Intravenously administered vasodilatory prostaglandins increase retinal and choroidal blood flow in rats

We established an experimental system for measuring blood flow in the rat fundus and examined whether intravenously administered vasodilatory prostaglandins (PGE₁, PGE₂, and PGI₂), 8-(4-chlorophenylthio)-cAMP (a cAMP analogue), and nicardipine (a Ca ²⁺-channel blocker) increase fundus blood flow (FBF). Under artificial ventilation, rats were injected with tetrodotoxin (50 μg/kg, i.v.) to eliminate any nerve activity and prevent movement of the eye. After tetrodotoxin, the rats were infused with norepinephrine (0.3 - 0.5 μg·kg^-1 ·min^-1) and epinephrine (2.7 - 4.5 μg·kg^-1·min^-1) simultaneously to maintain adequate systemic circulation. We found that intravenous infusion of PGE₁ (2 - 10 μg·kg^-1·min^-1), PGE₂ (3 - 30 μg·kg^-1·min^-1), and PGI₂ (1 - 10 μg·kg^-1·min ^-1) increased the FBF in a dose-dependent manner. The vasodilatory PGs decreased arterial pressure, whereas they did not affect heart rate. Like vasodilatory PGs, 8-(4-chlorophenylthio)-cAMP (30 μmol/kg, i.v.) increased FBF and decreased arterial pressure. While infusion of nicardipine (0.3 - 3 μg·kg^-1·min^-1) produced comparable depressor responses with those to vasodilatory PGs and the cAMP analogue, it did not increase FBF. These results suggest that vasodilatory PGs and cAMP act more selectively than Ca²⁺-channel blockers on retinal /choroidal blood vessels. Therefore, the vasodilatory PGs might be considered to be possible candidates for the therapeutics to treat disorders of retinal/choroidal circulation.