Department of Animal Sciences, University of Arizona, Tucson 85721, USA.
J Dairy Sci. 2010 Jan;93(1):170-9. doi: 10.3168/jds.2009-2469.
Heat stress (HS) is a multibillion-dollar problem for the global dairy industry, and reduced milk yield is the primary contributor to this annual economic loss. Feed intake declines precipitously during HS but accounts for only about 35% of the decreased milk synthesis, indicating that the physiological mechanisms responsible for decreased milk production during HS are only partly understood. Thus, our experimental objectives were to characterize the direct effects of HS on the somatotropic axis, a primary regulator of metabolism and milk yield. We recently reported no differences in mean growth hormone (GH) concentrations, GH pulsatility characteristics, or GH response to growth hormone releasing factor in HS versus pair-fed (PF) thermoneutral controls. Despite similarities in circulating GH characteristics, plasma insulin-like growth factor (IGF)-I concentrations were reduced during heat stress conditions but not in PF animals, suggesting that uncoupling of the hepatic GH-IGF axis may occur during HS. We investigated this possibility by measuring proximal indicators of hepatic GH signaling following a GH bolus. Heat stress but not PF decreased abundance of the GH receptor and GH-dependent signal transducer and activator of transcription (STAT)-5 phosphorylation. Consistent with reduced GH signaling through STAT-5, basal hepatic IGF-I mRNA abundance was lower in HS cows. Thus, the reduced hepatic GH responsiveness (in terms of IGF-I gene expression) observed during HS appears to involve mechanisms at least partially independent of reduced nutrient intake. The physiological significance of reduced hepatic GH receptor abundance during HS is unclear at this time. Aside from reducing IGF-I production, it may reduce other GH-sensitive bioenergetic processes such as gluconeogenesis.
热应激（HS）是全球奶制品行业的一个数十亿美元的问题，而产奶量下降是造成这种年度经济损失的主要原因。在热应激期间，饲料摄入量急剧下降，但仅占牛奶合成减少的约 35%，这表明导致热应激期间产奶量下降的生理机制仅部分被理解。因此，我们的实验目的是描述 HS 对生长激素轴的直接影响，生长激素轴是新陈代谢和产奶量的主要调节者。我们最近报道，在 HS 与配对喂养（PF）的等热对照组中，平均生长激素（GH）浓度、GH 脉冲特性或 GH 对生长激素释放因子的反应没有差异。尽管循环 GH 特征相似，但在热应激条件下血浆胰岛素样生长因子（IGF）-I 浓度降低，而在 PF 动物中则没有，这表明在 HS 期间肝 GH-IGF 轴可能发生解偶联。我们通过测量 GH 脉冲后肝 GH 信号的近端指标来研究这种可能性。HS 而不是 PF 降低了 GH 受体和 GH 依赖性信号转导和转录激活因子（STAT）-5 磷酸化的丰度。与通过 STAT-5 降低 GH 信号一致，HS 奶牛的基础肝 IGF-I mRNA 丰度较低。因此，在 HS 期间观察到的肝 GH 反应性降低（以 IGF-I 基因表达为衡量标准）似乎涉及至少部分独立于减少营养摄入的机制。目前尚不清楚 HS 期间肝 GH 受体丰度降低的生理意义。除了减少 IGF-I 的产生外，它还可能减少其他 GH 敏感的生物能量过程，如糖异生。
