Hormonal control of casein synthesis in organ culture of the bovine lactating mammary gland

Expiants from lactating bovine mammary gland were cultured in vitro in serum-free medium through 1-9 d. Casein synthesiswas determined by [32P] incorporation into newly synthesized Ca rennin precipitable fraction. High correlation (r = 0-98) was found between incorporation of [32P] and [3H]amino acids in expiants cultured under different hormonal regimes, thus indicating that post-translational phosphorylation is not a rate-limiting step in casein synthesis.Hormonal effects on casein synthesis were studied by supplementing the incubating medium with insulin (I), prolactin (PRL), Cortisol (F), thyroxine (T4) and triiodothyronine (T3). It was found that both PRL and I were required absolutely for maximal synthesis and almost maximal effect was achieved with 50 ng/ml. The effect of F was less clear, but some increase was achieved at the 200-1000 ng/ml range. T4 and T3 did not affect casein synthesis at a range of 10-u-10-7 m while a significant inhibition was observed at 2 x 10-5 m. A time-course study of casein synthesis further substantiated the dominant role of PRL in maintenance or even elevation of the initial rate of casein synthesis in the expiants, through the first 4 d of incubation.