Two types of chloride channel in the apical membrane of rat choroid plexus epithelial cells.

The patch clamp technique has been used to study ion channel activity in the apical (ventricular) membrane of epithelial cells from the rat choroid plexus. Two different classes of Cl(-)-selective channel were identified. A low conductance (26 pS) channel which was the predominant feature in cell-attached and inside-out patches. The occurrence of this channel appeared to increase in tissue bathed in forskolin. It was activated in inside-out patches by increasing the Ca2+ concentration at the intracellular face of the membrane and by depolarising potentials. The second class of channel was observed infrequently (2% of patches) and appeared to be similar to 'maxi'-Cl- channels which have been described in many other cell types. It had a conductance of 320 pS, opened to sub-conductance levels and displayed a marked voltage dependence in inside-out patches. The possible contribution of these channels to Cl- transport during the production of cerebrospinal fluid (CSF) is discussed.