Ascorbate-stimulated active Na+ transport in rabbit ciliary epithelium

Area of Science:

• Ocular Physiology
• Epithelial Transport
• Biochemistry

Background:

• Sodium (Na+) transport is crucial for aqueous humor secretion.
• High physiological Na+ concentrations obscure active transport mechanisms in the ciliary epithelium.
• Active Na+ transport saturates at lower concentrations, necessitating experiments at reduced Na+ levels.

Purpose of the Study:

• To investigate net sodium (Na+) transport across the isolated rabbit ciliary epithelium.
• To determine the role of ascorbic acid in modulating Na+ transport.
• To elucidate the relationship between Na+ transport and aqueous humor production.

Main Methods:

• Experiments were conducted on isolated rabbit ciliary epithelium.
• Reduced Na+ concentrations (30 mM) were used to isolate active transport.
• Fluxes of Na+ were measured in both directions (blood-to-aqueous and aqueous-to-blood).
• The effects of ascorbic acid and ouabain on Na+ transport were assessed.

Main Results:

• A net blood-to-aqueous Na+ flux was detected at reduced Na+ concentrations.
• Ascorbic acid significantly increased blood-to-aqueous Na+ flux.
• Ouabain inhibited both basal and ascorbate-stimulated Na+ transport.
• The ascorbate-mediated increase in Na+ flux was also observed at near-physiological Na+ levels (100 mM).

Conclusions:

• The rabbit ciliary epithelium actively transports Na+ into the posterior chamber.
• Aqueous ascorbate stimulates this Na+ transport, implicating it in aqueous humor secretion.
• While Na+ transport is stimulated by ascorbate, it accounts for only a small fraction of total aqueous humor production.