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

Abstract

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 32PO4 was six times higher than in the culture medium after a 4-h treatment with32PO4. Of the principal lactogenic hormones [insulin (I), cortisol (H), and prolactin (PRL)], only I and PRL (in the presence of H and I) stimulated 32PO4uptake into cultured mammary tissues; H, by itself or in the presence of I or PRL, inhibited 32PO4 uptake. All three lactogenic hormones together effected the greatest stimulation of32PO4 uptake. Similar hormone effects were observed with regard to 32PO4 incorporation into lipids and trichloroacetic acid-insoluble molecules. In a time course study, the onset of the PRL stimulation of32PO4 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 PO4uptake was determined to be ∼7.7 mM · h−1 · l cell water−1; the apparent Michaelis-Menten constant was ∼3–5 mM. The PRL effect on 32PO4 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 32PO4 uptake and incorporation into macromolecules in cultured mouse mammary tissues.