Active transport of iodide and other anions across the choroid plexus

1. An in vitro preparation of the frog choroid plexus was used to study mechanisms of anion transport.2. It was observed that, in the absence of electrochemical potential gradients, there were net fluxes of I(-), SCN(-), TcO(4) (-), and Br(-) across the plexus, from the ventricular to the serosal surface. The net flux of I(-) reached a maximum at a concentration of 250 muM.3. On the basis of competition effects it was concluded that the affinity of the transport process for anions was: ClO(4) > ReO(4) approximately BF(4) > SCN approximately SeCN > I > NO(3) > Br > Cl.4. Ouabain, oligomycin, phloretin and 2,4-DNP inhibited the net transport of anions, but phlorrhizin, furosemide, 2,4,6-trinitro-m-cresolate, reducing agents, and antithyroid agents did not. Ouabain and phloretin were only effective on the ventricular side of the preparation.5. Anion transport required the presence of both Na and K. The requirement for Na was specific, but Rb, and to a lesser extent Cs, could substitute for K. Na in either the ventricular or the serosal fluids could partially stimulate anion transport, but K was only effective in the ventricular solution.6. TcO(4) (-), SCN(-) and I(-) were accumulated within the choroidal epithelium from the ventricular fluid, but not from the serosal fluid. Accumulation was inhibited by ouabain and ClO(4) (-).7. The unidirectional influx of I(-) across the apical cell membrane was about an order of magnitude greater than the flux across the epithelium. This flux was inhibited by ClO(4) (-), ouabain, and Na-free solutions.8. These experiments suggest the following mechanism for anion transport across the plexus: anions are actively transported into the epithelium, by a ouabain sensitive, Na/K dependent pump located in the brush border membrane. The anions are accumulated within the epithelium, and, finally, they pass into the serosal fluid down their electrochemical potential gradient. Relations between anion transport, Na/K transport, and Na/K ATPases are discussed.