Essentiality of insulin and prolactin for accumulation of rat casein mRNAs.

Fetal calf serum (20%) does not substitute for insulin in terms of enhanced accumulation of rat casein mRNAs, but it can sustain the mammary tissue in culture. This property of fetal calf serum was utilized to show that in the presence of cortisol and 1) insulin or 2) prolactin, the delayed addition of prolactin or insulin, respectively, produced similar enhancement of 25 K and 42 K casein mRNAs. The results indicate that both insulin and prolactin are required for accumulation of rat casein mRNAs, the accumulation is independent of the sequence of addition of these hormones, and that the response cannot be ascribed entirely to the hormone added last.