Circadian variations of transforming growth factor-β2 and basic fibroblast growth factor in the rabbit aqueous humor

Purpose. To compare levels of transforming growth factor-beta2 (TGF-beta2) and basic fibroblast growth factor (bFGF) in the ocular fluids of rabbits at various times of the day. Methods. Seventy-two young adult New Zealand albino rabbits were entrained to a daily 12-hour light (6 AM to 6 PM) and 12-hour dark cycle. Eight rabbits were sacrificed at each of 6 time points: 4 AM, 8 AM, noon, 4 PM, 8 PM, and midnight. Under an acute 24-hour constant dark condition, 8 additional rabbits were sacrificed at each of 3 time points in the accustomed light period (8 AM, noon, and 4 PM). Samples of aqueous humor and vitreous humor were collected from both eyes. Using enzyme immunoassay, samples from one eye were assayed for total TGF-beta2 and active form of TGF-beta2 and samples from the other eye were assayed for bFGF. Concentrations at various time points were analyzed using one-way ANOVA and post-hoc Bonferroni t-test for multiple comparisons. Results. Aqueous humor levels of total TGF-beta2, active TGF-beta2, and bFGF changed during the 24-hour period. Total TGF-beta2 and bFGF levels increased between 4 AM and 8 AM. Active TGF-beta2 level increased between 8 PM and midnight. Similar patterns of changes were found when considering aqueous humor samples collected under the acute constant dark condition. In the vitreous humor, total TGF-beta2, active TGF-beta2, and bFGF remained unchanged throughout the 24 hours. Conclusions. Endogenous circadian rhythms of total TGF-beta2 ,active TGF-beta2, and bFGF occur in the rabbit aqueous humor. These two extracellular signaling molecules may be associated with specific physiological functions in the anterior segment according to the time of the day.