Effect of Prolactin on Sodium and Potassium Concentrations in Mammary Alveolar Tissue

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Abstract
A series of investigations is described in which changes in mammary tissue content of monovalent ions are measured following treatment with prolactin. These effects of prolactin are compared with the effects of ouabain and prolactin together. In vitro experiments were carried out using slices of lactating mammary gland from rabbits, held in continuously gassed (95% O2:5% CO2) Krebs bicarbonate buffer for 1 h in the presence or absence of prolactin (43.5 nM [1 .mu.g/ml], ouabain (0.1 mM) or both. In the presence of prolactin [Na+] decreased and [K+] increased in the whole slices, and also when calculated as intracellular ion concentrations. Ouabain had the reverse effect; ouabain and prolactin together also had the reverse effect to prolactin alone. The tissue slices were sensitive to prolactin concentrations as low as 2.2 nM (50 ng/ml) in the incubation medium. The decreased [Na+] in the tissue slices showed a significant dose/response relationship up to a prolactin concentration of 6.5 nM, above which no further decreases were observed. In vivo studies utilized rabbits in mid-pseudopregnancy, which were injected intraductally with prolactin (0.435 nmol [10 .mu.g]/duct), ouabain (20 nmol/duct) or both. At 7.5 h later [14C-]-lactose was administered i.v. to provide measurements of extracellular volume, and at 8 h the rabbits were killed. Similarly to the in vitro experiment, prolactin caused a decrease in whole tissue and calculated intracellular [Na+] and an increase in [K+] compared to control glands. These effects were reversed by treatment with ouabain plus prolactin. Prolactin activates the extrusion of Na+ from, and the entry of K+ into, mammary cells both in lactating and pre-lactating tissue. This effect can be reversed by ouabain.