Effect of adrenocorticotrophic hormone on gonadotrophin releasing hormone-induced luteinizing hormone secretion in vitro.

Abstract

An in vitro perifusion study investigated the effect of different forms of adrenocorticotrophic hormone (ACTH) on gonadotrophin releasing hormone (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 nonpregnant female sheep (without corpora lutea, unless otherwise stated). The 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 duplicate set of chambers, ACTH1-24 (5 x 10(-7) M) was included in the perifusate 0.5 h before the first GnRH challenge. Potassium chloride (KCl; 100 mM) was administered 2 h after the second GnRH challenge to assess the viability of the tissue and the size of the releasable LH pool. Results were expressed as percentage of LH secretion. The influence of ACTH1-24 on oestradiol sensitisation was also examined using pituitaries obtained during the luteal phase. Pituitary tissues were perifused throughout with 1 x 10(-9) M or 6 x 10(-11) M oestradiol in the medium. The LH response to the second GnRH challenge (GnRH 2) was significantly greater (p < 0.01) than after the first (GnRH 1) at the highest dose of GnRH (2.5 x 10(-10) M; 2547 +/- 804 vs. 4547 +/- 1013%), but at the lower doses (1 x 10(-10) M or 5 x 10(-11) M), the self-priming effect of GnRH was not evident (3016 +/- 550 vs. 2932 +/- 490% and 841 +/- 205 vs. 711 +/- 87%). Treatment with ACTH1-24 (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 KCl was also similar from control and ACTH1-24-treated tissue at all GnRH doses. Both lower doses of GnRH (1 x 10(-10) M or 5 x 10(-11) M) produced the self-priming effect when the pituitary tissue 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, ACTH1-24 treatment did not affect LH secretion (neither basal nor potassium-induced). The effect of ACTH1-39 (1 x 10(-8) M or 5 x 10(-7) M; n = 6 chambers per combination) on GnRH-induced LH secretion was examined 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). At the lower dose (1 x 10(-8) M), ACTH1-39 influenced neither tonic nor GnRH-induced LH secretion. The LH released by KCl was also similar to the control and ACTH-treated tissue. In contrast, the higher dose of ACTH1-39 (5 x 10(-7) M) increased tonic LH secretion immediately after inclusion in the medium (104 +/- 3 vs. 161 +/- 20%), but suppressed the GnRH self-priming effect after 2.5 x 10(-10) M, i.e., the LH responses 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. 2638 +/- 431%). When the effect of this higher dose of ACTH1-39 on oestradiol-priming was investigated, ACTH increased tonic LH secretion but 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 KCl compared to the controls (1803 +/- 409 vs. 4302 +/- 1017%). In summary, in vitro, ACTH1-24 did not affect either tonic LH secretion, the GnRH self-priming effect, or oestradiol sensitisation. The entire ACTH1-39 increased tonic LH secretion, but reduced GnRH self-priming and oestradiol sensitisation. (ABSTRACT TRUNCATED)