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

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.

Inhibition of a cardiac sarcoplasmic reticulum chloride channel by tamoxifen

Anion and cation channels present in the sarcoplasmic reticulum (SR) are believed to be necessary to maintain the electroneutrality of SR membrane during Ca(2+) uptake by the SR Ca(2+) pump (SERCA). Here we incorporated canine cardiac SR ion channels into lipid bilayers and studied the effects of tamoxifen and other antiestrogens on these channels. A Cl(-) channel was identified exhibiting multiple subconductance levels which could be divided into two primary conductance bands. Tamoxifen decreases the time the channel spends in its higher, voltage-sensitive band and the mean channel current. The lower, voltage-insensitive, conductance band is not affected by tamoxifen, nor is a K(+) channel present in the cardiac SR preparation. By examining SR Ca(2+) uptake, SERCA ATPase activity, and SR ion channels in the same preparation, we also estimated SERCA transport current, SR Cl(-) and K(+) currents, and the density of SERCA, Cl(-), and K(+) channels in cardiac SR membranes.

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.

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.

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.

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.

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.

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

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

Release of ATP by a human retinal pigment epithelial cell line: potential for autocrine stimulation through subretinal space

1. Stimulation of purinergic receptors on retinal pigment epithelial (RPE) cells can increase the rate of fluid transport or decrease phagocytosis. This study aims to: determine whether the purine ATP can be released from RPE cells, begin probing the mechanism of any release and test whether cells degrade ATP extracellularly. 2. ATP release was monitored from cultured human ARPE-19 cells with the luciferin-luciferase assay. Biphasic release of ATP was triggered by basic fibroblast growth factor (bFGF), by the pyrimidine uridine triphosphate (UTP) and by hypotonicity. 3. The Cl(-) channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) inhibited release of ATP, suggesting that release was associated with Cl(-) channels. 4. Elevating intracellular Ca(2+) directly with ionomycin was insufficient to trigger ATP release. 5. UTP induced a biphasic elevation in intracellular Ca(2+). NPPB inhibited the second phase, suggesting autostimulation by released ATP. 6. Cells grown on permeable supports showed apical release of ATP, analogous to release into subretinal space in vivo. 7. The presence of ecto-ATPases on ARPE-19 cell membranes was suggested by the degradation of ATP added to intact cells. 8. Phagocytosis of fluorescent beads was inhibited by ATP, but the ecto-5'-nucleotidase inhibitor alpha, beta-methylene ADP prevented this, suggesting that inhibition was mediated by extracellular conversion of ATP to adenosine. 9. These results suggest that growth factors, pyrimidines and changes in tonicity could trigger ATP release into subretinal space. The levels of ATP released may be capable of autocrine stimulation of ATP receptors, while conversion to adenosine by ecto-enzymes could alter phagocytosis.