Action And Clinical Pharmacology
Epinephrine acts on both alpha- and beta-adrenergic receptors. Through its action on alpha- adrenergic receptors, epinephrine counters the vasodilation and high vascular permeability that occurs during an anaphylactic reaction that can lead to loss of intravascular fluid volume and hypotension. Through its action on beta-adrenergic receptors, epinephrine causes bronchial smooth muscle relaxation that helps alleviate bronchospasm, wheezing, and dyspnea that may occur during anaphylaxis.
Epinephrine also helps to alleviate pruritus, urticaria, and angioedema, and may be effective in relieving gastrointestinal and genitourinary symptoms of anaphylaxis because of its relaxant effects on the smooth muscle of the stomach, intestine, and urinary bladder. Epinephrine contracts the smooth muscle of the uterus.
Epinephrine, when given intramuscularly or subcutaneously, has a rapid onset and short duration of action.
In a pharmacokinetic study in 35 healthy adult subjects, grouped by varying degrees of thickness in the subcutaneous fat layer of the thigh and stratified by gender, a single 0.3 mg/0.3 ml injection at the anterolateral aspect of the mid-thigh was made with an EpiPen® Auto-Injector and was compared in crossover design to a manual syringe-delivered dose with needles individualized for delivery to muscle layer. The results indicate no significant differences between subjects with varying thicknesses of subcutaneous fat layer (<15 mm, ≥15 mm and ≤20 mm, or >20 mm skin to muscle distance under maximum compression). Overall adrenaline exposure from 0 to 30 min (AUC0-30min) for all groups of subjects receiving EpiPen® exceeded exposures resulting from syringe delivery. Both inter-subject and intra-subject variability was, however, high in this study and therefore robust conclusions cannot be drawn.