CSF is produced primarily by the choroid plexus via an active secretory process. Which ion transport mechanism is the rate-limiting step in CSF secretion?

• A Carbonic anhydrase-dependent HCO3 secretion and Na+ cotransport into the ventricle
• B Passive filtration driven by choroidal capillary hydrostatic pressure
• C Na+/K+-ATPase on the apical (ventricular) surface of choroid plexus epithelium ✓
• D Aquaporin-1 (AQP1) water channels on basolateral surface

Explanation

CSF secretion by the choroid plexus epithelium is energetically active and depends critically on Na+/K+-ATPase located on the apical (ventricular-facing) membrane. This pump extrudes Na+ into the ventricle, creating an osmotic gradient that drives water secretion via AQP1 channels. Carbonic anhydrase contributes by supplying HCO3 and H+, facilitating ion transport, but Na+/K+-ATPase is the primary driver. Acetazolamide, a carbonic anhydrase inhibitor, reduces CSF production (used in idiopathic intracranial hypertension) but so do ouabain-related inhibitors of the ATPase.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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