Coupled Na/K/Cl efflux. "Reverse" unidirectional fluxes in squid giant axons

Breakdown of blood brain barrier as a mechanism of post-traumatic epilepsy

Traumatic brain injury (TBI) accounts for approximately 16% of acute symptomatic seizures which usually occur in the first week after trauma. Children are at higher risk for post-traumatic seizures than adults. Post-traumatic seizures are a risk factor for delayed development of epilepsy. Delayed, chronic post-traumatic epilepsy is preceded by a silent period during which therapeutic interventions may arrest, revert or prevent epileptogenesis. A number of recent review articles summarize the most important features of post-traumatic seizures and epilepsy; this review will instead focus on the link between cerebrovascular permeability, epileptogenesis and ictal events after TBI. The possibility of acting on the blood-brain barrier (BBB) and the neurovascular unit to prevent, disrupt or treat post-traumatic epilepsy is also discussed. Finally, we describe the latest quest for biomarkers of epileptogenesis which may allow for a more targeted intervention.

Trigeminal ganglion neurons of mice show intracellular chloride accumulation and chloride-dependent amplification of capsaicin-induced responses

Intracellular Cl⁻ concentrations ([Cl⁻]_i) of sensory neurons regulate signal transmission and signal amplification. In dorsal root ganglion (DRG) and olfactory sensory neurons (OSNs), Cl⁻ is accumulated by the Na⁺-K⁺-2Cl⁻ cotransporter 1 (NKCC1), resulting in a [Cl⁻]_i above electrochemical equilibrium and a depolarizing Cl⁻ efflux upon Cl⁻ channel opening. Here, we investigate the [Cl⁻]_i and function of Cl⁻ in primary sensory neurons of trigeminal ganglia (TG) of wild type (WT) and NKCC1^−/− mice using pharmacological and imaging approaches, patch-clamping, as well as behavioral testing. The [Cl⁻]_i of WT TG neurons indicated active NKCC1-dependent Cl⁻ accumulation. Gamma-aminobutyric acid (GABA)_A receptor activation induced a reduction of [Cl⁻]_i as well as Ca²⁺ transients in a corresponding fraction of TG neurons. Ca²⁺ transients were sensitive to inhibition of NKCC1 and voltage-gated Ca²⁺ channels (VGCCs). Ca²⁺ responses induced by capsaicin, a prototypical stimulus of transient receptor potential vanilloid subfamily member-1 (TRPV1) were diminished in NKCC1^−/− TG neurons, but elevated under conditions of a lowered [Cl⁻]_o suggesting a Cl⁻-dependent amplification of capsaicin-induced responses. Using next generation sequencing (NGS), we found expression of different Ca²⁺-activated Cl⁻ channels (CaCCs) in TGs of mice. Pharmacological inhibition of CaCCs reduced the amplitude of capsaicin-induced responses of TG neurons in Ca²⁺ imaging and electrophysiological recordings. In a behavioral paradigm, NKCC1^−/− mice showed less avoidance of the aversive stimulus capsaicin. In summary, our results strongly argue for a Ca²⁺-activated Cl⁻-dependent signal amplification mechanism in TG neurons that requires intracellular Cl⁻ accumulation by NKCC1 and the activation of CaCCs.

A potential antimicrobial treatment against ESBL-producing Klebsiella pneumoniae using the tellurium compound AS101

Due to the extensive spread of antibiotic-resistant Klebsiella pneumoniae, the non-toxic immunomodulator, ammonium trichloro (dioxoethylene-o, o') tellurate (AS101), was introduced for the first time in this study. Eleven strains of K. pneumoniae were tested: five were extended spectrum beta lactamase (ESBL)-producing strains and six were non-ESBL-producing strains. The MIC and MBC of ten strains were 9 microg/ml AS101 and 18 microg/ml for one strain. AS101 treatment inhibited bacterial growth in a dose-dependent manner on protein-rich media. No inhibition by AS101 was observed on poorer media. In combination with beta-mercaptoethanol (2-ME) or cysteamine, AS101 inhibited bacterial growth in both types of media. Growth inhibition was also shown following AS101 treatment at both lag and log phases. Our data indicate that AS101 enters the bacterium through its porins, causing bacterial destruction. The mechanism of cell death was characterized using several techniques: (a) scanning electron microscopy showed that bacteria treated with AS101 or in combination with cysteamine exhibited evidence of cell-wall damage; (b) X-ray microanalysis demonstrated damage to Na/K pumps; and (c) transmission electron microscopy demonstrated cell lysis. These phenomena suggest that AS101 has antibacterial potential against K. pneumoniae infections.

A biophysical model of an inner hair cell

Whole-cell patch-clamp recordings on isolated inner hair cells (IHCs) of guinea pig cochleae have revealed the presence of voltage-gated potassium channels. A biophysical model of an IHC is presented that indicates activation of slow voltage-gated potassium channels may lead to receptor potentials whose dc component decreases during the stimulus, and membrane potential hyperpolarizes when the stimulus is turned off. Both the decreasing dc and the hyperpolarization are, respectively, consistent with rapid adaptation and suppression of spontaneous rate in the auditory nerve. Receptor potentials recorded in vivo do not show these features, and when a nonspecific leak is included in the model to simulate microelectrode impalement, the model's receptor potentials become similar to those in vivo. The nonspecific leak creates an electrical shunt that masks slow channel activity and allows the cell to depolarize. Both the decreasing dc and the hyperpolarization are sensitive to the resting potential. Because the reported resting potentials in vivo and in vitro differ greatly, the model is used to investigate homeostatic mechanisms responsible for the resting potential. It is found that the voltage-gated potassium channels have the greatest influence on the resting potential, but that the standing transducer current may be sufficient to eliminate the decreasing dc and after-stimulus hyperpolarization.

Macromolecular crowding and its role as intracellular signalling of cell volume regulation

Macromolecular crowding has been proposed as a mechanism by means of which a cell can sense relatively small changes in volume or, more accurately, the concentration of intracellular solutes. According to the macromolecular theory, the kinetics and equilibria of enzymes can be greatly influenced by small changes in the concentration of ambient, inert macromolecules. A 10% change in the concentration of intracellular proteins can lead to changes of up to a factor of ten in the thermodynamic activity of putative molecular regulatory species, and consequently, the extent to which such regulator(s) may bind to and activate membrane-associated ion transporters. The aim of this review is to examine the concept of macromolecular crowding and how it profoundly affects macromolecular association in an intact cell with particular emphasis on its implication as a sensor and a mechanism through which cell volume is regulated.

Sodium-potassium-chloride cotransport

Obligatory, coupled cotransport of Na(+), K(+), and Cl(-) by cell membranes has been reported in nearly every animal cell type. This review examines the current status of our knowledge about this ion transport mechanism. Two isoforms of the Na(+)-K(+)-Cl(-) cotransporter (NKCC) protein (approximately 120-130 kDa, unglycosylated) are currently known. One isoform (NKCC2) has at least three alternatively spliced variants and is found exclusively in the kidney. The other (NKCC1) is found in nearly all cell types. The NKCC maintains intracellular Cl(-) concentration ([Cl(-)](i)) at levels above the predicted electrochemical equilibrium. The high [Cl(-)](i) is used by epithelial tissues to promote net salt transport and by neural cells to set synaptic potentials; its function in other cells is unknown. There is substantial evidence in some cells that the NKCC functions to offset osmotically induced cell shrinkage by mediating the net influx of osmotically active ions. Whether it serves to maintain cell volume under euvolemic conditons is less clear. The NKCC may play an important role in the cell cycle. Evidence that each cotransport cycle of the NKCC is electrically silent is discussed along with evidence for the electrically neutral stoichiometries of 1 Na(+):1 K(+):2 Cl- (for most cells) and 2 Na(+):1 K(+):3 Cl(-) (in squid axon). Evidence that the absolute dependence on ATP of the NKCC is the result of regulatory phosphorylation/dephosphorylation mechanisms is decribed. Interestingly, the presumed protein kinase(s) responsible has not been identified. An unusual form of NKCC regulation is by [Cl(-)](i). [Cl(-)](i) in the physiological range and above strongly inhibits the NKCC. This effect may be mediated by a decrease of protein phosphorylation. Although the NKCC has been studied for approximately 20 years, we are only beginning to frame the broad outlines of the structure, function, and regulation of this ubiquitous ion transport mechanism.

Activation of Na+,K+,Cl- cotransport in squid giant axon by extracellular ions: evidence for ordered binding

Activation of the influx mode of the Na+,K+,Cl- cotransporter (NKCC) by extracellular Na+, K+ and Cl- was studied using the internally dialyzed squid giant axon. Cooperative interactions among the three transported ions were assessed using ion activation of NKCC-mediated 36Cl influx under two sets of experimental conditions. The first, or control condition, used high, non-limiting concentrations of two of the cotransported ions (the co-ions) while activating cotransport with the third ion. Under this non-limiting co-ion condition the calculated Vmax of the cotransporter was between 57 and 60 pmol/cm2/s. The apparent activation (KApp, or half-saturation) constants were: K+, 9 mM; Na+, 52 mM; and Cl-, 146 mM. The second condition used limiting co-ion concentration conditions. In this case, activation by each ion was determined when one of the other two co-ions was present at or near its apparent half-saturation concentration as determined above. Under these limiting conditions, the KApp values for all three co-ions were significantly increased regardless of which co-ion was present at a limiting concentration. The effects on the apparent Vmax were more complicated. When K+ was the limiting co-ion, there was little effect on the Vmax for Na+ or Cl- activation. In contrast, limiting concentrations of Na+ or Cl- both resulted in a large reduction of the apparent Vmax when activating with the other two co-ions. These results are consistent with an ordered binding mechanism for the NKCC in which K+ binds before Na+ or Cl-. Physiological implications for these results are discussed.

Characterization of chloride efflux from GT1-7 neurons: lack of effect of ethanol on GABAA response

The purpose of this study of GT1-7 neurons was to partially characterize basal Cl- transport and GABAA mediated Cl- efflux and to test the effect of ethanol on a GABAA receptor that lacks a gamma subunit. We measured GABAA function and Cl- transport with 36Cl-. Our results show that basal 36Cl- efflux varied with temperature at 4 degrees C, 23 degrees C, and 37 degrees C. At 23 degrees C, DIDS, an inhibitor of anion exchange, reduced basal 36Cl- efflux maximally by 79.6% with an IC50 of 42.1 microM, whereas bumetanide, an inhibitor of (Na-K-Cl) cotransport, had no effect on basal 36Cl- efflux at concentrations up to 150 microM. At 4 degrees C, muscimol, a GABAA receptor agonist, stimulated 36Cl- efflux with an EC50 of 1.47 microM. Bicuculline, a GABAA receptor antagonist, completely reversed the effect of 20 microM muscimol with an IC50 of 6.08 microM. Ethanol, at concentrations up to 87 mM (0.4% (w/v)), had no effect on muscimol-induced 36Cl- efflux at 4 degrees C or at 32 degrees C. Our results indicate that stimulation of GABAA receptors causes an efflux of Cl- from GT1-7 neurons. This finding is consistent with the concept that stimulation of GABAA receptors produces depolarization of the plasma membrane, increase in cytosolic [Ca2+], and GnRH release. Our results represent the first description of chloride transport in GT1-7 neurons and suggest the presence of a Cl- exchange, but not (Na-K-Cl), transporter mechanism. Furthermore, the lack of an effect of ethanol observed in this study is consistent with the idea that a gamma 2L subunit may be necessary for the effects of low concentrations of ethanol at GABAA receptors.

Elevated [Cl-]i, and [Na+]i inhibit Na+, K+, Cl- cotransport by different mechanisms in squid giant axons

Bumetanide-sensitive (BS) unidirectional fluxes of (36)Cl- or (22)Na+ were measured in internally dialyzed squid giant axons while varying the intra- or extracellular concentrations of Na+ and/or Cl-. Raising either [Cl-]i or [Na+]i resulted in a concentration-dependent reduction of the BS influx of both (36)Cl- and (22)Na+. Raising [Cl-]i above 200 mM completely blocked BS influxes. However, raising [Na+]i to 290 mM resulted in saturable but incomplete inhibition of both BS Na+ influx and BS Cl- influx. The consequences of varying intracellular Cl- on cotransporter effluxes were complex. At lower [Cl-]i values (below 100 mM) intracellular Cl- activated cotransporter effluxes. Surprisingly, however, raising [Cl-]i levels > 125 mM resulted in a [Cl-]i-dependent inhibition of BS effluxes of both Na+ and Cl-. On the other hand, raising [Na+]i resulted only in the activation of the BS Na+ efflux; intracellular Na+ did not inhibit BS efflux even at 290 mM. The inhibitory effects of intracellular Na+ on cotransporter-mediated influxes, and lack of inhibitory effects on BS effluxes, are consistent with the trans-side inhibition expected for an ordered binding/release model of cotransporter operation. However, the inhibitory effects of intracellular Cl- on both influxes and effluxes are not explained by such a model. These data suggest that Cl may interact with an intracellular site (or sites), which does not mediate Cl transport, but does modulate the transport activity of the Na+, K+, Cl- cotransporter.

Potassium cotransport at the rat choroid plexus

The choroid plexuses are involved in cerebrospinal fluid (CSF) secretion and CSF K homeostasis. We examined K transport mechanisms present in the isolated rat choroid plexus that may be involved in these functions, predominantly using 86Rb as a marker for K. The study demonstrates that there are two primary uptake mechanisms. Ouabain-sensitive Na-K-adenosinetriphosphatase and bumetanide-sensitive cotransport, probably of the Na-K-2Cl form, account for 48 and 46% of uptake, respectively. Efflux studies demonstrate that the primary K efflux mechanism is also bumetanide-sensitive cotransport with the other major component probably being by K channels as it is inhibitable by barium or quinidine. Efflux via the cotransporter was not inhibited by R(+)-butylindazone, a KCl cotransport inhibitor, but it was enhanced in the presence of ouabain (P < 0.001) or increased extracellular Na concentration (P < 0.01). Furthermore, Na efflux was bumetanide sensitive (P < 0.05). In all, these data suggest that the efflux cotransporter is also of the Na-K-Cl form and that it is the same transporter as the influx mechanism operating in both directions. The evidence presented leads us to hypothesize that this cotransporter is on the apical membrane of the choroid plexus and that it may have a central role in CSF secretion and perhaps CSF K homeostasis.

Extracellular Mg(2+)-dependent Na+, K+, and Cl- efflux in squid giant axons

An extracellular Na+ (Nao)-dependent Mg2+ efflux process that requires intracellular ATP has been proposed as the sole mechanism responsible for Mg2+ extrusion in internally dialyzed squid axons (12). We have shown that this exchanger can also "reverse" and mediate an extracellular Mg2+ (Mgo)-dependent Na+ efflux (16). We have extended these studies and found that, in the presence of ouabain, bumetanide, tetrodotoxin, and K+ channel blockers and in the absence of extracellular Na+, K+, and bicarbonate, intracellular K+ and Cl- are also involved in the Mgo-dependent Na+ efflux process. Two main observations support this view: 1) operation of the Mgo-dependent Na+ efflux requires the presence of intracellular K+ and Cl-, and 2) Mgo removal produces a reversible and nearly identical reduction in the magnitude of the simultaneous efflux of the ionic pairs K(+)-Na+ and Cl(-)-Na+. These results suggest that the putative bumetanide-insensitive Na-Mg exchanger also transports K+ and Cl-.

Alpha-chymotrypsin deregulation of the sodium-calcium exchanger in barnacle muscle cells

To gain insight into the mechanism by which the protease alpha-chymotrypsin (alpha-chym) activates the Na-Ca exchanger in muscle cells we studied 1) the ability of this enzyme to remove the intracellular "catalytic" Ca2+ requirement for activation of all the modes of exchange mediated by the Na-Ca exchanger (i.e., Nao-Cai, Nai-Cao, Nao-Nai, and Cao-Cai, where the subscripts o and i represent extracellular and intracellular, respectively), and 2) the ability of certain monovalent cations to stimulate Cao-Cai exchange after activation of the exchanger by alpha-chym. Barnacle muscle cells were used as models because these cells are so large that they can be internally perfused and voltage clamped. The results show that alpha-chym produces a highly activated Na-Ca exchanger able to operate in all its modes of exchange independently of catalytic Cai. The concentration-dependent effect of alpha-chym was biphasic; maximal activation occurred at 0.5 mg alpha-chym/ml perfusate for 20 min of perfusion at a perfusion rate of 2.5 microliters/min. The results are discussed in terms of the possible effects of alpha-chym on the kinetic modulation of the exchanger.

Regulation by cell volume of Na(+)-K(+)-2Cl- cotransport in vascular endothelial cells: role of protein phosphorylation

Na(+)-K(+)-2Cl- cotransport in aortic endothelial cells is activated by cell shrinkage, inhibited by cell swelling, and is responsible for recovery of cell volume. The role of protein phosphorylation in the regulation of cotransport was examined with two inhibitors of protein phosphatases, okadaic acid and calyculin, and a protein kinase inhibitor, K252a. Both phosphatase inhibitors stimulated cotransport in isotonic medium, with calyculin, a more potent inhibitor of protein phosphatase I, being 50-fold more potent. Neither agent stimulated cotransport in hypertonic medium. Stimulation by calyculin was immediate and was complete by 5 min, with no change in cell Na + K content, indicating that the stimulation of cotransport was not secondary to cell shrinkage. The time required for calyculin to activate cotransport was longer in swollen cells than in normal cells, indicating that the phosphorylation step is affected by cell volume. Activation of cotransport when cells in isotonic medium were placed in hypertonic medium was more rapid than the inactivation of cotransport when cells in hypertonic medium were placed in isotonic medium, which is consistent with a shrinkage-activated kinase rather than a shrinkage-inhibited phosphatase. K252a, a nonspecific protein kinase inhibitor, reduced cotransport in both isotonic and hypertonic media. The rate of inactivation was the same in either medium, indicating that dephosphorylation is not regulated by cell volume. These results demonstrate that Na(+)-K(+)-2Cl- cotransport is activated by protein phosphorylation and is inactivated by a Type I protein phosphatase. The regulation of cotransport by volume is due to changes in the rate of phosphorylation rather than dephosphorylation, suggesting the existence of a volume-sensitive protein kinase. Both the kinase and the phosphatase are constitutively active, perhaps to allow for rapid changes in cotransport activity.

Development of GABA-mediated, chloride-dependent inhibition in CA1 pyramidal neurones of immature rat hippocampal slices

1. gamma-Aminobutyric acid (GABA)-mediated, Cl(-)-dependent inhibitory postsynaptic potentials (IPSPs) and GABA currents in immature rat hippocampal CA1 neurones were studied using the whole-cell recording technique in brain slices. 2. IPSPs evoked by electrical stimulation were observed in postnatal 2- to 5- (PN2-5), 8- to 13-(PN8-13) and 15- to 20-(PN15-20)day-old CA1 neurones. In the presence of glutamate receptor blockers 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D-2-amino-5-phosphonovaleric acid (APV), the reversal potential for the IPSP (EIPSP) was near the resting membrane potential (RMP) in the PN2-5 neurones, but 13 and 25 mV more negative than the RMP in PN8-13 and PN15-20 neurones respectively. IPSPs and GABA currents were blocked by the GABAA-receptor antagonists bicuculline or picrotoxin. 3. The reversal potential for somatic GABA currents (EGABA) was examined in the presence of tetrodotoxin (TTX). There was a strong dependence of the EGABA upon the patch pipette [Cl-] ([Cl-]p). indicating that the GABA currents were mediated by a Cl- conductance. In PN2-5 neurones, EGABA agreed with the value predicted by the Goldman-Hodgkin-Katz equation at given concentrations of internal and external anions permeable through GABA-activated Cl- channels, whereas EGABA in older neurones was 8-18 mV more negative. 4. Examination of the relations between EGABA, holding potential, [Cl-]p and resting conductance indicated that the membrane of the PN2-5 neurones was readily permeable to Cl- which followed a passive Donnan equilibrium. Passive distribution of Cl- played a decreasing role in PN8-13 neurones and in PN15-20 neurones. 5. To assess the contribution of outward Cl- co-transport, bath applications of high K+ or furosemide were performed. High K+ and furosemide caused a reversible positive shift of EGABA in PN15-20 neurones. Raising the temperature moved EGABA to a more negative potential, with a Q10 of 5 mV. A similar change of EGABA in response to high K+, but not to furosemide, was found in PN8-13 neurones. 6. The present data indicate the existence of GABAA-mediated inhibitory synaptic connections in CA1 neurones at the earliest stages of postnatal life. During the first postnatal week, Cl- ions are passively distributed and the EIPSP and EGABA are near the RMP.(ABSTRACT TRUNCATED AT 400 WORDS)

Extracellular magnesium-dependent sodium efflux in squid giant axons

Experiments were designed to determine whether the putative Na(+)-Mg2+ exchanger previously demonstrated to mediate Mg2+ efflux (R. DiPolo and L. Beagué. Biochim. Biophys. Acta 946: 424-428, 1988) could also mediate the efflux of Na+ (presumably a Na+ efflux-Mg2+ influx exchange) in squid giant axons. The effects of external Mg2+ (Mg(o)) on 22Na efflux were measured in internally dialyzed, ATP-fueled axons in which the contribution to Na+ efflux by other pathways was inhibited. To facilitate measurement of Mg(o)-dependent Na+ efflux, the intracellular concentration of Na+ was increased. To prevent Na(+)-Na+ exchange, external Na+ was replaced by tris(hydroxymethyl)aminomethane. To assess the effect of Mg(o) on Na+ efflux without altering the total divalent cation concentrations, Mg(o) was replaced mole-for-mole by external Ba2+ (Ba(o)). This manipulation produced reversible reductions in Na+ efflux. These reductions were neither due to membrane hyperpolarization nor to a direct effect of Bao but were due instead to the reduction in Mg(o). The Mg(o)-dependent Na+ efflux was inhibited by external amiloride but was spared by bumetanide. In the absence of external Na+, the Mgo-dependent Na+ efflux increased as a function of external Mg2+ with Michaelis-Menten kinetics. These results indicate that the Na(+)-Mg2+ exchange can mediate the efflux of Na+ (operate in Na+ efflux-Mg2+ influx mode of exchange).

Binding of bumetanide to microsomes from optic ganglia of the squid, Loligo pealei

Saturable high-affinity binding of [3H] bumetanide [dissociation constant (KD) = 80 nM] was measured in microsomal membranes prepared from squid optic ganglia. Under control conditions, the maximal specific binding of labeled bumetanide (Bmax) was approximately 6-7 pmol/mg protein. Binding had a higher relative affinity for bumetanide than for furosemide and depended on the presence of Cl- and K+, but not Na+, in the incubation media. In the case of K+, [3H]bumetanide binding was half-saturated at [K+] = 100 mM. The Cl- effect was biphasic. At [Cl-] between 0 and 150 mM, [3H]bumetanide binding increased with increasing [Cl-]. However, when [Cl-] was increased above 150 mM, [3H]bumetanide binding was progressively reduced. ATP acted as a nonessential activator [mean affinity constant (K0.5) approximately 1 microM] of the ion-dependent [3H]bumetanide binding by increasing the apparent binding capacity. The activation by ATP did not require Mg2+. Other adenosine analogues also stimulated the binding of bumetanide.

Osmotic stimulation of Na(+)-K(+)-Cl- cotransport in squid giant axon is [Cl-]i dependent

The effects of increasing extracellular osmolality on unidirectional Cl- fluxes through the Na(+)-K(+)-Cl- cotransporter were studied in internally dialyzed squid giant axons. Hyperosmotic seawater stimulated bumetanide-sensitive Cl-influx at 150 mM intracellular Cl- concentration ([Cl-]i), whereas Cl- efflux was unaffected under comparable ionic conditions. Stimulation of bumetanide-sensitive Cl- influx was proportional to the increase in extracellular osmolality. Bumetanide-sensitive Cl- influx began to increase after a latency of approximately 20 min after a stepwise increase of extracellular osmolality and continued to increase for at least 70 min. The increased bumetanide-sensitive Cl- influx measured after 65 min of exposure to hyperosmotic external fluid was a function of the intracellular Cl- concentration; stimulation by hyperosmotic external fluids was observed at physiological [Cl-]i levels (greater than 100 mM) but not at lower [Cl-]i levels. Under both normo- and hyperosmotic conditions, intracellular Cl- inhibited Na(+)-K(+)-Cl- cotransport influx in a concentration-dependent manner. However, in hyperosmotic seawater, the dose dependence of inhibition by intracellular Cl- was shifted to higher [Cl-]i values. Therefore, we conclude that hyperosmotic extracellular fluids stimulate influx via the Na(+)-K(+)-Cl- cotransport by resetting the relation between [Cl-]i and transport activity.

Effect of Na+ and K+ on Cl- distribution in guinea-pig vas deferens smooth muscle: evidence for Na+, K+, Cl- co-transport

1. Smooth muscle cells of the guinea-pig vas deferens after Cl- depletion actively reaccumulate ions to a level many times higher than that predicted by a passive distribution, even when anion exchange (largely responsible for Cl- movements in this preparation) is inhibited by DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid). The cells therefore must possess a second mechanism for Cl- accumulation. We have now investigated the ionic requirement of this mechanism using a combination of ion analysis, 36Cl fluxes and direct measurement of the intracellular Cl- activity (aiCl). 2. In the steady state, the Cl- content of tissues was 12-16% less in Na(+)-free solution than in normal Krebs solution. 3. Loss of 36Cl into Cl(-)-free solution was slowed by the absence of Na+ and accelerated on its readdition. Uptake of 36Cl by Cl(-)-depleted tissues was also reduced in the absence of extracellular Na+, particularly at longer time intervals as uptake reached completion. These effects occurred in the presence and absence of CO2-HCO3- and in the presence of DIDS. 4. The initial rate of rise of aiCl on readdition of Cl- to Cl(-)-depleted cells was not significantly affected by the absence of Na+ in the presence of a functional anion exchange, but aiCl stabilized at a lower value than in normal solution. Readdition of Na+ stimulated a rise in aiCl to the control level. Removal and readdition of K+ under these conditions had negligible effects. 5. When anion exchange was inhibited by the presence of DIDS, removal and readdition of Na+ caused, respectively, a marked inhibition and stimulation of the rise in aiCl during Cl- reaccumulation. Under these conditions removal and readdition of K+ had similar effects. 6. The results suggest that Na+, K+, Cl- co-transport is involved in transmembrane movements of Cl- at least when the anion exchange mechanism is blocked. 7. The possibility that the marked effects of changes in external Na+ on transmembrane Cl- movements in the presence of a functional anion exchange mechanism are caused by secondary effects due to changes in intracellular pH as well as to suppression of Na+, K+, Cl- co-transport is discussed.

In vitro studies of theophylline-induced changes in Na, K and Cl transport in hen (Gallus domesticus) colon suggesting bidirectional, basolateral NaK2Cl cotransport

1. In isolated mucosa from a NaCl-loaded hen theophylline stimulates both unidirectional chloride fluxes (JmsCl and JsmCl). Conductive and electroneutral exchange processes, besides a bumetanide-sensitive, rheogenic process contribute. 2. The bumetanide-sensitive fraction of the theophylline-induced delta JcmCl is sodium-dependent. 3. Incubation in nominally K(+)-free solutions reduces the bumetanide-sensitive fraction delta JsmCl more than treatment with ouabain. 4. With respect to chloride the bumetanide-sensitive fraction of delta JsmCl has a Hill coefficient of 1.93 +/- 0.03, a Jmax of 12.9 +/- 0.2 mumol/cm2.hr and a K 1/2 of 73 +/- 1 mmol/l. 5. After ouabain treatment delta JmsCl and delta JsmCl are equally sensitive to bumetanide, while delta JmsCl is bumetanide insensitive without ouabain treatment.

Intracellular chloride and the mechanism for its accumulation in rat lumbrical muscle

1. Double-barrelled Cl(-)-sensitive microelectrodes have been used to measure the intracellular Cl- activity (aCli) and membrane potential (Em) in rat lumbrical muscles. The mean Cl- equilibrium potential (ECl), calculated from the measured aCli in sixty fibres, was 2.9 +/- 2.5 mV (S.D. of an observation) less negative than Em. The value of aCli was higher than would be expected for a passive distribution, by a mean 1.4 +/- 1.2 mM. The mean Em was -59.5 +/- 8.2 mV. 2. Removal of external Cl- (Cl-(o)) resulted in a rapid fall in aCli and a transient depolarization. aCli stabilized at an apparent level of 1.7 +/- 1.0 mM (n = 24) while Em became substantially more negative than in normal Krebs solution (mean, -80.1 +/- 12.4 mV). Readdition of Cl-(o) caused a rapid rise in aCli and transient hyperpolarization. ECl quickly became less negative than Em and both then fell in parallel towards the levels previously recorded in normal Krebs solution. 3. If lack of selectivity of the Cl(-)-sensitive ion exchanger and the intracellular presence of interfering anions, assumed to be responsible for the apparent aCli recorded in Cl(-)-depleted fibres, were also responsible for the apparently non-passive Cl- distribution recorded under normal conditions, the difference between the calculated ECl and Em would increase at more negative potentials. This was not observed over a range of Em values between -46 and -84 mV. 4. Inhibition of the Cl- permeability by application of 9-anthracene carboxylic acid (9-AC) resulted in an immediate rise in aCli and hyperpolarization. An aCli up to 40 mM higher, or eleven times higher, than that predicted by a passive distribution was recorded. Application of 9-AC after depletion of intracellular Cl- in Cl(-)-free solution had no effect on either the apparent aCli or Em. 5. It is concluded that Cl- ions are actively accumulated by the skeletal muscle fibre and that the Cl- distribution therefore normally exerts a depolarizing influence. 6. In the presence of 9-AC and nominal absence of CO2 and HCO3-, readdition of Cl-(o) to Cl(-)-depleted fibres resulted in a substantial rise in aCli and a small, maintained depolarization. This clear demonstration of active accumulation was used to investigate the mechanism responsible for inward transport of Cl- ions. 7. Neither application of CO2 and HCO3- nor application of DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid) had any effect on the accumulation of Cl- ions. This suggests that Cl(-)-HCO3- exchange is not involved.(ABSTRACT TRUNCATED AT 400 WORDS)

Vanadate and fluoride effects on Na+-K+-Cl- cotransport in squid giant axon

The effects of vanadate and fluoride on the Na+-K+-Cl- cotransporter of the squid giant axon were assessed. In axons not treated with these agents, intracellular dialysis with ATP-depleting fluids caused bumetanide-inhibitable 36Cl influx to fall with a half time of approximately 16 min. In the presence of either 40 microM vanadate or 5 mM fluoride, the decay of bumetanide-inhibitable 36Cl influx was significantly slowed; half time for vanadate-treated axons is 45 min and for fluoride-treated axons is 37 min. These agents are not exerting their effects on Na+-K+-Cl- cotransport by influencing the rate of ATP depletion of the axon, since they had no effect on the ATP hydrolysis rate of an optic ganglia homogenate. We therefore suggest that these data support the hypothesis that Na+-K+-Cl- cotransport in squid axons is regulated by a phosphorylation-dephosphorylation mechanism and that vanadate and fluoride reduce the rate of dephosphorylation by inhibiting a protein phosphatase.

Asymmetry of Na-K-Cl cotransport in human erythrocytes

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