A chloride dependent K+ flux induced by N-ethylmaleimide in genetically low K+ sheep and goat erythrocytes

Kinetics of Cl-dependent K influx in human erythrocytes with and without external Na: effect of NEM

The majority of the ouabain-insensitive K influx in human erythrocytes is dependent on the presence of Cl. Recent studies have shown that a portion of the Cl-dependent K influx persists in the absence of external Na (Nao). It has been suggested that this Nao-independent component represents (K + Cl) cotransport, whereas the remainder of the Cl-dependent K influx seen on addition of external Na represents (Na + K + 2Cl) cotransport. In the present studies, the kinetics of Cl-dependent K influx were examined in the presence and absence of external Na, by varying external K and external Cl. Our studies suggest that the Nao-independent Cl-dependent pathway has a relatively low affinity for external K (Km 17-30 mM) in contrast to the high affinity of the Nao-augmented component (Km 3-4 mM). N-ethylmaleimide (NEM) stimulates the maximal velocity of the Nao-independent Cl-dependent K influx achievable without alteration of intracellular solutes but does not alter its Km for external K. In contrast, NEM has no stimulatory effect on the Nao-augmented component. The Cl dependence of the Nao-independent K influx is best described by a relatively flat curve with a mild upward concavity. The kinetic properties of the Nao-independent component of Cl-dependent K transport are very similar to those of the putative (K + Cl) cotransport pathway seen in low-K sheep erythrocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Passive K+-Cl- fluxes in low-K+ sheep erythrocytes: modulation by A23187 and bivalent cations

A fraction of the ouabain-resistant (OR) K+ flux of low-K+ (LK) sheep erythrocytes is Cl- dependent (K+-Cl- transport) and is activated reversibly by cell swelling or irreversibly by treatment with N-ethylmaleimide (NEM). The effect of the ionophore A23187 plus bivalent cations (Me2+) or ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA) was studied on K+-Cl- transport in control or NEM-treated LK cells. The following observations were made. 1) A23187 (6 microM), at a hematocrit of 10% (vol/vol) and in the presence of 1 mM EGTA, activated severalfold OR K+-Cl- transport in shrunken or swollen cells but failed to stimulate further K+-Cl- flux in NEM-treated cells. 2) In the absence of EGTA, but at very low external Ca2+ concentrations [( Ca2+]o = 10(-7) M), A23187 stimulated OR K+-Cl- flux in controls less than with EGTA and inhibited it slightly in NEM-treated cells. 3) When [Ca2+]o was raised to 10(-3) M, an almost complete inhibition of OR K+-Cl- fluxes occurred in shrunken, swollen, or NEM-treated cells. 4) Other Me2+ inhibited OR K+-Cl- flux in the presence of A23187 in the following order of decreasing potency: Mn2+ much greater than Ca2+ greater than Mg2+ greater than Sr2+ much much greater than Ba2+. 5) Stimulation of OR K+-Cl- flux by A23187 +/- EGTA and inhibition by A23187 + Ca2+ were reversible and did not alter significantly cellular ATP. 6) The stimulatory effect of A23187 plus EGTA, perhaps by Me2+ removal, on K+-Cl- flux and its inhibition by Ca2+ were reversibly abolished in metabolically depleted cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of N-ethylmaleimide on ouabain-insensitive cation fluxes in human red cell ghosts

In red cells of several species, the sulfhydryl reagent N-ethylmaleimide activates a Cl- -dependent, ouabain-resistant K+ transport pathway. Here we report our attempts to demonstrate ouabain-resistant Cl- -dependent K+ fluxes stimulated by N-ethylmaleimide in resealed human red cell ghosts using Rb+ as a K+ analogue. In contrast to intact cells, the rate constants of the base level Rb+ efflux in ghosts were similar in NaNO3 and NaCl (okRb = 0.535 +/- 0.079 h-1 and 0.534 +/- 0.085 h-1, respectively), while 1 mM N-ethylmaleimide stimulated Rb+ efflux strongly in NaNO3 (okRb = 14.26 +/- 1.32 h-1) and moderately in NaCl (okRb = 2.73 +/- 0.54 h-1). This effect was dependent on the presence of internal ATP. Stimulation of Rb+ efflux was observed in the presence of greater than or equal to 0.2 mM N-ethylmaleimide and increased at pH values approaching 8.0, consistent with titration of SH groups. N-Ethylmaleimide-stimulated Rb+ efflux was approx. 50% inhibited by 100 microM quinine sulfate whereas 1 microM bumetanide had no effect. In NaCl the N-ethylmaleimide-stimulated efflux saturated with initial internal ghost Rb+ concentration, but rates increased linearly in NaNO3. Replacement of external Na+ with glucamine or choline decreased the N-ethylmaleimide-stimulated Rb+ efflux, suggesting a role for external Na+. N-Ethylmaleimide-stimulated Rb+ efflux was greater in buffers with lipophilic anions such as SCN- or NO3- than in solutions with Cl- or acetate. However, the cation selectivity of the pathway studied was low, as Li+ efflux was also stimulated by N-ethylmaleimide. We conclude that the effect of N-ethylmaleimide on ouabain-resistant cation effluxes of human red cell ghosts is very different from the selective action of N-ethylmaleimide on Rb+ influxes in intact red cells.

Cell volume and metabolic dependence of NEM-activated K+-Cl- flux in human red blood cells

The effects of incubation in anisosmotic media and of metabolic depletion on ouabain-resistant (OR) Cl--dependent K+ influxes stimulated by N-ethylmaleimide (NEM) were studied in human red blood cells using Rb+ as K+ analogue. The NEM-stimulated but not the basal Rb+-Cl- influx measured in phosphate-buffered anisosmotic media was found to be cell volume dependent. When cellular ATP, [ATP]c, was lowered to less than 0.10 of its initial level by exposure to nonmetabolizable 2-deoxy-D-glucose, the NEM-stimulated but not the basal Cl--dependent Rb+ influxes were abolished. Metabolically depleted red blood cells subsequently repleted by incubation in glucose plus inosine regained the NEM-inducible Rb+ (K+) transport activity. The difference in the time course of ATP breakdown and Rb+ influx inhibition suggests that energization of the NEM-stimulated Rb+ flux by metabolism may involve factors additional to ATP.

Role of passive potassium fluxes in cell volume regulation in cultured HeLa cells

Cultured HeLa cells behave as ideal osmometers when subjected to hyperosmolar media, and show no volume regulatory behavior. In hypoosmolar solutions, cell swelling is not as great as predicted, and this is due largely to a loss of intracellular KCl. In hyperosmolar solutions there is a stimulation of the ouabain-insensitive but loop diuretic-sensitive 86Rb+ (K+) pathway. Analysis of the K+, Na+ and Cl- dependency of this K+ flux pathway demonstrates that the increase is principally due to an increase in its maximal velocity (Vmax). The sensitivity of this pathway to diuretic inhibition is unchanged in hyperosmolar media. Diuretic-sensitive 86Rb+ (K+) efflux stimulated by hypertonicity shows no marked dependence on external K+. The K+ loss observed in hypoosmolar media is distinct from the K+ transport pathway stimulated by hyperosmolar media on the basis of its sensitivity to furosemide and anion dependence.

Racial differences in bumetanide-sensitive cotransport and N-ethylmaleimide-stimulated potassium efflux

Racial differences in erythrocyte potassium effluxes mediated by two loop-diuretic sensitive modes of cotransport were compared. In red cells loaded to contain approximately equimolar amounts of sodium and potassium, black subjects had lower bumetanide-sensitive sodium-dependent net potassium effluxes as compared to whites. In fresh, washed erythrocytes pretreated with N-ethylmaleimide (NEM), maximal net potassium efflux was greater in blacks than in whites. NEM-stimulated potassium efflux was partially inhibited by bumetanide but only at very high concentrations. The quantitative differences in these two modes of potassium efflux suggest that NEM-stimulated potassium efflux is not an altered mode of sodium-dependent potassium efflux.

p-Chloromercuribenzenesulfonic acid stimulation of chloride-dependent sodium and potassium transport in human red blood cells

The organic mercurial p-chloromercuribenzensulfonic acid (PCMBS) reversibly increases fluxes of sodium and potassium across the human red blood cell membrane. We examined the effect of different monovalent anions on cation fluxes stimulated by PCMBS. A substantial portion of the fluxes of both cations was found to have a specific anion requirement for chloride or bromide, and was not observed when chloride was replaced by nitrate, acetate or methylsulfate. The chloride-dependent component of the cation fluxes was only observed when the cells were exposed to PCMBS concentrations of 0.5 mM or greater. Furosemide (1 mM) did not inhibit the PCMBS-stimulated cation fluxes. The observed anion specificity is directly associated with the transport process rather than PCMBS binding to the membrane. A portion of the potassium transport stimulated by PCMBS appears to involve K+-K+ exchange; however, Na+ + K+ cotransport is not stimulated by this sulfhydryl reagent.

Effects of A23187 and Ca2+ on volume- and thiol-stimulated, ouabain-resistant K+C1- fluxes in low K+ fluxes in low K+ sheep erythrocytes

Ouabain-resistant (OR), C1- -dependent K+ (K+C1-) transport measured by Rb+ influx in isosmotic and anisosmotic media was stimulated by the Ca2+ ionophore A23187 and EGTA (ethylene-glycol-tetracetic acid) in low K+ (LK) but not in high K+ (HK) sheep red cells. Increasing external Ca2+ concentrations, [Ca2+]o, from about 10(-7) to 10(-3)M in presence of A23187 and in absence of EGTA inhibited OR Rb+ influx, in LK red cells osmotically shrunken or swollen as well as treated with the thiol reagent N-ethylmaleimide (NEM). Hence the volume- and the NEM-stimulated K+C1- transport system in LK cells can be experimentally modulated by cellular Ca2+ or other Me2+, which may interact with sites on the K+C1- transporter under the control of membrane sulfhydryl (SH) groups.

Ouabain-resistant Na+, K+ transport system in mouse NIH 3T3 cells

It is shown that the ouabain-resistant (OR) furosemide-sensitive K+(Rb+) transport system performs a net efflux of K+ in growing mouse 3T3 cells. This conclusion is based on the finding that under the same assay conditions the furosemide-sensitive K+(Rb+) efflux was found to be two- to threefold higher than the ouabain-resistant furosemide-sensitive K+(Rb+) influx. The ouabain-resistant furosemide-sensitive influxes of both 22Na and 86Rb appear to be Cl- dependent, and the data are consistent with coupled unidirectional furosemide-sensitive influxes of Na+, K+ and Cl- with a ratio of 1:1:2. However, the net efflux of K+ performed by this transport system cannot be coupled to a ouabain-resistant net efflux of Na+ since the unidirectional ouabain-resistant efflux of Na+ was found to be negligible under physiological conditions. This latter conclusion was based on the fact that practically all the Na+ efflux appears to be ouabain-sensitive and sufficient to balance the Na+ influx under such steady-state conditions. Therefore, it is suggested that the ouabain-resistant furosemide-sensitive transport system in growing cells performs a facilitated diffusion of K+ and Na+, driven by their respective concentration gradients: a net K+ efflux and a net Na+ influx.

Thiol-dependent passive K+Cl- transport in sheep red blood cells: VI. Functional heterogeneity and immunologic identity with volume-stimulated K+(Rb+) fluxes

Ouabain-resistant (OR), volume- or N-ethylmaleimide (NEM)-stimulated K+(Rb+)Cl- fluxes were measured in low-K+ sheep red cells and found to be functionally separate but immunologically similar. In anisosmotic solutions both K+ effluxes and Rb+ influxes of NEM-treated and control cells were additive. In contrast to the NEM-stimulated K+Cl- flux, metabolic depletion did not reduce K+Cl- flux of normal or swollen cells. The anion preference of OR K+ efflux in NEM-treated cells was Br- greater than Cl- greater than HCO3- = F- much greater than I- = NO3- = CNS-, and hence consistent with a reported Br- greater than Cl- greater than NO3- sequence of the volume-dependent K+Cl- transport. Alloimmune anti-L1 antibodies known to decrease passive K+ fluxes in low K+ cells reduced by 51% both volume- and NEM-stimulated, furosemide-sensitive Rb+Cl- fluxes suggesting their immunologic identity, a conclusion also supported by anti-L1 absorption studies. Since pretreatment with anti-L1 prevented the activation of Rb+ influx by NEM, and the impermeant glutathionmaleimide-I did not stimulate Rb+Cl- influx, the NEM reactive SH groups must be located apart from the L1 antigen either within the membrane or on its cytoplasmic face. A model is proposed consisting of a K+Cl- transport path(s) regulated by a protein with two functional subunits or domains: a chemically (Cs) and a volume (Vs)-stimulated domain, both interfacing with the L1 surface antigen. Attachment of alloanti-L1 from the outside reduces K+Cl- transport stimulated through Cs by NEM or Vs by cell swelling.

Activation by N-ethylmaleimide of a latent K+-Cl- flux in human red blood cells

Twenty to fifty percent of the ouabain-insensitive Na+ and K+ fluxes in human red blood cells are mediated by Cl(-) -dependent coupled transport (cotransport). In this paper we report on the effect of the sulfhydryl group reagent N-ethylmaleimide (NEM) on Cl(-) -dependent ouabain-insensitive Na+ and K+ fluxes in human red blood cells. We found that NEM altered Na+ -K+ cotransport and activated a latent Cl(-) -dependent K+ transport mode normally apparently silent. This conclusion was based on the following observations. 1) At low concentrations (0.25 mM) NEM abolished the bumetanide-sensitive Na+ efflux and had no effect, even at a 10-fold higher concentration, on the bumetanide-sensitive K+ efflux. 2) At concentrations above 0.1 mM, NEM stimulated Cl(-) -dependent K+ efflux that was only partially inhibited by high concentrations of bumetanide or furosemide. In experiments using Rb+ as a K+ analogue, NEM activated Rb+ influx by stimulating the maximum velocity and lowering the apparent external cation affinity. The data suggest the presence of chemically reactive groups in human red blood cells for both Cl(-) -dependent K+ transport activated by NEM and Cl(-) -dependent coupled Na+-K+ movements.

Thiol-dependent passive K/Cl transport in sheep red cells: IV. Furosemide inhibition as a function of external Rb+, Na+, and Cl-

The effect of the loop diuretic furosemide (4-chloro-N-furfuryl-5-sulfamoyl-anthranilic acid) on the thiol-dependent, ouabain-insensitive K(Rb)/Cl transport in low K+ sheep red cells was studied at various concentrations of extracellular Rb+, Na+ and Cl-. In Rb+-free NaCl media, 2 X 10(-3) M furosemide inhibited only one-half of thiol-dependent K+ efflux. In the presence of 23 mM RbCl, however, the concentration of furosemide to produce 50% K+ efflux inhibition (IC50) was 5 X 10(-5) M. In Rb+ containing NaCl media, the inhibitory effect of 10(-3) M furosemide was equal to that caused by NO-3 replacement of Cl- in the medium. The apparent synergistic action of furosemide and external Rb+ on K+ efflux was also seen in the ouabain-insensitive Rb+ influx. A preliminary kinetic analysis suggests that furosemide binding alters both maximal K+(Rb+) transport and apparent external Rb+ affinity. In the presence of external Rb+, Na+ (as compared to choline) exerted a small but significant augmentation of the furosemide inhibition of K+(Rb+) fluxes. There was no effect of Cl- on the IC50 value of furosemide. As there is no evidence for coupled Na+K+ cotransport in low K+ sheep red cells, furosemide may modify thiol-dependent K+(Rb+)/Cl flux or Rb+ (and to a slight degree Na+) modulate the effect of furosemide.

Volume-induced increase of K+ and Cl- permeabilities in Ehrlich ascites tumor cells. Role of internal Ca2+

Ehrlich ascites tumor cells resuspended in hypotonic medium initially swell as nearly perfect osmometers , but subsequently recover their volume within 5 to 10 min with an associated KCl loss. 1. The regulatory volume decrease was unaffected when nitrate was substituted for Cl-, and was insensitive to bumetanide and DIDS. 2. Quinine, an inhibitor of the Ca2+- activated K+ pathway, blocked the volume recovery. 3. The hypotonic response was augmented by addition of the Ca2+ ionophore A23187 in the presence of external Ca2+, and also by a sudden increase in external Ca2+. The volume response was accelerated at alkaline pH. 4. The anti-calmodulin drugs trifluoperazine, pimozide, flupentixol, and chlorpromazine blocked the volume response. 5. Depletion of intracellular Ca2+ stores inhibited the regulatory volume decrease. 6. Consistent with the low conductive Cl- permeability of the cell membrane there was no change in cell volume or Cl- content when the K+ permeability was increased with valinomycin in isotonic medium. In contrast, addition of the Ca2+ ionophore A23187 in isotonic medium promoted Cl- loss and cell shrinkage. During regulatory volume decrease valinomycin accelerated the net loss of KCl, indicating that the conductive Cl- permeability was increased in parallel with and even more than the K+ permeability. It is proposed that separate conductive K+ and Cl- channels are activated during regulatory volume decrease by release of Ca2+ from internal stores, and that the effect is mediated by calmodulin.

Inactivation of regulatory volume decrease in human peripheral blood lymphocytes by N-ethylmaleimide

The sulfhydryl group reagent N-ethylmaleimide was found to inhibit in a dose dependent manner regulatory volume decrease of human peripheral lymphocytes swollen in buffered hyposmotic NaCl media. In hyposmotic KCl media NEM treated lymphocytes prevented an additional secondary swelling seen in control lymphocytes. The data suggest that N-ethylmaleimide acts on ion transport mechanisms involved in volume regulatory changes. This effect contrasts with the stimulation by N-ethylmaleimide of apparently volume sensitive K/Cl fluxes in certain mammalian red cells.

Passive transport of K+ and Na+ in human red blood cells: sulfhydryl binding agents and furosemide

Passive transport pathways for K+ and Na+ were studied in fresh human red blood cells (pretreated with ouabain) by measuring unidirectional influxes. The effects of the sulfhydryl binding agents N-ethylmaleimide (NEM) and p-chloromercuribenzene sulfonate (p-CMBS) and the loop diuretic furosemide were studied. Influxes were measured at equimolar K+ and Na+ concentrations (50 mM) with both ions present and also in K+-free or Na+-free media. Some experiments were carried out in Cl--free media (with NO-3 as the substitute). NEM stimulated K+ influx twofold; the stimulation required Cl- but not Na+. NEM inhibited Na+ influx 20%. Furosemide inhibited both K+ and Na+ influxes. All of furosemide-inhibitable Na+ influx required the presence of K+. However 30% of furosemide-inhibitable K+ influx did not require Na+. All of furosemide-inhibitable K+ influx required Cl-. The ratio of Na+-dependent K+ influx to K+-dependent Na+ influx was 3:1. p-CMBS stimulated both Na+ and K+ influxes. K+ influx in p-CMBS cells required neither Na+ nor Cl-. Likewise p-CMBS-promoted Na+ influx did not require K+. These various results are consistent with two Cl--dependent pathways for K+ transport, one requiring Na+ [perhaps (Na + K + Cl) cotransport] and one independent of Na+ [perhaps (K + Cl) cotransport]. The pathways promoted by p-CMBS are probably independent of the apparent cotransport systems.

Effect of ouabain upon diuretic-sensitive K+ transport in cultured cells. Evidence for separate modes of operation of the transporter

(1) Unidirectional K+ (86Rb) influx and efflux were measured in subconfluent layers of MDCK renal epithelial cells and HeLa carcinoma cells. (2) In both MDCK and HeLa cells, the furosemide-inhibitable and chloride-dependent component of K+ influx/efflux was stimulated 2-fold by a 30 min incubation in 1 . 10(-3) M ouabain. (3) Measurements of net K+ loss and Na+ gain in ouabain-treated cells at 1 h failed to show any diuretic sensitive component, confirming the exchange character of the diuretic-sensitive fluxes. (4) Prolonged incubations for 2.5 h in ouabain revealed a furosemide- and anion-dependent K+ (Cl-) outward net flux uncoupled from net Na+ movement. Net K+ (Cl-) outward flux was half-maximally inhibited by 2 microM furosemide. (5) After 2.5 h ouabain treatment, the anion and cation dependence of the diuretic-sensitive K+ influx/efflux were essentially unchanged when compared to untreated controls.

Thiol-dependent passive K/Cl transport in sheep red cells: III. Differential reactivity of membrane SH groups with N-ethylmaleimide and iodoacetamide

Treatment with N-ethylmaleimide (NEM) is known to stimulate ouabain-insensitive, Cl- -dependent K+ transport in low K+ (LK) but not in high K+ (HK) sheep red cells (Lauf, P.K., and Theg, B.E., 1980, Biophys. Biochem. Res. Commun. 92:1422-1428). The dependence of this effect on the pH of pretreatment with NEM and/or iodoacetamide (IAA) was studied. Maximum stimulation of Cl- -dependent K+ transport in LK red cells was produced by prior treatment with 1-5 mM NEM at pH 6 at which only about 30-40% of the 10(7) SH groups present per membrane reacted. At pH 6 no NEM effect was seen on Nap K+ fluxes in HK red cells. Treatment with NEM below pH 6 enhanced Cl- -independent K+ transport in both LK and HK red cells. At higher pH values or higher concentrations the NEM-stimulation of K+ transport was reduced and absent at pH 8.7. Exposure of LK cells to 5 mM IAA prior to NEM abolished the stimulatory effect of NEM on K+ transport. Hence at least two different chemical groups were reacting with NEM, and more alkaline SH or NH2 groups whose reaction with NEM leads to an inhibition of the NEM effect brought about at pH 6.

Thiol-dependent passive K/Cl transport in sheep red cells: II. Loss of Cl- and N-ethylmaleimide sensitivity in maturing high K+ cells

A fraction of the passive, ouabain-insensitive K+ fluxes in mature low Kr(LK) but not in high K+ (HK) sheep red cells requires the presence of Cl- anions and can be stimulated by volume expansion (Dunham, P.B., Ellory, J.C., J. Physiol (London) 318:511-530, 1981) or treatment with 2 mM N-ethylmaleimide (NEM) (Lauf, P.K., Theg., B.E., Biochem. Biophys. Res. Commun. 92:1422-1428, 1980). In the present study it is shown that reticulocytes of both anemic LK and HK sheep possess the Cl- -dependent K+ transport system which subsequently remains functional in mature LK but not in HK red cells. Kinetically, Cl- -mediated K+ fluxes of reticulocytes of LK sheep are different from mature red cells only in their Vmax values as measured in Na+ or choline+ media, while there is an apparently much lower affinity for external K+ ions in reticulocytes of HK sheep. N-ethylmaleimide stimulated K+ transport in the reticulocytes of both sheep genotypes suspended in Cl- but failed to do so in NO3- media. The data are interpreted in terms of their biochemical, physiologic, and genetic implications for the HK/LK transition in sheep red cells.

Thiol-dependent passive K/Cl transport in sheep red cells: I. Dependence on chloride and external ions

Treatment with 2 mM N-ethylmaleimide (NEM) caused a marked increase in K+ permeability of low K+ but not of high K+ sheep red cells suspended in isosmotic Cl- media with 10(-4) M ouabain. The Na+ permeability was unaltered. Kinetic analysis by K+ efflux and K+ or Rb+ influx measurements suggests that NEM primarily increased the bidirectional fluxes of K+ and Rb+, since (a) no significant change in the apparent external affinities of these ions was found, and (b) below unity, the observed flux ratios were close to those calculated from the Ussing relationship. Replacement of Cl- by NO3 abolished the NEM-stimulated and reduced the basal K+ flux rates. Similarly, 10(-3) M furosemide inhibited Cl- -dependent K+ fluxes in both control and NEM-treated LK red cells. Exposure of LK cells to hyposmotic but not to hyperosmotic salt solutions increased the basal Cl- dependent K+ flux twofold as reported by Dunham and Ellory (J. Physiol. (London) 318:511-530, 1981) but did not affect its fractional stimulation by NEM. The action of NEM is interpreted as a stimulation of a temperature-dependent and Cl- -requiring K+ transport pathway genetically preserved in adult LK but turned off in HK sheep red cells. In addition, common to both LK and HK sheep red cells was a basal K+ flux that operated in the presence of either Cl- or NO3-.

Na+,Cl- cotransport in Ehrlich ascites tumor cells activated during volume regulation (regulatory volume increase)

Ehrlich ascites cells were preincubated in hypotonic medium with subsequent restoration of tonicity. After the initial osmotic shrinkage the cells recovered their volume within 5 min with an associated KCl uptake. The volume recovery was inhibited when NO-3 was substituted for Cl-, and when Na+ was replaced by K+, or by choline (at 5 mM external K+). The volume recovery was strongly inhibited by furosemide and bumetanide, but essentially unaffected by DIDS. The net uptake of Cl- was much larger than the value predicted from the conductive Cl- permeability. The undirectional 36Cl flux, which was insensitive to bumetanide under steady-state conditions, was substantially increased during regulatory volume increase, and showed a large bumetanide-sensitive component. During volume recovery the Cl- flux ratio (influx/efflux) for the bumetanide-sensitive component was estimated at 1.85, compatible with a coupled uptake of Na+ and Cl-, or with an uptake via a K+,Na+,2Cl- cotransport system. The latter possibility is unlikely, however, because a net uptake of KCl was found even at low external K+, and because no K+ uptake was found in ouabain-poisoned cells. In the presence of ouabain a bumetanide-sensitive uptake during volume recovery of Na+ and Cl- in nearly equimolar amounts was demonstrated. It is proposed that the primary process during the regulatory volume increase is an activation of an otherwise quiescent, bumetanide-sensitive Na+,Cl- cotransport system with subsequent replacement of Na+ by K+ via the Na+/K+ pump, stimulated by the Na+ influx through the Na+,Cl- cotransport system.

Sodium and chloride co-transport by preimplantation rabbit blastocysts

Influxes of Na+ and Cl- across the trophectodermal epithelium of 6 day post coitum (p.c.) rabbit blastocysts are partially interrelated. This relationship was investigated under conditions in which either the external [Na+] or [Cl-] concentration was varied. Our results indicate that an ouabain-insensitive, furosemide-sensitive, Na+:Cl- co-transport system exists in the 6 and 6.5 day p.c. rabbit embryo, but not in the 5 day p.c. blastocyst. There was no dependence of NaCl uptake on the presence of K+ in the bathing medium. No evidence of Na+/H+ or Cl-/HCO3- exchange systems in the rabbit blastocyst was found. The effects of furosemide, exogenous cyclic AMP, and various exogenous prostaglandins on Na+ and Cl- movements were studied. Cyclic AMP and/or prostaglandin E1 may regulate the operation of this Na+:Cl- co-transport system.

Na+ K+ pump and passive K+ transport in large and small red cell populations of anemic high and low K+ sheep

Reticulocytes, isolated by centrifugal elutriation from massively bled sheep and identified by cytometric techniques, were analyzed with respect to their cation transport properties. In sheep with genetically high K+ (HK) or low K+ (LK) red cells, two reticulocyte types were distinguished by conventional or fluorescence-staining techniques 5-6 days after hemorrhage: Large reticulocytes as part of a newly formed macrocytic (M) erythrocyte population, and small reticulocytes present among the adult red cell population (volume population III of normal sheep blood, Valet et al., 1978). Although cellular reticulin disappeared within a few days, the M-cell population persisted throughout weeks in the peripheral circulation permitting a transport study of in vivo maturation. At all times, M cells of LK sheep had lower K+ and higher Na+ contents than M cells of HK sheep. Regardless of the sheep genotypes, M cells apparently reduced their volume during their first days in circulation; however, throughout the observation period, they did not attain that characteristic for adult red cells. Both ouabain-sensitive K+ pump and ouabain-insensitive K+ leak fluxes were elevated in M cells of both HK and LK sheep. The increased K+ pump flux was mainly due to higher K+ pump turnover rather than to the modestly increased number of pumps as measured by [3H]ouabain binding. In contrast, small reticulocytes enriched from separated volume population III cells by a Percoll-density gradient exhibited transport parameters close to their prospective mature HK or LK red cells. The data support the concept that the M cells derived from emergency reticulocytes while the small reticulocytes represented precursors of normal red cell maturation. The Na+ and K+ composition found in M cells of HK and LK sheep, respectively, suggest development of the LK steady state at or prior to the reticulocyte state, a finding consistent with that of Lee and Kirk (1982) on low K+ dog red cells.

Passive potassium transport in LK sheep red cells. Modification by N-ethyl maleimide

Passive K transport, as modified by N-ethyl maleimide (NEM), was studied in erythrocytes of the low-K (LK) phenotype of sheep. Brief (5-min) treatment with NEM at less than 0.5 mM caused inhibition of passive K influx; NEM at concentrations greater than 0.5 mM caused stimulation of K influx. NEM had similar effects on K efflux. The treatments with NEM did not affect cell volumes (passive K transport in LK cells is sensitive to changes in cell volume). The stimulation of K transport by high [NEM] was also not a consequence of an effect on the metabolic state of the cells. Passive K transport in LK cells is dependent on Cl (it is inhibited in Cl-free media; it may be K/Cl cotransport). NEM had no effect on K influx in Cl-free (NO3-substituted) media. Pretreatment of the cells with anti-L antiserum (L antigen is found on LK cells and not on HK cells) prevented stimulation of K influx by NEM, but did not prevent inhibition. Therefore, NEM modifies the Cl-dependent K transport pathway at two separate sites, a low-affinity site, at which it stimulates, and a high-affinity site, at which it inhibits. Anti-L antibody prevents NEM's action, but only at the low-affinity site.

Differentiation between serum stimulation of ouabain-resistant and sensitive Rb influx in quiescent NIH 3T3 cells

The addition of serum to quiescent NIH 3T3 mouse cell cultures resulted in a 10- to 20-fold increase of Rb influx which was resistant to ouabain, and only a three- to fourfold activation of ouabain-sensitive Rb influx. Stimulation of the ouabain-resistant Rb influx following serum addition reached its maximum within 2 min. The stimulation of ouabain-resistant Rb influx was a result of Vm increase while the Km for Rb was unchanged. Ouabain-resistant Rb influx, after serum addition, was resistant to amiloride and sensitive to ethacrynic acid. Replacing chloride in the medium by NO3-, CO3- and CH3COO- resulted in a drastic decrease in the ouabain-resistant Rb influx. It appeared, therefore, that the ouabain-resistant Rb influx in NIH 3T3 cells was Cl--dependent.

Proton-sulfate co-transport: mechanism of H+ and sulfate addition to the chloride transporter of human red blood cells

Proton and sulfate inhibition of the obligatory chloride-chloride exchange of human erythrocytes was measured at 0 degrees C to determine their mechanism of reaction with the anion transporter. The proton and sulfate that are co-transported by this mechanism at higher temperatures behaved as nontransported inhibitors at 0 degrees C. We analyzed the data in terms of four molecular mechanisms: (1) HSO4- addition to the transporter; (2) ordered addition with the proton first; (3) ordered addition with the sulfate first; (4) random addition to the transporter. The Dixon plots of 1/MCl vs. [SO4] at different proton concentrations were not parallel. Thus protons and sulfate ions were not mutually exclusive inhibitors. The slope of these Dixon plots was independent of pH above 7.0, which indicates that sulfate could bind to the unprotonated carrier and excludes the first two mechanisms. Protons were inhibitors of chloride flux in the absence of sulfate, which indicates that protons could bind to the unloaded carrier and excludes mechanism 3. The KI for sulfate was 4.35 +/0 0.36 mM. The pK for the protonatable group was 5.03 +/- 0.02. The binding of either a proton or sulfate to the carrier decreased the KI of the other by ninefold. The only simple mechanism consistent with the data is a random-ordered mechanism with more transporters loaded with a sulfate than loaded with a proton at the pH and sulfate concentrations of plasma.

Passive potassium transport in low potassium sheep red cells: dependence upon cell volume and chloride

The major pathway of passive K influx (ouabain-insensitive) was characterized in low-K type (LK) red cells of sheep. 1. Passive K transport in these cells was highly sensitive to variations in cell volume; it increased threefold or more in cells swollen osmotically by 10%, and decreased up to twofold in cells shrunken 5-10%. Active K influx was insensitive to changes in cell volume. Three different methods for varying cell volume osmotically all gave similar results. 2. The volume-sensitive pathway was specific for K in that Na influx did not vary with changes in cell volume. 3. The volume-sensitive K influx was a saturable function of external K concentration. It was slightly inhibited by Na, whereas K influx in shrunken cells was unaffected by Na. 4. Passive K influx was dependent on the major anion in the medium in that replacement of Cl with any of six other anions resulted in a reduction of K influx by 50-80% (replacement of Cl by Br caused an increase in K influx). The activation of K influx by Cl followed sigmoid kinetics. 5. Passive K influx is inhibited by anti-L antibody. The antibody affected only that portion of influx which was Cl-dependent and volume-sensitve. Of the subfractions of the antibody, it is anti-L1 which inhibits passive K transport. 6. Pretreatment of cells with iodoacetamide reduced the sensitivity of K influx to cell volume in that the influx was reduced in swollen IAA-treated cells and increased in shrunken IAA-cells. 7. Intracellular Ca has no role in altering passive K transport in LK sheep cells. Therefore, the major pathway of passive K transport in LK sheep red cells is sensitive to changes in cell volume, specific for K, dependent on Cl, and inhibited by anti-L1 antibody, The minor pathway, observed in shrunken cells, has none of these properties.

Hormone-induced diuretic-sensitive potassium transport in turkey erythrocytes is anion dependent

In turkey erythrocytes, the hormone-induced, diuretic-sensitive components of both unidirectional K+ influx and net salt uptake were Cl- dependent, with Br- partially able, and NO3(-) and SO42(-) unable to substitute as the main anion. Since the measured fluxes involve involve Na+/+ cotransport, these observations indicate that the unifying concept of an Na+/K+/2Cl- cotransport system (Geck, P., Pietrzyk, C., Burckhardt, B.-C., Pfeiffer, B. and Heinz, E. (1980) Biochim. Biophys. Acta 600, 432-447) may be extended to hormone-induced ion transport in avian erythrocytes.