Ž .
had a significantly greater P - 0.001 cortisol response than VH, most prominent
Ž .
Ž P s 0.06 in the peak values within an hour after the treatment samples no. 4 and 5; 30
. and 60 min after the ACTH injection, respectively . Progesterone levels after treatment
Ž .
did not differ P s 0.45 between the groups, although it was complementary to the
release of cortisol. Compared with VH, RBH had a higher peak in cortisol but a smaller rise in progesterone.
The duration of significantly elevated hormone concentrations after treatment did not differ between the groups. In RBH as a group, progesterone concentration was elevated
Ž .
for 204 min range: 150–300 min
and the VH group returned to pretreatment Ž
. Ž .
progesterone level after 246 min range: 180–300 min , P s 0.30 . Cortisol concentra- Ž
. tions were elevated for a longer period, with a mean of 276 min range: 240–300 min
Ž .
Ž .
in RBH and 240 min range: 210–300 min in VH P s 0.08 .
4. Discussion
4.1. Stress and repeat breeding Oestrus is an extremely stress-sensitive period whose carefully regulated events aim
to produce a fertilisable oocyte at ovulation and to convey gametes with precise timing in the oviduct. If any of these events are disrupted or delayed, fertilisation failure will
occur. In RBH, it is suggested that a hormonal asynchrony during oestrus, induced by suprabasal progesterone concentrations, is the underlying cause for an unsuitable oocyte
maturation, a malfunction in sperm transport and an improper timing of the encounter of spermatozoa and the delayed, newly ovulated oocyte. Experimentally induced suprabasal
progesterone concentrations in normal heifers caused asynchronies similar to those observed in RBH: prolonged oestrous duration, prolonged and increased oestradiol-17b
Ž release, delayed LH-peak, resulting in a subsequent lower fertility Duchens et al., 1994;
. 1995a,b . The suprabasal plasma progesterone concentrations could either be a conse-
quence of an incomplete luteolysis or an increased progesterone release from extra- gonadal sources. Treatment of RBH with luteolytic drugs has not proven effective
according to field practice experience. A second alternative to increased progesterone levels is a sustained adrenal stimulation associated with environmental or social stress.
Under experimental conditions, heifers treated with ACTH in the pre-ovulatory period presented increased progesterone concentrations with a inhibition of the LH-surge and
Ž .
oestrous behaviour Stoebel and Moberg, 1982 , fairly similar to what has been detected Ž
. in RBH Gustafsson et al., 1986; Bage et al., 1997 .
˚
4.2. Adrenal steroid secretion in RBH The present study indicates that there is a difference in steroid secretion from the
adrenals when ovariectomised RBH and normal heifers are subjected to simulated acute stress by an ACTH-challenge. From the results, it is not possible to show that the
suprabasal progesterone levels are of adrenal origin since the ACTH-challenge did not result in a higher progesterone secretion in RBH than VH. The capacity for cortisol
secretion from the adrenals was, however, definitely higher in RBH. This is obvious already when looking at the hormone baseline concentrations, which were significantly
higher for cortisol in RBH, and after treatment, with a significantly higher cortisol peak. Progesterone baseline concentrations were equally low in both groups, with many
samples below the detection limit, making it difficult to draw any conclusions. When considering the conversion of pregnenolone to progesterone, there are two pathways
Ž .
available, depending on which of two possible enzymes acts first review: Hall, 1994 . It might be so that in RBH, the cortisol-synthesising pathway is mostly employed in the
adrenal glands. Bearing in mind that cortisol and progesterone are not only products deriving from the same precursor pregnenolone, but that progesterone itself is a
well-known precursor for cortisol in an additional synthesis pathway, it is suggested Ž
. although not further investigated in this study that RBH might possibly have a higher
progesterone production capacity when cortisol is not secreted. The duration of signifi- cantly increased cortisol concentrations after treatment was notably longer in RBH than
Ž .
VH P s 0.08 . ACTH administration generally gives a peak response independent of Ž
. dose and higher doses result in a more prolonged effect Verkerk et al., 1994 . The
significantly heavier RBH were de facto given a lower dose than VHrkg body weight. When taking into account the dilution effect from their high body weight, large body
mass and blood volume, one might speculate that their adrenal response could be even higher than the actual results shown in this study. To clarify this, ACTH should be
administered on unitsrkg body weight in a future study. In the light of the current results, the advantages of a CRH-challenge should also be considered for an overall
physiological reaction that besides adrenal steroid release also includes suppressed immune function and central nervous system-mediated behavioural stress responses.
4.3. Chronic stress and indiÕidual stress sensitiÕity Ž
. da Rosa and Wagner
1981 reported that when adrenal-ectomised cows were
continuously infused with ACTH from day 2 of the oestrous cycle, plasma concentra- tions of progesterone exhibited a transitory increase and then remained suppressed.
Similar observations have been reported in ewes treated twice daily with ACTH on days Ž
. 12–16 of the oestrous cycle Cooke and Benhaj, 1989 . These authors found that while
secretion of cortisol remained relatively unchanged, progesterone concentrations de- clined progressively with an increasing number of ACTH injections. The results of these
experiments, together with the present findings, suggest that one component of the repeat breeding syndrome may be related to frequent stimulation of the HPA axis.
Frequent ACTH administration and chronic stress might change the microsomal environ- ment and the enzyme affinity, and therefore result in a shift in the pathways. Chronic
stress may magnify the efficiency of the ACTH-secreting mechanism, expressed by a faster increase in corticosterone in stressed rats compared with controls when additional
Ž .
stress was introduced Sakellaris and Vernikos-Danellis, 1975 . The same response pattern was exhibited in cows already exposed to stressors that showed increased HPA
Ž .
axis response when novel stimuli were presented Munksgaard and Simonsen, 1996 . Response to stress or ACTH treatment may differ markedly among individuals,
although evidence is available for a consistency in the response within one individual.
There was a consistent ability in cows for the cortisol response to increase, as the ACTH Ž
. dose increased Verkerk et al., 1998 . In a behavioural study, growing cattle showed a
Ž .
consistency in baseline ACTH and individual stereotypy levels over age Redbo, 1998 . Individual stress sensitivity is thought to be acquired in early postnatal life when stable
Ž individual differences in HPA responsiveness to stressful stimuli develops Meaney et
. al., 1996 . This was clearly illustrated in a study where 10 pairs of monozygous twin
cows responded differently within pairs when they were subjected to the same ACTH treatment, suggesting that environmental and management experiences rather than
Ž genetic similarities may modulate an individual’s adrenal response Verkerk et al.,
. 1994 .
In intensive management of dairy cows, genetic selection has increased production levels considerably. However, apart from a favourable increase in production, animals
that have been selected for high production efficiency seem to be more at risk for Ž
behavioural, physiological, and immunological problems for review, see Rauw et al., .
1998 . Chronic stress and stereotypies can develop under sub-optimal life conditions where normal behaviour cannot be expressed to a full extent. Many acute stress
situations are inevitable in livestock management such as palpation per rectum at oestrus and AI, which causes significant increase in plasma cortisol and may be potential
Ž .
stressors for cows Nakao et al., 1994 .
5. Conclusions
