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 th e 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; tota l 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, we ighed and used to isolate granulosa and thecal/interstitial cells for assessment of basal and gonadotrophin-responsive steroidogenesis in vi tro, or homogenized to extract total RNA for Northern analysis of 17-h ydroxylase/C-17-20-lyase (cytochrome P-450(c17 alpha)) mRNA, Serum oes tradiol and uterine weight were measured as indices of ovarian oestrog en production; androstenedione was measured to reflect ovarian androge n production, Consistent with the two-cell, two-gonadotrophin model of oestrogen synthesis, increased ovarian oestrogen secretion only occur red if both rhFSH and rhLH were given simultaneously, Treatment with r hFSH alone stimulated ovarian weight gain and granulosa cell aromatase activity without oestrogen secretion, whereas rhLH alone stimulated t hecal androgen synthesis and androgen secretion, When the total rhLH d ose was fixed at 1 IU, giving rise to an unmeasurably low serum concen tration of rhLH, additional treatment with rhFSH (30-72 IU) dose-depen dently stimulated serum androgen concentrations as well as oestrogen c oncentrations, The similar to 2.0 kb-sized P-450(c17 alpha) mRNA trans cript was undetectable in the ovaries of untreated control animals but was abundant in the ovaries of positive controls treated with 15 IU o f pregnant mare serum gonadotrophin, Treatment with 1 IU of rhLH alone barely induced a P-450(c17 alpha) mRNA signal and treatment with 30 I U of rhFSH alone was completely ineffective, However, combined treatme nt 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 v ia granulosa-on-theca paracrine signalling, and (ii) tonic stimulation by rhLH is required to facilitate thecal responsiveness to this rhFSH -activated paracrine signal(s).