HORMONAL REGULATION OF RNA AND PROTEIN SYNTHESIS IN THE MOUSE MAMMARY GLAND BEFORE AND DURING LACTATION

SUMMARY As measured by [3H]uridine incorporation in vivo, the low rate of RNA synthesis in the mammary gland of virgin C3H and BALB/c mice increased sixfold in the mammary tissue of 15-day pregnant mice. In the 5-day lactating gland, RNA synthesis was ten times higher than that in virgin mammary tissue. On the 10th day of lactation this increased RNA synthetic activity in the mammary gland was considerably reduced but was still twice that of the mammary tissue of virgin mice. Twenty-four hours after adrenalectomy, RNA synthesis in lactating glands was reduced by over 80%, whereas in the mammary gland before lactation, it was reduced by 20–30% only. A single i.p. injection of 250 μg of cortisol led to a threefold increase of RNA synthesis within 1 to 2 h in lactating glands of adrenalectomized mice; this was followed by a decline. Incorporation of [3H]leucine into trichloroacetic acid-insoluble material from lactating mammary tissue was used as a measure of'total protein' synthesis, and [3H]leucine radioactivity determined in Ca2+−rennin precipitate of 105000 g supernatant of lactating mammary tissue homogenate was used as a measure of casein synthesis. Adrenalectomy caused a 50% reduction of 'total protein' synthesis, whereas synthesis of 'casein-like' phosphoprotein virtually stopped after the operation. The injection of cortisol into adrenalectomized mice induced a selective increase of [3H]leucine incorporation into the casein of lactating glands. The results indicate that RNA synthesis in the mammary tissue is more dependent on adrenal hormones during the functional than the structural state of differentiation. The hormonal regulation of RNA synthesis and its role in milk protein synthesis in the mammary gland in vivo is discussed.