Plasma relaxin levels during suckling and oxytocin stimulation in the lactating sow.

Plasma relaxin levels were measured in animals at different stages of lactation and related to the amount of nuzzling and suckling behavior exhibited by the piglets. Only in some acute suckling episodes was relaxin secreted rapidly and episodically in spiteof normal pigletand sow behavior and interaction. However, when the piglets were removed from the dams 6 h before suckling, the sows were very restless and the relaxin response to suckling was delayed. Oxytocin injection in lactatingbut nonsuckled sows caused an episodic secretion of relaxin similar to suckling itself. The source of relaxin in the lactatingsow may be the old corpus luteum, since progesterone levels increasedacutely,somewhat reflectingthe profileof relaxinincrease over the suckling episode. BIOLOGY OF REPRODUCTION 33, 705-714 (1985)