Steroid secretion by perifused rabbit follicles: effect of repeated gonadotropin challenges.

Abstract

The capacity of a follicle to respond to a second challenge of LII is lost after exposure to a high dose of LII as indicated by a loss of hormone-stimulable adenylyl cyclase activity. To determine if a similar depression of steroidogenesis occurs, large follicles (1.5-2.0 mm) were isolated from estrous rabbits, placed in a perifusion system, and challenged at 6 h intervals for a 24-36 h period with LII:FSH at one of the following doses: 10 ng:2.5 Mg; 40 ng:2.5 Mg; 1 Mg:2.5 Mg; or phosphate-buffered saline (PBS). Progesterone (P), testosterone (T), and estradiol-1 7 3 (E) were measured in the perifusate. At the completion of the perifusion, follicles were fixed in Bouin’s solution, embedded, sectioned, and stained with hematoxylin and eosin for histological studies. Basal steroid secretion decreased gradually during the first 12 h and was then maintained at a constant low level for the remaining 24 h. Follicles stimulated with 1 Mg: 2.5 g of LI-I: FSH at 12, 18, 24, and 30 h produced a significant (P<0.05) increase in T and E after only the first challenge of LII: FSH, whereas P increased significantly (P<0.05) after each challenge. The repeated challenge of follicles with 1 Mg:2.5 g of LH:FSH at 0, 6, 12, 18, 24, and 30 h resulted in a significant (P<0.05) increase in P production after each gonadotropin stimulation while T and E secretion was elevated significantly (P<0.01) after only the first challenge. Follicles stimulated with 10 ng: 2.5 g of LH-FSII at 0, 6, 12, 18, 24, and 18 h produced a significant (P<0.05) increase P after only the fourth challenge and in T after the second, third, and fourth challenge. There was no increase in E. Repeated stimulation with 40 ng:2.5 g of LII:FSH at 0. 6, 12, and 18 h resulted in a significant (P<0.05) increase in T after the second challenge and in P after the third and fourth challenges. Luteal cells were present only in follicles challenged with the 1 Mg:2.5 g dose of LH:FSH, regardless of whether the follicles were challenged 4 or 6 times. These results indicate, first, that the steroid secretion profile depends upon the dose of the gonadotropin. While low (10 ng) and medium (40 ng) doses of LH with a constant high dose of FSII (2.5 Mg) elevated significantly the secretion of P and T, only a high dose of LH (1.0 Mg) with FSH (2.5 g) increased E secretion. Second, P secretion, unlike T and E secretion, is not depressed following a challenge with a maximum dose of LH: FSH. Third, luteinization of perifused follicles apparently depends upon exposure to a high dose of gonadotropin.