Regarding the cellular mechanisms of insulin action, all of the following are true EXCEPT:

Explanation

## Mechanisms of Insulin Action ### Correct Statements (Options 0, 1, 2) **Option 0: Tyrosine kinase receptor mechanism** - Insulin binds to its receptor (a transmembrane tyrosine kinase) - This triggers autophosphorylation of the receptor's intracellular domain - Activates downstream signalling cascades (PI3K/Akt, MAPK pathways) **Option 1: GLUT4 translocation** - Insulin promotes translocation of GLUT4 glucose transporters from intracellular vesicles to the cell membrane - Occurs in skeletal muscle and adipose tissue (insulin-responsive tissues) - Increases glucose uptake by 10–20 fold in these tissues **Option 2: Glycogen regulation via phosphodiesterase** - Insulin activates phosphodiesterase (PDE), which breaks down cAMP - Decreased cAMP → reduced PKA activity - PKA normally phosphorylates phosphorylase kinase (activating glycogenolysis) - Therefore, insulin **inhibits** glycogen breakdown — **TRUE** ### Incorrect Statement (Option 3) **Option 3: Insulin and lipolysis — THE TRAP** **Key Point:** Insulin is an **anabolic hormone** that **inhibits** lipolysis, not promotes it. - Insulin **decreases** hormone-sensitive lipase (HSL) activity by dephosphorylation - This occurs because insulin ↓ cAMP → ↓ PKA → HSL remains dephosphorylated (inactive) - Insulin **increases** triglyceride synthesis (esterification of fatty acids) - Insulin **decreases** fatty acid mobilization from adipose tissue The statement claims insulin **increases** lipolysis — this is **backwards**. Glucagon and catecholamines increase lipolysis; insulin opposes this. ### Summary Table: Insulin vs. Glucagon Effects | Process | Insulin | Glucagon | | --- | --- | --- | | Glycogenolysis | ↓ Inhibits | ↑ Promotes | | Gluconeogenesis | ↓ Inhibits | ↑ Promotes | | Lipolysis | ↓ Inhibits | ↑ Promotes | | Lipogenesis | ↑ Promotes | ↓ Inhibits | | Protein synthesis | ↑ Promotes | ↓ Inhibits | | Glucose uptake (muscle, fat) | ↑ Promotes | No effect | **High-Yield:** Remember insulin as the **storage hormone** — it promotes anabolism (synthesis) and inhibits catabolism (breakdown). Lipolysis is a **catabolic** process, so insulin opposes it.