Volume-sensitive chloride conductance in bovine chromaffin cell membrane

The tyrosine kinase p56lck mediates activation of swelling-induced chloride channels in lymphocytes

Osmotic cell swelling activates Cl- channels to achieve anion efflux. In this study, we find that both the tyrosine kinase inhibitor herbimycin A and genetic knockout of p56lck, a src-like tyrosine kinase, block regulatory volume decrease (RVD) in a human T cell line. Activation of a swelling-activated chloride current (ICl-swell) by osmotic swelling in whole-cell patch-clamp experiments is blocked by herbimycin A and lavendustin. Osmotic activation of ICl-swell is defective in p56lck-deficient cells. Retransfection of p56lck restores osmotic current activation. Furthermore, tyrosine kinase activity is sufficient for activation of ICl-swell. Addition of purified p56lck to excised patches activates an outwardly rectifying chloride channel with 31 pS unitary conductance. Purified p56lck washed into the cytoplasm activates ICl-swell in native and p56lck-deficient cells even when hypotonic intracellular solutions lead to cell shrinkage. When whole-cell currents are activated either by swelling or by p56lck, slow single-channel gating events can be observed revealing a unitary conductance of 25-28 pS. In accordance with our patch-clamp data, osmotic swelling increases activity of immunoprecipitated p56lck. We conclude that osmotic swelling activates ICl-swell in lymphocytes via the tyrosine kinase p56lck.

Vinblastine enhancement of hyposmosis-induced catecholamine release in cultured adrenal chromaffin cells: lack of relation to cell swelling and microtubule disruption

Exposure of chromaffin cells to hyposmotic solution has been shown to cause catecholamine release through the elevation of intracellular Ca2+ level. While cell volume change observed under hyposmotic conditions has been shown to be accompanied by the movement of various ions and suggested to be associated with the reorganization of cytoskeletons. In the present study, the effects of cytoskeleton-disrupting agents on hyposmosis-induced catecholamine release were examined to investigate a possible relationship between catecholamine release and cell volume change under hyposmotic conditions. Hyposmosis-induced catecholamine release was enhanced by pre-treatment of the cells with a microtubule-disrupting agent vinblastine, but not significantly altered by a microfilament-disrupting agent cytochalasin B. Vinblastine also caused an additional increase in the intracellular Ca2+ but failed to affect the cell volume change under hyposmotic conditions. In contrast, the hyposmosis-induced release was not significantly altered by either colchicine, another microtubule-disrupting agent, or taxol, a microtubule-stabilizing agent. These results indicate that vinblastine enhances hyposmosis-induced catecholamine release through an additional increase in the intracellular Ca2+ and furthermore suggest that this effect of vinblastine on the hyposmosis-induced release is unassociated with the disruption of the microtubule system, providing evidence for a lack of the direct relationship between catecholamine release and the cell volume change observed under hyposmotic conditions.

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.

Swelling-activated Gd3+-sensitive cation current and cell volume regulation in rabbit ventricular myocytes

The role of swelling-activated currents in cell volume regulation is unclear. Currents elicited by swelling rabbit ventricular myocytes in solutions with 0.6-0.9x normal osmolarity were studied using amphotericin perforated patch clamp techniques, and cell volume was examined concurrently by digital video microscopy. Graded swelling caused graded activation of an inwardly rectifying, time-independent cation current (ICir,swell) that was reversibly blocked by Gd3+, but ICir,swell was not detected in isotonic or hypertonic media. This current was not related to IK1 because it was insensitive to Ba2+. The PK/PNa ratio for ICir,swell was 5.9 +/- 0.3, implying that inward current is largely Na+ under physiological conditions. Increasing bath K+ increased gCir,swell but decreased rectification. Gd3+ block was fitted with a K0.5 of 1.7 +/- 0.3 microM and Hill coefficient, n, of 1.7 +/- 0.4. Exposure to Gd3+ also reduced hypotonic swelling by up to approximately 30%, and block of current preceded the volume change by approximately 1 min. Gd3+-induced cell shrinkage was proportional to ICir,swell when ICir,swell was varied by graded swelling or Gd3+ concentration and was voltage dependent, reflecting the voltage dependence of ICir,swell. Integrating the blocked ion flux and calculating the resulting change in osmolarity suggested that ICir,swell was sufficient to explain the majority of the volume change at -80 mV. In addition, swelling activated an outwardly rectifying Cl- current, ICl,swell. This current was absent after Cl- replacement, reversed at ECl, and was blocked by 1 mM 9-anthracene carboxylic acid. Block of ICl,swell provoked a 28% increase in swelling in hypotonic media. Thus, both cation and anion swelling-activated currents modulated the volume of ventricular myocytes. Besides its effects on cell volume, ICir,swell is expected to cause diastolic depolarization. Activation of ICir, swell also is likely to affect contraction and other physiological processes in myocytes.

Volume expansion-sensing outward-rectifier Cl- channel: fresh start to the molecular identity and volume sensor

The maintenance of a constant volume in the face of extracellular and intracellular osmotic perturbation is essential for the normal function and survival of animal cells. Osmotically swollen cells restore their volume, exhibiting a regulatory volume decrease by releasing intracellular K+, Cl-, organic solutes, and obligated water. In many cell types, the volume regulatory effluxes of Cl- and some organic osmolytes are known to be induced by swelling-induced activation of anion channels that are characterized by their moderate outward rectification, cytosolic ATP dependency, and intermediate unitary conductance (10-100 pS). Recently, simultaneous measurements of cell size by light microscopy and whole cell Cl- current have shown that the Cl- current density is proportionally increased with an increase in the outer surface area, which is mainly achieved through unfolding of membrane invaginations by volume expansion. Thus this anion channel can somehow sense volume expansion and can be called the volume expansion-sensing outwardly rectifying (VSOR) anion channel. Its molecular identity and activation mechanism are yet to be elucidated. Three cloned proteins, ClC-2, P-glycoprotein, and pIcln, have been proposed as candidates for the VSOR anion channel. The unitary conductance, voltage dependency, anion selectivity, pH dependency, and pharmacology of the VSOR anion channel are distinct from the ClC-2 Cl- channel, which is also known to be sensitive to volume changes. Recent patch-clamp studies in combination with molecular biological techniques have shown that P-glycoprotein is not itself the channel protein but is a regulator of its volume sensitivity. Although there is still debate about another candidate protein, pIcln, the most recent study has suggested that this is likely to be a regulator of some other distinct Cl- channel. Identification of the VSOR anion channel protein per se, its volume-sensing mechanism, and its accessory/regulatory proteins at the molecular level is currently a subject of utmost physiological importance.

A swelling-activated chloride current in rat sympathetic neurones

1. We have tested whether neurones show a swelling-induced Cl- current following hypotonic shock, by recording membrane current responses and cell volume changes in voltage clamped isolated rat sympathetic neurones during application of hypotonic solutions. 2. Using both whole-cell and perforated patch recording methods, hypotonic solution caused cell swelling and the activation of an inward Cl- current at -60 mV. This current showed weak outward rectification with no obvious time dependence. It was inhibited by SITS (0.3-1 mM), NPPB (30-300 microM) and niflumic acid (50-200 microM), but not by tamoxifen (10 microM). 3. Hypotonic solution did not cause a rise in intracellular Ca2+ concentration as measured by simultaneous indo-1 fluorescence. Also, neither the volume change nor Cl- current were affected by the removal of external Ca2+ or internal Ca2+ buffering to < or = 1 nM with EGTA. 4. The Cl- current was unaffected by an inhibitor of protein kinase C (PKC; GF109203X, 3 microM) or by omission of ATP from the pipette solution. 5. Cells exhibited a regulatory volume decrease during sustained exposure to hypotonic solution. This was completely inhibited by 0.5 mM niflumic acid. 6. It is concluded that osmotic swelling induces an outwardly rectifying, Ca2(+)- and PKC-independent Cl- current in these nerve cells. It is suggested that this current may be involved in volume regulatory mechanisms.

Evidence that outwardly rectifying Cl- channels underlie volume-regulated Cl- currents in heart

Swelling-induced Cl- current (ICl.swell) is present in most cardiac tissues, but the unitary channel underlying ICls.well is unknown. We used the cell-attached patch-clamp technique to assess the properties of single channels underlying ICls.well and the basally active Cl- current (ICl.b) in rabbit atrial myocytes. Under isotonic conditions, single outwardly rectifying Cl- channels (ORCCs) with a slope conductance of 28 +/- 1 pS at the reversal potential were observed in 21 (5.7%) of 367 patches. Unconditional kinetic analysis revealed at least three open and four closed-channel states. Hypotonic superfusion-induced swelling resulted in the appearance of active channels in 41 (15.5%) of 265 patches without channel activity under isotonic conditions and caused a second active channel to appear in 3 of 14 patches showing a single channel under isotonic conditions. Overall, channels were seen in 54 of 336 patches under hypotonic conditions (16.1%, P < .001 versus isotonic conditions). The current-voltage relations, reversal potential-[Cl-]o relations, open probability, and kinetics of swelling-induced channels were indistinguishable from those of ORCCs under isotonic conditions. Unitary ORCCs, ICl.b, and ICl.swell were strongly and similarly inhibited by tamoxifen. Swelling-induced increases in macroscopic Cl- current were attributable to an increase in the number of active ORCCs with no significant effects on single-channel amplitude or open probability. Estimated macroscopic currents based on cell surface area, patch dimensions, single-channel ORCC current amplitude, open probability, and density were consistent with measured values of ICl.b and ICl.swell. We conclude that ORCCs underlie volume-regulated basal and swelling-induced Cl- currents in isolated rabbit atrial myocytes.

Dual mode of N-methyl-D-aspartate-induced neuronal death in hippocampal slice cultures in relation to N-methyl-D-aspartate receptor properties

Hippocampal slices prepared from neonatal rats were cultured for several weeks, and excitotoxicity induced in CA1 pyramidal neurons by N-methyl-D-aspartate was evaluated at different culture stages. CA1 neurons cultured for one week exhibited cell death predominantly within 1-3 h after a 15-min N-methyl-D-aspartate insult (early death), whereas those cultured for three weeks showed cell death mainly a few hours to 24 h after the insult (delayed death). CA1 neurons cultured for two weeks were in a transitional state, expressing only weak early and delayed forms of cell death in response to N-methyl-D-aspartate. The N-methyl-D-aspartate-induced early cell death observed in the one-week group depended on external Cl- but did not require external Ca2+; rather, early cell death was enhanced in Ca2+-free solution. This early cell death was accompanied by cell swelling, but cell swelling alone produced by osmotic changes failed to induce cell death. There was no evidence that CA1 neurons in the one-week group were more responsive to N-methyl-D-aspartate than those in the two other groups. Delayed cell death examined in the three-week group depended on external Ca2+ but was independent of Cl-. The density of N-methyl-D-aspartate-induced whole-cell currents recorded from CA1 neurons in Mg2+-free solution remained unchanged during three weeks of culture. However, the N-methyl-D-aspartate receptor channel in the three-week group was more resistant to Mg2+ block than that in the one- or two-week group. The incidence of N-methyl-D-aspartate-induced delayed cell death was higher in the three-week group than in the two-week group in normal solution but not in Mg2+-free solution. Thus, Mg2+ block-resistant properties of the N-methyl-D-aspartate receptor channel acquired during prolonged culture may account for the high incidence of N-methyl-D-aspartate-induced delayed cell death in the three-week group. However, the N-methyl-D-aspartate receptor subunits expressed in the CA1 subfield did not show any feature specific to the three-week group. These results show that two mechanistically distinct modes of N-methyl-D-aspartate-induced neuronal death are manifested differentially at different culture stages, depending on the intrinsic neuron properties (i.e. early cell death) and on the properties or the responsiveness of the N-methyl-D-aspartate receptor (i.e. delayed cell death).

Volume-sensitive chloride current in pigmented ciliary epithelial cells: role of phospholipases

The whole cell recording technique was used to examine an outwardly rectifying chloride current activated by hypotonic shock in bovine pigmented ciliary epithelial (PCE) cells. Removal of internal and external Ca2+ did not affect the activation of these currents, but they were abolished by the phospholipase C inhibitor neomycin. The current was blocked by 5-nitro-2-(3-phenylpropylamino)benzoic acid, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid, and 4,4'-disothiocyanostilbene-2,2'-disulfonic acid (DIDS) in a voltage-dependent manner, but tamoxifen, dideoxyforskolin, and quinidine did not affect it. This blocking profile differs from that of the volume-sensitive chloride channel in neighboring nonpigmented ciliary epithelial cells (Wu, J., J. J. Zhang, H. Koppel, and T. J. C. Jacob, J. Physiol, Lond. 491: 743-755, 1996), and this difference implies that the volume responses of the two cell types are mediated by different chloride channels (Jacob, T. J. C., and J. J. Zhang. J. Physiol. Lond. In press). Intracellular administration of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) to PCE cells induced a transient, time-independent, outwardly rectifying chloride current that closely resembled the current activated by hypotonic shock. DIDS produced a voltage-dependent block of the GTP gamma S-activated current similar to the block of the hypotonically activated current. Intracellular neomycin completely prevented activation of this current as did incubation of the cells in calphostin C. and inhibitor of protein kinase C (PKC). Removal of Ca2+ did not affect activation of the current by GTP gamma S but extended the duration of the response. Inhibition of phospholipase A2 (PLA2) with p-bromophenacyl bromide prevented the activation of the hypotonically induced current and also inhibited the current once activated by hypotonic solution. The findings imply that the hypotonic response in PCE cells is mediated by both phospholipase C (PLC) and PLA2. Both phospholipases generate arachidonic acid, and, in addition, the PLC pathway regulates the PLA2 pathway via a PKC-dependent phosphorylation of PLA2.

Volume-activated Cl- channels

1. An increase in cell volume activates, in most mammalian cells, a Cl- current, ICl,vol. This current is involved in a variety of cellular functions, such as the maintenance of a constant cell volume, pH regulation, and control of membrane potential. It might also play a role in the regulation of cell proliferation and in the processes that control transition from proliferation to differentiation. This review focuses on various aspects of this current, including its biophysical characterisation and its functional role for various cell processes. 2. Volume-activated Cl- channels show all outward rectification. Iodide is more permeable than chloride. In some cell types, ICl,vol inactivates at positive potentials. Single channel conductance can be divided mainly into two groups: small (< 5 pS) and medium conductance channels (around 50 pS). 3. The pharmacology and modulation of these channels are reviewed in detail, and suggest the existence of an heterogeneous family of multiple volume-activated Cl- channels. 4. Molecular candidates for this channel (i.e. ClC-2, a member of the ClC-family of voltage-dependent Cl- channels, the mdr-1 encoded P-glycoprotein, the nucleotide-sensitive pICln protein and phospholemman) will be discussed.

Distinct voltage-dependent gating behaviours of a swelling-activated chloride current in human epithelial cells

1. The swelling-activated chloride current is critical in the homeostatic regulatory volume decrease (RVD) of both excitable and non-excitable cells. Although not activated by voltage, it displays kinetic behaviour similar to voltage-gated Shaker-type potassium currents. We have studied the voltage-dependent properties of this current in single T84 human cell line epithelial cells using whole-cell patch clamp methodology. 2. An external anion permeability sequence of I- > Cl- > methanesulphonate (MeSO3-) was observed for the swelling-activated current. Extracellular application of the chloride channel blocker DIDS (100 microM) resulted in approximately 50% block of the current in a voltage-dependent manner. 3. At positive membrane potentials, the swelling-activated chloride current undergoes time-dependent inactivation. Following such inactivation, recovery of both the inward and outward components of the macroscopic current was found to be voltage dependent. The time constants describing these two individual recovery processes were identical over a range of membrane potentials. In addition, the magnitude of current recovery was directly dependent upon the degree prior inactivation of current at positive voltage. 4. We further observed that the swelling-activated current undergoes a form of steady-state, voltage-dependent inactivation that appears to differ from the inactivation observed at positive potentials. This steady-state inactivation occurred over the physiological voltage range, with a membrane potential at half-maximal inactivation (V1/2) of -72 mV, and differed from the time-dependent inactivation observed at positive membrane potentials, which occurred with a V1/2 of 40 mV. These observations demonstrate two distinct forms of voltage-dependent inactivation, probably reflecting two separate gating processes at the level of the channel. 5. These latter properties are thus anticipated to regulate voltage-dependent chloride efflux under cell swelling conditions and further influence RVD and membrane excitability in cells generating action potentials.

Cell swelling activates ATP-dependent voltage-gated chloride channels in M-1 mouse cortical collecting duct cells

In the present study we used whole-cell patch clamp recordings to investigate swelling-activated Cl-currents (ICl-swell) in M-1 mouse cortical collecting duct (CCD) cells. Hypotonic cell swelling reversibly increased the whole-cell Cl- conductance by about 30-fold. The I-V relationship was outwardly-rectifying and ICl-swell displayed a characteristic voltage-dependence with relatively fast inactivation upon large depolarizing and slow activation upon hyperpolarizing voltage steps. Reversal potential measurements revealed a selectivity sequence SCN- > I- > Br- > Cl- > > gluconate. ICl-swell was inhibited by tamoxifen, NPPB (5-nitro-2(3-phenylpropylamino)-benzoate), DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid), flufenamic acid, niflumic acid, and glibenclamide, in descending order of potency. Extracellular cAMP had no significant effect. ICl-swell was Ca2+ independent, but current activation depended on the presence of a high-energy gamma-phosphate group from intracellular ATP or ATP gamma S. Moreover, it depended on the presence of intracellular Mg2+ and was inhibited by staurosporine, which indicates that a phosphorylation step is involved in channel activation. Increasing the cytosolic Ca2+ concentration by using ionomycin stimulated Cl- currents with a voltage dependence different from that of ICl-swell. Analysis of whole-cell current records during early onset of ICl-swell and during final recovery revealed discontinuous step-like changes of the whole-cell current level which were not observed under nonswelling conditions. A single-channel I-V curve constructed using the smallest resolvable current transitions detected at various holding potentials and revealed a slope conductance of 55, 15, and 8 pS at +120, 0, and -120 mV, respectively. The larger current steps observed in these recordings had about 2, 3, or 4 times the size of the putative single-channel current amplitude, suggesting a coordinated gating of several individual channels or channel subunits. In conclusion we have functionally characterized ICl-swell in M-1 CCD cells and have identified the underlying single channels in whole-cell current recordings.

Single channel properties of a volume sensitive anion channel: lessons from noise analysis

Swelling activated anion channels have recently been recognized to play an important role in not only volume regulatory electrolyte movement, but also in organic osmolyte transport. A swelling-activated, outwardly rectifying anion channel termed VSOAC (volume-sensitive organic osmolyte/anion channel) is a major pathway for swelling-induced loss of organic osmolytes and organic anions from mammalian cells. VSOAC has been described in numerous cell types. Until recently, however, the unitary conductance and gating kinetics of VSOAC were uncertain. Stationary noise analysis and single channel measurements have produced estimates for the unitary conductance of swelling-activated, outwardly rectifying anion channels that vary by > 15-fold. This review describes our current understanding of the single channel properties of VSOAC.

Hypotonicity-induced anion fluxes in cells expressing the multidrug-resistance-associated protein, MRP

Anion transport in human multidrug-resistant large cell lung tumour cells (COR-L23/R) which overexpress the multidrug-resistance-associated protein (MRP) has been compared with that in cells of the parent line (COR-L23/P). Whole-cell patch-clamp recordings reveal variability between individual cells in basal anion conductance and in anion conductance increases following exposure to hypotonic media. The increase of stimulated over basal conductance is significantly larger for resistant cells than for parent cells. The chloride channel blocker, diisothiocyanatostilbene-2-2'-disulphonic acid (DIDS), rapidly and reversibly inhibits the increase in outward but not inward conductance when applied externally at 10(-4) M during recording, but it is without effect when introduced into the cells via the patch pipette. Preincubation with DIDS greatly reduces both inward and outward conductance. 125I- efflux has been used to measure anion movement in cell populations. Basal efflux is similar in the two cell lines, but following a hypotonic challenge, the increase in rate constant for efflux from COR-L23/R cells is at least double that from COR-L23/P cells. This increase in efflux is greatly reduced by incubation with DIDS at 10(-4) M. Replacement of external chloride by gluconate does not affect efflux, thus excluding the possible involvement of DIDS-sensitive chloride exchange. Results from both techniques suggest that DIDS-sensitive, hypotonicity-induced anion channel activity is augmented in COR-L23/R multidrug-resistant variant cells which overexpress MRP. This augmentation may be caused by MRP itself or by other genes coexpressed with MRP.

MDR1/P-glycoprotein function. I. Effect of hypotonicity and inhibitors on rhodamine 123 exclusion

The MDR1 protein (P-glycoprotein) is a membrane ATPase whose expression results in resistance to several anti-tumor drugs. It has been proposed that the MDR1 protein, in addition to its pumplike properties, can function as (Gill et al. Cell 71: 23-32, 1992; Altenberg et al. Cancer Res. 54:618-622, 1994) or mediate the activity of (Hardy et al. EMBO J. 14: 68-75, 1995) a hypotonic stress-induced Cl- current. In addition, one study found that drug transport and Cl- channel-associated functions of MRD1 were separable and mutually exclusive and that, when cells were swelled, the MDR1 protein could not transport substrate. This hypothesis was tested in four pairs of isogenic cell lines with MDR1 transfectants expression 8,000-55,000 MDR1 antibody binding sites per cell. Cytoplasmic exclusion of rhodamine 123 was used as an indicator of MDR1 function to measure the effect of hypotonic stress, MDR1 inhibitors, and Cl- channel blockers on MRD1 transport function. It was found that MDR1 activity and its inhibition by cyclosporine A or flufenamic acid were unaffected by hypotonicity alone or in combination with Cl- channel blockers.

Chloride-channel block inhibits T lymphocyte activation and signalling

Both large- and small-conductance chloride (Cl-) channels have been found in human T lymphocytes; however, apart from possible roles in mediating regulatory volume decrease, their functions are not understood. We have used patch-clamp electrophysiology, Ca2+ spectrofluorometry, and Western blot assay for phosphotyrosine to investigate the effects of blocking Cl- channels on proliferation and on specific events in the activation of normal human T cells. Four chemically distinct Cl- channel blockers inhibited both the small-conductance Cl- channels and phytohemagglutinin (PHA)-induced lymphocyte proliferation in a similar dose-dependent manner; their order of potency was 5-nitro-2(3-phenylpropylamino)-benzoic acid (NPPB) > 4,4'-diisothiocyano-2,2'-disulfonic acid (DIDS) > flufenamic acid >> IAA-94. The Cl- channel blockers inhibited both the PHA-induced mobilization of Ca2+ and the rapid tyrosine phosphorylation of several polypeptides. Cell proliferation was not rescued by the Ca+ ionophore ionomycin or by addition of exogenous interleukin-2 (IL-2). Moreover, the blockers also inhibited phosphotyrosine expression in IL-2-treated, activated lymphoblasts. Thus, our results support a role for Cl- channels in early, PHA-evoked signalling and in later, II-2-dependent stages of lymphocyte activation and proliferation.

Three distinct chloride channels control anion movements in rat parotid acinar cells

1. We used the whole-cell configuration of the patch clamp technique to examine the different macroscopic Cl- currents present in single rat parotid acinar cells. 2. Cell swelling produced by negative osmotic pressure (hypotonic bath solutions) induced a large outwardly rectifying Cl- current with little or no time and voltage dependence. In contrast, an increase in intracellular [Ca2+] induced by ionomycin activated Cl- currents with very different properties. Ca(2+)-activated Cl- currents showed outward rectification, relatively slow activation kinetics and marked voltage dependence. These results are consistent with the existence of two different outwardly rectifying Cl- channels in rat parotid cells. 3. In conditions designed to eliminate the activation of these two Cl- currents, a third type of current was observed. This third current was activated in a time-dependent manner by hyperpolarized potentials and was about equally permeant to Cl-, I- and Br-. 4. The properties of the hyperpolarization-activated current were similar to those of the cloned ClC-2 channel. Polymerase chain reaction-based methods and ribonuclease protection analyses indicated the presence in parotid gland of mRNA homologous to ClC-2. 5. Individual parotid acinar cells expressed all three types of Cl- channels. Each type of channel may contribute to Cl- efflux in distinct stages of the secretion process depending on the intracellular [Ca2+], cell volume and membrane potential.

Swelling-induced catecholamine secretion recorded from single chromaffin cells

We have studied osmotically induced catecholamine secretion from bovine adrenal chromaffin cells by combining patch-clamp measurements, electrochemical detection of secretion, and Fura-2 measurements of intracellular free calcium concentration ([Ca2+]i). We find that osmotically induced catecholamine release is exocytotic and calcium dependent. Furthermore, we demonstrate that cell swelling is coupled to such secretion via a volume-activated current, carrying predominantly chloride, which causes a plateau depolarization of the cell membrane potential and thus promotes voltage-activated calcium influx. Therefore, cell volume changes may modulate the secretory activity.

Ca2+ entry and vasoconstriction during osmotic swelling of vascular smooth muscle cells

Exposure of aortic strips from guinea-pigs to hypotonic extracellular fluid is followed by marked vasoconstriction, which is inhibited by D-600 (3 microM), a blocker of voltage-sensitive Ca2+ channels. Conventional electrophysiology, patch-clamp studies, pH determination with 2',7' bis(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF) and Ca2+ measurements with Fura-2 have been performed on smooth muscle cells cultured either from rat or human aorta to further elucidate the underlying mechanisms. Exposure of the cells to a 25% hypotonic extracellular fluid leads to a rapid and fully reversible depolarization, paralleled by an increase of the selectivity and conductance of the cell membrane to Cl-, an acidification of the cytoplasm and an increase of intracellular Ca2+ concentration ([Ca2+]i). The latter is inhibited by the Ca2+ channel blocker D-600 (1-3 microM). It is concluded that osmotic cell swelling leads to the activation of an anion channel. The subsequent depolarization of the cell membrane activates voltage-sensitive Ca2+ channels which increases [Ca2+]i, thus stimulating the contraction of vascular smooth muscle cells.

Volume-sensitive chloride channel activity does not depend on endogenous P-glycoprotein

To determine whether endogenous P-glycoprotein, the MDR1 gene product that functions as a drug transport pump, is a volume-sensitive Cl- channel molecule or a protein kinase C-mediated regulator of the Cl- channel, whole-cell patch-clamp and molecular biological experiments were carried out in a human small intestinal epithelial cell line. Endogenous expression of P-glycoprotein was confirmed by Northern blot analysis, reverse transcription-polymerase chain reaction, Western blot analysis, and immunostaining. The P-glycoprotein expression was abolished by the antisense (but not sense) oligonucleotide for the MDR1 gene, whereas the magnitude of the Cl- current activated by osmotic swelling was not distinguishable between both antisense- and sense-treated cells. The volume-sensitive Cl- currents were not specifically affected by the anti-P-glycoprotein monoclonal antibodies, MRK16, C219, and UIC2. An inhibitor of P-glycoprotein-mediated pump activity, verapamil, was found to never affect the Cl- current. A substrate for the P-glycoprotein-mediated drug pump, vincristine or daunomycin, did not prevent swelling-induced activation of the Cl- current. Furthermore, the Cl- current was not affected by an activator of protein kinase C (12-O-tetradecanoylphorbol-13-acetate or 1-oleoyl-2-acetyl-sn-glycerol). Thus, it is concluded that the endogenous P-glycoprotein molecule is not itself a volume-sensitive Cl- channel nor a protein kinase C-mediated regulator of the channel in the human epithelial cells.

A voltage-gated chloride channel in ascidian embryos modulated by both the cell cycle clock and cell volume

1. Eggs of the ascidian Boltenia villosa have an inwardly rectifying Cl- current whose amplitude varies by more than 10-fold during each cell cycle, the largest amplitude being at exit from M-phase. We examined whether this current was also sensitive to changes in cell volume. 2. Cell swelling, produced by direct inflation through a whole-cell recording pipette, greatly increased the amplitude of the Cl- current at all stages of the cell cycle in activated eggs. Swelling was much less effective in unfertilized eggs. 3. The increase in Cl- current amplitude continued for 10-20 min after an increase in diameter that was complete in 10 s, suggesting the involvement of a second messenger system in the response. 4. Treatment of unfertilized eggs with 6-dimethylaminopurine (DMAP), an inhibitor of cell cycle-dependent protein kinases, increased the amplitude of the Cl- current and its sensitivity to swelling to levels characteristic of fertilized eggs. 5. Osmotically produced swelling also increased Cl- current amplitude in unfertilized eggs. 6. We propose that dephosphorylation renders the Cl- channel functional, and that swelling or activation of the egg increases the sensitivity of the channel to dephosphorylation, perhaps by disrupting its links to the cytoskeleton.

A mini Cl- channel sensitive to external pH in the basolateral membrane of guinea-pig parietal cells

1. Voltage-independent whole-cell Cl- currents were recorded from both single, isolated parietal cells and parietal cells within gastric glands obtained from the fundus of guinea-pig stomach. 2. The Cl- currents were rapidly suppressed by a Cl- channel blocker, NPPB (5-nitro-2-(3-phenylpropylamino)-benzoate), added to the (basolateral) bathing solution in a concentration-dependent manner with a half-maximal inhibition concentration of 12 microM. 3. The selectivity sequence among anions was I- > Br- > Cl- > F-, corresponding to Eisenman's sequence I. 4. The Cl- currents were independent of cytosolic Ca2+, cyclic AMP, cyclic GMP, GTP-gamma-S and cell volume, and were not affected by application of acid secretagogues, omeprazol, arachidonic acid or prostaglandin E2. 5. Reduction of pH in the (basolateral) bathing solution immediately inhibited the Cl- current with a pK (-log of KD) of 6.3, whereas changes in intracellular pH had no effect. 6. The single-channel conductance was estimated to be 0.46-0.6 pS by variance noise analysis during inhibition of whole-cell Cl- currents by NPPB or acidic pH. 7. It is concluded that pH-sensitive 'mini' Cl- channels, with a sub-picosiemens unitary conductance, exist in the basolateral membrane of guinea-pig parietal cells.

Volume-activated chloride channels in HL-60 cells: potent inhibition by an oxonol dye

When swollen in hypotonic media, HL-60 cells exhibit a regulatory volume decrease (RVD) response as a result of net losses of K+ and Cl-. This is primarily caused by a dramatic increase in Cl- permeability, which may reflect the opening of volume-sensitive channels (11). To test this hypothesis, we measured volume-activated Cl- currents in HL-60 cells using the patch-clamp technique. The whole cell Cl- conductance (in nS/pF at 100 mV) increased from 0.09 +/- 0.06 to 1.15 +/- 0.19 to 1.64 +/- 0.40 as the tonicity (in mosmol/kgH2O) of the external medium was decreased from 334 to 263 to 164, respectively. Cl- currents showed no significant inactivation during 800-ms pulses. Current-voltage curves exhibited outward rectification and were identical at holding potentials of 0 or -50 mV, suggesting that the gating of the channels is voltage independent. The selectivity sequence, based on permeability ratios (PX/PCl) calculated from the shifts of the reversal potentials, was SCN- > I- approximately NO3- > Br- > Cl- >> gluconate. 4-Acetamido-4'- isothiocyanostilbene-2,2'-disulfonic acid (SITS; 0.5 mM) inhibits HL-60 Cl- channels in a voltage-dependent manner, with approximately 10-fold increased affinity at potentials greater than +40 mV. Voltage-dependent blockade by SITS indicates that the binding site is located near the outside, where it senses 20% of the membrane potential. These Cl- channels were also inhibited in a voltage-independent manner by the oxonol dye bis-(1,3-dibutylbarbituric acid)pentamethine oxonol [diBA-(5)-C4] with a concentration that gives half inhibition (IC50) of 1.8 microM at room temperature. A similar apparent IC50 value (1.2 microM) was observed for net 36Cl- efflux into a Cl(-)-free hypotonic medium at 21 degrees C. It seems likely, therefore, that the volume-activated Cl- channels are responsible for the net Cl- efflux during RVD. These Cl- channels have properties similar to the "mini-Cl-" channels described in lymphocytes and neutrophils and are strongly inhibited by low concentrations of diBA-(5)-C4.

Modulation of a volume-regulated chloride current by F-actin

We have examined whether F-actin integrity is involved in activation of a volume-regulated Cl- current (VRChlC) in B-lymphocytes. VRChlC activation was initiated in response to establishing a whole cell recording in the presence of a hyposmotic gradient. Parallel confocal microscopy experiments using Rhodamine-Phalloidin (R-P) as a specific marker of F-actin showed that the submembrane actin ring is reversibly disrupted in response to an hyposmotic gradient. Disruptions of cortical F-actin integrity by 50 microM cytochalasin B (CB) does not trigger activation of VRChlC under isosmotic conditions or potentiate the rate of activation when the osmolarity of the extracellular solution was decreased by 75%. However, incubation with CB increased the rate of VRChlC activation in response to a 90% hyposmotic gradient. Phalloidin, a stabilizer of F-actin, decreases the rate of VRChlC activation in response to a 90% gradient, but has no effect in response to a 75% gradient. These observations suggest that disassembly of cortical F-actin is not critical for VRChlC activation in B-lymphocytes. The integrity of cortical F-actin, however, can exert a modulatory effect on the rate of VRChlC activation in the presence of a hyposmotic gradient.

Swelling activates chloride current and increases internal calcium in nonpigmented epithelial cells from the rabbit ciliary body

Membrane current and [Ca]i in rabbit nonpigmented ciliary body epithelial cells (NPE cells) were monitored with combined patch-clamp and fura-2 measurements during cell swelling induced by anisosmotic conditions. In the presence of K-channel blockers, cell swelling produced an increase in membrane current, accompanied by an increase in [Ca]i. Structural changes in the cell, associated with membrane deformation, may be the cause of the increase in [Ca]i during swelling. The conductance activated by swelling was permeable to Cl: it was dependent on the Cl concentration gradient across the cell membrane, and it was blocked by the Cl-channel blockers DIDS, SITS, NPPB, and DIOA. Although swelling increased both Cl current and [Ca]i, there was no evidence that Ca was involved in the regulation of the Cl conductance. Cell swelling activated the current even when [Ca]i was strongly buffered at an elevated level (500 nM) or at a low level (approximately 0) with internal Ca-BAPTA/Cs-BAPTA mixtures. In addition, Cl conductance was unaffected when [Ca]i was increased with a Ca ionophore. There was also no evidence that cAMP participates in the regulation of the Cl conductance: swelling activation of the current occurred in the presence of cAMP inhibitor (Rp-cAMP-S) and cAMP mimic (Sp-cAMP-S). The data suggest independent involvement of Cl conductance and internal Ca in the regulation of cell volume in NPE cells.

Pharmacology of volume regulation following hypotonicity-induced cell swelling in clonal N1E115 neuroblastoma cells

When exposed to hypotonic solutions, clonal N1E115 neuroblastoma cells initially swell and later undergo a regulatory volume decrease (RVD). We studied the effects of a variety of transport inhibitors on the time course of cross-sectional area of N1E115 cells exposed to a solution of reduced osmolarity (pi = 186 mosm). Application to the bath of either: (i) blockers of net K efflux through K channels (e.g. isotonic KCl or 20 mM TEA); or (ii) blockers of net efflux through anion channels (e.g. isotonic methanesulfonate, 10 microM DIDS or 100 microM IAA-94) all prevent RVD. In contrast, ouabain (a Na+/K+ pump blocker), bumetanide (a Na+/K+/Cl- cotransporter blocker) and SITS (a HCO3-/Cl- exchange blocker) do not. These data support the involvement of these channels over pumps or exchangers in solute exit during RVD. Only variable block of RVD was achieved using blockers of stretch activated non-selective cation C+ (SA) channels (i.e., amiloride and gadolinium, Gd3+) or a membrane permeant Ca chelator (BAPTA-AM) suggesting that neither the opening of C+ (SA) channels nor a global rise in cytosolic Ca2+ is critical for triggering RVD.

Regulation of whole cell currents by cytosolic cAMP, Ca2+, and Cl- in rat fetal distal lung epithelium

The whole cell patch-clamp technique was used to study ionic conductances in fetal distal lung epithelial (FDLE) cells. In unstimulated FDLE cells, K+ conductances were detected in lowered intracellular Cl- concentration ([Cl-]i, < or = 50 mM). The whole cell currents of FDLE cells were increased by elevation of intracellular Ca2+ concentration ([Ca2+]i) or intracellular adenosine 3',5'-cyclic monophosphate (cAMP) concentration ([cAMP]i). The elevation of [Ca2+]i activated the K+ currents. The amiloride-blockable whole cell currents were activated by [cAMP]i of 1 mM with [Cl-]i of 20 mM and were more frequently detected in the pipette solution without ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) than with it (0.5 mM). When the [Cl-]i was fixed at 50 or 145 mM, however, the increase in these currents was not detected even with cAMP and without EGTA. The amiloride-blockable currents were detected in both the Na+ and K+ pipette solutions. Thus the increase in amiloride-blockable whole cell currents was due to the activation of nonselective cation channels. In FDLE cells treated with terbutaline, which is a beta 2-adrenergic receptor agonist, or forskolin, these currents were detected in the pipette solution containing 20 mM Cl- but were suppressed with time when the pipette solution contained 50 or 145 mM Cl-. It seems likely that maintenance of [Cl-]i at the lowered level is an important requirement for the FDLE cells to activate the amiloride-blockable whole cell currents. It is proposed that cellular mechanisms, such as cell shrinkage, exist to reduce the [Cl-]i in response to cAMP.

Small-conductance chloride channels in human peripheral T lymphocytes

During whole-cell patch-clamp recording from normal (nontransformed) human T lymphocytes a chloride current spontaneously activated in > 98% of cells (n > 200) in the absence of applied osmotic or pressure gradients. However, some volume sensitivity was observed, as negative pressure pulses reduced the current. With iso-osmotic bath and pipette solutions the peak amplitude built up (time constant approximately 23 sec at room temperature), a variable-duration plateau phase followed, then the current ran down spontaneously (time constant approximately 280 sec). The anion permeability sequence, calculated from reversal potentials was I-, Br- > NO3-, Cl- > CH3SO3-, HCO3- > CH3COO- > F- > aspartate, gluconate, SO4(2-) and there was no measurable monovalent cation permeability. The Cl- current was independent of time during long voltage steps and there was no evidence of voltage-dependent gating; however, the current showed intrinsic outward rectification in symmetrical Cl- solutions. The conductance of the channels underlying the whole-cell current was calculated from fluctuation analysis, using power-spectral density and variance-vs.-mean analysis. Both methods yielded a single channel conductance of about 0.6 pS at -70 mV (close to the normal resting potential of T lymphocytes). The power spectral density function was best fit by the sum of two Lorentzian functions, with corner frequencies of 30 and 295 Hz, corresponding to mean open times of 0.54 and 5.13 msec. The pharmacological profile included rapid block by external application of flufenamic acid (50 microM), 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB, 100 microM), [6,7-dichloro-2-cyclopentyl-2,3- dihydro-2-methyl-1-oxo-1H-inden-5-yl)oxy] acetic acid (IAA-94, 250 microM) or 100 microM 1,9-dideoxyforskolin. The stilbene derivatives DIDS (4,4'-diisothiocyano-2,2' disulphonic acid stilbene, 500 microM) and SITS (4-acetamido-4'-isothiocyano-2,2'-disulphonic acid stilbene, 500 microM) prevented buildup of Cl- current after a 30-min preincubation at 500 microM. When tested in a mitogenic assay, DIDS, flufenamic acid, NPPB and IAA-94 all inhibited T-cell proliferation, suggesting a physiological function in addition to the observed volume sensitivity.

Volume-activated chloride channels in rat parotid acinar cells

1. Rat parotid acinar cells undergo a regulatory volume decrease in response to hypotonically induced cell swelling that is sensitive to K+ and Cl- gradients. To investigate the potential mechanisms involved, the whole-cell patch-clamp technique was used to characterize a volume-sensitive Cl- channel in rat parotid acinar cells. 2. Exposure of cells to a hyposmotic gradient induced large Cl- currents that exhibited outward rectification and were not affected by membrane potential or the absence of intracellular Ca2+. Low external pH increased the currents at all potentials without affecting current kinetics. These currents were nearly abolished when the cells were in hypertonic conditions. This decrease in the current amplitude was correlated with a decrease in the cell size. 3. The volume-sensitive currents displayed little or no time dependence, whereas Ca(2+)-activated Cl- channels, present in the same cells, displayed slow activation kinetics and large, time-dependent tail currents upon repolarization to the holding potential. 4. The reversal potential of the osmotically activated channels was close to the predicted chloride equilibrium potential and was sensitive to the physiological extracellular Cl- concentration ([Cl-]o). The relationship between reversal potential and [Cl-]o was fitted to a modified Nernst equation with a slope of 51 mV per decade, consistent with a Cl- selective conductance. 5. The anion permeability sequence of the channel, obtained from the shifts of the reversal potentials of the volume-sensitive Cl- current, was: SCN- > I- > NO-3 > Br- > Cl- > formate > propionate = methanesulphonate = acetate > or = F- > or = butyrate > valerate > gluconate = glucuronate = glutamate. 6. The current through the volume-sensitive channels was inhibited by the Cl- channel blocker SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid) in a voltage-dependent manner. 7. We conclude that rat parotid acinar cells express an outwardly rectifying Cl- current that can be activated by swelling under hypotonic conditions. This Cl- conductance may be an element of the cellular mechanisms of volume regulation in exocrine glands.

Single-channel properties of a volume-sensitive anion conductance. Current activation occurs by abrupt switching of closed channels to an open state

Swelling-induced loss of organic osmolytes from cells is mediated by an outwardly rectified, volume-sensitive anion channel termed VSOAC (Volume-Sensitive Organic osmolyte/Anion Channel). Similar swelling-activated anion channels have been described in numerous cell types. The unitary conductance and gating kinetics of VSOAC have been uncertain, however. Stationary noise analysis and single-channel measurements have produced estimates for the unitary conductance of swelling-activated, outwardly rectified anion channels that vary by > 15-fold. We used a combination of stationary and nonstationary noise analyses and single-channel measurements to estimate the unitary properties of VSOAC. Current noise was analyzed initially by assuming that graded changes in macroscopic current were due to graded changes in channel open probability. Stationary noise analysis predicts that the unitary conductance of VSOAC is approximately 1 pS at 0 mV. In sharp contrast, nonstationary noise analysis demonstrates that VSOAC is a 40-50 pS channel at +120 mV (approximately 15 pS at 0 mV). Measurement of single-channel events in whole-cell currents and outside-out membrane patches confirmed the nonstationary noise analysis results. The discrepancy between stationary and nonstationary noise analyses and single-channel measurements indicates that swelling-induced current activation is not mediated by a graded increase in channel open probability as assumed initially. Instead, activation of VSOAC appears to involve an abrupt switching of single channels from an OFF state, where channel open probability is zero, to an ON state, where open probability is near unity.

Drug-transport and volume-activated chloride channel functions in human erythroleukemia cells: relation to expression level of P-glycoprotein

The characteristics of volume-activated chloride currents, drug transport function and levels of P-glycoprotein (PgP) expression were compared between two human chronic erythroleukemia cell lines: a parental (K562) cell line and a derivative obtained by vinblastine selection (K562 VBL400). Parental K562 cells showed no detectable P-glycoprotein expression, measured at the protein level (immunofluorescence labeling with monoclonal antibodies), and had very low levels of MDR-1 mRNA expression (RT-PCR analysis), when compared with levels measured in K562 VBL400. Differences in Pgp-mediated transport were estimated by comparing the rates of Fluo3 accumulation. The higher drug-transport function of K562 VBL400 cells (e.g., lower Fluo3 accumulation) correlated with their elevated levels of MDR-1. The rate of dye transport was sensitive to verapamil but was not affected by the tonicity of the extracellular medium. In contrast to the clear differences in transport function, the characteristics of chloride currents induced by cell swelling were indistinguishable between the two cell lines. Currents measured in the whole-cell configuration were outwardly rectifying, had a higher permeability to iodide than to chloride (SCN- > I- > Cl- > gluconate), were potently blocked by NPPB and were unresponsive to verapamil. The percentage of responding cells and the mean current density were nearly identical in both cell lines. In addition, activation of the volume-sensitive current was not prevented during whole-cell recordings obtained with pipettes containing high concentration of cytotoxic drugs (vincristine or vinblastine). These results do not lend support to the previously reported association between Pgp expression and volume-sensitive chloride channels, and suggest that a different protein is responsible for this type of chloride channel in K562 cells.

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.

Hypotonically activated chloride current in HSG cells

Hypotonically induced changes in whole-cell currents and in cell volume were studied in the HSG cloned cell line using the whole-cell, patch clamp and Coulter counter techniques, respectively. Exposures to 10 to 50% hypotonic solutions induced dose-dependent increases in whole-cell conductances when measured using K+ and Cl- containing solutions. An outward current detected at 0 mV, corresponded to a K+ current which was transiently activated, (usually preceding activation of an inward current and had several characteristics in common with a Ca(2+)-activated K+ current we previously described in these cells. The hypotonically induced inward current had characteristics of a Cl- current. This current was inhibited by NPPB (5-nitro-2-(3-phenyl-propylamino)-benzoate) and SITS (4-acetamido-4'-isothiocyanostilbene), and its reversal potentials corresponded to the Cl- equilibrium potentials at high and low external Cl- concentrations. The induced current inactivated at voltages greater than +80 mV, and the I-V curve was outwardly rectifying. The current was unaffected by addition of BAPTA or removal of GTP from the patch pipette, but was inhibited by removal of ATP or by the presence of extracellular arachidonic acid, quinacrine, nordihydroguairetic acid, and cytochalasin D. Moreover, exposure of HSG cells to hypotonic media caused them to swell and then to undergo a regulatory volume decrease (RVD) response. Neither NPPB, SITS or quinine acting alone could inhibit RVD, but NPPB and quinine together totally inhibited RVD. These properties, plus the magnitudes of the induced currents, indicate that the hypotonically induced K+ and Cl- currents may underlie the RVD response. Cytochalasin D also blocked the RVD response, indicating that intact cytoskeletal F-actin may be required for activation of the present currents. Hence, our results indicate that hypotonic stress activates K+ and Cl- conductances in these cells, and that the activation pathway for the K+ conductance apparently involves [Ca2+], while the activation pathway for the Cl- conductance does not involve [Ca2+] nor lipoxygenase metabolism, but does require intact cytoskeletal F-actin.

Role of the cytoskeleton in the regulation of Cl- channels in human embryonic skeletal muscle cells

The effects of volume change and cytoskeleton manipulation on the Cl- channels in human embryonic skeletal muscle cells were studied. Trypsination, used for production of myoballs, changes the channel properties only a little. When the external osmolarity was reduced from 300 to 270 mosmol/l, the specific Cl- conductance, gCl, (at -80 mV) of myoballs increased from 5.1 +/- 1.9 to 30.4 +/- 12.2 microS/cm2 (SD; n = 6) within 15 min. Concomitantly, the kinetics of Cl- currents, elicited by clamping the membrane potential from a negative to positive values, changed from activation and subsequent slow inactivation to instantaneous activation with fast inactivation. G protein activation, protein kinase action or [Ca2+]i elevation seemed not to be involved in these effects. Similar changes were produced in the absence of a transmembrane osmotic gradient by 500 nM intracellular cytochalasin D (gCl = 34.3 +/- 10.3 microS/cm2; n = 6) or 12.5 microM colchicine (gCl = 15.4 +/- 1.4 microS/cm2; n = 5). When the external osmolarity was increased to 418 mosmol/l, 1 microM cytochalasin D did not affect gCl. In four of six cell-attached patches the open probability of the intermediate Cl- channel was increased after reduction of the bath osmolarity. In inside-out patches, the drugs increased the open probability of the channels. It is concluded that the Cl- channels are under control of the cytoskeleton.

Volume-activated Cl- currents in different mammalian non-excitable cell types

The existence and properties of volume-activated Cl- currents were studied in 15 different cell types (endothelium: human umbilical vein, human aorta, bovine pulmonary artery; fibroblasts: Swiss 3T3, L, C3H 10T1/2 and COS-1; epithelium: KB3, HeLa and A6; blood cells: RBL-2H3 and Jurkat; endothelioma cells derived from both subcutaneous and thymic hemangiomas; skin: IGR1 melanoma). Volume-activated Cl- currents with common characteristics, i.e. small conductance, outward rectification, higher permeability for iodide than for chloride and sensitivity to block by 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) could be elicited in all cells. The block of this current by tamoxifen and dideoxyforskolin is different for the various cell types, as well as the time course and the amplitude of the responses induced by repetitive applications of hypotonicity. Volume-activated Cl- channels with similar biophysical properties are therefore wide-spread among mammalian cells. This may reflect either a single Cl- channel that is ubiquitously expressed or a family of functionally related Cl- channels with cell specific expression patterns.

Mechanosensitivity of NMDA receptors in cultured mouse central neurons

Changes in osmotic and hydrostatic pressure were found to modulate NMDA responses of cultured embryonic mouse neurons recorded in various patch-clamp configurations. In nucleated patches, NMDA currents were potentiated by reductions in external osmolarity and were reduced in hyper-osmotic solutions. These changes, which were greater for low concentrations of NMDA, were not observed for responses to kainate, glycine, or GABA. They could be mimicked by directly changing the pipette pressure in nucleated, outside-out, inside-out, and cell-attached patches. Osmosensitivity of NMDA responses was also observed in the whole-cell mode, but only after prolonged dialysis. Mechanosensitivity of NMDA receptors could have an important role in neuronal regions experiencing changes in membrane tension, such as spines or growth cones.

Permeation properties and modulation of volume-activated Cl(-)-currents in human endothelial cells

1. We have studied the permeation and pharmacological properties of a recently described volume-activated, calcium-insensitive, small-conductance Cl(-)-channel in endothelial cells from human umbilical vein. 2. The relative permeability for various anions was I- > Cl- approximately Br- > F- > gluconate- (1.63 +/- 0.36: 1:0.95 +/- 0.16:0.46 +/- 0.04:0.19 +/- 0.07, n = 10). 3. 5-Nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) induced a fast and reversible block of the current (Ki = 29 mumol l-1). 4. Extracellular ATP induced a low-affinity block of the current, that showed a small voltage-dependence (K1 = 4.9 mmol l-1 at +80 mV and K1 = 8.2 mmol l-1 at -80 mV). 5. Extracellularly applied arachidonic acid (10 mumol l-1) irreversibly blocked the current in 5 out of 9 cells. This block seems to be non-specific, because other ionic currents, e.g. inwardly rectifying K+ currents, were blocked as well. 6. Tamoxifen induced a high affinity block of the current (K1 = 2.9 mumol l-1). Block and reversal of block were however much slower than with NPPB. 7. Cytotoxic compounds, which are substrates of the P-glycoprotein multidrug transporter, loaded into endothelial cells via the patch pipette, exerted only minor effects on the volume-activated current. Vinblastine and colcemid did not affect the volume-activated current, whereas daunomycin and vincristine induced a slow 'run-down' of the current. 8. The similarity between permeation and pharmacological properties of volume-activated Cl--currents in endothelial cells and those in many other cell types may suggest that they all belong to the same family of volume-activated small-conductance Cl--channels. Evidence that they belong to the class of P-glycoprotein associated Cl--channels is however only marginal, whereas their biophysical characteristics differ significantly from those of the CIC-2 volume-activated Cl--channels.

Delayed activation of single mechanosensitive channels in Lymnaea neurons

Some stretch-activated (SA) channels challenged with suction jumps exhibit adaptation, a dynamic behavior that can be overlooked because of its mechanical fragility. In previous studies of neuronal SA K channels, we detected no adaptation, but the protocols used were not designed to detect dynamics. Here, we reproduce the adaptation seen by others in Xenopus SA cationic (Cat) channels but show that, with the same protocol, no adaptation occurs with SA K channels. Instead, SA K channels exhibit a different dynamic behavior, delayed activation. Lymnaea SA K channels subjected to pressure jumps responded after a 1- to 4-s delay with a gradual, rather than abrupt, onset of activation. The delay was pressure dependent and was longer for patches from older cultured neurons. Delayed responses were fragile like SA Cat channel adaptation; they disappeared with repeated stimuli. Cytochalasin D decreased the delay and increased the stretch activation of SA K channels. Unlike SA Cat channel adaptation, which occurs only at hyperpolarized potentials, SA K channel delay was not voltage dependent. We note that once SA Cat and SA K channels are "stripped" of their fragile (cytoskeleton-dependent?) dynamics, however, their gating behaviors show little fundamental difference; both are stretch activatable and have a higher open probability at depolarized potentials.

Selection for MDR1/P-glycoprotein enhances swelling-activated K+ and Cl- currents in NIH/3T3 cells

The relationship between multidrug resistance (MDR) P-glycoprotein expression and swelling-activated Cl- and K+ conductance was investigated in mouse NIH/3T3 fibroblasts and their colchicine-selected counterparts (COL1000, high P-glycoprotein). Whole cell patch-clamp and isotopic flux experiments confirmed that swelling-activated Cl- currents were induced by 20-30% bath dilution only in the MDR-expressing cell line. However, at bath dilutions > 30%, both cell lines developed Cl- currents that reached similar large magnitudes at higher dilution levels. Thus the apparent absolute difference in cell lines at lower dilutions is due to a shift in the response curve relating hypotonicity to Cl- conductance. At all dilutions and in both cell lines, the swelling-activated Cl- currents were outwardly rectifying, active at negative cell voltages, and inactivated at positive voltages. Verapamil (100 microM) and 1,9-dideoxyforskolin (100 microM), which inhibit P-glycoprotein drug transport, did not significantly inhibit the swelling-activated Cl- conductance efflux in the COL1000 cells also showed a leftward shift in the response curve to hypotonicity. These results indicate that response curve to hypotonicity. These results indicate that colchicine-selection for increased P-glycoprotein expression did not lead to the expression of swelling-activated Cl- channels, but instead enhanced a step in the pathway from bath dilution to regulatory volume decrease that is common to both K+ and Cl- channels.

Activation of hyperpolarization and atypical osmosensitivity of a Cl- current in rat osteoblastic cells

During whole-cell recording of rat osteoblastic cells with high-Cl- internal solutions, 10 sec hyperpolarizing jumps from 0 mV induce a slow inward current relaxation, which is shown to be carried by hyperpolarization-activated Cl- channels. This relaxation increases and becomes faster with stronger hyperpolarizations. It is insensitive to Cs+ ions but is blocked in a voltage-dependent manner by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) 1 mM and is reduced by 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) 0.1 mM. Cd2+ ions are potent blockers of this current, blocking completely above 300 microM. The amplitude of the Cl- current activated by a given hyperpolarization increases during the first 10-20 min of whole-cell recording. This evolution and the fact that some recently cloned Cl- channels have been reported to be activated both by hyperpolarization and by external hyposmolarity led us to investigate the effects of external osmolarity. Reducing the external osmolarity induces a large Cl- current. However, this hyposmolarity-induced Cl- current and the hyperpolarization-activated Cl- current are shown to be distinct; 1,9-dideoxy forskolin selectively blocks the hyposmolarity-activated current. We show that the hyperpolarization-activated Cl- current is osmosensitive, but in an unusual way: it is reduced by external hyposmolarity and is increased by external hyperosmolarity. Furthermore, these modulations are more pronounced for small hyperpolarizations. The osmosensitivity of the hyperpolarization-activated Cl- current suggests a mechanosensitivity (activation by positive external pressure) that is likely to be physiologically important to bone cells.

Activation of a Cl- current by hypotonic volume increase in human endothelial cells

We have used whole-cell and perforated patches to study ionic currents induced by hypotonic extracellular solutions (HTS, 185 mOsm instead of 290 mOsm) in endothelial cells from human umbilical veins. These currents activated within 30-50 s after application of HTS, reached a maximum value after approximately 50-150 s and recovered completely after re-exposing the cells to normal osmolarity. They slowly inactivated at potentials positive to +50 mV. The same current was also activated by breaking into endothelial cells with a hypertonic pipette solution (377 mOsm instead of 290 mOsm). The reversal potential of these volume-induced currents using different extracellular and intracellular Cl- concentrations was always close to the Cl(-)-equilibrium potential. These currents are therefore mainly carried by Cl-. DIDS only weakly blocked the current (KI = 120 microM), while another Cl(-)-channel blocker, DCDPC (20 microM) was ineffective. We were unable to record single channel activity in cell-attached patches but we always observed an increased current variance during HTS. From the mean current-variance relation of the whole-cell current records, we determined a single channel conductance of 1.1 pS. The size and kinetics of the current were not correlated with the concomitant changes in intracellular calcium. Furthermore, the currents could still be activated in the presence of 10 mmol/liter intracellular EGTA and are thus Ca2+ independent. A similar current was also activated with iso-osmotic pipette solutions containing 300 mumol/liter GTP gamma S. Neomycin (1 mmol/liter), a blocker of PLC, did not prevent activation of this current. TPA (4 mumol/liter) was also ineffective in modulation of this current. The HTS-induced current was completely blocked by 10 mumol/liter pBPB, a PLA2 inhibitor. NDGA (4 mumol/liter) and indomethacin (5 mumol/liter), blockers of lipoxygenase and cyclo-oxygenase respectively, did however not affect the current induced by hypotonic solutions. The effects of arachidonic acid (10 mumol/liter) were variable. In 12 out of 40 cells it either directly activated a Cl- current or potentiated the current activated by HTS. The membrane current was decreased at all potentials in 18 cells, and was not affected in 10 cells. The HTS-induced currents may therefore be modulated by cleavage products of PLA2, but not by messengers downstream of arachidonic acid. Loading the cells with a segment of the heat stable protein kinase A inhibitor PKI (5-24) did not prevent activation of the HTS-induced current.(ABSTRACT TRUNCATED AT 400 WORDS)

Outwardly rectifying chloride current in rabbit osteoclasts is activated by hyposmotic stimulation

1. We characterized chloride currents in freshly isolated rabbit osteoclasts using whole-cell and single channel patch-clamp recording configurations. Depolarization activated an outwardly rectifying current in 40-50% of cells, distinct from the inwardly rectifying K+ current we have previously reported in osteoclasts. 2. The outwardly rectifying current persisted under conditions where all K+ currents were blocked. Furthermore, the outward current was reversibly inhibited by Cl- transport blockers 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS); 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS); 4,4'-dinitrostilbene-2,2'-disulphonic acid (DNDS); and niflumic acid. The blocked current had a reversal potential close to the predicted chloride equilibrium potential and was dependent on the chloride concentration gradient. 3. In those osteoclasts in which outwardly rectifying current was not initially apparent, exposure to hyposmotic extracellular solution resulted in its reversible activation. The induced current was due to Cl-, based on its reversal close to the chloride equilibrium potential and sensitivity to blockade by Cl- channel inhibitors. The hyposmotically induced current could be activated in Ca(2+)-free solutions containing 0.2 mM EGTA. 4. When studied in the current-clamp configuration, hyposmotic stimulation caused depolarization from -76 +/- 5 to -5 +/- 6 mV (mean +/- S.D., n = 7). 5. Unitary Cl- currents were recorded in the cell-attached patch configuration at positive potentials. Single channels had a slope conductance of 19 +/- 3 pS (n = 5). Reduction of the external [Cl-] shifted the current-voltage relationship in the positive direction, supporting the conclusion that these were Cl- currents. Like the whole-cell currents, single channel Cl- currents were activated by exposure of cells to hyposmotic bathing solution. 6. We conclude that rabbit osteoclasts express an outwardly rectifying Cl- current that can be activated by osmotic stress. Cl- channels may play a role in cell volume regulation and may also provide conductive pathways for dissipating the potential difference that arises from electrogenic proton transport during bone resorption.

Capsaicin-sensitive stretch responses in ferret trachealis muscle

1. Stretch-induced electrical and mechanical responses in segments of ferret trachealis muscle were studied. Stretches and post-stretch length changes were quantified by measuring distances between two marker spheres placed on the muscle surface. Electrical responses were determined by measuring membrane potential in the muscle cell syncytium. 2. Smooth muscle mechanical and electrical responses to the stretch manoeuvre were characterized by an initial shortening and depolarization phase and a reversal-repolarization phase. Both phases were resistant to atropine and tetrodotoxin. During the initial phase, the membrane depolarized to potentials as low as -20 mV. For stretches to 1.0 Lmax, from a holding length of 0.75 Lmax, 50% repolarization occurred at 6.8 +/- 0.4 min post-stretch; 50% reversal of shortening of the stretched segment occurred at 6.9 +/- 0.8 min post-stretch. 3. Depolarizing currents generated within muscle cells in the stretched segment spread into cells in non-stretched muscle. Space constants in the transverse and longitudinal directions averaged 480 +/- 46 and 146 +/- 50 microns, respectively. 4. During infusion of capsaicin (10 microM), muscle cells depolarized by 5.5 +/- 2.3 mV. Maximal depolarization was achieved after 15-20 min. After inhibition of neutral enkephalinase, capsaicin-evoked depolarization occurred more rapidly. Muscles depolarized by 11.2 +/- 2.1 mV after about 10 min of capsaicin and then slowly repolarized during continued treatment. When muscle segments were stretched during administration of capsaicin, the initial phase was similar to that observed before capsaicin, but the reversal-repolarization phase was prolonged. Following wash exposure to capsaicin, maximal stretch-induced depolarization was unchanged, but the time for 50% repolarization (t50-repolarization) decreased from the pre-capsaicin value of 8.4 +/- 1.3 to 4.1 +/- 0.5 min. The t50-reversal of stretch-evoked muscle shortening decreased to 54% of control values. 5. Short exposures (< 2 min) to substance P (SP, 1-7.5 microM) depolarized smooth muscle cells. Maximal depolarization was delayed, and occurred after [SP] had decreased to < 10 nM. Repolarization was delayed as long as 6 min following wash-out of SP. Stretches performed when SP-induced depolarization had nearly reversed showed no changes in the initial mechanical or electrical responses, but t50-repolarization increased to 162% of control values. 6. Immunochemical studies showed networks of neurones which react with SP antibodies. 7. These findings suggest that stretch induces SP release from capsaicin-sensitive C fibres, and that released SP affects smooth muscle ionic mechanisms which control and delay the reversal of stretch-induced membrane depolarization and shortening.

Molecular characterization of a swelling-induced chloride conductance regulatory protein, pICln

Cells maintain control of their volume by the passage of KCl and water across their membranes, but the regulatory proteins are unknown. Expression in Xenopus oocytes of a novel protein, pICln, activated a chloride conductance. We have cloned analogs of pICln from rat heart and Xenopus ovary. pICln was identified as an abundant soluble cytosolic protein (approximately 40 kd) that does not immunolocalize with the plasma membrane. pICln was found in epithelial and cardiac cells, brain, and Xenopus oocytes, forming complexes with soluble actin and other cytosolic proteins. Monoclonal antibodies recognizing pICln blocked activation of a native hypotonicity-induced chloride conductance (ICl.swell) in Xenopus oocytes, suggesting that pICln may link actin-bound cytoskeletal elements to an unidentified volume-sensitive chloride channel. The high degree of sequence conservation and widespread expression of pICln suggest that it is an important element in cellular volume regulation.

Hypotonicity activates a native chloride current in Xenopus oocytes

Xenopus oocytes are frequently utilized for in vivo expression of cellular proteins, especially ion channel proteins. A thorough understanding of the endogenous conductances and their regulation is paramount for proper characterization of expressed channel proteins. Here we detail a novel chloride current (ICl.swell) responsive to hypotonicity in Xenopus oocytes using the two-electrode voltage clamp technique. Reducing the extracellular osmolarity by 50% elicited a calcium-independent chloride current having an anion conductivity sequence identical with swelling-induced chloride currents observed in epithelial cells. The hypotonicity-activated current was blocked by chloride channel blockers, trivalent lanthanides, and nucleotides. G-protein, cAMP-PKA, and arachidonic acid signaling cascades were not involved in ICl.swell activation. ICl.swell is distinct from both stretch-activated nonselective cation channels and the calcium-activated chloride current in oocytes and may play a critical role in volume regulation in Xenopus oocytes.

Membrane chloride conductance and capacitance in Jurkat T lymphocytes during osmotic swelling

Video microscopy and whole-cell patch-clamp recording were used to monitor changes in relative cell volume (V/Vo), chloride conductance (gCl), and membrane capacitance (Cm) during osmotically induced swelling in Jurkat T lymphocytes. Cellular swelling was initiated with hyperosmotic pipette solutions. Simultaneous evaluation of V/Vo and gCl revealed a 59-s delay between the inception of swelling and the activation of outwardly rectifying, ATP-dependent Cl- channels. Following the delay, increases in V/Vo and gCl progressed in parallel. In contrast, Cm, a measure of cell surface area, fell gradually at a rate of approximately 150 fF/min after whole-cell access was achieved. The decline in Cm lasted 200 s and was followed by a rapid rise (approximately 750 fF/min). The rise in Cm coincided with a variable increase in "leak" current, gCl increased at a slower rate and reached lower peak values in experiments performed without ATP; ATP had no effect on the biphasic Cm time course. The temporal separation of conductance and capacitance during swelling suggests that gCl and Cm vary independently, supporting the hypothesis that a large portion, if not all, of the whole-cell Cl- conductance activated during swelling is provided by volume-sensitive Cl- channels preexisting in the plasma membrane.