Relationships between the thyroid and somatotropic axes in steers I: Effects of propylthiouracil-induced hypothyroidism on growth hormone, thyroid stimulating hormone and insulin-like growth factor I

The effects of propylthiouracil (PTU)-induced thyroid hormone imbalance on GH, TSH and IGF-I status in cattle were examined. In the first study, four crossbred steers (avg wt 350 kg) were fed a diet dressed with PTU (0, 1, 2 or 4 mg/kg/d BW) in a Latin square design with four 35-d periods. On day 29 in each period, steers were challenged with an intrajugular bolus of thyrotropin releasing hormone (TRH, 1.0 μg/kg). Blood samples were obtained to assess the change in plasma GH and TSH as affected by PTU. Plasma IGF-I was measured from blood samples obtained before and after (every 6 hr for 24 hr) intramuscular injection of bovine GH (0.1 mg/kg, day 31). Doses of 1 and 2 mg/kg PTU increased plasma T₄ (P<.01). At 4 mg/kg, PTU depressed T₄ concentrations to 30% of control (P<.01). Plasma T₃ linearly decreased with increasing doses of PTU (P<.01). Plasma TSH increased when PTU was fed at 4 mg/kg (P<.05) while the TSH response to TRH declined with increasing PTU (P<.02). Neither basal nor TRH-stimulated plasma concentration of GH was affected by PTU; the IGF-I response to GH tended to increase at the 1 and 2 mg/kg PTU (P<.01). In a second study 24 crossbred steers were fed PTU (1.5 mg/kg) for 119 d in a 2 × 2 factorial design with implantation of the steroid growth effector, Synovex-S (200 mg progesterone + 20 mg estradiol), as the other main effect. Basal plasma GH and IGF-I were not affected by PTU treatment. Synovex increased plasma concentration (P<.01) of IGF-I without an effect on plasma GH. The data suggest that mild changes in thyroid status associated with PTU affects regulation of T₃, T₄ and TSH more than GH or IGF-I in steers.     

Relationship of thyroid status to growth hormone and insulin-like growth factor-I (IGF-I) in plasma and IGF-I mRNA in liver and skeletal muscle of cattle

Steers were made hyperthyroid or hypothyroid to study the effects of physiological alterations in thyroid hormone status on plasma growth hormone (GH) profiles, plasma insulin-like growth factor-I (IGF-I) concentrations, and relative abundance of IGF-I mRNA in skeletal muscle and liver. Eighteen yearling crossbred steers (360 to 420 kg) were randomly allotted to hyperthyroid (subcutaneous injection 0.6 μg/kg BW L-thyroxine for 10 d), hypothyroid (oral thiouracil; 0.25% diet plus 12.5 g capsule/d for 17 d), or control (subcutaneous injection 0.9% NaCl) treatment groups. Blood samples were taken for measurement of GH, IGF-I, thyroxine (T₄) and triiodothyronine (T₃) by RIA. Samples of liver and skeletal muscle were taken by biopsy for measurement of IGF-I mRNA by solution hybridization. Steers receiving thiouracil had 57 and 53% (P<.05) lower T₄ and T₃, respectively, than control steers (84.1 and 1.7 ng/ml). The hyperthyroid steers had 228 and 65% greater (P<.05) T₄ and T₃ than control steers. Neither increased nor decreased thyroid status had any significant effects on plasma GH profiles, liver IGF-I mRNA, or plasma concentration of IGF-I. There was no effect of thyroid hormone alteration on skeletal muscle IGF-I mRNA concentrations. The results of this study suggest that short-term changes in thyroid status of cattle had no major impact on the GH-IGF-I axis or skeletal muscle IGF-I mRNA.     

Effects of winter nutrition and summer pasture or a feedlot diet on plasma Insulin-like Growth Factor I (IGF-I) and the relationship between circulating concentrations of IGF-I and thyroid hormones in steers

Effects of two winter nutritional levels (LOW, MOD) and two summer pastures (bahiagrass, BG; perennial peanut, PP) on plasma IGF-I, and the relationship between IGF-I and average daily gain (ADG), thyroid hormones, plasma urea, packed cell volume (PCV) and steer type were determined in 101 steers (217 kg) varying in breed composition, frame size and initial condition. Relationships between body composition or composition of gain and IGF-I were determined in 11 contemporary steers assigned directly to the feedlot. Initial IGF-I (57.9 ± 3.5 ng/ml) was positively correlated (P<.05) to initial condition, estimated percentage of Brahman and plasma T₃, but not related to subsequent ADG. During the 126-day wintering period, ADG was .21 kg for the LOW winter treatment and .47 kg for the MOD winter treatment. Concentration of IGF-I in the wintering period was affected (P<.01) by nutritional level (LOW = 71.8 ng/ml, MOD = 150.6 ng/ml) and was positively related to winter ADG in MOD steers (P<.01) but not in LOW steers. Concentration of IGF-I in winter was also positively related to condition at the end of the winter period (P<.01), T₃ (P<.05) and T₄ (P<.05). There were no effects of winter treatment on IGF-I during the subsequent summer pasture period. During the 145-d summer period, ADG was .53 kg for BG and .68 kg for PP. Concentration of IGF-I during the summer period was affected (P<.05) by pasture treatment (BG = 138.6 ng/ml, PP = 181.9 ng/ml), was positively related (P<.01) to PCV and percentage of Brahman, and was negatively related (P<.05) to estimated percentage of English breeding. In steers assigned directly to the feedlot, IGF-I was correlated with empty body (EB) weight (r=−.59, P<.10), EB water (r=−.59, P<.10) and EB protein (r=−.60, P<.10) at slaughter, and with days on feed (r=−.65, P<.05), but was not correlated with ADG or rate of component gain. These data indicate that IGF-I is related to nutritional status in steers as in other species, that there may be significant breed or cattle type differences in circulating concentrations of IGF-I, and that circulating concentration of IGF-I may be functionally related to plasma concentration of thyroid hormones.     

Effect of progressive cachectic parasitism and growth hormone treatment on hepatic 5'-deiodinase activity in calves

Thyroid status is compromised in a variety of acute and chronic infections. Conversion of thyroxine (T₄) into the metabolically active hormone, triiodothyronine (T₃), is catalyzed by 5′-deiodinase (5′D) mainly in extrathyroidal tissues. The objective of this study was to examine the effect of protozoan parasitic infection (Sarcocystis cruzi) on hepatic 5′D (type I) activity and plasma concentrations of T₃ and T₄ in placebo- or bovine GH (bGH)-injected calves. Holstein bull calves (127.5±2.0 kg BW) were assigned to control (C, ad libitum fed), infected (I, 250,000 S. cruzi sporocysts per os, ad libitum fed), and pair-fed (PF, non-infected, fed to intake of I treatment) groups placebo-injected, and three similar groups injected daily with pituitary-derived bGH (USDA-B-1, 0.1 mg/kg, i.m.) designated as C_GH, I_GH and PF_GH. GH injections were initiated on day 20 post-infection (PI), 3–4 days prior to the onset of clinical signs of the acute phase response (APR), and were continued to day 56 PI at which time calves were euthanized for liver collection. Blood samples were collected on day 0, 28, and 55 PI. Alterations in nutritional intake did not affect type I 5′D in liver. Treatment with bGH increased (P<0.05) 5′D activity in C (24.6%) and PF (25.5%) but not in I calves. Compared to PF calves, infection with S. cruzi reduced 5′D activity 25% (P<0.05) and 47.8% (P<0.01) in placebo- and bGH-injected calves, respectively. Neither nutrition nor bGH treatment significantly affected plasma concentrations of T₄ and T₃ on day 28 and 55 PI. However, plasma thyroid hormones were reduced by infection. On day 28 PI, the average plasma concentrations of T₃ and T₄ were reduced in infected calves (I and I_GH) 36.4% (P<0.01) and 29.4% (P<0.05), respectively, compared to pair-fed calves (PF and PF_GH). On day 55 PI, plasma T₃ still remained lower (23.7%, P<0.01 versus PF) in infected calves while plasma T₄ returned to control values. The data suggest that parasitic infection in growing calves inhibits both thyroidal secretion and extrathyroidal T₄ to T₃ conversion during the APR. After recovery from the APR, thyroidal secretion returns to normal but basal and bGH-stimulated generation of T₃ in liver remains impaired.     

Effects of cimaterol administration on plasma concentrations of various hormones and metabolites in friesian steers

The aim of the experiment was to determine the acute and chronic effects of the β-agonist, cimaterol, on plasma hormone and metabolite concentrations in steers. Twelve Friesian steers (liveweight = 488 ± 3 kg) were randomly assigned to receive either 0 (control; n=6) or .09 mg cimaterol/kg body weight/day (treated; n=6). Steers were fed grass silage ad libitum. Cimaterol, dissolved in 140 ml of acidified distilled water (pH 4.2), was administered orally at 1400 hr each d. After 13 d of treatment with cimaterol or vehicle (days 1 to 13), all animals were treated with vehicle for a further 7 d (days 14 to 20). On days 1, 13 and 20, blood samples were collected at 20 min-intervals for 4 hr before and 8 hr after cimaterol or vehicle dosing. All samples were assayed for growth hormone (GH) and insulin, while samples taken at −4, −2, 0, +2, +4, +6 and +8 hr relative to dosing were assayed for thyroxine (T₄), triiodothyronine (T₃), cortisol, urea, glucose and non-esterified fatty acids (NEFA). Samples taken at −3 and +3 hr relative to dosing were assayed for IGF-I only. On day 1, cimaterol acutely reduced (P<.05) GH and urea concentrations (7.6 vs 2.9 ± 1.4 ng/ml; and 6.0 vs 4.9 ± 0.45 mmol/l, respectively; mean control vs mean treated ± pooled standard error of difference), and increased (P<.05) NEFA, glucose and insulin concentrations (160 vs 276 ± 22 μmol/l, 4.1 vs 6.2 ± 0.15 mmol/l and 29.9 vs 179.7 ± 13.9 μU/ml, respectively). Plasma IGF-I, T₃, T₄ and cortisol concentrations were not altered by treatment. On day 13, cimaterol increased (P<.05) GH and NEFA concentrations (7.7 vs 14.5 ± 1.4 ng/ml and 202 vs 310 ± 22 mEq/l, respectively) and reduced (P<.05) plasma IGF-I concentrations (1296 vs 776 ± 227 ng/ml). Seven-d withdrawal of cimaterol (day 20) returned hormone and metabolite concentrations to control values. It is concluded that : 1) cimaterol acutely increased insulin, glucose and NEFA and decreased GH and urea concentrations, 2) cimaterol chronically increased GH and NEFA and decreased IGF-I concentrations, and 3) there was no residual effect of cimaterol following a 7-d withdrawal period.     

Plasma and tissue concentrations and molecular forms of somatostatin in calves infected with Sarcocystis cruzi

The effects of parasitic infection on plasma and tissue content of immunoreactive somatostatin (SRIF) were studied in 4-mo old male calves inoculated with the protozoan Sarcocystis cruzi. Because feed intake significantly decreased (70%) in infected calves around day 28 postinfection (pi), concomitant with the asexual replication of S. cruzi and outward expression of clinical signs, the relative contributions of infection and associated reduction in nutrition on plasma SRIF were evaluated. Treatment groups were: noninfected ad libitum fed (C), infected (250,000 S. cruzi oocysts per os) ad libitum fed (I) and noninfected calves pairfed to the level of intake of each infected calf (PF). Mean plasma concentrations of SRIF (pg/ml) on day 30 pi were: C, 224 ± 22; I, 742 ± 150; PF, 246 ± 31 (effect of infection P<.05). In another study, SRIF was measured in plasma and in pancreatic, duodenal, jejunal and ileal tissue extracts from normal and S. cruzi infected calves. Plasma and tissue samples were collected on day 42 pi. Mean plasma SRIF were 2.5 times higher in infected than control calves. Plasma insulin and insulin-like growth factor 1 was lower in infected v control calves (P<.02). Plasma glucagon was similar between groups. Duodenal (P<.05) and jejunal (P<.02) SRIF content was higher in infected than control calves. Chromatography of tissue extracts on Sephadex G-50 revealed that the increase in SRIF was accounted for, in part, by molecular forms larger than cyclic SRIF-14. Data suggest that peripheral SRIF is increased in calves during S. cruzi infection. The increase in SRIF is not solely related to plane of nutrition. Altered levels of gut SRIF(s) may be associated with perturbed metabolic regulation in parasitized animals through direct effects on the gut.     

鲜饲金荞麦对热应激蛋鸡生产性能、蛋品质和血清指标的影响

本试验旨在研究鲜饲金荞麦对热应激条件下的长顺绿壳蛋鸡生产性能、蛋品质和血清指标的影响。采用完全随机试验设计,将300只鸡分为5个处理组:C组(基础饲粮)、T₁组(基础饲粮+5%金荞麦)、T₂组(基础饲粮+10%金荞麦)、T₃组(基础饲粮+15%金荞麦)和T₄组(基础饲粮+20%金荞麦),每个处理5个重复,每个重复12只鸡。将所有用于试验的鸡只暴露于(31.4±1.5) ℃的高温环境下发生自然热应激。记录和统计每月平均采食量、蛋重和产蛋数,并测定试验末各处理鸡蛋的表观品质、全蛋氨基酸和蛋黄脂肪酸含量,以及蛋鸡的血清激素水平、炎症因子和内毒素浓度。结果表明,T₁组在试验第2个月的产蛋率显著高于其余处理组(P<0.05),C、T₁和T₂的产蛋率在第3个月和全期差异不显著(P>0.05);C和T₁的蛋黄颜色等级显著低于其余处理组(P<0.05);T₃组全蛋中的异亮氨酸含量最高,T₃和T₄组的蛋氨酸含量较高(P<0.05),而C组的多不饱和脂肪酸显著高于其他处理组(P<0.05);C组蛋鸡的肾上腺皮质激素和皮质酮浓度显著高于试验组(P<0.05),相反,C组的皮质酮和黄体生成素显著低于试验组(P<0.05),试验组白细胞介素-6、肿瘤坏死因子α和血清内毒素均显著低于对照组(P<0.05)。因此,饲粮中添加10%的鲜饲金荞麦对热应激条件下长顺绿壳蛋鸡的平均产蛋率无负面影响,加深了蛋黄颜色;改善了全蛋氨基酸组成和含量,但对蛋黄脂肪酸组成有负面影响;提高了血清促卵泡素和黄体生成素含量;降低了血清炎症因子和内毒素水平。     

Opioid modulation of gonadotropin releasing hormone release from the hypothalamic preoptic area in the pig

Two experiments (Exp) were conducted to examine in vitro the release of gonadotropin releasing hormone (GnRH) from the hypothalamus after treatment with naloxone (NAL) or morphine (MOR). In Exp 1, hypothalamic-preoptic area (HYP-POA) collected from 3 market weight gilts at sacrifice and sagitally halved were perifused for 90 min prior to a 10 min pulse of morphine (MOR; 4.5 × 10⁻⁶ M) followed by NAL (3.1 × 10⁻⁵ M) during the last 5 min of MOR (MOR + NAL; N=3). The other half of the explants (n=3) were exposed to NAL for 5 min. Fragments were exposed to KCl (60 mM) at 175 min to assess residual GnRH releasability. In Exp 2, nine gilts were ovariectomized and received either oil vehicle im (V; n=3); 10 μg estradiol-17β/kg BW im 42 hr before sacrifice (E; n=3); .85 mg progesterone/kg BW im twice daily for 6 d prior to sacrifice (P₄; n=3). Blood was collected to assess pituitary sensitivity to GnRH (.2 μg/kg BW) on the day prior to sacrifice. On the day of sacrifice HYP-POA explants were collected and treated as described in Exp 1 except tissue received only NAL. In Exp 1, NAL increased (P<.05) GnRH release. This response to NAL was attenuated (P<.05) by coadministration of MOR. Cumulative GnRH release after NAL was greater (P<.05) than after MOR + NAL. All tissues responded similarly to KCl with an increase (P<.05) in GnRH release. In Exp 2, pretreatment luteinizing hormone (LH) concentrations were lower (P<.05) in E gilts compared to V and P₄ animals with P₄ being lower (P<.05) than V gilts. LH response to GnRH was lower (P<.05) in E pigs than in V and P₄ animals, while the responses was similar between V and P₄ gilts. NAL increased GnRH release in all explants, whereas, KCl increased GnRH release in 6 of 9 explants. These results indicate that endogenous opioid peptides may modulate in vitro GnRH release from the hypothalamus in the gilt.     

Effect of exogenous thyrotropin-releasing hormone (TRH) administration on plasma levels of triiodothyronine (T3), thyroxine (T4) and growth hormone (GH) in chronically catheterized suckling piglets.

The purpose of the present study was to determine experimental conditions to stimulate secretion of thyroid hormones (T₃ and T₄) with thyrotropin-releasing hormone (TRH) injections in suckling piglets during the first weeks of postnatal life. Three consecutive experiments were conducted. Four 10–20 d old piglets were i.m. injected with 0, 20, 100, 500 μg (experiment 1) or 0, 4, 20, 100 μg TRH/kg BW (experiment 2) according to a 4 × 4 latin square design involving different litters in each experiment. Blood samples were taken −15, −1, 15, 30, 45, 60, 90, 120 180 and 300 min after TRH injection in experiment 1, and −.25, −.08, .25, .5, 1, 2, 4, 6, 8, 12, 24, 30, 36, 48, 60 and 72 hr after TRH injection in experiment 2. T₃ and T₄ levels were significantly (P<.01) increased as soon as 30 and 45 min after TRH injection, respectively. Maximal levels of T₃ and T₄ were obtained 2 and 4 hr after the injection of 100 μg TRH. T₃ and T₄ returned to basal levels within 6 and 8 hr post injection, respectively. Plasma pGH levels were significantly (P<.001) increased 15 min after TRH injection in piglets injected with 500 μg. In experiment 3, 100 μg TRH/kg BW were injected i.m. either daily or every other day from .0 to 23 days of age. Results showed that T₄ response to TRH did not decrease after repeated injections. These results indicate that daily i.m. injections of 100 μg TRH/kg BW can be used to increase thyroid hormone levels for at least 13 d in the young suckling piglet.     

Extrathyroidal thyroxine-5′-monodeiodinase activity in cattle

The monodeiodination of thyroxine (T₄) to triiodothyronine (T₃) was studied in vitro using liver, kidney, and muscle obtained from two-year old Angus and Hereford steers. Tissues were homogenized in .1 M phosphate buffer-.25 M sucrose - 5 mM EDTA, pH 7.5, and centrifuged at 2000 × g for 30 min. Supernatants were incubated with T4 (1.3 μM) at 37 C and T₃ generated was measured by radioimmunoassay of an ethanol extract of the incubation mixture. The T₄ to T₃ conversion in Angus liver homogenate was dependent upon pH, temperature, duration of incubation (5–120 min), homogenate (.025–.20 g-eq tissue/ml), and substrate concentration (.32–6.43 μM T₄). The apparent K_m and V_max of the conversion were .64 μM T₄ and 1.87 ng T₃ generated/hr/mg protein, respectively. Mean T₄ to T₃ conversion in Angus liver and kidney was 1.37 and .22 ng T₃/hr/mg protein. The presence of 2 mM dithiothreitol (DTT), a sulfhydryl protective agent, significantly increased T₃ generation in liver and kidney (5.12 and 4.58 ng/hr/mg protein) and also revealed activity in muscle (05 ng/hr/mg protein). In liver and kidney from Hereford steers conversion activity was 2.89 and .48 in absence and 10.91 and 5.38 ng T₃/hr/mg protein in presence of DTT, respectively. These results demonstrate the presence of a very active enzymatic system responsible for the peripheral 5′-monodeiodination of T₄ to T₃ in cattle.     

Dietary fatty acids modulate hormone repsonses in lactating cows: Mechanistic role for 5′-deiodinase activity in tissue

Supplemental dietary fat provides excess fatty acids (FA), which can alter circulating concentrations of several hormones. To test the effects of fatty acid isomer type and possible sites of regulation, we abomasally infused fat mixtures high in cis-C_18:1 FA (iTRS) or no infusion (NI) and performed intravenous arginine (ARG) and intramuscular thyrotropin-releasing hormone (TRH) challenges. The experimental design was a replicated 3 × 3 Latin square. Challenges were conducted on Days 10 (ARG) and 12 (TRH) after initiation of fat infusion on each of three 4-wk experimental periods. Plasma concentrations of IGF-I were lower (P < 0.01) when cows received iCIS or iTRS compared with NI. Plasma insulin concentrations increased with ARG but responses were not affected by FA. Plasma growth hormone (GH) was unchanged after ARG. Peak plasma GH and thyroid-stimulating hormone (TSH) responses to TRH were blunted (P < 0.05 and P < 0.1, respectively), whereas thyroxine (T₄) and triiodothyronine (T3) responses were augmented post-TRH (P < 0.01) when cows received either FA isomer. Prolactin responses to TRH were not different between infusion treatments, although basal plasma concentrations before TRH were higher in cows infused with iTRS (P < 0.05). To focus on fat regulation of the thyroid axis, we tested directly in vitro the ability of fatty acids dissolved with sodium taurocholate to affect Type-I 5'-deiodinase (5'D) activity in bovine liver homogenates. Homogenate 5'D was not affected by C_2:0---C10:0 fatty acids, but decreased linearly (P < 0.01) with increasing concentrations of C_12:0---C_16:0 and C_18:1 isomers. CisC_18:1 decreased 5′D more than the trans-isomer (P < 0.01), the difference was only apparent at concentrations greater than 0.25 mM. The data suggest that various aspects of pituitary hormone regulation are differentially affected by FA composition. Fatty acid infusion may accentuate end organ responses in the thyroid axis and decrease IGF-I in the somatotropic axis. The data also suggest that FA isomer may alter patterns of extrathyroidal generation of thyroid hormones via direct influences on 5′D.     

Dose effect of human growth hormone-releasing factor and thyrotropin-releasing factor on hormone concentrations in lactating dairy cows

Two experiments were conducted to study the effects of growth hormone-releasing factor (GRF) and thyrotropin-releasing factor (TRF) administration on hormone concentrations in dairy cows. In the first trial, 12 cows were used on 5 consecutive days to determine the effect of four sc doses of GRF (0, 1.1, 3.3 and 10 μg•kg⁻¹ BW) and three sc doses of TRF (0, 1.1 and 3.3 μg•kg⁻¹ BW) combined in a factorial arrangement. GRF and TRF acted in synergy (P = .02) on serum growth hormone (GH) concentration even at the lowest dose tested and GH response to the two releasing factors was higher than the maximal response observed with each factor alone. TRF increased (P<.01) prolactin (Prl), thyrotropin (TSH), triiodothyronine (T₃) and thyroxine (T₄) concentrations similarly at the 1.1 and 3.3 μg•kg⁻¹ doses and GRF did not interact (P>.40) with TRF on the release of these hormones. In the second trial, the effect of GRF (3.3 μg•kg⁻¹ BW, sc) and TRF (1.1 μg•kg⁻¹ BW, sc) was tested at three stages (18, 72 and 210 days) of lactation on serum Prl and TSH concentrations. Eighteen cows (n = 6 per stage of lactation) were used in two replicates of a 3 × 3 latin square. The TRF and GRF-TRF treatments were equipotent (P>.05) in increasing Prl and TSH concentrations. Prl and TSH responses were similar (P>.40) throughout lactation. In summary, GRF at doses ranging from 1.1 to 10.0 μg•kg⁻¹ and TRF at doses ranging from 1.1 to 3.3 μg•kg⁻¹ act in synergy on GH release and do not interact on Prl, TSH, T₃ and T₄ concentrations in dairy cows. Furthermore, Prl and TSH response to TRF are not affected by stage of lactation.     

Total and free iodothyronine levels of growing Thoroughbred foals: Effects of weaning and gender

The aim of this study was investigate the effect of growing associated with different gender on circulating total and free iodothyronine concentrations during the first 13 mo of age in foals. In addition, we investigated the evolution of circulating concentrations of thyroid hormones during the first 3 d of weaning. Blood was collected from 13 clinically healthy Thoroughbred foals every month. All foals were weaned at the 4 mo and blood samples were taken also at 24, 48 and 72 h after weaning. The results obtained showed growing effects for tri-iodothyronine (T₃), thyroxine (T₄), free tri-iodothyronine (fT₃) and free thyroxine (fT₄) values (P < .001).

Serum T₃ concentrations averaged respectively 2.89 and 0.29 nmol/L at 7 and 9 mo. Serum T₄ concentrations averaged respectively 100.17 and 21.77 nmol/L at 1 and at 10 mo. Serum fT₃ concentrations averaged respectively 6.96 and 1.50 pmol/L at 1 and 4 mo. Serum fT₄ concentrations averaged respectively 31.40 and 4.93 pmol/L at 1 and 9 mo. Significant correlations between T₃, T₄, fT₃ and fT₄ with body weight (BW) and between T₃, T₄ and fT₄ with age were observed.

Weaning effects (P < .001) were shown for T₃ and fT₄ levels. No differences (P > .05) in T₄ and fT₃ levels were observed over the 3-day period. Gender effects (P < .001) were shown for T₃, T₄, fT₃, and fT₄ levels. Significant correlations between T₄ and fT₄ with BW and age were observed in colts and fillies. T₃ concentrations were correlated with age only in colts and fT₃ with BW only in colts. The results obtained seem to lend support to the recognized effects of growing and weaning in modulating the thyroid function of Thoroughbred foals. In fact, significant and differentiated effects of growing and weaning on total and free iodothyronine levels have been demonstrated.     

江淮地区不同轮茬作物对苜蓿产量及根际土壤质量的影响

为阐明江淮地区不同轮茬作物对苜蓿产量及根际土壤质量的影响,以苜蓿生长4年翻耕灭茬重新种植1年为对照(CK),研究了不同轮茬处理(3年苜蓿-玉米-苜蓿记作T 1,3年苜蓿-高粱-苜蓿记作T 2),利用传统法测定苜蓿产量及不同土层土壤理化性质,使用试剂盒测定根际土壤酶活性,通过高通量测序法对根际土壤细菌群落多样性进行了分析。结果表明:轮茬玉米(T 1)及轮茬高粱(T 2)后苜蓿产量分别是苜蓿连种(CK)的1.27和1.13倍;轮茬高粱使土壤pH、有机质含量显著增加(P<0.05),轮茬玉米土壤总氮含量显著增加(P<0.05),轮茬玉米及轮茬高粱显著提高了土壤速效钾含量(P<0.05);不同轮茬均能减轻土壤容重,且轮茬高粱更为显著;与苜蓿连种相比,轮茬玉米与轮茬高粱土壤过氧化氢酶及土壤脲酶活性均显著高于苜蓿连种土壤(P<0.05);借助高通量测序法对根际土壤细菌群落结构多样性分析表明,各处理土壤前3位高丰度表达优势细菌门均为变形菌、拟杆菌和厚壁菌;轮茬玉米与轮茬高粱处理根际土壤细菌种群数量显著高于CK,OTU数量分别是苜蓿连种土壤的1.25和1.39倍,且轮茬高粱处理的土壤细菌Shannon、Chao、Sobs多样性指数显著高于苜蓿连作土壤(P<0.05);而在细菌相对丰度上,苜蓿连作土壤中变形菌及厚壁菌相对丰度显著高于轮茬处理(P<0.05),而轮茬土壤中绿湾菌及浮霉菌要显著高于连作土壤(P<0.05)。综上表明,江淮地区轮茬较苜蓿连作更有助于提高土壤肥力及土壤酶活性,稳定细菌群落结构,改良土壤,提高土地生产力进而提高产量。     

Effect of daily injections of growth hormone-releasing factor and thyrotropin-releasing hormone on growth and endocrine parameters in chickens

The control of growth is a complex mechanism regulated by several metabolic hormones including growth hormone (GH) and thyroid hormones. In avian species, as well as in mammals, GH secretion is regulated by hypothalamic hypophysiotropic hormones. Since thyrotropin-releasing hormone (TRH) and growth hormone-releasing factor (GRF) are potent GH secretagogues in poultry, we were interested in determining the influence of daily intravenous administration of either peptide or both simultaneously on circulating GH and IGF-I concentrations and whether an improvement in growth rate or efficiency would be obtained.

Male broiler chicks were injected once daily for a period of 21 days with either GRF (10 μg/kg), TRH (1 μg/kg) or both GRF and TRH (10 and 1 μg/kg respectively) between four and seven weeks of age. On the last day of the experiment, following intravenous injection of TRH, GRF or a combination of GRF and TRH, plasma GH levels were significantly (P<.05) increased to a similar extent in control chicks and in those which had received daily peptide injections for the previous 21 days. Circulating GH levels between 10 and 90 min post-injection were significantly (P<.05) greater and more than additive than GH levels in chicks injected with both GRF and TRH when compared to those injected with either peptide alone. Mean plasma T₃ concentrations during that same time period were significantly elevated (P<.05) above saline-injected control chick levels in birds treated with TRH or GRF and TRH respectively, regardless of whether the chicks had received peptide injections for the previous 21 days. There was no evidence of pituitary refractoriness to chronic administration of either TRH or GRF injection in terms of growth or thyroid hormone secretion.

Despite the large elevation in GH concentration each day, growth rate, feed efficiency and circulating IGF-I concentrations were not enhanced. Thus the quantity or secretory pattern of GH secretion induced by TRH or GRF administration was not sufficient to increase plasma IGF-I concentration or growth.     

Hormonal changes following an acute stress in control and somatostatin-immunized pigs

Sixteen Yorkshire pigs (49 ± 2 kg BW at 17 weeks) were immunized against somatostatin (SRIF; 4 males, 4 females) or its conjugated protein, bovine serum albumin (BSA; controls; 4 males, 4 females). Immunizations were done at 10, 12 and 14 weeks of age. Jugular vein cannulae were surgically inserted at 17 weeks of age. Five d later, half of each sex from the control and SRIF-immunized groups were stressed. The other half were subjected to the same stress 48 hr later. On both days, remaining animals were used as unstressed controls. The stress consisted of 5 min of snare restraint. Blood samples were collected from all pigs on both days at −20, −15, −10, −5, 0 (beginning of stress), 2, 6, 10, 15, 20, 30, 40, 60, 90, 120, 150, 180 and 240 min. Samples were radioimmunoassayed for cortisol, growth hormone (GH), prolactin (Prl), insulin, triiodothyronine (T₃), thyroxine (T₄) and insulin-like growth factor I (IGF-I). Mean antibody titers against SRIF (1:150 dilution) at 15 weeks were 0.49 ± .09% and 54.5 ± 4.9% for control and SRIF immunized pigs, respectively. Gender and immunization against SRIF had no effect on any of the variables measured (P>0.05), except for T₃ levels which were greater in females than in males (P<0.05). The stress by time of sampling interaction was significant (P<0.01) for all hormones measured. Cortisol values almost tripled within 15 min of stress, reaching concentrations above 100 ng/mL. Maximal increases were seen at 2 min for T₄ (14%), at 6 min for T₃ (36%), at 15 min for Prl (46%) and at 10 min for insulin (141%). An increase of 129% in GH concentration was present at 20 min in stressed pigs; however, an increase of 97% was also seen at 120 min in control pigs. Concentrations of IGF-I decreased (21%) by 60 min in the stressed pigs and remained depressed for up to 150 min. Stress associated with snare restraint, therefore, induces major changes in the concentrations of a series of hormones in growing pigs. On the other hand, immunization against SRIF did not alter any of the hormonal profiles measured. Since snare restraint is widely used to handle pigs during jugular puncture, any study of hormonal secretion in this species should be carried out under carefully controlled conditions in terms of blood sampling technique.     

Effects of pituitary adenylate cyclase-activating polypeptide (PACAP), prostaglandin E2 (PGE2) and growth hormone releasing factor (GRF) on the release of growth hormone from cultured bovine anterior pituitary cells in vitro

The effect of pituitary adenylate cyclase-activating polypeptide (PACAP) on growth hormone (GH) release was compared with that of prostaglandin E₂ (PGE₂) and growth hormone releasing factor (GRF) from cultured bovine anterior pituitary cells in vitro. Both PACAP and PGE₂ stimulated GH release at concentrations as low as 10⁻⁹ and 10⁻⁸ M, respectively, (P<0.01). However, GRF released GH at a concentration as low as 10⁻¹³ M (P<0.01). Percent increases of GH compared with controls were not significantly different among GRF, PACAP, and PGE₂ at 10⁻⁷ M; however, the increases of GH by the 10⁻⁸ M GRF, PACAP and PGE₂ were 196, 118, and 27%, respectively, (P<0.01), and 124, 65, and 1% in the 10⁻⁹ M media, respectively, (P<0.01). When GRF and somatostatin (SS) were added together, the GH releasing effect of GRF was blunted (P<0.01). Similar bluntness were observed in PACAP and PGE₂, when SS was added. The stimulatory effects of GRF and PGE₂ together were similar to that by either GRF or PGE₂ alone. When GRF and PACAP were added together, the GH released by both secretagogues was greater than that by PACAP alone (P<0.01); however, a synergistic effect was not clear when compared with GRF alone.

These findings suggest that PACAP and PGE₂ may modulate the release of GH in cattle.     

Plasma growth hormone, insulin, glucocorticoids and thyroid hormones in large, medium and small breeds of steers with and without an estradiol implant

Growth hormone secretory patterns and plasma concentrations of insulin, adrenal corticosteroids, thyroxine and triiodothyronine were measured in three types of steers with and without estradiol implants. The three types of cattle were large (Simmental), medium (Limousin) and small (Angus) frame size with medium, low and high propensity to deposit fat. Simmental steers had higher (P<.01) concentrations of growth hormone and lower (P<.01) concentrations of insulin as compared with Angus. Implanting the steers with estradiol increased (P<.01) growth hormone concentrations in all three groups, tended to decrease glucocorticoids and had no effect on insulin or thyroid hormone concentrations. There tended to be less frequent release of growth hormone, with secretory spikes of greater magnitude in the Simmental steers. The implants tended to increase the frequency of release of growth hormone in all three groups.     

Effect of a nonsteroidal aromatase inhibitor on in vitro and in vivo secretion of estradiol and on the estrous cycle in ewes.

Three experiments were performed to study effects of decreased concentrations of estradiol-17β (E₂) on lifespan and function of ensuing ovine corpora lutea (CL). In experiment 1, 52 follicles were collected from 10 ewes and placed into individual culture with 0 or .01 μCi ³H-androstenedione (10 ng; ³H-A) and 0, 10⁻¹¹, 10⁻⁹, 10⁻⁷, or 10⁻⁵ M of a nonsteroidal aromatase inhibitor, CGS16949A (CGS). Concentrations of E₂ secreted into the medium, and synthesis of estrogens as estimated by formation of ³H-water from ³H-A were decreased by 10⁻⁵ and 10⁻⁷ (P<.01), but not 10⁻⁹ or 10⁻¹¹ M CGS. In experiment 2, luteolysis was induced in 24 ewes by injection of PGF₂ on days 5 to 10 of the estrous cycle (0 hr). Ewes received 0, 0.5, 1.0, 2.0 or 4.0 mg CGS per kg BW i.v. at −12, 0, 12 and 24 hr, and an ovulatory dose of hCG at 36 hr. Jugular (P<.001) and vena caval (P<.001) concentrations of E₂ were decreased by CGS at all doses tested for 8 to 10 hr, but had returned to levels similar to control ewes by the time of the next injection. Concentrations of E₂ around the time of the LH surge were similar in control and treated ewes. During the subsequent luteal phase, concentrations of progesterone (P₄) were similar in control and treated ewes. Thus, transient decreases in E₂ during the follicular phase were not deleterious to the subsequent luteal phase. In experiment 3, luteolysis was induced in 18 ewes by injection of PGF₂ on days 6 or 7 (0 hr) of the estrous cycle. Ewes received 0 or 1 mg CGS per kg BW i.v. every 8 hr from 0 to 40 hr. Ovulation was induced with hCG at 36 hr. CGS reduced jugular (P<.001) and vena caval (P<.001) concentrations of E₂, prevented an endogenous surge of LH (P<.05) and increased (P<.001) concentrations of FSH. All ewes had ovulated a marked follicle by 72 hr, but onset of the luteal phase, as assessed by concentrations of P₄, was delayed (P<.01) in ewes receiving CGS. Delayed luteal phases were not solely attributable to the presence of new CL or to luteinization of follicular cysts. When data were aligned according to the day ewes were observed in estrus, profiles of P₄ did not differ with treatment. Therefore, normal luteal function ensued following estrus whether or not ewes re-ovulated. In conclusion, decreased secretion of E₂ by the preovulatory follicle was not involved in the ontogeny of CL of short lifespan or subnormal function. Instead, adequate production of E₂ or precisely timed E₂ secretion may be required during follicular development for subsequent functional luteinization.     

Effects of supplemental Bermuda grass hay or corn on intake, digestion and performance of cattle consuming endophyte-infected fescue

Effects of supplemental Bermuda grass hay (BG) or ground corn on intake, digestion and performance of cattle consuming endophyte-infected fescue (I) were studied. In Exp. 1, a Latin square study, five growing Holstein steers (158.1 kg) consumed I ad libitum and were offered 0, .3, .6, .9 or 1.2% body weight (BW) of BG daily. Total dry matter (DM) intake rose linearly (P less than .05) with increasing BG, although intake was numerically similar with .6, .9 and 1.2% BW of BG. Digestibility was constant with diet (P greater than .10). Six growing Holstein steers used in Exp. 2, a Latin square with a 2 x 3 factorial arrangement of treatments, ingested I or noninfected (NI) fescue hay ad libitum with 0, .5 or 1.0% BW of ground corn. Total DM intake increased linearly as the level of corn rose (P less than .05). Total intake with I increased more with the first than with the second addition of corn, and the opposite occurred with NI (interaction between fescue infection and the quadratic effect of corn level, P less than .10). Organic matter digested (g/d) was greater for NI than for I and rose linearly with increasing corn ingestion (P less than .05). Ninety-six crossbred beef heifers and steers (184.2 kg avg initial live weight) were used in a 77-d fall grazing experiment (Exp. 3) with a 2 x 3 factorial treatment arrangement. Cattle grazed I or NI paddocks and were given no supplement or .34% BW of BG or .65% BW of ground corn on a daily basis (DM).(ABSTRACT TRUNCATED AT 250 WORDS)