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

Pulmonary Circulation Transvascular Fluid Fluxes Do Not Change during General Anesthesia in Dogs

General anesthesia (GA) can cause abnormal lung fluid redistribution. Pulmonary circulation transvascular fluid fluxes (J_VA) are attributed to changes in hydrostatic forces and erythrocyte volume (EV) regulation. Despite the very low hydraulic conductance of pulmonary microvasculature it is possible that GA may affect hydrostatic forces through changes in pulmonary vascular resistance (PVR), and EV through alteration of erythrocyte transmembrane ion fluxes (_ionJ_VA). Furosemide (Fur) was also used because of its potential to affect pulmonary hydrostatic forces and _ionJ_VA. A hypothesis was tested that J_VA, with or without furosemide treatment, will not change with time during GA. Twenty dogs that underwent castration/ovariectomy were randomly assigned to Fur (n = 10) (4 mg/kg IV) or placebo treated group (Con, n = 10). Baseline arterial (BL) and mixed venous blood were sampled during GA just before treatment with Fur or placebo and then at 15, 30 and 45 min post-treatment. Cardiac output (Q) and pulmonary artery pressure (P_AP) were measured. J_VA and _ionJ_VA were calculated from changes in plasma protein, hemoglobin, hematocrit, plasma and whole blood ions, and Q. Variables were analyzed using random intercept mixed model (P < 0.05). Data are expressed as means ± SE. Furosemide caused a significant volume depletion as evident from changes in plasma protein and hematocrit (P < 0.001). However; Q, P_AP, and J_VA were not affected by time or Fur, whereas erythrocyte fluid flux was affected by Fur (P = 0.03). Furosemide also affected erythrocyte transmembrane K⁺ and Cl⁻, and transvascular Cl⁻ metabolism (P ≤ 0.05). No other erythrocyte transmembrane or transvascular ion fluxes were affected by time of GA or Fur. Our hypothesis was verified as J_VA was not affected by GA or ion metabolism changes due to Fur treatment. Furosemide and 45 min of GA did not cause significant hydrostatic changes based on Q and P_AP. Inhibition of Na⁺/K⁺/2Cl⁻ cotransport caused by Fur treatment, which can alter EV regulation and J_VA, was offset by the Jacobs Stewart cycle. The results of this study indicate that the Jacobs Stewart cycle/erythrocyte Cl⁻ metabolism can also act as a safety factor for the stability of lung fluid redistribution preserving optimal diffusion distance across the blood gas barrier.

Effect of frusemide on transvascular fluid fluxes across the lung in exercising horses

REASONS FOR PERFORMING STUDY

Frusemide (Fru) is widely prescribed for management of racehorses experiencing EIPH. The effect of Fru in the lung appears to be a reduction in transcapillary pressures and inhibition of the erythrocyte anion exchange, which may lead to attenuation of transpulmonary fluid fluxes during exercise.

HYPOTHESIS

Treatment with Fru will attenuate transpulmonary fluid fluxes in horses during high intensity exercise.

METHODS

In a crossover study, 6 race-fit Standardbred horses were treated with 250 mg of Fru i.v. (FruTr) or placebo (Con) 4 h before exercise on a high speed treadmill until fatigue. Arterial and central mixed venous blood, as well as CO(2) elimination and O(2) uptake, were sampled. Volume changes across the lung and transvascular fluid fluxes were calculated from changes in haemoglobin, packed cell volume, plasma protein and cardiac output (Q).

RESULTS

During exercise, Q increased in both Con and FruTr, with Q being significantly lower in FruTr (mean ± s.e. 301.8 ± 8.5 l/min at fatigue) compared to Con (336.5 ± 15.6 l/min) (P<0.01). At rest frusemide had no effect on erythrocyte (J(ER)) and transvascular (J(V-A)) fluid fluxes across the lung. Exercise had a significant effect on J(ER) and J(V-A) (P ≤ 0.02). During exercise, J(ER) (at fatigue 14.6 ± 2.3 l/min and 11.6 ± 2.2 l/min in Con and FruTr, respectively) and J(V-A) (at fatigue 14.9 ± 2.3 l/min and 12.0 ± 2.2 l/min in Con and FruTr, respectively) were not significantly different between Con and FruTr (P = 0.6 and P = 0.8 for J(ER) and J(V-A), respectively).

CONCLUSIONS AND CLINICAL IMPORTANCE

Fru does not have a measurable effect on J(ER) and J(V-A). Cardiac output was reduced in FruTr, suggesting that there were also smaller changes in the capillary recruitment and transvascular transmural hydrostatic pressures; however, this did not effect J(V-A). Therefore, Fru at the dose of 250 mg does not appear to be an effective treatment for regulating pulmonary transvascular forces during exercise in horses.

Multiple transport functions of a red blood cell anion exchanger, tAE1: its role in cell volume regulation

1. It was previously shown that expressed in Xenopus oocyte the mouse (mAE1) and the trout (tAE1) anion exchanger behave differently: both elicit anion exchange activity but only tAE1 induces a transport of organic solutes correlated with a chloride channel activity. The present data, obtained by measurement of Xenopus oocyte membrane permeability and conductance, provide evidence that tAE1 also induces a large increase in Na(+) and K(+) permeability inhibited by several AE1 inhibitors. 2. This inhibition does not result from an effect on the driving force for electrodiffusion but represents a direct effect on the cation pathway. 3. As a control, expression of cystic fibrosis transmembrane conductance regulator (CFTR) having, once stimulated by 3-isobutyl-1-methylxanthine (IBMX), the same anion conductance magnitude as tAE1 did not induce any cation movement. 4. Chloride exchange, channel activity and cation transport induced by anion exchanger expression are inhibited by free or covalently bound H2DIDS as well. This covalent inhibition is reversed by the point mutation of Lys-522, the covalent binding site of H2DIDS to the protein. These data reveal that tAE1 itself acts both as an anion exchanger and as a channel of broad selectivity. 5. All results obtained by expression of AE1 isoforms in Xenopus oocytes and those obtained in erythrocytes are consistent with the proposal that, in nucleated erythrocytes, tAE1 functions as the swelling-activated osmolyte anion channel involved in cell volume regulation. In contrast AE1 from mammalian red cells, which do not regulate their volume, lacks swelling-activated osmolyte channel properties. 6. tAE1 illustrates the ability of a specific transport system to be a multifunctional protein exhibiting other transport functions when submitted to regulation.

The non-selective voltage-activated cation channel in the human red blood cell membrane: reconciliation between two conflicting reports and further characterisation

Using the patch-clamp technique, the non-selective, voltage-activated cation channel in the human red blood cell (RBC) membrane was further characterised. Activity of the cation channel could be demonstrated at a range of salt concentrations with the current-voltage characteristics for monovalent cations going from linear to superlinear functions, depending on the cation concentration in the range of 100-500 mM. The non-selective voltage-activated cation channel was demonstrated to be permeable to the divalent cations Ca2+ and Ba2+, and even Mg2+. The current-voltage relations for the divalent cations were superlinear even at 75 mM salt concentration, but indicated outward rectification in contrast to the I-V curve for monovalent cations. The degree of activation at a given membrane potential depended strongly on the prehistory of the channel. The gating exhibited hysteretic-like behaviour, since the quasi steady-state deactivation and activation curves were displaced by approximately 25 mV. This result fully explains apparent discrepancies between V0.5-values previously obtained by slightly different experimental protocols. The possible physiological/pathophysiological role of the channel is discussed in the context of the demonstrated permeability for divalent cations.

Failure of spermatogenesis in mouse lines deficient in the Na(+)-K(+)-2Cl(-) cotransporter

The Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) carries 1 molecule of Na(+) and K(+) along with 2 molecules of Cl(-) across the cell membrane. It is expressed in a broad spectrum of tissues and has been implicated in cell volume regulation and in ion transport by secretory epithelial tissue. However, the specific contribution of NKCC1 to the physiology of the various organ systems is largely undefined. We have generated mouse lines carrying either of 2 mutant alleles of the Slc12a2 gene, which encodes this cotransporter: a null allele and a mutation that results in deletion of 72 amino acids of the cytoplasmic domain. Both NKCC1-deficient mouse lines show behavioral abnormalities characteristic of mice with inner ear defects. Male NKCC1-deficient mice are infertile because of defective spermatogenesis, as shown by the absence of spermatozoa in histological sections of their epididymides and the small number of spermatids in their testes. Consistent with this observation, we show that Slc12a2 is expressed in Sertoli cells, pachytene spermatocytes, and round spermatids isolated from wild-type animals. Our results indicate a critical role for NKCC1-mediated ion transport in spermatogenesis and suggest that the cytoplasmic domain of NKCC1 is essential in the normal functioning of this protein.

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 a novel organic solute transporter in mammalian red blood cells

1. Suspending human red blood cells in isotonic sucrose (low ionic strength, LIS) medium induces a significant increase in membrane transport of glutamine, glutamate, lactate, histidine, taurine, glycine, serine, choline and carnitine but not sorbitol or sucrose. 2. Progressive lowering of ionic strength by sucrose or NaCl replacement gave a similar activation profile for taurine influx as found earlier for residual K+(86Rb+) flux. 3. The induced taurine transport could be measured as enhanced influx and efflux. Influx was linear with external concentration up to 10 mM, largely insensitive to alteration in cell volume, and did not vary with red blood cell age. 4. Unlike previous results for residual K+ transport, altering transmembrane potential with gluconate or glucuronate media led to an increase in taurine influx similar to that observed in LIS media. Varying medium pH confirmed the effect was not due to alteration in pH. 5. The LIS-induced flux was sensitive to a variety of 'classical' anion transport inhibitors in the order of potency DNDS, DIDS, NPPB, DIOA, niflumic acid, furosemide (frusemide), glibenclamide, nitrendipine and bumetanide. 6. The taurine flux showed a temperature dependence similar to that of the LIS-induced residual K+ flux. High hydrostatic pressure (40 MPa), however, inhibited taurine flux but stimulated residual K+ influx in LIS media. 7. A significant enhanced taurine flux could be demonstrated in red blood cells of other species, including horse, cattle, pig and high and low potassium type sheep. 8. It is concluded that lowering ionic strength activates a transport pathway for organic molecules sharing some similarities with background Cl- channels and LIS-induced residual K+ fluxes. In the latter context, however, there are certain significant differences (effect of transmembrane potential; volume; pressure sensitivity; species distribution) which may be important, and the unequivocal identity of the two transport processes remains to be confirmed.

Urea inhibits NaK2Cl cotransport in human erythrocytes

We examined the effect of urea on NaK2Cl cotransport in human erythrocytes. In erythrocytes from nine normal subjects, the addition of 45 mM urea, a concentration commonly encountered in uremic subjects, inhibited NaK2Cl cotransport by 33 +/- 7%. Urea inhibited NaK2Cl cotransport reversibly, and in a concentration-dependent fashion with half-maximal inhibition at 63 +/- 10 mM. Acute cell shrinkage increased, and acute cell swelling decreased NaK2Cl cotransport in human erythrocytes. Okadaic acid (OA), a specific inhibitor of protein phosphatase 1 and 2A, increased NaK2Cl cotransport by nearly 80%, suggesting an important role for these phosphatases in the regulation of NaK2Cl cotransport. Urea inhibited bumetanide-sensitive K influx even when protein phosphatases were inhibited with OA, suggesting that urea acted by inhibiting a kinase. In cells subjected to shrinking and OA pretreatment, maneuvers expected to increase the net phosphorylation, urea inhibited cotransport only minimally, suggesting that urea acted by causing a net dephosphorylation of the cotransport protein, or some key regulatory protein. The finding that concentrations of urea found in uremic subjects inhibited NaK2Cl cotransport, a widespread transport pathway with important physiological functions, suggests that urea is not only a marker for accumulation of other uremic toxins, but may be a significant uremic toxin itself.

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.

The membrane defect in hereditary stomatocytosis

Hereditary stomatocytosis and allied conditions represent a series of diseases in which abnormal movements of univalent cations across the plasma membrane play an important part in cellular disease. The primary problem lies not in the active transporters but in the basal permeability of the membrane, which is always increased, and the extent of the increase correlates with the cellular dysfunction. A number of structural abnormalities have been described in these membranes, but the most consistent and convincing is the deficiency of a hitherto uncharacterized integral membrane protein of molecular weight 31 kDa in the severe, 'overhydrated' form of the disease. The true function of this protein remains enigmatic, but its deficiency in this condition indicates that it may have a role in the regulation of cation transport.

K+ fluxes mediated by Na(+)-K(+)-Cl- cotransport and Na(+)-K(+)-ATPase pumps in renal tubule cell lines transformed by wild-type and temperature-sensitive strains of Simian virus 40

The relative contributions of Na(+)-K(+)-ATPase pumps and Na(+)-K(+)-Cl- cotransport to total rubidium (Rb+) influx into primary cultures of renal tubule cells (PC.RC) and cells transformed either with the wild-type or a temperature-sensitive mutant of the simian virus 40 (SV40), were measured under various growth conditions. The Na(+)-K(+)-ATPase-mediated component represented 74% and 44-48% of total Rb+ influx into PC.RC and SV40-transformed cells, respectively. Proliferating transformed cells showed substantial ouabain-resistant bumetanide-sensitive (Or-Bs) Rb+ influx (41-45% of total) which indicated the presence of a Na(+)-K(+)-Cl- cotransport. The Or-Bs component of Rb+ influx was greatly reduced when temperature-sensitive transformed renal cells (RC.SVtsA58) grown in Petri dishes or on permeable filters were shifted from the permissive (33 degrees C) to the restrictive temperature (39.5 degrees C) to arrest cell growth. The ouabain-sensitive Rb+ influx mediated by the Na(+)-K(+)-ATPase, the total and amiloride-sensitive Na+ uptakes were not modified following inhibition of cell proliferation. A similar fall in the Or-Bs influx was obtained when renal tubule cells transformed by the wild-type SV40 (RC.SV) were incubated with the K+ channel blocker, tetraethylammonium (TEA) ion, which we had previously shown to arrest cell growth without affecting cell viability (Teulon et al.: J. Cell. Physiol., 151:113-125, 1992). Reinitiation of cell growth by removal of TEA or return to 33 degrees C of the temperature-sensitive cells restored the Or-Bs component of Rb influx. Taken together, these results indicate that the Na(+)-K(+)-Cl- cotransport activity is critically dependent on cell growth conditions.

Hemisodium, a novel selective Na ionophore. Effect on normal human erythrocytes

Hemisodium is a novel Na ionophore that belongs to the class of compounds called cryptands. These compounds possess an electron-rich cavity for binding of cations and are conformationally organized during synthesis to favor the selective binding of one cation over another. In media containing 145 mM NaCl and 5 mM KCl, hemisodium (10(-5) M) increased erythrocyte Na content from 23 to 345 mmol/kg.dry cell solid (dcs) over 4 h and increased water content from 1.8 to 3.5 liter/kg.dcs over the same period. K content decreased somewhat over the same time period, but this fall in K content was prevented entirely by incubation in either low Na media (to prevent net Na entry) or in Cl free media. Thus, the decrease in K content in high NaCl media was due to cell swelling, which activated KCl cotransport, and not due to a direct action of hemisodium on K permeability. Hemisodium-mediated Na transport was conductive, because erythrocyte membrane potential (Vm), determined by diS-C3-5 fluorescence, changed from -9 to +22 mV in high Na media in the presence of hemisodium and DIDS. In cells equilibrated with sulfamate, an anion with low conductive permeability, Vm changed 54 mV per 10-fold change in external Na concentration with the addition of hemisodium. In contrast, a 10-fold change in the external concentration of K, Rb, Cs, or T1 failed to alter Vm in the presence of hemisodium, suggesting a high Na specificity of the ionophore. Na conductance determined from net fluxes increased from 0.04 to 5.2 microS/cm2 with 10 microM hemisodium, and with that concentration the ratio of Na to K conductance was 45:1. Among the Na ionophores available so far, hemisodium appears to have the greatest specificity. Hemisodium may be a valuable tool in membrane transport studies.

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)

Voltage-activated cation permeability in high-potassium but not low-potassium red blood cells

We have recently reported that voltage-activated fluxes of Na, K, and Ca occur in human red blood cells [J.A. Halperin, C. Brugnara, M. Tosteson, T. Van Ha, and D. C. Tosteson. Am. J. Physiol. 257 (Cell Physiol. 26): C986-C996, 1989]. The cation permeability increases progressively as the membrane potential becomes more inside positive above +20 mV. In this paper we show that this effect also occurs in high-potassium (HK), but not in low-potassium (LK), sheep and dog red blood cells. This result suggests that the voltage-activated cation transport pathway is not the result of nonspecific dielectric breakdown of the lipid bilayer but, rather, relates to some membrane component, presumably a protein, that is expressed in HK human and sheep but not in LK sheep and dog red blood cells.

Asymmetry of Na-K-Cl cotransport in human erythrocytes

The Na-K-Cl cotransport system in human erythrocytes was studied by measuring net influxes and effluxes of Na and K. The influx of K was shown to be stimulated by Na and the influx of Na was stimulated by K, satisfying the fundamental criterion of cotransport. In addition, these mutually stimulating cation influxes had a stoichiometry of 1:1 and were entirely inhibited by furosemide; these results are also consistent with cotransport. Furthermore, the mutually stimulating influxes were entirely dependent on Cl, since they were abolished when nitrate was substituted for Cl. In contrast, cotransport, defined by mutual dependence of fluxes, was not detected in the outward direction over a range of cellular Na and K concentrations from 0 to 50 mmol/l cells. The cotransport pathway did, however, appear to mediate a Na-stimulated K efflux (but no K-stimulated Na efflux), and furosemide-inhibitable effluxes of both Na and K. Nitrate (but not sulfate) appeared to substitute for chloride in promoting Na-stimulated K efflux. Thus the Na-K-Cl cotransport system in human red cells is intrinsically asymmetric, and mediates coupled cation fluxes readily only in the inward direction.