Bunazosin reduces intraocular pressure in rabbits by increasing uveoscleral outflow

The mechanism of the ocular hypotensive effect of bunazosin hydrochloride (an alpha1-adrenergic antagonist) and the possible intermediary role of prostaglandins were studied in New Zealand albino rabbits. Aqueous flow, outflow facility and uveoscleral outflow were determined by fluorophotometry, and intraocular pressure (IOP) was measured by pneumatonometry on the fourth day of twice daily topical treatment with 0.1% bunazosin. Uveoscleral outflow was measured with a tracer infusion technique at 1 to 2 hours after one dose of 0.1% bunazosin. Total outflow facility was measured by a two-level constant-pressure infusion method before and at one hour after one dose of 0.1% bunazosin. The effect of topically applied cyclooxygenase inhibitors, including 0.25% indomethacin and 0.03% flurbiprofen, on the IOP reduction after bunazosin was evaluated. At 3 hours after the seventh consecutive dose given twice-daily, bunazosin significantly (P<0.001) reduced IOP to 13.4+/-0.8 mm Hg (mean +/- SEM) from a baseline of 19.6+/-1.1 mm Hg. Indomethacin significantly inhibited the IOP reduction after one dose of bunazosin, whereas flurbiprofen did not (repeated measures ANOVA). Bunazosin significantly increased uveoscleral outflow (P<0.05) and total outflow facility (P<0.02), but not fluorophotometric outflow facility or aqueous flow. It is concluded that, in rabbits, 0.1% bunazosin reduces IOP predominantly by increasing uveoscleral outflow. The role of prostaglandins in this effect is equivocal.