PROPERTIES OF THE CAMP-ACTIVATED CL- CURRENT IN CHOROID-PLEXUS EPITHELIAL-CELLS ISOLATED FROM THE RAT

1. This study used whole-cell patch clamp and RNA in situ hybridizatio n experiments to determine whether the cAMP-activated Cl- current expr essed in choroid plexus epithelial cells was carried by the cystic fib rosis transmembrane conductance regulator (CFTR) channel. 2. In patch clamp experiments, inclusion of 0.25 mM cAMP and 375 nM protein kinase A catalytic subunit (PKA) in the electrode solution caused activation of an inwardly rectifying current (21/23 cells). This current was Cl- selective, since the current reversal potential (E(rev)) was -31 +/- 3 mV with equilibrium potential values for Cl- (E(Cl)) and Na+ (E(Na)) of -44 and 0 mV, respectively. 3. In anion substitution experiments, the relative anion permeability sequence for the inward rectifier was: I- (3.5) > HCO3- (1.5) = Cl- (1.0) > Br- (0.6) > aspartate (0.2). 4. The inward rectifier was sensitive to inhibition by a range of known c hannel inhibitors, including: glibenclamide (100 mu M), DIDS (100 and 500 mu M), NPPB (100 mu M) and Ba2+ (1 mM). 5. In RNA in situ hybridiz ation experiments, using two independent rat CFTR cRNA probes, express ion of CFTR could not be detected in epithelial cells from the rat cho roid plexus. 6. In conclusion, the cAMP-dependent whole-cell Cl- curre nt present in choroid plexus epithelial cells from the rat has propert ies which are distinctly different from those of CFTR.