Cellular Receptors

Question 1

Gs proteins

Answer 1

l Binding of agonists to receptors linked to Gs proteins increases cAMP
catecholamines (beta), dopamine (D1), glucagon, histamine (H2), prostacyclin, and some serotonin subtype

Question 2

Gi protein

Answer 2

l Binding of agonists to receptors linked to Gi proteins decreases cAMP production.

adrenoreceptors (alpha2), ACh (M2), dopamine (D2 subtypes), and several opioid and serotonin subtypes.

Question 3

Gq protein

Answer 3

Other receptor systems are coupled via GTP-binding proteins (Gq), which activate phospholipase C. Activation of this enzyme releases the second messengers inositol triphosphate (IP3) and diacylglycerol (DAG) from the membrane phospholipid phosphatidylinositol bisphosphate (PIP2). The IP3 induces release of Ca2+ from the sarcoplasmic reticulum (SR), which, together with DAG, activates protein kinase C. The protein kinase C serves then to phosphorylate a set of tissue-specific substrate enzymes, usually not phosphorylated by protein kinase A, and thereby affects their activity.
l These signaling mechanisms are invoked following activation of recep- tors for ACh (M1 and M3), norepinephrine (alpha1), angiotensin II, and several serotonin subtypes.