Kinetics of DIDS inhibition of swelling-activated K-Cl cotransport in low K sheep erythrocytes

Advances in the development of novel compounds targeting cation-chloride cotransporter physiology

For about half a century, the pharmacology of electroneutral cation-chloride cotransporters has been dominated by a few drugs that are widely used in clinical medicine. Because these diuretic drugs are so good at what they do, there has been little incentive in expanding their pharmacology. The increasing realization that cation-chloride cotransporters are involved in many other key physiological processes and the knowledge that different tissues express homologous proteins with matching transport functions have rekindled interest in drug discovery. This review summarizes the methods available to assess the function of these transporters and describe the multiple efforts that have made to identify new compounds. We describe multiple screens targeting KCC2 function and one screen designed to find compounds that discriminate between NKCC1 and NKCC2. Two of the KCC2 screens identified new inhibitors that are 3-4 orders of magnitude more potent than furosemide. Additional screens identified compounds that purportedly increase cell surface expression of the cotransporter, as well as several FDA-approved drugs that increase KCC2 transcription and expression. The technical details of each screen biased them toward specific processes in the life cycle of the transporter, making these efforts independent and complementary. In addition, each drug discovery effort contributes to our understanding of the biology of the cotransporters.

The structural basis of function and regulation of neuronal cotransporters NKCC1 and KCC2

NKCC and KCC transporters mediate coupled transport of Na++K++Cl- and K++Cl- across the plasma membrane, thus regulating cell Cl- concentration and cell volume and playing critical roles in transepithelial salt and water transport and in neuronal excitability. The function of these transporters has been intensively studied, but a mechanistic understanding has awaited structural studies of the transporters. Here, we present the cryo-electron microscopy (cryo-EM) structures of the two neuronal cation-chloride cotransporters human NKCC1 (SLC12A2) and mouse KCC2 (SLC12A5), along with computational analysis and functional characterization. These structures highlight essential residues in ion transport and allow us to propose mechanisms by which phosphorylation regulates transport activity.

OEsophageal Ion Transport Mechanisms and Significance Under Pathological Conditions

Ion transporters play an important role in several physiological functions, such as cell volume regulation, pH homeostasis and secretion. In the oesophagus, ion transport proteins are part of the epithelial resistance, a mechanism which protects the oesophagus against reflux-induced damage. A change in the function or expression of ion transporters has significance in the development or neoplastic progression of Barrett’s oesophagus (BO). In this review, we discuss the physiological and pathophysiological roles of ion transporters in the oesophagus, highlighting transport proteins which serve as therapeutic targets or prognostic markers in eosinophilic oesophagitis, BO and esophageal cancer. We believe that this review highlights important relationships which might contribute to a better understanding of the pathomechanisms of esophageal diseases.

Cryo-EM structures of DrNKCC1 and hKCC1: a new milestone in the physiology of cation-chloride cotransporters

New milestones have been reached in the field of cation-Cl- cotransporters with the recently released cryo-electron microscopy (EM) structures of the Danio rerio (zebrafish) Na+-K+-2Cl- cotransporter (DrNKCC1) and the human K+-Cl- cotransporter (hKCC1). In this review we provide a brief timeline that identifies the multiple breakthroughs in the field of solute carrier 12 transporters that led to the structure resolution of two of its key members. While cation-Cl- cotransporters share the overall architecture of carriers belonging to the amino acid-polyamine-organocation (APC) superfamily and some of their substrate binding sites, several new insights are gained from the two individual structures. A first major feature relates to the largest extracellular domain between transmembrane domain (TMD) 5 and TMD6 of KCC1, which stabilizes the dimer and forms a cap that likely participates in extracellular gating. A second feature is the conservation of the K+ and Cl- binding sites in both structures and evidence of an unexpected second Cl- coordination site in the KCC1 structure. Structural data are discussed in the context of previously published studies that examined the basic and kinetics properties of these cotransport mechanisms. A third characteristic is the evidence of an extracellular gate formed by conserved salt bridges between charged residues located toward the end of TMD3 and TMD4 in both transporters and the existence of an additional neighboring bridge in the hKCC1 structure. A fourth feature of these newly solved structures relates to the multiple points of contacts between the monomer forming the cotransporter homodimer units. These involve the TMDs, the COOH-terminal domains, and the large extracellular loop for hKCC1.

Vasorelaxation elicited by endogenous and exogenous hydrogen sulfide in mouse mesenteric arteries

H₂S causes vasorelaxation however there is considerable heterogeneity in the reported pharmacological mechanism of this effect. This study examines the contribution of endogenously released H₂S in the regulation of vascular tone and the mechanism of H₂S-induced vasorelaxation in small resistance-like arteries. Mesenteric arteries from C57 and eNOS^-/- mice were mounted in myographs to record isometric force. Vasorelaxation responses to NaHS were examined in the presence of various inhibitors of vasorelaxation pathways. Expression and activity of the H₂S-producing enzyme, cystathionine-γ-lyase (CSE), were also examined. CSE was expressed in vascular smooth muscle and perivascular adipose cells from mouse mesenteric artery. The substrate for CSE, L-cysteine, caused a modest vasorelaxation (35%) in arteries from C57 mice and poor vasorelaxation (10%) in arteries from eNOS^-/- mice that was sensitive to the CSE inhibitor DL-propargylglycine. The fast H₂S donor, NaHS, elicited a full and biphasic vasorelaxation response in mesenteric arteries (EC₅₀ (1) 8.7 μM, EC₅₀ (2) 0.6 mM), which was significantly inhibited in eNOS^-/- vessels (P < 0.05), unaffected by endothelial removal, or blockers at any point in the NO via soluble guanylate cyclase and cGMP (NO-sGC-cGMP) vasorelaxation pathway. Vasorelaxation to NaHS was significantly inhibited by blocking K⁺ channels of the K_Ca and K_V subtypes and the Cl^-/HCO₃^- exchanger (P < 0.05). Further experiments showed that NaHS can significantly inhibit voltage-gated Ca²⁺ channel function (P < 0.05). The vasorelaxant effect of H₂S in small resistance-like arteries is complex, involving eNOS, K⁺ channels, Cl^-/HCO₃^- exchanger, and voltage-gated Ca²⁺ channels. CSE is present in the smooth muscle and periadventitial adipose tissue of these resistance-like vessels and can be activated to cause modest vasorelaxation under these in vitro conditions.

Challenges of Finding Novel Drugs Targeting the K-Cl Cotransporter

Human disease-causing mutations and genetically modified mouse models have established the importance of KCC2 and KCC3 in nervous system physiology. These two proteins mediate the electroneutral cotransport of K+ and Cl- ions across the neuronal membrane. Disruption of KCC2 function affects inhibitory synaptic transmission with consequences for epilepsy, pain perception, and potentially some neuropsychiatric disorders, whereas disruption of KCC3 affects both central and peripheral nervous systems, resulting in psychosis and peripheral neuropathy. Until recently, the KCC field has suffered from an almost complete lack of pharmacological tools with which to probe cotransporter function. The only available tools being the very poorly potent loop diuretics (e.g., furosemide EC50 = 6 × 10-4 M). To address this deficiency, efforts that focused on the discovery of KCC modulators have been undertaken. This work has resulted in the discovery of novel inhibitory compounds that are up to four orders of magnitude more potent (EC50 = 6 × 10-7 M) and with increased specificity. While useful for ex vivo studies, these tools possess poor pharmacokinetic properties, severely limiting their utility in vivo. In addition, only a few agents acting on regulatory molecules have been identified as putative KCC activators. Thus, further research is required to develop tools suitable to advance our understanding of how KCC modulation may be useful for the treatment of disease.

A novel mechanism for the anticonvulsant effect of furosemide in rat hippocampus in vitro

Though both in vivo and in vitro studies have demonstrated an anticonvulsant effect of the loop diuretic furosemide, the precise mechanism behind this effect is still debated. The current study investigates the effect of furosemide on Cs-induced epileptiform activity (Cs-FP) evoked in area CA1 of rat hippocampal slices in the presence of Cs(+) (5mM) and ionotropic glutamatergic and GABAergic receptor antagonists. As this model diverges in several respects from other epilepsy models it can offer new insight into the mechanism behind the anticonvulsive effect of furosemide. The present study shows that furosemide suppresses the Cs-FP in a dose-dependent manner with a near complete block at concentrations ≥ 1.25 mM. Because furosemide targets several types of ion transporters we examined the effect of more selective antagonists. Bumetanide (20 μM), which selectively inhibits the Na-K-2Cl co-transporter (NKCC1), had no significant effect on the Cs-FP. VU0240551 (10 μM), a selective antagonist of the K-Cl co-transporter (KCC2), reduced the ictal-like phase by 51.73 ± 8.5% without affecting the interictal-like phase of the Cs-FP. DIDS (50 μM), a nonselective antagonist of Cl(-)/HCO3(-)-exchangers, Na(+)-HCO3(-)-cotransporters, chloride channels and KCC2, suppressed the ictal-like phase by 60.8 ± 8.1% without affecting the interictal-like phase. At 500 μM, DIDS completely suppressed the Cs-FP. Based on these results we propose that the anticonvulsant action of furosemide in the Cs(+)-model is exerted through blockade of the neuronal KCC2 and Na(+)-independent Cl(-)/HCO3(-)-exchanger (AE3) leading to stabilization of the activity-induced intracellular acidification in CA1 pyramidal neurons.

Negative shift in the glycine reversal potential mediated by a Ca2+- and pH-dependent mechanism in interneurons

Cartwheel cells are glycinergic auditory interneurons which fire Na(+)- and Ca(2+)-dependent spike bursts, termed complex spikes, and which synapse on both principal cells and one another. The reversal potential for glycine (E(gly)) can be hyperpolarizing or depolarizing in cartwheel cells, and many cells are even excited by glycine. We explored the role of spike activity in determining E(gly) in mouse cartwheel cells using gramicidin perforated-patch recording. E(gly) was found to shift toward more negative potentials after a period of complex spiking or Ca(2+) spiking induced by depolarization, thus enhancing glycine's inhibitory effect for approximately 30 s following cessation of spiking. Combined perforated patch electrophysiology and imaging studies showed that the negative E(gly) shift was triggered by a Ca(2+)-dependent intracellular acidification. The effect on E(gly) was likely caused by bicarbonate-Cl(-) exchanger-mediated reduction in intracellular Cl(-), as H(2)DIDS and removal of HCO(3)(-)/CO(2) inhibited the negative E(gly) shift. The outward Cl(-) flux underlying the negative shift in E(gly) opposed a positive shift triggered by passive Cl(-) redistribution during the depolarization. Thus, a Ca(2+)-dependent mechanism serves to maintain or enhance the strength of inhibition in the face of increased excitatory activity.

Small-molecule screen identifies inhibitors of the neuronal K-Cl cotransporter KCC2

KCC2, a neuronal-specific K-Cl cotransporter, plays a major role in maintaining intracellular Cl(-) concentration in neurons below its electrochemical equilibrium potential, thus favoring robust GABA hyperpolarizing or inhibitory responses. The pharmacology of the K-Cl cotransporter is dominated by loop diuretics such as furosemide and bumetanide, molecules used in clinical medicine because they inhibit the loop of Henle Na-K-2Cl cotransporter with much higher affinity. To identify molecules that affect KCC2 activity, we developed a fluorescence-based assay suitable for high-throughput screening (HTS) and used the assay to screen a library of 234,000 small molecules. We identified a large number of molecules that either decrease or increase the activity of the cotransporter. Here, we report the characterization of a small number of inhibitors, some of which inhibit KCC2 activity in the submicomolar range without substantially affecting NKCC1 activity. Using medicinal chemistry, we synthesized a number of variants, tested their effect on KCC2 function, and provide an analysis of structure/activity relationships. We also used one of the compounds to demonstrate competitive inhibition in regard to external [K(+)] versus noncompetitive inhibition in respect to external [Cl(-)].

Oxygen-dependent ion transport in erythrocytes

The present contribution reviews current knowledge of apparently oxygen-dependent ion transport in erythrocytes and presents modern hypotheses on their regulatory mechanisms and physiological roles. In addition to molecular oxygen as such, reactive oxygen species, nitric oxide, carbon monoxide, regional variations of cellular ATP and hydrogen sulphide may play a role in the regulation of transport, provided that they are affected by oxygen tension. It appears that the transporter molecules themselves do not have direct oxygen sensors. Thus, the oxygen level must be sensed elsewhere, and the effect transduced to the transporter. The possible pathways involved in the regulation of transport, including haemoglobin as a sensor, and phosphorylation/dephosphorylation reactions both in the transporter and its upstream effectors, are discussed.

Characterization of the deoxyhemoglobin binding site on human erythrocyte band 3: implications for O2 regulation of erythrocyte properties

Band 3, the major protein of the human erythrocyte membrane, associates with multiple metabolic, ion transport, and structural proteins. Functional studies demonstrate that the oxygenation state of the erythrocyte regulates cellular properties performed by these and/or related proteins. Because deoxyhemoglobin, but not oxyhemoglobin, binds band 3 reversibly with high affinity, these observations raise the hypothesis that hemoglobin might regulate erythrocyte properties through its reversible, oxygenation-dependent association with band 3. To explore this hypothesis, we have characterized the binding site of deoxyHb on human erythrocyte band 3. We report that (1) deoxyHb binds to residues 12-23 of band 3; (2) mutation of residues on either side of this sequence greatly enhances affinity of deoxyHb for band 3, suggesting that evolution of a higher affinity interaction would have been possible had it been beneficial for survival; (3) Hb does not bind to 2 other sequences in band 3 despite their high sequence homology to residues 12-23, and (4) the Hb binding site on band 3 lies proximal to binding sites for glycolytic enzymes, band 4.1 and ankyrin, suggesting possible mechanisms through which multifarious erythrocyte properties might be regulated by the oxygenation state of the cell.

Mechanisms of neuronal chloride accumulation in intact mouse olfactory epithelium

When olfactory receptor neurons respond to odours, a depolarizing Cl(-) efflux is a substantial part of the response. This requires that the resting neuron accumulate Cl(-) against an electrochemical gradient. In isolated olfactory receptor neurons, the Na(+)-K(+)-2Cl(-) cotransporter NKCC1 is essential for Cl(-) accumulation. However, in intact epithelium, a robust electrical olfactory response persists in mice lacking NKCC1. This response is largely due to a neuronal Cl(-) efflux. It thus appears that NKCC1 is an important part of a more complex system of Cl(-) accumulation. To identify the remaining transport proteins, we first screened by RT-PCR for 21 Cl(-) transporters in mouse nasal tissue containing olfactory mucosa. For most of the Cl(-) transporters, the presence of mRNA was demonstrated. We also investigated the effects of pharmacological block or genetic ablation of Cl(-) transporters on the olfactory field potential, the electroolfactogram (EOG). Mice lacking the common Cl(-)/HCO(3)(-) exchanger AE2 had normal EOGs. Block of NKCC cotransport with bumetanide reduced the EOG in epithelia from wild-type mice but had no effect in mice lacking NKCC1. Hydrochlorothiazide, a blocker of the Na(+)-Cl(-) cotransporter, had only a small effect. DIDS, a blocker of some KCC cotransporters and Cl(-)/HCO(3)(-) exchangers, reduced the EOG in epithelia from both wild-type and NKCC1 knockout mice. A combination of bumetanide and DIDS decreased the response more than either drug alone. However, no combination of drugs completely abolished the Cl(-) component of the response. These results support the involvement of both NKCC1 and one or more DIDS-sensitive transporters in Cl(-) accumulation in olfactory receptor neurons.

Blockade of chloride conductance antagonizes PMA-induced ramification in the murine microglial cell line, BV-2

In microglial cells, activation of ion channels and ion transporters is associated with the transformation from an amoeboid to a ramified phenotype and vice versa. In the present study, we evaluated the contributions of protein kinase C (PKC) activity and ion conductance to the phorbol 12-myristate 13-acetate (PMA)-dependent ramification in the murine microglial cell line, BV-2. In a first set of experiments, we showed that PMA, a commonly used activator of PKC, but not the bioinactive analog 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD), induces ramification in BV-2 cells. Surprisingly, the PKC inhibitors calphostin C, chelerythrine, or bisindolylmaleimide II did not antagonize PMA-induced ramification. In a further set of experiments, we found that 4,4'-diisocyanatostilbene-2,2' disulfonic acid (DIDS), 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS), which block chloride channels and K-Cl cotransporters, and SKF 96365, a non-selective ion channel blocker, consistently suppressed PMA-induced ramification in BV-2 cells. Additional ion channel blockers, including lanthanides, amiloride, Ba2+, 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), and flufenamic acid did not affect PMA-induced ramification in BV-2 cells. Cs+ accentuated the PMA-dependent ramification in BV-2 cells. Thus, our results indicate (1) that a PMA-binding protein, excluding PKC isoforms, is critical in structural remodeling of microglial cells and (2) that chloride conductance plays a pivotal role in induction of ramification in microglial cells.

Regulation of K-Cl cotransport: from function to genes

This review intends to summarize the vast literature on K-Cl cotransport (COT) regulation from a functional and genetic viewpoint. Special attention has been given to the signaling pathways involved in the transporter's regulation found in several tissues and cell types, and more specifically, in vascular smooth muscle cells (VSMCs). The number of publications on K-Cl COT has been steadily increasing since its discovery at the beginning of the 1980s, with red blood cells (RBCs) from different species (human, sheep, dog, rabbit, guinea pig, turkey, duck, frog, rat, mouse, fish, and lamprey) being the most studied model. Other tissues/cell types under study are brain, kidney, epithelia, muscle/smooth muscle, tumor cells, heart, liver, insect cells, endothelial cells, bone, platelets, thymocytes and Leishmania donovani. One of the salient properties of K-Cl-COT is its activation by cell swelling and its participation in the recovery of cell volume, a process known as regulatory volume decrease (RVD). Activation by thiol modification with N-ethylmaleimide (NEM) has spawned investigations on the redox dependence of K-Cl COT, and is used as a positive control for the operation of the system in many tissues and cells. The most accepted model of K-Cl COT regulation proposes protein kinases and phosphatases linked in a chain of phosphorylation/dephosphorylation events. More recent studies include regulatory pathways involving the phosphatidyl inositol/protein kinase C (PKC)-mediated pathway for regulation by lithium (Li) in low-K sheep red blood cells (LK SRBCs), and the nitric oxide (NO)/cGMP/protein kinase G (PKG) pathway as well as the platelet-derived growth factor (PDGF)-mediated mechanism in VSMCs. Studies on VSM transfected cells containing the PKG catalytic domain demonstrated the participation of this enzyme in K-Cl COT regulation. Commonly used vasodilators activate K-Cl COT in a dose-dependent manner through the NO/cGMP/PKG pathway. Interaction between the cotransporter and the cytoskeleton appears to depend on the cellular origin and experimental conditions. Pathophysiologically, K-Cl COT is altered in sickle cell anemia and neuropathies, and it has also been proposed to play a role in blood pressure control. Four closely related human genes code for KCCs (KCC1-4). Although considerable information is accumulating on tissue distribution, function and pathologies associated with the different isoforms, little is known about the genetic regulation of the KCC genes in terms of transcriptional and post-transcriptional regulation. A few reports indicate that the NO/cGMP/PKG signaling pathway regulates KCC1 and KCC3 mRNA expression in VSMCs at the post-transcriptional level. However, the detailed mechanisms of post-transcriptional regulation of KCC genes and of regulation of KCC2 and KCC4 mRNA expression are unknown. The K-Cl COT field is expected to expand further over the next decades, as new isoforms and/or regulatory pathways are discovered and its implication in health and disease is revealed.

Disparity between ionic mediators of volume regulation and apoptosis in N1E 115 mouse neuroblastoma cells

Cellular volume loss or shrinkage is a ubiquitous feature of apoptosis and thus may contribute to this form of degeneration. Chloride (Cl(-)) and potassium (K(+)) efflux has been shown to participate in volume regulation and several recent reports have implicated K(+) efflux in apoptotic neuronal death. Here pharmacological inhibitors of various K(+) and Cl(-) channels and transporters were used to decipher the relationship between cellular volume regulation and apoptosis. Following exposure to a hypotonic media, cells swell but over time gradually recover, returning to their original cell volume in a process known as regulatory volume decrease (RVD). RVD in N1E 115 neuroblastoma cells was monitored using time-lapse videomicroscopy, cell size and DNA degradation were followed using flow cytometry and fragmented apoptotic nuclei were visualized using Hoechst staining. RVD was blocked by high K(+), TEA and 4-AP (K(+) channel blockers), DIDS and niflumic acid but not SITS (Cl(-) channel blockers), ethacrynic acid (Cl(-) pump blocker), bumetanide (Na(+)/K(+)/Cl(-) cotransporter blocker) and furosemide (K(+)/Cl(-) cotransport blocker). In contrast, only DIDS and SITS (blockers of the Cl(-)/HCO(3) exchanger) inhibited apoptosis, suggesting that a common mechanistic link between RVD and apoptosis is the Cl(-)/HCO(3) exchanger. Thus, this study does not support the notion that K(+) channels are universal anti-apoptotic targets. Instead, the Cl(-)/HCO(3) exchanger may prove to be a viable target of therapeutic intervention for treating pathological apoptosis and neurodegeneration.

Molecular, functional, and genomic characterization of human KCC2, the neuronal K-Cl cotransporter

The expression level of the neuronal-specific K-Cl cotransporter KCC2 (SLC12A5) is a major determinant of whether neurons will respond to GABA with a depolarizing, excitatory response or a hyperpolarizing, inhibitory response. In view of the potential role in human neuronal excitability we have characterized the hKCC2 cDNA and gene. The 5.9 kb hKCC2 transcript is specific to brain, and is induced during in vitro differentiation of NT2 teratocarcinoma cells into neuronal NT2-N cells. The 24-exon SLC12A5 gene is on human chromosome 20q13, and contains a polymorphic dinucleotide repeat within intron 1 near a potential binding site for neuron-restrictive silencing factor. Expression of hKCC2 cRNA in Xenopus laevis oocytes results in significant Cl(-)-dependent (86)Rb(+) uptake under isotonic conditions; cell swelling under hypotonic conditions causes a 20-fold activation, which is blocked by the protein phosphatase inhibitor calyculin-A. In contrast, oocytes expressing mouse KCC4 do not mediate isotonic K-Cl cotransport but express much higher absolute transport activity than KCC2 oocytes under hypotonic conditions. Initial and steady state kinetics of hKCC2-injected oocytes were performed in both isotonic and hypotonic conditions, revealing K(m)s for K(+) and Cl(-) of 9.3+/-1.8 mM and 6.8+/-0.9 mM, respectively; both affinities are significantly higher than KCC1 and KCC4. The K(m) for Cl(-) is close to the intracellular Cl(-) activity of mature neurons, as befits a neuronal efflux mechanism.

Novel effects of minocycline on Ca(2+)-dependent Cl(-) secretion in human airway epithelial Calu-3 cells

The present study concerns previously unreported effects of the antibiotic minocycline on the transepithelial Cl(-) transport in Calu-3 cells, which display electrophysiological properties consistent with human airway serous cells. Basolateral 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS, 200 microM) augmented Cl(-) secretion, which was detected as a 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB, 100 microM, a Cl(-) channel blocker)-sensitive short-circuit current (I(sc)). The DIDS-induced I(sc) was composed of Ca(2+)-activated K(+) (K(Ca)) channel-dependent and -independent components. The former was selectively inhibited by 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl)ester (BAPTA/AM, 10 microM), charybdotoxin (ChTx, 100 nM), clotrimazole (10 microM), basolateral Ca(2+) removal, and basolateral minocycline (IC(50) = 20 microM). The latter was attenuated by basolateral BaCl (5 mM). In contrast, forskolin (10 microM)-induced I(sc), which is insensitive to BAPTA/AM and ChTx, was unaffected by minocycline (100 microM). ATP-induced I(sc) was partially inhibited by basolateral but not by apical minocycline. I(sc) due to basolateral application of ionomycin (1 microM) was markedly suppressed by NPPB and basolateral Ca(2+) removal. These inhibitory effects were mimicked by minocycline applied only from the basolateral side of the monolayer. In the basolateral absence of Ca(2+), 1-ethyl-2-benzimdazolinone (500 microM), a K(Ca) channel opener, generated a sustained I(sc) sensitive to ChTx. Minocycline had no significant effect on the ChTx-sensitive component of the I(sc). It is concluded that minocycline inhibits K(Ca) channel-dependent Cl(-) secretion via a blockade of Ca(2+) influx across the basolateral membrane from the extracellular side.

Direct estimate of 1:1 stoichiometry of K(+)-Cl(-) cotransport in rabbit erythrocytes

This work was undertaken to obtain a direct measure of the stoichiometry of Na(+)-independent K(+)-Cl(-) cotransport (KCC), with rabbit red blood cells as a model system. To determine whether (86)Rb(+) can be used quantitatively as a tracer for KCC, (86)Rb(+) and K(+) effluxes were measured in parallel after activation of KCC with N-ethylmaleimide (NEM). The rate constant for NEM-stimulated K(+) efflux into isosmotic NaCl was smaller than that for (86)Rb(+) by a factor of 0.68 +/- 0.11 (SD, n = 5). This correction factor was used in all other experiments to calculate the K(+) efflux from the measured (86)Rb(+) efflux. To minimize interference from the anion exchanger, extracellular Cl(-) was replaced with SO, and 4,4'-diisothiocyanothiocyanatodihydrostilbene-2,2'-disulfonic acid was present in the flux media. The membrane potential was clamped near 0 mV with the protonophore 2,4-dinitrophenol. The Cl(-) efflux at 25 degrees C under these conditions is approximately 100,000-fold smaller than the uninhibited Cl(-)/Cl(-) exchange flux and is stimulated approximately 2-fold by NEM. The NEM-stimulated (36)Cl(-) flux is inhibited by okadaic acid and calyculin A, as expected for KCC. The ratio of the NEM-stimulated K(+) to Cl(-) efflux is 1.12 +/- 0.26 (SD, n = 5). We conclude that K(+)-Cl(-) cotransport in rabbit red blood cells has a stoichiometry of 1:1.

Perturbation of the pump-leak balance for Na+ and K+ in malaria-infected erythrocytes

In human erythrocytes infected with the mature form of the malaria parasite Plasmodium falciparum, the cytosolic concentration of Na+ is increased and that of K+ is decreased. In this study, the membrane transport changes underlying this perturbation were investigated using a combination of86Rb+, 43K+, and22Na+ flux measurements and a semiquantitative hemolysis technique. From >15 h postinvasion, there appeared in the infected erythrocyte membrane new permeation pathways (NPP) that caused a significant increase in the basal ion permeability of the erythrocyte membrane and that were inhibited by furosemide (0.1 mM). The NPP showed the selectivity sequence Cs+ > Rb+ > K+ > Na+, with the K+-to-Na+permeability ratio estimated as 2.3. From 18 to 36 h postinvasion, the activity of the erythrocyte Na+/K+ pump increased in response to increased cytosolic Na+ (a consequence of the increased leakage of Na+ via the NPP) but underwent a progressive decrease in the latter 12 h of the parasite's occupancy of the erythrocyte (36–48 h postinvasion). Incorporation of the measured ion transport rates into a mathematical model of the human erythrocyte indicates that the induction of the NPP, together with the impairment of the Na+/K+pump, accounts for the altered Na+ and K+levels in the host cell cytosol, as well as predicting an initial decrease, followed by a lytic increase in the volume of the host erythrocyte.

Functional comparison of the K+-Cl- cotransporters KCC1 and KCC4

The K(+)-Cl(-) cotransporters (KCCs) are members of the cation-chloride cotransporter gene family and fall into two phylogenetic subgroups: KCC2 paired with KCC4 and KCC1 paired with KCC3. We report a functional comparison in Xenopus oocytes of KCC1 and KCC4, widely expressed representatives of these two subgroups. KCC1 and KCC4 exhibit differential sensitivity to transport inhibitors, such that KCC4 is much less sensitive to bumetanide and furosemide. The efficacy of these anion inhibitors is critically dependent on the concentration of extracellular K(+), with much higher inhibition in 50 mm K(+) versus 2 mm K(+). KCC4 is also uniquely sensitive to 10 mm barium and to 2 mm trichlormethiazide. Kinetic characterization reveals divergent affinities for K(+) (K(m) values of approximately 25.5 and 17.5 mm for KCC1 and KCC4, respectively), probably due to variation within the second transmembrane segment. Although the two isoforms have equivalent affinities for Cl(-), they differ in the anion selectivity of K(+) transport (Cl(-) > SCN(-) = Br(-) > PO(4)(-3) > I(-) for KCC1 and Cl(-) > Br(-) > PO(4)(-3) = I(-) > SCN(-) for KCC4). Both KCCs express minimal K(+)-Cl(-) cotransport under isotonic conditions, with significant activation by cell swelling under hypotonic conditions. The cysteine-alkylating agent N-ethylmaleimide activates K(+)-Cl(-) cotransport in isotonic conditions but abrogates hypotonic activation, an unexpected dissociation of N-ethylmaleimide sensitivity and volume sensitivity. Although KCC4 is consistently more volume-sensitive, the hypotonic activation of both isoforms is critically dependent on protein phosphatase 1. Overall, the functional comparison of these cloned K(+)-Cl(-) cotransporters reveals important functional, pharmacological, and kinetic differences with both physiological and mechanistic implications.

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.

Cl- transport in the lobster stretch receptor neurone

Experiments were performed to identify mechanisms underlying non-leakage and non-H+/HCO3--linked transmembrane Cl- transports in the slowly adapting stretch receptor neurone of the European lobster, using intracellular microelectrode and pharmacological techniques. In methodological tests, it was established that direct estimates of intracellular Cl- with ion-sensitive microelectrodes are statistically identical with indirect estimates by means of a GABA method, where 1-2 mM GABA is transforming the cell's membrane voltage into its Cl- equilibrium voltage from which the Cl- concentration is inferred by the Nernst equation. From experiments using sodium orthovanadate and ethacrynic acid, supposed to block primary Cl- pumps, and bumetanide, supposed to block Na-K-Cl co-transporters, it appeared that neither of the two Cl- transport systems exists in the stretch receptor neurone. It could be shown, however, that the cell is equipped with an electroneutral K-Cl co-transporter that (a) is blockable by furosemide in high (Km approximately 350 microM), by 4-acetamido-4'-isothiocyanato-stilbene-2,2-disulphonic acid (SITS) in medium-high (Km approximately 35 microM), and by 4, 4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS) in low (Km approximately 15 microM) doses, (b) is (transiently) activatable by (1 mM) n-ethylmaleimide, (c) is not suppressed by extracellular Rb+ or NH4+, and (d) is not directly coupled to any transmembrane transports of Na+, H+ or HCO3-. From functional tests, with varying transmembrane K+ and Cl- gradients, evidence obtained that the K-Cl co-transporter is able to reverse its transport direction and to adjust its transport rate in a considerable range. As a whole, the results speak in favour of the K-Cl co-transporter being responsible (a) for normally keeping the intracellular Cl- concentration at low levels, for an optimization of the cell's inhibitory system, and (b) for achieving fast transmembrane shifts of K+ (and Cl-), as a means of stabilizing the cell's membrane excitability in conditions of varying extracellular K+ concentrations.

K-Cl cotransporter expression in the human kidney

The K-Cl cotransporter protein KCC1 is a membrane transport protein that mediates the coupled, electroneutral transport of K and Cl across plasma membranes. The precise cell type(s) in the kidney that express the K-Cl cotransporter have remained unknown. The aim of the present investigation was to define the distribution of KCC1 mRNA in the human kidney. We used in situ hybridization with a nonradioactive digoxigenin-labeled riboprobe. We identified abundant KCC1 mRNA expression in the epithelial cells throughout the distal and proximal renal tubular epithelium. The transporter was also expressed in glomerular mesangial cells and endothelial cells of the renal vessels. These findings suggest that the K-Cl cotransporter may have an important role in transepithelial K and Cl reabsorption.

Volume-activated DIDS-sensitive whole-cell chloride currents in trout red blood cells

1. The nystatin-perforated whole-cell recording mode of the patch-clamp technique was used to investigate the membrane conductance of trout (Oncorhynchus mykiss) red blood cells in the steady state, 5 min after exposure to hyposmotic medium and 10 min after return to normal isosmotic medium. 2. Whole-cell I-V relations showed outward rectification when red blood cells were bathed in isosmotic (320 mosmol l-1) saline solution and the patch pipette was filled with 117 mM KCl. The membrane conductance was 2.58 +/- 0.59 nS (number of experiments, n = 18) between 0 and 100 mV and 1.32 +/- 0.19 nS (n = 18) between 0 and -100 mV. Removal of Cl- from the extracellular side or incubation with the Cl- channel blocker DIDS caused a reduction in whole-cell membrane conductance by more than 50%, indicating that the membrane current was generated by Cl- ions. The remaining conductance was voltage independent and probably due to non-selective cation conductance. 3. The membrane conductance increased approximately 2-fold after cell swelling induced by exposure to hyposmotic saline solution (215 mosmol l-1). This effect was abolished in Cl(-)-free hyposmotic medium or in the presence of DIDS. 4. The return to isosmotic solution produced a fall in membrane conductance to, or below, control values. 5. We conclude that trout red blood cells possess a significant Cl- conductance in the steady state which is reversibly activated during cell swelling and contributes to volume recovery.

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.

DIDS inhibition of deformation-induced cation flux in human erythrocytes

The permeability of human erythrocytes to sodium, potassium and calcium increases when the cells are deformed by shear. We now report that the anion-exchange inhibitor DIDS (4,4'-diisothiocyanostilbene-2,2'-disulfonic acid) inhibited 55-60% of the deformation-induced flux with an apparent K1/2 of 1 microM. Covalently bound DIDS was also effective. In cells partially derivatized at 0 degrees C (pH 7.4), anion exchange and the deformation flux were inhibited in parallel, implying that lysine a is the site of inhibition for both fluxes. Ektacytometry showed that DIDS does not inhibit by lowering the cell's ability to deform. Crosslinking of lysines in Band 3 was not required for inhibition of the stress flux, as demonstrated by electrophoretic analysis of chymotrypsin-cleaved Band 3 after DIDS treatment. Chymotrypsin cleavage itself did not affect the cation flux rates. DNDS, an anion exchange inhibitor that binds to the chloride site on Band 3 but is unable to derivatize lysine a, is an ineffective inhibitor of the deformation flux. Other high-affinity inhibitors of anion exchange were also relatively ineffective against the deformation flux, and anion exchange itself was unchanged by shear. These results suggest that 55-60% of the deformation-induced cation movement traverses a route that includes Band 3, but is distinct from the pathway utilized by anion exchange. Chloride-dependent cation pathways do not participate in the stress induced cation flux, since complete exchange of intracellular chloride for sulfate had no effect on the rates. Deformation of erythrocytes by laminar shear appears to increase the non-specific cation permeability.