SYMPATHOMIMETIC DRUG (Epinephrine)

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  • The smooth muscle of blood vessels that supply skeletal muscles has both beta-2 and alpha receptors; activation of beta-2 receptors causes vasodilation, and stimulation of alpha receptors constricts these vessels.
  • In such vessels, the threshold concentration for activation of beta-2 receptors by Epinephrine is lower than that for alpha receptors, but when both types of receptors are activated at high concentrations of Epi, the response to a receptors predominates; physiological concentrations of Epi primarily cause vasodilation.
  • The integrated response of an organ to sympathomimetic amines results not only from their direct effects, but also from reflex homeostatic adjustments.
  • when a drug (e.g., a beta-2 agonist) lowers mean blood pressure at the mechanoreceptors of the carotid sinus and aortic arch, the baroreceptor reflex works to restore pressure by reducing parasympathetic (vagal) outflow from the CNS to the heart, and increasing sympathetic outflow to the heart and vessels. Conversely occurs when alpha agonist are used.
  • The baroreceptor reflex effect is of special importance for drugs that have little capacity to activate beta receptors directly. With diseases (e.g., atherosclerosis) that may impair baroreceptor mechanisms, effects of sympathomimetic drugs may be magnified.
  • Coronary blood flow is enhanced by Epinephrine or by cardiac sympathetic stimulation under physiological conditions. The increased flow, which occurs even with doses that do not increase the aortic blood pressure, is the result of two factors. The first is the increased relative duration of diastole at higher heart rates; this is partially offset by decreased blood flow during systole because of more forceful contraction of the surrounding myocardium and an increase in mechanical compression of the coronary vessels. The increased flow during diastole is further enhanced if aortic blood pressure is elevated by Epi; as a consequence, total coronary flow may be increased. The second factor is a metabolic dilator effect that results from the increased strength of contraction and myocardial O2 consumption due to direct effects of Epi on cardiac myocytes. This vasodilation is mediated in part by adenosine released from cardiac myocytes, which tends to override a direct vasoconstrictor effect of Epi that results from activation of a receptors in coronary vessels.
  • Epi elevates the concentrations of glucose and lactate in blood , and can inhibit (alpha-2 effect) or stimulate (beta-2 effect) insulin secretion; inhibition is the predominant effect. Glucagon secretion is enhanced via activation of beta receptors of the a cells of pancreatic islets. Epi also decreases the uptake of glucose by peripheral tissues, in part because of its effects on the secretion of insulin, but also possibly due to direct effects on skeletal muscle.
  • Epi raises the plasma concentration of free fatty acids by stimulating beta receptors in adipocytes, activating triglyceride lipase and accelerating triglyceride breakdown to free fatty acids and glycerol.
  • Epi injection is available in 1 mg/mL (1:1000), 0.1 mg/mL (1:10,000), and 0.5 mg/mL (1:2000) solutions. The usual adult dose given subcutaneously ranges from 0.3 to 0.5 mg. The intravenous route is used cautiously if an immediate and reliable effect is mandatory. If the solution is given by vein, it must be adequately diluted and injected very slowly. The dose is seldom as much as 0.25 mg, except for cardiac arrest, when larger doses may be required. Epi suspensions are used to slow subcutaneous absorption and must never be injected intravenously.

  • THERAPEUTIC USES:
  1. Rapid relief of hypersensitivity reactions, including anaphylaxis, to drugs and other allergens.
  2. prolong the action of local anesthetics, presumably by vasoconstriction and a consequent reduction in absorption.
  3. restore cardiac rhythm in patients with cardiac arrest.
  4. topical hemostatic agent on bleeding surfaces such as in the mouth or in bleeding peptic ulcers during endoscopy of the stomach and duodenum.

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