Aqueous ascorbate concentration in hereditary buphthalmic rabbits

Ascorbate in aqueous humor augments nitric oxide production by macrophages

Immunosuppressive molecules within the aqueous humor (AqH) are thought to preserve ocular immune privilege by inhibiting proinflammatory NO production by macrophages (MΦs). Consistent with previous observations, we observed that although MΦs stimulated in the presence of AqH expressed NO synthase 2 (NOS2) protein, nitrite concentrations in culture supernatants, an indirect measure of NO production, did not increase. Interestingly, NOS2 enzymatic activity, as measured by the conversion of L-arginine (L-Arg) into L-citrulline, was augmented in lysates of MΦs stimulated in the presence of AqH. These data suggested that intracellular L-Arg may have been limited by AqH. However, we observed increased mRNA expression of the L-Arg transporter, cationic amino acid transporter 2B, and increased L-Arg uptake in MΦs stimulated in the presence of AqH. Arginases were expressed by stimulated Ms, but competition for L-Arg with NOS2 was excluded. Expression of GTP cyclohydrolase, which produces tetrahydrobiopterin (H(4)B), an essential cofactor for NOS2 homodimerization, increased after M stimulation in the presence or absence of AqH and NOS2 homodimers formed. Taken together, these data provided no evidence for inhibited NOS2 enzymatic activity by AqH, suggesting that a factor within AqH may have interfered with the measurement of nitrite. Indeed, we observed that nitrite standards were not measurable in the presence of AqH, and this effect was due to ascorbate in AqH. Controlling for interference by ascorbate revealed that AqH augmented NO production in MΦs via ascorbate, which limited degradation of H(4)B. Therefore, AqH may augment NO production in macrophages by stabilizing H(4)B and increasing intracellular L-Arg.

Tear film and ocular surface changes in a rabbit model of neurotrophic keratitis

The authors studied the tear film and ocular surface in a rabbit model of neurotrophic keratitis to determine the extent to which the surface disease of neurotrophic keratitis resembled keratoconjunctivitis sicca. After denervation, tear film osmolarity increased and remained significantly elevated for 14 weeks. The ocular surface developed decreased conjunctival goblet cell density, decreased corneal epithelial glycogen, and morphologic changes similar to those seen in keratoconjunctivitis sicca. Although the conjunctival changes were consistent with the increases in tear film osmolarity and the surface disease of keratoconjunctivitis sicca, the corneal changes observed with denervation, including slit-lamp findings, morphologic changes, and decreases in glycogen, were too severe and rapid in onset to be accounted for by osmolarity alone. Neurotrophic "keratitis" is an ocular surface disease composed in part of the surface disease of keratoconjunctivitis sicca. However, the data also support an additional mechanism for corneal disease that could be due to the trophic influence of the trigeminal nerve.

Light-induced avian glaucoma as an animal model for human primary glaucoma

Glaucoma can be induced in domestic chicks at the will of the investigator, by the simple device of rearing the chicks under continuous light. This light-induced avian glaucoma (LIAG) is presented as an animal model system for human open-angle glaucoma. A number of morphological and physiological findings in LIAG are reviewed, and the LIAG system is compared with several other glaucoma model systems, in dogs, rabbits and monkeys. Intraocular pressure in LIAG has been demonstrated to be responsive to several anti-glaucoma drugs, and the system could be used for further drug testing. Thus it is suggested that LIAG may be especially useful in studies seeking to understand human glaucoma, and how to forestall it, or treat it. As well, a prolonged "pre-glaucoma" period is available to the investigator working with LIAG, during which a pathological course is already underway in the eye, but intraocular pressure has not yet gone up.

Evidence for an hereditary defect in taurine transport in the ciliary epithelium of an inbred strain of rabbits

The concentration of taurine in the aqueous humour and serum of 21 rabbits with hereditary buphthalmia (Bu rabbits-genotype bu/bu) was compared with the aqueous and serum taurine levels of eight strain-related normal rabbits (JAX) and nine non-strain-related normal rabbits (MCV). There was a significant difference in the mean aqueous taurine concentration in each of the three groups. The Bu rabbits had only 29% of the MCV rabbits' level while the JAX rabbits were intermediate with 56% of the MCV level. It is suggested that some of the JAX rabbits may be heterozygous and the Bu rabbits homozygous for a semi-dominant allele of a gene that is less efficient in taurine transport in the ciliary epithelium than the normal allele represented by the MCV animals.

Diurnal mitochondrial changes in avian retinal pigment epithelium: a search for correlation with thyroid state

The avian retinal pigment epithelium (RPE) is well endowed with mitochondria, 10-15% of which appear to be "donut-shaped". The number of such mitochondria presenting ring profiles varies with the photoperiod, with a peak early in the dark period and a low early in the light period. This cycle is inversely correlated with the diurnal rhythm of plasma thyroxine (T4), but is in phase with the T3 cycle. We could detect no effect on the numbers of ring mitochondria when exogenous TSH, T4 or T3 was injected. Numbers and appearance of ring mitochondria were likewise unaltered in chicks breathing pure oxygen or spent air for 15 min. before sacrifice. We speculate that this aberrant mitochondrial form may have some relationship to the processing of shed outer segment discs, also a photoperiod-driven phenomenon. It has been suggested that the daily manufacture of hydrolytic enzymes for breakdown of disc proteins is probably highly energy-expensive. We also note that the peroxidation of the lipid components of these membranes must be another aspect of disc disposal. Ascorbic acid, whose photooxidation in the eye generates hydrogen peroxide, may play a role in this process.