P-glycoprotein regulates a volume-activated chloride current in bovine non-pigmented ciliary epithelial cells

Ocular Drug Delivery

Although the eye is an accessible organ for direct drug application, ocular drug delivery remains a major challenge due to multiple barriers within the eye. Key barriers include static barriers imposed by the cornea, conjunctiva, and retinal pigment epithelium and dynamic barriers including tear turnover and blood and lymphatic clearance mechanisms. Systemic administration by oral and parenteral routes is limited by static blood-tissue barriers that include epithelial and endothelial layers, in addition to rapid vascular clearance mechanisms. Together, the static and dynamic barriers limit the rate and extent of drug delivery to the eye. Thus, there is an ongoing need to identify novel delivery systems and approaches to enhance and sustain ocular drug delivery. This chapter summarizes current and recent experimental approaches for drug delivery to the anterior and posterior segments of the eye.

Recent advances in topical nano drug-delivery systems for the anterior ocular segment

Over the past decade, there has been a rise in the number of clinical cases of moderate to severe anterior segment ocular diseases. Conventional topical ophthalmic formulations have several limitations - to address which, novel drug-delivery systems are needed. Additionally, formidable physiological barriers limit ocular bioavailability through the topical route of application. During the last decade, various nano-scaled ocular drug-delivery strategies have been reported. Some of these exploratory, topical, noninvasive approaches have shown promise in improving penetration into the anterior segment tissues of the eye. In this article, we review the available literature with respect to the safety, efficiency and effectiveness of these nano systems.

Recent advances in drug and nutrient transport across the blood-retinal barrier

INTRODUCTION

The blood-retinal barrier (BRB) is the barrier separating the blood and neural retina, and transport systems for low-weight molecules at the BRB are expected to be useful for developing drugs for the treatment of ocular neural disorders and maintaining a healthy retina. Areas covered: This review discusses blood-to-retina and retina-to-blood transport of drugs and nutrients at the BRB. In particular, P-gp (ABCB1/MDR1) has low impact on the transport of cationic drugs at the BRB, suggesting a significant role of novel organic cation transporters in influx and efflux transport of lipophilic cationic drugs between blood and the retina. The transport of pravastatin at the BRB involves transporters including organic anion transporting polypeptide 1a4 (Oatp1a4). Recent studies have shown the involvement of solute carrier transporters in the blood-to-retina transport of nutrients including riboflavin, L-ornithine, β-alanine, and L-histidine, implying that dipeptide transport at the BRB is minimal. Expert opinion: Novel organic cation transport systems and the elimination-dominant transport of pravastatin at the BRB are expected to be useful in systemic drug delivery to the neural retina without CNS side effects. The mechanism of nutrient transport at the BRB is expected to provide a new strategy for delivery of nutrient-mimetic drugs.

Ocular Drug Delivery Barriers-Role of Nanocarriers in the Treatment of Anterior Segment Ocular Diseases

Ocular drug delivery is challenging due to the presence of anatomical and physiological barriers. These barriers can affect drug entry into the eye following multiple routes of administration (e.g., topical, systemic, and injectable). Topical administration in the form of eye drops is preferred for treating anterior segment diseases, as it is convenient and provides local delivery of drugs. Major concerns with topical delivery include poor drug absorption and low bioavailability. To improve the bioavailability of topically administered drugs, novel drug delivery systems are being investigated. Nanocarrier delivery systems demonstrate enhanced drug permeation and prolonged drug release. This review provides an overview of ocular barriers to anterior segment delivery, along with ways to overcome these barriers using nanocarrier systems. The disposition of nanocarriers following topical administration, their safety, toxicity and clinical trials involving nanocarrier systems are also discussed.

Expression, activity and pharmacokinetic impact of ocular transporters

The eye is protected by several tissues that limit the permeability and entry of potentially harmful substances, but also hamper the delivery of drugs in the treatment of ocular diseases. Active transport across the ocular barriers may affect drug distribution, but the impact of drug transporters on ocular drug delivery is not well known. We have collected and critically reviewed the literature for ocular expression and activity of known drug transporters. The review concentrates on drug transporters that have been functionally characterized in ocular tissues or primary cells and on transporters for which there is available expression data at the protein level. Species differences are highlighted, since these may explain observed inconsistencies in the influence of specific transporters on drug disposition. There is variable evidence about the pharmacokinetic role of transporters in ocular tissues. The strongest evidence for the role of active transport is available for the blood-retinal barrier. We explored the role of active transport in the cornea and blood retinal barrier with pharmacokinetic simulations. The simulations show that the active transport is important only in the case of specific parameter combinations.

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 Nanomicellar Formulation Approaches for Anterior and Posterior Segment Ocular Drug Delivery

One of the most challenging areas of pharmaceutical research is ocular drug delivery. The unique anatomy and physiology of the eye impedes drug permeation to deeper ocular tissues. Nanosized carrier systems such as nanoparticles, liposomes, suspensions, dendrimers, and nanomicelles are being explored for ocular drug delivery. In this review, we have focused on application of emerging nanomicellar carrier systems in ocular drug delivery. Nanomicelles are nanosized vesicular carriers formed from amphiphilic monomer units. Surfactant and polymeric micellar nanocarriers provide an amenable means to improve drug solubilization, develop clear aqueous formulations and deliver drugs to anterior and posterior ocular tissues. Nanomicelles due to their amphiphilic nature encapsulate hydrophobic drugs and aid in drug delivery. Various methods are employed to develop nanosized micellar formulations depending upon the physicochemical properties of the drug. Nanomicellar carriers appear to be promising vehicles with potential applications in ocular drug delivery. In this review, we attempted to discuss about the progress in ocular drug delivery research using nanomicelles as carriers from the published literature and issued patents. Also, with regards to ocular static and dynamic barriers which prevent drug permeation, a brief discussion about nanomicelles, types of nanomicelles, their methods of preparation and micellar strategy to overcome ocular barriers, delivering therapeutic levels of drugs to anterior and posterior ocular tissues are discussed.

Single-channel properties of a stretch-sensitive chloride channel in the human mast cell line HMC-1

A stretch-activated (SA) Cl(-) channel in the plasma membrane of the human mast cell line HMC-1 was identified in outside-out patch-clamp experiments. SA currents, induced by pressure applied to the pipette, exhibited voltage dependence with strong outward rectification (55.1 pS at +100 mV and an about tenfold lower conductance at -100 mV). The probability of the SA channel being open (P (o)) also showed steep outward rectification and pressure dependence. The open-time distribution was fitted with three components with time constants of tau(1o) = 755.1 ms, tau(2o) = 166.4 ms, and tau(3o) = 16.5 ms at +60 mV. The closed-time distribution also required three components with time constants of tau(1c) = 661.6 ms, tau(2c) = 253.2 ms, and tau(3c) = 5.6 ms at +60 mV. Lowering extracellular Cl(-) concentration reduced the conductance, shifted the reversal potential toward chloride reversal potential, and decreased the P (o) at positive potentials. The SA Cl(-) currents were reversibly blocked by the chloride channel blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) but not by (Z)-1-(p-dimethylaminoethoxyphenyl)-1,2-diphenyl-1-butene (tamoxifen). Furthermore, in HMC-1 cells swelling due to osmotic stress, DIDS could inhibit the increase in intracellular [Ca(2+)] and degranulation. We conclude that in the HMC-1 cell line, the SA outward currents are mediated by Cl(-) influx. The SA Cl(-) channel might contribute to mast cell degranulation caused by mechanical stimuli or accelerate membrane fusion during the degranulation process.

Ocular Drug Delivery; Impact of in vitro Cell Culture Models

Normal vision depends on the optimal function of ocular barriers and intact membranes that selectively regulate the environment of ocular tissues. Novel pharmacotherapeutic modalities have aimed to overcome such biological barriers which impede efficient ocular drug delivery. To determine the impact of ocular barriers on research related to ophthalmic drug delivery and targeting, herein we provide a review of the literature on isolated primary or immortalized cell culture models which can be used for evaluation of ocular barriers. In vitro cell cultures are valuable tools which serve investigations on ocular barriers such as corneal and conjunctival epithelium, retinal pigment epithelium and retinal capillary endothelium, and can provide platforms for further investigations. Ocular barrier-based cell culture systems can be simply set up and used for drug delivery and targeting purposes as well as for pathological and toxicological research.

Enhanced corneal absorption of erythromycin by modulating P-glycoprotein and MRP mediated efflux with corticosteroids

PURPOSE

The objectives were (i) to test in vivo functional activity of MRP2 on rabbit corneal epithelium and (ii) to evaluate modulation of P-gp and MRP2 mediated efflux of erythromycin when co-administered with corticosteroids.

METHODS

Cultured rabbit primary corneal epithelial cells (rPCECs) was employed as an in vitro model for rabbit cornea. Cellular accumulation and bi-directional transport studies were conducted across Madin-Darby Canine Kidney (MDCK) cells overexpressing MDR1 and MRP2 proteins to delineate transporter specific interaction of steroids. Ocular pharmacokinetic studies were conducted in rabbits following a single-dose infusion of erythromycin in the presence of specific inhibitors and steroids.

RESULTS

Bi-directional transport of erythromycin across MDCK-MDR1 and MDCK-MRP2 cells showed significant difference between BL-AP and AP-BL permeability, suggesting that erythromycin is a substrate for P-gp and MRP2. Cellular accumulation of erythromycin in rPCEC was inhibited by steroids in a dose dependent manner. MK571, a specific MRP inhibitor, modulated the aqueous humor concentration of erythromycin in vivo. Even, steroids inhibited P-gp and MRP2 mediated efflux with maximum increase in k(a), AUC(0-infinity), C(max) and C(last) values of erythromycin, observed with 6alpha-methyl prednisolone.

CONCLUSION

MRP2 is functionally active along with P-gp in effluxing drug molecules out of corneal epithelium. Steroids were able to significantly inhibit both P-gp and MRP2 mediated efflux of erythromycin.

Recent perspectives in ocular drug delivery

Anatomy and physiology of the eye makes it a highly protected organ. Designing an effective therapy for ocular diseases, especially for the posterior segment, has been considered as a formidable task. Limitations of topical and intravitreal route of administration have challenged scientists to find alternative mode of administration like periocular routes. Transporter targeted drug delivery has generated a great deal of interest in the field because of its potential to overcome many barriers associated with current therapy. Application of nanotechnology has been very promising in the treatment of a gamut of diseases. In this review, we have briefly discussed several ocular drug delivery systems such as microemulsions, nanosuspensions, nanoparticles, liposomes, niosomes, dendrimers, implants, and hydrogels. Potential for ocular gene therapy has also been described in this article. In near future, a great deal of attention will be paid to develop non-invasive sustained drug release for both anterior and posterior segment eye disorders. A better understanding of nature of ocular diseases, barriers and factors affecting in vivo performance, would greatly drive the development of new delivery systems. Current momentum in the invention of new drug delivery systems hold a promise towards much improved therapies for the treatment of vision threatening disorders.

Ocular novel drug delivery: impacts of membranes and barriers

BACKGROUND

Ocular drug delivery is an extremely challenging area due to its restrictive barrier functionalities.

OBJECTIVE

Drug transport via corneal/non-corneal routes involves several intricate biological processes such as drug penetration across the ocular barriers and transfer to the anterior or posterior chambers, thus the influence of these processes on the pharmacotherapy of the eye should be fully addressed.

METHODS

To pursue the impacts of such impediments in novel drug therapy, recent publications were reviewed regarding advanced strategies such as nanomedicines.

CONCLUSION

The ocular barriers are highly specialized and selectively control the inward/outward traverse of compounds, hence a better understanding of these biological obstacles would provide a platform to advance ophthalmic drug therapy towards specified delivery/targeting with minimal adverse consequences.

Role of Calcium in Volume-Activated Chloride Currents in a Mouse Cholangiocyte Cell Line

Volume-activated Cl(-) channels (VACCs) play vital roles in many cells including cholangiocytes. Previously, we characterized the VACCs in mouse cholangiocytes. Since calcium plays an important role in VACC regulation in many cells, we have studied the effect of calcium modulation on the regulatory volume decrease (RVD) and VACC currents in mouse bile duct cells (MBDCs). Cell volume measurements were assessed by a Coulter counter with cell sizer, and conventional whole-cell patch-clamp techniques were used to study the role of calcium on RVD and VACC currents. Cell volume study indicated that MBDCs exhibited RVD, which was inhibited by 5-nitro-2'-(3-phenylpropylamino)-benzoate (NPPB), 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS) and 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra-acetoxymethyl ester (BAPTA-AM) but not by removal of extracellular calcium. During hypotonic challenge, MBDCs exhibited an outwardly rectified current, which was significantly inhibited by administration of classical chloride channel inhibitors such as NPPB and tamoxifen. Chelation of the intracellular calcium with BAPTA-AM or removal of extracellular calcium and calcium channel blocker had no significant effect on VACC currents during hypotonic challenge. In addition to VACC, MBDC had a calcium-activated chloride channel, which was inhibited by NPPB. The present study is the first to systemically study the role of calcium on the VACC and RVD in mouse cholangiocytes and demonstrates that a certain level of intracellular calcium is necessary for RVD but the activation of VACC during RVD does not require calcium. These findings suggest that calcium does not have a direct regulatory role on VACC but has a permissive role on RVD in cholangiocytes.

Roles of volume-activated Cl- currents and regulatory volume decrease in the cell cycle and proliferation in nasopharyngeal carcinoma cells

OBJECTIVES

Previously it has been shown, that the volume-activated plasma membrane chloride channel is associated with regulatory volume decrease (RVD) of cells and may play an important role in control of cell proliferation. We have demonstrated that both expression of the channel and RVD capacity are actively regulated in the cell cycle. In this study, we aimed to further study the role of the volume-activated chloride current and RVD in cell cycle progression and overall in cell proliferation.

MATERIALS AND METHODS

Whole-cell currents, RVD, cell cycle distribution, cell proliferation and cell viability were measured or detected with the patch-clamp technique, the cell image analysis technique, flow cytometry, the MTT assay and the trypan blue assay respectively, in nasopharyngeal carcinoma cells (CNE-2Z cells).

RESULTS

The Cl- channel blockers, 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) and tamoxifen, inhibit the volume-activated chloride current, RVD and proliferation of CNE-2Z cells in a dose-dependent manner. Analysis of relationships between the current, RVD and cell proliferation showed that both the current and RVD were positively correlated with cell proliferation. NPPB (100 microM) and tamoxifen (20 microM) did not significantly induce cell death, but inhibited cell proliferation, implying that the blockers may inhibit cell proliferation by affecting cell cycle progression. This was verified by the observation that tamoxifen (20 microM) and NPPB (100 microM) inhibited cell cycle progress and arrested cells at the G0/G1 phase boundary.

CONCLUSIONS

Activity of the volume-activated chloride channel is one of the important factors that regulate the passage of cells through the G1 restriction point and that the Cl- current associated with RVD plays an important role in cell proliferation.

Swelling-activated chloride channels in aqueous humour formation: on the one side and the other

Aqueous humour is secreted by the ciliary epithelium comprising pigmented and non-pigmented cell layers facing the stroma and aqueous humour respectively. Net chloride secretion likely limits the rate of aqueous humour formation and proceeds in three steps: stromal chloride entry into pigmented cells, diffusion through gap junctions and final non-pigmented cell secretion. Swelling-activated chloride channels function on both epithelial surfaces. At the stromal surface, swelling- and cyclic adenosine monophosphate-activated maxi-chloride channels can recycle chloride, reducing net chloride secretion. At the aqueous-humour surface, swelling- and A3 adenosine receptor-activated chloride channels subserve chloride release into the aqueous humour. The similar macroscopic properties of the two non-pigmented cell chloride currents suggest that both flow through a common conduit. In addition, measurements of intraocular pressure (IOP) in living wild-type and mutant mice have confirmed that A3 adenosine receptor-activated agonists and antagonists increase and lower IOP respectively. Isolated ciliary epithelial cells are commonly perfused with hypotonic solution to probe and characterize chloride channels, but the physiological role of swelling-activated channels has been unclear without knowing their epithelial distribution. Recently, hypotonic challenge has been found to stimulate the chloride-sensitive short-circuit current across the intact bovine ciliary epithelium, suggesting that the net effect of the swelling-activated chloride currents is oriented to enhance aqueous humour formation. Taken together, the results suggest that swelling-activated chloride channels are predominantly oriented to enhance aqueous humour secretion, and these chloride channels at the aqueous surface may be identical with adenosine receptor-activated chloride channels which likely modulate aqueous inflow and IOP in the living mouse.

Single nucleotide polymorphisms in human P-glycoprotein: its impact on drug delivery and disposition

Drug efflux pumps belong to a large family of ATP-binding cassette transporter proteins. These pumps bind their substrate and export it through the membrane using energy derived from ATP hydrolysis. P-glycoprotein, the main efflux pump in this family, is expressed not only in tumour cells but also in normal tissues with excretory function (liver, kidney and the intestine). It has a broad specificity of substrates and plays an important role in drug delivery and disposition. Recently, genetic screening of P-glycoprotein has yielded multiple single nucleotide polymorphisms, which seem to alter transporter function and expression. This review discusses the various polymorphisms of this gene and its impact on drug disposition and diseases.

Hyposmotic activation of ICl,swell in rabbit nonpigmented ciliary epithelial cells involves increased ClC-3 trafficking to the plasma membrane

In mammalian nonpigmented ciliary epithelial (NPE) cells, hyposmotic stimulation leading to cell swelling activates an outwardly rectifying Cl(-) conductance (I(Cl,swell)), which, in turn, results in regulatory volume decrease. The aim of this study was to determine whether increased trafficking of intracellular ClC-3 Cl channels to the plasma membrane could contribute to the I(Cl,swell) following hyposmotic stimulation. Our results demonstrate that hyposmotic stimulation reversibly activates an outwardly rectifying Cl(-) current that is inhibited by phorbol-12-dibutyrate and niflumic acid. Transfection with ClC-3 antisense, but not sense, oligonucleotides reduced ClC-3 expression as well as I(Cl,swell). Intracellular dialysis with 2 different ClC-3 antibodies abolished activation of I(Cl,swell). Immunofluorescence microscopy showed that hyposmotic stimulation increased ClC-3 immunoreactivity at the plasma membrane. To determine whether this increased expression of ClC-3 at the plasma membrane could be due to increased vesicular trafficking, we examined membrane dynamics with the fluorescent membrane dye FM1-43. Hyposmotic stimulation rapidly increased the rate of exocytosis, which, along with ICl,swell, was inhibited by the phosphoinositide-3-kinase inhibitor wortmannin and the microtubule disrupting agent, nocodazole. These findings suggest that ClC-3 channels contribute to I(Cl,swell) following hyposmotic stimulation through increased trafficking of channels to the plasma membrane.

Volume-sensitive Cl- current in bovine adrenocortical cells: comparison with the ACTH-induced Cl- current

In a previous study performed on zona fasciculata (ZF) cells isolated from calf adrenal glands, we identified an ACTH-induced Cl- current involved in cell membrane depolarization. In the present work, we describe a volume-sensitive Cl- current and compare it with the ACTH-activated Cl- current. Experiments were performed using the whole-cell patch-clamp recording method, video microscopy and cortisol-secretion measurements. In current-clamp experiments, hypotonic solutions induced a membrane depolarization to -22 mV. This depolarization, correlated with an increase in the membrane conductance, was sensitive to different Cl- channel inhibitors. In voltage-clamp experiments, hypotonic solution induced a membrane current that slowly decayed and reversed at -21 mV. This ionic current displayed no time dependence and showed a slight outward rectification. It was blocked to variable extent by different conventional Cl- channel inhibitors. Under hypotonic conditions, membrane depolarizations were preceded by an increase in cell volume that was not detected under ACTH stimulation. It was concluded that hypotonic solution induced cell swelling, which activated a Cl- current involved in membrane depolarization. Although cell volume change was not observed in the presence of ACTH, biophysical properties and pharmacological profile of the volume-sensitive Cl- current present obvious similarities with the ACTH-activated Cl- current. As compared to ACTH, hypotonic solutions failed to trigger cortisol production that was weakly stimulated in the presence of high-K+ solution. This shows that in ZF cells, membrane depolarization is not a sufficient condition to fully activate secretory activities.

Pharmacokinetics of erythromycin in rabbit corneas after single-dose infusion: role of P-glycoprotein as a barrier to in vivo ocular drug absorption

Efflux pump like P-glycoprotein (P-gp) is known to be a major barrier to drug delivery. Functional P-glycoprotein has been recently identified in cornea and corneal cell lines. Thus, it is probable that P-glycoprotein may restrict in vivo ocular drug absorption, resulting in low ocular bioavailability. Experiments were designed using New Zealand albino (New Zealand White) rabbits to assess inhibitors of P-gp efflux to increase drug absorption. Anesthetized rabbits were given constant topical infusions of [(14)C]erythromycin in the presence and absence of inhibitors. Testosterone, verapamil, quinidine, and cyclosporine A were selected as P-gp inhibitors. Transport experiments were conducted in Madin-Darby canine kidney cells transfected with the human mdr1 gene (MDCK-MDR1). Erythromycin exhibited significant efflux out of MDCK-MDR1 cells, suggesting that erythromycin is a good substrate for P-gp. Ocular pharmacokinetic studies were conducted using a topical single-dose infusion method. Maximum inhibition of P-gp mediated efflux was observed with 500 microM testosterone. Area under the curve (AUC)(0- infinity ) of erythromycin with 500 microM testosterone was almost 4 times higher than AUC(0- infinity ) without any inhibitor. Rate of elimination (k(10)) for erythromycin and those with inhibitors was found to be similar (141 +/- 23 min), suggesting that elimination pathways were not altered. All the inhibitors were found to be nontoxic. Verapamil also inhibited the efflux pump with moderate change in AUC(0- infinity ) and C(max) compared with control. Thus, P-gp is found to be active in vivo, and it restricts topical erythromycin absorption across the cornea, which can be inhibited by known P-gp inhibitors. Therefore, ocular bioavailability of P-gp substrates can be significantly enhanced by proper selection of P-gp inhibitors.

Water transport in the brain: Role of cotransporters

It is generally accepted that cotransporters transport water in addition to their normal substrates, although the precise mechanism is debated; both active and passive modes of transport have been suggested. The magnitude of the water flux mediated by cotransporters may well be significant: both the number of cotransporters per cell and the unit water permeability are high. For example, the Na(+)-glutamate cotransporter (EAAT1) has a unit water permeability one tenth of that of aquaporin (AQP) 1. Cotransporters are widely distributed in the brain and participate in several vital functions: inorganic ions are transported by K(+)-Cl(-) and Na(+)-K(+)-Cl(-) cotransporters, neurotransmitters are reabsorbed from the synaptic cleft by Na(+)-dependent cotransporters located on glial cells and neurons, and metabolites such as lactate are removed from the extracellular space by means of H(+)-lactate cotransporters. We have previously determined water transport capacities for these cotransporters in model systems (Xenopus oocytes, cell cultures, and in vitro preparations), and will discuss their role in water homeostasis of the astroglial cell under both normo- and pathophysiologal situations. Astroglia is a polarized cell with EAAT localized at the end facing the neuropil while the end abutting the circulation is rich in AQP4. The water transport properties of EAAT suggest a new model for volume homeostasis of the extracellular space during neural activity.

2-APB inhibits volume-regulated anion channels independently from intracellular calcium signaling modulation

It has previously been suggested that volume-regulated anion channels (VRACs) and store-operated channels (SOCs) interact with each other according to their expected colocalization in the plasma membrane of LNCaP cells. In order to study interactions between these two channels, we used 2-aminoethoxydiphenyl borate (2-APB) as a regular SOC inhibitor. Surprisingly 2-APB reduced VRAC activity in a dose-dependent manner (IC(50)=122.8 microM), but not 2,2-diphenyltetrahydrofuran (a structural analog of 2-APB). This effect was also present in keratinocytes. We conclude that 2-APB is an inhibitor of the VRAC family, and is also a potent tool to study the SOC-VRAC interaction in LNCaP cells.

Potentiation of glycine responses by dideoxyforskolin and tamoxifen in rat spinal neurons

Dideoxyforskolin, a forskolin analogue unable to stimulate adenylate cyclase, and tamoxifen, an antioestrogen widely used against breast cancer, are both known to block some Cl- channels. Their effects on Cl- responses to glycine or GABA have been tested here by using whole-cell recording from cultured spinal neurons. Dideoxyforskolin (4 or 16 microm) and tamoxifen (0.2-5 microm) both potentiate responses to low glycine concentrations. They also induce blocking effects, predominant at high glycine concentrations. At 5 microm, tamoxifen increased responses to 15 microm glycine by a factor >4.5, reaching 20 in some neurons. Potentiation by extracellular dideoxyforskolin or tamoxifen persisted after intracellular application of the modulator and was not due to Zn2+ contamination. Potentiation by tamoxifen also persisted in a Ca2+-free extracellular solution, after intracellular Ca2+ buffering and protein kinase C blockade. Thus, the critical sites of action are not intracellular. The EC50 for glycine was lowered 6.6-fold by 5 microm tamoxifen. The kinetics and voltage-dependence of the effects of tamoxifen on glycine responses support the idea that this hydrophobic drug may act from a site located within the membrane. Tamoxifen (5 micro m) also increased responses to 2 micro m GABA by a factor of 3.5, but barely affected peak responses to 20 microm GABA. The demonstration that tamoxifen affects some of the main inhibitory receptors should be useful for better evaluating its neurological effects. Furthermore, the results identify a new class of molecules that potentiate glycine receptor function.

Molecular mechanisms of water transport in the eye

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

Cell cycle-dependent expression of volume-activated chloride currents in nasopharyngeal carcinoma cells

Patch-clamping and cell image analysis techniques were used to study the expression of the volume-activated Cl(-) current, I(Cl(vol)), and regulatory volume decrease (RVD) capacity in the cell cycle in nasopharyngeal carcinoma cells (CNE-2Z). Hypotonic challenge caused CNE-2Z cells to swell and activated a Cl(-) current with a linear conductance, negligible time-dependent inactivation, and a reversal potential close to the Cl(-) equilibrium potential. The sequence of anion permeability was I(-) > Br(-) > Cl(-) > gluconate. The Cl(-) channel blockers tamoxifen, 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), and ATP inhibited I(Cl(vol)). Synchronous cultures of cells were obtained by the mitotic shake-off technique and by a double chemical-block (thymidine and hydroxyurea) technique. The expression of I(Cl(vol)) was cell cycle dependent, being high in G(1) phase, downregulated in S phase, but increasing again in M phase. Hypotonic solution activated RVD, which was cell cycle dependent and inhibited by the Cl(-) channel blockers NPPB, tamoxifen, and ATP. The expression of I(Cl(vol)) was closely correlated with the RVD capacity in the cell cycle, suggesting a functional relationship. Inhibition of I(Cl(vol)) by NPPB (100 microM) arrested cells in G(0)/G(1). The data also suggest that expression of I(Cl(vol)) and RVD capacity are actively modulated during the cell cycle. The volume-activated Cl(-) current associated with RVD may therefore play an important role during the cell cycle progress.

Regulatory volume decrease is actively modulated during the cell cycle

Nasopharyngeal carcinoma cells, CNE-2Z, when swollen by 47% hypotonic solution, exhibited a regulatory volume decrease (RVD). The RVD was inhibited by extracellular applications of the chloride channel blockers tamoxifen (30 microM; 61% inhibition), 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB, 100 microM; 60% inhibition), and ATP (10 mM; 91% inhibition). The level and time constant of RVD varied greatly between cells. Most cells conducted an incomplete RVD, but a few had the ability to recover their volume completely. There was no obvious correlation between cell volume and RVD capacity. Flow cytometric analysis showed that highly synchronous cells were obtained by the mitotic shake-off technique and that the cells progressed through the cell cycle synchronously when incubated in culture medium. Combined application of DNA synthesis inhibitors, thymidine and hydroxyurea arrested cells at the G1/S boundary and 87% of the cells reached S phase 4 h after being released. RVD capacity changed significantly during the cell cycle progression in cells synchronized by shake-off technique. RVD capacity being at its highest in G1 phase and lowest in S phase. The RVD capacity in G1 (shake-off cells sampled after 4 h of incubation), S (obtained by chemical arrest), and M cells (selected under microscope) was 73, 33, and 58%, respectively, and the time constants were 435, 769, and 2,000 sec, respectively. We conclude that RVD capacity is actively modulated in the cell cycle and RVD may play an important role in cell cycle progress.

Ca2+ modulation of volume-regulated anion channels: evidence for colocalization with store-operated channels

Ca2+ regulation of Cl- current induced by cell swelling (I(CI,swell)) in response to hypotonicity was studied in human prostate cancer epithelial cells (LNCaP) by using the patch-clamp technique. Increase of global intracellular Ca2+ ([Ca2+]in) to 1 mM as well as variations of the extracellular Ca2+ ([Ca2+]out) in the 0 to 10 mM range did not affect time course of the development, maximal amplitude, rectification properties, and kinetics of I(CI,swell). However, the presence of 0.1 mM thapsigargin (TG), an inhibitor of endoplasmic reticulum (ER) Ca2+ pump, resulted in a more than 50% inhibition of ICI,swell. The blockade of plasma membrane store-operated channels (SOCs), activated in the presence of TG, by 2 mM Ni2+ prevented TG-conferred I(CI,swell) inhibition by extracellular Ca2+. In the presence of TG and Ca2+, the cells failed to exhibit regulatory volume decrease. We conclude that interaction between volume-regulated anion channels (VRACs) carrying I(CI,swell) and Ca2+ occurs in the microdomains from the inner surface of the membrane that are not accessible to the changes in [Ca2+]in, but can be readily reached by Ca2+ entering the cell via plasma membrane, especially through SOCs. Preferred access of SOC-transported Ca2+ to VRAC suggests colocalization of these channels in the cell membrane.

PGE(2), Ca(2+), and cAMP mediate ATP activation of Cl(-) channels in pigmented ciliary epithelial cells

Purines regulate intraocular pressure. Adenosine activates Cl(-) channels of nonpigmented ciliary epithelial cells facing the aqueous humor, enhancing secretion. Tamoxifen and ATP synergistically activate Cl(-) channels of pigmented ciliary epithelial (PE) cells facing the stroma, potentially reducing net secretion. The actions of nucleotides alone on Cl(-) channel activity of bovine PE cells were studied by electronic cell sorting, patch clamping, and luciferin/luciferase ATP assay. Cl(-) channels were activated by ATP > UTP, ADP, and UDP, but not by 2-methylthio-ATP, all at 100 microM. UTP triggered ATP release. The second messengers Ca(2+), prostaglandin (PG)E(2), and cAMP activated Cl(-) channels without enhancing effects of 100 microM ATP. Buffering intracellular Ca(2+) activity with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'- tetraacetic acid or blocking PGE(2) formation with indomethacin inhibited ATP-triggered channel activation. The Rp stereoisomer of 8-bromoadenosine 3',5'-cyclic monophosphothioate inhibited protein kinase A activity but mimicked 8-bromoadenosine 3',5'-cyclic monophosphate. We conclude that nucleotides can act at >1 P2Y receptor to trigger a sequential cascade involving Ca(2+), PGE(2), and cAMP. cAMP acts directly on Cl(-) channels of PE cells, increasing stromal release and potentially reducing net aqueous humor formation and intraocular pressure.

Expression and function of P-glycoprotein and absence of multidrug resistance-related protein in rat and beige mouse peritoneal mast cells

To clarify the function of the multidrug transporter P-glycoprotein in mast cells we used the green fluorescent compound Bodipy-FL-verapamil, which is a substrate of P-glycoprotein. This compound is also transported by Multidrug Resistance-related Protein (MRP), another membrane transport protein expressed in many tumour resistant cells as well as in normal cells. When rat peritoneal mast cells were incubated with Bodipy-verapamil, a rapid uptake of this compound was observed. Pretreatment with modulators of P-glycoprotein activity, such as verapamil and vinblastine, increased Bodipy-verapamil intracellular concentrations. In addition, Bodipy-verapamil efflux from these cells was rapid and also inhibited by verapamil and vinblastine. In contrast, no effect was observed when cells were treated with agents, such as probenecid and indomethacin, that are known inhibitors of MRP. Methylamine and monensin, substances that modify the pH values in the granules, were able to lower the concentrations of Bodipy-verapamil. Microscopical observations, conducted in both rat and beige mouse mast cells, demonstrated that the fluorochrome accumulated in the cytoplasmic secretory granules. RT-PCR performed on rat peritoneal mast cells revealed the presence of MDR1a and MDR1b mRNAs; on the contrary, MRP mRNA was not expressed. Mast cells were further treated with the fluorescent probe LysoSensor Blue, a weak base that becomes fluorescent when inside acidic organelles. This substance accumulated in mast cell granular structures and its fluorescence was reduced either by treatment with P-glycoprotein modulators or with agents that disrupt pH gradients. In conclusion, these data further confirm the presence of an active P-glycoprotein, but not of MRP, in rat peritoneal mast cells. These findings, coupled with previous ultrastructural data, lend further support to the assumption that this protein is located on the mast cell perigranular membrane. The functional role of P-glycoprotein in these cells is at present unclear, but a possible involvement in the transport of molecules from the granules to the cytosol can be hypothesized. Alternatively, this protein might be indirectly implicated in changes of pH values inside secretory granules.

Volume-regulated chloride conductance in the LNCaP human prostate cancer cell line

Patch-clamp recordings were used to study ion currents induced by cell swelling caused by hypotonicity in human prostate cancer epithelial cells, LNCaP. The reversal potential of the swelling-evoked current suggested that Cl(-) was the primary charge carrier (termed I(Cl,swell)). The selectivity sequence of the underlying volume-regulated anion channels (VRACs) for different anions was Br(-) approximately I(-) > Cl(-) > F(-) > methanesulfonate >> glutamate, with relative permeability numbers of 1.26, 1.20, 1.0, 0.77, 0.49, and 0.036, respectively. The current-voltage patterns of the whole cell currents as well as single-channel currents showed moderate outward rectification. Unitary VRAC conductance was determined at 9.6 +/- 1.8 pS. Conventional Cl(-) channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid (100 microM) and DIDS (100 microM) inhibited whole cell I(Cl,swell) in a voltage-dependent manner, with the block decreasing from 39.6 +/- 9.7% and 71.0 +/- 11. 0% at +50 mV to 26.2 +/- 7.2% and 14.5 +/- 6.6% at -100 mV, respectively. Verapamil (50 microM), a standard Ca(2+) antagonist and P-glycoprotein function inhibitor, depressed the current by a maximum of 15%. Protein tyrosine kinase inhibitors downregulated I(Cl,swell) (genistein with an IC(50) of 2.6 microM and lavendustin A by 60 +/- 14% at 1 microM). The protein tyrosine phosphatase inhibitor sodium orthovanadate (500 microM) stimulated I(Cl,swell) by 54 +/- 11%. We conclude that VRACs in human prostate cancer epithelial cells are modulated via protein tyrosine phosphorylation.

Tamoxifen and ATP synergistically activate Cl- release by cultured bovine pigmented ciliary epithelial cells

Purines alter aqueous humour secretion by the bilayered ciliary epithelium. Adenosine but not ATP shrinks non-pigmented ciliary epithelial (NPE) cells by activating Cl- channels. We now report effects of ATP on pigmented ciliary epithelial (PE) cells. Cultured bovine PE cells were studied volumetrically by electronic cell sorting. ATP and tamoxifen acted synergistically to shrink PE cells. Neither ATP nor tamoxifen alone had a consistent effect on cell volume. The tamoxifen, ATP-activated shrinkage required Cl- release since the response was blocked by removing Cl- and was inhibited by the Cl- channel blockers 5-nitro-2-(3-phenylpropylamino)-benzoate and 4,4'-diisothiocyano-2,2'-disulfonic acid. The modulating effect of tamoxifen could have reflected many actions of tamoxifen. Our data do not support the suggestion that tamoxifen inhibits protein kinase C (PKC) or calcium-calmodulin, or that it acts on histamine or carbachol receptors. The shrinkage produced by ATP and tamoxifen was blocked by 17beta-oestradiol, but not 17alpha-oestradiol. The cooperative interaction between tamoxifen and ATP was not mediated by an enhanced rise in [Ca2+]i. The results indicate that tamoxifen interacts synergistically with ATP to activate Cl- release by the PE cells.

Regulation of volume-activated chloride channels by P-glycoprotein: phosphorylation has the final say!

P-glycoprotein (Pgp) is a transmembrane transporter causing efflux of a number of chemically unrelated drugs and is responsible for resistance to a variety of anticancer drugs during chemotherapy. Pgp overexpression in cells is also associated with volume-activated chloride channel activity; Pgp is thought to regulate such activity. Reversible phosphorylation is a possible mechanism for regulating the transport and chloride channel regulation functions of Pgp. Protein kinase C (PKC) is a good candidate for inducing such phosphorylation. Hierarchical multiple phosphorylation (e.g. of different serines and with different PKC isoforms) may shuttle the protein between its different states of activity (transport or channel regulation). Cell volume changes may trigger phosphorylation of Pgp at sites causing inhibition of transport. The possible regulation of chloride channels by Pgp and the potential involvement of reversible phosphorylation in such regulation is reviewed.

The role of ClC-3 in volume-activated chloride currents and volume regulation in bovine epithelial cells demonstrated by antisense inhibition

1. A chloride current with mild outward rectification was induced in the native bovine non-pigmented ciliary epithelial (NPCE) cells by a 23 % hypotonic solution. The current showed no or little inactivation at depolarized steps. 2. ATP blocked 88 and 61 % of the outward and inward components of the volume-activated chloride current (ICl,vol) with an IC50 of 5.3 and 9.6 mM, respectively. 3. The volume-activated chloride current was decreased and the activation of the current was delayed by inhibiting endogenous ClC-3 expression using a ClC-3 antisense oligonucleotide. The inhibition of the current as a function of antisense concentration was asymptotic with a maximum about 60 %. The remaining current was probably not derived from ClC-3 and was inhibited by ATP. 4. ClC-3 expression in the bovine NPCE cells was verified by immunofluorescence studies. ClC-3 immunofluorescence was distributed throughout the cells but with the predominant location within the nucleus. The expression of ClC-3 protein was diminished by the ClC-3 antisense oligonucleotide with the greatest diminution occurring in the nuclear region. 5. The size of the volume-activated chloride current was positively correlated with the ClC-3 immunofluorescence level. 6. Regulatory volume decrease of the NPCE cells was reduced by ClC-3 antisense oligonucleotide. 7. We conclude that endogenous ClC-3 is associated with the volume-activated chloride current and is involved in cell volume regulation, but that it can only contribute towards a proportion of the current in NPCE cells. 8. The nuclear predominance of ClC-3 immunofluorescence in NPCE cells, the absence of basal activity of chloride current and the marked pharmacological differences between IClC-3 and ICl,vol argue against ClC-3 being the only, or even the main, volume-activated chloride channel in NPCE cells.

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

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

Modulation of volume-sensitive Cl - channels and cell volume by actin filaments and microtubules in human cervical cancer HT-3 cells

Hypotonicity activates volume-sensitive Cl- currents, which are implicated in the regulatory volume decrease (RVD) responses and transport of taurine in human cervical cancer HT-3 cells. In this study, the role of cytoskeleton in the regulation of volume-sensitive Cl- channels and RVD responses in HT-3 cells was studied. Cells were incubated with various compounds, which depolymerized or polymerized cytoskeletal elements, i.e. actin filaments and microtubules. The hypotonicity-induced changes in Cl- conductance and in cell volume were measured by whole-cell voltage clamping and cell size monitoring, respectively. Our results show that in HT-3 cells hypotonicity activated an outward rectified Cl- current that was abrogated by Cl- channel blockers. Cytochalasin B, an actin-depolymerizing compound, induced a substantial increase in Cl- conductance under isotonic condition and potentiated the expression of Cl- currents in hypotonic stress. Phorbol 12-myristate 13-acetate (PMA) significantly inhibited the cytochalasin B-induced activation of Cl- conductance under isotonic condition. On the other hand, treatment with cytochalasin B significantly prolonged the RVD responses. Phalloidin, a stabilizer of actin polymerization, did not change the basal currents under isotonic condition, but completely abolished the increase in whole-cell Cl- conductance elicited by hypotonicity and retarded the cell volume recovery. Colchicine, a microtubule-assembly inhibitor, had no effect on either basal Cl- conductance or volume-sensitive Cl- current and was unable to inhibit the RVD responses. Taxol, a microtubule-stabilizing compound, did not alter the basal Cl- conductance, but inhibited the activation of volume-sensitive Cl- channels as well as the process of RVD in a dose-dependent manner. These data support the notion that functional integrity of actin filaments and microtubules plays critical roles in maintaining the RVD responses and activation of Cl- channels in human cervical cancer HT-3 cells.

Effect of coupling on volume-regulatory response of ciliary epithelial cells suggests mechanism for secretion

The ciliary epithelium of the eye secretes the aqueous humor. It is a double epithelium arranged so that the apical surfaces of the nonpigmented ciliary epithelial (NPCE) and pigmented ciliary epithelial (PCE) cells face each other and the basolateral membranes face the inside of the eye and the blood, respectively. We have investigated the volume responses of both single cells and coupled pairs from this tissue to osmotic challenge. Both NPCE and PCE cells undergo regulatory volume increase (RVI) and decrease (RVD) when exposed to hyper- and hyposmotic solution, respectively. In hyposmotic solution single cells swell and return to their original volumes within approximately 3 min. In nonpigmented cells RVD could be inhibited by blockers of volume-activated Cl- channels [tamoxifen (100%) > quinidine (87%) > DIDS (84%) > 5-nitro-2-(3-phenylpropylamino)benzoic acid (80%) > SITS (58%)] and K+ channels [Ba2+ (31%)]. However, in PCE cells these inhibitors and additionally tetraethylammonium and Gd3+ were without effect. Only bumetanide, an inhibitor of Na+-K+-2Cl- cotransport, was found to have any effect on RVD in PCE cells. NPCE-PCE cell coupled pairs also underwent RVD, but with altered kinetics. The onset of RVD of the PCE cell in a pair occurred approximately 80 s before that of the NPCE cell, and the peak swell was reduced. This is consistent with fluid movement from the PCE to the NPCE cell. The effect of the volume-activated Cl- channel inhibitor tamoxifen was to eliminate this difference in the times of onset of RVD in coupled cell pairs and to inhibit RVD in both the NPCE and PCE cells partially. On the basis of these observations we suggest that fluid is transferred from the PCE to the NPCE cell in coupled pairs during cell swelling and the subsequent RVD. Furthermore, we speculate that reciprocal RVI-RVD could underlie aqueous humor secretion.

Effects of hypotonic swelling on the cellular distribution and expression of pI(Cln) in human nonpigmented ciliary epithelial cells

PURPOSE

It has been proposed that pI(Cln), a highly acidic protein, is a candidate gene product related to the swelling-activated chloride (Cl-) channel Icl.swell in mammalian cells. However, no consensus has been reached as to whether this relationship is direct or indirect. Recently the cDNA for pI(Cln) was isolated from human ciliary epithelial cells. To learn more about the structure-function of pI(Cln) we attempted to: i) overexpress pI(Cln) as a fusion protein in bacteria; ii) carry out its purification; iii) generate polyclonal antibodies to study its expression and cellular localization in the ciliary epithelial cells; and iv) determine whether cell-swelling affects pI(Cln) expression in ciliary epithelial cells.

METHODS

The open reading frame (ORF) of human pI(Cln) was subcloned in the pET-20b(+) plasmid and established as a recombinant vector in E. coli BL21(DE3)pLysS cells. Upon induction with iso-propyl-beta-thio-galactopyranoside (IPTG), pI(Cln) was isolated as a His-Tag fusion protein and purified to homogeneity. Polyclonal antibodies were raised in rabbits after immunization with pI(Cln) purified protein, and its expression and cellular distribution in ciliary epithelial cells determined by Western blot, immunoprecipitation and indirect immunofluorescence respectively. Cell-swelling effect on ciliary epithelial cells was carried out upon treatment of cultured cells with hypotonic solution up to 60 min and pI(Cln) expression measured by Northern and Western blot analysis.

RESULTS

By Western blot analysis or immunoprecipitation, pI(Cln) antisera recognized a main band of 37-kDa in total cell extracts from ciliary body or metabolically labeled ciliary epithelial cells. By indirect immunofluorescence, pI(Cln) antibodies stained the cytoplasm of NPE in the intact tissue, and the perinuclear region of cultured ciliary epithelial cells. When subjected to hypotonic treatment, NPE cells did not induce translocation of pI(Cln) protein from the cytoplasm into the plasma membrane, nor changes in pI(Cln) expression at the protein level, but did down regulate up to 30% the level of pI(Cln) mRNA in continued hypotonic treatment.

CONCLUSIONS

These observations indicate that, contrary to previous suggestions, the pI(Cln) protein is not likely to be in contact with the plasma membrane of ciliary epithelial cells, and its influence on Cl- -channel activity is more likely to be expressed indirectly, (i.e. through cytoskeletal restructuring).

Functional and molecular characterization of a volume-sensitive chloride current in rat brain endothelial cells

1. Volume-activated chloride currents in cultured rat brain endothelial cells were investigated on a functional level using the whole-cell voltage-clamp technique and on a molecular level using the reverse transcriptase-polymerase chain reaction (RT-PCR). 2. Exposure to a hypotonic solution caused the activation of a large, outward rectifying current, which exhibited a slight time-dependent decrease at strong depolarizing potentials. The anion permeability of the induced current was I- (1.7) > Br- (1.2) > Cl- (1.0) > F- (0. 7) > gluconate (0.18). 3. The chloride channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB, 100 microM) rapidly and reversibly inhibited both inward and outward currents. The chloride transport blocker 4,4'-diisothiocyanatostilbene-2, 2'-disulphonic acid (DIDS, 100 microM) also blocked the hypotonicity-induced current in a reversible manner. In this case, the outward current was more effectively suppressed than the inward current. The volume-activated current was also inhibited by the antioestrogen tamoxifen (10 microM). 4. The current was dependent on intracellular ATP and independent of intracellular Ca2+. 5. Activation of protein kinase C by phorbol 12,13-dibutyrate (PDBu, 100 nM) inhibited the increase in current normally observed following hypotonic challenge. 6. Extracellular ATP (10 mM) inhibited the current with a more pronounced effect on the outward than the inward current. 7. Verapamil (100 microM) decreased both the inward and the outward hypotonicity-activated chloride current. 8. RT-PCR analysis was used to determine possible molecular candidates for the volume-sensitive current. Expression of the ClC-2, ClC-3 and ClC-5 chloride channels, as well as pICln, could be shown at the mRNA level. 9. We conclude that rat brain endothelial cells express chloride channels which are activated by osmotic swelling. The biophysical and pharmacological properties of the current show strong similarities to those of ClC-3 channel currents as described in other cell types.

Role of P-glycoprotein in distribution of rhodamine 123 into aqueous humor in rabbits

PURPOSE

Examination of the contribution of functional P-glycoprotein (P-gp), an ATP-dependent efflux pump, in blood-aqueous barrier in rabbits.

METHODS

Rhodamine-123 (Rho-123), a P-gp substrate, was administered intravenously via the marginal ear vein of rabbits. Rhodamine B (Rho-B), an analogue of Rho-123, was also injected with the same dose, as a reference compound. Quinidine at different concentrations was applied topically to the corneal surface by eye drops prior to the intravenous administration of a Rho compound. The aqueous distribution (a ratio of concentration in aqueous humor to that in plasma) of these Rho compounds was analyzed in relation to the aqueous concentration of quinidine. Transport study across Caco-2 cell monolayers was carried out to examine the involvement of P-gp in Rho-B transport.

RESULTS

It was proved that Rho-B is not a P-gp substrate by a transport study across Caco-2 cell monolayers, in contrast to Rho-123 (a P-gp substrate). The aqueous distribution of Rho-123 given intravenously was significantly lower than that of Rho-B. Topical quinidine (a P-gp inhibitor) markedly increased the aqueous distribution of Rho-123, depending on the aqueous concentrations of quinidine, though it did not affect the aqueous distribution of Rho-B.

CONCLUSIONS

The contribution of functional P-gp in blood-aqueous barrier was clearly demonstrated by analyzing the aqueous distribution of Rho-123 in the presence or absence of quinidine. These experiments only allow us to address one part of the blood-aqueous barrier, the capillary endothelium, and, to do so by using different substrates for P-gp, a sort of chemical analogy with the presumed blood-aqueous barrier across capillary endothelia. The alteration of P-gp function by pharmacotherapy or in pathological state should be considered in the ophthalmic medical treatment.

Association of intrinsic pICln with volume-activated Cl- current and volume regulation in a native epithelial cell

We investigated the relationship between pICln, the volume-activated Cl- current, and volume regulation in native bovine nonpigmented ciliary epithelial (NPCE) cells. Immunofluorescence studies demonstrated the presence of pICln protein in the NPCE cells. Exposure to hypotonic solution activated a Cl- current and induced regulatory volume decrease (RVD) in freshly isolated bovine NPCE cells. Three antisense oligonucleotides complementary to human pICln mRNA were used in the experiments. The antisense oligonucleotides were taken up by the cells in a dose-dependent manner. The antisense oligonucleotides, designed to be complementary to the initiation codon region of the human pICln mRNA, "knocked down" the pICln protein immunofluorescence, delayed the activation of volume-activated Cl- current, diminished the value of the current, and reduced the ability of the cells to volume regulate. We conclude that pICln is involved in the activation pathway of the volume-activated Cl- current and RVD following hypotonic swelling.

Pharmacology of CFTR chloride channel activity

Pharmacology of CFTR Chloride Channel Activity. Physiol. Rev. 79, Suppl.: S109-S144, 1999. - The pharmacology of cystic fibrosis transmembrane conductance regulator (CFTR) is at an early stage of development. Here we attempt to review the status of those compounds that modulate the Cl- channel activity of CFTR. Three classes of compounds, the sulfonylureas, the disulfonic stilbenes, and the arylaminobenzoates, have been shown to directly interact with CFTR to cause channel blockade. Kinetic analysis has revealed the sulfonylureas and arylaminobenzoates interact with the open state of CFTR to cause blockade. Suggestive evidence indicates the disulfonic stilbenes act by a similar mechanism but only from the intracellular side of CFTR. Site-directed mutagenesis studies indicate the involvement of specific amino acid residues in the proposed transmembrane segment 6 for disulfonic stilbene blockade and segments 6 and 12 for arylaminobenzoate blockade. Unfortunately, these compounds (sulfonylureas, disulfonic stilbenes, arylaminobenzoate) also act at a number of other cellular sites that can indirectly alter the activity of CFTR or the transepithelial secretion of Cl-. The nonspecificity of these compounds has complicated the interpretation of results from cellular-based experiments. Compounds that increase the activity of CFTR include the alkylxanthines, phosphodiesterase inhibitors, phosphatase inhibitors, isoflavones and flavones, benzimidazolones, and psoralens. Channel activation can arise from the stimulation of the cAMP signal transduction cascade, the inhibition of inactivating enzymes (phosphodiesterases, phosphatases), as well as the direct binding to CFTR. However, in contrast to the compounds that block CFTR, a detailed understanding of how the above compounds increase the activity of CFTR has not yet emerged.

Characterization of cell volume-sensitive chloride currents in freshly prepared and cultured pancreatic acinar cells from early postnatal rats

1. In freshly prepared and cultured exocrine pancreatic acinar cells from 5- to 7-day-old rats a chloride-selective membrane conductance could be activated by intracellular application of GTPgammaS (40-100 microM), by application of positive pressure (5 cmH2O) to the pipette interior or by challenging the cells with a hyposmolar bath solution. Hyperosmolar bath solutions inhibited the cell volume-sensitive chloride currents. 2. The anion permeability sequence of the cell volume-sensitive chloride conductance was I- > Cl- approximately Br- > F- > methanesulphonate- > glutamate-. I- had a higher permeability but lower conductance than Cl-. The permeability ratio for Pglutamate/PCl was 0.12. 3. The cell volume-sensitive chloride conductance showed outward rectification. Membrane depolarization to high positive voltages (>= +60 mV) caused a time-dependent decay in outward currents. 4. DIDS (4, 4'-diisothiocyanatostilbene-2,2'-disulphonic acid) and SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid) reversibly inhibited the cell volume-sensitive chloride current in a voltage-dependent manner. NPPB (5-nitro-2-(3-phenylpropylamino)-benzoic acid), quinidine, quinine and tamoxifen caused voltage-independent current inhibition. 5. Combined fura-2 and whole-cell current measurements showed that activation of the cell volume-sensitive chloride current does not involve cytosolic Ca2+ signals. Furthermore, there is no evidence that Ca2+-activated chloride currents play a significant role in cultured pancreatic acinar cells from 5- to 7-day-old rats. 6. Polymerase chain reaction followed by DNA sequence analysis indicated the presence of mRNA homologous to the ClC-3 chloride channel in pancreatic tissue from 5-day-old rats.

Antisense to MDR1 mRNA reduces P-glycoprotein expression, swelling-activated C1- current and volume regulation in bovine ciliary epithelial cells

Native ciliary epithelial cells from the ciliary epithelium of the eye exhibit anti-P-glycoprotein (P-gp) immunofluorescence. We have used an antisense 'knock-down' approach to investigate the relationship between P-gp and the volume-activated chloride current (IC1,swell) and its role in volume regulation. An antisense oligonucleotide to the human multidrug resistance (MDR1) gene, taken up by the cells in a dose-dependent manner, reduced P-gp immunofluorescence, inhibited IC1,swell and significantly increased the latency of activation of IC1,swell. Increasing the hypotonic stress did not result in an increased activation of ICl,swell. MDR1 antisense 'knock-down' also reduced the ability of the cells to volume regulate following a hypotonic challenge. These cells are known to express at least two volume-activated chloride channels, and the data suggest that P-gp is involved in the activation pathway of a subset of channels that contribute to whole-cell IC1,swell and participate in volume regulation.

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

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

Development of an intrinsic P-glycoprotein-mediated doxorubicin resistance in quiescent cell layers of large, multicellular prostate tumor spheroids

Growing multicellular prostate tumor spheroids develop quiescent cell subpopulations in central regions with features of intrinsic multicell-mediated drug resistance. Doxorubicin (dox) uptake was significantly reduced in large spheroids (diameter 400+/-70 microm), which consist predominantly of quiescent cells, as compared to small spheroids (diameter 100+/-50 microm), which consist entirely of proliferating cells. After removal of dox from the incubation medium, dox fluorescence declined more efficiently in large spheroids, which led to a decreased dox toxicity as revealed by colony-forming assays. Verapamil significantly increased dox retention in large spheroids and, consequently, augmented dox toxicity. At a depth 80 microm from the spheroid periphery, a significantly decreased dox fluorescence was observed in the deep, quiescent cell layers of large spheroids. The P-glycoprotein-mediated multidrug resistance (MDR)-reversing agents verapamil, cyclosporin A, quinidine, sodium orthovanadate and tamoxifen significantly increased dox fluorescence at this depth, whereas genistein, indomethacin, probenecid and brefeldin A, which reverse multidrug-resistance-associated protein (MRP) function, exerted no effect. Anti-P-glycoprotein immunohistochemistry of multicellular tumor spheroids revealed an increase of P-glycoprotein expression in large speroids as compared to small spheroids, which was most prominent in the Ki-67-negative, quiescent cell layers 60 to 100 microm distant from the periphery of the spheroid, indicating that the MDR phenotype is related to cell quiescence. This was corroborated by whole-cell patch-clamp experiments, where the C219 antibody, which is directed against the ATP-binding site of P-glycoprotein, significantly inhibited P-glycoprotein-associated, volume-activated chloride currents in quiescent, but not proliferating cells from multicellular tumor spheroids.