Weber W J, Baumgard L H, Kazmer G W, Zinn S A, Hansen L B, Chester-Jones H, Crooker B A
Department of Animal Science, University of Minnesota, St. Paul 55108, USA.
J Dairy Sci. 2005 May;88(5):1723-31. doi: 10.3168/jds.S0022-0302(05)72845-9.
Holstein heifer, steer, and bull calves from control (CL) and select (SL) lines of cows that differed by more than 4000 kg of milk during a 305-d lactation (SL > CL) were used to determine growth hormone (GH) response to 5 doses of GH releasing hormone (GHRH) and how this response was affected by gender, period (age), and genetic merit for milk yield. Doses (0, 2.5, 5, 10, and 20 microg/100 kg of BW) of a GHRH analog were assigned randomly to each heifer (4 CL, 4 SL), steer (4 CL, 4 SL), and bull (3 CL, 3 SL) calf and administered on consecutive days at approximately 3, 6, and 10 mo of age (periods; P1, P2, and P3). Jugular blood samples (n = 15) collected between -30 and 240 min relative to GHRH administration were used to quantify area under the GH response curve (AUC) after subtracting mean prechallenge GH concentrations. Estimates of maximum response (Rmax) and sensitivity (ED50) to GHRH were obtained from the hyperbolic dose response curves (AUC vs. dose). Data were analyzed for effects of dose, line, period, gender, and their interactions with period as the repeated effect. Prechallenge GH concentrations were not affected by genetic line, gender, or period. The AUC was not affected by line, but decreased with period and increased with GHRH dose. The Rmax did not differ between lines or among genders, but decreased with period. The ED50 did not differ between lines or among periods, but heifers were more sensitive to GHRH than steers or bulls. Although GH response to GHRH has been identified as a potential indicator of genetic merit, it did not differ between these substantially different genetic lines.
选取在305天泌乳期内产奶量相差超过4000千克（SL > CL）的对照（CL）系和选择（SL）系奶牛所产的荷斯坦小母牛、阉牛和公牛犊，用于测定生长激素（GH）对5种剂量生长激素释放激素（GHRH）的反应，以及性别、时期（年龄）和产奶量遗传价值对该反应的影响。将一种GHRH类似物的剂量（0、2.5、5、10和20微克/100千克体重）随机分配给每头小母牛（4头CL系、4头SL系）、阉牛（4头CL系、4头SL系）和公牛犊（3头CL系、3头SL系），并在大约3、6和10月龄（时期；P1、P2和P3）连续给药。在相对于GHRH给药的 -30至240分钟之间采集颈静脉血样（n = 15），用于在减去激发前GH平均浓度后量化GH反应曲线下面积（AUC）。从双曲线剂量反应曲线（AUC与剂量）中获得对GHRH的最大反应（Rmax）和敏感性（ED50）估计值。分析数据以研究剂量、品系、时期、性别及其与时期的交互作用（作为重复效应）的影响。激发前GH浓度不受遗传品系、性别或时期的影响。AUC不受品系影响，但随时期下降且随GHRH剂量增加。Rmax在品系之间或性别之间没有差异，但随时期下降。ED50在品系之间或时期之间没有差异，但小母牛对GHRH比阉牛或公牛更敏感。尽管GH对GHRH的反应已被确定为遗传价值的潜在指标，但在这些差异很大的遗传品系之间没有差异。
