AROMATASE-ACTIVITY IN THE MARE OVARY DURING ESTROUS-CYCLE - MEASUREMENT OF ENDOGENOUS STEROIDS AND OF THEIR IN-VITRO INHIBITORY EFFECT

This present study was undertaken to clarify estrogen synthesis in the mare ovary. First of all, an evaluation of endogenous steroid content s was carried out in the follicular fluid and in the luteal tissue at different stages of the luteal phase. Radioimmunoassays were performed after separation and purification of each hormone by chromatography. High amounts of conjugated (0.9 mg/l) and unconjugated (4 mg/l) estrad iol-17 beta were found in the follicular fluid of the large follicules (50 mm). These concentrations of estrogens decreased drasticaly in th e luteal tissue, and only low levels of circulating estrogens are foun d during the luteal phase. On the other hand, a high aromatization abi lity has been evidenced in the cyclic corpus luteum in vitro. In an at tempt to clarify the regulation of estrogen synthesis, we have tested the inhibitory effect of several endogenous steroids on equine ovarian aromatase activity. 5 alpha-Dihydrotestosterone appeared to be the mo st potent competitive inhibitor (K-i = 181 nmol/l) of aromatase activi ty, while the addition of a 3-sulfate group induced a slump in the inh ibitory potency of estrone (K-i = 397 nmol/l vs 2206 nmol/l) and dehyd roepiandrosterone (K-i = 291 nmol/l vs 6157 nmol/l). The physiological role of these conjugated steroids has not been known until now; we su ggest that they would play a role in protecting aromatase from inhibit ion, in vivo. The high amounts of progesterone found in the luteal tis sue (1.3 g/kg of proteins) might play a role in the regulation of estr ogen production either by suppressing the induction of aromatase synth esis or by inhibiting the activity of the enzyme complex.