Hurley W L, Convey E M, Leung K, Edgerton L A, Hemken R W
J Anim Sci. 1980 Aug;51(2):374-9. doi: 10.2527/jas1980.512374x.
Serum concentrations of prolactin (PRL), thyrotropin (TSH), thyroxine (T4) and triiodothyronine (T3) were compared between Holstein calves fed a toxic fescue (G1-307) and Holstein calves fed a less toxic fescue (G1-306) at one of three environmental temperatures (low, 10 to 13 C; medium, 21 to 23 C; high, 34 to 35 C). Three calves were assigned to each treatment combination, and hormone concentrations were determined before (basal) and after thyrotropin-releasing hormone (TRH) was injected at 15 microgram/100 kg body weight via jugular cannula. Calves fed the toxic G1-307 had lower basal PRL concentrations than those fed G1-306 (1.8 +/- .1 vs 6.0 +/- 1.2 ng/ml). Increasing ambient temperature increased basal PRL levels in calves fed G1-306 (i.e., low [2.3 +/- .3] < medium [3.6 +/- .9] < high [12.1 +/- 1.3 ng/ml]) but not in calves fed G1-307 (i.e. low [1.8 +/- .1] = medium [2.0 +/- .1] = high [1.6 +/- .1]). Similarly, after TRH injection, PRL release was greater in calves fed the less toxic G1-306 (35.7 +/- 5.2 ng/ml) than in those fed G1-307 (5.6 +/- .6 ng/ml) and was positively affected by temperature (i.e., low [6.9 +/- 1.2] < medium [18.8 +/- 5.6] < high [36.3 +/- 5.9 ng/ml]). In contrast, basal and TRH-induced TSH concentrations were unaffected by temperature or fescue. However, T3 (time 0) was inversely related to temperature (1.06 +/- .1, 1.40 +/- .1 and 1.87 +/- .2 ng/ml for high, medium and low temperature, respectively). After TRH injections, T3 increased linearly and at the same rate (.007 ng/ml/min) at all temperatures, but T4 concentrations increased only in low temperature groups. Neither T4 nor T3 concentrations were affected by forage fed. We conclude that some factor(s) in toxic fescue inhibits PRL secretion, especially at high temperature, an environmental condition closely associated with fescue summer toxicosis in cattle. Increased body temperature and respiration rates associated with fescue summer toxicosis apparently are not mediated by TSH or thyroid hormone secretion.
比较了在三种环境温度（低温，10至13摄氏度；中温，21至23摄氏度；高温，34至35摄氏度）之一条件下，采食有毒羊茅（G1 - 307）的荷斯坦犊牛与采食毒性较小羊茅（G1 - 306）的荷斯坦犊牛血清中催乳素（PRL）、促甲状腺激素（TSH）、甲状腺素（T4）和三碘甲状腺原氨酸（T3）的浓度。每种处理组合分配三头犊牛，通过颈静脉插管以15微克/100千克体重注射促甲状腺激素释放激素（TRH）之前（基础值）和之后测定激素浓度。采食有毒G1 - 307的犊牛基础PRL浓度低于采食G1 - 306的犊牛（1.8±0.1对6.0±1.2纳克/毫升）。环境温度升高使采食G1 - 306的犊牛基础PRL水平升高（即低温[2.3±0.3]<中温[3.6±0.9]<高温[12.1±1.3纳克/毫升]），但采食G1 - 307的犊牛未出现这种情况（即低温[1.8±0.1]=中温[2.0±0.1]=高温[1.6±0.1]）。同样，注射TRH后，采食毒性较小G1 - 306的犊牛PRL释放量（35.7±5.2纳克/毫升）高于采食G1 - 307的犊牛（5.6±0.6纳克/毫升），且受温度正向影响（即低温[6.9±1.2]<中温[18.8±5.6]<高温[36.3±5.9纳克/毫升]）。相反，基础和TRH诱导的TSH浓度不受温度或羊茅的影响。然而，T3（0时刻）与温度呈负相关（高温、中温和低温分别为）。注射TRH后，T3在所有温度下均呈线性增加且速率相同（0.007纳克/毫升/分钟），但T4浓度仅在低温组增加。T4和T3浓度均不受所喂饲草料的影响。我们得出结论，有毒羊茅中的某些因素抑制PRL分泌，尤其是在高温下，高温是与牛的羊茅夏季中毒密切相关的环境条件。与羊茅夏季中毒相关的体温和呼吸频率增加显然不是由TSH或甲状腺激素分泌介导的。
