Intracellular concentrations of sodium, potassium and chloride in the lactating mammary gland and their relation to the secretory mechanism

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Abstract
1. The intracellular ( I.C.) concentrations of Na, K and Cl in mammary cells from lactating guinea-pigs have been calculated from the analysis of fresh tissue and the measurement of the extracellular ( E.C.) space with [14C]sucrose and the milk content with [14C]lactose. 2. Assuming that alveolar milk has the same concentration as teat milk, the intracellular concentrations were calculated to be K 115, Na 42 and Cl 66 m-equiv. l−1 intracellular water. 3. Intracellular concentrations were also calculated in slices incubated in Krebs—bicarbonate medium plus glucose. There was a large increase in the sucrose ( E.C.) space and a rise in total tissue [Na] and [Cl]. On the assumption that the medium had equilibrated with the milk space as well as the E.C. space, the calculated I.C. concentrations of Na (43 m-equiv. l−1), and Cl (62) were very similar while [K] was somewhat higher (143 m-equiv. l−1 I.C. water). 4. The calculated I.C. concentrations of all three ions are all higher than in milk but the ratios between them are almost identical. 5. Similar figures for the I.C. concentrations of Na, K and Cl have been obtained in the goat, cow and sheep mammary tissue incubated in vitro. 6. Moderate changes in the concentrations of Na, K and Cl in the external medium had no effect on cell composition but during incubation without ions [14C]sucrose became distributed throughout the total tissue water indicating that sucrose had entered the I.C. compartment. 7. Acetazolamide (10−2 M), aldosterone (1·4 × 10−6 M) and, in some experiments, lack of glucose lowered I.C. [Cl−], but oxytocin, vasopressin and low doses of insulin had no effect. 8. The data are difficult to reconcile with the hypothesis of Zaks, Natochin, Sokolova, Tanasiichuk & Tverskoi (1965) that freshly secreted milk has the ionic composition of plasma. 9. Comparison of I.C. ion concentrations and the membrane potential between the cells and milk suggests that Na+ and K+ are passively distributed across the apical membrane but that Cl− must be actively held in the cells. Across the basal membrane the data are consistent with the presence of a Na+ pump and with Kinura's (1969) detection of a Na:K ATPase on the basal and lateral membranes. In addition another inward-facing Cl− pump may exist at this site.