[14C]Carbon-dioxide fixation by isolated leaf epidermes with stomata closed or open

Following small hypo-osmotic shocks, ion concentrations (Na⁺, K⁺, Cl^-) in Platymonas subcordiformis decreased; this was due mainly to an increase of cell volume. With larger hypo-osmotic stresses, the decrease of ion concentration continued and, additionally, extrusion of mannitol was observed. The ion and mannitol concentrations were not regained after 240 min. In contrast, following hyperosmotic shocks, the ion concentrations increased transitorily during the first 20–40 min. The same was true for K⁺ following small hyperosmotic stresses and for Na⁺ and — partially — Cl^- with larger shocks. Large hyperosmotic stresses caused permanent accumulation of mannitol, which levelled off after 60–80 min. Thus the transient increase of ions bridged the concentration gap until mannitol was accumulated to a high enough concentration to account for the osmotic adaptation of Platymonas, together with a basal level of the ions K⁺, Na⁺, Cl^-.Abbreviations PS photosynthesis - Resp respiration     

Solute Accumulation in Tobacco Cells Adapted to NaCl

Cells of Nicotiana tabacum L. var Wisconsin 38 adapted to NaCl (up to 428 millimolar) which have undergone extensive osmotic adjustment accumulated Na⁺ and Cl⁻ as principal solutes for this adjustment. Although the intracellular concentrations of Na⁺ and Cl⁻ correlated well with the level of adaptation, these ions apparently did not contribute to the osmotic adjustment which occurred during a culture growth cycle, because the concentrations of Na⁺ and Cl⁻ did not increase during the period of most active osmotic adjustment. The average intracellular concentrations of soluble sugars and total free amino acids increased as a function of the level of adaptation; however, the levels of these solutes did not approach those observed for Na⁺ and Cl⁻. The concentration of proline was positively correlated with cell osmotic potential, accumulating to an average concentration of 129 millimolar in cells adapted to 428 millimolar NaCl and representing about 80% of the total free amino acid pool as compared to an average of 0.29 millimolar and about 4% of the pool in unadapted cells. These results indicate that although Na⁺ and Cl⁻ are principal components of osmotic adjustment, organic solutes also may make significant contributions.     

Sodium,potassium, chloride and proline concentrations of chloroplasts isolated from a halophyte,Mesembryanthemum crystallinum L.

Concentrations of four major solutes (Na⁺, K⁺, Cl^-, proline) were determined in isolated, intact chloroplasts from the halophyte Mesembryanthemum crystallinum L. following long-term exposure of plants to three levels of NaCl salinity in the rooting medium. Chloroplasts were obtained by gentle rupture of leaf protoplasts. There was either no or only small leakage of inorganic ions from the chloroplasts to the medium during three rapidly performed washing steps involving precipitation and re-suspension of chloroplast pellets. Increasing NaCl salinity of the rooting medium resulted in a rise of Na⁺ und Cl^- in the total leaf sap, up to approximately 500 and 400 mM, respectively, for plants grown at 400 mM NaCl. However, chloroplast levels of Na⁺ und Cl^- did not exceed 160–230 and 40–60 mM, respectively, based upon a chloroplast osmotic volume of 20–30 l per mg chlorophyll. At 20 mM NaCl in the rooting medium, the Na⁺/K⁺ ratio of the chloroplasts was about 1; at 400 mM NaCl the ratio was about 5. Growth at 400 mM NaCl led to markedly increased concentrations of proline in the leaf sap (8 mM) compared with the leaf sap of plants grown in culture solution without added NaCl (proline 0.25 mM). Although proline was fivefold more concentrated in the chloroplasts than in the total leaf sap of plants treated with 400 mM NaCl, the overall contribution of proline to the osmotic adjustment of chloroplasts was small. The capacity to limit chloroplast Cl^- concentrations under conditions of high external salinity was in contrast to an apparent affinity of chloroplasts for Cl^- under conditions of low Cl^- availability.Abbreviation Chl chlorophyll     

Water relations and xylem transport of nutrients in pepper plants grown under two different salts stress regimes

Two iso-osmotic concentrations of NaCl and Na₂SO₄ were used for discriminating between the effects of specific ion toxicities of salt stress on pepper plants (Capsicum annuum L.) grown in hydroponic conditions, in a controlled-environment greenhouse. The two salts were applied to plants at different electrical conductivities, and leaf water relations, osmotic adjustment and root hydraulic conductance were measured. Leaf water potential (_w), leaf osmotic potential (_o) and leaf turgor potential (_p) decreased significantly when EC increased, but the decrease was less for NaCl- than for Na₂SO₄-treated plants. The reduction in stomatal conductance was higher for NaCl-treated plants. There were no differences in the effect of both treatments on the osmotic adjustment, and a reduction in root hydraulic conductance and the flux of solutes into the xylem was observed, except for the saline ions (Na⁺, Cl^– and SO₄ ^2–). Therefore, pepper growth decreased with increasing salinity because the plants were unable to adjust osmotically or because of the toxic effects of Cl^–, SO₄ ^2– and/or Na⁺. However, turgor of NaCl-treated plants was maintained at low EC (3 and 4 dS m^–1) probably due to the maintenance of water transport into the plant (decrease of stomatal conductance), which, together with the lower concentration of Na⁺ in the plant tissues compared with the Na₂SO₄ treatment, could be the cause of the smaller decrease in growth.     

Sodium chloride toxicity and the cellular basis of salt tolerance in halophytes

Background Halophytes are the flora of saline soils. They adjust osmotically to soil salinity by accumulating ions and sequestering the vast majority of these (generally Na⁺ and Cl⁻) in vacuoles, while in the cytoplasm organic solutes are accumulated to prevent adverse effects on metabolism. At high salinities, however, growth is inhibited. Possible causes are: toxicity to metabolism of Na⁺ and/or Cl⁻ in the cytoplasm; insufficient osmotic adjustment resulting in reduced net photosynthesis because of stomatal closure; reduced turgor for expansion growth; adverse cellular water relations if ions build up in the apoplast (cell walls) of leaves; diversion of energy needed to maintain solute homeostasis; sub-optimal levels of K⁺ (or other mineral nutrients) required for maintaining enzyme activities; possible damage from reactive oxygen species; or changes in hormonal concentrations.Scope This review discusses the evidence for Na⁺ and Cl⁻ toxicity and the concept of tissue tolerance in relation to halophytes.Conclusions The data reviewed here suggest that halophytes tolerate cytoplasmic Na⁺ and Cl⁻ concentrations of 100–200 mm, but whether these ions ever reach toxic concentrations that inhibit metabolism in the cytoplasm or cause death is unknown. Measurements of ion concentrations in the cytosol of various cell types for contrasting species and growth conditions are needed. Future work should also focus on the properties of the tonoplast that enable ion accumulation and prevent ion leakage, such as the special properties of ion transporters and of the lipids that determine membrane permeability.     

Intracellular compartmentation of Na+, K+ and Cl− in the Ehrlich ascites tumor cell: Correlation with the membrane potential

Summary The intracellular distribution of Na⁺, K⁺, Cl^– and water has been studied in the Ehrlich ascites tumor cell. Comparison of the ion and water contents of whole cells with those of cells exposed to La³⁺ and mechanical stress indicated that La³⁺ treatment results in selective damage to the cell membrane and permits evaluation of cytoplasmic and nuclear ion concentrations. The results show that Na⁺ is sequestered within the nucleus, while K⁺ and Cl^– are more highly concentrated in the cell cytoplasm. Reduction of the [Na⁺] of the incubation medium by replacement with K⁺ results in reduced cytoplasmic [Na⁺], increased [Cl^–] and no change in [K⁺]. Nuclear concentrations of these ions are virtually insensitive to the cation composition of the medium. Concomitant measurements of the membrane potential were made. The potential in control cells was –13.7 mV. Reduction of [Na⁺] in the medium caused significant depolarization. The measured potential is describable by the Cl^– equilibrium potential and can be accounted for in terms of cation distributions and permeabilities. The energetic implications of the intracellular compartmentation of ions are discussed.     

Intracellular compartmentation of ions in salt adapted tobacco cells

Na⁺ and Cl⁻ are the principal solutes utilized for osmotic adjustment in cells of Nicotiana tabacum L. var Wisconsin 38 (tobacco) adapted to NaCl, accumulating to levels of 472 and 386 millimolar, respectively, in cells adapted to 428 millimolar NaCl. X-ray microanalysis of unetched frozen-hydrated cells adapted to salt indicated that Na⁺ and Cl⁻ were compartmentalized in the vacuole, at concentrations of 780 and 624 millimolar, respectively, while cytoplasmic concentrations of the ions were maintained at 96 millimolar. The morphometric differences which existed between unadapted and salt adapted cells, (cytoplasmic volume of 22 and 45% of the cell, respectively), facilitated containment of the excited volume of the x-ray signal in the cytoplasm of the adapted cells. Confirmation of ion compartmentation in salt adapted cells was obtained based on kinetic analyses of ²²Na⁺ and ³⁶Cl⁻ efflux from cells in steady state. These data provide evidence that ion compartmentation is a component of salt adaptation of glycophyte cells.     

On the interrelations of the Na+-and Cl?-influxes in the pulmonate freshwater snailLymnaea stagnalis

Summary In the freshwater snailLymnaea stagnalis the influxes of Na⁺ and Cl^– were studied at different external concentrations of these ions. The characteristies of the Na⁺- and Cl^–-influxes are similar with respect to saturation kinetics,K _m (0.1 mM) and activation by low-salt adaptation. In short-term experiments the Na⁺- and Cl^–-influxes are independent. Because of the counter-ions (H⁺ and HCO ₃ ^– ) involved, this indicates a potential acid-base regulatory capacity. Low-salt adaptation, due to either Na⁺-or Cl^–-depletion, activates both the Na⁺- and the Cl^–-influx. It is suggested that under both conditions the number of active integumental pumps, involved in Na⁺- as well as in Cl^–-uptake, is increased.     

Ion composition of unicellular marine and fresh-water algae,with special reference to Platymonas subcordiformis cultivated in media with different osmotic strengths

Summary Ion compositions (K⁺, Na⁺ Mg²⁺, Ca²⁺, Cl^-, phosphate) of the euryhaline algae, Platymonas subcordiformis, Chlorella salina, grown in media with a salinity range from 0.1 to 0.6 M NaCl and of the fresh-water algae, Ankistrodesmus braunii and Scenedesmus obliquus, were compared. Enhancement of ion concentrations with increasing salinity in Platymonas was attributed largely to decreasing cell volume. In both the euryhaline algae, Na⁺ and — partially — Cl^- content per cell increased significantly with rising salinity. The contents per cell of the other ions were not affected. Considering the relevance of ions and mannitol (Platymonas) and proline (Chlorella) as osmotically active particles, it was found that the ions alone maintained osmotic balance with low external salinity. With increasing salinity the organic compounds contributed up to 20–30% of the cellular solute potential. The main cation, K⁺, was the main contributor to the osmotic balance; the accumulation of organic compounds as well as of Na⁺ and Cl^- contributes further to the ability of the algae to adapt to high salinity. The results confirm the hypothesis of low Cl^- concentrations in nonvacuolate cells in comparison to vacuolate cells.     

Abscisic Acid Stimulated Osmotic Adjustment and Its Involvement in Adaptation of Tobacco Cells to NaCl

Osmotic adjustment of cultured tobacco (Nicotiana tabacum L. var Wisconsin 38) cells was stimulated by 10 micromolar (±) abscisic acid (ABA) during adaptation to water deficit imposed by various solutes including NaCl, KCl, K₂SO₄, Na₂SO₄, sucrose, mannitol, or glucose. The maximum difference in cell osmotic potential (Ψπ) caused by ABA treatment during adaptation to 171 millimolar NaCl was about 6 to 7 bar. The cell Ψπ differences elicited by ABA were not due to growth inhibition since ABA stimulated growth of cells in the presence of 171 millimolar NaCl. ABA caused a cell Ψπ difference of about 1 to 2 bar in medium without added NaCl. Intracellular concentrations of Na⁺, K⁺, Cl⁻, free amino acids, or organic acids could not account for the Ψπ differences induced by ABA in NaCl treated cells. However, since growth of NaCl treated cells is more rapid in the presence of ABA than in its absence, greater accumulation of Na⁺, K⁺, and Cl⁻ was necessary for ion pool maintenance. Higher intracellular sucrose and reducing sugar concentrations could account for the majority of the greater osmotic adjustment of ABA treated cells. More rapid accumulation of proline associated with ABA treatment was highly correlated with the effects of ABA on cell Ψπ. These and other data indicate that the role of ABA in accelerating salt adaptation is not mediated by simply stimulating osmotic adjustment.     

Salinity induced differences in growth,ion distribution and partitioning in barley between the cultivar Maythorpe and its derived mutant Golden Promise

Dry matter changes and ion partitioning in two near isogenic barley cultivars Maythorpe (relatively salt sensitive) and Golden Promise (relatively salt tolerant) were studied in response to increasing salinity. Although the growth of both cultivars was significantly reduced by exposure to NaCl, the effect was greater in Maythorpe, whilst Golden Promise maintained an increased ratio of young to old leaf blade. Golden Promise maintained significantly lower Na⁺ concentrations in young expanding tissues compared with Maythorpe. Partitioning of Cl^– was evident in that both varieties maintained lower Cl^– concentrations in mesophyll than in epidermal cells. Golden Promise maintained higher K⁺/Na⁺ and Ca²⁺/Na⁺ ratios in young leaf blade and young sheath tissues than Maythorpe when exposed to salt. Differences in ion partitioning and the maintenance of higher K⁺ and Ca²⁺ to Na⁺ ratios, especially in young growing and recently expanded tissues, would appear to be important mechanisms contributing to the improved salt tolerance of Golden Promise.     

Salt sensitivity in chickpea is determined by sodium toxicity

Main conclusion

Salt sensitivity in chickpea is determined by Na⁺ toxicity, whereas relatively high leaf tissue concentrations of Cl^? were tolerated, and the osmotic component of 60-mM NaCl was not detrimental.Chickpea (Cicer arietinum L.) is sensitive to salinity. This study dissected the responses of chickpea to osmotic and ionic components (Na⁺ and/or Cl^?) of salt stress. Two genotypes with contrasting salt tolerances were exposed to osmotic treatments (?0.16 and ?0.29 MPa), Na⁺-salts, Cl^?-salts, or NaCl at 0, 30, or 60 mM for 42 days and growth, tissue ion concentrations and leaf gas-exchange were assessed. The osmotic treatments and Cl^?-salts did not affect growth, whereas Na⁺-salts and NaCl treatments equally impaired growth in either genotype. Shoot Na⁺ and Cl^? concentrations had markedly increased, whereas shoot K⁺ had declined in the NaCl treatments, but both genotypes had similar shoot concentrations of each of these individual ions after 14 and 28 days of treatments. Genesis836 achieved higher net photosynthetic rate (64–84 % of control) compared with Rupali (35–56 % of control) at equivalent leaf Na⁺ concentrations. We conclude that (1) salt sensitivity in chickpea is determined by Na⁺ toxicity, and (2) the two contrasting genotypes appear to differ in ‘tissue tolerance’ of high Na⁺. This study provides a basis for focus on Na⁺ tolerance traits for future varietal improvement programs for salinity tolerance in chickpea.

     

Effect of salinity on osmotic adjustment characteristics of <Emphasis Type="Italic">Kandelia candel</Emphasis>

To elucidate the osmotic adjustment characteristics of mangrove plants, inorganic ion and organic solute contents of intermediate leaves were investigated in 3-month-old Kandelia candel (L.) Druce seedlings during 45 days of NaCl treatments (0, 200, and 500 mM NaCl). The contents of Na⁺, Cl⁻, total free amino acids, proline, total soluble sugars, pinitol and mannitol increased to different degree by salinity, whereas, K⁺ content decreased by salinity compared with control. NaCl treatment induced an increase of inorganic ion contribution while a decrease of organic solute contribution. It was concluded that accumulating a large amount of inorganic ions was used as the main osmotic adjustment mechanism under salinity treatment. However, accumulation of organic osmolytes might be considered to play much more important role in osmoregulation under severe salinity (500 mM NaCl) than under moderate salinity (200 mM NaCl), thus the damage caused by high toxic ions (Na⁺ and Cl⁻) concentration in K. candel leaves could be avoided.     

Three-year field response of young olive trees (Olea europaea L., cv. Arbequina) to soil salinity: Trunk growth and leaf ion accumulation

Irrigated olive is rapidly increasing in arid and semiarid areas, many of which may be negatively affected by soil salinity. We evaluated changes in trunk growth and leaf Cl^–, Na⁺ and K⁺ concentrations in young Arbequina olives (Olea europaea L.) grown in a saline-sodic field over a three-year period. The trunk diameter was measured at the beginning and the end of the 1999 (70 trees), 2000 (59 trees) and 2001 (42 trees) growing periods. Leaves, sampled in August of each year, were analyzed for Cl^–, Na⁺ and K⁺ concentrations. Soil salinity (apparent electrical conductivity, ECa) of each monitored tree was measured 14 times during the 1999–2001 experimental period with an electromagnetic sensor and converted to root zone electrical conductivity of the soil saturation extract (ECe) based on ECa–ECe calibration curves. Salinity tolerance was determined using the Maas and Hoffman threshold–slope response model. Based on salinity thresholds (ECe_thr), the tolerance of olive in terms of trunk growth was high in 1999 (ECe_thr = 6.7 dS m^–1), but declined with age and time of exposure to salts by 30% in 2000 (ECe_thr = 4.7 dS m^–1) and by 55% in 2001 (ECe_thr = 3.0 dS m^–1). Based on the high absolute slopes obtained in all years (values between 16% and 23% dS^–1 m), olive was classified as very sensitive to ECe values above the threshold. Trunk growth thresholds based on leaf ion concentrations varied, depending on years, between 2.6 and 4.0 mg g^–1 (Cl_thr) and between 1.0 and 1.2 mg g^–1 (Na_thr), indicating that Arbequina olive was less sensitive to leaf Cl^– and much more sensitive to leaf Na⁺ than values reported as toxic in greenhouse studies. Leaf K⁺ slightly decreased with increasing salinity, whereas the K⁺/Na⁺ ratio sharply decreased with increasing salinity. We concluded that the initial salinity tolerance of olive was high, but declined sharply with time of exposure to salts and became quite sensitive due primarily to increasing toxic concentrations of Na⁺ in the leaves.     

Response to short-term inundation with isoosmotic solutions of seawater and sorbitol in a C4 nonhalophyte: evidence for a salt tolerance mechanism

Summary To examine the importance of Na⁺ and Cl^- to osmotic adjustment in a salt-tolerant ecotype of the C₄ nonhalophyte Andropogon glomeratus, plants were watered with sorbitol, a neutral osmoticum, and synthetic seawater, for five days. Gas exchange measurements were made during the course of the watering treatment and during a recovery period following treatment. Leaf osmotic adjustment occurred only in plants watered with seawater, and was associated with an increase in Na⁺ and Cl^- concentrations. Estimates of the molar concentrations indicated these ions could account for 95% of the leaf osmotic adjustment. Net photosynthetic CO₂ uptake was less effected during the watering treatment, and photosynthetic recovery was greater following the treatment in plants watered with seawater. Photosynthetic inhibition was related primarily to metabolic factors, including a decrease in carboxylation efficiency. A model is presented for a mechanism promoting tolerance to transient seawater inundation in A. glomeratus.     

Stimulation by HCO 3 − of Na+ transport in rabbit gallbladder

Summary Bicarbonate presence in the bathing media doubles Na⁺ and fluid transepithelial transport and in parallel significantly increases Na⁺ and Cl^– intracellular concentrations and contents, decreases K⁺ cell concentration without changing its amount, and causes a large cell swelling. Na⁺ and Cl^– lumen-to-cell influxes are significantly enhanced, Na⁺ more so than Cl^–. The stimulation does not raise any immediate change in luminal membrane potential and cannot be due to a HCO ₃ ^– -ATPase in the brush border. The stimulation goes together with a large increase in a Na⁺-dependent H⁺ secretion into the lumen. All of these data suggests that HCO ₃ ^– both activates Na⁺–Cl^– cotransport and H⁺–Na⁺ countertransport at the luminal barrier.Thiocyanate inhibits Na⁺ and fluid transepithelial transport without affecting H⁺ secretion and HCO ₃ ^– -dependent Na⁺ influx. It reduces Na⁺ and Cl^– concentrations and contents, increases the same parameters for K⁺, causes a cell shrinking, and abolishes the lumen-to-cell Cl^– influx. It enters the cell and is accumulated in the cytoplasm with a process which is Na⁺-dependent and HCO ₃ ^– -activated. Thus, SCN^– is likely to compete for the Cl^– site on the cotransport carrier and to be slowly transferred by the cotransport system itself.     

Effects of an hypo-osmotic shock on Na+, K+ and Cl? levels in isolated axons ofCarcinus maenas

Summary Application of an hypo-osmotic shock to isolated axons ofCarcinus maenas induces a decrease in the intracellular content of K⁺, Na⁺ and Cl^–. The changes in Na⁺ and Cl^– levels are only transitory while the K⁺ level reaches new steady-state value much lower than the control. The modification of K⁺ concentration cannot be ascribed only to a simple dilution process and it is proposed that the regulation of intracellular K⁺ plays an important role in limiting the swelling which occurs in this tissue.Hypo-osmotic conditions also induce an increase in Na⁺ permeability.The results are discussed in relation to changes in ion transport and interactions with intracellular organic compounds that could arise during the process of volume regulation.     

Characterization of esophageal desalination in the seawater eel,Anguilla japonica

To characterize mechanisms of esophageal desalination, osmotic water permeability and ion fluxes were measured in the isolated esophagus of the seawater eel. The osmotic permeability coefficient in the seawater eel esophagus was 2·10^-4 cm·s^-1. This value was much lower than those in tight epithelial, although the eel esophagus is a leaky epithelium with a tissue resistance of 77 ohm·cm^-2. When the esophagus was bathed in normal Ringer solutions on both sides no net ion and water fluxes were observed. However, when mucosal NaCl concentration was increased by a factor of 3, Na⁺ und Cl^- ions were transferred from mucosa to serosa (desalination). If only Na⁺ or Cl^- concentration in the mucosal fluid was increased by a factor of 3, net Na⁺ and Cl^- fluxes were reduced to 30–40%, indicating that 60–70% of the net Na⁺ and Cl^- fluxes are coupled mutually. The coupled NaCl transport seems to be effective in desalting the luminal high NaCl. The remaining 30–40% of the total Na⁺ and Cl^- fluxes seems to be due to a simple diffusion, because these components are independent of each other and follow their electrochemical gradients, and also because these fluxes remain even after treatment with NaCN or ouabain. A half of the coupled NaCl transport could be explained by a Na⁺/H⁺–Cl^-/HCO ₃ ^- double exchanger on the apical membrane of the esophageal epithelium, because mucosal amiloride and 4.4-diisothiocyanatostilbene-2,2-disulphonic acid inhibited the net Na⁺ and Cl^- fluxes by approximately 30%. The other half of the coupled NaCl transport, which follows their electrochemical gradients, still remains to be explained.Abbreviations DIDS 4,4-diisothiocyanatostilbene-2,2-disulphonic acid - NMDG N-methyl-d-glucosamine - P _Cl Cl^- permeability coefficient - PD transepithelial potential difference - P _Na Na⁺ permeability coefficient - P _osm osinotic permeability coefficient - TALH thick ascending limb of Henle's loop     

Ionic and Water Relations Responses of Two Populations of a Non-Halophyte to Salinity

Bowman, W. D. 1988. Ionic and water relations responses of twopopulations of a non-halophyte to salinity.–J. exp. Bot39: 97–105 Salinity-induced changes in the ionic and water relations inplants from two naturally-occurring populations of the C₄ non-halophyteAndropogon glomeratus were measured to detect differences inthe capacity to adjust osmotic potentials and in ion contentpotentially responsible for the osmotic adjustment Pressure-volumecurves and leaf ion content were measured in plants from twopopulations, salt marsh and inland, after long-term exposureto three salinity levels. Osmotic adjustment and decreases inthe bulk tissue elasticity occurred to a similar extent in bothpopulations with increasing salinity. Cl^– concentrationsincreased with increasing salinity in both populations, whereasleaf Na⁺ concentrations increased only in the inland population,but were higher at all salinities in the marsh population. K⁺concentrations changed little with increasing salinity. Prolineconcentrations increased only at the highest salinity level,and did not difler significantly between populations. Theseresults suggest a role for Na⁺ uptake and regulation in osmoticadjustment in the marsh population, contrasting with studiesof salt tolerance in other nonhalophytic grasses     

The nature of the neutral Na+−Cl−-coupled entry at the apical membrane of rabbit gallbladder epithelium: II. Na+−Cl− symport is independent of K+

Summary In the epithelium of rabbit gallbladder, in the nominal absence of bicarbonate, intracellular Cl^– activity is about 25mm, about 4 times higher than intracellular Cl^– activity at the electrochemical equilibrium. It is essentially not affected by 10^–4 m acetazolamide and 10^–4 m 4-acetamido-4-isothiocyanostilbene-2,2-disulfonate (SITS) even during prolonged exposures; it falls to the equilibrium value by removal of Na⁺ from the lumen without significant changes of the apical membrane potential difference. Both intracellular Cl^– and Na⁺ activities are decreased by luminal treatment with 25mm SCN^–; the initial rates of change are not significantly different. In addition, the initial rates of change of intracellular Cl^– activity are not significantly different upon Na⁺ or Cl^– entry block by the appropriate reduction of the concentration of either ion in the luminal solution. Luminal K⁺ removal or 10^–5 m bumetanide do not affect intracellular Cl^– and Na⁺ activities or Cl^– influx through the apical membrane. It is concluded that in the absence of bicarbonate NaCl entry is entirely due to a Na⁺–Cl^– symport on a single carrier which, at least under the conditions tested, does not cotransport K⁺.