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R. Gerometta, O. A. Candia, C. W. Kong, A. C. Zamudio; Ascorbate and K Concentration Differences Between the Anterior and Posterior Chambers of Live Cows and Rabbits. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3708.
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Current interpretations of aqueous humor dynamics indicate that all fluid entering the anterior chamber (AC) comes from the posterior chamber (PC) via the pupil. Thus, the solute composition of fluid in the AC should equal that of the PC. This equality will not hold for any solute with a higher concentration in the PC than plasma, due to active transport by the ciliary epithelium (CE), if there is a dilution in the AC by fluid directly entering the AC across the anterior surface of the iris (because such entering fluid would have a solute content resembling plasma). Given that the CE actively transports ascorbate (Asc) so that aqueous levels are 25 to 50-fold higher than plasma in most species, and we gave evidence of the active transport of K across the bovine CE (To et al, Am J Physiol 280, 2001), we measured AC v PC levels of Asc in rabbit and cow, and of K in the cow to detect a possible difference that will support a flow from the iris.
Under topical anesthesia, aqueous samples of AC and PC were obtained with 2 separate needles inserted through the cornea; the tip of the first needle was positioned at the angle and the second needle was placed into the PC under the iris via the pupil. 20 µl samples were taken from each chamber for the Asc assay (Liu et al, Clin Chem 28, 1982), while 100 µl samples of bovine AC and PC were used for [K] determination with a K-sensitive electrode.
Rabbit Asc levels were 1.05 ± 0.03 and 1.42 ± 0.06 mM (± SE; n= 14; P< 0.001 as paired data) in the AC and PC, or 36% higher in the PC. With the bovine samples, the Asc values were 0.96 ± 0.07 and 1.12 ± 0.07 mM (± SE; n= 6; P< 0.01), or 17% higher in the PC. In the case of the bovine [K], the respective levels were 3.58 ± 0.13 and 4.56 ± 0.23 mM (n= 22, P< 0.001), or 22% higher in the PC.
The difference in Asc concentration between PC and AC is consistent with works by Kinsey (AMA Arch Ophthalmol 50, 1953), who showed that PC [Asc] is higher than the AC and by Freddo (Exp Eye Res 73, 2001), who showed that plasma-derived proteins are conveyed directly to the AC from the ciliary body stroma, via the anterior surface of the iris. This direct pathway to the AC implies that fluid can also passively enter the eye via this route and contribute towards "aqueous humor formation".
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