Pharmacokinetics of the fluocinolone/5-fluorouracil codrug in the gas-filled eye

PURPOSE

To determine the effect of intraocular gas tamponade on drug levels achieved with the intravitreal sustained-release fluocinolone (FL)/5-fluorouracil (5-FU) codrug pellet.

METHODS

After insertion of a 10-mg codrug pellet into the right eyes of 43 New Zealand white rabbits, perfluoropropane (0.4 mL of 100% C3F8) or a control sham was then injected into the midvitreous cavity. On postoperative days 2, 4, 7, 21, and 42, aqueous samples were collected, the rabbits were killed, and the right eyes were enucleated. The vitreous and remaining codrug pellet were then isolated. Pellet and intravitreal drug levels were determined by high-pressure liquid chromatography.

RESULTS

No measurable drug levels were detected in any of the aqueous samples. Maximal gas expansion occurred by day 4 and partial resorption was observed by days 14 to 21. Vitreous FL and 5-FU levels during C3F8 expansion (day 2) were statistically significantly higher in the gas-filled eyes. On postoperative days 4, 7, 21, and 42, there were no statistically significant differences between FL and 5-FU drug levels in eyes containing C3F8 as compared with control eyes. Pellet codrug, FL, and 5-FU levels over time were similar in gas-filled and control eyes.

CONCLUSIONS

Intraocular gas tamponade does not significantly affect the sustained intravitreal drug levels achieved with the FL/5-FU codrug. If clinically efficacious, the FL/5-FU codrug formulation does not need to be altered to treat proliferative vitreoretinopathy in the presence of intraocular gas.

Features of macular hole closure in the early postoperative period using optical coherence tomography

PURPOSE

Studying the retina in the early postoperative period after macular hole surgery is difficult because of the limitation of imaging of the fundus through a gas tamponade. Silicone oil was shown recently to be an effective alternative to gas for macular hole repair. The authors hypothesized that optical coherence tomography (OCT) could be performed through silicone oil to study early macular hole closure.

METHODS

Fourteen patients with idiopathic full-thickness macular holes underwent repair, including vitrectomy and silicone fill of the vitreous cavity. Silicone oil was removed 5 to 18 weeks later in a second operative procedure. Optical coherence tomography images were obtained preoperatively (n = 14), on the first postoperative day (n = 7), after 1 week (n = 4), after 1 month (n = 14), and after silicone oil removal (n = 14).

RESULTS

By the first postoperative day, the retina was flat and the hole was open or closed in all patients. By 1 month, 9 of 14 patients had flat retinas and closed holes, whereas the other 5 patients had flat retinas and open holes. All patients who had an open hole after 1 month had a reopening of the hole after silicone oil removal.

CONCLUSIONS

Flattening of the retina with resolution of the foveal cysts after macular hole repair occurs by the first postoperative day. Closure of the horizontal retinal separation is variable but occurs by the first postoperative month in cases of successful macular hole repair. Persistent retinal separation after 1 month may be predictive of hole reopening.

Exudative retinal detachment and retinitis associated with acquired syphilitic uveitis

PURPOSE

To describe three cases of exudative retinal detachment and focal retinitis associated with acquired syphilitic uveitis.

METHODS

Three patients who were referred for evaluation of uveitis were examined. Slit-lamp examination, ophthalmoscopy, B-scan ultrasonography, fundus photography, and fluorescein angiography were performed before and after therapy.

RESULTS

Each patient had uveitis with exudative retinal detachment, periphlebitis, and focal retinitis. Laboratory testing (fluorescent treponemal antibody absorption) revealed positive serology for active syphilis in all cases. Human immunodeficiency virus antibody testing was negative in all patients. Retinal detachment resolved in all cases after treatment with intravenous penicillin. Despite resolution of subretinal fluid, visual acuity remained poor in eyes in which the macula was detached.

CONCLUSION

Syphilis is a cause of exudative retinal detachment. Antibiotic therapy can lead to retinal reattachment. Early recognition and treatment may prevent severe vision loss.

Diagnosis of vitreoretinal adhesions in macular disease with optical coherence tomography

PURPOSE

To compare the relative incidence of vitreoretinal adhesions associated with partial vitreous separation within the macula diagnosed with optical coherence tomography (OCT) with that of those diagnosed with biomicroscopy.

METHODS

The authors obtained linear cross-sectional retinal images using OCT in patients with selected macular diseases. Additional studies included biomicroscopy, fundus photography, fluorescein angiography, and B-scan ultrasonography.

RESULTS

Optical coherence tomography was performed on 132 eyes of 119 patients. Vitreoretinal adhesions within the macula were identified using OCT in 39 eyes (30%) with the following diagnoses: idiopathic epiretinal membrane (n = 13), diabetic retinopathy (n = 7), idiopathic macular hole (n = 7), cystoid macular edema (n = 7), and vitreomacular traction syndrome (n = 5). Biomicroscopy identified vitreoretinal adhesions in only 11 eyes (8%). Two distinct vitreoretinal adhesion patterns were identified with OCT, each associated with partial separation of the posterior hyaloid face: focal (n = 25) and multifocal (n = 14).

CONCLUSIONS

Optical coherence tomography is more sensitive than biomicroscopy in identifying vitreoretinal adhesions associated with macular disease.

Use of isobutyl cyanoacrylate tissue adhesive to stabilize mucous membrane grafts in total socket reconstruction

PURPOSE

To describe a technique to secure a mucous membrane graft to a custom conformer during reconstruction of the conjunctival fornices and socket.

METHODS

A cyanoacrylate-based tissue glue was used instead of sutures to secure the mucous membrane graft to the conformer.

RESULTS

The adhesion between the graft and the conformer weakens over time, permitting easy removal of the conformer from the socket 6 to 12 weeks postoperatively. No complications were encountered in six patients in whom this technique was used.

CONCLUSION

Using a cyanoacrylate-based tissue glue is simpler and quicker than suturing a mucous membrane graft.

Intravitreal sustained-release cyclosporine in the treatment of experimental uveitis

OBJECTIVE

Uveitis often is a chronic disease requiring long-term medical therapy. Despite treatment, the disease may be difficult to control and may produce serious, vision-threatening ocular complications. In this study, the authors determined whether an intravitreal cyclosporine-sustained delivery device was effective in the treatment of ocular inflammation in a rabbit model of uveitis.

METHODS

New Zealand White rabbits were immunized subcutaneously with Mycobacterium tuberculosis H37Ra antigen. Fourteen days later, sustained-release cyclosporine devices were implanted into the vitreous cavity of the right eye of experimental rabbits. Control animals received sham devices. Seven days after device implantation, rabbits were challenged with an intravitreal injection of tuberculin antigen. To simulate chronic inflammation with exacerbations, some animals were rechallenged with intravitreal antigen on day 21 after device implantation. Inflammation was assessed clinically by a masked observer who graded anterior chamber cells, flare, corneal neovascularization, iris congestion, and vitreous opacity daily until day 7 and on day 13 after the initial intravitreous challenge, and on days 1 and 2 after the rechallenge. Retinal function was evaluated by electroretinography. Animals were killed 3, 6, 8, and 14 days after the initial intravitreal challenge and on the second day after rechallenge for aqueous leukocyte count, protein measurement, and histologic examination. The number of aqueous and peripheral blood proliferating lymphocytes and the subset of CD4+ T cells were determined by flow cytometry. High-performance liquid chromatography was used to measure cyclosporine A levels in vitreous and peripheral blood. Light microscopy was used to evaluate the eyes histopathologically.

RESULTS

By clinical criteria, treated eyes had significantly less inflammation than untreated eyes. The number of aqueous cells and protein concentration determined quantitatively paralleled the clinical assessment of anterior chamber cells and flare. The electroretinography B-wave was depressed significantly in untreated eyes compared with that of treated eyes (P < 0.02). Histopathologic examination results showed marked inflammation and tissue disorganization in untreated eyes, whereas cyclosporine-treated eyes had preserved architecture and greatly reduced inflammatory cells. Intravitreal cyclosporine remained at therapeutic levels for at least 6 months after intravitreal device implantation, whereas blood levels were low to nondetectable.

CONCLUSIONS

The intravitreal cyclosporine A device effectively suppresses ocular inflammation in a rabbit model of uveitis. This device may be useful in the treatment of patients with severe chronic uveitis who are intolerant to currently available therapies.

Chloride transport in cultured fetal human retinal pigment epithelium

This study identifies bumetanide-sensitive chloride transport in cultured fetal human retinal pigment epithelium (RPE). Ion flux and electrophysiologic studies were performed on fetal human RPE grown to confluence on microporous culture wells mounted in modified Ussing chambers. Unidirectional transepithelial Cl- fluxes were measured along with the transepithelial potential (TEP), resistance (Rt), and short circuit current (Isc). In separate experiments the apical and basal membrane voltages (Vap and Vba) and membrane resistance ratio (Rap/Rba) were measured using standard electrophysiological techniques. The average electrical parameters under control conditions were TEP = 3.6 +/- 2.1 mV, Rt = 385 +/- 98 ohm cm2, and Isc = 8.8 +/- 3.6 microA cm-2. Apical bumetanide (10(-5) M) reduced the TEP and Isc an average of 0.6 +/- 0.4 mV and 1.6 +/- 0.8 microA cm-2, respectively. Under open-circuit conditions, we measured a net apical to basal 36Cl flux of 0.30 microEq cm-2 hr-1. Bumetanide reduced this net Cl- flux by 83% by reducing the unidirectional apical-to-basal Cl- flux. Apical bumetanide hyperpolarized Vba from -58 +/- 14 to -66 +/- 17 mV and reduced Rap/Rba from 1.17 +/- 0.50 to 0.85 +/- 0.38, consistent with blockade of Cl- uptake and reduction of the Cl- equilibrium potential at a basal membrane Cl- channel. Basal bumetanide had no effect on electrical parameters. We identified a bumetanide sensitive Cl- uptake mechanism at the apical membrane which may represent the Na-K-2Cl cotransporter reported in explant and other cultured preparations of mammalian RPE.

Potassium conductances in cultured bovine and human retinal pigment epithelium

PURPOSE

To identify K+ conductances on the apical and basolateral membranes in cultured monolayers of fetal bovine and human retinal pigment epithelium (RPE).

METHODS

Bovine and human RPE cells were grown on a permeable substrate for an average of 4 and 25 months, respectively, mounted in a modified Ussing chamber that allowed rapid solution changes at both membranes, and perfused with modified Ringer's solutions. Conventional microelectrode recording techniques were used to record intracellularly from RPE cells.

RESULTS

Electrical parameters under control conditions for bovine (n = 11) and human (n = 7) cultures respectively, were: transepithelial potential, 0.6 +/- 0.2 and 3.2 +/- 0.5 mV; Rt, 70 +/- 12 and 227 +/- 30 ohm.cm2; and Vap, -61 +/- 3 and -51 +/- 2 mV (mean +/- SEM). The relative K+ conductance (TK) was estimated from responses to 10-fold increases in [K+]o. For the apical and basal membranes, the values for TK were 0.65 and 0.37, respectively, in bovine and 0.33 and 0.45, respectively, in human RPE. Barium applied to either surface of the cultures produced membrane depolarizations and suppressed the responses significantly to K+. In bovine, a 10-fold decrease in basal [K+]o hyperpolarized Vba by 2 +/- 1 mV; a similar decrease in apical [K+]o hyperpolarized Vap by 14 +/- 1 mV. At both membranes, perfusion with Ba2+ unmasked a significant membrane depolarization induced by lowering [K+]o; this phenomenon, observed previously at the apical membrane in fresh explant bovine, frog, and toad RPE, appears to be an unmasking of Na+/K+ pump modulation by K+.

CONCLUSIONS

These results indicate the presence of Ba2+ sensitive K+ conductances at the apical and basal membranes of confluent monolayers of cultured fetal human and bovine RPE and the presence of Na+/K+ pump sites at both membranes of cultured bovine cells.

Penetrating keratoplasty with vitreoretinal surgery using the Eckardt temporary keratoprosthesis: modified technique allowing use of larger corneal grafts

We report our experience with the Eckardt temporary keratoprosthesis including a technique modification allowing use of larger corneal grafts. We combined penetrating keratoplasty with vitreoretinal surgery using the Eckardt keratoprosthesis in 24 eyes of 24 patients in two patient groups: trauma (n = 11) and nontrauma (n = 13). Our technique modification involved centering a larger partial trephination for the donor cornea around the smaller trephination used for the keratoprosthesis. After the keratoprosthesis was removed, we excised host cornea graft. With the exception of visual acuity (VA), the outcomes were similar for both trauma and nontrauma groups, and combined results were as follows. Corneal grafts remained clear in 71% of patients at a mean follow-up period of 16 +/- 3 (SE) months. For the trauma and nontrauma groups, VA improved in 82 and 23%, and decreased in 18% and 38.5%, respectively. Postoperative loss of VA was due primarily to retinal scarring (47%) and phthisis bulbi (37%). Only one case of graft rejection occurred. Postoperatively, the larger corneal grafts (8.0 and 8.5 mm) were less steep, and residual astigmatism was more easily managed than with the "recommended" (7.2 mm) size.

CONCLUSIONS

Larger corneal grafts allow less astigmatism and a more normal corneal curvature. Theoretical advantages also include faster rehabilitation of vision, better predictability for intraocular lens implant power calculations, and transplantation of more endothelial cells. The Eckardt will continue to be a useful tool and our technique modification allowing use of larger corneal grafts should enhance its utility.

Localization of NaK ATPase on cultured human retinal pigment epithelium

PURPOSE

To localize NaK ATPase sites on cultured human retinal pigment epithelium (RPE).

METHODS

Cultured human RPE from fetal, 2-year-old, and 21-year-old donors was grown to confluence in microporous culture wells for 4 months to 2 years, mounted in a small-volume Ussing chamber, and perfused with growth medium. Ouabain (10(-5)-M) was applied to the basal and apical sides of the RPE. Changes in transepithelial resistance (Rt), transepithelial potential (TEP), and apical and basal membrane potentials were measured.

RESULTS

Application of ouabain to the basal side of RPE produced a small sustained increase in TEP after 6 minutes and, simultaneously, small depolarizations of both apical and basal membranes. During the continued presence of ouabain on the basal side, application of ouabain to the apical side produced a significantly larger TEP decrease and greater depolarization of both membranes. Significant changes in Rt were not observed.

CONCLUSIONS

These results indicate that NaK ATPase sites are present on both the apical and basolateral membranes of cultured human RPE. The greater effect of ouabain when applied to the apical side suggests that functional NaK ATPase sites are more abundant on the apical membrane.

Calcium gradients and light-evoked calcium changes outside rods in the intact cat retina

We have studied light-evoked changes in extracellular Ca2+ concentration ([Ca2+]o) in the intact cat eye using ion-sensitive double-barreled microelectrodes. Two prominent changes in Ca2+ concentration were observed that differed in retinal location. There was a light-evoked increase in [Ca2+]o, accompanied by brief ON and OFF transients, which was maximal in the inner plexiform layer and was not further studied. There was an unexpected sustained light-evoked decrease in [Ca2+]o, of relatively rapid onset and offset, which was maximal in the distalmost region of the subretinal space (SRS). [Ca2+]o in the SRS was 1.0 mM higher than in the vitreous humor during dark adaptation and this transretinal gradient disappeared during rod-saturating illumination. After correcting for the light-evoked increase in the volume of the SRS, an increase in the total Ca2+ content of the SRS during illumination was revealed, which presumably represents the Ca2+ released by rods. To explain the light-evoked [Ca2+]o changes, we used the diffusion model described in the accompanying paper (Li et al., 1994b), with the addition of light-dependent sources of Ca2+ at the retina/retinal pigment epithelium (RPE) border and rod outer segments. We conclude that a drop in [Ca2+]o around photoreceptors, which persists during illumination and reduces a transretinal Ca2+ gradient, is the combined effect of the light-evoked SRS volume increase, Ca2+ release from photoreceptors, and an unidentified mechanism(s), which is presumably Ca2+ transport by the RPE. The relatively rapid onset and offset of the [Ca2+]o decrease remains unexplained. These steady-state shifts in [Ca2+]o should have significant effects on photoreceptor function, especially adaptation.

Light-dependent hydration of the space surrounding photoreceptors in chick retina

PURPOSE

To characterize the light-evoked increase in the volume (or hydration) of the subretinal space (SRS) of chick retina and to investigate its retinal pigment epithelial (RPE) origin.

METHODS

The authors perfused chick retina-RPE-choroid preparations with the extracellular space marker, tetramethylammonium (TMA+), and estimated its concentration ([TMA+]o) using double-barreled, ion-selective electrodes. They recorded dark-light changes in subretinal [TMA+]o and studied the effects of drugs that suppress RPE ion transport and conductances.

RESULTS

The authors observed a slow, light-evoked decrease in subretinal [TMA+]o, about 7% in amplitude, that slowly returned to baseline and then increased at light OFF. It was 40% to 65% inhibited by bumetanide and furosemide, known to inhibit RPE apical membrane Na-K-Cl cotransport; 70% to 80% inhibited by DIDS, an inhibitor of RPE basal membrane chloride conductance, and 50% inhibited by 0.5 mM BaCl2, a blocker of the K+ conductance of the RPE apical membrane. Decreases in subretinal [TMA+]o also were produced when the authors lowered [Cl-]o or [K+]o in the SRS by reducing concentrations in the retinal perfusate. The effect on [TMA]o was larger for [Cl-]o than for [K+]o and was inhibited by bumetanide and DIDS.

CONCLUSIONS

Changes in subretinal ion concentrations in light and darkness, most likely [K+]o, modify SRS hydration by modulating RPE apical membrane Na-K-Cl cotransport, as well as the conductive effluxes of K+ across the apical membrane and Cl- across the basal membrane. These changes depend principally on the movement of water into or out of the RPE. In turn, subretinal hydration modifies local ion concentrations and the composition of the interphotoreceptor matrix.

Basolateral membrane Cl- and K+ conductances of the dark-adapted chick retinal pigment epithelium

1. We characterized the basolateral membrane Cl- and K+ conductances of the dark-adapted chick neural retina-retinal pigment epithelium (RPE)-choroid preparation. Conventional microelectrodes were used to measure apical (V(ap)) and basolateral (Vba) membrane voltage, and double-barreled Cl- and K+ selective microelectrodes were used to follow the time course and magnitude of ion concentration changes outside the basolateral (basal) membrane. 2. In response to a fivefold decrease in basal [Cl-]o, Vba rapidly depolarized by 6.4 +/- 0.7 (SE) mV, and the apparent resistance of the basolateral membrane (Rba) increased. The Cl- channel blocker 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS) suppressed the Vba depolarization by 40% and blocked the Rba increase. Estimates of the relative Cl- conductance (transference number, TCl) from the DIDS-sensitive component of the Cl- diffusion potential gave an average value for TCl of 0.22 +/- 0.03. 3. Further evidence for a Cl- conductance was obtained by measuring changes in intracellular Cl- activity (aCli) induced by transtissue current. Depolarizing Vba elevated aiCl, whereas hyperpolarizing Vba had the opposite effect, consistent with conductive Cl- movement across the basal membrane. TCl estimated from these data averaged 0.23 +/- 0.02. 4. In response to a sixfold increase in basal [K+]o, Vba depolarized 6.1 +/- 0.8 mV. The amplitude of this K+ diffusion potential was inhibited 44 and 67% by 5 and 10 mM Ba2+, respectively. TK was estimated to be 0.61 +/- 0.05. 5. The rapid c-wave membrane hyperpolarizations in response to the light-evoked decrease in subretinal [K+]o were used to calculate the equivalent resistances of the apical membrane (R(ap)), basolateral membrane (Rba), and the paracellular shunt pathway (Rs). They were 152 +/- 10, 615 +/- 38, and 138 +/- 7 omega.cm2 (n = 11 tissues), respectively. From these data the equivalent electromotive force for the basal (Eba) and apical (Eap) membranes were estimated to be -45 +/- 2 and -77 +/- 1 mV, respectively. This estimate of Eba is in the range of that predicted from our estimates of TCl and TK, indicating that, in the dark-adapted chick retina, the resting conductance of the basal membrane can largely be accounted for by the Cl- and K+ conductances described here.

Light-evoked modulation of basolateral membrane Cl- conductance in chick retinal pigment epithelium: the light peak and fast oscillation

1. We studied the ionic mechanism of the light-peak voltage of the DC electroretinogram (DC ERG) in an in vitro preparation of chick neural retina-retinal pigment epithelium (RPE)-choroid. The light peak originates from a depolarization of the RPE basolateral (basal) membrane, associated with an increase in its conductance. Using conventional and Cl(-)-selective microelectrodes, we tested the hypothesis that the light-peak voltage is generated by an increase in Cl- conductance (gCl) of the basolateral (basal) membrane. 2. Perfusion of the RPE basal membrane with 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS), a known blocker of gCl in chick RPE, suppressed both the light-peak depolarization and the accompanying conductance increase of the basal membrane. 3. Using sustained transepithelial current to clamp the basal membrane potential at different levels, we estimated the reversal potential of the light peak. At membrane potentials above the equilibrium potential for Cl- (ECl = -40 +/- 10 mV mean +/- SE), light-peak polarity was reversed. Current-voltage (I-V) curves measured in the dark and at the peak of the light peak also gave a reversal potential in the same range as ECl. In addition, shifting ECl by changing intracellular Cl- (aCli) via passage of transepithelial current or perfusing the apical side of the RPE with the Cl- uptake blocker, furosemide, shifted the light-peak reversal potential in the same direction as the change in ECl. 4. The transference number for Cl-, TCl, was estimated from step decreases in basal Cl- and increased from 0.20 +/- 0.01 in the dark to 0.31 +/- 0.01 during the light peak. These results indicate an average increase of 55% in the relative conductance of the basal membrane for Cl-. 5. Light-evoked changes in aCli, measured with Cl(-)-selective microelectrodes, were too small to account for the change in basal membrane potential during the light peak. These data strongly support the hypothesis that the light peak originates from an increase in RPE basal membrane permeability to Cl-. 6. We also obtained support for the model of Joseph and Miller that the fast-oscillation trough of the DC ERG, generated by a delayed basal membrane hyperpolarization of the RPE, originates from light-evoked modulation of the Cl- transport pathway. Perfusing either the apical side of the RPE with furosemide or the basal side with DIDS suppressed the fast oscillation. The delayed basal hyperpolarization reversed polarity at membrane potentials positive to ECl.(ABSTRACT TRUNCATED AT 400 WORDS)

Direct evidence for a basolateral membrane Cl- conductance in toad retinal pigment epithelium

There is now evidence that a Cl- conductance on the basal membrane of the retinal pigment epithelium (RPE) is involved in the generation of both the fast oscillation and the light peak of the direct-current electroretinogram as well as being critical for transepithelial fluid and salt movement. In the present study, we characterized the basolateral membrane Cl- conductance of an in vitro preparation of toad RPE-choroid using conventional and Cl(-)-selective microelectrodes. Under control conditions, the potential across the apical (Vap) and basal (Vba) membranes averaged -60 +/- 2 and -45 +/- 2 mV, respectively (n = 40). Intracellular Cl- activity (aiCl = 20 +/- 1 mM) was distributed above equilibrium across both membranes, consistent with active accumulation of Cl-. A sixfold decrease in Cl- in the basal bath depolarized Vba by 12 +/- 1 mV (n = 17) and increased the apparent basal membrane resistance. By sequential measurement of aiCl and subepithelial Cl- activity during a step decrease in basal Cl-, we constructed the change in Cl- equilibrium potential (ECl) across the basal membrane. Estimation of the change in basal membrane electromotive force during the change in ECl gave an average value for the Cl- transference number (TCl) of 0.45. Further evidence for a Cl- conductance was obtained by measuring changes in aiCl induced by transepithelial current. Depolarizing Vba elevated aiCl, whereas hyperpolarizing Vba had the opposite effect, consistent with conductive Cl- movement across the basal membrane. Both the amplitude of the Cl- diffusion potential and the current-induced changes in aiCl were reduced by basal perfusion with 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (250-500 microM), a blocker of Cl- channels in some epithelia.

Cobalt increases photoreceptor-dependent responses of the chick retinal pigment epithelium

While using cobalt (Co2+) to block synaptic transmission in an in vitro preparation of chick retina, the authors observed significant changes in the DC electroretinogram (DC ERG). Cobalt (3.0 mM) increased the amplitudes of all three photoreceptor-dependent responses of the retinal pigment epithelium (RPE): the c-wave, fast-oscillation trough, and light peak. Intracellular recordings from RPE cells revealed that Co2+ increased those light-evoked changes in RPE membrane potentials that contribute to each of these responses. Monitoring of subretinal [K+]o with K(+)-selective microelectrodes showed that Co2+ increased the amplitude of the light-evoked [K+]o decrease, and this must contribute to the observed increase in c-wave and fast-oscillation trough because both are generated by this [K+]o change. Cobalt also increased the initial rate of [K+]o decrease at light-onset, the rate of subretinal [K+]o reaccumulation during maintained illumination, and the amplitude of the [K+]o overshoot at light-offset. The Co(2+)-induced increases in light-evoked RPE responses and subretinal [K+]o changes may arise from a direct effect on photoreceptors because (1) blockade of postphotoreceptoral activity did not block these effects of Co2+; and (2) Co2+ did not alter significantly the electrical properties of isolated RPE-choroid tissues. The authors conclude that, in the cone-dominated chick retina, Co2+ may act directly on the photoreceptors to increase the light response.

Effects of cAMP and IBMX on the chick retinal pigment epithelium. Membrane potentials and light-evoked responses

cAMP is known to alter electrical measures of retinal pigment epithelial (RPE) function (standing potential and DC ERG. To locate the origin of these effects, studies were performed on in vitro preparations of chick retina-RPE-choroid and RPE-choroid, which were separately perfused with cAMP or db-cAMP on the retinal (apical) and choroidal (basal) tissues surfaces. Similar studies were performed using the phosphodiesterase inhibitor IBMX. cAMP and db-cAMP produced essentially identical results. cAMP placed in the retinal or choroidal baths hyperpolarized the RPE basal membrane, increased the apparent basal membrane resistance (Rba), and decreased the ERG c-wave. Experiments in RPE-choroid preparations suggested that these effects of cAMP were not secondary to effects on the neural retina. Effects were observed at 1.0 microM and increased with concentration (1.0 microM-500 microM), with choroidal application producing smaller effects than retinal. The c-wave decrease with retinal cAMP was due principally to the increase in Rba. However, the light-evoked subretinal K+ ([K+]0) decrease, measured with K(+)-specific microelectrodes, showed a small reduction that may have contributed to the c-wave change and that explained an observed decrease in slow PIII. Retinal cAMP also led to a small decrease in ERG a-wave amplitude, which in view of the effects on [K+]0, is consistent with a small reduction in the light response of photoreceptors. The cAMP-induced hyperpolarization of the RPE basal membrane may result from a decrease in basal membrane anion conductance. The results of retinal or choroidal perfusion with IBMX (0.1 mM) in the retina-RPE-choroid preparation were similar and were complicated by effects on photoreceptors. IBMX increased the amplitude of the light response of photoreceptors as indicated by an increase in amplitude of the light-evoked [K+]0 decrease and the a-wave of the ERG. IBMX also produced a transient increase in dark-adapted [K+]0 that altered the membrane potentials of the RPE and obscured any direct effects of IBMX on the RPE. However, IBMX perfusion of the RPE-choroid suggested that the effect on the RPE was similar to that of cAMP perfusion. The IBMX-induced increase in the photoresponse also led to increases in other components of the DC ERG, including the c-wave, fast oscillation and light peak.

Effects of dopamine on the chick retinal pigment epithelium. Membrane potentials and light-evoked responses

Dopamine, a retinal neurotransmitter, is known to affect electrical measures of retinal pigment epithelial (RPE) function: the standing potential and the DC ERG. To locate the origin of these effects, studies were performed on in vitro preparations of chick retina-RPE-choroid, which were separately perfused on the retinal and choroidal tissue surfaces. Dopamine (250 micrograms) in the retinal bath depolarized the RPE basal membrane, decreased the apparent basal membrane resistance (Rba) and increased the ERG c-wave. At concentrations less than or equal to 100 microM, retinal dopamine often caused a transient basal membrane hyperpolarization, accompanied by an apparent increase in Rba and decrease in c-wave. Surprisingly, 20-100 microM choroidal dopamine induced similar changes in basal membrane potential, resistance and c-wave amplitude, and the transient hyperpolarization and increase in Rba were often more pronounced than at comparable concentrations of retinal dopamine. Experiments in RPE-choroid preparations suggested that the effects of retinal dopamine were not secondary to effects on the neural retina. The effects of retinal and choroidal dopamine in the same tissue often were distinct, suggesting separate receptor populations on the apical and basolateral membranes of the RPE. The c-wave changes could be explained by the changes in Rba, and not by an effect on the light-evoked decrease in subretinal [K+]0. Choroidal perfusion with 50 microM 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS), which appears to block a Cl- conductance in chick RPE, blocked the effects of dopamine perfusion on either side of the tissue. These results suggest that perfusion with either retinal or choroidal dopamine leads to electrical effects on the RPE basal membrane, possibly via a second-messenger system affecting a basal membrane Cl- conductance. Dopamine could suppress the "light-peak" depolarization of the RPE basal membrane. When either retinal or choroidal dopamine induced a large net change in trans-tissue potential (originating as a change in basal membrane potential), the light peak was severely depressed, while smaller changes produced correspondingly smaller decreases in light-peak amplitude. We found, however, that light-peak amplitude was not significantly reduced when there was little net change in the trans-tissue potential, even though dopamine may have produced sizable transient effects. Thus, despite apparent occupation of dopamine receptors on the RPE, the light peak persisted under these conditions. Similar relations between light-peak amplitude and net change in trans-tissue potential have been observed for a variety of different conditions, suggesting that the effect of dopamine on the light peak is nonspecific.

Effects of melatonin on the chick retinal pigment epithelium: membrane potentials and light-evoked responses

The indolamine hormone melatonin, synthesized in the retina, is thought to participate in modulating day-night cyclic variations in photoreceptor and retinal pigment epithelium (RPE) function. It has also been shown to alter the electrical activity of the RPE of the mammalian eye (Dawis and Niemeyer, 1985, Soc. Neurosci. 11, 1079:1988, Clin. Vis. Sci. 3, 109-118: Textorius and Nilsson, 1987, Doc. Ophthalmol, 65, 97-111). To determine the origin of such electrical effects studies were performed on in vitro preparations of both chick retina-RPE-choroid and RPE-choroid. In retina-RPE-choroid preparations choroidal perfusion with melatonin (2.10(-6) M) hyperpolarized the basal membrane, increased its apparent resistance, and diminished the amplitude of the c-wave of the electroretinogram (ERG). Retinal perfusion with melatonin (2.10(-6) M) first depolarized the RPE apical membrane and increased its apparent resistance and this gave way to a basal membrane hyperpolarization accompanied by an increase in basal membrane resistance. Both phases were accompanied by c-wave decreases. Experiments in RPE-choroid preparations suggested that the choroidal effect of melatonin was independent of the neural retina, while the retinal effect was more complex and probably included a neural retinal component. Retinal or choroidal melatonin (2.10(-6) M) had little or no effect on the amplitude of the light peak of the DC ERG. These results show that chick RPE membrane potentials and resistances at either the apical or basal membrane can be affected by melatonin directly, or indirectly via effects on other cells.

Effects of DIDS on the chick retinal pigment epithelium. II. Mechanism of the light peak and other responses originating at the basal membrane

4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS) appears to block a Cl- conductance in the basal membrane of the chick retinal pigment epithelium (RPE) (Gallemore and Steinberg, 1989). The present paper describes the effect of DIDS, as well as effects of anion movement blockade with other pharmacological agents, on 3 responses that originate as depolarizations of the RPE basal membrane: the light peak of the DC electroretinogram, the response to sodium azide, and the response to a small transtissue hyperosmotic gradient. Intracellular and extracellular potentials, and measurements of subretinal [K+]o were obtained from an in vitro preparation of chick retina-RPE-choroid. Choroidal perfusion with each of 4 anion transport and Cl- channel blockers (DIDS, 4-acetamido-4'-isothiocyanostilbene, phenylanthranylic acid, and thiocyanate) suppressed the light peak. DIDS, the most potent agent, abolished the basal membrane light-peak depolarization. Choroidal perfusion with azide depolarized the basal membrane and decreased its apparent resistance, Rba, as in cat (Linsenmeier and Steinberg, 1987). The basal membrane depolarization, recorded extracellularly as an increase in the transtissue potential (TTP), was blocked by DIDS. Azide also increased RPE c-wave amplitude, which could be explained by the decrease in Rba, and it suppressed the light peak. As previously described (Shirao and Steinberg, 1987), a 25 mOsm transtissue hyperosmotic load (retinal side hyperosmotic) increased the TTP, increased the c-wave and decreased the transtissue resistance. DIDS suppressed each of these effects. We conclude that the light peak, the response to azide, and the response to a retinal hyperosmotic load may each originate as an increase in a basal membrane anion conductance that may be permeable to chloride.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of DIDS on the chick retinal pigment epithelium. I. Membrane potentials, apparent resistances, and mechanisms

While little is known about the transport properties of the retinal pigment epithelium (RPE) basal membrane, mechanisms for anion movement across the basal membrane appear to be present (Miller and Steinberg, 1977; Hughes et al., 1984; Miller and Farber, 1984). This work examines the electrophysiological effects of the anion conductance blocker, 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS) on the basal membrane of an in vitro preparation of chick retina-RPE-choroid. DIDS (10-125 microM), added to the choroidal bath, decreased the transtissue potential by decreasing the potential across the RPE. Intracellular RPE recordings showed that DIDS affected the membrane potential in 2 phases, initially hyperpolarizing the basal membrane and then, after prolonged exposure, depolarizing the apical membrane. Resistance assessment by transtissue current pulses and intracellular c-wave recordings suggested that DIDS increased basal membrane resistance (Rba) during the first phase and increased apical membrane resistance (Rap) during the second phase. Measurements of intracellular Cl- activity (aiCl) showed that Cl- was actively accumulated by the chick RPE since it was distributed above equilibrium across both the apical and basal membranes. Perfusion of the basal membrane with 50 microM DIDS significantly increased aiCl-. The DIDS-induced basal membrane hyperpolarization, apparent increase in Rba, and increase in aiCl- are all consistent with Cl- -conductance blockade. During the second phase, apical membrane responsiveness to the light-evoked decrease in subretinal [K+]o (Oakley, 1977) was reduced an average of 58%. This finding, given the second-phase apical membrane depolarization and apparent increase in Rap, is consistent with a decrease in apical membrane K+ conductance.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence in support of a photoreceptoral origin for the "light-peak substance"

The light peak of the DC ERG was recorded in in vitro preparations of chick neural retina-RPE-choroid. Either the pharmacological agents Co2+, Mg2+, and low Ca2+, known to block calcium-dependent synaptic transmission or 2-amino-4-phosphonobutyric acid (APB), which blocks transmission in the ON pathway, were added to the solution perfusing the retina. These agents suppressed the b-wave of the ERG, thus indicating a severe suppression of synaptic transmission in the neural retina. The retina was also perfused with the OFF pathway inhibitor cis-2,3-piperidine dicarboxylic acid (PDA). None of these agents, however, significantly depressed the light peak, supporting the hypothesis that the light peak is generated by a photoreceptor-RPE interaction.