The effects of ouabain on endothelial function in human and rabbit corneas

These experiments were undertaken to study the effects of ouabain on endothelial function in isolated perfused human and rabbit corneas. Both human and rabbit corneas swell at rates which are dose-dependent when perfused with ouabain (10(-8) to 10(-5) M). The human tissue, however, requires tenfold higher concentrations to achieve swelling rates comparable to rabbit. Dose-dependent ultrastructural changes are seen in ouabain-perfused endothelia progressing from moderate endothelial edema (7.5 X 10(-7) M) to marked edema with altered subcellular organelles and vacuolization (10(-5) M). Ouabain-perfused human corneas show marked endothelial edema even at the lowest concentrations of ouabain tested. Apical junctions remain intact, however, in both human and rabbit endothelia at all concentrations. These results suggest that endothelial Na/K ATPase is essential to pump function in both human and rabbit corneas.

Pharmacokinetics of topical ocular phenylephrine HCl

The rates of corneal penetration and efflux of phenylephrine (PE) and its metabolites were found to be limited by the epithelium. The rate constant for corneal penetration as measured in a lucite block perfusion system was 1.06 X 10(-3) hr-1 when the epithelium was present and 1.25 X 10(-2) hr-1 when the epithelium was denuded. Epithelial removal reduced the half-time (t1/2) for corneal efflux of PE from 24 min to 6 min. Ocular absorption of topically applied 0.1% PE (three 30-microliters instillations at five-minute intervals) was increased when the corneal epithelium was removed prior to application. Corneal concentrations of PE increased threefold, aqueous humor concentrations increased ten to 13-fold and iris/ciliary body concentrations increased sixfold upon epithelial removal. HPLC analysis suggested that the corneal epithelium was responsible for the metabolic degradation of PE, which occurred following topical instillation of PE.

The transcorneal permeability of sulfonamide carbonic anhydrase inhibitors and their effect on aqueous humor secretion

Eleven sulfonamide carbonic anhydrase inhibitors of varied chemical and physical types were studied with respect to transcorneal permeability and reduction of intraocular flow and pressure. Using the isolated rabbit cornea, a constant drug concentration on the epithelial side and 6 ml solution in the endothelial chamber, first order rate constants (kin) ranged from 0 . 1-40 X 10(-3)/hr, roughly proportional to their lipid solubility. Drugs on the high side of this range were generally water insoluble and had pKa's too high to yield sodium salts at useful pH; therefore, the actual amount of drug delivered was small. We sought compounds which combined low pKa, good lipid solubility, and high activity against the enzyme. Trifluormethazolamide (TFM) has a pKa of 6 . 6, ether partition coefficient of 6, and a K1 of 2 X 10(-8)M. kin is 3 X 10(-3)/hr. TFM and five other compounds were also studied in vivo for their ability to penetrate the eye into the anterior and posterior chambers. These rate constants were roughly proportional to those measured in vitro; however, significant differences in accession to the two chambers were observed, as a function of varying physico-chemical properties of the drugs. A 3% solution of TFM (100 mM) applied to the rabbit eye for 25 min generated 0 . 7 mM in the anterior chamber and 0 . 07 mM in the posterior. Tissue distribution of TFM (and its metabolite) showed a relatively high concentration in the ciliary body 6 hr after dose. Intraocular pressure was reduced by 4 mmHg. With 10 min exposure this concentration of TFM reduced pressure by about 1 . 7 mmHg. Although the use of this drug is limited by its chemical instability and the length of exposure needed, the principle of treating glaucoma by the topical use of carbonic anhydrase inhibitors appears feasible.

The effects of sodium hyaluronate, chondroitin sulfate, and methylcellulose on the corneal endothelium and intraocular pressure

Sodium hyaluronate (Healon), chondroitin sulfate, and methylcellulose have been used to protect the corneal endothelium from intraocular lens trauma. A study of the efficacy and toxicity of these compounds showed that 1% sodium hyaluronate, 0.4% methylcellulose, and 20% chondroitin sulfate were nontoxic to the corneal endothelium, but that 20% chondroitin sulfate caused a marked decrease in corneal thickness because of its hypertonicity. Anterior chamber injection of these viscous substances resulted in an increase in intraocular pressure. Within one to four hours the maximum intraocular pressure with 1% sodium hyaluronate was 67 +/- 4.1 mm Hg and that with 20% chondroitin sulfate was 55 +/- 3.5 mm Hg. The intraocular pressure did not increase to these high levels with 10% chondroitin sulfate or 0.4% methylcellulose or when the test substances were washed out of the anterior chamber. The corneal endothelium was protected from injury with 1% sodium hyaluronate and 20% chondroitin sulfate, but 10% chondroitin sulfate and 0.4% methylcellulose provided only minimal protection.

Healing of experimental corneal wounds treated with topically applied retinoids

We treated experimental corneal epithelial wounds in rabbits with topical retinoids. Treatment with 0.1% all-trans-retinoic acid three times per day resulted in a 21% increase in the healing rate compared to the control eyes. Treatment five times a day resulted in a 35% increase in healing rate. Treatment with topical retinoic acid also promoted corneal deturgescence. Retinyl palmitate, retinyl acetate, retinol, and 13-cis-retinoic acid had no effect on corneal wound healing. These data suggested that topically applied all-trans-retinoic acid may be effective in promoting corneal healing after surgery and in the treatment of persistent and recurring corneal epithelial defects.

Retinoid permeability and uptake in corneas of normal and vitamin A-deficient rabbits

In vitro perfusion of corneas of normal and vitamin A-deficient rabbits provided a model in which to study the pharmacokinetics of corneal permeability and uptake of retinoic acid and retinol. The permeability coefficients of retinoic acid and retinol were 1.49 x 10(-5) and 0.61 x 10(-5) cm/s, respectively. Removal of the corneal epithelium did not affect the permeability of these lipid-soluble retinoids; however, diffusion through xerophthalmic, vitamin A-deficient corneas was significantly reduced. The corneal uptake of retinoic acid and retinol was reduced by 50% on removal of the epithelium, was nonspecific, and was not affected by xerophthalmia. High-performance liquid chromatography indicated that these retinoids were not metabolized during diffusion through the cornea. These results show that topical application of retinoids is a rational approach to the treatment of such corneal diseases as xerophthalmia and epithelial defects.

The effect of intraocular irrigating solutions on lens clarity in normal and diabetic rabbits

We conducted a series of experiments on normal and diabetic rabbits to evaluate the effect of intraocular irrigating solutions on the lens. Lenticular clarity was studied in vivo in eyes undergoing vitreous cavity perfusions with various irrigating solutions. The clarity of isolated lenses incubated in these same solutions was also evaluated. In order to quantitate the cataractous changes observed, we measured the uptake of inulin (tagged with radioactive carbon) by isolated lenses. We found that balanced salt solution (BSS) and BSS Plus maintain excellent lens clarity in normal rabbits; that diabetic lenses are more likely to develop opacification than normal lenses, becoming cataractous when exposed to BSS Plus; and that adding supplemental glucose to BSS Plus allows this solution to maintain excellent lens clarity in diabetic rabbits.

A comparison of healing of corneal epithelial wounds stained with fluorescein or Richardson's stain

The effects of fluorescein and Richardson's stain on corneal epithelial wound healing were compared in eyes of rabbits whose corneas had the epithelium removed by scraping or by n-heptanol. One eye of each rabbit was stained with fluorescein and the other eye was stained with Richardson's stain at intervals throughout the healing process, and the wounds were photographed for planimetry and determination of re-epithelialization rate. Corneal thickness was also measured throughout the re-epithelialization. These studies showed that Richardson's stain, as compared with fluorescein, decreases re-epithelialization rate, delays wound closure, and slows the return of the edematous cornea to normal thickness. Therefore fluorescein rather than Richardson's stain should be used to stain epithelial defects in corneal wound healing studies and in the evaluation of the corneal toxicity of chemical agents.

The corneal endothelium. Normal and pathologic structure and function

A summary of normal and abnormal endothelial structure and function is presented. Endothelium originates from neural crest and it elaborates a banded basement membrane in utero. It is involved in mesenchymal dysgenesis of the anterior segment, like the central defect of Peters' anomaly. Cytoplasmic organelles include mitochondria that provide energy for the metabolic pump, rough endoplasmic reticulum that participate in secretion of extracellular matrix, and a terminal web that may participate in cell migration. The endothelium's main function is to control corneal hydration and nutrition with a leaky barrier formed by the apical gap and macula occludens junctions that keep some water out of the stroma but allow nutrients to pass, and with an ATPase-dependent metabolic pump that is located in the lateral plasma membranes. Endothelial wound healing involves flattening and enlargement of cells to maintain an intact monolayer as well as production of abnormal collagenous material posterior to Descemet's membrane. HLA antigens located in the plasma membrane may participate in corneal endothelial graft rejection. Clinical assessment of the endothelium involves three modalities: specular microscopy to study endothelial morphology, fluorophotometry to measure barrier function, and pachymetry to measure corneal thickness.

Corneal edema and the intraocular use of epinephrine

Commercially prepared dilutions (1:10,000) of epinephrine can cause marked increases in corneal thickness and loss of corneal endothelial cells when injected into the anterior chamber of the eye. Endothelial toxicity is related to the buffer capacity of the epinephrine solution, which is in turn controlled by the concentration of the antioxidant (sodium bisulfite) as well as by the vehicle formulation and a low pH value.

Healing of corneal epithelial wounds in marine and freshwater fish

The corneal epithelium of a fish is in direct contact with the aquatic environment and is a barrier to movement of ions and water into and through the cornea. This tissue layer is thus important in maintenance of corneal transparency. When the epithelium is wounded, its protective function is lost and corneal transparency remains compromised until the epithelial barrier is re-established. This study was undertaken to investigate the healing response of the fish cornea to epithelial abrasion. Wounds were stained with fluorescein and photographed during healing. Wound areas were measured by planimetry. The cornea of the sculpin, a marine teleost, becomes edematous after wounding and heals at 2.54 to 3.42 mm2/hr. Nonswelling corneas of the elasmobranchs--dogfish shark and skate--heal at 1.29 mm2/hr, respectively. The wounded eye of the rainbow trout, a freshwater teleost, is stressed by the low osmolality of the environment. Severe corneal edema and cataracts develop following epithelial wounding, and the cornea heals at 0.64 mm2/hr. Although the healing rates in teleosts differ from those in mammals, histology shows that the corneal healing mechanism is essentially the same in fish and mammals.

Osmotic tolerance of rabbit and human corneal endothelium

Rabbit and human corneas were mounted in a specular microscope and perfused with a balanced salt solution of varying osmolality (200 to 500 mOsm). Measurements of corneal thickness were made throughout the perfusion period, and at selected times the corneas were fixed and prepared for scanning and transmission electron microscopy. A hypo-osmotic perfusion medium caused an increase in corneal thickness; by comparison, a hyperosmotic perfusion medium decreased corneal thickness in both rabbit and human corneas. Despite the marked changes in corneal thickness and the water movement that occurred across the endothelium, the cellular ultrastructure remained intact. In reversal studies (return to 300-mOsm perfusion medium), corneal thickness returned to control values with no marked changes in endothelial cell structure. These data indicate that the corneal endotheium can tolerate a wide range of solution osmolalities (200 to 400 mOsm) without marked endotheial cell breakdown if the essential ions are present.

Effects of ionophores X537a and A23187 and calcium-free medium on corneal endothelial morphology

Past studies have shown that apical junctional complexes (AJCs) of corneal endothelial cells break down in the presence of a Ca++-free medium. The purpose of this study was to examine the ability of Ca++ ionophores to maintain the AJCs in the Ca++-free media in both isolated perfused corneas and cultured endothelial cells. In addition, the ability of disintegrated AJCs to re-form when the endothelium is returned to a medium containing calcium ws also examined. Rabbit corneas were mounted in an in vitro specular microscope and perfused with a Ca++-free medium, or a Ca++-free medium containing 10(-5)M X537A or A23187 calcium ionophore. Also, confluent monolayer cultures of bovine corneal endothelial cells were placed in a Ca++-free medium or a Ca++-free medium containing 10(-5)M X537A or A23187 Ca++ ionophore and incubated for selected time periods. When junctional breakdown occurred, one cornea or culture plate was fixed for scanning and transmission electron microscopy (SEM and TEM), and the other was returned to a medium containing Ca++ and subsequently fixed for SEM and TEM. Both isolated perfused and cultured corneal endothelial cell AJCs exhibited marked disintegration in the presence of Ca++-free medium. The presence of an ionophore in the medium cultured cells. When returned to a medium containing Ca++, the corneas that had been perfused with Ca++-free medium containing an ionophore re-formed the junctions sooner than did those that had been perfused with a Ca++-free medium alone. These results suggests that the ionophores may be capable of mobilizing intracellular calcium to protect the AJCs.

Effects of nucleoside antivirals and their metabolites on the corneal endothelium

The endothelial surface of rabbit corneas were perfused in vitro with bicarbonate Ringer's containing 5 mM glucose, 0.3 mM reduced glutathione, and various concentrations of nucleoside antivirals or their metabolites. During three hour perfusions, the swelling rates of corneas perfused with buffer containing either antivirals or metabolites were not significantly different from controls. Scanning electron microscopy of the endothelial cell layer revealed no structural abnormalities in any treatment group. One metabolite, fluoride ion, reduced endothelial glucose oxidation by about 60 percent when incubated with corneal tissue in vitro. The inhibition of glucose metabolism by fluoride ions was observed only at concentrations at least sixty times greater than would be anticipated in the anterior chamber of patients receiving topical F3TdR therapy. These studies indicate that 5-trifluoromethyl-2'-deoxyuridine, 5-iodo-2'-deoxyuridine, 9-(2-hydroxyethoxymethyl)-guanine, and their metabolites do not alter endothelial function when studied at physiological concentrations over a short term of exposure.

Glucose metabolism in the cornea and lens in elasmobranchs, teleosts and mammals: response to thiol-oxidation

Various structural and metabolic adaptations that have occurred in elasmobranchs, in salt and fresh water teleosts, and in mammals have enabled these species to adapt to varied environments. In all cases the corneas of these species remain transparent and expend metabolic energy to maintain this transparency. The studies reported in this paper describe the structural and the metabolic adaptations that have occurred in these corneas. Included are measurements of corneal oxygen consumption, Q10 and corneal hydration. Thiol-oxidation of the intracellular glutathione with diamide has been shown to produce marked stimulation of the hexose-monophosphate shunt in the component layers of the cornea and the lens of rabbit, dogfish, and sculpin. Activities of glycolytic, citric acid cycle, and pentose phosphate shunt enzymes in the corneal and muscle tissue were also studied. Species variations were found between elasmobranchs, marine teleosts, and rabbits. In each of these species, the corneal endothelium was distinguished from the epithelium by much lower enzyme activities. It can be concluded that the enzyme activities and metabolic differences represent functional adaptations that have occurred to insure transparency under these extremely varied osmotic conditions.

Intraocular fluid dynamics. Measurements following vitrectomy and intraocular sulfur hexafluoride administration

Partial vitrectomies were performed in rabbits, replacing 40% of initial vitreous volume with 100% sulfur hexafluoride, and concentrations of sodium, potassium, calcium, magnesium, ascorbate, and protein were determined in aqueous and vitreous at 10, 14, 21, 28, 56, and 110 days. There were significant increases of potassium, calcium, magnesium, and protein above control values at varying times throughout the study. Vitreous potassium concentration was decreased at 14 and 21 days. All other measurements were similar to control values. Results of studies indicate that, following vitrectomy in rabbits, the blood-retinal barrier can re-form in 14 days and blood-aqueous barrier in ten days. However, a greater than normal exchange diffusion of these measured substances occurs between aqueous and vitreous through 21 days. Four eyes with cataracts, vitreous membranes, and/or detached retinas demonstrated elevated aqueous and vitreous protein concentrations through 110 days, although other measurements returned to normal levels.

Cytotoxicity of pivalylphenylephrine and pivalic acid to corneal endothelium

We examined the effects of topically applied pivalylphenylephrine (PPE) and pivalic acid (PA) on the corneal endothelium of rabbits and the direct effects of PPE and PA on monolayer cultures of bovine corneal endothelium. The PPE-treated corneas without epithelium significantly increased in thickness, whereas no change in thickness was observed in corneas with epithelium intact. The PA did not alter the thickness of corneas with or without epithelium. Although 0.001% PE had no noticeable effect in two hours, 0.01% PPE caused breakdown of intercellular junctions in cultured cells in five minutes. Higher concentrations of PPE caused the cells to detach from the culture dishes within 30 minutes of treatment. Only 1% PA caused cell elongation and loss of intercellular contact after 60 to 90 minutes of exposure; lower concentrations did not effect cultured cells.

The effect of phenylephrine on the cornea

Rabbit corneas were treated with three drops of phenylephrine hydrochloride with the epithelium intact or denuded. Corneal thickness was measured before and after drug treatment, and at various times after treatment the corneas were fixed for scanning and transmission electron microscopic observation. The results of this study show that phenylephrine caused a dramatic increase in corneal thickness (drug-induced edema) and cellular vacuolation within the keratocytes and endothelial cells in the corneas without the epithelium. Corneal thickness did not change and the ultrastructural changes were minimal following drug application in those corneas with the epithelium intact. Results of this study also suggest that phenylephrine has a cytotoxic effect on the corneal endothelium and keratocytes when used in corneas where the epithelium has been removed. In coreas with intact epithelium, the damage was less severe and limited to the epithelium.

The pH tolerance of rabbit and human corneal endothelium

The endotheliums of rabbit corneas were perfused in an in vitro perfusion specular microscope up to 3 hr with solutions varying in pH from 3.5 to 10.0. Corneal thickness was monitored throughout the experiment, and at appropriate times the corneas were prepared for SEM and TEM. Analysis of the corneal thickness data and interpretation of the electron micrographs reveals that outside of the pH range of 6.5 to 8.5, structural and functional alterations occur. Direct cellular damage, as well as disruption of junctional complexes, lead to a breakdown in the barrier function of the corneal endothelium. The extent of this breakdown is dependent upon both the magnitude of the pH change and the exposure time. Further experiments on banked human eyes support this finding.

Effect of phenylephrine on normal and regenerated endothelial cells in cat cornea

Topical commercial phenylephrine HCl (Neo-Synephrine 10%) has been shown to cause an increase in corneal thickness and reversible vacuolization of corneal endothelial cells in rabbits. Using an in vivo model of regenerated corneal endothelial cells in the cat, we compared the cytotoxicity of phenylephrine-HCl 10% to regenerated and to normal, nonregenerated cells. Following removal of the epithelium, topical application of the drug causes the appearance of anterior and posterior bands of stromal edema and reversible vacuolization in both normal and regenerated endothelial cells. Phenylephrine was not more damaging to the regenerated cells. Polymorphonuclear leukocytes infiltrated between the regenerating cells 24 hr after treatment but did not appear to destroy them. Phenylephrine may therefore be implicated as a causative factor of corneal edema and postoperative inflammation.

The pentose phosphate pathway in developing chick cornea

Embryonic chick corneas at different stages of development were evaluated for activity of the pentose phosphate pathway. The appearance of activity was concurrent with the onset of corneal transperancy (stage 40). Highest values were found after complete transparency is achieved (stage 45 and after hatching). Phenazine methosulfate, an artificial electron acceptor, increased activity at all stages studied even before endogenous activity was measurable; however, no increase in glucose uptake was observed. Thus, the enzymes for the pathway are present at early stages (i.e., stage 38 and 40) although in latent form. The pathway probably functions in the developing cornea to generate NADPH rather than sugar moieties for macromolecular incorporation.