Determination of the Source of Equine Relaxin1

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Abstract
The profile of relaxin immunological activity in plasma throughout gestation in the mare was reported. Based upon the appearance of relaxin activity in relation to the corpora lutea (CL) (which regress during pregnancy in the mare), the CL were not thought to be the sole significant source of relaxin in the mare, relaxin was suggested to be of placental origin. If the ovaries made any contribution to relaxin activity during pregnancy or if the placenta is the sole source of relaxin was determined. Thoroughbred mares were ovariectomized on about Day 50 of gestation (Day 50 OVEX) before detectable relaxin secretion, or on Day 100 of gestation (Day 100 OVEX) after the initiation of secretion. In Day 50 ovariectomized animals, pregnancy was terminated at Day 100, 125 or 150 by caesarean section for tissue collection. Daily plasma samples were collected from Day 45 to the termination of pregnancy, or Day 150. In Day 50 ovariectomized animals, the time of initiation of relaxin secretion was similar to that of control animals. The increase in relaxin levels was not less than, but greater than that seen in normal animals. However, plasma progesterone and estrone were not significantly altered in these animals during the time that relaxin levels diverged. In Day 100 ovariectomized animals, the profile of plasma relaxin activity was similar to the profile in control animals. Tissue samples were collected from CL (Day 50 or 100), fetal gonads, endometrium, endometrial cups, placenta, and amnion. Relaxain activity was determined by radioimmunoassay. The highest amounts of relaxin activity were found in placental tissue. Activity in CL, endometrium, fetal gonads, and amnion were comparatively low. Endometrial cups contained intermediate amounts. Relaxin was found in a Day 65 placenta which is before relaxin is detectable in plasma. Relaxin activity was highest in the Day 150 placenta. Based upon the above studies, the CL of pregnant mares were not a significant source of relaxin. The placenta evidently is the main source of relaxin activity in the pregnant mare.

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