Effects of forskolin, prostaglandin F2 alpha, and Ba2+ on the short-circuit current of the isolated rabbit iris-ciliary body

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.

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.

Role of ion channels in aqueous humor formation

The aqueous humor is secreted by the ciliary epithelium, a bilayered syncytial epithelium comprising a pigmented (PE) cell layer abutting the stroma and a nonpigmented (NPE) cell layer facing the aqueous phase. As in other epithelia, secretion depends on the transfer of solute, with water passively following. Na+, K+, and Cl- enter the syncytium principally through a Na(+)-K(+)-2Cl- symport, diffusing to the aqueous surface of the NPE cells. The Na+, K+, and Cl- are secreted into the aqueous humor through the Na+/K+ exchange pump, K+ channels, and Cl- channels, respectively. Na+ is also secreted between the cells in response to a small transepithelial potential. The K+ channels are critical not only for K+ release but also for hyperpolarizing the membrane, providing an electrical driving force for Cl- secretion. Some of the K+ channels are Ca2+ sensitive and can be activated by Ca2+ entry through T- and L-type Ca2+ channels. The roles of the ciliary epithelial nonselective and Na+ channels are less clear. This review describes the ion channels thus far identified in the ciliary epithelium in terms of the activation and inactivation of their macroscopic currents, the open probabilities and conductances of the single channels, and their locations and regulation. The review relates each class of channel to known families of channels and indicates how those channels can contribute to the secretion of the aqueous humor.

Prostaglandin A2 increases uveoscleral outflow and trabecular outflow facility in the cat

Prostaglandins (PG) are very effective ocular hypotensive agents. It is generally agreed that these drugs reduce intraocular pressure primarily by increasing uveoscleral outflow. They may also increase trabecular outflow facility though available evidence is less convincing. It has been hypothesized that PGs may increase facility of uveoscleral outflow in addition to their other mechanisms, but this has not yet been tested. To help clarify the ocular hypotensive mechanism of action of a derived PG of the A type, cats were treated twice daily for one week with PGA2 (0.01%) to one eye and vehicle to the other. Measurements were made of aqueous flow and outflow facility with fluorophotometry and of intraocular pressure with pneumatonometry. From these values, uveoscleral outflow was calculated. In addition, total outflow facility, uveoscleral outflow, and uveoscleral outflow facility were determined with invasive methods. PGA2 significantly reduced IOP by a mean of at least 4.7 mmHg in all experiments with all P-values less than 0.01. Compared with contralateral vehicle-treated control eyes, uveoscleral outflow in the treated eye was significantly (P < 0.05) increased by at least 50% using two different methods of measurement. Compared with baseline day, PGA2 significantly (P < or = 0.05) increased aqueous flow by 1.8 microliters min-1, fluorophotometric outflow facility by 0.36 microliter min-1 mmHg-1 and fluorophotometric uveoscleral outflow by 2.0 microliters min-1. Total outflow facility was not significantly different comparing treated with contralateral control eyes. Facility of uveoscleral outflow was < or = 0.02 microliters min-1 mmHg-1 for both control and treated eyes. It is concluded that PGA2 decreases IOP in cats by increasing uveoscleral outflow and trabecular outflow facility as measured with fluorophotometry. A significant increase in aqueous flow reduces the ocular hypotensive effect.

Effect of bicarbonate on intracellular potential of rabbit ciliary epithelium

Extracellular HCO3- hyperpolarizes the intracellular potential and makes the aqueous medium negative with respect to the stromal surface of the rabbit ciliary epithelial syncytium. The bases for these observations have been unclear. We have been studying the bicarbonate-induced hyperpolarization (BIH) with sustained intracellular recordings for periods as long as 1-2 hrs. The BIH was observed [6.0 +/- 0.4 mV (mean +/- SE, N = 22)] even when the external pH was clamped constant by appropriately changing the CO2 tension. External HCO3- was required since aeration with CO2 at low external pH did not replicate the BIH. DIDS [4,4'-diisothiocyano-2,2'-disulfonic acid] did not abolish the effect. The hyperpolarization is unlikely to reflect the pH dependence of K+ channels alone, since the effect was not reduced by either 2 mM Ba2+ alone or 2 mM Ba2+ together with 50-100 microM quinidine. The BIH depends directly or indirectly on external Na+, since the sign of the polarization response was reversed either by replacing Na+ with N-methyl-D-glucamine or by blocking the Na+,K(+)-exchange pump with 50-100 microM ouabain. Replacement of external Cl- with NO3- or application of the Cl(-)channel blocker NPPB [5-nitro-2-(3-phenylpropylamino)-benzoate] depolarized the membrane and reversed the sign of the BIH. The response of the ciliary epithelium to HCO3- is complex and may arise from several mechanisms. We suggest that one important element is an anion channel whose conductance is reduced by bicarbonate and whose reversal potential is indirectly dependent on the operations of the Na+,K(+)-pump and a Cl(-)-linked symport.

Detection and characterization of Ca(2+)-activated K+ channels in transformed cells of human non-pigmented ciliary epithelium

Cell-attached and excised inside-out membrane patches were used to study single channel currents in a cell line derived from human non-pigmented ciliary epithelium. Most of the patches contained a Ca(2+)-dependent K+ channel with large unitary conductance (200 pS in symmetrical K+ solutions). Single channel current in cell-attached patches exposed to high K+ solution in the pipette showed a null potential of -36 mV. This value, which should yield an approximate estimation of cell membrane potential, was reversibly increased by -30 to -40 mV in the presence of Ca2+ ionophores. Tetraethylammonium up to 10 mM applied at the membrane cytoplasmic face had no effect on the channel. Addition of 1 mM BaCl2 to excised patches caused a voltage-dependent blockade of the channel. In the presence of barium the unit currents were not altered, but the channel remained closed for long periods of time and the open state probability decreased with depolarization. The possibility that this channel participates in regulation of transepithelial ciliary body secretion is discussed.

Ascorbate-stimulated active Na+ transport in rabbit ciliary epithelium

In a physiological medium (134 mM Na+ concentration), unidirectional blood-to-aqueous and aqueous-to-blood Na+ fluxes across the isolated rabbit ciliary epithelium are large, rendering the detection of a net transport difficult. At 134 mM an active component for Na+ may be obscured by diffusional fluxes and a bidirectional Na(+)-Cl- cotransport. Considering that the active transport saturates at about 30 mM, experiments were performed at this reduced Na+ concentration to minimize the influence of diffusional pathways. A net blood-to-aqueous Na+ flux that ranged from 0.25 to 0.81 mu eq/hr was obtained. Addition of ascorbic acid to the aqueous side under this condition increased the blood-to-aqueous flux with little effect on the flux in the opposite direction. Ouabain inhibited both the Na+ and ascorbate-stimulated Na+ transport. The increase in blood-to-aqueous Na+ flux by ascorbate was also observed in tissues bathed with [Na+] closer to physiological levels (100 mM). These results indicate that the rabbit ciliary epithelium transports Na+ into the posterior chamber. Since aqueous ascorbate stimulates Na+ transport, it may be implicated in both Na+ movement and aqueous humor secretion. However, the rate of Na+ transport can only account for a small fraction of total aqueous humor production.

Mechanically stripped pigmented and non-pigmented epithelium of the shark ciliary body: morphology and transepithelial electrical properties

Sections of intact ciliary epithelium and mechanically stripped non-pigmented (NPE) and pigmented (PE) cell layers of adult sharks (Squalus acanthias) were mounted in Ussing-type chambers (area 0.1 cm2). Addition of 10(-5) M forskolin to the aqueous side of intact epithelium significantly increased short-circuit current (Isc) within 15 min and a maximum of approx. 30 microA cm-2 was reached after 45-60 min. Transepithelial potential difference (V) increased from -0.8 mV (aqueous side negative as compared with blood/stromal side) to -1.5 mV, whereas resistance (R) was unchanged (50 omega cm2). Forskolin was without effect when applied to the blood side. In stripped PE preparations (R 15 omega cm2), 10(-5) M forskolin applied to the apical side induced a qualitatively similar change of Isc and V compared with the intact tissue. The forskolin-induced effects were fully reversed by 10(-4) M bumetanide and were not dependent on pretreatment of the tissue with 10(-3) M BaCl2. In stripped NPE preparations resistance was usually less than 10 omega cm2 and was not stable. This is consistent with the morphologic observation that although tight junctions were still demonstrable in stripped NPE cells, the apical membranes were damaged. In preparations taken for light and electron microscopy the stripped PE layer revealed intact epithelial cells. In particular, the basal thirds of the stripped PE cells were in very close contact with each other. These attachment zones may have the appearance of tight junctions. Thus the PE cells of the shark ciliary epithelium can be successfully isolated for transepithelial transport studies. The adenylate cyclase system is present in PE cells, and transepithelial transport of chloride may be regulated by intracellular cAMP.

Voltage-activated currents recorded from rabbit pigmented ciliary body epithelial cells in culture

1. The whole-cell recording mode of the patch-clamp technique was used to investigate the presence of voltage-activated currents in the isolated pigmented cells from the rabbit ciliary body epithelium grown in culture. 2. In Ringer solution with composition similar to that of the rabbit aqueous humour, depolarizing voltage steps activated a transient inward current and a delayed outward current, while hyperpolarization elicited an inwardly rectified current. 3. The depolarization-activated inward current was mainly carried by Na+ and was blocked by submicromolar concentrations of tetrodotoxin. This current in many cells was sufficiently large to produce a regenerative Na+ spike. 4. The depolarization-activated outward current was carried by K+ and blocked by external TEA and Ba2+. Its activation appeared to be Ca2(+)-independent. 5. The hyperpolarization-activated inward current was almost exclusively carried by K+ and was blocked by Ba2+ and Cs+. For large hyperpolarizations below -120 mV, this current exhibited a biphasic activation with a fast transient peak followed by a slower sag, that appeared to be due to K+ depletion. 6. The voltage-dependent K+ conductances probably act to stabilize the cell membrane resting potential and may also play a role in ion transport. The function of the Na(+)-dependent inward current is unclear, but it may permit the electrically coupled epithelial cells of the ciliary body to conduct propagated action potentials.

Volume regulation of cultured, transformed, non-pigmented epithelial cells from human ciliary body

Electronic cell sizing has been used to measure the volume of cells suspended in isosmotic and in hyposmotic solutions of identical ionic composition. Without inhibitors, the cells displayed a regulatory volume decrease (RVD) following anisosmotic cell swelling with a time constant (tau) of 6.3 +/- 0.9 min (mean +/- S.E.). The RVD was markedly impaired by substituting gluconate for external Cl-, and tau was prolonged by: (i) reducing the chemical gradient favoring K+ loss (by elevating the external [K+] and blocking the Na. K-exchange pump), (ii) blocking the K+ channels with Ba2+, (iii) blocking Cl- channels and Cl-/HCO3 = exchange with DIDS, and (iv) removing external HCO3-. Withdrawing HCO3- may have altered the RVD either directly by inhibiting a Cl-/HCO3- antiport, or indirectly by affecting intracellular pH. The regulatory volume response of ODM Cl-2/SV40 cells is in several respects qualitatively similar to that of non-pigmented epithelial cells of the intact ciliary body. These common characteristics suggest that the cultured cells can serve as a useful model for studying solute and fluid transport across the human ciliary epithelium. The basis for the RVD is likely to be activation of separate K+ and Cl- channels, with or without the parallel operation of coupled K+/H+ and Cl-/HCO3- antiports.

Intracellular voltage recordings in bovine non-pigmented ciliary epithelial cells in primary culture

Bovine non-pigmented ciliary epithelial cells (NPE) have been isolated by a technique of selective adhesion to tissue culture plastic. NPE cells in primary culture proliferated and maintained epithelial-like morphology for about 4 weeks in tissue culture medium containing 10% fetal calf serum. If grown for longer than 4 weeks in serum-containing medium, cells changed their morphology and became elongated and spindle-shaped. Membrane potentials were measured using conventional microelectrodes. In NPE cells of epithelial-like shape, replacing extracellular Na+ induced a transient hyperpolarization of the membrane potential, while in elongated cells of spindle-shaped morphology an immediate depolarization was observed. We therefore only used epithelial-like NPE for further experiments. In these cells the mean membrane potential was -40.3 +/- 0.5 mV (n = 36). Relative K+ conductance was increased by extracellular alkalinization. Removing extracellular K+ led to a depolarization and readdition of K+ to K+ depleted cells resulted in a hyperpolarization. Both voltage responses were sensitive to ouabain, indicating that Na+/K+ ATPase is inhibited by K+ replacement, and that there is overshoot-activation of the pump when K+ is readded. Extracellular Cl- replacement led to a DIDS sensitive, transient depolarization, which is compatible with a stilbene-sensitive Cl(-)-conductance. Removing HCO3- led to a Na+ dependent and DIDS-sensitive depolarization. However, the electrical response on replacement of extracellular Na+ was not influenced by DIDS or the extracellular HCO3(-)-concentration.

K+-conductance and electrogenic Na+/K+ transport of cultured bovine pigmented ciliary epithelium

Using intracellular microelectrode technique, we investigated the changes in membrane voltage (V) of cultured bovine pigmented ciliary epithelial cells induced by different extracellular solutions. (1) V in 213 cells under steady-state conditions averaged -46.1 +/- 0.6 mV (SEM). (2) Increasing extracellular K+ concentration [( K+]o) depolarized V. Addition of Ba2+ could diminish this response. (3) Depolarization on doubling [K+]o was increased at higher [K+]o (or low voltage). (4) Removing extracellular Ca2+ decreased V and reduced the V amplitude on increasing [K+]o. (5) V was pH sensitive. Extra- and intracellular acidification depolarized V; alkalinization induced a hyperpolarization. V responses to high [K+]o were reduced at acidic extracellular pH. (6) Removing K+o depolarized, K+o readdition after K+ depletion transiently hyperpolarized V. These responses were insensitive to Ba2+ but were abolished in the presence of ouabain or in Na+-free medium. (7) Na+ readdition after Na+ depletion transiently hyperpolarized V. This reaction was markedly reduced in the presence of ouabain or in K+-free solution but unchanged by Ba2+. It is concluded that in cultured bovine pigmented ciliary epithelial cells K+ conductance depends on Ca2+, pH and [K+]o (or voltage). An electrogenic Na+/K+-transport is present, which is stimulated during recovery from K+ or Na+ depletion. This transport is inhibited by ouabain and in K+- or Na+-free medium.

Aerobic and anaerobic metabolism of bovine ciliary process: effects of metabolic and transport inhibitors

In the present study we have measured the oxygen consumption and lactic acid production, under aerobic and anaerobic conditions, in the bovine ciliary process epithelium (CPE) in the presence and absence of transport modifiers. Basal oxygen consumption was 8-15 microliters O2 consumed/mg protein/hr and decreased by 35% when sodium was removed or ouabain was added to the media. Anaerobic metabolism as measured by lactate production was also attenuated by sodium-free or ouabain treatment. When O2 consumption was severely limited by cyanide, lactic acid production increased significantly ("Pasteur effect"), whereas 2-deoxyglucose reduced lactate formation. Both chloride-free and acetazolamide treated CPE increased their dependency on aerobic glycolysis, and this response was also observed under anaerobic conditions, suggesting the presence of an anion transport mechanism. A net lactate production was also found to occur across the aqueous epithelium under aerobic and anaerobic conditions. These results are consistent with the presence of a bicarbonate-sensitive anion transport system located in the ciliary process epithelium.