Which feature best distinguishes gluconeogenesis in the liver from that in the kidney during prolonged fasting?

Explanation

## Gluconeogenesis: Hepatic vs Renal Contribution ### Temporal Contribution Pattern **Key Point:** During the first 24 hours of fasting, the liver accounts for ~90% of gluconeogenesis. After 48 hours, the kidney progressively takes over and eventually contributes 40% of total glucose production, while the liver contribution decreases. ### Enzymatic Capacity Comparison | Feature | Liver | Kidney | |---------|-------|--------| | **PEPCK expression** | High, constitutive | Low initially; ↑ with prolonged fasting | | **Glucose-6-phosphatase** | Present; releases free glucose | Present; releases free glucose | | **FBPase-1** | High activity | Moderate activity | | **Glutaminase** | Low | High; key for amino acid catabolism | | **Glycogenolysis** | Primary early response | Minimal glycogen stores | ### Substrate Utilization **High-Yield:** - **Liver:** Primarily uses lactate (Cori cycle), amino acids, and glycerol; glycogenolysis dominates first 12 hours. - **Kidney:** Predominantly uses glutamine (40–50% of renal gluconeogenesis substrate after 48 hours) and other amino acids; minimal glycogen. ### Temporal Shift Mechanism 1. Early fasting (0–24 h): Liver glycogenolysis + hepatic gluconeogenesis dominate. 2. Intermediate (24–48 h): Glycogen depleted; renal PEPCK expression begins to increase. 3. Prolonged fasting (>48 h): Kidney gluconeogenesis becomes the major source; renal PEPCK is upregulated by cortisol and glucagon. **Clinical Pearl:** In starvation ketoacidosis, the kidney's contribution to gluconeogenesis is critical for maintaining blood glucose when hepatic glycogen is exhausted. This is why renal dysfunction in prolonged fasting increases hypoglycemia risk. ### Why Other Options Are Incorrect - **Option 1 (Acetyl-CoA inhibits liver gluconeogenesis):** Acetyl-CoA actually *activates* pyruvate carboxylase (first committed step of gluconeogenesis), promoting hepatic gluconeogenesis. - **Option 2 (Kidney lacks glucose-6-phosphatase):** The kidney *does* express glucose-6-phosphatase, allowing it to release free glucose into the bloodstream. - **Option 3 (Liver uses only amino acids):** The liver uses multiple substrates: lactate, glycerol, amino acids, and glycogenolysis; it is not restricted to amino acids alone.