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 solution (0, 26 and 80 mM). At constant Na+ concentration, Na+ fluxes increased on the average from 8.0 to 11.9 mu eq/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 mu eq/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 calculated 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.