Na+ + K+ + 2Cl- cotransport in animal cells--its role in volume regulation

Decrease in AQP4 expression level in atrophied skeletal muscles with innervation

Functional interaction between the selective water channel AQP4 and several ion channels, such as TRPV4, NKCC1, and Na⁺/K⁺‐ATPase, closely participate to regulate osmotic homeostasis. In the skeletal muscles, the decrease in APQ4 expression due to denervation was followed by the restoration of AQP4 expression during reinnervation. These findings raised the possibility that innervation status is an essential factor to regulate AQP4 expression in the skeletal muscles. This study investigated this hypothesis using disuse muscle atrophy model with innervation. Adult female Fischer 344 rats (8 weeks of age) were randomly assigned to either control (C) or cast immobilization (IM) groups (n = 6 per group). Two weeks after cast immobilization, the tibialis anterior muscles of each group were removed and the expression levels of some target proteins were quantified by western blot analysis. The expression level of AQP4 significantly decreased at 2 weeks post‐immobilization (p < 0.05). Moreover, the expression levels of TRPV4, NKCC1, and Na⁺/K⁺‐ATPase significantly decreased at 2 weeks post‐immobilization (p < 0.05). This study suggested that innervation status is not always a key regulatory factor to maintain the expression of AQP4 in the skeletal muscles. Moreover, the transport of water and ions by AQP4 may be changed during immobilization‐induced muscle atrophy.

Water Homeostasis and Cell Volume Maintenance and Regulation

From early unicellular organisms that formed in salty water environments to complex organisms that live on land away from water, cells have had to protect a homeostatic internal environment favorable to the biochemical reactions necessary for life. In this chapter, we will outline what steps were necessary to conserve the water within our cells and how mechanisms have evolved to maintain and regulate our cellular and organismal volume. We will first examine whole body water homeostasis and the relationship between kidney function, regulation of blood pressure, and blood filtration in the process of producing urine. We will then discuss how the composition of the lipid-rich bilayer affects its permeability to water and salts, and how the cell uses this differential to drive physiological and biochemical cellular functions. The capacity to maintain cell volume is vital to epithelial transport, neurotransmission, cell cycle, apoptosis, and cell migration. Finally, we will wrap up the chapter by discussing in some detail specific channels, cotransporters, and exchangers that have evolved to facilitate the movement of cations and anions otherwise unable to cross the lipid-rich bilayer and that are involved in maintaining or regulating cell volume.

The extracellular space and epileptic activity in the adult brain: explaining the antiepileptic effects of furosemide and bumetanide

Treatments that modulate the size of the extracellular space (ECS) also block epileptiform activity in adult brain tissue. This includes the loop diuretics furosemide and bumetanide, and alterations of the osmolarity of the ECS. These treatments block epileptiform activity in a variety of laboratory adult seizure models regardless of the underlying synaptic and physiologic mechanisms generating the seizure activity. Optical imaging studies on adult hippocampal slices show that the blockade of epileptiform activity by these treatments is concomitant with their blockade of activity-driven changes of the ECS. Here we develop and analyze the hypothesis that activity-driven changes in the size of the ECS are necessary for the maintenance of hypersynchronous epileptiform activity. In support of this hypothesis is an accumulation of data from a number of studies suggesting that furosemide and bumetanide mediate antiepileptic effects through their blockade of cell swelling, dependent on their antagonism of the glial Na+-K-2Cl cotransporter (NKCC1).

Molecular determinants of hyperosmotically activated NKCC1-mediated K+/K+ exchange

Na(+)-K(+)-2Cl(-) cotransport (NKCC) mediates the movement of two Cl(-) ions for one Na(+) and one K(+) ion. Under isosmotic conditions or with activation of the kinases SPAK/WNK4, the NKCC1-mediated Cl(-) uptake in Xenopus laevis oocytes, as measured using (36)Cl, is twice the value of K(+) uptake, as determined using (86)Rb. Under hyperosmotic conditions, there is a significant activation of the bumetanide-sensitive K(+) uptake with only a minimal increase in bumetanide-sensitive Cl(-) uptake. This suggests that when stimulated by hypertonicity, the cotransporter mediates K(+)/K(+) and Cl(-)/Cl(-) exchange. Although significant stimulation of K(+)/K(+) exchange was observed with NKCC1, a significantly smaller hyperosmotic stimulatory effect was observed with NKCC2. In order to identify the molecular determinant(s) of this NKCC1-specific activation, we created chimeras of the mouse NKCC1 and the rat NKCC2. Swapping the regulatory amino termini of the cotransporters neither conferred activation to NKCC2 nor prevented activation of NKCC1. Using unique restrictions sites, we created additional chimeric molecules and determined that the first intracellular loop between membrane-spanning domains one and two and the second extracellular loop between membrane-spanning domains three and four of NKCC1 are necessary components of the hyperosmotic stimulation of K(+)/K(+) exchange.

Furosemide potentiates the anticonvulsant action of valproate in the mouse maximal electroshock seizure model

Accumulating evidence indicates that furosemide (FUR, a loop diuretic) exerts the anticonvulsant action in various in vitro and in vivo experiments. Therefore, the aim of this study was to assess the influence of FUR on the protective action of numerous conventional and newer antiepileptic drugs (carbamazepine [CBZ], lamotrigine [LTG], oxcarbazepine [OXC], phenobarbital [PB], topiramate [TPM] and valproate [VPA]) in the mouse maximal electroshock seizure (MES) model. Results indicate that FUR (up to 100mg/kg, i.p., 30 min before the test) neither altered the threshold for electroconvulsions nor protected the animals against MES-induced seizures in mice. FUR (100 mg/kg, i.p.) enhanced the anticonvulsant effects of VPA in the MES test by reducing its ED(50) value from 230.4 to 185.4 mg/kg (P<0.05). In contrast, FUR at 100 mg/kg had no significant effect on the antielectroshock action of the remaining drugs tested (CBZ, LTG, OXC, PB, and TPM) in mice. Estimation of free plasma and total brain VPA concentrations revealed that the observed interaction between FUR and VPA in the MES test was pharmacodynamic in nature because neither free plasma nor total brain VPA concentrations were altered after i.p. administration of FUR. In conclusion, one can ascertain that the selective potentiation of the antielectroshock action of VPA by FUR and lack of any pharmacokinetic interactions between drugs, make this combination of pivotal importance for epileptic patients treated with VPA and received FUR from other than epilepsy reasons.

The Na+/K+/2Cl- cotransporter in the sea bass Dicentrarchus labrax during ontogeny: involvement in osmoregulation

This study combines a cellular and molecular analysis of the Na(+)/K(+)/2Cl(-) cotransporter (NKCC) to determine the osmoregulatory role of this protein in different tissues during the ontogeny of the sea bass. We have characterized the complete sequence of the NKCC1 isoform isolated from the sea bass gills and have identified, by immunofluorescence, NKCC1, and other isoforms, within the epithelium of the major osmoregulatory organs. Different (absorptive and secretory) functions have been attributed to this protein according to the tissue and salinity. The effects of short- (1-4 days), medium- (7-21 days) and long (6 months)-term freshwater (FW) adaptations were investigated, in comparison with seawater (SW)-maintained sea bass. In adult sea bass after long-term adaptation to FW and SW, the gills had the highest expression of NKCC mRNA compared with the median/posterior kidney and to the posterior intestine. Expression of NKCC mRNA in the kidney was 95% (SW) and 63% (FW) lower, and in the intestine 98% (SW) and 77% (FW) lower. Compared to SW-maintained sea bass, long-term FW adaptation induced a significant 5.6-fold decrease in the branchial NKCC gene expression whereas the intestinal and renal expressions did not vary significantly. The cells of the intestine and collecting ducts as well as a part of the epithelium lining the urinary bladder expressed NKCC apically. Within the gill chloride cells, NKCC was found basolaterally in SW-acclimated fish; some apically stained cells were detected after 7 days of FW exposure and their relative number increased progressively following FW acclimation. The appearance of FW-type chloride cells induces a functional shift of the gills from a secretory to an absorptive epithelium, which was only completed after long-term exposure to FW. Short- and medium-term exposure to FW induced a progressive decrease in total NKCC content and an increase in functionally different branchial chloride cells. During development, the cotransporter was already expressed in tegumentary ionocytes and along the digestive tract of late embryos. NKCC was recorded in the branchial chamber and along the renal collecting ducts in prelarvae and also in the dorsal part of the urinary bladder in larvae. The expression of NKCC along the osmoregulatory epithelial cells and the presence of Na(+)/K(+)-ATPase within these cells contribute to the increase of the osmoregulatory capacity during sea bass ontogeny.

Fate of hypertonicity-stressed corneal epithelial cells depends on differential MAPK activation and p38MAPK/Na-K-2Cl cotransporter1 interaction

The capacity of the corneal epithelium to adapt to hypertonic challenge is dependent on the ability of the cells to upregulate the expression and activity of cell membrane-associated Na-K-2Cl cotransporter1 (NKCC1). Yet, the signaling pathways that control this response during hypertonic stress are still unclear. We studied stress-induced changes in proliferation and survival capacity of SV40-immortalized human (HCEC) and rabbit (RCEC) corneal epithelial cells as a function of (i) the magnitude of the hypertonic challenge, (ii) differential changes in activation of mitogen-activated protein kinase (MAPK), and (iii) the extent of p38MAPK interaction with NKCC1. Cells were incubated in hypertonic (up to 600 mOsm) media for varying time periods up to 24 h. Phosphorylated forms of p44/42, p38, and stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) MAPK were immunoprecipitated from cell lysates, and the amount of each activated NKCC1-associated MAPK was evaluated by Western blot/ECL assay. DNA integrity was assessed by electrophoresis in a 2% agarose gel. Cell survival and proliferation were evaluated based on three criteria: protein content, cell count, and the MTT assay. Exposure to media of 325-350 mOsm increased proliferation of HCEC up to 75%, whereas this response was limited to <16% in RCEC. At higher osmolarities, cell proliferation decreased in both species. SAPK/JNK activity increased 150-fold in HCEC and <10-fold in RCEC, while DNA fragmentation occurred only in HCEC. Compared to HCEC, the better RCEC survival rate was associated with higher p38MAPK activity and near complete restoration of p44/42MAPK activity after the first 30 min. In both cell lines, the amount of phospho-NKCC1 that coimmunoprecipitated with phospho-p38MAPK was proportional to the magnitudes of their respective activation levels. However, no such associations occurred between amounts of phosphorylated p44/42MAPK or SAPK/JNK and phospho-NKCC1. Under isotonic conditions, with bumetanide-induced inhibition of RCEC and HCEC NKCC1 activities, p44/42MAPK activity declined by 40 and 60%, respectively. Such declines led to proportional decreases in cell proliferation. Survival of hypertonicity-stressed corneal epithelial cells depends both on p38MAPK activation capacity and the ability of p38MAPK to stimulate NKCC1 activity through protein-protein interaction. The level of NKCC1 activation affects the extent of cell volume recovery and, in turn, epithelial survival capacity.

Cholangiocyte anion exchange and biliary bicarbonate excretion

Primary canalicular bile undergoes a process of fluidization and alkalinization along the biliary tract that is influenced by several factors including hormones, innervation/neuropeptides, and biliary constituents. The excretion of bicarbonate at both the canaliculi and the bile ducts is an important contributor to the generation of the so-called bile-salt independent flow. Bicarbonate is secreted from hepatocytes and cholangiocytes through parallel mechanisms which involve chloride efflux through activation of Cl^- channels, and further bicarbonate secretion via AE2/SLC4A2-mediated Cl^-/HCO₃^- exchange. Glucagon and secretin are two relevant hormones which seem to act very similarly in their target cells (hepatocytes for the former and cholangiocytes for the latter). These hormones interact with their specific G protein-coupled receptors, causing increases in intracellular levels of cAMP and activation of cAMP-dependent Cl^- and HCO₃^- secretory mechanisms. Both hepatocytes and cholangiocytes appear to have cAMP-responsive intracellular vesicles in which AE2/SLC4A2 colocalizes with cell specific Cl^- channels (CFTR in cholangiocytes and not yet determined in hepatocytes) and aquaporins (AQP8 in hepatocytes and AQP1 in cholangiocytes). cAMP-induced coordinated trafficking of these vesicles to either canalicular or cholangiocyte lumenal membranes and further exocytosis results in increased osmotic forces and passive movement of water with net bicarbonate-rich hydrocholeresis.

Na-K-2Cl cotransporter inhibition impairs human lung cellular proliferation

The widespread presence of the Na-K-2Cl (NKCC) cotransporter protein suggests that chronic administration of inhibitors may result in adverse effects. Inhibition of the NKCC cotransporter by loop diuretics is felt to underlie the diuretic and the pulmonary smooth muscle relaxant effects of this drug class. However, the fundamental regulation of salt and water movement by this cotransporter suggests that it may also mediate cell volume changes occurring during cell cycle progression. Thus we hypothesized that NKCC cotransporter inhibition by loop diuretics would decrease cellular proliferation. Normal human bronchial smooth muscle cells (BSMC) showed a significant concentration-dependent decrease in cell counts after 7 days of exposure to both bumetanide ( n = 5–10) and furosemide ( n = 6–16) compared with controls. Proliferation was similarly inhibited in normal human lung fibroblasts ( n = 5–9). To determine whether this was due to loss of cells, we performed apoptosis assays on BSMC. Both annexin V-propidium iodide staining ( n = 5–10) and single cell gel electrophoresis assays ( n = 4) were negative for necrosis and apoptosis in BSMC exposed to 10 μM bumetanide. Subsequent analysis of the cell cycle by flow cytometry showed that bumetanide-exposed BSMC were delayed in G1 phase compared with controls ( n = 4–8). This is the first evidence for loop diuretic inhibition of airway smooth muscle cell proliferation. NKCC cotransporter inhibition impeded G1-S phase transition without facilitating cell death. Thus although inhibition by loop diuretics relaxes airway smooth muscle, the NKCC cotransporter may have a more important role in cell proliferation regulation.

Cl(-)-dependent secretory mechanisms in isolated rat bile duct epithelial units

Cholangiocytes absorb and secrete fluid, modifying primary canalicular bile. In several Cl(-)-secreting epithelia, Na(+)-K(+)-2Cl(-) cotransport is a basolateral Cl(-) uptake pathway facilitating apical Cl(-) secretion. To determine if cholangiocytes possess similar mechanisms independent of CO2/HCO, we assessed Cl(-)-dependent secretion in rat liver isolated polarized bile duct units (IBDUs) by using videomicroscopy. Without CO2/HCO, forskolin (FSK) stimulated secretion entirely dependent on Na(+) and Cl(-) and inhibited by Na(+)-K(+)-2Cl(-) inhibitor bumetanide. Carbonic anhydrase inhibitor ethoxyzolamide had no effect on FSK-stimulated secretion, indicating negligible endogenous CO2/HCO transport. In contrast, FSK-stimulated secretion was inhibited approximately 85% by K(+) channel inhibitor Ba(2+) and blocked completely by bumetanide plus Ba(2+). IBDU Na(+)-K(+)-2Cl(-) cotransport activity was assessed by recording intracellular pH during NH4Cl exposure. Bumetanide inhibited initial acidification rates due to NH entry in the presence and absence of CO2/HCO. In contrast, when stimulated by FSK, a 35% increase in Na(+)-K(+)-2Cl(-) cotransport activity occurred without CO2/HCO. These data suggest a cellular model of HCO-independent secretion in which Na(+)-K(+)-2Cl(-) cotransport maintains high intracellular Cl(-) concentration. Intracellular cAMP concentration increases activate basolateral K(+) conductance, raises apical Cl(-) permeability, and causes transcellular Cl(-) movement into the lumen. Polarized IBDU cholangiocytes are capable of vectorial Cl(-)-dependent fluid secretion independent of HCO. Bumetanide-sensitive Na(+)-K(+)-2Cl(-) cotransport, Cl(-)/HCO exchange, and Ba(2+)-sensitive K(+) channels are important components of stimulated fluid secretion in intrahepatic bile duct epithelium.

The Na-K-Cl cotransporter of secretory epithelia

The Na-K-Cl cotransporters are a class of ion transport proteins that transport Na, K, and Cl ions into and out of cells in an electrically neutral manner, in most cases with a stoichiometry of 1Na:1K:2Cl. To date, two Na-K-Cl cotransporter isoforms have been identified: NKCC1, which is present in a wide variety of secretory epithelia and non-epithelial cells; and NKCC2, which is present exclusively in the kidney, in the epithelial cells of the thick ascending limb of Henle's loop and of the macula densa. Both NKCC isoforms represent part of a diverse family of cation-chloride cotransport proteins that share a common predicted membrane topology; this family also includes Na-Cl cotransporters and multiple K-Cl cotransporter isoforms. In secretory epithelia, the regulation of NKCC1, which is typically present on the basolateral membrane, is tightly coordinated with that of other transporters, including apical Cl channels, to maintain cell volume and integrity during active salt and fluid secretion. Changes in intracellular [Cl] ([Cl]i) appear to be involved in this regulation of NKCC1, which is directly phosphorylated by an unknown protein kinase in response to various secretagogues as well as reductions in [Cl]i and cell volume. This review focuses on structure-function relationships within NKCC1 and on recent developments pertaining to NKCC1 regulation at cellular and molecular levels.

Induction of apoptosis using sphingolipids activates a chloride current in Xenopus laevis oocytes

The purpose of this study was to investigate whether the cell shrinkage that occurs during apoptosis could be explained by a change of the activity in ion transport pathways. We tested whether sphingolipids, which are potent pro-apoptotic compounds, can activate ionic currents in Xenopus laevis oocytes. Apoptosis was characterized in our model by a decrease in cell volume, a loss of cell viability, and DNA cleavage. Oocytes were studied using voltage-clamp after injection with N,N-dimethyl-D-erythrosphingosine (DMS) or D-sphingosine (DS). DMS and DS activated a fast-activating, slowly inactivating, outwardly rectifying current, similar to I(Cl-swell), a swelling-induced chloride current. Lowering the extracellular chloride dramatically reduced the current, and the channel was more selective for thiocyanate and iodide (thiocyanate > iodide) than for chloride. The current was blocked by 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) and lanthanum but not by niflumic acid. Oocytes injected with a pseudosubstrate inhibitor of protein kinase C (PKC), PKC-(19-31), exhibited the same current. DMS-activated current was abolished by preexposure with phorbol myristate acetate. Our results suggest that induction of apoptosis in X. laevis oocytes, using sphingolipids or PKC inhibitors, activates a current similar to swelling-induced chloride current previously described in oocytes.

Activation of inositol 1,4,5-trisphosphate receptors induces transient changes in cell shape of fertilized Xenopus eggs

Injection of inositol 1,4,5-trisphosphate (InsP3) into fertilized Xenopus eggs induced transient changes in cell shape. The region around the injected site contracted during the first 2 min, followed by swelling. These changes which initiated at the injected site extended toward the opposite side. Injection of adenophostin B, a potent InsP3 receptor agonist, also induced similar morphological changes, which suggested that InsP3 receptor activation, and not the action of InsP3 metabolites, is responsible for these changes. To determine whether these changes correlate to InsP3 receptor-mediated calcium release, we examined the morphological changes and those in intracellular free calcium concentrations ([Ca2+]i). A calcium wave was observed to precede the propagation of changes in cell shape by about 2 min. The extent of propagation of cell shape changes varied with the eggs but consistently depended on the extent of the calcium wave propagation. Changes in cell shape were inhibited in eggs injected with the calcium chelator, BAPTA, indicating that calcium released from the InsP3-sensitive calcium store is required for cell shape changes. During the cell shape changes, the contracted region was strongly stained with rhodamine-phalloidin, which suggests that structural changes of actin filaments are involved in the cortical changes. We propose that spatiotemporally controlled elevation of intracellular calcium induces successive cortical cytoskeletal changes that are responsible for changes in cell shape. These observations provide insight into the potency of InsP3/calcium signaling in the regulation of cortical cytoskeleton.

Sodium pump activity, not glial spatial buffering, clears potassium after epileptiform activity induced in the dentate gyrus

A number of mechanisms have been proposed to play a role in the regulation of activity-dependent variations in extracellular potassium concentration ([K(+)](o)). We tested possible regulatory mechanisms for [K(+)](o) during spontaneous recurrent epileptiform activity induced in the dentate gyrus of hippocampal slices from adult rats by perfusion with 8 mM potassium and 0-added calcium medium in an interface chamber. Local application of tetrodotoxin blocked local [K(+)](o) changes, suggesting that potassium is released and taken up locally. Perfusion with barium or cesium, blockers of the inward rectifying potassium channel, did not alter the baseline [K(+)](o), the ceiling level of [K(+)](o) reached during the burst, or the rate of [K(+)](o) recovery after termination of the bursts. Decreasing gap junctional conductance did not change the baseline [K(+)](o) or the half-time of recovery of the [K(+)](o) after the bursts but did cause a decrease in the ceiling level of [K(+)](o). Perfusion with furosemide, which will block cation/chloride cotransporters, or perfusion with low chloride did not change the baseline [K(+)](o) or the half-time of recovery of the [K(+)](o) after the bursts but did increase the ceiling level of [K(+)](o). Bath or local application of ouabain, a Na(+)/K(+)-ATPase inhibitor, increased the baseline [K(+)](o), slowed the rate of [K(+)](o) recovery, and induced spreading depression. These findings suggest that potassium redistribution by glia only plays a minor role in the regulation of [K(+)](o) in this model. The major regulator of [K(+)](o) in this model appears to be uptake via a Na(+)/K(+)-ATPase, most likely neuronal.

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.

Molecular and functional evidence for Na(+)-K(+)-2Cl(-) cotransporter expression in rat skeletal muscle

Doubt has been raised about the expression of a functional Na(+)-K(+)-2Cl(-) cotransporter in rat skeletal muscle. In this study we present molecular and functional evidence for expression of a protein having the characteristics of a cotransporter. RT-PCR of RNA isolated from rat soleus muscle with primers to a conserved putative membrane-spanning domain resulted in a single product of predicted size. Sequencing of the product showed that it bears >90% homology with known rodent NKCC1 (BSC2) cotransporters. RNase protection assay of RNA isolated from the rat soleus muscle also identified this sequence. Immunologic detection of the cotransporter with two different antibodies indicated the presence of cotransporter protein, perhaps more than one, in blots of total muscle protein. Immunohistochemical detection by confocal microscopy localized the majority of expression of the protein to the muscle fibers. Functional studies of cotransport activity also indicate the appropriate sensitivity to inhibitors and ion dependence. Taken together, these data support the presence and function of Na(+)-K(+)-2Cl(-) cotransporter activity in the soleus muscle of the rat.

Quantitative determination of free intracellular amino acids in single human polymorphonuclear leucocytes. Recent developments in sample preparation and high-performance liquid chromatography

The described procedure allows quantitative, highly precise and reproducible analysis of free amino acid concentrations in single polymorphonuclear leucocytes (PMLs). This method is superior to previously described procedures with regard to sample size, PML separation, sample preparation and stability, as well as the chosen fluorescence high-performance liquid chromatography procedure, and can satisfy the high demands for ultra-sensitive and comprehensive amino acid analysis, especially for the continuous surveillance of severe diseases and organ dysfunction.

Physiological significance of volume-regulatory transporters

Research over the past 25 years has identified specific ion transporters and channels that are activated by acute changes in cell volume and that serve to restore steady-state volume. The mechanism by which cells sense changes in cell volume and activate the appropriate transporters remains a mystery, but recent studies are providing important clues. A curious aspect of volume regulation in mammalian cells is that it is often absent or incomplete in anisosmotic media, whereas complete volume regulation is observed with isosmotic shrinkage and swelling. The basis for this may lie in an important role of intracellular Cl- in controlling volume-regulatory transporters. This is physiologically relevant, since the principal threat to cell volume in vivo is not changes in extracellular osmolarity but rather changes in the cellular content of osmotically active molecules. Volume-regulatory transporters are also closely linked to cell growth and metabolism, producing requisite changes in cell volume that may also signal subsequent growth and metabolic events. Thus, despite the relatively constant osmolarity in mammals, volume-regulatory transporters have important roles in mammalian physiology.

Extracellular chloride and the maintenance of spontaneous epileptiform activity in rat hippocampal slices

Previous studies showed that furosemide blocks spontaneous epileptiform activity without diminishing synaptic transmission or reducing hyperexcited field responses to electrical stimuli. We now test the hypothesis that the antiepileptic effects of furosemide are mediated through its blockade of the Na+,K+,2Cl- cotransporter and thus should be mimicked by a reduction of extracellular chloride ([Cl-]o). In the first set of experiments, field recordings from the CA1 cell body layer of hippocampal slices showed that spontaneous bursting developed within 10-20 min in slices perfused with low-[Cl-]o (7 mM) medium but that this spontaneous epileptiform activity ceased after a further 10-20 min. Intracellular recordings from CA1 pyramidal cells showed that normal action potential discharge could be elicited by membrane depolarization, even after the tissue was perfused with low-[Cl-]o medium for >2 h. In a second set of experiments, spontaneous bursting activity was induced in slices by perfusion with high-[K+]o (10 mM), bicuculline (100 microM), or 4-aminopyridine (100 microM). In each case, recordings from the CA1 region showed that reduction of [Cl-]o to 21 mM reversibly blocked the bursting within 1 h. Similar to previous observations with furosemide treatment, low-[Cl-]o medium blocked spontaneous hypersynchronous discharges without reducing synaptic hyperexcitability (i.e., hyperexcitable field responses evoked by electrical stimulation). In a third set of experiments, prolonged exposure (>1 h after spontaneous bursting ceased) of slices to systematically varied [Cl-]o and [K+]o resulted in one of three types of events: 1) spontaneous, long-lasting, and repetitive negative field potential shifts (7 mM [Cl-]o; 3 mM [K+]o); 2) oscillations consisting of 5- to 10-mV negative shifts in the field potential, with a period of approximately 1 cycle/40 s (16 mM [Cl-]o; 12 mM [K+]o); and 3) shorter, infrequently occurring negative field shifts lasting 20-40 s (21 mM [Cl-]o; 3 mM [K+]o). Our observations indicate that the effects of low [Cl-]o on neuronal synchronization and spontaneous discharge are time dependent. Similar effects were seen with furosemide and low [Cl-]o, consistent with the hypothesis that the antiepileptic effect of furosemide is mediated by the drug's effect on chloride transporters. Finally, the results of altering extracellular potassium along with chloride suggest that blockade of the Na+, K+,2Cl- cotransporter, which normally transports chloride from the extracellular space into glial cells, is key to these antiepileptic effects.

Cardiac cell volume: crystal clear or murky waters? A comparison with other cell types

The osmolarity of bodily fluids is strictly controlled so that most cells do not experience changes in osmotic pressure under normal conditions, but osmotic changes can occur in pathological states such as ischemia, septic shock, and diabetic coma. The primary effect of a change in osmolarity is to acutely alter cell volume. If the osmolarity around a cell is decreased, the cell swells, and if increased, it shrinks. In order to tolerate changes in osmolarity, cells have evolved volume regulatory mechanisms activated by osmotic challenge to normalise cell volume and maintain normal function. In the heart, osmotic stress is encountered during a period of myocardial ischemia when metabolites such as lactate accumulate intracellularly and to a certain degree extracellularly, and cause cell swelling. This swelling may be exacerbated further on reperfusion when the hyperosmotic extracellular milieu is replaced by normosmotic blood. In this review, we describe the theory and mechanisms of volume regulation, and draw on findings in extracardiac tissues, such as kidney, whose responses to osmotic change are well characterised. We then describe cell volume regulation in the heart, with particular emphasis on the effect of myocardial ischemia. Finally, we describe the consequences of osmotic cell swelling for the cell and for the heart, and discuss the implications for antiarrhythmic drug efficacy. Using computer modelling, we have summated the changes induced by cell swelling, and predict that swelling will shorten the action potential. This finding indicates that cell swelling is an important component of the response to ischemia, a component modulating the excitability of the heart.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Occlusion of K+ in the Na+/K+/2Cl- cotransporter of Ehrlich ascites tumor cells

Proteins of n-octyl glucoside solubilized membrane vesicles derived from Ehrlich ascites tumor cells can occlude 86Rb+.K+ displaces 86Rb+ and it is assumed that 86Rb+ can be used as a tracer to measure K+ occlusion. The following observations indicate that the Na+/K+/2Cl- cotransporter is responsible for this occlusion: (1) Na+ does not compete for the K+ binding site, but rather stimulates 86Rb+ occlusion. (2) K+ occlusion saturates with increasing [Na+] and [K+], the respective K0.5 values being 50 +/- 7 microM for Na+ and 371 +/- 63 microM for K+. (3) Preincubation with 1 mM ouabain does not inhibit 86Rb+ occlusion, arguing against the Na+/K+-ATPase as being responsible for the occlusion. This notion is supported by the K0.5 value for K+ being higher than reported for Na+/K+-ATPase and by the stimulatory effect of Na+. (4) The K+ occlusion is sensitive to [Cl-], and the occluded ion is protected by the presence of bumetanide during cation exchange chromatography. Our results suggest that occlusion measurements of substrate ions could be a profitable way to study the ion binding mechanism(s) of the Na+/K+/2Cl- cotransporter.

Restoration of responsiveness to phorbol ester by reconstitution of a functional Na/K/Cl cotransporter in cotransporter-deficient BALB/c 3T3 cells

Previous studies in this laboratory have implicated the membrane transport protein Na/K/Cl cotransporter (NKCC1) as an important component of the signaling pathways activated by phorbol esters in BALB/c 3T3 cells. The NKCC1 protein functions as a Na/K/Cl cotransporter in BALB/c 3T3 cells and many other cell types. Loss of NKCC1 function has been associated with loss of mitogenic responsiveness to phorbol ester. Here we report that expression of a cloned NKCC1 cDNA fused to a tetracycline-regulated promoter in BALB/c 3T3 cells deficient in Na/K/Cl cotransport activity (clone E12a cells) restored cotransport function. Compared with parental cotransport-deficient cells, transfected clones expressing the exogenous NKCC1 gene responded like typical BALB/c 3T3 cells to 12-O-tetradecanoylphorbol-13-acetate: loop diuretic-sensitive 86Rb+ flux was inhibited, cell volume was decreased, and cell growth was stimulated. These results support our previous conclusion that the loss of responsiveness of E12a cells to phorbol ester is caused by mutation of the endogenous NKCC1 gene.

Basolateral membrane chloride permeability of A6 cells: implication in cell volume regulation

The permeability to Cl- of the basolateral membrane (blm) was investigated in renal (A6) epithelial cells, assessing their role in transepithelial ion transport under steady-state conditions (isoosmotic) and following a hypoosmotic shock (i.e. in a regulatory volume decrease, RVD). Three different complementary studies were made by measuring: (1) the Cl- transport rates (delta F/Fo s-1 (x10(-3))), where F is the fluorescence of N-(6-methoxyquinoyl) acetoethyl ester, MQAE, and Fo the maximal fluorescence (x10(-3)) of both membranes by following the intracellular Cl- activities (ai Cl-, measured with MQAE) after extracellular Cl- substitution (2) the blm 86Rb and 36Cl uptakes and (3) the cellular potential and Cl- current using the whole-cell patch-clamp technique to differentiate between the different Cl- transport mechanisms. The permeability of the blm to Cl- was found to be much greater than that of the apical membranes under resting conditions: aiCl- changes were 5.3 +/- 0.7 mM and 25.5 +/- 1.05 mM (n = 79) when Cl- was substituted by NO3(-) in the media bathing apical and basolateral membranes. The Cl- transport rate of the blm was blocked by bumetanide (100 microM) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB, 50 microM) but not by N-phenylanthranilic acid (DPC, 100 microM). 86Rb and 36Cl uptake experiments confirmed the presence of a bumetanide- and a NPPB-sensitive Cl- pathway, the latter being approximately three times more important than the former (Na/K/2Cl cotransporter). Appli-cation of a hypoosmotic medium to the serosal side of the cell increased delta F/Fo s-1 (x10(-3)) after extracellular Cl- substitution (1.03 +/- 0.10 and 2.45 +/- 0.17 arbitrary fluorescent units s-1 for isoosmotic and hypoosmotic conditions respectively, n = 11); this delta F/Fo s-1 (x10(-3)) increase was totally blocked by serosal NPPB application; on the other hand, cotransporter activity was decreased by the hypoosmotic shock. Cellular Ca2+ depletion had no effect on delta F/Fo s-1 (x10(-3)) under isoosmotic conditions, but blocked the delta F/Fo s-1 (x10(-3)) increase induced by a hypoosmotic stress. Under isotonic conditions the measured cellular potential at rest was -37.2 +/- 4.0 mV but reached a maximal and transient depolarization of -25.1 +/- 3.7 mV (n = 9) under hypoosmotic conditions. The cellular current at a patch-clamping cellular potential of -85 mV (close to the Nernst equilibrium potential for K+) was blocked by NPPB and transiently increased by hypoosmotic shock (&ap;50% maximum increase). This study demonstrates that the major component of Cl- transport through the blm of the A6 monolayer is a conductive pathway (NPPB-sensitive Cl- channels) and not a Na/K/2Cl cotransporter. These channels could play a role in transepithelial Cl- absorption and cell volume regulation. The increase in the blm Cl- conductance, inducing a depolarization of these membranes, is proposed as one of the early events responsible for the stimulation of the 86Rb efflux involved in cell volume regulation.

Membrane mechanisms and intracellular signalling in cell volume regulation

Recent work on selected aspects of the cellular and molecular physiology of cell volume regulation is reviewed. First, the physiological significance of the regulation of cell volume is discussed. Membrane transporters involved in cell volume regulation are reviewed, including volume-sensitive K+ and Cl- channels, K+, Cl- and Na+, K+, 2Cl- cotransporters, and the Na+, H+, Cl-, HCO3-, and K+, H+ exchangers. The role of amino acids, particularly taurine, as cellular osmolytes is discussed. Possible mechanisms by which cells sense their volumes, along with the sensors of these signals, are discussed. The signals are mechanical changes in the membrane and changes in macromolecular crowding. Sensors of these signals include stretch-activated channels, the cytoskeleton, and specific membrane or cytoplasmic enzymes. Mechanisms for transduction of the signal from sensors to transporters are reviewed. These include the Ca(2+)-calmodulin system, phospholipases, polyphosphoinositide metabolism, eicosanoid metabolism, and protein kinases and phosphatases. A detailed model is presented for the swelling-initiated signal transduction pathway in Ehrlich ascites tumor cells. Finally, the coordinated control of volume-regulatory transport processes and changes in the expression of organic osmolyte transporters with long-term adaptation to osmotic stress are reviewed briefly.

The Na-K-Cl cotransporters

The Na-K-Cl cotransporters are a class of membrane proteins that transport Na, K, and Cl ions into and out of cells in an electrically neutral manner, in most cases with a stoichiometry of 1Na:1K:2Cl. Na-K-Cl cotransporters are present in a wide variety of cells and tissues, including reabsorptive and secretory epithelia, nerve and muscle cells, endothelial cells, fibroblasts, and blood cells. Na-K-Cl cotransport plays a vital role in renal salt reabsorption and in salt secretion by intestinal, airway, salivary gland, and other secretory epithelia. Cotransport function also appears to be important in the maintenance and regulation of cell volume and of ion gradients by both epithelial and nonepithelial cells. Na-K-Cl cotransport activity is inhibited by "loop" diuretics, including the clinically efficacious agents bumetanide and furosemide. The regulation of Na-K-Cl cotransport is mediated, at least in some cases, through direct phosphorylation of the cotransport protein. Cotransporter regulation is highly tissue specific, perhaps in part related to the presence of different Na-K-Cl cotransporter isoforms. In epithelia, both absorptive (kidney-specific) and secretory isoforms have been identified by cDNA cloning and sequencing and Northern blot analysis; alternatively spliced variants of the kidney-specific isoform have also been identified. The absorptive and secretory isoforms exhibit approximately 60% identity at the amino acid sequence level; these sequences in turn show approximately 45% overall homology with those of thiazide-sensitive, bumetanide-insensitive, Na-Cl cotransport proteins of winter flounder urinary bladder and mammalian kidney. This review focuses on recent developments in the identification of Na-K-Cl cotransport proteins in epithelial and on the regulation of epithelial Na-K-Cl cotransporter function at cellular and molecular levels.

Characterization of the endogenous Na(+)-K(+)-2Cl- cotransporter in Xenopus oocytes

Over time, Xenopus laevis changed from producing stage V and VI oocytes with little native Na(+)-K(+)-2Cl- cotransport activity to those with substantial activity. In oocytes with high endogenous activity, K+ uptake, using the tracer 86Rb+ was approximately 20 pmol.min-1.oocyte-1 in the presence of blockers of Na(+)-K(+)-ATPase and conductive K+ transport. Bumetanide (10 microM) inhibited > 90% of this uptake, suggesting involvement of Na(+)-K(+)-2Cl- cotransport. This was confirmed by two observations that are found in this cotransporter in other tissues: 1) The related diuretics, thiobenzmetanide [50% inhibitory concentration (IC50), 2 x 10(-11) M] > bumetanide (IC50, 7 x 10(-8) M) > furosemide (IC50, 2.5 x 10(-6) M) inhibited the cotransporter in a dose-dependent manner. 2) There was little uptake of K+ in the absence of extracellular Na+ or Cl-. Halving medium osmolarity to 92 mosM decreased bumetanide-sensitive K+ uptake by approximately 75%, whereas a doubling of medium osmolarity increased it by approximately 50%. The cotransport activity was increased fourfold by the phosphatase inhibitor calyculin A (200 nM) but was unaffected by 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate, 8-bromoguanosine 3',5'-cyclic monophosphate, ATP, ionomycin, or okadaic acid. Both the photoaffinity bumetanide analogue, 4-[3H]benzoyl-5-sulfamoyl-3-(3-thenyloxy)benzoic acid, and an antiserum raised against Ehrlich ascites cell cotransporter specifically labeled an approximately 140-kDa oocyte membrane protein. These results demonstrated that, in addition to the Na+ pump and K+ channels, K+ uptake in Xenopus oocytes occurs via a loop-diuretic-sensitive Na(+)-K(+)-2Cl- cotransporter.(ABSTRACT TRUNCATED AT 250 WORDS)

Na+, K+, Cl- cotransport and its regulation in Ehrlich ascites tumor cells. Ca2+/calmodulin and protein kinase C dependent pathways

Net Cl- uptake as well as unidirectional 36Cl influx during regulatory volume increase (RVI) require external K+. Half-maximal rate of bumetanide-sensitive 36Cl uptake is attained at about 3.3 mM external K+. The bumetanide-sensitive K+ influx found during RVI is strongly dependent on both Na+ and Cl-. The bumetanide-sensitive unidirectional Na+ influx during RVI is dependent on K+ as well as on Cl-. The cotransporter activated during RVI in Ehrlich cells, therefore, seems to transport Na+, K+ and Cl-. In the presence of ouabain and Ba+ the stoichiometry of the bumetanide-sensitive net fluxes can be measured at 1.0 Na+, 0.8 K+, 2.0 Cl- or approximately 1:Na, 1:K, 2:Cl. Under these circumstances the K+ and Cl- flux ratios (influx/efflux) for the bumetanide-sensitive component were estimated at 1.34 +/- 0.08 and 1.82 +/- 0.15 which should be compared to the gradient for the Na+, K+, 2Cl- cotransport system at 1.75 +/- 0.24. Addition of sucrose to hypertonicity causes the Ehrlich cells to shrink with no signs of RVI, whereas shrinkage with hypertonic standard medium (all extracellular ion concentrations increased) results in a RVI response towards the original cell volume. Under both conditions a bumetanide-sensitive unidirectional K+ influx is activated. During hypotonic conditions a small bumetanide-sensitive K+ influx is observed, indicating that the cotransport system is already activated. The cotransport is activated 10-15 fold by bradykinin, an agonist which stimulates phospholipase C resulting in release of internal Ca2+ and activation of protein kinase C. The anti-calmodulin drug pimozide inhibits most of the bumetanide-sensitive K+ influx during RVI. The cotransporter can be activated by the phorbol ester TPA. These results indicate that the stimulation of the Na+, K+, Cl- cotransport involves both Ca2+/calmodulin and protein kinase C.

Ca(2+)-dependent heat production by rat skeletal muscle in hypertonic media depends on Na(+)-Cl- co-transport stimulation

1. The rate of energy dissipation (E) in isolated, superfused soleus muscles from young rats was continuously measured under normosmotic and 100-mosM hyperosmotic conditions. The substantial increase of E with respect to basal level in hyperosmolarity (excess E), which is entirely dependent on the presence of extracellular sodium, was largely prevented or inhibited by bumetanide, a potent inhibitor of Na(+)-Cl- co-transport system, or by the removal of chloride from the superfusate (isethionate substitution). Bumetanide or the removal of chloride also acutely decreased basal E, by about 7%. 2. Bumetanide almost entirely suppressed the major, Ca(2+)-dependent part of excess E in hyperosmolarity, as well as the concomitant increase of 45Ca2+ efflux and small increase in resting muscle tension; in contrast, the part of excess E associated with stimulation of Na(+)-H+ exchange in hyperosmolarity was left unmodified. 3. Reduction of 22Na+ influx by bumetanide was more marked in hyperosmolarity than under control conditions, although stimulation of total 22Na+ influx by a 100-mosM stress was not statistically significant. Inhibition of Ca2+ release into the sarcoplasm using dantrolene sodium did not prevent the stimulation of bumetanide-sensitive 22Na+ influx, but rather increased it about fourfold. 4. It is concluded that the largest part of excess E in hyperosmolarity, which is Ca(2+)-dependent energy expenditure, is suppressed when steady-state stimulation of a Na(+)-Cl- co-transport system is inhibited either directly by bumetanide or the removal of extracellular chloride, or indirectly by the blocking of active Na(+)-K+ transport. How the stimulation of Na(+)-Cl- co-transport, by as little as 1 nmol s-1 (g wet muscle weight)-1 during a 100-mosM stress, enhances Ca(2+)-dependent heat by as much as 2.5 mW (g wet muscle weight)-1 remains to be clarified.

Analysis of volume regulation in an epithelial cell model

An epithelial cell is modeled as a single compartment, bounded by apical and basolateral cell membranes, and containing two nonelectrolyte solute species, nominally NaCl and KCl. Membrane transport of these species may be metabolically driven, or it may follow the transmembrane concentration gradients, either singly (a channel) or jointly (a cotransporter). To represent the effect of stretch-activated channels or shrinkage-activated cotransporters, the membrane permeabilities and cotransport coefficients are permitted to be functions of cell volume. When this epithelium is considered as a dynamical system, conditions are indicated which guarantee the uniqueness and stability of equilibria. Experimentally, many epithelial cells can regulate their volume, and such volume regulatory capability is defined for this model. It is clearly distinct from dynamical stability of the equilibrium and requires more stringent conditions on the volume-dependent permeabilities and cotransporters. For a previously developed model of the toad urinary bladder (Strieter et al., 1990, J. gen. Physiol. 96, 319-344) the uniqueness and stability of its equilibria are indicated. The analysis also demonstrates that under some conditions a second stable equilibrium may appear, along with a saddle-node bifurcation. This is illustrated numerically in a modified model of the epithelium of the thick ascending limb of Henle.

Regulation of vascular endothelial cell volume by Na-K-2Cl cotransport

The relationship between cell volume and Na-K-2Cl cotransport was studied in cultured bovine aortic endothelial cells. Hypertonic cell shrinkage increased bumetanide-sensitive, Na- or Cl-dependent K influx without altering bumetanide-insensitive influx. Greater stimulation of cotransport was observed in cells shrunken isosmotically either by preincubation in K-free and Na-free medium or by preincubation in hypotonic medium. Cell swelling, produced by preincubation in isotonic high-K medium, inhibited bumetanide-sensitive K influx. Simultaneous measurements of [3H]bumetanide binding and K influx revealed an increased number of binding sites without an increased influx per binding site in shrunken cells. Bumetanide did not alter the volume or ion content of cells in isotonic or hypertonic medium, indicating that no net influx of ions occurs through cotransport under these conditions. In isosmotically shrunken cells, there was greater stimulation of bumetanide-sensitive influx than of bumetanide-sensitive efflux, resulting in net bumetanide-sensitive influx. Rapid recovery of cell K, Na, and water occurred over 10-20 min and was inhibited by bumetanide or by the removal of external Na or Cl. These data demonstrate that Na-K-2Cl cotransport in aortic endothelial cells is regulated by cell volume, possibly through changes in the number of functional cotransporters, and mediates a brisk regulatory volume increase in isosmotically shrunken cells. Although thermodynamically favored, no net influx occurs through Na-K-2Cl cotransport in cells of normal volume or in hypertonically shrunken cells. This suggests additional regulation of cotransport, perhaps through trans-inhibition by intracellular Cl. Regulation of cell volume by Na-K-2Cl cotransport may be important in maintaining endothelial integrity.

Cl- channels in basolateral renal medullary membrane vesicles: IV. Analogous channel activation by Cl- or cAMP-dependent protein kinase

We examined the interactions of cAMP-dependent protein kinase and varying aqueous Cl- concentrations in modulating the activity of Cl- channels obtained by fusing basolaterally enriched renal outer medullary vesicles into planar lipid bilayers. Under the present experimental conditions, the cis and trans solutions face the extracellular and intracellular aspects of these Cl- channels, respectively. Raising the trans Cl- concentration from 2 to 50 mM increased the channel open-time probability, raised the unit channel conductance, and affected the voltage-independent determinant (delta G) of channel activity but not the gating charge (Winters, C.J., Reeves, W.B., Andreoli, T.E. 1990. J. Membrane Biol. 118:269-278). With 2 mM trans KCl, trans addition of the catalytic subunit of PKA (C-PKA) plus ATP increased channel open-time probability and altered the voltage-independent determinant of channel activity without affecting either unit channel conductance or gating charge. The effect was ATP specific, did not occur with (C-PKA plus ATP) addition to cis solutions, and was abolished by denaturing C-PKA. Finally, (C-PKA plus ATP) activation of channel activity was not detected with relatively high (50 mM) trans Cl- concentrations. These data indicate that (C-PKA plus ATP) might modulate Cl- channel activity by phosphorylation at or near the Cl(-)-sensitive site on the intracellular face of these channels.

Inability of Ehrlich ascites tumor cells to volume regulate following a hyperosmotic challenge

Ehrlich cells shrink when the osmolality of the suspending medium is increased and behave, at least initially, as osmometers. Subsequent behavior depends on the nature of the hyperosmotic solute but in no case did the cells exhibit regulatory volume increase. With hyperosmotic NaCl an osmometric response was found and the resultant volume maintained relatively constant. Continuous shrinkage was observed, however, with sucrose-induced hyperosmolality. In both cases increasing osmolality from 300 to 500 mOSM initiated significant changes in cellular electrolyte content, as well as intracellular pH. This was brought about by activation of the Na+/H+ exchanger, the Na/K pump, the Na+ + K+ + 2Cl cotransporter and by loss of K+ via a Ba-sensitive pathway. The cotransporter in response to elevated [Cl-]i (approximately 100 mM) and/or the increase in the outwardly directed gradient of chemical potential for Na+, K+ and Cl-, mediated net loss of ions which accounted for cell shrinkage in the sucrose-containing medium. In hyperosmotic NaCl, however, the net Cl- flux was almost zero suggesting minimal net cotransport activity. We conclude that volume stability following cell shrinkage depends on the transmembrane gradient of chemical potential for [Na+ + K+ + Cl-], as well as the ratio of intra- to extracellular [Cl-]. Both factors appear to influence the activity of the cotransport pathway.

Regulation of cellular volume in rabbit ventricular myocytes: bumetanide, chlorothiazide, and ouabain

Video microscopy was used to study the regulation of cell volume in isolated rabbit ventricular myocytes. Myocytes rapidly (less than or equal to 2 min) swelled and shrank in hyposmotic and hyperosmotic solutions, respectively, and this initial volume response was maintained without a regulatory volume decrease or increase for 20 min. Relative cell volumes (normalized to isosmotic solution, 1T) were as follows: 1.41 +/- 0.01 in 0.6T, 1.20 +/- 0.04 in 0.8T, 0.71 +/- 0.04 in 1.8T, and 0.57 +/- 0.03 in 2.6T. These volume changes were significantly less than expected if all of the measured volume was osmotically active water. Changes in width and thickness were significantly greater than changes in cell length. The idea that cotransport contributes to cell volume regulation was tested by inhibiting Na(+)-K(+)-2Cl- cotransport with bumetanide (BUM) and Na(+)-Cl- cotransport with chlorothiazide (CTZ). Under isotonic conditions, a 10-min exposure to BUM (1 microM), CTZ (100 microM), or BUM (10 microM) plus CTZ (100 microM) decreased relative cell volume to 0.87 +/- 0.01, 0.86 +/- 0.02, and 0.82 +/- 0.04, respectively. BUM plus CTZ also modified the response to osmotic stress. Swelling in 2.6T medium was 76% greater and shrinkage in 0.6T medium was 29% less than in the absence of diuretics. In contrast to the rapid effects of diuretics, inhibition of the Na(+)-K+ pump with 10 microM ouabain for 20 min did not affect cell volume in 1T solution. Nevertheless, ouabain decreased swelling in 0.6T medium by 52% and increased shrinkage in 1.8T medium by 34%. These data suggest that under isotonic conditions Na(+)-K(+)-2Cl- and Na(+)-Cl- cotransport are critical in establishing cell volume, but osmoregulation can compensate for Na(+)-K+ pump inhibition for at least 20 min. Under anisotonic conditions, the Na(+)-K+ pump and Na(+)-K(+)-2Cl- and/or Na(+)-Cl- cotransport are important in myocyte volume regulation.

Volume regulation of nerve terminals

Pinched-off presynaptic nerve terminals (synaptosomes) possess significant regulatory volume increase (RVI) and regulatory volume decrease (RVD) capabilities. Following a swelling induced by a hypotonic challenge, the synaptosomes regulate their volume and adjust it, in 2 min, to within 5% of its initial value (RVD) at an initial rate of -0.77 +/- 0.10%/s (mean +/- SEM). Following a shrinking induced by a hypertonic challenge, the synaptosomes also regulate their volume at an initial rate of 0.18 +/- 0.02%/s (RVI), resulting in a new steady state, reached within 5-10 min, with a synaptosomal volume below the original volume. The omission of Na+ or K+ ions from the extrasynaptosomal medium reduces the initial rate of RVI by 72.5 and 66.5%, respectively. The "loop diuretics" bumetanide and furosemide significantly inhibited the RVI of the synaptosomes. In contrast, ouabain, amiloride, or 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid did not have any significant effect on RVI parameters. Furthermore, bumetanide-sensitive 86Rb uptake by rat brain synaptosomes was stimulated threefold by a hypertonic perturbation of 30%. Thus we conclude that the RVI of synaptosomes is mainly due to a stimulation of the Na+, K+, Cl- co-transport system induced by the synaptosomal shrinking following the hypertonic challenge.

The Na+,K+,2Cl-cotransport system in HeLa cells: aspects of its physiological regulation

We have previously reported on the biochemical properties of a Na+,K+,2Cl-cotransport system in HeLa cells and here we deal with aspects of its physiological regulation. Na+,K+,2Cl-cotransport in HeLa cells was studied by 86Rb+ influx and 86Rb+/22Na+ efflux measurements. The effects of rat atrial natriuretic peptide (ANP), isoproterenol, and amino acids on 86Rb+ flux, mediated by the bumetanide-sensitive Na+,K+,2Cl-cotransport system and the ouabain-sensitive Na+/K(+)-pump, were investigated. ANP reduced bumetanide-sensitive 86Rb+ influx under isotonic as well as under hypertonic conditions. Similar decrease of bumetanide-sensitive 86Rb+ influx was observed in the presence of 8-bromo-cGMP, while neither isoproterenol as a beta-receptor agonist nor 8-bromo-cAMP-could alter bumetanide-sensitive 86Rb+ influx. Furthermore, efflux of 86Rb+ and 22Na+ was greatly reduced in the presence of bumetanide and ANP. Together with our recent findings, showing functionally active, high affinity receptors for ANP on HeLa cells (Kort and Koch, Biochim. Biophys. Res. Commun. 168: 148-154, 1990), this study indicates that ANP participates in the regulation of the Na+,K+,2Cl-cotransport system in HeLa cells. Further measurements revealed that amino acids as present in the growth medium (Joklik's minimal essential medium) and the amino acid derivative alpha-methyl-aminoisobutyric acid (metAIB, 1 and 5 mM, respectively) also reduced Na+,K+,2Cl-cotransport-mediated 86Rb+ uptake and diminished the stimulatory effect of hypertonicity on the contransporter. In addition, the Na+/K(+)-pump was markedly stimulated in the presence of amino acids, while neither ANP and 8-Br-GMP nor isoproterenol and 8-Br-cAMP had a significant effect on the activity of the Na+/K(+)-pump.

The nature of the neutral Na(+)-Cl- coupled entry at the apical membrane of rabbit gallbladder epithelium: III. Analysis of transports on membrane vesicles

In rabbit gallbladder epithelium, a Na+/H+, Cl-/HCO3- double exchange and a Na(+)-Cl- symport are both present, but experiments on intact tissue cannot resolve whether the two transport systems operate simultaneously. Thus, isolated apical plasma membrane vesicles were prepared. After preloading with Na+, injection into a sodium-free medium caused a stable intravesicular acidification (monitored with the acridine orange fluorescence quenching method) that was reversed by Na+ addition to the external solution. Although to a lesser extent, acidification took place also in experiments with an electric potential difference (PD) equal to 0. If a preset pH difference (delta pH) was imposed [( H+]in greater than [H+]out, PD = 0), the addition of Na-gluconate to the external solution caused delta pH dissipation at a rate that followed saturation kinetics. Amiloride (10(-4) M) reduced the delta pH dissipation rate. Taken together, these data indicate the presence of Na+ and H+ conductances in addition to an amiloride-sensitive, electroneutral Na+/H+ exchange. An inwardly directed [Cl-] gradient (PD = 0) did not induce intravesicular acidification. Therefore, in this preparation, there was no evidence for the presence of a Cl-/OH- exchange. When both [Na+] and [Cl-] gradients (outwardly directed, PD = 0) were present, fluorescence quenching reached a maximum 20-30 sec after vesicle injection and then quickly decreased. The decrease was not observed in the presence of a [Na+] gradient alone or the same [Na+] gradient with Cl- at equal concentrations at both sides. Similarly, the decrease was abolished in the presence of both Na+ and Cl- concentration gradients and hydrochlorothiazide (5 x 10(-4) M). The decrease was not influenced by an inhibitor of Cl-/OH- exchange (10(-4) M furosemide) or of Na(+)-K(+)-2Cl- symport (10(-5) M bumetanide). We conclude that a Na+/H+ exchange and a Na(+)-Cl- symport are present and act simultaneously. This suggests that in intact tissue the Na(+)-Cl- symport is also likely to work in parallel with the Na+/H+ exchange and does not represent an induced homeostatic reaction of the epithelium when Na+/H+ exchange is inhibited.

Regulatory volume increase in Ehrlich ascites tumor cells

Ehrlich ascites tumor cells, shrunken as a result of KCl-depletion and Na+ loading, re-establish normal ionic concentrations by the combined activity of the Na+/K+ pump and the (2Cl- + K+ + Na+) cotransport system. Restoration of cell volume, however, correlates only with the increase in intracellular Cl-. This along with the finding that the equilibrium volume is linearly related to the steady state [Cl-] suggests that the extent to which cell volume increases is determined by Cl- transport. Net Cl- uptake, which is mediated almost exclusively by the cotransport system, is ultimately responsible for establishing the steady-state intracellular Cl- concentration. Transport mediated by this pathway ceases when the sum of the chemical potentials for Na+, K+ and Cl- approaches zero and corresponds with the establishment of a steady state for Cl-. These findings suggest that Cl- plays a key role in the regulation of net cotransport activity and thereby cell volume.

Effects of potassium or potassium/magnesium supplementation on potassium content of body tissues and fluids in furosemide-treated rats on magnesium-deficient or magnesium-sufficient diet

Persistent Mg2+ deficiency may interfere with restoration of normal tissue K+ levels. This study examined: a) the effects of chronic furosemide treatment on K+ of sartorius, aorta and ventricle of rats fed Mg2(+)-deficient (100 ppm) or Mg2(+)-sufficient (400 ppm) diet and deionized water; b) whether normal tissue K+ is restored by oral K+ or K+/Mg2+ supplementation with continued furosemide therapy. Levels of Mg2+ were also measured. Furosemide (20 mg/kg i.p.) decreased K+ in sartorius, aorta and ventricle by 5.5, 4.3 and 19.9 microEq/gm (p less than .05), respectively, in rats fed 100 ppm Mg2+ diet. Furosemide did not alter K+ levels in rats fed 400 ppm Mg2+ diet. K+ supplementation (1 mEq/kg for 7 days) restored K+ to normal in sartorius but the addition of Mg2+ supplementation was necessary to restore K+ levels to normal in ventricle and aorta. These data indicate that furosemide can decrease tissue K+ in rats on a Mg2(+)-deficient diet. This decrease can be reversed during diuretic administration by K+ supplementation in sartorius, or K+ plus Mg2+ supplementation in ventricle and aorta.