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

Estrone (E₁) and 17β-estradiol (E₂) 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 E₁ and E₂ in plasma and milk and to determine the correlations between plasma and milk E₁ and E₂ and with milk components throughout pregnancy. Blood and milk samples were collected from 13 cows every 28 d throughout pregnancy. The E ₁ and E₂ were quantified in plasma and milk using RLA 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 E₁ concentrations in milk averaged 0.6, 7.9, and 27.1 pg/mL. The E₂ concentrations in plasma averaged 0.5, 0.9, and 2.0 pg/mL; milk E₂ averaged 0.3, 0.9, and 5.0 pg/mL. Plasma and milk E₂ concentrations were greater in trimester 3 compared with trimesters 1 and 2. The E₁ concentrations in milk were significantly correlated with plasma E₁ concentrations (r = 0.77), percentage of milk fat (r = 0.50), and milk yield (r = -0.43). The E₂ concentrations in milk were significantly correlated with plasma E₂ 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 E₂ in milk may change over the course of pregnancy.