Effects of a sustained muscular contraction on human intraocular pressure

1. Normal subjects performed fatiguing static hand-grip contraction at tensions of 20% and 55% of their maximum voluntary contraction (MVC). Intraocular pressure (IOP) was measured by applanation tonometry before, during and after the isometric exercise. Forearm blood samples were taken from the antecubital vein in both the exercised and non-exercised arm before and 2 min post-exercise for measurement of plasma lactate, osmolality, Pv,o2, Pv,co2 and pH.

2. During hand grip the heart rate and blood pressure increased significantly, whereas the IOP remained unchanged from control in both the 20% and 55% MVC experiments.

3. In the recovery period heart rate and blood pressure returned to control values within 3 min and the IOP decreased significantly from control in both the 20% and 55% MVC experiments.

4. When an occlusion cuff was inflated on the exercising arm just before release of the 55% MVC grip, the decreased IOP could be delayed until the cuff was released.

5. Post-exercise blood samples showed elevated lactate concentrations and Pv,o2 and decreased pH in the exercised arm; however, the values remained unchanged in the non-exercised arm. The decreased IOP after exercise may be related to an increased blood lactate concentration.

Corneal diffusion and metabolism of 12(R)-hydroxyeicosatetraenoic acid (12(R)HETE)

PURPOSE

To quantify the corneal diffusion and metabolism of tritiated 12(R)-hydroxyeicosatetraenoic acid (12(R)HETE) in the in vitro-mounted rabbit cornea to determine if this compound or one of its metabolites can diffuse across the stroma to the corneal endothelium.

METHODS

The studies were performed in a Lucite block perfusion chamber by placing tritiated 12(R)HETE on the tear side of the cornea under the following conditions: (A) cornea completely intact (endothelium and epithelium present); (B) cornea with epithelium removed; and (C) cornea with both epithelium and endothelium removed. Radioactivity of 12(R)HETE and metabolites were measured in the different corneal layers and in the corneal perfusates using scintillation spectroscopy. 12(R)-hydroxyeicosatetraenoic and its metabolites were then quantified in the tissue perfusates using high performance liquid chromatography (HPLC) analysis.

RESULTS

12(R)HETE is rapidly taken up and metabolized by the intact cornea to a number of more polar compounds including a metabolite which has spectral and retention time characteristics of 8(R)-hydroxyhexadecatrienoic acid (8(R)HHDTrE). Both 12(R)HETE and 8(R)HHDTrE can diffuse through the stroma to the endothelium. Corneas having the epithelium removed also allow diffusion of 12(R)HETE across the stroma; however, there is significantly less metabolism. When both the epithelium and endothelium are removed, 12(R)HETE is capable of diffusing across the stroma; however, there is little metabolism, which suggests that the majority of 12(R)HETE metabolism occurs in the epithelium and, to a lesser degree, in the endothelium and stroma.

CONCLUSIONS

12(R)HETE and its metabolites are capable of diffusing from the epithelium through the cornea where they may adversely affect the endothelium.

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.

In vivo metabolism of topically applied retinol and all-trans retinoic acid by the rabbit cornea

Corneas of normal and vitamin A-deficient rabbits were treated topically with [11, 12-3H] retinol or [11, 12-3H] all-trans retinoic acid. Methanol extracts of these corneas were analyzed by high pressure liquid chromatography. Radiolabeled compounds were extracted from the corneas which co-migrated chromatographically with known retinoid standards. In agreement with studies on other tissues and organs, retinol was metabolized to retinoic acid and more polar compounds by corneas of normal and vitamin A-deficient rabbits. All-trans retinoic acid was isomerized to 13-cis retinoic acid in normal rabbit corneas; however, this trans-cis isomerization did not occur in vitamin A-deficient, xerophthalmic corneas.

Choroidal rete mirabile function and resistance to retinal oxygen toxicity in fish

Previous studies have suggested that resistance to O2 toxicity in teleost fish retina is directly related to choroidal rete mirabile function and to the chronically elevated PO2 at the retina. We tested this hypothesis in several species of fish with varying degrees of rete development. The effect of hyperbaric oxygen on O2 consumption, Na+-, K+-ATPase activity nd the ERG was recorded. Hyperbaric oxygen had no effect on the ERG of either the flounder, which has a large rete and elevated PO2, or the eel, which has no rete. Exposure to hyperbaric oxygen for 6 hr attenuated by 50% the ERG of the goosefish which has a small rete (retinal PO2 = 90 mmHg). Hyperbaric oxygen abolished the b-wave of the dogfish ERG and attenuated the skate b-wave by 80%. Elasmobranchs have no rete and low retinal PO2. Hyperbaric oxygen had no effect on O2 consumption and ATPase activity in any species studied. The ERG data supported the hypothesis that resistance to oxygen toxicity by the teleost retina is an adaptation to a chronically elevated PO2 generated by the choroidal rete.

A method for the in vitro determination of feline corneal endothelial permeability

A mounting block for in vitro perfusion of the cat cornea is described. Using this apparatus and the techniques of Araie, the permeability (Pac) of the normal cat corneal endothelium to carboxyfluorescein was determined to be 2.5 +/- 0.2 x 10(-4) cm/min. To assess the sensitivity of this technique in determining changes in Pac associated with alterations in endothelial, morphology, three cats underwent 2 successive unilateral, central, 10 mm diameter circular areas of endothelial debridement 6 weeks apart. Six weeks following the second wounding all 3 animals underwent morphometric analysis and Pac determination. A trend toward an elevation in Pac with extreme reductions in cell density was observed.

Comparison of rabbit and human corneas stored in Optisol-GS: changes in stromal sodium

PURPOSE

To evaluate the hydration, and the levels of free, total and bound sodium in fresh rabbit corneal stromas and also those preserved for up to 21 days in Optisol-GS. The effect of epithelial removal on stromal sodium and hydration parameters was also evaluated. Trends in stromal hydration and sodium environment were compared to results we previously obtained using human eyes stored under identical conditions.

METHODS

Stromal hydration was evaluated thermogravimetrically. A sodium-specific electrode and an atomic absorption spectrophotometer were used to determine the amounts of free and total stromal sodium, respectively. In one cornea of each pair, the epithelium was removed prior to placement in the storage media. After 3, 7, 14 or 21 days at 4 degrees C, corneas were removed from the Optisol-GS, at which time sodium and hydration measurements were obtained.

RESULTS

With an intact epithelium, the hydration of the rabbit stromas was elevated significantly at each day of storage compared to fresh corneas. Free and total sodium levels of rabbit stromas did not differ statistically from fresh values, however the bound sodium values did increase during storage. In the absence of the epithelium, the stromal hydration and sodium content (free, total and bound) were significantly elevated and the increase was much greater than in corneas stored with an intact epithelium. These findings differ from those we measured previously using human tissue.

CONCLUSIONS

Rabbit corneas responded differently from human corneas to storage in Optisol-GS. The hydration levels increased to a greater level in rabbit than human corneas under both storage conditions. The trends in amounts of both free and total sodium were similar between the species, although the absolute amounts differed. The largest discrepancy was observed in the amount of bound sodium, with the rabbit corneas experiencing large increases not documented in the human tissue. These results suggest that direct comparisons of stromal hydration and ionic environment between the species should be approached with caution.

Effect of intraocular irrigating solutions on the viability of cultured retinal vascular endothelial cells

PURPOSE

To compare the abilities of balanced salt solution, BSS Plus and Hartmann's lactated Ringer's (HLR) solution to maintain the viability of retinal vascular endothelial cells (RVEC) in vitro.

METHODS

Cultured retinal vascular endothelial cells were suspended in each irrigating solution for four hours. Viability was determined by trypan blue exclusion at 30 minute intervals. Regression analysis was used to determine the rate of viability loss. Additional studies were performed to determine the effectiveness of lactate in maintaining cell viability.

RESULTS

Retinal vascular endothelial cells lost viability at a greater rate (p < 0.001) in BSS (8.7%/hr) compared with BSS Plus (3.3%/hr). Cells in Hartmann's lactated Ringer's lost viability at a significantly lower rate (4.4%/hr) than retinal vascular endothelial cells in lactate-free Hartmann's lactated Ringer's solution (8.4%/hr). Lactate was as effective as glucose in preserving RVEC viability. By comparison, the viability of corneal endothelial cells was not effectively maintained by lactate. For these cells, BSS Plus was clearly superior to Hartmann's lactated Ringer's solution in maintaining viability.

CONCLUSIONS

BSS Plus and Hartmann's lactated Ringer's solution are both superior to balanced salt solution in maintaining retinal vascular endothelial cell viability. For retinal vascular endothelial cells, Hartmann's lactated Ringer's solution preserves cell viability as well as BSS Plus, since the retinal vascular endothelial cells, unlike corneal endothelial cells, can apparently utilize lactate as an energy source.

BSS Plus: a potential irrigating solution for neurosurgery

BSS Plus is a pH-stable balanced salt solution similar to glutathione bicarbonate Ringer's solution. Extensively used in ophthalmology, it is of potential value in neurosurgery. In comparative tests of its effectiveness, 28 cats underwent bilateral irrigation of the surface of the cerebral cortex with normal saline on one side and BSS Plus on the other. After 2 hours, a marked decrease was seen in the surface pH of the hemisphere irrigated with normal saline but not of the hemisphere treated with BSS Plus. Blood-brain barrier changes (measured with Evans blue dye techniques) were more evident following saline irrigation. Somatosensory evoked potentials and cerebral blood flow were not significantly altered. Conventional light microscopy using three standard stains did not reveal a significant difference. Transmission electron microscopy studies were performed in 14 animals and scanning electron microscopy in six. In five animals both transmission and scanning electron microscopy studies were conducted after irrigation with both agents without a cottonoid cover and with immediate harvest of superficial layers from the living brain and immersion-fixation in glutaraldehyde. Tissue preservation was superior on the BSS Plus side in all studies. This agent may represent an improved irrigation solution for neurosurgery, but further studies are required.

Maturation of the corneal endothelial tight junction

Apical tight junctional formation of the rabbit corneal endothelium was examined by freeze-fracture analysis and measurement of paracellular permeability to 5(6)-carboxyfluorescein. Freeze-fracture analysis indicated that apical tight junction formation of the rabbit corneal endothelium is a dynamic process. At birth, there are few tight junctional strands present and a minimal barrier for paracellular diffusion. As the rabbit matures, a more complex network of anastomosing tight junctional strands begins to encircle the cell perimeter under the apical folds. However, even in the mature animal (3 months), there are discontinuities and free ends in the network, thus suggesting that the barrier is not complete even at this stage. Paracellular permeability measurements using 5(6)-carboxyfluorescein as a tracer corroborate these anatomic findings. Endothelial paracellular flux measurements steadily decrease as the rabbit matures from birth to young adult. This indicates that the tight junctional network is increasing in complexity and progressively limiting the flow of substances through the intercellular space.

Effect of the ophthalmic preservative thimerosal on rabbit and human corneal endothelium

Widespread use of the mercurial-containing preservative thimerosal as an antibacterial agent in ophthalmic drugs and solutions warranted an investigation into its possible cytotoxic effects on the functional and ultrastructural integrity of the corneal endothelium. No changes in corneal thickness were observed during 5 hours' perfusion of the endothelium of rabbit and human corneas with 0.0001 and 0.0005 percent thimerosal in glutathione bicarbonate Ringer's solution (GBR). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of the endothelium of the 0.0001 percent group revealed normal ultrastructure. SEM and TEM of the endothelium of corneas perfused with 0.0005 percent thimerosal for 5 hours revealed condensed mitochondria, cytoplasmic vacuoles, and cytoplasmic flaps at the apical end of the cellular junctions. Perfusion of higher concentrations (0.001 and 0.005 perecnt) of thimerosal in GBR resulted in increases in corneal thickness after 2 hours and irreversible ultrastructural damage to the endothelial cells by 5 hours. Corneas perfused with 0.01 and 0.1 percent thimerosal in GBR showed a rapid and immediate increase in corneal thickness and endothelial cell death and necrosis within 1 hour. It is postulated that the mercury in thimerosal becomes bound to the cell membrane protein sulfhydryl groups, causing an increase in cellular permeability; These results suggest that the prolonged exposure of the corneal endothelium to thimerosal in the accepted antimicrobial dosage of 0.005 to 0.001 percent may result in functional and structural damage to the endothelium.

The effect of intraocular pressure on human and rabbit scleral permeability

PURPOSE

The purpose of this study was to evaluate the effects of intraocular pressure on the permeability of human and rabbit sclera to water, dexamethasone, and carboxyfluorescein.

METHODS

Scleral sections excised from moist-chamber-stored human globes or eyes obtained from euthanatized New Zealand White rabbits were mounted in a perfusion chamber that can create a transscleral pressure that simulates an intraocular pressure. A small depot of drug (100 microl) was added to the episcleral surface while perfusing an irrigating solution slowly across the choroidal side. The perfusate was collected and scleral permeability calculated. Experiments were performed at 0, 15, 30, and 60 mm Hg for each compound in human and rabbit tissue.

RESULTS

Analysis of variance showed a significant effect of intraocular pressure on both human and rabbit scleral permeability. Human scleral permeability was decreased by as much as a factor of two for water (P = 0.0004), dexamethasone (P<0.0001), and carboxyfluorescein (P = 0.0064) at elevated intraocular pressures. Rabbit scleral permeability was similarly affected by elevated intraocular pressure for water (P = 0.0039), dexamethasone (P = 0.0001), and carboxyfluorescein (P = 0.0016).

CONCLUSIONS

This study shows that simulated intraocular pressure ranging from 15 to 60 mm Hg can decrease scleral permeability to small molecules by one half when compared with the sclera with no pressure applied.

Glutathione in rabbit corneal endothelia: the effects of selected perfusion fluids

Although the ameliorating effect of glutathione on corneal deturgescence is known, its chemical mechanism is not understood. An endeavor toward the latter was made by perfusing freshly excised rabbit corneas with selected perfusion fluids, measuring corneal thickness, and assaying the endothelial cells for reduced and oxidized glutathione after 2 and 5 hr of perfusion. Ringer's solution, containing either lactate or bicarbonate, caused significant decreases in both forms of glutathione after perfusion. The corneas increased in thickness considerably during these periods. When 5 mM glucose was added to bicarbonate-Ringer's solution, the corneas swelled about half as much as before. However, glutathione levels were as depressed as with simple Ringer's fluid. Adenosine (0.5 mM) in the presence of glucose (bicarbonate-Ringer's) caused a further swelling decrease so that the corneas were maintained at near normal thickness. The levels of glutathione were 84% of control values compared to 35% to 45% for Ringer's solutions (+/- glucose). The addition of glutathione to glucose (bicarbonate-Ringer's) caused intracellular glutathione levels to be higher than control values while allowing minimal tissue swelling. Glutathione in combination with adenosine, glucose, and bicarbonate produced the highest intracellular glutathione levels and a slight corneal deswelling. After oxidation of intracellular glutathione with t-butyl hydroperoxide in glucose (bicarbonate-Ringer's), endothelial cells were destroyed within 1 hr. The oxidant, however, may have had a direct effect upon the endothelial cell membranes.

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.

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.

Long-term observation of morphologic and functional features of cat corneal endothelium after wounding

PURPOSE

(1) To test the hypothesis that corneas with enlarged endothelial cells (and thus less intercellular space) have decreased endothelial permeability to small polar solutes. (2) To measure corneal endothelial ouabain binding (Na+/K+ ATPase "pump site" density) and Descemet's membrane production after endothelial wounding.

METHODS

Bilateral specular microscopy and anterior segment fluorophotometry were performed at 2-month intervals for 1 year in ten cats after mechanically damaging the corneal endothelium in one eye of each. The measurements were repeated at 2 years in four cats and at 3 years in two cats. Eighteen months after wounding, endothelial ouabain binding was measured in both eyes of six cats. Transmission electron micrographs of Descemet's membrane were analyzed in both eyes of six cats at 18 months, two cats at 2 years, and two cats at 3 years after wounding.

RESULTS

From 6 to 12 months after wounding, the endothelial permeability to carboxyfluorescein was significantly decreased (P < 0.05), and the mean endothelial cell size was significantly increased (P < 0.001) in the damaged eyes. The enlarged endothelial cells persisted in the few cats observed 2 and 3 years after wounding. There was no significant difference in endothelial ouabain binding between the damaged and control corneas in six cats tested 18 months after wounding. On subsequent histologic examination, a layer of abnormal Descemet's membrane was present in all ten wounded eyes, with additional normal Descemet's membrane posterior to it, between the abnormal layer and the endothelial cells.

CONCLUSIONS

The results are consistent with the hypothesis that corneal endothelial permeability to small polar solutes varies directly with the amount of intercellular space available for diffusion across the monolayer. The results also confirm clinical reports of decreased endothelial permeability in corneas with enlarged endothelial cells. In histopathologic specimens, a layer of abnormal Descemet's membrane can be a historical marker for a period of endothelial damage and corneal decompensation.