<b>Heat stress responses and adaptations of peripheral tissues in lactation Holstein dairy cow revealed by transcriptomic analysis</b>

Heat stress can have detrimental effects on the milk synthesis process by reducing the availability of precursors to the mammary gland. This study aimed to investigate the metabolism and thermoregulation in heat-stressed lactating dairy cows by using transcriptomic and bioinformatic analyses of adipose tissue and skeletal muscle. Eight multiparous Holstein dairy cows with rumen cannula were randomly divided into two groups (n = 4) and subjected to either heat stress or thermal neutrality with pair-fed. An experimental period of 25 days was conducted, with the first 7 days allotted for adaptation and the subsequent 18 days divided into a control phase (9 days) and a trial phase (9 days). During the adaptation and control phase, all cows were kept in thermal neutral conditions with ad libitum feeding. During the trial phase, four cows were exposed to cyclical heat stress conditions and fed ad libitum, while the other four cows were maintained in thermal neutral conditions but were pair-fed. Transcriptomic analysis was conducted on both adipose tissue and skeletal muscle tissues collected at the end of the trial phase in each group. The RNA-Seq results revealed that heat stress affected the expression of 60 and 237 genes in adipose tissue and skeletal muscle tissues, respectively. The DIA analysis highlighted the overall adaptations of metabolism pathways in adipose tissue, as well as the activation of extracellular matrix remodeling and the elevation of insulin resistance by inhibiting glycosphingolipid biosynthesis, which may contribute to the decline in fat mobilization during heat stress and be associated with obesity. Additionally, heat stress was found to promote glycogenolysis in skeletal muscle by upregulating the activity of serine/threonine kinase and regulation of glucose uptake in the muscle. The findings of this study provide insights for future research endeavors aiming to better comprehend the physiological and productivity effects of heat stress on lactating cows as well as their adaptations, and support the development of effective mitigation strategies.