In a patient with chronic metabolic acidosis, ammonium excretion rises several fold above baseline. Which cell type and substrate account for this adaptive increase?

• A Alpha-intercalated cells metabolizing beta-hydroxybutyrate to yield ammonium
• B Thick ascending limb cells metabolizing citrate to yield ammonium
• C Proximal tubular cells metabolizing glutamine to yield ammonium and new bicarbonate ✓
• D Collecting duct principal cells metabolizing alanine to yield ammonium

Explanation

Chronic acidosis induces glutaminase and related enzymes in proximal tubular cells, increasing deamination of glutamine to ammonium plus bicarbonate. Each glutamine molecule yields two ammonium ions, excreted into the lumen as NH4+, and two new bicarbonate ions returned to blood via Na/HCO3 cotransport. This adds truly new base, unlike filtered bicarbonate reclamation. Intercalated cells secrete pre-existing protons and cannot generate new bicarbonate from glutamine.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.