17β-estradiol and estrone concentrations in plasma and milk during bovine pregnancy

Estrone (E-1) and 17 beta-estradiol (E-2) are present in milk, but the mechanism(s) that regulate their appearance in milk are not known. The objectives of this study were to determine the impact of stage of pregnancy on the concentrations of E-1 and E-2 in plasma and milk and to determine the correlations between plasma and milk E-1 and E-2 and with milk components throughout pregnancy. Blood and milk samples were collected from 13 cows every 28 d throughout pregnancy. The E-1 and E-2 were quantified in plasma and milk using RIA after organic solvent extractions and Sephadex LH-20 column chromatography. Plasma E1 concentrations averaged 0.8, 16.9, and 41.8 pg/mL in trimesters 1, 2, and 3, respectively. The respective E-1 concentrations in milk averaged 0.6, 7.9, and 27.1 pg/mL. The E-2 concentrations in plasma averaged 0.5, 0.9, and 2.0 pg/mL; milk E-2 averaged 0.3, 0.9, and 5.0 pg/mL. Plasma and milk E-2 concentrations were greater in trimester 3 compared with trimesters 1 and 2. The E-1 concentrations in milk were significantly correlated with plasma E-1 concentrations (r = 0.77), percentage of milk fat (r = 0.50), and milk yield (r = -0.43). The E-2 concentrations in milk were significantly correlated with plasma E-2 concentrations (r = 0.93), percentage of milk protein (r = 0.63), and milk yield (r = -0.57). The milk-to-plasma ratio of E-2 increased from 0.4 during trimester 1 to 2.2 in trimester 3, which suggested that the mechanism(s) regulating the appearance of E-2 in milk may change over the course of pregnancy.