Measurement of amino acid levels in the vitreous humor of rats after chronic intraocular pressure elevation or optic nerve transection

Purpose: To investigate whether the levels of free amino acids and protein in the vitreous of rat eyes are altered with chronic intraocular pressure (IOP) elevation or after optic nerve transection. Materials and Methods: The concentrations of 20 amino acids in the vitreous humor were measured by high-performance liquid chromatography in both eyes of 41 rats with unilateral IOP elevation induced by translimbal photocoagulation. Eyes were studied 1 day and 1, 2, 4, and 9 weeks after initial IOP elevation. The same amino acids were measured in 41 rats I day and 2, 4, and 9 weeks after unilateral transection of the orbital optic nerve. The intravitreal protein level was assayed in additional 22 rats with IOP elevation and 12 rats after nerve transection. Two masked observers evaluated the amount of optic nerve damage with a semiquantitative, light-microscopic technique. Results: In rats with experimental glaucoma, amino acid concentrations were unchanged I day after treatment. At I week, 4 of 20 amino acids (aspartate, proline, alanine, and lysine) were higher than in control eyes (P less than or equal to 0.01), but this difference was nonsignificant after Bonferroni correction for multiple simultaneous amino acid comp risons (none achieved P < 0.0025). No amino acid was significantly different from control in the nerve transection groups (all P > 0.05). Vitreous protein level was significantly higher in glaucomatous eyes than their paired controls at I day (P < 0.0001) and I week (P < 0.002). One day and I week after optic nerve transection, vitreal proteins were significantly elevated compared with control eyes from untreated animals (P < 0.0020 and P < 0.0022, respectively), though not compared with their fellow eyes (P = 0.25 and 0.10). Conclusion: Chronic experimental glaucoma and transection of the optic nerve increase the amount of protein in the rat vitreous above control levels. In the vitreous of rats with experimental glaucoma, a number of free amino acids were transiently elevated to a modest degree, but no significant difference in vitreous glutamate concentration was detected (P > 0.01).