Electrically silent Na⁺ and Cl^- fluxes across the rabbit ciliary epithelium

Unidirectional Na⁺, Cl^-, and mannitol fluxes were measured across the isolated and short-circuited rabbit iris-ciliary body as functions of the Cl^- concentration in the bathing solutions (0, 26 and 80 mM). At constant Na⁺ concentration, Na⁺ fluxes increased on the average from 8.0 to 11.9 μeq/hr as the Cl^- concentration was raised and vice versa. Cl^- fluxes also increased more than expected from simple diffusion; from 2.0 (26 mM) to 8.1 μeq/hr (80 mM). Mannitol permeability (9.0 x 10^-7 cm/sec) was independent of the Cl^- concentration and similar to that measured in 'tight' epithelia. In Cl^--free solutions, there was good agreement between the measured electrical resistance and that calculared with the partial conductance equation. In Cl^--rich solutions, the calculated resistance was smaller than the measured resistance, suggesting electrically silent fluxes of Na⁺ and Cl^-. These silent fluxes were of similar magnitude and possibly coupled with each other. This mechanism may provide an additional pathway for electrolyte movement across the ciliary epithelium.