Chloride Self-exchange Research Articles

Chloride and bicarbonate transport in chick embryonic red blood cells.

1. Unidirectional efflux of 36Cl- and H14CO3- from erythrocytes of 4- to 16-day-old chick embryos was measured under steady-state conditions at 37 degrees C and pH 7.7. The efflux rates were high, > 3 s-1, and were, therefore, measured by means of the continuous flow tube method. 2. At day 4 of development the range of permeability coefficients for bicarbonate and chloride (PHCO3 and PCl was 1-30 x 10(-4) cm s-1, with average values of respectively 10 x 10(-4) and 8 x 10(-4) cm s-1. However, the results can be divided into two groups, one with PHCO3 and PCl above 12 x 10(-4) cm s-1, and one with values below 5 x 10(-4) cm s-1. The same range of values was also obtained for day 6 erythrocytes, but the overlap is more conspicuous. At day 16, PHCO3 and PCl were respectively 9 x 10(-4) and 6 x 10(-4) cm s-1 (37 degrees C, pH 7.7). In adult chicken red blood cells PHCO3 and PCl were respectively 7 x 10(-4) and 4 x 10(-4) cm s-1, and in human red blood cells the respective values were 5.6 x 10(-4) and 4 x 10(-4) cm s-1. 3. Chloride self-exchange, measured at 0 degrees C, was almost completely inhibited by addition of 1 mM 4,4'-diisothiocyanostilbene-2,2'-disulphonate (DIDS) at both days 6 and 16 of embryonic development, supporting the finding that the embryonic chick erythrocytes also have a transmembrane anion exchanger similar to that of other red cells.(ABSTRACT TRUNCATED AT 250 WORDS)

The human erythrocyte anion transport protein, band 3. Characterization of exofacial alkaline titratable groups involved in anion binding/translocation.

Chloride self-exchange across the human erythrocyte membrane at alkaline extracellular pH (pHO) and constant neutral intracellular pH (pH(i)) can be described by an exofacial deprotonatable reciprocating anion binding site model. The conversion of the transport system from the neutral to the alkaline state is related to deprotonation of a positively charged ionic strength- and substrate-sensitive group. In the absence of substrate ions ([ClO] = 0) the group has a pK of approximately 9.4 at constant high ionic strength (equivalent to approximately 150 mM KCl) and a pK of approximately 8.7 at approximately zero ionic strength. The alkaline ping-pong system (examined at constant high ionic strength) demonstrates outward recruitment of the binding sites with an asymmetry factor of approximately 0.2, as compared with the inward recruitment of the transport system at neutral pHO with an asymmetry factor of approximately 10. The intrinsic half-saturation constant for chloride binding, with [Cli] = [Clo], increased from approximately 30 mM at neutral to approximately 110 mM at alkaline pHO. The maximal transport rate was a factor of approximately 1.7 higher at alkaline pHO. This increase explains the stimulation of anion transport, the "modifier hump," observed at alkaline pHO. The translocation of anions at alkaline pHO was inhibited by deprotonation of another substrate-sensitive group with an intrinsic pK of approximately 11.3. This group together with the group with a pK of approximately 9.4 appear to form the essential part of the exofacial anion binding site. The effect of extracellular iodide inhibition on chloride transport as a function of pHO could, moreover, be simulated if three extracellular iodide binding constants were included in the model: namely, a competitive intrinsic iodide binding constant of approximately 1 mM in the neutral state, a self-inhibitor binding constant of approximately 120 mM in the neutral state, and a competitive intrinsic binding constant of approximately 38 mM in the alkaline state.

Inhibition of chloride self-exchange with stilbene disulphonates in depolarized skeletal muscle of Rana temporaria.

1. The inhibition of 36Cl efflux with stilbene disulphonates, SD, has been studied under conditions of chloride equilibrium in depolarized fibre bundles from frog semitendinosi. The chosen probes were the aminoreactive derivative SITS and the derivative DNDS with no aminoreactive group. SD were added to the medium during 36Cl efflux allowing the estimation of fractional inhibition after a single 36Cl loading. 2. Both probes inhibited chloride self-exchange reversibly within the pH range 5.5-9.5 under study. 3. At SD concentrations above the half-inhibition concentration the inhibition reached a steady level with a time lag equal to that required for extracellular fluid change. The time constant for reversibility upon the removal of SD increased with decreasing pH, but rapid reversibility always appeared upon an increase of pH to 7.2. These findings suggest that SD may enter the membrane at low pH, but that the inhibitory action is confined to superficial membrane sites. 4. The inhibitory power of both probes showed a pronounced pH dependence, pK approximately 7. The half-inhibition concentration increased about 6-7 times when pH was lowered one unit from the pK value. 5. The apparent affinity of SITS to the transport system was about 5 times higher than that of DNDS. The apparent dissociation constants at neutral pH were 8.5 x 10(-5) M (SITS) and 4.5 x 10(-4) M (DNDS). Both probes showed a maximal inhibition close to 100% at neutral pH and approximately 85% at pH 5.5. 6. The inhibition depended on the chloride concentration in a way consistent with competitive inhibition in both neutral and acid media. 7. The results are consistent with the classical model of anion transport in frog muscle, suggesting that SD and chloride may compete for binding to a site with increasing anion affinity upon protonation; the results do not, however, exclude that the conductive and the non-conductive chloride transport modes in frog muscle are mediated by separate SD-sensitive transport pathways.

Selective inhibition of sodium-linked and sodium-independent bicarbonate/chloride antiport in Vero cells.

The effects of compounds previously described to inhibit anion transport were tested for their ability to inhibit anion antiport in Vero cells as measured by uptake of 36Cl- by chloride self-exchange and as bicarbonate-linked uptake of 22Na+. While 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid inhibited both processes, ethacrynic acid and probenecid selectively inhibited the uptake of 36Cl-. Low concentrations of pyridoxal phosphate and picrylsulfonic acid selectively inhibited the bicarbonate linked uptake of 22Na+, while higher concentrations of these compounds also inhibited the uptake of 36Cl-. Measurements of the internal pH indicated that ethacrynic acid inhibits Na+-independent HCO-3/Cl- exchange, while it has no measurable effect on Na+-linked bicarbonate-dependent regulation of the internal pH. Conversely, picrylsulfonic acid selectively inhibits the latter process. The results indicate that anion antiport in Vero cells occurs by two independent processes.

Influence of chloride concentration and pH on the 36Cl efflux from depolarized skeletal muscle of Rana temporaria.

1. 36Cl- efflux rates were measured in depolarized fibre bundles (predominantly fast fibres) from m. semitendinosus of Rana temporaria in order to investigate the influence of chloride concentration (20-400 mM) and external pH (pHo, 5.5-11.6) on chloride self-exchange. Usually, the bundles were depolarized to a membrane potential of virtually zero ([Cl-]i = [Cl-]o = [Cl-]) in 140 mM-K+ and 20 mM-Cl- combined with either 60 mM-SO4(2-) (+52 mM-sucrose) or 120 mM-methyl sulphate, gluconate or glucuronate. Various values of [Cl-] were obtained by adding KCl to the medium, keeping constant the concentration of relatively impermeant solutes and thus the fibre volume. 2. The chloride permeability, PCl, at pH 7.5 decreased with increasing [Cl-] above 50 mM. At [Cl-] less than 50 mM, PCl was about 4 X 10(-6) cm s-1. 3. PCl at pHo 7.5 and 5.6 was a hyperbolic function of [Cl-1] (greater than or equal to 50 mM), consistent with saturation kinetics of chloride self-exchange. 4. Methyl sulphate inhibited chloride self-exchange and may enter depolarized fibre membranes slowly. Comparison of PCl estimates in SO4(2-), gluconate, and glucuronate suggested that SO4(2-) and gluconate may interfere with chloride self-exchange at low [Cl-], and that glucuronate interferes the least. 5. A change from chloride equilibrium to an inward KCl gradient increased the 36Cl- efflux rate in acid media (pH less than 6.5) and decreased the efflux rate at pHo greater than or equal to 7. The effects were not due to changes of membrane potential and demonstrate a change of transport mode consistent with single-filing at high pH and dominant exchange diffusion at low pH. This result makes interpretation of the saturation kinetics difficult. 6. At constant [Cl-], PCl showed a bell-shaped dependence on pHo with a maximum around pH 8.5. The decrease of chloride permeability as the pH is decreased below 8.5 showed an apparent pK of about 7. PCl fell at very alkaline pH with an apparent pK of about 10. 7. The results are in agreement with previous observations of an external pH-sensitive control of PCl (pK approximately 7) and suggest that, in conditions close to chloride equilibrium, this control includes a transition between a dominant conductive channel conformation at high pH and a dominant non-conductive conformation at low pH.(ABSTRACT TRUNCATED AT 400 WORDS)

Concentration dependence of the chloride selfexchange and homoexchange fluxes in human red cell ghosts.

The concentration dependence of the unidirectional chloride flux in human red cell ghosts was studied under selfexchange and under homoexchange conditions. Under selfexchange conditions the intracellular concentration of chloride [Cl]in is equal to the extracellular concentration [Cl]ex and [Cl]in and [Cl]ex are raised concomitantly. Under homoexchange conditions [Cl]in or [Cl]ex were varied separately at a fixed trans-concentration of chloride. The chloride fluxes were calculated from the rate of the tracer efflux and the intracellular chloride. All experiments were executed in isotonic (330 mosM) KCl/K-citrate/sorbitol solutions containing 0-100 mM KCl, 40 mM K-citrate and different concentrations of sorbitol for isoosmotic substitution. The chloride selfexchange and the chloride homoexchange fluxes exhibit a pure saturation kinetics. The halfsaturation constant for the chloride selfexchange was approximately 20 mM, the maximal flux was approx. 3.5 X 10(-4) mol/(min . g cells). The apparent chloride halfsaturation constants from the homoexchange experiments were in the range of 0.9-4.5 mM for the outer and of 5.5-14.5 mM (0 degree C, pH 7.3) for the inner membrane surface, both halfsaturation constants increase with increasing trans-concentrations. At infinite trans-concentrations of chloride, the halfsaturation constant for the outer and the inner membrane surface amounts to approximately 5 mM and approximately 15 mM, respectively. The slope of the double reciprocal plots of flux versus cis-chloride concentration decreases with increasing trans-concentration of chloride. The kinetics of the chloride transport provides evidence for a carrier mediated transport mechanism with a single reciprocating transport site. The translocation of the loaded carrier appears to be much faster than the translocation of the unloaded carrier.(ABSTRACT TRUNCATED AT 250 WORDS)

Chloride self-exchange in toad skeletal muscle in vivo and in vitro.

The exchange of cellular Cl with 36Cl has been measured in saline-perfused hindlimb muscles of the pithed toad and compared with cellular Cl exchange in isolated muscles incubated in the vitro either in toad Ringer solution or in toad plasma. In the perfused hindlimb, the rate of 36Cl efflux from muscle cells [17.0 +/- 0.9 pmol Cl.(cm2 plasma membrane.s)-1] was only 40% as fast as that of the 36Cl influx. The discrepancy between Cl influx and efflux was accompanied by a cellular accumulation of Cl against the electrochemical gradient for this anion. Concurrently, the cells took up Na in amounts at least equal to the accumulated Cl. During this accumulation of Na and Cl, the mean resting potential remained constant at a value of -89.2 +/- 1.9 (SE) mV. Na and Cl were taken up by the muscle cells of the perfused hindlimb without a concomitant decrease in cellular K content; i.e., without evidence of inhibition of the Na-K pump. The rate constant for cellular 36Cl efflux from isolated toad muscles preincubated for 3 h in vitro in toad Ringer solution was about five times faster than that of muscles in the perfused hindlimb and similar in magnitude to published values for Cl fluxes in frog muscle. Cellular Cl efflux from muscles briefly preincubated in vitro for 15 min instead of 3 h was significantly slower than after prolonged preincubation. In vitro incubation of isolated toad muscles in toad plasma slowed the cellular 36Cl efflux to values approaching those measured in the perfused hindlimb, without comparably depressing the 36Cl influx. It is suggested that the uptake of NaCl by the cells of perfused hindlimb muscle may proceed by an electroneutral inward cotransport of Na and Cl on the same carrier.

Bicarbonate exchange through the human red cell membrane determined with [14C] bicarbonate.

1. Bicarbonate transport across human red cell membranes was studied between 0 and 10 degrees C at alkaline pH values by determining the efflux of 14C-labelled bicarbonate from resealed erythrocyte ghosts. Transfer of labelled CO2 was eliminated as a source of error, when formation of intracellular 14CO2 was inhibited with carbonic anhydrase inhibitors. The study showed that there are no fundamental differences between the characteristics of bicarbonate and of chloride self-exchange as has been inferred from previous studies of chloride-bicarbonate exchange. 2. Efflux of radioactivity could be reduced more than 99% by reversible and irreversible inhibitors of anion transport. Inhibition of both chloride and bicarbonate self-exchange was linearly related to the binding of 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS) to the membranes. Complete (i.e. greater than 99%) inhibition was obtained after binding of 1.2 x 10(6) DIDS molecules per cell. 3. Bicarbonate self-exchange proved a saturable function of bicarbonate concentration, with a maximum at external and internal concentrations of approximately 100 mM, showing self-depression at higher bicarbonate concentrations, and half-maximum exchange flux at a concentration of 10 mM. The results were consistent with the hypothesis that the exchange mechanism has two anion binding sites, one mediating ion transport and the other causing transport inhibition. 4. Maximum exchange flux of bicarbonate was about 30% larger thant that of chloride, and the affinity of bicarbonate for the transport site was about three times larger than that of chloride. The apparent activation energy of bicarbonate exchange was 28 kcal/mole, the same order of magnitude as found for other inorganic anions between 0 and 10 degrees C. 5. The ability of other inorganic anions to exchange with bicarbonate decreased in the sequence Cl greater than NO3 greater than F greater than Br greater than or equal to I, corresponding to the sequence of the rate of self-exchange of halides. 6. Counter-transport of bicarbonate could be driven by a chloride gradient, when ghosts containing KCl were suspended in a medium containing traces of labelled bicarbonate in addition to a non-permeating anion. Concentration ratios (ci/co) up to about 1000 could be obtained. 7. It is concluded that bicarbonate is transported by the inorganic anion exchange mechanism of the erythrocyte membrane. The slight differences between the exchange kinetics of chloride and bicarbonate were explained by differing affinities of the two anions for the two anion binding sites of the transport system.

Temperature-dependent changes of chloride transport kinetics in human red cells.

Chloride self-exchange in human red cells was studied between 0 degrees C and 38 degrees C. At higher temperatures the flow-tube method was used. Although the general features of chloride transport at 0 degrees C and 38 degrees C are similar, the following differences were found: (a) the maximum pH of chloride self-exchange flux was lowered 0.6 pH unit from 7.8 to 7.2 when temperature was increased from 0 degrees C to 38 degrees C; (b)the apparent half-saturation constant increased from 28 mM at 0 degrees C to 65 mM at 38 degrees C; (c) chloride transport at body temperature is slower than predicted by other investigators by extrapolation from low-temperature results. Chloride transport increased only 200 times when temperature was raised from 0 degrees C to 38 degrees C, because the apparent activation energy decreased from 30 kcal mol(-1) to 20 kcal mol(-1) above a temperature of 15 degrees C; (d) a study of temperature dependence of the slower bromide self-exchange showed that a similar change of activation energy occurred around 25 degrees C. Both in the case of Cl(-) (15 degrees C) and in the case of Br(-) (25 degrees C), critical temperature was reached when the anion self-exchange had a turnover number of about 4x10(9) ions cell (-1)s(-1); (e) inhibition of chloride transport by DIDS (4,4'- diisothiocyano-stilbene-2,2'-disulfonate)revealed that the deflection persisted at 15 degrees C at partial inhibition (66 percent) presumably because DIDS inactivated 66 percent of the transport sites. It is suggested that a less temperature- dependent step of anion exchange becomes rate limiting at the temperature where a critical turnover number is reached.

Anion transport and membrane morphology.

Freeze-fracture electronmicroscopy has been used to examine the membrane ultrastructure of human red blood cells in the presence of inhibitors of chloride exchange. The extent of inhibition was correlated with a decrease of intramembrane particle density on the B-fracture face. Dimethylsulfoxide (DMSO) and glycerol, which markedly and reversibly reduced the intramembrane particle density, were shown to drastically and reversibly inhibit chloride self-exchange. DMSO was shown to be a noncompetitive inhibitor of chloride flux.

Furosemide inhibition of chloride transport in human red blood cells.

The chloride self-exchange flux across the human red cell membrane is rapidly and reversibly inhibited by 10(-4) M furosemide, a potent chloruretic agent. Furosemide reduces the chloride flux at all chloride concentrations and increases the cellular chloride concentration at which the flux is half-maximum. Kinetic analysis of the flux measurements made at several furosemide and chloride concentrations yields a pattern of mixed inhibition with a dissociation constant for the inhibitor-transport mechanism complex of 5 X 10(-5) M. From this pattern of inhibition and other observations, including that the percent inhibition is independent of pH (range 5.6-8.9), we conclude that the anionic form of furosemide interacts primarily with the chloride transport mechanism at a site separate from both the transport site and the halide-reactive modifier site.

The role of carbonic anhydrase inhibitors on anion permeability into ox red blood cells.

1. Organic anion permeability in ox red blood cell was measured by studying steady-state self-exchange of oxalate, chosen as a prototypical substrate of the organic anion transport system previously described; chloride self-exchange measured the inorganic anion permeability. 2. Carbonic anhydrase inhibitors of the sulphonamide class inhibit both organic anion self-exchange (A-/A-) and chloride self-exchang (CL-/CL-) although carbonic anhydrase plays no role in these exchanges. These results confirm the conclusions already published that sulphonamides can act directly on the cellular membrane as specific inhibitors of anion transport. 3. There is a correlation between the chemical structure of the sulphonamides and their capacity for inhibiting transmembrane anionic exchange. It is of significance that N-sulphamyl substitution, which abolishes the carbonic anhydrase inhibitory potency, does not destroy anionic inhibitory capacity and may even increase it. 4. For each sulphonamide the capacities for inhibiting chloride transport and oxalate transport are strictly identical. Inhibition appears non-competitive. 5. The temperature sensitivity of oxalate self-exchange is exactly the same as that of chloride self-exchange. From this, and from the nature of their inhibition by sulphonamides, it is proposed that chloride and organic anions share the same transport mechanism. 6. In the light of the present results the chloruretic action of sulphonamides in various tissues, in particular the kidney, is discussed.

Effects of halides and bicarbonate on chloride transport in human red blood cells.

Chloride self-exchange was determined by measuring the rate of 36Cl efflux from human red blood cells at pH 7.2 (0 degrees C) in the presence of fluoride, bromide, iodide, and bicarbonate. The chloride concentration was varied between 10--400 mM and the concentration of other halides and bicarbonate between 10--300 mM. Chloride equilibrium flux showed saturation kinetics. The half-saturation constant increased and the maximum flux decreased in the presence of halides and bicarbonate: the inhibition kinetics were both competitive and noncompetitive. The competitive and the noncompetitive effects increased proportionately in the sequence: fluoride less than bromide less than iodide. The inhibitory action of bicarbonate was predominantly competitive. The noncompetitive effect of chloride (chloride self-inhibition) on chloride transport was less dominant at high inhibitor concentrations. Similarly, the noncompetitive action of the inhibitors was less dominant at high chloride concentrations. The results can be described by a carrier model with two anion binding sites: a transport site, and a second site which modifies the maximum transport rate. Binding to both types of sites increases proportionately in the sequence: fluoride less than chloride less than bromide less than iodide.

Chloride transport in human red cells.

1. The chloride equilibrium flux (chloride self-exchange) was determined by measuring the rate of 36Cl efflux from radioactively labelled human red cells. The cellular chloride concentration was varied between 5 and 700 mM by the nystatin technique (Cass & Dalmark, 1973). The chloride transport capacity was not affected by the nystatin technique. 2. The chloride equilibrium flux showed saturation kinetics in the pH range between 6-2 and 9-2 (0 degrees C). The chloride transport decreased at chloride concentrations higher than those which gave the maximum transport. 3. The apparent half-saturation constant, (K1/2), depended on the pH and whether the chloride transport was perceived as a function of the chloride concentration in the medium or in the cell water. The (K1/2)m increased and the (K1/2)c decreased with increasing pH. The dependence of the chloride transport on the chloride concentration was described by Michaelis-Menten kinetics at pH 7-2, but at values of pH outside pH 7-8 S-shaped or steeper graphs were observed. 4. The chloride equilibrium flux varied with the pH at constant chloride concentration in the medium (pH 5-7-9-5). The transport had a bell-shaped pH dependence at chloride concentrations below 200 mM. At chloride concentrations between 300 and 600 mM the chloride transport increased with increasing pH to reach a plateau around pH 8. The position of the acidic branches of the pH graphs was independent of the chloride concentration (25-600 mM), but the position of the alkaline branches moved towards higher values of pH with increasing chloride concentration (5-150 mM). Thus, the position of the pH optimum increased with increasing chloride concentration. The chloride transport at low pH values was a function of the inverse second power of the hydrogen ion concentration. The pK of the groups which caused the inhibition was approximately 6 and independent of the temperature (0-18 degrees C). 5. The chloride equilibrium flux as a function of chloride concentration, pH, and temperature could be described by a transport model with a mobile, positively charged, chloride binding carrier with a single chloride dissociation constant of 33 mM, a transport capacity of 900 m-mole/3 x 10(13) cells.min (pH 7-2, 0 degrees C), and an Arrhenius activation energy of 30 kcal/mole. The pH dependence of the transport of inorganic monovalent and divalent anions is discussed in relation to the suggested model.

Some effects of low pH on chloride exchange in human red blood cells.

In order to test the range of pH values over which the titratable carried model for inorganic anion exchange is valid, chloride self-exchange across human red blood cells was examined between pH 4.75 and 5.7 at 0 decrees c. It was found that chloride self-exchange flux had a minimum near pH 5 and increased again with further increase in hydrogen ion activity. The Arrhenius activation energy for chloride exchange was greatly reduced at low pH values. The chloride flux at pH 5.1 did not show the saturation kinetics reported at higher pH values but was proportional to the value of the chloride concentration squared. In addition, the extent of inhibition of chloride self-exchange flux by phloretin was reduced at low pH. Our interpretation of these findings is that the carrier-mediated flux becomes a progressively smaller fraction of the total flux at lower pH values and that a different transport mode requiring two chloride ions to form the permeant species and having a low specificity and temperature dependence becomes significant below pH5. A possible mechanism for this transport is that chloride crosses red cell membranes as dimers of HCl at these very low pH values.

Characteristics of chloride transport in human red blood cells.

The efflux of chloride-36 from human erythrocytes under steady-state conditions is a saturable process that is competitively inhibited by bicarbonate and noncompetitively inhibited by acetate. This chloride self-exchange flux is reversibly dependent on the pH of the medium between 5.7 and 9.6 with a maximum flux at pH 7.8. The increase in chloride flux between pH 5.7 and 7.8 is inexplicable by the fixed charge hypothesis. The interpretations are made that chloride transport in human erythrocytes is carrier mediated, that bicarbonate utilizes the same transport mechanism, and that the mechanism can be titrated with hydrogen ions into less functional forms for chloride transport.

Temperature dependence of chloride, bromide, iodide, thiocyanate and salicylate transport in human red cells.

1. The temperature dependence of the steady-state self-exchange of chloride between human red cells and a plasma-like electrolyte medium has been studied by measuring the rate of (36)Cl(-) efflux from radioactively labelled cells. Between 0 and 10 degrees C the rate increased by a factor of eight corresponding to an Arrhenius activation energy of 33 kcal/mole.2. The rate of chloride exchange decreased significantly in experiments where 95% of the chloride ions in cells and medium were replaced by other monovalent anions of a lyotropic series. The rate of chloride self-exchange was increasingly reduced by bromide, bicarbonate, nitrate, iodide, thiocyanate, and salicylate. The latter aromatic anion was by far the most potent inhibitor, reducing the rate of chloride self-exchange to 0.2% of the value found in a chloride medium.3. The temperature sensitivity of the chloride self-exchange was not affected significantly by the anionic inhibitors. The Arrhenius activation energies of chloride exchange were between 30 and 40 kcal/mole in the presence of the six inhibitory anions mentioned above.4. The rate of self-exchange of bromide, thiocyanate, and iodide between human red cells and media was determined after washing and labelling cells in media containing 120 mM bromide, thiocyanate, or iodide respectively. The rate of self-exchange of the three anions were 12, 3, and 0.4% of the rate of chloride self-exchange found in the chloride medium.5. The Arrhenius activation energies of the self-exchange of bromide, iodide, and thiocyanate were all between 29 and 37 kcal/mole, the same magnitude as found for the self-exchange of chloride.6. Although approximately 40% of the intracellular iodide and salicylate ions appeared to be adsorbed to intracellular proteins, the rate of tracer anion efflux followed first order kinetics until at least 98% of the intracellular anions had been exchanged.7. The self-exchange of salicylate across the human red cell membrane occurred by a different mechanism than the one utilized by the inorganic monovalent anions. The activation energy of salicylate exchange (13.2 kcal/mole) was significantly lower than that of inorganic anion exchange. Salicylate exchange increased with decreasing pH in contrast to the exchange of chloride, which decreases when pH is lowered.