Spontaneous fluid transport across isolated rabbit and bovine ciliary body preparations

PURPOSE. To quantify spontaneous fluid transport across the isolated ciliary bodies of rabbit and bovine and to determine their osmotic permeabilities. METHODS. A complete annulus of ciliary body was mounted in a custom-designed chamber appropriate for detecting net fluid movement across the in vitro preparation. RESULTS. A net fluid flow in the blood-to-aqueous direction was measured. It was generally observed that tissue freshness is a critical parameter for detection of such flow. The spontaneous, baseline fluid transport rate lasted, on average, ≈4 hours. This flow solely reflects the secretory activity of the isolated ciliary epithelium, since the in vitro arrangement precludes contributions from ultrafiltration. Both the isolated rabbit and bovine ciliary body epithelia transported fluid in the absence of an external osmotic or pressure gradient. After corrections for area and possible collapse of the processes, a total flux rate of approximately 23 μL/hour or 13% of the in vivo flow in rabbit was estimated. This value agrees with predictions of ionic fluxes and short-circuit current measurements, which are also obtained in vitro. The fluid flow is bicarbonate dependent in rabbit and chloride dependent in bovine, consistent with ionic transport mechanisms described in these species. Ouabain inhibited the fluid flow across both species, indicating dependence on active ionic transport. Irrespective of the spontaneous fluid transport, a flow elicited by an osmotic gradient allowed for a calculation of the osmotic permeability coefficient (Pf; ≈10-3 cm/s) in line with reports in other epithelia. In addition, mannitol permeability (5.6 × 10-6 cm/sec) was similar to that measured in "tight" epithelia, as determined by measurements of radiolabeled fluxes of the sugar across rabbit ciliary bodies mounted in the chambers used for the present fluid transport study. CONCLUSIONS. This work demonstrates that isolated ciliary epithelial preparations transport fluid in the blood-to-aqueous direction. The present observations suggest that mounting arrangements for measuring volumetric fluid flow across the ciliary epithelium is suitable for future studies directed toward the pharmacological control of secretion.