Oxytocin causes muscle contraction through-

Ans. is 'c' i.e., IP3-DAG * Oxytocin acts on G-protein coupled IP3-DAG system receptorsPhospholipase IP3 - DAG system* Activation of phospholipase (by stimulatory G protein) hydrolyses the membrane phospholipid phosphatidyl inositol 4, 5 bisphosphate (PIP2) to generate the second messenger inositol I, 4, 5 - triphosphate (IP3) and diacylglycerol (DAG).* IP3 mobilizes Ca+2 from intracellular organelles - increased cytosolic Ca+2. DAG enhances Protein Kinase 'C' activation by Ca+2. Ca+2 acts as third messenger in this type of transduction mechanism and mediates the physiological effects of drugs.* Protein kinase-C phosphorylates various intracellular proteins (threonine, serine or tyrosine residue), causing their activation or inactivation.* Example - Growth hormone releasing hormone (GHRH), Thyrotropin releasing hormone (TRH), GnRh, ADH Vasopressin (VI receptor, vasopressor action), oxytocin, cholecystokinin, PDGF, Gastrinf Catecholamines (some actions via a1 receptors), Angiotensin II (vascular smooth muscle), Substance P, Histamine - H1, muscarinic M1, M3