alpha1 - Adrenergic Receptors (NE)

Activation of alpha1-adrenergic responses.
Stimulation of alpha1-adrenergic receptors by norepinephrine leads to the activation of a Gq coupling protein. The alpha subunit of this G protein activates the effector, phospholipase C, which leads to the release of IP3 (inositol 1,4,5-trisphosphate) and DAG (diacylglycerol) from phosphatidylinositol 4,5-bisphosphate (PtdIns 4,5-P2). IP3 stimulates the release of sequestered stores of calcium, leading to an increased concentration of cytoplasmic Ca2+. Ca2+ may then activate Ca2+-dependent protein kinases, which in turn phosphorylate their substrates. DAG activates protein kinase C.
alpha1-adrenergic receptors cause contraction of smooth muscle in the following organs:

• Eye: iris radial dilator muscle contraction (mydriasis - dilation)
• Arterioles: (skin, cerebral, abdominal, and salivary gland)
• Stomach and Intestine: contraction of sphincters
• Glandular Secretion (lacrimal(increase), salivary(increase), bronchial(decrease), pancreatic(decrease), mucosal(decrease), and sweat(palms sympathetic increase))
• Pilomotor muscles: contraction (goose pimples)
• Bladder neck: trigone and sphincter contraction
• Sex Organs, Male: ejaculation
• Sex Organs, Female: if pregnant uterus contraction.

Adapted from Figure 9-1: StatRef Online Library: Basic and Clinical Pharmacology 8th ed. (2001)
by Michael Bolger, 8/26/01
Copyright(c) 2001 The McGraw Hill Companies, Inc. All rights reserved