Progesterone control of the initiation of lactose synthesis in the rat

The in vitro incorporation of [14c]glucose into lactose in mammary tissue, the concentration of lactose in the mammary tissue and the concentration of lactose in the mammary secretion were determined during late pregnancy and lactation in the rat. These changes were related to the decline in blood progesterone during late pregnancy. The incorporation of [14C]glucose into lactose was detected on day 20 of pregnancy; it increased gradually until day 22 and then increased rapidly just prior to term (day 23 of pregnancy) to reach mean +/- s.e.m. maximum hourly values of 12.0 +/- 1.5 cpm x 10(4)/g by day 3 of lactation, and then declined to lower values (6.1 +/- 0.6 cpm x 10(4)/g) by day 20 of lactation. The concentration of lactose in both in the mammary tissue and in the mammary secretion increased rapidly over the last 24h of pregnancy and then more gradually after birth to reach mean +/- s.e.m. maximum values of 6.85 +/- 1.11 mg/g tissues and 43.1 +/-0 2.1 g/l respectively on day 15 of lactation. The decline in plasma progesterone to low levels between days 21 and 22 of pregnancy preceded the rapid increase in the concentration of lactose in the mammary tissue. A similar relationship was observed between the decline in progesterone and the increase in lactose in mammary tissue in rats Caesarean-sectioned on day 19 of pregnancy, and the administration of progesterone immediately following Caesarean section significantly depressed the accumulation of lactose in the mammary tissue. The results support the involvement of progesterone withdrawal in lactogenesis in the rat and indicate that [14C]glucose incorporation into lactose and the concentration of lactose both in the mammary tissue and in mammary secretion are useful indicators of the synthetic activity of the mammary gland.