Animal Molecular Breeding 2024, Vol.14, No.3, 196-206 http://animalscipublisher.com/index.php/amb 197 2 Mechanisms of Heat Stress in Dairy Cattle 2.1 Physiological impact of heat stress Heat stress in dairy cattle leads to significant physiological changes that adversely affect their productivity and overall health. One of the primary impacts is a reduction in feed intake, which subsequently leads to decreased milk production. This reduction in feed intake is a direct response to the increased body temperature and the need to minimize metabolic heat production. Additionally, heat stress induces various physiological stress responses, including alterations in blood flow, increased respiration rate, and elevated heart rate, all aimed at dissipating excess body heat (Garner et al., 2020). Chronic heat stress can also lead to changes in the expression of genes related to inflammation, lipid metabolism, and the cardiovascular system, further exacerbating the physiological strain on the animals (Liu et al., 2020). 2.2 Behavioral responses to heat stress Behavioral adaptations are crucial for dairy cattle to cope with heat stress. Cows often seek shade and increase their water intake to cool down. They may also reduce their physical activity to lower metabolic heat production. These behavioral changes are essential for maintaining homeostasis but can lead to reduced feed intake and, consequently, lower milk yield (Choi et al., 2021). The behavioral responses are often accompanied by physiological changes, such as increased sweating and panting, which help in thermoregulation but can also lead to dehydration and electrolyte imbalances. 2.3 Cellular and molecular responses At the cellular and molecular levels, heat stress triggers a complex network of responses aimed at protecting cellular integrity and function. One of the key responses is the upregulation of heat shock proteins (HSPs), which play a critical role in protein folding and protection against thermal damage (Sengar et al., 2017). Genes involved in oxidative stress, apoptosis, and glucose metabolism are also differentially expressed under heat stress conditions, indicating a broad cellular response to mitigate damage and maintain cellular function (Fang et al., 2021). Additionally, miRNAs have been identified as important regulators of the heat stress response, targeting genes involved in stress signaling pathways and protein synthesis. For instance, miRNAs such as bta-miR-423-5p and bta-miR-2898 have been shown to regulate the expression of heat shock proteins and other stress-related genes, highlighting their role in the post-transcriptional regulation of the heat stress response (Deb and Sengar, 2020; Liu et al., 2020). 3 Gene Expression Changes in Heat-Stressed Dairy Cattle 3.1 Key genes affected by heat stress Heat stress significantly impacts the expression of various genes in dairy cattle. Notably, genes encoding heat shock proteins (HSPs) such as HSP70, HSP90, and HSP27 are upregulated in response to elevated temperatures. For instance, the expression of HSP70 and HSP90 genes was significantly higher in Hanwoo calves exposed to high temperature-humidity indices (THIs) (Kim et al., 2020). Similarly, in Sahiwal and Tharparkar breeds of zebu cattle, HSP70 family genes (HSPA1A, HSPA1B, and HSPA8) showed maximal induction during summer (Kumar et al., 2015). These findings suggest that HSP genes are crucial markers for assessing heat stress in dairy cattle. 3.2 Heat shock proteins (HSPs) and their roles Heat shock proteins (HSPs) play a vital role in protecting cells from heat-induced damage. HSP70, one of the most studied HSPs, is highly sensitive to heat stress and is primarily responsible for cellular protection. In bovine mammary epithelial cells, HSP70 expression peaked at 14 times the control level after 1 hour of heat exposure (Hu et al., 2016). HSP90 also plays a significant role, with its expression being higher in Sahiwal cattle compared to Frieswal cattle under both in vitro and environmental heat stress conditions (Deb et al., 2014). These proteins help maintain cellular homeostasis by preventing protein aggregation and assisting in protein refolding. 3.3 Impact on metabolic pathways Heat stress affects various metabolic pathways in dairy cattle, leading to altered nutrient utilization and metabolic disorders. For example, heat-stressed cows exhibit decreased ruminal pH and acetate concentration, along with increased ruminal lactate levels (Kim et al., 2022). This shift in rumen fermentation is associated with changes in
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