NEW YORK, July 19 — That familiar jolt of fear or excitement, the racing heart and quickened breath, is the work of a single molecule: adrenaline. Also known as epinephrine, this hormone and medication is far more than just a biological alarm bell. For families and patients facing allergic reactions, cardiac emergencies, or even childhood croup, it can be a lifeline.
Adrenaline is produced naturally by the adrenal glands and a small number of neurons in the medulla oblongata. It acts as a master switch for the fight-or-flight response, increasing blood flow to muscles, boosting heart output by acting on the SA node, dilating pupils, and raising blood sugar.
These effects come from binding to alpha and beta receptors throughout the body. Interestingly, the substance is not unique to humans; it appears in many animals, some single-celled organisms, and has even been isolated from the plant Scoparia dulcis in Northern Vietnam. The term “adrenergic” refers to responsiveness to adrenaline, though the main sympathetic neurotransmitter is actually noradrenaline, a discovery credited to Ulf von Euler in 1946.
Walter Bradford Cannon originally proposed the concept of the adrenal medulla working with the sympathetic nervous system in the flight, fight, and fright response. Yet the adrenal medulla is not required for survival; patients who have had adrenalectomies maintain normal responses to hypoglycemia and exercise.
One physiological trigger for adrenaline secretion is exercise, first demonstrated by measuring dilation of a denervated cat pupil on a treadmill and later confirmed by biological assay of urine.
From Lab to Bedside: Medical Uses
Medically, adrenaline is a versatile drug. It is used to treat allergic anaphylaxis, cardiac arrest, and superficial bleeding. Inhaled adrenaline can improve symptoms of croup and may be used for asthma when other treatments are ineffective.
It is administered intravenously, by intramuscular injection, by inhalation, or subcutaneously. For open-angle glaucoma, epinephrine helps lower intraocular pressure by increasing aqueous humor outflow.
A case has also been made for adrenaline infusion in preterm infants with cardiovascular compromise, though more trials are needed to confirm reductions in morbidity and mortality. Common side effects include shakiness, anxiety, and sweating. It can cause a fast heart rate and high blood pressure, and occasionally an abnormal heart rhythm.
Safety during pregnancy and breastfeeding is unclear, so benefits must be weighed carefully. Pharmacological doses stimulate α1, α2, β1, β2, and β3 adrenoceptors.
The adrenal medulla contributes over 90% of circulating adrenaline; after adrenalectomy, adrenaline disappears from the bloodstream. Adrenaline has a β2-mediated effect on metabolism and airway, with no direct neural connection from sympathetic ganglia to the airway. Biochemical methods for measuring plasma catecholamines have been available since 1950, but fluorimetric assays are too non-specific and insensitive for precise adrenaline measurement.
For researchers and clinicians, this means the full picture of how adrenaline behaves in individual patients is still being refined. Consult your doctor for medical advice.
Sources
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