Effects of ethacrynic acid on intraocular pressure of anesthetized rats

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.