Abstract
Purpose: :
Based on our previous findings that sildenafil increases IOP and protein content in the AC of sheep1, we investigated the possibility that it also accelerates the refilling of the AC after AH withdrawal. We posit that the refilling occurs via our hypothesized "ciliary stroma-AC" (CS-AC) pathway, bypassing the transport mechanisms of the ciliary epithelium.
Methods: :
Twelve Corriedale sheep were selected. IOP was measured with the Perkins tonometer1. After IOP measurement, either 60 or 120 µl of AH was withdrawn from 1 eye of 6 sheep. The refilling of the AC was followed by observation using the oblique illumination, and IOP measurements were continued as the AC formed. After IOP stabilization, 100 mg of sildenafil was orally administered to the sheep. Eighty min later, 60 or 120 µl was withdrawn from the contralateral eye. Refilling time was considered the point at which both eyes exhibited the same IOP. AH turnover was estimated from previously determined facility measurements and normal 10 mm Hg IOP2. In a second group of 6 sheep, IOP was measured in both eyes and immediately 300-320 µl of AH was withdrawn from one eye and the sheep were given 100 mg oral sildenafil. Sixty min later, the AC from which AH was withdrawn was reconstituted and IOP was re-measured in both eyes.
Results: :
IOP recovered to control values in 52 and 58 min after the 60 and 120 µl withdrawals. Sildenafil increased IOP from 10 to 20 mm Hg in 80-90 min. The refilling times of the contralateral eye after ingestion of sildenafil were 18 and 26 min after the 60 and 120 µl AH withdrawals respectively. We can estimate the AH turnover to be at least 3.3 and 4.6 µl/min after the 60 and 120 µl withdrawals in the presence of sildenafil. In contrast, the rate of refilling without sildenafil was about 1-2 µl/min. These are minimum estimates since simultaneous with refilling there is outflow of AH consequent to the build up in pressure in the AC. After the 320 µl withdrawal, IOP recovered in 60 min indicating a minimum 6 µl/min refilling rate.
Conclusions: :
The rate of refilling of the AC in control conditions may be accounted for by the osmotic-driven flow across the ciliary epithelium, although its value may be influenced by the variable pressure-dependent outflow that occurs simultaneously. The minimum value for the rate of AC refilling after sildenafil far exceeds the capability of the ciliary epithelium transport system and may be accounted for by a parallel flow directly into the AC. These results support the idea that a CS-AC flow may exist when pressure in the AC or CS is altered. 1Invest Ophthalmol Vis Sci 2010;51:3139; 2Arch Ophthalmol.2010;128(3):338-343.
Keywords: anterior chamber • intraocular pressure • inflow/ciliary body