Acute Effects of Estradiol and Follicle-Stimulating Hormone on Specific Binding of Human [125I]Iodo-Follicle-Stimulating Hormone to Rat Ovarian Granulosa Cellsin vivoandin vitro*
Twenty-four day old hypophysectomized and intact immature female rats were used to determine the time dependent effects of estradiol on FSH [follicle stimulating hormone] stimulation of FSH receptor in granulosa cells. In hypophysectomized rats, a single 2 .mu.g injection of human (h) FSH (LER-1577) given alone, concomitantly with, or 6 h after 1.5 mg estradiol caused a 50% increase in the available FSH receptor content in granulosa cells by 24 h. The same dose of hFSH given 12 or 24 h after estradiol caused a 300% increase in available FSH receptor by 24 h. To establish the accuracy of these in vitro FSH-binding assays, it was imperative to determine if the hFSH administered in vivo occupied binding sites and thereby masked increases in total available FSH receptor occurring within the first 24 h. Therefore, 20 .times. 106 cpm of 125I-labeled hFSH, equivalent to 250 ng hFSH LER-1577, was administered s.c. to estradiol-primed hypophysectomized rats 1 h after an injection of saline or 2 .mu.g hFSH. The amount of labeled hormone bound in vivo to 30,000 .times. g crude membrane pellets was measured at 2, 6, 12 and 24 h. These same pellets were resuspended and incubated with 125I-labeled hormone in vitro to measure available FSH-binding sites. The 2 .mu.g dose of hFSH did not reduce the uptake of hormone in vivo nor did it mask the early increases in available FSH receptor as measured in vitro. Estradiol alone did not affect the rate of ovarian uptake of labeled hFSH, the number of FSH-binding sites, or ovarian weight within 24 h. In intact rats, estradiol administered 24 h before hFSH did not enhance FSH stimulation of FSH receptor but did enhance FSH stimulation of ovarian weight. Specific effects of estradiol on granulosa cell differentiation in small preantral follicles are probably time-dependent. Estradiol-mediated enhancement of FSH stimulation of FSH receptor occurs rapidly and may serve as 1 mechanism for selecting follicles which will be more responsive to FSH.