Ascorbate-stimulated active na⁺ transport in rabbit ciliary epithelium

In a physiological medium (134 mM Na⁺ concentration), unidirectional blood-to-aqueous and aqueous-to-blood Na⁺ fluxes across the isolated rabbit ciliary epithelium are large, rendering the detection of a net transport difficult. At 134 mM an active component for Na⁺ may be obscured by diffusional fluxes and a bidirectional Na⁺-Cl^- cotransport. Considering that the active transport saturates at about 30 mM, experiments were performed at this reduced Na concentration to minimize the influence of diffusional pathways. A net blood-to-aqueous Na⁺ flux that ranged from 0.25 to 0.81 /xeq/hr was obtained. Addition of ascorbic acid to the aqueous side under this condition increased the blood-to-aqueous flux with little effect on the flux in the opposite direction. Ouabain inhibited both the Na⁺ and ascorbate-stimulated Na⁺ transport. The increase in blood-to-agueous Na flux by ascorbate was also observed in tissues bathed with [Na⁺] closer to physiological levels (100 mM). These results indicate that the rabbit ciliary epithelium transports Na into the posterior chamber. Since aqueous ascorbate stimulates Na⁺ transport, it may be implicated in both Na movement and aqueous humor secretion. However, the rate of Na⁺ transport can only account for a small fraction of total aqueous humor production.