Furosemide-sensitive K+ (Rb+) transport in human erythrocytes: modes of operation, dependence on extracellular and intracellular Na+, kinetics, pH dependency and the effect of cell volume and N-ethylmaleimide

Is post-hypertonic lysis of human red blood cells caused by excessive cell volume regulation?

Human red blood cells (RBC) exposed to hypertonic media are subject to post-hypertonic lysis - an injury that only develops during resuspension to an isotonic medium. The nature of post-hypertonic lysis was previously hypothesized to be osmotic when cation leaks were observed, and salt loading was suggested as a cause of the cell swelling upon resuspension in an isotonic medium. However, it was problematic to account for the salt loading since the plasma membrane of human RBCs was considered impermeable to cations. In this study, the hypertonicity-related behavior of human RBCs is revisited within the framework of modern cell physiology, considering current knowledge on membrane ion transport mechanisms - an account still missing. It is recognized here that the hypertonic behavior of human RBCs is consistent with the acute regulatory volume increase (RVI) response - a healthy physiological reaction initiated by cells to regulate their volume by salt accumulation. It is shown by reviewing the published studies that human RBCs can increase cation conductance considerably by activating cell volume-regulated ion transport pathways inactive under normal isotonic conditions and thus facilitate salt loading. A simplified physiological model accounting for transmembrane ion fluxes and membrane voltage predicts the isotonic cell swelling associated with increased cation conductance, eventually reaching hemolytic volume. The proposed involvement of cell volume regulation mechanisms shows the potential to explain the complex nature of the osmotic response of human RBCs and other cells. Cryobiological implications, including mechanisms of cryoprotection, are discussed.

Stable potassium isotopes (41K/39K) track transcellular and paracellular potassium transport in biological systems

As the most abundant cation in archaeal, bacterial, and eukaryotic cells, potassium (K⁺) is an essential element for life. While much is known about the machinery of transcellular and paracellular K transport–channels, pumps, co-transporters, and tight-junction proteins—many quantitative aspects of K homeostasis in biological systems remain poorly constrained. Here we present measurements of the stable isotope ratios of potassium (⁴¹K/³⁹K) in three biological systems (algae, fish, and mammals). When considered in the context of our current understanding of plausible mechanisms of K isotope fractionation and K⁺ transport in these biological systems, our results provide evidence that the fractionation of K isotopes depends on transport pathway and transmembrane transport machinery. Specifically, we find that passive transport of K⁺ down its electrochemical potential through channels and pores in tight-junctions at favors ³⁹K, a result which we attribute to a kinetic isotope effect associated with dehydration and/or size selectivity at the channel/pore entrance. In contrast, we find that transport of K⁺ against its electrochemical gradient via pumps and co-transporters is associated with less/no isotopic fractionation, a result that we attribute to small equilibrium isotope effects that are expressed in pumps/co-transporters due to their slower turnover rate and the relatively long residence time of K⁺ in the ion pocket. These results indicate that stable K isotopes may be able to provide quantitative constraints on transporter-specific K⁺ fluxes (e.g., the fraction of K efflux from a tissue by channels vs. co-transporters) and how these fluxes change in different physiological states. In addition, precise determination of K isotope effects associated with K⁺ transport via channels, pumps, and co-transporters may provide unique constraints on the mechanisms of K transport that could be tested with steered molecular dynamic simulations.

Cation transport by the neuronal K(+)-Cl(-) cotransporter KCC2: thermodynamics and kinetics of alternate transport modes

Both Cs(+) and NH(4)(+) alter neuronal Cl(-) homeostasis, yet the mechanisms have not been clearly elucidated. We hypothesized that these two cations altered the operation of the neuronal K(+)-Cl(-) cotransporter (KCC2). Using exogenously expressed KCC2 protein, we first examined the interaction of cations at the transport site of KCC2 by monitoring furosemide-sensitive (86)Rb(+) influx as a function of external Rb(+) concentration at different fixed external cation concentrations (Na(+), Li(+), K(+), Cs(+), and NH(4)(+)). Neither Na(+) nor Li(+) affected furosemide-sensitive (86)Rb(+) influx, indicating their inability to interact at the cation translocation site of KCC2. As expected for an enzyme that accepts Rb(+) and K(+) as alternate substrates, K(+) was a competitive inhibitor of Rb(+) transport by KCC2. Like K(+), both Cs(+) and NH(4)(+) behaved as competitive inhibitors of Rb(+) transport by KCC2, indicating their potential as transport substrates. Using ion chromatography to measure unidirectional Rb(+) and Cs(+) influxes, we determined that although KCC2 was capable of transporting Cs(+), it did so with a lower apparent affinity and maximal velocity compared with Rb(+). To assess NH(4)(+) transport by KCC2, we monitored intracellular pH (pH(i)) with a pH-sensitive fluorescent dye after an NH(4)(+)-induced alkaline load. Cells expressing KCC2 protein recovered pH(i) much more rapidly than untransfected cells, indicating that KCC2 can mediate net NH(4)(+) uptake. Consistent with KCC2-mediated NH(4)(+) transport, pH(i) recovery in KCC2-expressing cells could be inhibited by furosemide (200 microM) or removal of external [Cl(-)]. Thermodynamic and kinetic considerations of KCC2 operating in alternate transport modes can explain altered neuronal Cl(-) homeostasis in the presence of Cs(+) and NH(4)(+).

Exercise-induced changes in plasma composition increase erythrocyte Na+,K+-ATPase, but not Na+-K+-2Cl- cotransporter, activity to stimulate net and unidirectional K+ transport in humans

We tested the hypothesis that exercise-induced changes in plasma composition result in peak stimulation of erythrocyte unidirectional K+ (JK,in) and net K+ (JK,net) transport within the first 120 s. In experimental series 1 (7 men; 2 women), plasma [K+] was continuously measured in vitro (37 degrees C) after the addition of red blood cells (RBCs) obtained from rested subjects (resting RBCs) into an exercise-simulated plasma (ESP; increased plasma osmolality, [K+], [H+], [lactate] and [adrenaline] (epinephrine)), and JK,net calculated. In experimental series 2 (7 men; 4 women), resting RBCs were incubated in true exercise plasma (TEP) obtained after two 30 s bouts of high intensity leg cycling exercise to determine JK,net and JK,in (via RBC 86Rb accumulation). JK,net of resting RBCs increased from 0.9 +/- 28.7 in resting plasma to 285 +/- 164 mmol (l RBCs)-1 h-1 in ESP and to 178 +/- 60 mmol (l RBCs)-1 h-1 after 10 s in TEP. Both JK,net and JK,in peaked within 10 s of incubation and decreased rapidly during the initial 120 s. The use of inhibitors for the Na+,K+-ATPase (ouabain) and the Na+-K+-2Cl- cotransporter (NKCC; bumetanide) indicated that rapid increases in JK,in and JK,net upon incubation of resting RBCs in TEP were due primarily to increased Na+,K+-ATPase activity; the NKCC appeared to be involved only when the Na+,K+-ATPase was blocked. It is concluded that RBCs rapidly increase JK,in and JK,net in response to exercise-induced changes in plasma composition.

Mechanism generating endocochlear potential: role played by intermediate cells in stria vascularis

The endocochlear DC potential (EP) is generated by the stria vascularis, and essential for the normal function of hair cells. Intermediate cells are melanocytes in the stria vascularis. To examine the contribution of the membrane potential of intermediate cells (E(m)) to the EP, a comparison was made between the effects of K(+) channel blockers on the E(m) and those on the EP. The E(m) of dissociated guinea pig intermediate cells was measured in the zero-current clamp mode of the whole-cell patch clamp configuration. The E(m) changed by 55.1 mV per 10-fold changes in extracellular K(+) concentration. Ba(2+), Cs(+), and quinine depressed the E(m) in a dose-dependent manner, whereas tetraethylammonium at 30 mM and 4-aminopyridine at 10 mM had no effect. The reduction of the E(m) by Ba(2+) and Cs(+) was enhanced by lowering the extracellular K(+) concentration from 3.6 mM to 1.2 mM. To examine the effect of the K(+) channel blockers on the EP, the EP of guinea pigs was maintained by vascular perfusion, and K(+) channel blockers were administered to the artificial blood. Ba(2+), Cs(+) and quinine depressed the EP in a dose-dependent manner, whereas tetraethylammonium at 30 mM and 4-aminopyridine at 10 mM did not change the EP. A 10-fold increase in the K(+) concentration in the artificial blood caused a minor decrease in the EP of only 10.6 mV. The changes in the EP were similar to those seen in the E(m) obtained at the lower extracellular K(+) concentration of 1.2 mM. On the basis of these results, we propose that the EP is critically dependent on the voltage jump across the plasma membrane of intermediate cells, and that K(+) concentration in the intercellular space in the stria vascularis may be actively controlled at a concentration lower than the plasma level.

Cation transport and cell volume changes in maturing rat reticulocytes

During maturation, reticulocytes lose membrane material, including transporters, and this is accompanied by a loss of cell water and volume. Here we determined a possible role of ion transport in adjusting cell volume during maturation. Reticulocytes and red blood cells of different ages were prepared from erythropoietin-treated rats by density gradient fractionation. Cell volume and ion transport were measured in freshly prepared cells and in reticulocytes during in vitro maturation. Reticulocytes had an increased K content and cell volume, whereas intracellular Na was decreased. All parameters approached whole blood values after 2 days in culture. Na-K pump was elevated in reticulocytes and decreased during maturation. Na-K-2Cl cotransport (NKCC) activity was lower in reticulocytes and was activated 8- and 20-fold by shrinkage and okadaic acid, respectively, whereas stimulation was barely detectable in high-buoyant density red blood cells. The ouabain- and bumetanide-insensitive Na flux in reticulocytes decreased on maturation. Most of it was inhibited by amiloride, indicating the presence of Na/proton exchange. Our results show that, although the Na-K-pump activity in reticulocytes is very much increased, the enhanced capacity of NKCC is essentially cryptic until stimulated. Both types of capacities (activities) decrease during maturation, indicating a possible loss of transport protein. The decrease was constrained to the period of reticulocyte maturation. Loss of transport capacity appears to exceed the loss of membrane surface area. Reticulocyte age-related changes in the net electrochemical driving force indicate that the increasing NKCC activity might contribute to the reduction in cell water.

Sodium-potassium-chloride cotransport

Obligatory, coupled cotransport of Na(+), K(+), and Cl(-) by cell membranes has been reported in nearly every animal cell type. This review examines the current status of our knowledge about this ion transport mechanism. Two isoforms of the Na(+)-K(+)-Cl(-) cotransporter (NKCC) protein (approximately 120-130 kDa, unglycosylated) are currently known. One isoform (NKCC2) has at least three alternatively spliced variants and is found exclusively in the kidney. The other (NKCC1) is found in nearly all cell types. The NKCC maintains intracellular Cl(-) concentration ([Cl(-)](i)) at levels above the predicted electrochemical equilibrium. The high [Cl(-)](i) is used by epithelial tissues to promote net salt transport and by neural cells to set synaptic potentials; its function in other cells is unknown. There is substantial evidence in some cells that the NKCC functions to offset osmotically induced cell shrinkage by mediating the net influx of osmotically active ions. Whether it serves to maintain cell volume under euvolemic conditons is less clear. The NKCC may play an important role in the cell cycle. Evidence that each cotransport cycle of the NKCC is electrically silent is discussed along with evidence for the electrically neutral stoichiometries of 1 Na(+):1 K(+):2 Cl- (for most cells) and 2 Na(+):1 K(+):3 Cl(-) (in squid axon). Evidence that the absolute dependence on ATP of the NKCC is the result of regulatory phosphorylation/dephosphorylation mechanisms is decribed. Interestingly, the presumed protein kinase(s) responsible has not been identified. An unusual form of NKCC regulation is by [Cl(-)](i). [Cl(-)](i) in the physiological range and above strongly inhibits the NKCC. This effect may be mediated by a decrease of protein phosphorylation. Although the NKCC has been studied for approximately 20 years, we are only beginning to frame the broad outlines of the structure, function, and regulation of this ubiquitous ion transport mechanism.

Exercise-induced stimulation of K(+) transport in human erythrocytes

The hypothesis was tested that exercise-induced changes in plasma composition stimulate unidirectional K(+) transport (J(in)K) in human red blood cells (RBCs). Ten men performed two 30-s high-intensity leg-cycling tests separated by 4 min of rest. Antecubital venous blood was sampled before exercise and at the end of the second exercise bout. RBCs were separated from true exercise plasma, (42)K was added to plasma, and RBC K(+) transport was studied in vitro at 37 degrees C. In the second part of the study, blood from nine healthy men studied in vitro at 37 degrees C was used to test the hypothesis that exercise-simulated (ES) plasma stimulates net K(+) transport and J(in)K (measured using (86)Rb) in human RBCs. The J(in)K of resting RBCs added to true exercise plasma was 1,574 +/- 200 (SE) micromol. h(-1). l(-1) vs. 1,236 +/- 256 micromol. h(-1). l(-1) in true resting plasma at 2 min (controls). In true exercise and ES plasma, J(in)K was increased through activation of the ouabain-sensitive Na(+)-K(+) pump and the bumetanide-sensitive Na(+)-K(+)-2Cl(-) cotransporter. Increases in plasma osmolality and K(+), H(+), and epinephrine concentrations independently and in combination stimulated K(+) transport into human RBCs. In a third series of experiments, in which ES plasma K(+) concentration was continuously measured during the first 5 min of incubation of RBCs, a 1.6 +/- 0.3 mmol/l decrease in plasma K(+) concentration occurred during the first 2 min. It is concluded that RBCs transport K(+) at elevated rates in response to exercise-induced changes in plasma composition.

Activation of Na+,K+,Cl- cotransport in squid giant axon by extracellular ions: evidence for ordered binding

Activation of the influx mode of the Na+,K+,Cl- cotransporter (NKCC) by extracellular Na+, K+ and Cl- was studied using the internally dialyzed squid giant axon. Cooperative interactions among the three transported ions were assessed using ion activation of NKCC-mediated 36Cl influx under two sets of experimental conditions. The first, or control condition, used high, non-limiting concentrations of two of the cotransported ions (the co-ions) while activating cotransport with the third ion. Under this non-limiting co-ion condition the calculated Vmax of the cotransporter was between 57 and 60 pmol/cm2/s. The apparent activation (KApp, or half-saturation) constants were: K+, 9 mM; Na+, 52 mM; and Cl-, 146 mM. The second condition used limiting co-ion concentration conditions. In this case, activation by each ion was determined when one of the other two co-ions was present at or near its apparent half-saturation concentration as determined above. Under these limiting conditions, the KApp values for all three co-ions were significantly increased regardless of which co-ion was present at a limiting concentration. The effects on the apparent Vmax were more complicated. When K+ was the limiting co-ion, there was little effect on the Vmax for Na+ or Cl- activation. In contrast, limiting concentrations of Na+ or Cl- both resulted in a large reduction of the apparent Vmax when activating with the other two co-ions. These results are consistent with an ordered binding mechanism for the NKCC in which K+ binds before Na+ or Cl-. Physiological implications for these results are discussed.

Potassium fluxes in contracting human skeletal muscle and red blood cells

The present study examined K+ fluxes in red blood cells and muscle during muscle contractions. Seven subjects performed two-legged submaximal knee-extensor exercise for 30 min. After 10 min of leg exercise (L1), intense arm exercise was also performed for 10 min (L2+A). Plasma epinephrine and norepinephrine concentrations were higher (P < 0.05) in L2+A compared with L1. Arterial plasma K+ at the end of L2+A was higher than in L1 (5.6 vs. 4.4 mM, P < 0.05) and returned to the L1 level on cessation of arm exercise. A net K+ release of 0.16 mmol/min from the active legs during L1 was turned to a net K+ uptake of 0.79 mmol/min during L2+A. Both arterial and venous red blood cell K+-to-hemoglobin ratios were constant during exercise. The present data suggest that contracting muscle can take up K+ probably by a combination of K+ and hormone activation of the Na+-K+ pump. Furthermore, changes in red blood cell K+ concentrations during muscle activity appear to be due to water movements and not transmembrane fluxes of K+.

Red blood cells do not contribute to removal of K+ released from exhaustively working forearm muscle

K+ released from exercising muscle via K+ channels needs to be removed from the interstitium into the blood to maintain high muscle cell membrane potential and allow normal muscle contractility. Uptake by red blood cells has been discussed as one mechanism that would also serve to regulate red blood cell volume, which was found to be constant despite increased plasma osmolality and K+ concentration ([K+pl]). We evaluated exercise-related changes in [K+pl], pH, osmolality, mean cellular Hb concentration, cell water, and red blood cell K+ concentration during exhaustive handgrip exercise. Unidirectional 86Rb+ (K+) uptake by red blood cells was measured in media with elevated extracellular K+, osmolarity, and catecholamines to simulate particularly those exercise-related changes in plasma composition that are known to stimulate K+ uptake. During exercise [K+pl] increased from 4.4 +/- 0.7 to 7.1 +/- 0.5 mmol/l plasma water and red blood cell K+ concentration increased from 137.2 +/- 6.0 to 144.6 +/- 4.6 mmol/l cell water (P </= 0.05), but the intracellular K+-to-mean cellular Hb concentration ratio did not change. 86Rb+ uptake by red blood cells was increased by approximately 20% on stimulation, caused by activation of the Na+-K+ pump and Na+-K+-2Cl- cotransport. Results indicate the K+ content of red blood cells did not change as cells passed the exhaustively exercising forearm muscle despite the elevated [K+pl]. The tendency for an increase in intracellular K+ concentration was due to a slight, although statistically not significant, decrease in red blood cell volume. K+ uptake, although elevated, was too small to move significant amounts of K+ into red blood cells. Our results suggest that red blood cells do not contribute to the removal of K+ released from muscle and do not regulate their volume by K+ uptake during exhaustive forearm exercise.

The Na-K-Cl cotransporters

The Na-K-Cl cotransporters are a class of membrane proteins that transport Na, K, and Cl ions into and out of a wide variety of epithelial and nonepithelial cells. The transport process mediated by Na-K-Cl cotransporters is characterized by electroneutrality (almost always with stoichiometry of 1Na:1K:2Cl) and inhibition by the "loop" diuretics bumetanide, benzmetanide, and furosemide. Presently, two distinct Na-K-Cl cotransporter isoforms have been identified by cDNA cloning and expression; genes encoding these two isoforms are located on different chromosomes and their gene products share approximately 60% amino acid sequence identity. The NKCC1 (CCC1, BSC2) isoform is present in a wide variety of tissues; most epithelia containing NKCC1 are secretory epithelia with the Na-K-Cl cotransporter localized to the basolateral membrane. By contrast, NKCC2 (CCC2, BSC1) is found only in the kidney, localized to the apical membrane of the epithelial cells of the thick ascending limb of Henle's loop and of the macula densa. Mutations in the NKCC2 gene result in Bartter's syndrome, an inherited disease characterized by hypokalemic metabolic alkalosis, hypercalciuria, salt wasting, and volume depletion. The two Na-K-Cl cotransporter isoforms are also part of a superfamily of cation-chloride cotransporters, which includes electroneutral K-Cl and Na-Cl cotransporters. Na-K-Cl cotransporter activity is affected by a large variety of hormonal stimuli as well as by changes in cell volume; in many tissues this regulation (particularly of the NKCCI isoform) occurs through direct phosphorylation/dephosphorylation of the cotransport protein itself though the specific protein kinases involved remain unknown. An important regulator of cotransporter activity in secretory epithelia and other cells as well is intracellular [Cl] ([Cl]i), with a reduction in [Cl]i being the apparent means by which basolateral Na-K-Cl cotransport activity is increased and thus coordinated with that of stimulated apical Cl channels in actively secreting epithelia.

A model of Na-K-2Cl cotransport based on ordered ion binding and glide symmetry

In the duck red blood cell, Na-K-2Cl cotransport exhibits two modes of ion movement: net cotransport and obligate cation exchange. In high-K cells, the predominant exchange is K/K (or K/Rb). In high-Na cells, it becomes Na/Na (or Na/Li). Both represent partial reactions in which a fully loaded carrier releases part of its cargo, rebinds fresh ions, and returns back across the membrane fully loaded. Net cotransport occurs when the carrier unloads completely and returns empty. This mode has a fixed stoichiometry of 1Na:1K:2Cl under all conditions tested. The ion requirements of the two exchanges differ: K/K exchange requires only K and Cl outside but all three ions inside. Na/Na exchange requires all three ions outside but only Na inside. We propose a simple model in which the carrier can only move when either fully loaded or completely empty and in which the ions bind in a strictly ordered sequence. For example, externally, a Na binds first and then a Cl, followed by a K and a second Cl. Internally, the first on is the first off (glide symmetry), so the Na is released first and then the first Cl, followed by the K and finally by the second Cl. Only then can the empty form return to the outside to start a new cycle.

Occlusion of K+ in the Na+/K+/2Cl- cotransporter of Ehrlich ascites tumor cells

Proteins of n-octyl glucoside solubilized membrane vesicles derived from Ehrlich ascites tumor cells can occlude 86Rb+.K+ displaces 86Rb+ and it is assumed that 86Rb+ can be used as a tracer to measure K+ occlusion. The following observations indicate that the Na+/K+/2Cl- cotransporter is responsible for this occlusion: (1) Na+ does not compete for the K+ binding site, but rather stimulates 86Rb+ occlusion. (2) K+ occlusion saturates with increasing [Na+] and [K+], the respective K0.5 values being 50 +/- 7 microM for Na+ and 371 +/- 63 microM for K+. (3) Preincubation with 1 mM ouabain does not inhibit 86Rb+ occlusion, arguing against the Na+/K+-ATPase as being responsible for the occlusion. This notion is supported by the K0.5 value for K+ being higher than reported for Na+/K+-ATPase and by the stimulatory effect of Na+. (4) The K+ occlusion is sensitive to [Cl-], and the occluded ion is protected by the presence of bumetanide during cation exchange chromatography. Our results suggest that occlusion measurements of substrate ions could be a profitable way to study the ion binding mechanism(s) of the Na+/K+/2Cl- cotransporter.

Elevated [Cl-]i, and [Na+]i inhibit Na+, K+, Cl- cotransport by different mechanisms in squid giant axons

Bumetanide-sensitive (BS) unidirectional fluxes of (36)Cl- or (22)Na+ were measured in internally dialyzed squid giant axons while varying the intra- or extracellular concentrations of Na+ and/or Cl-. Raising either [Cl-]i or [Na+]i resulted in a concentration-dependent reduction of the BS influx of both (36)Cl- and (22)Na+. Raising [Cl-]i above 200 mM completely blocked BS influxes. However, raising [Na+]i to 290 mM resulted in saturable but incomplete inhibition of both BS Na+ influx and BS Cl- influx. The consequences of varying intracellular Cl- on cotransporter effluxes were complex. At lower [Cl-]i values (below 100 mM) intracellular Cl- activated cotransporter effluxes. Surprisingly, however, raising [Cl-]i levels > 125 mM resulted in a [Cl-]i-dependent inhibition of BS effluxes of both Na+ and Cl-. On the other hand, raising [Na+]i resulted only in the activation of the BS Na+ efflux; intracellular Na+ did not inhibit BS efflux even at 290 mM. The inhibitory effects of intracellular Na+ on cotransporter-mediated influxes, and lack of inhibitory effects on BS effluxes, are consistent with the trans-side inhibition expected for an ordered binding/release model of cotransporter operation. However, the inhibitory effects of intracellular Cl- on both influxes and effluxes are not explained by such a model. These data suggest that Cl may interact with an intracellular site (or sites), which does not mediate Cl transport, but does modulate the transport activity of the Na+, K+, Cl- cotransporter.

Membrane mechanisms and intracellular signalling in cell volume regulation

Recent work on selected aspects of the cellular and molecular physiology of cell volume regulation is reviewed. First, the physiological significance of the regulation of cell volume is discussed. Membrane transporters involved in cell volume regulation are reviewed, including volume-sensitive K+ and Cl- channels, K+, Cl- and Na+, K+, 2Cl- cotransporters, and the Na+, H+, Cl-, HCO3-, and K+, H+ exchangers. The role of amino acids, particularly taurine, as cellular osmolytes is discussed. Possible mechanisms by which cells sense their volumes, along with the sensors of these signals, are discussed. The signals are mechanical changes in the membrane and changes in macromolecular crowding. Sensors of these signals include stretch-activated channels, the cytoskeleton, and specific membrane or cytoplasmic enzymes. Mechanisms for transduction of the signal from sensors to transporters are reviewed. These include the Ca(2+)-calmodulin system, phospholipases, polyphosphoinositide metabolism, eicosanoid metabolism, and protein kinases and phosphatases. A detailed model is presented for the swelling-initiated signal transduction pathway in Ehrlich ascites tumor cells. Finally, the coordinated control of volume-regulatory transport processes and changes in the expression of organic osmolyte transporters with long-term adaptation to osmotic stress are reviewed briefly.

The Na-K-Cl cotransporters

The Na-K-Cl cotransporters are a class of membrane proteins that transport Na, K, and Cl ions into and out of cells in an electrically neutral manner, in most cases with a stoichiometry of 1Na:1K:2Cl. Na-K-Cl cotransporters are present in a wide variety of cells and tissues, including reabsorptive and secretory epithelia, nerve and muscle cells, endothelial cells, fibroblasts, and blood cells. Na-K-Cl cotransport plays a vital role in renal salt reabsorption and in salt secretion by intestinal, airway, salivary gland, and other secretory epithelia. Cotransport function also appears to be important in the maintenance and regulation of cell volume and of ion gradients by both epithelial and nonepithelial cells. Na-K-Cl cotransport activity is inhibited by "loop" diuretics, including the clinically efficacious agents bumetanide and furosemide. The regulation of Na-K-Cl cotransport is mediated, at least in some cases, through direct phosphorylation of the cotransport protein. Cotransporter regulation is highly tissue specific, perhaps in part related to the presence of different Na-K-Cl cotransporter isoforms. In epithelia, both absorptive (kidney-specific) and secretory isoforms have been identified by cDNA cloning and sequencing and Northern blot analysis; alternatively spliced variants of the kidney-specific isoform have also been identified. The absorptive and secretory isoforms exhibit approximately 60% identity at the amino acid sequence level; these sequences in turn show approximately 45% overall homology with those of thiazide-sensitive, bumetanide-insensitive, Na-Cl cotransport proteins of winter flounder urinary bladder and mammalian kidney. This review focuses on recent developments in the identification of Na-K-Cl cotransport proteins in epithelial and on the regulation of epithelial Na-K-Cl cotransporter function at cellular and molecular levels.

Insulin activates furosemide-sensitive K+ and Cl- uptake system in BC3H1 cells

Insulin augments the activity of Na(+)-K(+)-adenosinetriphosphatase (ATPase) in skeletal muscles. This study shows that when furosemide- and bumetanide-inhibitable 86Rb+ uptake is measured in the skeletal muscle-like BC3H1 cell line, insulin and insulin-like growth factor I (IGF-I) activate a loop diuretic-sensitive K+ and Cl- transport system but have no effect on Na(+)-K(+)-ATPase. The insulin-stimulated K+ transport system is extracellular Na+ concentration ([Na+]o) independent and extracellular Cl- concentration ([Cl-]o) dependent. Na(+)-independent K(+)-Cl- cotransport systems have been identified in other cells, but their sensitivity to insulin or growth factors has not been described. The affinities of the insulin-stimulated K+ uptake in BC3H1 cells for K+ (0.9 +/- 0.1 mM) and loop diuretics (5.9 x 10(-7) and 10(-7) M for furosemide and bumetanide, respectively) are higher than those of K(+)-Cl- cotransporters in other cells. Thus the insulin-stimulated K+ and Cl- transport system in BC3H1 seems kinetically different from K(+)-Cl- cotransporters in other cells. Insulin and IGF-I may activate a unique K(+)-Cl- cotransporter or activate a [Na+]o-independent K(+)-Cl- cotransport mode of Na(+)-K(+)-Cl- cotransporter in BC3H1 cells.

Genetic control of erythrocyte volume regulation: effect of a single gene (rol) on cation metabolism

In laboratory mice we previously defined a gene, rol (resistance to osmotic lysis), based on its effect on erythrocyte osmotic fragility. Here we report a physiological characterization of rol gene action utilizing congenic strains developed for the purpose; these two strains have a common genetic background and differ only by the two alleles of rol, susceptible (rols) or resistant (rolr). In comparison to rols/s erythrocytes, rolr/r cells have a reduced mean cell volume, a higher mean corpuscular hemoglobin concentration and hemolytic volume, and respond differently to swelling induced by ion influx. Rolr/r erythrocytes also have reduced cell water and K, which are associated with a threefold higher activity of the Na-K-Cl cotransporter (measured as ouabain-resistant, bumetanide-sensitive 86Rb influx) and 30% higher Na pump activity. Apart from differences in ion transport and water content, the content of 2,3-diphosphoglycerate (2,3-DPG) in rolr/r cells is 15% lower than in rols/s ones. Analyses of membrane structural components revealed no rol-associated differences in their phospholipid or fatty acid content, nor were strain differences evident among the membrane and cytoskeletal proteins and their posttranslational modifications (phosphorylation and fatty acylation). Rol is not the structural gene for either the alpha- or the beta-chain of hemoglobin and has no effect on erythrocyte production or destruction. The concerted effect of rol variation on erythrocyte volume, water and cation content, cation cotransport, and 2,3-DPG levels is similar in many ways to the variation observed among individual humans for the same characteristics.(ABSTRACT TRUNCATED AT 250 WORDS)

K-Cl cotransport, pH, and role of Mg in volume-clamped low-K sheep erythrocytes: three equilibrium states

Ouabain-resistant K efflux and Rb influx in Cl and NO3 media were studied in volume-clamped low-K (LK) sheep red blood cells (SRBC) with normal and experimentally reduced cytoplasmic Mg (Mgi) levels as function of pH and at 37 degrees C. Sucrose was added to solutions with constant ionic strength and variable pH to maintain normal cell volume. Cl-dependent ouabain-resistant K(Rb) fluxes (K-Cl cotransport) at unity relative cell volume exhibited a maximum at pH approximately 7 in normal-Mgi LK cells consistent with the apparent acid pH activation reported for human erythrocytes. However, in LK SRBC with Mgi lowered by A-23187 and an external Mg chelator, K(Rb)-Cl cotransport was reversibly activated as the pH was raised from 6.5 to 9. The alkaline pH effect on Cl-dependent Rb influx in low-Mgi LK SRBC was due to a 10-fold rise in the maximum velocity values without a major change in the Km values. The pH dependence of the experimental flux reversal point, i.e., the extracellular Rb concentration at which no net K-Cl cotransport occurs, approximately paralleled that of the flux reversal point predicted from the ratio of the ion products, in both control and low-Mgi LK cells, albeit with a small displacement to higher extracellular Rb concentration at all pH values. The kinetic data can be explained by a general minimum three-state equilibrium in which deprotonation recruits transporters from a resting R state into the active A state modified by Mgi to an inactive I state.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulatory interaction of ATP Na+ and Cl- in the turnover cycle of the NaK2Cl cotransporter

To probe the mechanism by which intracellular ATP, Na+, and Cl- influence the activity of the NaK2Cl cotransporter, we measured bumetanide-sensitive (BS) 86Rb fluxes in the osteosarcoma cell line UMR-106-01. Under physiological gradients of Na+, K+, and Cl-, depleting cellular ATP by incubation with deoxyglucose and antimycin A (DOG/AA) for 20 min at 37 degrees C reduced BS 86Rb uptake from 6 to 1 nmol/mg protein per min. Similar incubation with 0.5 mM ouabain to inhibit the Na+ pump had no effect on the uptake, excluding the possibility that DOG/AA inhibited the uptake by modifying the cellular Na+ and K+ gradients. Loading the cells with Na+ and depleting them of K+ by a 2-3-h incubation with ouabain or DOG/AA increased the rate of BS 86Rb uptake to approximately 12 nmol/mg protein per min. The unidirectional BS 86Rb influx into control cells was approximately 10 times faster than the unidirectional BS 86Rb efflux. On the other hand, at steady state the unidirectional BS 86Rb influx and efflux in ouabain-treated cells were similar, suggesting that most of the BS 86Rb uptake into the ouabain-treated cells is due to K+/K+ exchange. The entire BS 86Rb uptake into ouabain-treated cells was insensitive to depletion of cellular ATP. However, the influx could be converted to ATP-sensitive influx by reducing cellular Cl- and/or Na+ in ouabain-treated cells to impose conditions for net uptake of the ions. The BS 86Rb uptake in ouabain-treated cells required the presence of Na+, K+, and Cl- in the extracellular medium. Thus, loading the cells with Na+ induced rapid 86Rb (K+) influx and efflux which, unlike net uptake, were insensitive to cellular ATP. Therefore, we suggest that ATP regulates a step in the turnover cycle of the cotransporter that is required for net but not K+/K+ exchange fluxes. Depleting control cells of Cl- increased BS 86Rb uptake from medium-containing physiological Na+ and K+ concentrations from 6 to approximately 15 nmol/mg protein per min. The uptake was blocked by depletion of cellular ATP with DOG/AA and required the presence of all three ions in the external medium. Thus, intracellular Cl- appears to influence net uptake by the cotransporter. Depletion of intracellular Na+ was as effective as depletion of Cl- in stimulating BS 86Rb uptake.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Red cell ion transport abnormalities in experimental hypertension

The original attractive hypothesis on the important role of elevated cell Na+ concentration in the pathogenesis of hypertension stimulated a search for generalized membrane defects and ion transport abnormalities in various easily accessible cells including erythrocytes. An attempt is made here to compare this hypothesis with the data on red cell ion transport alterations that were observed in experimental hypertension over the last 15 years. Several methodological (presence of extracellular Na+ in incubation media, kinetic approach to the evaluation of transport systems) and physiological problems (potassium depletion, age-dependent changes) are discussed in more detail because they can substantially modify the results obtained. Available data suggest a possible contribution of augmented Na+ leak to the development of both genetic and salt-dependent experimental hypertension. The role of alterations in the activity of the Na(+)-K+ pump or the Na(+)-K+ cotransport system still remains unclear.

Na+, K+, Cl- cotransport and its regulation in Ehrlich ascites tumor cells. Ca2+/calmodulin and protein kinase C dependent pathways

Net Cl- uptake as well as unidirectional 36Cl influx during regulatory volume increase (RVI) require external K+. Half-maximal rate of bumetanide-sensitive 36Cl uptake is attained at about 3.3 mM external K+. The bumetanide-sensitive K+ influx found during RVI is strongly dependent on both Na+ and Cl-. The bumetanide-sensitive unidirectional Na+ influx during RVI is dependent on K+ as well as on Cl-. The cotransporter activated during RVI in Ehrlich cells, therefore, seems to transport Na+, K+ and Cl-. In the presence of ouabain and Ba+ the stoichiometry of the bumetanide-sensitive net fluxes can be measured at 1.0 Na+, 0.8 K+, 2.0 Cl- or approximately 1:Na, 1:K, 2:Cl. Under these circumstances the K+ and Cl- flux ratios (influx/efflux) for the bumetanide-sensitive component were estimated at 1.34 +/- 0.08 and 1.82 +/- 0.15 which should be compared to the gradient for the Na+, K+, 2Cl- cotransport system at 1.75 +/- 0.24. Addition of sucrose to hypertonicity causes the Ehrlich cells to shrink with no signs of RVI, whereas shrinkage with hypertonic standard medium (all extracellular ion concentrations increased) results in a RVI response towards the original cell volume. Under both conditions a bumetanide-sensitive unidirectional K+ influx is activated. During hypotonic conditions a small bumetanide-sensitive K+ influx is observed, indicating that the cotransport system is already activated. The cotransport is activated 10-15 fold by bradykinin, an agonist which stimulates phospholipase C resulting in release of internal Ca2+ and activation of protein kinase C. The anti-calmodulin drug pimozide inhibits most of the bumetanide-sensitive K+ influx during RVI. The cotransporter can be activated by the phorbol ester TPA. These results indicate that the stimulation of the Na+, K+, Cl- cotransport involves both Ca2+/calmodulin and protein kinase C.

Cation transport and volume regulation in sickle red blood cells

Cellular dehydration is one of several pathological features of the sickle cell. Cation depletion is quite severe in certain populations of sickle cells and contributes to the rheological dysfunction that is the root cause of vascular occlusion in this disease. The mechanism of dehydration of sickle cells in vivo has not been ascertained, but three transport pathways may play important roles in this process. These include the deoxygenation-induced pathway that permits passive K+ loss and entry of Na+ and Ca2+; the K(+)-Cl- cotransport pathway, activated by acidification or cell swelling; and the Ca(2+)-activated K+ channel, or Gardos pathway, presumably activated by deoxygenation-induced Ca2+ influx. Recent evidence suggests that these pathways may interact in vivo. Heterogeneity exists among sickle cells as to the rate at which they become dense, suggesting that other factors may affect the activity or interactions of these pathways. Understanding the mechanism of dehydration of sickle cells may provide opportunities for pharmacological manipulation of cell volume to mitigate some of the symptoms of sickle cell disease.

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.

Association of red blood cell sodium leak with blood pressure in recombinant inbred strains

Red blood cell Na+ content as well as ouabain-resistant Na+ and Rb+ (K+) transport (susceptible or resistant to inhibition by loop diuretics) were determined in spontaneously hypertensive rats (SHR) and normotensive Brown Norway (BN) rats the erythrocytes of which were incubated in either saline or Mg(2+)-sucrose medium. Elevated ouabain-resistant Na+ net uptake contrasted with slightly decreased red blood cell Na+ content in SHR compared with BN rats. Acceleration of furosemide- and bumetanide-sensitive Na+ fluxes contributed to enhanced ouabain-resistant Na+ influx into SHR erythrocytes in saline medium, whereas higher furosemide- or bumetanide-resistant Na+ efflux caused greater ouabain-resistant Na+ efflux in Mg(2+)-sucrose medium. Furosemide- and bumetanide-resistant Rb+ leaks were augmented in SHR erythrocytes. The association of the disclosed ion transport alterations with blood pressure was examined in 20 recombinant inbred strains derived from F2 SHR x BN hybrids. Ouabain-resistant Na+ uptake as well as furosemide- and bumetanide-resistant Na+ inward leaks (but not red blood cell Na+ content or furosemide- and bumetanide-sensitive Na+ net uptake) cosegregated with systolic and pulse pressures but not diastolic pressure of the recombinant inbred strains. In contrast, neither ouabain-resistant Na+ efflux nor any component of ouabain-resistant Rb+ uptake correlated positively with blood pressure of the recombinant inbred strains. Increased ouabain-resistant Na+ influx was compensated for by accelerated ouabain-sensitive Na+ extrusion because red blood cell Na+ content was not elevated in the hypertensive strains. Thus, high cell Na+ turnover rates might be related to genetic hypertension if an altered Na+ inward leak would be less effectively compensated for in tissues involved in cardiovascular regulation.

Internal magnesium, 2,3-diphosphoglycerate, and the regulation of the steady-state volume of human red blood cells by the Na/K/2Cl cotransport system

This study is concerned with the relationship between the Na/K/Cl cotransport system and the steady-state volume (MCV) of red blood cells. Cotransport rate was determined in unfractionated and density-separated red cells of different MCV from different donors to see whether cotransport differences contribute to the difference in the distribution of MCVs. Cotransport, studied in cells at their original MCVs, was determined as the bumetanide (10 microM)-sensitive 22Na efflux in the presence of ouabain (50 microM) after adjusting cellular Na (Nai) and Ki to achieve near maximal transport rates. This condition was chosen to rule out MCV-related differences in Nai and Ki that might contribute to differences in the net chemical driving force for cotransport. We found that in both unfractionated and density-separated red cells the cotransport rate was inversely correlated with MCV. MCV was correlated directly with red cell 2,3-diphosphoglycerate (DPG), whereas total red cell Mg was only slightly elevated in cells with high MCV. Thus intracellular free Mg (Mgifree) is evidently lower in red cells with high 2,3-DPG (i.e., high MCV) and vice versa. Results from flux measurements at their original MCVs, after altering Mgifree with the ionophore A23187, indicated a high Mgi sensitivity of cotransport: depletion of Mgifree inhibited and an elevation of Mgifree increased the cotransport rate. The apparent K0.5 for Mgifree was approximately 0.4 mM. Maximizing Mgifree at optimum Nai and Ki minimized the differences in cotransport rates among the different donors. It is concluded that the relative cotransport rate is regulated for cells in the steady state at their original cell volume, not by the number of copies of the cotransporter but by differences in Mgifree. The interindividual differences in Mgifree, determined primarily by differences in the 2,3-DPG content, are responsible for the differences in the relative cotransport activity that results in an inverse relationship with in vivo differences in MCV. Indirect evidence indicates that the relative cotransport rate, as indexed by Mgifree, is determined by the phosphorylated level of the cotransport system.

Activation of ion transport pathways by changes in cell volume

Swelling-activated K+ and Cl- channels, which mediate RVD, are found in most cell types. Prominent exceptions to this rule include red cells, which together with some types of epithelia, utilize electroneutral [K(+)-Cl-] cotransport for down-regulation of volume. Shrinkage-activated Na+/H+ exchange and [Na(+)-K(+)-2 Cl-] cotransport mediate RVI in many cell types, although the activation of these systems may require special conditions, such as previous RVD. Swelling-activated K+/H+ exchange and Ca2+/Na+ exchange seem to be restricted to certain species of red cells. Swelling-activated calcium channels, although not carrying sufficient ion flux to contribute to volume changes may play an important role in the activation of transport pathways. In this review of volume-activated ion transport pathways we have concentrated on regulatory phenomena. We have listed known secondary messenger pathways that modulate volume-activated transporters, although the evidence that volume signals are transduced via these systems is preliminary. We have focused on several mechanisms that might function as volume sensors. In our view, the most important candidates for this role are the structures which detect deformation or stretching of the membrane and the skeletal filaments attached to it, and the extraordinary effects that small changes in concentration of cytoplasmic macromolecules may exert on the activities of cytoplasmic and membrane enzymes (macromolecular crowding). It is noteworthy that volume-activated ion transporters are intercalated into the cellular signaling network as receptors, messengers and effectors. Stretch-activated ion channels may serve as receptors for cell volume itself. Cell swelling or shrinkage may serve a messenger function in the communication between opposing surfaces of epithelia, or in the regulation of metabolic pathways in the liver. Finally, these transporters may act as effector systems when they perform regulatory volume increase or decrease. This review discusses several examples in which relatively simple methods of examining volume regulation led to the discovery of transporters ultimately found to play key roles in the transmission of information within the cell. So, why volume? Because it's functionally important, it's relatively cheap (if you happened to have everything else, you only need some distilled water or concentrated salt solution), and since it involves many disciplines of experimental biology, it's fun to do.

X-linked recessive nephrolithiasis with renal failure

BACKGROUND AND METHODS

Nephrolithiasis may occur as a consequence of a number of hereditary disorders. We describe a large kindred from northern New York with hereditary nephrolithiasis accompanied by urinary concentrating defects, nephrocalcinosis, renal insufficiency, and renal wasting of potassium, phosphate, calcium, and uric acid. The pattern of inheritance was established by examining the patients and their records and interviewing family members. Selected members of the family were evaluated in detail, with measurements of erythrocyte cation fluxes and carbonic anhydrase (carbonate dehydratase) activity.

RESULTS

The kindred consisted of 162 family members from six generations. All nine affected persons were male and appeared to have inherited the disease from their mothers. No affected man transmitted the gene to a son, but the daughters of affected men were carriers. The patients presented in childhood with calcium nephrolithiasis and proteinuria, with progression to nephrocalcinosis, urinary concentrating defects, and renal insufficiency. Renal biopsies revealed tubular atrophy, interstitial fibrosis, and glomerulosclerosis; the characteristic features of other forms of hereditary nephritis were absent. Abnormalities in the renal excretion of calcium, phosphate, potassium, and uric acid were found only in the adult members of the kindred, although renal biopsies were abnormal even in younger members. In one patient who has had a renal transplant for seven years, the disease has not recurred.

CONCLUSIONS

This kindred manifested an X-linked recessive nephrolithiasis with renal failure, a new form of hereditary renal disease. Most of the identifiable physiologic abnormalities occurred after the development of nephrolithiasis and renal insufficiency and may not be of pathogenetic importance.

[3H]bumetanide binding to mouse kidney membranes: identification of corresponding membrane proteins

Crude plasma membranes from whole mouse kidneys have two classes of [3H]bumetanide binding sites. High-affinity sites (K1/2 approximately equal to 0.04 microM; Bmax = 1-2 pmol/mg protein) are similar to those identified on dog kidney membranes (B. Forbush and H.C. Palfrey. J. Biol. Chem. 258: 11787-11792, 1983) both with respect to affinity and in that Na, K, and Cl are required for [3H]bumetanide binding. Low-affinity sites (K1/2 approximately equal to 1 microM; Bmax = 7-14 pmol/mg) are unaffected by removal of these ions; such sites are not seen with dog kidney. When mouse kidney membranes are photolabeled with 4-[3H]benzoyl-5-sulfamoyl-3-(3-thenyloxy)benzoic acid [( 3H]BSTBA), a photoreactive bumetanide analogue, specific incorporation of the label is seen in two regions. As with dog kidney [M. Haas and B. Forbush. Am. J. Physiol. 253 (Cell Physiol. 22): C243-C252, 1987], an approximately 150-kDa protein is labeled with high affinity (K1/2 approximately equal to 0.05 microM). This labeling also requires Na, K, and Cl and appears to correspond to the high-affinity [3H]bumetanide binding sites and to the Na-K-Cl cotransport system. A second peak of [3H]BSTBA photolabeling, centered at approximately 75 kDa, incorporates the label with lower affinity (K1/2 = 2-3 microM). The photolabeling at approximately 75 kDa is unaffected by Na, K, and Cl concentrations and thus may correspond, at least in part, to the low-affinity [3H]bumetanide binding sites. Western blot analysis of [3H]BSTBA-labeled mouse kidney membranes was performed using an antiserum raised to proteins of approximately 82 and approximately 39 kDa isolated from mouse Ehrlich ascites tumor cells using a bumetanide affinity gel (P. B. Dunham, F. Jessen, and E. K. Hoffmann. Proc. Natl. Acad. Sci. USA 87: 6828-6832, 1990). This antiserum cross-reacts with a approximately 150-kDa mouse kidney protein, the staining profile of which on Western blot corresponds very closely to the peak of specific [3H]BSTBA incorporation in this region. The antiserum also reacts with proteins in the range of 65-85 kDa, overlapping the low-affinity peak of [3H]BSTBA incorporation.

Characteristics of 86Rb+ transport in human erythrocytes infected with Plasmodium falciparum

Human red cells infected in vitro with Plasmodium falciparum showed a significant increase in the rate of both ouabain-sensitive and ouabain-insensitive 86Rb+ influx. The increase in ouabain-insensitive 86Rb+ influx was due, in part, to increased transport via a bumetanide-sensitive system and, in part to transport via a pathway that was absent (or at least inactive) in uninfected cells. The parasite-induced pathway was inhibited by piperine and had a dose response very similar to that of the Gardos channel of uninfected cells but was less sensitive than the Gardos channel to inhibition by quinine.

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.

The ATPase activity of saponin-treated rat erythrocytes: regulation by monovalent cations, calcium, ouabain, and furosemide

The ATPase activities were studied in rat erythrocytes permeabilized with saponin. The concentrations of calcium and magnesium ions were varied within the range of 0.1-60 microM and 50-370 microM, respectively, by using EGTA-citrate buffer. The maximal activity of Ca2(+)-ATPase of permeabilized erythrocytes was by one order of magnitude higher, whereas the Ca2(+)-binding affinity was 1.5-2 times higher than that in erythrocyte ghosts washed an isotonic solution containing EGTA. Addition of the hemolysate restored the kinetic parameters of ghost Ca2(+)-ATPase practically completely, whereas in the presence of exogenous calmodulin only part of Ca2(+)-ATPase activity was recovered. Neither calmodulin nor R24571, a highly potent specific inhibitor of calmodulin-dependent reactions, influenced the Ca2(+)-ATPase activity of permeabilized erythrocytes. At Ca2+ concentrations below 0.7 microM, ouabain (0.5-1 mM) activated whereas at higher Ca2+ concentrations it inhibited the Ca2(+)-ATPase activity. Taking this observation into account the Na+/K(+)-ATPase was determined as the difference of between the ATPase activities in the presence of Na+ and K+ and in the presence of K+ alone. At physiological concentration of Mg2+ (370 microM), the addition of 0.3-1 microM Ca2+ increased Na+/K(+)-ATPase activity by 1.5-3-fold. Higher concentrations of this cation inhibited the enzyme. At low Mg2+ concentration (e.g., 50 microM) only Na+/K(+)-ATPase inhibition by Ca2+ was seen. It was found that at [NaCl] less than 20 mM furosemide was increased ouabain-inhibited component of ATPase in Ca2(+)-free media. This activating effect of furosemide was enhanced with a diminution of [Na+] upto 2 mM and did not reach the saturation level unless the 2 mM of drug was used. The activating effect of furosemide on Na+/K(+)-ATPase activity confirmed by experiments in which the ouabain-inhibited component was measured by the 86Rb+ influx into intact erythrocytes.

Thiol-dependent passive K: Cl transport in sheep red blood cells: IX. Modulation by pH in the presence and absence of DIDS and the effect of NEM

Recently we proposed that cytoplasmic acidification of low K+ (LK) sheep erythrocytes may stimulate ouabain-resistant Cl(-)-dependent K+ flux (K+: Cl- contransport), also known to be activated by cell swelling, treatment with N-ethylmaleimide (NEM), or removal of cellular bivalent cations. Here we studied the dependence of K+ transport on intracellular and extracellular pH (pHi, pHo) varied either simultaneously or independently using the Cl-/HCO3- exchange inhibitor 4,4, diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS). In both control and NEM-treated LK cells volumes were kept near normal by varying extracellular sucrose. Using DIDS as an effective pH clamp, both K+ efflux and influx of Rb+ used as K+ congener were strongly activated at acid pHi and alkaline pHo. A small stimulation of K+ (Rb+) flux was also seen at acid pHi in the absence of DIDS, i.e., when pHi approximately pHo. Anti-Ll serum, known to inhibit K+: Cl-cotransport, prevented the pHi-stimulated K+ (Rb+) fluxes. Subsequent to NEM treatment at pH 6, K+ (Rb+) fluxes were activated only by raising pH, and thus were similar to the pH activation profile of K+ (Rb+) fluxes in DIDS-treated cells with pHo varied at constant physiologic pHi. Anti-Ll, which inhibited NEM-stimulated K+ (Rb+) fluxes, failed to do so in NEM-plus DIDS-treated cells. Thus, NEM treatment interferes with the internal but not with the external pH-sensitive site.

K-Cl cotransport in LK sheep erythrocytes: kinetics of stimulation by cell swelling

The effects of osmotic cell swelling were studied on the kinetics of Cl-dependent K+ influx, K-Cl cotransport, in erythrocytes from sheep of the low K+ (LK) phenotype. Swelling approximately 25% stimulated transport by increasing maximum velocity (Jmax) approximately 1.5-fold and by increasing apparent affinity for external K (Ko) nearly twofold. Dithiothreitol (DTT) was shown to be a partial, reversible inhibitor of K-Cl cotransport. It inhibited in cells of normal volume by reducing Jmax more than twofold; apparent affinity for Ko was increased by DTT, suggesting that DTT stabilizes the transporter-Ko complex. Cell swelling reduced the extent of inhibition by DTT: Jmax was inhibited by only about one-third in swollen cells, and apparent affinity was only slightly affected. This result suggested that DTT does not act directly on the transporter, but on a hypothetical regulator, an endogenous inhibitor. Swelling relieves inhibition by the regulator, and reduces the effect of DTT. Reducing intracellular Mg2+, Mgc, stimulated cotransport. Swelling of low-Mg2+ cells stimulated transport further, but only by raising apparent affinity for Ko nearly threefold: Jmax was unaffected. Thus effects of swelling on Jmax and apparent affinity are separable processes. The inhibitory effects of Mgc and DTT were shown to be additive, indicating separate modes of action. There appear to be two endogenous inhibitors: the hypothetical regulator, which holds affinity for Ko, low; and Mgc, which affects Jmax, perhaps by holding some transporters in an inactive form. Swelling stimulates transport by relieving both types of inhibition.

Kinetics of Na+ and K+ transport in red blood cells of Dahl rats. Effects of age and salt

Blood pressure response to chronic high salt intake and kinetics of red blood cell Na+ and K+ (Rb+) transport were studied in salt-sensitive (DS) and salt-resistant (DR) Dahl rats fed a high salt diet (8% NaCl) for 7 weeks from the fifth (young), 12th (adult), or 23rd (old) week of age. The kinetics of ouabain-sensitive Rb+ uptake and Na+ extrusion were determined in Na+ media as a function of both intracellular Na+ (Na+i, 2-8 mmol/l cells) and extracellular Rb+ (Rb+o). In addition, the kinetics of furosemide-sensitive Rb+ uptake (related to Rb+o) and the magnitude of the Na+ and Rb+ leaks were assessed. High salt induced hypertension in young and adult but not in old DS rats although red blood cell Na+ was slightly increased in all age groups of DS rats fed a high salt diet. The kinetic parameters of the Na(+)-K+ pump were similar in DS and DR rats fed a low salt diet. Ouabain-sensitive transport rates were not suppressed in erythrocytes of salt hypertensive Dahl rats. Maximal velocities of the Na(+)-K+ pump (related to Na+i) decreased significantly with age in all groups except in DS rats fed a high salt diet. This was compensated by an age-dependent increase in the affinity for Na+i so that no substantial differences in transport rates between young and old rats were seen at physiological cell Na+ and plasma K+ levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of bumetanide to microsomes from optic ganglia of the squid, Loligo pealei

Saturable high-affinity binding of [3H] bumetanide [dissociation constant (KD) = 80 nM] was measured in microsomal membranes prepared from squid optic ganglia. Under control conditions, the maximal specific binding of labeled bumetanide (Bmax) was approximately 6-7 pmol/mg protein. Binding had a higher relative affinity for bumetanide than for furosemide and depended on the presence of Cl- and K+, but not Na+, in the incubation media. In the case of K+, [3H]bumetanide binding was half-saturated at [K+] = 100 mM. The Cl- effect was biphasic. At [Cl-] between 0 and 150 mM, [3H]bumetanide binding increased with increasing [Cl-]. However, when [Cl-] was increased above 150 mM, [3H]bumetanide binding was progressively reduced. ATP acted as a nonessential activator [mean affinity constant (K0.5) approximately 1 microM] of the ion-dependent [3H]bumetanide binding by increasing the apparent binding capacity. The activation by ATP did not require Mg2+. Other adenosine analogues also stimulated the binding of bumetanide.

Characterization of a Na(+)-K(+)-2Cl- cotransport system in oocytes from Xenopus laevis

In order to characterize the transport systems mediating K+ uptake into oocytes, flux studies employing 86Rb were performed on Xenopus oocytes stripped of follicular cells by pretreatment with Ca2(+)-Mg2(+)-free Barth's medium. Total Rb+ uptake consisted of an ouabain-sensitive and an ouabain-insensitive flux. In the presence of 100 mmol/l NaCl and 0.1 mmol/l ouabain the ouabain-insensitive flux amounted to 754.7 +/- 59.9 pmol/oocyte per h (n = 30 cells, i.e., 10 cells each from three different animals). In the absence of Na+ (Na+ substituted by N-methylglucamine) or when Cl- was replaced by NO3- the ouabain-insensitive flux was reduced to 84.4 +/- 42.9 and 79.2 +/- 12.1 pmol/oocyte per h, respectively (n = 50 cells). Furthermore, this Na(+)- and Cl(-)-dependent flux was completely inhibited by 10(-4) mol/l bumetanide, a specific inhibitor of the Na(+)-K(+)-2Cl- cotransport system. These results suggest that K+ uptake via a bumetanide-sensitive Na(+)-K(+)-2Cl- cotransport system represents a major K+ pathway in oocytes.

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.

Kinetics of volume-sensitive K transport in human erythrocytes: evidence for asymmetry

The kinetic properties of volume-sensitive K fluxes in swollen human erythrocytes were investigated. Swelling-activated Cl-dependent K influx was a saturable function of external K concentration with a low affinity (apparent Km of 115-130 mM) and high capacity [maximal velocity (Vmax) = 20-30 mmol.l original cells-1.h-1 (mmol.loc-1.h-1)]. The Vmax and apparent Km for Cl-dependent K efflux were lower (Km = 47 mM; Vmax = 2.2 mmol.loc-1.h-1). The Hill coefficients for both K efflux and influx were close to unity, suggesting a single binding site for K. The increase of external K trans-stimulated K efflux, but the increase of intracellular K had no effect on Cl-dependent K influx in swollen cells. Under zero trans conditions, the Vmax (18 vs. 3 mmol.loc-1.h-1) and Km (138 vs. 32) were markedly different for influx and efflux, respectively. These results provide evidence for intrinsic functional asymmetry, such that the transporter is more prevalent and stable in the outward-facing conformation. The mean ratio of Km to Vmax for efflux (12.1) was 1.56 times larger than the same ratio for influx (7.8), but the difference between the means did not reach statistical significance. These kinetic observations are analyzed in terms of the simple carrier and the cotransport models.

Cl-dependent K transport in a pure population of volume-regulating human erythrocytes

Swelling of human red cells activates a putative K-Cl cotransport that is not present at normal cell volume and that disappears after several hours. To determine whether regulatory volume decrease (RVD) is occurring in human erythrocytes and is responsible for the inactivation of K-Cl cotransport, the relationship between cell volume and the inactivation and reactivation of volume-sensitive (VS) K-Cl cotransport was studied. VS K influx into high K cells was transient, whereas influx into low K cells (prepared with nystatin), which are unable to shrink via K efflux, remained fully activated. Likewise, VS K efflux into hypotonic medium disappeared after 100 min in a low K medium but remained activated in a high K medium that prevented cell shrinkage. Cells that had been preincubated in hypotonic medium to inactivate VS K-Cl cotransport showed no significant recovery of VS cotransport after a 6-h incubation in isotonic medium but showed full restoration of VS cotransport after treatment with nystatin in isotonic medium to reequilibrate cell water. A pure fraction of volume-regulating (VR) cells was subsequently isolated by preincubating red cells in hypotonic medium and then subjecting them to further hypotonicity to lyse all non-VR cells. The 2.5% of cells that remained consisted of 16% reticulocytes and exhibited a Cl-dependent RVD in hypotonic medium. VS K-Cl cotransport was enriched 10-fold and Na-K-Cl cotransport was enriched 12-fold in these cells, whereas the enrichment of N-ethylmaleimide (NEM)-activated K-Cl cotransport was only threefold.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of K-C1 cotransport in rat red cells by a hemolytic anemia-producing metabolite of dapsone

Dapsone, a sulfone compound used in the treatment of leprosy and, more recently, Pneumocystis carinii pneumonia, produces as a major side effect a hemolytic anemia. This anemia is characterized by oxidation of hemoglobin to methemoglobin and increased splenic uptake of red blood cells. Using a rat model, Grossman and Jollow (J. Pharmacol. Exp. Ther. 244: 118-125, 1988) found that dapsone hydroxylamine (DDS-NOH), a dapsone metabolite, is responsible for its hemolytic effect in vivo. DDS-NOH also promotes hemoglobin binding to SH groups on rat red cell membrane proteins (Budinsky et al., FASEB J. 2: A801, 1988). Since the binding of hemoglobin and other reagents (e.g., N-ethylmaleimide) to membrane SH groups has been associated with increased K transport in red blood cells, we examined the effect of DDS-NOH on K efflux from rat red blood cells in vitro. Cells shrink when exposed to DDS-NOH (100 microM) in media with plasma-like ionic composition. This shrinkage is prevented if extracellular K is raised to 110 mM or if intra- and extracellular Cl are replaced by methylsulfate (MeSO4), suggesting involvement of a K-Cl cotransport pathway. Indeed, 100 microM DDS-NOH produces a 4- to 5-fold increase in K efflux in cells containing Cl but less than a 2-fold increase in cells containing MeSO4. This stimulatory effect is specific for K; Na efflux is slightly inhibited by 100 microM DDS-NOH. The concentrations of DDS-NOH required for half-maximal stimulation of Cl-dependent K efflux (53 microM) is similar to its half-maximal hemolytic concentration in rats (approximately 100 microM). Furthermore, the stimulation of Cl-dependent K efflux by DDS-NOH is greater than 80% reversed by subsequent treatment of the cells with dithiothreitol, suggesting involvement of SH groups. Our results indicate that DDS-NOH exposure stimulates an apparent K-Cl cotransport in rat red blood cells, resulting in cell shrinkage under physiological ionic conditions. Since shrinkage of red blood cells renders them less deformable (Mohandas et al., J. Clin. Invest. 66: 563-573, 1980), this suggests a pathophysiological mechanism whereby DDS-NOH exposure in vivo could promote increased splenic uptake of red blood cells and hemolytic anemia.

Volume-sensitive, Cl-dependent K transport in resealed human erythrocyte ghosts

Potassium influx and efflux in Cl and NO3 media were measured in resealed ghosts prepared from human red cells. Cl-dependent K influx was three times that in intact cells and, as in intact cells, was partially supported by Br but not by thiocyanate (SCN). In other properties, this flux differed from that in intact cells: substitution of N-methylglucamine for Na did not decrease but rather increased Cl-dependent K influx, the affinity for external K was reduced, with a Km of 21.3 +/- 12.5 mM, and inhibition by furosemide and bumetanide was incomplete. Furosemide at 1 mM inhibited Cl-dependent influx by 26 and 51% at 4 and 20 mM K, respectively. Bumetanide inhibited Cl-dependent K influx by 0 and 55% at concentrations of 10 microM and 1 mM, respectively, in 4 mM K, with no further inhibition at 20 mM K. Neither the magnitude nor the properties of the flux were altered by preparing ghosts in the presence of 1,4-dithiothreitol, indicating that sulfhydryl oxidation was not responsible for the altered flux in ghosts. Treatment with N-ethylmaleimide (NEM) either before or after ghost preparation did not increase Cl-dependent K influx. However, Cl-dependent influx in ghosts could be augmented by increasing ghost volume or ATP content. Resealed human erythrocyte ghosts thus exhibit a volume- and ATP-sensitive, Cl-dependent K flux that differs substantially from the putative Na-K-Cl cotransport in intact cells in that it is independent of Na, is relatively resistant to furosemide and bumetanide, and has a low affinity for K.(ABSTRACT TRUNCATED AT 250 WORDS)

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.