Relaxin Activity in the Pregnant Mare1

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Abstract
Plasma relaxin activity was measured daily in the mare during the estrous cycle and throughout pregnancy. A radioimmunoassay (RIA) for highly purified porcine relaxin was utilized. While competitive binding curves of dilutions of mare plasma and porcine relaxin standards were not found to be parallel, the material measured by the RIA eluted as one band from gel filtration and had a molecular weight similar to porcine relaxin. Relaxin immunoactivity was not detected during the seven estrous cycles studied. Relaxin activity was below detection in all mares for the first part of gestation and began to increase at about Day 80. Relaxin activity then progressively rose (P<0.05) to peak levels (5.9 ng/ml) between 176-200 days of gestation. Levels of relaxin subsequently declined (P<0.05) to ∼ 4.2 ng/ml over the next 60 days. Relaxin activity increased (P<0.05) steadily from Day 240 to the day of foaling. Relaxin activity rapidly fell following foaling.