Technical Note
Adrenocorticotrophic hormone and dexamethasone failed to
affect milk yield in dairy goats: comparative aspects
A. Shamay, S.J. Mabjeesh, F. Shapiro, N. Silanikove
*Agricultural Research Organization (ARO), The Volcani Center, Institute of Animal Science, P.O. Box 6, Bet Dagan 50250, Israel
Received 6 December 1999; accepted 18 April 2000
Abstract
The ability of adrenocorticotrophic hormone (ACTH; single i.v. injection of 2.5 IU/kg BW) and dexamethasone (single i.m. injection of 36 mg/kg BW) to affect milk production was studied in mid-lactating Israeli Saanen goats. None of these treatments produced changes in milk yield and composition of the goats. The effects of ACTH on blood cortisol levels, and the effects of ACTH and dexamethasone on blood plasma concentrations of glucose, however, were consistent with previous reports in goats and cows. These responses suggest that ACTH and dexamethasone treatments produced their expected glucocorticoid effects. It is suggested that obstructing the axis: stress-ACTH-glucocorticoid-down regulation of milk yield, which was demonstrated in dairy cows, re¯ects the adaptation of goats to harsh conditions, and the selection pressure to produce milk under conditions which are considered stressful for other ruminants.#2000 Elsevier Science B.V. All rights reserved.
Keywords:ACTH; Dexamethasone; Dairy goats; Stress; Milk yield
1. Introduction
In bovines, an adrenocorticotrophic hormone (ACTH)-induced elevation in corticosteroids has been reported to negatively affect milk secretion (Flux et al., 1954; Cambell et al., 1964; Bremel and Gangwer, 1978; Varner and Johnson, 1983). The decrease in milk secretion is most likely related to the elevation of blood corticosteroids since injection of exogenous synthetic corticoids also produce marked depression in milk yield in dairy cows (Braun et al., 1970; Hartmann and Kronfeld, 1973; Shamay et al., 2000).
Results on the effects of ACTH and corticosteroids on milk yield in goats are few and inconsistent. Stewart and Thompson (1984) reported that ACTH and cortisol might increase milk yield, whereas Anderson et al. (1991) found no effect of dexametha-sone on milk yield in goats. Goats are better adapted to harsh environmental conditions than other domesti-cated ruminants, which is re¯ected in attenuated response to stressful conditions (Silanikove, 1992, 2000a,b). The response of animals exposed to stress can be separated into two phases: phase 1 gradually grades into phase 2 (Silanikove, 2000b). In phase 1, activation of the hypothalamic±pituitary±adrenal axis and the consequent increase of plasma cortisol con-centration are the most prominent responses of an animal (Silanikove, 2000b). Phase 2 is the adapted response to stress, and is characterized by decline in plasma cortisol activity to basal levels (Silanikove,
*Corresponding author. Tel.:972-8-9484436;
fax:972-8-9475075
E-mail address: nsilanik@agri.huji.ac.il (N. Silanikove).
2000b). Our objective was to ®nd how treatment with ACTH, or with synthetic corticosteroid (dexametha-sone) would affect the yield and composition of goat milk in comparison to established effects of these treatments in dairy cows. This information may con-tribute to better insight into the physiological capacity of goats to produce milk immediately following expo-sure to stressful conditions.
2. Materials and methods
2.1. Animals and general procedures
Non-pregnant Israeli Saanen goats (British Saanen crossed with local goats and back crossed with British Saanen bucks for three to six generations) in their second, third or fourth lactation at mid-lactation, and weighing approximately 70 kg, were used in this experiment. The experiments were carried out during October and November. Goats were machine milked twice daily at 07:00 and 16:00 hours, and milk yield was routinely recorded. Goats were fed the concen-trate portion of their diet during milking. Goats were fed cubes of alfalfa hay (40% NDF and 14% protein) ad libitum, and 1.5 kg concentrates (18% protein). Basic milk yield and composition was recorded for 3 days before treatments. Animals were accustomed to all procedures of sampling and treatment before the start of the experiment. The dose of ACTH chosen was based on Toerien et al. (1999) experiment and that of dexamethasone based on plasma glucose responses to various doses of dexamethasone.
2.2. ACTH treatment
Five goats were injected i.v. with 2.5 IU/kg of ACTH (Corticotropin A; Sigma Chemical Co., St. Louis, MO) after milking at 16:00 hours. Blood samples were taken before and after injection as described by Toerien et al. (1999). Control goats (n5) were injected at the same time with 40 ml saline.
2.3. Dexamethasone treatment
The experimental goats (n5) were injected i.m. into the neck with 36 mg/kg of dexamethasone
(dex-amethasone±HCl, Vitamed, Haifa, Israel) dissolved in 40 ml of saline after milking at 16:00 hours. Jugular blood samples were taken a day before, just before the injection, and in intervals of 24 h for 4 days after the injection.
2.4. Chemical analysis
The concentrations of lactose, protein (total, whey, and casein), and minerals (Na, K, Ca) in milk, and glucose in blood plasma samples were determined as described by Shamay et al. (2000). Cortisol concen-trations in blood plasma were determined by radio-immunoassay using commercial kit (Coat-A-Count DPC, Los Angeles, CA).
2.5. Statistical analysis
Variations were analyzed according to the General Linear Model of SAS (1988) for repeated measures over time. The model tested the post-treatment results if they differed signi®cantly from pre-treatments, using the pre-treatment results as co-variables, and compared between experimental (ACTH, or dexa-methasone injected) and control (saline injected) ani-mals.
3. Results
Goats in this experiment produced on an average 1.5(0.3) l per day of milk, which contained 18.2(2.6) g/l of protein, 12.6(3.1) g/l of casein, 6.3(2.8) g/l of whey proteins, 127(7) mM of lac-tose, 62(8) mM of potassium, 27(6) mM of sodium, and 25(6) mM of calcium. None of the dosages of ACTH or dexamethasone produced any signi®cant changes, or even signs for trends of changes, in milk yield and milk composition.
post-treatment in the range of 80±120 ng/ml. By 60 min post-treatment cortisol concentration dropped to 40±60 ng/ml and by 3 h post-treatment plasma cortisol concentrations dropped back to the basal level. Kinetics of plasma cortisol following ACTH treatment resembled very much the one described recently by Toerien et al. (1999) in Angora and Spanish goats.
Basal glucose concentration in the present experi-ments was 45(3.1) mg/100 ml, which was a typical value for blood glucose in ruminants. ACTH treatment induced transient increase in blood glucose concen-trations, which lagged behind the changes in plasma cortisol concentrations (Fig. 1). Peak blood glucose concentrations (822.1 mg/100 ml) were measured at 4 h post-treatment. By 8 h post-treatment glucose concentration (702.8 mg/100 ml) was still above the basal level, and by 24 h post-treatment it dropped back to the basal level.
In preliminary trial we found that a dose of 12 mg dexamethasone/kg elevated peak glucose concentra-tion to 63 mg/100 ml, a dose of 24 mg/kg to 100 mg/ 100 ml, and a dose of 36 mg/kg elevated plasma glucose concentrations to 130 mg/100 ml. The highest dose was chosen because it induced changes in plasma glucose concentration which were similar to those found in cows (Shamay et al., 2000). Dexamethasone treatment induced transient increase in blood plasma glucose concentration (Fig. 2). The increase in glucose concentration was measured at 24 h post-treatment, and the elevated concentrations were increased further at 48 h post-treatment. At 72 h
post-treatment plasma glucose concentration dropped back to the basal level.
4. Discussion
ACTH treatment did not negatively affect milk as found in dairy cows, nor positively affected it, as found by Stewart and Thompson (1984) in dairy goats. One possible explanation is that the ACTH that was used was not effective. However, this possibility may be ruled out because the peak and kinetics of cortisol concentrations following stimulation by ACTH found in the Saanen does in the present experi-ment were similar to those found in other breeds of goats (see Toerien et al., 1999, and references cited therein). Furthermore, the increase in blood plasma glucose concentration is consistent with the increase in cortisol and with the kinetics of glucose in the plasma of goats stressed by transportation (Nwe et al., 1996).
Dexamethasone is a potent synthetic glucocorti-coid, and this was re¯ected in the extended period of hyperglycemia in comparison to treatments with ACTH (Fig. 1 versus Fig. 2), or cortisol (Stewart and Thompson, 1984). The positive interrelationship between the dosage of dexamethasone and the induc-tion of hyperglycemia is consistent with the results of Maddux et al. (1988) in goats. The kinetics of changes in blood plasma glucose concentration found in the present experiment in goats are similar to those described in cows after single intramuscular injection
Fig. 1. Effect of ACTH treatment on jugular vein blood plasma glucose concentrations. Values are meanS.E.M., the response at 4 and 8 h post-treatment was signi®cant atp<0.01.
of dexamethasone (Hartmann and Kronfeld, 1973; Shamay et al., 2000). Thus, our results con¯ict with those of Stewart and Thompson (1984), who reported that cortisol may increase the milk yield in goats. On the other hand, our results are consistent with those of Anderson et al. (1991), that milk production in goats does not response to glucocorticoids administration as in the cow. Notwithstanding, the highest dose that was used in the present experiment was similar in absolute terms to the effective dose in cows (Hartmann and Kronfeld, 1973; Shamay et al., 2000), indicating that on a body weight basis the dose was 15 times higher in the goats.
Various stressful situations, whether environmental (heat stress, water deprivation; Silanikove, 1992), psychological (movement in truck and reallocation; Bremel and Gangwer, 1978), or the trauma of surgery (Bremel and Gangwer, 1978) and diseases such as mastitis (Shuster et al., 1991) negatively affect the milk yield in dairy cows. The common denominator in most of these situations is activation of the hypotha-lamic±pituitary±adrenal axis (Varner and Johnson, 1983). Goats are adapted to harsh tropical and sub-tropical environmental conditions better than any other domesticated ruminants (Silanikove, 1992, 2000a). Although, breeds indigenous to desert envir-onments are better adapted to harsh conditions than non-desert breeds, the differences are related to the level of adaptation and not to lack of adaptation in the non-desert breeds (Silanikove, 2000a). Exotic breeds are crossed with indigenous breeds to increase their adaptation to new environments. The fact that the Saanen goats used in the present experiment were crossed with local goats suggests that through the selection scheme important features of their adapta-tion to the local condiadapta-tions were preserved. Domestic ruminants are raised to produce milk and meat and, therefore, are selected to attain maximal productivity, or maximal reproduction combined with production in a given ecosystem (Landau et al., 2000; Silanikove, 2000a). Thus, the attenuated milk production responses to heat stress and water deprivation in goats re¯ects their adaptation to such conditions. Our data suggest that the attenuated milk yield response upon exposure to stressful conditions depends on completely ignoring the axis: stress-ACTH-glucocorticoid-down regulation of milk yield, which was demonstrated in dairy cows. In dairy cows,
which are much less adapted to stressful conditions, the activation of stress-down regulation of milk production is potentially of existential survival impor-tance (Shamay et al., 2000).
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