Urea and the vitreous barrier of the eye

Urea and ocular surface: Synthesis, secretion and its role in tear film homeostasis

Urea has been detected in the tear film, aqueous humor, and vitreous of the eye. While most of the urea in the aqueous humor and vitreous is considered to be an ultrafiltrate from the blood vessels, the presence of urea transporters and urea-synthesizing enzymes in the lacrimal gland, meibomian glands, conjunctiva, and cornea suggests ureagenesis occurring at the ocular surface. This review summarizes the distribution and function of urea transporters, urea and its synthesizing enzymes at the ocular surface to analyze their role in the tear film homeostasis. Urea transporters (UT)-A- and UT-B-as well as the enzymes arginase I, II, and agmatinase are located at the ocular surface. Urea concentration on the ocular surface is influenced by blood urea concentration, the amount of urea released by the tear fluid, tear evaporation, and arginase concentration in the tears. There are conflicting reports on the relationship between tear and plasma urea levels though a linear correlation exists between their levels. Urea protects the ocular surface from osmotic stress and is thought to maintain a lipid-water interface in the lamellar phase of the tear film. The reduction of urea levels in the tears of patients with evaporative dry eye suggests its possible role in tear film stability. Other than mitigating osmotic stress, urea has hydrating properties as well. Animal studies have demonstrated the healing effects of urea on the corneal epithelium. Future studies examining the variations in urea content in tears from different ocular surfaces, at different times of day, and under different environmental conditions would further solidify the role of urea in tear film stability.

Pharmacokinetics of sparfloxacin in the serum and vitreous humor of rabbits: physicochemical properties that regulate penetration of quinolone antimicrobials

We have used a recently described animal model to characterize the ocular pharmacokinetics of sparfloxacin in vitreous humor of uninfected albino rabbits following systemic administration and direct intraocular injection. The relationships of lipophilicity, protein binding, and molecular weight to the penetration and elimination of sparfloxacin were compared to those of ciprofloxacin, fleroxacin, and ofloxacin. To determine whether elimination was active, elimination rates following direct injection with and without probenecid or heat-killed bacteria were compared. Sparfloxacin concentrations were measured in the serum and vitreous humor by a biological assay. Protein binding and lipophilicity were determined, respectively, by ultrafiltration and oil-water partitioning. Pharmacokinetic parameters were characterized with RSTRIP, an iterative, nonlinear, weighted, least-squares-regression program. The relationship between each independent variable and mean quinolone concentration or elimination rate in the vitreous humor was determined by multiple linear regression. The mean concentration of sparfloxacin in the vitreous humor was 59.4% +/- 12.2% of that in serum. Penetration of sparfloxacin, ciprofloxacin, fleroxacin, and ofloxacin into, and elimination from, the vitreous humor correlated with lipophilicity (r2 > 0.999). The linear-regression equation describing this relationship was not improved by including the inverse of the square root of the molecular weight and/or the degree of protein binding. Elimination rates for each quinolone were decreased by the intraocular administration of probenecid. Heat-killed Staphylococcus epidermidis decreased the rate of elimination of fleroxacin. Penetration of sparfloxacin into the noninflamed vitreous humor was greater than that of any quinolone previously examined. There was an excellent correlation between lipophilicity and vitreous entry or elimination for sparfloxacin as well as ciprofloxacin, fleroxacin, and ofloxacin. There are two modes of quinolone translocation into and out of the vitreous humor: diffusion into the eye and both diffusion and carrier-mediated elimination out of the vitreous humor.

Topical urea as a treatment for non-infectious keratopathy

The purpose of this study was to evaluate topical urea ophthalmic ointment as an agent to promote epithelial healing. Corneal epitheliopathy and epithelial defects were chemically induced in rabbits. Urea ophthalmic ointment was administered to one eye and control ointment to the fellow eye. The rabbits were examined by a masked observer for eleven days and points were awarded for steps of improvement in their ocular surface disease. The eyes receiving the urea ointment improved an average of 5.73 points while the control eyes improved 4.2 points (p < 0.0001). In conclusion, topical urea dissolved in a bland ointment encourages corneal reepithelialization and limits epithelial damage after toxic injury to corneal epithelium.

Tear urea nitrogen and creatinine levels in renal patients

Tear and blood urea nitrogen, creatinine, and glucose levels were quantitatively analyzed by an enzymatic method in 30 normal patients and in 10 patients with end stage renal disease. The tear and blood samples collected from the end stage renal patients before and after hemodialysis showed that the change in urea nitrogen and creatinine levels in the tears correlated with the change in blood urea nitrogen and creatinine. Tear samples may serve as an alternative, less invasive indicator of serum metabolites in renal patients.

Clinical biochemistry of tears

The composition of tear fluid as it exists in the conjunctival fornix and in the precorneal tear film is of a complicated nature. The precorneal tear film is a physically inhomogenous system, produced by the lacrimal glands, the accessory lacrimal glands and the goblet cells of the conjunctiva and the Meibomian glands of the lid margin. The method of collection is crucial for the quantity and concentration of a great number of different compounds that have been demonstrated in tears. A survey is given of the literature concerning tears as they are composed of proteins, enzymes, lipids, metabolites, electrolytes and drugs, the latter secreted during therapy. Clinical applications of the determination of several compounds for diagnosis or for monitoring drug therapy are summarized.

Collection method dependent concentrations of some metabolites in human tear fluid, with special reference to glucose in hyperglycaemic conditions

Concentrations of the metabolites glucose, lactate, pyruvate, and urea were determined in human tear fluid. Collection of the tears in glass capillaries, without mechanical irritation, permitted the estimation of naturally occurring levels of these metabolites. Glucose concentrations were very low, also in diabetics with high blood glucose levels. Lactate was present at higher levels and pyruvate and urea in about the same concentrations as normally for blood. Collection of the tears with filter paper strips as absorbent material caused slight epithelial damage and consequently a loss of the barrier function of the epithelium. In filter paper eluates glucose concentrations were found to be much higher, especially in diabetics with high blood glucose levels. Lactate and pyruvate concentrations were not influenced, whereas the urea concentrations decreased. The value of clinical tear glucose tests is discussed.