Main hormone acting upon uterus to initiate labor-

Ans-A i.e., Oxytocin Oxytocino Oxytocin is a nonapeptide,o It is secreted by the posterior pituitary along with vasopressin (ADH).o Both oxytocin and ADH are synthesized within the nerve cell bodies in the supraoptic and paraventricular nuclei of the hypothalamus; are transported down the axon and stored in the nerve endings within the neurohypophysis.o They are stored in separate neurons as complexes with their specific binding proteins (Neurophysin).o ActionsUterusIt increases the force and frequency of uterine contraction.Estrogens sensitize the uterus to oxytocin, while progestins decrease the sensitivity.The increased contractility is restricted to the fundus and body,, lower segment is not contracted, may even be relaxed at term.BreastOxytocin contracts myoepithelium of mammary alveoli and forces milk into bigger sinusoids - Milk ejection or milk letdown reflex.CVSHigher doses cause vasodilation - Brief fall in BP, reflex tachycardia, and flushing.The umbilical vessels are constricted oxytocin may help their closure at birth.KidneyOxytocin in high doses exerts ADH like action -->> water retention can occur.