Resistance to blood flow in the rabbit ophthalmic artery after topical treatment with timolol

Purpose: The aim of this study was to investigate the resistance to blood flow in the ophthalmic artery of rabbits receiving topical treatment with timolol. Methods: Eight (8) New Zealand albino rabbits received 20 mu l of timolol treatment (vehicle, 0.1%, 0.33%, 1%, and 3.3%) on the right eye. Blood-flow velocity in the ophthalmic artery was determined in the treated eye using color Doppler imaging (CDI) with a 12-MHz linear ultrasound transducer prior to the treatment and at 0.5, 1, 1.5, 2, and 3 h after the treatment. Intraocular pressure (IOP) was measured in both eyes, using a pneurnatonometer at the same time points. Pourcelot's resistive index of blood flow was calculated, using the peak systolic velocity and the end diastolic velocity. A control experiment was performed with CDI obtained from the right eye when the left eye was treated with 1% timolol. Results: In the eye treated with 1% and 3.3% timolol, a dose-dependent increase in the resistive index of blood flow occurred in the ophthalmic artery. No change in the resistive index occurred when the contralateral eye was treated with 1% timolol. Changes of IOP were not different between the two eyes under all the experimental conditions. Timolol, at all concentrations, caused a significant reduction of heart rate. A similar reduction of heart rate occurred when either eye was treated with 1% timolol. Conclusions: Topical treatment with timolol in rabbits can increase the resistance to blood flow in the ophthalmic artery. This effect is caused by a mechanism local to the eye and is not dependent on an IOP change.