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

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.

Roles of external and cellular Cl- ions on the activation of an apical electrodiffusional Cl- pathway in toad skin

This study is concerned with the short-circuit current, Isc, responses of the Cl(-)-transporting cells of toad skin submitted to sudden changes of the external Cl- concentration, [Cl]o. Sudden changes of [Cl]o, carried out under apical membrane depolarization, allowed comparison of the roles of [Cl]o and [Cl]cell on the activation of the apical Cl- pathways. Equilibration of short-circuited skins symmetrically in K-Ringer's solutions of different Cl- concentrations permitted adjustment of [Cl]cell to different levels. For a given Cl- concentration (in the range of 11.7 to 117 mM) on both sides of a depolarized apical membrane, this structure exhibits a high Cl- permeability, P(Cl)apical. On the other hand, for the same range of [Cl]cell but with [Cl]o = 0, P(Cl)apical is reduced to negligible values. These observations indicate that when the apical membrane is depolarized P(Cl)apical is modulated by [Cl]o; in the absence of external Cl- ions, intracellular Cl- is not sufficient to activate P(Cl)apical. Computer simulation shows that the fast Cl- currents induced across the apical membrane by sudden shifts of [Cl]o from a control equilibrium value strictly follow the laws of electrodiffusion. For each experimental group, the computer-generated Isc versus [( Cl]cell - [Cl]o) curve which best fits the experimental data can only be obtained by a unique pair of P(Cl)apical and Rb (resistance of the basolateral membrane), thus allowing the calculation of these parameters. The electrodiffusional behavior of the net Cl- flux across the apical membrane supports the channel nature of the apical Cl- pathways in the Cl(-)-transporting cells. Cl- ions contribute significantly to the overall conductance of the basolateral membrane even in the presence of a high K concentration in the internal solution.

Comparative roles of voltage and Cl ions upon activation of a Cl conductive pathway in toad skin

(1) Combined use of external Cl concentration pulses and apical membrane depolarization permitted to compare the roles of apical voltage and Cl ions upon the activation of a skin Cl conductance, GCl, which is assumed to reflect activation of the permeability of a Cl pathway. (2) Apical membrane depolarization induced by skin hyperpolarization, or by short-circuiting skins with high K Ringer's on the inner side, failed to activate GCl in the absence of external Cl, GCl remaining negligible. Under apical membrane depolarization, a step elevation of [Cl]0 slowly activated GCl as characterized by a sigmoidal current response of slow onset concomitant to a slow conductance increase. External Cl removal had the reverse effect, slowly inactivating GCl. (3) With the apical membrane in the normal polarized state, a step increase of [Cl]0 slowly activated GCl to submaximal values. This indicates that the interaction of Cl ions with the apical membrane partially activates GCl in the absence of apical membrane depolarization. (4) Activation of GCl was interpreted on the basis of a direct effect of Cl ions upon the apical membrane, having been attributed to the apical membrane voltage an indirect role. Voltage would affect the Cl distribution across the apical membrane, and, as a result, the Cl concentration at a proposed regulatory site which modulates the apical membrane permeability to Cl ions.

Ion transport by mitochondria-rich cells in toad skin

The optical sectioning video imaging technique was used for measurements of the volume of mitochondria-rich (m.r.) cells of the isolated epithelium of toad skin. Under short-circuit conditions, cell volume decreased by about 14% in response to bilateral exposure to Cl-free (gluconate substitution) solutions, apical exposure to a sodium-free solution, or to amiloride. Serosal exposure to ouabain resulted in a large increase in volume, which could be prevented either by the simultaneous application of amiloride in the apical solution or by the exposure of the epithelium to bilateral Cl-free solutions. Unilateral exposure to a Cl-free solution did not prevent ouabain-induced cell swelling. It is concluded that m.r. cells have an amiloride-blockable Na conductance in the apical membrane, a ouabain-sensitive Na pump in the basolateral membrane, and a passive Cl permeability in both membranes. From the initial rate of ouabain-induced cell volume increase the active Na current carried by a single m.r. cell was estimated to be 9.9 +/- 1.3 pA. Voltage clamping of the preparation in the physiological range of potentials (0 to -100 mV, serosa grounded) resulted in a cell volume increase with a time course similar to that of the stimulation of the voltage-dependent Cl conductance. Volume increase and conductance activation were prevented by exposure of the tissue to a Cl-free apical solution. The steady-state volume of the m.r. cells increased with the clamping voltage, and at -100 mV the volume was about 1.15 times that under short-circuit conditions. The rate of volume increase during current passage was significantly decreased by lowering the serosal K concentration (Ki) to 0.5 mM, but was independent of whether Ki was 2.4, 5, or 10 mM. This indicates that the K conductance of the serosal membrane becomes rate limiting for the uptake of KCl when Ki is significantly lower than its physiological value. It is concluded that the voltage-activated Cl currents flow through the m.r. cells and that swelling is caused by an uptake of Cl ions from the apical bath and K ions from the serosal bath. Bilateral exposure of the tissue to hypo- or hypertonic bathing solutions changed cell volume without detectable changes in the Cl conductance. The volume response to external osmotic perturbations followed that of an osmometer with an osmotically inactive volume of 21%.(ABSTRACT TRUNCATED AT 400 WORDS)

Concentration dependence of halide fluxes and selectivity of the anion pathway in toad skin

The isolated toad (Bufo bufo) skin was mounted under voltage-clamp conditions in a chamber shown to cause no significant edge damage. The serosal side of the skin was bathed with NaCl-Ringer's, and the passive voltage-sensitive anion conductance studied in its fully voltage activated state, V = -80 mV (apical bath negative). The active sodium currents were eliminated by replacing external Na+ with K+. With [Cl-]o varying between 1.45 mM and 110 mM (gluconate substitution) and [I-]o = 3 mM, the total clamping current (y) and the sum of halide currents (x), estimated from flux measurements, were related by y = 1.0x-3.7 microA cm-2 (r2 = 0.98, n = 50 preparations). The increase in [Cl-]o produced a sigmoidal increase in Cl- influx and clamping current, with the rate coefficient for the influx increasing with [Cl-]o for 1.45 less than [Cl-]o less than 60 mM, but decreasing slightly again as [Cl-]o was further raised to 110 mM. A similar relationship was obtained between the rate coefficient for the Br- influx and [Br-]o, and the I- influx and [Cl-]o, indicating that these three ions are transported by a pathway that is activated by Cl-o and Br-o. The rate coefficients for the influxes ranked as follows, I-:Cl-:Br- = 0.7:1:1.3. The I-/Cl- selectivity was shown to be independent of the degree of Cl-o activation of the anion pathway, and identical with the I-/Cl- selectivity of a furosemide-sensitive, conductive pathway. With [Cl-]o, [Br-]o, or [I-]o = 110 mM, the currents ranked as follows, Cl-:Br-:I- = 1:0.68:0.06, indicating that Cl-, to a lesser extent Br-, and I-, poorly activate the conductive anion pathway. External I- was a potent inhibitor of the Cl-o activation of the Cl- conductance. The unidirectional I- fluxes ([I-]o = [I-]i = 3 mM, [Cl-]o = [Cl-]i = 110 mM) revealed passive transport for V less than -50 mV, active transport for V = o mV, and exchange diffusion for V = 50 mV, confirming our previous finding that depending on the transepithelial potential, the toad skin exhibits three modes of anion transport. A model that shares some properties with that of the anion transport system of the red cell membrane accounts for our findings, and for an inwardly directed active Cl- flux in terms of Cl-/HCO3- exchange.

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.

Influence of extracellular Cl concentration on Cl transport across isolated skin or Rana pipiens

The effect of changes in Cl concentration in the external and/or serosal bath on Cl transport across short-circuited frog skin was studied by measurements of transepithelial Cl influx (J13Cl) and efflux (J31Cl), short-circuit current, transepithelial potential, and conductance (Gm). J14Cl as well as J31Cl were found to have a saturating component and a component which is apparently linear with Cl concentration. The linear component of J31Cl appears only upon addition of Cl to external medium, and about 3/4 of this component does not contribute to Gm. The saturating component of J31Cl is only 5% of total J31Cl with 115mM Cl in the serosal medium. Replacement of 115 mM Cl- in external medium by SO4=, NO3-, HCO3- or I- results in 87-97% reduction of J31Cl, whereas replacement with Br- has no effect. As external Cl concentration is raised in steps from 2 to 115 mM, J13Cl and J31Cl increase by the same amount but J13Cl is persistently 0.15 mueg/cm2hr larger than J31Cl. These results indicate that at least 3/4 of linear components of J13Cl and J31Cl proceed via an exchange diffusion mechanism which seems to be located at the outer cell border. The saturating component of J13Cl is involved in active Cl transport in an inward direction, and there is evidence suggesting that Cl uptake across outer cell border, which proceeds against an electrochemical gradient, is electroneutral but not directly linked to Na.

Influence of extracellular Cl concentration on Cl transport across isolated skin of Rana pipiens

The effect of changes in Cl concentration in the external and/or serosal bath on Cl transport across short-circuited frog skin was studied by measurements of transepithelial Cl influx (JCl13) and efflux (JCl31), short-circuit current, transepithelial potential, and conductance (Gm). JCl13 as well as JCl31 were found to have a saturating component and a component which is apparently linear with Cl concentration. The linear component of JCl31 appears only upon addition of Cl to external medium, and about 3/4 of this component does not contribute to Gm. The saturating component of JCl31 is only 5% of total JCl31 with 115 mM Cl in the serosal medium. Replacement of 115 mM Cl- in external medium by SO4=, NO3-, HCO-3 or I- results in 87-97% reduction of JCl31, whereas replacement with Br- has no effect. As external Cl concentration is raised in steps from 2 to 115 mM, JCl13 and JCl31 increase by the same amount but JCl13 is persistently 0.15 mu eq/cm2hr larger than JCl31. These results indicate that at least 3/4 of linear components of JCl13 and JCl31 proceed via an exchange diffusion mechanism which seems to be located at the outer cell border. The saturating component of JCl13 is involved in active Cl transport in an inward direction, and there is evidence suggesting that Cl uptake across outer cell border, which proceeds against an electrochemical gradient, is electroneutral but not directly linked to Na.

Studies on chloride permeability of the skin of Leptodactylus ocellatus: II. Na+ and Cl- effect of inward movements of Cl-

At low concentration (1 mM) of Cl- in the outer solution, the influx of chloride through the isolated skin (JCl13) of the South American frog Leptodactylus ocellatus (L.) seems to be carried by two mechanisms: (i) a passive one that exhibits the characteristics of an exchange diffusion process, and (ii) an active penetration. Studies of the influx and efflux of chloride (JCl13 and JCl31) indicate that the presence of a high (107 mM) concentration of Cl- in the outer solution activates the translocation of this ion through the cells. Studies of the unidirectional flux of Cl- across the outer barrier (JCl12) indicate that Na+ out stimulates the penetration of Cl- at this level. Cl- out, in turn, stimulates the JNa12, but this effect is only detected at low concentrations of Na+ out.

Studies on chloride permeability of the skin of Leptodactylus ocellatus: I. Na+ and Cl- effect on passive movements of Cl-

The outflux of chloride through the isolated skin (JCl31) of the South American frog Leptodactylus ocellatus (L.) is carried by a mechanism that saturates at high concentration of chloride on the inside, and is stimulated by the presence of Cl- in the outer solution (trans side). The presence of Na+ on the outside, by itself, does not increase JCl31. However, when JCl31 is already increased by chloride on the trans side, the addition of Na+ produces a significant further increase. At low concentration of Cl- on the outside JCl31 is carried by an exchange diffusion mechanism. At high concentrations of Cl- outside, JCl31 proceeds through a route which involves changes in electrical parameters. The results suggest that both mechanisms are located on the cell membranes and, therefore, that the fluxes would cross through the cytoplasm of the cells. Na+ stimulates the second mechanism only.