Transepithelial transport of ascorbic acid by the isolated intact ciliary epithelial bilayer of the rabbit eye

Expression Profiling of Ascorbic Acid-Related Transporters in Human and Mouse Eyes

Purpose

Ascorbic acid (AsA) is an important antioxidant in the eye. Ascorbic acid is usually transported by sodium-dependent AsA transporters (SVCTs), and dehydroascorbic acid (DHA) by glucose transporters (GLUTs). This study investigates these AsA-related transporters in human compared with mouse eyes.

Methods

Five pairs of human donor eyes and 15 pairs of mouse eyes were collected. Immunofluorescence and in situ hybridization were performed to detect SVCTs and GLUTs expression in the ciliary epithelium, retina, and lens epithelial cells (LECs). These tissues were isolated with laser microdissection followed by extraction of total RNA. Quantitative PCR (qPCR) was performed to examine the mRNA level of SVCTs and GLUTs in human and mouse ocular tissues.

Results

Immunofluorescence and in situ hybridization showed SVCT2 and GLUT1 expression in human ciliary epithelium with varied distributions. Sodium-dependent AsA transporter 2 is expressed only in the pigmented epithelium (PE), and GLUT1 is predominately expressed in the nonpigmented epithelium (NPE). However, SVCT2 was not identified in mouse ciliary epithelium, whereas GLUT1 expressed in both PE and NPE. Laser microdissection and qPCR revealed high levels of SVCT2 mRNA in human RPE cells and murine neural retina. Sodium-dependent AsA transporter 1 mRNA could be detected only in human and murine LECs. Glucose transporter 3 and GLUT4 mRNA could not be detected in either the human or mouse ciliary processes or in the lens epithelium.

Conclusions

These fundamental findings indicate AsA transporter expression in eyes of humans is significantly different compared with mice. This may explain why human aqueous and vitreous humors contain higher AsA levels compared with other animals.

Differential effects of ascorbate on endothelium-derived hyperpolarizing factor (EDHF)-mediated vasodilatation in the bovine ciliary vascular bed and coronary artery

1. The ability of ascorbate to inhibit endothelium-derived hyperpolarizing factor (EDHF)-mediated vasodilatation was compared in the bovine perfused ciliary vascular bed and isolated rings of coronary artery. 2. Acetylcholine-induced, EDHF-mediated vasodilatation of the ciliary circulation was blocked following inclusion of ascorbate (50 micro M, 120 min) in the perfusion fluid. The blockade was highly selective since ascorbate had no effect on the vasodilator actions of the K(ATP) channel opener, levcromakalim, nor on the tonic vasodepressor action of basally released nitric oxide. 3. The possibility that concentration of ascorbate by the ciliary body was a prerequisite for blockade to occur was ruled out, since EDHF was still blocked when the anterior and posterior chambers were continuously flushed with Krebs solution or when both the aqueous and vitreous humour were drained. 4. Ascorbate at 50 micro M failed to affect bradykinin- or acetylcholine-induced, EDHF-mediated vasodilatation in rings of bovine coronary artery. Raising the concentration to 3 mM did produce blockade of EDHF, but this was nonselective, since vasodilator responses to endothelium-derived nitric oxide were also inhibited. 5. Thus, ascorbate (50 micro M) is not a universal blocker of EDHF. Whether its ability to block in the bovine ciliary circulation, but not in the coronary artery, is due to differences in the nature of EDHF at the two sites, differences in vessel size (resistance arterioles versus conduit artery), the presence or absence of flow, or to some other factor remains to be determined.

Fluid transport by human nonpigmented ciliary epithelial layers in culture: a homeostatic role for aquaporin-1

We report for the first time that cultured nonpigmented human ciliary epithelial (NPE) cell layers transport fluid. Cells were grown to confluence on permeable membrane inserts, and fluid transport across the resulting cell layers was determined by volume clamp at 37 degrees C. These cell layers translocated fluid from the apical to the basal side at a steady rate of 3.6 microl x h(-1) x cm(-2) (n = 4) for 8 h. This fluid movement was independent of hydrostatic pressure and was completely inhibited by 1 mM ouabain, suggesting it arose from fluid transport. Mercuric chloride, a nonspecific but potent blocker of Hg(2+)-sensitive aquaporins, and aquaporin-1 antisense oligonucleotides both partially inhibited fluid transport across the cell layers, which suggests that water channels have a role in NPE cell homeostasis. In addition, these results suggest that of the two ciliary epithelial layers in tandem, the NPE layer by itself can transport fluid. This cultured layer, therefore, constitutes an interesting model that may be useful for physiological and pharmacological characterization of ciliary epithelial fluid secretion.

Microdialysis and drug delivery to the eye

The eye presents unique challenges in both the development of tools for elucidating drug disposition as well as for the development of modes of drug delivery for treatment of ocular diseases. In this paper, we present a discussion of the anatomical and physiological characteristics and limitations present in the eye for microdialysis sampling of endogenous substrates and xenobiotics. To date, over twenty papers describing microdialysis approaches for assessment of ocular drug delivery and endogenous substrate characterization have been published. Although the majority of papers describe sampling of vitreous humor, recent efforts have been directed towards ocular anterior segment sampling using microdialysis. With this approach, an appreciable reduction in animal use has been realized. In addition, simultaneous examination of administered drug and endogenous substrates modulated by the drug is possible with this approach, facilitating construction of ocular pharmacokinetic/pharmacodynamic relationships through use of relevant surrogate markers.

Novel paracrine signaling mechanism in the ocular ciliary epithelium

The ciliary body contains an epithelial bilayer consisting of an outer pigmented cell layer (PE) and an inner nonpigmented cell layer (NPE) responsible for aqueous humor secretion. Secretion may be mediated in part by cytosolic Ca2+ concentration ([Ca2+]i), but whether or how the two layers could coordinate their Ca2+ signals to regulate secretion is unclear. To investigate interactions between PE and NPE, we examined [Ca2+]i signaling in isolated intact ciliary epithelial bilayers using confocal microscopy. Phenylephrine selectively increased [Ca2+]i in PE and acetylcholine increased [Ca2+]i in NPE, but epinephrine increased [Ca2+]i in both layers. This increase spread from PE to NPE, and [Ca2+]i signaling across the bilayer remained coordinated during [Ca2+]i oscillations. All epinephrine-induced [Ca2+]i signaling was blocked by the alpha1-adrenergic antagonist prazosin, whereas signaling in the NPE but not PE was blocked by the beta-adrenergic antagonist propranolol, the gap junction blockers octanol and 18alpha-glycyrrhetinic acid, or the A kinase inhibitor Rp diastereomer of adenosine 3',5'-cyclic monophosphothioate. The beta-adrenergic agonist isoproterenol failed to increase Ca2+ by itself, but isoproterenol plus phenylephrine-induced [Ca2+]i signals across the bilayer similar to those induced by epinephrine. Finally, isoproterenol increased cell-to-cell spread of lucifer yellow via gap junctions, whereas cell-to-cell spread of [Ca2+]i signals could be induced by photorelease of caged inositol 1,4,5-trisphosphate. Thus, calcium signals are coordinated in the epithelial bilayer so that adrenergic stimulation can increase [Ca2+]i in NPE, but only if NPE are primed by activation of endogenous adenylyl cyclase, whereupon they receive stimulation from adjacent PE via gap junctions. This novel interplay between endocrine and paracrine pathways may coordinate [Ca2+]i signaling across the ciliary epithelial bilayer.