Potassium uptake by the dog erythrocyte

The inward transport of potassium by separated dog erythrocytes has been studied at concentrations of potassium in the medium from 2.9 to 25.0 m.eq./liter and at 38.0 and 33.0 degrees C. At the physiological concentration of external potassium (4.06 m.eq./liter medium), the inward potassium flux is 0.11 m.eq./liter cells hour and the glucose consumption is 2.0 mM/liter cells hour. The dependence of potassium influx on extracellular potassium concentration is given by the following equation, K influx (m.eq./liter cells hour) = 0.028 [K](amb.) - 0.003 in which [K](amb.) refers to the potassium concentration in the medium. In a single 93 hour experiment, 94 per cent of the intracellular potassium was exchanged at an apparently uniform rate. The average apparent activation energy for the process is 7,750 calories +/- 2,000 calories/mol and there is some indication that the apparent activation energy of inward K transport decreases with increasing external K concentration.     

The permeability of the human erythrocyte to sodium and potassium

Measurements have been made on the permeability of the human erythrocyte to Na and K in vitro, using radioactive tracers to observe the system in the steady state. The average inward K flux is 1.67 m.eq./liter cells hour, and the apparent activation energy is 12,300 ± 1300 calories/mol. The inward K flux is independent of the external K concentration in the range of concentrations studied (4 to 16 m.eq. K/liter plasma). Rb appears to compete with K for transport into the cell, whereas Na and Li do not. The average inward Na flux is 3.08 ± 0.57 m.eq. Na/liter cells hour, and the apparent activation energies are 20,200 ± 2700 calories/mol for inward transport, and 14,900 ± 3,400 calories/mol for outward transport. The inward Na flux is dependent on the external Na concentration, but not in a linear fashion. Li appears to compete with Na for inward transport, whereas K and Rb do not. An approximate maximum estimate shows that the energy required for cation transport is only 8.8 calories/mol liter cells hour of the 110 calories/mol liter cells hour available from the consumption of glucose. A working hypothesis for the transport of Na and K is presented.     

Potassium transport in human erythrocytes: evidence for a three compartment system

Whole human blood is incubated for periods of ½ to 3 hours with K⁴² at 37°C. At the close of this period, called pre-incubation, the plasma is removed from the cells and the cells, now become radioactive, are again incubated in a mixture of plasma and buffer for periods of up to 10 additional hours. The time course of the K⁴² activity of the incubating medium is followed. Characteristically, after 2 hours of pre-incubation, the activity in the medium rises to a peak about 1 and ½ hours after resuspension, and then falls slowly until at 10 hours it is very close to its initial value at the beginning of the resuspension interval. This transient rise in K⁴² activity in the medium is taken to indicate that the red cell does not consist of a single uniform K compartment, but contains at least two compartments. Thus one cellular compartment contains a reservoir of high specific activity K which provides the specific activity gradient necessary to drive the K⁴² content of the medium to its transient peak. Experiments with Na indicate that its behavior in this respect is unlike that of K. The experimental data are matched to a simple model system which is capable of theoretical analysis with the aid of an analogue computer. The model system, whose characteristics agree fairly well with those observed experimentally on red cell suspensions, comprises two intracellular compartments, one containing 2.35 m.eq. K/liter blood, and the other 44.1 m.eq. K/liter blood. The plasma K content is 2.64 m.eq./liter blood. The flux between plasma and the smaller intracellular compartment is 0.65 m.eq. K/liter blood hour; that between the smaller and the larger intracellular compartment, 1.77 m.eq. K/liter blood hour; and that between the larger intracellular compartment and the plasma is 0.34 m.eq. K/liter blood hour.     

The transport of sodium into human erythrocytes in vivo

The relative Na²⁴ specific activity of red cells and plasma was measured at periods up to 30 hours following a single intravenous injection of Na²⁴ in normal healthy young adults. The average specific activity of the red cells relative to that of the plasma at 24 hours and beyond was found to average 0.83 ± 0.05 in a series of five normal individuals, significantly different from 1.0. This indicates that all the intracellular Na is not exchangeable in 24 hours, and confirms earlier in vitro results. The red cell Na concentration in man was shown to be 12.1 ± 1.1 m.eq. Na/liter red cell, as measured in a series of nineteen normal healthy young adults. A theoretical analysis of the data on exchangeable cell Na suggests that the red cell Na (5.3 m.eq. Na/liter blood) is divided into a fast compartment comprising 4.25 m.eq. Na/liter blood, and a slow compartment comprising 1.07 m.eq. Na/liter blood. If these compartments are arranged in parallel, the flux between plasma and fast compartment is 1.32 m.eq. Na/liter blood hour, and that between plasma and slow compartment is 0.016 m.eq. Na/liter blood hour. Results of experiments on two patients with congenital hemolytic jaundice suggest that the fraction of slowly exchanging Na may increase with the age of the red cell.     

Effect of metabolic depletion on the furosemide-sensitive Na and K fluxes in human red cells

Summary We report in this paper the effect of metabolic depletion on several modes of furosemide-sensitive (FS) Na and K transport in human red blood cells. The reduction of ATP content below 100 mol/liter cells produced a marked decrease in the maximal activation (V _max) of the outward. FS transport of Na and K into choline medium in the presence of ouabain (0.1 mM) and 1 mM MgCl₂. TheK _0.5 for internal Na to activate the FS Na efflux was not altered by metabolic depletion. However, metabolic depletion markedly decreased the K _i for external K (K _o ) to inhibit the FS Na efflux into choline medium (from 25 to 11 mM). Repletion of ATP content by incubation of cells in a substraterich medium recovered control levels ofV _max of the FS Na and K fluxes and of K _i for external K to inhibit FS Na efflux. TheV _max of FS Na and K influxes was also markedly decreased when the ATP content dropped below 100 mol/liter cells. This was mainly due to a decrease in the inward-coupled transport of K and Na (Na _o -stimulated K influx and the K _o -stimulated Na influx). The FS K _i /K _o exchange pathway of the Na–K cotransport, estimated from the FS K influx from choline-20 mM K _o medium into cells containing 22 mmol Na/liter cells, was also reduced by starvation. Starvation did not inhibit the FS Na _i /Na _o exchange pathway, estimated as FS Na influx from a medium containing 130 mM NaCl into cells containing 22 mmol Na/liter cells. The unidirectional FS²²Na efflux and influx were also measured in control and starved cells containing 22 mmol Na/liter cells, incubated in a Na medium (130 mM) at varying external K (0 to 20 mM). In substrate-fed cells, incubated in the absence of external K, FS Na efflux was larger than Na influx. This FS net Na extrusion (400 to 500 mol/liter cells·hr) decreased when external K was increased, approaching zero around 15 mM K _o . In starved cells the net Na extrusion was markedly decreased and it approached zero at lower K _o than in substrate-fed cells. Our results indicate that the FS Na and K fluxes, and their major component, the gradient driven Na–K–Cl cotransport system, are dependent on the metabolic integrity of the cells.     

Temperature Adaptation of Active Sodium-Potassium Transport and of Passive Permeability in Erythrocytes of Ground Squirrels

Unidirectional active and passive fluxes of ⁴²K and ²⁴Na were measured in red blood cells of ground squirrels (hibernators) and guinea pigs (nonhibernators). As temperature is lowered, "active" (ouabain-sensitive) K influx and Na efflux were more greatly diminished in guinea pig cells than in those of ground squirrels. The fraction of total K influx which is ouabain sensitive in red blood cells of ground squirrels was virtually constant at all temperatures, whereas it decreased abruptly in guinea pig cells as temperature was lowered. All the passive fluxes (i.e., Na influx, K efflux, and ouabain-insensitive K influx and Na efflux) decreased logarithmically with decrease in temperature in both species, but in ground squirrels the temperature dependence (Q₁₀ 2.5–3.0) was greater than in guinea pig (Q₁₀ 1.6–1.9). Thus, red blood cells of ground squirrel are able to resist loss of K and gain of Na at low temperature both because of relatively greater Na-K transport (than in cells of nonhibernators) and because of reduced passive leakage of ions.     

The effects of sickling on ion transport. II. The effect of sickling on sodium and cesium transport

When the red cells from patients with sickle cell anemia (S-S) were kept in the disk shape by incubation in O₂, they maintained cell sodium in the steady state for at least 10 hours. The sodium flux in such cells at 37°C. was 6.0 ± 1.5 m.eq./ (liters RBC) x (hours). When S-S cells were sickled by incubation in N₂, sodium outflux increased two- to threefold, while influx increased four- to fivefold and the cells gained net sodium. A small but undetermined fraction of the sodium in disk and sickle shaped S-S cells exchanges at one or more rates which are substantially slower than those calculated here from the initial rate of transfer of tracer from cells to the medium. The penetration of tracer Cs into normal and both disk and sickled S-S cells was markedly inhibited by increasing the K concentration in the medium, indicating that Cs and K compete for an entrance pathway in all three cell types. The ratio of the inward rate constant for tracer Cs to that for K⁴² in normal and disk-shaped S-S cells increased only slightly when the K concentration in the medium was increased, indicating that almost all the Cs entered such cells in competition with K. Sickling accelerated the entrance of tracer cesium into S-S cells. Furthermore, the rate constant ratio increased with increasing external K concentration in sickled cells, suggesting the simultaneous presence of a non-competitive route for cesium influx in this cell type. The results are interpreted to support the view that sickling (a) accelerates inward transport of K and Cs and outward transport of Na by a non-diffusion, assumed carrier, process and (b) opens pathways for the diffusion of all three ions.     

Action of Cortisol on Sodium Transport in Canine Erythrocytes

Incubation of blood from deoxycorticosterone-treated, adrenalectomized dogs with glucose, ²²NaCl, and cortisol, added in vitro, revealed log dose-related acceleration of sodium influx, of glucose utilization, and of lactate formation by cortisol in concentrations between 150 and 1000 µg/liter. Addition of 2-deoxyglucose, or preincubation of the blood until blood glucose concentration had fallen below 2.0 mg per 100 ml, reduced or abolished the acceleratory action of added cortisol on sodium influx but had no effect on sodium influx in the absence of added cortisol. Cortisol did not change the ATP or ATPase content of erythrocytes, or the metabolism of glucose via the pentose phosphate pathway, or the rate of efflux of ²²Na from the erythrocytes. The acceleratory actions of cortisol on sodium, influx, glucose utilization, and lactate formation were significantly correlated. Cortisol (1000 µg/liter) enhanced sodium influx by approximately 8.7 mmole per liter erythrocytes per hour for each 1 mmole cortisol-induced increment in ATP production. It is concluded that sodium influx in canine erythrocytes comprises a passive component, unchanged by cellular metabolism, and a second component which is accelerated and inhibited in proportion to prevailing plasma concentrations of cortisol and aldosterone, and which (for cortisol) depends upon accelerated ATP production via glycolysis. These steroid actions probably result from effects on enzyme activity rather than on new enzyme induction.     

The non-correlation of bioelectric potentials with ionic gradients

The sartorius muscles of 320 toads have been analyzed for Na⁺ and K⁺. There is a wide variation in the Na⁺ content which when calculated intracellularly varied from 0 m.eq./kg. to 58 m.eq./kg. In particular it was found that the distribution of internal Na⁺ in the intact animal was such that only 17 per cent of the muscles should give from the Nernst equation the observed overshoot of 37 mv. In contrast to this wide variation the K⁺ content is comparatively constant, the range being 71 to 112 m.eq./kg. The mean observed resting potential of 87 mv. agreed well with the potential calculated from the mean intracellular K⁺ by the Nernst equation. Analyses of plasma show that the Na⁺ content is constant at 130 m.eq./liter, and the K⁺ is 3.0 m.eq./liter. The resting and action potentials of 77 muscles have been recorded and then the muscles have been analyzed. The results have shown that there is no correlation between the level of intracellular Na⁺ and the overshoot. Furthermore the apparent correlation between the average K⁺ content and the average resting potential has been shown to be fortuitous, when the correlation in individual muscles is considered. When a muscle is soaked in Ringer solution for several hours there is a gain of Na⁺ and a loss of K⁺. These shifts should result in changes in the respective potentials, but such changes were not found. The above findings have been discussed in the light of the present theories that the resting potential and the action potential are directly related to the ionic ratio across the membrane. Our results very definitely do not support the theory that the overshoot is related to the Na⁺ gradient, and this also applies with respect to the K⁺ gradient and the resting potential.     

Quinine and quinidine inhibit and reveal heterogeneity of K-Cl cotransport in low K sheep erythrocytes

Low K (LK) sheep red blood cells (SRBCs) serve as a model to study K-Cl cotransport which plays an important role in cellular dehydration in human erythrocytes homozygous for hemoglobin S. Cinchona bark derivatives, such as quinine (Q) and quinidine (QD), are effectively used in the treatment of malaria. In the present study, we investigated in LK SRBCs, the effect of various concentrations of Q and QD on Cl-dependent K efflux and Rb influx (K(Rb)-Cl flux), activated by either swelling in hyposmotic media, thiol alkylation with N-ethylmaleimide (NEM), or by cellular Mg (Mg _i ) removal through A23187 in the presence of external chelators. K efflux or Rb influx were determined in Cl and NO₃ medium and K(Rb)-Cl flux was defined as the Cl-dependent (Cl minus NO₃) component. K(Rb)-Cl flux stimulated by all three interventions was inhibited by both Q and QD in a dose-dependent manner. Maximum inhibition of K(Rb)-Cl flux occurred at Q and QD concentrations ?1 mm. The inhibitory effect of Q was manifested in Cl, but not in NO₃, whereas QD reduced K and Rb fluxes both in Cl and NO₃ media. The mean 50% inhibitory concentration (IC⁵⁰) of Q and QD to inhibit K(Rb)-Cl flux varied between 0.23 and 2.24 mm. From determinations of the percentages of inhibition of the different components of K and Rb fluxes, we found that SRBCs possess a Cl-dependent QD-sensitive and a Cl-dependent QD-insensitive K efflux and Rb influx. These two components vary in magnitude depending on the manipulation and directional flux, but in average they are about 50% of the total Cl-dependent flux. This study raises the possibility that, in SRBCs, the Cl-dependent K(Rb) fluxes are heterogeneous. This work was supported by a grant from the National Institutes of Health (NIH DK5RO1 37,160).     

Concentrative accumulation of choline by human erythrocytes

Influx and efflux of choline in human erythrocytes were studied using ¹⁴C-choline. When incubated at 37°C with physiological concentrations of choline erythrocytes concentrate choline; the steady-state ratio is 2.08 ± 0.23 when the external choline is 2.5 µM and falls to 0.94 ± 0.13 as the external concentration is raised to 50 µM. During the steady state the influx of choline is consistent with a carrier system with an apparent Michaelis constant of 30 x 10^-6 and a maximum flux of 1.1 µmoles per liter cells per min. For the influx into cells preequilibrated with a choline-free buffer the apparent Michaelis constant is about 6.5 x 10^-6 M and the maximum flux is 0.22 µmole per liter cells per min. At intracellular concentrations below 50 µmole per liter cells the efflux in the steady state approximates first order kinetics; however, it is not flux through a leak because it is inhibited by hemicholinium. Influx and efflux show a pronounced exchange flux phenomenon. The ability to concentrate choline is lost when external sodium is replaced by lithium or potassium. However, the uphill movement of choline is probably not coupled directly to the Na⁺ electrochemical gradient.     

Effect of staphylococcal alpha-hemolysin upon anion transport in the rabbit erythrocyte

Equilibrium exchange of SO₄^2? was measured prior to and during hemolysis in rabbit erythrocytes exposed to staphylococcal α-hemolysin. The anion-transport protein of the rabbit erythrocyte has also been identified. Equilibrium exchange of SO₄^2? was measured by both efflux and influx of ³⁵SO₄^2?. The rate of influx of SO₄^2? in rabbit erythrocytes exposed to α-hemolysin was twice that of the untreated cells. The rate of SO₄^2? efflux was unchanged by α-hemolysin. Inhibition of anion exchange with 4,4′-diisothiocyano-2,2′-stilbenedisulfonic acid (DIDS) did not inhibit hemolysis, therefore, the increased influx of SO₄^2? may occur through a DIDS-insensitive pathway.     

Erythrocyte lipid composition and sodium transport in human liver disease

In patients with liver disease there are usually increases in erythrocyte cholesterol and phosphatidylcholine concentrations. This increase in membrane lipid changes the shape of the erythrocyte and “spur” or “target” cells may be present. Sodium fluxes were measured in erythrocytes from 17 patients with a variety of liver diseases and from 17 normal subjects and the values related to the lipid content of the membrane. Ouabain-insensitive and ouabain-sensitive effluxes were lower in patients than in normal subjects and the reduction in ouabain-insensitive efflux was more marked. Sodium influx was also significantly lower in erythrocytes from patients than controls. Ouabain-sensitive and ouabain-insensitive effluxes and sodium influx did not correlate with the cholesterol content of erythrocytes from patients. Significant negative correlations were noted between ouabain-insensitive sodium efflux (r = ?0.63, P < 0.01), sodium influx (r = ?0.61, P < 0.01) and intracellular sodium concentration (r = ?0.66, P < 0.01) and the cholesterol : phospholipid molar ratio of the cell but there was no significant correlation between this ratio and the ouabain-sensitive sodium efflux (r = 0.41, P > 0.05). These results support the hypothesis that an altered lipid composition may affect the permeability of the erythrocyte membrane in patients with liver disease.     

Structural requirements for anion substrates of the methotrexate transport system in L1210 cells

A broad spectrum of structurally diverse anions reversibly inhibits the influx of methotrexate in L1210 cells. Several of the more effective anions and their respective inhibition constants (K_i values) were: 5-methyltetrahydrofolate (0.3 μm), bromosulfophthalein (2 μm), thiamine pyrophosphate (3 μm), 8-anilino-1-naphthalene sulfonate (7 μm), phthalate (20 μm), and AMP (50 μm). Moderate inhibition was observed with P_i (K_i = 400 μm) and other divalent inorganic anions, while small monovalent anions such as Cl^? (K_i = 30 mm) were the least effective. When these same anions were tested for an effect on methotrexate efflux, stimulation was observed with some anions, while others had no effect. Enhancement was produced by folate compounds and p-aminobenzoylglutamate, small monovalent (e.g., Cl^?, acetate, and lactate) and divalent (e.g., phosphate and succinate) anions, a few nucleotides (e.g., AMP), and thiamine pyrophosphate, while little or no effect was associated with trivalent anions (e.g., citrate), most nucleotides, and large organic anions (e.g., bromosulfophthalein, NAD, and NADP). Anions with the ability to promote methotrexate efflux in control cells lost this capacity upon exposure of the cells to an irreversible inhibitor of methotrexate influx. These results support the hypothesis that methotrexate transport proceeds via an anion-exchange mechanism and moreover provide evidence that anion substrates for this system can be identified by their ability to promote methotrexate efflux. Anions which appear most likely to participate in this exchange cycle in vivo are P_i and AMP.     

Cation movements in the high sodium erythrocyte of the cat

The uptake of ⁴²K and ²⁴Na by cat erythrocytes was investigated. Under steady-state conditions, the nontransient component of ⁴²K influx was found to be 0.18 ± 0.01 meq/liter RBC/hr and insensitive to ouabain (100 µM); the corresponding value for ²⁴Na was 17 ± meq/liter RBC/hr. A study was made of the effects of anions upon cation movements in these and other mammalian red cells. Iodide was found to inhibit markedly (>50%) Na inward movements in cat and dog but not in the other erythrocytes. An increase (15–30%) in K uptake in the presence of iodide was noted in all the mammalian cells studied.     

Characteristics of the volume- and chloride-dependent K transport in human erythrocytes homozygous for hemoglobin C

Summary In human red cells homozygous for hemoglobin C (CC), cell swelling and acid pH increase K efflux and net K loss in the presence of ouabain (0.1mm) and bumetanide. We report herein, that K influx is also dependent on cell volume in CC cells: cell swelling induces a marked increase in the maximal rate (from 6 to 18 mmol/liter cell × hr) and in the affinity for external K (from 77±16mm to 28±3mm) of K influx. When the external K concentration is varied from 0 to 140mm, K efflux from CC and normal control cells is unaffected. Thus, K/K exchange is not a major component of this K movement. K transport through the pathway of CC cells is dependent on the presence of chloride or bromide; substitution with nitrate, acetate or thiocyanate inhibits the volume- and pH-dependent K efflux. When CC cells are separated according to density, a sizable volume-dependent component of K efflux can be identified in all the fractions and is the most active in the least dense fraction. N-ethylmaleimide (NEM) markedly stimulates K efflux from CC cells in chloride but not in nitrate media, and this effect is present in all the fractions of CC cells separated according to density. The persistence of this transport system in denser CC cells suggests that not only cell age, but also the presence of the positively charged C hemoglobin is an important determinant of the activity of this system. These data also indicate that the K transport pathway of CC cells is not an electrodiffusional process and is coupled to chloride.     

The effects of alterations in the external sodium concentration on human leucocyte sodium and potassium transport in vitro

Human leucocytes incubated in tissue culture fluid of low-sodium concentration (2 mM; iso-osmolarity maintained with choline chloride) reached a new equlibrium within 1 hour and lost approximately 25% of intracellular potassium and 70% of intracellular sodium. The rate constant for ouabainsensitive sodium efflux fell by more than 50% and the ouabain-insensitive rate constant increased nearly threefold in the low-sodium medium. Total sodium efflux fell in proportion to internal sodium whereas ouabain-insensitive sodium efflux remained unchanged. A reduction in external sodium from 140 to 2 mM was associated with a 75% fall in sodium influx. In the low-sodium medium ouabainsensitive potassium influx exceeded ouabain-sensitive sodium efflux and no ouabain-sensitive potassium efflux could be demonstrated. Ouabain-insensitive potassium influx and that portion of potassium efflux which is dependent on external potassium fell in parallel in low-sodium cells, suggesting reduced activity of a ouabain-insensitive K:K exchange system.     

Intracellular Potassium : A Determinant of the Sodium-Potassium Pump Rate

Normal human red cells which have had their intracellular sodium (Na_c) reduced have a diminished Na-K pump rate, but only if intracellular potassium (K_c) is high. If most of the K_c is replaced by tetramethylammonium or choline, both ouabain-sensitive Na efflux and K influx are significantly increased even with Na_c below normal. Cells with reduced Na_c and high K_c have an unchanged Na efflux if external potassium (K_ext) is removed. In contrast, low-Na, low-K cells have a large ouabain-sensitive Na efflux which shows a normal response to removal of K_ext. Neither low-K nor high-K cells have an altered ouabain-sensitive K efflux. Measurement at constant low Na_c and varying K_c shows the pump Na efflux to be an inverse function of K_c. Thus, in low-Na cells, K_c appears to act as an inhibitor of the pump. Inhibition by high K_c can be seen even when Na_c is normal. The effects attributed to K_c are distinguished experimentally from other variables such as cell volume, adenosine triphosphate concentration, effects of the replacement cations, and the method used to alter intracellular cation concentrations. A role is proposed for K_c, in cooperation with Na_c, in regulating the pump rate of normal human red cells.     

Transient Activation of Plasmalemma K Efflux and H Influx in Tobacco by a Pectate Lyase Isozyme from Erwinia chrysanthemi

A purified pectate lyase isozyme derived from Erwinia chrysanthemi induced rapid net K⁺ efflux and H⁺ influx in suspension-cultured tobacco cells. Comparable fluxes of other ions (Na⁺, Cl⁻) were not observed. The K⁺ efflux/H⁺ influx response began within 15 minutes after addition of enzyme to cell suspensions and continued for approximately 1 hour after which cells resumed the net H⁺ efflux exhibited prior to enzyme treatment. The response was not prolonged by a second enzyme dose 1 hour after the first. The K⁺/H⁺ response was characterized by saturation at low enzymic activity (2 × 10⁻³ units per milliliter), and inhibition by the protonophore, carbonyl cyanide m-chlorophenylhydrazone, and was not associated with membrane leakiness caused by structural cell wall damage. The total K⁺ loss and H⁺ uptake induced by enzyme was one-fourth to one-third that induced by Pseudomonas syringae pv. pisi and did not reduce cell viability. These results indicate that pectate lyase induces a K⁺ efflux/H⁺ influx response in tobacco similar to but of shorter duration than that induced by P. syringae pv. pisi during the hypersensitive response. Pectate lyase or other cell wall degrading enzymes may therefore influence the induction of hypersensitivity.     

Chloride transport and compartmentation within main and lateral roots of two grapevine rootstocks differing in salt tolerance

Root Cl^? transport was investigated using ³⁶Cl^? flux analysis in two grapevine (Vitis sp.) rootstock hybrids differing in salt tolerance; 1103 Paulsen (salt-tolerant) and K 51–40 (salt sensitive). Initial ³⁶Cl^? influx to the root was greater in Paulsen than K 51–40. This flux, attributed to the Cl^? influx to the cytoplasm (Φ _oc) increased with increasing external concentrations of Cl^? for plants adapted to growth in 30 mM NaCl. The concentration kinetics in this high concentration range could be fit to a Michaeils–Menton equation. There was no significant difference between genotypes in Km (28.68 ± 15.76 and 24.27 ± 18.51 mM for Paulsen and K 51–40, respectively), but Paulsen had greater V _max (0.127 ± 0.042) compared to K 51–40 (0.059 ± 0.026 μm g^?1 FW min^?1). In Paulsen, the main root had greater contribution to ³⁶Cl^? uptake than lateral roots, there being no significant difference in lateral root influx between the genotypes. ³⁶Cl^? transport to the shoot of K 51–40 was greater than for Paulsen. It was estimated that efflux rate from the xylem parenchyma cells to the xylem vessels (Φ _cx) in K 51–40 was twice that of Paulsen. Compartmental analysis from ³⁶Cl^? efflux kinetics confirmed the larger Φ _oc and the higher ratio of main to lateral root Φ _oc for Paulsen. Efflux from the cytoplasm (Φ _co) was higher than 95 % of Φ _oc indicating a high degree of cycling across the plasma membrane in roots at these high external Cl^? concentrations. Paulsen appears to keep the cytoplasmic Cl^? concentration in roots lower than K 51–40 via greater efflux to the vacuole and to the outside medium. The difference in salt tolerance between the genotypes can be attributed to different Cl^? transport properties at the plasma membrane and tonoplast and particularly in Cl^? efflux to the xylem.