The epithelial cells of the choroid plexus secrete cerebrospinal fluid (CSF
), by a process that involves the movement of Na+, Cl- and HCO3- from the b
lood to the ventricles of the brain. This creates the osmotic gradient, whi
ch drives the secretion of H2O. The unidirectional movement of the ions is
achieved due to the polarity of the epithelium, i.e., the ion transport pro
teins in the blood-facing (basolateral) are different to those in the ventr
icular (apical) membranes. Saito and Wright (1983) proposed a model for sec
retion by the amphibian choroid plexus, in which secretion was dependent on
activity of HCO3- channels in the apical membrane. The patch clamp method
has now been used to study the ion channels expressed in rat choroid plexus
. Two potassium channels have been observed that have a role in maintaining
the membrane potential of the epithelial cell, and in regulating the trans
port of K+ across the epithelium. An inward-rectifying anion channel has al
so been identified, which is closely related to ClC-2 channels, and has a s
ignificant HCO3- permeability. This channel is expressed in the apical memb
rane of the epithelium where it may play an important role in CSF secretion
. A model of CSF secretion by the mammalian choroid plexus is proposed that
accommodates these channels and other data on the expression of transport
proteins in the choroid plexus. (C) 2001 Wiley-Liss, Inc.