Effects of heat stress on plasminogen activator related in milk protein in bovine mammary epithelial cells and mid lactating Holstein cow milk

Heat stress (HS) is a significant factor in reducing milk yield and protein in dairy cows. However, studies on the mechanisms underlying HS-induced reductions in milk protein are limited. This study investigated the effects of HS on the plasminogen activator (PA) in bovine mammary alveolar cells (MAC-T) and cow milk to investigate milk protein reduction under hyperthermia. In an in vivo study, milk samples were collected from thirty mid-lactating Holstein cows in different seasons with varying temperature–humidity index (severe–moderate, SM; moderate–mild, MM; and mild–comfortable, MC). The average daily milk yield of cows was 33.93 ± 0.98 kg; cows were fed a TMR diet, delivered twice daily, and ad libitum access to water. The SM group had the highest (P < 0.05) milk uPA concentrations (1.48 ng/mL), followed by the MM group (0.43 ng/mL). Milk protein levels were lower (P < 0.05) in the SM group (3.1 %), which was exposed to more HS than the MM (3.35 %) and MC (3.46 %) groups. To determine whether HS also alters PA expression in MAC-T cells, cells were subjected to HS at 41 °C for 2 h, with recovery at 37 °C for 8 h. HS increased (P < 0.05) mRNA expression levels of heat shock protein 70 (HSP70), urokinase-PA (uPA), and tissue-PA (tPA). HS upregulated HSP70 expression (P < 0.05) and tended to raise uPA protein expression (P = 0.096). Although HS did not affect protein synthesis in MAC-T cells, cortisol, insulin, and prolactin did not affect uPA or tPA expressions in MAC-T cells. These results suggest that HS contributes to decreased milk protein content through elevated PA in the alveolar lumen rather than impaired protein synthesis in MAC-T cells. Additionally, these HS-induced mechanisms in PA provide an additional clue explaining dairy cows' decreased milk protein levels.