Interactions of insulin, corticosterone and prolactin in promoting milk-fat synthesis by mammary explants from pregnant rabbits

Abstract

1. The rate of fatty acid synthesis by mammary explants from rabbits pregnant for 16 days or from rabbits pseudopregnant for 11 days was stimulated up to 15-fold by culturing for 2-4 days with prolactin. This treatment initiated the predominant synthesis of C(8:0) and C(10:0) fatty acids, which are characteristic of rabbit milk. 2. Inclusion of insulin in the culture medium increased the rate of synthesis of these medium-chain fatty acids. By contrast the inclusion of corticosterone led to the predominant synthesis of long-chain fatty acids. When explants were cultured for 2-4 days with insulin, corticosterone and prolactin, the rate of fatty acid synthesis increased up to 42-fold, but both medium- and long-chain fatty acids were synthesized. 3. These results show that the stimulus to mammary-gland lipogenesis and the initiation of synthesis of medium-chain fatty acids observed between days 16 and 23 of pregnancy in the rabbit can be simulated in vitro by prolactin alone. 4. When mammary explants from rabbits pregnant for 23 days were cultured for 2 days with insulin, corticosterone and prolactin, the rate of fatty acid synthesis increased fivefold, but there was a preferential synthesis of long-chain fatty acids. Culture with prolactin alone had little effect on the rate or pattern of fatty acids synthesized. 5. The results are compared with findings in vivo on the control of lipogenesis in the rabbit mammary gland, and are contrasted with the known effects of hormones in vitro on the mammary gland of the mid-pregnant mouse.

Metabolic adaptations during lactogenesis. Lactose synthesis in rabbit mammary tissue during pregnancy and lactation

“…There do not appear to be any reports of changes in the plasma concentration of prolactin in the rabbit during lactogenesis, but it seems likely that prolactin is involved as well as cortisol. Experiments both in vivo and in vitro indicate that prolactin is capable of inducing the synthesis of lactose and fatty acids in pseudopregnant rabbits (Palmiter, 1969a;DeLouis & Denamur, 1972;Forsyth et al, 1972).…”

Lactogenic Activity in the Serum of Rabbits During Pregnancy and Early Lactation

“…Numerous aspects of the mammary physiology of the rabbit are well established; these include changes in the nucleic acid content of the mammary gland during pregnancy and lactation (Denamur, 1963), hormonal requirements for mammary growth and secretion in vivo (Cowie, Hartmann & Turvey, 1969;Denamur, 1970) and in vitro (Denamur, 1971), control of lactose synthesis Mellenberger & Bauman, 1974 ), fatty acid synthesis (Forsyth, Strong & Dils, 1972;Mellenberger & Bauman, 1974e), and regulation of protein biosynthesis (Houdebine & Gaye, 1975). Djiane, Durand & Kelly (1977) have recently shown that the number of prolactin receptors increases strikingly when lactation is initiated.…”

The insulin receptor plays an important role in secretory differentiation in the mammary gland

“…The same concern extends to the use of transgenic and knockout mice that have been used to great effect to study steroid hormones. Although genetically modified mice can be used for studying the insulin system, as we will see the experimental design is crucial if complicating systemic effects are to be avoided.Much early data demonstrated a role for insulin in maintenance of milk secretion in the lactating animal (1,3,15,18,19,22,25,33,35). The earliest studies showed that insulin stimulated lipid synthesis in mammary slices from lactating ruminants (13).…”