2 Epinephrine / norepinephrine receptor types

Norepinephrine, Epinephrine, and their Adrenergic receptors

Norpinephrine (NE) and epinephrine (E) are the principal sympathetic neurotransmitters. (ASIDE: The word “Adrenaline” was registered in a US patent in 1901 and blocked from generic use in the US. Therefore, in the US we are technically supposed to say epinephrine for the compound that all the rest of the world, as well as our popular culture, calls adrenaline. The words “epi nephros” mean over the kidney in Greek, and the words “ad renal” mean next to the kidney in Latin.)

• NE is released locally on tissue targets by most sympathetic postganglionic nerve fibers at their varicosities. The released NE is taken up again into the sympathetic nerve fibers by a transporter for reuse.
• Both E and NE are released by the adrenal medulla (4:1 ratio) by exocytosis from the secretory chromaffin granules of chromaffin cells. Like other typical endocrine hormones, these hormones are secreted into the circulatory system. This is the source of circulating E.
• Release of NE from sympathetic nerves along visceral blood vessels is the principal source of circulating NE.

E and NE bind with different affinities to adrenergic receptors, which are GPCRs coupled to different signaling G proteins and stimulating different intracellular pathways. The table below shows that each of the major families of heterotrimeric G proteins is activated by one of the adrenergic receptors. This material will be essential in preparation for the ANS Pharmacology sessions that will follow soon. More complete tables are at the end of the chapter.

Location and coupling of ADRENERGIC receptors