Hormone regulation of choline uptake and incorporation in mouse mammary gland explants

Choline is a nutrient in milk that is essential for the nourishment and growth of newborns. In rat milk, choline is present in concentrations that are more than an order of magnitude higher than in maternal serum. Using cultured mammary tissues taken from 12-14-day pregnant mice, the effects of the three primary lactogenic hormones--prolactin (PRL), insulin (I), and cortisol (H)--on choline uptake and incorporation into lipids were determined. By itself or in the presence of H and/or PRL, I was the only hormone that increased the accumulation of choline in aqueous tissue fractions. In contrast, PRL, when present with I plus H, was the only hormone that stimulated the incorporation of choline into the lipid fraction of tissues. Choline uptake was found to be sodium and time dependent; maximum distribution ratios >18 were achieved after a 6-hr uptake time. In kinetic studies the apparent Km for choline uptake was calculated to be approximately 2.7 mM, whereas the Vmax was 7.4 mM intracellular water per 30 mins. These results suggest the existence of a sodium-dependent active transporter for choline in the mouse mammary gland that is specifically stimulated by I. PRL, in contrast, only stimulates the incorporation of choline into lipids.