Cell volume regulation in hemoglobin CC and AA erythrocytes

Regional Prevalence of Hemoglobin C Across Saudi Arabia: An Epidemiological Survey

INTRODUCTION

HbC is a common structural hemoglobinopathy especially in West Africa. Prevalence and regional distribution of HbC in Saudi Arabia are widely undocumented. Patients with homozygous HbC disease may have mild hemolytic anemia whereas combination with hemoglobin S (HbS) leads to a clinically severe phenotype.

AIM

The current epidemiological study, considered the largest from Saudi Arabia, aimed to evaluate the regional prevalence of the HbC variant among the couples participating in the premarital screening program from 2011 to 2018.

METHODS

Data from the PMSGC program were obtained for premarital screening and genetic counseling. The collected data were then entered into the SEHA platform, a centralized electronic repository for the 13 designated regions in Saudi Arabia. Hemoglobin electrophoresis samples are analyzed using either HPLC, capillary electrophoresis, or a combination of both methods to confirm the presence of abnormal hemoglobin bands.

RESULTS

This study included 1,871,184 individuals from 2011 to 2018. Of those, 49.8% were males and 50.2% were females. 112,618 (6.0%) had an abnormal test. Total number of Hb C cases were 778 (0.04%). HbC trait (HbAC) was detected in 764 participants while homozygous HbC (HbCC) and combined heterozygous (HbSC) were found in 9 and 5 cases, respectively. The regions near the Red Sea have higher rates than the central and eastern regions.

CONCLUSION

HbC is a rare variant in Saudi Arabia with varying regional frequencies. HbC variant is more common in Mecca and Madina regions. The geographic area of HbC distribution differs from the areas with high prevalence of HbS, which explains why HbSC disease cases are overwhelmingly rare.

Water Homeostasis and Cell Volume Maintenance and Regulation

From early unicellular organisms that formed in salty water environments to complex organisms that live on land away from water, cells have had to protect a homeostatic internal environment favorable to the biochemical reactions necessary for life. In this chapter, we will outline what steps were necessary to conserve the water within our cells and how mechanisms have evolved to maintain and regulate our cellular and organismal volume. We will first examine whole body water homeostasis and the relationship between kidney function, regulation of blood pressure, and blood filtration in the process of producing urine. We will then discuss how the composition of the lipid-rich bilayer affects its permeability to water and salts, and how the cell uses this differential to drive physiological and biochemical cellular functions. The capacity to maintain cell volume is vital to epithelial transport, neurotransmission, cell cycle, apoptosis, and cell migration. Finally, we will wrap up the chapter by discussing in some detail specific channels, cotransporters, and exchangers that have evolved to facilitate the movement of cations and anions otherwise unable to cross the lipid-rich bilayer and that are involved in maintaining or regulating cell volume.

Non-opsonising aggregates of IgG and complement in haemoglobin C erythrocytes

Haemoglobin C (HbC) differs from normal HbA by a lysine for glutamate substitution at position 6 of beta-globin. Heterozygous AC and homozygous CC phenotypes are associated with shortened erythrocyte life spans and mild anaemia. AC and CC erythrocytes contain elevated amounts of membrane-associated haemichromes, band 3 clusters, and immunoglobulin G (IgG) in vivo. These findings led us to investigate whether AC and CC erythrocytes might expose elevated levels of IgG and complement, two opsonins that have been implicated in the phagocytic clearance of senescent and sickle erythrocytes. Surprisingly, we found IgG, complement, and other plasma proteins co-localised in aggregates beneath the membrane of circulating AC and CC erythrocytes. These observations, and our finding of similar aggregates in erythrocytes heterozygous or homozygous for haemoglobin S (sickle-cell haemoglobin), suggest that the vast majority of membrane-associated IgG and complement detected in these abnormal erythrocytes is intracellular and does not contribute to the eventual opsonic clearance of these cells. Phagocytosis studies with macrophages provide evidence in support of this suggestion. Studies of erythrocyte clearance that involve the detection of membrane-associated IgG and complement as putative opsonins should investigate the possibility that these plasma proteins reside in the erythrocyte interior, and not on the cell surface.

Red cell osmotic fragility studies in hemoglobin C-β thalassemia: osmotically resistant microspherocytes

Typically certain features of red cell morphology predict the results of osmotic fragility testing. Microspherocytes generally have increased and target cells decreased fragility. Blood smears in homozygous hemoglobin C disease show an interesting admixture of microspherocytes and target cells. Yet osmotic fragility studies generally show only reduced fragility and no population of fragile cells to correspond with the spherocytes. The present study demonstrates that the red cells of patients with hemoglobin C-beta thalassemia share many characteristics with hemoglobin C red cells, including the decreased osmotic fragility of all cells despite the presence of both spherocytes and target cells. These paradoxically osmotically resistant spherocytes probably arise because of cellular dehydration due to a K-Cl transport system which may be activated by binding of hemoglobin C to the red cell membrane.

Regulatory volume decrease in a renal distal tubular cell line (A6). I. Role of K+ and Cl-

Changes in volume of A6 epithelial cells were monitored by recording cell thickness (Tc). The response of Tc to a reduction of the basolateral osmolality from 260 to 140 mosmol/kg was recorded while transepithelial Na+ transport was inhibited by 20 microM amiloride. With Cl--containing bathing media, this osmotic challenge elicited a rapid rise in Tc followed by a regulatory volume decrease (RVD). Substitution of SO4(2-) or gluconate for Cl- markedly reduced the RVD, whereas cells completely maintained their ability to regulate their volume after replacing Cl- by NO3(-). A conductive pathway for Cl- excretion is suggested, which is insensitive to NPPB [5-nitro-2-(3-phenylpropylamino)benzoic acid], an inhibitor of some types of Cl- channels. Ba2+ (5 or 20 mM) reduced the RVD. A more pronounced inhibition of the RVD was obtained with 500 microM quinine, a potent blocker of volume-activated K+ channels. K+-induced depolarization of the basolateral membranes of tissues incubated with SO4(2-)-containing solutions completely abolished the RVD. Noise analysis in the presence of Ba2+ showed the activation of an apical K+ conductive pathway. These results demonstrate that cell volume regulation is controlled by processes involving Cl- and K+ excretion through conductive pathways.

Potassium efflux from infant and adult rat choroid plexuses: effects of CSF anion substitution, N-ethylmaleimide and Cl transport inhibitors

To analyze interdependent transport of K and Cl, we investigated Rb (K) efflux from in vitro choroid plexus (CP) in isotonic artificial CSF (aCSF) medium containing anions or agents that alter KCl transport. Lateral ventricle CP was loaded with 86Rb for release to enable calculation of the efflux rate coefficient, k. With Cl as the main anion in control aCSF, the k value for 86Rb (K) in CP of 1 week infant rats (0.177 min-1) was 19% lower than in adults (0.218 min-1) (P < 0.005). Replacing CSF Cl with NO3 or SCN, respectively, reduced k for K in infant CP by 73% and 43%; similar anion selectivity was observed in adult tissues (P < 0.05). N-Ethylmaleimide (NEM), which stimulates KCI cotransport, significantly enhanced K efflux in infants and adults. In adult CP, the KCl cotransport inhibitor, furosemide (1 mM), decreased K efflux by 23% or 65%, respectively, when aCSF had Cl or NO3 as the main anion. In infant rat CP, 0.1 mM bumetanide (another KCl cotransport inhibitor), reduced k for K by 65%, whereas the Cl channel blocker diphenylamine carboxylate (1 mM) did not significantly alter K efflux. The collective findings for rat CP indicate a substantial component of K efflux that is associated with Cl concentration and the Cl transport protein sensitive to loop diuretics and NEM. The Cl-dependent K efflux is present in infants.

Swelling-stimulated passive potassium transport in camel erythrocytes: inhibitory effects of furosemide and sodium fluoride

The inhibitory effects of furosemide, sodium fluoride, and age on volume-dependent, ouabain-resistant K+ influx were investigated in camel red blood cells. Swelling of young camel erythrocytes hypotonically stimulates ouabain-resistant potassium influx, a response that was lacking in old camel erythrocytes. The swelling-stimulated influx was partially inhibited by 1 mM furosemide and by 10 and 20 mM sodium fluoride. The inhibitory effect of furosemide was significantly increased if rubidium was added to the flux media. There was a significant correlation between potassium influx in normo- and hypotonic media which might indicate that the anion-dependent transport system operates, to some extent, to regulate cell volume.

Swelling activation of K-Cl cotransport in LK sheep erythrocytes: a three-state process

K-Cl cotransport in LK sheep erythrocytes is activated by osmotic swelling and inhibited by shrinkage. The mechanism by which changes in cell volume are transduced into changes in transport was investigated by measuring time courses of changes in transport after osmotic challenges in cells with normal and reduced Mg concentrations. When cells of normal volume and normal Mg are swollen, there is a delay of 10 min or more before the final steady-state flux is achieved, as there is for swelling activation of K-Cl cotransport in erythrocytes of other species. The delay was shown to be independent of the extent of swelling. There was also a delay after shrinkage inactivation of cotransport. Reducing cellular Mg concentration activates cotransport. Swelling of low-Mg cells activates cotransport further, but with no measurable delay. In contrast, there is a delay in shrinkage inactivation of cotransport in low-Mg cells. The results are interpreted in terms of a three-state model: [formula see text] in which A state, B state, and C state transporters have relatively slow, intermediate, and fast transport rates, respectively. Most transporters in shrunken cells with normal Mg are in the A state. Swelling converts transporters to the B state in the rate-limiting process, followed by rapid conversion to the C state. Reducing cell Mg also promotes the A-->B conversion. Swelling of low-Mg cells activates transport rapidly because of the initial predominance of B state transporters. The results support the following conclusions about the rate constants of the three-state model: k21 is the rate constant for a Mg-promoted process that is inhibited by swelling; k12 is not volume sensitive. Both k23 and k32 are increased by swelling and reduced by shrinkage; they are rate constants for a single process, whereas k12 and k21 are rate constants for separate processes. Finally, the A-->B conversion entails an increase in Jmax of the transporters, and the B-->C conversion entails an increase in the affinity of the transporters for K.

Membrane properties of erythrocytes in subjects undergoing multiple blood donations with or without recombinant erythropoietin

To examine the characteristics of 'young' human red cells, we studied blood from seven healthy male volunteers who developed systemic reticulocytosis during a 3-week blood donation period. Each of these subjects donated a total of 6 units (450 ml/unit) of blood (2 units/week for 3 weeks) with subcutaneous recombinant erythropoietin (SC rEPO; 200 U/kg daily for 3 weeks). Two of these subjects were also studied with a similar protocol in the absence of rEPO (4.5 +/- 0.5 units donated). SC rEPO administration was associated with an increased K content of the erythrocyte and with the appearance of hypochromic cells, which were initially normocytic and then became normochromic and microcytic. Measurements of cation transport revealed that, with the exception of the Na-K-Cl cotransport, all the systems studied increased their activities following blood donations with or without SC rEPO. The increase was highest in the K-Cl cotransport (2- and 5-fold for control and rEPO parts of the study, respectively), while the Na-K pump increased slightly in control and 40% with rEPO. The Na-Li countertransport increased 40% and 100% in the control and rEPO parts of the study, respectively. Concomitant with increased ion transport activity, electron microscopic studies of plasma and red cells of subjects receiving SC rEPO showed the presence of circulating exosomes and cytoplasmic multivesicular bodies. The transferrin receptor was detected in the circulating exosomes, thereby providing evidence that, as do nonhuman red cells, maturing human reticulocytes shed exosome-associated transferrin receptors.

Volume-dependent potassium transport in camel red blood cells

In this study the volume-dependent, ouabain-resistant K+ influx and efflux in camel red blood cells were measured with the tracer 86Rb+. The results showed that the camel erythrocytes do not have the Na(+)-K+ cotransport. The cell swelling increases a ouabain-resistant K+ influx and shrinkage decreases it nearly two-fold. The swelling-stimulated K+ influx and efflux were chloride dependent. The anion dependence of K+ influx in swollen cells was as follows: Br- > Cl- > NO3. The pH-dependent curve for swelling-stimulated potassium influx, and the active K+ influx in camel erythrocytes were determined. The findings indicate that camel erythrocytes' potassium transport system has many similarities to other mammalian species.

Effect of osmolality on 86Rb+ uptake and release by Leishmania donovani

Promastigotes from late-log phase cultures of Leishmania donovani were washed and resuspended in Hanks' Balanced Salt Solution without glucose or phenyl red but with 20 mM (N-[2-hydroxyethyl] piperazine-N'-[2-ethanesulfonic acid]) (HEPES) (HBSS-, 305 mOsm/kg). They were then added to a solution containing 86Rb such that the final osmolality and ionic composition was as desired. Samples were taken at known times and the amount of intracellular 86Rb was measured. Similarly, experiments were performed in which 86Rb was added to the cultures about 18 hr before collection, and the amount of 86Rb released from the washed cells was measured. Under iso-osmotic conditions only about 1.3% of the intracellular 86Rb was released in 900 sec. This increased about 4-fold if the osmolality was reduced from 305-153 mOsm/kg. This is much slower than the very rapid release of alanine in response to hypo-osmotic stress, indicating that alanine release is not via a non-specific pore. Reducing the temperature from 26 degrees C to 3-4 degrees C completely inhibits 86Rb release under iso-osmotic conditions and largely inhibits it under hypo-osmotic conditions. The rate of 86Rb release was not sensitive to K+ concentration and was not altered if chloride was replaced by sulfamate. Ouabain had no effect on either 86Rb uptake or release, but carbonylcyanide P-trifluoromethoxyphenylhydrazone (FCCP) reduced the rate of 86Rb release and, after about a 300 sec exposure, completely inhibited 86Rb uptake. Amiloride partially inhibited 86Rb release, but had no effect on uptake. A decrease in pH from 7.1-5.9 had little effect on 86Rb release under iso-osmotic conditions and slightly increased the rate of release under hypo-osmotic conditions, but it decreased the rate of uptake under both iso-osmotic and hypo-osmotic conditions. Cells taken from 3-day stationary phase cultures released 86Rb more slowly under iso-osmotic conditions than cells from late log phase cultures, but were more responsive to hypo-osmotic stress than were log phase cells. These data appear to rule out an [Na-K-Cl] transporter or a [K-Cl] cotransporter as the means of K+ release, but are consistent with the possibility that a K+/H+ exchanger is present. The possibility that other carrier systems may be present is also discussed.

The electrolytes in hyponatremia

It is commonly taught that retention of free water is the dominant factor reducing the serum sodium concentration in hyponatremia. To determine whether the concentrations of other electrolytes are similarly diluted, we identified 51 patients with hyponatremia (Na = 121 +/- 1 mmol/L [mEq/L]) and compared electrolyte and laboratory values at the time of hyponatremia with values at a time when serum sodium was in the normal range (138 +/- 1 mmol/L). The medium interval between these measurements was 12 days. At the time of hyponatremia, serum sodium and chloride were substantially and significantly reduced by 12% to 15%. Although many hyponatremic patients had overtly increased or decreased concentrations of the other measured electrolytes, there were no significant changes in the mean concentration for any of these at the time of hyponatremia. Unchanged mean values were found for the plasma concentration of bicarbonate (26.1 +/- 0.6 normal v 25.2 +/- 0.8 mmol/L at the time of hyponatremia), potassium (4.31 +/- 0.10 v 4.33 +/- 0.15 mmol/L), albumin, phosphate, and creatinine. The stability of these laboratory values was observed both in patients with clinically normal extracellular fluid (ECF) volume and in those with true or effective ECF depletion. The urinary sodium (UNa) concentration was found to be a reliable predictor of the ECF volume status, whereas the fractional sodium excretion (FENa) was not. Electrolyte derangements are common in patients with hyponatremia, but are usually confined to patients on diuretics or who have an abnormal ECF volume. In the absence of these complicating situations, the plasma electrolytes are typically normal and are not reduced by dilution to the same extent as Na and CI. Based on a review of both the classic and recent knowledge concerning electrolyte regulation in hyponatremia, we propose that two factors explain these observations. First, the degree of dilution is overestimated because of Na losses in urine and perhaps Na shift into cells. Second, both renal and extrarenal adaptive mechanisms are activated by hyponatremia that stabilizes the concentration of other ions. One of these mechanisms is cell swelling, which triggers a volume-regulatory response leading to the release of ions and water into the ECF. Other adaptive mechanisms are mediated by antidiuretic hormone (ADH) per se, and by atrial natriuretic peptide (ANP).

A mathematical model of the volume, pH, and ion content regulation in reticulocytes. Application to the pathophysiology of sickle cell dehydration

We developed a mathematical model of the reticulocyte, seeking to explain how a cell with similar volume but much higher ionic traffic than the mature red cell (RBC) regulates its volume, pH, and ion content in physiological and abnormal conditions. Analysis of the fluxbalance required by reticulocytes to conserve volume and composition predicted the existence of previously unsuspected Na(+)-dependent Cl- entry mechanisms. Unlike mature RBCs, reticulocytes did not tend to return to their original state after brief perturbations. The model predicted hysteresis and drift in cell pH, volume, and ion contents after transient alterations in membrane permeability or medium composition; irreversible cell dehydration could thus occur by brief K+ permeabilization, transient medium acidification, or the replacement of external Na+ with an impermeant cation. Both the hysteresis and drift after perturbations were shown to depend on the pHi dependence of the K:Cl cotransport, a major reticulocyte transporter. This behavior suggested a novel mechanism for the generation of irreversibly sickled cells directly from reticulocytes, rather than in a stepwise, progressive manner from discocytes. Experimental tests of the model's predictions and the hypothesis are described in the following paper.

Thiol-dependent passive K: Cl transport in sheep red blood cells: X. A hydroxylamine-oxidation induced K: Cl flux blocked by diethylpyrocarbonate

Hydroxylamine, a potent oxidizing agent used to reverse carbethoxylation of histidine by diethylpyrocarbonate, activated Cl-dependent K flux (K: Cl cotransport) of low K sheep red blood cells almost sixfold. When K: Cl cotransport was already stimulated by N-ethylmaleimide, hydroxylamine caused an additional twofold activation suggesting modification of sites different from those thiol alkylated. This conclusion was supported by the finding that hydroxylamine additively augmented also the diamide-induced K: Cl flux (Lauf, P.K. 1988. J. Membrane Biol. 101: 179-188) with dithiothreitol fully reversing the diamide but not the hydroxylamine effect. Stimulation of K: Cl cotransport by hydroxylamine was completely inhibited by treatment with diethylpyrocarbonate also known to prevent K: Cl cotransport stimulation by N-ethylmaleimide, both effects being independent of the order of addition. Hence, although the effect of carbethoxy modification of K: Cl flux cannot be reversed by hydroxylamine and thus excludes histidine as the target for diethylpyrocarbonate, our finding reveals an important chemical determinant of K: Cl cotransport stimulation by both hydroxylamine oxidation and thiol group alkylation.

Kinetics of activation and inactivation of swelling-stimulated K+/Cl- transport. The volume-sensitive parameter is the rate constant for inactivation

Red blood cells of several species are known to exhibit a ouabain-insensitive, anion-dependent K+ (Rb+) flux that is stimulated by cell swelling. We have used rabbit red cells to study the kinetics of activation and inactivation of the flux upon step changes in tonicity. Sudden hypotonic swelling (210 mosmol) activates the flux after a lag period of 10 min at 37 degrees C and 30-50 min at 25 degrees C. In cells that were preswollen to activate the transporter, sudden shrinkage (by addition of hypertonic NaCl) causes a rapid inactivation of the flux; the time lag for inactivation is less than 2 min at 37 degrees C. A minimal model of the volume-sensitive KCl transport system requires two states of the transporter. The activated (A) state catalyzes transport at some finite rate (turnover number unknown because the number of transporters is unknown). The resting (R) state has a much lower or possibly zero transport rate. The interconversion between the states is characterized by unimolecular rate constants R k12 in equilibrium with k21 A. The rate of relaxation to any new steady state is equal to the sum of the rate constants k12 + k21. Because the rate of transport activation in a hypotonic medium is lower than the rate of inactivation in an isotonic medium, we conclude that the volume-sensitive rate process is inactivation (the A to R transition); that is, cell swelling activates transport by lowering k21. Three phosphatase inhibitors (fluoride, orthovanadate, and inorganic phosphate) all inhibit the swelling-activated flux and also slow down the rate of approach to the swollen steady state. This finding suggests that a net dephosphorylation is necessary for activation of the flux and that the net dephosphorylation takes place as a result of swelling-induced inhibition of a kinase rather than stimulation of a phosphatase.

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

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 (0.1 mM) 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 X hr) and in the affinity for external K (from 77 +/- 16 mM to 28 +/- 3 mM) of K influx. When the external K concentration is varied from 0 to 140 mM. 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.

Cation transport in oxidant-stressed human erythrocytes: heightened N-ethylmaleimide activation of passive K+ influx after mild peroxidation

Normal and chronically dehydrated (hereditary xerocytosis) human red cells were subjected to mild peroxidative treatment (315 microM hydrogen peroxide (H2O2), 15 min) in the presence of azide. The subsequent expression of passive (ouabain-resistant) K+ transport activities was analyzed by measurement of 86Rb+ influx. Peroxidation of normal red cells did not affect basal K+ transport activity, but the increment in K+ influx elicited by 0.5 mM N-ethylmaleimide (NEM) was increased 3-fold. The enhanced K+ influx was chloride-dependent, but only partially inhibited by 0.1 mM furosemide. Stimulated activity declined progressively after NEM activation, but could be restored by a second NEM treatment. Prior conversion of hemoglobin to the carbonmonoxy form abolished the response to peroxide, while 200 microM butylated hydroxytoluene (BHT) exerted only partial inhibition, suggesting that the effect of H2O2 requires interaction of activated, unstable hemoglobin species with the membrane, but that lipid peroxidation is not sufficient. Peroxidation following NEM treatment also enhanced NEM activation, indicating that enhancement does not require altered NEM reactions with stimulatory or inhibitory sites. Passive K+ transport in hereditary xerocytosis red cells was not activated by NEM, with or without H2O2 pretreatment. The results demonstrate that modest peroxidative damage to red cells can heighten the activation of a transport system that is thought to be capable of mediating net K+ efflux and volume reduction in cells that express it. Models are proposed in which the effects of NEM, H2O2, cell swelling and other factors are mediated by conformational changes in a postulated subpopulation of anion channel (Band 3) molecules that bind the K+ transporter.

Cation-anion cotransport

Two methods have been described for the study of cation-chloride cotransport systems. The zero-trans efflux method is designed to determine stoichiometric relationships between cotransported ions under conditions where ion exchanges cannot occur. These exchanges (e.g., Na+/Na+, K+/K+) may occur as partial or incomplete reactions of a cotransport process and can lead to erroneous determinations of the stoichiometry of the cotransport process. The zero-trans efflux method can also be used to study the effects of cell volume, pH, and intracellular ion concentrations on cotransport processes. The valinomycin method is used to determine the electrogenicity or electroneutrality of transport, and in this regard can be used in conjunction with other methods such as those employing potential-sensitive dyes or microelectrodes. Other, more recently developed ionophores with specificity for lithium rather than potassium have now been used to study the effect of Em on the ATP-dependent Na+/K+ pump. It may be possible to use such ionophores to confirm the suspected electroneutrality of (K+ + Cl-) cotransport systems, as well as for other studies of specific potassium transport processes in which valinomycin obviously cannot be used. Both methods discussed in detail in this chapter, and particularly the valinomycin method, were originally devised for use in red blood cells in order to take advantage of (or circumvent) properties of the red cell membrane, such as its low permeability to sodium and potassium and relatively high permeability to chloride. However, valinomycin has been used successfully to demonstrate the electroneutrality of (Na+ + K+ + 2Cl-) cotransport in MDCK cells, and the zero-trans efflux method should be applicable to the study of transport processes in other types of cells in suspension, so long as the transport system being studied can be accurately defined (e.g., as an inhibitor-sensitive or chloride-dependent cation flux) and comprises a significant fraction of the total salt efflux.