Hormonal Changes Associated with Estrogen Induced Luteolysis in the Pregnant Hamster: A Reevaluation of the Luteotropic Complex

Abstract

Hamsters were injected sc on Day 1 of pregnancy (sperm positive) with 50 μg estradiol cyclopentylpropionate (ECP) or peanut oil. On Day 5, serum progesterone (P₄) was 10.6 ng/ml in controls vs 3.1 ng/ml after ECP. In the ECP group, serum prolactin (PRL) and follicle stimulating hormone (FSH) did not differ from controls but serum luteinizing hormone (LH) was significantly lower than that of the controls, and usually below the sensitivity of the radioimmunoassay (RIA). After ECP, structural signs of luteolysis (weight and histology) and absence of antral follicles characterized the ovary. Injection of an anti-LH serum on Day 4 halved serum P₄ levels on Day 5 in control animals but caused no further lowering of P₄ in ECP-treated hamsters. Treatment on Days 1-5 with 1.0 IU hCG or 10 μg LH plus ECP on Day 1 restored, by the afternoon of Day 5, serum P₄ to the control range (9-10 ng/ml) and antral follicles were now present. The results indicate that a large dose of ECP causes luteolysis by reducing LH levels and reinforce the concept of a luteotropic complex in the hamster with PRL and FSH constituting the minimal components and LH serving as a synergist.