During a short, intense sprint, epinephrine is released and acts on skeletal muscle cells to rapidly increase the rate of glycolysis for ATP generation. Which mechanism is most critical for coupling the epinephrine signal to this massive increase in glycolytic flux?
- PKA-mediated activation of phosphofructokinase-1, directly increasing glycolytic flux through the rate-limiting step.
- PKA-mediated phosphorylation and activation of glycogen phosphorylase, providing abundant substrate for glycolysis. (correct answer)
- PKA-mediated phosphorylation and inactivation of the muscle isoform of pyruvate kinase, redirecting flux to lactate production.
- Epinephrine-mediated activation of protein phosphatases, which dephosphorylate and activate key glycolytic enzymes.
Explanation: The primary role of epinephrine in muscle is to mobilize internal glycogen stores for immediate energy needs. The epinephrine-cAMP-PKA cascade activates glycogen phosphorylase, which rapidly breaks down glycogen into glucose-6-phosphate. This massive increase in substrate concentration drives glycolytic flux forward through substrate-level control, providing ATP for muscle contraction. Note that the muscle isoform of pyruvate kinase is not subject to PKA-mediated phosphorylation like the liver isoform.