Effect of ACTH-challenge on progesterone and cortisol levels in ovariectomised repeat breeder heifers

In order to investigate the potential influence of stress as a component of the repeat breeding syndrome, the adrenocortical capacity for steroid prod uction was evaluated in ovariectomised dairy heifers, In repeat breeder hei fers (RBH), marginally elevated plasma progesterone levels during oestrus, so-called suprabasal progesterone levels, have earlier been measured and ar e believed to impair fertility. The aim was to distinguish if this progeste rone could be of extra-gonadal or in this case, adrenal origin. Baseline le vels of plasma cortisol and progesterone were determined as well as the cor responding response after induced acute stress in the form of an adrenocort icotropin (ACTH)-challenge. Comparisons were made between strictly selected RBH, n = 5 and virgin heifers (VH), n = 5 of the Swedish Red and White bre ed. The heifers were used as their own pre-challenge controls in a 2-day tr ial. On the control day, saline was injected i.v. and on the treatment day, a synthetic analogue of ACTH (60 mu g Synachten(R)). Via a jugular vein ca theter, blood samples were collected every 30 min for 6 h each day of the e xperiment. Analyses for plasma progesterone and cortisol were made. RBH had a significantly higher (P < 0.01) pretreatment baseline cortisol level (10 .1 +/- 2.3 nmol l(-1)) than VH (2.6 +/- 0.2 nmol l(-1)). Moreover, the cort isol response after stimuli was stronger in RBH than VH, especially concern ing total hormone production (P < 0.001), but there was also a tendency tow ards higher peak values (P = 0.06) and longer duration of significantly inc reased hormone concentrations (P = 0.08). Progesterone concentrations, howe ver, did not differ between the groups. Both baseline levels (P = 0.25) and posttreatment production(P = 0.45) were of the same magnitude in RBH and V H. In conclusion, the study could not confirm that suprabasal progesterone concentrations during oestrus in RBH derive from the adrenal glands. Still, apparent differences were found in adrenocortical activity when ovariectom ised heifers, VH and RBH, were subjected to an ACTH-challenge. It is sugges ted that a sustained adrenal stimulation associated with environmental or s ocial stress could be one factor in the repeat breeding syndrome. (C) 2000 Elsevier Science B.V. All rights reserved.