Alpha-lactalbumin production by rat mammary gland: prepartum emergence of resistance to glucocorticoids and progestins.

Explants of mammary glands from pregnant or lactating rats were shown to differ markedly in their response to glucocorticoid and progestin in terms of alpha-lactalbumin production. Tissue from day 14 pregnant rats had low initial tissue levels and basal production of alpha-lactalbumin, and showed a dose-biphasic response to glucocorticoids, with significantly higher production at 3 nM than 300 nM RU26988; the progestin ORG 2058 inhibited production. In explants from lactating rats (day 1 and day 5) tissue content and basal production of alpha-lactalbumin were markedly increased compared with day 14 pregnant, but were unaltered by glucocorticoid or progestin. Animals near parturition (day 19, day 20) showed a transitional pattern, with considerable variation between animals. Basal production and tissue content correlated closely; as levels rose, the tissues became progressively less responsive to the effects of both classes of steroid. We interpret these data as evidence for a switch, in terms of glucocorticoid control of alpha-lactalbumin gene expression pre-partum; the mechanism(s) underlying this switch remain to be explored.