The Effects of a Gonadotropin-Releasing Hormone Agonist on Ovulation and Steroidogenesis during Perfusion of Rabbit and Rat Ovariesin vitro*

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Abstract
The ability of a [gonadotropin-releasing hormone] agonist (GnRHa) to exert direct effects on rat and rabbit ovaries were examined in vitro. Ovaries of estrous rabbits and immature, PMSG[pregnant mare serum gonadotropin]-primed rats were surgically removed and perfused with a defined medium via an aortic cannula. The ovary remained viable and capable of undergoing ovulation in response to LH[luterinizing hormone]. Samples of perfusion medium were taken for steroid measurements and the number of ovulations determined by direct observation (rabbit) or oocyte recovery (rat). Follicles of ovaries perfused with medium alone rarely ovulated. GnRHa (0.1 .mu.g/ml) induced ovulations in 6 of 7 rat ovaries (4-22 ovulations per ovulating ovary); this effect was blocked by a GnRH antagonist. A much higher dose of the agonist (10 .mu.g/ml) induced ovulations in only 7 of 15 rabbit ovaries. GnRHa caused small but significant increases in progesterone levels in the perfusion medium in both species in comparison to no treatment. Mean estradiol levels also tended to be higher in the GnRHa groups in comparison to controls but the differences were not significant. GnRHa appears to act directly on both the rabbit and rat ovary but the rat ovary, is much more sensitive to its ovulation-inducing effects.