Epinephrine, commonly known as adrenaline, is a hormone released by the body in response to stress. Its primary function is to prepare the body for action, often called the “fight or flight” response. This preparation involves rapidly increasing glucose in the bloodstream. This surge provides fuel for muscles and the brain to react swiftly to challenging situations.
Epinephrine’s Urgent Role
Epinephrine serves as both a hormone and a neurotransmitter. It is primarily produced and released by the adrenal glands, located on top of each kidney. Small amounts are also produced by neurons in the brain. Its release is triggered by various stimuli, including physical or emotional stress, fear, and strenuous exercise.
This release is a central component of the sympathetic nervous system’s “fight or flight” response. When faced with a threat, the body requires a rapid increase in energy to either confront or escape it.
Glucose Mobilization Pathways
Epinephrine increases blood glucose through two main processes: glycogenolysis and gluconeogenesis, primarily acting on the liver and muscles. Glycogenolysis involves the breakdown of glycogen, a stored form of glucose. In the liver, epinephrine stimulates the rapid conversion of liver glycogen into glucose, which is then released directly into the bloodstream to raise blood glucose levels.
The glucose-6-phosphate produced from muscle glycogenolysis is metabolized within muscle cells to provide energy for muscle contraction, not released into circulation. Gluconeogenesis, the second pathway, involves the synthesis of new glucose from non-carbohydrate sources. The liver and kidneys convert precursors such as amino acids, lactate, and glycerol into glucose, contributing to elevated blood glucose.
The Cellular Signaling Cascade
Epinephrine initiates its effects by binding to adrenergic receptors on target cells, such as those in the liver and muscle. These receptors are part of the G-protein coupled receptor family. When epinephrine binds, it changes their shape, activating an associated G-protein.
The activated G-protein stimulates an enzyme called adenylate cyclase. This enzyme converts adenosine triphosphate (ATP) into cyclic adenosine monophosphate (cAMP), a “second messenger” within the cell. Increased cAMP activates protein kinase A (PKA). PKA then phosphorylates, or adds a phosphate group to, various enzymes involved in glucose metabolism, activating them to promote glycogenolysis and gluconeogenesis, leading to increased glucose release into the blood.