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.
Bibliographical noteFunding Information:
The authors express their appreciation to Sara Sanders, Rosemarie Burgos-Zimbleman, Laura Odens, Gilad Shwartz, Shannon Baker, Sarah Hartman, and Jennifer Ernest for assistance at the Agriculture Research Complex (University of Arizona, Tuscon) and to Wanda Weber (Department of Animal Science, University of Minnesota) for the GH analyses. This work was partially funded by the University of Arizona Experiment Station , #ARZT-136339-H-24-130 , the University of Minnesota Agriculture Experiment Station (project number 16-46 ) and National Research Initiative Competitive Grants no. 2005-35203-16041 and 2008-35206-18817 from the USDA Cooperative State Research, Education, and Extension Service.
- Heat stress