Intracellular chloride activity in glial cells of the leech central nervous system

Chloride/anion channels in glial cell membranes

At least seven different chloride/anion currents have now been identified in astrocytes, oligodendrocytes/Schwann cells, and microglia. Only for two of these currents is the corresponding gene known. One of these genes is not encoding for a chloride channel, but for a class of mitochondria-like pores also found in cell membranes. Astrocytes and oligodendrocytes differ in their resting properties: astrocytes accumulate chloride but do not have a significant permeability. Oligodendrocytes have a close to passive distribution and a significant permeability. Under certain circumstances, astrocytes can express a resting chloride conductance. Reactive and neoplastic astrocytes as well as astrocytes with an altered shape exhibit a resting conductance. The function of these channels certainly involves volume regulation. Other possible functions are potassium homeostasis, migration, proliferation (in microglia), and involvement in spreading depression waves. Of greatest interest are two phenomena discovered in situ: The ClC-2 channel is only found in astrocytic endfeet near blood capillaries adjacent to neuronal GABA(A) receptors. In the supraoptic nucleus of the hypothalamus, there is an osmosensitive astrocytic taurine release. This released taurine interacts with glycine receptors in neighboring neurons, causing inhibition. It is assumed that with the future availability of more in situ, rather than in vitro, studies, an increased number of such complex interactions between glial cells, neurons, and blood vessels will be discovered.

Characterization of a synaptiform transmission between a neuron and a glial cell in the leech central nervous system

The cross-talk between neurons and glial cells is receiving increased attention because of its potential role in information processing in nervous systems. Stimulation of a single identifiable neuron, the neurosecretory Leydig interneuron in segmental ganglia of the leech Hirudo medicinalis, which modulates specific behaviors in the leech, evokes membrane hyperpolarization directly in the giant glial cell (Schmidt and Deitmer. Eur J Neurosci 11:3125-3133, 1999). We have studied the neuron-to-glia signal transmission in the voltage-clamped giant glial cell to determine whether this interaction exhibits properties of a chemical synapse. The glial response had a mean latency of 4.9 s and was dependent on the action potential frequency; the glial cell responded to as few as five Leydig neuron action potentials in 50% of the trials. The glial current was sustained for minutes during repetitive Leydig neuron activity without any sign of desensitization. The current was sensitive to tetraethylammonium, and its reversal potential of -78 mV shifted with the external K+ concentration. The glial response increased with the duration of the neuronal action potentials and was sensitive to the external Ca2+/Mg2+ concentration ratio. The results suggest that Leydig neuron activity leads to a Ca2+-dependent release of transmitter from the neuronal dendrites, evoking an K+ outward current in the giant glial cell, implying a synapse-like transmission between a neuron and a glial cell.

Role of bicarbonate and chloride in GABA- and glycine-induced depolarization and [Ca2+]i rise in fetal rat motoneurons in situ

Ca(2+) imaging and (perforated) patch recording were used to analyze the mechanism of GABA- and glycine-induced depolarizations in lumbar motoneurons of spinal cord slices from fetal rats. In fura-2 ester-loaded cells, the agonist-induced depolarizations increased [Ca(2+)](i) by up to 100 nm. The GABA- and glycine-evoked [Ca(2+)](i) transients were suppressed by bicuculline and strychnine, respectively. Their magnitude decreased by approximately 50% between embryonic days 15.5 and 19.5. The [Ca(2+)](i) increases were abolished by Ca(2+)-free superfusate and attenuated by approximately 65% by nifedipine, showing that the responses were mediated by voltage-activated Ca(2+) channels. The [Ca(2+)](i) rises were potentiated by >300% immediately after removal of Cl(-) from the superfusate but recovered to values of 50-200% of control during repeated agonist administration in Cl(-)-free saline. Bumetanide gradually suppressed the [Ca(2+)](i) increases by >75%. Subsequent removal of Cl(-) reconstituted the responses and increased, upon repeated agonist application, the peak [Ca(2+)](i) rises to values above control. Removal of HCO(3)(-) from the Cl(-)-free (bumetanide-containing) superfusate reversibly abolished both the agonist-induced [Ca(2+)](i) rises and depolarizations that were reestablished by formate anions. In Cl(-)-containing superfusate, removal of HCO(3)(-) decreased both the peak and duration of the agonist-evoked membrane depolarization and [Ca(2+)](i) response. Our findings show that HCO(3)(-) efflux has a major contribution to depolarizations mediated by GABA(A) and glycine receptor-coupled anion channels in prenatal neurons. We hypothesize that the HCO(3)(-)-dependent depolarizing component, which is likely to produce an intracellular acidosis, might play an important role during the early postnatal period when the Cl(-)-dependent component gradually shifts to hyperpolarization.

Role of bicarbonate and chloride in GABA- and glycine-induced depolarization and [Ca2+]i rise in fetal rat motoneurons in situ

Ca(2+) imaging and (perforated) patch recording were used to analyze the mechanism of GABA- and glycine-induced depolarizations in lumbar motoneurons of spinal cord slices from fetal rats. In fura-2 ester-loaded cells, the agonist-induced depolarizations increased [Ca(2+)](i) by up to 100 nm. The GABA- and glycine-evoked [Ca(2+)](i) transients were suppressed by bicuculline and strychnine, respectively. Their magnitude decreased by approximately 50% between embryonic days 15.5 and 19.5. The [Ca(2+)](i) increases were abolished by Ca(2+)-free superfusate and attenuated by approximately 65% by nifedipine, showing that the responses were mediated by voltage-activated Ca(2+) channels. The [Ca(2+)](i) rises were potentiated by >300% immediately after removal of Cl(-) from the superfusate but recovered to values of 50-200% of control during repeated agonist administration in Cl(-)-free saline. Bumetanide gradually suppressed the [Ca(2+)](i) increases by >75%. Subsequent removal of Cl(-) reconstituted the responses and increased, upon repeated agonist application, the peak [Ca(2+)](i) rises to values above control. Removal of HCO(3)(-) from the Cl(-)-free (bumetanide-containing) superfusate reversibly abolished both the agonist-induced [Ca(2+)](i) rises and depolarizations that were reestablished by formate anions. In Cl(-)-containing superfusate, removal of HCO(3)(-) decreased both the peak and duration of the agonist-evoked membrane depolarization and [Ca(2+)](i) response. Our findings show that HCO(3)(-) efflux has a major contribution to depolarizations mediated by GABA(A) and glycine receptor-coupled anion channels in prenatal neurons. We hypothesize that the HCO(3)(-)-dependent depolarizing component, which is likely to produce an intracellular acidosis, might play an important role during the early postnatal period when the Cl(-)-dependent component gradually shifts to hyperpolarization.

Glutamate receptor 5/6/7-like and glutamate transporter-1-like immunoreactivity in the leech central nervous system

Previous physiological and pharmacological evidence has suggested a neurotransmitter role for the excitatory amino acid glutamate in the leech central nervous system (CNS). In the present study, we sought to localize glutamate receptor (GluR) subunits (GluR 5/6/7, GluR 2/3 and N-methyl-D-aspartate receptor 1 [NMDAR 1]) and a glutamate transporter subtype [GLT-1] within the leech CNS using mono- and polyclonal antibodies. In whole-mounted tissue, small cells of the outer capsule and putative microglia labeled with both GluR 5/6/7 and GluR 2/3 but not NMDAR 1 subunit antisera. In general, GluR 5/6/7-like immunofluorescence was both more intense and more widespread than GluR 2/3-like immunolabeling. Cryostat-sectioned tissue revealed extensive GluR 5/6/7-like immunoreactivity throughout the neuropil as well as labeling within a few neuronal somata. GLT-1-like immunoreactivity localized to the inner capsule, which is the interface between neuronal somata and the neuropil and is deeply invested by processes of neuropil glia. These results complement previous physiological and pharmacological findings indicating that the leech CNS possesses the cellular machinery to respond to glutamate and to transport glutamate from extracellular spaces. Together, they provide further evidence for glutamate's role as a neurotransmitter within the leech CNS.

Macroscopic and single-channel chloride currents in neuropile glial cells of the leech central nervous system

In patch-clamp experiments we characterized four Cl- channels (42 pS, 70 pS, 80 pS and 229 pS) underlying the large Cl- conductance of leech neuropile glial cells. They differed with respect to their gating, their rectification and their activity in the cell-attached configuration, showed the selectivity sequence I->Cl->/=Br->F- and were impermeable to SO42-. The four channels were blocked by NPPB, DPC, niflumic acid and DIDS and exhibited either three or four sublevel states. The outward rectifying 42 pS, 70 pS and 80 pS Cl- channels were classified as intermediate conductance Cl- channels and they could contribute to the high Cl- conductance of the glial membrane, which stabilizes the glial membrane potential. The inward rectifying 229 pS Cl- channel is very similar to vertebrate high conductance Cl- channels, which are assumed to be part of an emergency system that is activated under pathophysiological conditions. In voltage-clamp experiments we calculated that the Cl- conductance amounts to one-third of the total membrane conductance. Reduction of this Cl- conductance by Cl- channel inhibitors markedly depolarized the glial cell membrane. These prominent depolarizations depended on Na+ influx and in most cases the glial cells failed to regulate their membrane potential following wash-out of the inhibitors.

Potassium homeostasis and glial energy metabolism

Since capillaries appear not to contribute significantly to rapid removal of K+ from brain tissue, the K+ released into extracellular clefts by neurons at the onset of electrical activity is presumably removed either by redistribution in the clefts or by uptake into cells. What appear to be the three major processes require no energy from the glial cells. These are diffusion through the extracellular clefts, spatial buffering by glial cells, and net uptake of K+ into glial cells through glial K+ channels associated with uptake of Cl- through an independent Cl- conductance. There is a relatively slow uptake by the Na+/K+-ATPase, which directly consumes ATP. In addition, some glial cells take up K+ on the Na+/K+/2Cl- cotransporter, which leads indirectly to energy consumption when the Na+ is subsequently pumped out. Currently available data suggest that the glial energy metabolism devoted to K+ homeostasis is less than a tenth of the total tissue energy metabolism, even under conditions of pathologically high extracellular [K+]. Hence, in situ, it is possible that glial cells could function with much less ATP than neurons do. All the various routes of muffling of changes in extracellular [K+] can be modulated, directly or indirectly, by transmitters liberated by neurons. A consequence of this could be regulation of the entry of Na+ into glial cells such that the Na+/K+-ATPase is activated. The degree of activation might be adjusted so that the resulting activation of the glial glycolytic pathway is appropriate to the provision of the quantity of metabolic substrates required by the neurons.

Effect of extracellular K+ on the intracellular free Ca2+ concentration in leech glial cells and Retzius neurones

The effects of extracellular K+ on the intracellular free Ca2+ concentration ([Ca2+]i) of neuropile glial cells and Retzius neurones in intact segmental ganglia of the leech Hirudo medicinalis were investigated by using iontophoretically injected fura-2. In both cell types, an elevation of the extracellular K+ concentration ([K+]o) caused an increase in [Ca2+]i, which was blocked by Co2+, Ni2+ and menthol, whereas nicardipine, flunarizine, omega-conotoxin GVIA and omega-agatoxin IVA were ineffective. In Ca(2+)-free solution, the K(+)-induced [Ca2+]i increase was largely suppressed in neuropile glial cells and completely abolished in Retzius neurones. The results indicate that the K(+)-induced [Ca2+]i increase was mainly due to Ca2+ influx through voltage-dependent Ca2+ channels. The Ca2+ channels of the two cell types were activated at different membrane potentials but at the same [K+]o. In both cell types, the recovery from a K(+)-induced [Ca2+]i increase was unaltered in Na(+)-free solution, indicating that active Ca2+ transport across the plasma membrane is mediated by Na(+)-independent mechanisms.

Ca2+ influx into leech neuropile glial cells mediated by nicotinic acetylcholine receptors

The effect of cholinergic agonists and antagonists on the intracellular free Ca2+ concentration ([Ca2+]i) of leech neuropile glial cells was investigated by use of iontophoretically injected fura-2. In neuropile glial cells, cholinergic agonists induced a marked increase in [Ca2+]i that was inhibited by d-tubocurarine, alpha-bungarotoxin, strychnine, and atropine. The efficacy of the various agonists and antagonists indicates that the [Ca2+]i increase is mediated by the nicotinic acetylcholine (ACh) receptors that have been characterized previously in these cells by using electrophysiological methods. In the presence of high agonist concentrations, [Ca2+]i partly recovered, suggesting that the ACh receptors desensitize. The [Ca2+]i increase induced by cholinergic agonists was abolished in Ca2(+)-free solution, which indicates that it is caused by Ca2+ influx from the external medium. The agonist-induced [Ca2+]i increase was partly preserved in Na(+)-free solution, whereas the agonist-induced membrane depolarization was strongly suppressed. The agonist-induced [Ca2+]i increase was also partly preserved in the presence of 5 mM Ni2+, which almost abolished the K(+)-induced [Ca2+]i increase mediated by voltage-dependent Ca2+ channels. It is concluded that at low agonist concentrations the [Ca2+]i increase in leech neuropile glial cells is mediated exclusively by the ion channels associated with the nicotinic ACh receptors. At high agonist concentrations, voltage-dependent [Ca2+]i increase in leech neuropile glial cells is mediated exclusively by the ion channels associated with the nicotinic ACh receptors. At high agonist concentrations, voltage-dependent Ca2+ channels activated by the concomitant membrane depolarization also contribute to the agonist-induced Ca2+ influx.

Intracellular chloride activity of leech neurones and glial cells in physiological, low chloride saline

Leech blood apparently contains considerably less chloride than generally used in physiological experiments. Instead of 85-130 mM Cl- used in experimental salines, leech blood contains around 40 mM Cl- and up to 45 mM organic anions, in particular malate. We have reinvestigated the distribution of Cl- across the cell membrane of identified glial cells and neurones in the central nervous system of the leech Hirudo medicinalis L., using double-barrelled Cl(-)- and pH-selective microelectrodes, in a conventional leech saline, and in a saline with a low Cl- concentration (40 mM), containing 40 mM malate. The interference of anions other than Cl- to the response of the ion-selective microelectrodes was estimated in Cl(-)-free salines (Cl- replaced by malate and/or gluconate). The results show that the absolute intracellular Cl- activities (aCli) in glial cells and neurones, but not the electrochemical gradients of Cl- across the glial and the neuronal cell membranes, are altered in the low Cl-, malate-based saline. In Retzius neurones, aCli is lower than expected from electrochemical equilibrium, while in pressure neurones and in neuropil glial cells, aCli is distributed close to its equilibrium in both salines, respectively. The steady-state intracellular pH values in the glial cells and Retzius neurones are little affected (< or = 0.1 pH units) in the low Cl-, malate-based saline.

Chloride-dependent pH regulation in connective glial cells of the leech nervous system

We used double-barreled pH-sensitive microelectrodes to study the mechanisms by which the intracellular pH is regulated in the connective glial cells of the medicinal leech. The experiments indicate that a Cl(-)- and HCO3(-)-dependent mechanism mediates some recovery from intracellular alkalosis even in the absence of external Na+. This suggests the presence of Na(+)-independent Cl-/HCO3- exchange in the connective glial membrane. At alkaline pHi, this exchange most likely operates in the direction of net acid loading (i.e. HCO3- efflux).

Sodium-bicarbonate cotransport current in identified leech glial cells

1. The membrane current associated with the cotransport of Na+ and HCO3- was investigated in neuropil glial cells in isolated ganglia of the leech Hirudo medicinalis L. using the two-electrode voltage-clamp technique. 2. The addition of 5% CO2-24 mM HCO3- evoked an outward current, which slowly decayed, and which was dependent upon the presence of external Na+. Removal of CO2-HCO3- elicited a transient inward current. Re-addition of Na+ to Na(+)-free saline in the presence of CO2-HCO3- also produced an outward current. Under these conditions an intracellular alkalinization and a rise in intracellular [Na+] were recorded using triple-barrelled, ion-sensitive microelectrodes. Addition or removal of HCO3-, in the absence of external Na+, caused little or no change in membrane voltage, membrane current and intracellular pH, indicating that the glial membrane has a very low HCO3- conductance. 3. Voltage steps revealed nearly linear current-voltage relationships both in the absence and presence of CO2-HCO3-, with an intersection at the assumed reversal potential of the HCO(3-)-dependent current. These results suggest a cotransport stoichiometry of 2HCO3-: 1 Na+. The HCO(3-)-dependent current could be inhibited by diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS). 4. Simultaneous recording of current and intracellular pH showed a correlation of the maximal acid-base flux with the transient HCO(3-)-dependent current during voltage steps in the presence of CO2-HCO3-. The maximum rate of acid-base flux and the HCO(3-)-dependent peak current showed a similar dependence on membrane voltage. Lowering the external pH from 7.4 to 7.0 produced an inward current, which increased twofold in the presence of CO2-HCO3-. This current was largely inhibited by DIDS, indicating outward-going electrogenic Na(+)-HCO3- cotransport during external acidification. 5. When external Na+ was replaced by Li+, a similar outward current and intracellular alkalinization were observed in the presence of CO2-HCO3-. The Li(+)-induced intracellular alkalinization was not inhibited by amiloride, a blocker of Na+(Li+)-H+ exchange, but was sensitive to DIDS. These results suggest that Li+ could, at least partly, substitute for Na+ at the cotransporter site. 6. Our results indicate that the Na(+)-HCO3- cotransport produces a current across the glial cell membrane in both directions with a reversal potential near the membrane resting potential, rendering pHi a function of the glial membrane potential.

Intracellular chloride activity and the effect of 5-hydroxytryptamine on the chloride conductance of leech Retzius neurons

Intracellular Cl- activity (aCli) and 5-hydroxytryptamine (5-HT)-induced membrane currents of Retzius neurons in the central nervous system of the medicinal leech were measured using Cl- sensitive microelectrodes and a two-microelectrode voltage-clamp technique. At the membrane of Retzius neurons Cl- ions were not passively distributed. Under different conditions the chloride equilibrium potential (ECl, -60.1 mV for isotonic saline and -57.8 mV for a hypertonic saline) was negative with respect to the membrane potential (Em, -55 +/- 3.8 and -47 +/- 3.4 mV respectively). The endogenous neurohormone 5-HT always polarized the membrane of Retzius neurons in the direction of ECl. When voltage-clamping the membrane of Retzius neurons near the resting potential both in situ and in primary culture, application of 5-HT produced an outward current (I5-HT) and an increase in membrane conductance. Current-voltage relationships for I5-HT showed a slight outward rectification and reversal potentials of -61.6 +/- 3.1 mV in situ and -66 +/- 3.1 mV in primary culture, both values being comparable to the ECl of Retzius neurons as measured in situ. The results indicate that 5-HT increases the Cl- conductance of Retzius neurons, thereby hyperpolarizing the cell membrane and affecting both the excitability of the neuron and 5-HT release from it. This could affect the feeding and swimming behaviour of the leech.

Single ion channel currents in neuropile glial cells of the leech central nervous system

The patch-clamp technique was used to investigate the activity of single ion channels in neuropile glial (NG) cells in the central nervous system (CNS) of the medicinal leech, Hirudo medicinalis. We found evidence for two distinct Cl- channels that could be distinguished by their basic electrical properties and their responses to different inhibitors on single ion channel currents. In the inside-out configuration in symmetrical Cl- solutions, these channels showed current-voltage relationships with slight outward rectification, mean conductances of 70 and 80 pS, and reversal potentials near 0 mV. Significant permeability to Na+, K+, or SO4(2-) could not be detected. The open-state probability of the 70 pS Cl- channel increased with membrane depolarization, whereas the open-state probability of the 80 pS Cl- channel was voltage-independent. The application of the stilbene derivative DIDS (100 microM) to the cytoplasmic side of the glial cell membrane blocked both Cl- channels. The activity of the 70 pS channel was blocked irreversibly by DIDS, whereas the activity of the 80 pS channel reappeared after wash-out of DIDS. Both channels were blocked reversibly by 1 mM Zn2+. K+ channels could only be observed occasionally in the soma membrane of the NG cells. We have characterized a 60 pS K+ channel with a high selectivity for K+ over Na+. The low density of K+ channels in the soma membrane may indicate a non-uniform distribution of this channel type in NG cells.

Effects of potassium on the anion and cation contents of primary cultures of mouse astrocytes and neurons

In astrocytes, as [K+]o was increased from 1.2 to 10 mM, [K+]i and [Cl-]i were increased, whereas [Na+]i was decreased. As [K+]o was increased from 10 to 60 mM, intracellular concentration of these three ions showed no significant change. When [K+]o was increased from 60 to 122 mM, an increase in [K+]i and [Cl-]i and a decrease in [Na+]i were observed. In neurons, as [K+]o was increased from 1.2 to 2.8 mM, [Na+]i and [Cl-]i were decreased, whereas [K+]i was increased. As [K+]o was increased from 2.8 to 30 mM, [K+]i, [Na+]i and [Cl-]i showed no significant change. When [K+]o was increased from 30 to 122 mM, [K+]i and [Cl-]i were increased, whereas [Na+]i was decreased. In astrocytes, pHi increased when [K+]o was increased. In neurons, there was a biphasic change in pHi. In lower [K+]o (1.2-2.8 mM) pHi decreased as [K+]o increased, whereas in higher [K+]o (2.8-122 mM) pHi was directly related to [K+]o. In both astrocytes and neurons, changes in [K+]o did not affect the extracellular water content, whereas the intracellular water content increased as the [K+]o increased. Transmembrane potential (Em) as measured with Tl-204 was inversely related to [K+]o between 1.2 and 90 mM, a ten-fold increase in [K+]o depolarized the astrocytes by about 56 mV and the neurons about 52 mV. The Em values measured with Tl-204 were close to the potassium equilibrium potential (Ek) except those in neurons at lower [K+]o. However, they were not equal to the chloride equilibrium potential (ECl) at [K+]o lower than 30 mM in both astrocytes and neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of glia in K+ and pH homeostasis in the neonatal rat spinal cord

Stimulation-evoked transient changes in extracellular potassium ([K+]e) and pH (pHe) were studied in the neonatal rat spinal cords isolated from 3-13-day-old pups. In unstimulated pups the [K+]e baseline was elevated and pHe was more acid than that in Ringer's solution (3.5 mM K+, pH 7.3-7.35). The [K+]e and pHe in 3-6-day-old pups was 3.91 +/- 0.12 mM and pHe 7.19 +/- 0.01, respectively, while in 10-13-day-old pups it was 4.35 +/- 0.15 mM and 7.11 +/- 0.01, respectively. The [K+]e changes evoked in the dorsal horn by a single electrical stimulus were as large as 1.5-2.5 mM. Such changes in [K+]e are evoked in the adult rat spinal cord with stimulation at a frequency of 10-30 Hz. The maximal changes of 2.1-6.5 mM were found at a stimulation frequency of 10 Hz in 3-6-day-old animals. In older animals the [K+]e changes progressively decreased. The poststimulation K(+)-undershoot was found after a single stimulus as well as after repetitive stimulation. In 3-8-day-old pups, the stimulation evoked an alkaline shift, which was followed by a smaller poststimulation acid shift when the stimulation was discontinued. In pups 3-4-days-old the stimulation evoked the greatest alkaline shifts, i.e., by as much as 0.05 pH units after a single pulse and by about 0.1 pH units during stimulation at a frequency of 10 Hz. In 5-8-day-old pups, the alkaline shift became smaller and the poststimulation acid shift increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological measurements of volume changes in leech neuropile glial cells

Double-barrelled microelectrodes, sensitive to quaternary ammonium ions, were used for simultaneous measurements of the intracellular free concentrations of choline ([Ch]i) or tetramethylammonium ([TMA]i) as well as membrane potential (Em) in neuropile glial cells of the leech, Hirudo medicinalis. Bath application of Ch or TMA (5 mM, 1 min) resulted in a transient membrane depolarization accompanied by a long-lasting (0.5-1 h) intracellular accumulation of these compounds to levels of between 5 and 15 mM. Changes in [Ch]i or [TMA]i were used for the calculation of changes in relative cell volume. Elevation of the extracellular K+ concentration [( K+]e) from 4 to 9, 15, 21, 27.5, or 40 mM elicited a membrane depolarization and a reversible cell swelling by about 7.5, 14, 18.5, 27 and 50%, whereas reduction of [K+]e to 1.5 mM as well as bath application of serotonin (5-HT) produced a membrane hyperpolarization and a concomitant shrinkage by about 6 and 14.3%, respectively. The measured alterations in cell volume were compared with calculated data based on the assumption of an osmotic equilibrium disturbed by potential-dependent changes of the intracellular Cl- concentration. The results indicate, that K(+)- and serotonin-induced changes in the cell volume of the neuropile glial cells are due to passive KCl and water fluxes.