Identification and measurement of testosterone in plasma and follicular fluid of the mare, using gas chromatography–mass spectrometry associated with isotope dilution

Abstract

Testosterone has been identified by mass spectrometry in blood and follicular fluid aspirated from mature Graafian follicles of mares. Quantitative measurements made by gas chromatography–mass spectrometry have validated the determination of plasma testosterone made by radioimmunoassay. However, because of high levels of epitestosterone (17α-hydroxyandrost-4-en-3-one) in the follicular fluid, radioimmunoassay overestimates the true concentrations of testosterone. The occurrence of testosterone in mare follicular fluid at a concentration which is two orders of magnitude higher than that in peripheral plasma suggests that the follicle may contribute to the production of circulating testosterone. A biosynthetic pathway for oestradiol-17β which involves testosterone is therefore likely to occur in the mare ovary as in many other mammalian species.