Ion movements in human red cells independent of the sodium pump

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.

Interaction of inhibitin with the human erythrocyte Na+(Li+)i/Nao+ exchanger

The kinetic interactions of inhibitin, a peptide isolated from cultured leukaemic promyelocytes, with erythrocyte Na+/Na+ and Na+/Li+ exchanges have been investigated. Inhibitin (1 microM) reduced the ouabain- and bumetanide-resistant sodium efflux and influx by equivalent amounts indicating an inhibitin-sensitive exchange component of 0.52 mmol/l per h. This value was not significantly different from that measured as the difference in sodium-rich (140 mM) and sodium-free media (0.49 mmol/l per h). Similarly, the inhibitin-sensitive lithium efflux was equivalent to the sodium/lithium countertransport component (0.36 vs. 0.34 mmol/l per h), indicating that both exchanges were mediated by the same transport process, which is inhibitin-sensitive. The dose-response curve revealed the presence of a single inhibitin binding site per exchanger with a Ki of 2.10(-7) M. In kinetic inhibition studies, inhibitin (0.1 microM) decreased the Vmax of ouabain- and bumetanide-resistant sodium efflux with no effect on the Km for external sodium, i.e., inhibitin displayed a non-competitive mechanism of action. These findings indicate that inhibitin interacts with the Na+(Li+)i/Nao+ exchanger at a site distinct from the sodium binding site.

Characterization of Na+ transport in normal human fibroblasts and neoplastic H.Ep.2 cells and the role of inhibitin

Na+ transport was characterized in normal human fibroblasts and neoplastic H.Ep.2 cells in order to investigate the role of the endogenous peptidic factor 'inhibitin' that is secreted by a variety of neoplastic cells (including H.Ep.2) and inhibits Na+/Na+ exchange in human erythrocytes. Although active (Na+,K+-ATPase mediated) Na+ fluxes were similar in the two cell types, H.Ep.2 cells maintained higher intracellular Na+ concentration (26 mM) compared to fibroblasts (12 mM). An analysis of passive Na+ fluxes showed a difference in the handling of Na+ via ouabain and bumetanide-insensitive transport between the two cell types: H.Ep.2 cells achieved net Na+ influx via an amiloride-sensitive pathway that was only demonstrated in fibroblasts when 10% fetal calf serum (FCS) was present. Kinetic studies were undertaken to investigate the interaction between Na+ flux via Na+/H+ and Na+/Na+ exchanges. For this purpose, an outwardly directed Na+ gradient was created by loading the cells with Na+ (Nai greater than 100 mM) to activate the reverse functioning of Na+/H+ exchange (i.e., Na+out H+in). The rates of ouabain- and bumetanide-insensitive Na+ efflux were measured over a range of extracellular Na+ concentrations (Na+o 14-140 mM). In the presence of 10% FCS, the two cell types showed different responses: in fibroblasts the Na+ efflux rate showed an inverse correlation with extracellular Na+ concentration, while H.Ep.2 cells significantly increased their rate of Na+ efflux as extracellular Na+ concentration increased. So although the thermodynamic force would direct net Na+ efflux when Na+i greater than Na+o, H.Ep.2 cells were under kinetic control to perform Na+/Na+ exchange. When exogenous inhibitin was tested on fibroblasts, the steady-state intracellular Na+ concentration increased from 14 to 19 mM (p less than 0.01). In Na+-loaded fibroblasts, serum-stimulated Na+ efflux was partially inhibitin sensitive and the maximal inhibitory effect was seen when extracellular Na+ concentration was 14 mM and presumably the Na+/H+ exchanger operating in the reverse mode. This study demonstrated that, in contrast to fibroblasts, H.Ep.2 cells have a modified Na+/H+ exchange system whereby it acts in the Na+in H+out mode without exogenous growth factor activation and resists functioning in the reversed mode. It is proposed that inhibitin is the endogenous modifier of this transport system in H.Ep.2 cells with the result that H.Ep.2 cells maintain a higher concentration of intracellular Na+ compared to fibroblasts.

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.

Inhibitin: a specific inhibitor of sodium/sodium exchange in erythrocytes

An inhibitor of ouabain-insensitive sodium/sodium exchange in erythrocytes has been isolated from leukemic promyelocytes. To explore the specific effects of this inhibitor, named inhibitin, sodium transport experiments were carried out in human erythrocytes. Inhibitin reduced ouabain-insensitive bidirectional sodium transport. It did not change net sodium fluxes, had no significant effect on rubidium influx, and did not inhibit sodium-potassium-ATPase activity. The inhibitory effect of inhibitin was studied on sodium/sodium exchange and on sodium/lithium countertransport in 140 mM sodium and in sodium-free media. In the presence of sodium, inhibitin reduced sodium and lithium efflux to that observed in sodium-free medium. Inhibitin showed no reduction in sodium or lithium efflux when sodium was replaced by choline chloride or Mg2+. When inhibitin was combined with one or more of the other transport inhibitors (i.e., ouabain, furosemide, or bumetanide and amiloride), its inhibitable component remained distinct and it did not overlap with that of the other inhibitors. These studies show that inhibitin is a specific inhibitor of carrier-mediated sodium/sodium exchange and sodium/lithium countertransport processes in human erythrocytes.

Basic characterization of an ouabain-resistant, bumetanide-sensitive K+ carrier-mediated transport system in J774.2 mouse macrophage-like cell line and in variants deficient in adenylate cyclase and cAMP-dependent protein kinase activities

86Rb(K+) transport across the plasma membrane of macrophage-like cells was studied. The cells used were the wild-type J774.2 and its two variants, CT2 cells, deficient in adenylate cyclase, and J7H1 cells, deficient in cAMP-dependent protein kinase. In the three cell lines about 15% of the total 86Rb(K+) influx is transported by the K+ carrier-mediated transport system. The 86Rb(K+) efflux carried by the same transporter is negligible when measured in the absence of ouabain in the medium. Therefore this carrier conducts a net inward flux of K+ under the experimental conditions used. The transporter is sensitive to extracellular Na+ and inhibited by 'loop' diuretics; bumetanide inhibits ouabain-resistant 86Rb(K+) influx with IC50 of 0.1, 5.0, and 0.05 microM for J774.2, CT2 and J7H1 macrophages, respectively. The membrane potential of the three cells was measured, using the distribution of [3H]tetraphenylphosphonium [( 3H]TPP+) across the plasma membrane, and found to be -80.1, -108.5 and -105.1 mV for J774.2, CT2 and J7H1 cells, respectively. The addition of bumetanide to the cell medium does not alter [3H]TPP+ uptake indicating that the transporter is electrically silent. It is concluded that despite the differences in cAMP metabolism by the three macrophages, the basic characteristics of K+ carrier-mediated transport system of the three cells are very similar.

Diuretics. Clinical pharmacology and therapeutic use (Part I)

25 years have elapsed since the introduction of the first effective oral diuretic, chlorothiazide. Diuretics are now amongst the most widely prescribed drugs in clinical practice worldwide. Availability of these drugs has not only brought therapeutic benefit to countless numbers of patients but it has at the same time provided valuable research tools with which to investigate the functional behaviour of the kidney and other electrolyte-transporting tissues. Despite many remaining gaps in our knowledge of the biochemical processes involved in diuretic drug action, available compounds can be divided into 5 groups on the basis of their preferential effects on different segments of the nephron involved in tubular reabsorption of sodium chloride and water. Firstly, there is heterogeneous group of chemicals that share the common property of powerful, short-lived diuretic effects that are complete within 4 to 6 hours. These agents act on the thick ascending limb of Henle's loop and are known as 'high ceiling' or 'loop' diuretics. The second group are the benzothiadiazines and their many related heterocyclic variants, all of which localise their effects to the early portion of the distal tubule. The third group comprises the potassium-sparing diuretics which act exclusively on the Na+-K+/H+ exchange mechanisms in the late distal tubule and cortical collecting duct. The action of drugs in groups 2 and 3 is prolonged to between 12 and 24 hours. The fourth group consists of diuretics that are chemically related to ethacrynic acid but have the unusual property of combining within the same molecule the property of saluresis and uricosuria. These compounds have actions, to different individual extents, in the proximal tubule, thick ascending limb, and early distal tubule and are known as 'polyvalent' diuretics. Finally, there is a mixed group of weak or adjunctive diuretics which includes the vasodilator xanthines such as aminophylline, and the osmotically active compounds such as mannitol. Available evidence on the molecular mechanisms of action of diuretics in each group is reviewed. The haemodynamic, humoral and physical factors involved in control of electrolyte and fluid handling by the kidney in normal conditions and pathological states are discussed in relation to rational choices of different diuretics in the treatment of various oedematous and non-oedematous conditions.

Role of the furosemide-sensitive Na+/K+ transport system in determining the steady-state Na+ and K+ content and volume of human erythrocytes in vitro and in vivo

To study the physiological role of the bidirectionally operating, furosemide-sensitive Na+/K+ transport system of human erythrocytes, the effect of furosemide on red cell cation and hemoglobin content was determined in cells incubated for 24 hr with ouabain in 145 mM NaCl media containing 0 to 10 mM K+ or Rb+. In pure Na+ media, furosemide accelerated cell Na+ gain and retarded cellular K+ loss. External K+ (5 mM) had an effect similar to furosemide and markedly reduced the action of the drug on cellular cation content. External Rb+ accelerated the Na+ gain like K+, but did not affect the K+ retention induced by furosemide. The data are interpreted to indicate that the furosemide-sensitive Na+/K+ transport system of human erythrocytes mediates an equimolar extrusion of Na+ and K+ in Na+ media (Na+/K+ "cotransport"), a 1:1 K+/K+ (K+/Rb+) and Na+/Na+ "exchange" progressively appearing upon increasing external K+ (Rb+) concentrations to 5 mM. The effect of furosemide (or external K+/Rb+) on cation contents was associated with a prevention of the cell shrinkage seen in pure Na+ media, or with a cell swelling, indicating that the furosemide-sensitive Na+/K+ transport system is involved in the control of cell volume of human erythrocytes. The action of furosemide on cellular volume and cation content tended to disappear at 5 mM external K+ or Rb+. The in vivo red cell K+ content was negatively correlated to the rate of furosemide-sensitive K+ (Rb+) uptake, and a positive correlation was seen between mean cellular hemoglobin content and furosemide-sensitive transport activity. The transport system possibly functions as a K+ and water-extruding mechanism under physiological conditions in vivo. The red cell Na+ content showed no correlation to the activity of the furosemide-sensitive transport system.

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

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

Abnormal sodium efflux in erythrocytes of patients with essential hypertension

Erythrocyte sodium efflux as well as sodium, potassium, and water content were studied in 12 untreated men with uncomplicated essential hypertension and in 18 normotensive control subjects. In the patients with essential hypertension, the rate constant for total sodium efflux was significantly lower than in the normotensives (5.96.10(-3) +/- 0.45.10(-3) min-1 vs 6.69.10(-3) +/- 0.49.10(-3) min-1; p less than 0.005), which was due to a reduced ouabain-sensitive sodium efflux rate constant. Significant differences in total sodium efflux and ouabain-sensitive sodium efflux, however, could not be demonstrated, since intracellular sodium concentrations, although insignificant, were higher in the patients with essential hypertension (6.11 +/- 0.74 mmole/liter vs 5.97 +/- 0.66 mmole/liter. The rate constants for ouabain-insensitive sodium efflux, for ouabain-insensitive furosemide-sensitive sodium efflux, and for passive (ouabain-insensitive furosemide-insensitive) sodium efflux were similar in hypertensives and in normotensives. The cause of the reduced rate constant for ouabain-sensitive sodium efflux is not clear. However, as suggested for other types of altered erythrocyte transport mechanisms described recently, it might be determined genetically.

Occurrence of passive furosemide-sensitive transmembrane potassium transport in cultured cells

Furosemide (1 x 10(-4) M) inhibits a proportion of the total passive (ouabain-insensitive) K+ influx into primary chick heart cell cultures (85%), BC3H1 cells (75%), MDCK cells (40%) and HeLa cells (57%). This action of furosemide upon K+ influx is independent of (Na+ + K+)-pump inhibition since the furosemide-sensitive component of the K+ influx is identical in the presence and absence of ouabain (1 x 10(-3) M). For HeLa cells the passive, furosemide-sensitive component of K+ influx is markedly dependent upon the external K+, Na+ and Cl- content. Acetate, iodide and nitrate are ineffective as substitutes for Cl-, whereas Br- is partially effective. Partial Cl- replacement by NO3- gave an apparent affinity of 100 mM [Cl]. Na+ replacement by choline+ abolishes the furosemide-sensitive component, whereas Li+ replacement reduces this component by 48%. Partial Na+ replacement by choline+ gives an apparent affinity of 25 mM [Na+]. Variation in the external K+ content gives an affinity for the furosemide-sensitive component of approx. 1.0 mM. Furosemide inhibition of the passive K+ influx is of high affinity, half-maximal inhibition being observed at 5 x 10(-6) M furosemide. Piretanide (1 x 10(-4) M) and phloretin (1 x 10(-4) M) inhibit the same component of passive K+ influx as furosemide; ethacrynic acid and amiloride (both 1 x 10(-4) M) partially so. The stilbene, SITS (1 x 10(-6) M), was ineffective as an inhibitor for the furosemide-sensitive component.

Cation movements across mouse red blood cells

This paper describes some features of the Na and K transport systems in red cells obtained from B10.A mice. When mouse erythrocytes were incubated in a plasma-like control medium, the scillaren-sensitive Na efflux was 3.6 +/- 0.4 mmol/l red blood cells per h while the scillaren-sensitive K influx was 3.1 +/- 0.3, values not significantly different from ech other. Scillaren had no significant effect on either Na influx or K efflux. There was a large (approx. 3 mmol/l red blood cells per h) scillaren-sensitive, Na-Na exchange diffusion component present under K-free conditions. When K was present in the incubation medium, this exchange system was suppressed.

Studies on lithium transport across the red cell membrane. V. On the nature of the Na+-dependent Li+ countertransport system of mammalian erythrocytes

Ouabain-resistant Na+-Li+ countertransport was studied on erythrocytes of man, sheep, rabbit, and beef. A transport system, exchanging Li+ for Na+ in a ratio of 1:1, was present in all four species. Li+ uptake by the exchange system increased 30-fold in the order man less than HK-sheep less than LK-sheep less than rabbit less than LK-beef. This order is identical to that of ouabain-resistant Na+-Na+ exchange in these species, but bears no relation to the Na+-K+ pump activity. The activity of the Na+-Li+ exchange system varied up to 7 and 16-fold among individual red cell specimens from man and beef, the variability being much smaller in sheep and rabbit erythrocytes. The affinities of the system for Li+ and Na+ were similar among the species and individuals (half saturation of the external site at about 1 mM Li+ and 50 mM Na+, respectively). 50-60% of Na+-Li+ exchange was blocked by N-ethylmaleimide in all species. p-Chloromercuribenzene sulfonate inhibited the exchange only in beef and sheep erythrocytes (60-80%). The two SH-reagents act by decreasing the maximum activity of the system, whilst leaving its affinity for Li+ unaltered. Phloretin was a potent inhibitor in all species. 1 mM each of furosemide, ethacrynic acid, and quinidine induced only a slight inhibition. The Na+-Li+ exchange of human and beef erythrocytes increased 3.5-fold upon elevation of the extracellular pH from 6 to 8.5, the pH-dependence arising from a change in affinity of the system for the cations and being similar to that reported for ouabain-resistant Na+-Na+ exchange in beef erythrocytes. It is concluded that a transport system exists in the red cell membranes of the four species which can mediate ouabain-resistant exchange of either Na+ for Na+, Na+ for Li+, or Li+ for Li+. The exchange system exhibits essentially identical transport characteristics in the four species, but shows a marked inter- and intra-species variability in maximum transport capacity and some differences in susceptibility towards inhibitors. A similar transport system is probably present also in other tissues. The exchange system seems to be distinct from the conventional Na+-K+ pump and shows no clear relation to one of the furosemide-sensitive, ouabain-resistant Na+ transport systems described in the literature.

Sodium outflux and influx within nerve terminals from chronic epileptogenic foci

The Na+-pump (oMpNa) or maximal sodium outflux inhibited by ouabain was studied in nerve terminals isolated from the primary and mirror epileptogenic foci of freezing lesions. In addition, the Na+ for Na+ exchange diffusion that is sensitive to ethacrynic acid and furosemide was also analyzed. oMpNa in control and epileptogenic states had similar and indistinguishable activation curves for K0 (extracellular K) in a sodium medium except for a two-fold difference in magnitude. However, the apparent affinity of the Na+-pump for K0 as measured by K1/2 was shifted to the right or decreased in epileptogenic foci (2 mM K0 compared to 0.5 mM in controls) when measured in a magnesium medium. A decrease in the apparent affinity for internal sodium was also observed. Of the total sodium outflux (67.7 nmol/mg/min), Na+ movements which are insensitive to ouabain and external K+ but stimulated by external Na+ and inhibited by furosemide and ethacrynic acid (20 nmol/mg/min or 32% of sodium outflux) represented the Na+ for Na+ exchange diffusion in nerve terminals. Na+ influx rising with increasing internal sodium in the presence of ouabain and blocked by furosemide represent the corresponding inward Na+ movement. No differences were observed between controls and epileptogenic states.

Inhibition of anion permeability by amphiphilic compounds in human red cell: evidence for an interaction of niflumic acid with the band 3 protein

In human erythrocyte, permeability to the anion is instantaneously, reversibly, and noncompetitively inhibited by the nonsteroidal anti-inflammatory drug, niflumic acid. The active form of this powerful inhibitor (I50 = 6 X 10(-7) M) is the ionic form. We demonstrated that: (i) The binding of niflumic acid to the membrane of unsealed ghosts show one saturable and one linear component over the concentration range studied. The saturable component vanishes when chloride transport is fully inhibited by covalently bound 4-acetamido-4'-isothiocyano stilbene-2,2'-disulfonic acid (SITS). Our estimate of these SITS protectable niflumate binding sites (about 9 x 10(5) per cell) agrees with the number of protein molecules per cell in band 3. These sites are half-saturated with 10(-6) M niflumic acid, a concentration very close to I50. (ii) Niflumic acid inhibits the binding reaction of SITS with anion controlling transport sites. These results indicate that niflumic acid and SITS are mutually exclusive inhibitors, suggesting that niflumic acid interacts with the protein in band 3. Niflumic acid also decreases glucose and ouabain-insensitive sodium permeabilities. However, these effects are produced at a very high concentration of niflumic acid (in millimolar range), suggesting unspecific action, possibly through lipid phase.

Studies on the lithium transport across the red cell membrane. I. Li+ uphill transport by the Na+-dependent Li+ counter-transport system of human erythrocytes

Li+ net-transfer across cell membranes was studied on human erythrocytes and ghosts preloaded with 1-2 mM Li+ and incubated in saline media of varying composition at initial thermodynamic equilibrium for Li+. The following results were obtained: 1. Li+ is extruded from glycolyzing erythrocytes against an electrochemical gradient until a steady-state Li+ distribution is established after 24-28 h. 2. The initial rate of Li+ extrusion is not altered by ouabain or by reduction of ATP levels to less than 25% of the normal value. 3. Replacement of external Na+ by K+ or choline+ abolishes the establishment of an electrochemical Li+ gradient. 4. The Li+ distribution ratio Lie+/Lii+ increases proportional to the ratio Nae+/Nai+ at constant extravellular K+ concentrations. 5. In ghost suspension an uphill Li+ transport is driven by an oppositely directed Na+ gradient. The direction of the Li+ uphill transport can be reversed by reversing the Na+ gradient. From the results it is concluded that the Li+ uphill transport across human red cell membranes is mediated by a Na+-dependent Li+ counter-transport system. This system is not inhibited by ouabain and does not appear to be identical to the Na+-Na+ exchange system described by Garrahan and Glynn.

Possible relationship between membrane proteins and phospholipid asymmetry in the human erythrocyte membrane

After incubation of human erythrocytes at 37 degrees C in the absence of glucose (A) for 24 h, (B) for 4 h with 8 mM hexanol or (C) for 3 h with SH reagents, phosphatidylethanolamine becomes partly susceptible to hydrolysis by phospholipase A2 from Naja naja. The presence of glucose during the pretreatments suppresses this effect, except in the case of SH reagents that inhibit glycolysis. After incubation with tetrathionate, up to 45% of the phosphatidylethanolamine is degraded by the enzyme, an amount considerably in excess of the 20% attacked in fresh erythrocytes. Pancreatic phospholipase A2, an enzyme unable to hydrolyse the phospholipids of intact erythrocytes, partially degrades phosphatidylcholine and phosphatidylethanolamine of erythrocytes pretreated with hexanol or SH reagents. Reagents capable of oxidizing SH groups to disulfides (tetrathionate, o-iodosobenzoate and hydroquinone) even render susceptible to pancreatic phospholipase A2 phosphatidylserine, a phospholipid supposed to be entirely located in the inner lipid layer of the membrane. Alkylating or acylating SH reagents have no such effect. It is postulated that disulfide bond formation between membrane protein SH groups leads to an alteration in protein-phospholipid interactions and consequently induces a reorientation of phospholipids between the inner and the outer membrane lipid layer.

A new model for regulation of sodium transport in high resistance epithelia

A new three barrier, four compartment model for sodium transport in high resistance urinary epithelia is presented. This model provides a unified and simplified mechanistic explanation for sodium transport and its quantitative regulation. Sodium enters the epithelial cell by passive diffusion. Active extrusion occurs across the lateral cell membrane into the lateral intercellular space (LICS). Sodium movement from the LICS into the serosal compartment is not free and unobstructed as in the models for low resistance epithelia, but rather occurs through a regulatory channel of the LICS passing through desmosomes and the basilar slit. The exact configuration of this regulatory channel controls the rate of sodium movement from the LICS into the serosal compartment. Thus, the configuration of the regulatory channel controls the afterload on the sodium pump and thus ultimately controls the rate of transepithelial sodium transport. Antidiuretic hormone could act by increasing the effective width of this regulatory channel by contraction of intracellular microtubules or microfilaments. Present theories for regulation of transepithelial sodium transport in high resistance epithelia invoke a regulatory barrier at the apical cell membrane or at the active sodium pump located in the basolateral cell membrane. The hypothetical model presented here invokes a new alternative: regulation of the active pump rate by the sodium concentration in the LICS serving as an afterload on the pump; sodium escape from the LICS into the serosal compartment thus becomes the regulatory step for transepithelial transport.

The use of ionophores of rapid loading of human red cells with radioactive cations for cation-pump studies

Techniques are described for the rapid loading of intact human red cells with radioactive isotopes of alkali cations or Ca2+ by using ionophorous compounds (nigericin, gramicidin D and A 23187). Loading was rapid and efficient if the membrane potential of the cells was rendered more negative inside. After cation loading the ionophores could be bound to albumin and removed by repeated washings. The ATP and 2,3-DPG contents of the cells were practically unaltered by this treatment. Passive membrane permeability to Na+ and Ca2+ returned to normal. Loaded erythrocytes pumped out Na+ in a ouabain-sensitive and Ca2+ in a lanthanum-sensitive way. Ca2+ -loaded red cells were microspherocytes and exhibited a rapid K+ -efflux. Parallel with the extrusion of Ca2+ cells regained their biconcave shape and normal passive permeability to K+.

Deviating flux ratios for Na+ in ouabain-treated frog skin

Unidirectional Na+ fluxes across ouabain-treated frog skins were measured at different applied voltages. The calculated influx/efflux ratios appear to deviate markedly from Ussing's flux-ratio equation. This means that interactions of Na+ ions with some component in the system occur. Possible mechanisms, responsible for this phenomenon, are indicated.

Non-pumped sodium fluxes in human red blood cells. Evidence for facilitated diffusion

Unidirectional and net Na+ fluxes modified by changes in internal Na+ concentration ([Na+]i) were studied in human red blood cells incubated in K+-free solutions containing 10-minus 4 m ouabain. An increase in [Na+]i brought about (a) a reduction in net Na+ gain, (b) no change in Na+ influx, (c) a reduction in the rate constant for Na+ effux and (d) an increase in Na+ efflux. Similar reductions in net Na+ gain were observed when the changes in [Na+]i were carried out at constant [K+]i. In addition, the rate constant for 42K+ efflux was not affected by changes in [Na+]i. The electrical membrane potential (as determined from the chloride distribution ratio) was also constnat. Furosemide (10-minus 3 M) increased the net Na+ gain in concentration reduced Na+ efflux and increased Na+ influx: the magnitude of these effects was dependent onthe intracellular Na+. The reduction in the net Na+ gain as [Na+]i increased was unaffected by depletion of cellular ATP to values below 10 mumol/1 cells, and this effect was independent of the depletion method used

Cation flux in the ehrlich ascites tumor cell. Evidence for Na+-for-Na+ and K+-for-K+ exchange diffusion

In a previous study, evidence was presented for an external Na+-dependent, ouabain-insensitive component of Na+ efflux and an external K+-dependent component of K+ efflux in the Ehrlich ascites tumor cell. Evidence is now presented that these components are inhibited by the diuretic furosemide and that under conditions of normal extracellular Na+ and K+ they represent Na+-for-Na+ and K-+for-K+ exchange mechanisms. Using 86Rb to monitor K+ movements, furosemide is shown to inhibit an ouabain-insensitive component of Rb+ influx and a component of Rb+ efflux, both representing approx. 30 percent of the total flux. Inhibition of Rb+ efflux is greatly reduced by removal of extracellular K+. Furosemide does not alter steady-state levels of intracellular K+ and it does not prevent cells depleted of K+ by incubation in the cold from regaining K+ upon warming. Using 22Na to monitor Na+ movements, furosemide is shown to inhibit an ouabain-insensitive component of unidirectional Na+ efflux which represents approx. 22 percent of total Na+ efflux. Furosemide does not alter steady-state levels of intracellular Na+ and does not prevent removal of intracellular Na+ upon warming from cells loaded with Na+ by preincubation in the cold. The ability of furosemide to affect unidirectional Na+ and K+ fluxes but not net fluxes is consistent with the conclusion that these components of cation movement across the cell membrane represent one-for-one exchange mechanisms. Data are also presented which demonstrate that the uptake of alpha-aminoisobutyrate is not affected by furosemide. This indicates that these components of cation flux are not directly involved in the Na+-dependent amino acid transport system A.

The freezing lesion. III. The effects of diphenylhydantoin on potassium transport within nerve terminals from the primary foci

Possible mechanisms by which dephenylhydantoin (DPH) controls seizures were examined. The effects of intraperitoneal DPH on seizure discharges within epileptogenic freeze lesions were correlated with DPH action on in vitro potassium uptake within synaptosomes isolated from the same freeze foci. When in vivo DPH suppressed seizure discharges, it stimulated in vitro potassium uptake within synaptosomes incubated in a high-Kplus (10 mM) media. With 2-5 mM Naplus and 10mM Kplus, DPH stimulation of synaptosome potassium uptake was reversed by ouabain. With 50 mM Naplus and 10 mM Kplus, DPH stimulation of potassium uptake was not reversed by ouabain. In low-Kplus (0.2-5 mM) media, DPH did not affect potassium uptake even when sodium concentrations were varied at 10-100 mM. In sham-operated controls and in non-epileptogenic lesions, the effects of DPH on synaptosome potassium uptake were identical to those previously reported in normal brains. These results strongly suggest that DPH controls the epileptogenic state by stimulating potassium uptake within synaptic terminals. DPH controls the epileptogenic state by stimulating potassium uptake within synaptic terminals. DPH enhances synaptic potassium uptake by stimulating the (Naplus-Kplus) pump and a second potassium uptake process which is insensitive to ouabain.

Intracellular potassium. A determinant of the sodium-potassium pump rate

Normal human red cells which have had their intracellular sodium (Na(c)) reduced have a diminished Na-K pump rate, but only if intracellular potassium (K(c)) is high. If most of the K(c) is replaced by tetramethylammonium or choline, both ouabain-sensitive Na efflux and K influx are significantly increased even with Na(c) below normal. Cells with reduced Na(c) and high K(c) have an unchanged Na efflux if external potassium (K(ext)) is removed. In contrast, low-Na, low-K cells have a large ouabain-sensitive Na efflux which shows a normal response to removal of K(ext). Neither low-K nor high-K cells have an altered ouabain-sensitive K efflux. Measurement at constant low Na(c) and varying K(c) shows the pump Na efflux to be an inverse function of K(c). Thus, in low-Na cells, K(c) appears to act as an inhibitor of the pump. Inhibition by high K(c) can be seen even when Na(c) is normal. The effects attributed to K(c) are distinguished experimentally from other variables such as cell volume, adenosine triphosphate concentration, effects of the replacement cations, and the method used to alter intracellular cation concentrations. A role is proposed for K(c), in cooperation with Na(c), in regulating the pump rate of normal human red cells.

A furosemide-sensitive cotransport of sodium plus potassium in the human red cell

The influxes of Na(+) and K(+) into the human red cell appear to be interrelated. This relationship was investigated under conditions in which either Na(+) or K(+) concentration outside the cell was varied or one cation was replaced by Mg(2+), choline(+), or Li(+). The effects of furosemide on Na(+) and K(+) movements were studied in the presence of ouabain. When ouabain was present, Na(+) influx was higher with K(+) ions externally than with other cations externally. Furosemide inhibited this K(+)-stimulated Na(+) influx, but it had little effect when K(+) was absent. Ouabain-insensitive K(+) influx was stimulated two-fold by external Na(+) compared with other cations. Furosemide also inhibited this stimulation, but it had little effect when Mg(2+) or choline(+) replaced external Na(+). Thus it was confirmed that synergism exists between the ouabain-insensitive influxes of Na(+) and K(+) and it was demostrated that furosemide inhibits this cooperative effect. The ouabain-insensitive influx of both K(+) and Na(+) showed a hyperbolic "saturating" dependence on the external concentration of the transported cation. Furosemide therefore eliminates a saturable component of influx of each cation. The net uptake of Na(+) in the presence of ouabain was stimulated by K(+) ions. A similar effect was observed with red cells, in which Li(+) replaced nearly all the internal Na(+) plus K(+) ions. In these cells, net Na(+) uptake was stimulated by external K(+), and net K(+) uptake was stimulated by external Na(+). Furosemide inhibited this mutual stimulation of net cation entries. The inhibitory action of furosemide was not limited to inward flux and net movement of Na(+) and K(+). Furosemide also inhibited the efflux of Na(+) into Na(+)-free media and the efflux of K(+) into K(+)-free media. It appeared, therefore, that the action of furosemide was not explained by inhibition of exchange diffusion. These data are consistent with an ouabain-insensitive transport process that facilitates the inward cotransport of Na(+) plus K(+)-ions, and that can produce a net movement of both ions. Although this process under some conditions mediates an equal bidirectional flux of both Na(+) and K(+), it cannot be defined as exchange diffusion. The contransport process is inhibited by furosemide.

Sodium movements in high-sodium beef red cells: properties of a ouabain-insensitive exchange diffusion

1. The relative importance of the Na efflux components in beef red cells has been evaluated. The component which is insensitive to ouabain, which does not require external K but depends on the presence of external Na, accounts for about 90% of the total Na efflux.2. The experiments reported in this paper are consistent with the presence of an ouabain-insensitive Na(+)-Na(+) exchange process accounting for this ouabain-insensitive external Na dependent efflux.3. A strictly parallel behaviour of influx and efflux is observed when the pH is altered. The exchange diffusion process is inhibited over 90% by a decrease in pH from range pH 8.0-5.5.4. Both Na efflux and influx are markedly increased by raising the temperature from 27 to 37 degrees C.5. Energy depleted cells and fresh cells behave similarly in respect to Na movements. In depleted resealed ghosts, a large Na-dependent efflux occurs. No chemical energy and no special nucleotide is required for the Na(+)-Na(+) exchanges.6. When the external or internal Na concentrations are changed, a parallel behaviour of influx and efflux is observed.7. The relation between the magnitude of the exchange diffusion flux and the external or internal Na concentration fits quite well the Michaelis-Menten equation suggesting that only one Na(+) reacts with the transport mechanism. The affinity for Na is lower however at the outer surface than at the inner border of the membrane.8. The relation between this exchange process, the ouabain-insensitive Na-Na exchanges found in human red cell, and Ussing's model of exchange diffusion is discussed.

Ouabain-uninhibited sodium transport in human erythrocytes. Evidence against a second pump

Others have concluded that a second Na "pump" (active Na outflux) exists in human erythrocytes. This second pump was said to be ouabain-insensitive, unlike the classic ouabain-sensitive Na-K pump. An alternative explanation is that "pump II" is Na exchange diffusion. These hypotheses were examined in the present experiments, utilizing (22)Na influx and outflux measurements, net Na fluxes, and ATPase determinations. Ouabain-uninhibited Na outflux was reduced 0.58+/-0.05 mmol/liter cells per h when extracellular Na (Na(o)) was replaced by Mg. Ethacrynic acid or furosemide produced similar decrements of outflux (0.50 mmol) in the presence of ouabain and Na(o). However, these diuretics had minimal inhibitory effects on outflux in the absence of Na(o) suggesting that they inhibited principally the Na(o)-dependent outflux. Whereas this ouabain-uninhibited portion of outflux was dependent on Na(o), it was independent of K(o). Contrary to expectations, Na influx did not change when intracellular Na was altered. No uphill, net Na transport (ouabain-uninhibited) could be demonstrated under a variety of circumstances. Furosemide at high concentrations inhibited ATPase, reducing both ouabain-sensitive and ouabain-insensitive enzyme at 1.0 mM concentration while showing no effect on ATPase at 0.05-0.1 mM concentration. The effects of furosemide on ATPase and on Na flux were dissociable on a dose-response curve. Energy depletion for 22 h practically eliminated the Na(o)-dependent, diuretic-inhibited Na outflux. Activation energies and temperature coefficients for the diuretic-inhibited outflux were one-half the values for the classic ouabain-inhibited pump. These data are interpreted as evidence against a second Na pump. Exchange diffusion accounts adequately for most of these observations; however, the ouabain-insensitive fluxes may be complex and composed of several processes.

Stimulation of paraventricular neurosecretory cells by oxytocin applied iontophoretically

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Diphenylhydantoin and potassium transport in isolated nerve terminals

THE ANTIEPILEPTIC ACTION OF DIPHENYLHYDANTOIN (DPH) HAS BEEN EXPLAINED BY TWO DIFFERENT THEORIES: (a) that DPH stimulates the Na-K pump; (b) that DPH specifically blocks the passive translocation of sodium. Since electrophysiological experiments have recently suggested abnormal synaptic mechanisms as the basis for epileptogenic discharges, the action of DPH on K transport within synaptic terminals isolated from "normal" rat brain cortex was examined directly. A rapid filtration technique was used to assess in vitro potassium transport within synaptosomes. In vivo DPH did not significantly change endogenous K content within synaptosomes. With sodium (50 mM) and potassium (10 mM) concentrations optimal for Na-K pump activity, in vivo and in vitro DPH (10(-4) M) had minimal or no effects on total K uptake. DPH stimulated potassium uptake within synaptosomes under two situations: (a) at high sodium (50-100 mM) and low potassium (less than 2 mM) concentrations; (b) when synaptosomes were incubated with ouabain (10(-4) M) 50 mM Na and 10 mM K. In both situations, K was leaking out of synaptic terminals and the enhancement in net K uptake roughly corresponded to the ouabain inhibitable segment. In the absence of ouabain, the stimulatory effects of DPH were not observed when K was 2 mM or higher and when Na was 10 mM or lower. The stimulatory effects of in vitro DPH appeared over a range of concentrations from 10(-4) to 10(-10) M while single intraperitoneal injections of DPH had to be administered for 2 days before its effects were observed on synaptosomal K transport. The present data provided direct evidence for DPH stimulation of active potassium transport within synaptosomes under ionic conditions simulating the depolarized state. At other ionic conditions, DPH had inhibitory or no effects on K uptake. Although the results do not specify whether the effects of DPH on the Na-K pump are direct or indirect, they suggest that the action of DPH depends upon the state of the membrane and the specific ionic environment.

The influence of the chloride gradient across red cell membranes on sodium and potassium movements

1. A study has been made to see whether active and passive movements of sodium and potassium in human red blood cells are influenced by changing the chloride gradient and hence the potential difference across the cell membrane.2. Chloride distribution was measured between red cells and isotonic solutions with a range of concentrations of chloride and non-penetrating anions (EDTA, citrate, gluconate). The cell chloride concentration was greater than that outside with low external chloride, suggesting that the sign of the membrane potential was reversed. The chloride ratio (internal/external) was approximately equal to the inverse of the hydrogen ion ratio at normal and low external chloride, and inversely proportional to external pH. These results show that chloride is passively distributed, making it valid to calculate the membrane potential from the chloride ratio.3. Ouabain-sensitive (pump) potassium influx and sodium efflux were decreased by not more than 20 and 40% respectively on reversing the chloride gradient, corresponding to a change in membrane potential from -9 to +30 mV. In contrast, passive (ouabain-insensitive) movements were reversibly altered - potassium influx was decreased about 60% and potassium efflux was increased some tenfold. Sodium influx was unaffected by the nature of the anion and depended only on the external sodium concentration, whereas ouabain-insensitive sodium efflux was increased about threefold. When external sodium was replaced by potassium there was a decrease in ouabain-insensitive sodium efflux with normal chloride, but an increase in low-chloride medium.4. Net movements of sodium and potassium were roughly in accord with the unidirectional fluxes.5. The results suggest that reversing the chloride gradient and, therefore, the sign of the membrane potential, had little effect on the sodium pump, but caused a marked increase in passive outward movements of both sodium and potassium ions.

The kinetics of ouabain inhibition and the partition of rubidium influx in human red blood cells

IN THE DEVELOPMENT OF OUABAIN INHIBITION OF RUBIDIUM INFLUX IN HUMAN RED BLOOD CELLS A TIME LAG CAN BE DETECTED WHICH IS A FUNCTION OF AT LEAST THREE VARIABLES: the concentrations of external sodium, rubidium, and ouabain. The inhibition is antagonized by rubidium and favored by sodium. Similar considerations could be applied to the binding of ouabain to membrane sites. The total influx of rubidium as a function of external rubidium concentration can be separated into two components: (a) a linear uptake not affected by external sodium or ouabain and not requiring an energy supply, and (b) a saturable component. The latter component, on the basis of the different effects of the aforementioned factors, can be divided into three fractions. The first is ouabain-sensitive, inhibited by external sodium at low rubidium, and requires an energy supply; this represents about 70-80% of the total uptake and is related to the active sodium extrusion mechanism. The second is ouabain-insensitive, activated by external sodium over the entire range of rubidium concentrations studied, and dependent on internal ATP; this represents about 15% of the total influx; it could be coupled to an active sodium extrusion or belong to a rubidium-potassium exchange. The third, which can be called residual influx, is ouabain-insensitive, unaffected by external sodium, and independent of internal ATP; this represents about 10-20% of the total influx.

The control by internal calcium of membrane permeability to sodium and potassium

1. A study has been made of the relationship between the concentration of internal calcium and the permeability of human red cell membranes to sodium and potassium.2. Fresh red cells contain very little calcium, but after being depleted of ATP by ageing they took up calcium from Ringer solution. The entry was unaffected by external sodium and potassium but was markedly pH dependent. When supplied with energy, calcium-loaded cells actively extruded calcium by a saturable process which was also unaffected by the distribution of sodium and potassium across the membranes. The activity of the calcium pump was sufficient to maintain the low internal concentration found under physiological conditions.3. Raising internal calcium in metabolically poor cells caused a loss of cell potassium which was greater than the concomitant sodium gain. These changes were reversed when ATP was supplied. External calcium had no effect. The increase in permeability to sodium and potassium was enhanced by the simultaneous addition of fluoride, and, even more so, of iodoacetate. These inhibitors had no effect on membrane permeability unless calcium was also present. Inosine potentiated the action of fluoride and iodoacetate in causing potassium loss, by allowing more calcium to enter the cells.4. The results suggest that the permeability of human red cell membranes to sodium and potassium is regulated by internal calcium, which in turn is controlled by a calcium pump that utilizes ATP.

The effects of ethacrynic acid and other sulphydryl reagents on sodium fluxes in frog muscle

1. Ethacrynic acid (2 mM) increased the sodium efflux from freshly dissected frog sartorius muscles. This increase was not observed in muscles previously treated with strophanthidin.2. In strophanthidin-treated muscles, the addition of ethacrynic acid (2 mM) caused a reduction of sodium efflux. The value of efflux reached in these muscles is similar to that observed in muscles immersed in sodium-free solutions containing strophanthidin.3. Ethacrynic acid reduced sodium influx into strophanthidin-treated muscles. Potassium influx was not affected by this substance. These findings suggest that the inhibitor blocks an exchange of sodium for sodium.4. The increase in sodium efflux caused by ethacrynic acid did not result from depolarization nor from an increase in [Na](i).5. Ethacrynic acid caused only a reduction of sodium efflux in muscles previously loaded with sodium by prolonged immersion in potassium-free solutions at low temperatures.6. A derivative of ethacrynic acid that lacks the ability to combine with sulphydryl (SH) groups did not reduce sodium efflux from muscles treated with strophanthidin.7. Para-chloromercuribenzoic acid (PCMB) reduced sodium efflux from control and strophanthidin-treated muscles. This reduction seems to be restricted to the sodium dependent component of the efflux.8. The inhibition of the sodium-dependent component of sodium efflux caused by ethacrynic acid and PCMB appears to have a similar mechanism, namely, the combination with SH groups.