Effect of prolactin on phosphate transport and incorporation in mouse mammary gland explants

Inorganic phosphate is present in milk at a concentration that is severalfold higher than in maternal plasma. In cultured mammary tissues from 12- to 14-day-pregnant mice, the intracellular concentration of (32)PO(4) was six times higher than in the culture medium after a 4-h treatment with (32)PO(4). Of the principal lactogenic hormones [insulin (I), cortisol (H), and prolactin (PRL)], only I and PRL (in the presence of H and I) stimulated (32)PO(4) uptake into cultured mammary tissues; H, by itself or in the presence of I or PRL, inhibited (32)PO(4) uptake. All three lactogenic hormones together effected the greatest stimulation of (32)PO(4) uptake. Similar hormone effects were observed with regard to (32)PO(4) incorporation into lipids and trichloroacetic acid-insoluble molecules. In a time course study, the onset of the PRL stimulation of (32)PO(4) uptake and incorporation occurred 8-12 h after PRL addition; in dose-response studies, the PRL effect was manifested with PRL concentrations of 50 ng/ml and above. From kinetic studies, the apparent maximal velocity of PO(4) uptake was determined to be approximately 7.7 mM x h(-1) x l cell water(-1); the apparent Michaelis-Menten constant was approximately 3-5 mM. The PRL effect on (32)PO(4) uptake was abolished when sodium was absent from the uptake medium. These studies thus demonstrate a complex interaction of three hormones (I, H, and PRL) in the regulation of (32)PO(4) uptake and incorporation into macromolecules in cultured mouse mammary tissues.