Effect of bicarbonate on intracellular potential of rabbit ciliary epithelium

Species variation in biology and physiology of the ciliary epithelium: similarities and differences

Glaucoma is a leading cause of irreversible blindness worldwide. Lowering intraocular pressure (IOP) is the only strategy documented to delay the appearance and retard the progression of vision loss. One major approach for lowering IOP is to slow the rate of aqueous humor formation by the ciliary epithelium. As discussed in the present review, the transport basis for this secretion is largely understood. However, several substantive issues are yet to be resolved, including the integrated regulation of secretion, the functional topography of the ciliary epithelium, and the degree and significance of species variation in aqueous humor inflow. This review discusses species differences in net secretion, particularly of Cl(-) and HCO(3)(-) secretion. Identifying animal models most accurately mimicking aqueous humor formation in the human will facilitate development of future novel initiatives to lower IOP.

Fluid transport phenomena in ocular epithelia

This article discusses three largely unrecognized aspects related to fluid movement in ocular tissues; namely, (a) the dynamic changes in water permeability observed in corneal and conjunctival epithelia under anisotonic conditions, (b) the indications that the fluid transport rate exhibited by the ciliary epithelium is insufficient to explain aqueous humor production, and (c) the evidence for fluid movement into and out of the lens during accommodation. We have studied each of these subjects in recent years and present an evaluation of our data within the context of the results of others who have also worked on electrolyte and fluid transport in ocular tissues. We propose that (1) the corneal and conjunctival epithelia, with apical aspects naturally exposed to variable tonicities, are capable of regulating their water permeabilities as part of the cell-volume regulatory process, (2) fluid may directly enter the anterior chamber of the eye across the anterior surface of the iris, thereby representing an additional entry pathway for aqueous humor production, and (3) changes in lens volume occur during accommodation, and such changes are best explained by a net influx and efflux of fluid.

Basis of chloride transport in ciliary epithelium

The aqueous humor is formed by the bilayered ciliary epithelium. The pigmented ciliary epithelium (PE) faces the stroma and the nonpigmented ciliary epithelium (NPE) contacts the aqueous humor. Cl(-) secretion likely limits the rate of aqueous humor formation. Many transport components underlying Cl(-) secretion are known. Cl(-) is taken up from the stroma into PE cells by electroneutral transporters, diffuses to the NPE cells through gap junctions and is released largely through Cl(-) channels. Recent work suggests that significant Cl(-) recycling occurs at both surfaces of the ciliary epithelium, providing the basis for modulation of net secretion. The PE-NPE cell couplet likely forms the fundamental unit of secretion; gap junctions within the PE and NPE cell layers are inadequate to maintain constancy of ionic composition throughout the epithelium under certain conditions. Although many hormones, drugs and signaling cascades are known to have effects, a persuasive model of the regulation of aqueous humor formation has not yet been developed. cAMP likely plays a central role, potentially both enhancing and reducing secretion by actions at both surfaces of the ciliary epithelium. Among other hormone receptors, A(3) adenosine receptors likely alter intraocular pressure by regulating NPE-cell Cl(-) channel activity. Recently, functional evidence for the regional variation in ciliary epithelial secretion has been demonstrated; the physiologic and pathophysiologic implications of this regional variation remain to be addressed.

cAMP-activated maxi-Cl(-) channels in native bovine pigmented ciliary epithelial cells

The eye's aqueous humor is secreted by a bilayered ciliary epithelium comprising pigmented (PE) and nonpigmented (NPE) epithelial cell layers. Stromal Cl(-) enters the PE cells and crosses gap junctions to the NPE cells for release into the aqueous humor. Maxi-Cl(-) channels are expressed in PE cells, but their physiological significance is unclear. To address this question, excised patches and whole native bovine PE cells were patch clamped, and volume was monitored by calcein fluorescence. In symmetrical 130 mM NaCl, cAMP at the cytoplasmic surface of inside-out patches produced concentration-dependent activation of maxi-Cl(-) channels with a unitary conductance of 272 +/- 2 pS (n = 80). Voltage steps from 0 to +/-80 mV, but not to +/-40 mV, produced rapid channel inactivation consistent with the typical characteristics of maxi-Cl(-) channels. cAMP also activated the maxi-Cl(-) channels in outside-out patches. In both cases, maxi-Cl(-) channels were reversibly inhibited by SITS and 5-nitro-2-(phenylpropylamino)benzoate (NPPB). Decreasing cytoplasmic Cl(-) concentration reduced both open-channel probability and unitary conductance. Similarly, the membrane-permeant 8-bromo-cAMP stimulated outward and inward whole cell currents; the stimulation was larger at higher intracellular Cl(-) concentration. As with unitary currents, cAMP-triggered whole cell currents displayed inactivation at +/-80 but not at +/-40 mV. Moreover, cAMP triggered NPPB-sensitive shrinkage of PE cells. The results suggest that cAMP directly activates maxi-Cl(-) channels of native PE cells that contribute to Cl(-) release particularly from Cl(-)-loaded cells. These cAMP-activated channels provide a potential mechanism for reducing and modulating net aqueous humor secretion by facilitating Cl(-) reabsorption into the ciliary stroma.

Electron microprobe analysis of ouabain-exposed ciliary epithelium: PE-NPE cell couplets form the functional units

Aqueous humor is secreted by the bilayered ciliary epithelium. Solutes and water enter the pigmented ciliary epithelial (PE) cell layer, cross gap junctions into the nonpigmented ciliary epithelial (NPE) cell layer, and are released into the aqueous humor. Electrical measurements suggest that heptanol reduces transepithelial ion movement by interrupting PE-NPE communication and that gap junctions may be a regulatory site of aqueous humor formation. Several lines of evidence also suggest that net ciliary epithelial transport is strongly region dependent. Divided rabbit iris-ciliary bodies were incubated in chambers under control and experimental conditions, quick-frozen, cryosectioned, and freeze-dried. Elemental intracellular contents of NPE and PE cells were determined by electron probe X-ray microanalysis. With or without heptanol, ouabain produced concentration- and time-dependent changes more markedly in anterior than in posterior epithelium. Without heptanol, there were considerable cell-to-cell variations in Na gain and K loss. However, contiguous NPE and PE cells displayed similar changes, even when nearby cell pairs were little changed by ouabain in aqueous, stromal, or both reservoirs. In contrast, with heptanol present, ouabain added to aqueous or both reservoirs produced much larger changes in NPE than in PE cells. The results indicate that 1) heptanol indeed interrupts PE-NPE junctions, providing an opportunity for electron microprobe analysis of the sidedness of modification of ciliary epithelial secretion; 2) Na and K undergo faster turnover in anterior than in posterior epithelium; and 3) PE-NPE gap junctions differ from PE-PE and NPE-NPE junctions in permitting ionic equilibration between adjoining ouabain-stressed cells.

Molecular mechanisms of water transport in the eye

The four major sites for ocular water transport, the corneal epithelium and endothelium, the ciliary epithelium, and the retinal pigment epithelium, are reviewed. The cornea has an inherent tendency to swell, which is counteracted by its two surface cell layers, the corneal epithelium and endothelium. The bilayered ciliary epithelium secretes the aqueous humor into the posterior chamber, and the retinal pigment epithelium transports water from the retinal to the choroidal site. For each epithelium, ion transport mechanisms are associated with fluid transport, but the exact molecular coupling sites between ion and water transport remain undefined. In the retinal pigment epithelium, a H+-lactate cotransporter transports water. This protein could be the site of coupling between salt and water in this epithelium. The distribution of aquaporins does not suggest a role for these proteins in a general model for water transport in ocular epithelia. Some water-transporting membranes contain aquaporins, others do not. The ultrastructure is also variable among the cell layers and cannot be fitted into a general model. On the other hand, the direction of cotransport in symporters complies with the direction of fluid transport in both the corneal epi- and endothelium, as well as the ciliary epithelium and retinal pigment epithelium.

Timolol may inhibit aqueous humor secretion by cAMP-independent action on ciliary epithelial cells

The beta-adrenergic antagonist timolol reduces ciliary epithelial secretion in glaucomatous patients. Whether inhibition is mediated by reducing cAMP is unknown. Elemental composition of rabbit ciliary epithelium was studied by electron probe X-ray microanalysis. Volume of cultured bovine pigmented ciliary epithelial (PE) cells was measured by electronic cell sizing; Ca(2+) activity and pH were monitored with fura 2 and 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, respectively. Timolol (10 microM) produced similar K and Cl losses from ciliary epithelia in HCO/CO(2) solution but had no effect in HCO/CO(2)-free solution or in HCO/CO(2) solution containing the carbonic anhydrase inhibitor acetazolamide. Inhibition of Na(+)/H(+) exchange by dimethylamiloride in HCO/CO(2) solution reduced Cl and K comparably to timolol. cAMP did not reverse timolol's effects. Timolol (100 nM, 10 microM) and levobunolol (10 microM) produced cAMP-independent inhibition of the regulatory volume increase (RVI) in PE cells and increased intracellular Ca(2+) and pH. Increasing Ca(2+) with ionomycin also blocked the RVI. The results document a previously unrecognized cAMP-independent transport effect of timolol. Inhibition of Cl(-)/HCO exchange may mediate timolol's inhibition of aqueous humor formation.

Molecular basis of ocular abnormalities associated with proximal renal tubular acidosis

Proximal renal tubular acidosis associated with ocular abnormalities such as band keratopathy, glaucoma, and cataracts is caused by mutations in the Na(+)-HCO(3)(-) cotransporter (NBC-1). However, the mechanism by which NBC-1 inactivation leads to such ocular abnormalities remains to be elucidated. By immunological analysis of human and rat eyes, we demonstrate that both kidney type (kNBC-1) and pancreatic type (pNBC-1) transporters are present in the corneal endothelium, trabecular meshwork, ciliary epithelium, and lens epithelium. In the human lens epithelial (HLE) cells, RT-PCR detected mRNAs of both kNBC-1 and pNBC-1. Although a Na(+)-HCO(3)-cotransport activity has not been detected in mammalian lens epithelia, cell pH (pH(i)) measurements revealed the presence of Cl(-)-independent, electrogenic Na(+)-HCO(3)-cotransport activity in HLE cells. In addition, up to 80% of amiloride-insensitive pH(i) recovery from acid load in the presence of HCO(3)(-)/CO(2) was inhibited by adenovirus-mediated transfer of a specific hammerhead ribozyme against NBC-1, consistent with a major role of NBC-1 in overall HCO(3)-transport by the lens epithelium. These results indicate that the normal transport activity of NBC-1 is indispensable not only for the maintenance of corneal and lenticular transparency but also for the regulation of aqueous humor outflow.

Model of ionic transport for bovine ciliary epithelium: effects of acetazolamide and HCO

The possible existence of transepithelial bicarbonate transport across the isolated bovine ciliary body was investigated by employing a chamber that allows for the measurement of unidirectional, radiolabeled fluxes of CO2 + HCO. No net flux of HCO was detected. However, acetazolamide (0.1 mM) reduced the simultaneously measured short-circuit current (I(sc)). In other experiments in which (36)Cl- was used, a net Cl- flux of 1.12 microeq. h(-1). cm(-2) (30 microA/cm(2)) in the blood-to-aqueous direction was detected. Acetazolamide, as well as removal of HCO from the aqueous bathing solution, inhibited the net Cl- flux and I(sc). Because such removal should increase HCO diffusion toward the aqueous compartment and increase the I(sc), this paradoxical effect could result from cell acidification and partial closure of Cl- channels. The acetazolamide effect on Cl- fluxes can be explained by a reduction of cellular H+ and HCO (generated from metabolic CO2 production), which exchange with Na+ and Cl- via Na+/H+ and Cl-/HCO exchangers, contributing to the net Cl- transport. The fact that the net Cl- flux is about three times larger than the I(sc) is explained with a vectorial model in which there is a secretion of Na+ and K+ into the aqueous humor that partially subtracts from the net Cl- flux. These transport characteristics of the bovine ciliary epithelium suggest how acetazolamide reduces intraocular pressure in the absence of HCO transport as a driving force for fluid secretion.

Similarity of A(3)-adenosine and swelling-activated Cl(-) channels in nonpigmented ciliary epithelial cells

Chloride release from nonpigmented ciliary epithelial (NPE) cells is a final step in forming aqueous humor, and adenosine stimulates Cl(-) transport by these cells. Whole cell patch clamping of cultured human NPE cells indicated that the A(3)-selective agonist 1-deoxy-1-(6-[([3-iodophenyl]methyl)amino]-9H-purin-9-yl)-N-methyl-be ta-D-ribofuranuronamide (IB-MECA) stimulated currents (I(IB-MECA)) by approximately 90% at +80 mV. Partial replacement of external Cl(-) with aspartate reduced outward currents and shifted the reversal potential (V(rev)) from -23 +/- 2 mV to -0.0 +/- 0.7 mV. Nitrate substitution had little effect. Perfusion with the Cl(-) channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and niflumic acid inhibited the currents. Partial Cl(-) replacement with aspartate and NO(3)(-), and perfusion with NPPB, had similar effects on the swelling-activated whole cell currents (I(Swell)). Partial cyclamate substitution for external Cl(-) inhibited inward and outward currents of both I(IB-MECA) and I(Swell). Both sets of currents also showed outward rectification and inactivation at large depolarizing potentials. The results are consistent with the concept that A(3)-subtype adenosine agonists and swelling activate a common population of Cl(-) channels.

Formation of the aqueous humor

1. Glaucoma is a worldwide disease affecting approximately 1-2% of the population aged over 35 years in industrial countries and is a major cause of blindness. 2. Glaucoma is usually associated with an increased intraocular pressure reflecting an imbalance between the rate of production of fluid (the aqueous humor) by the ciliary epithelial cells and its drainage from the eye. Therefore, it is important to understand how this secretion is produced. This requires a knowledge of ciliary epithelial cell composition, which has, in the past, proved difficult to obtain in mammalian preparations. 3. We have recently used the technique of electron-probe X-ray microanalysis to determine this composition under a variety of in vitro conditions. 4. Our results have led to a new model for this secretion that emphasizes the potential secretory role of the Na+/K+/2Cl- cotransporter.

Potential contribution of epithelial Na+ channel to net secretion of aqueous humor

The aqueous humor of the eye is secreted by the bilayered ciliary epithelium, consisting of the pigmented (PE) cell layer facing the stroma and the nonpigmented (NPE) cell layer facing the aqueous humor. Cells within each layer and between the two layers are linked by gap junctions, forming a ciliary epithelial syncytium. Unidirectional secretion from the stroma to the aqueous proceeds both through the cells (the transcellular pathway) and between the cells (the paracellular pathway). Net formation of aqueous humor must, however, be the algebraic sum of unidirectional secretion and unidirectional reabsorption from the aqueous humor back into the stoma. The mechanisms potentially underlying reabsorption of aqueous humor by the NPE cells have recently been addressed by studying the regulatory response (RVI) of anisosmotically shrunken NPE cells. The results indicated that epithelial Na+ channels with a high affinity to amiloride likely contribute to reabsorption of solute from the aqueous humor. We have substantiated this possibility by using Northern analysis to identify in human ciliary body RNA a 3.7-kb transcript corresponding to the alpha-subunit of the amiloride-sensitive, alpha beta gamma-ENaC epithelial sodium channel. We have also found that the Na(+)-channel inhibitor benzamil inhibits the RVI without affecting the cell volume of isotonic cell suspensions. This observation supports the hypothesis that the low conductance, highly selective epithelial Na+ channel is activated by shrinkage and contributes to unidirectional reabsorption as aqueous humor. Examples are provided of how the integrative regulation of aqueous humor formation can involve conjugate actions on both unidirectional secretion and reabsorption.

Vacuolar H+-ATPase in ocular ciliary epithelium

The mechanisms controlling the production of aqueous humor and the regulation of intraocular pressure are poorly understood. Here, we provide evidence that a vacuolar H+-ATPase (V-ATPase) in the ocular ciliary epithelium is a key component of this process. In intracellular pH (pHi) measurements of isolated ciliary epithelium performed with 2',7-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF), the selective V-ATPase inhibitor bafilomycin A1 slowed the recovery of pHi in response to acute intracellular acidification, demonstrating the presence of V-ATPase in the plasma membrane. In isolated rabbit ciliary body preparations examined under voltage-clamped conditions, bafilomycin A1 produced a concentration-dependent decrease in short-circuit current, and topical application of bafilomycin A1 reduced intraocular pressure in rabbits, indicating an essential role of the V-ATPase in ciliary epithelial ion transport. Immunocytochemistry utilizing antibodies specific for the B1 isoform of the V-ATPase 56-kDa subunit revealed localization of V-ATPase in both the plasma membrane and cytoplasm of the native ciliary epithelium in both rabbit and rat eye. The regional and subcellular distribution of V-ATPase in specific regions of the ciliary process was altered profoundly by isoproterenol and phorbol esters, suggesting that change in the intracellular distribution of the enzyme is a mechanism by which drugs, hormones, and neurotransmitters modify aqueous humor production.

Effect of heptanol on the short circuit currents of cornea and ciliary body demonstrates rate limiting role of heterocellular gap junctions in active ciliary body transport

Rabbit ciliary body and cornea were mounted in Ussing-type chambers in Tyrode's under voltage clamp and the effects of heptanol, a gap junction inhibitor, on the short circuit current generated by each of the respective epithelia were determined. Studies were carried out either in control conditions or following amphotericin B permeabilization of either the basolateral membrane of the nonpigmented epithelium of the ciliary body or the apical membrane of the corneal epithelium, respectively. Previous studies have shown that, following these permeabilizations, short circuit currents are established, reflecting aqueous (or tear)-to-serosa Na+ fluxes, and that Na+ translocation through gap junctions connecting the individual layers of these tissues constitutes the major rate limiting step. Heptanol inhibited most of the short circuit current of the amphotericin B-modified ciliary body and cornea and of the unmodified ciliary body epithelium (control). In all these cases, the apparent IC50 was about 0.8 M. In the unmodified corneal epithelium, where ion translocation across the apical membrane constitutes the main rate limiting step for active secretion, 0.4 or 0.8 mM heptanol induced short circuit current increases; partial inhibition was observed only at high concentrations known to cause maximal inhibition of junctional permeability. Heptanol also enhanced the volume regulatory decrease of cultured human NPE cells, a process dependent on cell swelling-induced stimulation of Cl- and K+ permeabilities. Combined with our previous results demonstrating the lack of heptanol effects on other epithelial functions, these data suggest that the effect of heptanol on the active ciliary body transepithelial transport is primarily due to inhibition of the nonpigmented-pigmented junctional path and that this path is a potential site of rate limitation for the secretory process.

Role of ion channels in aqueous humor formation

The aqueous humor is secreted by the ciliary epithelium, a bilayered syncytial epithelium comprising a pigmented (PE) cell layer abutting the stroma and a nonpigmented (NPE) cell layer facing the aqueous phase. As in other epithelia, secretion depends on the transfer of solute, with water passively following. Na+, K+, and Cl- enter the syncytium principally through a Na(+)-K(+)-2Cl- symport, diffusing to the aqueous surface of the NPE cells. The Na+, K+, and Cl- are secreted into the aqueous humor through the Na+/K+ exchange pump, K+ channels, and Cl- channels, respectively. Na+ is also secreted between the cells in response to a small transepithelial potential. The K+ channels are critical not only for K+ release but also for hyperpolarizing the membrane, providing an electrical driving force for Cl- secretion. Some of the K+ channels are Ca2+ sensitive and can be activated by Ca2+ entry through T- and L-type Ca2+ channels. The roles of the ciliary epithelial nonselective and Na+ channels are less clear. This review describes the ion channels thus far identified in the ciliary epithelium in terms of the activation and inactivation of their macroscopic currents, the open probabilities and conductances of the single channels, and their locations and regulation. The review relates each class of channel to known families of channels and indicates how those channels can contribute to the secretion of the aqueous humor.

Cell-attached patch clamping of the intact rabbit ciliary epithelium

Following thorough removal of adhering aqueous humor, we have succeeded in patch clamping the intact rabbit ciliary epithelium in the cell-attached and inside-out excised-patch modes. Rapidly fluctuating currents ("chatter activity') were observed during recordings conducted for as long as 1 h. Chatter activity did not reflect seal instability since interconversion was noted between chatter activity and transitions between stable open and closed states, excision of patches into the bath was associated with substantial shifts in the reversal potential, and chatter activity could be triggered by sustained hyperpolarization, but was insensitive to stretch. The chatter channel was identified as cation-nonselective from the reversal potentials both in the cell-attached and excised-patch modes. The channel's kinetics were similar to those of the cGMP-activated phototransduction channel. The results of PCR amplifications of fragments in cDNA libraries from both human ciliary body and human nonpigmented ciliary epithelial (NPE) cells indicated that human ciliary epithelial cells transcribe message for the retinal phototransduction channel. The possible role of the phototransduction channel in expressing chatter activity was further explored by perfusing preparations with a known activator of that channel (cGMP) and with a known inhibitor (L-cis-diltiazem). Neither agent significantly affected chatter behavior. We conclude that: (1) this is the first demonstration of the feasibility of patch-clamping the intact ciliary epithelium; (2) the NPE cells display chatter activity arising from rapidly fluctuating transitions of a cation-nonselective channel; (3) NPE cells can transcribe message for the cation-nonselective phototransduction channel; and (4) if the observed chatter activity is from a homologue of the photo-transduction channel, the homologue is pharmacologically distinct.

Mechanisms of regulatory volume decrease in nonpigmented human ciliary epithelial cells

To study the net solute and water efflux pathways of the ciliary epithelium we employed a cultured human NPE cell line. Because of the possible relationship between transepithelial ion and water flux and cell volume regulation, the ion efflux pathways mediating regulatory volume decrease (RVD) were investigated. Osmotic swelling of NPE cells was followed by a volume recovery. Volume recovery was K+ dependent and inhibited by K+ channel blockers such as quinine (1 mM). After osmotic swelling, a Cl(-)-dependent membrane depolarization occurred that was inhibited by Cl- channel blockers such as 5-nitro-2-(3-phenylpropylamino)benzoic acid (100 microM) or Ca2+ chelators such as ethylene glycolbis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA, 2.0 mM). Cell swelling was also accompanied by an increase in intracellular Ca2+ concentration ([Ca2+]i) of approximately 200 nM. The swelling-induced rise in [Ca2+]i and RVD were diminished in the presence of 10 microM La3+, 50 nM 12-O-tetradecanoylphorbol 13-acetate, and nominally Ca(2+)-free medium. Near total blockage of RVD occurred after pretreatment of NPE cells with Ca(2+)-free EGTA-1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) acetoxymethyl ester-containing solutions. The inhibition of RVD by EGTA-BAPTA treatment was overcome by increasing K+ conductance with gramicidin. The above findings indicate that RVD in NPE cells is mediated by separate K+ and Cl- conductances (channels). These data also show that swelling-induced increases in [Ca2+]i help modulate net ion efflux during regulation.