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To obtain direct evidence for FSH-stimulated paracrine signaling in the ovary, 21-day-old intact or hypophysectomized female Wistar rats received four sc injections of recombinant human FSH (rhFSH; total dose, 16-72 IU) at 12-h intervals. Ovaries were removed 48 h after the first injection to extract total RNA for Northern analysis of 17-hydroxylase/C17-20-lyase (cytochrome P450c17 alpha) mRNA or to isolate thecal/interstitial cells for assessment of basal and hLH-responsive androgen synthesis in vitro. In situ hybridization with a 35S-labeled cytochrome P450c17 alpha cRNA probe confirmed that expression of the cytochrome P450c17 alpha gene was specific to thecal/interstitial cells. The approximately 2.0-kilobase P450c17 alpha mRNA signal in ovarian total RNA from intact animals was increased approximately 5-fold by treatment with rhFSH (total dose, 72 IU) or PMSG (15 IU). This effect was shown to be dose dependent, with a approximately 2-fold increase in response to 16 IU (total dose) rhFSH. P450c17 alpha mRNA levels in isolated granulosa and thecal/interstitial cell total RNA from intact animals were compared to establish which was the principal cellular site of P450c17 alpha mRNA expression. The P450c17 alpha mRNA signal was undetectable in control granulosa cells and only barely discernible after treatment with 72 IU (total dose) rhFSH. In contrast, P450c17 alpha mRNA was abundant in control thecal/interstitial mRNA, and its level was increased 4- to 6-fold by treatment with rhFSH. Treatment of hypophysectomized animals with rhFSH did not consistently alter ovarian P450c17 alpha mRNA levels. During culture for 48 h in serum-free medium, basal androgen (androstenedione plus androsterone) production by thecal/interstitial cells from intact animals was unaffected by treatment with rhFSH in vivo, but hLH-stimulated androgen production by these cells was enhanced approximately 2-fold. Neither basal nor hLH-responsive androgen production by thecal/interstitial cells from hypophysectomized animals was altered by previous treatment with rhFSH in vivo. Treatment of thecal/interstitial cell cultures from both intact and hypophysectomized animals with inhibin (0.1-30 ng/ml), a putative granulosa-derived paracrine factor, did not measurably affect basal androgen synthesis, but potently enhanced LH-responsive androgen synthesis in vitro. Similarly, treatment of thecal/interstitial cell cultures with conditioned medium from FSH-treated granulosa cell cultures significantly enhanced LH-responsive, but not basal, androgen production. We conclude that treatment of pituitary-intact rats with "pure" FSH modulates thecal/interstitial cell androgen synthesis. Granulosa cells, but not thecal cells, possess FSH receptors, and thecal/interstitial cells are the principal ovarian sites of P450c17 alpha expression.(ABSTRACT TRUNCATED AT 400 WORDS) more...