= Adrenaline
{wiki}
= Epinephrine
{synonym}
<Adrenaline>, also called <epinephrine>, is an endogenous <catecholamine> messenger: the body makes it and can release it into blood as a <hormone>. The <adrenal medulla>, the inner region of an <adrenal gland>, secretes <adrenaline> in response to <sympathetic nervous system> stimulation. A circulating <hormone> broadcasts a signal, but each receiving <biological cell> interprets it through its own <receptors> and intracellular machinery. <Adrenaline> activates both <alpha-adrenergic receptors> and <beta-adrenergic receptors>; it does not command one identical response everywhere.<Adrenal Gland by National Cancer Institute>[Hormones of the Adrenal Medulla: secretion in response to sympathetic stimulation]{ref}<Epinephrine MeSH descriptor by National Library of Medicine>[Epinephrine, D004837: Scope Note, entry term Adrenaline and Catecholamines tree placement]{ref}
Fuel mobilization is a useful example. A <beta-adrenergic receptor> can increase <adenylyl cyclase> activity through a stimulatory <G protein>, increasing the intracellular <second messenger> <cAMP>. <Protein kinase A> responds to <cAMP>; a <phosphorylation> relay through <phosphorylase kinase> activates <glycogen phosphorylase>, favoring <glycogenolysis>. Here <adrenaline> is the extracellular message, <cAMP> is the intracellular message, and <glycogen> is the reserve being mobilized. These are three different roles.<The regulatory component of adenylate cyclase. Purification and properties by P. C. Sternweis and others>[Abstract, properties 2-4: purified guanine-nucleotide regulatory component reconstitutes hormone-stimulated adenylyl cyclase]{ref}<An adenosine 3′,5′-monophosphate-dependant protein kinase from rabbit skeletal muscle by D. A. Walsh and others>[Walsh, Perkins and Krebs, 1968, printed p. 3763 Summary and introductory paragraphs: cAMP-dependent kinase and phosphorylase kinase activation]{ref}<Pharmacological Characterizations of Adrenergic Receptors in Human Adipocytes by Thomas W. Burns and others>[Abstract, second paragraph: positive beta-receptor coupling to adenylyl cyclase]{ref}
\Image[/adrenaline-beta-camp-fuel-mobilization.png]
{title=<Adrenaline> reception and <cAMP>-dependent fuel mobilization}
{description=Original pathway schematic. Solid arrows show a selected activating relay to <glycogenolysis>; the dashed branch summarizes promotion of <lipolysis> in responsive <adipocytes>, omitting its intermediate steps. Arrows denote causal influences, not transport of <adrenaline> into the <biological cell>, reaction rates or measured yields.}
The two reserves provide different material. <Glycogenolysis> draws on <glycogen>; <lipolysis> breaks down stored <triglycerides>, releasing <fatty acids> and <glycerol>. A <skeletal muscle> <biological cell>'s own reserve and fuel released by <adipose tissue> are different supplies. Increasing a signal cannot mobilize an empty reserve: receptor activation changes the use of available material rather than creating fuel. The <adrenergic adipocyte assay> below gives direct human evidence for the <lipolysis> branch.<Pharmacological Characterizations of Adrenergic Receptors in Human Adipocytes by Thomas W. Burns and others>[Abstract: epinephrine-stimulated glycerol release as an index of lipolysis]{ref}
Receptor context can even oppose this branch. In the human <adipocyte> preparation below, <beta-adrenergic receptors> promoted <cAMP> accumulation, whereas <alpha-2 adrenergic receptors> opposed it. <Alpha-1 adrenergic receptors> were associated with a different <phospholipid> readout. The balance of these responses matters more than the name of the messenger alone.<Pharmacological Characterizations of Adrenergic Receptors in Human Adipocytes by Thomas W. Burns and others>[Abstract: opposing beta and alpha-2 coupling; separate alpha-1 phospholipid response]{ref}
\Image[/adrenaline-adrenergic-receptor-context.png]
{title=Different <adrenergic receptors> interpret the same <adrenaline> signal}
{description=Original qualitative receptor-context schematic based on the human <adrenergic adipocyte assay>. An arrow denotes promotion; a flat-ended line denotes opposition. The separate <alpha-1 adrenergic receptor> branch represents the <phospholipid> readout, not a second activating arrow to <cAMP>. No response magnitude, universal tissue distribution or experimental curve is implied.}
This separation is useful when explaining <signal transduction>: ask which <receptor> is present, which relay it controls, which reserve or target is available, and which response was actually measured. <Receptor binding occupancy>, <second messenger> production and final fuel release are distinct observations. A stronger <cAMP> signal need not imply an equally large change in every downstream process; substrate availability and the rest of the receiving <biological cell> still matter.
The extract-transfer experiment under <second messenger> supplies the earlier experimental separation between <hormone> reception and an intracellular intermediary. Its existing account includes the preparations and controls. The complementary <adrenergic adipocyte assay> asks a different question: can the same <adrenaline> stimulus engage opposing <receptors> within a human preparation?
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