Epinephrine (also called adrenaline) is a catecholamine hormone and neurotransmitter synthesized and secreted by chromaffin cells in the adrenal medulla. It is released in response to perceived danger or physiological stress and is the primary effector of the acute fight-or-flight response in the horse.
On release, epinephrine binds alpha- and beta-adrenergic receptors throughout the body. The net effect is rapid mobilization for exertion: cardiac output rises as heart rate and stroke volume increase; blood is redistributed away from the skin and gastrointestinal tract toward skeletal muscle; bronchial smooth muscle relaxes to widen the airway; and glycogen in the liver is converted to glucose to fuel the muscles. Sweat glands in the horse are activated via beta-adrenergic receptor stimulation, which is why epinephrine drives the sweating that normally accompanies intense work or acute stress. Chronic overstimulation of these receptors is the proposed mechanism behind anhydrosis, the failure to sweat that develops in horses working in persistently hot, humid climates.
Epinephrine is also used therapeutically in equine practice: as a vasopressor in anaphylactic shock and cardiac arrest, and as a local vasoconstrictor added to regional anaesthetic solutions to extend their duration and reduce systemic absorption. Clinically administered doses and the management of anaphylaxis are outside the scope of this reference. For the broader hormonal system that epinephrine operates within, see hormone.
Further Reading
- Epinephrine: Wikipedia overview of the catecholamine hormone, its biosynthesis, receptors, and physiological roles.