Timolol effects on aqueous humor dynamics in eyes of anesthetized rats

Anterior chambers of the eyes of male rats were cannulated under pentobarbi tal anesthesia for intracameral infusions of balanced salt solution (BSS) a nd intraocular pressure (IOP) recording. Blood pressure was recorded from a femoral artery. IOP was recorded during a 2-h intracameral infusion compos ed of a constant component (0.05 mu l/min) and a periodic component (0.25 m u l/min), cycling at 4 min on and then 4 min off. After a 20-min baseline p eriod, 1 drop of timolol (0.5%) or BSS was applied to the cornea and repeat ed 1 h later. Intracameral infusions of BSS and 0.05% timolol were also com pared. Topical timolol slightly delayed the BSS-induced IOP rise (p <.05). Complex demodulation and the estimated gain parameter of a second-order tra nsfer function fit to the periodic responses revealed that topical timolol also reduced (p <.05) passive outflow resistance. Intracameral timolol mark edly delayed the BSS-induced rise in IOP. Initially, timolol decreased both outflow impedance and nonresistive components do <.05) of IOP, but these e ffects dissipated by 2 h when IOPs were similar. In all experiments, within -group blood pressure was unchanged. Topical and intracameral timolol have different effects on IOP. The data support the opinion that, in vivo, timol ol acts at p-receptors that control both outflow impedance and nonresistive mechanisms, probably vascular, to lower IOP.