A Role for Medium-Chain Fatty Acids in the Regulation of Lipid Synthesis in Milk Stasis?

Abstract

It is well-established from farming practice that frequent milking increases milk yield in cow and goat. As milk is stored within the lumen of the mammary gland after secretion from the epithelial cells, it implies that once the lumen is full there is a feed-back mechanism to control further milk synthesis and secretion. The finding that the secretory rate increased when milk stored in autotransplanted glands of lactating goats was diluted with an inert solution provided strong evidence for the existence of an inhibitor(s) in milk (Henderson and Peaker, 1984; 1987). A milk fraction has been identified that rapidly, but reversibly, inhibits lactose and casein synthesis in a concentration dependent way. The inhibitor is a heat-labile constituent of whey proteins and has been shown to be effective in goat and rabbit in vivo (Wilde et al., 1987; reviewed in Wilde and Peaker, 1990). Interestingly, the active material did not inhibit fatty acid synthesis (lipogenesis) from acetate (Wilde et al., 1987). This implies that inhibition of protein and lactose synthesis results in a linked inhibition of lipid synthesis. The evidence is somewhat ambiguous on this point in that inhibition of rat mammary gland protein synthesis in vivo with cycloheximide did not decrease the rate of lipogenesis in the gland for at least 2h after administration, but then it declined markedly (Roberts et al., 1982). Alternatively, it is possible that milk contains a specific inhibitor(s) of milk lipid synthesis that acts independently of the whey fraction protein inhibitor. Some 30 years ago a fraction from rat milk was described which inhibited fatty acid synthesis in vitro, and in particular, the activity of acetyl-CoA carboxylase (Levy, 1964), a key enzyme in the lipogenic pathway. Later work by Levy (Miller et al., 1970) identified the inhibitory components of this fraction as free fatty acids. The aim of this contribution is to assess whether medium-chain fatty acids (MCFA), present in milk at low concentration (Miller et al., 1970), play a regulatory role in mammary gland lipid metabolism of non-ruminants and in particular the rat.