Glucocorticoids in Mammary Tissue and Milk

Mammary glands are target organs for glucocorticoids which are bound with high affinity (Kd≌10−8 to 10−10M) to specific sites in the cytoplasm and nucleus of the cell. The mammary glucocorticoid binding site is unique from the corticoid binding globulin (transcortin) of blood. In one study each mammary cell of lactating cows bound approximately 1 ,300 glucocorticoid molecules, whereas mammary tissue of heifers and dry cows bound approximately 300 to 400 molecules per cell. The mammary tissues binding sites are probably essential for glucocorticoids to exert their biological function within the mammary cell because binding activity is correlated with inhibition of glucose incorporation into mammary tissue slices in lactating cows. A small portion of the glucocorticoids are excreted into the milk, although their concentrations in milk (x̄ range from .19 to .62 ng/ml) averaged only 3.8% of those in serum (x̄ range from 6.8 to 13.4 ng/ml). The milk glucocorticoids decrease with advancing lactation but are unaffected by stage of estrous cycle. The glucocorticoids are associated primarily with the protein fractions of milk, whereas the ovarian steroids are associated principally with the lipid fractions of milk.