Summary. The role of equine chorionic gonadotrophin (CG, formerly termed Pregnant Mare Serum Gonadotrophin, PMSG) in maintaining equine pregnancy was investigated by examining the effects of this hormone on the maternal ovaries during early gestation and relating these findings to the receptor binding activities of CG in vitro. Measurement of plasma progestagen profiles in mares and donkeys carrying horse, donkey, mule (9 horse × ♂ donkey) and hinny ♀ donkey × ♂ horse) conceptuses confirmed that CG induced several secondary ovulations and thus maintained maternal progestagen concentrations. However, in mares carrying horse and mule conceptuses and in donkeys carrying donkey conceptuses the growth of the follicles that gave rise to the secondary corpora lutea occurred before CG was secreted and the CG did not express any FSH-like activity. Nevertheless, in donkeys carrying hinny pregnancies the CG secreted by the hinny conceptus stimulated massive follicular growth in addition to luteinization, presumably because of an enhanced sensitivity of donkey ovaries to hinny CG which, as demonstrated in previous studies, is a mixture of horse and donkey CG and hence has considerably more FSH-like activity than normal donkey CG.
In-vitro receptor binding experiments showed that both horse and donkey gonadal tissues possessed a low binding affinity for horse CG compared to that exhibited by equivalent gonadal tissues of other species. Furthermore, horse CG bound with low, but significant, affinity to horse and donkey LH receptors and donkey FSH receptors, but exhibited negligible binding to horse FSH receptors.
We suggest that in equids the receptor-mediated refractoriness to the gonadotrophic activities of the intraspecific chorionic gonadotrophin protects the ovaries during pregnancy.