The effect of phenylephrine on the cornea

Phenylephrine induces necroptosis and apoptosis in corneal epithelial cells dose- and time-dependently

In the present study, the toxicity of phenylephrine, a selective α1-adrenergic receptor agonist, in corneal epithelial cells and its underlying mechanisms were investigated using an in vitro model of human corneal epithelial cells (HCEPCs) and an in vivo model of New Zealand white rabbit corneas. The HCEPCs treated with phenylephrine at concentrations from 10% to 0.078125% displayed abnormal morphology, decline of cell viability and elevation of plasma membrane permeability time- and dose-dependently. Moreover, 10%-1.25% phenylephrine induce necrosis characteristics of marginalization and uneven distribution of chromatin through up-regulation of RIPK1, RIPK3 and MLKL along with inactivation of caspase-8 and caspase-2, whereas 0.625% phenylephrine induced condensed chromatin, S phase arrest, phosphatidylserine externalization, DNA fragmentation and apoptotic body formation in the HCECs through activation of caspase-2, -8, -9 and -3 as well as down-regulation of Bcl-2, up-regulation of Bad, ΔΨm disruption and release of cytochrome c and AIF into cytosol. At last, 10% phenylephrine induced destruction of the corneal epithelia and apoptosis of corneal epithelial cells in rabbit corneas. In conclusion, 10% to 1.25% phenylephrine cause necroptosis via RIPK1-RIPK3-MLKL axis and 0.625% phenylephrine induce apoptosis via a mitochondrion-dependent and death receptor-mediated signal pathway in HCEPCs.

A new, specular reflection-based, precorneal tear film stability measurement technique in a rabbit model: viscoelastic increases tear film stability

PURPOSE

To develop a safe, noninvasive, noncontact, continuous in vivo method to measure the dehydration rate of the precorneal tear film and to compare the effectiveness of a viscoelastic agent in maintaining the precorneal tear film to that of a balanced salt solution.

METHODS

Software was designed to analyze the corneal reflection produced by the operating microscope's coaxial illumination. The software characterized the shape of the reflection, which became distorted as the precorneal tear film evaporated; characterization was accomplished by fitting an ellipse to the reflection and measuring its projected surface area. Balanced salt solution Plus (BSS+) and a 2% hydroxypropylmethylcellulose viscoelastic were used as the test agents. The tear film evaporation rate was characterized and compared over a period of 20 minutes in 20 eyes from 10 New Zealand white rabbits.

RESULTS

The ellipse axes ratio and surface area were found to decrease initially after each application of either viscoelastic or BSS+ and then to increase linearly as the tear film began to evaporate (P < 0.001) for eyes treated with BSS+ only. Eyes treated with BSS+ required 7.5 ± 2.7 applications to maintain sufficient corneal hydration during the 20-minute test period, whereas eyes treated with viscoelastic required 1.4 ± 0.5 applications. The rates of evaporation differed significantly (P < 0.043) between viscoelastic and BSS+.

CONCLUSIONS

The shape and surface area of the corneal reflection are strongly correlated with the state of the tear film. Rabbits' corneas treated with viscoelastic remained hydrated significantly longer than corneas treated with BSS+.

Management of intraoperative iris prolapse: stepwise practical approach

Iris prolapse is not an uncommon occurrence during cataract surgery. It usually occurs through the main incision during hydrodissection and is commonly associated with floppy-iris syndrome; however, it can manifest in cases with no known predisposition and can occur at any stage during surgery. The mechanism is explained by the Bernoulli principle and its effect on iris position during the movement of fluid within the eye. Predisposing factors are iris configuration, anterior chamber depth, and position and architecture of the corneal tunnel. Strategies for prevention and management include the use of pharmacological agents, ophthalmic viscosurgical devices, and iris retractors. These strategies can be augmented by alteration and adaptation of the surgeon's technique.

Cation trapping by cellular acidic compartments: beyond the concept of lysosomotropic drugs

"Lysosomotropic" cationic drugs are known to concentrate in acidic cell compartments due to low retro-diffusion of the protonated molecule (ion trapping); they draw water by an osmotic mechanism, leading to a vacuolar response. Several aspects of this phenomenon were recently reexamined. (1) The proton pump vacuolar (V)-ATPase is the driving force of cationic drug uptake and ensuing vacuolization. In quantitative transport experiments, V-ATPase inhibitors, such as bafilomycin A1, greatly reduced the uptake of cationic drugs and released them in preloaded cells. (2) Pigmented or fluorescent amines are effectively present in a concentrated form in the large vacuoles. (3) Consistent with V-ATPase expression in trans-Golgi, lysosomes and endosomes, a fraction of the vacuoles is consistently labeled with trans-Golgi markers and protein secretion and endocytosis are often inhibited in vacuolar cells. (4) Macroautophagic signaling (accumulation of lipidated and membrane-bound LC3 II) and labeling of the large vacuoles by the autophagy effector LC3 were consistently observed in cells, precisely at incubation periods and amine concentrations that cause vacuolization. Vacuoles also exhibit late endosome/lysosome markers, because they may originate from such organelles or because macroautophagosomes fuse with lysosomes. Autophagosome persistence is likely due to the lack of resolution of autophagy, rather than to nutritional deprivation. (5) Increased lipophilicity decreases the threshold concentration for the vacuolar and autophagic cytopathology, because simple diffusion into cells is limiting. (6) A still unexplained mitotic arrest is consistently observed in cells loaded with amines. An extended recognition of relevant clinical situations is proposed for local or systemic drug administration.

Barriers to glaucoma drug delivery

Topical medications remain the mainstay of glaucoma treatment. This review will aim to cover the pharmacokinetics of topically applied drops, the ocular barriers to drug delivery, and the role of ophthalmic drug formulation in enhancing drug delivery to the target tissue while minimizing side effects and increasing patient compliance. Recent advances in surgical techniques, therapeutic approaches, and material sciences have produced exciting new therapies for ocular diseases. The development of new vehicles and drug formulations that require less patient compliance is also discussed, as are the routes of drug delivery for neuroprotection.

The human corneal endothelium: new insights into electrophysiology and ion channels

The corneal endothelium is a monolayer that mediates the flux of solutes and water across the posterior corneal surface. Thereby, it plays an essential role to maintain the transparency of the cornea. Unlike the epithelium, the human endothelium is an amitotic cell layer with a critical cell density and the risk of corneal decompensation. The number of endothelial cells subsequently decreases with age. Moreover, the endothelial cell loss is accelerated after various impairments such as surgical trauma (e.g. cataract extraction) and following corneal transplantation. This cell loss is associated with programmed cell death (apoptosis) and changed ion channel activity. However, little is known about the electrophysiology and ion channel expression (in particular Ca2+ channels) in corneal endothelial cells. This article reviews our current knowledge about the electrophysiology of the corneal endothelium. It highlights ion channel expression, which may have a major role in corneal cell physiology and pathological events. A better understanding of the (electro)physiological function of the cornea may lead to the development of clinical relevant new therapeutic and preventive measures.

Nonhypotensive autonomic agents in veterinary ophthalmology

The parasympathetic and sympathetic divisions of the autonomic nervous system are involved in homeostatic control of a wide variety of ocular functions, including accommodation, pupillomotor control, lacrimation, eyelid position, and aqueous humor production. Familiarity with the functional anatomy of the autonomic nervous system is paramount to the understanding and application of the large number of autonomic drugs used in veterinary ophthalmology. The cholinergic and adrenergic agents discussed in this article are commonly employed to facilitate routine ophthalmic examination, in the diagnosis of autonomic dysfunction, and in the treatment of a variety of ocular diseases.

Cataract surgery without preoperative eyedrops

We present a technique that uses intracameral lidocaine to induce pupil dilation without using preoperative mydriatic eyedrops. After 1 or 2 drops of topical lidocaine hydrochloride 1% (Xylocaine-MPF 1%) are applied to the ocular surface, a 1.0 mm side-port incision is created through which Xylocaine-MPF 1% is injected into the anterior chamber. The lidocaine paralyzes the pupil sphincter, and adequate mydriasis occurs within 90 seconds. Epinephrine (0.3 cc of 1:1000) is added to the irrigation fluid comprising balanced salt solution (BSS), and standard phacoemulsification with intraocular lens implantation is performed. Pupil dilation is maintained or increased during the procedure. Postoperatively, the pupil returns more quickly to normal size and reaction. Using lidocaine for mydriasis instead of standard dilating drops eliminates the cardiac risk of topical sympathetic agents, decreases the time patients wait in the holding area before surgery, reduces the risk of superficial punctate keratopathy, and provides faster recovery of normal pupil function.

Effect of corneal wetting solutions on corneal thickness during ophthalmic surgery

PURPOSE

To measure the changes in corneal thickness with wetting solutions used in ophthalmic surgery.

SETTINGS

Ege University, School of Medicine, Department of Ophthalmology, Izmir, Turkey.

METHODS

Thirty-one adult pigmented rabbits that weighed about 2.5 kg each were anesthetized with pentobarbital sodium. The rabbits were randomly divided into 3 groups to receive 3 wetting solutions: Group 1, Ringer's lactate; Group 2, balanced salt solution (BSS); and Group 3, BSS with glutation (BSS Plus). The solutions were dropped on the right cornea of the rabbits at a rate of 6.0 cc in 12 minutes using an intravenous infusion pump. Corneal thickness was measured by ultrasonic pachymetry before and after the procedure, and the between-group changes in corneal thickness were compared.

RESULTS

The corneal thickness before and after the procedure was 361.27 microm +/- 19.3 (SD) and 380.00 +/- 25.0 microm, respectively, in Group 1 (P =.000); 372.10 +/- 18.8 microm and 388.60 +/- 24.1 microm, respectively, in Group 2 (P =.003); and 358.10 +/- 26.5 microm and 360.10 +/- 24.1 microm, respectively, in Group 3 (P =.316).

CONCLUSION

As a corneal wetting solution, BSS Plus resulted in significantly fewer changes in corneal thickness than Ringer's lactate or BSS. This should be considered in cases involving long intraocular surgery.

The resiliency of the corneal endothelium to refractive and intraocular surgery

PURPOSE

To describe stress factors (phenylephrine and contact lenses) from the corneal epithelium that can affect the corneal endothelium, and to describe the effects of refractive and intraocular surgery on the corneal endothelial structure and function.

METHODS

Significant clinical and experimental publications are reviewed and recent experiments conducted in the author's laboratory to describe the corneal endothelial stresses.

RESULTS

The corneal epithelium serves as a barrier to topical phenylephrine (2.5-10%). In a compromised epithelium, topical phenylephrine will cause drug-induced stromal edema and endothelial vacuolization. Contact lenses are capable of stimulating the epithelial arachidonic acid cascade to release 12(R)hydroxyeicosatetraenoic acid (12(R)HETE) and 8(R)hydroxy-hexadecatrienoic acid (8(R)HHDTrE) to cause endothelial Na+/K+ adenosine triphosphatase (ATPase)-inhibition and polymegethism. Specular microscopy of the corneal endothelial cells after refractive surgery (photorefractive keratectomy [PRK], laser in situ keratomileusis [LASIK], intrastromal rings [INTACs]) has shown that there is minimal effect. However, laser ablation of the stroma within 200 microm of the corneal endothelium will result in endothelial cell structural changes and the formation of the amorphous substance deposited onto Descemet's membrane. Phacoemulsification with a high flow of the irrigation solution can alter the endothelial surface glycoprotein layer. Lidocaine hydrochloride (1%) used as intracameral anesthesia readily diffuses through the corneal endothelium, resulting in stromal uptake and endothelial cell swelling. With phacoemulsification, however, the washout of lidocaine from the cornea (T1/2, 5 minutes) and iris (T1/2, 9 minutes) occurs quickly. Corneal endothelial wound healing after keratoplasty occurs in the following sequence: migration of endothelial cells, development of tight junctions, and the formation of Na+/K+ ATPase pump sites.

CONCLUSIONS

Corneal endothelial resiliency is due to the increased peripheral endothelial cell number for migration, the ability of endothelial cells to form tight junctions to maintain the endothelial barrier, the increase in endothelial Na+/K+ ATPase pump sites under stress, and the ability of the corneal endothelial cells to shift their metabolism of glucose to the hexose monophosphate shunt for the production of nicotinamide adenine dinucleotide phosphate (NADPH) and membrane repair. All of these factors are important, along with the aqueous humor sodium concentration, which establishes the osmotic gradient for corneal deturgescence and transparency.

Effects of calcium channel blockers on rabbit corneal endothelial function

The effects of calcium channel antagonists and agents that alter intracellular Ca2+ mobilization on corneal endothelial function have been examined. All experiments, except where specifically designated, were performed in the continuous presence of extracellular Ca2+. Verapamil (at 50 microM) increased the swelling rate of corneas bathed in normal Ringer solution whereas nifedipine and diltiazem (both up to 100 microM) were without effect. The nifedipine analog nisoldipine caused corneal swelling at 10 microM and 50 microM but nimodipine was without effect. When briefly exposed to a Ca(2+)-free solution corneal swelling was enhanced after subsequent exposure to 50 microM verapamil in normal Ringer but not after 50 microM diltiazem in normal Ringer, indicating that Ca2+ entry from the bathing solution into the cell was important and was apparently impeded by verapamil. Cadmium (0.6 and 1 mM) but not nickel (up to 250 microM) caused swelling of corneas bathed in normal Ringer. A Ca2+ channel agonist, BAY-K-8644, alone did not influence corneal thickness but when presented to the endothelium with 50 microM verapamil the swelling rate was much reduced compared to verapamil alone. The agonist, therefore, presumably maintained some Ca2+ channels open in face of the Ca2+ channel blocker. An agent that inhibited the release of intracellular Ca2+ stores (TMB-8) caused an initial corneal swelling over the first 1.5 hr of perfusion but thereafter had no effect on corneal thickness. In the presence of continued extracellular Ca2+ one explanation for the results is that modulation of intracellular Ca2+ by agents that alter plasma membrane transfer of Ca2+ influences apical junction permeability.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid toxicological model for use in assessing ocular drugs

Nonsteroidal antiinflammatory drugs (NSAIDs) were applied to corneas either by in vitro or in vivo methods. The in vitro method involved excising and mounting corneas in a perfusion system at 37 degrees and exposing drug for 2.5 h. The in vivo methods represent either topical administration to the rabbit eye or topical in vivo infusion using a fixed well which permitted a constant concentration (0.05 per cent) to be applied to the eye of anesthetized rabbits for up to 120 min. An overlay grid procedure using scanning electron microscopy (SEM) showed less per cent endothelial damage with in vivo methods than with the in vitro method of administration, but per cent damage depended on which section was viewed. Damage to the epithelium and endothelium were also assessed by quantitative carboxyfluorescein and Janus green staining and uptake procedures, respectively, following drug exposure by the in vivo infusion method. Results for the epithelium indicated that the more lipophilic NSAIDs damaged the epithelial layer to a greater degree than newly synthesized hydrophilic NSAIDs. Damage to the epithelium correlated to the surface activity of the NSAIDs. Qualitative assessment of epithelial and endothelial toxicity can be performed with SEM and transmission electron microscopy (TEM) while vital staining procedures and the SEM grid procedure can be used to quantitatively assess corneal toxicity. The staining methods, however, possess advantages over SEM and TEM procedures in that they are rapid and do not require laborious preparation. As a result of these characteristics, the vital staining procedures could be used as part of a biopharmaceutical screening technique in evaluating new ophthalmic drugs.

A mechanistic study on the enhancement of corneal penetration of phenylephrine by flurbiprofen in the rabbit

The effect of flurbiprofen on the corneal penetration of phenylephrine was investigated using isolated albino rabbit corneas mounted in the Ussing diffusion chamber. The corneal penetration of phenylephrine was increased 5-11 times by 4-16 mM flurbiprofen. Such an increase in the corneal penetration of phenylephrine by flurbiprofen appeared to correlate with the increase in the lipophilicity of phenylephrine by flurbiprofen due to ion pair formation. The magnitude of increase at 16 mM flurbiprofen was 2.4 times less than that afforded by 0.5% EDTA and 16.3 times less than that afforded by deepithelizing the cornea. Since flurbiprofen also increased the corneal penetration of benzoic acid and sorbitol with which it cannot form ion pairs, increased lipophilicity of phenylephrine due to ion pair formation with flurbiprofen was unlikely to be the only mechanism that enhanced corneal penetration. An additional mechanism under our experimental conditions of prolonged exposure of the cornea to the phenylephrine-flurbiprofen combination was lowering of the barrier function of the corneal epithelium by concentrations of flurbiprofen greater than that used therapeutically.

Adrenergic regulation of sodium and chloride transport in the isolated cornea of rabbit and man

Isolated corneas were mounted in Ussing-Zerahn-type chambers and short circuit current (SCC) was measured before and after application of drugs (5 X 10(-5) mol X 1(-1)) interfering with adrenergic receptors. Epinephrine increased SCC in the rabbit cornea and decreased SCC in the human cornea. alpha-Adrenergic stimulation or inhibition did not affect SCC. The increase in SCC observed after terbutaline (beta 2-agonist) was similar to the increase after isoproterenol (beta 1- and beta 2-agonist). SCC was not influenced by the beta 1-antagonist atenolol but was modified, although differently in rabbit and man, by the beta 1- and beta 2-antagonist propranolol. Thus, the catecholamine response of the rabbit and human cornea is mainly mediated by beta 2-adrenergic receptors. However, species differences were observed when testing the effect of propranolol on the transcorneal flux of 22Na and 36Cl. In the rabbit cornea the net Cl flux (directed from the aqueous to the tear side) was inhibited by propranolol, whereas net Na flux (from the tear to the aqueous side) was not influenced by the drug. In the human cornea propranolol reduced unidirectional Na flux from the aqueous to the tear side. Thus, the regulatory effect of propranolol on corneal transparency is different in man and the rabbit.

Complications of vitreous surgery for diabetic retinopathy. II. Postoperative complications

One hundred seventy-nine eyes were analyzed to determine the incidence of postoperative complications after vitrectomy for proliferative diabetic retinopathy. One hundred twenty-nine (72%) of the 179 eyes achieved improved vision, and 117 (65%) were considered visual successes with final vision in the functional range of 5/200 or better. Corneal epithelial defects occurred in 51 eyes (28%), but severe corneal complications were rare, and no eye developed corneal clouding as the sole cause of later visual loss. The lens was retained in 128 eyes (75%), and visually significant lens opacities occurred later in 17% of the phakic eyes. Postoperative iris neovascularization was reduced by not removing the lens, and rubeosis iridis occurred in 15 (13%) of 114 phakic eyes and in 21 (32%) of 65 aphakic eyes (P = 0.012). Vitreous hemorrhage was present in 75% of eyes immediately after surgery and cleared in an average of 6.2 weeks in phakic eyes and 5.4 weeks in aphakic eyes. Fifty-two eyes (29%) had recurrent vitreous hemorrhage after the initial postoperative period. New retinal detachment occurred after surgery in 16% of eyes and was treated successfully in 38%. Reoperations were done in 45 eyes (25%), and 32 (71%) of these were for repair of retinal detachment or removal of nonclearing vitreous hemorrhage. Six eyes (3%) developed phthisis bulbi, and one other eye was enucleated.

The effect of ophthalmic preservatives on the shape of corneal endothelial cells

The mean area (293.6 +/- 53.6 micrometers 2) and perimeter (65.5 +/- 5.8 micrometers) of individual cells were determined from the photographed endothelium of a total of 54 guinea pig corneas, after staining with silver nitrate and removal of the epithelium and stroma. In addition a quantitative assessment of cell shape S (= P2/A) was calculated. The mean cell density (3406 +/- 619 cells per mm2) was similar to that for young humans, and the S value (14.76 +/- 0.73) was as expected for slightly irregular hexagonal and pentagonal shapes. Incubation of the corneas in glutathione-bicarbonate-Ringer solution for periods up to 4 h resulted in only slight changes in the endothelial cell shape (S = 15.21 +/- 0.89). However, incubation with thimerosal (0.01%) for 2 h (S = 16.78 +/- 1.79) or with disodium ethylenediaminetetraacetate (Na2EDTA) (0.2%) for 1 h (S = 17.20 +/- 2.36) gave a marked increase in the tortuosity of the cell 'outlines'. Longer periods of contact with Na2EDTA resulted in a rounding up and separation of endothelial cells.

Effect of phenylephrine on aqueous humor flow

The effect of topical 2.5% phenylephrine hydrochloride, an alpha adrenergic agonist, on the rate of aqueous humor flow was studied in the eyes of 22 normal human subjects. The drug and a placebo were administered by random assignment from coded containers with the fellow eye serving as the control. Intraocular pressure, anterior chamber volume, corneal thickness and the rate of clearance of topically applied fluorescein from the anterior chamber were measured. Four of the 22 subjects underwent a study of the effect of phenylephrine on the permeability to fluorescein of the blood-aqueous barrier. No statistically significant effect of the drug was found on the rate of aqueous humor flow, intraocular pressure, anterior chamber volume, corneal thickness or permeability of the blood-aqueous barrier.

Pupillary dilatation with single eyedrop mydriatic combinations

We studied the mydiatic effect of three solutions containing a combination of two mydriatic drugs in 80 adult patients. The solutions tested were cyclopentolate HCl 0.5% with phenylephrine 2.5%, tropicamide 0.5% with phenylephrine 2.5%, and tropicamide 1.0% with phenylephrine 2.5%. We evaluated the effect of prior instillation of proparacaine 0.5% eyedrops. All three mydriatic combination solutions evaluated produced pupillary dilatation of about 7 mm within 60 minutes. Additional pupillary dilatation of 1 mm occurred when proparacaine was instilled before the mydriatic combination eyedrop. Mydriasis was resistant to bright light during indirect ophthalmoscopy in all patients. Pupils of younger patients dilated better than those of older patients, but sex and iris color were factors in the amount of pupillary dilatation obtained. Wide and sustained pupillary dilatation can be obtained for satisfactory indirect ophthalmoscopy by the instillation of one drop of proparacaine solution followed by a single drop of any of the three mydriatic combination solutions evaluated. By eliminating the need for multiple instillations of drugs, the use of a single eyedrop mydriatic combination is convenient in terms of time saved and also lessens the change of systemic drug toxicity.