In Vivo and In Vitro Investigations on the Synthesis of Estradiol‐17β by the Bovine Mammary Gland near Term

Abstract

Contents Concentration of estradiol‐17β in the milk and tail vein were determined by radio‐immunoassay in 40 dairy cows from day 20 prepartum to day 4 postpartum. The animals were divided into two groups according to their milk production in the previous lactation: group 1 (n = 20)‐high‐yielding cows (6500 kg milk per lactation), group 2 (n = 20) low yielding cows (3700 kg milk per lactation). The levels of estradiol‐17β were higher (p 0.01) in the milk vein than in the tail vein from about day 12 prepartum to parturition. In comparison with low‐yielding cows the high‐yielding cows had significantly higher (p 0.01) estradiol‐17β values in the mammary venous plasma, but not in peripheral plasma. In an in vitro experiment, homogenates of bovine mammary tissue collected on day 7 prepartum from two cows were incubated with 3H‐androstendione. After incubation the samples were extracted and 3H‐estradiol‐17β was separated by high‐performance liquid chromatography. Radioactivity of fractions containing 3H‐estradiol‐17β was determined and calculated as a percentage of total radioactivity. Androstendione was metabolized into estradiol‐17β to the extent of 37%. These results indicate that the mammary secretion of estradiol‐17β starts about day 12 prepartum and is associated with milk yield. Furthermore, the results of a preliminary in vitro study with a limited number of animals suggest that the bovine mammary tissue is able to synthesize estradiol‐17β near term.