Influence of the conceptuses and the maternal pituitary on the distribution of multiple components of serum relaxin immunoactivity during pregnancy in the rat.

Relaxin isolated from the ovaries of pregnant rats has a mol wt of 6,500; however, relaxin immunoactivity (RI) in the peripheral serum of pregnant rats is associated with three major components. Essentially all RI is associated with a large component (C1, mol wt approximately 60,000) on day 15 of pregnancy (day 15), but the distribution shifts progressively so that about 50% of the RI is associated with two smaller components (C2, mol wt approximately 13,000; C3, mol wt approximately 6,500) by day 19. The present study examined factors that might influence the distribution of RI among these three components. The distribution of RI in sera from ovarian vein and abdominal aorta on days 16 and 19 was compared after gel filtration through Sephacryl S-200. The distribution of RI among the three major components was similar for sera collected from the two sources. It is concluded that the progressive shift in the distribution of RI among the three major components during late pregnancy is attributable to dynamic changes in their secretion, rather than peripheral metabolism of relaxin after its release from the ovary. This study also determined the influence of the conceptuses and the maternal pituitary on the distribution of RI. The distribution of RI in peripheral sera obtained on days 16 and 19 from intact and hypophysectomized full complement-bearing rats as well as pituitary-intact and hypophysectomized one conceptus-bearing rats was compared after filtration through Sephacryl S-200. The shift in distribution of RI from C1 to smaller components C2 and C3 was markedly attenuated in rats bearing one conceptus, whereas hypophysectomy was without effect. It is concluded that the progressive shift in the distribution of serum RI among the three major components during late pregnancy is influenced by the number of conceptuses but not the maternal pituitary.