Membrane receptors

Physiology · Basic cellular physiology

There are four main types of membrane receptor: ligand-gated ion channels, tyrosine kinase receptors, guanylate cyclase receptors and G protein-coupled receptors

Ligand-gated ion channel receptors

generally mediate fast responses
e.g. nicotinic acetylcholine, GABA-A & GABA-C, glutamate receptors

Tyrosine kinase receptors

intrinsic tyrosine kinase: insulin, insulin-like growth factor (IGF), epidermal growth factor (EGF)
receptor-associated tyrosine kinase: growth hormone, prolactin, interferon, interleukin

Guanylate cyclase receptors

contain intrinsic enzyme activity
e.g. atrial natriuretic factor, brain natriuretic peptide

G protein-coupled receptors

generally mediate slow transmission and affect metabolic processes
activated by a wide variety of extracellular signals e.g. Peptide hormones, biogenic amines, lipophilic hormones, light
7-helix membrane-spanning domains
consist of 3 main subunits: alpha, beta and gamma
the alpha subunit is linked to GDP.Ligand binding causes conformational changes to receptor, GDP is phosphorylated to GTP,and the alpha subunit is activated
G proteins are named according to the alpha subunit (G_s, G_i, G_q)

	G_s	G_i	G_q
Mechanism	Stimulates adenylate cyclase → increases cAMP → activates protein kinase A	Inhibits adenylate cyclase → decreases cAMP → inhibits protein kinase A	Activates phospholipase C → splits PIP₂ to IP₃ & DAG → activates protein kinase C
Examples	• Beta-1 receptors (epinephrine, norepinephrine, dobutamine) • Beta-2 receptors (epinephrine, salbuterol) • H2 receptors (histamine) • D1 receptors (dopamine) • V2 receptors (vasopressin) • Receptors for ACTH, LH, FSH, glucagon, PTH, calcitonin, prostaglandins	• M2 receptors (acetylcholine) • Alpha-2 receptors (epinephrine, norephinephrine) • D2 receptors (dopamine) • GABA-B receptor	• Alpha-1 receptors (epinephrine, norepinephrine) • H1 receptors (histamine) • V1 receptors (vasopressin) • M1, M3 receptors (acetylcholine)