The mechanisms of calcium homeostasis and signalling in the lens

Excessive Ca²⁺ can be detrimental to cells and raised levels of Ca²⁺ in human lenses with cortical cataract have been found to play a major role in the opacification process. Ca²⁺ homeostasis is therefore, recognised as having fundamental importance in lens pathophysiology. Furthermore, Ca²⁺ plays a central role as a second messenger in cell signalling and mechanisms have evolved which give cells exquisite control over intracellular Ca²⁺ ([Ca²⁺]_i) via an array of specialised regulatory and signalling proteins. In this review we discuss these mechanisms as they apply to the lens. Ca²⁺ levels in human aqueous humour are approximately 1 mM and there is a large, 10,000 fold, inwardly directed gradient across the plasma membrane. In the face of such a large gradient highly efficient mechanisms are needed to maintain low [Ca²⁺]_i. The Na⁺/Ca²⁺ exchanger (NCX) and plasma membrane Ca²⁺-ATPase (PMCA) actively remove Ca²⁺ from the cells, whereas the sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) sequesters Ca²⁺ in the endoplasmic reticulum (ER) Ca²⁺ store. In lens epithelial cells the dominant role is played by the ATPases, whilst in the fibre cells NCX activity appears to be more important. Usually, [Ca²⁺]_i can be increased in a number of ways. Ca²⁺ influx through the plasma membrane, for example, is mediated by an array of channels with evidence in the lens for the presence of voltage-operated Ca²⁺ channels (VOCCs), receptor-operated Ca²⁺ channels (ROCCs) and channels mediating store-operated Ca²⁺ entry (SOCE). Ca²⁺ signalling is initiated via activation of G-protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTK) of which the lens expresses a surprisingly diverse array responding to various neurotransmitters, hormones, growth factors, autocoids and proteases. Downstream of plasma membrane receptors are IP₃-gated channels (IP₃Rs) and ryanodine receptors (RYRs) located in the ER, which when activated cause a rapid increase in [Ca²⁺]_i and these have also been identified in the lens. Through an appreciation of the diversity and complexity of the mechanisms involved in Ca²⁺ homeostasis in normal lens cells we move closer to an understanding of the mechanisms which mediate pathological Ca²⁺ overload as occurs in the process of cataract formation.     

Lens epithelial cell response to isoforms of platelet-derived growth factor

It has been reported previously that platelet derived growth factor (PDGF) may play an important role in the regulation of lens growth and differentiation. To evaluate PDGF-induced effects at the cellular level, we investigated the response of cultured bovine lens epithelial cells (BLEC) to PDGF-AB, -AA, and -BB isoforms at the cellular level. Stimulation of BLEC with PDGF isoforms showed no increase in cell proliferation under the culture conditions of this study. In contrast, measurement of cytosolic free calcium concentration ([Ca²⁺]_i), which has been shown to be an important second messenger for controlling multiple cellular processes in the lens, revealed a dose-dependent rise in [Ca²⁺]_i upon stimulation with PDGF-AB and -BB isoforms. PDGF-AA used in similar concentrations was not effective. Our data suggest that PDGF-AB and -BB may play a role in the regulation of cellular functions in BLEC via modulation of intracellular calcium homeostasis.Presented in part at the annual meeting of the Association for Research in Vision and Ophthalmology, Sarasota, Florida, May 1991     

Rapid increase of intracellular Ca2+ concentration caused by aminoadipic acid enantiomers in retinal Müller cells and neurons in vitro

The ability of the gliotoxic compounds D,L-, D- or L-2-aminoadipic acid (AAA) to increase selectively the intracellular concentration of free calcium ion ([Ca²⁺]_i) was examined in Müller cells cultured with or without retinal neurons. The monitoring of [Ca²⁺]_i following exposure to 0.06 to 6 mM AAA was performed by a microfluorometry using a fluorescent Ca²⁺ indicator, Fura-2 acetoxymethyl ester. A rapid increase of [Ca²⁺]_i occurred in the Müller cells following exposure to a relatively low concentration of the L-isomer. This is compatible with the known strong gliotoxicity of this isomer. The D,L- and D-forms of AAA activated neurons at low concentrations and activated the Müller cells at higher concentrations. The D-isomer appears to act selectively on retinal neurons and may be an agonist of an excitatory amino acid receptor. These results indicate that the ability of AAA to elevate cytosolic [Ca²⁺]_i depends on the stereospecificity of the AAA and on cell type.     

Characterization of transient receptor potential vanilloid channel 4 (TRPV4) in human corneal endothelial cells

The transient receptor potential vanilloid 4 (TRPV4) is a Ca²⁺-and Mg²⁺ permeable cation channel that might be a cellular osmosensor since it is activated upon hypotonic cell swelling. TRPV4 is also thermosensitive and responds to moderate heat (from 24 to 27 °C) as well as to phorbol esters (4α-PDD) and several endogenous substances including arachidonic acid (AA), the endocannabinoids anandamide and 2-AG, and cytochrome P-450 metabolites of AA, such as epoxyeicosatrienoic acids. The resulting Ca²⁺ influx occurring in response to swelling induces regulatory volume decrease (RVD) behavior. As regulation of cell volume is essential for corneal endothelial function, we determined whether human corneal endothelial cells have functional TRPV4 channel activity. RT-PCR identified TRPV4 gene expression in the HCEC-12 cell line as well as two clonal daughter cell lines (HCEC-H9C1, HCEC-B4G12). [Ca²⁺]_i transients were monitored in fura-2 loaded cells. Nonselective cation channel currents were recorded in the whole-cell mode of the planar patch-clamp technique. TRPV4 mRNA was found in HCEC-12 and the clonal daughter cell lines. TRPV4 channel agonists (4α-PDD and GSK1016790A; both 5 μmol/l) as well as moderate heat (<40 °C) elicited [Ca²⁺]_i transients. Hypotonicity increased [Ca²⁺]_i and nonselective cation channel currents in HCEC-12 cells. There is functional TRPV4 expression in HCEC-12 and in its clonal daughter cell lines based on Ca²⁺ transients and underlying currents induced by known activators of this channel.     

P1-/P2-purinergic receptors on cultured rabbit retinal Müller cells

Adenosine 5′-triphosphate (ATP) and its metabolic products function as neurotransmitters or neuromodulators under the control of P₁/P₂-urinergic receptors. To determine the presence of these receptors on retinal Müller cells, spectrofluorometry was carried out on intracellular calcium mobilization, using Fura-2 images. Müller cells were cultured from adult rabbit retinas. Cytosolic calcium ([Ca²⁺]_i) increased dose dependently with the application of ATP. This response was not blocked when a calcium channel blocker, nifedipine, was present, but this response was blocked, for the most part, when a P₂ receptor antagonist, pyridoxalphosphate-6-azophenyl-2′,4′-disulfonic acid (PPADS) was present. Increase in [Ca²⁺]_i was noted by the A₁ or A₂ agonist, which was blocked completely by each antagonist. Response to the a₁ agonist was apparent only at high concentrations. Increase in [Ca²⁺]_i was seen in some cells following administration of the P_2x agonist, methylene ATP, only at a high concentration (100 μM) but not in the presence of PPADS (50 nM). The greatest increase in [Ca²⁺]_i was induced by a P_2y agonist, methyl thio ATP at 1 to 10 μm, which was completely blocked by PPADS. Cultured Müller cells are thus shown quite likely to possess the P₁-/P₂- purinergic receptors including A₂ and P_2y.     

Increase in intracellular free calcium concentration of limulus photoreceptors caused by a metabolic inhibitor

Using a calcium indicator dye (arsenazo III), we detected a reversible rise in [Ca^2?]_i to about 2 mM in Limulus ventral photoreceptors when the cells were poisoned with the metabolic inhibitor DNP. Our results provide direct evidence for the hypothesis linking the action of metabolic inhibitors to an increase in [Ca^2?]_i in invertebrate photoreceptors.     

Role of the Phospholipase C Pathway and Calcium Mobilization in Oxytocin-Induced Contraction of Lacrimal Gland Myoepithelial Cells

PurposeWe reported that oxytocin (OXT), added to freshly prepared lacrimal gland lobules, induced myoepithelial cell (MEC) contraction. In other systems, OXT activates phospholipase C (PLC) generating Inositol 1,4,5-trisphosphate (IP₃) which increases intracellular calcium concentration ([Ca²⁺]_i) causing contraction. The aim of the current study was to investigate the role of this pathway in OXT-induced contraction of MEC.MethodsTear volume was measured using the cotton thread method. Lacrimal gland MEC were isolated and propagated from α-smooth muscle actin (SMA)-green fluorescent protein (GFP) mice, in which MEC express GFP making them easily identifiable. RNA and protein samples were prepared for RT-PCR and Western blotting for G protein expression. Changes in [Ca²⁺]_i were measured in Fura-2 loaded MEC using a ratio imaging system. MEC contraction was monitored in real time and changes in cell size were quantified using ImageJ software.ResultsOXT applied either topically to surgically exposed lacrimal glands or delivered subcutaneously resulted in increased tear volume. OXT stimulated lacrimal gland MEC contraction in a dose-dependent manner, with a maximum response at 10⁻⁷ M. MEC express the PLC coupling G proteins, Gαq and Gα11, and their activation by OXT resulted in a concentration-dependent increase in [Ca²⁺]_i with a maximum response at 10⁻⁶ M. Furthermore, the activation of the IP₃ receptor to increase [Ca²⁺]_i is crucial for OXT-induced MEC contraction since blocking the IP₃ receptor with 2-APB completely abrogated this response.ConclusionsWe conclude that OXT uses the PLC/Ca²⁺ pathway to stimulate MEC contraction and increase lacrimal gland secretion.     

Rapid increase in cytosolic calcium ion concentration mediated by acetylcholine receptors in cultured retinal neurons and Müller cells

· Purpose: This study was conducted to detect the presence of muscarinic or nicotinic receptors in cultured retinal neurons and Müller cells. · Methods: Pure Müller cell cultures and cocultures of retinal neurons and Müller cells were used; the former, obtained from adult rabbit retinas, and the latter, retinal neurons from neonatal rats, were cocultured with Müller cells. Intracellular calcium ion concentration ([Ca²⁺]_i) following the administration of acetylcholine, a cholinesterase inhibitor (trichlorfon), nicotine or muscarinic agonist with or without a receptor antagonist was monitored using the calcium ion indicator, fura-2. · Results: Acetylcholine and trichlorfon induced rapid increase in [Ca²⁺]_i in half of either cell type. Trichlorfon induced positive response in coculture but not in the pure Müller cell cultures. This positive response was blocked only partially in the presence of atropine. Approximately 30–40% of neurons responded to nicotine at 5 μM, which was significantly blocked by α-bungarotoxin at 50 nM. No response to nicotine could be detected in Müller cells. Approximately 50% of neurons responded to muscarine at 50 μM, but 500 μM was required for the formation of calcium transients in 50% of Müller cells. The muscarine inducement of rapid increase in [Ca²⁺]_i was blocked by atropine. The agonist of M1 (a muscarinic receptor subtype), McN-A-343, at 0.5 μM induced the most significant and rapid increase in [Ca²⁺]_i both in neurons and Müller cells. McN-A-343 administration at 0.05 μM induced positive response in half the neurons, but only in approximately 10% of Müller cells. Such positive response was not observed following preincubation with the M1 antagonist, pirenzepine, at 50 μM. · Conclusions: Cocultured retinal neurons enhance the release of acetylcholine following anticholinesterase administration, and approximately half the neurons were found to possess muscarinic and nicotinic receptors. However, Müller cells appeared to possess only the less sensitive muscarinic receptor. Muscarinic receptor subtypes on either type of cell contained at least M1. Received: 17 November 1997 Revised version received: 20 April 1998 Accepted: 21 April 1998     

Human and porcine anterior lens capsule as support for growing and grafting retinal pigment epithelium and iris pigment epithelium

· Purpose: To establish a method for transplantation of cultured monolayers of RPE and IPE into the subretinal space, anterior lens capsule was evaluated for its suitability to serve as growth support and carrier for transplantation procedures. · Materials and methods: Twenty-four anterior lens capsules were obtained from porcine eyes. The same number of human lens capsules was obtained during cataract surgery. Six lens capsules of each species were stored at –80°C. Subsequently, the capsules were transferred onto type-I collagen. A second set of six lens capsules was treated identically except for the cryo treatment. A third set of six capsules was initially exposed to 0.05% trypsin for 30 min. Suspended porcine RPE and IPE cells (5×10⁴ cells/well) were seeded on the top of each capsule. The remaining six lens capsules served as controls and were incubated in uncoated 12-well dishes without undergoing experimental treatment. The cultures were maintained in a water-saturated atmosphere at 37°C with 5% CO₂. Six days later, viability, morphology, and cell density were determined. The capsules covered by a confluent monolayer of cells were transferred into uncoated wells and cultivated for another 10 days. At the end of the experiment, light and phase-contrast microscopy was performed on all capsules. · Results: Storage at –80°C and exposure to trypsin resulted in significant reduction of cellular contamination. The highest cell density was found after 5 days when capsules which had undergone cryopreservation or trypsin exposure served as support for RPE and IPE. The pigment cell layer was firmly attached to the capsules and permitted a transfer to other culture flasks without significant cell loss. The IPE cell layer remained confluent after transfer to uncoated culture flasks, while the RPE cell layer ceased to proliferate 10 days after transfer. · Conclusions: Lens capsules may be suitable for growing and supporting monolayers of pigment epithelial cells. Especially IPE cells formed stable monolayers on anterior lens capsules which could be transferred to secondary culture flasks without inflicting damage on the cells. Received: 6 October 1998 Revised version received: 18 December 1998 Accepted: 30 March 1999     

Effects of Rho-associated protein kinase inhibitors Y-27632 and Y-39983 on isolated rabbit ciliary arteries

Purpose

In normotensive eyes, reduced ocular blood flow can lead to glaucoma pathogenesis. Drugs that reduce intraocular pressure (IOP) often cause vasodilation of the ciliary arteries and improve blood flow to the eye. A novel class of drugs called Rho-associated coiled coil-forming protein kinase (ROCK) inhibitors can lower IOP. Therefore, we tested the ability of two ROCK inhibitors, Y-27632 and Y39983, to relax rabbit ciliary arteries.

Methods

We measured in vitro ciliary artery smooth muscle contractions by isometric tension recordings and changes of intracellular free calcium concentration ([Ca²⁺]_i) by fluorescence photometry.

Results

Both Y-27632 and Y-39983 induced a concentration-dependent relaxation in rabbit ciliary arteries precontracted with a high-potassium (high-K) solution. The amplitude of relaxation induced by Y-27632 and Y-39983 was not affected by either 100 ??M N ^G-nitro-l-arginine methyl ester (l-NAME) or 10 ??M indomethacin. In Ca²⁺-free solution, Y-27632 and Y-39983 significantly inhibited the transient contraction of ciliary arteries induced by 10 ??M histamine. However, neither Y-27632 nor Y-39983 affected the elevation of [Ca²⁺]_i induced by high-K solution and histamine.

Conclusions

We concluded that Y-27632 and Y-39983 relaxed isolated rabbit ciliary artery segments in vitro. The mechanism of relaxation was not dependent on endothelial-derived factors such as nitric oxide (NO) or prostacyclin, nor was it dependent on changes in intracellular Ca²⁺ concentration.     

miR-181调控人晶状体上皮细胞凋亡的机制研究

目的：探讨 miR-181在白内障晶状体组织中的表达情况,及其对人晶状体上皮细胞凋亡的调控机制。
　　方法：利用Real time q-PCR方法,检测年龄相关性白内障患者晶状体前囊膜和人晶状体上皮细胞凋亡模型中miR-181的表达情况;利用Lipofectamine 2000瞬时转染miR-181 mimic 和 inhibitor 调节人晶状体上皮细胞中miR-181的表达,利用Real time q-PCR方法验证转染效率,利用流式细胞仪检测细胞凋亡率的变化。
　　结果：与对照组相比,年龄相关性白内障患者晶状体前囊膜组和人晶状体上皮细胞凋亡模型组,miR-181的表达显著升高;miR-181 mimic转染组,miR-181的表达显著升高,细胞凋亡率显著升高;miR-181 inhibitor转染组, miR-181的表达显著降低,细胞凋亡率显著降低,差异均有统计学意义(P<0.01)。
　　结论：miR-181在年龄相关性白内障晶状体组织中呈高表达,miR-181能够促进人晶状体上皮细胞凋亡,从而可能在年龄相关性白内障的发病过程中发挥一定作用,miR-181可能成为白内障非手术治疗的一种新途径,但具体机制有待进一步研究。     

Lens epithelial cell death secondary to acanthamoeba keratitis: absence of capsular bag opacification six years after cataract surgery

Purpose

To show the evolution of anterior chamber structures 6 years after cataract surgery in a case with Acanthamoeba keratitis (AK).

Methods

A 37-year-old woman with AK receiving long-term treatment with chlorhexidine, propamidine isethionate and steroids developed a white cataract and iris atrophy. Penetrating keratoplasty and cataract surgery were performed with subsequent intraocular pressure elevation requiring Molteno shunt implantation. Two years after the last surgery, endothelial decompensation developed and another penetrating keratoplasty was performed. Intraoperatively, the anterior and posterior capsules were completely transparent.

Results

Six years after cataract surgery, the intraocular lens was centered with clear anterior and posterior capsules without lens epithelial cells proliferation. No Soemmering''s ring formation or posterior capsule opacification was found. Also, no zonular damage or pseudophacodonesis was observed.

Conclusions

This case suggests that AK infection and AK treatment not only cause white progressive cataract but also lens epithelial cell death. The capsules may be completely clear 6 years after cataract surgery, with a good quality of vision regardless of intraocular lens material or design.Key words: Acanthamoeba keratitis, Lens epithelial cells, Penetrating keratoplasty, Glaucoma, Cataract, Iris atrophy     

Molecular and functional mapping of regional differences in P2Y receptor expression in the rat lens

Extracellular ATP has been shown to mobilize intracellular Ca²⁺ in cultured ovine lens epithelial cells and in human lens epithelium, suggesting a role for purines in the modulation of lens transparency. In this study, we characterized the expression profiles of P2Y receptor isoforms throughout the rat lens at both the molecular and the functional levels. RT-PCR indicated that P2Y₁, P2Y₂, P2Y₄ and P2Y₆ are expressed in the lens, while P2Y₁₂, P2Y₁₃ and P2Y₁₄ are not. Immunohistochemistry, using isoform specific antibodies, indicated that the epithelium does not express P2Y₁ and P2Y₂, but that the underlying fiber cells, which differentiate from the epithelial cells, exhibit strong membranous labeling. Although co-expressed in fiber cells, differences in P2Y₁ and P2Y₂ expression were apparent. P2Y₁ expression extended deeper into the lens than P2Y₂, and its expression co-localized with Cx50 gap junction plaques, while P2Y₂ did not. Labeling for P2Y₄ and P2Y₆ receptors were observed in both epithelial cells and fiber cells, but the labeling was predominantly cytoplasmic in nature. While purine agonist (ATP, ADP, UTP and UDP) application to the lens induced mobilization of intracellular Ca²⁺ in cortical fiber cells, little to no effect was observed in the anterior and equatorial epithelium. Thus the inability of UTP and UDP to mobilize intracellular Ca²⁺ in the epithelium and the predominately cytoplasmic location of P2Y₄ and P2Y₆ suggests that these receptors may represent an inactive pool of receptors that may be activated under non-physiological conditions. In contrast, our results indicated that P2Y₁ and P2Y₂ are functionally active in fiber cells and their differential subcellular expression patterns suggest they may regulate distinct processes in the lens under steady state conditions.     

Effects of low [Ca2+]0 upon [K+]0 during and after maintained illumination of the isolated retina of the toad

Using K^?-selective microelectrodes, [K⁺]₀ was measured in the receptor layer of the isolated retina of the toad, Bufo marinus, during and after maintained adapting illumination. Lowering [Ca^2?]₀ to one-tenth of its control value produced larger, more rapid changes in [K^?]₀ at light onset and offset than under control conditions. Lowering [Ca²⁺]₀ also produced rod membrane depolarization in the dark and larger light-evoked changes in rod membrane voltage. The observed effects are consistent with a mechanism by which lowering [Ca^2?]₀ increases sodium conductance in rods in the dark, which in turn increases [Na⁺], and stimulates the rods Na⁺/K⁺ pump. This putative mechanism may be used to explain several effects of low [Ca²⁺]₀ upon rod function observed previously.     

Partial characterization of a putative new growth factor present in pathological human vitreous

Background Several growth factors have been implicated in the development of proliferative eye diseases, and some of those are present in human vitreous (HV). The effects of HV on cellular responses which modulate proliferative cell processes were studied. This study describes the partial characterization of a vitreous factor activity which does not correspond to any of the previously reported growth factors in pathological HV. Methods Vitreous humour was obtained from medical vitrectomies, from patients with PDR and PVR. The biological activity of the vitreous factor was determined by its ability to increase cytosolic calcium concentration ([Ca²⁺ _i]), increase production of inositol phosphates, and induce cell proliferation in the cell line EGFR T17. In some experiments other cell lines, such as NIH 3T3, 3T3-L1, FRTL5, A431, PC 12, Y79, and GH₃, were also employed. Measurement of [Ca²⁺]_i in cell suspensions was performed using the fluorescent Ca²⁺ indicator fura-2. The activity of the factor present in HV was compared with other growth factors by means of: (a) [Ca²⁺]_i mobilization pattern, (b) sequential homologous and heterologous desensitization of receptors, (c) effects of phorbol esters on their action, and (d) inactivation after treatment with different proteolytic enzymes. Results The HV-induced cell proliferation and increases in [Ca²⁺]_i concentration were characterized by a peculiar time pattern. The different approaches used ruled out its identity with PDGF, bFGF, EGF, TGF-, IGFs, TNF-, NGF, and other compounds such as ATP, angiotensin I, and bradykinin. Vitreous factor actions are mediated by specific receptors apparently regulated by PKC. This factor is able to induce [Ca²⁺]_i mobilization in most of the cell lines studied, indicating that its effects are not tissue specific. Conclusions These results suggest the presence of a growth factor activity in pathological HV which may be due to the presence of an undescribed growth factor in the eye.     

胰蛋白酶预防后发性白内障的实验研究

目的 探讨应用晶状体上皮细胞清除的方法预防后发性白内障。方法 老年性白内障手术中前囊连续环形撕囊后取得人晶状体前囊，应用不同浓度胰蛋白酶对前囊进行消化处理。观察上皮细胞的清除情况。结果 0.25%，0.2%胰蛋白酶溶液处理的晶状体前囊，晶状体上皮细胞可以完全脱落，0.1%胰蛋白酶溶液处理晶状体前囊，上皮细胞部分脱落，结论 该结果可为临床应用胰蛋白酶清除晶状体上皮细胞防治后发生白内障提供理论依据。     

不同类型白内障晶状体上皮细胞密度分析

目的 研究人晶状体上皮细胞密度与白内障类型的关系,探讨晶状体上皮细胞病变对白内障发病的意义。方法 随机收集60例白内障手术患者的晶状体前囊膜,固定后铺片,行HE染色后测定上皮细胞密度。按不同类型白内障进行分组,分析晶状体上皮细胞密度与白内障类型及患者年龄以及性别的关系。结果 先天性白内障组晶状体上皮细胞密度最高,平均(4316.63±479.65)个·mm~(-2),平均年龄(9.88±8.16)岁;其次为外伤性白内障组,平均细胞密度(3899.81±537.00)个·mm~(-2),平均年龄(29.43±19.66)岁;并发性白内障组平均细胞密度(3838.46±454.72)个·mm~(-2),平均年龄(54.22±7.63)岁;老年性白内障组平均细胞密度(3740.45±526.33)个·mm~(-2),平均年龄(67.39±11.08)岁。老年性白内障晶状体上皮细胞密度50～59岁组、60～69岁组、70～79岁组、80岁以上组均数间差异无显著性意义,男女患者间差异无显著性意义。结论 白内障晶状体上皮细胞密度与白内障类型有关;老年性白内障晶状体上皮细胞密度与患者年龄及性别无明显相关。     

硅油眼晶状体前囊膜及上皮细胞的病理学改变及Caspase-3表达

目的 观察硅油充填眼晶状体前囊膜及上皮细胞的病理学改变及Caspase—3表达，探讨硅油引起白内障的病因及与凋亡的关系。方法 收集27只晶状体前囊膜，通过HE染色和免疫组化方法对硅油眼的晶状体前囊膜和上皮细胞及Caspase—3在其上的表达进行观察。结果 前囊膜下许多空间被硅油滴占据；Caspase—3在晶状体上皮细胞有很好的表达，27例中有23例呈阳性表达。结论 硅油滴与白内障的发生、发展有关；凋亡在硅油眼并发白内障中存在。