Ethacrynic acid (ECA) lowers intraocular pressure (IOP) by an effect usuall
y ascribed to increased drainage of aqueous humor by the trabecular meshwor
k. Here, we describe the effects of a continuous 2-hr intracameral infusion
of balanced salt solution (BSS), with or without 2 mM ECA (sodium salt), o
n IOP of pentobarbital anesthetized rats. The infusion was divided into a c
onstant (0.05 mu l/min) and a periodic (0.25 mu l/min) component that cycle
d 4 min on then 4 min off. This permitted the calculation of dynamic change
s in resistive (trabecular and uveoslceral drainage) and nonresistive (aque
ous synthesis, episcleral venous pressure) components of IOP by fitting a s
econd-order transfer function to the responses. ECA markedly blunted the BS
S-induced rise in IOP (P < 0.01). The rise in resistive mechanisms (ocular
impedance) was transiently blunted by ECA (P < 0.05) during the third and f
ourth 8-min cycles, and nonresistive mechanisms were reduced by ECA from cy
cles 3-10 (P < 0.05). Then, at the end of the infusion, the control and EGA
dynamic values were similar (P > 0.05), although IOP of EGA-treated rats w
as still slightly reduced (P < 0.05). The most likely explanation is a summ
ation of small changes in both resistive and nonresistive components of IOP
dynamics. Systemic blood pressure was unchanged within either group. The w
ell-known effects of ECA on the trabecular meshwork, alone, are insufficien
t to explain the dynamic changes in IOP observed in this model.