Dynamic Changes of Multiple Forms of Serum Immunoactive Relaxin during Pregnancy in the Rat*

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Abstract
Relaxin isolated from the ovaries of pregnant rats has a MW of 6500. The size(s) of the relaxin-immunoactive component(s) in the serum of rats during pregnancy were determined. Peripheral sera from rats on days 15, 17, 19 and 21 of pregnancy were filtered through Sephacryl S-200 columns at 37.degree. C and 4.degree. C. Relaxin immunoactivity eluted in 3 components designated C1, C2 and C3, which had MW of approximately 60,000, 13,000 and 6500, respectively. The distribution of relaxin immunoactivity among C1, C2 and C3 changed with the day of pregnancy. On day 15 of pregnancy essentially all relaxin immunoactivity was associated with C1. By day 17 approximately 20% of the relaxin immunoactivity was in C2 plus C3, and by day 21 the percentage in each of these 2 components increased to about 25%. C1 appears to possess relaxin bioactivity since the frequency of intrauterine pressure cycles in nonpregnant rats declined during cross-circulation with day 15 pregnant rats. C1 may not be attributable to the binding of 6500 MW relaxin to a binding protein(s) since relaxin immunoactivity remained associated with a component(s) having a MW greater than 28,000 after incubation and gel filtration of day 15 serum in the presence of 2.0 M potassium thiocyanate at 37.degree. C. Moreover, the levels of C1 as well as those of C2 and C3 were markedly increased in the peripheral serum of day-21 rats within 15 min of LH administration. C3 may be the 6500-MW form of relaxin and C2 may be an intermediate in the processing of 20,000-MW rat relaxin precursor to the 6500-MW form of relaxin. The physiological significance, if any, of the multiplicity of relaxin-immunoactive components is unknown.