Mechanistic Effects of Baicalein on Aqueous Humor Drainage and Intraocular Pressure

Elevated intraocular pressure (IOP) is a major risk factor for glaucoma that results from impeded fluid drainage. The increase in outflow resistance is caused by trabecular meshwork (TM) cell dysfunction and excessive extracellular matrix (ECM) deposition. Baicalein (Ba) is a natural flavonoid and has been shown to regulate cell contraction, fluid secretion, and ECM remodeling in various cell types, suggesting the potential significance of regulating outflow resistance and IOP. We demonstrated that Ba significantly lowered the IOP by about 5 mmHg in living mice. Consistent with that, Ba increased the outflow facility by up to 90% in enucleated mouse eyes. The effects of Ba on cell volume regulation and contractility were examined in primary human TM (hTM) cells. We found that Ba (1–100 µM) had no effect on cell volume under iso-osmotic conditions but inhibited the regulatory volume decrease (RVD) by up to 70% under hypotonic challenge. In addition, Ba relaxed hTM cells via reduced myosin light chain (MLC) phosphorylation. Using iTRAQ-based quantitative proteomics, 47 proteins were significantly regulated in hTM cells after a 3-h Ba treatment. Ba significantly increased the expression of cathepsin B by 1.51-fold and downregulated the expression of D-dopachrome decarboxylase and pre-B-cell leukemia transcription factor-interacting protein 1 with a fold-change of 0.58 and 0.40, respectively. We suggest that a Ba-mediated increase in outflow facility is triggered by cell relaxation via MLC phosphorylation along with inhibiting RVD in hTM cells. The Ba-mediated changes in protein expression support the notion of altered ECM homeostasis, potentially contributing to a reduction of outflow resistance and thereby IOP.

cAMP Stimulates Transepithelial Short-Circuit Current and Fluid Transport Across Porcine Ciliary Epithelium

Purpose

To investigate the effects of cAMP on transepithelial electrical parameters and fluid transport across porcine ciliary epithelium.

Methods

Transepithelial electrical parameters were determined by mounting freshly isolated porcine ciliary epithelium in a modified Ussing chamber. Similarly, fluid movement across intact ciliary body was measured with a custom-made fluid flow chamber.

Results

Addition of 1, 10, and 100 μM 8-Br-cAMP (cAMP) to the aqueous side (nonpigmented ciliary epithelium, NPE) induced a sustained increase in short-circuit current (Isc). Addition of niflumic acid (NFA) to the aqueous surface effectively blocked the cAMP-induced Isc stimulation. The administration of cAMP to the stromal side (pigmented ciliary epithelium, PE) triggered a significant stimulation of Isc only at 100 μM. No additive effect was observed with bilateral application of cAMP. Likewise, forskolin caused a significant stimulation of Isc when applied to the aqueous side. Concomitantly, cAMP and forskolin increased fluid transport across porcine ciliary epithelium, and this stimulation was effectively inhibited by aqueous NFA. Depleting Cl- in the bathing solution abolished the baseline Isc and inhibited the subsequent stimulation by cAMP. Pretreatment with protein kinase A (PKA) blockers (H89/KT5720) significantly inhibited the cAMP- and forskolin-induced Isc responses.

Conclusions

Our results suggest that cAMP triggers a sustained stimulation of Cl- and fluid transport across porcine ciliary epithelium; Cl- channels in the NPE cells are potentially a cellular site for this PKA-sensitive cAMP-mediated response.

Regulatory Roles of Anoctamin-6 in Human Trabecular Meshwork Cells

Purpose

Trabecular meshwork (TM) cell volume is a determinant of aqueous humor outflow resistance, and thereby IOP. Regulation of TM cell volume depends on chloride ion (Cl-) release through swelling-activated channels (ICl,Swell), whose pore is formed by LRRC8 proteins. Chloride ion release through swelling-activated channels has been reported to be regulated by calcium-activated anoctamins, but this finding is controversial. Particularly uncertain has been the effect of anoctamin Ano6, reported as a Ca2+-activated Cl- (CaCC) or cation channel in other cells. The current study tested whether anoctamin activity modifies volume regulation of primary TM cell cultures and cell lines.

Methods

Gene expression was studied with quantitative PCR, supplemented by reverse-transcriptase PCR and Western immunoblots. Currents were measured by ruptured whole-cell patch clamping and volume by electronic cell sizing.

Results

Primary TM cell cultures and the TM5 and GTM3 cell lines expressed Ano6 3 to 4 orders of magnitude higher than the other anoctamin CaCCs (Ano1 and Ano2). Ionomycin increased cell Ca2+ and activated macroscopic currents conforming to CaCCs in other cells, but displayed significantly more positive mean reversal potentials (+5 to +12 mV) than those displayed by ICl,Swell (-14 to -21 mV) in the same cells. Nonselective CaCC inhibitors (tannic acid>CaCCinh-A01) and transient Ano6 knockdown strongly inhibited ionomycin-activated currents, ICl,Swell and the regulatory volume response to hyposmotic swelling.

Conclusions

Ionomycin activates CaCCs associated with net cation movement in TM cells. These currents, ICl,Swell, and cell volume are regulated by Ano6. The findings suggest a novel clinically-relevant approach for altering cell volume, and thereby outflow resistance, by targeting Ano6.

Ocular Purine Receptors as Drug Targets in the Eye

Agonists and antagonists of various subtypes of G protein coupled adenosine receptors (ARs), P2Y receptors (P2YRs), and ATP-gated P2X receptor ion channels (P2XRs) are under consideration as agents for the treatment of ocular diseases, including glaucoma and dry eye. Numerous nucleoside and nonnucleoside modulators of the receptors are available as research tools and potential therapeutic molecules. Three of the 4 subtypes of ARs have been exploited with clinical candidate molecules for treatment of the eye: A1, A2A, and A3. An A1AR agonist is in clinical trials for glaucoma, A2AAR reduces neuroinflammation, A3AR protects retinal ganglion cells from apoptosis, and both A3AR agonists and antagonists had been reported to lower intraocular pressure (IOP). Extracellular concentrations of endogenous nucleotides, including dinucleoside polyphosphates, are increased in pathological states, activating P2Y and P2XRs throughout the eye. P2YR agonists, including P2Y2 and P2Y6, lower IOP. Antagonists of the P2X7R prevent the ATP-induced neuronal apoptosis in the retina. Thus, modulators of the purinome in the eye might be a source of new therapies for ocular diseases.

Temperature oscillations drive cycles in the activity of MMP-2,9 secreted by a human trabecular meshwork cell line

PURPOSE

Aqueous humor inflow falls 50% during sleeping hours without proportional fall in IOP, partly reflecting reduced outflow facility. The mechanisms underlying outflow facility cycling are unknown. One outflow facility regulator is matrix metalloproteinase (MMP) release from trabecular meshwork (TM) cells. Because anterior segment temperature must oscillate due to core temperature cycling and eyelid closure during sleep, we tested whether physiologically relevant temperature oscillations drive cycles in the activity of secreted MMP.

METHODS

Temperature of transformed normal human TM cells (hTM5 line) was fixed or alternated 12 hours/12 hours between 33°C and 37°C. Activity of secreted MMP-2 and MMP-9 was measured by zymography, and gene expression by RT-PCR and quantitative PCR.

RESULTS

Raising temperature to 37°C increased, and lowering to 33°C reduced, activity of secreted MMP. Switching between 37°C and 33°C altered MMP-9 by 40% ± 3% and MMP-2 by 22% ± 2%. Peripheral circadian clocks did not mediate temperature-driven cycling of MMP secretion because MMP-release oscillations did not persist at constant temperature after 3 to 6 days of alternating temperatures, and temperature cycles did not entrain clock-gene expression in these cells. Furthermore, inhibiting heat shock transcription factor 1, which links temperature and peripheral clock-gene oscillations, inhibited MMP-9 but not MMP-2 temperature-driven MMP cycling. Inhibition of heat-sensitive TRPV1 channels altered total MMP secretion but not temperature-induced modulations. Inhibiting cold-sensitive TRPM-8 channels had no effect.

CONCLUSIONS

Physiologically relevant temperature oscillations drive fluctuations of secreted MMP-2 and MMP-9 activity in hTM5 cells independent of peripheral clock genes and temperature-sensitive TRP channels.

The role of activated adenosine receptors in degranulation of human LAD2 mast cells

Mast cell degranulation triggers hypersensitivity reactions at the body-environment interface. Adenosine modulates degranulation, but enhancement and inhibition have both been reported. Which of four adenosine receptors (ARs) mediate modulation, and how, remains uncertain. Also uncertain is whether adenosine reaches mast cell ARs by autocrine ATP release and ecto-enzymatic conversion. Uncertainties partly reflect species and cell heterogeneity, circumvented here by focusing on homogeneous human LAD2 cells. Quantitative PCR detected expression of A2A, A2B, and A3, but not A1, ARs. Nonselective activation of ARs with increasing NECA monotonically enhanced immunologically or C3a-stimulated degranulation. NECA alone stimulated degranulation slightly. Selective AR antagonists did not affect C3a-stimulated degranulation. NECA's enhancement of C3a-triggered degranulation was partially inhibited by separate application of each selective antagonist, and abolished by simultaneous addition of antagonists to the three ARs. Only the A2A antagonist separately inhibited NECA's enhancement of immunologically stimulated degranulation, which was abolished by simultaneous addition of the three selective antagonists. Immunological or C3a activation did not stimulate ATP release. NECA also enhanced immunologically triggered degranulation of mouse bone marrow derived mast cells (BMMCs), which was partially reduced only by simultaneous addition of the three antagonists or by the nonselective antagonist CGS15943. BMMCs also expressed A2A, A2B, and A3 ARs. but not A1AR detectably. We conclude that (a) A1AR is unnecessary for LAD2 degranulation or AR enhancement; (b) A2A, A2B, and A3 ARs all contribute to pharmacologic AR enhancement of LAD2 and BMMC degranulation; and (c) LAD2 cells depend on microenvironmental adenosine to trigger AR modulation.

Purines in the eye: recent evidence for the physiological and pathological role of purines in the RPE, retinal neurons, astrocytes, Müller cells, lens, trabecular meshwork, cornea and lacrimal gland

This review highlights recent findings that describ how purines modulate the physiological and pathophysiological responses of ocular tissues. For example, in lacrimal glands the cross-talk between P2X7 receptors and both M3 muscarinic receptors and α1D-adrenergic receptors can influence tear secretion. In the cornea, purines lead to post-translational modification of EGFR and structural proteins that participate in wound repair in the epithelium and influence the expression of matrix proteins in the stroma. Purines act at receptors on both the trabecular meshwork and ciliary epithelium to modulate intraocular pressure (IOP); ATP-release pathways of inflow and outflow cells differ, possibly permitting differential modulation of adenosine delivery. Modulators of trabecular meshwork cell ATP release include cell volume, stretch, extracellular Ca(2+) concentration, oxidation state, actin remodeling and possibly endogenous cardiotonic steroids. In the lens, osmotic stress leads to ATP release following TRPV4 activation upstream of hemichannel opening. In the anterior eye, diadenosine polyphosphates such as Ap4A act at P2 receptors to modulate the rate and composition of tear secretion, impact corneal wound healing and lower IOP. The Gq11-coupled P2Y1-receptor contributes to volume control in Müller cells and thus the retina. P2X receptors are expressed in neurons in the inner and outer retina and contribute to visual processing as well as the demise of retinal ganglion cells. In RPE cells, the balance between extracellular ATP and adenosine may modulate lysosomal pH and the rate of lipofuscin formation. In optic nerve head astrocytes, mechanosensitive ATP release via pannexin hemichannels, coupled with stretch-dependent upregulation of pannexins, provides a mechanism for ATP signaling in chronic glaucoma. With so many receptors linked to divergent functions throughout the eye, ensuring the transmitters remain local and stimulation is restricted to the intended target may be a key issue in understanding how physiological signaling becomes pathological in ocular disease.

The cellular specificity of the effect of vasopressin on toad urinary bladder

Phase and electron micrographs of toad bladders were obtained following dilution of bathing media in the presence and absence of vasopressin. Dilution of the mucosal medium alone resulted in no morphologic changes. Subsequent addition of vasopressin produced an increase in the cell volume of the granular cells, manifested by some or all of the following changes: increased area of granular cell profiles as observed in sections, rounding of the cell nucleus, displacement of the two components of the nuclear envelope, loss of nuclear heterochromatin, sacculation of the endoplasmic reticulum and the Golgi apparatus, and reduction in the electron density of the cell cytoplasm. No such morphologic changes were noted in the other cell types comprising the mucosal epithelium - the mitochondria-rich, the goblet, and the basal cells. On the other hand, dilution of the serosal bathing medium in the absence of vasopressin caused a marked increase in the cell volume of all these cell types. The results demonstrate that the action of vasopressin to enhance bulk water flow across toad bladder is exerted specifically on the apical surface of the granular cells. It is suggested that the hormonal effect on sodium transport may also be limited to the granular cells. The route of osmotic water flow and the possible role of the other mucosal epithelial cells is discussed.

Determination of the driving force of the Na(+) pump in toad bladder by means of vasopressin

Vasopressin stimulates Na(+) transport across toad bladder largely or entirely by decreasing the resistance to Na(+) entry into the transporting epithelial cells. Therefore, the hormone should induce proportional changes in short circuit current (I S ) and tissue conductance; the ratio of these changes should equal the driving force (E Na) of the Na(+) pump.Administration of vasopressin provided a rapid, reversible and reproducible technique for the measurement ofE Na. Values calculated forE Na ranged from 74 to 186 mV, in agreement with previously published estimates. The results were not dependent on the vasopressin concentration over a wide range of concentrations.Ouabain, an agent thought to inhibit specifically the Na(+) pump, decreased bothI S andE Na. On the other hand, amiloride, a diuretic thought to block specifically Na(+) entry, markedly reducedI S , without reducingE Na.It is concluded that vasopressin constitutes a probe for the rapid reproducible determination ofE Na under a wide variety of physiological conditions.

CALHM1 ion channel mediates purinergic neurotransmission of sweet, bitter and umami tastes

Recognition of sweet, bitter and umami tastes requires the non-vesicular release from taste bud cells of ATP, which acts as a neurotransmitter to activate afferent neural gustatory pathways. However, how ATP is released to fulfil this function is not fully understood. Here we show that calcium homeostasis modulator 1 (CALHM1), a voltage-gated ion channel, is indispensable for taste-stimuli-evoked ATP release from sweet-, bitter- and umami-sensing taste bud cells. Calhm1 knockout mice have severely impaired perceptions of sweet, bitter and umami compounds, whereas their recognition of sour and salty tastes remains mostly normal. Calhm1 deficiency affects taste perception without interfering with taste cell development or integrity. CALHM1 is expressed specifically in sweet/bitter/umami-sensing type II taste bud cells. Its heterologous expression induces a novel ATP permeability that releases ATP from cells in response to manipulations that activate the CALHM1 ion channel. Knockout of Calhm1 strongly reduces voltage-gated currents in type II cells and taste-evoked ATP release from taste buds without affecting the excitability of taste cells by taste stimuli. Thus, CALHM1 is a voltage-gated ATP-release channel required for sweet, bitter and umami taste perception.

Effects of cardiotonic steroids on trabecular meshwork cells: search for mediator of ouabain-enhanced outflow facility

Lowering intraocular pressure (IOP) is currently the only strategy documented to slow the onset and progression of glaucomatous blindness. Ouabain, a cardiotonic glycoside inhibitor of Na(+), K(+)-activated ATPase, was recently reported to enhance outflow facility in porcine anterior segments at concentrations as low as 30 nM for ≥4 h, suggesting a novel approach to lowering IOP. The underlying mechanism is unknown, but associated cytoskeletal changes were observed in porcine trabecular meshwork cells. We have previously found that changes in ATP release and subsequent ectoenzymatic conversion to adenosine may play a role in linking cytoskeletal remodeling with modulation of outflow resistance. We now tested whether altered ATP release might also be a mediator of ouabain's effect on outflow facility. ATP release from transformed human TM5 and explant-derived human trabecular meshwork cells was measured by the luciferin-luciferase reaction. Matrix metalloproteinases (MMPs) were studied by zymography, cell Na(+) concentration by SBFI fluorometry, gene expression of ATP-release pathways by real-time PCR, cell volume by electronic cell sorting and cell viability by the LDH and MTT methods. Actin was examined by confocal microscopy of phalloidin-stained cells. Contrary to expectation, ouabain at concentrations ≥10 nM inhibited swelling-triggered ATP release from TM5 cells after ≥4 h of exposure. Inhibition was enhanced by increasing ouabain concentration and exposure time. Similar effects were produced by the reversible cardiac aglycone strophanthidin. Ouabain also inhibited swelling-activated ATP release from explant-derived native human TM cells. Ouabain (4 h, 30 nM and 100 nM) did not alter gene expression of the ATP-release pathways, and cell viability was unchanged by exposure to ouabain (30 nM-1 μM). Preincubation with 30 nM ouabain for 4 h did not detectably change Na(+) level, the regulatory volume decrease (RVD) or the actin cytoskeleton of TM5 cells, but did inhibit hypotonicity-elicited ATP release. Moreover, even when N-methyl-d-glucosamine replaced Na(+) in the extracellular fluid, ouabain still inhibited swelling-initiated ATP release at 100 nM. In the absence of ouabain, extracellular ATP stimulated MMP secretion, which was largely blocked by inhibiting conversion of ATP to adenosine, as expected. In contrast, ouabain reduced ATP release, but did not alter secretion of MMP-2 and MMP-9 from cells pretreated for ≤4 h. The results suggest that: (1) ouabain can trigger enhancement of outflow facility independent of its transport and actin-restructuring effects exerted at higher concentration and longer duration; (2) ouabain exerts parallel independent effects on ATP release and outflow facility; and (3) these effects likely reflect ouabain-induced changes in the scaffolding and/or signaling functions of Na(+), K(+)-activated ATPase.

Mechanisms of ATP release, the enabling step in purinergic dynamics

The only effective intervention to slow onset and progression of glaucomatous blindness is to lower intraocular pressure (IOP). Among other modulators, adenosine receptors (ARs) exert complex regulation of IOP. Agonists of A(3)ARs in the ciliary epithelium activate Cl(-) channels, favoring increased formation of aqueous humor and elevated IOP. In contrast, stimulating A(1)ARs in the trabecular outflow pathway enhances release of matrix metalloproteinases (MMPs) from trabecular meshwork (TM) cells, reducing resistance to outflow of aqueous humor to lower IOP. These opposing actions are thought to be initiated by cellular release of ATP and its ectoenzymatic conversion to adenosine. This view is now supported by our identification of six ectoATPases in trabecular meshwork (TM) cells and by our observation that external ATP enhances TM-cell secretion of MMPs through ectoenzymatic formation of adenosine. ATP release is enhanced by cell swelling and stretch. Also, enhanced ATP release and downstream MMP secretion is one mediator of the action of actin depolymerization to reduce outflow resistance. Inflow and outflow cells share pannexin-1 and connexin hemichannel pathways for ATP release. However, vesicular release and P2X(7) release pathways were functionally limited to inflow and outflow cells, respectively, suggesting that blocking exocytosis might selectively inhibit inflow, lowering IOP.

Cytoskeletal dependence of adenosine triphosphate release by human trabecular meshwork cells

PURPOSE

To test whether adenosine triphosphate (ATP) release links cytoskeletal remodeling with release of matrix metalloproteinases (MMPs), regulators of outflow facility and intraocular pressure.

METHODS

ATP release was measured by luciferin-luciferase. Ecto-ATPases from transformed human trabecular meshwork (TM) cells (TM5) and explant-derived TM cells were identified by RT-PCR. Actin was visualized by phalloidin staining. Cell viability was assayed by lactate dehydrogenase and thiazolyl blue tetrazolium bromide methods and propidium iodide exclusion, gene expression by real-time PCR, and MMP release by zymography. Cell volume was monitored by electronic cell sorting.

RESULTS

Hypotonicity (50%) and mechanical stretch increased ATP release with similar pharmacologic profiles. TM cells expressed ecto-ATPases E-NPP1-3, E-NTPD2, E-NTPD8, and CD73. Prolonged dexamethasone (DEX) exposure (≥ 2 weeks), but not brief exposure (3 days), increased cross-linked actin networks and reduced swelling-triggered ATP release. Cytochalasin D (CCD) exerted opposite effects. Neither DEX nor CCD altered the cell viability, gene expression, or pharmacologic profile of ATP-release pathways. DEX accelerated, and CCD slowed, the regulatory volume decrease after hypotonic exposure. Activating A(1) adenosine receptors (A(1)ARs) increased total MMP-2 and MMP-9 release. DEX reduced total A(1)AR-triggered MMP release, and CCD increased the active form of MMP-2 release. The A(1)AR agonist CHA and the A(1)AR antagonist DPCPX partially reversed the effects of DEX and CCD, respectively.

CONCLUSIONS

Cytoskeletal restructuring modulated swelling-activated ATP release, in part by changing the duration of cell swelling after hypotonic challenge. Modifying ATP release is expected to modulate MMP secretion by altering ecto-enzymatic delivery of adenosine to A(1)ARs, linking cytoskeletal remodeling and MMP-mediated modulation of outflow facility.

Pathways for ATP release by bovine ciliary epithelial cells, the initial step in purinergic regulation of aqueous humor inflow

ATP release by nonpigmented (NPE) and pigmented (PE) ciliary epithelial cells is the enabling step in purinergic regulation of aqueous humor formation, but the release pathways are unknown. We measured ATP release from primary cultures of bovine mixed NPE and PE (bCE) cells and transformed bovine NPE and PE cells, using the luciferin-luciferase reaction. Hypotonicity-triggered bCE ATP release was inhibited by the relatively selective blocker of pannexin-1 (PX1) hemichannels (probenecid, 1 mM, 47 ± 2%), by a connexin inhibitor (heptanol, 1 mM, 49 ± 4%), and by an inhibitor of vesicular release (bafilomycin A1, 25 ± 2%), but not by the P2X(7) receptor (P2RX(7)) antagonist KN-62. Bafilomycin A1 acts by reducing the driving force for uptake of ATP from the cytosol into vesicles. The reducing agent dithiothreitol reduced probenecid-blockable ATP release. Similar results were obtained with NPE and PE cell lines. Pannexins PX1-3, connexins Cx43 and Cx40, and P2RX(7) were identified in native cells and cell lines by RT-PCR. PX1 mRNA expression was confirmed by Northern blots; its quantitative expression was comparable to that of Cx43 by real-time PCR. Heterologous expression of bovine PX1 in HEK293T cells enhanced swelling-activated ATP release, inhibitable by probenecid. We conclude that P2RX(7)-independent PX1 hemichannels, Cx hemichannels, and vesicular release contribute comparably to swelling-triggered ATP release. The relatively large response to dithiothreitol raises the possibility that the oxidation-reduction state is a substantial regulator of PX1-mediated ATP release from bovine ciliary epithelial cells.

Regulation of gap junction coupling in bovine ciliary epithelium

Aqueous humor is formed by fluid transfer from the ciliary stroma sequentially across the pigmented ciliary epithelial (PE) cells, gap junctions, and nonpigmented ciliary epithelial (NPE) cells. Which connexins (Cx) contribute to PE-NPE gap junctional formation appears species specific. We tested whether small interfering RNA (siRNA) against Cx43 (siCx43) affects bovine PE-NPE communication and whether cAMP affects communication. Native bovine ciliary epithelial cells were studied by dual-cell patch clamping, Lucifer Yellow (LY) transfer, quantitative polymerase chain reaction with reverse transcription (qRT-PCR), and Western immunoblot. qRT-PCR revealed at least 100-fold greater expression for Cx43 than Cx40. siCx43 knocked down target mRNA expression by 55 +/- 7% after 24 h, compared with nontargeting control siRNA (NTC1) transfection. After 48 h, siCx43 reduced Cx43 protein expression and LY transfer. The ratio of fluorescence intensity (R(f)) in recipient to donor cell was 0.47 +/- 0.09 (n = 11) 10 min after whole cell patch formation in couplets transfected with NTC1. siCx43 decreased R(f) by approximately 60% to 0.20 +/- 0.07 (n = 13, P < 0.02). Dibutyryl-cAMP (500 microM) also reduced LY dye transfer by approximately 60%, reducing R(f) from 0.41 +/- 0.05 (n = 15) to 0.17 +/- 0.05 (n = 20) after 10 min. Junctional currents were lowered by approximately 50% (n = 6) after 10-min perfusion with 500 microM dibutyryl-cAMP (n = 6); thereafter, heptanol abolished the currents (n = 5). Preincubation with the PKA inhibitor H-89 (2 microM) prevented cAMP-triggered current reduction (n = 6). We conclude that 1) Cx43, but not Cx40, is a major functional component of bovine PE-NPE gap junctions; and 2) under certain conditions, cAMP may act through PKA to inhibit bovine PE-NPE gap junctional communication.

Nucleoside-derived antagonists to A3 adenosine receptors lower mouse intraocular pressure and act across species

The purpose of the study was to determine whether novel, selective antagonists of human A3 adenosine receptors (ARs) derived from the A3-selective agonist Cl-IB-MECA lower intraocular pressure (IOP) and act across species. IOP was measured invasively with a micropipette by the Servo-Null Micropipette System (SNMS) and by non-invasive pneumotonometry during topical drug application. Antagonist efficacy was also assayed by measuring inhibition of adenosine-triggered shrinkage of native bovine nonpigmented ciliary epithelial (NPE) cells. Five agonist-based A3AR antagonists lowered mouse IOP measured with SNMS tonometry by 3-5 mm Hg within minutes of topical application. Of the five agonist derivatives, LJ 1251 was the only antagonist to lower IOP measured by pneumotonometry. No effect was detected pneumotonometrically over 30 min following application of the other four compounds, consonant with slower, smaller responses previously measured non-invasively following topical application of A3AR agonists and the dihydropyridine A3AR antagonist MRS 1191. Latanoprost similarly lowered SNMS-measured IOP, but not IOP measured non-invasively over 30 min. Like MRS 1191, agonist-based A3AR antagonists applied to native bovine NPE cells inhibited adenosine-triggered shrinkage. In summary, the results indicate that antagonists of human A3ARs derived from the potent, selective A3 agonist Cl-IB-MECA display efficacy in mouse and bovine cells, as well. When intraocular delivery was enhanced by measuring mouse IOP invasively, five derivatives of the A3AR agonist Cl-IB-MECA lowered IOP but only one rapidly reduced IOP measured non-invasively after topical application. We conclude that derivatives of the highly-selective A3AR agonist Cl-IB-MECA can reduce IOP upon reaching their intraocular target, and that nucleoside-based derivatives are promising A3 antagonists for study in multiple animal models.

Depression of intraocular pressure following inactivation of connexin43 in the nonpigmented epithelium of the ciliary body

PURPOSE

Conditional inactivation of connexin43 (Cx43) in the pigmented epithelium of the mouse eye results in a reduction in aqueous humor production and complete loss of the vitreous chamber. It was proposed that gap junctions between pigmented and nonpigmented epithelia of the ciliary body are critical for the production of the aqueous humor. To form such junctions, Cx43 in the pigmented epithelium must interact with connexin(s) present in the adjacent cells of the nonpigmented epithelium. The importance of Cx43 expression in the nonpigmented epithelium for the establishment of gap junctions and the regulation of intraocular pressure was tested.

METHODS

To inactivate Cx43 in the nonpigmented epithelium of the mouse eye, a mouse line was crossed with a floxed Cx43 locus (Cx43(flox/flox)) and a transgenic mouse line expressing cre recombinase under the control of the Pax6alpha promoter. General eye structure was evaluated by light microscopy, gap junctions were analyzed by electron microscopy, and intraocular pressure was directly assessed with micropipettes.

RESULTS

In Pax6alpha-cre/Cx43(flox/flox) mice, Cx43 was partially inactivated in the nonpigmented epithelium of the ciliary body and iris. Animals developed dilatations between the pigmented and nonpigmented epithelia and displayed a significant reduction in intraocular pressure. However, gap junctions between the ciliary epithelial layers were decreased but not eliminated.

CONCLUSIONS

Cx43 expression in the nonpigmented epithelium of the ciliary body contributes to the formation of gap junctions with the cells of the pigmented epithelium. These gap junctions play a critical role in maintaining the physical integrity of the ciliary body epithelium. Although the partial loss of Cx43 from the nonpigmented epithelium was correlated with a measurable drop in intraocular pressure, possible changes in Cx43 in the aqueous outflow pathway may provide an additional contribution to the observed phenotype.

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

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

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

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

Electron microprobe analysis of rabbit ciliary epithelium indicates enhanced secretion posteriorly and enhanced absorption anteriorly

The rate of aqueous humor formation sequentially across the pigmented (PE) and nonpigmented (NPE) ciliary epithelial cell layers may not be uniform over the epithelial surface. Because of the tissue's small size and complex geometry, this possibility cannot be readily tested by conventional techniques. Rabbit iris-ciliary bodies were divided, incubated, quick-frozen, cryosectioned, and freeze-dried for electron probe X-ray microanalysis of the elemental contents of the PE and NPE cells. We confirmed that preincubation with ouabain to block Na(+),K(+)-ATPase increases Na(+) and decreases K(+) contents far more anteriorly than posteriorly. The anterior and posterior regions were the iridial portion of the primary ciliary processes and the pars plicata, respectively. Following interruption of gap junctions with heptanol, ouabain produced smaller changes in anterior PE cells, possibly reflecting higher Na(+) or K(+) permeability of anterior NPE cells. Inhibiting Na(+) entry selectively with amiloride, benzamil, or dimethylamiloride reduced anterior effects of ouabain by approximately 50%. Regional dependence of net secretion was also assessed with hypotonic stress, which stimulates ciliary epithelial cell regulatory volume decrease (RVD) and net Cl(-) secretion. In contrast to ouabain's actions, the RVD was far more marked posteriorly than anteriorly. These results suggest that 1) enhanced Na(+) reabsorption anteriorly, likely through Na(+) channels and Na(+)/H(+) exchange, mediates the regional dependence of ouabain's actions; and 2) secretion may proceed primarily posteriorly, with secondary processing and reabsorption anteriorly. Stimulation of anterior reabsorption might provide a novel strategy for reducing net secretion.

Barrier qualities of the mouse eye to topically applied drugs

The mouse eye displays unusually rapid intraocular pressure (IOP) responses to topically applied drugs as measured by the invasive servo-null micropipette system (SNMS). To learn if the time course reflected rapid drug transfer across the thin mouse cornea and sclera, we monitored a different parameter, pupillary size, following topical application of droplets containing 40 microM (0.073 microg) carbachol. No miosis developed from this low carbachol concentration unless the cornea was impaled with an exploring micropipette as used in the SNMS. We also compared the mouse IOP response to several purinergic drugs, measured by the invasive SNMS and non-invasive pneumotonometry. Responses to the previously studied non-selective adenosine-receptor (AR) agonist adenosine, the A(3)-selective agonist Cl-IB-MECA and the A(3)-selective antagonist MRS 1191 were all enhanced to varying degrees, in time and magnitude, by corneal impalement. We conclude that the thin ocular coats of the mouse eye actually present a substantial barrier to drug penetration. Corneal impalement with even fine-tipped micropipettes can significantly enhance entry of topically-applied drugs into the mouse aqueous humor, reflecting either direct diffusion around the tip or a more complex impalement-triggered change in ocular barrier properties. Comparison of invasive and non-invasive measurement methods can document drug efficacy at intraocular target sites even if topical drug penetration is too slow to manifest convincing physiologic effects in intact eyes.

Cell-specific differential modulation of human trabecular meshwork cells by selective adenosine receptor agonists

Activation of A1 and A2A subtype adenosine receptors (AR) likely exert opposing effects on outflow of aqueous humor, and thereby, on intraocular pressure. Selective agonists of adenosine receptor (AR) subtypes have previously been applied to trabecular meshwork (TM) and Schlemm's canal (SC) cells to identify the site(s) of differential purinergic modulation. However, the apparent changes in volume monitored by previously measuring projected cell area might have partially reflected cell contraction and relaxation. In addition, whole-cell current responses of the TM cells previously described were highly variable following application of selective A1, A2A and A3 agonists. The complexity of the electrophysiologic responses may have reflected cell heterogeneity of the populations harvested from collagenase digestion of TM explants. We now report measurements of TM-cell volume using calcein fluorescence quenching, an approach independent of contractile state. Furthermore, we have applied selective AR agonists to a uniform population of human TM cells, the hTM5 cell line. A1, but not A2A or A3, AR agonists triggered TM-cell shrinkage. Both A1 and A2A AR agonists produced reproducible increases in TM-cell whole-cell currents of similar magnitude. The results suggest that previous measurements of explant-derived TM cells may have reflected a range of responses from phenotypically different cell populations, and that the opposing effects of A1 and A2A agonists on outflow resistance are not likely to be mediated by actions on a single population of TM cells. These opposing effects might reflect AR responses by two or more subpopulations of TM cells, by TM and SC cells or by inner-wall SC cells, alone.

Swelling-activated Cl- channels support Cl- secretion by bovine ciliary epithelium

PURPOSE

To determine whether swelling-activated Cl- currents (ICl,swell) observed in isolated nonpigmented ciliary epithelial (NPE) cells contribute to Cl- secretion across the ciliary epithelium.

METHODS

Ion transport across intact bovine ciliary epithelium was monitored electrically. Native isolated bovine NPE cells were harvested enzymatically. Cell volume changes were measured by calcein-fluorescence quenching.

RESULTS

Bilateral reduction in osmolality transiently increased short-circuit current (Isc), averaging 60% to 70%. Bilateral pretreatment with 5-nitro-2-(phenylpropylamino)-benzoate (NPPB), a Cl- channel blocker, reduced Isc stimulation by approximately 60%, suggesting that transcellular ICl,swell largely mediates the increased current. The hypotonically-triggered Isc stimulation was also inhibited by phloretin, a blocker of swelling-activated Cl- channels and by flufenamic acid, a blocker of Cl- and nonselective cation channels. Cyclamate substitution for bath Cl- reduced the baseline Isc and the increase in hypotonically-triggered Isc. In that case, addition of either NPPB or flufenamic acid did not produce further inhibition. The transepithelial responses were correlated with regulatory volume responses of freshly harvested NPE cells. Hypotonicity elicited a regulatory volume decrease (RVD) over a period comparable to that of the hypotonicity-triggered increase in Isc. The RVD was also inhibited by Cl--channel blockers and by Cl- substitution.

CONCLUSIONS

ICl,swell of NPE cells is functionally expressed in intact ciliary epithelium and is oriented to subserve aqueous humor formation. NPE cell volume can be measured with calcein-fluorescence quenching. ICl,swell may be stimulated by increased stromal fluid uptake and delivery to the NPE cells, facilitating Cl- secretion and increasing fluid release into the posterior chamber.

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.

NUCLEOSIDE PRODRUGS OF A3 ADENOSINE RECEPTOR AGONISTS AND ANTAGONISTS

9-(β-D-Ribosfuranosyluronamide)adenine derivatives that are selective agonists and antagonists of the A₃ adenosine receptor (AR) have been derivatized as prodrugs for in vivo delivery. The free hydroxy groups at the 2' and 3' positions of the agonists 2-chloro-N ⁶-(3-iodobenzyl)-9-(N-methyl-(β-D-ribosfuranosyluronamide)adenine 2b, the corresponding 4'-thio nucleoside 2c, and antagonists 4a and 4b (5'-N,N-dimethylamides related to 2b and 2c, respectively) were derivatized through simple acylation reactions. The prodrug derivatives were tested in radioligand binding assays at ARs and in a functional assay of adenylate cyclase at the A₃AR and found to be considerably less active than the parent drugs. The hydrolysis of nucleoside 2',3'-diesters to regenerate the parent compound in the presence of human blood was demonstrated. 2',3'-Dipropionate esters of 2b and 4a were readily cleaved in a two-step reaction to regenerate the parent drug, on a time scale of two hours. The cleavage of a 2',3'-dihexanoate ester occurred at a slower rate. This indicates that the prodrugs are suitable as masked forms of the biologically active A₃AR agonists and antagonists for future evaluation in vivo.

Noninvasive intraocular pressure measurements in mice by pneumotonometry

PURPOSE

To develop a reliable, noninvasive, continuous, and easily implemented system for measuring intraocular pressure (IOP) in mice.

METHODS

Pneumotonometry was adapted for measurement of mouse IOP. Measurements were compared with those obtained with the servo-null micropipette system (SNMS) and with direct anterior chamber cannulation. Heart rate was monitored by the precordial pulse, EKG, or tail pulse in anesthetized mice. The characteristic ocular hypotensive response to mannitol was assessed as an additional validation of the

METHOD

RESULTS

Measurements of IOP obtained using pneumotonometry agreed closely with values measured by SNMS or by direct cannulation. IOP oscillations were synchronous with the heart rate, with a coherence peak between them of approximately 2 Hz, equal to the pulse frequency. Hypertonic mannitol reduced IOP from 13.7 +/- 0.9 mm Hg by 7.7 +/- 0.7 mm Hg after 15 minutes.

CONCLUSIONS

Pneumotonometry is a reliable and noninvasive method for the measurement of IOP in mice and may permit comparisons of IOP to hemodynamic factors. This system is simpler and more adaptable for glaucoma research than previously reported methodologies for measuring IOP in mice.

The cross-species A3 adenosine-receptor antagonist MRS 1292 inhibits adenosine-triggered human nonpigmented ciliary epithelial cell fluid release and reduces mouse intraocular pressure

PURPOSE

Antagonists to A3 adenosine receptors (ARs) lower mouse intraocular pressure (IOP), but extension to humans is limited by species variability. We tested whether the specific A3AR antagonist MRS 1292, designed to cross species, mimicks the effects of other A3AR antagonists on cultured human nonpigmented ciliary epithelial (NPE) cells and mouse IOP.

METHODS

NPE cell volume was monitored by electronic cell sorting. Mouse IOP was measured with the Servo-Null Micropipette System.

RESULTS

Adenosine triggered A3AR-mediated shrinkage of human NPE cells. Shrinkage was blocked by MRS 1292 (IC50 = 42 +/- 11 nM, p < 0.01) and by another A3AR antagonist effective in this system, MRS 1191. Topical application of the A3AR agonist IB-MECA increased mouse IOP. MRS 1292 reduced IOP by 4.0 +/- 0.8 mmHg at 25-microM droplet concentration (n = 10, p < 0.005).

CONCLUSIONS

MRS 1292 inhibits A3AR-mediated shrinkage of human NPE cells and reduces mouse IOP, consistent with its putative action as a cross-species A3 antagonist.

Basis of chloride transport in ciliary epithelium

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

Inhibition of swelling-activated Cl- currents by functional anti-ClC-3 antibody in native bovine non-pigmented ciliary epithelial cells

PURPOSE

To determine the potential role of ClC-3, a PKC-inhibitable Cl(-) channel, in mediating the swelling-activated Cl(-) current (I(Cl,swell)) of native bovine nonpigmented ciliary epithelial (NPE) cells.

METHODS

Native bovine NPE cells were freshly harvested by enzymatic digestion. Whole-cell currents were recorded by patch-clamp measurements either in the presence or absence of a functional anti-ClC-3 antibody.

RESULTS

Baseline whole-cell currents were small under isotonic conditions. Hypotonic cell swelling stimulated outwardly rectifying I(Cl,swell), which was reversibly inhibited by the Cl(-) channel blockers, phloretin (300 microM) or 5-nitro-2-(phenylpropylamino)-benzoate (NPPB, 100 microM). Intracellular dialysis with anti-ClC-3 C(670-687) antibody did not affect baseline currents, but significantly delayed and inhibited hypotonic stimulation of I(Cl,swell). Preabsorption of the antibody with its antigen prevented the inhibition of I(Cl,swell) by antibody. In addition, intracellular dialysis with control Ex(133-148) antibody did not affect the I(Cl,swell). Moreover, activation of PKC by pretreatment with 100 nM phorbol 12,13-dibutyrate (PDBu) significantly inhibited the initial stimulation of I(Cl,swell), but had no effects on the steady state currents.

CONCLUSIONS

The results suggest that endogenous ClC-3 is involved in mediating I(Cl,swell) of native bovine NPE cells. The delayed stimulation of I(Cl,swell) by PDBu may reflect upregulation of swelling-activated Cl(-) channels of different subtypes, especially when the function of ClC-3 is blocked. This information will be useful in understanding the mechanisms controlling aqueous humor formation and thereby intraocular pressure.

Differential P1-purinergic modulation of human Schlemm's canal inner-wall cells

Intraocular pressure is directly dependent on aqueous humor flow into, and resistance to flow out of, the eye. Adenosine has complex effects on intraocular pressure. Stimulation of A1and A2Aadenosine receptors changes intraocular pressure oppositely, likely through opposing actions on the outflow of aqueous humor. While the cellular sites regulating outflow resistance are unknown, the cells lining the inner wall of Schlemm's canal (SC) are a likely regulatory site. We applied selective adenosine receptor agonists to SC cells in vitro to compare the responses to A1and A2Astimulation. Parallel studies were conducted with human inner-wall SC cells isolated by a novel enzyme-assisted technique and with cannula-derived mixed inner- and outer-wall SC cells. A1agonists increased whole cell currents of both inner-wall and cannula-derived SC cells. An A2Aagonist reduced currents most consistently in specifically inner-wall SC cells. Those currents were also increased by A2B, but not consistently affected by A3, stimulation. A1, A2A, and A3agonists all increased SC-cell intracellular Ca2+. The electrophysiological results are consistent with the possibility that inner-wall SC cells may mediate the previously reported modulatory effects of adenosine on outflow resistance. The results are also consistent with the presence of functional A2B, as well as A1, A2A, and A3adenosine receptors in SC cells.

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

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

The ins and outs of aqueous humour secretion

The intraocular pressure (IOP) reflects a balance between inflow and outflow of aqueous humour. A major strategy in the medical treatment of glaucoma is to reduce inflow and thereby IOP. Understanding the mechanisms and regulation of inflow is thus of clear clinical relevance. Many mechanisms underlying inflow have been identified. The integration and regulation of these mechanisms is less clear. Aqueous humour is secreted across the ciliary epithelium by transferring solute, chiefly NaCl, from the stroma to the posterior chamber of the eye, with water passively following. The epithelium consists of two layers: the pigmented ciliary epithelial (PE) cells abutting the stroma, and the non-pigmented ciliary epithelial (NPE) cells facing the aqueous humour. Gap junctions link adjacent cells within and between these layers. Secretion proceeds in three steps: (1) uptake of NaCl from stroma to PE cells by electroneutral transporters, (2) passage of NaCl from PE to NPE cells through gap junctions, and (3) release of Na+ and Cl- through Na+,K+-activated ATPase and Cl- channels, respectively. Most of our understanding of inflow mechanisms has been obtained by studying in vitro preparations at subcellular, cellular and tissue levels. A particularly productive approach has been the electron probe X-ray microanalysis (EPMA) of the elemental composition of excised ciliary epithelium. This technique permits analysis of adjacent cells within different regions of the ciliary epithelium. EPMA of rabbit preparations has supported the idea that paired activity of Na+/H+ and Cl-/HCO3- antiports can be the dominant mechanism underlying the first step in secretion, stromal NaCl uptake by PE cells. EPMA also indicates that Cl- turnover is faster in the anterior than the posterior region of the epithelium. At the opposite epithelial surface, release of Na+ through Na+,K+-activated ATPase of NPE cells is also greater anteriorly than posteriorly. The accompanying release of Cl- through ion channels is enhanced by agonists of A3 adenosine receptors (ARs). The concepts that paired antiport activity is important in stromal NaCl uptake and that A3ARs modulate NaCl release into the aqueous humour were based on in vitro studies. The potential relevance of these conclusions to in vivo conditions has been tested by measurements of IOP in the living mouse. The results have confirmed the predictions that inhibitors of Na+/H+ antiports lower IOP, and that A3AR agonists and antagonists raise and lower IOP, respectively.

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

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

The fall and rise of active chloride transport: implications for regulation of intraocular pressure

Early study of transepithelial salt transfer focused on Cl(-) and not Na(+), partly because Cl(-) was readily measureable. The advent of flame photometry and tracer techniques brought Na(+) to the fore, especially since short-circuited frog skin (Rana temporaria) produces baseline net movement of Na(+) and not of Cl(-). Zadunaisky was among the first to describe what is currently termed secondary active Cl(-) transport, helping stimulate interest in Cl(-) handling by other tissues, notably the thick ascending limb of the loop of Henle important in renal counter-current multiplication. More recently, molecules responsible for electroneutral and electrogenic Cl(-) transfer have been cloned, and specific diseases resulting from their faulty expression have been identified. The clinical importance of transepithelial Cl(-) transfer is illustrated by studies of aqueous humor formation by the eye's bilayered ciliary epithelium. NaCl is taken up from the stroma by the pigmented ciliary epithelial (PE) layer, diffuses through gap junctions into the nonpigmented ciliary epithelial (NPE) layer, and is released into the aqueous humor largely through Na(+) pumps and Cl(-) channels. ATP released by NPE cells can be ecto-enzymatically metabolized to adenosine. Adenosine can mediate paracrine/autocrine stimulation of Cl(-) channels and aqueous humor secretion by occupying A(3) adenosine receptors (ARs). A(3)AR agonists indeed elevate, and A(3)AR antagonists lower, intraocular pressure (IOP) in wild-type mice. A(3)AR knockout mice have low IOP and their responses to A(3)AR agonists and antagonists are blunted; this suggests that reducing Cl(-)-channel activity with A(3)AR antagonists may provide a novel approach for treating glaucoma.

Common actions of adenosine receptor agonists in modulating human trabecular meshwork cell transport

A(1) adenosine receptors (ARs) reduce, and A(2)ARs increase intraocular pressure, partly by differentially altering resistance to aqueous humor outflow. It is unknown whether the opposing effects of A(1)AR and A(2)AR agonists are mediated at different outflow-pathway cell targets or by opposing actions on a single cell target. We tested whether a major outflow-pathway cell, the trabecular meshwork (TM) cell might constitute the primary AR-agonist target and respond differentially to A(1), A(2A) and A(3)AR agonists. Receptor activation in human TM cells was identified by applying subtype-selective AR agonists: CPA and ADAC for A(1)ARs, CGS 21680 and DPMA for A(2A)ARs, and Cl-IB-MECA and IB-MECA for A(3)ARs. Stimulation of A(1), A(2A) and A(3)ARs elevated Ca(2+), measured with fura-2. Whole-cell patch clamping indicated that AR agonists activated ion channels non-uniformly, possibly reflecting variability in magnitude of agonist-triggered second-messenger responses. A(1), A(2A) and A(3)AR agonists all reduced volume, determined by calcein cell imaging. The endogenous source of adenosine delivery to the outflow pathway could be the TM cells since these cells were stimulated to release ATP by hypotonic perfusion. We conclude that: (1) TM cells express functional A(1), A(2A) and A(3)ARs; and (2) the reported differential effects of AR agonists on aqueous humor outflow are not mediated by differential actions on TM-cell Ca(2+) and volume, but likely by actions on separate cell targets.

Noninvasive assessment of aqueous humor turnover in the mouse eye

PURPOSE

To develop a noninvasive test for monitoring changes in aqueous humor turnover in the mouse eye.

METHODS

After topical instillation of fluorescein, the rate of decay of fluorescence from aqueous humor and cornea was monitored in Black Swiss, C57 Bl6, and DBA 2J mice with a microscope equipped with epifluorescence and a charge-coupled device (CCD) camera.

RESULTS

The rate of decay of fluorescence was identical in right and left eyes over an approximately 70-minute measurement period. The rate of decay was similar in normal mice aged 2 and 18 months. Pilocarpine and latanoprost, known to enhance aqueous humor outflow in humans, accelerated the decay of fluorescence. Levobunolol, known to inhibit aqueous humor inflow in humans, slowed decay. Dimethylamiloride, an inhibitor of the Na(+),H(+) exchanger that is known to act on cultured cells of both the ciliary epithelium and trabecular meshwork and to lower mouse intraocular pressure (IOP), enhanced decay. DBA 2J mice, in which secondary glaucoma develops, displayed a slower decay of fluorescence at 18 months of age than age-matched unaffected animals.

CONCLUSIONS

Monitoring decay of fluorescence provides a noninvasive index of aqueous humor dynamics in the mouse eye that facilitates study of ocular hypotensive drugs and mouse models of glaucoma. Coupled with knowledge of IOP, it permits semiquantitative conclusions about the relative roles of aqueous humor inflow and outflow in conditions with altered IOP. Based on this approach, dimethylamiloride appears to lower mouse IOP primarily by enhancing outflow of aqueous humor.

Knockout of A3 adenosine receptors reduces mouse intraocular pressure

PURPOSE

To test the putative role of A(3) adenosine receptors (ARs) in modulating intraocular pressure (IOP).

METHODS

IOP was monitored for up to 32 minutes in A3-knockout (A3AR-/-) and A3AR+/+ control mice by the servo-null approach. The IOP responses to adenosine, A3AR agonists and A3AR antagonists were studied singly or in combination in both strains.

RESULTS

IOP was significantly lower in A3AR-/- mice (12.9 +/- 0.7 mm Hg) than in A3AR+/+ control animals (17.4 +/- 0.6 mm Hg). The nonselective AR agonist adenosine produced a much smaller increase in IOP (2.2 +/- 0.8 mm Hg) in the knockout than in A3AR+/+ control mice (14.9 +/- 2.4 mm Hg). The A3-selective agonist IB-MECA did not affect IOP in A3-knockout mice, but raised it in A3AR+/+ mice. The highly selective A3AR antagonist MRS 1191 did not affect IOP in A3AR-/- mice, but lowered it in A3AR+/+ control mice. Preadministering MRS 1191 did not affect the small adenosine-triggered increase in IOP in A3AR-/- mice, but markedly attenuated adenosine's effects on IOP in A3AR+/+ control mice. MRS 1523, an A3AR antagonist less selective than MRS 1191 in rats, decreased IOP in both A3AR-/- and A3AR+/+ animals. As in black Swiss outbred mice and other mammalian species, reducing aqueous humor inflow with acetazolamide lowered IOP and administering water intraperitoneally increased IOP in both A3AR-/- and A3AR+/+ mice.

CONCLUSIONS

The reduced IOP and altered purinergic responses of IOP in A3AR knockout mice support the conclusion that A3ARs contribute to the regulation of IOP.

Human trabecular meshwork cell volume regulation

The volume of certain subpopulations of trabecular meshwork (TM) cells may modify outflow resistance of aqueous humor, thereby altering intraocular pressure. This study examines the contribution that Na+/H+, Cl-/HCO exchange, and K+-Cl- efflux mechanisms have on the volume of TM cells. Volume, Cl- currents, and intracellular Ca2+ activity of cultured human TM cells were studied with calcein fluorescence, whole cell patch clamping, and fura 2 fluorescence, respectively. At physiological bicarbonate concentration, the selective Na+/H+ antiport inhibitor dimethylamiloride reduced isotonic cell volume. Hypotonicity triggered a regulatory volume decrease (RVD), which could be inhibited by the Cl- channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB), the K+ channel blockers Ba2+ and tetraethylammonium, and the K+-Cl- symport blocker [(dihydroindenyl)oxy]alkanoic acid. The fluid uptake mechanism in isotonic conditions was dependent on bicarbonate; at physiological levels, the Na+/H+ exchange inhibitor dimethylamiloride reduced cell volume, whereas at low levels the Na+-K+-2Cl- symport inhibitor bumetanide had the predominant effect. Patch-clamp measurements showed that hypotonicity activated an outwardly rectifying, NPPB-sensitive Cl- channel displaying the permeability ranking Cl- > methylsulfonate > aspartate. 2,3-Butanedione 2-monoxime antagonized actomyosin activity and both increased baseline [Ca2+] and abolished swelling-activated increase in [Ca2+], but it did not affect RVD. Results indicate that human TM cells display a Ca2+-independent RVD and that volume is regulated by swelling-activated K+ and Cl- channels, Na+/H+ antiports, and possibly K+-Cl- symports in addition to Na+-K+-2Cl- symports.

Inhibitors of NHE-1 Na+/H+ exchange reduce mouse intraocular pressure

PURPOSE

To test whether blocking the Na+/H+ antiport reduces intraocular pressure (IOP) in the mouse.

METHODS

The electrophysiologic approach (the servo-null micropipette system, SNMS) that had been adapted for continuously monitoring IOP in the mouse was used in a study of the effects of a series of transport inhibitors.

RESULTS

Topical application of three direct blockers of Na+/H+ exchangers produced comparable reductions in mouse IOP: dimethylamiloride (DMA, -5.0 +/- 0.7 mm Hg), ethylisopropylamiloride (EIPA, -4.1 +/- 1.0), and BIIB723 (-4.9 +/- 1.7 mm Hg). These effects were mediated locally, not systemically, because adding DMA to one eye had no effect on IOP in the contralateral eye. In contrast to the actions of selective inhibitors of Na+/H+ exchange, neither the low-potency inhibitor amiloride nor the inhibitor of Na+-K+-2Cl- cotransport bumetanide by itself was effective. Dorzolamide, which slows delivery of H+ and HCO3- to Na+/H+ and Cl-/HCO3- antiports, also reduced IOP by 2.9 +/- 0.6 mm Hg. After first blocking Na+/H+ exchange with DMA, EIPA, BIIB723, or dorzolamide, application of bumetanide produced an additional reduction in IOP of 3.8 to 4.0 mm Hg.

CONCLUSIONS

The first step in formation of aqueous humor is uptake of NaCl by the ciliary epithelial cells from the stroma, possibly by both paired Na+/H+ and Cl-/HCO3- antiports and a bumetanide-sensitive Na+-K+-2Cl- symport. The present data are consistent with electron probe x-ray microanalyses of rabbit ciliary epithelium indicating that the antiports are the dominant mechanism. That bumetanide can produce a previously unobserved lowering of IOP when the Na+/H+ antiport is also inhibited substantiates a dominant antiport mechanism.