Fig. 4. Trimethylamine oxide (TMAO)-sensitivity of zinc-induced change in active cerebrospinal fluid-to-blood (CSF-to-blood) [¹⁴C]2,4-dichlorophenoxyacetic acid (2,4-D) fluxes across paired halves of shark IVth choroid plexus. Fluxes of 2,4-D were determined after tissues were exposed, ± 72 mmol l^–1 TMAO (13.5°C), to treatment as follows. Control, 6 h in zinc-free medium + 1.5 h recovery in L-15E; Zinc-treated, 6 h in 50 µmol l^–1 ZnSO₄ + 1.5 h recovery in L-15E. Tabulated data show the mean unidirectional fluxes from which net data were derived; B to C, blood-to-CSF flux; C to B, CSF-to-blood flux. Data are means ± s.e.m. (N=6). ^*Significant effect of zinc at P<0.05.