The role of the mammary gland during heat stress

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] Disruptive effects of climate change, such as increasing environmental temperature, have direct impacts on economic viability and efficiency of food production. In lactating dairy cows, heat stress reduces milk production and alters function of mammary secretory cells, at least partly by disturbing local protein metabolism. We hypothesized that hyperthermia would not only reduce mammary blood flow but would also reduce mammary extraction of nutrients from blood. In addition, we hypothesized that transcriptional profiling of mammary tissue would reveal disruption of cellular homeostasis. Our objective was to determine the effects of hyperthermia on mammary function. More specifically, we aimed to profile mammary blood flow and the changes in mammary transcriptome of heat-stressed lactating dairy cows. We investigated the effects of early and prolonged exposure of lactating dairy cows to hyperthermia by exposing cows to programmed constantly elevated temperature and humidity to induce and maintain body temperature approximately 1[degree]C above normal. Experiments were conducted to evaluate the production responses of hyperthermic lactating dairy cows, to characterize total and nutritive mammary blood flow, and to elucidate the regulation of mammary function during early and prolonged exposure to hyperthermia. Results from these studies established that 1) hyperthermia reduces total and nutritive mammary blood flow, limiting nutrient disappearance across the mammary gland; 2) hyperthermia does not induce shunting of blood away from the gland; 3) hyperthermia affects mammary tissue transcriptome, mainly altering processes associated with ECM and cell adhesion; 4) the effects of exposure to prolonged heat stress on mammary gene expression are distinct from the effects of feed restriction, in lactating dairy cows; and 5) mammary function is reestablished within 8 days after cessation of heat stress.