Adrenaline
Mechanism of action

As a hormone, adrenaline acts on nearly all body tissues by binding to adrenergic receptors. Its effects on various tissues depend on the type of tissue and expression of specific forms of adrenergic receptors. For example, high levels of adrenaline cause smooth muscle relaxation in the airways but causes contraction of the smooth muscle that lines most arterioles.

Adrenaline is a nonselective agonist of all adrenergic receptors, including the major subtypes α, α, β, β, and β. Adrenaline's binding to these receptors triggers a number of metabolic changes. Binding to α-adrenergic receptors inhibits insulin secretion by the pancreas, stimulates glycogenolysis in the liver and muscle, and stimulates glycolysis and inhibits insulin-mediated glycogenesis in muscle. β adrenergic receptor binding triggers glucagon secretion in the pancreas, increased adrenocorticotropic hormone (ACTH) secretion by the pituitary gland, and increased lipolysis by adipose tissue. Together, these effects increase blood glucose and fatty acids, providing substrates for energy production within cells throughout the body. Binding of β adrenergic receptor also increases the production of cyclic AMP.

Adrenaline causes liver cells to release glucose into the blood, acting through both alpha and beta-adrenergic receptors to stimulate glycogenolysis. Adrenaline binds to β receptors on liver cells, which changes conformation and helps G, a heterotrimeric G protein, exchange GDP to GTP. This trimeric G protein dissociates to G alpha and G beta/gamma subunits. G alpha stimulates adenylyl cyclase, thus converting adenosine triphosphate into cyclic adenosine monophosphate (AMP). Cyclic AMP activates protein kinase A. Protein kinase A phosphorylates and partially activates phosphorylase kinase. Adrenaline also binds to α adrenergic receptors, causing an increase in inositol trisphosphate, inducing calcium ions to enter the cytoplasm. Calcium ions bind to calmodulin, which leads to further activation of phosphorylase kinase. Phosphorylase kinase phosphorylates glycogen phosphorylase, which then breaks down glycogen leading to the production of glucose.

Adrenaline also has significant effects on the cardiovascular system. It increases peripheral resistance via α receptor -dependent vasoconstriction and increases cardiac output by binding to β receptors. The goal of reducing peripheral circulation is to increase coronary and cerebral perfusion pressures and therefore increase oxygen exchange at the cellular level. While adrenaline does increase aortic, cerebral, and carotid circulation pressure, it lowers carotid blood flow and end-tidal CO or E CO levels. It appears that adrenaline improves microcirculation at the expense of the capillary beds where perfusion takes place.
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