A patient with type 2 diabetes on metformin develops lactic acidosis post-contrast CT scan. The mechanism by which metformin predisposes to lactic acidosis under contrast nephropathy is:

• A Contrast dye combines with metformin in the renal tubules to form a toxic compound that destroys mitochondria directly
• B Contrast dye competitively inhibits OCT2 (organic cation transporter-2) renal tubular secretion of metformin, raising plasma levels
• C Metformin directly inhibits complex I of the mitochondrial respiratory chain, impairing NADH oxidation and shifting pyruvate to lactate; renal impairment allows metformin accumulation above the threshold for this effect ✓
• D Metformin increases glucagon-like peptide secretion which stimulates hepatic gluconeogenesis from lactate, depleting lactate clearance pathways

Explanation

Metformin inhibits mitochondrial complex I (NADH dehydrogenase) in hepatocytes, which reduces ATP production and increases NADH/NAD+ ratio, diverting pyruvate to lactate. Additionally, metformin inhibits hepatic gluconeogenesis from lactate. At therapeutic doses, plasma concentrations are low and the effect is manageable. However, in acute kidney injury (as can occur with nephrotoxic contrast agents), renal clearance of metformin falls (OCT2 and MATE1/2 are responsible for tubular secretion), metformin accumulates, and intrahepatic concentrations rise to levels that significantly inhibit mitochondrial respiration, triggering lactic acidosis.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.