Ca2+ mobilization and interlayer signal transfer in the heterocellular bilayered epithelium of the rabbit ciliary body

Terbutaline, forskolin and cAMP reduce secretion of aqueous humour in the isolated bovine eye

In order to elucidate involvement of cyclic AMP and intracellular Ca²⁺,[Ca²⁺]_i, in the modulation of aqueous humour formation (AHF), we studied the effects of terbutaline, forskolin and 8-Br-cAMP in the isolated bovine eye. We also studied the interaction of cAMP on calcium signaling in cultured ciliary epithelial (CE) cells. Drug effects on AHF were measured by fluorescein dilution. Drug effects on [Ca²⁺]_i were studied by the fura-2 fluorescence ratio technique. Terbutaline (100 nmol-100 M), forskolin (30 nM-100 M) or 8-Br-cAMP (100 nM– 10 μM), administered in the arterial perfusate produced significant reductions in AHF. The AH reducing effect of terbutaline was blocked by a selective inhibitor of protein kinase A (KT-5720). ATP (100 M) caused a rapid, transient (peak) increase in [Ca²⁺]_i followed by a sustained plateau phase lasting more than 5 minutes. Preincubation of the cells (6 min) with terbutaline, forskolin or 8-Br-cAMP significantly reduced the peak calcium response to ATP. The sustained plateau phase of the response, on the other hand, was augmented by each of the agents. KT-5720 partially reversed the inhibitory effect of terbutaline on the peak and totally inhibited its effect on the plateau phase. These data indicate: (a) that AHF in the bovine eye can be manipulated through cyclic AMP, operating via protein kinase A, (b) that protein kinase A can affect [Ca²⁺]_i homeostasis, (c) that calcium release from the intracellular store, not the entry, affects AHF, and (d) that interaction of [Ca²⁺]_i with cAMP plays a role in modulating AH secretion.

Purinergic signaling in the rabbit ciliary body epithelium

In the past forty years, a wealth of information has accumulated that points to the presence of adenosine and adenine nucleotides in the anterior segment of the eye and a number of hypotheses have been introduced to describe the possible role of these agents in the regulation of aqueous humor flow. However, in the absence of a generally accepted model for the cellular and molecular mechanisms of aqueous humor formation by the ciliary body epithelium, efforts to identify the signal transduction pathway(s) responsible for regulation of the ion and water transport have not been successful. This article briefly reviews the evidence for (i). the presence in aqueous humor of adenine nucleotides, cyclic adenosine monophosphate and adenosine, their metabolic product, (ii). the possible role of these agents in the regulation of aqueous humor dynamics, and (iii). the expression of ecto-nucleotidases, receptors, and second messengers that may mediate such regulation. Finally, a model for the regulation of aqueous humor formation by adenosine and ATP is proposed.

Molecular profiling and cellular localization of connexin isoforms in the rat ciliary epithelium

The functionally distinct epithelial layers of the ciliary body act as a syncitium to produce the aqueous humour. Ultrastructural studies have shown that the pigmented (PE) and non-pigmented (NPE) cell layers of the ciliary epithelium are connected by gap junctions. However the molecular composition of gap junctions both between and within the two cell layers has not been comprehensively studied. To address this issue the authors have performed an extensive molecular screening of connexin (Cx) expression patterns in ciliary epithelium of the rat. Initially, mRNA was extracted from rat ciliary bodies, reverse-transcribed, and subjected to two rounds of PCR using primer sets designed against each of the 14 Cx isoforms known to be expressed in the rat. This initial screening protocol amplified eight candidate Cx isoforms (Cxs 26, 31, 33, 37, 40, 43, 45 and 46). The Cx isoforms identified in this initial screen were then first assigned to the ciliary epithelium itself (Cxs 26, 31, 40 and 43) or structures outside the epithelium (Cxs 37, 40, and 45) using immunohistochemistry performed on ciliary body whole mounts. No convincing evidence for either Cx 33 or 46 labelling was found in the ciliary body. Then the four Cx isoforms localized to the epithelium were further localized to specific membrane domains within the epithelial cell layers by performing high resolution imaging of the antibody labeling patterns obtained in cryosections. This enabled Cx26 and 31 to be specifically localized to spatially different gap junctions between NPE cells. Cx31 labeled gap junctions associated with an extensive network of membrane interdigitations found between NPE cells at their basal surfaces. In contrast Cx26 labeling in NPE cells was restricted to the basolateral membranes of adjacent NPE cells. Cx40 and Cx43 were both localized to the PE-NPE interface where they formed discrete homomeric/homotypic gap junction plaques. No convincing evidence was found for antibody labeling between PE cells. Thus it appears that intercellular communication, both within the NPE layer and between the PE and NPE cell layers, is mediated by gap junction channels that have distinctive permeability properties. In particular the results raise the possibility that the permeability of PE-NPE gap junctions can be modulated by changing the Cx43 : Cx40 expression ratio. Whether such a change in Cx expression ratios occurs and what effect it has on aqueous humour production and composition remains to be determined.

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.

Intercellular Ca2+ signalling: the artery wall

The fact that many cells in the cardiovascular system are coupled via gap junctions enables direct intercellular Ca2+ signalling. Ca2+ ions and/or inositol trisphosphate (IP3) can pass through these aqueous pores. Cell-cell coupling can occur between cells of the same type, or via special junctions between adjacent cells of different types. Homocellular coupling acts to amplify and prolong Ca2+ signals, whereas heterocellular coupling allows complex interactions between the cells, including the modulation of their respective responses. This review will focus on the interactions between cells that form the blood vessel wall, illustrating how cell-cell communication defines important physiological functions.

Response of rabbit ciliary process cyclic nucleotides to specific phosphodiesterase inhibitors

PURPOSE

To determine which cyclic nucleotide phosphodiesterase family activities can be identified in rabbit ciliary processes.

METHODS

Freshly excised rabbit ciliary processes were incubated in vitro with family selective phosphodiesterase inhibitors in the absence or presence of activators of soluble or membrane bound guanylate cyclase. The resulting increases of cyclic AMP and cyclic GMP were measured by RIA.

RESULTS

Rabbit ciliary process cyclic AMP levels were increased by the phosphodiesterase 1 and 4 selective inhibitors, 8-methoxymethyl-IBMX and rolipram, respectively. Cyclic GMP levels were increased by 8-methoxymethyl-IBMX; whereas the phosphodiesterase 5 and 6 selective inhibitor, zaprinast, increased cyclic GMP little. Nitric oxide donors increased cyclic AMP in addition to cyclic GMP, and inhibition of soluble guanylate cyclase eliminated the increase of cyclic AMP. The effects of sodium nitroprusside and the phosphodiesterase 3 selective inhibitor, cilostamide, on cyclic AMP were not additive suggesting that the sodium nitroprusside mediated increase of cyclic GMP, like cilostamide, inhibited phosphodiesterase 3.

CONCLUSIONS

Cyclic AMP in rabbit ciliary processes is hydrolyzed primarily by phosphodiesterases 1 and 4, and, when cyclic GMP levels are low, by phosphodiesterase 3; cyclic GMP is hydrolyzed primarily by phosphodiesterase 1.

Multiple receptor activation elicits synergistic IP formation in nonpigmented ciliary body epithelial cells

We have examined the interaction between muscarinic and alpha(2)-adrenergic receptor activation on inositol phosphate (IP) formation in the nonpigmented cells of the ciliary body epithelium (NPE cells) of the rabbit. We have compared these changes with those previously observed in the intracellular free Ca(2+) concentration. Whereas muscarinic receptor activation causes an increase in intracellular Ca(2+) and IP formation, activation of alpha(2)-receptors does not significantly increase either intracellular Ca(2+) or IPs over basal levels. However, simultaneous activation of muscarinic and alpha(2)-adrenergic receptors with the specific agonists carbachol and UK-14304 produces massive Ca(2+) increases and results in a synergistic increase in IP formation. This synergistic IP formation is inhibited by both muscarinic and alpha(2)-adrenergic receptor antagonists as well as by pertussis toxin and an inhibitor of phospholipase C. IP formation is predominantly independent of intracellular Ca(2+), because it is decreased but not prevented by blocking the entry of Ca(2+) with LaCl(3) or chelating intracellular Ca(2+) with 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. Thus synergistic IP formation underlies, at least in part, the synergistic increase in intracellular Ca(2+) resulting from simultaneous activation of muscarinic and alpha(2)-adrenergic receptors.

Similarity of A(3)-adenosine and swelling-activated Cl(-) channels in nonpigmented ciliary epithelial cells

Chloride release from nonpigmented ciliary epithelial (NPE) cells is a final step in forming aqueous humor, and adenosine stimulates Cl(-) transport by these cells. Whole cell patch clamping of cultured human NPE cells indicated that the A(3)-selective agonist 1-deoxy-1-(6-[([3-iodophenyl]methyl)amino]-9H-purin-9-yl)-N-methyl-be ta-D-ribofuranuronamide (IB-MECA) stimulated currents (I(IB-MECA)) by approximately 90% at +80 mV. Partial replacement of external Cl(-) with aspartate reduced outward currents and shifted the reversal potential (V(rev)) from -23 +/- 2 mV to -0.0 +/- 0.7 mV. Nitrate substitution had little effect. Perfusion with the Cl(-) channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and niflumic acid inhibited the currents. Partial Cl(-) replacement with aspartate and NO(3)(-), and perfusion with NPPB, had similar effects on the swelling-activated whole cell currents (I(Swell)). Partial cyclamate substitution for external Cl(-) inhibited inward and outward currents of both I(IB-MECA) and I(Swell). Both sets of currents also showed outward rectification and inactivation at large depolarizing potentials. The results are consistent with the concept that A(3)-subtype adenosine agonists and swelling activate a common population of Cl(-) channels.

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.

Gap junctions in the eye: evidence for heteromeric, heterotypic and mixed-homotypic interactions

Some of the best evidence that different types of gap junction proteins (connexins) interact with each other in vivo has been found in the eye. This review focuses on three diverse ocular tissues that may contain heterotypic or heteromeric gap junction channels. Each of the tissues uses gap junctions in a superlative fashion: The crystalline lens has an exceptionally high density of gap junctions; the ciliary body expresses a surprising variety of connexins; the neural retina shows remarkable specificity in the patterns of intercellular coupling.

Adrenergic regulation of calcium-activated potassium current in cultured rabbit pigmented ciliary epithelial cells

1. The effects of adrenergic agonists on K+ currents were studied in cultured rabbit pigmented ciliary epithelial (PCE) cells. 2. Outward K+ current (IK) was reduced by tetraethylammonium chloride, the Ca2+-activated K+ (K(Ca)) channel blocker iberiotoxin (IbTX), or Ca2+-free external Ringer solution. The calcium ionophore ionomycin increased an IbTX-sensitive IK in PCE cells. 3. The adrenergic agonists adrenaline and phenylephrine increased IK in PCE cells. The induced current was blocked by IbTX and the alpha1-antagonist prazosin, suggesting that adrenergic agonists activate IK(Ca) via alpha1-adrenoreceptors. 4. Internal dialysis of D-myo-inositol 1,4, 5-trisphosphate (IP3) increased IK, whilst pre-incubation of PCE cells with thapsigargin or the phospholipase C (PLC) inhibitor U-73122 reduced phenylephrine-induced increases in IK(Ca). Adrenergic increases in IK(Ca) were mediated by a pertussis toxin-insensitive G protein. 5. These results demonstrate that IK(Ca) channels in rabbit PCE cells are coupled to alpha1-adrenergic receptors and a PLC/IP3 signalling pathway. Activation of these channels may modulate fluid secretion by the ciliary epithelium.

Novel paracrine signaling mechanism in the ocular ciliary epithelium

The ciliary body contains an epithelial bilayer consisting of an outer pigmented cell layer (PE) and an inner nonpigmented cell layer (NPE) responsible for aqueous humor secretion. Secretion may be mediated in part by cytosolic Ca2+ concentration ([Ca2+]i), but whether or how the two layers could coordinate their Ca2+ signals to regulate secretion is unclear. To investigate interactions between PE and NPE, we examined [Ca2+]i signaling in isolated intact ciliary epithelial bilayers using confocal microscopy. Phenylephrine selectively increased [Ca2+]i in PE and acetylcholine increased [Ca2+]i in NPE, but epinephrine increased [Ca2+]i in both layers. This increase spread from PE to NPE, and [Ca2+]i signaling across the bilayer remained coordinated during [Ca2+]i oscillations. All epinephrine-induced [Ca2+]i signaling was blocked by the alpha1-adrenergic antagonist prazosin, whereas signaling in the NPE but not PE was blocked by the beta-adrenergic antagonist propranolol, the gap junction blockers octanol and 18alpha-glycyrrhetinic acid, or the A kinase inhibitor Rp diastereomer of adenosine 3',5'-cyclic monophosphothioate. The beta-adrenergic agonist isoproterenol failed to increase Ca2+ by itself, but isoproterenol plus phenylephrine-induced [Ca2+]i signals across the bilayer similar to those induced by epinephrine. Finally, isoproterenol increased cell-to-cell spread of lucifer yellow via gap junctions, whereas cell-to-cell spread of [Ca2+]i signals could be induced by photorelease of caged inositol 1,4,5-trisphosphate. Thus, calcium signals are coordinated in the epithelial bilayer so that adrenergic stimulation can increase [Ca2+]i in NPE, but only if NPE are primed by activation of endogenous adenylyl cyclase, whereupon they receive stimulation from adjacent PE via gap junctions. This novel interplay between endocrine and paracrine pathways may coordinate [Ca2+]i signaling across the ciliary epithelial bilayer.

Connexin50, a gap junction protein of macrogliaP6n the mammalian retina and visual pathway

Reverse transcriptase-polymerase chain reaction (RT-PCR), Western blotting and immunocytochemistry were used to study the expression of gap junction proteins (connexins; Cx) in the rat and rabbit retina. RT-PCR of rabbit total retinal RNA using primers selected for the human Cx50 (alpha 8 Cx) DNA template yielded cDNA fragments of the predicted base pair size. Western blots of rat and rabbit retinal membrane preparations probed with a monoclonal antibody which recognizes Cx50 in the lens of several mammalian species revealed a single band (MW 50 kD), identical to that recognized in lens membrane extracts. In frozen retinal sections of both species, the same monoclonal antibody as well as two polyclonal antisera raised against a synthetic peptide from the C-terminal region of the human Cx50 polypeptide labeled Müller cells and astrocytes. In Müller cells, labeling was strongest in the endfeet and in the filamentous processes ensheathing the photoreceptors. Extending from the neural retina, Cx50-like immuno-reactivity was detected in astrocytes of the optic nerve and along retinal projections within the CNS. Our data indicate that Müller cells and astrocytes of mammalian retinas and throughout the visual pathway are coupled through gap junctions composed of connexin50.

Functional coupling in bovine ciliary epithelial cells is modulated by carbachol

The functional coupling of the ciliary epithelium was studied in isolated pairs (couplets) of pigmented ciliary epithelial (PCE) and nonpigmented ciliary epithelial (NPCE) cells using the whole cell patch clamp and the fluorescent dye lucifer yellow. One cell of the pair (usually the NPCE cell of a NPCE-PCE cell couplet) was accessed with a 2-5 M omega electrode, containing 1-2 mM lucifer yellow, in the whole cell configuration of the patch clamp. After voltage-clamp experiments were completed, cells were viewed under a fluorescent microscope to confirm that the cells were coupled. The electrical coupling of the cells was also studied by calculating the capacitance (using the time-domain technique), assuming a "supercell" model for coupled cells. The mean capacitance of coupled pairs was 79.8 +/- 4.3 (SE) pF (n = 47) compared with single cell capacitances of 36.8 +/- 3.4 pF (n = 10) for PCE cells and 38.1 +/- 3.1 pF (n = 15) for NPCE cells. Octanol, carbachol (CCh), and raised extracellular Ca2+ concentration ([Ca2+]o) all caused uncoupling in pairs (couplets) of coupled NPCE and PCE cells. At room temperature (22-24 degrees C), the capacitance of the couplets decreased from 70.5 +/- 8.0 to 48.0 +/- 5.2 pF (n = 5) when exposed to octanol (1 mM), from 73.8 +/- 9.2 to 43.2 +/- 9.5 pF (n = 4) when exposed to CCh (100 microM), and from 80.5 +/- 6.7 to 49.9 +/- 7.8 pF (n = 4) when exposed to 10 mM [Ca2+]o. The response to CCh was dose dependent; at higher temperatures of 34-37 degrees C, 10 microM CCh caused a 38% reduction in capacitance, from 53.7 +/- 9.7 to 33.5 +/- 3.3 pF (n = 7) with a half-time of 249 s, and 100 microM CCh caused a 49% reduction in capacitance, from 51.3 +/- 5.6 to 26.0 +/- 2.4 pF (n = 7) with a half-time of 124 s. After pairs uncoupled and the uncoupling agent was washed out, the cell pairs often exhibited an increase in capacitance that we interpreted as "recoupling" or a reopening of the gap junctional communication pathway; the half-time for this process was 729 s after uncoupling with 100 microM CCh and 211 s after uncoupling with 10 microM CCh. This interpretation was confirmed optically by the spread of lucifer yellow into both cells of an uncoupled pair with a time course corresponding to the increase in electrical coupling. The controllable coupling of ciliary epithelial cells extends the idea of a functional syncytium involved in active transport. PCE cells take up solute and water from the blood, which then cross to NPCE cells via gap junctions and from there are secreted into the posterior chamber of the eye. Modulation of the coupling between NPCE and PCE cells may provide a mechanism to control secretion.

Effect of heptanol on the short circuit currents of cornea and ciliary body demonstrates rate limiting role of heterocellular gap junctions in active ciliary body transport

Rabbit ciliary body and cornea were mounted in Ussing-type chambers in Tyrode's under voltage clamp and the effects of heptanol, a gap junction inhibitor, on the short circuit current generated by each of the respective epithelia were determined. Studies were carried out either in control conditions or following amphotericin B permeabilization of either the basolateral membrane of the nonpigmented epithelium of the ciliary body or the apical membrane of the corneal epithelium, respectively. Previous studies have shown that, following these permeabilizations, short circuit currents are established, reflecting aqueous (or tear)-to-serosa Na+ fluxes, and that Na+ translocation through gap junctions connecting the individual layers of these tissues constitutes the major rate limiting step. Heptanol inhibited most of the short circuit current of the amphotericin B-modified ciliary body and cornea and of the unmodified ciliary body epithelium (control). In all these cases, the apparent IC50 was about 0.8 M. In the unmodified corneal epithelium, where ion translocation across the apical membrane constitutes the main rate limiting step for active secretion, 0.4 or 0.8 mM heptanol induced short circuit current increases; partial inhibition was observed only at high concentrations known to cause maximal inhibition of junctional permeability. Heptanol also enhanced the volume regulatory decrease of cultured human NPE cells, a process dependent on cell swelling-induced stimulation of Cl- and K+ permeabilities. Combined with our previous results demonstrating the lack of heptanol effects on other epithelial functions, these data suggest that the effect of heptanol on the active ciliary body transepithelial transport is primarily due to inhibition of the nonpigmented-pigmented junctional path and that this path is a potential site of rate limitation for the secretory process.