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A.C. Zamudio, R. Gerometta, L. Barrios, O.A. Candia; Fluid Transport across the Isolated Rabbit and Bovine Ciliary Body . Invest. Ophthalmol. Vis. Sci. 2003;44(13):3445.
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Purpose. The relative contribution of secretion (active transport) and utrafiltration (pressure gradient) to aqueous humor (AH) production remains controversial. Furthermore, the predominantly transported ion in the isolated rabbit iris-ciliary body (ICB) is HCO3, whereas in bovine ICB it is Cl. It is expected that in the isolated ICB, without a pressure gradient across it, only active transport will drive fluid movement giving a measure of secretion. Previous measurements of ionic transport by labeled fluxes or short-circuit current predict a relatively low (≈15%) contribution of secretion in both rabbit and bovine tissues. However, direct measurements of fluid secretion (Jv) across the isolated ICB had not been made. Methods. We designed a chamber in which the complete ring of ICB can be clamped between 2 pairs of O-rings, one just external to the pupil, and the other pair internal to the ora serrata, exposing most of the ciliary processes to the bathing solutions. Results. The actually measured Jv (3.5 µl/hr . cm2) was corrected by a factor of 8 that accounted for the fraction of ICB exposed to the bathing solutions, and the possible collapse of the processes. Even after these incremental corrections, Jv was 28 µl/hr per tissue in the rabbit and 27 µl/hr per tissue in bovine, or about 19 and 14% respectively of the in vivo production of AH, in line with the previous prediction based on ionic transport measurements. Importantly, Jv was inhibited by removals of HCO3 in rabbit and of Cl in bovine. It was reduced by 60% in both species by ouabain. Conclusion. The contribution of secretion as determined in the isolated rabbit and bovine ICBs represents only a fraction of the in vivo AH production. Different active transport mechanisms seem to drive a similar rate of fluid transport in the two species.
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