Intraocular fluid dynamics. III. The site and mechanism of prostaglandin transfer across the blood intraocular fluid barriers

Abstract

When a mixture of 3H labelled prostaglandin E 1(PGE 1) and 14C-sucrose is injected into the vitreous body, 3H leaves the vitreous at a substantially higher rate ( t 1 2 = 3 hr ) than 14C ( t 1 2 = 15 hr ). High levels of 14C were observed in the aqueous humour 4 to 20 hr following intravitreal injection, indicating that much of the sucrose leaves the eye by the bulk flow. 3H was not detected in the aqueous humour at any time following the intravitreal injection; therefore, PGE 1 must be lost from the eye before it can enter the anterior chamber. Less than 3% of the originally injected activity was detected in intraocular tissues, indicating that the high rate of 3H loss is due to rapid transfer of PGE 1 across some region of the blood-ocular fluid barriers, and consequent loss into the circulation. Although it was not possible to accurately determine rates of loss of 3H and 14C from the posterior fluid following injection of 3H-PGE 1 and 14C-sucrose into the posterior chamber, such attempts showed that 3H leaves the posterior chamber several times faster than sucrose. When the same mixture was injected into the anterior chamber the rates of loss of 3H and 14C from the aqueous humour were essentially identical; thus, in contrast to posterior chamber, there is no specific mechanism to facilitate the removal of PGs from the anterior chamber of the eye. When a mixture of [ 3H]PGE 1 and [ 14C] sucrose or [ 14C] thiourea was presented to the blood side of the blood intraocular fluid barrier by short-term close arterial infusion, the 3H/ 14C ratios in the anterior and posterior aqueous and in the vitreous humour were less than that of the infusion medium. Thus, either the passive permeability of these barriers to PGE 1 is less than that of thiourea or sucrose or that, some of the PGE 1 molecules which cross these barriers are transported out again by an active process. These and previous results suggest that the ciliary processes (and/or the posterior surface of the iris) and possibly the blood-vitreal barrier possess specific transport mechanisms to facilitate the removal of prostaglandins from the extracellular fluid compartments of the posterior segment of the eye.