Metabolic clearance rate, production rate, and source of progesterone in donkeys with fetuses of different genotypes

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Abstract
Fetal genotype has a pronounced influence upon progestagen and gonadotropin (PMSG) concentrations in the peripheral blood of mares and donkeys during the first half of pregnancy. Plasma progestagen and PMSG concentrations in donkeys carrying hinny (.male. horse .times. .female. donkey) conceptuses are appreciably higher than in donkeys carrying normal donkey conceptuses, or in mares carrying normal horse conceptuses. In contrast, PMSG concentrations are low in mares carrying mule (.male. donkey .times. .female. horse) conceptuses compared with mares carrying normal horse conceptuses. The unusually high concentration of peripheral plasma progesterone in donkeys carrying a hinny conceptus compared with that in the other equine pregnancies was due to a marked increase in progesterone production rate rather than a decrease in metabolic clearance rate. This influence of fetal genotype on progesterone production rate is associated with a marked stimulation of ovarian function at a time when high concentrations of PMSG are present in maternal blood.

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