Assignment 4

Citation:

Dewachter, P., Raëth-Fries, I., Jouan-Hureaux, V., Menu, P., Vigneron, C., Longrois, D., and Mertes, P. M. 2007. A Comparison of Epinephrine Only, Arginine Vasopressin Only, and Epinephrine Followed by Arginine Vasopressin on the Survival Rate in a Rat Model of Anaphylactic Shock. Anesthesiology. 106: 977-983. PMID: 17457129

Pubmed link here!


Figure 1: Symptoms of Anaphylaxis (From Medline Plus)


Summary:

Introduction, Materials and Methods

It has been well established in scientific literature as well as medical practice that epinephrine is the most effective in treating anaphylactic shock [1]. Anaphylactic shock is an acute systemic, severe reaction leading to systemic vasodilation (associated with a sudden drop in blood pressure) and edema of bronchial mucosa (leading to bronchoconstriction and difficulty breathing) [3]. If left untreated, this condition can result in death in a matter of minutes. Epinephrin serves to counteract these effects by directly acting on α- and β-adrenergic receptors in an agonistic fashion. Its actions on α1-adrenergic receptors increases left ventricular preload by reducing venous capacitance; this serves to increase blood pressure. Its actions on β-adrenergic receptors reverse the bronchoconstriction and increase cardiac inotropy (contractility) and chronotropy (heart rate); this serves to both increase blood pressure and relieve breathing difficulty [1, 2].

Figure 2: Symptoms of Anaphylaxis (From Ambulance Technician Study)

Recent experimental studies have also shown that arginine vasopressin (AVP) serves to increase mean arterial pressure (MAP) values in animal models of anaphylactic shock. This increase in MAP was comparable to the increase in MAP observed when anaphylactic shock is treated with epinephrine [1]. However, even the highest AVP doses were associated with a much lower skeletal muscle oxygen pressure (PtiO2) value, a parameter previously shown in human and animal models to be correlated with survival [1].

Figure 3: Molecular model of arginine vasporessin (AVP) (From 3D Chem)

The purpose of this study are twofold: first, to compare the effects of epinephrine only and AVP only on the resolution of anaphylaxis in terms of MAP, heart rate, Ptio2 value, and survival. Second, to investigate the effects of using AVP after epinephrine treatment on anaphylaxis in terms of MAP, heart rate, PtiO2 value, and survival [1].

This was tested using ovalbumin-sensitized Brown Norway rats (Rattus norvegicus; see photo at left from the City of Berkeley); ovalbumin (see computer generated structure below) is the main protein found in egg whites [1]. These rats were anesthetized, intubated, and shock induced with ovalbumin (see photo below, from UBC Faculty of Food and Land Systems). This animal model was arranged to represent the conditions in which a human would undergo anaphylactic shock occurring during anesthesia. Groups of rats (n=6) were randomly allocated to receive a certain treatment five minutes after shock onset. A five minute interval between anaphylaxis onset and initiation of treatment was chosen as this best represented the time, in a human operating room, between clinical diagnosis of anaphylaxis and subsequent preparation of and treatment with epinephrine [1]. Each group of rats were treated with one of the following: saline (no-treatment, control group); two boluses of epinephrine followed by continuous epinephrine diffusion; AVP bolus followed by continuous infusion of AVP; and, epinephrine bolus followed by continuous infusion of AVP. After treatment, MAP and PtiO2 were measured while survival was analyzed [1].




Results and Discussion



Graph A: Profile of Mean Arterial Pressure (MAP) before and after onset of treatment in each of the three treatment groups and the control (no treatment) group. Note that T=0 corresponds to the time in which anaphylaxis was induced. T=5 corresponds to the time at which treatment began. Note that epinephrine significantly increases MAP, as does epinephrine plus AVP but at a slower rate. AVP only causes no significant change in MAP [1] (from Dewachter et al. (2007)).



Graph B: Profile of skeletal muscle tissue oxygen partial pressure (PtiO2) after onset of treatment in each of the three treatment groups and the control (no treatment) group. Note that T=0 corresponds to the time in which anaphylaxis was induced. T=5 corresponds to the time at which treatment began. Note that epinephrine significantly increases PtiO2, as does epinephrine plus AVP but at a slower rate. AVP only does not result in any increase in PtiO2 [1] (from Dewachter et al. (2007)).

The main findings of this study were as follows: therapy with epinephrine only was associated with a high rate of survival (84%), where AVP treatment only had a 100% mortality rate; epinephrine therapy followed by continuous infusion of AVP was associated with a 100% survival rate [1]. When comparing the epinephrine only, AVP only, and epinephrine plus AVP groups, the MAP and PtiO2 profiles are very similar. The epinephrine only and epinephrine plus AVP groups were characterized by a partial restoration of MAP and a full recovery of PtiO2 associated with tachycardia and unchanged heart rate, respectively. The AVP group, on the other hand, was characterized by a decrease in MAP and PtiO2 and a low heart rate. The survival of the three groups differed as well: there was a high rate of survival in the epinephrine only group and the epinephrine plus AVP group [1]. The survival rate in the AVP only group was nonexistent; there was a 100% mortality rate. Despite the initial increase of MAP caused by AVP, it decreases more over time and is associated with a lower heart rate and a significant decrease of PtiO2 [1].

When AVP was injected as a continuous infusion after an initial dose of epinephrine, AVP restored MAP, just as epinephrine only treatment did, and was associated with an unchanged heart rate. Tachycardia was, however, observed with the epinephrine only group [1].

It is noteworthy that, when comparing the epinephrine plus AVP group to the AVP only group, there was a lower infusion of AVP in the epinephrine plus AVP group than in the AVP only group. Despite this lower infusion, the MAP and PtiO2 values of the epinephrine and AVP group were significantly higher [1]. The drug administration duration of the epinephrine plus AVP group was also less than the duration of the AVP only group. Taken together, these observations suggest that the epinephrine administration before the infusion of AVP cause a major change in the effects of the AVP that probably explains the highly improved survival rate of this group [1].

Figure 4: Surgery being performed on a rat (From ResearchTraining.org)

The main results of this paper showed that a single treatment of epinephrine caused an increase in MAP accompanied by simultaneous increase in cardiac output and mean pulmonary capillary wedge pressure compared with the control group. A continued dosage of epinephrine produces a sustained improvement of in MAP and PtiO2 [1, 4]. It acts by increasing cardiac output and stroke work due to the effects epinephrine binding to β-adrenergic receptors. The mechanism by which AVP acts as a vasopressor is unclear. AVP, in vasoplegic shock, has been shown to restore vascular tone by at least four mechanisms. The activation of V1 receptors; the closure of ATP-sensitive K+ channels which normally produce vasodilation via cellular hyperpolarization; the modulation of nitric oxide via antagonization; and the potentiation of adrenergic and other vasoconstrictor agents [1].

Considering the results, Dewachter et al. (2007) show that epinephrine must still be considered the first line of drug treatment for anaphylaxis. They also show, however, that AVP can be used to supplement epinephrine treatment after epinephrine has been given [1].

Critique:

This paper was very clear and concise. Anesthesia-induced anaphylaxis was generated and modeled well in the Norway Rat; ultimately, this means it was an excellent model for the human system. The anaphylactic state and the mechanism of action in which it is relieved by epinephrine was explained in a clear, precise fashion. Although the mechanism through which AVP acts as a vasopressor is unclear, the authors still give several possibilities of how this mechanism may work. This was much appreciated, as it really provided a better overall picture of how AVP would fit into the anaphylaxis treatment regime. The purpose of this paper was to determine if AVP could be potentially used to supplement epinephrine as a treatment for anaphylaxis; ultimately, the authors claimed AVP could supplement epinephrine but not replace it, and the results supported this conclusion.

The figures presented in the results section were clear and well organized. They presented enough information to be helpful and show trends but not so much that they became unclear. The legends to each of the figures were excellent and easy to understand. There was, however, a problem with clarity with the table indicating the heart rate measured in the rats during anaphylaxis. It seemed rather meaningless to simply present a table of numbers. It would have been far more useful to present the heart rates in a graphical format, such as a bar graph or a line graph. This way the reader would be able to easily view trends in the heart rate, such as tachycardia or a decrease back to baseline.

It was also useful for the authors to explain the materials and methods as thoroughly as they did. Since this experiment is an animal model for a human condition, it was important to explain the function and purpose of each of the steps in a human context. For example, after anaphylaxis was induced the authors waited five minutes before administering treatment. This was meant to simulate, in a human operating room environment, the time needed for the diagnosis of anaphylaxis and the preparation of the treatment.

Overall this paper was incredibly thorough and well informed. The authors validated the continued use of epinephrine as the primary treatment for anaphylaxis and indicated the benefits of using AVP with this treatment.

Future Experiments:

Future experiments are required to validate the information obtained by the Dewachter et al. (2007) results. Although the addition of AVP to traditional epinephrine treatment of anaphylaxis may help increase survival rate, it is important to realize that these results were for rats and may not be of clinical importance in humans. The most logical step at this point would be to organize a human trial. Although it would be ethically irresponsible to induce anaphylaxis in humans in order to test the effects of AVP on epinephrine treatment, it is possible to conduct a study on patients already scheduled for surgery who may be at a high risk for anaphylaxis. If consent is obtained, these patients would agree to receive a continuous intravenous injection of AVP along with epinephrine treatment in the occurrence of anaphylaxis. Surgery requires monitoring of blood pressure, heart rate, respiratory rate, and oxygen saturation levels so no additional equipment would have to be used. After surgery, if anaphylaxis occurred, the information from these monitors can be easily downloaded and the effects of the epinephrine and AVP administration can be determined. Standards can be obtained from other surgeries where epinephrine was used on anaphylaxis to establish a baseline rate of recovery and survival rate. If the AVP in the epinephrine treatment is not successful, the statistics obtained from the surgery will closely resemble those of the standard epinephrine treatment. If the AVP enhances the epinephrine treatment, the surgery statistics will show a more rapid improvement (meaning a faster increase in blood pressure, a faster increase in oxygen saturation, and the absence of tachycardia). Only when AVP has been tested in human trials and found to be successful can we truly incorporate it into the treatment of anaphylaxis.

Photo from the Daily Mail Online

References:

[1] Dewachter, P., Raëth-Fries, I., Jouan-Hureaux, V., Menu, P., Vigneron, C., Longrois, D., and Mertes, P. M. (2007). A Comparison of Epinephrine Only, Arginine Vasopressin Only, and Epinephrine Followed by Arginine Vasopressin on the Survival Rate in a Rat Model of Anaphylactic Shock. Anesthesiology. 106: 977-983.

[2] Kindt, T., Goldsby, R., Osborne, B. (2007). Kuby Immunology.New York, NY: W. H. Freeman and Company.

[3] Silverthorn, D. (2007). Human Physiology: An Integrated Approach. San Francisco, USA: Benjamin Cummings.

[4] Stolk, Jon M., U’Prichard, David C., Fuxe, Kjell. (Eds.). (1988). Epinephrine in the Central Nervous System. Oxford: Oxford University Press.

Assignment 3

Epinephrine Function: A Vital Role in the Stress Response

Figure 1: Molecular model of epinephrine (From World of Molecules.)

Epinephrine is a catecholamine, released from the adrenal gland, and a vital hormone in the short term stress response, or the fight or flight response as it is more commonly known. This state was initially described by Walter Cannon, an American physiologist, in 1915. The theory he developed states that animals react to cases of acute stress by a generalized discharge of the sympathetic nervous system. This, in turn, prepares the animal to fight or flee [1].

Normally animals maintain homeostasis, another state described by Walter Cannon in which a constant internal environment is maintained in lieu of changes in the external environment. In this state, the neurons in the locus ceruleus (a nucleus in the brain stem involved with the stress response; see photo to the right, from MindBlog) fire in regular intervals [1, 3, 4]. When a stressful stimulus is perceived, a signal is relayed from the sensory cortex of the brain via the hypothalamus to the brain stem. This signaling route increases the rate of noradrenergic activity in the locus ceruleus. This increase in activity causes a corresponding increase in alertness in the organism. Catecholamines are released and their abundance at the locus ceruleus neuroreceptors facilitate behaviors related to combat or escape [3, 4].

A different sort of response occurs when the stimulus is perceived as a threat. A signal is still relayed from the sensory cortex of the brain via the hypothalamus to the brain stem, but it is a more intense and prolonged discharge [3]. This then activates the sympathetic division of the autonomic nervous system. Acetylcholine release is triggered from preganglionic sympathetic nerves which then triggers a release of epinephrine from the chromaffin cells in the medulla of the adrenal glands [3, 4]. The epinephrine causes a boost in the oxygen and glucose delivered to the brain and muscle as well as an increase in heart rate and stroke volume. It suppresses both the immune system and non-emergency bodily systems, such as the gastrointestinal tract [4].

Epinephrine acts through non selective binding of α1, α2, β1, and β2 adrenergic receptors on the cell plasma membrane. In the liver it binds the α1 receptors and starts the inositol-phospholipid signaling pathway [4]. This signals the phosphorylation of glycogen synthase, inactivating it, and phosphorylase kinase, activating it [3, 4]. The activation of phosphorylase kinase activates another enzyme called glycogen phosphorylase, which catalyzes the breakdown of glycogen. Glucose is then released into the bloodstream. β2 receptors are primarily found in skeletal muscle blood vessels and activation of these receptors causes vasodilation. In contrast, α adrenergic receptors are found in most smooth muscles cause vasoconstriction when activated by epinephrine [4, 5].


Figure 2: The Role of Epinephrine in the Fight or Flight Response (from Fight nor Flight).


Effects caused by epinephrine in the fight or flight response include:

• Acceleration of heart rate and ventilation rate
• Inhibition of stomach and intestinal action
• Constriction of blood vessels
• Liberation of nutrients for muscular action
• Inhibition of lacrimal gland and inhibition of salivation
• Dilation of pupils
• Relaxation of bladder
• Inhibition of erection
• Auditory exclusion
• Tunnel vision [3, 4]

Interestingly, the fight or flight response in humans has been shown to be disruptive and damaging. In modern situations, humans do not normally encounter emergencies which require the physical readiness provided by the stress response; ultimately, the stress response will be activated in situations where physical action is inappropriate [3].

Related Pathology: Anaphylaxis

Anaphylactic shock is an allergic reaction that is classified as a Type I hypersensitivity reaction. The reaction may involve the skin, eyes, bronchopulmonary tissues and gastrointestinal tract and can result in death. After an initial exposure to the allergen, a person's immune system becomes sensitized to that allergen. On subsequent exposures, an allergic reaction occurs [2].


Figure 3: Type I Hypersensitivity Reaction (from The University of South Carolina School of Medicine.)


This reaction is mediated by IgE antibodies while the primary cell component is the mast cell. Mast cells are found in connective tissue throughout the body, and in especially high concentrations in areas that are vascularized [2, 3]. In their cytoplasm are large granules which store chemical compounds, the most important of which are histamine and tumor necrosis factor-α (TNF-α). Histamine is an amine which causes dilation of local blood vessels and smooth muscle contraction while TNF-α causes vasodilation and further promotes the inflammatory response [2].

Figure 4: Activation of the Mast Cell in the Anaphylactic Response (from Davidson College.)


Figure 5: Chemical Factors Released by Mast Cells During Anaphylaxis (from Davidson College.)


When a person is exposed to a certain allergen, IgE antibodies are preferentially produced. IgE has a very high affinity for its receptor on mast cells [2]. Subsequent exposures to the same allergen cross links the cell-bound IgE and triggers the release of chemical substances, or degranulation. These substances include histamine (causing bronchoconstricion, mucus secretion, vasodilation, and vascular permeability), tryptase (causing proteolysis), and prostoglandins (causing edema and pain) [2].

These substances enter circulation and have a profound system wide effect, and cause systematic vasodilation (associated with a sudden drop in blood pressure) and edema of bronchial mucosa (resulting in bronchoconstriction and difficulty breathing) [2]. If left untreated, anaphylactic shock can lead to death in a matter of minutes [2].

Treatment

Figure 6: Epinephrine for treatment of anaphylaxis. (From VaxServe.)

Primary treatment of anaphylaxis is epinephrine [2, 4]. Epinephrine prevents worsening of the airway constriction and stimulates the heart to continue beating by binding non selectively to α1, α2, β1, and β2 adrenergic receptors on the cell plasma membrane [4].

Epinephrine helps control the relaxation and contraction of smooth muscle cells.

Muscle contraction occurs through binding of calmodulin to calcium ions. Contractions in smooth muscle are initiated by chemicals that increase the levels of intracellular calcium. The calcium then binds with calmodulin, which then binds to and activates myosin-light chain kinase. This complex then phosphorylates myosin and activates the myosin ATPase. This causes the muscle to contract [4]. When epinephrine binds to an epinephrine receptor it activates adenylyl cyclase, which produces cyclic AMP (cAMP) from ATP. Then cAMP acticates a protein kinase, which phosphorylates myosin light chain kinase. Phosphorylation inactivates this kinase so it has a lower affinity for the calcium-calmodulin complex. This stops the downstream signal for muscle contraction. This relaxed the smooth muscle tissue. Epinephrine also decreases the release and membrane permeability of histamine to reduce the effects of histamine [6].

Epinephrine helps raise blood pressure.

To raise blood pressure, epinephrine binds β-adrenergic receptors, which change shape and activate G-Proteins, which activate adenylyl cyclase to convert ATP to cAMP [4]. cAMP activates cAMP-dependent protein kinase (PKA) [4]. In cardiac muscle, PKA phosphorylates calcium channels in the plasma membrane and myosin heads [4, 7]. Phosphorylated calcium channels remain open longer and thus allow more calcium into the myocardial cell. This, in turn, allows more myosin-binding sites on actin to be uncovered. More cross bridges can be formed, allowing for a stronger contractile force and a higher blood pressure [7].

Figure 7: Mechanism of Action of Epinephrine on Muscle Contraction During Anaphylaxis (from Davidson College.)

Most people who have an allergy severe enough to cause anaphylaxis carry a dose of epinephrine with them at all times. Typically, this is in the form of an EpiPen (see photo on right; from Wikipedia). This small device will, when used, inject enough epinephrine into the casualty to give approximately 15 to 20 minutes of relief from the symptoms of anaphylaxis [2, 4]. This is usually enough time for the casualty to be taken to the hospital for further treatment. It is worthwhile to note that the effects of epinephrine are only a temporary relief from anaphylaxis and that further treatment is required. In a hospital setting, physicians aim to treat the cell hypersensitivity as well as the symptoms. Antihistamine drugs are given as well as corticosteroids. The hypotension is treated with intravenous fluids while bronchospasms are treated with bronchodilators [2].

Epinephrine Overdose

An interesting side effect of the increased employment of EpiPens is a rise in the number of epinephrine overdoses. This often occurs when a helpful civilian is attempting to use an EpiPen (or similar device) on a casualty suffering from anaphylaxis and mistakenly injects him- or herself with the drug. Symptoms, including intense vasoconstriction leading to tissue ischemia and tachycardia, appear within seconds or minutes of injection [3, 4]. If treated promptly with phentolamine, a drug which causes vasodilation, recovery is usually swift with no lasting effects [5].


References

[1] Donnerer, Josef and Lembeck, Fred. (2006). The Chemical Languages of the Nervous System: History of Scientists and Substances. Basel, Switzerland: Reinhardt Druck.

[2] Kindt, T., Goldsby, R., Osborne, B. (2007). Kuby Immunology.New York, NY: W. H. Freeman and Company.

[3] Norris, David O. (2007). Vertebrate Endocrinology. Burlington, MA, USA: Elsevier Academic Press.

[4] Silverthorn, D. (2007). Human Physiology: An Integrated Approach. San Francisco, USA: Benjamin Cummings.

[5] Stolk, Jon M., U’Prichard, David C., Fuxe, Kjell. (Eds.). (1988). Epinephrine in the Central Nervous System. Oxford: Oxford University Press.

[6] Witcher, R. (2006). Anaphylaxis. Accessed November 4, 2008 http://www.bio.davidson.edu/courses/immunology/Students/spring2006/Witcher/Anaphylaxis.html

Assignment 2

Epinephrine is a catecholamine, meaning it is derived from the amino acids phenylalanine and tyrosine. Since this assignment requires a peptide hormone, I have chosen Phenylethanolamine N-Methyltransferase (or PNMT). This enzyme is found in the adrenal medulla and catalyses the reaction which converts norepinephrine to epinephrine [6, 8].


Figure 1: Conversion of norepinephrine to epinephrine by Phenylethanolamine N-Methyltransferase in the adreal medulla (from The Bello Lectures)


Phenylethanolamine N-Methyltransferase

Gene Map and Alignment


Figure 2: Gene map of human PNMT (from Kepp et al. (2007); reference 5)

The human PNMT has been mapped to chromosome 17q22-q24 and consists of three exons and two introns spanning about 2100 basepairs [1]. This gene encodes a protein with 282 amino acid residues and a predicted molecular weight of 30 853 kDa, including the initial methionine. Interestingly, this amino acid sequence was 88% homologous to that of bovine PNMT [1, 2].

Figure 3: Alignment of Sus (Pig), Danio (Zebrafish), Homo (Human) and Mus (Mouse) PNMT (generated by ClustalW)

Key (from ClustalW):
"*" means that the residues or nucleotides in that column are identical in all sequences in the alignment.
":" means that conserved substitutions have been observed.
"." means that semi-conserved substitutions are observed.

Color Key (from ClustalW):

AVFPMILW	RED	Small (small+ hydrophobic (incl.aromatic -Y))
DE	BLUE	Acidic
RK	MAGENTA	Basic
STYHCNGQ	GREEN	Hydroxyl + Amine + Basic - Q
Others	Gray

Figure 4: Cladogram showing relationship between PNMT of pig, human, mouse, and zebrafish (generated by ClustalW)

Figure 5: Score table of alignment. Note that the higher the score, the higher the similarity between proteins. From this table, human and mouse PNMT are the most similar. (Generated from ClustalW)


Active Site

PNMT converts norepinephrine to epinephrine by transferring a methyl group from S-adenosyl-L-methionine to the primary amine of norepinephrine to form epinephrine and the cofactor product S-adenosyl-L-homocysteine [2, 4, 5]. X-ray structures show that the active site of PNMT is covered and a significant conformational change is required to make the active site accessible for the binding of substrates [4, 5]. Several amino acid residues have been discovered to be responsible for substrate positioning and binding, including Asp267 which is important in positioning the substrate but does not participate directly in catalysis [3, 5]. This amino acid presumably works through its interaction with the side-chain hydroxyl of the phenylethanolamines. Experimental results have shown S-adenosyl-L-methionine causes the conformational change necessary for conversion of norepinephrine to epinephrine [2, 3, 5].


Figure 6: Human PNMT amino acids (in yellow) interacting with an inhibitor (the white central molecule) at the active site (From Grunewald et al. (2007); Reference 3)



References

[1] Baetge, E. E., Behringer, R. R., Messing, A., Brinster, R. L., Palmiter, R. D. (1988). Transgenic mice express the human phenylethanolamine N-methyltransferase gene in adrenal medulla and retina. Proc. Nat. Acad. Sci. 85: 3648-3652

[2] Goldstein, D. S., Eisenhofer, G., and Kopin, I. J. (2006). Clinical Catecholamine Neurochemistry: A Legacy of Julius Axelrod. Cellular and Molecular Neurobiology. 26: 695-702

[3] Grunewald, G. L., Seim, M. R., Regier, R. C., and Criscione, K. R. (2007). Exploring the Active Site of Phenylethanolamine N-Methyltransferase with 1,2,3,4-Tetrahydrobenz[h]isoquinoline Inhibitors. Bioorg Med Chem. 15(3): 1298-1310

[4] Hilbert, P., Lindpaintner, K., Beckmann, J. S., Serikawa, T., Soubrier, F., Dubay, C., Cartwright, P., De Gouyon, B., Julier, C., Takahasi, S., Vincent, M., Ganten, D., Georges, M., Lathrop, G. M. (1991). Chromosomal mapping of two genetic loci associated with blood-pressure regulation in hereditary hypertensive rats. Nature. 353: 521-529

[5] Kepp, K., Juhanson, P., Kozich, V., Ots, M., Viigimaa, M., and Laan, M. (2007). Resequencing PNMT in European hypertensive and normotensive individuals: no common susceptibility variants for hypertension and purifying selection on intron I. BMC Medical Genetics. 8:(47)

[6] Norris, David O. (2007). Vertebrate Endocrinology. Burlington, MA, USA: Elsevier Academic Press.

[7] Silverthorn, D. (2007). Human Physiology: An Integrated Approach. San Francisco, USA: Benjamin Cummings.

[8] Stolk, Jon M., U’Prichard, David C., Fuxe, Kjell. (Eds.). (1988). Epinephrine in the Central Nervous System. Oxford: Oxford University Press.

Assignment 1

Epinephrine

Epinephrine, also called adrenaline, is the hormone and neurotransmitter found in many vertebrates responsible for the stress response. It is a catecholamine, meaning it is derived from the amino acid tyrosine and the amino acid phenylalanine [2, 4]. It was originally isolated by Napoleon Cybulski in 1895 and repeat discoveries occurred in May 1896 by William Bates, in 1897 by John Jacob Abel and in 1900 by Jokichi Takamine. Frederich Stolz was the first to artificially synthesize this hormone in 1904 [1].


Figure 1: Chemical and 3D Structure of Epinephrine. (From Wikipedia)



Synthesis and Regulation

Epinephrine is synthesized from norepinephrine in a synthetic pathway shared by all catecholamines. However, only the epinephrine secreting cells posses the enzyme phenylethanolamine N-methyltransferase (PNMT) necessary for converting norepinephrine to epinephrine [2, 3]. This is accomplished by the addition of a methyl group donated by S-adenosylmethionine. After synthesis and secretion, epinephrine circulates in the blood bound to the plasma protein albumin [2]. It is a very short lived hormone, with a biological half life of about five minutes. Inactivation occurs in the liver by liver monoamine oxidase, which produces inactive metabolites that appear in the urine [2, 3].



Figure 2: Synthesis Pathway of Catecholamines, Including Epinephrine (From The Kanji Foundry Press)


Regulation of epinephrine is primarily by the sympathetic nervous system. When cholinergic sympathetic nerve fibers in the medulla of the adrenal gland are stimulated it causes local release of acetylcholine. Acetylcholine causes the adrenal medulla chromaffin cells to uptake calcium ions and release epinephrine [2, 4].


Figure 3: Chromaffin cells of the Adrenal Medulla (From The University of Oklahoma Health Sciences Center)



Mechanism of Action and Effects

Epinephrine acts through non selective binding of α1, α2, β1, and β2 adrenergic receptors on the cell plasma membrane. In the liver it binds the α1 receptors and starts the inositol-phospholipid signaling pathway [4]. This signals the phosphorylation of glycogen synthase, inactivating it, and phosphorylase kinase, activating it [2, 3]. The activation of phosphorylase kinase activates another enzyme called glycogen phosphorylase, which catalyzes the breakdown of glycogen. Glucose is then released into the bloodstream. β2 receptors are primarily found in skeletal muscle blood vessels and activation of these receptors causes vasodilation. In contrast, α adrenergic receptors are found in most smooth muscles cause vasoconstriction when activated by epinephrine [2, 4].

Epinephrine is perhaps one of the most well known hormones because of its role in the stress response, also known as the “fight or flight” response. When a a stimulus is perceived as a threat or an emergency, epinephrine is released from the adrenal glands and has a number of effects on the body to prepare the organism for an emergency [2, 4]. These effects include:

• Acceleration of heart rate and ventilation rate
• Inhibition of stomach and intestinal action
• Constriction of blood vessels
• Liberation of nutrients for muscular action
• Inhibition of lacrimal gland and inhibition of salivation
• Dilation of pupils
• Relaxation of bladder
• Inhibition of erection
• Auditory exclusion
• Tunnel vision [3, 4]

Figure 4: Binding and Effects of Epinephrine on the Liver (From The Biology Project)


Uses

In medicine, epinephrine is used to treat anaphylaxis, a life threatening medical emergency which causes constriction of the airway. Administration of epinephrine prevents worsening of airway constriction and stimulates the heart to continue beating [4]. People with severe allergies which cause anaphylaxis usually carry a dose of epinephrine (typically in a device called an EpiPen) in case of emergencies. You can learn more about anaphylaxis, its symptoms, and epinephrine administration via EpiPen here.

Epinephrine is also used to treat cardiac arrest. It increases peripheral resistance via α1-adrenoreceptor vasoconstriction so that blood is shunted to the heart and lungs and increases the β1-adrenoceptor response which increases cardiac rate and output [4].


References


[1] Donnerer, Josef and Lembeck, Fred. (2006). The Chemical Languages of the Nervous System: History of Scientists and Substances. Basel, Switzerland: Reinhardt Druck.

[2] Norris, David O. (2007). Vertebrate Endocrinology. Burlington, MA, USA: Elsevier Academic Press.

[3] Silverthorn, D. (2007). Human Physiology: An Integrated Approach. San Francisco, USA: Benjamin Cummings.

[4] Stolk, Jon M., U’Prichard, David C., Fuxe, Kjell. (Eds.). (1988). Epinephrine in the Central Nervous System. Oxford: Oxford University Press.