Inhibition of casein and fat synthesis and alpha-lactalbumin secretion by progesterone in explants from bovine lactating mammary glands.

The effect of progesterone on casein and fat synthesis, on alpha-lactalbumin secretion and on accumulation of casein mRNA was studied in explants from bovine lactating mammary glands, cultured in the presence of insulin, cortisol or prolactin. All four biological activities were inhibited down to or below the level obtained in medium without prolactin. However, inhibition was only achieved by using concentrations of progesterone exceeding by 1000-fold those required for similar effects in tissue from non-lactating animals. A similar inhibitory effect was also found using high levels of other steroids without 11 beta-hydroxyl, such as deoxycorticosterone and dehydroepiandrosterone, and to some extent with oestradiol-17 beta, but not with cortisol or corticosterone. The inhibition by progesterone did not result from a decrease in explant viability, as determined by glucose uptake.