Mechanisms of regulatory volume decrease in nonpigmented human ciliary epithelial cells

Swelling-Activated K+ Efflux and Regulatory Volume Decrease Efficiency in Human Bronchial Epithelial Cells

This study describes the correlation between cell swelling-induced K+ efflux and volume regulation efficiency evaluated with agents known to modulate ion channel activity and/or intracellular signaling processes in a human bronchial epithelial cell line, 16HBE14o(-1). Cells on permeable filter supports, differentiated into polarized monolayers, were monitored continuously at room temperature for changes in cell height (T(c)), as an index of cell volume, whereas (86)Rb efflux was assessed for K+ channel activity. The sudden reduction in osmolality of both the apical and basolateral perfusates (from 290 to 170 mosmol/kg H(2)O) evoked a rapid increase in cell volume by 35%. Subsequently, the regulatory volume decrease (RVD) restored cell volume almost completely (to 94% of the isosmotic value). The basolateral (86)Rb efflux markedly increased during the hyposmotic shock, from 0.50 +/- 0.03 min(-1) to a peak value of 6.32 +/- 0.07 min(-1), while apical (86)Rb efflux was negligible. Channel blockers, such as GdCl(3) (0.5 mM), quinine (0.5 mM) and 5-nitro-2-(3-phenyl-propylamino) benzoic acid (NPPB, 100 microM), abolished the RVD. The protein tyrosine kinase inhibitors tyrphostin 23 (100 microM) and genistein (150 microM) attenuated the RVD. All agents decreased variably the hyposmosis-induced elevation in (86)Rb efflux, whereas NPPB induced a complete block, suggesting a link between basolateral K(+) and Cl(-1) efflux. Forskolin-mediated activation of adenylyl cyclase stimulated the RVD with a concomitant increase in basolateral (86)Rb efflux. These data suggest that the basolateral extrusion of K+ and Cl(-1) from 16HBE14o(-1) cells in response to cell swelling determines RVD efficiency.

Measurement of rapid changes in cell volume by forward light scattering

Light scattering is an empirical technique employed to measure rapid changes in cell volume. This study describes a new configuration for the method of light scattering and its corroboration by measurements of cell height (as a measure of cell volume). Corneal endothelial cells cultured on glass cover-slips were mounted in a perfusion chamber on the stage of an inverted microscope. A beam of light was focused on the cells from above the stage at an angle of 40 degrees to the plane of the stage. The scattered light intensity (SLI), captured by the objective and referred to as forward light scatter (FLS), increased and decreased in response to hyposmotic and hyperosmotic shocks, respectively. The rapid increase and decrease in SLI corresponded to cell swelling and shrinkage, respectively. Subsequently, SLI decreased and increased as expected for a regulatory volume decrease (RVD) and increase (RVI), respectively. These data are in agreement with measurements of cell height, demonstrating that the method of light scatter in FLS mode is useful for monitoring rapid changes in cell volume of cultured cells. Changes in SLI caused by gramicidin were consistent with cell volume changes induced by equilibration of NaCl and KCl concentrations across the cell membrane. Similarly, an additional decrease in SLI was recorded during RVD upon increasing K+ conductance by valinomycin. Decreasing K+ conductance of the cell membrane with Ba2+ changed the time course of SLI consistent with the effect of the K+ channel blocker on RVD. Bumetanide and dihydro-ouabain inhibited increases in SLI during RVI. In conclusion, FLS is a valid method for qualitative analysis of cell volume changes with a high time resolution.

Regulation of intracellular calcium in N1E-115 neuroblastoma cells: the role of Na(+)/Ca(2+) exchange

In fura 2-loaded N1E-115 cells, regulation of intracellular Ca(2+) concentration ([Ca(2+)](i)) following a Ca(2+) load induced by 1 microM thapsigargin and 10 microM carbonylcyanide p-trifluoromethyoxyphenylhydrazone (FCCP) was Na(+) dependent and inhibited by 5 mM Ni(2+). In cells with normal intracellular Na(+) concentration ([Na(+)](i)), removal of bath Na(+), which should result in reversal of Na(+)/Ca(2+) exchange, did not increase [Ca(2+)](i) unless cell Ca(2+) buffer capacity was reduced. When N1E-115 cells were Na(+) loaded using 100 microM veratridine and 4 microg/ml scorpion venom, the rate of the reverse mode of the Na(+)/Ca(2+) exchanger was apparently enhanced, since an approximately 4- to 6-fold increase in [Ca(2+)](i) occurred despite normal cell Ca(2+) buffering. In SBFI-loaded cells, we were able to demonstrate forward operation of the Na(+)/Ca(2+) exchanger (net efflux of Ca(2+)) by observing increases (approximately 6 mM) in [Na(+)](i). These Ni(2+) (5 mM)-inhibited increases in [Na(+)](i) could only be observed when a continuous ionomycin-induced influx of Ca(2+) occurred. The voltage-sensitive dye bis-(1,3-diethylthiobarbituric acid) trimethine oxonol was used to measure changes in membrane potential. Ionomycin (1 microM) depolarized N1E-115 cells (approximately 25 mV). This depolarization was Na(+) dependent and blocked by 5 mM Ni(2+) and 250-500 microM benzamil. These data provide evidence for the presence of an electrogenic Na(+)/Ca(2+) exchanger that is capable of regulating [Ca(2+)](i) after release of Ca(2+) from cell stores.

Hyposmotically activated chloride channels in cultured rabbit non-pigmented ciliary epithelial cells

1. We used whole-cell patch-clamp recording techniques and noise analysis of whole-cell current to investigate the properties of hyposmotic shock (HOS)-activated Cl- channels in SV40-transformed rabbit non-pigmented ciliary epithelial (NPCE) cells. 2. Under conditions designed to isolate Cl- currents, exposure of cells to hyposmotic external solution reversibly increased the whole-cell conductance. 3. The whole-cell current activated with a slow time course (> 15 min), exhibited outward rectification and was Cl- selective. 4. The disulphonic stilbene derivatives 4, 4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS, 0.5 mM), 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS, 0. 5 mM) and 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS, 0.5 mM) produced a voltage-sensitive block of HOS-activated Cl- current at depolarized potentials, whereas niflumic acid produced a voltage-independent block of the current. 5. Under Ca2+-free conditions, HOS stimulation still reversibly activated the Cl- current, but the amplitude of current was reduced and the time course of current activation was slower compared with control (P < 0. 05). 6. The non-specific kinase inhibitor H-7 (100 microM), upregulated HOS-activated Cl- current amplitude in all cells tested (P < 0.05). 7. Noise analysis of whole-cell Cl- current indicated that cell swelling activated a high density of small conductance Cl- channels (< 1 pS). 8. We conclude that HOS primarily activates a high density of volume-sensitive small conductance Cl- channels in rabbit NPCE cells, and that Ca2+ and phosphorylation are involved in channel regulation.

Modeling cell volume regulation in nonexcitable cells: the roles of the Na+ pump and of cotransport systems

The purpose of this study is to contribute to understanding the role of Na+-K+-ATPase and of ionic cotransporters in the regulation of cell volume, by employing a model that describes the rates of change of the intracellular concentrations of Na+, K+, and Cl-, of the cell volume, and of the membrane potential. In most previous models of dynamic cellular phenomena, Na+-K+-ATPase is incorporated via phenomenological formulations; the enzyme is incorporated here via an explicit kinetic scheme. Another feature of the present model is the capability to perform short-term cell volume regulation mediated by cotransporters of KCl and NaCl. The model is employed to perform numerical simulations for a "typical" nonpolarized animal cell. Basically, the results are consistent with the view that the Na+ pump mainly plays a long-term role in the maintenance of the electrochemical gradients of Na+ and K+ and that short-term cell volume regulation is achieved via passive transport, exemplified in this case by the cotransport of KCl and NaCl.

Regulatory volume decrease and intracellular Ca2+ in murine neuroblastoma cells studied with fluorescent probes

The possible role of Ca2+ as a second messenger mediating regulatory volume decrease (RVD) in osmotically swollen cells was investigated in murine neural cell lines (N1E-115 and NG108-15) by means of novel microspectrofluorimetric techniques that allow simultaneous measurement of changes in cell water volume and [Ca2+]i in single cells loaded with fura-2. [Ca2+]i was measured ratiometrically, whereas the volume change was determined at the intracellular isosbestic wavelength (358 nm). Independent volume measurements were done using calcein, a fluorescent probe insensitive to intracellular ions. When challenged with approximately 40% hyposmotic solutions, the cells expanded osmometrically and then underwent RVD. Concomitant with the volume response, there was a transient increase in [Ca2+]i, whose onset preceded RVD. For hyposmotic solutions (up to approximately -40%), [Ca2+]i increased steeply with the reciprocal of the external osmotic pressure and with the cell volume. Chelation of external and internal Ca2+, with EGTA and 1,2-bis-(o -aminophenoxy) ethane-N,N,N ',N '-tetraacetic acid (BAPTA), respectively, attenuated but did not prevent RVD. This Ca2+-independent RVD proceeded even when there was a concomitant decrease in [Ca2+]i below resting levels. Similar results were obtained in cells loaded with calcein. For cells not treated with BAPTA, restoration of external Ca2+ during the relaxation of RVD elicited by Ca2+-free hyposmotic solutions produced an increase in [Ca2+]i without affecting the rate or extent of the responses. RVD and the increase in [Ca2+]i were blocked or attenuated upon the second of two approximately 40% hyposmotic challenges applied at an interval of 30-60 min. The inactivation persisted in Ca2+-free solutions. Hence, our simultaneous measurements of intracellular Ca2+ and volume in single neuroblastoma cells directly demonstrate that an increase in intracellular Ca2+ is not necessary for triggering RVD or its inactivation. The attenuation of RVD after Ca2+ chelation could occur through secondary effects or could indicate that Ca2+ is required for optimal RVD responses.

Hypotonicity activates a lanthanide-sensitive pathway for K+ release in A6 epithelia

The nature of the pathway for K+ release activated during regulatory volume decrease (RVD) in A6 epithelia was investigated by measuring cell thickness (Tc) as an index of cell volume and by probing K+ efflux with 86Rb as tracer for K+ (RRb). Cell swelling was induced by sudden reduction of basolateral osmolality (from 260 to 140 mosmol/kgH2O). Experiments were performed in the absence of Na+ transport. Apical RRb was negligible in iso- and hyposmotic conditions. On the other hand, osmotic shock increased basolateral RRb (RblRb) rapidly, reaching a maximum 7 min after the peak in Tc. Quinine (0.5 mM) completely inhibited RVD and RblRb. Also verapamil (0.2 mM) impeded volume recovery considerably; lidocaine (0.2 mM) did not exert a noticeable effect. The K+ channel blocker Ba2+ (30 mM) delayed RVD but could not prevent complete volume recovery. Cs+ inhibited RVD noticeably at concentrations <40 mM. With large Cs+ concentrations (>40 mM), the initial osmometric swelling was followed by a gradual increase of Tc, suggesting activation of Cs+ influx. Chronic exposure of the basolateral surface to 0.5 mM La3+ or Gd3+ completely abolished RVD and RblRb. Acute administration of lanthanides at the time of osmolality decrease did not affect the initial phase of RVD and reduced RblRb only slightly. Apical Gd3+ exerted an inhibitory effect on RVD and RblRb. The effect of Gd3+ should therefore be localized at an intracellular site. The role of Ca2+ entry could be excluded by failure of extracellular Ca2+ removal to inhibit volume recovery. In contrast to lanthanides, chronically and acutely administered Mg2+ (0.5 mM) inhibited RVD and RblRb by approximately 50%. These data suggest that K+ excretion during RVD occurs through a rather poorly selective pathway that does not seem to be directly activated by membrane stretch.

pICln can regulate swelling-induced Cl- currents in either layer of rabbit ciliary epithelium

Swelling-induced Cl- currents were investigated in freshly prepared non-pigmented epithelial (NPE) and pigmented epithelial (PE) cells of the rabbit ciliary body using the whole-cell patch clamp technique. Exposure of both NPE and PE cells to hypotonic stress induced Cl- currents that exhibited outward rectification and were insensitive to Ca+2. We found that swelling-induced Cl- currents in PE cell are observed shortly after isolation. The swelling-induced Cl- current showed little or no inactivation at positive membrane voltages and was sensitive to 100 microM NPPB and 100 microM DIDS. Injection of cRNA encoded rabbit pICln into Xenopus oocytes produced an outwardly rectifying Cl- current displaying features consistent with the swelling-induced Cl- current in epithelium. pICln is ubiquitous in the ciliary epithelium. It participates in the equilibration of short term tonicity alterations, a phenomenon underlying mechanisms with larger and slower amplitudes for aqueous secretion by these cells.

The role of Cl- channels in volume regulation in bovine pigmented epithelial cells

The following is the abstract of the article discussed in the subsequent letter:

Mitchell, Claire H., Jin Jun Zhang, Liwei Wang, and Tim J. C. Jacob. Volume-sensitive chloride current in pigmented ciliary epithelial cells: role of phospholipases. Am. J. Physiol. 272 ( Cell Physiol. 41): C212–C222, 1997.—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′-diisothiocyanostilbene-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) (GTPγS) 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 GTPγS-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, an inhibitor of protein kinase C (PKC). Removal of Ca2+ did not affect activation of the current by GTPγS 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.

Synergism between hypotonically induced calcium release and fatty acyl-CoA esters induced calcium release from intracellular stores

The non-mitochondrial Ca2+ stores in permeabilized A7r5 cells responded to a decrease in Mg-ATP concentration with a pronounced Ca2+ release if 20 microM CoA was present. This release was rather specific for the preincubation or removal of ATP. ATP gamma S was much less effective and AMP-PNP, GTP, ITP, CTP, UTP, ADP, AMP, adenosine and adenine had no effect. CoA activated with an EC50 of 6 microM. Dephospho-CoA was a less effective cofactor and desulfo-CoA was ineffective. The release induced by Mg-ATP removal did not occur in the presence of 2% fatty acid-free bovine serum albumin and did not develop at 4 degrees C. All these findings suggest that CoA had to be acylated by endogenous fatty-acyl-CoA synthetase to become effective. Myristoyl- and palmitoyl-CoA esters were identified as the most effective cofactors for the release. Ca2+ release induced by removing Mg-ATP did not occur if the osmolality of the medium was kept constant by addition of mannitol, sucrose, KCl, MgCl2 or Mg-GTP, indicating that the decrease in tonicity was the trigger for the release. Mg-ATP plus CoA also synergized with Ca2+ release induced by a hypotonic shock imposed by diluting the medium with H2O. Osmolality changes induced by decreasing the Mg-ATP concentration were more effective in releasing Ca2+ than equal decreases in concentration of all solutes. We conclude that fatty acyl-CoA esters sensitize the hypotonically induced Ca2+ release from the non-mitochondrial Ca2+ stores.

The role of swelling-induced anion channels during neuronal volume regulation

Regulation of cell volume is an essential function of most mammalian cells. In the cells of the central nervous system, maintenance of cell osmolarity and, hence, volume, is particularly crucial because of the restrictive nature of the skull. Cell volume regulation involves a variety of pathways, with considerable differences between cell types. One common pathway activated during hypo-osmotic stress involves chloride (Cl-) channels. However, hypo-osmotically stimulated anion permeability can be regulated by a diverse array of second messengers. Although neuronal swelling can occur in a number of pathological and nonpathological conditions, our understanding of neuronal volume regulation is limited. This article summarizes our current understanding of the role of anion channels during neuronal volume regulation.

Hypotonically induced calcium release from intracellular calcium stores

Osmotic cell swelling induced by hypotonic stress is associated with a rise in intracellular Ca2+ concentration, which is at least partly due to a release of Ca2+ from internal stores. Since osmotic influx of water dilutes the cytoplasmic milieu, we have investigated how nonmitochondrial Ca2+ stores in permeabilized A7r5 cells respond to a reduction in cytoplasmic tonicity. We now present experimental evidence for a direct Ca2+ release from the stores when exposed to a hypotonic medium. The release is graded, but does not occur through the inositol trisphosphate or the ryanodine receptor. Ca2+ seems to be released through the passive leak pathway, and this phenomenon can be partially inhibited by divalent cations in the following order of potency: Ni2+ = Co2+ > Mn2+ > Mg2+ > Ba2+. This release also occurs in intact A7r5 cells. This novel mechanism of hypotonically induced Ca2+ release is therefore an inherent property of the stores, which can occur in the absence of second messengers. Intracellular stores can therefore act as osmosensors.