EFFECT OF LUTEINIZING-HORMONE ON FOLLICLE-STIMULATING HORMONE-ACTIVATED PARACRINE SIGNALING IN RAT OVARY

'Pure' follicle stimulating hormone (FSH) and luteinizing hormone (LH) are expected shortly to become available for pharmaceutical use in the clinical setting, To test the contribution of LH to optimal ovarian responsiveness to FSH, 21-day-old hypophysectomized, immature, female rats received four s.c. injections of recombinant human LH (rhLH; total dose 1-10 IU) and/or rhFSH (total dose 30-72 IU) given at 12-hourly intervals, At 48 h after the first injection, ovaries were removed, weighed and used to isolate granulosa and thecal/interstitial cells for assessment of basal and gonadotrophin-responsive steroidogenesis in vitro, or homogenized to extract total RNA for Northern analysis of 17-hydroxylase/C-17-20-lyase (cytochrome P-450(c17 alpha)) mRNA, Serum oestradiol and uterine weight were measured as indices of ovarian oestrogen production; androstenedione was measured to reflect ovarian androgen production, Consistent with the two-cell, two-gonadotrophin model of oestrogen synthesis, increased ovarian oestrogen secretion only occurred if both rhFSH and rhLH were given simultaneously, Treatment with rhFSH alone stimulated ovarian weight gain and granulosa cell aromatase activity without oestrogen secretion, whereas rhLH alone stimulated thecal androgen synthesis and androgen secretion, When the total rhLH dose was fixed at 1 IU, giving rise to an unmeasurably low serum concentration of rhLH, additional treatment with rhFSH (30-72 IU) dose-dependently stimulated serum androgen concentrations as well as oestrogen concentrations, The similar to 2.0 kb-sized P-450(c17 alpha) mRNA transcript was undetectable in the ovaries of untreated control animals but was abundant in the ovaries of positive controls treated with 15 IU of pregnant mare serum gonadotrophin, Treatment with 1 IU of rhLH alone barely induced a P-450(c17 alpha) mRNA signal and treatment with 30 IU of rhFSH alone was completely ineffective, However, combined treatment with 1 IU of rhLH and 30 IU of rhFSH markedly enhanced the P450(c17 alpha) mRNA signal to a level approaching the positive control, Since P-450(c17 alpha) mRNA is expressed exclusively in thecal cells, which do not possess FSH receptors, we conclude that (i) rhFSH upregulates thecal P-450(c17 alpha) mRNA and hence follicular androgen synthesis via granulosa-on-theca paracrine signalling, and (ii) tonic stimulation by rhLH is required to facilitate thecal responsiveness to this rhFSH-activated paracrine signal(s).