Electron microprobe analysis of rabbit ciliary epithelium indicates enhanced secretion posteriorly and enhanced absorption anteriorly

Abstract

The rate of aqueous humor formation sequentially across the pigmented (PE) and nonpigmented (NPE) ciliary epithelial cell layers may not be uniform over the epithelial surface. Because of the tissue's small size and complex geometry, this possibility cannot be readily tested by conventional techniques. Rabbit iris-ciliary bodies were divided, incubated, quick-frozen, cryosectioned, and freeze-dried for electron probe X-ray microanalysis of the elemental contents of the PE and NPE cells. We confirmed that preincubation with ouabain to block Na(+),K(+)-ATPase increases Na(+) and decreases K(+) contents far more anteriorly than posteriorly. The anterior and posterior regions were the iridial portion of the primary ciliary processes and the pars plicata, respectively. Following interruption of gap junctions with heptanol, ouabain produced smaller changes in anterior PE cells, possibly reflecting higher Na(+) or K(+) permeability of anterior NPE cells. Inhibiting Na(+) entry selectively with amiloride, benzamil, or dimethylamiloride reduced anterior effects of ouabain by approximately 50%. Regional dependence of net secretion was also assessed with hypotonic stress, which stimulates ciliary epithelial cell regulatory volume decrease (RVD) and net Cl(-) secretion. In contrast to ouabain's actions, the RVD was far more marked posteriorly than anteriorly. These results suggest that 1) enhanced Na(+) reabsorption anteriorly, likely through Na(+) channels and Na(+)/H(+) exchange, mediates the regional dependence of ouabain's actions; and 2) secretion may proceed primarily posteriorly, with secondary processing and reabsorption anteriorly. Stimulation of anterior reabsorption might provide a novel strategy for reducing net secretion.

Responses of Sodium–Hydrogen Exchange to Nitric Oxide in Porcine Cultured Nonpigmented Ciliary Epithelium

“…There is direct evidence that NO donors SNP, SNAP, and l -arginine reduce aqueous formation 8. The evidence of an effect of NHE inhibitors DMA and EIPA on aqueous secretion is less direct,22 but McLaughlin et al44 have reported clear effects of NHE inhibitors on the ciliary epithelial bilayer, as evidenced by changes in cytoplasmic ion levels detected by x-ray microanalysis.…”

Mechanism of Aqueous Humor Secretion, Its Regulation and Relevance to Glaucoma

“…The situation is complex, however, because NPE cells also may reabsorb some of the solute and water from the aqueous humor back into the stroma (Hu et al, 2011;McLaughlin et al, 2007). The algebraic sum of these secretion and reabsorption processes determines the net secretion rate of AH into the posterior chamber.…”

Differential volume regulation and calcium signaling in two ciliary body cell types is subserved by TRPV4 channels

“…The aqueous fluid supplies nutrients and oxygen to nonvascularized tissues (lens, cornea, and trabecular meshwork) and is ultimately drained through the ciliary muscle and the trabecular meshwork in the anterior chamber of the eye. Aqueous secretion is subserved by the unidirectional transport of ions and water through gap junctions between PE cells and NPE cells (3,4) and is driven by the osmotic gradient generated by Na + /K + exchange across basolateral NPE membranes (2)(3)(4)(5). Despite the critical dependence of aqueous humor secretion on osmotic pressure (1,4,6), the molecular mechanism through which NPE and PE cells sense and regulate changes in volume is not well understood.…”