A decrease in the permeability of aquaporin zero as a possible cause for presbyopia

The crystalline lens appears to be a simple organ with the sole role of focusing light upon the retina. However, numerous studies have underscored its dynamic nature with a host of compartmentalized physiological processes. As the individual ages, the normal lens develops two inescapable processes, presbyopia and cataracts. Yet, to date, there is no uniform explanation for presbyopia and many factors have been proposed as contributors including continuous enlargement of the lens, loss of power of the ciliary muscle and hardening of the lens fibers. Proposed explanations are incomplete and need experimental confirmation. This paper analyzes the possible causes for presbyopia and proposes a new one for it: a decrease in the permeability of aquaporin zero (AQP-0) also known as major intrinsic protein (MIP). Based on original findings of our laboratory, this paper proposes that a fluid flow exists inside the avascular lens. This fluid enters and leaves the lens during the accommodation process. We believe that for this to occur the lens utilizes the permeability of aquaporin zero which is abundant in the membrane of the fiber cells. Volume change due to fluid traversing the surface of the lens occurs during accommodation. We present the hypothesis that increasing the permeability of AQP-0 would facilitate accommodation. Therefore, defects in AQP-0 permeability may be a cause for presbyopia. We would also like to propose that it is possible to visualize and measure the fluid volume lost during un-accommodation and determine if the fluid is lost across the anterior, posterior or both surfaces. An age-related loss in lens water permeability could reduce fluid fluxes during the shape changes of accommodation potentially contributing to presbyopia.

Tissue plasminogen activator reduces the elevated intraocular pressure induced by prednisolone in sheep

We have previously shown that tissue plasminogen activator (tPA) injected in the vitreous of sheep, reduced or prevented the elevation of the intraocular pressure (IOP) normally produced by the instillation of 1% prednisolone. We now report the effect of tPA when injected into the anterior chamber (AC) in amounts of 0.01, 0.001 and 0.0001 μg diluted in a volume of 50 μL. Lyophilized tPA, obtained as Actilyse 50 mg from Boehringer Ingelheim containing arginine was utilized. The Actilyse was diluted in balanced salt solution to obtain the desired amount of tPA in 50 μL. An identical solution containing only arginine was prepared to inject into the contralateral eye as a control. Six sheep of the Corriedale breed were selected. At the beginning of the study all eyes received instillation of 1% prednisolone 3 times/day for 10 days to elevate their IOP from 10 mm Hg to about 23 mm Hg. Then, 0.0001 μg was injected into one of the eyes and its effect was followed for up to 55:00 h while the instillation of prednisolone continued in both eyes. The same protocol was implemented for the 0.001 and 0.01 μg amounts after extended washout and IOP was over 22 mm Hg. The injection of 0.0001 μg into the AC had no effect on an IOP of 23.0 mm Hg at 6:00 and 30:00 h after injection. 0.001 μg of tPA reduced IOP from 23.1 to 18.6 mm Hg at 6:00 h but IOP recovered to 22.3 mm Hg at 30:00 h. Injection of 0.01 μg produced a marked and prolonged reduction of IOP. From a baseline of 23.0, IOP was reduced to 14.0, 14.7, 21.2, and 20.9 mm Hg at 5.0, 23.0, 27.0 and 45.5 h, respectively. The 0.423 μg of arginine, which is associated with 0.01 μg tPA, was injected alone and had no effect. Recombinant human tPA injected in the AC is effective in reversing steroid-induced IOP elevation in sheep. The reduction of IOP elevation may be the result of an effect on extra-cellular matrix turnover in the TM. These findings suggest that tPA may by useful as a therapeutic agent in steroid-induced glaucomas.

Sildenafil stimulates aqueous humor turnover in rabbits

Sildenafil citrate increases ocular blood flow and accelerates the rate of anterior chamber refilling after paracentesis. The latter effect could have resulted from a reduction in outflow facility or from an increase in aqueous humor (AH) production. In this study, we used scanning ocular fluorophotometry to examine the effects of sildenafil on AH turnover, and thus, AH production in eyes of live normal rabbits. For this, the rate of aqueous humor flow (AHF) was quantified with a commercially available fluorophotometer that measured the rate of fluorescein clearance from the anterior segment, which predominantly occurs via the trabecular meshwork. After ≈2 h of control scans to determine the baseline rate of AHF, the rabbits were fed 33 mg of sildenafil and allowed ≈45 min for the drug to enter the systemic circulation. Thereafter, fluorescence scans were retaken for an additional 90-120 min. Sildenafil ingestion increased AHF by about 36%, from 2.31 μL/min to 3.14 μL/min (P < 0.001, as two-tailed paired data, n = 20 eyes). This observation indicates that sildenafil citrate, which is a phosphodiesterase type-5 inhibitor currently marketed as a vasodilator (e.g., Viagra, Revatio), stimulates AHF in rabbits. Our results seem consistent with reports indicating that the drug dilates intraocular arteries and augments intraocular vascular flow. These physiological responses to the agent apparently led to increased fluid entry into the anterior chamber. As such, the drug might have utility in patients with ocular hypotony resulting from insufficient AH formation.

Fluid circulation determined in the isolated bovine lens

PURPOSE

In 1997, a theoretical model was developed that predicted the existence of an internal, Na(+)-driven fluid circulation from the poles to the equator of the lens. In the present work, we demonstrate with a novel system that fluid movement can be measured across the polar and equatorial surface areas of isolated cow lenses. We have also determined the effects of ouabain and reduced bath [Na(+)].

METHODS

Lenses were isolated in a chamber with three compartments separated by two thin O-rings. Each compartment, anterior (A), equatorial (E), and posterior (P), was connected to a vertical capillary graduated in 0.25 μL. Capillary levels were read every 15 minutes. The protocols consisted of 2 hours in either open circuit or short circuit. The effects of ouabain and low-Na(+) solutions were determined under open circuit.

RESULTS

In 21 experiments, the E capillary increased at a mean rate of 0.060 μL/min while the A and P levels decreased at rates of 0.044 and 0.037 μL/min, respectively, closely accounting for the increase in E. The first-hour flows under short circuit were approximately 40% larger than those in open-circuit conditions. The first-hour flows were always larger than those during the second hour. Preincubation of lenses with either ouabain or low-[Na(+)] solutions resulted in reduced rates of fluid transport. When KCl was used to replace NaCl, a transitory stimulation of fluid transport occurred.

CONCLUSIONS

These experiments support that a fluid circulation consistent with the 1997 model is physiologically active.

Sildenafil accelerates anterior chamber refilling after paracentesis in sheep and rabbits

PURPOSE

Sildenafil increases ocular blood flow. Thus, the authors investigated if it also increases anterior chamber (AC) refilling after paracentesis.

METHODS

Corriedale sheep and albino rabbits were used as animal models. Intraocular pressure (IOP) was measured, paracentesis performed on one eye, and AC refilling followed by observation using oblique illumination. IOP measurements continued as the AC formed. After IOP stabilization, sildenafil (100 mg) was orally administered. Forty to 60 minutes later, AH was withdrawn from the contralateral eye. The point at which IOP recovered was used to determine refilling time. Paracentesis volumes were either 60, 120, or 300 μL in sheep, and 50 or 100 μL in rabbits.

RESULTS

IOP recovered in approximately 49, 56, and 50 minutes after the 60, 120, and 300 μL withdrawals in sheep. The refilling times of the contralateral eye after sildenafil ingestion were approximately 19, 26, and 37 minutes for the respective AH withdrawals. With rabbits, IOP recovered in approximately 13 minutes after the 50 and 100 μL AH withdrawals. After sildenafil, the IOP recovery times of the fellow eye were approximately 6 minutes. AH refilling rates were estimated by dividing the paracentesis volume by IOP recovery time. After sildenafil, such rates were larger than the AH formation rate attributed to secretion by the ciliary epithelium.

CONCLUSIONS

Sildenafil accelerates the rate of AC refilling and might have beneficial utility as an agent enhancing fluid entry into the AC of patients who experienced AH loss during eye surgery, as well as in some cases of ocular hypotony.

Gene expression changes in steroid-induced IOP elevation in bovine trabecular meshwork

PURPOSE

To determine whether gene expression changes occur in the trabecular meshwork (TM) of cow eyes with steroid-induced intraocular pressure (IOP) elevation.

METHODS

Adult female Braford cows (n = 4) were subjected to uniocular prednisolone acetate treatment for 6 weeks. IOP was monitored with an applanation tonometer. At the conclusion of the experiment, animals were euthanized, eyes were enucleated, and the TM was dissected and stored in an aqueous nontoxic tissue storage reagent. RNA was extracted and subjected to microarray analysis using commercial oligonucleotide bovine arrays. Some of the genes differentially expressed between control and experimental eyes were confirmed by quantitative RT-PCR and some of the respective proteins were studied by immunoblotting.

RESULTS

IOP began to increase after 3 weeks of treatment, reaching a peak 2 weeks later. IOP differences between corticosteroid-treated and fellow control eyes were 6 ± 1 mm Hg (mean ± SD) at the conclusion of the study. Microarray analysis revealed that expression of 258 genes was upregulated, whereas expression of 187 genes was downregulated in the TM of eyes with steroid-induced IOP elevation. Genes identified to be differentially expressed include genes coding for cytoskeletal proteins, enzymes, growth and transcription factors, as well as extracellular matrix proteins and immune response proteins. A number of relevant gene networks were detected by bioinformatic analysis.

CONCLUSIONS

Steroid-induced IOP elevation alters gene expression in the bovine TM. Identification of genes with changing expression in this model of open-angle glaucoma may help elucidate the primary changes occurring at the molecular level in this condition.

Interaction between mechanical and osmotic forces in the isolated rabbit lens

Based on our previous work showing that cow and rabbit lenses isolated with their accommodation anatomical components intact change volume during simulated accommodation in vitro, and that hyposmolality and hyperosmolality also produce volume changes, we tested the idea that exerting these forces simultaneously may add or counteract each other. Further, we attempted to find a point at which osmotic and mechanical forces may cancel each other. Using previously described methodology, we found that combined stretching and anisotonic conditions applied to a lens always produced less of a volume change than that observed on its paired lens from the fellow eye that was only subjected to anisotonic conditions. Our results suggest that a stretching force that increases the equatorial diameter by 0.4% and reduces the lens volume by 1.8% could be canceled by a hyposmotic force of about -20 to -30 mOsM. Counter-intuitively, lenses that were subjected to stretching and hyperosmolality had less volume decrease than their paired lenses only exposed to hypertonicity. This latter observation is likely due to the prevention by the mechanical stretching forces of the shortening of the equatorial diameter, which normally occurs in hypertonic media.

An hypothesis on pressure transmission from anterior chamber to optic nerve

Few studies have characterized how pressure in the anterior chamber (AC) of the eye is transmitted via the vitreous to the vitreous-ganglion cell interface. We are aware of only one study that simultaneously measured the pressures in the AC and vitreous humor; and of only one study that simultaneously measured the pressures in the AC and the suprachoroidal space (SCS). The pressure in the AC is defined as the intraocular pressure (IOP), which when elevated beyond statistically normal limits is a recognized risk factor for glaucoma, a malady best described as an optic neuropathy with degeneration and eventual death of the retinal ganglion cells (GC's) and highly characteristic changes in the optic nerve head (ONH). Most investigators currently believe that the prevalent risk factor for GC apoptosis is ocular hypertension, but no one has demonstrated how an increase in IOP in the AC is transmitted to the GC's. In patients with primary open angle glaucoma, the pressure in the AC increases due to an increase in the resistance of the trabecular meshwork (TM) outflow pathway. We questioned how such increased pressure in the AC would be transmitted to the GC to produce the changes in the ONH seen in glaucoma. Based on our preliminary data and purview of the literature, we hypothesize that a pressure increase originating in the AC is likely transmitted via both the SCS and the vitreous, with transmission via the former pathway probably most efficient in affecting the GC. Independently of the mechanism that produces GC apoptosis, the ones that are first affected, as repeatedly shown by visual field tests, are the most peripheral ones; i.e., those whose axons are the most external as they form the ONH and enter the lamina cribrosa. There are no published reports explaining this peculiarity. The dogma is that the pressure transmitted via the vitreous is higher at the periphery because it is transmitted across a shorter distance, since the vitreous acts as a buffer that absorbs part of the pressure being transmitted. We propose that IOP is not only transmitted via the vitreous but also via the SCS. Increases in IOP could be efficiently applied via the SCS to the most external axons of the ONH as they leave the eye. Our hypothesis can also explain low-tension glaucoma in which the most peripheral GC's are also affected first, because pressure is transmitted without decay due to a reduced uveoscleral (UVS) flow.

Treatment of sheep steroid-induced ocular hypertension with a glucocorticoid-inducible MMP1 gene therapy virus

PURPOSE

To investigate whether intracameral injection of the adenovirus vector AdhGRE.MMP1 would reduce or prevent elevated intraocular pressure (IOP) induced by corticosteroids in living animals.

METHODS

Glucocorticoid-inducible adenovirus vectors carrying wild-type or mutant forms of human metalloproteinase 1 (MMP1 and mutMMP1) cDNAs were generated. An adenovirus carrying no gene (Ad5.CMV.Null) was used as an additional control. Sheep were injected intracamerally with 30 microL of each vector, either previously or after the induction of increased IOP with topical prednisolone or sub-Tenon triamcinolone under various protocols. IOP was measured with a Perkins tonometer. Inflammation was monitored by visual inspection.

RESULTS

In eyes in which IOP was already elevated to 24 to 30 mm Hg, injection of AdhGRE.MMP1 reduced IOP by 70% in 24 hours and to 10 to 13 mm Hg in 48 hours. In eyes with normal IOP (9-11 mm Hg), preinjection of the virus protected against the increase in IOP normally produced by the corticosteroid. IOP remained at a level of approximately 12 mm Hg for 5 days despite the continuous application of the corticosteroid. Injections of the control viruses had no hypotensive effects. There were no signs of ocular inflammation or discomfort to the animals.

CONCLUSIONS

A single dose of a gene therapy vector carrying an inducible metalloproteinase human gene can both protect against the IOP increase produced by corticosteroid instillation in the sheep model and quickly reverse the IOP increase previously elicited by the corticosteroid. These results are a first step toward a treatment of steroid-glaucoma with inducible overexpression of extracellular matrix modulator genes.

Suppression of corticosteroid-induced ocular hypertension in sheep by anecortave

OBJECTIVE

To confirm the ocular hypotensive effects of anecortave acetate on an ovine model for steroid-induced ocular hypertension. Eyes of normal sheep exhibit a robust steroid-induced ocular hypertensive response. Recent observations in an uncontrolled, interventional case series indicated that anecortave elicited hypotensive effects when administered as a sub-Tenon depot in the eyes of a small sample of patients with glaucoma.

METHODS

Intraocular pressure (IOP) was monitored by Perkins applanation tonometry in 16 normal sheep receiving topically administered prednisolone acetate, 0.5%, in both eyes, 3 times daily, a protocol that doubled IOP within 12 days. Half of the sheep had received a unilateral sub-Tenon injection of anecortave in 1 eye prior to the initiation of the bilateral prednisolone instillations, while the 8 remaining sheep received the unilateral anecortave sub-Tenon depot after the IOP was maximally elevated by the prednisolone instillations.

RESULTS

In these 2 sets of experiments, the presence of the anecortave depot suppressed the steroid-induced IOP elevation and reverted the elevated IOP to baseline levels. Measurements of aqueous outflow facility indicated that eyes treated with prednisolone plus anecortave exhibited a 5.8-fold higher outflow facility than the fellow eyes solely exposed to prednisolone, indicating that anecortave prevented the increase in outflow resistance produced by the corticosteroid.

CONCLUSION

Elucidation of the mechanisms of action of anecortave in animal models may prove relevant to the design of novel interventions for the management of primary open-angle glaucoma.

Effects of sildenafil and tadalafil on intraocular pressure in sheep: implications for aqueous humor dynamics

PURPOSE

To determine the effects of vasodilators on intraocular pressure (IOP) and the protein content of sheep aqueous humor (AH), because the vasodilators may increase fluid leakage from the fenestrated capillaries of the ciliary body to the extracellular tissue and directly to the anterior chamber (AC) via the iris, and some senior patients (older than 70) treated with sildenafil have exhibited elevated IOP.

METHODS

Experiments were performed on domestic sheep residing on a ranch in Argentina. These docile and compliant animals readily swallowed tablets of sildenafil (50 and 100 mg) and tadalafil (20 mg). IOP was monitored by Perkins applanation tonometry in 21 normal sheep receiving orally administered drugs. In addition, paracentesis was performed on six sheep to quantify changes in AH protein levels.

RESULTS

Ingestion of both sildenafil and tadalafil increased sheep IOP from normal levels of approximately 9 to 11 mm Hg within 1 hour. The IOP elevation was approximately 1.6-fold with both doses of sildenafil. IOP returned to control values within 4 hours. With the longer-lasting vasodilator tadalafil, IOP remained 1.6- to 1.9-fold higher than normal for at least 48 hours and returned to control levels within 4 days. The AH protein content was approximately 39% higher in sheep given 100 mg sildenafil.

CONCLUSIONS

These data are consistent with a vasodilator-evoked increase in plasma-like fluid in the AC, which likely accounts for the IOP elevation. The results are discussed with a model for AH dynamics that may be of importance to senior individuals treated for vascular diseases with these compounds.

Characterization of serotonergic receptors in rabbit, porcine and human conjunctivae

PURPOSE

To characterize the serotonin (5-HT) receptors linked to the modulation of adenylyl cyclase activity in rabbit, porcine and human conjunctivae.

METHODS

Serotonin receptor-subtype expression was examined using reverse transcription-polymerase chain reaction (RT-PCR) and receptor subtype-specific polyclonal antibodies for the immunofluorescent labeling of conjunctival cryosections. In addition, measurements of the effects of serotonergics on the short-circuit current (I(sc)) across rabbit and porcine conjunctivae were contrasted.

RESULTS

RT-PCR assays indicated the expression of 5-HT(1B ) and 5-HT(1D) receptors, subtypes negatively coupled to adenylyl cyclase, in the rabbit conjunctiva. This approach also suggested the co-expression of 5-HT(1B), 5-HT(1D), 5-HT(1F), 5-HT(4) and 5-HT(7) mRNA's in the porcine conjunctiva, and 5-HT( 1D), 5-HT(1F) and 5-HT(7) in the human conjunctiva. Since the 5-HT(4) and 5-HT(7) receptors are positively linked to adenylyl cyclase, these results implied that the porcine and human tissues exhibited subtypes both positively and negatively linked to the enzyme. However, immunohistochemical observations, using currently available antibodies solely localized the 5-HT(7) moiety in the porcine and human epithelia, suggested that the 1B/1D forms may be minor elements. Consistent with this prospect, 5-HT was a stimulant of the transepithelial I(sc) across the porcine conjunctiva, an opposite response from earlier findings that demonstrated inhibitory effects by 5-HT on the rabbit I(sc), which are now explained by the localization of the 1B/1D receptors in the rabbit stratified epithelium.

CONCLUSIONS

The 5-HT receptors expressed by mammalian conjunctivae are not identical. In terms of 5-HT receptor expression, the porcine tissue may be a more appropriate model for human, than is the rabbit, in that 5-HT may serve as a secretagogue in the human epithelium.

Changes in rabbit and cow lens shape and volume upon imposition of anisotonic conditions

In vivo, mammalian lenses have the capacity to effect fully reversible changes in shape, and possibly volume, during the accommodation process. Isolated lenses also change shape by readily swelling or shrinking when placed in anisotonic media. However, the manner by which the lens changes its shape when its volume is changed osmotically is not firmly established. Putatively, the lens could swell or shrink evenly in all directions, or manifest distinctive swelling and/or shrinking patterns when exposed to anisotonic media. The present study measured physical changes in lenses consistent with the latter alternative using methods we developed for determining rapid changes in lens shape and volume. It was found in isolated rabbit and cow lenses that the length of the axis between the anterior and posterior poles (A-P length) primarily increases under hypotonic conditions (-40 to -100 mOsM), with smaller, or no changes, in equatorial diameter (ED). Hypertonic conditions (+50 to +100 mOsM) on rabbit lenses elicited a predominant reduction in ED, while the A-P length was only marginally reduced. Hypertonic solutions of +150 mOsM were required to obtain similar changes in cow lens shape. The ratio of the A-P length to the ED was taken as a measure of "circularity". This ratio increased gradually in rabbit and cow lenses bathed in hypotonic solutions because of the increase in the A-P length. The calculated lens volume increased in tandem with the increase in "circularity". Lens circularity also increased under hypertonic conditions due to the decrease in ED, but this increase in circularity during shrinkage was not as pronounced as that which occurred during swelling. As such, the lens has a tendency upon swelling to change its shape by approaching the structure of a globular spheroid (as occurs during accommodation for near focusing), but lens shrinkage does not result in a flatter lens with a reduced A-P length as occurs during dis-accommodation for distance focusing. Moreover, osmotically evoked shape changes appear irreversible, in contrast to the mechanically elicited shape changes of accommodation.

Steroid-induced ocular hypertension in normal sheep

PURPOSE

To determine whether the ovine eye develops elevation of intraocular pressure (IOP) in response to corticosteroid applied topically.

METHODS

IOP was monitored by Perkins applanation tonometry in a group of 18 sheep receiving topically administered 0.5% prednisolone acetate in one eye (experimental), three times daily, for a period of 3 or four 4 weeks after the establishment of baseline IOP values. Perkins readings were converted to actual mm Hg using a calibration curve derived from in vivo manometric measurements. IOP was monitored for an additional 1 to 3 weeks after discontinuation of corticosteroid treatment.

RESULTS

Baseline IOP in normal sheep was 10.6+/-1.4 mm Hg (mean +/- SD; n=36 eyes). The IOP of the experimental eyes began to increase after 1 week of prednisolone treatment in all sheep and reached a peak 1 week later (27.5 mm Hg experimental vs. 11.7 mm Hg fellow, control eye; P<0.001). After the discontinuation of corticosteroid instillation, the IOP of the treated eyes declined to the baseline values over the course of 1 to 3 weeks.

CONCLUSIONS

Ovine eyes exhibit a robust steroid-induced ocular hypertensive response, with 100% occurrence in this trial. The mechanisms of steroid-induced glaucoma may be related to those involved in primary open-angle glaucoma and could provide insight into primary open-angle and clues to its treatment.

Inhibitions of chloride transport and gap junction reduce fluid flow across the whole porcine ciliary epithelium

PURPOSE

To study the effects of chloride transport and gap junction inhibitors on fluid formation across the porcine ciliary epithelium.

METHODS

A complete annulus of porcine iris-ciliary body preparation was mounted onto a modified Ussing type chamber to measure the fluid flow (FF) rate. The potential difference (PD) across the preparation was monitored simultaneously. The effects of several inhibitors on chloride transport and gap junction were studied. These included 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), 5-(N,N-dimethyl)amiloride hydrochloride (DMA), bumetanide, niflumic acid, and heptanol.

RESULTS

The average baseline FF rate was 2.56 +/- 0.07 microL/h per preparation (n = 33). DIDS (0.1 mM) or DMA (0.1 mM) showed no effect on both FF and PD when added to the blood side of the preparation. Bumetanide (0.1 mM), on the blood side, inhibited the FF by 46% and caused a slight depolarization of PD. Heptanol (3.5 mM) depolarized the PD and reduced FF by 45% and 78% through the blood and aqueous sides, respectively. Niflumic acid (1 mM at the aqueous side) also depolarized the PD and significantly inhibited the FF (62%).

CONCLUSIONS

The effects of the chloride transport inhibitors on fluid formation across the porcine iris-ciliary body were comparable to that in previous chloride transport studies. The results indicated that fluid secretion by the isolated porcine ciliary epithelium is mainly driven by chloride transport. However, there may be other unidentified ion movements that drive residual FF after chloride transport is inhibited.

Surface change of the mammalian lens during accommodation

Classical theories suggest that the surface area of the crystalline lens changes during accommodation while the lens volume remains constant. Our recent work challenged this view by showing that the lens volume decreases as the lens flattens during unaccommodation. In this paper we investigate 1) the magnitude of changes in the surface of the in vitro isolated cow lens during simulated accommodation, as well as that of human lens models, determined from lateral photographs and the application of the first theorem of Pappus; and 2) the velocity of the equatorial diameter recovery of prestretched cow and rabbit lenses by using a custom-built software-controlled stretching apparatus synchronized to a digital camera. Our results showed that the in vitro cow lens surface changed in an unexpected manner during accommodation depending on how much tension was applied to flatten the lens. In this case, the anterior surface initially collapsed with a reduction in surface followed by an increase in surface, when the stretching was applied. In the human lens model, the surface increased when the lens unaccommodated. The lens volume always decreases as the lens flattens. An explanation for the unexpected surface change is presented and discussed. Furthermore, we determined that the changes in lens volume, as reflected by the speed of the equatorial diameter recovery in in vitro cow and rabbit lenses during simulated accommodation, occurred within a physiologically relevant time frame (200 ms), implying a rapid movement of fluid to and from the lens during accommodation.

IBMX-elicited inhibition of water permeability in the isolated rabbit conjunctival epithelium

Agents expected to increase intracellular cAMP levels were tested on the diffusional water permeability (P(dw)) of isolated rabbit conjunctival epithelia given recent indications of the apical expression of AQP5, a water channel homologue regulated by cAMP in other cell systems. For these experiments, segments of conjunctivae were mounted between Ussing-type hemichambers under short-circuit conditions. Unidirectional water fluxes (J(dw)) were measured by adding (3)H(2)O to one hemichamber and sampling from the other, while the electrical parameters (I(sc) and R(t)) were recorded simultaneously. J(dw) were determined under control conditions and after the introduction of forskolin, dibutyryl-cAMP, rolipram and IBMX. All agents reduced J(dw), with rolipram and IBMX the most effective inhibitors (~28% reduction), while simultaneously evoking stimulations of the I(sc); suggesting that cAMP regulates ionic transport and P(dw) independently. This observation was consistent with the elimination of the IBMX-elicited I(sc) stimulations by the PKA inhibitor, H89, and the ineffectiveness of the sulfonamide in preventing the J(dw) reductions produced by the xanthine. Data from mannitol fluxes and Arrhenius plots indicated that the IBMX-elicited P(dw) reduction occurred at the level of water-transporting channels, but the specific moiety was not identified. Instead it was observed that lipophiles commonly used in other systems to uncouple cellular communication precluded the effects of IBMX on J(dw), but the mechanism for these results was not directly linked to gap-junction blockade in the conjunctiva, as assessed by the transepithelial electrical parameters. Putatively, agents such as heptanol, by also fluidizing the bilayer, may have changed the conformation of a water channel in a manner preventing down-regulation by IBMX. Nevertheless, this study uncovered an apparently unique response to cAMP elevation exhibited by the conjunctiva, namely that P(dw) declines via an H89-insensitive pathway under conditions whereby PKA-dependent electrolyte transport might be over stimulated due to excessive cAMP levels (e.g., PDE inhibition).

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.

Volume change of the ocular lens during accommodation

During accommodation, mammalian lenses change shape from a rounder configuration (near focusing) to a flatter one (distance focusing). Thus the lens must have the capacity to change its volume, capsular surface area, or both. Because lens topology is similar to a torus, we developed an approach that allows volume determination from the lens cross-sectional area (CSA). The CSA was obtained from photographs taken perpendicularly to the lenticular anterior-posterior (A-P) axis and computed with software. We calculated the volume of isolated bovine lenses in conditions simulating accommodation by forcing shape changes with a custom-built stretching device in which the ciliary body-zonulae-lens complex (CB-Z-L) was placed. Two measurements were taken (CSA and center of mass) to calculate volume. Mechanically stretching the CB-Z-L increased the equatorial length and decreased the A-P length, CSA, and lens volume. The control parameters were restored when the lenses were stretched and relaxed in an aqueous physiological solution, but not when submerged in oil, a condition with which fluid leaves the lens and does not reenter. This suggests that changes in lens CSA previously observed in humans could have resulted from fluid movement out of the lens. Thus accommodation may involve changes not only in capsular surface but also in volume. Furthermore, we calculated theoretical volume changes during accommodation in models of human lenses using published structural parameters. In conclusion, we suggest that impediments to fluid flow between the aquaporin-rich lens fibers and the lens surface could contribute to the aging-related loss of accommodative power.

Fluid transport across the isolated porcine ciliary epithelium

PURPOSE

To quantify spontaneous fluid transport across the isolated porcine ciliary epithelium and determine its sensitivity to the electrolyte transport inhibitors ouabain and bumetanide, as well as bath Cl(-) and HCO(3)(-) levels.

METHODS

A complete annulus of ciliary body was mounted in a custom-designed chamber appropriate for quantifying net fluid movement, as well as the transepithelial potential difference (PD) across the in vitro ciliary epithelium.

RESULTS

A spontaneous and stable fluid flow (FF) in the blood-to-aqueous direction was measured over a 4-hour period. This flux solely reflected the secretory activity of the isolated ciliary epithelium (CE), given the absence of externally applied osmotic or pressure gradients. In contrast to FF, the PD declined during the 4 hours in vitro, suggesting that the integrity of the tight junctions may have been compromised during this time so that an increased movement of counter ions via the paracellular pathway could have shunted the PD, while at the same time transcellular fluid transport remained unaffected. The FF in the blood-to-aqueous direction (2.3 +/- 0.2 muL/hr; n = 7) was eliminated by a unilateral reduction in the bath Cl(-) levels on the blood side of the preparation and restored on reintroducing the anion to the bathing medium. This linkage between FF and blood side [Cl(-)] is consistent with the existence of a net Cl(-) flux across the porcine CE in the same direction as the fluid transport. Addition of bumetanide to the blood-side bath inhibited FF by approximately 40%, whereas the removal of CO(2)/HCO(3)(-) from the blood-side bathing solution elicited a approximately 50% reduction in FF. Ouabain inhibited the FF from either side of the preparation, although the effects were more rapid when the glycoside was applied to the blood side of the tissue. Overall, these findings indicate the dependence of FF on active ionic transport by the isolated CE.

CONCLUSIONS

Isolated porcine ciliary epithelial preparations transport fluid in the blood-to-aqueous direction, indicating that measurements of volumetric fluid flow across this preparation may serve as a suitable model for future studies directed toward the pharmacological control of secretion.

Regulation of water permeability in rabbit conjunctival epithelium by anisotonic conditions

Effects of unilateral exposure to anisotonic conditions on diffusional water permeability of the isolated rabbit conjunctiva were determined. A segment of the bulbar-palpebral conjunctiva was mounted between Ussing-type hemichambers under short-circuit conditions. Unidirectional water fluxes ( Jdw) were measured in either direction by adding3H2O to one hemichamber and sampling from the other. Electrical parameters were measured simultaneously. Jdwwere determined under control isosmotic conditions and after introduction of either hyper- or hypotonic solutions against the tear or stromal sides of the preparations. In each of these four separate conditions, the anisotonic medium produced an ∼20–30% reduction in Jdwacross the tissue, with the exception that to obtain such reduction with increased tonicity from the stromal side (medium osmolality increased by adding sucrose), conditions presumptively inhibiting regulatory volume increase mechanisms (e.g., pretreatment with amiloride and bumetanide) were also required. All reductions in Jdwelicited by the various anisotonic conditions were reversible on restoration of control tonicity. In experiments in which preparations were pretreated with the protein cross-linking agent glutaraldehyde, anisotonicity-elicited reductions in Jdwwere not observed. Such reductions were also not observed in the presence of HgCl2, implying the involvement of aquaporins. However, it is possible that the mercurial may be toxic to the epithelium, preventing the tonicity response. Nevertheless, from concomitant changes in transepithelial electrical resistance, as well as [14C]mannitol fluxes, [14C]butanol fluxes, and Arrhenius plots, arguments are presented that the above effects are best explained as a cell-regulated reduction in membrane water permeability that occurs at the level of water-transporting channels. Presumably both apical and basolateral membranes can downregulate their water permeabilities as part of a protective mechanism to help maintain cell volume.

Cl− concentrations of bovine, porcine and ovine aqueous humor are higher than in plasma

Aqueous humor (AH) Cl- levels were measured from live cows and pigs to confirm the Cl(-)-secretory activities found across the ciliary bodies of these species in in vitro experiments. The values obtained were compared to those of the electrolyte in plasma (PL). In addition, Na+ and K+ concentrations were also measured in AH and PL. For analogy, the same measurements were made in sheep and rabbit. The results indicate that the Cl- AH-to-PL ratios of the bovine, porcine and ovine species, but not that of rabbit, are comparable to published values for humans. Thus, the transport activities of the ciliary epithelia of the former animals may be more adequate as a representative model for human than the commonly used rabbit, which exhibits strikingly different transport properties with a ciliary epithelium that is primarily a bicarbonate-transporting tissue.

Spontaneous Fluid Transport across Isolated Rabbit and Bovine Ciliary Body Preparations

PURPOSE

To quantify spontaneous fluid transport across the isolated ciliary bodies of rabbit and bovine and to determine their osmotic permeabilities.

METHODS

A complete annulus of ciliary body was mounted in a custom-designed chamber appropriate for detecting net fluid movement across the in vitro preparation.

RESULTS

A net fluid flow in the blood-to-aqueous direction was measured. It was generally observed that tissue freshness is a critical parameter for detection of such flow. The spontaneous, baseline fluid transport rate lasted, on average, approximately 4 hours. This flow solely reflects the secretory activity of the isolated ciliary epithelium, since the in vitro arrangement precludes contributions from ultrafiltration. Both the isolated rabbit and bovine ciliary body epithelia transported fluid in the absence of an external osmotic or pressure gradient. After corrections for area and possible collapse of the processes, a total flux rate of approximately 23 microL/hour or 13% of the in vivo flow in rabbit was estimated. This value agrees with predictions of ionic fluxes and short-circuit current measurements, which are also obtained in vitro. The fluid flow is bicarbonate dependent in rabbit and chloride dependent in bovine, consistent with ionic transport mechanisms described in these species. Ouabain inhibited the fluid flow across both species, indicating dependence on active ionic transport. Irrespective of the spontaneous fluid transport, a flow elicited by an osmotic gradient allowed for a calculation of the osmotic permeability coefficient (P(f); approximately 10(-3) cm/s) in line with reports in other epithelia. In addition, mannitol permeability (5.6 x 10(-6) cm/sec) was similar to that measured in "tight" epithelia, as determined by measurements of radiolabeled fluxes of the sugar across rabbit ciliary bodies mounted in the chambers used for the present fluid transport study.

CONCLUSIONS

This work demonstrates that isolated ciliary epithelial preparations transport fluid in the blood-to-aqueous direction. The present observations suggest that mounting arrangements for measuring volumetric fluid flow across the ciliary epithelium is suitable for future studies directed toward the pharmacological control of secretion.

Cl- secretory effects of EBIO in the rabbit conjunctival epithelium

Experiments were conducted to determine whether the Cl- secretagogue, 1-ethyl-2-benzimidazolinone (EBIO), stimulates Cl- transport in the rabbit conjunctival epithelium. For this study, epithelia were isolated in an Ussing-type chamber under short-circuit conditions. The effects of EBIO on the short-circuit current (I(sc)) and transepithelial resistance (R(t)) were measured under physiological conditions, as well as in experiments with altered electrolyte concentrations. Addition of 0.5 mM EBIO to the apical bath stimulated the control I(sc) by 64% and reduced R(t) by 21% (P < 0.05; paired data). Under Cl(-)-free conditions, I(sc) stimulation using EBIO was markedly attenuated. In the presence of an apical-to-basolateral K+ gradient and permeabilization of the apical membrane, the majority of the I(sc) reflected the transcellular movement of K+ via basolateral K+ channels. Under these conditions, EBIO in combination with A23187 elicited nearly instantaneous 60-90% increases in I(sc) that were sensitive to the calmodulin antagonist calmidazolium and the K+ channel blocker tetraethyl ammonium. In the presence of an apical-to-basolateral Cl- gradient and nystatin permeabilization of the basolateral aspect, EBIO increased the Cl(-)-dependent I(sc), an effect prevented by the channel blocker glibenclamide (0.3 mM). The latter compound also was used to determine the proportion of EBIO-evoked unidirectional 36Cl- fluxes in the presence of the Cl- gradient that traversed the epithelium transcellularly. Overall, EBIO activated apical Cl- channels and basolateral K+ channels (presumably those that are Ca2+ dependent), thereby suggesting that this compound, or related derivatives, may be suitable as topical agents to stimulate fluid transport across the tissue in individuals with lacrimal gland deficiencies.

Chloride channel gene expression in the rabbit cornea

PURPOSE

The maintenance of stromal hydration by the corneal endothelium relies on active transendothelial anion transport, with bicarbonate and chloride the major anions carrying the current. However, the ion transport pathways that operate to maintain stromal hydration have yet to be fully elucidated.

METHODS

We used RT-PCR to identify the gene expression profile of members of the ClC family of chloride channels in freshly isolated samples of rabbit corneal endothelium, stroma, and epithelium. The expression of a separate group of genes was also examined to confirm the purity of the sample collection protocol. The expression of the ClC-2 and ClC-3 channel protein in the cornea was also evaluated by light and electron microscopic immunolabelling.

RESULTS

The mRNA for ClC-2, ClC-3, ClC-5, ClC-6, and ClC-7 were expressed in both the corneal epithelium and endothelium, and in the stroma. The mRNA for the skeletal muscle specific channel ClC-1 and the kidney specific chloride channel ClC-Ka were not detectable. ClC-4 mRNA was not detected in any rabbit tissue examined. The expression pattern of the mRNAs for collagens V, VI, VII, and VIII demonstrated the absence of contamination in epithelial and endothelial samples. ClC-2 and ClC-3 immunolabelling confirmed the presence of these proteins in corneal endothelium, stroma, and epithelium.

CONCLUSIONS

Together with cystic fibrosis transmembrane conductance regulator (CFTR) and calcium activated chloride channel-1 (CLCA1), these results bring the number of chloride channel genes known to be expressed in the corneal endothelium and epithelium to seven. These channels are likely to be important for the maintenance of corneal transparency.

Steroid-induced ocular hypertension in normal cattle

OBJECTIVES

To determine whether the bovine eye develops elevation of intraocular pressure (IOP) in response to topical corticosteroid use and to develop a reliable model of steroid-induced elevation of IOP in an animal.

METHODS

Intraocular pressure was monitored by Perkins applanation tonometry in a group of 12 cows receiving topically administered prednisolone acetate in 1 eye 3 times a day for a period of 49 days after the establishment of baseline IOP values. Perkins readings were converted to IOP in mm Hg using calibration curves derived from in vitro cannulation manometric experiments and validated with in vivo manometric measurements. Intraocular pressure was also monitored for 50 days after the discontinuation of corticosteroid therapy.

RESULTS

Intraocular pressure began to increase after 3 weeks of treatment in 100% of the cow eyes receiving corticosteroid and reached a peak 1 week later. Peak interocular IOP differences between the corticosteroid-treated eye and the fellow control eye reached up to 15 mm Hg and began to decline after the discontinuation of treatment but remained significantly elevated for a period of 3 more weeks.

CONCLUSIONS

Bovine eyes exhibit a robust steroid-induced ocular hypertensive response, with 100% occurrence in this trial. The IOP elevation caused by corticosteroid slowly subsides after discontinuation of treatment. Clinical Relevance The mechanisms of steroid-induced glaucoma may be related to those involved in primary open-angle glaucoma and could provide the clues to elucidate the pathogenesis of the latter. The high prevalence of corticosteroid-induced elevation of IOP in the cow and the large amount of tissue available will permit studies on the mechanism of this phenomenon not previously possible.

Electrolyte and fluid transport across corneal, conjunctival and lens epithelia

All ocular epithelia examined to date transport fluid as a consequence of a sufficiently high water permeability bestowed by endogenous water channels (aquaporins) and transepithelial solute movement due to active transport mechanisms. This article provides a synopsis of the current understanding of electrolyte and fluid transport across corneal, conjunctival and lens epithelia.

Beta-adrenergic inhibition of rabbit lens anterior-surface K(+) conductance

PURPOSE

To characterize the effects of cAMP-elevating stimuli on the rabbit translens electrical parameters and examine the distribution of beta adrenoceptors about the epithelial surface.

METHODS

The electrophysiological experiments encompassed the isolation of lenses within a vertically arranged, Ussing-type chamber under short-circuit conditions, an approach that allowed for measurements of short-circuit current (I(sc)) across, in separate experiments, discrete surface regions. Epithelial beta receptors were localized by immunofluorescent labeling of lens cryosections primarily exposed to a polyclonal antibody against human beta( 2)-adrenoceptors. Reverse transcription - polymerase chain reaction (RT-PCR) was used to generate cDNA (using specific primers based upon the sequence of the previously cloned human beta(2) receptor) from rabbit lens RNA extracted from mechanically sequestered anterior and equatorial epithelial cells.

RESULTS

Asymmetrical I(sc) reductions with increases in translens resistance were elicited with epinephrine, isoproterenol, terbutaline, forskolin, and a lipid-permeable cAMP analogue. Electrical changes were recorded across the anterior aspect and not observed when the above compounds were applied to solutions bathing the equatorial and posterior surfaces. Immunohistochemical observations indicated the expression of beta receptors from the anterior epithelium to the equatorial region. RT-PCR yielded cDNA of expected basepair length for the apparent fragment of the beta(2)-adrenoceptor, which exhibited a sequence homology 90% identical with its human equivalent in both the anterior and equatorial epithelia.

CONCLUSIONS

The cAMP-sensitive conductance(s) appear limited to the anterior epithelium and undetectable equatorially. The asymmetrical I(sc) responses do not seem to arise from a spatial heterogeneity in epithelial receptor expression.

Beta-adrenergic stimulation of Na(+)-K(+)-2Cl(-) cotransport activity in the rabbit lens

Experimental maneuvers known to increase cellular cAMP levels evoked a stimulation in the K(+) influx across the anterior surfaces of isolated rabbit lenses, as measured by 86Rb(+) uptake. For this, the lenses were mounted in a modified Ussing-type chamber and exposed to the radiolabel under short-circuit conditions. The enhanced, cAMP-elicited flux was attributed to the basolateral Na(+)-K(+)-2Cl(-) cotransporter given its preclusion by bumetanide, a highly selective inhibitor of this symport, and the ineffectiveness of ouabain in mitigating the stimulation. The ouabain- plus bumetanide-insensitive K(+) uptake, which is about 10% of the total influx and represents passive entry of the radiolabel, was not affected by cAMP-elevating conditions. Forskolin, an activator of adenylyl cyclase; epinephrine, a non-selective adrenergic agonist; and the beta-selective agents, isoproterenol and terbutaline, were among the drugs used to elicit the increase in bumetanide-sensitive K(+) inflow. In experiments with isoproterenol, the stimulated influx evoked by the agonist was inhibited in lenses simultaneously exposed to propranolol. Other observations included that the stimulation of bumetanide-sensitive K(+) influx with forskolin was eliminated in lenses pretreated with the protein kinase inhibitors, staurosporine or H-89. However, these drugs were ineffective in preventing the increased influx produced by calyculin A, a phosphatase inhibitor, suggesting modulation of the cotransporter by at least two independent pathways. The cAMP-generating stimuli also produced an inhibition of the short-circuit current across the lens and an increase in translens resistance. These latter effects suggest that cAMP elevation also evokes an inhibition in an epithelial conductance(s) simultaneously to the stimulation of the cotransporter. As such, this study provides the first indication for the regulation of lens transport by adrenoceptors, presumably of the beta-2 subtype.

Cl secretagogues reduce basolateral K permeability in the rabbit corneal epithelium

The stromal-to-tear transport of Cl by the rabbit corneal epithelium is increased by pharmacological effectors (secretagogues) that raise cAMP. It is well established that such secretagogues increase the apical membrane permeability to Cl and thus facilitate the efflux of the anion. However, we and others have found that cAMP-elevating agents frequently decrease the transepithelial potential difference across the rabbit cornea. The mechanism underlying this latter phenomenon had not been characterized. In this report, transepithelial and microelectrode studies were combined with measurements of unidirectional fluxes of 36Cl, 22Na and 86Rb to show that secretagogues known to act via cAMP also decrease the K permeability of the basolateral membrane, which by cellular depolarization would decrease apical Cl secretion. This effect was increasingly pronounced as a function of concentration when agents (e.g., epinephrine, isoproterenol) were applied to the apical side of the preparations. The addition of these agonists to the basolateral bathing solution, or of forskolin to the apical side, solely elicited inhibitions of basolateral K permeability. It seems that apical Cl and basolateral K conductances are independently and inversely regulated by cAMP. The opposite effects that cAMP could have on fluid secretion and epithelial thickness, by increasing apical Cl permeability but decreasing basolateral K permeability, may serve as a mechanism to maintain epithelial thickness within a narrow range.

Distribution of acetylcholine-sensitive currents around the rabbit crystalline lens

The relative distribution of acetylcholine (ACh) receptors on the surface of the isolated ocular lens of the rabbit was determined from induced changes in translens short-circuit current (I(SC)) and the translenticular resistance (R(t)) at seven delineated, parallel zones from the anterior to the posterior pole. For this, one O-ring (from among several having different diameters) was used to separate two zones in a vertically arranged Ussing-type chamber. Different O-rings separated different zone pairs. Earlier experiments from this laboratory used a conventional divided chamber, which occluded the equatorial surface, to demonstrate that anterior applications of ACh transiently decreased the I(SC) due to an intracellular Ca(2+) release and inhibition of anteriorly located K(+) channels. Measurements obtained with the newly designed zonal arrangement determined that the entire epithelial surface from its anterior-most aspect to the equatorial region responds electrically to ACh exposure, while the posterior-most region does not. Furthermore, lens-mounting positions that resulted in separation of the epithelium so that portions of its surface were present in each hemichamber resulted in inverse current changes upon bilateral ACh addition to the bathing solutions. Reductions in outward cationic current across the anterior surface into the anterior bath upon ACh treatment were accompanied by an increase in translens resistance consistent with a closure of basolateral K(+) channels. Overall, these results suggest that the posterior fiber cells may lack ACh receptors, which are clearly present in the lens epithelium that covers about two-thirds of the rabbit lens surface area, and indicate that an ACh-evoked Ca(2+) signal does not spread throughout the epithelial layer. A functional role for lens acetylcholine receptors remains to be determined.

Presence of CFTR in the conjunctival epithelium

PURPOSE

To determine the presence of the cystic fibrosis transmembrane conductance regulator (CFTR) in the conjunctiva and examine the possibility of its regional expression in rabbit, rat and porcine conjunctival epithelia given distinct differences in morphological appearance between the bulbar and palpebral epithelia.

METHODS

Two specific anti-CFTR antibodies, against different epitopes in the R domain of the CFTR molecule were used in immunofluorescent labeling of frozen fixed sections isolated from the bulbar and palpebral regions of fresh rabbit, porcine and rat conjunctivae. CFTR expression was also determined in the rabbit conjunctival epithelium using RT-PCR methods.

RESULTS

CFTR immunoreactivity in the conjunctival epithelium exhibits polarized expression and is associated with the apical domain of conjunctival epithelial cells. An identical pattern of staining obtained in porcine cryosections using either of the anti-human CFTR antibodies confirmed the specificity of the positive apical staining. RT-PCR analysis produced bands at the predicted size for CFTR mRNA transcripts in both bulbar and palpebral portions of the rabbit conjunctival epithelium.

CONCLUSIONS

Apical localization of CFTR in the conjunctival epithelium is consistent with the function of this protein as a chloride channel or as a regulator of channel activity. The identification of CFTR in both bulbar and palpebral portions of the conjunctiva provides evidence that the mechanisms for Cl secretion reside throughout the conjunctiva. This finding suggests that manipulations of the CFTR Cl channel could affect transepithelial Cl transport rates and water movement into the tear film.

Regional distribution of the Na(+) and K(+) currents around the crystalline lens of rabbit

Early studies described asymmetrical electrical properties across the ocular lens in the anterior-to-posterior direction. More recent results obtained with a vibrating probe indicated that currents around the lens surface are not uniform by showing an outwardly directed K(+) efflux at the lens equator and Na(+) influx at the poles. The latter studies have been used to support theoretical models for fluid recirculation within the avascular lens. However, the existence of a nonuniform current distribution in the lens epithelium from the anterior pole to the equator has never been confirmed. The present work developed a modified short-circuiting technique to examine the net flows of Na(+) and K(+) across arbitrarily defined lens surface regions. Results indicate that passive inflows of Na(+) occur at both the anterior polar region and posterior lens surface, consistent with suggestions derived from the vibrating probe data, whereas K(+) efflux plus the Na(+)-K(+) pump-generated current comprise the currents at the equatorial surface and an area anterior to it. Furthermore, Na(+)-K(+) pump activity was absent at the posterior surface and its polar region in all lenses examined, as well as from the anterior polar region in most lenses. The latter unexpected observation suggests that the monolayered epithelium, which is confined to the anterior surface of the lens, does not express an active Na(+)-K(+) pump at its anterior-most aspect. Nevertheless, this report represents the first independent confirmation that positive currents leave the lens around the equator and reenter across the polar and posterior surfaces.

Localization of a Na(+)-K(+)-2Cl(-) cotransporter in the rabbit lens

Earlier work from this laboratory demonstrated a bumetanide-inhibitable K(+) uptake activity in cultured bovine lens epithelial cells, but not at the anterior surfaces of intact bovine lenses isolated in an Ussing-type chamber. Presently the distribution of the bumetanide-sensitive Na(+)-K(+)-2Cl(-) cotransporter within the lens was re-examined. To complement previous results, (86)Rb(+) uptake experiments were done in a chamber design that limited exposure of the radiolabel to specific surfaces of rabbit lenses under short-circuit conditions. In addition, the cotransporter protein (NKCC1, but not NKCC2) was immune-detected in Western blots. For the latter, membrane preparations were obtained from capsule-plus-epithelial specimens, and from three cortical fractions, i.e. the anterior, equatorial, and posterior regions, as well as a fifth, nuclear fraction. K(+) influxes across the anterior-polar, equatorial, and posterior-polar surfaces were 0.375, 0.348 and 0.056 microEq (hr cm(2))(-1) respectively, rates that were not significantly reduced by the presence of 0.1 mM bumetanide (P > 0.15, as unpaired data). In contrast, bumetanide-sensitive K(+) influx rates were measured across the anterior and equatorial surfaces under hypertonic, but not under hypo-osmotic conditions. In culture, bumetanide and ouabain were equipotent in reducing by approximately half the K(+) uptake of quiescent, rabbit lens epithelial cells under control, iso-osmotic conditions, indicating a cell-culture induced up-regulation of the cotransport activity by an undetermined mechanism. The immunoblotting of lens membrane proteins elicited approximately 170-180 kDa bands accordant with the identity of the NKCC1 isoform in the epithelial and cortical equatorial fractions. Thus, NKCC1 was readily demonstrated using membrane specimens taken from within the lens. Its activity in the intact organ may be activated by conditions fostering cell shrinkage, and perhaps, agents stimulating epithelial cell elongation, given its distribution within the lens.

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.

Identification and localization of acid-base transporters in the conjunctival epithelium

Acid-base transporters of rabbit and porcine conjunctival epithelia were identified and localized with immunoblotting and immunohistochemical techniques using specific antibodies against carriers commonly found in epithelia, i.e. the Cl(-)/HCO3(-)exchanger (AE2), Na(+)/H(+) exchanger (NHE-1, -2, -3) and the electrogenic Na(+)-(n)HCO3(-) cotransporter (NBC). Western blot analysis demonstrated that anti-AE2 reacted with an approximate 170 kDa protein in both rabbit and pig cell membranes prepared from separately isolated bulbar and palpebral conjunctivae. NHE1 was similarly identified in these distinct conjunctival regions but results with anti-NBC were ambiguous. Histochemical examinations indicated that the AE2 and NHE1 proteins reside on the basolateral surfaces of the plasma membrane throughout the multilayered tissue. The immunostaining of porcine cryosections for AE2 and rabbit sections for NHE1 was specific, because of its abolishment following either pre-absorption with the corresponding peptide or omission of the primary antibody. Screening with anti-NBC produced weak staining of the sections that appeared to be non-specific. For confirmation of these results, the acid-base transporters present in rabbit cell cultures of conjunctival epithelia were ascertained from the changes in intracellular pH (pH(i)) evoked upon sequential superfusion with media of altered composition. This approach readily obtained Na(+)- and Cl(-)-dependent pH(i)effects consistent with the existence of Cl(-)/HCO3(-) and Na(+)/H(+)exchange activities. Evidence for the presence of NBC could not be acquired, thereby substantiating the observations from the immunodetection techniques. The identity and location of the antiporters that were found suggested that these elements could contribute to transcellular Cl(-)transport in the basolateral-to-apical direction. To test this possibility, the effects of AE and/or NHE inhibition were determined on the bumetanide-insensitive Cl(-)-dependent short-circuit current across rabbit conjunctivae freshly isolated in Ussing-type chambers. These experiments revealed that such current is indeed sustained by the antiporters. Results with acetazolamide further suggested that the contribution of the acid-base transporters towards transepithelial Cl(-)secretion is variable and dependent upon individual rates of metabolic CO(2)production. Overall, the present study provides an initial identification of the acid-base transporters present in the conjunctiva. Besides their likely role in intracellular pH regulation, the parallel, basolateral expression of AE2 and NHE1 indicates that these elements do not directly contribute to the pH of the tear film but may complement the Na(+)-2Cl(-)-K(+)cotransporter in effectuating Cl(-)secretion.

Serotonin-elicited inhibition of Cl(-) secretion in the rabbit conjunctival epithelium

The effects of serotonin [5-hydroxytryptamine (5-HT)] on the transepithelial electrical properties of the short-circuited rabbit conjunctiva were examined. With this epithelium, the short-circuit current (I(sc)) measures Cl(-) secretion plus an amiloride-resistant Na(+) absorptive process. Apical addition of 5-HT (10 microM) elicited a prompt I(sc) reduction from 14.2 +/- 1.2 to 10.9 +/- 1.2 microA/cm(2) and increased transepithelial resistance from 0.89 +/- 0.05 to 1.03 +/- 0.06 kOmega. cm(2) (means +/- SE, n = 21, P<0.05). Similar changes were obtained with conjunctivae bathed without Na(+) in the apical bath, as well as with conjunctivae preexposed to bumetanide with the Cl(-)-dependent I(sc) sustained by the parallel activities of basolateral Na(+)/H(+) and Cl(-)/HCO(3)(-) exchangers. In contrast, the 5-HT-evoked effects were attenuated by the absence of Cl(-) (DeltaI(sc) = -0.5 +/- 0.2, n = 5), suggesting that reduced Cl(-) conductance(s) is an effect of 5-HT exposure. In amphotericin B-treated conjunctiva and in the presence of a transepithelial K(+) gradient, 5-HT addition reduced K(+) diffusion across the preparation by 13% and increased transepithelial resistance by 4% (n = 6, P < 0.05), indicating that an inhibition in K(+) conductance(s) was also detectable. Significant electrical responses also occurred under physiological conditions when 5-HT was introduced to epithelia pretreated with adrenergic agonists or protein kinase C, phospholipase C, phosphodiesterase, or adenylyl cyclase inhibitors or after perturbation of Ca(2+) homeostasis. Briefly, the conjunctiva harbors the only known Cl(-)-secreting epithelium in which 5-HT evokes Cl(-) transport inhibition; receptor subtype and signal transduction mechanism were not determined.

Immunolocalization of Na-K-ATPase, Na-K-Cl and Na-glucose cotransporters in the conjunctival epithelium

PURPOSE

To investigate possible regional expression of transport systems in the conjunctival epithelium given distinct differences in morphological appearance between the bulbar and palpebral epithelia as well as variations found in the proportions of Na absorptive versus Cl secretory activities in electrophysiological studies.

METHODS

Mouse monoclonal antibodies against the alpha1-subunit of Na-K-ATPase and Na-K-Cl cotransporter (NKCC1) and a rabbit polyclonal antibody against the Na-glucose cotransporter (SGLT1) were used in immunoblotting and immunofluorescent labeling of frozen fixed sections isolated from either the bulbar and palpebral regions of the conjunctiva.

RESULTS

Western blot analysis clearly demonstrated the presence of Na-K-ATPase, NKCC1 and SGLT1 proteins in both bulbar and palpebral conjunctiva. Indirect immunofluorescence studies on bulbar and palpebral conjunctival portions revealed intense staining by the anti-NKCC1 and anti-alpha1-Na-K-ATPase antibodies exclusively at the basolateral surfaces, whereas the anti-SGLT1 antibody was used with porcine conjunctiva to elicit strong and unambiguous staining along the apical plasma membrane.

CONCLUSIONS

Proteins that mediate the transport activities of the Na-K-ATPase and Na-K-Cl cotransporter are uniformly distributed throughout the palpebral and bulbar regions of the rabbit conjunctival epithelium. Although the Na-glucose cotransporter could be detected in immunoblots of the rabbit, this cotransporter appears to be uniformly distributed as well, based upon immunohistochemical sections of the pig conjunctiva. Thus, it appears likely that mechanisms for Cl secretion and Na absorption reside in both bulbar and palpebral segments of the conjunctival epithelium.

Cyclic AMP-dependent stimulation of basolateral K(+)conductance in the rabbit conjunctival epithelium

The regulation of Na(+)transport by cAMP in freshly isolated rabbit conjunctival epithelium, a tissue exhibiting both Cl(-)secretion and Na(+)absorption, was examined. Bulbar-palpebral segments of conjunctiva were mounted between Ussing-type hemichambers under short-circuit conditions in Cl(-)free media. In this situation, the short-circuit current (I(sc)) measures an amiloride-resistant Na(+)absorptive process in the apical-to-basolateral direction. Apical additions (each at 10 microm) of cAMP-elevating compounds, forskolin, rolipram, IBMX and epinephrine all stimulated the Na(+)-dependent I(sc)by approximately 3.5-4.5 microA cm(-2)(minimal 40% increase) and reduced transepithelial resistance (R(t)) by at least 7% (P<0.05). Pre-exposure (1 hr) to the protein kinase A (PKA) inhibitor H-89 (10 microm), which in itself inhibited the I(sc)by 0. 5 microA cm(-2), attenuated the I(sc)responses of the cAMP-elevating agents (P< 0.05, unpaired data). In reverse, H-89 promptly decreased the I(sc)by 1.5-2.5 microA cm(-2)and increased R(t)by 5% (P<0.05) in tissues pre-stimulated with either forskolin or an epinephrine plus IBMX combination. Additions of epinephrine or rolipram to apically permeabilized preparations using amphotericin B, increased the I(sc)by 12 and 22% respectively over baseline and reduced R(t)by 6% (P<0.05). Similarly, in the presence of a transepithelial K(+)gradient (apical to basolateral) and amphotericin B, cAMP elevation stimulated K diffusion across the preparation by at least 1.8 microA cm(-2)and decreased the R(t)by 4% (P<0.05), changes that were reversed by subsequent H-89 addition. Under Cl(-)rich conditions, pretreatment with 5 m m Ba(2+)reduced the basal I(sc)by 59% and blocked the cAMP-induced I(sc)stimulations typically seen in the presence of the anion. The results provide evidence for a PKA-regulated, Ba(2+)-inhibitable (voltage insensitive) basolateral K(+)conductance in rabbit conjunctival epithelial cells. The action of Cl(-)secretogogues acting via cAMP on basolateral K(+)channel activity indicates that endogenous levels of cAMP may play a role in the regulation of Cl(-)secretion and Na(+)absorption in the conjunctiva.

Phorbol ester modulation of active ion transport across the rabbit conjunctival epithelium

Protein kinase C (PKC) activation elicits diverse cell-type specific effects on key epithelial transporters. The present work examined the influence of phorbol esters, which are known activators of PKC isoenzymes, on the short-circuit current (Isc), a direct measure of net transcellular electrolyte transport, of the rabbit conjunctiva. In this preparation, the Iscmeasures a Na+-dependent, bumetanide-inhibitable Cl-transport in the basolateral-to-apical direction plus an amiloride-resistant Na+absorptive process in the opposite direction. Additions of phorbol 12-myristate-13-acetate (PMA) to the basolateral bathing media did not affect the transepithelial electrical parameters; but its introduction to the apical bath at 1 and 10 micrometers elicited a transient ( approximately 2 min duration) Iscspike followed by a sustained reduction relative to the control level. Such PMA-elicited Iscreductions were from 14. 0+/-2.0 to 3.1+/-0.8 microA cm-2(+/-s.e.m.'s, n =3) at 1 micrometer and from 16.5+/-1.9 to 4.6+/-0.7 microA cm-2(n =22) at 10 micrometers. The former concentration failed to produce extensive Iscreductions in 3 other experiments. Similar results were obtained with phorbol 12, 13-dibutyrate (PDBu). Its apical administration at 0.1 micrometer reduced the Iscfrom 18.5+/-4.1 to 7.8+/-2.0 (n =3), and from 16. 5+/-2.9 to 6.9+/-1.2 (n =7) when introduced at 1 micrometer. The phorbol-evoked Iscreductions occurred without a simultaneous change in transepithelial resistance (Rt). However, after about 15-20 min, Rtgradually declined by about 25%. In contrast to these results, treatment with a phorbol ester known not to activate PKC (4-alpha-PMA) did not affect the electrical parameters when added at 10 micrometers. PMA- and PDBu-evoked Iscreductions could be obtained with conjunctiva that were (1) pretreated with bumetanide, (2) bathed in Cl--free media, and (3) pretreated with amphotericin B, changes consistent with a likely inhibition of the basolateral Na+/K+pump. Such Iscinhibitions were attenuated with conjunctiva pre-exposed to 1 micrometer staurosporine, a nonselective kinase inhibitor known to suppress PKC activity. Staurosporine, in itself, produced a rapid 26% Iscinhibition (n =15) along with a 17% Rtincrease upon its apical introduction. These electrical responses were less extensive in Cl--free media and absent in amphotericin B-treated conjunctiva, suggesting the presence of a kinase-mediated regulation of apical channels for both Na+and Cl-. Overall, these results imply that in addition to previously demonstrated epinephrine-elicited, up-regulation of Cl-secretion, mechanisms may also exist, via PKC activation, to suppress Na+/K+pumping and consequently reduce transepithelial transport rates.

Reduction in water permeability of the rabbit conjunctival epithelium by hypotonicity

The effects of unilateral exposure to hypotonic media on the diffusional water permeability of the isolated rabbit conjunctiva were determined. For these experiments, a segment of the bulbar-palpebral conjunctiva was mounted between Ussing-type hemichambers under short-circuited conditions. Unidirectional diffusional water fluxes (Jdw) were measured in either direction by adding 3H2O to one hemichamber and sampling from the other. Electrical parameters were measured simultaneously. Jdw were determined in control isosmotic conditions and after dilution of one of the bathing solutions from 290 to 108 mOsMolar. This hypotonic condition reduced Jdw by 25-30% (n = 17) when applied basolaterally and by 25% (n = 6) apically. The effects were reversible and were also obtained when the opposite bathing solution contained amphotericin B, selectively permeabilizing the contralateral cell surface. From concomitant changes in transepithelial electrical resistance as well as 14C-mannitol fluxes completed under identical conditions, arguments are presented that the above effect is best explained as a cell regulated reduction in membrane water permeability. Presumably both apical and basolateral membranes can down-regulate their water permeabilities. This response, suggesting a protective mechanism to help maintain cell volume from hypotonicity, was also seen in other studies using the amphibian bladder and the frog cornea, in which the effect was only obtained basolaterally. Thus, regulation of epithelial water permeability appears to be a basic trait common to both amphibians and mammals, although tissue differences exist.

Reduction of water permeability by anisotonic solutions in frog corneal epithelium

PURPOSE

To study the effects of bathing solution osmolarity and Cl- secretagogues on the diffusional water permeability (Pdw) of the isolated frog corneal epithelium.

METHODS

Isolated frog corneas, with the endothelium scraped off, were mounted as a partition between Ussing-type hemichambers. Unidirectional diffusional water fluxes (Jdw) were measured by adding 3H2O to one hemichamber and sampling from the other. Electrical parameters were measured simultaneously. Jdw was determined in control isosmotic conditions and after either changes in osmolarity of the bathing solutions or the additions of amphotericin B, epinephrine, Ca2+ ionophore, and other agents.

RESULTS

Apical addition of 0.5 mM HgCl2 elicited an 11-fold increase in paracellular conductance and inhibited Jdw by 36%, suggesting that Jdw was predominantly transcellular and that there was a negligible contribution of the paracellular pathway. Pretreatment of corneas with 2-mercaptoethanol prevented the effects of Hg2+ on the paracellular conductance and Jdw. A hypotonic medium on the basolateral side reversibly reduced Jdw proportionately to the reduction in osmolarity, with 40 mOsm exerting a 29% decrease. Results from an Arrhenius plot suggest that water channels closed under this condition. Apical hypertonicity (350 mOsm) reduced Jdw by approximately 12%. Basolateral hypertonicity (450 mOsm), after permeabilization of the apical membrane with amphotericin B, reduced Jdw by 15%. Epinephrine was the only Cl- secretagogue that reduced Jdw, on average by 12%. This effect, which was also observed with amphotericin B-treated corneas, was not mediated by classical beta-receptors based on the results obtained with isoproterenol and propranolol.

CONCLUSIONS

Changes in basolateral osmolarity or the presence of an apical hypertonic solution decreased the diffusional water permeability (Pdw) of the corneal epithelium. Epinephrine also decreased Pdw, and this effect was localized to the basolateral membrane. The similarities, of a sequence motif found in potassium channels and beta-adrenergic receptor kinases that are regulated by the beta gamma subunit of G proteins with that found in aquaporins 2 and 5, could explain the link with epinephrine. Regardless of the mechanism, these results indicate that corneal epithelial water permeability can be regulated, presumably to protect cell volume from changes in solution osmolarity.

Potassium current oscillations across the rabbit lens epithelium

Rabbit lenses expressing spontaneous oscillations in translens short-circuit current (Isc) are obtained somewhat frequently, with this phenomenon observed in approximately 30% of isolated lenses as described earlier (Exp. Eye Res. 61, 129-140, 1995). Since pharmacological protocols to consistently elicit Isc oscillations were not found, characterizations of the underlying transport processes have been limited to the application of various inhibitors on the spontaneous phenomenon. The present report extends the initial observations by confirming that oscillations are immediately inhibited upon the anterior addition of the Ca2+ channel blocker nifedipine (10 microM), and by demonstrating that other treatments which should affect epithelial Ca2+ homeostasis are also inhibitory (e.g., Bay K 8644 (10 microM), diltiazem (10 microM), EGTA (2 mm), and Ca2+-free media). Furthermore, Isc oscillations are immediately inhibited by the K+ channel blocker, Ba2+, but not by the Na+-K+ pump inhibitor, ouabain. The intracellular Ca2+ mobilizing agents thapsigargin (0.1 microM) or acetylcholine (1 microM) modified but did not permanently inhibit the oscillations, confirming earlier observations. At 50 microM, however, acetylcholine addition was inhibitory, but reversible, for oscillations restarted upon its subsequent removal. In addition, lens oscillations were also characterized under open-circuit conditions with microelectrodes inserted in the superficial cells near the equator of lenses isolated in a divided chamber. The potential difference (PD) across each lens face was recorded, as was the translens PD (PDt), which equals the difference between the PDs across each lens surface. Oscillations in PDt were obtained in 7 of 26 lenses. The oscillations arose only from an oscillation in the PD across the anterior face (PDa). While PDa and PDt oscillated with the same amplitude (approximately 12 mV) and period (approximately 70 sec), the PD across the posterior surface remained stable. During these oscillations the conductance of the anterior surface was maximal at the most positive voltage of the anterior bath with respect to the lens interior (46 mV), whereas, minimal conductance occurred at the least positive PDa (34 mV). Overall, these observations are consistent with the likely presence of voltage-operated Ca2+ channels in parallel with various Ca2+-sensitive K+ channels in the epithelial basolateral membrane. A model to explain the oscillatory pattern across the anterior face while the PD across the posterior face remains unaltered is presented.