Effect of ACTH (tetracosactide) on steroid hormone levels in the mare: Part A: Effect in intact normal mares and mares with possible estrous related behavioral abnormalities

Ovariectomized mares and mares with inactive ovaries may show signs of estrus. The reason behind this phenomenon is not clear; however, steroid hormones of adrenal origin have been suggested. Moreover, aberrant adrenal hormone production has been implied as a reason why some intact mares may change behavior. In the present study, the effect of ACTH on plasma levels of cortisol, progesterone, androstenedione and testosterone was investigated in intact mares with normal estrous behavior (‘controls’, n = 5) and intact mares that according to their owners showed deviant estrous behavior (‘problem’ mares, n = 7). Blood samples were collected hourly from 12:00 h until 14:00 h the following day (half-hourly between 14:00 and 17:00 h) on two occasions (at two estruses), with saline or ACTH treatment (tetracosactide) at 14:00 h (saline treatment day or ACTH treatment day). ACTH treatment caused a significant increase in plasma levels of cortisol, progesterone, androstenedione and testosterone in all mares (P < 0.05). An overall significant difference in cortisol response to ACTH was found (P < 0.05), with ‘problem’ mares showing a significantly lower increase in cortisol levels 30 min to 3 h post ACTH treatment (P < 0.001). The ‘problem’ mares also showed a significantly higher increase than controls in progesterone levels in the same time period (P < 0.05). The reason for the reduced adreno-cortical reactivity, with a low cortisol response to the ACTH treatment, in the ‘problem’ mares is unknown, but may indicate a difference in adrenal function as compared to control mares.