Rabbit follicular adenylyl cyclase activity. I. Conditions of assay and gonadotropin sensitivity in granulosa cells and follicle shells.

Abstract

Studies were conducted to characterize further the gonadotropin-induced stimulation of the adenylyl cyclase (AC) of estrous rabbit follicles by luteinizing hormone (LH) and follicle stimulating hormone (FSH). AC activities were measured in homogenates of granulosa cells and follicle shells. Follicle shells contained the normal complement of thecal cells as well as peripheral granulosa cells which were not expressed when follicles were popped. Maximal AC stimulation of 10-fold and 13-fold in follicle shells and 11-fold and 18-fold in granulosa cells for LH and FSH, respectively, required mM ATP concentrations in the presence of very limiting (<1 mM) Mg2+ concentrations. GTP could not substitute for ATP, nor did it affect AC activity in the presence of ATP. The sensitivities to various LH and FSH fractions of the granulosa cell and the follicle shell AC were compared. Follicle shells and granulosa cells from estrous follicles were equally sensitive to low concentrations of LH and required 0.06 μg/ml LH to achieve half-maximal stimulation of the AC. The sensitivities of the AC of these follicular compartments were also tested with four different FSH fractions: a porcine FSH fraction, NIH-FSH-P1; two ovine FSH preparations, NIH-FSH-S13 and HP-FSH-G4–150C [the latter, a highly purified fraction supplied by Dr. Harold Papkoff (HP)]; and rabbit FSH, HP-FSH-ex. 130-D (a partially purified preparation). At least 30–200-fold higher concentrations of all of these FSH fractions compared with LH were required to stimulate half-maximally the AC of both follicle shells and the rabbit granulosa cells. However, the maximal degree of stimulation by FSH and LH of the granulosa cells and follicle shell AC systems was not different. A comparison of the gonadotropin sensitivity and maximal activity of the AC of small antral rabbit follicles with that of estrous follicle shells revealed that the AC of the small antral follicle shells was equally sensitive to LH and equally insensitive to FSH whereas the maximal degree of stimulation by these gonadotropins was markedly lowered. These data on the low sensitivity of the AC of rabbit follicle shells as well as granulosa cells raise questions concerning the role of FSH in follicular functions in rabbits.