A researcher administers a drug that inhibits Na+/K+-ATPase specifically in the choroid plexus. The expected primary effect on CSF composition is:

• A Increased CSF glucose due to reduced active transport of glucose into CSF
• B Increased CSF protein due to breakdown of tight junctions in choroid plexus
• C Increased ICP due to reduced CSF absorption
• D Reduced CSF sodium and volume, since active Na+ transport by choroidal epithelium drives osmotic water movement that generates CSF secretion ✓

Explanation

CSF is actively secreted by choroid plexus epithelial cells, not simply an ultrafiltrate. Na+/K+-ATPase on the basolateral membrane of choroidal epithelium pumps Na+ out of the cell into blood, keeping intracellular Na+ low. Na+ enters the cell from CSF side via apical Na+ channels and cotransporters. The primary secretion is driven by active Na+ transport across the basolateral membrane into the choroidal stroma/capillaries, creating an osmotic gradient that draws water. Carbonic anhydrase also contributes HCO3- and H2O. Inhibiting Na+/K+-ATPase would reduce Na+ transport, decrease CSF secretion rate and volume. Acetazolamide (carbonic anhydrase inhibitor) similarly reduces CSF production and is used to treat intracranial hypertension. CSF protein is normally low due to tight junctions, which are unaffected by Na+/K+-ATPase inhibition.

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

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