Extracellular calcium ion concentration required for prolactin to express its actions on casein, ribonucleic acid, and lipid biosynthesis in mouse mammary gland explants.

The depletion of extracellular calcium ions by EGTA abolishes the PRL stimulation of [14C]acetate incorporation into lipids in mouse mammary gland explants. This abolition is reversed by the addition of calcium, but not magnesium. The chelation of calcium ions was similarly shown in earlier studies to abolish PRL's actions on RNA and casein synthesis. When the actions of PRL on lipid, RNA, and casein biosynthesis were tested when the extracellular calcium ion concentration was varied, it was observed that 5-10 microM calcium was required for PRL to manifest its actions. The calcium ionophore A23187 was unable, however, to mimic the PRL stimulation of lipid, RNA, or casein synthesis. Trifluoperazine, an inhibitor of calmodulin action had no effect on the PRL-induced synthesis of lipids, RNA, or casein, except at pharmacological levels.