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ANIMAL GROWTH, PHYSIOLOGY, AND REPRODUCTION |
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* Teagasc, Grange Research Centre, Dunsany, Co. Meath, Ireland,
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Faculty of Veterinary Medicine and
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Conway Institute, University College Dublin 04, Dublin, Ireland
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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Key Words: Adrenocorticotropic Hormone • Corticotropin-Releasing Hormone • Cortisol • Hypothalamic-Pituitary-Adrenal Axis • Stress • Steers
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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). Under the influence of both internal and external stressors, activation of the HPA axis causes increased synthesis of corticotropin-releasing hormone (CRH) in the parvocellular neurons of the paraventricular nucleus of the hypothalamus and its release into portal circulation within a few seconds of the onset of stress (O’Connor et al., 2000). Hypothalamic CRH is a neuropeptide responsible for endocrine, autonomic, immunological, and behavioral responses of mammalian organisms to stress (Moberg and Minch, 2000). The major role of CRH is the regulation of the HPA axis through induction of the pro-opiomelanocortin gene and secretion of both basal and stress-induced release of ACTH from the anterior pituitary gland and glucocorticoids from the adrenal gland (Toates, 1995). Hypersecretion of CRH elicits anxiogenic-like effects and immunosuppressive activity, decreases food intake, reduces weight gain in man (O’Connor, 2000), modulates locomotor activity, and limits the efficiency of reproduction by decreasing secretory activity of cells producing GH and GnRH in cattle (Smith and Dobson, 2002). Exogenous administration of CRH in rats, guinea pigs, mice, or primates results in "general activation" of the sympathetic nervous system, as measured by increased locomotor activity, sniffing, and increased cortisol concentrations (Sutton et al., 1982).
The effect of CRH administration has been evaluated in young calves (Veissier et al., 1999) and cows (Fisher et al., 2002), but there are no data on the hormonal response of steers to exogenous CRH. Administration of CRH might provide a method to test the sensitivity of the pituitary and adrenal gland during chronic stress. Therefore, the objective of this study was to determine the appropriate dose and effects of exogenous bovine CRH (bCRH) as an indicator of pituitary-adrenal response in adult steers.
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Materials and Methods |
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Twenty 14-mo-old Holstein-Friesian steers were blocked by BW (443.7 ± 2.5 kg) and assigned within block to receive either saline (control; 2 mL, 0.89% [wt/vol] NaCl) or bCRH (0.1, 0.3, 1.0, or 1.5 µg/kg BW). The steers were housed in a slatted-floor facility, with one steer from each treatment in each pen (3.8 x 4.5 m; n = 5 per pen). Before assignment to treatment, steers were housed for a 2-wk acclimatization period, during which they were tethered on a halter for 2 h each morning. Steers were given ad libitum access to grass silage (DM = 887 g/kg) and supplemented with 2.5 kg of a barley/soybean mix (values on DM basis: CP = 155 g/kg, crude fiber = 41.9 g/kg, acid hydrolysable oil = 39 g/kg, and ash = 58.6 g/kg) per steer daily. Steers had free access to water troughs in each pen.
All procedures were conducted under experimental license from the Irish Department of Health in accordance with the Cruelty to Animals Act 1876 and the European Communities (Amendment of Cruelty to Animals Act 1876) Regulations 1994.
CRH Challenge
On d 0, the bCRH (American Peptide Co., Inc., Sunnyvale, CA) dose was dissolved in distilled water 1 h before treatment. Steers treated with bCRH received 0.1, 0.3, 1.0, or 1.5 µg of bCRH/kg BW, and control animals received normal saline i.v. through an indwelling jugular catheter between 0815 and 0827. At the time of bCRH or saline administration, steers were haltered and tied to the feed barrier in front of the pen. They were free to lie and stand. Steers did not have access to water during the 3.5-h blood-sampling period.
Blood Sample Collection
To facilitate intensive blood sampling, indwelling jugular catheters were fitted aseptically on d –1. Serial blood samples were collected at –15 and 0 min before and at 15, 30, 45, 60, 75, 90, 105, 120, 135, 150, 165, and 180 min after bCRH or saline administration. Blood samples were collected into tubes containing heparin and EDTA, as anticoagulants, for cortisol and ACTH assays, respectively. Whole blood was centrifuged at 1,600 x g for 15 min at 4°C and plasma was stored at –20 and –80°C until assayed for cortisol and ACTH, respectively.
Assay Procedures
Commercially available RIA kits were used to determine the plasma concentrations of cortisol (Corti-cote, ICN Pharmaceuticals, Orangeburg, NY) and ACTH (Diagnostic Products Corp., Los Angeles, CA) as described by Fisher et al. (1997). The intraassay CV (n = 6) for samples containing 4.4, 11.8, and 48.3 ng of cortisol/mL were 10.4, 9.1, and 6.9%, respectively. The interassay CV (n = 2) for the same samples were 17.8, 9.3, and 7.1%, respectively. For ACTH, the intraassay CV (n = 6 to 8) for samples containing 77.7 and 296.4 pg/mL were 12.4 and 17.2%, respectively. The interassay CV (n = 2) for the same samples were 3.1 and 13.7%, respectively.
Statistical Analyses
The degree of HPA axis function was obtained by determining the ACTH and cortisol responses after bCRH administration. The data were analyzed by ANOVA or rank ANOVA as appropriate using the GLM procedure of the SAS (SAS Inst., Inc., Cary, NC). For each steer, mean concentrations for ACTH and cortisol were calculated from the time of bCRH administration until the final blood sample at 180 min after treatment. Peak plasma concentration and time to reach peak concentrations for ACTH and cortisol after bCRH administration were calculated for each steer. The ratio of mean cortisol to mean ACTH was calculated as a measure of the adrenal response following bCRH challenge. Data on mean cortisol:mean ACTH ratio, and time to reach peak, were analyzed following rank transformation using the GLM procedure of SAS. The pretreatment baseline concentrations of ACTH and cortisol did not differ (ACTH, P = 0.722; cortisol, P = 0.389) among treatments, so no covariate adjustment was required. Pairwise comparisons of means were tested using a Fisher’s (protected) least significant difference test, for the peak and mean for ACTH and cortisol concentrations (Zar, 1999). Median data for the time to reach peak and mean cortisol to mean ACTH were analyzed using the Kruskall-Wallis procedure (Zar, 1999).
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Results |
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). Administration of 0.3, 1.0, and 1.5 µg of bCRH/kg BW increased peak ACTH (P < 0.05) above pretreatment concentrations. The peak ACTH concentration occurred between 15 to 30 min following bCRH administration, and peak concentrations were, on average, three to seven times greater than basal concentrations (Table 1; Figure 1). The greatest dose of bCRH (1.5 µg) elicited a greater (P 
0.01) ACTH response than the lowest dose (0.1 µg). Mean ACTH concentration following 0.1 µg of bCRH/kg BW did not differ significantly from controls, whereas mean ACTH responses following administration of 0.3, 1.0, and 1.5 µg of bCRH/kg BW were greater (P < 0.05) than in controls.
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). Mean cortisol responses were greater (P < 0.005) in all bCRH treated steers than in controls, but differences were not significant in mean cortisol responses (P = 0.099 to 0.777) among bCRH doses (Table 1
). Mean cortisol to mean ACTH for all bCRH doses were greater (P = 0.038) than for controls.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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reported an increase in ACTH and cortisol concentrations in young calves following administration of 0.1 µg of bCRH/kg BW. In humans, the dose response range to exogenous ovine CRH was between 0.001 and 30 µg of ovine CRH/kg BW (Orth et al., 1983). Zhang et al. (1990) observed that pigs responded to exogenous administration of 0.01 µg of human CRH/kg BW. Therefore, our results suggest that the HPA axis of steers is less sensitive to CRH than that of humans, or pigs.
In this study, peak concentrations of ACTH ranged between 126 and 269 pg/mL for doses between 0.1 and 1.5 µg of bCRH/kg BW. Peak concentrations of ACTH reached 30 pg/mL following a dose of 1 µg of ovine CRH/kg BW in humans (Orth et al., 1983), 120 pg/mL following a dose of 0.55 µg of porcine CRH/kg BW in pigs (Minton and Parson, 1993), 64.7 pg/mL following a dose of 100 µg of ovine CRH/kg BW in sheep (Saphier et al., 1992), and 99 pg/mL following a dose of 0.1 µg of bCRH/kg BW in calves (Veissier et al., 1999). The differences in concentrations of ACTH may reflect differences in species response to CRH, and/or different assay methods used in these studies compared with the RIA procedure that was used in the present study.
In contrast, the range for the cortisol peak (36 to 42 ng/mL) for all doses of bCRH administered to steers was lower than that in humans (120 ng/mL, Orth et al., 1983), but was similar to that found in sheep (34.2 ng/mL; Saphier et al., 1992). However, Veissier et al. (1999) reported a cortisol peak of 14 ng/mL in calves following administration of 0.1 µg of bCRH/kg BW. These differences may be attributed to lower sensitivity of the adrenal to ACTH or decreased capacity to produce cortisol in farm animals compared with humans. The greater concentrations of cortisol in steers than calves might be due to a changed responsiveness following castration or difference in age, BW, and metabolic activity. The longer duration of cortisol response compared with ACTH is similar to results found following acute stress, such as castration (Fisher et al., 1997). The effect of chronic stress in altering the dynamics of the ACTH and cortisol response following CRH remains to be elucidated.
Cortisol:ACTH ratio is a useful measure of adrenal reactivity to ACTH (Veissier et al., 1999; Fisher et al., 2002). Hauger et al. (1990) reported increased levels of cortisol in response to an acute stress, despite CRH receptor downregulation at the pituitary in response to a chronic intermittent stressor in rats. Greater values of cortisol:ACTH ratio in steers, indicating greater adrenal responsiveness, could be helpful in evaluating further the sensitivity of the adrenal gland during stressful events.
In conclusion, the results showed an overall increase in ACTH and cortisol concentrations following exogenous administration of bCRH. Steers responded to exogenous doses of bCRH with increased concentrations of ACTH from 0.3 to 1.5 µg of bCRH/kg BW. The optimal dose of bCRH to use in adult steers was 0.3 µg of bCRH/kg BW.
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Implications |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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1 Correspondence—phone: +353-46-9061100; fax: +353-46-9026154; e-mail: bearley{at}grange.teagasc.ie.
Received for publication November 3, 2003. Accepted for publication March 2, 2004.
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Literature Cited |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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production, leukocyte, acute-phase proteins, growth, and feed intake. J. Anim. Sci. 75:1899–1908.This article has been cited by other articles:
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  S. Gupta, B. Earley, S. T. L. Ting, and M. A. Crowe Effect of repeated regrouping and relocation on the physiological, immunological, and hematological variables and performance of steers J Anim Sci, August 1, 2005; 83(8): 1948 - 1958. [Abstract] [Full Text] [PDF]
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| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
—), 0.1 µg of bovine corticotropin-releasing hormone (bCRH)/kg BW (—
—), 0.3 µg of bCRH/kg BW (—
—), 1.0 µg of bCRH/kg BW (—x—), and 1.5 µg of bCRH/kg BW (—
—); n = 4 for all treatment groups. Mean plasma ACTH response following 0.1 µg of bCRH/kg BW administration did not differ significantly from saline-treated steers, whereas mean ACTH following administration of 0.3, 1.0, and 1.5 µg of bCRH/kg BW were greater (P < 0.005) than in saline-administrated steers.