THE BOVINE ARTERIALLY-PERFUSED EYE - AN IN-VITRO METHOD FOR THE STUDYOF DRUG MECHANISMS ON IOP, AQUEOUS-HUMOR FORMATION AND UVEAL VASCULATURE

A method is reported in which the isolated bovine eye is perfused thro ugh a long posterior ciliary artery with buffered physiological saline , to provide simultaneous monitoring of drug effects on intraocular pr essure (IOP), vascular resistance and the condition of the blood-aqueo us barrier. With perfusion under constant pressure of 45 mm Hg, perfus ate flows at 1.64 +/- 0.12 ml.min-1 (mean +/- SEM) and IOP in 7.26 +/- 0.16 mm Hg. Applying a constant flow rate of 2.25 ml.min-1, IOP avera ges 10.19 +/- 0.32 mm Hg and in both cases this can be maintained for around 2h. Increasing the perfusion flow rate from 1.5 to 3.5 ml.min-1 produces a 76% rise in perfusion pressure but IOP increases only insi gnificantly (<10%). The inclusion in the perfusion fluid of dextran an d albumin to maintain oncotic pressure similar to that of plasma makes no difference to the IOP achieved and does not affect the leakiness o f the barrier. The preparation shows a net consumption of oxygen, supp orting the hypothesis that the aqueous humour formed is secreted by ac tive transport processes. Timolol (in bolus doses of 1-300 nmol) injec ted into the perfusing fluid is shown to induce a dose-dependent fall in IOP within 5 min, reaching a steady state within 40 min. Timolol, h owever, causes no significant change in vascular resistance, whether t his is measured as perfusion flow rate under constant pressure or as p erfusion pressure at constant flow rate, nor does it alter the permeab ility of the barrier. Other beta-blockers such as oxprenolol and betax olol also induce dose-dependent decreases in IOP. By applying a fluore scein dilution technique, it is found that the aqueous formation rate (K(out) = 0.0046 min-1, or 12.9 mul.min-1) is also reduced by timolol and, in a dose-dependent manner, by the new carbonic anhydrase inhibit or, MK-927. The bovine perfused eye offers a useful method for studyin g the mechanisms of action of drugs on IOP and aqueous humour formatio n, in isolation from the complicating influences of the CNS and the ca rdiovascular system and without the necessity to kill animals for expe rimental purposes.