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

[Features of water-electrolyte component of the tear fluid]

Tear production is a complex multi-step process that can be arbitrarily divided into three stages: «primary» secretion by the acinar cells of the main lacrimal glands, formation of «secondary» lacrimal fluid in the ducts of the main lacrimal glands, and «tertiary» modification of the tear composition in the conjunctival sac. This article highlights mechanisms of water and electrolytes secretion in the process of tear fluid production and describes the particularities of distribution of the membrane transport proteins in the lacrimal gland and the ocular surface.

Association between tear and blood glucose concentrations: Random intercept model adjusted with confounders in tear samples negative for occult blood

AIMS/INTRODUCTION

To prevent diabetic complications, strict glucose control and frequent monitoring of blood glucose levels with invasive methods are necessary. We considered the monitoring of tear glucose levels might be a possible method for non-invasive glucose monitoring. To develop tear glucose monitoring for clinical application, we investigated the precise correlation between the blood and tear glucose concentrations.

MATERIALS AND METHODS

A total of 10 participants and 20 participants with diabetes were admitted, and blood and tear samples were collected. Before statistical analysis, we eliminated tear samples contaminated with blood. We observed the daily blood and tear glucose dynamics, and carried out a random intercept model analysis to examine the association between the blood and tear glucose concentrations.

RESULTS

Tear occult blood tests showed that the tear glucose concentrations and their variation increased in both participants with and without diabetes as contamination of blood increased. In both participants with and without diabetes, fluctuations of the plasma glucose concentrations were observed depending on the timing of collection of the samples, and the dynamics of the tear glucose concentrations paralleled those of the plasma glucose concentrations. The random intercept model analysis showed a significant association between the plasma and tear glucose concentrations in participants with diabetes (P < 0.001). This association still existed even after adjusting for the glycated hemoglobin levels and the prandial state (P < 0.001).

CONCLUSIONS

It is important to eliminate the tear samples contaminated with blood. Tear glucose monitoring might be a reliable and non-invasive substitute method for monitoring the blood glucose concentrations for diabetes patients, irrespective of glycated hemoglobin levels and timing of sample collection.

Utility of transporter/receptor(s) in drug delivery to the eye

The eye is a highly protected organ, and designing an effective therapy is often considered a challenging task. The anatomical and physiological barriers result in low ocular bioavailability of drugs. Due to these constraints, less than 5% of the administered dose is absorbed from the conventional ophthalmic dosage forms. Further, physicochemical properties such as lipophilicity, molecular weight and charge modulate the permeability of drug molecules. Vision-threatening diseases such as glaucoma, diabetic macular edema, cataract, wet and dry age-related macular degeneration, proliferative vitreoretinopathy, uveitis, and cytomegalovirus retinitis alter the pathophysiological and molecular mechanisms. Understanding these mechanisms may result in the development of novel treatment modalities. Recently, transporter/receptor targeted prodrug approach has generated significant interest in ocular drug delivery. These transporters and receptors are involved in the transport of essential nutrients, vitamins, and xenobiotics across biological membranes. Several influx transporters (peptides, amino acids, glucose, lactate and nucleosides/nucleobases) and receptors (folate and biotin) have been identified on conjunctiva, cornea, and retina. Structural and functional delineation of these transporters will enable more drugs targeting the posterior segment to be successfully delivered topically. Prodrug derivatization targeting transporters and receptors expressed on ocular tissues has been the subject of intense research. Several prodrugs have been designed to target these transporters and enhance the absorption of poorly permeating parent drug. Moreover, this approach might be used in gene delivery to modify cellular function and membrane receptors. This review provides comprehensive information on ocular drug delivery, with special emphasis on the use of transporters and receptors to improve drug bioavailability.

Novel strategies for anterior segment ocular drug delivery

Research advancements in pharmaceutical sciences have led to the development of new strategies in drug delivery to anterior segment. Designing a new delivery system that can efficiently target the diseased anterior ocular tissue, generate high drug levels, and maintain prolonged and effective concentrations with no or minimal side effects is the major focus of current research. Drug delivery by traditional method of administration via topical dosing is impeded by ocular static and dynamic barriers. Various products have been introduced into the market that prolong drug retention in the precorneal pocket and to improve bioavailability. However, there is a need of a delivery system that can provide controlled release to treat chronic ocular diseases with a reduced dosing frequency without causing any visual disturbances. This review provides an overview of anterior ocular barriers along with strategies to overcome these ocular barriers and deliver therapeutic agents to the affected anterior ocular tissue with a special emphasis on nanotechnology-based drug delivery approaches.

Tear Dynamics Model

PURPOSE

Quantitative understanding of tear dynamics may help in developing better ophthalmic drug delivery vehicles and dry eye treatments. This paper attempts to develop a comprehensive model that can predict the effect of physiological parameters on various issues related to tear dynamics.

METHODS

The model is based on mass balances of water and solutes such as glucose, sodium, potassium, and chloride. The mass balances require models for the drainage of fluid through the canaliculi and the transport of tears and solutes through the conjunctiva. The model parameters are obtained by simulating Ussing-chamber experiments. The differential equations for the unsteady balances are solved numerically.

RESULTS

The model predicts that under normal conditions, the tear volume, tear osmolarity and potential are 7.1 microl, 297.6 mM, and -15.1 mV, respectively. The model also predicts that the conjunctiva is secretory and contributes about 25% of the total tear production. We also predict the effect of evaporation on tear physiology and show that an increase in evaporation increases osmolarity, reduces tear volume, and increases conjuctival secretion. Additionally, the new tear dynamics model helps assess the effect of osmolarity of the instilled drops, insertion of punctum plugs and use of moisture chamber as treatments for dry eyes. Furthermore, the model is used to predict the effect of modulation of specific transport pathways, which is proposed as a potential remedy for dry eyes, on conjuctival secretion and total tear volume.

CONCLUSIONS

Most of the predicted results agree with the reported experimental results, at least qualitatively. However, some predictions disagree with experiments suggesting that further improvements in the model are needed. The model developed in this paper can improve our understanding of tear dynamics and also serve as a tool to evaluate the efficacy of various modalities at treating dry eyes.

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.

Tear glucose analysis for the noninvasive detection and monitoring of diabetes mellitus

One approach to the noninvasive monitoring of blood glucose concentration is to monitor glucose concentrations in tear fluid. While several methods for sensing glucose in tear fluid have been proposed, controversy remains as to the precise concentrations of tear glucose in normal and diabetic subjects and as to whether tear fluid glucose concentrations correlate with blood glucose concentrations. This review covers the present understanding of the physiology of glucose transport in tears, the regulation of the aqueous tear fraction, and studies of tear glucose concentration over the last 80 years. The various tear collection methods employed greatly influence the measured tear glucose concentrations. Studies that involve mechanical irritation of the conjunctiva during sampling measure the highest tear glucose concentrations, while studies that avoid tear stimulation measure the lowest concentrations. Attempts to monitor tear glucose concentration in situ by using contact lens-based sensing devices are discussed, and new observations are presented of tear glucose concentration obtained by a method designed to avoid tear stimulation. These studies indicate the importance of the sampling method in determining tear glucose concentrations. On the basis of these results, we discuss the future of in vivo tear glucose sensing and outline the studies needed to resolve the remaining questions about the relationship between tear and blood glucose concentrations.

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.

Roles of the conjunctiva in ocular drug delivery: a review of conjunctival transport mechanisms and their regulation

Conjunctiva plays many roles including protection of ocular surface, production of tear film, and a conduit for drug clearance (depending on drug properties) into the systemic circulation or for drug transport to the deep tissues of the eye. The conjunctiva, which is a moderately tight epithelium, endowed with various transport processes for the homeostasis of ions, solutes, and water in the conjunctival surface and tear film. Modulation of ion transport in the conjunctiva leads to alterations in transconjunctival fluid flow that may become useful for treatment of dry-eye state in the eye. As a possible drug delivery route to the posterior portion of the eye, conjunctiva is an attractive route due to both larger surface area than that of cornea and expression of several key transport processes. Tear contains D-glucose and many amino acids, in addition to the usual ions in the body fluids. Several ion-coupled solute transport processes for absorption of amino acids, D-glucose, monocarboxylate, nucleosides, and dipeptides are expressed in the conjunctiva. Thanks to the rich endowment of these transport processes, drug transport across the conjunctiva into the intraocular tissues may become quite feasible. Subconjunctival injection of microparticles and matrix materials (which allows sustained release of drugs) is shown to maintain reasonable levels of various drugs in the vitreous, perhaps attesting to the fact that conjunctiva per se may contribute as a part of multiple transport barrier(s) in ocular drug delivery. In addition, several conjunctival approaches have been investigated to optimize treatment of dry-eye syndrome and intraocular diseases, and more can be accomplished in the coming years.

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.

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.

Transporters/receptors in the anterior chamber: pathways to explore ocular drug delivery strategies

Membrane transporters/receptors are involved in drug transport processes and play a key role in intestinal absorption, tissue distribution and elimination. Drug targeting to specific transporters and receptors using carrier-mediated absorption has immense clinical significance. Ocular drug delivery is a challenging task since it involves drug transport across various barriers in the eye. Specialised transport processes exist at these barriers, which control the entry of drugs and xenobiotics. Ocular drug therapy involving topical or systemic administration of drugs has various limitations. Transport processes in the eye have been targeted in an effort to increase ocular bioavailability of drugs following topical instillation. This review discusses various transport processes in the eye and drug delivery strategies utilising these transporters/receptors.

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.

Regulation of mucin and fluid secretion by conjunctival epithelial cells

Tears play a vital role in the health and protection of the cornea and conjunctiva. The tear film consists of multiple layers and different glands secrete each layer. Because of many and varied requirements of the ocular surface cells, the volume, composition and structure of the tear film must be exquisitely controlled. If any layer of the tear film is disrupted or altered, the entire tear film is affected, often with deleterious effects. This chapter reviews the current knowledge of the neural and growth factor regulation of electrolyte, water and protein secretion from the goblet and stratified squamous cells of the conjunctiva as well as the mechanisms used for fluid secretion. The evidence presented in this review suggests that parasympathetic nerves stimulate goblet, but not stratified squamous, cell secretion. Sympathetic nerves stimulate stratified squamous, but not goblet, cell secretion, while P2Y(2) agonists stimulate secretion from both cell types. Growth factors regulate goblet cell secretion, but their effects on stratified squamous cell secretion are unknown.

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.

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.

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.