Rumsey T S, McLeod K, Elsasser T H, Kahl S, Baldwin R L
Growth Biology Laboratory, USDA, Livestock and Poultry Sciences Institute, Beltsville, MD 20705-2350, USA.
J Anim Sci. 1999 Aug;77(8):2079-87. doi: 10.2527/1999.7782079x.
The objective of this study was to determine the effect of a subtherapeutic level of chlortetracycline (CTC) fed to growing beef steers under conditions of limited and adequate dietary protein on plasma concentrations of GH, thyroid-stimulating hormone (TSH), and thyroid hormones before and after an injection of thyrotropin-releasing hormone (TRH) + GHRH. Young beef steers (n = 32; average BW = 285 kg) were assigned to a 2x2 factorial arrangement of treatments of either a 10 or 13% crude protein diet (70% concentrate, 15% wheat straw, and 15% cottonseed hulls) and either a corn meal carrier or carrier + 350 mg of CTC daily top dressed on the diet. Steers were fed ad libitum amounts of diet for 56 d, and a jugular catheter was then placed in each steer in four groups (two steers from each treatment combination per group) during four consecutive days (one group per day). Each steer was injected via the jugular catheter with 1.0 microg/kg BW TRH + .1 microg/kg BW GHRH in 10 mL of saline at 0800. Blood samples were collected at -30, -15, 0, 5, 10, 15, 20, 30, 45, 60, 120, 240, and 360 min after releasing hormone injection. Plasma samples were analyzed for GH, TSH, thyroxine (T4), and triiodothyronine (T3). After 84 d on trial, the steers were slaughtered and the pituitary and samples of liver were collected and analyzed for 5'-deiodinase activity. Feeding CTC attenuated the GH response to releasing hormone challenge by 26% for both area under the response curve (P<.03) and peak response (P<.10). Likewise, CTC attenuated the TSH response to releasing hormone challenge for area under the response curve by 16% (P<.10) and peak response by 33% (P<.02), and attenuated the T4 response for area under the curve by 12% (P<.08) and peak response by 14% (P<.04). Type II deiodinase activity in the pituitary was 36% less (P<.02) in CTC-fed steers than in steers not fed CTC. The results of this study are interpreted to suggest that feeding subtherapeutic levels of CTC to young growing beef cattle attenuates the release of GH and TSH in response to pituitary releasing hormones, suggesting a mechanism by which CTC may influence tissue deposition in cattle.
本研究的目的是确定在日粮蛋白质有限和充足的条件下，给生长中的肉用公牛饲喂低于治疗剂量的金霉素（CTC），对注射促甲状腺激素释放激素（TRH）+生长激素释放激素（GHRH）前后血浆中生长激素（GH）、促甲状腺激素（TSH）和甲状腺激素浓度的影响。将年轻肉用公牛（n = 32；平均体重 = 285千克）分配到2×2析因试验设计的处理组中，分别饲喂10%或13%粗蛋白日粮（70%精料、15%小麦秸秆和15%棉籽壳），且日粮中要么添加玉米粉载体，要么添加载体+每天350毫克的金霉素。公牛自由采食日粮56天，然后在连续四天里，每组（每组来自每个处理组合的两头公牛）的每头公牛都放置颈静脉导管。在0800时，通过颈静脉导管给每头公牛注射1.0微克/千克体重的TRH + 0.1微克/千克体重的GHRH，溶于10毫升生理盐水中。在注射激素后-30、-15、0、5、10、15、20、30、45、60、120、240和360分钟采集血样。分析血浆样本中的GH、TSH、甲状腺素（T4）和三碘甲状腺原氨酸（T3）。试验84天后，屠宰公牛，采集垂体和肝脏样本并分析5'-脱碘酶活性。饲喂金霉素使生长激素对释放激素刺激的反应在反应曲线下面积方面降低了26%（P<0.03），在峰值反应方面降低了26%（P<0.10）。同样，金霉素使促甲状腺激素对释放激素刺激的反应在反应曲线下面积方面降低了16%（P<0.10），在峰值反应方面降低了33%（P<0.02），并使甲状腺素在曲线下面积方面的反应降低了12%（P<0.08），在峰值反应方面降低了14%（P<0.04）。饲喂金霉素的公牛垂体中的II型脱碘酶活性比未饲喂金霉素的公牛低36%（P<0.02）。本研究结果表明，给年轻生长肉用公牛饲喂低于治疗剂量的金霉素会减弱生长激素和促甲状腺激素对垂体释放激素刺激的释放，提示金霉素可能影响牛体组织沉积的一种机制。
