Which is an important signaling molecule activated during both high-intensity interval training and submaximal endurance exercise?

• A. 5' adenosine monophosphate activated protein kinase
• B. Nuclear factor kappa B
• C. PGC-1α
• D. mTOR

Answer: (A)

During exercise, the muscle cell faces a sudden surge in energy demand as ATP is rapidly used. The key signal that rises when energy becomes scarce is AMPK, an energy-sensing enzyme that acts as a cellular thermostat. When the ATP to AMP/ADP ratio shifts toward energy depletion, AMPK becomes activated by phosphorylation through upstream kinases and then coordinates the metabolic response.

Once active, AMPK promotes processes that generate ATP and conserves energy. It enhances glucose uptake and utilization, increases fatty acid oxidation, and inhibits energy-consuming anabolic processes. Importantly, AMPK also reinforces mitochondrial biogenesis by activating PGC-1α, helping the muscle adapt to repeated workouts by expanding its oxidative capacity. This makes AMPK a central messenger for adaptations to both high-intensity interval training, which imposes strong, acute energy stress, and submaximal endurance exercise, which sustains energy demand over a longer period.

While PGC-1α plays a crucial role in driving mitochondrial biogenesis, it does so downstream of AMPK and other signals, serving as a transcriptional coactivator rather than the initial energy-sensing alert. mTOR, on the other hand, tends to be more prominent when energy and nutrients are abundant and is suppressed by energy stress typical of endurance and HIIT. NF-κB is linked more to inflammatory responses than to the core metabolic adaptations driving endurance training.