Corticosterone concentrations in the serum and milk of lactating rats: parallel changes after induced stress.

Transfer of corticosterone from serum to milk was studied in lactating rats after ether and pentobarbital anesthesia. Two minutes after the onset of anesthesia stress, total serum and milk corticosterone concentrations were not significantly different. By 30 min, corticosterone concentrations in serum increased to a peak and plateaued at about this level during the 2-h experimental period, while concentrations in milk reached a maximal plateau only at 50 min poststress. The peak corticosterone concentrations in serum and milk were 76 +/- 6 and 36 +/- 4 micrograms/dl, respectively. By centrifugal ultrafiltration dialysis, the percentages of free corticosterone in serum and milk were not significantly different at the maximal concentrations; thus, free corticosterone in serum was about 200% of that in milk. In contrast, anesthesia stress had no effect on serum and milk corticosteroid-binding globulin concentrations; the serum to milk ratio was approximately 10:1. No accumulation of corticosterone in milk was observed after repeated ether stress. Decreases in milk corticosterone levels after recovery from ether stress paralleled that of its serum counterpart. These results indicate that corticosterone concentrations in milk increase rapidly after ether stress, but are limited to a level significantly lower than that in serum. The possible effects of corticosterone in milk on the development of infant rats remains to be defined.