Epinephrine mobilizes glucose from hepatic glycogen stores during stress. Glucagon performs the same function in liver but has no glycogenolytic effect in skeletal muscle. The reason for this tissue specificity is:

• A Skeletal muscle lacks receptors for glucagon ✓
• B Muscle glycogen phosphorylase cannot be phosphorylated
• C Muscle cAMP is degraded too rapidly by phosphodiesterase
• D Muscle contains only the dephospho form of phosphorylase kinase

Explanation

Skeletal muscle simply does not express glucagon receptors, so the hormone cannot initiate a cAMP cascade there despite having all the downstream machinery intact. Muscle glycogenolysis is instead regulated by epinephrine via beta2-adrenergic receptors and by intracellular calcium released during contraction, which activates phosphorylase kinase directly. Muscle also lacks glucose-6-phosphatase, so it cannot export free glucose, another reason hepatic glycogen serves systemic blood glucose while muscle glycogen is used locally.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.