During prolonged exercise, alanine released from skeletal muscle is taken up by the liver and converted to glucose, which returns to muscle. Beyond carbon transfer, what additional physiological role does the glucose-alanine cycle serve?

• A It transports toxic ammonia generated in muscle to the liver as nontoxic alanine nitrogen ✓
• B It regenerates NAD+ within skeletal muscle to sustain glycolysis
• C It supplies ATP directly to hepatocytes during hypoxic stress
• D It buffers hydrogen ions produced by muscle lactate formation

Explanation

In muscle, pyruvate receives amino groups via transamination from branched-chain amino acid breakdown, forming alanine. The liver converts alanine back to pyruvate and channels the nitrogen into urea synthesis. Thus the cycle simultaneously delivers gluconeogenic carbon and safely exports muscle-derived nitrogen for urea disposal. Unlike the Cori cycle, it does not regenerate NAD+ in muscle; redox balance in muscle during exercise depends on lactate formation.

Reference: Harper Illustrated Biochemistry, 32nd ed.

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