Topical verapamil lowers outflow facility in the rabbit eye

Results of studies examining the mechanism of the ocular hypotensive effect of topical calcium channel blockers are controversial. Whereas evidence obtained in perfused human eyes indicates that these drugs lower intraocular pressure by increasing the aqueous humor outflow, tonographic studies in rabbits have revealed that they reduce both the aqueous humor outflow and inflow. In order to clarify such a discrepancy, the aim of this study was to assess whether the effect of topical verapamil on the facility of aqueous humor outflow in the rabbit eye was dose-related. Total outflow facility was determined by two-level constant pressure perfusion in anesthetized rabbits. The effect of 5 different concentrations on aqueous humor outflow at 60 minutes postdrug was studied in groups of 10 rabbits each. Baseline outflow facility was also determined in a group of 15 rabbits. In order to check the reliability of the method for detecting drug-induced changes in aqueous outflow, the effect of pilocarpine was also tested. Topical verapamil was shown to lower outflow facility in the rabbit eye in a dose-related fashion. On the contrary, topical pilocarpine was found to significantly increase outflow facility. Our data indicate that topical verapamil reduces outflow facility in the rabbit eye.