Extracellular Calcium Ion Concentration Required for Prolactin to Express Its Actions on Casein, Ribonucleic Acid, and Lipid Biosynthesis in Mouse Mammary Gland Explants*

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Abstract
The depletion of extracellular Ca ions by EGTA [ethyleneglycol bis(.beta.-aminoethyl ether)N,N,N'',N''-tetraacetic acid] stimulation of [14C]acetate incorporation into lipids in mouse mammary gland explants. This abolition is reversed by the addition of Ca, but not Mg. The chelation of Ca ions was similarly shown in earlier studies to abolish PRL actions on RNA and casein synthesis. When the actions of PRL on lipid, RNA and casein biosynthesis were tested when the extracellular Ca ion concentration was varied, it was observed that 5-10 .mu.M Ca was required for PRL to manifest its actions. The calcium ionophore A23187 [calcimycin] 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.