The Gonadotropin-Primed Rat: A Model for Testing Agents that Influence Steroid Biosynthesis

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Abstract
Immature female rats were injected with 50 IU pregnant mare serum gonadotropin (PMSG) on day 0 and 25 IU human chorionic gonadotropin (hCG) on day 3. Plasma progestin rose to concentrations ranging from 350 to 625 ng/ml from day 5 to day 17. Expression of plasma progestin concentrations in ng/ml/mg ovary weight showed maximal concentrations of approximately 3 ng/ml/mg ovary weight on days 7 to 11 followed by a gradual decrease through day 19. Although the pituitary glands of D5GPR were significantly larger (5.20 mg ± 0.49 mg SE) than controls (3.65 mg ± 0.15 mg SE), radioimmunoassay indicated serum LH levels were not different than controls. Administration of LHreleasing factor (LRF) to D5GPR caused a significant increase in serum LH and a significant decrease in plasma progestin concentrations 20 min after LRF administration. Injection of ergocornine methanesulfonate did not alter serum LH or plasma progestin concentrations. A minimum iv dose of 3 to 5 μg prostaglandin F2α caused a maximal decrease (approximately 50%) in peripheral plasma progestin concentrations within 30 min in D5GPR. Injection of 1.5 × 10-5 mole of aminoglutethimide phosphate (AG) caused a decrease in plasma progestin to approximately 25% of control concentrations within 30 min in D5GPR. Injection of 1.5 × 10-5 mole of metyrapone increased plasma progestin to 175% of control concentrations within 30 min of administration in D5GPR. It is concluded that the immature female rat primed with PMSG and hCG is a convenient in vivo biological model for the study of agents which influence steroid biosynthesis. (Endocrinology93: 723, 1973)

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