Circadian variations of prostaglandins in the rabbit aqueous humor

Forty-eight young adult New Zealand albino rabbits were entrained to a daily 12-hr light and 12-hr dark cycle. Under a constant dark environment, rabbits were sacrificed at 4-hr intervals, beginning at 2 hr before the accustomed lights-on period. Eight rabbits were used for each of the 6 time points within 24 hr. Aqueous humor and vitreous humor samples were collected. Samples from one eye of each rabbit were assayed for prostaglandins D-2, E-2, and F-2 alpha by enzyme immunoassay. Total protein concentrations in the samples were also determined. Concentrations of prostaglandins D-2 and F-2 alpha in the aqueous humor were found to vary in a consistent pattern. The prostaglandin F-2 alpha concentration increased in the early subjective light period, remained high in this period, and decreased in the subjective dark period. The prostaglandin D-2 concentration was elevated only in the early subjective light period. No significant change of aqueous humor prostaglandin E-2 level was found. Total protein concentration in the aqueous humor varied in a similar pattern as the aqueous humor prostaglandin F-2 alpha, indicating a close association between the permeability of protein across the blood-aqueous barrier and the endogenous prostaglandin F-2 alpha level. In the vitreous humor, concentrations of all three prostaglandin subtypes remained unchanged, as did the total protein concentration. However, vitreous humor concentrations of prostaglandin D-2 and F-2 alpha were significantly higher than their concentrations in the aqueous humor.