In vitro and in vivo temporal aspects of ACTH secretion: stimulatory actions of corticotropin-releasing hormone and vasopressin in cattle

The objective of the present study was to evaluate the temporal aspects associated with corticotropin-releasing hormone (CRH) and vasopressin (VP) stimulated bovine adrenocorticotropic hormone (ACTH) secretion in vitro and in vivo. For the in vitro studies, bovine anterior pituitary glands were enzymatically dispersed to establish primary cultures. On day 5 of culture, cells were challenged for 3 h with medium alone (Control) or various combinations and concentrations of bovine CRH (bCRH) and VP. Both CRH and VP each increased (P < 0.05) ACTH secretion. Maximal increases in ACTH secretion occurred in response to 0.1 microM CRH (5.5-fold) and 1 microM VP (3.7-fold), relative to Control cells. The in vivo portion of the study examined possible temporal differences in the activation of the pituitary-adrenal axis by CRH and VP. Jersey cows were randomly assigned to one of four groups (n = 8 cows/group): (i) Control (saline); (ii) bCRH (0.3 microg/kg BW); (iii) VP (1 microg/kg BW) and (iv) bCRH (0.3 microg/kg BW) + VP (1 microg/kg BW). Jugular blood samples were collected at 15-min intervals for 4 h pre- and for 6 h post-treatment; samples were also taken at 1, 5 and 10 min post-treatment. Plasma concentration of ACTH did not differ among treatment groups for the 4-h pre-treatment period. At 1 min post-treatment, bCRH + VP, VP and bCRH increased ACTH secretion by 22.4-, 9.6- and 2.2-fold, respectively, relative to Control (32.7 +/- 7.2 pg/ml). Maximal plasma concentration of ACTH occurred at 5, 10 and 15 min post-treatment for the VP (1017.7 +/- 219.9 pg/ml), bCRH + VP (1399.8 +/- 260.1 pg/ml) and bCRH (324.8 +/- 126.2 pg/ml) treatment groups respectively. Both the in vitro and in vivo data demonstrated that while VP acutely activates the bovine pituitary-adrenal axis, CRH-induced ACTH secretion is slower in onset but of longer duration. The present study also provides insight into the dynamics of ACTH and cortisol (CS) responsiveness to CRH and VP in cattle.     

Adrenocorticotropic hormone and cortisol in calves after corticotropin-releasing hormone.

The aim for this study was to analyze responsiveness of the hypothalamo-pituitary-adrenocortical axis to exogenous bovine corticotropin-releasing hormone (bCRH) in calves. Two dose-response studies were carried out, using either bCRH alone (dose rates of 0, .01, .03, and .1 microg bCRH/kg live weight) or in combination with arginine-vasopressin (bCRH:AVP, 0:0, .1:.05, .5:.25, and 1:.5 microg kg live weight). The bCRH was administered i.v. to calves (n = 5 to 7 per dose) housed individually or in groups. Serial blood samples were obtained from before to 300 min after injection and analyzed for plasma ACTH and cortisol concentrations. The lowest bCRH dose that produced a response in all calves was .1 microg/kg. In the experiment using bCRH with AVP, increasing the bCRH dose from .1 to 1 microg/kg resulted in an increase in peak ACTH concentration (321 vs. 2,003 pg/mL) but did not significantly affect the peak cortisol concentration (37 vs. 40 ng/mL). The time to reach the peak cortisol concentration increased with the dose of bCRH with AVP (from 38 to 111 min). The ACTH and cortisol concentrations determined at any time between 20 and 90 min after bCRH injection were correlated to the integrated responses calculated as areas under the ACTH and the cortisol curves (r between .61 and .99, P<.05). In comparison with results from studies in humans, pigs, and sheep, our data showed that the pituitary of calves seems less sensitive to CRH than that of other mammals, despite a greater capacity to produce ACTH. Moreover, the calf's adrenals seem to have a lower capacity to produce cortisol than adrenals of other mammals. As in other species, it seems that AVP enhances the release of ACTH and cortisol. For CRH challenge to be used in calves, we suggest injecting at least .1 microg of bCRH/kg live weight either with or without AVP and taking several blood samples before injection and between 20 and 90 min after injection.     

Pituitary adenylate cyclase-activating polypeptide induces secretion of growth hormone in cattle.

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a hypothalamic neuropeptide that stimulates release of growth hormone (GH) from cultured bovine anterior pituitary gland cells, but the role of PACAP on the regulation of in vivo secretion of GH in cattle is not known. To test the hypothesis that PACAP induces secretion of GH in cattle, meal-fed Holstein steers were injected with incremental doses of PACAP (0, 0.1, 0.3, 1, 3, and 10 microg/kg BW) before feeding and concentrations of GH in serum were quantified. Compared with saline, injection of 3 and 10 microg PACAP/kg BW increased peak concentrations of GH in serum from 11.2 ng/ml to 23.7 and 21.8 ng/ml, respectively (P < 0.01). Peak concentrations of GH in serum were similar in steers injected with 3 or 10 microg PACAP/kg BW. Meal-fed Holstein steers were then injected with 3 microg/PACAP/kg BW either 1 hr before feeding or 1 hr after feeding to determine if PACAP-induced secretion of GH was suppressed after feeding. Feeding suppressed basal concentrations of GH in serum. Injection of PACAP before feeding induced greater peak concentrations of GH in serum (19.2 +/- 2.6 vs. 11.7 +/- 2.6 ng/ml) and area under the response curve (391 +/- 47 vs. 255 +/- 52 ng. ml(-1) min) than injection of PACAP after feeding, suggesting somatotropes become refractory to PACAP after feeding similar to that observed by us and others with growth hormone-releasing hormone (GHRH). We concluded that PACAP induces secretion of GH and could play a role in regulating endogenous secretion of GH in cattle, perhaps in concert with GHRH.     

Bovine acute-phase response after different doses of corticotropin-releasing hormone challenge

The objective was to compare the acute-phase response of steers receiving different doses of bovine corticotropin-releasing hormone (CRH). Fourteen weaned Angus steers (BW = 191 ± 2.1 kg, age = 167 ± 4.7 d) fitted with an indwelling jugular catheter and a rectal temperature (RT) monitoring device were assigned to receive 1 of 3 treatments (intravenous infusion): 1) 0.1 μg of CRH/kg of BW (CRH1; n = 5), 2) 0.5 μg of CRH/kg of BW (CRH5; n = 5), and 3) 10 mL of saline (0.9%; n = 4). Blood samples were collected via catheters, relative to treatment infusion (0 h), hourly from -2 to 0 h and 4 to 8 h and every 30 min from 0 to 4 h. Rectal temperatures were recorded every 30 min from -2 to 8 h. Blood samples were also collected via jugular venipuncture and rectal temperatures assessed using a digital thermometer every 6 h from 12 to 72 h and every 24 h from 96 to 168 h. All plasma samples collected during the study were analyzed for concentrations of haptoglobin. All plasma samples collected from -2 to 8 h were analyzed for cortisol concentrations. Serum samples collected hourly from -2 to 8 h were analyzed for concentrations of NEFA, IL-6, tumor necrosis factor (TNF)-α, and interferon-γ. Cortisol peaked at 0.5 h for CRH1 steers but returned to baseline concentrations at 1 h relative to infusion (time effect; P < 0.01). In CRH5 steers, cortisol peaked at 0.5 h and returned to baseline concentrations 3.5 h relative to infusion (time effect; P < 0.01). Cortisol concentrations did not change after treatment infusion for saline steers (time effect; P = 0.42). In CRH1 steers, NEFA concentrations peaked 5 h after treatment infusion (time effect; P = 0.01). Conversely, serum NEFA concentrations did not change for CRH5 and saline steers after treatment infusion (time effect; P > 0.37). Mean serum TNF-α concentrations in CRH1 steers after treatment infusion were greater compared with saline (P = 0.02), tended to be greater (P = 0.08) compared with CRH5, and were similar (P = 0.40) between CRH5 and saline steers. Mean RT in CRH1 steers after treatment infusion were greater (P < 0.04) compared with saline and CRH5 and similar (P = 0.50) between CRH5 and saline steers. Haptoglobin increased and peaked 72 h after treatment infusion for CRH1 steers (time effect; P = 0.01) but did not change for CRH5 and saline steers (time effect; P > 0.45). In conclusion, the bovine acute-phase response stimulated by CRH infusion is dependent on the CRH dose and the subsequent response in circulating cortisol.     

Effects of adrenocorticotropic hormone and flunixin meglumine on pregnancy retention in beef cows

Pregnancy loss in beef cattle after d 28 of gestation is variable, but it has been reported to be as great as 14% and has been related to transportation or handling stress. The primary objective of this study was to determine whether activation of the hypophyseal-adrenal axis with ACTH would mimic a stressful response and cause pregnancy loss in beef cattle. A secondary objective was to determine if a single injection of the PG synthesis inhibitor flunixin meglumine would attenuate the stress response and suppress serum PGF(2α) concentrations to prevent pregnancy loss. Forty nonlactating beef cows that were 34 ± 0.33 d pregnant were used for this study. In a 2 × 3 factorial arrangement, cows were randomly assigned to receive ACTH [0 or 0.5 IU/kg of BW, intramuscularly (i.m.)] at 0 and 2 h of the study and flunixin meglumine (0, 1.1, or 2.2 mg/kg of BW, i.m.) at 0 h. Blood samples were collected from all cows at 0 h and every 30 min for 4 h to measure serum cortisol and PGF(2α) metabolite (PGFM) concentrations. Rectal temperature was collected for each cow at 0, 120, and 240 min. Pregnancy exams were conducted 31 and 58 d after treatment by transrectal ultrasonography, and the presence of a fetal heartbeat was used as an indicator of fetal viability. Serum cortisol concentration was affected (P < 0.01) by ACTH, time, and the interaction of ACTH × time, but not by flunixin meglumine (P ≥ 0.14) or any other interactions. Cortisol concentrations increased (P < 0.01) in the serum of ACTH-treated cows immediately after ACTH treatment and remained increased (P < 0.01) throughout the 4-h sampling period. Serum PGFM concentration was not affected by ACTH (P = 0.97) or by any interactions (P > 0.35) with ACTH, but was affected (P < 0.01) by flunixin meglumine, time, and the interaction of flunixin meglumine × time. Regardless of dosage (1.1 or 2.2 mg/kg of BW), flunixin meglumine decreased (P < 0.01) serum PGFM concentrations in both ACTH-treated and control cows for the duration of the study. Although ACTH treatment induced a prolonged increase in serum cortisol concentration, none of the cows used in this study lost a pregnancy. In conclusion, the activation of the hypophyseal-adrenal axis with ACTH increased serum cortisol concentrations but did not increase serum concentrations of PGFM or cause pregnancy loss during early gestation in cows. Flunixin meglumine treatment suppressed serum PGFM concentrations in control and ACTH-treated cows.     

Effects of parachlorophenylalanine,quipazine and cyproheptadine on growth hormone and adrenocorticotropin secretion in steers

Adrenergic and perhaps dopaminergic neurons provide inhibitory regulation of growth hormone (GH) secretion in ruminants. This suggests that either serotonergic or other neurons regulate the stimulatory release of GH. The nature of neurotransmitter control of adrenocorticotropin (ACTH) secretion in ruminants has not been determined. Parachlorophenylalanine (PCPA; serotonin synthesis inhibitor), quipazine (serotonin receptor agonist) and cyproheptadine (serotonin receptor antagonist) were utilized in Holstein steers to determine whether serotonin receptors mediate stimulatory actions on GH and ACTH secretion. PCPA (100 mg/kg BW) administered each day at 1900 hr for three successive days did not alter mean GH concentrations, amplitude of GH peaks, nor the number of GH peaks. Likewise, PCPA altered none of these parameters for ACTH. Quipazine injected iv at .1 or .5 mg/kg BW increased plasma GH (P<.05) and ACTH (P<.001) concentrations. There was a dose effect of quipazine on both GH (P<.05) and ACTH (P<.001) secretion. Pretreatment of steers with cyproheptadine (.06 and .6 mg/kg BW) reduced the stimulation of GH by quipazine (P<.0001) and decreased basal GH concentrations (P<.0004). Cyproheptadine at .06 mg/kg BW did not alter quipazine effects on ACTH, however, the higher dose decreased the peak ACTH response (P<.02) to quipazine. Studies with quipazine and cyproheptadine indicated that serotonergic mechanisms are likely involved in the regulation of GH and ACTH secretion in steers.     

Duration of pituitary and adrenocortical suppression after long-term administration of anti-inflammatory doses of prednisone in dogs.

Duration and magnitude of hypothalamic-pituitary-adrenal axis suppression caused by daily oral administration of a glucocorticoid was investigated, using an anti-inflammatory dose of prednisone. Twelve healthy adult male dogs were given prednisone orally for 35 days (0.55 mg/kg of body weight, q 12 h), and a control group of 6 dogs was given gelatin capsule vehicle. Plasma cortisol (baseline and 2-hour post-ACTH administration) and plasma ACTH and cortisol (baseline and 30-minutes post corticotropin-releasing hormone [CRH] administration) concentrations were monitored biweekly during and after the 35-day treatment period. Baseline plasma ACTH and cortisol and post-ACTH plasma cortisol concentrations were significantly (P less than 0.05) reduced in treated vs control dogs after 14 days of oral prednisone administration. By day 28, baseline ACTH and cortisol concentrations remained significantly (P less than 0.05) reduced and reserve function was markedly (P less than 0.0001) reduced as evidenced by mean post-CRH ACTH, post-CRH cortisol, and post-ACTH cortisol concentrations in treated vs control dogs. Two weeks after termination of daily prednisone administration, significant difference between group means was not evident in baseline ACTH or cortisol values, post-CRH ACTH or cortisol values, or post-ACTH cortisol values, compared with values in controls. Results indicate complete hypothalamic-pituitary-adrenal axis recovery 2 weeks after oral administration of an anti-inflammatory regimen of prednisone given daily for 5 weeks.     

Effects of single intravenously administered doses of dexamethasone on response to the adrenocorticotropic hormone stimulation test in dogs

The effects of single IV administered doses of dexamethasone on response to the adrenocorticotropic hormone (ACTH) stimulation test (baseline plasma ACTH, pre-ACTH cortisol, and post-ACTH cortisol concentrations) performed 1, 2, and 3 days (experiment 1) or 3, 7, 10, and 14 days (experiment 2) after dexamethasone treatment were evaluated in healthy Beagles. In experiment 1, ACTH stimulation tests were carried out after administration of 0, 0.01, 0.1, 1, and 5 mg of dexamethasone/kg of body weight. Dosages greater than or equal to 0.1 mg of dexamethasone/kg decreased pre-ACTH plasma cortisol concentration on subsequent days, whereas dosages greater than or equal to 1 mg/kg also decreased plasma ACTH concentration. Treatment with 1 or 5 mg of dexamethasone/kg suppressed (P less than 0.05) post-ACTH plasma cortisol concentration (on day 3 after 1 mg of dexamethasone/kg; on days 1, 2, and 3 after 5 mg of dexamethasone/kg). In experiment 2, IV administration of 1 mg of dexamethasone/kg was associated only with low (P less than 0.05) post-ACTH plasma cortisol concentration in dogs on day 3. In experiment 2, pre-ACTH plasma cortisol and ACTH concentrations in dogs on days 3, 7, 10, and 14 and post-ACTH plasma cortisol concentration on days 7, 10, and 14 were not affected by dexamethasone administration. The results suggest that, in dogs, a single IV administered dosage of greater than or equal to 0.1 mg of dexamethasone/kg can alter the results of the ACTH stimulation test for at least 3 days. The suppressive effect of dexamethasone is dose dependent and is not apparent 7 days after treatment with 1 mg of dexamethasone/kg.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of repeated regrouping and relocation on the physiological, immunological, and hematological variables and performance of steers

To investigate the effect of repeated regrouping and repenning (R&R) on the hypothalamic-pituitary-adrenal axis, immune function, blood biochemical and hematological variables, and ADG, 72 Holstein-Friesian (14-mo-old; 441 +/- 3.2 kg) steers were assigned to either the control (C; n = 30) or regrouped (R; n = 42) treatments and housed six per pen in 12 pens. The R steers were exposed to six R&R over 84 d. New pen cohorts were allowed to stabilize for 14 d, and none of the R steers was allowed to share the same pen or penmates where or with whom they were previously housed. Control steers were housed in the same pen with the same penmates. Steers were blood sampled 2 h before and 2 h after the first, third, and sixth R&R. Steers were weighed the day before each R&R. Median area under the plasma cortisol curve (AUC) was greater (P < 0.05) in R than C steers after the first R&R. Following the first, third, and sixth R&R, the median ACTH AUC did not differ between the treatments. Cortisol AUC in R steers decreased (P < 0.001) following the third and sixth compared with the first R&R, however, cortisol AUC in response to exogenous ACTH (following administration of dexamethasone at -12 h) after the third R&R was greater in C than R steers (P < 0.05). Corticotropin-releasing hormone-induced cortisol and ACTH AUC were not different in C vs. R after the sixth R&R. There were no differences among treatments in haptoglobin, fibrinogen, and concanavalin A-induced interferon-gamma after the first, third, and sixth R&R. Albumin, urea, and NEFA were greater (P < 0.05) in R than C steers after the first R&R. beta-Hydroxy-butyrate and glucose concentrations were greater (P < 0.05) in R than C, whereas no changes in the protein and globulin concentrations were found in C vs. R after the sixth R& R. White blood cell, differential and total count, red blood cell, and platelet numbers did not differ in C vs. R after the first and third R&R. Lymphocyte numbers and mean corpuscular volume were greater (P < 0.05) in R than C steers after the sixth R&R. Monocyte numbers were greater (P < 0.05) in R than C steers following first R&R. There was no difference in the overall ADG in C vs. R; however, there was a tendency (P = 0.10) for lesser ADG by R than C steers following second R& R. In conclusion, steers exposed to R&R responded with increased plasma cortisol, albumin, urea, and NEFA. Repeated R&R did not have a sustained detrimental effect on immune and production measurements.     

Treadmill exercise is not an effective methodology for producing the dark-cutting condition in young cattle

Holstein steer calves (n = 25) were used to evaluate the effects of treadmill exercise (TME) on blood metabolite status and formation of dark-cutting beef. Calves were blocked by BW (156 +/- 33.2 kg) and assigned randomly within blocks to 1 of 5 TME treatments arranged in a 2 x 2 factorial design (4 or 8 km/h for a duration of 10 or 15 min) with a nonexercised control. Venous blood was collected via indwelling jugular catheters at 10, 2, and 0 min before TME and at 2-min intervals during exercise. Nonexercised steers were placed on the treadmill but stood still for 15 min. Serum cortisol levels, as well as plasma concentrations of glucose, lactate, and NEFA, were similar (P > 0.05) before TME. Serum cortisol concentrations were unaffected (P > 0.05) during the first 6 min of TME, but between 8 and 15 min of TME, cortisol concentrations were greater (P < 0.05) in steers exercised at 8 km/h than those exercised at 4 km/h or controls (speed x time, P < 0.001). Although TME did not affect (P > 0.05) plasma glucose levels, plasma lactate concentrations in steers exercised at 8 km/h increased (P < 0.05) sharply with the onset of the TME treatment and remained elevated compared with steers exercised at 4 km/h or unexercised controls (speed x time, P < 0.001). Exercised steers had the lowest (P < 0.05) plasma NEFA concentrations during the first 6 min of TME compared with unexercised steers; however, NEFA concentrations were similar after 10 and 12 min of TME, and by the end of TME, steers exercised at 8 km/h had greater (P < 0.05) NEFA levels than nonexercised controls or steers exercised at 4 km/h (speed x time, P < 0.001). Even though muscle glycogen levels and pH decreased (P < 0.001) and muscle lactate concentrations increased (P < 0.001) with increasing time postmortem, neither treadmill speed nor TME duration altered postmortem LM metabolism. Consequently, there were no (P > 0.05) differences in the color, water-holding capacity, shear force, or incidences of dark-cutting carcasses associated with preslaughter TME. It is apparent that preslaughter TME, at the speeds and durations employed in this study, failed to alter antemortem or postmortem muscle metabolism and would not be a suitable animal model for studying the formation of the dark-cutting condition in ruminants.     

ACTH stimulation test in the captive cheetah (Acinonyx jubatus)

Serum cortisol response was assessed in 8 captive cheetahs, of varying ages, after the intravenous administration of 500 microg of tetracosactide (Synacthen Depot, Novartis, Kempton Park) while maintained under general anaesthesia. In addition, 8 cheetahs were anaesthetised and given an equal volume of saline in order to establish baseline cortisol concentrations at similar stages of anaesthesia. A significant difference in the median cortisol concentration measured over time was found following ACTH administration in the ACTH group (P < 0.001). There was no difference between the median cortisol concentrations in the ACTH group at time-points 120, 150 and 180 min after ACTH stimulation (P = 0.867). Thus it appears appropriate to collect serum 120 to 180 min after tetracosactide administration to assess maximal stimulation of the adrenal in the cheetah. No statistically significant rise was seen in the anaesthetised control group following the injection of saline (P = 0.238).     

Effects of adrenocorticotropic hormone challenge and age on hair cortisol concentrations in dairy cattle

Dairy cattle suffer stress from management and production; contemporary farming tries to improve animal welfare and reduce stress. Therefore, the assessment of long-term hypothalamic-pituitary-adrenal function using non-invasive techniques is useful. The aims in this study were: to measure cortisol concentration in cow and calves hair by radioimmunoassay (RIA), to test cortisol accumulation in bovine hair after adrenocorticotropic hormone (ACTH) challenges, and determine the influence of hair color on cortisol concentrations. Fifteen Holstein heifers were allotted to 3 groups (n = 5 each): in control group (C), just the hair was sampled; in the saline solution group (SS), IV saline solution was administered on days 0, 7, and 14; and the ACTH group was challenged 3 times with ACTH (0.15 UI per kg of body weight) on days 0, 7, and 14. Serum samples from the SS and ACTH groups were obtained 0, 60 and 90 min post-injection. Serum cortisol concentration was greater 60 and 90 min after injection with ACTH. Hair was clipped on days 0, 14, 28, and 44. Hair cortisol was methanol extracted and measured by RIA. Hair cortisol was preserved for 11 mo. Hair cortisol concentrations in the ACTH group were greater than in the saline and control groups on days 14 and 28, but not on day 44. Concentrations were greater in calves than in cows and greater in white hair than in black hair. Cortisol accumulated in bovine hair after ACTH challenges, but the concentration was affected by both age and hair color. If hair color effects are taken into account, assessing cortisol concentration in hair is a potentially useful non-invasive method for assessing stress in cattle.     

Growth hormone response to a novel growth hormone-releasing tripeptide in horses: interaction with gonadotropin-releasing hormone, thyrotropin-releasing hormone, and sulpiride

A series of experiments was performed to determine the factor(s) responsible for an apparent inhibition of GH secretion in mares administered the GH secretagogue EP51389 in combination with GnRH, thyrotropin-releasing hormone (TRH), and sulpiride. Experiment 1 tested the repeatability of the original observation: 10 mares received EP51389 at 10 microg/kg BW; five received TRH (10 microg/kg BW), GnRH (1 microg/kg BW), and sulpiride (100 microg/kg BW) immediately before EP51389, and five received saline. The mixture of TRH, GnRH, and sulpiride reduced (P = 0.0034) the GH response to EP51389, confirming the inhibitory effects. Experiment 2 tested the hypothesis that sulpiride, a dopamine antagonist, was the inhibitory agent. Twelve mares received EP51389 as in Exp. 1; six received sulpiride before EP51389 and six received saline. The GH responses in the two groups were similar (P > 0.1), indicating that sulpiride was not the inhibitory factor. Experiment 3 tested the effects of TRH and(or) GnRH in a 2 x 2 factorial arrangement of treatments. Three mares each received saline, TRH, GnRH, or the combination before EP51389 injection. There was a reduction (P < 0.0001) in GH response in mares receiving TRH, whereas GnRH had no effect (P > 0.1). Given those results, Exp. 4 was conducted to confirm that TRH was inhibitory in vivo as opposed to some unknown chemical interaction of the two compounds in the injection solution. Twenty mares received TRH or saline and(or) EP51389 or saline in a 2 x 2 factorial arrangement of treatments. Injections were given separately so that the two secretagogues never came in contact before injection. Again, TRH reduced (P < 0.0001) the GH response to EP51389. In addition, TRH and EP51389 each resulted in a temporary increase in cortisol concentrations. Experiment 5 tested whether TRH would alter the GH response to GHRH itself. Twelve mares received porcine GHRH at 0.4 microg/kg BW; six received TRH prior to GHRH and six received saline. After adjustment for pretreatment differences between groups, the GHRH-induced GH response was completely inhibited (P = 0.068) by TRH. Exp. 6 was a repeat of Exp. 5, except geldings were used (five per group). Again, pretreatment with TRH inhibited (P < 0.0001) the GH response to GHRH. In conclusion, TRH inhibits the GH response not only to EP51389 but also to GHRH in horses, and in addition to its known secretagogue action on prolactin and TSH it may also stimulate ACTH at the dosage used in these experiments.     

Effects of dexamethasone,glucose infusion,adrenocorticotropin, and propylthiouracil on plasma leptin concentrations in horses

In experiment 1, nine light horse geldings (three 3 x 3 Latin squares) received dexamethasone (DEX; 125 microg/kg BW, i.m.), glucose (0.2 g/kg BW, i.v.), or nothing (control) once per day for 4 days. DEX increased (P < 0.001) glucose, insulin, and leptin concentrations and resulted in a delayed increase (P < 0.001) in IGF-I concentrations. In experiment 2, mares were similarly treated with DEX (n = 6) or vehicle (n = 6). DEX again increased (P < 0.01) glucose, insulin, and leptin concentrations; the delayed elevation in IGF-I concentrations occurred on day 10, 12, and 19, relative to the first day of treatment. In experiment 3, six light horse geldings received either 200 IU of adrenocorticotropin (ACTH) i.m. or vehicle twice daily for 4 days. ACTH increased (P < 0.001) cortisol concentrations. Further, ACTH resulted in increases (P < 0.01) glucose, insulin, and leptin concentrations. In experiment 4, plasma samples from four light horse stallions that were fed 6-n-propyl-2-thiouracil (PTU) at 6 mg/kg BW for 60 days to induce hypothyroidism were compared to samples from control stallions. On day 52, stallions receiving PTU had lower concentrations of thyroxine (P < 0.05) and triiodothyronine (P < 0.01) and higher (P < 0.01) concentrations of TSH. Leptin concentrations were higher (P < 0.01) in PTU-fed stallions from day 10 through 52. In conclusion, circulating concentrations of leptin in horses was increased by administering DEX. Treatment with ACTH increased cortisol and resulted in lesser increases in leptin, glucose, and insulin. In addition, PTU feeding results in lesser increases in leptin concentrations.     

Technical note: Use of near-infrared reflectance spectroscopy to predict intake and digestibility in bulls and steers

Multiple fecal samples were collected from growing Angus bulls (264 to 419 kg of BW, 3.0 to 11.4 kg/d of DMI) to predict DMI of a corn-silage-based diet. Contemporaneous digestion trials were conducted with the same diet in 12 steers in yr 1 to 3 and bulls in yr 4. Near-infrared spectra from fecal samples (n = 730 from 282 growing bulls, n = 240 from 36 steers and 12 bulls for digestion trials) were obtained from dried and ground fecal samples, and modified partial least squares regression was used to develop equations to predict DMI and DM digestibility (DMD). Although mean predicted DMI of the growing bulls (7.52 ± 0.04 kg/d or 22.4 ± 0.1 g/kg of BW) was within 2% of mean measured DMI (7.63 ± 0.06 kg/d or 22.7 ± 0.1 g/kg of BW), the mean of paired differences within samples (0.11 ± 0.04 kg/d or 0.3 ± 0.1 g/kg of BW) was greater (P < 0.01) than zero. Measured DMD (72.3 ± 0.5%) was identical (P < 0.97) to predicted DMD (72.3 ± 0.5%), and DMD for bulls in the digestion trial did not differ (P < 0.27) from DMD for steers. Prediction of intake requires incorporation of some measured values from the set of fecal samples to be predicted. Lack of similarity between spectra of fecal grab samples from the growing bulls and daily fecal collection of steers and bulls in the digestion trials in this study indicates the need for further verification before prediction of DMD with fecal grab samples.     

Induced gonadotropin release in adrenocorticotropin-treated bulls and steers

The effect of adrenocorticotropin hormone (ACTH) on plasma cortisol and on gonadotropin releasing hormone (GnRH)-induced release of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and testosterone was determined in nine Holstein bulls and 12 Holstein steers. Treatments consisted of animals receiving either GnRH (200 micrograms, Group G), ACTH (.45 IU/kg BW, Group A) or a combination of ACTH followed 2 h later by GnRH (Group AG). Group G steers and bulls had elevated plasma LH and FSH within .5 h after GnRH injection and plasma testosterone was increased by 1 h after GnRH injection in bulls. In Group A, plasma cortisol was elevated by .5 h after ACTH injection in both steers and bulls, but plasma LH and FSH were unaffected. In Group A bulls, testosterone was reduced after ACTH injection. In Group AG, ACTH caused an immediate increase in plasma cortisol in both steers and bulls, but did not affect the increase in either plasma LH or FSH in response to GnRH in steers. In Group AG bulls, ACTH did not prevent an increase in either plasma LH, FSH or testosterone in response to GnRH compared with basal concentrations. However, magnitude of systemic FSH response was reduced compared with response in Group G bulls, but plasma LH and testosterone were not reduced. The results indicate that ACTH caused an increase in plasma cortisol, but did not adversely affect LH or FSH response to GnRH in steers and bulls. Further, while testosterone was decreased after ACTH alone, neither ACTH nor resulting increased plasma cortisol resulted in decreased testosterone production in the bull after GnRH stimulation.     

Adrenocortical response to ACTH in Angora and Spanish goat wethers.

Angora goats do not cope well with stress compared with goats of other breeds. Our hypothesis that this involves subclinical primary hypoadrenocorticism associated with low cortisol release in response to ACTH stimulation was tested by measuring adrenocortical response (plasma cortisol) in six Spanish (37 +/- 2 kg BW) and six Angora wethers (39 +/- 3 kg BW) under simulated acute and chronic ACTH challenges. In Exp. 1 (acute ACTH challenge), wethers were dosed i.v. with high (2.5 IU/kg BW) or low (.4 IU/kg BW) quantities of ACTH. In Exp. 2 (chronic ACTH challenge), ACTH at the rate of .015 IU/(kg BW x min) or saline (.15 M NaCl) was infused i.v. at 15 mL/h for 6 h. The mean baseline plasma cortisol concentration before ACTH stimulation was similar (P > .05) between Angora and Spanish goats in Exp. 1 (averaged over days) and in Exp. 2. The cortisol concentration response area (ng/ (mL x min) x 10(-3)) above the baseline was similar (P > .05) between Angora and Spanish goats during low (7.6 +/- .5 and 9.0 +/- 1.7, respectively) and high (12.8 +/- 1.0 and 16.0 +/- 1.8, respectively) levels of acute ACTH challenge (Exp. 1) and during chronic ACTH challenge (45.1 +/- 5.9 and 41.8 +/- 7.3, respectively; Exp. 2). In conclusion, these data indicate that, under the conditions of this study, adrenocortical responsiveness to ACTH stimulation is not different between Angora and Spanish goat wethers and, thus, may not contribute to stress susceptibility in Angora goats.     

Plasma cortisol and progesterone responses to low doses of adrenocorticotropic hormone in ovariectmized lactating cows

The objective of this study was to describe the responses of the plasma progesterone and cortisol concentrations in ovariectomized lactating cows to low doses of adrenocorticotropic hormone (ACTH). The estrous cycles in 3 lactating cows were synchronized, and the cows were ovariectomized in the luteal phase. ACTH challenge tests were conducted at doses of 3, 6, 12 and 25 IU. Blood samples were collected at 30 min intervals, and the plasma progesterone and cortisol concentrations were analyzed by EIA. A concomitant rise in plasma progesterone and plasma cortisol was observed in cows treated with 12 IU or higher doses of ACTH. Significant increments in the plasma cortisol concentrations were observed at all doses of ACTH. The means (+/- SE) of the peak plasma progesterone concentrations after the 3, 6, 12 and 25 IU ACTH challenge tests were 0.6 +/- 0.1, 1.3 +/- 0.4, 1.5 +/- 0.3 and 2.4 +/- 0.3 ng/ml, respectively. The means of the peak plasma cortisol concentrations in the 3 cows after the ACTH challenge were 14.0 +/- 1.5, 17.0 +/- 2.5, 23.3 +/- 3.0, and 33.3 +/- 7.0 ng/ml, respectively. The effects of the doses, time after treatment, and their interaction on the plasma progesterone concentrations after the ACTH challenge were significant (P<0.01). Likewise, the effects of the doses, time after treatment, and their interaction on the plasma cortisol concentrations after the ACTH challenge were significant (P<0.01). The mean AUC values for the plasma progesterone and cortisol concentrations after the ACTH treatments were also significantly affected by the dose of ACTH (P<0.01 and P<0.05, respectively). A significantly positive correlation was obtained between the peak plasma progesterone and cortisol concentrations after different doses of ACTH (r=0.7, P<0.05). The results suggest that lactating dairy cows are capable of secreting a significant amount of adrenal progesterone, reaching up to the minimal concentration necessary to cause suppression of estrus in response to 12 IU ACTH (P<0.01). The concomitant plasma cortisol concentration was 23.3 ng/ml.     

Adrenocorticotropic hormone stimulation tests in healthy foals from birth to 12 weeks of age

The purpose of this study was to investigate total baseline plasma cortisol and adrenocorticotropic hormone (ACTH) concentrations, and ACTH-stimulated cortisol concentrations in foals from birth to 12 wk of age. Plasma (baseline) cortisol and ACTH concentrations were measured in 13 healthy foals at birth and at 1, 2, 3, 4, 5, 7, 10, 14, 21, 28, 42, 56, and 84 d of age. Each foal received cosyntropin (0.1 μg/kg) intravenously. Plasma cortisol concentrations were measured before (baseline), and 30, and 60 min after cosyntropin administration at birth and at 3, 5, 7, 10, 14, 21, 28, 42, 56, and 84 d of age. Compared with baseline, cortisol concentration increased significantly 30 min after administration of cosyntropin on all days. Cortisol concentration was highest at birth, measured at 30 and 60 min after cosyntropin administration, compared with all other days. With the exception of birth measurements, cortisol concentration was significantly higher on day 84, measured at 30 and 60 min after cosyntropin administration, when compared with all other days. Baseline plasma ACTH was lowest at birth when compared with concentrations on days 2, 3, 4, 5, 7, 10, 14, 42, 56, and 84. Administration of 0.1 μg/kg of cosyntropin, IV, reliably induces cortisol secretion in healthy foals. Differences in the magnitude of response to cosyntropin are observed depending on the age of the foal. These data should serve as a reference for the ACTH stimulation test in foals and should be useful in subsequent studies to evaluate the hypothalamic-pituitary-adrenal axis in healthy and critically ill foals.     

Chronic effects of recombinant porcine growth hormone on adrenal weight and activity in pigs

This study was conducted to examine serum cortisol concentrations and adrenal cortisol output in pigs treated chronically with recombinant pGH (rpGH) at a maximally effective anabolic dose. Recombinant pGH (140 micrograms/kg body weight) was administered daily to eight barrows for 77 d. At slaughter, adrenal glands were removed, weighed and sliced; slices of fresh adrenal tissue were incubated for 1 h in the presence or absence of ACTH. Recombinant pGH increased adrenal weight by 39%. This change was accompanied by an inversely proportional reduction of in vitro cortisol output per gram of tissue, with the net result that total cortisol output per adrenal per kilogram of BW was unaltered, as was cortisol output in the presence of ACTH. Serum cortisol concentrations were measured in 10 barrows fitted with femoral artery catheters and treated daily with 0 or 140 micrograms rpGH/kg BW for 8 d. Basal and ACTH-stimulated cortisol concentrations were not altered by rpGH treatment. These results do not support our earlier speculation that a pGH-dependent increase in adrenal weight is associated with a chronic increase in adrenal activity, but rather demonstrate that corticosteroid output is tightly regulated and remains constant despite a marked increase in the size of the adrenal glands.