EFFECT OF ADRENOCORTICOTROPIC HORMONE ON GONADOTROPIN-RELEASING HORMONE-INDUCED LUTEINIZING-HORMONE SECRETION IN-VITRO

An in vitro perifusion study investigated the effect of different form s of adrenocorticotrophic hormone (ACTH) on gonadotrophin releasing ho rmone (GnRH)-induced luteinizing hormone (LH) secretion, particularly GnRH self-priming, and oestradiol sensitisation of the ovine pituitary . Fragments of pituitaries were obtained from mixed-breed adult nonpre gnant female sheep (without corpora lutea, unless otherwise stated). T he amount of LH released by different doses of GnRH (2.5 x 10(-10) M ( n = 9 chambers), 1 x 10(-10) M (n = 9), or 5 x 10(-11) M (n = 6)) was evaluated by giving two GnRH pulses (5 min each) 2 h apart. In a dupli cate set of chambers, ACTH(1-24) (5 x 10(-7) M) was included in the pe rifusate 0.5 h before the first GnRH challenge. Potassium chloride (KC l; 100 mM) was administered 2 h after the second GnRH challenge to ass ess the viability of the tissue and the size of the releasable LK pool . Results were expressed as percentage of LH secretion. The influence of ACTH(1-24) on oestradiol sensitisation was also examined using pitu itaries obtained during the luteal phase. Pituitary tissues were perif used throughout with 1 x 10(-9) M or 6 x 10(-11) M oestradiol in the m edium. The LH response to the second GnRH challenge (GnRH 2) was signi ficantly greater (p < 0.01) than after the first (GnRH 1) at the highe st dose of GnRH (2.5 x 10(-10) M; 2547 +/- 804 vs. 4547 +/- 1013%), bu t at the lower doses (1 x 10(-10) M or 5 x 10(-11) M), the self-primin g effect of GnRH was not evident (3016 +/- 550 vs. 2932 +/- 490% and 8 41 +/- 205 vs. 711 +/- 87%). Treatment with ACTH(1-24) (5 x 10(-7) M) did not affect tonic LH secretion nor the LH response to the first or second GnRH challenge at any of the GnRH doses tested. The LH released in response to KCI was also similar from control and ACTH(1-24)-treat ed tissue at all GnRH doses. Both lower doses of GnRH (1 x 10(-10) M o r 5 x 10(-11) M) produced the self-priming effect when the pituitary t issue was sensitised with the higher dose of oestradiol (1 x 10(-9) M; 1711 +/- 239 vs. 5085 +/- 1307%, and 1502 +/- 376 vs. 2619 +/- 629%). In the presence of lower concentrations of oestradiol (6 x 10(-11) M) , self-priming was observed only after the higher dose of GnRH (1 x 10 (-10) M; 1293 +/- 214 vs. 2865 +/- 436%), not the lower dose (5 x 10(- 11) M; 985 +/- 203 vs. 1271 +/- 436%). In spite of these differences, ACTH(1-24) treatment did not affect LH secretion (neither basal nor po tassium-induced). The effect of ACTH(1-39) (1 x 10(-8) M or 5 x 10(-7) M; n = 6 chambers per combination) on GnRH-induced LH secretion was e xamined using the higher (2.5 x 10(-10) M) or lower dose of GnRH (1 x 10(-10) M), with or without oestradiol sensitisation (1 x 10(-9) M). A t the lower dose (1 x 10(-8) M), ACTH(1-39) influenced neither tonic n or GnRH-induced LH secretion. The LH released by KCI was also similar to the control and ACTH-treated tissue. In contrast, the higher dose o f ACTH(1-39) (5 x 10(-7) M) increased tonic LH secretion immediately a fter inclusion in the medium (104 +/- 3 vs. 161 +/- 20%), but suppress ed the GnRH self-priming effect after 2.5 x 10(-10) M, i.e., the LH re sponses to GnRH 1 and 2 were similar (1786 +/- 294 vs. 1553 +/- 373%). However, the LH response to KCl was not significantly different (p > 0.05) between the control and ACTH-treated tissues (2333 +/- 286 vs. 2 638 +/- 431%). When the effect of this higher dose of ACTH(1-39) on oe stradiol-priming was investigated, ACTH increased tonic LH secretion b ut suppressed the self-priming effect of GnRH (1 x 10(-10) M GnRH; 945 +/- 274 vs. 922 +/- 323%; p > 0.05), and decreased (p < 0.05) the LH released in response to KCI compared to the controls (1803 +/- 409 vs. 4302 +/- 1017%). In summary, in vitro, ACTH(1-24) did not affect eith er tonic LH secretion, the GnRH self-priming effect, or oestradiol sen sitisation. The entire ACTH(1-39) increased tonic LH secretion, but re duced GnRH self-priming and oestradiol sensitisation. The initial effe ct of the entire ACTH(1-39) may be exerted via an interference with Gn RH receptors, whereas the latter effect may cause disruption of oestra diol-induced LH synthesis. (C) 1997 Elsevier Science B.V.