The Fight-or-Flight Response: How the Body Mobilizes Energy

When an organism encounters danger, it must decide whether to fight or flee. This rapid decision-making is supported by a series of physiological changes, including increased heart rate, faster breathing, and a surge in blood sugar levels. These responses are controlled by the release of epinephrine (adrenaline), which triggers a cascade of biochemical events to provide energy for immediate action. This article explores the critical role of signal transduction, the discovery of cyclic AMP (cAMP), and its broader significance in cellular processes.

Epinephrine and the Three-Stage Signaling Process

When epinephrine is released, it binds to receptor proteins on the surface of liver and muscle cells, initiating a three-stage process:

1. Reception – Epinephrine binds to a receptor on the cell surface.
2. Transduction – A biochemical signal inside the cell is triggered (the missing link in early research).
3. Response – Glycogen is broken down into glucose for energy.

The second stage, transduction, remained a mystery until the groundbreaking work of Earl Sutherland.

Earl Sutherland’s Discovery of cAMP

In the 1950s, Sutherland investigated how epinephrine stimulated glucose production but found that simply mixing epinephrine and enzymes in a test tube did not yield glucose. He hypothesized the presence of an intermediate chemical messenger—later identified as cyclic adenosine monophosphate (cAMP).

Sutherland discovered that when epinephrine binds to a receptor, it activates an enzyme called adenylyl cyclase, which converts ATP to cAMP. This second messenger then triggers enzyme-driven reactions that lead to glycogen breakdown and glucose release. His findings earned him the 1971 Nobel Prize in Physiology or Medicine.

The Broader Role of cAMP in Cellular Function

Beyond energy metabolism, cAMP is a vital second messenger in many biological processes across diverse organisms, including:

• Cell division and differentiation
• Ion transport and movement
• Muscle contractions
• Signal transduction in bacteria, fungi, plants, and animals

When confronted by an adversary, the choice is “fight or flight.” In either case, the body prepares itself by releasing epinephrine, which activates an increase in blood glucose. The second messenger (cAMP) provides the link between the activation of a receptor on the surface of a liver cell and the release of energy-providing glucose.