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

Kinetic studies of K-Cl cotransport in cultured rat vascular smooth muscle cells

During aging, and development of atherosclerosis and cardiovascular disease (CVD), aortic vascular smooth muscle cells (VSMCs) transition from healthy contractile to diseased synthetic phenotypes. K-Cl cotransport (KCC) maintains cell volume and ion homeostasis in growth and differentiation, and hence is important for VSMC proliferation and migration. Therefore, KCC activity may play a role in the contractile-to-synthetic VSMC phenotypic transition. Early, medium, and late synthetic passage VSMCs were tested for specific cytoskeletal protein expression. KCC-mediated ouabain- and bumetanide-insensitive Rb+ (a K+ congener) influx was determined as Cl--dependent Rb+ influx at different external Rb+ and Cl- ion concentrations, [Rb+]o and [Cl-]o. Expressions of the cytoskeletal proteins α-actin, vimentin, and desmin fell from early through late synthetic VSMCs. KCC kinetic parameters, such as maximum velocity ( Vm), and apparent Cl- and Rb+ affinities ( Km), were calculated with Lineweaver-Burk, Hanes-Woolf, and Hill approximations. Vm values of both Rb+- and Cl--dependent influxes were of equal magnitude, commensurate with a KCC stoichiometry of unity, and rose threefold from early to late synthetic VSMCs. Hill coefficients for Rb+ and Cl- correlated with cell passage number, suggesting increased KCC ligand cooperativity. However, Km values for [Cl-]o were strikingly bimodal with 60-80 mM in early, ~20-30 mM in medium, and 60 mM in late passage cells. In contrast, Km values for [Rb+]o remained steady at ~17 mM. Since total KCC isoform expression was similar with cell passage, structure/function changes of the KCC signalosome may accompany the transition of aortic VSMCs from a healthy to a diseased phenotype.

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.

K-Cl cotransport function and its potential contribution to cardiovascular disease

K-Cl cotransport is the coupled electroneutral movement of K and Cl ions carried out by at least four protein isoforms, KCC1-4. These transporters belong to the SLC12A family of coupled cotransporters and, due to their multiple functions, play an important role in the maintenance of cellular homeostasis. Significant information exists on the overall function of these transporters, but less is known about the role of the specific isoforms. Most functional studies were done on K-Cl cotransport fluxes without knowing the molecular details, and only recently attention has been paid to the isoforms and their individual contribution to the fluxes. This review summarizes briefly and updates the information on the overall functions of this transporter, and offers some ideas on its potential contribution to the pathophysiological basis of cardiovascular disease. By virtue of its properties and the cellular ionic distribution, K-Cl cotransport participates in volume regulation of the nucleated and some enucleated cells studied thus far. One of the hallmarks in cardiovascular disease is the inability of the organism to maintain water and electrolyte balance in effectors and/or target tissues. Oxidative stress is another compounding factor in cardiovascular disease and of great significance in our modern life styles. Several functions of the transporter are modulated by oxidative stress, which in turn may cause the transporter to operate in either "overdrive" with the purpose to counteract homeostatic changes, or not to respond at all, again setting the stage for pathological changes leading to cardiovascular disease. Intracellular Mg, a second messenger, acts as an inhibitor of K-Cl cotransport and plays a crucial role in regulating the activity of protein kinases and phosphatases, which, in turn, regulate a myriad of cellular functions. Although the role of Mg in cardiovascular disease has been dealt with for several decades, this chapter is evolving nowadays at a faster pace and the relationships between Mg, K-Cl cotransport, and cardiovascular disease is an area that awaits further experimentation. We envision that further studies on the role of K-Cl cotransport, and ideally on its specific isoforms, in mammalian cells will add missing links and help to understand the cellular mechanisms involved in the pathophysiology of cardiovascular disease.

Evidence supporting a role for KCl cotransporter in the thick ascending limb of Henle's loop

BACKGROUND

A basolateral Ba(2+)-sensitive KCl cotransporter has previously been proposed as participating in basolateral K+ recycling and transepithelial NaCl reabsorption in the thick ascending limb of Henle's loop (TAL). The aim of the present study was to answer the question as to whether this cotransporter plays a role in transepithelial K+ reabsorption and whether dietary Mg(2+) deficiency, known to regulate the KCl cotransporter in erythrocytes, also regulates KCl transport in the TAL.

METHODS

The effects of a low-Mg(2+) diet were investigated on urinary and plasma K+ concentration in control mice and Mg(2+)-deficient mice. Transepithelial Na+, Cl- and K+ net fluxes (J(Na), J(Cl), J(K)), determined in isolated perfused TALs with electron probe analysis or cation-exchange high-performance liquid chromatography (HPLC) and electrophysiological parameters (V(te), R(te)), were measured in both animal groups. Expression of transcripts for the KCl cotransporter and its possible regulation by low-Mg(2+) were studied by RT-PCR in microdissected mouse cortical TAL (CTAL) and medullary TAL (MTAL) segments.

RESULTS

In isolated perfused CTALs, basolateral Ba(2+) and amiloride induced a large K+ net secretion towards the tubular lumen, paralleled by a 50% decrease in transepithelial NaCl reabsorption. KCC1 transcripts were found in the mouse CTAL and MTAL. A low-Mg(2+) diet led to diminished urinary K+ excretion, lowered plasma K+ concentration and up-regulation of KCC1 transcripts in the TAL. For low-Mg(2+) diet, this upregulation was associated with increased transepithelial K+ reabsorption in the in vitro-perfused CTAL.

CONCLUSIONS

Our study provides evidence that the KCl cotransporter, which is functionally expressed in the TAL, plays an important role in transepithelial K+ reabsorption. Direct inhibition of this transporter by Ba(2+) and its indirect inhibition by amiloride lead to a strong transepithelial K+ secretion and diminished NaCl reabsorption in the TAL. Up-regulation of KCC1 mRNA by dietary Mg(2+) restriction is associated with an increased K+ reabsorption in the in vitro perfused CTAL.

K-Cl co-transport: immunocytochemical and functional evidence for more than one KCC isoform in high K and low K sheep erythrocytes

K-Cl co-transport (COT) is significantly higher in low K (LK), L-antigen (L) positive, than in high K (HK), M-antigen (M) positive, sheep red blood cells (SRBCs) and is inhibited by sheep allo-anti-L1 antibody. To answer the question of whether this difference in K-Cl co-transport activity resides at the level of the transporter or its regulation, a combined immunocytochemical and functional approach was taken. At least four isoforms of K-Cl COT encoded by different KCC genes are known, with 12 transmembrane domains and cytoplasmic C- and N-terminal domains (Ctd and Ntd, respectively). Polyclonal anti-rat (rt)KCC1 antibodies against a fusion peptide with 77 amino acids from the Ctd of rtKCC1 and anti-human (h)KCC3 against an 18-aa peptide from the Ntd of hKCC3, were prepared in rabbits (rb). Two distinctly separate protein bands of 180 and 145 kDa molecular mass were detected in hemoglobin-free ghosts from RBCs of two LK (one homozygous LL and one heterozygous LM) and one HK (homozygous MM) sheep by Western blots with rb anti-rtKCC1 and rb anti-hKCC3. Confocal microscopy showed specific immunostaining of KCC1 with rb anti-rtKCC1, and of KCC3 with rb anti-hKCC3, in white ghosts from both LK and HK SRBCs. To test the functional heterogeneity of K-Cl COT, the effect of the anti-L1 antibody was assessed on K-Cl COT activated by the kinase inhibitor staurosporine. Incubation of LK SRBCs with anti-L1 serum inhibited by 30% staurosporine-stimulated K-Cl COT suggesting that approximately two-thirds of the transport activity is independent of the L1 antigen. That staurosporine altered the L1 antigen/antibody reaction is unlikely since the action of another antibody, anti-Lp, stimulating the Na/K pump flux, was not modified. The present results, in conjunction with earlier work, lead to the hypothesis that the partial anti-L1 inhibition of K-Cl COT may be related to the molecular KCC dimorphism, seen in these cells with anti-KCC1 and anti-KCC3 antibodies.

K-Cl cotransport in vascular smooth muscle and erythrocytes: possible implication in vasodilation

K-Cl cotransport, the electroneutral-coupled movement of K and Cl ions, plays an important role in regulatory volume decrease. We recently reported that nitrite, a nitric oxide derivative possessing potent vasodilation properties, stimulates K-Cl cotransport in low-K sheep red blood cells (LK SRBCs). We hypothesized that activation of vascular smooth muscle (VSM) K-Cl cotransport by vasodilators decreases VSM tension. Here we tested this hypothesis by comparing the effects of commonly used vasodilators, hydralazine (HYZ), sodium nitroprusside, isosorbide mononitrate, and pentaerythritol, on K-Cl cotransport in LK SRBCs and in primary cultures of rat VSM cells (VSMCs) and of HYZ-induced K-Cl cotransport activation on relaxation of isolated porcine coronary rings. K-Cl cotransport was measured as the Cl-dependent K efflux or Rb influx in the presence and absence of inhibitors for other K/Rb transport pathways. All vasodilators activated K-Cl cotransport in LK SRBCs and HYZ in VSMCs, and this activation was inhibited by calyculin and genistein, two inhibitors of K-Cl cotransport. KT-5823, a specific inhibitor of protein kinase G, abolished the sodium nitroprusside-stimulated K-Cl cotransport in LK SRBCs, suggesting involvement of the cGMP pathway in K-Cl cotransport activation. Hydralazine, in a dose-dependent manner, and sodium nitroprusside relaxed (independently of the endothelium) precontracted arteries when only K-Cl cotransport was operating and other pathways for K/Rb transport, including the Ca-activated K channel, were inhibited. Our findings suggest that K-Cl cotransport may be involved in vasodilation.

H2O2 activates red blood cell K-Cl cotransport via stimulation of a phosphatase

K-Cl cotransport is involved in volume regulation in a number of cell types. Cell swelling stimulates K-Cl cotransport, probably by inhibition of a volume-sensitive kinase. K-Cl cotransport can also be activated by oxidants and thiol reagents. We investigated the effect of H2O2 on K-Cl cotransport of LK sheep red blood cells in an attempt to identify the target of oxidants. H2O2 stimulated K-Cl cotransport. The stimulation was virtually abolished by subsequent incubation with calyculin, a protein phosphatase inhibitor. This suggests that H2O2 stimulates a calyculin-sensitive phosphatase and activates K-Cl cotransport by causing a decrease in phosphorylation of the transporter or a regulatory protein. The thiol reagent N-ethylmaleimide, which stimulates K-Cl cotransport, did not stimulate cotransport further in cells with cotransport activated by staurosporine but did stimulate cotransport further in cells with cotransport activated by H2O2. These results suggest that there are at least two distinct phosphorylation sites on the transporter or a regulator. The results also suggest that the phosphatase is associated with the membrane.

Glutathione removal reveals kinases as common targets for K-Cl cotransport stimulation in sheep erythrocytes

K-Cl cotransport is activated by swelling, lowering of cellular free Mg (Mgi), and thiol modification of erythrocytes. Direct actions by thiol reagents on the K-Cl cotransport complex were separated from indirect effects through nonoxidative changes in cellular glutathione (GSH). We used 1-chloro-2,4-dinitrobenzene (CDNB), which, conjugated to GSH, is extruded from the erythrocyte as a thioether. CDNB caused a small biphasic effect (inhibition and stimulation) on K-Cl cotransport and, at 1 mM, abolished its stimulation by N-ethylmaleimide (NEM), diazenedicarboxylic acid bis[N,N-dimethylamide], methyl methanethiosulfonate, and staurosporine, a kinase inhibitor, independent of the order of treatment. Hence, NEM and other activating-thiol reagents, and perhaps GSH removal itself, target unidentified kinases involved in activation of K-Cl cotransport. CDNB also abrogated K-Cl cotransport stimulation by Mgi depletion independent of the order of treatment, indicating inhibition at a second site nearer to the transporter. Furthermore, CDNB treatment elevated and rendered K-Cl cotransport insensitive to osmotic shrinkage, suggesting uncoupling from the regulator.

Effects of urea on K-Cl cotransport in sheep red blood cells: evidence for two signals of swelling

The activation proceeds with a delay, like activation by swelling. Swelling of cells in urea activates K uptake further, but with no delay. Inactivation after removal of urea also proceeds without delay. With cotransport partially activated by reducing intracellular Mg concentration ([Mg]i) or with staurosporine, urea did not activate cotransport further. However, swelling activated cotransport further in these two types of cells. In terms of the three-state process for swelling-activation of K-Cl cotransport (P. B. Dunham, J. Klimczak, and P. J. Logue, J. Gen. Physiol. 101: 733-765, 1993), these results indicate that urea activates the first conversion, A-->B, and does so by inhibiting the reverse reaction promoted by a kinase, just as reducing [Mg]i does. Stimulation of cotransport by urea is nearly completely reversed by shrinkage, whereas activation by reducing [Mg]i is reversed only approximately 35%. Therefore urea inhibits the kinase indirectly, like swelling, by reducing macromolecular crowding of cytoplasmic proteins (A. P. Minton, G. C. Coleclasure, and J. C. Parker. Proc. Natl. Acad. Sci. USA 89: 10504-10506, 1992). Since swelling activates cotransport in two ways, one mimicked by urea and one not, there must be two signals of swelling, one a reduction of macromolecular crowding and the other probably a mechanical signal.

Effect of cell age and phenylhydrazine on the cation transport properties of rabbit erythrocytes

We studied the effect of cell age on the cation transport systems of rabbit erythrocytes by increasing the proportion of circulating young erythrocytes with either repeated bleeding or with phenylhydrazine (PHZ) treatment. We found that when the reticulocyte content of rabbit blood is increased by bleeding (from 1 to 40-50% of the circulating red cells), the response of the various transport pathways differs. The largest increase (fivefold) was found in the activity of K-Cl cotransport, which peaked 3 days after the last bleeding. The Na-K pump activity peaked at a similar time, but the % increase was twofold less than the K-Cl cotransport. There was a very small increase in the activity of the Na-Li exchange, whereas the Na-H exchange reached peak values 10 days after the last bleeding (twofold increase), when activities of K-Cl cotransport and Na-K pump had returned to almost normal levels. In vivo PHZ treatment resulted in anemia and marked reticulocytosis (80-90% of circulating cells). Transport rates were markedly increased (Na-K pump 9.6-fold, Na-H exchange 6.8-fold, Na-Li exchange 2.75-fold; K-Cl cotransport: 10-20-fold). When blood from PHZ-treated rabbits was incubated in vitro for 24-48 hours, red cell volume and K content decreased. This process was associated with a 70% reduction in the activity of the K-Cl cotransport after 24 hours and a 90% reduction after 48 hours. The activity of the other systems also declined and approached baseline values after 48 hours. Loss of transport activity was not affected by 10 microM E-64, whereas 10 mM methylamine reduced the inactivation of the Na-H exchange and of the Na-Li exchange. PHZ treatment of rabbit red cells in vitro resulted in marked increase of the K-Cl cotransport and inhibition of Na-K pump, Na-H exchange, and Na-Li exchange. These effects were abolished by DTT, with the exception of the Na-K pump inhibition, which was DTT insensitive. Thus both cell age and oxidative damage are important determinants of cation transport in rabbit red cells.

Erythrocyte K-Cl cotransport: properties and regulation

Erythrocytes possess a Cl-dependent, Na-independent K transport system cotransporting K and Cl in a 1:1 stoichiometry that is membrane potential independent. This K-Cl cotransporter is stimulated by cell swelling, acidification, Mg depletion, and thiol modification. Cell shrinkage, elevation of cellular divalent ions, thiol alkylation, phosphatase inhibitors, and derivatives of certain loop diuretics and stilbenes are inhibitory. Thus regulation of K-Cl cotransport at the membrane and cytoplasmic levels is highly complex. Basal K-Cl cotransport decreases with cellular maturation, whereas its modes of stimulation and inhibition are variable between species. The physiological inactivation appears to be prevented in low-K animal erythrocytes. In certain human hemoglobinopathies, K-Cl cotransport may be the cause of cellular dehydration and volume decrease. K-Cl cotransport occurs also in nonerythroid cells, such as in epithelial and liver cells of other species. At the threshold of molecular characterization, this comprehensive review places our present understanding of the mechanisms modulating K-Cl cotransport physiologically and pathophysiologically into kinetic and thermodynamic perspectives.

Several cation transporters and volume regulation in high-K dog red blood cells

Normal dog red blood cells lack the Na-K pump, and their cation composition is low K and high Na (LK). Recently, a dog was found with red blood cells containing high K and low Na concentrations (HK) due to the existence of the Na-K pump. In the present study, cation transport and volume regulation in HK cells were compared with those of LK cells. HK cells showed not only Rb influx through a Na-K pump, but also Rb influx through a Cl-dependent K transporter. The Rb influx rate through the Na-K pump was 0.65-1.44 mmol.l cells-1.h-1 in Cl and 1.75-2.24 mmol.l cells-1.h-1 in NO3, in HK cells, but only trace activities are found in LK cells. In HK cells, the Rb influx rate through Cl-dependent K transport was 0.36-0.96 mmol.l cells-1.h-1, and it was enhanced in swollen cells but vanished in shrunken cells. In LK cells, the transport was evident only in swollen cells. The original volume of swollen HK cells was restored by water extrusion promoted by Cl-dependent transport. The Na-Ca exchange transporter, which works as a volume regulator in LK cells, functioned in HK cells only when they were loaded with Na. Hence, the exchange transporter is latent in HK cells under physiological conditions. Moreover, the exchange transporter could restore the cell volume in swollen and Na-loaded HK cells. However, the volume in HK cells was still larger than that in LK cells, while the Na-Ca exchange transporter was working.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics of Cl-dependent K fluxes in hyposmotically swollen low K sheep erythrocytes

A detailed kinetic study of K:Cl cotransport in hyposmotically swollen low K sheep red blood cells was carried out to characterize the nature of the outwardly poised carrier. The kinetic parameters were determined from the rate of K efflux and influx under zero-K-trans conditions in red cells with cellular K altered by the nystatin method and with different extracellular K or Rb concentrations. Although apparent affinities for efflux and influx were quite similar, the maximal velocity for K efflux was approximately two times greater than for influx. Furthermore, at thermodynamic equilibrium (i.e., when the ion product of K and Cl within the cell was equal to that outside) a temperature-dependent net K efflux was observed, approaching zero only when the external product reached approximately two times the internal product. The binding order of the ions to the transporter was asymmetric, being ordered outside (Cl binding first, followed by K) and random inside. K efflux but not influx was trans-inhibited by KCl. Trans inhibition of K efflux was used to verify the order of binding outside: trans inhibition by external Cl occurred in the absence of external K, but not vice versa. Thus K:Cl cotransport is kinetically asymmetric in hyposmotically swollen low K sheep red cells.

Low potassium-type but not high potassium-type sheep red blood cells show passive K+ transport induced by low ionic strength

Low potassium-type (LK) sheep red blood cells show a significant increase of the residual (i.e., ouabain-insensitive) K+ influx when the ionic strength of the solution is decreased. This effect is absent from high potassium-type (HK) sheep red blood cells. The KCl cotransport system is not involved since three different manoeuvres to suppress the KCl cotransport (replacement of Cl- by NO3-, volume-decrease, inhibition by anti-L1 antibodies) have no effect on the low ionic strength-stimulated K+ influx.

Differential effect of pH on the density and volume of rat reticulocytes and erythrocytes. Relevance to their fractionation by centrifugation

Rats were injected with 59Fe-ferrous citrate and bled thereafter at different times (16 h to 49 d). This gave rise to red cell populations in which cells corresponding in age to the time elapsed between injection and bleeding were labeled. The anticoagulant used was either acid-citrate-dextrose (ACD) with a pH adjusted to 7.3 or ACD (pH 5.1). Final pH of the collected blood was about 7.2-7.4 in the former case and 6.4-6.7 in the latter. Red cells were then centrifuged (5) and approximately 7-10% of the packed cells from the top and 7-10% from the bottom of the cell column collected. When reticulocytes are the predominant labeled red cell population, as in blood obtained for about 24 h after isotope injection, a fractionation of these cells and mature erythrocytes is in evidence only when blood is collected at the higher pH. Thus, at pH 7.2-7.4 ratios of specific radioactivities of cells in top fraction/cells in an unfractionated sample are about 3, whereas at pH 6.4-6.7, the analogous ratios are 1 or less. These differences in specific activity ratios, as a function of pH at collection, virtually disappear after about 4 d following isotope injection. The lower pH is known to increase the volume and decrease the density of mature red blood cells. The marked effect of pH on cellular fractionation could be correlated with the smaller change in rat reticulocyte density and volume in acid medium. At pH 6.4-6.7, the densities of mature erythrocytes and reticulocytes are so close that their physical separation by centrifugation is not feasible.

Activation of a Cl-dependent K flux by cAMP in pig red cells

Activation of a Cl-dependent K flux by adenosine 3',5'-cyclic monophosphate (cAMP) was characterized in pig red cells, a cell type that lacks both the Ca-activated K channel and the Na-K-Cl cotransport pathway. As in other red cells, both Cl-dependent K efflux and K influx are stimulated on cell swelling. Although pig red cells fail to respond to beta-adrenergic stimuli, it is possible to raise the intracellular cAMP content by preincubating cells in the presence of 1 mM cAMP. The Cl-dependent K flux was compared in cells having a basal cAMP content of approximately 0.29 nmol/g hemoglobin vs. cAMP-loaded cells having approximately 8.4 nmol cAMP/g hemoglobin. Loading with cAMP stimulated both Cl-dependent K efflux and influx of hypotonically swollen cells. In maximally swollen cells whose volume was increased by approximately 17%, the Cl-dependent Rb influx occurs with a maximum velocity (Vmax) of 17.9 +/- 3.2 mumol.g hemoglobin (Hb)-1.h-1 and Km for Rb of 22.9 +/- 4.1 mM. In cAMP-loaded cells, both Vmax and Km were increased to 59.8 +/- 8.5 mumol.g Hb-1.h-1 and 63.1 +/- 8.8 mM, respectively. The Cl-dependent Rb influx is much larger in young cells than in old cells. However, both cell types respond to cAMP activation. Whereas cAMP and its analogues, 8-bromoadenosine 3',5'-cyclic monophosphate and dibutyryl adenosine 3',5'-cyclic monophosphate are stimulatory, AMP and guanosine 3',5'-cyclic monophosphate (cGMP) are not. These findings suggest that, like other ion transport systems, the Cl-dependent K flux of pig red cells is endowed with the capacity to respond to cAMP.

Volume and anion dependency of ouabain-resistant K-Rb fluxes in sheep red blood cells

The effect of six different anions on the volume response of ouabain-resistant K transport was systematically studied at extracellular pH (pHo) = 7.4 in sheep red blood cells of both low and high K genotype before and after treatment with the sulfhydryl (SH) reagent N-ethylmaleimide (NEM). In methanesulfonate (CH3SO3), both the apparent Rb permeability (P(app)Rb), calculated from ouabain-resistant Rb influx), and K permeability (PK, calculated from the rate constants of ouabain-resistant zero-trans K efflux, 0k(OR)K) were volume independent and close to 10(-10) cm/s for both cell types, but in Cl, Br, I, SCN, and NO3 they were significantly different in low and high K cells with altered cell volumes. Thus, in 15% osmotically shrunken low K cells, P(app)Rb) and PK were similar regardless of the anions present, but upon 10-15% swelling, they increased to approximately 4-6 X 10(-9) cm/s in Br and 2 X 10(-9) cm/s in Cl and also increased with comparatively small increments in I, SCN, and NO3. Treatment with NEM enhanced both P(app)Rb) and PK, particularly in shrunken low K cells, to approximately 10(-8) cm/s in Br and Cl but not in I, SCN, and NO3. In shrunken or isotonic high K cells, P(app)Rb) and PK were close to 10(-10) cm/s in all anions except for SCN. Swelling and/or NEM increased PK and P(app)Rb) in Cl and Br only two- to threefold.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of K+ transport in resealed human erythrocyte ghosts

We report here our studies on K+ transport in resealed human red cell ghosts (RG) in the presence of 0.1 mM ouabain and 0.01 mM bumetanide, inhibitors of the Na+-K+ pump and Na+-K+-Cl- cotransport, respectively. RG were obtained with the gel-filtration method. K+ efflux from RG was dependent on the pH used in the lysis buffer and increased when the pH used in the lysis buffer and increased when the pH was raised from 5.5 to 8.0. As in intact red cells, RG made from cells of the least dense fraction had a much higher K+ efflux than RG made from cells of the densest fraction. This K+ flux is volume independent and increases when the pH of the flux medium is increased from 6.0 to 8.0. K+ efflux (60-70%) at pH 7.40 from RG made from cells of the least dense fraction is inhibited when Cl- is substituted by nitrate or when the ghosts are resealed in the absence of ATP. This chloride- and ATP-dependent component is markedly reduced in RG made from cells of the densest fraction. An increase in the internal Mg2+ concentration in RG from the least dense fraction induced marked inhibition of K+ efflux. Contrary to intact cells, N-ethylmaleimide (NEM) did not affect K+ efflux from RG. Thus the effects of pH, osmolarity, and NEM on K+ transport in RG are markedly different from those reported in intact erythrocytes.

Kinetic comparison of ouabain-resistant K:Cl fluxes (K:Cl [Co]-transport) stimulated in sheep erythrocytes by membrane thiol oxidation and alkylation

The stimulatory effects of two thiol (SH) group oxidants, methylmethane thiosulfonate (MMTS) and diazene dicarboxylic acid bis [N,N-dimethylamide] (diamide), on the kinetics of ouabain-resistant (OR) K:Cl [co]-transport in low K (LK) sheep red blood cells were compared with the effects of alkylating agents, notably N-ethylmaleimide (NEM). At low concentrations, both MMTS and diamide stimulated K:Cl [co]-transport, and with a latency period, as measured by OR zero-trans K efflux and OR uptake of external Rb, Rbo, as K congener in Cl and NO3 media. At high concentrations the effect of diamide saturated, and that of MMTS disappeared. The stimulatory effect of MMTS was partially reversed by the reducing agent dithiothreitol (DTT) known to fully restore the diamide-activated K flux (Lauf, J. Memb. Biol. 101:179-188, 1988). In diamide preequilibrated LK sheep red cells, the Km of K:Cl [co]-transport for external Cl, Clo, was 84.3 mM, and 18.7 mM for Rbo, with nearly identical Vmax values around 4 mmol Rb/L cells x h for K (Rb) fluxes in Cl and after correction for the small Cl-independent component. Zero net K (Rb) flux existed at Kc (cell K)/Rbo concentration ratios, [K]c/[Rb]c, of 0.8 i.e. when the electrochemical driving forces across the membrane were about equal. The measured K efflux/Rb influx ratios were almost twice those predicted from [K]c/[Rb]o and the Cl equilibrium potential suggesting that the diamide-stimulated K (Rb) flux may occur through non-diffusional, carrier-mediated transport.(ABSTRACT TRUNCATED AT 250 WORDS)

Human red cell volume regulation in hypotonic media

1. There is substantial evidence for a volume-sensitive KCl cotransport system in young human RBC. 2. The KCl cotransport system becomes latent on cell maturation. 3. There is a correlation between the activation of the KCl cotransporter by either pressure, NEM, ghosting or in certain anemias with disc/cup cell shape change. 4. The stretch-activated KCl transporter may be coupled to some component of the cell cytoskeleton.

Effect of N-ethylmaleimide on K transport in density-separated human red blood cells

N-ethylmaleimide (NEM) is a sulfhydryl-reacting agent known to stimulate chloride-dependent K transport in a variety of red cells. In high K sheep red cells, NEM-induced K movements are greater in magnitude in young cells compared with old cells. We hypothesized that human red cells might respond to NEM like high K sheep red cells. To test this idea, cells of various age were exposed to 0.5 mM NEM. We found that, after a 4-h incubation, young cells lost 50% of cell K, compared with 10% K loss in older cells. K loss in all fractions was inhibited by chloride replacement or furosemide.

Volume-stimulated, Cl(-)-dependent K+ efflux is highly expressed in young human red cells containing normal hemoglobin or HbS

We report here that a Cl(-)-dependent K+ (K:Cl) efflux, which is stimulated by N-ethylmaleimide (NEM) and by increased red cell volume, exists in young red cells of individuals with normal hemoglobin A (AA) and in those homozygous for hemoglobin S (SS). We have investigated this K:Cl efflux in several density-defined red cell fractions obtained from Percoll-Stractan continuous density gradients. We found high activity of the NEM-stimulated K:Cl transport in reticulocytes and young red cells from nine sickle cell (SS) patients (43 +/- 27 mean +/- SD mmol K+/liter of cells/hr = flux units (FU)) and in the young cell fraction of three AA individuals with high reticulocytosis recuperating from nutritional anemias (41.7 +/- 10 FU). In addition, we observed significant interindividual variation of this K:Cl efflux in the discocyte fraction of SS blood. Cell swelling markedly stimulated the K:Cl efflux, in SS whole blood (9.8 +/- 7.4 FU, in SS young cells (13 +/- 13 FU), and in AA young cells (21.4 +/- 11 FU). The activity of the Na-K-Cl cotransport, as estimated by the bumetanide sensitive K+ efflux was not found to be cell-age dependent in either AA or SS cells. Measurements of red cell density by isopycnic gradients indicated that 27% of the young cells reduce their volume by a Cl(-)-dependent process in hypotonic or low pH-induced swelling. The large volume-stimulated K:Cl efflux in AA young cells raises the possibility that these fluxes may be involved in the maturation of erythropoietic precursors.(ABSTRACT TRUNCATED AT 250 WORDS)

Direct evidence for chloride-dependent volume reduction in macrocytic sheep reticulocytes

Cytometric analysis of the volume-distribution of macrocytic reticulocytes from 6-8 days acutely anemic sheep of both high and low potassium erythrocyte type revealed hyposmotically induced cell volume reduction in K-free NaCl but not in Na-methane sulfonate (CH3SO3Na) media. Furthermore N-ethylmaleimide, known to stimulate K:Cl efflux in these cells, and low extracellular pH caused cell shrinkage in isosmotic NaCl but not in CH3SO3Na. These data suggest that cell volume reduction, physiologically occurring during reticulocyte maturation, is a Cl-dependent process most likely involving electro-neutral K:Cl transport known to exist in reticulocytes of both sheep cation genotypes.

Cell volume, K transport, and cell density in human erythrocytes

We report here studies on the regulation of cell volume and K transport in human erythrocytes separated according to density. When cell volume was increased (isosmotic swelling, nystatin technique), erythrocytes of the least dense but not of the densest fraction shrunk back toward their original volume. This process was due to a ouabain (0.1 mM) and bumetanide (0.01 mM) (OB)-resistant K loss. OB-resistant K+ efflux from the least dense fraction was stimulated by hypotonic swelling and had a bell-shaped dependence on pH (pH optimum 6.75-7.0). These pH and volume effects were not evident in the densest fraction. The swelling-induced K+ efflux from the least dense fraction was inhibited when chloride was substituted by nitrate, thiocyanate, and acetate, whereas it was stimulated by bromide. Increasing cell Mg2+ content also markedly inhibited K+ efflux from isosmotically swollen cells. N-ethylmaleimide (NEM, 1 mM) greatly increased OB-resistant K+ efflux from the least dense fraction but not from the densest fraction. These data reveal the presence, in the lease dense fraction of normal human erythrocytes, of a pathway for K+ transport that is dependent on volume, pH, and chloride, is inhibited by internal Mg2+, and possibly plays a role in determining the erythrocyte water and cation content.

Swelling, NEM, and A23187 activate Cl(-)-dependent K+ transport in high-K+ sheep red cells

In low K+ (LK) sheep red cells a significant fraction of the total ouabain-resistant (OR) K+ flux is inhibited when Cl- is replaced by other anions of the Hofmeister series except Br- (Cl(-)-dependent K+ flux). In contrast, high K+ (HK) sheep red cells in isosmotic media did not possess any significant OR Cl(-)-dependent K+ flux when Cl- was replaced by NO3- or I-. However, exposure to hyposmotic solutions, treatment with the sulfhydryl (SH) group reagent N-ethylmaleimide (NEM) or with the bivalent metal ion (Me2+) ionophore A23187 in absence of external Me2+ caused a significant activation of Cl(-)-dependent K+ transport as measured with Rb+ as K+ congener. There was no Cl(-)-dependent Rb+ flux in A23187-treated cells when Mn2+, Mg2+, and Ca2+ were present at 1 mM concentrations, suggesting that cellular accumulation of these Me2+ is inhibitory. Similar to LK red cells, HK red cells failed to respond to A23187 when pretreated with NEM supporting the hypothesis proposed recently (Lauf, P. K. J. Membr. Biol. 88: 1-13, 1985) of a common mechanism of Cl(-)-dependent K+ transport activation. The magnitudes of the Cl(-)-dependent Rb+ fluxes in HK cells were much smaller than those elicited by identical treatments in LK red cells, and the effect of all interventions was not due to the presence of reticulocytes known to possess Cl(-)-dependent K+ transport.(ABSTRACT TRUNCATED AT 250 WORDS)

Anion transport systems in the plasma membrane of vertebrate cells

In the case of the red blood cell, anion transport is a highly specific one-for-one exchange catalyzed by a major membrane protein known as band 3 or as capnophorin. This red cell anion-exchange system mediates the Cl-(-)HCO3- exchange responsible for most of the bicarbonate transport capacity of the blood. The rapidly expanding knowledge of the molecular biology and the transport kinetics of this specialized transport system is very briefly reviewed in Section III. Exchange diffusion mechanisms for anions are found in many cells other than erythrocytes. The exchange diffusion system in Ehrlich cells has several similarities to that in red cells. In several cell types (subsection IV-B), there is evidence that intracellular pH regulation depends on Cl-(-)HCO3- exchange processes. Anion exchange in other single cells is described in Section IV, and its role in pH regulation is described in Section VII. Anion exchange mechanism operating in parallel with, and only functionally linked to Na+-H+ or K+-H+ exchange mechanisms can also play a role in cell volume regulation as described in Section VII. In the Ehrlich ascites cell and other vertebrate cells, electroneutral anion transfer has been found to occur also by a cotransport system for cations and chloride operating in parallel with the exchange diffusion system. The cotransport system is capable of mediating secondary active chloride influx. In avian red cells, the cotransport system has been shown to be activated by adrenergic agonists and by cyclic AMP, suggesting that the cotransport is involved in regulatory processes (see subsection V-A.). In several cell types, cotransport systems are activated and play a role during volume regulation, as described in Section V and in Section VII. It is also likely that this secondary active cotransport of chloride plays a significant role for the apparently active extrusion of acid equivalents from certain cells. If a continuous influx of chloride against an electrochemical gradient is maintained by a cotransport system, the chloride disequilibrium can drive an influx of bicarbonate through the anion exchange mechanism, as described in Section VII. Finally, even the electrodiffusion of anions is shown to be regulated, and in Ehrlich cells and human lymphocytes an activation of the anion diffusion pathway plays a major role in cell volume regulation as described in Section VI and subsection VII-B.(ABSTRACT TRUNCATED AT 250 WORDS)

Volume-dependent and NEM-stimulated K+,Cl- transport is elevated in oxygenated SS, SC and CC human red cells

Mechanisms involved in cell volume regulation are important in SS, SC cells as they might be involved in determining the extent of sickling and the generation of dense cells and irreversibly sickled cells. We have studied in these cells the response to cell swelling of the K+,Cl- transporter. We found that Hb SS, SC and CC red cells have higher values of a ouabain-resistant, chloride-dependent and NEM-stimulated K+ efflux than AA red cells. In contrast, the Na+,K+,Cl- cotransport estimated from the bumetanide-sensitive component of K+ efflux was not significantly different in SS, SC and CC red cells. The (ouabain + bumetanide)-resistant K+ efflux from SS, SC and CC red cells was stimulated by cell swelling induced by reduction of the osmotic pressure (300 to 220 mosmol/l) and pH (8 to 7) of the flux media (140 mM NaCl). The Cl--dependent K+ efflux stimulated by osmotic swelling highly correlated with the NEM-stimulated component (r = 0.8, p less than 0.001, n = 22) and the acid-pH-induced swelling (r = 0.969, p less than 0.001, n = 22), indicating that it is driven by the K+,Cl- transporter.

Effects of high hydrostatic pressure on 'passive' monovalent cation transport in human red cells

The effects of high hydrostatic pressure (up to 400 ATA) on the 'passive' (defined as ouabain + bumetanide + EGTA-insensitive) influx and efflux of radiotracer cations (K+ Rb+, Na+, Cs+) has been studied in human red cells suspended at different medium tonicities giving altered cell volumes. Under all conditions studied, cation permeability was raised at pressure, and at least two distinct components were found to comprise this flux. Thus, increasing pressure caused a generalized increase in cation permeability which was unaffected by the anion present, demonstrated linear concentration dependence, and was reduced with cell swelling, and stimulated a specific KCl pathway which was Cl- dependent, demonstrated saturation kinetics with raised [K]0 and was increased with cell swelling. High hydrostatic pressure caused a significant alteration to red cell morphology from the normal biconcave disc to cup-shaped forms and it is proposed that this is associated with the unmasking of the volume-sensitive KCl system.

Pig reticulocytes. V. Development of Rb+ influx during in vitro maturation

Influx of the K+ analogue Rb+ was measured through the ouabain-sensitive Na+/K+ pump and the ouabain-insensitive "leak" pathways in Cl- or NO3- in mature red cells from adult pigs and in reticulocytes naturally occurring in 7-day-old piglets. In reticulocytes, Rb+ influxes by the two pathways were of about equal magnitude in Cl- (13 and 10 mmoles/liter cells X hr) and at least 25-fold larger than in mature red cells (0.5 and 0.4 mmoles/liter cells X hr). In Na+ media, a portion of the ouabain-insensitive "leak" flux of Rb+ was Cl(-)-dependent (Rb+Cl- transport) as NO3- replacement reduced Rb+ influx by 90% in reticulocytes and by 40% in mature red cells. The sulfhydryl reagent N-ethylmaleimide (NEM) stimulated Rb+Cl- transport about twofold in reticulocytes and up to 13-fold in mature red cells. When reticulocytes matured to erythrocytes during in vitro incubation, about 90% of both ouabain-sensitive Rb+ pump and ouabain-insensitive Rb+Cl- influx were lost. In contrast, the NEM-stimulated Rb+Cl- transport changed much less throughout this period, suggesting an entity operationally but not necessarily structurally distinct from the basal Rb+Cl- transport. Although the experimental variability precluded a full assessment of significant changes in the small Na+/K+ (Rb+) pump and Rb+Cl- fluxes in mature pig red cells kept for the same time period in vitro, Rb+ flux changes in reticulocytes appear to be maturational in nature, reflecting parallel activity transitions of Na+/K+ pump and Cl(-)-dependent K+ fluxes in vivo.

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 blood cells. V. Dependence on metabolism

Measurement of ouabain-insensitive K+ efflux and Rb+ influx in low-K+ sheep red blood cells with artificially altered cellular ATP levels revealed that Cl--dependent K+ transport was activated by N-ethylmaleimide (NEM) in cells with ATP concentrations above 0.5 mM. Depletion of ATP by starvation at 37 degrees C either in glucose-free media for 16 h or in 2-deoxy-D-glucose-containing media for 4 h completely abolished the response of K+-Cl-transport to NEM but did not reduce the basal Cl--dependent K+ flux. On repletion of cellular ATP by a second incubation in media containing glucose, inosine, and inorganic phosphate, the NEM-stimulated K+ (Rb+) flux reappeared. The magnitude of flux reactivation varied directly and monotonically with the ATP level. The data constitute the first unequivocal evidence for a reversible ATP dependence of thiol groups functionally involved in the thiol-dependent K+-Cl- transporter, suggesting that thiol-dependent and volume-sensitive K+-Cl- transport systems are operationally different.

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.