Evidence against Na+-pump mediation of Ca2+-activated K+ transport and diuretic-sensitive (Na+/K+)-cotransport

The effect of cellular calcium on Na+/K+ cotransport in human red blood cells

The increase in Ca2+ permeability by addition of ionophore A23187 in the presence of external Ca2+ did not alter the bumetanide-sensitive Na+/K+ effluxes in human red blood cells. An inhibition of this pathway by cellular Ca2+ could be observed only under conditions in which the cellular ATP content was drastically depleted.

Na fluxes in human mononuclear leucocytes

Unidirectional 22Na fluxes were investigated in human peripheral mononuclear leucocytes. Ouabain inhibited about 60% of Na efflux and the addition of bumetanide further reduced Na efflux rate by about 45%, suggesting the presence of a transport pathway capable of extruding Na against its gradient. Prostaglandins E1 and E2 and exogenous cAMP were found to be potent inhibitors of the bumetanide-sensitive Na efflux without affecting the ouabain-sensitive or the ouabain and bumetanide resistant Na effluxes.

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)

Influence of loop diuretics and anions on passive potassium influx into human red cells

Passive K influx into human red cells was measured with and without Cl ions, Na ions and loop diuretics. Ouabain and loop diuretics appear to inhibit specifically and respectively the Na pump and (Na+K) 'co-transport'. Inhibitors of other pathways, e.g. 4,4'-diisothiocyantostilbene-2,2'-disulphonic acid or amiloride did not inhibit passive K influx. Loop diuretics inhibited with high apparent affinity in Na-containing media and with low apparent affinity in Na-free media where there was a substantial Cl-dependent component. The Cl concentration dependence was measured using six anion substitutions for Cl. With NO3, acetate and gluconate, the curves were sigmoidal and not fully saturable at 150 mM-Cl; with iodide and thiocyanate, the curves were convex; with sulphate, there was saturation at 120 mM-Cl. The half-maximal K influx as a function of [Na]0 was 40 mM for the Cl-dependent flux component and 12 mM for the diuretic-sensitive flux.

Potassium movements associated with amino acid and sugar transport in enterocytes isolated from rabbit jejunum

Active transport of amino acids and the sugar, alpha-methyl-D-glucoside (alpha-MG) caused an increase in the rate of K efflux from isolated rabbit enterocytes. These effects were inhibited by apamin (5 X 10(-7) M), quinidine (10(-3) M), Ba (5 X 10(-3) M) and trifluoperazine (5 X 10(-5) M) but not by the loop diuretic furosemide (10(-4) M). None of these drugs affected the basal rate of K efflux. The stimulatory effects of amino acids or alpha-MG on K efflux are too great to be explained in terms of an increase in the electrical driving force across the plasma membrane of these cells and a change in membrane permeability is envisaged. An apparent Ca-dependent K permeability in isolated enterocytes can be demonstrated using the Ca ionophore A23187. The effect of the ionophore on K efflux is abolished by apamin or Ba. It is proposed that Ca-dependent K channels mediate the sugar and amino acid induced increases of K efflux. Under control conditions there is a decrease in intracellular K concentration during accumulation of alanine or alpha-MG. Ba by itself does not alter K concentration but it did produce a marked increase when used in conjunction with alanine or alpha-MG. The accumulation of alpha-MG was inhibited in the presence of Ba. This is consistent with an interference with the driving force for sugar accumulation. It is suggested that the increase in K permeability described has a role in both maintaining ion homoeostasis during Na-coupled transport and contributing to the driving force for sugar and amino acid absorption.

Evidence against involvement of the human erythrocyte plasma membrane Ca2+-ATPase in Ca2+-dependent K+ transport

Two tests were performed to assess the relationship between the Ca2+-activated K+ channel and the Ca2+-pumping ATPase in human erythrocytes. Antibodies against the purified ATPase inhibited the ATPase in resealed erythrocytes, but had no effect on the K+ channel (as assessed by Rb+ efflux). Reconstituted liposomes containing the purified active Ca2+-pumping ATPase showed no Ca2+-activated Rb+ influx. Both of these results suggest that some molecule other than the Ca2+-ATPase is responsible for the K+ channel.

Purification from human plasma of endogenous sodium transport inhibitor(s)

Na+,K+-ATPase inhibitors extracted from plasma of healthy human subjects displaced 3H-ouabain binding to human erythrocytes and inhibited the Na+ efflux catalyzed by the Na+,K+-pump and unexpectedly the Na+,K+-cotransport system without alteration of the Na+,Na+-exchange or the Na+ passive permeability. This suggests the presence in healthy human plasma of endogenous factors with ouabain-like and furosemide-like activities.

Furosemide-sensitive potassium efflux in cultured mouse fibroblasts

Transfer of LM(TK-) cells from normal growth medium to medium lacking K+ leads to a rapid loss of intracellular K+, which is 50-70% inhibited by furosemide or bumetanide. The diuretic-sensitive component of K+ efflux requires both Na+ and Cl-, and is presumably mediated by a K+, Na+, Cl- cotransport system of the kind described in avian erythrocytes and Ehrlich ascites cells. It can be calculated that such a system should be near equilibrium under normal growth conditions but should mediate net efflux (as observed) when the driving force is altered by reducing extracellular K+. The diuretic-sensitive component of net K+ efflux is also sensitive to amiloride. This effect is probably indirect, however, with amiloride acting to block the Na+ influx that supplies Na+ to the cotransport system. At the low extracellular K+ concentrations employed in these studies, the diuretic-sensitive system is a physiologically important pathway of K+ loss. The rate of growth in low-K+ medium can be increased (or the rate of cell lysis decreased) by adding diuretic or by reducing external Na+ or Cl-.

Sodium and potassium transport in trout (Salmo gairdneri) erythrocytes

The principal pathways of Na+ and K+ transport in trout erythrocytes have been characterized. Approximately 50% of K+ influx in steady-state erythrocytes was inhibited by ouabain (1 mM) and 46% by furosemide (1 mM). Furosemide-sensitive K+ influx was a saturable function of external K+ concentration with a Km of 25 mM. This flux component was also inhibited by SITS (4-acetamido-4'-isothiocyanatostilbene-2'2-disulphonate) (concentration required for 50% inhibition, I50 = 7.6 X 10(-6)M) and by the removal of external Cl-. An increase in cell volume stimulated furosemide-sensitive K+ influx and cell shrinkage inhibited this flux. K+ efflux was mainly furosemide-sensitive (64% of total). This pathway was unaffected by variations in extracellular K+ concentration and is therefore not exchange diffusion. However, it was affected by variations in cell volume in a similar way to the furosemide-sensitive K+ influx. Na+ influx was only slightly sensitive to furosemide (13% of total) but this component was very sensitive to changes in cell volume; decreased cell volume increased Na+ influx whilst increased cell volume inhibited Na+ influx. Furosemide-sensitive K+ influx was unaffected by variations in external Na+ concentration. Similarly, furosemide-sensitive Na+ influx was unaffected by variations in external K+ concentration. This indicates that the passive influxes of Na+ and K+ were not coupled, in contrast to the situation in avian erythrocytes. The opposite effects of cell volume upon passive Na+ and K+ fluxes are in good agreement with the net movements of these cations during volume regulation in erythrocytes of the flounder (Cala, 1977) and the toadfish (Lauf, 1982).

Passive transport of K+ and Na+ in human red blood cells: sulfhydryl binding agents and furosemide

Passive transport pathways for K+ and Na+ were studied in fresh human red blood cells (pretreated with ouabain) by measuring unidirectional influxes. The effects of the sulfhydryl binding agents N-ethylmaleimide (NEM) and p-chloromercuribenzene sulfonate (p-CMBS) and the loop diuretic furosemide were studied. Influxes were measured at equimolar K+ and Na+ concentrations (50 mM) with both ions present and also in K+-free or Na+-free media. Some experiments were carried out in Cl--free media (with NO-3 as the substitute). NEM stimulated K+ influx twofold; the stimulation required Cl- but not Na+. NEM inhibited Na+ influx 20%. Furosemide inhibited both K+ and Na+ influxes. All of furosemide-inhibitable Na+ influx required the presence of K+. However 30% of furosemide-inhibitable K+ influx did not require Na+. All of furosemide-inhibitable K+ influx required Cl-. The ratio of Na+-dependent K+ influx to K+-dependent Na+ influx was 3:1. p-CMBS stimulated both Na+ and K+ influxes. K+ influx in p-CMBS cells required neither Na+ nor Cl-. Likewise p-CMBS-promoted Na+ influx did not require K+. These various results are consistent with two Cl--dependent pathways for K+ transport, one requiring Na+ [perhaps (Na + K + Cl) cotransport] and one independent of Na+ [perhaps (K + Cl) cotransport]. The pathways promoted by p-CMBS are probably independent of the apparent cotransport systems.

Effect of ouabain upon diuretic-sensitive K+ transport in cultured cells. Evidence for separate modes of operation of the transporter

(1) Unidirectional K+ (86Rb) influx and efflux were measured in subconfluent layers of MDCK renal epithelial cells and HeLa carcinoma cells. (2) In both MDCK and HeLa cells, the furosemide-inhibitable and chloride-dependent component of K+ influx/efflux was stimulated 2-fold by a 30 min incubation in 1 . 10(-3) M ouabain. (3) Measurements of net K+ loss and Na+ gain in ouabain-treated cells at 1 h failed to show any diuretic sensitive component, confirming the exchange character of the diuretic-sensitive fluxes. (4) Prolonged incubations for 2.5 h in ouabain revealed a furosemide- and anion-dependent K+ (Cl-) outward net flux uncoupled from net Na+ movement. Net K+ (Cl-) outward flux was half-maximally inhibited by 2 microM furosemide. (5) After 2.5 h ouabain treatment, the anion and cation dependence of the diuretic-sensitive K+ influx/efflux were essentially unchanged when compared to untreated controls.

Incorporation of Na+ - Ca2+ antiporter and of (Na+ + K+)-ATPase into liposomes and demonstration of their non-identity

(Na+ + K+)-ATPase was isolated from the grey matter of brain and incorporated into liposomes. Most of the reconstituted enzyme was oriented 'inside-out' with respect to its in vivo orientation and externally added ATP promoted Na+ uptake that was inhibitable by internally trapped ouabain. Using the same proteoliposomes, an Na+ - Ca2+ exchange system was observed as indicated by the following pieces of evidence. (1) The Na+ gradient provided the only readily apparent driving force for acceleration of Ca2+ accumulation into proteoliposomes. (2) The antiporter was specific for Ca2+, high Mg2+ excess did not inhibit Ca2+ antiport. (3) The Na+ efflux was dependent on the extravesicular Ca2+ concentration. (4) The Na+ efflux was not inhibited by tetrodotoxin. The demonstrated Na+ - Ca2+ exchange could not be related to (Na+ + K+)-ATPase protein, since it was not purified with (Na+ + K+)-ATPase, as followed from transport studies with liposomes containing (Na+ + K+)-ATPase of different specific activity. The results strongly indicate that plasma membranes isolated from the grey matter of brain contain an Na+ - Ca2+ exchange system and that the proteoliposomes are suitable for further purification of the carrier molecule.

K+ transport in 'tight' epithelial monolayers of MDCK cells. Evidence for a calcium-activated K+ channel

Measurements of 86Rb efflux across the apical and basal-lateral aspects of intact monolayers of 'high-resistance' MDCK cells mounted in Ussing chambers have been made. A transient increase in 86Rb efflux across both epithelial borders upon stimulation with adrenalineeeeeee or ionophore A23187 is observed. The increased 86Rb across the basal cell aspects is of greatest quantitative importance. Measurements of total cellular K+ contents by flame photometry of tissue extracts indicate a net loss of K+ following adrenalin addition. The effects of adrenalin and ionophore A23187 upon 86Rb efflux are abolished in 'Ca2+ -free' media. The properties of the Ca2+ -dependent increase in 86Rb efflux show similarities to Ca2+ -activated K+ conductances in other tissues, notably human red cells, including inhibition by quinine (1 mM), tetraethylammonium (25 mM) and insensitivity to bee venom toxin (apamin) (25 nM). Adrenalin is only effective when applied to the basal bathing solution suggesting that the receptors mediating adrenalin action are located upon the basal-lateral membranes. Half maximal stimulation of 86Rb efflux by adrenalin is observed at 9.1 X 10(-7) M. The action of various adrenergic receptor agonists and antagonists are consistent with adrenalin action being mediated by an alpha-adrenergic receptor.

Effects of ouabain and osmolarity on bumetanide-sensitive potassium transport in simian virus-transformed 3T3 cells

Unidirectional potassium influx in simian virus-transformed 3T3 cells was dissected into a ouabain-inhibitable "pump' component, a bumetanide-sensitive and chloride-dependent "cotransport' component, and a residual "leak' flux. The bumetanide-sensitive component was stimulated 2-3-fold by a 60-min preincubation with ouabain. Subsequent washing of the cells and incubation in ouabain-free saline reversed both the inhibition of the Na+ pump and the stimulation of bumetanide-sensitive flux. Bumetanide-sensitive potassium influx was also stimulated by hypertonic cell shrinkage (induced by 0.1 M or 0.2 M sorbitol). This latter observation suggests that the bumetanide-sensitive system may play a role in cellular volume regulation.