Induction of transient functional luteolysis in cyclic sheep by a 3 beta-hydroxysteroid dehydrogenase inhibitor

The mechanism by which prostaglandin F2 alpha terminates luteal function in the sheep is unclear even though it is used extensively in animal husbandry. At the time of luteal regression, a decrease in 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) activity is apparent in the corpus luteum, but it is not known whether the decrease in enzyme activity is the primary cause of structural luteolysis. The effect of trilostane, a 3 beta-HSD inhibitor, on luteal function and morphology has therefore been investigated. Intravenous injection of trilostane in the mid-luteal phase of the oestrous cycle caused a decrease in ovarian tissue progesterone content. A transient decrease in peripheral and utero-ovarian vein plasma progesterone was observed but there was no significant effect on the length of the luteal phase of the cycle. There was no significant change in plasma 13,14-dihydro-15-oxo-prostaglandin F2 alpha during the period when plasma progesterone was depressed. Morphological examination of the corpora lutea revealed a decrease in the concentration of electron-dense granules without any other features of impending luteal regression. When plasma progesterone was reduced for more than 10 h by two injections of trilostane 4h apart, there was again no subsequent effect on the length of the oestrous cycle or on the return to oestrus. Plasma progesterone returned to preinjection levels within 24 h of injection. This evidence suggests that competitive inhibition of 3 beta-HSD activity, per se, is ineffective in bringing about structural luteolysis.