the initial value. This decrease was accompanied by a proportionate decrease in apical Na permeability, estimated from the change in amiloride-sensitive resistance in depolarized preparations. In contrast, the paracellular resistance was unaffected by trypsinization. Amiloride, a specific blocker of the apical Na channels, prevented inactivation by trypsin. Inhibition of Na transport by substitution of mucosal Na, however, had no effect on the response to trypsin. Trypsinization of the apical membrane was also used to study regulation of Na transport by anti-diuretic hormone (ADH) and aldosterone. Prior exposure of the apical surface to trypsin did not reduce the response to ADH, which indicates that the ADH-induced Na channels were inaccessible to trypsin before addition of the hormone. On the other hand, stimulation of short-circuit current by aldosterone or pyruvate (added to substrate-depleted, aldosterone-repleted bladders) was substantially reduced by prior trypsinization of the apical surface. Thus, the increase in apical Na permeability elicited by aldosterone or substrate involves activation of Na channels that are continuously present in the apical membrane in nonconductive but trypsin-sensitive forms.

Calcium reduces the sodium permeability of luminal membrane vesicles from toad bladder. Studies using a fast-reaction apparatus

Regulation of the sodium permeability of the luminal membrane is the major mechanism by which the net rate of sodium transport across tight epithelia is varied. Previous evidence has suggested that the permeability of the luminal membrane might be regulated by changes in intracellular sodium or calcium activities. To test this directly, we isolated a fraction of the plasma membrane from the toad urinary bladder, which contains a fast, amiloride-sensitive sodium flux with characteristics similar to those of the native luminal membrane. Using a flow-quench apparatus to measure the initial rate of sodium efflux from these vesicles in the millisecond time range, we have demonstrated that the isotope exchange permeability of these vesicles is very sensitive to calcium. Calcium reduces the sodium permeability, and the half-maximal inhibitory concentration is 0.5 microM, well within the range of calcium activity found in cells. Also, the permeability of the luminal membrane vesicles is little affected by the ambient sodium concentration. These results, when taken together with studies on whole tissue, suggest that cell calcium may be an important regulator of transepithelial sodium transport by its effect on luminal sodium permeability. The effect of cell sodium on permeability may be mediated by calcium rather than by sodium itself.

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.

Kinetics of amiloride action in the hen coprodaeum in vitro

The kinetics of amiloride action on the isolated epithelium of the hen coprodaeum are reported. Tissues were taken from birds fed on low salt diets for 9-10 days, conditions which induce a high resting short circuit current due to sodium and sensitive to amiloride. The relation between the inhibition of amiloride sensitive short circuit current and blocker concentration obeyed simple mass laws with an apparent stoichiometry of 1:1 between amiloride and the sodium entry sites. The concentration of amiloride producing its half maximal effect (Ki) was 1.77 +/- 0.20 microM at a sodium concentration of 130 mM. There was a shallow dependence of Ki on sodium concentration, the value of Ki falling to 0.78 +/- 0.1 microM at 1.3 mM Na. The relation of Ki to Na concentration was linear indicating competitive antagonism. The sodium concentration which half saturates the amiloride site (KNa) was 80 mM. This value is very different from the concentration of sodium which half saturates SCC (Kscc = 5-7 mM) suggesting there are at least two sites at which sodium can modify the transporting characteristics. These data are compared to those for other epithelia where Kscc and KNa are rather similar. The benzyl derivative of amiloride (benzamil) was found to be 11.6 times more potent than amiloride on this tissue. The potency ration is similar to that for other sodium transporting epithelia suggesting that the structure of the ion translocation mechanism is partly conserved between species although the Ki for amiloride may vary by an order of magnitude.

Cellular lithium and transepithelial transport across toad urinary bladder

Toad urinary bladders were exposed on either their mucosal or serosal surfaces, or on both surfaces, to medium in which sodium was replaced completely by lithium. With mucosal lithium Ringer's, serosal sodium Ringer's, short-circuit current (SCC) declined by about 50 percent over the first 60 min and was then maintained over a further 180 min. Cellular lithium content was comparable to the sodium transport pool. With lithium Ringer's serosa, SCC was abolished over 60 to 120 min whether the mucosal cation was sodium or lithium. Measurements of cellular ionic composition revealed that the epithelial cells gained lithium from both the mucosal and serosal media. With lithium Ringer's mucosa and serosa, cells lost potassium and gained lithium and a little chloride and water, but these changes in cellular ions could not account for the current flow across the tissue under these conditions, which must, therefore, have been carried by a transepithelial movement of lithium itself. The inhibition by serosal lithium of SCC was overcome by exposure of the mucosal surface of the bladders to amphotericin B. Thus it reflected, predominantly, an inhibition of lithium entry to the cells across the apical membrane. It is suggested that this inhibition is a consequence of cellular lithium accumulation.

Ions and water in the epithelial cells of rabbit descending colon

1. Isolated sheets of rabbit descending colon epithelial cells stripped from their underlying muscle coats were incubated in chambers at 37 degrees C with oxygenated media, and their non-inulin space water, sodium, potassium and chloride contents were subsequently determined. 2. With sodium Ringer bathing both surfaces, amiloride, 10(-4) M, decreased non-inulin space sodium content by 76 mmol/kg dry wt. Ouabain, 10(-3) M, caused loss of non-inulin space potassium which was not completely compensated for by uptake of sodium over 30 min incubation. Chloride and water, therefore, decreased. Amiloride, 10(-4) M, inhibited but did not prevent this uptake of sodium after ouabain. 3. Tissues exposed to sodium-free choline Ringer rapidly exchanged non-inulin space sodium for choline and, more slowly, lost potassium, chloride and water. The equilibration of sodium in the non-inulin space when sodium Ringer was restored to the mucosal medium alone was largely amiloride-insensitive. For restoration of non-inulin space potassium to normal levels, sodium was required in the serosal but not the mucosal medium. 4. Neither the absence of glucose nor the absence of chloride from the mucosal medium affected the non-inulin space sodium content when sodium was restored to the mucosal medium bathing sodium-depleted tissues. 5. It is argued that, whereas non-inulin space potassium and water contents are synonymous with their cellular values, only about one third of non-inulin space sodium is cellular when sodium Ringer bathes both surfaces, and the concentration of the sodium within the cellular transport pool approximated 20 mmol/kg H2O, consistent with estimates obtained from other techniques.