A 52-year-old man with type 2 diabetes mellitus presents with persistent hyperglycemia despite oral antidiabetic therapy. His fasting blood glucose is 180 mg/dL and HbA1c is 9.2%. However, his blood glucose improves significantly after 30 minutes of brisk walking. The structure marked **D** in the diagram represents GLUT4 vesicle translocation to the plasma membrane in skeletal muscle. Which of the following best explains why exercise-induced glucose uptake occurs in this patient despite impaired insulin signaling?

Explanation

## Why Exercise stimulates GLUT4 translocation via the AMPK pathway, which is insulin-independent and bypasses the defective insulin receptor signaling is right The clinical anchor states that **GLUT4 is the insulin-dependent glucose transporter**, but crucially, **exercise stimulates GLUT4 translocation by an INSULIN-INDEPENDENT mechanism (AMPK pathway)**. This explains the paradox: in type 2 diabetes, insulin resistance impairs the normal insulin-triggered translocation of GLUT4 (marked **D**) to the plasma membrane via the PI3K/Akt pathway. However, muscle contraction during exercise activates AMP-activated protein kinase (AMPK), which directly phosphorylates TBC1D4 (formerly AS160) and promotes GLUT4 vesicle translocation without requiring functional insulin signaling. This is why exercise lowers blood glucose even in insulin-resistant patients. (Harper 32e Ch 19; Guyton & Hall 14e Ch 79) ## Why each distractor is wrong - **Exercise increases circulating insulin levels, which overcomes the insulin resistance and restores normal GLUT4 translocation**: This is incorrect because the clinical anchor explicitly states that exercise stimulates GLUT4 translocation by an **insulin-independent mechanism**. Insulin resistance in type 2 diabetes is not overcome by exercise-induced insulin secretion; rather, exercise bypasses the defective insulin signaling entirely via AMPK. - **Exercise inhibits gluconeogenesis in the liver, reducing hepatic glucose output without requiring GLUT4 translocation**: While exercise does reduce hepatic glucose output, this does not explain the **direct mechanism of glucose uptake in skeletal muscle via GLUT4 translocation** marked **D**. The question specifically asks about the mechanism of glucose uptake in muscle, not hepatic glucose production. - **Exercise increases expression of GLUT1 in skeletal muscle, which is the primary glucose transporter during physical activity**: This is incorrect. GLUT1 is the basal glucose transporter in red blood cells and brain and is **not** the primary glucose transporter in skeletal muscle during exercise. **GLUT4 is the insulin-dependent glucose transporter in skeletal muscle**, and it is GLUT4 translocation (marked **D**), not GLUT1, that is responsible for exercise-stimulated glucose uptake in muscle. **High-Yield:** GLUT4 translocation in muscle can be triggered by **two independent pathways**: (1) insulin → PI3K/Akt (defective in type 2 DM), and (2) muscle contraction → AMPK (preserved in type 2 DM) — this is why exercise works even when insulin signaling fails. [cite: Harper 32e Ch 19; Guyton & Hall 14e Ch 79]