Epinephrine Triggers Rapid Body Responses Through cAMP Signaling
- Imagine you’re running a race. Your heart pounds, your breathing quickens, and energy surges through your muscles.
- This rapid response is orchestrated by epinephrine (also known as adrenaline), a hormone that prepares your body for action.
- But how does epinephrine trigger these changes so quickly?
- The answer lies in a sophisticated communication system involving receptors, G proteins, and a second messenger called cyclic AMP (cAMP).
- The terms "adrenaline" and "epinephrine" are used interchangeably.
- "Adrenaline" is more common globally, while "epinephrine" is prevalent in North America.
- “Adrenaline” comes from Latin ad = at and ren = kidney
- “Epinephrine” comes from old Greek epi = above and nephros = kidney, respectively.
Epinephrine Activates the Fight-or-Flight Response via Adrenergic Receptors
Definition
Epinephrine
Epinephrine, also known as adrenaline, is a hormone and neurotransmitter produced by the adrenal glands.
- Epinephrine plays a critical role in the fight or flight response, activating various physiological changes in response to stress or danger.
- The action of epinephrine is mediated through specific receptors, known as adrenergic receptors, which are part of the G protein-coupled receptor (GPCR) family.
Step 1: Epinephrine Binds to Its Receptor
- Epinephrine is a hormone secreted by the adrenal glands during stress or excitement.
- It travels through the bloodstream and targets specific cells, such as those in the heart, liver, and muscles.
- These target cells have specialized proteins called G-protein-coupled receptors (GPCRs) embedded in their plasma membranes.
- When epinephrine binds to its receptor, it triggers a conformational change (a change in shape) in the receptor.
- This change is the first step in transmitting the signal into the cell.
GPCRs are the most common type of receptor in humans, involved in detecting hormones, neurotransmitters, and even light.
Step 2: Activation of the G Protein
- Attached to the inner side of the plasma membrane is a G protein, which acts as a molecular switch.
- It consists of three subunits: alpha (α), beta (β), and gamma (γ).
- In its inactive state, the alpha subunit is bound to a molecule called GDP (guanosine diphosphate).
- When the receptor changes shape, it interacts with the G protein, causing the GDP to be released and replaced by GTP (guanosine triphosphate).
- This exchange activates the G protein, which then dissociates into two parts: the alpha subunit (now bound to GTP) and the beta-gamma dimer.
Don’t confuse GDP with GTP. GDP is the inactive form, while GTP is the active form that powers the G protein.
Step 3: Activation of Adenylyl Cyclase
- The activated alpha subunit of the G protein moves along the plasma membrane and binds to an enzyme called adenylyl cyclase.
