Epithelium corneal

In humans, cases of dermatitis have been described after contact with DHBs. Combined exposure to hydroquinone and quinone airborne concentrations causes eye irritation, sensitivity to light, injury of the corneal epithelium, and visual disturbances (126). Cases with an appreciable loss of vision have occurred (127). Long-term exposure causes staining due to irritation or allergy of the conjunctiva and cornea and also opacities. Resorcinol and catechol are also irritants for eyes. 

Toxicology. The acute oral and dermal toxicity of naphthalene is low with LD q values for rats from 1780—2500 mg/kg orally (41) and greater than 2000 mg/kg dermally. The inhalation of naphthalene vapors may cause headache, nausea, confusion, and profuse perspiration, and if exposure is severe, vomiting, optic neuritis, and hematuria may occur (28). Chronic exposure studies conducted by the NTP ia mice for two years showed that naphthalene caused irritation to the nasal passages, but no other overt toxicity was noted. Rabbits that received 1—2 g/d of naphthalene either orally or hypodermically developed changes ia the lens of the eye after a few days, foUowed by definite opacity of the lens after several days (41). Rare cases of such corneal epithelium damage ia humans have been reported (28). Naphthalene can be irritating to the skin, and hypersensitivity does occur. 

Direct eye contact with liquid produces injury, generally transient, to the corneal epithelium. The liquid is mildly imtating to the skin due to the degreasing effect repeated contact may cause dermatitis. Ingestion of substantial quantities of liquid can damage the mucous membranes, and produce acute effects ranging from mild discomfort to profound anaesdiesia. 

Injury (either physical or chemical) to the comeal endothelial cells has a marked efiect on occular function as these cells are responsible for maintaining the thickness and clarity of the cornea, yet they cannot be replaced if damaged. Immunohistochemical studies have revealed that enzymatic antioxidant defences, SOD, CAT and GSHPx, are similarly distributed in the corneal epithelium and endothelium (Rao etal., 1985 Attala et d., 1987, 1988). Other antioxidants include ascorbate, carotenoids and vitamin E (Fleath, 1962). 

The carbomer polymeric gel base itself has been used successfully to treat moderate to severe cases of dry eye (keratoconjunctivitis sicca) [282]. The dry eye syndrome is usually characterized by deficiency of tear production and, therefore, requires frequent instillation of aqueous artificial tear eyedrops to keep the corneal epithelium moist. The gel base applied in a small amount provides a prolonged lubrication to the external ocular tissues, and some patients have reduced the frequency of dosing to control their symptoms to three times a day or fewer. 

The corneal epithelium is a stratified squamous epithelium like the epidermis of the skin but is nonkeratinized like other mucosal epithelia such as the intestinal and airway epithelia. Although the corneal epithelium, five to seven cells thick, represents less than 10% of the entire corneal thickness, it provides as much as 99% resistance to the diffusion of small electrolytes such as Na+ and Cl [55-57],... 

Three principal cell types exist in the corneal epithelium, namely, superficial cells, wing cells, and basal cells. A single layer of cuboidal basal cells is the sole site of cell division in the corneal epithelium. By necessity, these cells have more prominent mitochondria and Golgi apparatuses and hence high levels of metabolic and synthetic activities. Immediately above the basal cells is a zone comprising two to three layers of wing cells that are in an intermediate state of differentiation. 

Figure 2 Schematic representation of the active ion transport processes in the rabbit corneal epithelium.

Br > Cl > I. This conductance was not time-dependent. On the other hand, Cl conductance stimulated by the Ca2+ ionophore A23187 (2.5 pM) or elevation in the free Ca2+ levels in the pipet from 100 to 500 nM was time-dependent and exhibited a nonlinear current-voltage relationship that was outwardly rectifying. The permselectivity of this Ca2+-stimulated conductance was 1 > Br > cr, distinguishing it from the cAMP-stimulated Cfr conductance. As both calcium ionophore A23187 and cAMP were shown to elevate the Cl conductance of the cornea, it is quite likely that these two types of Cr channels are present in the corneal epithelium [96,106,114],... 

Figure 3 Putative model for the mechanism by which biogenic amines stimulate CE secretion across the rabbit corneal epithelium. Epn = epinephrine Nep = norepinephrine Tim = Timolol Ser = serotonin Msg = methysergide Dop = dopamine Hal = haloperi-dol (E = (E-adrenoceptor AC = adenylate cyclase. The scheme is consistent with the observation that epithelial responsiveness to serotonin and dopamine can be blocked by their receptor antagonists haloperidol and methysergide, respectively, and by both timolol treatment and sympathectomy. The probable source of serotonin or dopamine is the sympathetic fibers that innervate the cornea. (From Ref. 284.)...

Evidence for the presence of a K+/H+ exchanger in the basal cells of corneal epithelium was also obtained by Bonanno [137], Elevated levels of intracellular K+ maintained by the basolateral Na+/K+-ATPase are utilized by K+/H+... 

While the lactate-H+ symporter and the K+/H+ exchanger are involved in acidification of the cell, the Na+/H+ exchanger present in the basal cells exports protons out of the cell in exchange for Na+ [139]. It was observed that removal of Na+ from the Ringer s solution decreased intracellular pH by 0.5 unit in basal cells, possibly due to inhibition of the Na+/H+ exchanger. As the basal cells are the precursors for the superficial cells of the corneal epithelium, it is quite likely that similar exchange processes are also present in the superficial layer, the principal barrier to ion and drug transport [99,103],... 

SD Klyce. (1972). Electrical profiles in the corneal epithelium. J Physiol 226 407-429. 

Y Wang, M Chen, JM Wolosin. (1993). ZO-1 in corneal epithelium Stratal distribution and synthesis induction by outer cell removal. Exp Eye Res 57 283-292. 

SD Klyce, RKS Wong. (1977). Site and mode of adrenaline action on chloride transport across the rabbit corneal epithelium. J Physiol 266 777-799. 

WS Marshall, JW Hanrahan. (1991). Anion channels in the apical membrane of mammalian corneal epithelium primary cultures. Invest Ophthalmol Vis Sci 32 1562-1568. 

WS Marshall, SD Klyce. (1984). Cellular mode of serotonin action on Cl transport in the rabbit corneal epithelium. Biochim Biophys Acta 778 139-143. 

RL Shih, VHL Lee. (1990). Rate limiting barrier to the penetration of ocular hypotensive beta-blockers across the corneal epithelium in the pigmented rabbit. J Ocular Pharmacol 6 329-336. 

JM Wolosin. (1988). Regeneration of resistance and ion transport in rabbit corneal epithelium after induced surface cell exfoliation. J Membr Biol 104 45-55. 

JA Bonanno, SD Klyce, EJ Cragoe Jr. (1989). Mechanism of chloride uptake in rabbit corneal epithelium. Am J Physiol 257 C290-C296. 

OA Candia, L Grillone, TC Chu. (1986). Forskolin effects on frog and rabbit corneal epithelium ion transport. Am J Physiol 251 C448-C454. 

SD Klyce, KA Palkama, M Harkonen, WS Marshall, S Huhtaniitty, KP Mann, AH Neufeld. (1982). Neural serotonin stimulates chloride transport in the rabbit corneal epithelium. Invest Ophthalmol Vis Sci 23 181-192. 

RJ Walkenbach, RD LeGrand. (1982). Inhibition of adenylate cyclase activity in the corneal epithelium by anti-inflammatory steroids. Exp Eye Res 34 161-168. 

JW Huff, PS Reinach. (1985). Mechanism of inhibition of net ion transport across frog corneal epithelium by calcium channel antagonists. J Membr Biol 85 215-223. 

JA Bonanno. (1991). K+-H+ exchange, a fundamental cell acidifier in corneal epithelium. Am J Physiol 260 C618-C625. 

CMAW Festen, JFG Siegers, CH Van Os. (1983). Intracellular activities of chloride, potassium, and sodium ions in rabbit corneal epithelium. Biochim Biophys Acta 732 394-404. 

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