Histamine and prostacyclin. Primary and secondary release in allergic conjunctivitis

The relationship between the release of histamine, a major mast cell mediator of conjunctival type I reactions, and the production of a prostanoid, prostacyclin (prostaglandin I2, PGI2), was examined in a guinea pig model of allergic conjunctivitis. Guinea pigs were sensitized topically and challenged by repeated conjunctival instillation of fluoresceinyl ovalbumin. Histamine and 6-keto-PGF1 alpha, the stable product of the spontaneous degradation of PGI2, were measured in tears by radioimmunoassays. Clinical type I reactions and tear histamine appeared by 8 days and increased up to 22 days during the initial sensitization, with notable variations between animals. The kinetics of histamine and 6-keto-PGF1 alpha release in tears were examined over a 24-hr period after the antigen challenge. Histamine release was maximal during the first 10 min and returned to baseline values by 1 hr in all instances. The 6-keto-PGF1 alpha release also peaked during the first 10 min but continued for an extended period. The ratio of tear 6-keto-PGF1 alpha to histamine increased more than 16-fold over the 2 hr after antigen challenge. Late-phase reactions with second peaks of histamine or 6-keto-PGF1 alpha in the tears were observed in two different guinea pigs 4-8 hr after antigen challenge. Histamine applied to the eyes of naive guinea pigs also induced the release of 6-keto-PGF1 alpha in tears. Histamine appeared to act as a primary mediator, stimulating the secondary production and release of PGI2 by constitutive (eg, vascular) and possibly infiltrating inflammatory cells during an allergic conjunctival reaction.