Seasonal variations in the responses of luteal and follicular luteinizing hormone-stimulated adenylyl cyclases to estradiol implants and implant removal in pseudopregnant rabbits.

Summer rabbits appeared to be less sensitive to the presence of exogenous estradiol or to the withdrawal of exogenous estradiol than were Winter rabbits. Suppression of the luteinizing hormone(LH)-stimulated adenylyl cyclase of corpora lutea by estradiol-filled silastic capsules, which resulted in serum estradiol concentrations approximately 3.8 X control (high level), appeared to be greater in Winter rabbits than in Summer rabbits. Both high level and low level (2.1 X control) estradiol implants suppressed follicular LH-stimulated adenylyl cyclase in Winter rabbits but neither size capsule had an effect in Summer rabbits. Serum LH concentrations were equally low in Winter rabbits using either high or low level estradiol implants, while only the high level implants caused a decrease in serum LH in Summer rabbits. Withdrawal of the exogenous estradiol caused a precipitous fall in serum progesterone concentrations in Winter rabbits; estradiol withdrawal in Summer rabbits was without effect.