The role of ion channels in aqueous humor formation.
Jacob, T. J. C., and M. M. Civan.
Eye Research Lab, Physiology Unit, School of Molecular & Medical
Biosciences, University of Wales, Cardiff CF1 3US, UK and Departments
of Physiology and Medicine, University of Pennsylvania School of
Medicine, Philadelphia, PA 19104-6085
APStracts 3:0037C, 1996.
The aqueous humor is secreted by the ciliary epithelium, a bilayered
syncytial epithelium comprising a pigmented (PE) cell layer abutting
the stroma and a nonpigmented (NPE) cell layer facing the aqueous
phase. As in other epithelia, secretion depends on the transfer of
solute, with water passively following. Na+, K+ and Cl- enter the
syncytium principally through a Na+/K+/2Cl- symport, diffusing to the
aqueous surface of the NPE cells. The Na+, K+ and Cl- are secreted
into the aqueous humor through the Na,K-exchange pump, K+ channels
and Cl- channels, respectively. Na+ is also secreted between the
cells in response to a small transepithelial potential. The K+
channels are critical not only for K+ release but also for
hyperpolarizing the membrane, providing an electrical driving force
for Cl- secretion. Some of the K+ channels are Ca2+ sensitive, and
can be activated by Ca2+ entry through T- and L-type Ca2+ channels.
The roles of the ciliary epithelial nonselective and Na+ channels are
less clear. The review describes the ion channels thus far identified
in the ciliary epithelium in terms of the activation and inactivation
of their macroscopic currents, the open probabilities and
conductances of the single channels, and their locations and
regulation. The review relates each class of channel to known
families of channels, and indicates how those channels can contribute
to the secretion of the aqueous humor.
Received 2 January 1996; accepted in final form 4 January 1996.
APS Manuscript Number C48-6.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 8 February 96