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.