Periodic Change Of Bovine Lactation-related Hormones And The Effect Of Hormones On Gene Expression In Bovine Mammary Gland Explants

Our study investigated the variation of eight lactation-related hormones during the wholedairy production cycle, the correlation between the hormones and milk performance to revealthe importance of hormonal regulation for the lactation and to provide systematic theory andexperimental basis for the study of the molecular biology mechanism of lactation. Besides,the effect of each hormone alone or in combination with other hormones on gene expressionin bovine mammary gland explants had been investigated.Trial1Holstein cows （n=3） of the same lactational physiology were selected to obtainthe blood and milk samples periodically. The contents of hormones in serum were measuredby radioimmunoassay, and the milk composition was measured by automatic analyzer. Theresults indicated that the content of serum E₂changed significantly during the productioncycle （P<0.0001）. E₂remained high before parturition, and remained low relativelypostpartum until the dry period; the content of serum P₄changed significantly （P<0.05）. P₄remained at a high level before parturition, and remained low until15days postpartum.P₄remained high from lactation40days to the dry period. The content of serum PRL changedsignificantly （P<0.0001）. PRL remained relatively stable during the perinatal period, but itrapidly declined from the perinatal period to lactation40days. And there was a significantincrease from lactation40days to270days. The content of serum T₄changed significantly （P<0.0001）. T₄remained low during the perinatal period.Meanwhile it remained high andshowed a growth trend from the lactation period to the dry period. The content of serum GHchanged not significantly during the production cycle （P>0.05）. The content of serum INSchanged significantly during the production cycle （P<0.0001）. INS remained low during theperinatal period, and it remained high from lactation40days to270days. There was adecrease during the dry period. The content of serum G changed significantly during theproduction cycle （P<0.0001）. G remained high before parturition, and remained low relatively postpartum. It remained stable during the lactation, and there was an increase during the dryperiod. The content of serum IGF-I changed significantly during the production cycle （P<0.05）. IGF-I decreased after increasing during the perinatal period, and it remained stableduring40-270days of lactation. It also decreased after increasing during the dry period. Themilk yield, milk protein and lactose changed significantly during lactation （P<0.0001）, whilethe milk fat content did not change significantly （P>0.05）. There were a negative correlation（P<0.05） between milk production and E₂、PRL and G in serum, a positive correlation（P<0.05） between milk protein and E₂、P₄and G in serum, a negative correlation （P<0.05）between milk fat and P₄in serum and a negative correlation （P<0.01） between lactose and E₂,PRL, G and IGF-I in serum during lactation. These results suggest that the eight hormonesshow characteristic changes during the dairy production cycle, and correlation analysisindicate that: milk yield is mainly affected by E₂, PRL, and G; milk fat is mainly affected byP₄; milk protein is mainly affected by E₂, P₄and G; lactose is mainly affected by E₂, PRL, Gand IGF-I.Trial2Mammary gland explants were cultured in the presence of different hormones,and mRNA expression of several genes were relatively quantified by RT-PCR in bovinemammary gland explants. The results showed that concentration of prolactin in mediumsignificantly affected ACC, FAS and STAT5mRNA expression （P<0.01）, and which weresignificantly lower than other groups when the concentration of prolactin was at physiologicalconcentration.When the concentration was two times the physiological concentration, theexpression was highest. IGFBP5mRNA expression was greatly higher than other treatmentswhen the concentration of prolactin was lower than physiological concentration （P<0.05）.Butdifferent treatments did not affect αS1-casein mRNA expression （P>0.05）. Concentration ofgrowth hormone significantly affected the expression of ACC, FAS and STAT5mRNA（P<0.01）, and which were significantly decreased when the concentration was two timeshigher than physiological concentration, but the expression increased slightly with theconcentration. It was higher than other groups when the concentration was three times thephysiological concentration （P<0.05）, but then their expression was inhibited. Concentrationof hydrocortisone had a significant effect （P<0.05） on the expression of STAT5mRNA, andwhich was significantly lower than other groups when it was lower than physiological concentration. The IGFBP5mRNA expression was lowest when the concentration was lowerthan physiological concentration. Combination of Prolactin, growth hormone andhydrocortisone in medium made a significant effect on the expression of ACC, FAS andSTAT5mRNA （P <0.05）, and which were higher than other treatments when treated byGH+H. All the treatments had no significant effect on the αS1-casein and IGFBP5mRNAexpression （P>0.05）.The results indicate that fat synthesis in the mammary gland is mainlyregulated by PRL and GH, and the mammary cell proliferation and apoptosis are mainlyregulated by PRL, H, and GH.But the casein synthesis is only regulated by GH, and GH+Hsignificantly upregulate the expression of ACC, FAS and STAT5.