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R. Gerometta, O. A. Candia, A. C. Zamudio, C.-W. Kong, B. Wu, J. Leiva; Fluid Flow From the Ciliary Body to the Anterior Chamber. Invest. Ophthalmol. Vis. Sci. 2007;48(13):4892.
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It is widely assumed that all of the aqueous humor (AH) that flows into the anterior chamber (AC) comes from the posterior chamber (PC) (Bill A., Exp Eye Res, 1973). However, most measurements of AH flow are based on exit from the AC, not flow across the pupil. T. Freddo has shown that high molecular-weight substances can easily diffuse from the ciliary body (CB) to the AC across the anterior face of the iris (Exp Eye Res, 2001). It is then possible that part of the fluid leaving the capillaries of the CB flows directly to the AC as a parallel pathway to the secretion into the PC. If there is a flow from the CB to the AC, the [Cl] (Cl is actively transported into the PC in the cow) will be lower in the AC than in the PC and closer to that in the plasma. Our purpose was to investigate if such a [Cl] difference between the chambers exists.
Two protocols were implemented to test for a possible [Cl] difference. A) In 9 unconscious cows, a needle containing in its body 2 Ag/AgCl electrodes 7 mm apart, was introduced into the AC so that the electrodes detected the [Cl]’s of both the angle and an area anterior to the pupil simultaneously. The PD between the 2 electrodes should be proportional to the difference in [Cl]’s of the AH at the 2 sites. B) Cow eyes were excised from the orbit and frozen in liquid nitrogen within 30 seconds of the animal death. Upon dissection of adjacent tissues, the frozen AC and PC fluids were separated as 2 pieces of ice and collected in different tubes. After thawing, [Cl], [Na] and [K] were measured with a Nova Analyzer®. This latter protocol was also implemented in 10 rabbits, although instead of freezing the eyes, the fluids of the AC or PC were quickly aspirated from each contralateral eye.
A) On average [Cl] at the pupil was 9 mM higher (- 2mV difference) than at the angle. B) [Cl]’s of AC, PC and plasma were respectively 101.8, 120.8 and 106.2 mM. [K] was consistently higher in the PC (13.5 mM) than in the AC (4.1 mM).Contrary to the cow, the rabbit does not transport Cl and thus, the average [Cl] was larger in the AC than the PC: 106.3 and 101.4 mM respectively. Plasma [Cl] was 107.9 mM.
We propose that part of the fluid flow into the AC comes directly from that in the CB stroma, thus changing the [Cl] to approximate that of the plasma. This interpretation could also explain why inhibition of active transport mechanisms in the ciliary epithelium (Larsson L & Alm A, Arch Ophthalmol, 1998; Shahidullah M et. al., Invest Ophthalmol, 2003) cannot totally stop AH flow.
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