Effect of prolactin on casein and fat synthesis and casein secretion in explants of bovine mammary tissue pretreated in athymic nude mice.

Bovine mammary gland tissue from 4- to 6-mo-old calves was grafted s.c. (dorsum) to 7- to 9-wk-old female athymic "nude" mice. The graft-bearing mice were divided into three groups and were treated with the following hormonal combinations: 1) implantation of a pellet containing estrogen and progesterone followed by injection of estrogen and progesterone, 2) injections of estrogen and progesterone, and 3) injections of estrogen, progesterone, and prolactin. Following in vivo treatment, the grafts were removed and prepared for organ culture. Explants from each treatment were subdivided into two hormonal regimens: 1) insulin and cortisol; 2) insulin, cortisol, and prolactin. An increase in in vitro fat synthesis was observed in explants cultured in the presence of prolactin. This increase was not affected by previous in vivo hormonal treatment. In vitro casein synthesis was not stimulated by the presence of prolactin in the culture medium; however, casein secretion was elevated in explants from grafts from animals that were treated in vivo with estrogen, progesterone, and prolactin. Casein secretion was further increased in explants cultured in the presence of prolactin but rapidly dropped in its absence. Explants that were not implanted in mice responded in long-term organ culture similarly to explants pretreated in vivo.