Measurement and stoichiometry of bumetanide-sensitive (2Na:1K:3Cl) cotransport in ferret red cells

Molecular characteristics and physiological roles of Na+ -K+ -Cl- cotransporter 2

Na+ -K+ -Cl- cotransporter 2 (NKCC2; SLC12A1) is an integral membrane protein that comes as three splice variants and mediates the cotranslocation of Na+ , K+ , and Cl- ions through the apical membrane of the thick ascending loop of Henle (TALH). In doing so, and through the involvement of other ion transport systems, it allows this nephron segment to reclaim a large fraction of the ultrafiltered Na+ , Cl- , Ca2+ , Mg2+ , and HCO3- loads. The functional relevance of NKCC2 in human is illustrated by the many abnormalities that result from the inactivation of this transport system through the use of loop diuretics or in the setting of inherited disorders. The following presentation aims at discussing the physiological roles and molecular characteristics of Na+ -K+ -Cl- cotransport in the TALH and those of the individual NKCC2 splice variants more specifically. Many of the historical and recent data that have emerged from the experiments conducted will be outlined and their larger meaning will also be placed into perspective with the aid of various hypotheses.

Multiple Facets and Roles of Na+-K+-Cl−Cotransport: Mechanisms and Therapeutic Implications

The Na+-K+-Cl−cotransporters play key physiological and pathophysiological roles by regulating the membrane potential of many cell types and the movement of fluid across a variety of epithelial or endothelial structures. As such, they should soon become invaluable targets for the treatment of various disorders including pain, epilepsy, brain edema, and hypertension. This review highlights the nature of these roles, the mechanisms at play, and the unresolved issues in the field.

Molecular features and physiological roles of K+-Cl- cotransporter 4 (KCC4)

A K⁺-Cl^- cotransport system was documented for the first time during the mid-seventies in sheep and goat red blood cells. It was then described as a Na⁺-independent and ouabain-insensitive ion carrier that could be stimulated by cell swelling and N-ethylmaleimide (NEM), a thiol-reacting agent. Twenty years later, this system was found to be dispensed by four different isoforms in animal cells. The first one was identified in the expressed sequence tag (EST) database by Gillen et al. based on the assumption that it would be homologous to the Na⁺-dependent K⁺-Cl^- cotransport system for which the molecular identity had already been uncovered. Not long after, the three other isoforms were once again identified in the EST databank. Among those, KCC4 has generated much interest a few years ago when it was shown to sustain distal renal acidification and hearing development in mouse. As will be seen in this review, many additional roles were ascribed to this isoform, in keeping with its wide distribution in animal species. However, some of them have still not been confirmed through animal models of gene inactivation or overexpression. Along the same line, considerable knowledge has been acquired on the mechanisms by which KCC4 is regulated and the environmental cues to which it is sensitive. Yet, it is inferred to some extent from historical views and extrapolations.

Kinetics of hyperosmotically stimulated Na-K-2Cl cotransporter in Xenopus laevis oocytes

A detailed study of hypertonically stimulated Na-K-2Cl cotransport (NKCC1) in Xenopus laevis oocytes was carried out to better understand the 1 K(+):1 Cl(-) stoichiometry of transport that was previously observed. In this study, we derived the velocity equations for K(+) influx under both rapid equilibrium assumptions and combined equilibrium and steady-state assumptions and demonstrate that the behavior of the equations and curves in Lineweaver-Burke plots are consistent with a model where Cl(-) binds first, followed by Na(+), a second Cl(-), and then K(+). We further demonstrate that stimulation of K(+) movement by K(+) on the trans side is an intrinsic property of a carrier that transports multiple substrates. We also demonstrate that K(+) movement through NKCC1 is strictly dependent upon the presence of external Na(+), even though only a fraction of Na(+) is in fact transported. Finally, we propose that the larger transport of K(+), as compared with Na(+), is a result of the return of partially unloaded carriers, which masks the net 1Na(+):1K(+):2Cl(-) stoichiometry of NKCC1. These data have profound implications for the physiology of Na-K-2Cl cotransport, since transport of K-Cl in some conditions seems to be uncoupled from the transport of Na-Cl.

Regulation of Na-K-2Cl cotransport by phosphorylation and protein-protein interactions

The Na-K-2Cl cotransporter plays important roles in cell ion homeostasis and volume control and is particularly important in mediating the movement of ions and thus water across epithelia. In addition to being affected by the concentration of the transported ions, cotransport is affected by cell volume, hormones, growth factors, oxygen tension, and intracellular ionized Mg(2+) concentration. These probably influence transport through three main routes acting in parallel: cotransporter phosphorylation, protein-protein interactions and cell Cl(-) concentration. Many effects are mediated, at least in part, by changes in protein phosphorylation, and are disrupted by kinase and phosphatase inhibitors, and manoeuvres that reduce cell ATP content. In some cases, phosphorylation of the cotransporter itself on serine and threonine (but not tyrosine) is associated with changes in transport rate, in others, phosphorylation of associated proteins has more influence. Analysis of the stimulation of cotransport by calyculin A, arsenite and deoxygenation suggests that the cotransporter is phosphorylated by several kinases and dephosphorylated by several phosphatases. These kinases and phosphatases may themselves be regulated by phosphorylation of residues including tyrosine, with Src kinases possibly playing an important role. Protein-protein interactions also influence cotransport activity. Cotransporter molecules bind to each other to form high molecular weight complexes, they also bind to other members of the cation-chloride cotransport family, to a variety of cytoskeletal proteins, and to enzymes that are part of regulatory cascades. Many of these interactions affect transport and may override the effects of cotransporter phosphorylation. Cell Cl(-) may also directly affect the way the cotransporter functions independently of its role as substrate.

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.

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.

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.

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.

Endogenous Na(+)-K+ (or NH4+)-2Cl- cotransport in Rana oocytes; anomalous effect of external NH4+ on pHi

1. In Rana oocytes, measurements with chloride-sensitive microelectrodes show that the mean intracellular chloride activity (34.8 +/- 6.3 mM, n = 79) is three times higher than that expected for the passive distribution of chloride ions across the outer membrane (12.4 mM, mean membrane potential -43 +/- 8.8 mV, n = 79). 2. Reuptake of chloride into oocytes depleted by prolonged exposure to chloride-free saline takes place against the electrochemical gradient. 3. Chloride reuptake does not take place in sodium-free solution or in a sodium-substituted potassium-free solution. It is inhibited by bumetanide (10(-5) M) in the bathing medium. 4. The overall stoichiometry of the transport mechanism deduced from simultaneous measurements of intracellular sodium and chloride using ion-selective electrodes is 1Na+:1K+:2Cl-. 5. Ammonium ions substitute for potassium on the cotransporter. 6. In oocytes smaller than 0.9 mm in diameter, exposure to external ammonium causes an alkaline shift in intracellular pH as the NH3 enters and takes up H+ to form NH4+. We propose that chloride-dependent NH4+ transport contributes to the accumulation of NH4+ and causes the 'postexposure' acidification as the intracellular NH4+ releases H+ to form NH3 which is then lost from the cell. 7. In larger oocytes ammonium exposure produces a rapid reduction in pHi which may be explained in part by cotransport-mediated uptake of NH4+. Evidence is also provided for a second chloride-dependent NH4+ transport mechanism and a chloride-independent process.

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)

Ferret red cells: Na/Ca exchange and NaKCl cotransport

The primary pathway for K influx in ferret red cells is the Na-K-Cl cotransporter and the primary pathway for Ca influx is the Na/Ca exchanger. This makes ferret red cells favorable models for the study of these two transport systems. The evidence that Na/Ca exchange is of primary importance for steady state cell volume regulation and the Na-K-Cl cotransport has a minor role is presented. The approaches to, and results of, the determination of the stoichiometry, of the mechanism, and of the regulation by ATP and Mg, for Na/Ca exchange is contrasted with that taken for Na-K-Cl cotransport.

Potassium transport in lamprey (Lampetra fluviatilis) erythrocytes: Evidence for K+ channels

1. Unidirectional K+ (86Rb) influx in lamprey red blood cells was studied under different conditions. 2. The influx of 86Rb was markedly inhibited by 1 mM Ba2+ when cells were incubated in saline containing 4 mM K+. In K(+)-free media, the influx rate constant of 86Rb was lower, and 1 mM Ba2+ had no blocking effect. 3. Treatment of the red cells with 0.1 mM ouabain in the absence of external K+ resulted in the appearance of the component of 86Rb influx inhibited by 1 mM Ba2+, quinine, TEA or amiloride. 4. Similar results were obtained in red cells incubated in Na(+)-free media MgCl2-sucrose. 5. The results obtained provide evidence for the existence of K+ channels in the red cell membrane of the lamprey. Under physiological conditions (in the presence of 4 mM K+) the total rate constant for the 86Rb influx in erythrocytes was about 1.9/hr, including ouabain-sensitive (0.6/hr), Ba(2+)-sensitive (1.1/hr) and residual (0.2/hr) components.

The effects of metabolism on Na(+)-K(+)-Cl- co-transport in ferret red cells

1. The effects of altering metabolism on Na(+)-K(+)-Cl- co-transport were studied in ferret red cells. Na(+)-K(+)-Cl- co-transport was measured as the bumetanide-sensitive uptake of 86Rb. 2. Glucose, but not inosine or adenosine, sustained metabolism and maintained cell ATP content ([ATP]i) at the physiological level. [ATP]i could be reduced by prolonged incubation of cells in a substrate-free medium or more quickly by incubating cells with 2-deoxyglucose or with a mixture of iodoacetamide and glucose. 3. Na(+)-K(+)-Cl- co-transport activity was inhibited when [ATP]i was reduced to below 100 mumol (1 cell)-1 by starvation or by treatment with 2-deoxyglucose. However, a unique relationship between [ATP]i and activity could not be found. [ATP]i and the method and time course of ATP depletion all influenced activity. The inhibition of Na(+)-K(+)-Cl- co-transport, caused by reducing [ATP]i could be partially reversed by restoring [ATP]i to normal. 4. Increasing the concentration of intracellular ionized magnesium [( Mg2+]i) did not stimulate co-transport activity in ATP-depleted cells. This contrasts with the substantial stimulation seen in cells with normal [ATP]i. 5. Vanadate stimulated Na(+)-K(+)-Cl- co-transport activity in ATP-depleted cells but not in cells with normal [ATP]i. Fluoride did not affect activity at any [ATP]i. 6. The effects of some sulphydryl reagents on Na(+)-K(+)-Cl- co-transport were also examined. n-Ethylmaleimide (1 mM) inhibited Na(+)-K(+)-Cl- co-transport while it stimulated bumetanide-resistant potassium transport. Dithiothreitol (1 mM) did not affect activity. Iodoacetamide (6 mM) appeared to reduce the inhibition of cotransport activity seen at low [ATP]i but also greatly increased cell fragility. 7. The data suggest that activity of the Na(+)-K(+)-Cl- co-transport system is controlled by a cycle of phosphorylation and dephosphorylation with the phosphorylated form being active. Phosphorylation and transport appear to be almost maximal in ferret red cells with normal [ATP]i. Reduction of [ATP]i may allow changes in phosphatase activity to manifest as changes in transport rate. Differences in the balance between phosphorylation and dephosphorylation may explain tissue-dependent variations in the response of the system to various stimuli.

Effects of low ionic strength media on passive human red cell monovalent cation transport

1. The effect of low ionic strength media on the residual, i.e. (ouabain + bumetanide + Ca2+)-insensitive, K+ influx was characterized in human red blood cells. 2. This K+ flux was enhanced significantly in isotonic solutions of low ionic strength using sucrose to maintain constant osmolarity. This effect was found for fresh red blood cells as well as for stored (bank) red blood cells. However, the absolute magnitude of K+ influx in solutions of low ionic strength was halved for stored red blood cells. 3. Anion replacement of Cl- by CH3SO4- did not affect residual K+ fluxes, showing that Cl- -dependent transport pathways (e.g. the KCl co-transporter) are not involved in the low ionic strength effect. 4. The enhanced K+ influx in low ionic strength media was reversible when the cells were resuspended in a solution of physiological ionic strength. 5. K+ influx measured in light and dense fractions of erythrocytes (separated by centrifugation and corresponding to samples enriched with either 'young' or 'mature' red cells) showed that the low ionic strength effect does not change markedly with cell age. 6. Low ionic strength media elevated residual, i.e. (ouabain + bumetanide + Ca2+)-insensitive, influx of both K+ and Na+ by about the same amount. In both cases the flux was linear with concentration in the range investigated (0.25-10 mM). No significant increase in the uptake of the cations Ca2+ and lysine in low ionic strength solutions could be found. 7. In CH3SO4- -containing solutions of physiological ionic strength the residual K+ influx was almost independent of cell volume, whereas this flux in CH3SO4- -containing solutions of low ionic strength declined as cell volume was increased. 8. K+ flux measurements in solutions of different external pH, where NaCl was replaced by sodium gluconate or sodium glucuronate, showed that the reduced ionic strength is of more importance for the enhanced residual K+ influx than the changed transmembrane potential or the changed intracellular pH. However, a small pH dependence could be found, the K+ flux passing through a minimum around pHi 7.3. 9. Hydrostatic pressure enhanced the residual K+ flux in media of low ionic strength synergistically, so that very large fluxes (greater than 10 mmol (1 cells)-1 h-1) were obtained at 40 MPa. The apparent activation volumes (delta V*) for the pressure-sensitive K+ flux were -108 and -69 ml mol-1 in low ionic strength or physiological ionic strength solutions respectively.(ABSTRACT TRUNCATED AT 400 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.

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.

Membrane transport of potassium ions in erythrocytes of the American black bear, Ursus americanus

1. Membrane transport of K ions was investigated in red blood cells of bears by methods of measurement of unidirectional isotopic fluxes. 2. Unlike red cells of dogs, red cells of bears exhibited a significant, though small, component of ouabain-sensitive K influx. 3. Ouabain-insensitive K influx, as in other carnivore cells, was activated by swelling and inhibited by shrinkage. Swelling-induced K influx was dependent upon presence of chloride ions but was not inhibited by furosemide or bumetanide. 4. Ouabain-sensitive K influx was largest with ATP and with high concentration of Na in the cell, but it persisted in the absence of cytoplasmic Na or ATP. It was also resistant to the drug, harmaline, at a concentration that in other cells fully inhibits ouabain-sensitive K influx. 5. It was concluded that under such adverse conditions ouabain-sensitive K influx represents another mode of the Na/K pump not fully described elsewhere. 6. Also, as in low K red cells of sheep and goat, apparent absence of Na/K pump activity in carnivore red cells may represent suppression rather than elimination of activity. 7. Ouabain-insensitive K influx showed a seasonal pattern with minima occurring in early winter, earlier than for the minimum observed in Na influx. 8. Ouabain-sensitive K influx tended to be lower in the hibernation season of the bear, but the seasonal pattern was not consistent.

The coupled movements of sodium and chloride across the basolateral membrane of frog skin epithelium

1. When frog skin epithelium was exposed to a chloride-free solution bathing the basolateral side of the frog skin preparation the short-circuit current fell and there was a simultaneous loss of chloride and water from its cells. This effect was partially blocked by furosemide when this drug was added to the basolateral bathing solution. 2. Under control conditions and when added to the solution bathing the basolateral side of the preparation furosemide had no effect on the ion and water contents of the frog skin epithelium. 3. Furosemide but not SITS (4-acetamide-4'-isothiocyanate-stilbene-2,2'-disulphonic acid) or amiloride blocked the recovery of short-circuit current and the reuptake of chloride and water by preparations pre-incubated with chloride-free solution on the basolateral side. The recovery of the short-circuit current was also blocked by the replacement of basolateral potassium by sodium. 4. The effect on the short-circuit current of graded replacements by impermeant ions of sodium or chloride did not show saturation for concentrations of these ions up to their control values. 5. Replacement of basolateral potassium by sodium inhibited the short-circuit current and the recovery observed when potassium was reintroduced in the basolateral bathing solution was blocked by furosemide. 6. The replacement of basolateral sodium or chloride by impermeant ions induced an immediate fall in the intracellular concentrations of both sodium and chloride suggesting that the transport system coupling the movements of the two ions across the basolateral membrane is operative under control conditions. 7. It is proposed that the coupled movements of sodium and chloride across the basolateral membrane of the frog skin epithelium are mediated by a sodium-potassium-2 chloride co-transport system which under control conditions is very near equilibrium.

K-independent Na-Cl cotransport in bovine tracheal epithelial cells

Uptakes of 22Na, 36Cl, and 86Rb into isolated bovine tracheal epithelial cells were measured in the presence and absence of 10 microM bumetanide. Preincubation with ouabain (0.5 mM) did not alter initial rates of Na and Cl uptakes but prolonged from 0.5 or less to 2 min the period in which Na uptake is linear with time. Initial rates of bumetanide-inhibitable Na and Cl uptakes (influxes), measured for 2 min in identical solutions, were similar in magnitude (bumetanide-sensitive Cl influx/bumetanide-sensitive Na influx = 1.2). Omission of K did not affect bumetanide-sensitive Na or Cl influx. Cl influx was not affected by 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid. Amiloride (0.1 mM) partially inhibited the bumetanide-insensitive Na influx but had no effect on the bumetanide-sensitive Na influx. Rb influx was not affected by bumetanide but was markedly reduced by ouabain and slightly reduced by Ba, the combination being additive. Half-maximally inhibitory concentration values for inhibition of Cl influx by 4 sulfamoylbenzoic acid derivatives were as follows (in mumol/l): 0.125 benzmetanide; 0.64 bumetanide; 16 piretanide; and 26 furosemide. Affinities of Na and Cl for the bumetanide-inhibitable cotransport process were determined by measuring bumetanide-sensitive Cl influx at varying [Na] and bumetanide-sensitive Na influx at varying [Cl]. Both plots were hyperbolic, and the K0.5 values for Na and Cl were 4.1 and 53.9 mM, respectively. Bumetanide-inhibitable Cl influx was not altered by secretory stimuli (epinephrine, A23187) but was more than doubled by osmotic shrinkage (200 mM mannitol or sucrose).(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.

Characterization of Na+/K+/Cl- cotransport in cultured HT29 human colonic adenocarcinoma cells

A Na+/K+/Cl- cotransport pathway has been examined in the HT29 human colonic adenocarcinoma cell line using 86Rb as the K congener. Ouabain-resistant bumetanide-sensitive (OR-BS) K+ influx in attached HT29 cells was 17.9 +/- 0.9 nmol/min per mg protein at 25 degrees C. The identity of this pathway as a Na+/K+/Cl- cotransporter has been deduced from the following findings: (a) OR-BS K+ influx ceased if the external Cl- (Cl-o) was replaced by NO3- or the external Na+ (Na+o) by choline; (b) neither OR-BS 24Na+ nor 36Cl- influx was detectable in the absence of external K+ (K+o); and (c) concomitant measurements of 86Rb+, 22Na+, and 36Cl- influx indicated that the stoichiometry of the cotransport system approached a ratio of 1N+:1K+:2Cl-. In addition, OR-BS K+ influx was exquisitely sensitive to cellular ATP levels. Depletion of the normal ATP content of 35-40 nmol/mg protein to 10-15 nmol/mg protein, a concentration at which the ouabain-sensitive K+ influx was unaffected, completely abolished K+ cotransport. OR-BS K+ influx was slightly reduced by the divalent cations Ca2+, Ba2+, Mg2+ and Mn2+. Although changes in cell volume, whether shrinking or swelling, did not influence OR-BS K+ influx, ouabain-sensitive K+ influx was activated by cell swelling. As in T84 cells, we found that the OR-BS K+ influx in HT29 cells was stimulated by exogenous cyclic AMP analogues and by augmented cyclic AMP content in response to vasoactive intestinal peptide, forskolin, norepinephrine and forskolin or prostaglandin E1.

Furosemide and Ca2+ affect 86Rb+ efflux from pancreatic beta-cells by different mechanisms

The interaction between furosemide, calcium and D-glucose on the 86Rb+ efflux from beta-cell-rich mouse pancreatic islets was investigated in a perifusion system with high temporal resolution. Raising the glucose concentration from 4 to 20 mM induced an initial decrease in 86Rb+ efflux, which was followed by a steep increase and then a secondary decrease. Removal of extracellular calcium increased the 86Rb+ efflux at 4 mM D-glucose but reduced it at 20 mM. The initial biphasic changes in 86Rb+ efflux induced by 20 mM D-glucose were inhibited by calcium deficiency. Furosemide (100 microM) reduced the 86Rb+ efflux rate both at 4 and 20 mM D-glucose and the magnitudes appeared to be similar at either glucose concentration. Furosemide (100 microM) reduced the glucose-induced (10 mM) 45Ca+ uptake but did not affect the basal (3 mM D-glucose) 45Ca+ uptake. However, the ability of furosemide (100 microM) to reduce the 86Rb+ efflux at a high glucose concentration (20 mM) was independent of extracellular calcium. The inhibitory effects of furosemide and calcium deficiency on the 86Rb+ efflux rate appeared to be additive. It is concluded that the effect of furosemide on 86Rb+ efflux is not secondary to reduced calcium uptake and that the effects of furosemide and calcium deficiency are mediated by different mechanisms. The effect of furosemide is compatible with inhibition of loop diuretic-sensitive co-transport of Na+, K+ and Cl- and the effect of calcium deficiency with reduced activity of calcium-regulated potassium channels.

Co-ordinated variations in chloride-dependent potassium transport and cell water in normal human erythrocytes

1. The capacity of the loop-diuretic-sensitive Na+-K+-Cl- system in normal human erythrocytes shows tenfold interindividual variation between different donors, although the transport rate is constant from month to month for any one donor. 2. The present work shows that this variation in Na+-K+-Cl- transport is inversely correlated with a low-capacity loop-diuretic-insensitive K+ transport, which is chloride dependent and is stimulated by cell swelling in hypotonic media. 3. These variations in K+ transport from donor to donor are related to cell water. Those donors who show high loop-diuretic-sensitive Na+-K+-Cl- co-transport have lower cell water and vice versa.

Vanadate and fluoride effects on Na+-K+-Cl- cotransport in squid giant axon

The effects of vanadate and fluoride on the Na+-K+-Cl- cotransporter of the squid giant axon were assessed. In axons not treated with these agents, intracellular dialysis with ATP-depleting fluids caused bumetanide-inhibitable 36Cl influx to fall with a half time of approximately 16 min. In the presence of either 40 microM vanadate or 5 mM fluoride, the decay of bumetanide-inhibitable 36Cl influx was significantly slowed; half time for vanadate-treated axons is 45 min and for fluoride-treated axons is 37 min. These agents are not exerting their effects on Na+-K+-Cl- cotransport by influencing the rate of ATP depletion of the axon, since they had no effect on the ATP hydrolysis rate of an optic ganglia homogenate. We therefore suggest that these data support the hypothesis that Na+-K+-Cl- cotransport in squid axons is regulated by a phosphorylation-dephosphorylation mechanism and that vanadate and fluoride reduce the rate of dephosphorylation by inhibiting a protein phosphatase.

Photoaffinity labelling of a 150 kDa (Na + K + Cl)-cotransport protein from duck red cells with an analog of bumetanide

We have used a radiolabelled, benzophenone analog of bumetanide, 4-[3H]benzoyl-5-sulfamoyl-3-(3-thenyloxy)benzoic acid ([3H]BSTBA) to photolabel plasma membranes from duck red blood cells. BSTBA, like bumetanide, is a loop diuretic and a potent inhibitor of (Na + K + Cl) cotransport, and [3H]BSTBA binds to intact duck red cells with a high affinity similar to that of [3H]bumetanide (K 1/2 congruent to 0.1 microM). We incubated duck red cells with [3H]BSTBA, then lysed the cells and exposed the ghosts to ultraviolet light. The ghosting and photolysis was done at 0 degree C to prevent dissociation of the [3H]BSTBA. The ghosts were then sonicated to remove the nuclei and run on SDS-polyacrylamide gels. Analysis of H2O2-digested gel slices revealed [3H]BSTBA to be incorporated into a protein of approx. 150 kDa. This is the same molecular weight we obtain for a protein from dog kidney membranes which is photolabelled by [3H]BSTBA in a manner highly consistent with labelling of the (Na + K + Cl) cotransporter (Haas and Forbush (1987) Am. J. Physiol. 253, C243-C252). Several lines of evidence strongly suggest that the 150 kDa protein from duck red cell membranes is an integral component of the (Na + K + Cl)-cotransport system in these cells: (1) Photolabelling of this protein by [3H]BSTBA is blocked when 10 microM unlabelled bumetanide is included in the initial incubation medium with [3H]BSTBA; (2) Photoincorporation of [3H]BSTBA into the 150 kDa protein is markedly increased when the initial incubation medium is hypertonic or contains norepinephrine, conditions which similarly stimulate both (Na + K + Cl) cotransport and saturable [3H]bumetanide binding in duck red cells; (3) The photolabelling of this protein shows a saturable dependence on [3H]BSTBA concentration, with a K1/2 (0.06 microM) similar to that for the reversible, saturable binding of [3H]BSTBA and [3H]bumetanide to duck red cells; and (4) [3H]BSTBA photoincorporation into the 150 kDa protein, like saturable [3H]bumetanide binding to intact cells, requires the simultaneous presence of Na+, K+, and Cl- in the medium containing the radiolabelled diuretic.

The effects of magnesium on potassium transport in ferret red cells

1. The magnesium dependence of net and isotopic (using 86Rb as tracer) potassium transport was measured in fed ferret red cells. Bumetanide (0.1 mM) was used to dissect total flux into two components: bumetanide sensitive and bumetanide resistant. 2. Increasing the external magnesium concentration from zero (added) to 2 mM stimulated bumetanide-sensitive uptake by 16% but inhibited the bumetanide-resistant component by about 20%. 3. Ionophore A23187 was used to control internal magnesium concentration. A23187 was usually present in the cells during measurement of isotopic fluxes but was washed away before measurement of net fluxes. The magnesium-buffering characteristics of fed ferret red cells were assessed during these experiments. The cytoplasm acts as a high-capacity, low-affinity magnesium buffer over most of the range. Some high-affinity binding was seen in the presence of A23187 and 2 mM-EDTA. 4. A23187 itself slightly inhibits bumetanide-sensitive potassium transport. 5. Bumetanide-sensitive potassium transport is strongly dependent on the concentration of internal ionized magnesium. Transport is 35% maximal at 10(-7) M and increases up to the maximal rate at 1.3 mM. Further increase in ionized magnesium concentration to 3.5 mM has no additional effect. The curve relating activity to magnesium concentration is steepest at the physiological magnesium concentration. The effects of changing magnesium concentration are fully reversible. 6. Reduction of internal ionized magnesium concentration to 10(-7) M with A23187 and EDTA approximately doubles bumetanide-resistant potassium transport. 7. Bumetanide-sensitive fluxes occur via the sodium-potassium-chloride co-transport system under the conditions used. Results described in this paper thus suggest that internal magnesium may be an important physiological controller of sodium-potassium-chloride co-transport activity.

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.

Characteristics and functions of Na-K-Cl cotransport in epithelial tissues

This review summarizes our present understanding of Na-K-Cl cotransport and its physiological role in absorption and secretion of electrolytes and water in epithelial tissues. In the past several years an extensive literature about this cotransporter has developed due to its widespread distribution in a variety of cell types and its essential role in fluid and electrolyte transport in several epithelial tissues. We summarize this literature and speculate on the future characterization of this transport system. Although this review focuses on cotransport as it relates to absorptive and secretory processes in epithelia, important information concerning the pharmacology, stoichiometry, and regulation of Na-K-Cl cotransport in nonepithelial systems (i.e., erythrocytes, fibroblasts, squid axon, etc.) has been included to supplement areas that are less well established in the epithelial literature.

Effect of membrane potential on furosemide-inhibitable sodium influxes in human red blood cells

Furosemide-inhibitable Na influx (a measure of Na/K/Cl cotransport) was determined as a function of membrane potential in human red blood cells. The membrane potential was varied from -42 to +118 mV using valinomycin and gradients of K. The furosemide-inhibitable, unidirectional Na influx was independent of membrane potential over the entire range of potentials. The change in flux per mV, 0.443 mumol/(liter cells.hr.mV), was not significantly different from zero. The mean flux was 153 +/- 16 mumol/(liter cells.hr) (+/- SEM, n = 71). The ouabain and furosemide-resistant influxes of Na and K were also measured as functions of membrane potential using either valinomycin and K or a chloride-free, tartrate flux medium to vary membrane potential. The unidirectional Na influx decreased slightly as the membrane potential was increased from negative potentials to about +10 mV. At higher membrane potentials Na influx rose dramatically with potential. This increase was not reversible and was also observed with K influx.

The effects of calcium on potassium transport in ferret red cells

1. The effects of changes in the concentration of intracellular calcium on potassium transport were investigated in ferret red cells. Bumetanide was used to divide potassium transport into three components: total, bumetanide sensitive and bumetanide resistant. The bumetanide-sensitive component is equivalent to sodium-potassium-chloride co-transport. 2. Internal calcium concentration was controlled with the ionophore A23187 which was present throughout the experiments. 3. Changes in internal ionized calcium over the range 5 X 10(-10) M to 7 X 10(-7) M did not affect any component of potassium uptake. 4. Increasing the internal ionized calcium concentration above 10(-6) M stimulated bumetanide-resistant potassium transport. Half-maximal stimulation of this system was achieved with 3 X 10(-6) M-internal calcium. The system spontaneously inactivated after the initial activation by calcium and ionophore. Transport was inhibited by 1 mM-quinine. 5. Increasing the internal ionized calcium concentration to 10(-5) M had no effect on bumetanide-sensitive transport. 6. Concentrations of intracellular ionized calcium above 10(-5) M inhibited all three components of transport. Inhibition of the bumetanide-sensitive component was only slightly reversed when internal calcium concentration was reduced to normal. 7. Physiological changes in internal ionized calcium concentration do not affect sodium-potassium-chloride co-transport in ferret red cells. Very high concentrations of calcium inhibit transport, probably by an indirect mechanism. 8. In the course of the experiments the concentration of ionized intracellular magnesium in oxygenated ferret red cells was found to be about 0.65 mM.

Na-K-2Cl cotransport in winter flounder intestine and bovine kidney outer medulla: [3H] bumetanide binding and effects of furosemide analogues

The effects of several sulfamoyl benzoic acid derivatives on Na-K-Cl cotransport were investigated in winter flounder intestine. The relative efficacy (IC50 values) and order of potency of these derivatives were benzmetanide, 5 X 10(-8) M greater than bumetanide 3 X 10(-7) M greater than piretanide 3 X 10(-6) M greater than furosemide 7 X 10(-6) M greater than amino piretanide 1 X 10(-5) 3-amino-4-penoxy-5-sulfamoyl benzoic acid. Binding of [3H] bumetanide was studied in microsomal membranes from winter flounder intestine and compared to that in bovine kidney outer medulla. Binding was also studied in brush-border membranes from winter flounder intestine. The estimated values for Kd and number of binding sites (n) were: bovine kidney, Kd = 1.6 X 10(-7), n = 10.5 pmol/mg protein; winter flounder intestine, Kd 1.2 X 10(-7), n = 7.3 pmol/mg protein, and brush-border membranes from winter flounder, Kd = 5.3 X 10(-7), n = 20.4 pmol/mg protein. The estimated Kd for bumetanide binding to winter flounder brush-border membranes derived from association and dissociation kinetics was 6.8 X 10(-7) M. The similarity in magnitudes of IC50 and Kd for bumetanide suggests that the brush-border cotransporter is ordinarily rate-limiting for transmural salt absorption and that bumetanide specifically binds to the cotransporter. Measurement of bumetanide binding at various concentrations of Na, K and Cl showed that optimal binding required all three ions to be present at about 5 mM concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of anion transport in the passive movement of lead across the human red cell membrane

Passive Pb transport across the red cell membrane has been studied by measuring Pb uptake from Pb-buffered solutions into resealed ghosts containing EGTA. Over 90% of Pb uptake occurs by a pathway which is inhibited by drugs which block anion transport. The order of effectiveness is 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS) and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS) greater than phloretin greater than furosemide and bumetanide. Ouabain and cytochalasin B are ineffective. This implicates the anion-exchange mechanism in Pb uptake. The rate of Pb uptake by this route is directly proportional to external Pb2+ and HCO3- concentrations, and inversely proportional to the H+ concentration. These findings suggest that Pb transport depends on the formation of PbCO3 in solution. Pb transport depends upon the presence of a second anion. In the presence of HCO3-, the rate is stimulated in the order ClO4- less than NO3- and CH3CO2- less than F- less than Cl- less than Br- less than I-. The temperature dependence of Pb uptake is similar to that of HCO3-(-)Cl- exchange. Changes in membrane potential appear to influence Pb transport. The effects are small and somewhat variable, but in general a negative internal potential accelerates uptake and reduces exit. A positive internal potential reduces uptake and accelerates exit. These results suggest that Pb is transported on the anion exchanger. Exchange of PbCO3 for a monovalent anion best fits the experimental data, although transport of a ternary PbCO3(-)anion- complex is a possibility.

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)

Kinetic mechanism of Na+, K+, Cl--cotransport as studied by Rb+ influx into HeLa cells: effects of extracellular monovalent ions

Ouabain-insensitive, furosemide-sensitive Rb+ influx (JRb) into HeLa cells was examined as functions of the extracellular Rb+, Na+ and Cl- concentrations. Rate equations and kinetic parameters, including the apparent maximum JRb, the apparent values of Km for the three ions and the apparent Ki for K+, were derived. Results suggested that one unit molecule of this transport system has one Na+, one K+ and two Cl- sites with different affinities, one of the Cl- sites related with binding of Na+, and the other with binding of K+(Rb+). A 1:1 stoichiometry was demonstrated between ouabain-insensitive, furosemide-sensitive influxes of 22Na+ and Rb+, and a 1:2 stoichiometry between those of Rb+ and 36Cl-. The influx of either one of these ions was inhibited in the absence of any one of the other two ions. Monovalent anions such as nitrate, acetate, thiocyanate and lactate as substitutes for Cl- inhibited ouabain-insensitive Rb+ influx, whereas sulfamate and probably also gluconate did not inhibit JRb. From the present results, a general model and a specialized cotransport model were proposed: In HeLa cells, one Na+ and one Cl- bind concurrently to their sites and then one K+(Rb+) and another Cl- bind concurrently. After completion of ion bindings Na+, K+(Rb+) and Cl- in a ratio of 1:1:2 show synchronous transmembrane movements.

The number of chloride-cation cotransport sites on Ehrlich ascites cells measured with [3H]bumetanide

Simultaneous measurements were made of net Cl influxes and [3H]bumetanide binding to Ehrlich ascites tumor cells in which the chloride-cation cotransport pathway had been activated by hypertonic challenge. There was a good linear correlation between inhibition of Cl influx during regulatory volume increase and numbers of bumetanide molecules bound per cell, consistent with high specificity of bumetanide binding to cotransport sites. The extrapolation to the number of bumetanide binding sites per cell at maximal inhibition of Cl transport thus gives the number of cotransport sites per cell as 2.0 X 10(6). From this, and the fluxes measured (not necessarily maximum fluxes), the turnover number is calculated at 50 Cl ions per site per second. Unstimulated cells in isotonic medium, with negligible bumetanide-inhibitable fluxes, have the same number of bumetanide binding sites as the activated cells undergoing volume regulation.

Potassium-chloride cotransport in resealed human red cell ghosts

Furosemide-inhibitable K influx is threefold higher in resealed ghosts of human erythrocytes than in intact cells. The enhancement is specific for K in that furosemide-inhibitable Na influx is the same in resealed ghosts and intact cells. The enhanced K influx resembles K-Cl cotransport in intact cells in that it requires Cl but not Na. N-ethylmaleimide (NEM), which stimulates furosemide-inhibitable K influx in intact cells, is without effect (or slightly inhibitory) in resealed ghosts. The failure of NEM to enhance the flux was not due to low ATP in the ghosts. These findings suggest that enhancement of the K flux in ghosts occurs by oxidation of membrane protein sulfhydryl groups, known to occur with lysis, the same sulfhydryl groups at which NEM acts by alkylation. This conclusion is supported by two observations: dithiothreitol completely prevents the increase in K influx in ghosts; this agent inhibits both oxidation of sulfhydryl groups and alkylation of them by NEM; and K influx in resealed ghosts is sensitive to changes in cell volume, just as it is in NEM-treated intact cells.

Volume, pH, and ion-content regulation in human red cells: analysis of transient behavior with an integrated model

A basic mathematical model of human red cells is presented which integrates the charge and nonideal osmotic behavior of hemoglobin and of other impermeant cell solutes with the ion transport properties of the red cell membrane. The computing strategy was designed to predict the behavior of all measurable variables in time in ways that optimize comparison with experimentally determined behavior. The need and applications of such a model are illustrated in three separate examples covering different areas of experimentation in the physiology and pathophysiology of red cells.

Stoichiometry and ion affinities of the Na-K-Cl cotransport system in the intestine of the winter flounder (Pseudopleuronectes americanus)

Na-K-Cl cotransport stoichiometry and affinities for Na, K and Cl were determined in flounder intestine. Measurement of simultaneous NaCl and RbCl influxes resulted in ratios of 2.2 for Cl/Na and 1.8 for Cl/Rb. The effect of Na and Rb on Rb influx showed first order kinetics with K1/2 values of 5 and 4.5 mM and Hill coefficients of 0.9 and 1.2, respectively. The effect of Cl on rubidium influx showed a sigmoidal relationship with K1/2 of 20 mM and a Hill coefficient of 2.0. The effects of variations in Na and Cl concentration on short-circuit current (Isc) were also determined. The K1/2 for Na was 7 mM with a Hill coefficient of 0.9 and the K1/2 for Cl was 46 mM with a Hill coefficient of 1.9. Based on the simultaneous influx measurements, a cotransport stoichiometry of 1Na:1K:2Cl is concluded. The Hill coefficients for Cl suggest a high degree of cooperativity between Cl binding sites. Measurements of the ratio of net Na and Cl transepithelial fluxes under short-circuit conditions (using a low Na Ringer solution to minimize the passive Na flux) indicate that the Cl/Na flux ratio is approximately 2:1. Therefore, Na recycling from serosa to mucosa does not significantly contribute to the Isc. Addition of serosal ouabain (100 microM) inhibited Rb influx, indicating that Na-K-Cl cotransport is inhibited by ouabain. This finding suggests that a feedback mechanism exists between the Na-K-ATPase on the basolateral membrane and the apical Na-K-2Cl cotransporter.