Metabolic and endocrine adaptations to heat stress in lactating dairy cows

Heat stress (HS), a stress response in homeotherms mainly due to elevated ambient temperature and failure of effective heat dissipation, causes a substantial negative economic impact to livestock industry worldwide. Reduced feed intake, a typical phenomenon observed during HS, was thought to be the primary driver for the milk production loss. However, accumulating evidence indicates that HS influences animal metabolism and endocrine profiles independent of reduced feed intake. Previous studies comparing heat-stressed lactating cows with control group pair-fed (PF) to the intake of HS group but housed in thermoneutral conditions, in order to eliminate the confounding factors result from differentiated feed intakes, showed that HS increased circulating insulin and decreased plasma non-esterified fatty acid (NEFA) in lactating cow, the opposite responses typical of PF cohorts. Therefore, the present studies were performed in order to elucidate the mechanism(s) underlying these counterintuitive changes. In response to a glucose tolerance test (GTT), both HS and PF decreased whole body glucose disposal rate, a sign of insulin resistance. Only PF decreased skeletal muscle insulin sensitivity in terms of reduced protein kinase B (PKB/AKT) phosphorylation, a downstream protein of insulin receptor (IR), while HS group maintained similar intact insulin responsiveness in the liver and skeletal muscle as thermoneutral conditions. There was a global reduction in gene expression of the enzymes related to lipid metabolism in adipose tissue of heat-stressed cows. Similarly, β-adrenergic signaling, a major stimulator of lipid mobilization, was suppressed in terms of NEFA release response during a chronic epinephrine challenge in HS group. After the challenge, phosphorylations of cAMP-response element binding protein (CREB) and hormone sensitive lipase, both located downstream of β-adrenergic receptor, were decreased in HS, but not in thermoneutral conditions, another indicator of impaired adrenergic signaling. In contrast, IR and AKT phosphorylation were increased in HS conditions indicating insulin signaling may be elevated during HS in adipose. Collectively, HS reduces lipid mobilization and appears to favor glucose utilization via alterations of lipid metabolism and hormones signaling pathways. These unique alterations in HS might shed some light on developing counter-HS approaches in the future.