Epinephrine is a hormone and neurotransmitter.It is catecholomine, a sympathomimetic monoamine derived from the amino acids phenylalanine and tryosine,Epinephrine is often shortened to epi or to EP in American medical jargon.It is also referred to as adrenaline Epinephrine is a "fight or flight" hormone, and plays a central role in the short-term stress reaction. It is released from the adrenal glands when danger threatens or in an emergency. Such triggers may be threatening, exciting, or environmental stressor conditions such as high noise levels, or bright light and high ambient temperature Action of Epinephrine Epinephrine is one of many hormones that is water soluble(hydrophilic) and therefore unable to cross the hydrophobic plasma membranes of its target cells.Instead it binds to receptor proteins located in the plasma membrane and does not enter the cell. when Epinephrine binds to beta-adrenergic receptors on teh liver cell, G proteins on the inner side of the cell membrane are activated.Each G protein is composed to three subunits and the binding of epinephrine to its receptor protein causes one of the G protein subunits to dissociate from the other two. The G protein subunit which dissociates from the others carries a GDP,which is replaced by GTP when the subunit is activated The activated G protein subunit then diffuses within the plasma membrane until it encounters adenylyl cyclase, a membrane enzyme that is inactive until it interacts with the G protein subunit. When activated by the G protein subunit,adenylyl cyclase that formation of cAMP from ATP.The cAMP formed at the inner surface of the membrane diffuses within the cytoplasm,where it binds to and activates protein kinase-A, An enzyme that adds phosphate groups to specific cellular proteins. In liver cells, protein kinase-A phosphorylates and thereby activates another enzyme called phosphorylase,which converts glycogen into glucose-6-phosphate.The glucose-6-phosphate is then converted to glucose. Through this multistage mechanism,epinephrine causes the liver to secrete glucose into the blood during the fight-or-flight reaction. Epinephrine is used as a drug to treat cardiac arrest and other cardiac dysrhythmias resulting in diminished or absent cardiac output; its action is to increase peripheral resistance via α1-adrenoceptor vasoconstriction, so that blood is shunted to the body's core, and the β1-adrenoceptor response which is increased cardiac rate and output (the speed and pronouncement of heart beats). This beneficial action comes with a significant negative consequence—increased cardiac irritability—which may lead to additional complications immediately following an otherwise successful resuscitation. Alternatives to this treatment include vasopressin, a powerful antidiuretic which also increases peripheral vascular resistance leading to blood shunting via vasoconstriction, but without the attendant increase in myocardial irritability. Due to its suppressive effect on the immune system, epinephrine is the drug of choice for treating anaphylaxis. It is also useful in treating sepsis. Allergy patients undergoing immunotherapy may receive an epinephrine rinse before the allergen extract is administered, thus reducing the immune response to the administered allergen. It is also used as a bronchodilator for asthma if specific beta2-adrenergic receptor agonists are unavailable or ineffective. Because of various expression of α1 or β2-receptors, depending on the patient, administration of epinephrine may raise or lower blood pressure, depending whether or not the net increase or decrease in peripheral resistance can balance the positive inotropic and chronotropic effects of epinephrine on the heart, effects which respectively increase the contractility and rate of the heart.