Sex differences in response to adrenocorticotropin (ACTH) administration in sheep

This thesis summarises and discusses results of two separate studies on the effects of adrenocorticotropin (ACTH) on the secretion of steroid hormones in sheep of different sex and gonadal status. Furthermore, it includes a study on the effects of ACTH and progesterone on luteinising hormone (LH) secretion in castrated rams as well as a study on sex differences in oestrogen receptor (ER) content of adrenal glands of sheep. Testosterone concentrations of castrated males before and after ACTH administration were below the detection limit of the assay used. After ACTH administration cortisol, progesterone and 17α-hydroxy pregnenolone (17OHP5) were elevated for several hours and differed from pre-ACTH levels. Cortisol after ACTH administration was higher in females than in males in both the breeding and the non-breeding season. Gonadectomy eliminated the differences between the ewes and the rams. Rams showed a lower response in the breeding season than in the non-breeding season, which was not observed in the ewes. After ACTH administration the number of LH pulses in the first half of the experimental period was significantly lower than it was in the second half. No effect was seen on mean LH secretion. Progesterone treatment had no effect on either the number of LH pulses or LH secretion. Ewes had higher levels of cytosolic ERs in the adrenals than did the rams and gonadectomy increased ER levels. No differences were observed in messenger ribonucleic acid (mRNA) levels for ER sub-type α (ERα) and insulin-like growth factor I (IGF-I). Progesterone receptor (PR) mRNA levels were reduced in ovariectomised ewes and enhanced in castrated rams. All of the animals had positive nuclear staining for ERα in the adrenal cortex, and no differences were observed between the groups. The studies revealed progesterone and 17OHP5 secretion after ACTH administration, with the adrenal gland being the most likely source. Sex differences were observed in ACTH-induced cortisol secretion in intact sheep. Adrenocorticotropin decreased LH pulsatility in recently castrated rams. We demonstrated the existence of ER in the adrenal gland of sheep and found varying sensitivity to oestrogens as the ER levels differed among gender and gonadal status. Oestrogens may affect steroidogenesis directly at the adrenal cortex, which suggests that oestrogens are partly responsible for the sex differences in cortisol secretion in sheep.