Effects of prolactin withdrawal on activity of pyruvate dehydrogenase of rat mammary gland.

After the placenta is lost at parturition the concentration of placental lactogen in the blood declines rapidly (Grumbach et al., 1968), whereas that of prolactin shows a compensatory increase. I assume that this switch in lactogenic hormone results in a fall in ‘anti-insulin’ activity, a situation which would occur if either the serum concentration of prolactin were lower than that of placental lactogen (Hwang et al., 1971) or if prolactin were inherently less like growth hormone (Niall et al., 1971). Hence the restraint on carbohydrate metabolism is removed and milk synthesis proceeds. The idea of negative rather than positive control of the initiation of milk synthesis has previously been considered for progesterone, whose concentration also falls at parturition (Kuhn, 1971). Although progesterone probably regulates the switch from placental to pituitary lactogen, the possibility that it has a direct inhibitory effect on the mammary gland seems less likely. Thus although progesterone does inhibit the induction of alactalbumin in vitro, its effect appears to be specific to this protein, other proteins being unaffected (Turkington &Hill, 1969; Leader &Barry, 1969) In contrast, the hypothesis I propose suggests a way in which a single hormone, placental lactogen, through its separate biochemical effects on carbohydrate metabolism and specific protein synthesis might exert a general restraint on the synthesis ofmilk constituents, while simultaneously promoting growth and differentiation of the milk-producing tissue.