SOURCES OF RELAXIN IN THE RABBIT1,2

Abstract

Pregnant rabbits castrated on the 14th day and treated with progesterone showed normal levels of relaxin in the blood. A relaxin concn. of 20-30 guinea pig unit (g.p.u.) per g. equivalent of fresh tissue was obtained for the ovary, 15-50 g.p.u. for the uterus, 10-25 g.p.u. for the fetal placenta, and 250-350 g.p.u. for the maternal placenta. Since castration fails to affect normal pregnancy levels of relaxin in the blood and there is a relatively high concn. in maternal placenta, the placenta is probably the most significant source of relaxin during pregnancy. During pseudopregnancy both the ovary and uterus must be regarded as potential sources, though the data indicate that the uterus is the greater source. Analysis for relaxin revealed 5 g.p.u. per g. of ovary and 50 g.p.u. per g. of uterine tissue. Prolonged estrogenic treatment in a rabbit devoid of ovaries and uterus resulted in 0.2-0.3 g.p.u. per ml. of serum and 5 g.p.u. per g. of vaginal tissue. In the rabbit, under suitable conditions, the ovaries, uterus, placenta, and vagina may all secrete relaxin.