Corticosteroid-Binding Globulin in the Rat: Possible Role in the Initiation of Lactation

Corticosteroid-binding globulin (CBG) activity of rat serum was determined by multiple equilibrium dialysis with 14C-labeled corticosteroids at 37 C. CBG and corticosterone levels of immature rats (3,12 or 21 days old) were found to be lower than those of mature animals (60 days); the values observed in the mature female rat were about 2.5 times higher than those in the male. CBG activity was similar in pregnant and nonpregnant animals, although in pregnancy resting corticosterone levels were 40% lower than in the nonpregnant state. On the 3rd day of lactation a precipitous decrease in CBG activity occurred, the resting corticosterone values increased to the virgin level, and a marked decrease in thymus weight indicated an increased concentration of free corticosterone. CBG levels during lactation were inversely related to the number of pups per litter. Four days after weaning, CBG activity returned to the virgin level, but thymus weight and resting corticosterone concentration did not reach virgin levels until much later. It is suggested that CBG plays an important role in the initiation of lactation in the rat. (Endocrinology76: 1079, 1965)