Determination of the source of relaxin immunoreactivity during pregnancy in the dog

We investigated the source of immunoreactive relaxin (IR) in the dog during pregnancy using the following: (1) controls, (2) dogs ovariectomized during pregnancy and maintained on progesterone, (3) dogs hysterectomized during pregnancy or immediately postpartum, (4) corpora lutea, uteri, and placentas collected at various times during pregnancy for determination of IR, and (5) maternal (overian and uterine vein) and fetal (cardiac) blood and amniotic and allantoic fluid. Plasma IR patterns remained unchanged in animals subjected to ovariectomy, whereas in the controls IR was first detected at about Day 20 of pregnancy, peaked on Day 35, and was then remained at the same level until parturition. Hysterectomy, on the other hand, resulted in IR values becoming undetectable within 2 to 3 days. The highest tissue concentrations of IR were found in the placenta. These findings indicate that the source of relaxin in the pregnant dog is the placenta.