Cell volume response to hyposmotic shock and elevated cAMP in bovine trabecular meshwork cells

Astroglial cAMP signalling in space and time

To maintain a high level of specificity and normal cell function, the cyclic adenosine monophosphate (cAMP) pathway is tightly regulated in space and time. Recent advances in cAMP reporter technology have provided insights into spatio-temporal characteristics of cAMP signalling in individual living cells, including astrocytes. Astrocytes are glial cells in the central nervous system with many homeostatic functions. In contrast to neurons, astrocytes are electrically silent, but, in response to extracellular stimuli through activation of surface receptors, they can increase intracellular levels of secondary messengers, e.g. Ca²⁺ and cAMP. This enables them to communicate with neighbouring cells, such as neurons and endothelial cells of blood vessels. The dynamics of receptor-mediated Ca²⁺ signalling in astrocytes has been extensively studied in the past in contrast to cAMP signalling. Here, we present the first insights into the temporal dynamics of cAMP signalling in living astrocytes, which revealed that cAMP signals in astrocytes exhibit tonic dynamics and are slower than Ca²⁺ signals with phasic dynamics. We debate on the heterogeneity of basal cAMP levels in astrocytes and how hypotonicity-induced astrocyte swelling affects temporal dynamics of cAMP signalling. Understanding the spatio-temporal characteristics of cAMP signalling in astrocytes is of extreme importance because cAMP governs many important cellular processes and any malfunctions may lead to pathology.

Research and progress on ClC‑2 (Review)

Chloride channel 2 (ClC-2) is one of the nine mammalian members of the ClC family. The present review discusses the molecular properties of ClC‑2, including CLCN2, ClC‑2 promoter and the structural properties of ClC‑2 protein; physiological properties; functional properties, including the regulation of cell volume. The effects of ClC‑2 on the digestive, respiratory, circulatory, nervous and optical systems are also discussed, in addition to the mechanisms involved in the regulation of ClC‑2. The review then discusses the diseases associated with ClC‑2, including degeneration of the retina, Sjögren's syndrome, age‑related cataracts, degeneration of the testes, azoospermia, lung cancer, constipation, repair of impaired intestinal mucosa barrier, leukemia, cystic fibrosis, leukoencephalopathy, epilepsy and diabetes mellitus. It was concluded that future investigations of ClC‑2 are likely to be focused on developing specific drugs, activators and inhibitors regulating the expression of ClC‑2 to treat diseases associated with ClC‑2. The determination of CLCN2 is required to prevent and treat several diseases associated with ClC‑2.

Adrenergic activation attenuates astrocyte swelling induced by hypotonicity and neurotrauma

Edema in the central nervous system can rapidly result in life-threatening complications. Vasogenic edema is clinically manageable, but there is no established medical treatment for cytotoxic edema, which affects astrocytes and is a primary trigger of acute post-traumatic neuronal death. To test the hypothesis that adrenergic receptor agonists, including the stress stimulus epinephrine protects neural parenchyma from damage, we characterized its effects on hypotonicity-induced cellular edema in cortical astrocytes by in vivo and in vitro imaging. After epinephrine administration, hypotonicity-induced swelling of astrocytes was markedly reduced and cytosolic 3'-5'-cyclic adenosine monophosphate (cAMP) was increased, as shown by a fluorescence resonance energy transfer nanosensor. Although, the kinetics of epinephrine-induced cAMP signaling was slowed in primary cortical astrocytes exposed to hypotonicity, the swelling reduction by epinephrine was associated with an attenuated hypotonicity-induced cytosolic Ca(2+) excitability, which may be the key to prevent astrocyte swelling. Furthermore, in a rat model of spinal cord injury, epinephrine applied locally markedly reduced neural edema around the contusion epicenter. These findings reveal new targets for the treatment of cellular edema in the central nervous system.

Ion Channels in the Eye: Involvement in Ocular Pathologies

The eye is the sensory organ of vision. There, the retina transforms photons into electrical signals that are sent to higher brain areas to produce visual sensations. In the light path to the retina, different types of cells and tissues are involved in maintaining the transparency of avascular structures like the cornea or lens, while others, like the retinal pigment epithelium, have a critical role in the maintenance of photoreceptor function by regenerating the visual pigment. Here, we have reviewed the roles of different ion channels expressed in ocular tissues (cornea, conjunctiva and neurons innervating the ocular surface, lens, retina, retinal pigment epithelium, and the inflow and outflow systems of the aqueous humor) that are involved in ocular disease pathophysiologies and those whose deletion or pharmacological modulation leads to specific diseases of the eye. These include pathologies such as retinitis pigmentosa, macular degeneration, achromatopsia, glaucoma, cataracts, dry eye, or keratoconjunctivitis among others. Several disease-associated ion channels are potential targets for pharmacological intervention or other therapeutic approaches, thus highlighting the importance of these channels in ocular physiology and pathophysiology.

Functional and pharmacological characterization of volume-regulated anion channels in human normal and cystic fibrosis bronchial and nasal epithelial cells

Volume-regulated anion channels (VRACs) are widely present in various cell types and have important functions ranging from regulatory volume decrease to control of cell proliferation and apoptosis. Here we aimed to compare the biophysical features and pharmacological profiles of VRAC currents in healthy and cystic fibrosis (CF) respiratory epithelial cells in order to characterize these currents both functionally and pharmacologically. Whole-cell electrophysiology was used to characterize the VRAC current in normal (16HBE14o-; HBE) and CF cell lines (CFBE14o-; CFBE), as well as in native human nasal epithelial cells. Application of hypotonic solution produced current responses of similar sizes in both HBE and CFBE cells. Biophysical properties of VRACs, such as instantaneous activation and deactivation upon voltage step, some inactivation at potentials positive to 40 mV and outwardly-rectifying I-V curves, were indistinguishable in both cell types. Extensive pharmacological analysis of the currents revealed a similar pharmacological profile in response to three blockers--NPPB, DCPIB and DIDS. Native primary human nasal epithelial cells from both healthy and CF volunteers also showed typical VRAC responses of comparable sizes. VRACs in these cells were more sensitive to external solution hypotonicity compared to HBE and CFBE cells. In all cell types studied robust VRAC currents could be induced at constant cell volume by G-protein activation with GTPγS infusion. This study provides the first extensive comparative functional and pharmacological analysis of VRAC currents in normal and CF airway epithelial cells and shows that VRACs are unimpaired molecularly or functionally in CF.

Opening up of plasmalemma type-1 VDAC to form apoptotic "find me signal" pathways is essential in early apoptosis - evidence from the pathogenesis of cystic fibrosis resulting from failure of apoptotic cell clearance followed by sterile inflammation

Cell membrane-standing type-1 VDAC is involved in cell volume regulation and thus apoptosis. The channel has been shown to figure as a pathway for osmolytes of varying classes, ATP included. An early event in apoptotic cell death is the release of "find me signals" by cells that enter the apoptotic process. ATP is one of those signals. Apoptotic cells this way attract phagocytes for an immunologically silent cell clearance. Thus, whenever apoptosis fails by a blockade of plasmalemma type-1 VDAC processes of sterile inflammation must be assumed for cell elimination. This is evident from a close look on the pathogenetic process of cystic fibrosis (CF). However, in normal airway epithelia two different anion channels cooperate to guarantee an appropriate volume of airway surface liquid (ASL) necessary for surface clearing: the cystic fibrosis conductance regulator (CFTR) and the outwardly rectifying chloride channel (ORCC) complex also called "alternate chloride channel" and under the control of the CFTR. There are arguments, that type-1 VDAC forms the channel part of the ORCC complex, and it has been shown that CFTR and type-1 VDAC co-localize in the apical membranes of human surface respiratory epithelium. In cystic fibrosis, the central cAMP-dependent regulation of ion and water transport via functional CFTR is lost. Here, CFTR molecules do not reach the apical membranes of airway epithelia anymore or work in an insufficient way, respectively. In addition, type-1 VDAC is no longer available to work as a "find me signal" pathway. In consequence, clearing away of apoptotic cells is blocked. There are experimental data on the channel characteristics of type-1 VDAC under the anion channel blocker DIDS (4,4-diisothiocyanato-stilbenedisulphonic acid) that argue in favor of this hypothesis. Together, type-1 VDAC should be kept as a "find me signal" pathway, which may give way to several classes of such signals.

Mechanical stimulation-induced calcium wave propagation in cell monolayers: the example of bovine corneal endothelial cells

Intercellular communication is essential for the coordination of physiological processes between cells in a variety of organs and tissues, including the brain, liver, retina, cochlea and vasculature. In experimental settings, intercellular Ca(2+)-waves can be elicited by applying a mechanical stimulus to a single cell. This leads to the release of the intracellular signaling molecules IP3 and Ca(2+) that initiate the propagation of the Ca(2+)-wave concentrically from the mechanically stimulated cell to the neighboring cells. The main molecular pathways that control intercellular Ca(2+)-wave propagation are provided by gap junction channels through the direct transfer of IP3 and by hemichannels through the release of ATP. Identification and characterization of the properties and regulation of different connexin and pannexin isoforms as gap junction channels and hemichannels are allowed by the quantification of the spread of the intercellular Ca(2+)-wave, siRNA, and the use of inhibitors of gap junction channels and hemichannels. Here, we describe a method to measure intercellular Ca(2+)-wave in monolayers of primary corneal endothelial cells loaded with Fluo4-AM in response to a controlled and localized mechanical stimulus provoked by an acute, short-lasting deformation of the cell as a result of touching the cell membrane with a micromanipulator-controlled glass micropipette with a tip diameter of less than 1 μm. We also describe the isolation of primary bovine corneal endothelial cells and its use as model system to assess Cx43-hemichannel activity as the driven force for intercellular Ca(2+)-waves through the release of ATP. Finally, we discuss the use, advantages, limitations and alternatives of this method in the context of gap junction channel and hemichannel research.

Guanylate cyclase activators, cell volume changes and IOP reduction

Glaucoma afflicts millions of people worldwide and is a major cause of blindness. The risk to develop glaucoma is enhanced by increases in IOP, which result from deranged flow of aqueous humor. Aqueous humor is a fluid located in the front of the eye that gives the eye its buoyancy and supplies nutrients to other eye tissues. Aqueous humor is secreted by a tissue called ciliary processes and exits the eye via two tissues; the trabecular meshwork (TM) and Schlemm's canal. Because the spaces through which the fluid flows get smaller as the TM joins the area of the Schlemm's canal, there is resistance to aqueous humor outflow and this resistance creates IOP. There is a correlation between changes in TM and Schlemm's canal cell volume and rates of aqueous humor outflow; agents that decrease TM and Schlemm's canal cell volume, increase the rate of aqueous humor outflow, thus decreasing IOP. IOP is regulated by guanylate cyclase activators as shown in humans, rabbits and monkeys. There are two distinct groups of guanylate cyclases, membrane guanylate cyclase and soluble guanylate cyclase (sGC); activation of both have been shown to decrease IOP. Members of the membrane guanylate cyclase family of receptors bind to peptide ligands, while the sGC responds to gases (such as NO and CO(2)) and compounds (such as YC1, [3-(5'-hydroxymethyl-2'furyl)-1-benzyl indazole), a benzyl indazole derivative, and BAY-58-2667); activation of either results in formation of cyclic GMP (cGMP) and activation of protein kinase G (PKG) and subsequent phosphorylation of target proteins, including the high conductance calcium activated potassium channel (BKca channel). While activators of both membrane guanylate cyclase and sGC have the ability to lower IOP, the IOP lowering effects of sGC are noteworthy because sGC activators can be topically applied to the eye to achieve an effect. We have demonstrated that activators of sGC increase the rate at which aqueous humor exits the eye in a time course that correlates with the time course for sGC-induced decreases in TM and Schlemm's canal cell volume. Additionally, sGC-induced decrease in cell volume is accompanied by both K(+) and Cl(-) efflux induced by activation of K(+) and Cl(-) channels, including the BKca channel and/or K(+)Cl(-) symport. This suggests that parallel K(+)Cl(-) efflux, and resultant H(2)O efflux result in decreases in cell volume. These observations suggest a functional role for sGC activators, and suggest that the sGC/cGMP/PKG systems are potential therapeutic targets in the treatment of glaucoma.

Swelling-induced Ca²+ influx and K+ efflux in American alligator erythrocytes

The American alligator can hibernate during winter, which may lead to osmotic imbalance because of reduced kidney function and lack of food consumption during this period. Accordingly, we hypothesized that their red blood cells would have a well-developed regulatory volume decrease (RVD) to cope with the homeostatic challenges associated with torpor. Osmotic fragility was determined optically, mean cell volume was measured by electronic sizing, and changes in intracellular Ca²+ concentration were visualized using fluorescence microscopy and fluo-4-AM. Osmotic fragility increased and the ability to regulate volume was inhibited when extracellular Na+ was replaced with K+, or when cells were exposed to the K+ channel inhibitor quinine, indicating a requirement of K+ efflux for RVD. Addition of the ionophore gramicidin to the extracellular medium decreased osmotic fragility and also potentiated volume recovery, even in the presence of quinine. In addition, hypotonic shock (0.5 x Ringer) caused an increase in cytosolic Ca²+, which resulted from Ca²+ influx because it was not observed when extracellular Ca²+ was chelated with EGTA (ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid). Furthermore, cells loaded with BAPTA-AM (1,2-bis(2-aminophenoxymethyl)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl) ester) or exposed to a low Ca²+-EGTA hypotonic Ringer had a greater osmotic fragility and also failed to recover from cell swelling, indicating that extracellular Ca²+ was needed for RVD. Gramicidin reversed the inhibitory effect of low extracellular Ca²+. Finally, and surprisingly, the Ca²+ ionophore A23187 increased osmotic fragility and inhibited volume recovery. Taken together, our results show that cell swelling activated a K+ permeable pathway via a Ca²+-dependent mechanism, and this process mediated K+ loss during RVD.

Dynamic regulation of barrier integrity of the corneal endothelium

The corneal endothelium maintains stromal deturgescence, which is a prerequisite for corneal transparency. The principal challenge to stromal deturgescence is the swelling pressure associated with the hydrophilic glycosaminoglycans in the stroma. This negative pressure induces fluid leak into the stroma from the anterior chamber, but the rate of leak is restrained by the tight junctions of the endothelium. This role of the endothelium represents its barrier function. In healthy cornea, the fluid leak is counterbalanced by an active fluid pump mechanism associated with the endothelium itself. Although this pump-leak hypothesis was postulated several decades ago, the mechanisms underlying regulation of the balance between the pump and leak functions remain largely unknown. In the last couple of decades, the ion transport systems that support the fluid pump activity have been discovered. In contrast, despite significant evidence for corneal edema secondary to endothelial barrier dysfunction, the molecular aspects underlying its regulation are relatively unknown. Recent findings in our laboratory, however, indicate that barrier integrity (i.e., structural and functional integrity of the tight junctions) of the endothelium is sensitive to remodeling of its peri-junctional actomyosin ring, which is located at the apical junctional complex. This review provides a focused perspective on dynamic regulation of the barrier integrity of endothelium vis-à-vis plasticity of the peri-junctional actomyosin ring and its association with cell signaling downstream of small GTPases of the Rho family. Based on findings to date, it appears that development of specific pharmacological strategies to treat corneal edema in response to inflammatory stress would be possible in the near future.

Activation of the BK(Ca) channel increases outflow facility and decreases trabecular meshwork cell volume

PURPOSE

Inhibition of the BK(Ca) channel attenuated the nitric oxide-induced increase in outflow facility and decrease in trabecular meshwork (TM) cell volume suggesting the involvement of the BK(Ca) channel in TM cell function. This study examined the effects of activation of the BK(Ca) channel on outflow facility and TM cell volume and determined if the effects of NO and BK(Ca) channel activation on TM cell volume were additive.

METHODS

Porcine eyes were used to measure outflow facility using the anterior segment organ culture perfusion system. Cell volume was measured using Calcein AM fluorescent dye, detected by confocal microscopy, and quantified using NIH ImageJ software.

RESULTS

NS1619 increased outflow facility 86% over baseline. Additionally, there was a concentration-dependent decrease in TM cell volume in response to NS1619, which was abolished by iberiotoxin (IBTX). While NS1619 alone and DETA-NO alone decreased TM cell volume, together their effects were not additive. The time course for NS1619-induced increases in outflow facility correlated with the time course for NS1619-induced decreases in cell volume.

CONCLUSIONS

BK(Ca) channel activation increases outflow facility and decreases cell volume suggesting that K(+) efflux regulates TM cell function.

Cyclic mechanical stress and trabecular meshwork cell contractility

PURPOSE

Ocular pulse decreases outflow facility of perfused anterior segments. However, the mechanism by which conventional outflow tissues respond to cyclic intraocular pressure oscillations is unknown. The purpose of the present study was to examine responses of trabecular meshwork (TM) cells to cyclic biomechanical stress in the presence and absence of compounds known to affect cell contractility.

METHODS

To model flow in the juxtacanalicular region of the TM and to measure changes in transendothelial flow, human TM cell monolayers on permeable filters were perfused at a constant flow rate until reaching a stable baseline pressure and then were exposed to cyclic stress with an average amplitude of 2.7 mm Hg peak to peak at a 1-Hz frequency for 2 hours in the presence or absence of compounds known to affect cell contractility (isoproterenol, Y27632, pilocarpine, and nifedipine). Pressure was recorded continuously. Immunocytochemistry staining was used to determine filamentous actin stress fiber content, whereas Western blot analysis was used to measure the extent of myosin light chain (p-MLC) phosphorylation and ratio of filamentous to globular actin.

RESULTS

Human TM cells respond to cyclic pressure oscillations by increasing mean intrachamber pressure (decreasing hydraulic conductivity) (126.13% +/- 2.4%; P < 0.05), a response blocked in the presence of Y27632, a rho-kinase inhibitor (101.35 +/- 0.59; P = 0.234), but not isoproterenol, pilocarpine, or nifedipine. Although mechanical stress appeared to have no effect, Y27632 decreased phosphorylated myosin light chain, filamentous/globular actin ratio, and stress fiber formation in TM cells.

CONCLUSIONS

Human TM cells respond to cyclic mechanical stress by increasing intrachamber pressure. Pulse-mediated effects are blocked by Y27632, implicating a role for Rho-kinase-mediated signaling and cellular contractility in ocular pulse-associated changes in outflow facility.

Electron probe X-ray microanalysis of intact pathway for human aqueous humor outflow

Intraocular pressure (IOP) is regulated by the resistance to outflow of the eye's aqueous humor. Elevated resistance raises IOP and can cause glaucoma. Despite the importance of outflow resistance, its site and regulation are unclear. The small size, complex geometry, and relative inaccessibility of the outflow pathway have limited study to whole animal, whole eye, or anterior-segment preparations, or isolated cells. We now report measuring elemental contents of the heterogeneous cell types within the intact human trabecular outflow pathway using electron-probe X-ray microanalysis. Baseline contents of Na(+), K(+), Cl(-), and P and volume (monitored as Na+K contents) were comparable to those of epithelial cells previously studied. Elemental contents and volume were altered by ouabain to block Na(+)-K(+)-activated ATPase and by hypotonicity to trigger a regulatory volume decrease (RVD). Previous results with isolated trabecular meshwork (TM) cells had disagreed whether TM cells express an RVD. In the intact tissue, we found that all cells, including TM cells, displayed a regulatory solute release consistent with an RVD. Selective agonists of A(1) and A(2) adenosine receptors (ARs), which exert opposite effects on IOP, produced similar effects on juxtacanalicular (JCT) cells, previously inaccessible to functional study, but not on Schlemm's canal cells that adjoin the JCT. The results obtained with hypotonicity and AR agonists indicate the potential of this approach to dissect physiological mechanisms in an area that is extremely difficult to study functionally and demonstrate the utility of electron microprobe analysis in studying the cellular physiology of the human trabecular outflow pathway in situ.

Forskolin induces myosin light chain dephosphorylation in bovine trabecular meshwork cells

PURPOSE

Enhanced contractility of the actin cytoskeleton in trabecular meshwork (TM) cells is implicated in increased resistance to aqueous humor outflow. In this study, we have investigated effects of forskolin, which is known to elevate cAMP and also enhance aqueous humor outflow, on myosin light chain (MLC) phosphorylation, a biochemical marker of actin contractility.

METHODS

Experiments were performed using cultured bovine TM cells. Phosphorylated MLC (pMLC), expressed as the % of untreated cells, was assessed by urea-glycerol gel electrophoresis and Western blotting. RhoA activity was determined by affinity precipitation of RhoA-GTP to RhoA binding domain of an effector of RhoA. Intracellular cAMP levels were measured by ELISA.

RESULTS

Exposure to LPA (lysophosphatidic acid) led to increased MLC phosphorylation (LPA: pMLC=133%) and activation of RhoA. These responses of LPA were suppressed by co-treatment with forskolin (LPA+forskolin: pMLC=88%). Similarly, ET-1 and nocodazole-induced MLC phosphorylation (ET-1: pMLC=145%; nocodazole: pMLC=145%) as well as RhoA activation were suppressed by co-treatment with forskolin (ET-1+forskolin: pMLC=99%; nocodazole+forskolin: pMLC=107%). Exposure to forskolin alone led to MLC dephosphorylation (pMLC=68%). Forskolin alone led to a 4-fold increase in cAMP levels. This increase was not affected when co-treated with LPA or ET-1.

CONCLUSIONS

Forskolin prevents MLC phosphorylation induced by LPA, ET-1, and nocodazole through inhibition of RhoA-Rho kinase axis. MLC dephosphorylation and consequent relaxation of actin cytoskeleton in TM cells presumably underlies the increased outflow facility reported in response to forskolin.

NO-induced regulation of human trabecular meshwork cell volume and aqueous humor outflow facility involve the BKCa ion channel

Nitric oxide (NO) donors decrease intraocular pressure (IOP) by increasing aqueous outflow facility in the trabecular meshwork (TM) and/or Schlemm's canal. However, the cellular mechanisms are unknown. Cellular mechanisms known to regulate outflow facility include changes in cell volume and cellular contractility. In this study, we investigated the effects of NO donors on outflow facility and NO-induced effects on TM cell volume. We tested the involvement of soluble guanylate cyclase (sGC), cGMP, PKG, and the large-conductance Ca2+-activated K+ (BKCa) channel using inhibitors and activators. Cell volume was measured using calcein AM fluorescent dye, detected by confocal microscopy, and quantified using NIH ImageJ software. An anterior segment organ perfusion system measured outflow facility. NO increased outflow facility in porcine eye anterior segments (0.4884-1.3956 microl.min(-1).mmHg(-1)) over baseline (0.2373-0.5220 microl.min(-1).mmHg(-1)) within 10 min of drug application. These NO-induced increases in outflow facility were inhibited by the the BKCa channel inhibitor IBTX. Exposure of TM cells to NO resulted in a 10% decrease in cell volume, and these decreases were abolished by the sGC inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one and IBTX, suggesting the involvement of sGC and K+ eflux, respectively. NO-induced decreases in cell volume were mimicked by 8-Br-cGMP and abolished by the PKG inhibitor (RP)-8-Br-PET-cGMP-S, suggesting the involvement cGMP and PKG. Additionally, the time course for NO-induced decreases in TM cell volume correlated with NO-induced increases in outflow facility, suggesting that the NO-induced alterations in cell volume may influence outflow facility.

Mechanosensor Channels in Mammalian Somatosensory Neurons

Mechanoreceptive sensory neurons innervating the skin, skeletal muscles and viscera signal both innocuous and noxious information necessary for proprioception, touch and pain. These neurons are responsible for the transduction of mechanical stimuli into action potentials that propagate to the central nervous system. The ability of these cells to detect mechanical stimuli impinging on them relies on the presence of mechanosensitive channels that transduce the external mechanical forces into electrical and chemical signals. Although a great deal of information regarding the molecular and biophysical properties of mechanosensitive channels in prokaryotes has been accumulated over the past two decades, less is known about the mechanosensitive channels necessary for proprioception and the senses of touch and pain. This review summarizes the most pertinent data on mechanosensitive channels of mammalian somatosensory neurons, focusing on their properties, pharmacology and putative identity.

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.

Identification and functional characterization of ClC-2 chloride channels in trabecular meshwork cells

In the eye, trabecular meshwork (TM) cell volume may be an important determinant of aqueous humor outflow. Among their functions, ClC-2 chloride channels are thought to be involved in regulation of cellular volume and intracellular [Cl(-)]. We characterized the properties and modulation of an inwardly rectifying chloride current activated in these cells. Patch-clamp recordings revealed inwardly rectifying chloride currents activated by membrane hyperpolarization in primary cultures of both bovine (BTM) and human (HTM) TM cells. Electrophysiological properties and anion permeability sequence (Cl(-)>Br(-)>I(-)>F(-)) were in agreement with previous data for ClC-2 in other cells. The currents were blocked by Cd(2+) and enhanced by extracellular acidification, 8Br-cAMP and cell swelling, while extracellular alkalinization decreased them. RT-PCR experiments using total RNA revealed the molecular expression of ClC-2 channels. Previously we reported the involvement of swelling-activated chloride channels (Cl(swell)) and Ca(2+)-activated K(+) channels (BK(Ca)) in cell volume and outflow facility regulation. However, in the present analysis, cell volume experiments in calcein-loaded cells and outflow facility studies performed in bovine anterior segments revealed that ClC-2 channels do not make a significant contribution to the recovery of cellular volume or to the regulation of the outflow facility. Nevertheless, ClC-2 modulation by different stimuli may contribute to intracellular [Cl(-)] regulation and other cellular functions yet to be determined in the TM.

Mechanosensitivity and the eye: cells coping with the pressure

The cells of the various organ systems in humans are subject to mechanical forces to which they must respond. Here the authors review what is known of the ways in which the cells of animals, ranging from the prokaryotic to humans, sense and transduce mechanical forces to respond to such stimuli. In what way this pertains to the eye, especially with respect to axial myopia and the pressure related disease of glaucoma, is then surveyed.

Effects of dinucleoside polyphosphates on trabecular meshwork cells and aqueous humor outflow facility

The most important risk factor for the development of glaucoma is elevated intraocular pressure (IOP). Hypotensive drugs decrease IOP, preventing optic nerve damage and further vision loss. The balance between aqueous humor (AH) production and drainage determines IOP, and problems in AH outflow pathways are associated with open-angle glaucoma development. Previous studies have shown the presence of diadenosine tetraphosphate (Ap(4)A) and pentaphosphate (Ap(5)A) in the AH. Topic application of Ap(4)A to the cornea decreased IOP, whereas Ap(5)A increased it. Because dinucleoside polyphosphates stimulate P2Y purinergic receptors, we studied their presence in trabecular meshwork (TM) cells. Additionally, the effects of diadenosine polyphosphates (Ap(n)As; n = 3-5) and Up(4)U (P(1),P(4)-(diuridine 5')-tetraphosphate; INS365) in outflow facility were tested. P2Y(1), P2Y(2), and P2Y(4) receptors were detected in TM cells by Western blot and immunocytochemistry. In TM cells, Ap(3)A, Ap(4)A, and Ap(5)A induced discrete intracellular calcium concentration ([Ca(2+)](i)) mobilizations compared with higher and more sustained [Ca(2+)](i) mobilizations after Up(4)U application. In bovine ocular anterior segments perfused at constant pressure, 1 microM Ap(3)A or Ap(4)A increased outflow facility, whereas Up(4)U or Ap(5)A did not modify it. 2-MeSADP, a selective P2Y(1) agonist, induced outflow facility increases similar to those obtained after Ap(3)A and Ap(4)A, and these were prevented by addition of the selective P2Y(1) receptor antagonist MRS-2179 (2'-deoxy-N(6)-methyladenosine-3',5'-diphosphate). Our results demonstrate that the hypotensive effect of Ap(4)A and other dinucleotides is mediated, at least in part, by increasing trabecular outflow facility through activation of P2Y(1) receptors. The latter would seem to be an interesting target in the development of antiglaucomatous drugs to selectively increase AH outflow.

Differential expression of the human chloride channel genes in the trabecular meshwork under stress conditions

Among other channels, voltage-gated chloride channels (ClC) regulate cell volume, membrane potential and cellular transport. Because changes in trabecular meshwork (TM) cell volume influence outflow facility and because the relative abundance of a gene's transcript is an indication of the relevance of the gene's function, we investigated the presence and relative expression of seven members of the CLCN gene family in the human TM. To elucidate the role of ClC-2 and ClC-3 in cell swelling, we studied changes in their mRNA levels after hypotonic shock. In addition, to examine the potential involvement of these two channels in conditions associated with glaucoma, we determined their transcripts levels in response to elevated intraocular pressure (IOP) and dexamethasone (DEX). For our evaluations, we used non-transformed human TM cells and perfused human anterior segments from post-mortem donors. For hypotonic shock, cells were exposed to 260 mOsm kg(-1) medium for 15 and 30 min. For DEX, cells were treated with 0.1 microm DEX for 1, 4 and 10 days. For elevated IOP, one eye of each pair of perfused human anterior segments was subjected to DeltaP 38+/-4 mm Hg for 1 hr, 4 and 7 days while the contralateral remained at baseline pressure as a control. ClCs transcripts were determined by relative quantitative RT-PCR. Our results showed that all transcripts but ClC-1 were detected in HTM cells. ClC-2 and ClC-3 were the most abundant and comprised about twice the amount of ClC-6 and ClC-7 and four times that of ClC-4 and ClC-5. Hypotonic conditions consistently up regulated CLCN2 and slightly up regulated CLCN3. After short periods of elevated pressure, ClC-2 and ClC-3 transcripts were increased but ClC-2 induction was significantly higher than that of ClC-3. In contrast, after long pressure insults (7 days), ClC-3 mRNA was significantly increased while CLCN2 was not changed. DEX treatment markedly down regulated CLCN3 and little, if any, reduced ClC-2. The extent of response of the CLCN2 and CLCN3 to these conditions was markedly affected by individual traits but at all times maintained the relative expression pattern of both genes. CLCN2 gene expression was predominantly influenced by cell volume regulation while that of CLCN3 was preferentially affected by conditions associated with TM pathology.

Stimulation of cannabinoid (CB1) and prostanoid (EP2) receptors opens BKCa channels and relaxes ocular trabecular meshwork

Prostanoids and cannabinoids have ocular hypotensive and neuroprotective properties. The effect of the prostanoid AH13205 (EP2), the thromboxane-mimetic U46619, the cannabinoid (CB) agonists WIN55212-2 and CP 55,940, endothelin-1 (ET-1) and 8-bromo-cAMP on the membrane currents of trabecular meshwork (TM) cells were measured using the patch-clamp technique and compared to their effects on TM contractility. Previous studies show relaxation of TM to AH 13205 and other substances that elevate cAMP, while U46619 and endothelin-1 contract TM. This study shows that after contraction (100%) with carbachol (10(-6)m), the CB agonist CP 55,940 dose-dependently reduced contractility to 83+/-4% (n=9) (10(-6)m) and 61+/-10%, (n=7) (10(-5)m). In the presence of both the CB1 antagonist AM251 (10(-6)m) and CP 55,940 (10(-5)m), the contractile response to carbachol reached 84+/-3% (n=6) of the original level. In patch-clamp experiments, membrane permeable 8-bromo-cAMP (10(-4)m) had no effect on currents of TM cells. In contrast, AH 13205 and two cannabinoids reversibly enhanced outward current through high-conductance Ca(2+)-activated K(+) channels (BKCa, BK, maxi-K) to the following values (in % of the initial value at 100 mV): AH 13205 (10(-5)m): 200+/-28% (n=6), CP 55,940 (10(-6)m): 196+/-33% (n=7), CP 55,940 (10(-5)m): 484+/-113% (n=7), WIN55212-2 (10(-5)m): 205+/-41% (n=10). Iberiotoxin (10(-7)m) completely blocked these responses. The current response to CP 55,940 (10(-5)m) could be partially blocked by the CB1 antagonist AM251 (10(-6)m). Conversely, the contractile agents in this study either caused a transient reduction in outward current (ET-1(5x10(-8)m)) or had no effect (U46619 (10(-6)m)). We conclude that stimulation of EP2 and CB1 receptors in TM is coupled to the activation of BKCa channels via a non-diffusible second messenger cascade. This effect may contribute to the relaxant activity of EP2 and CB1 agonists in isolated TM strips, modulating ocular outflow.