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 mu eq/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-aqueous 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.
Candia
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Department of Ophthalmology, Mount Sinai School of Medicine, New York, NY 10029.
Shi
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Chu
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D000818:Animals; D001082:Aqueous Humor / Q000378:metabolism; D001205:Ascorbic Acid / Q000493:pharmacokinetics*; D001693:Biological Transport, Active; D001769:Blood / Q000378:metabolism; D002924:Ciliary Body / Q000378:metabolism*; D004848:Epithelium / Q000378:metabolism; D005260:Female; D008564:Membrane Potentials; D010042:Ouabain / Q000493:pharmacokinetics; D011817:Rabbits; D012964:Sodium / Q000378:metabolism*
D010042:Ouabain; D012964:Sodium; D001205:Ascorbic Acid