Fig. 1. Two examples of unidirectional and net fluxes of 10 µmol l^–1 [¹⁴C]2,4-dichlorophenoxyacetic acid (2,4-D) in freshly isolated shark IVth choroid plexus (CP) under short-circuited conditions. (A) Unidirectional absorptive [cerebrospinal fluid-to-blood (CSF-to-Bl)] and secretory [Bl-to-CSF] fluxes of 2,4-D in paired halves of IVth CP mounted in Ussing chambers under short-circuited conditions in elasmobranch Ringer solution. Fluxes were initiated at t=0 by addition of isotopically labeled 2,4-D. Steady-state flux was apparent by t=0.5 h. (B) Alternate method of determining unidirectional and net absorptive flux of 2,4-D in a single one-half IVth CP under short-circuited conditions. Flux was initiated at t=0 by addition of isotopically labeled 2,4-D to the CSF compartment without inhibitor. At t=1 h, 100 µmol l^–1 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) was added to both blood and CSF compartments, and 10 mmol l^–1 para-aminohippuric acid (PAH) was added only to the CSF compartment (arrow); unidirectional flux was measured for an additional hour. Net flux was calculated as the difference between the initial 1-h flux (total flux) and second 1-h flux (inhibitor-insensitive flux).