The transport of K+ (Rb+) by cell fragments isolated from goats' milk, which are believed to be pieces of mammary epithelial cell cytoplasm encapsulated by apical membrane, has been studied. K+ (Rb+) accumulation was time dependent and was virtually abolished by 0.1% Triton X-100 which indicates that K+ (Rb+) uptake is into a membrane-limited space rather than by binding. Replacing medium Cl- with SCN- increased whereas gluconate replacement of Cl- decreased K+ (Rb+) influx suggesting that uptake is sensitive to changes in membrane potential. Na+, K+, Li+ and Rb+ ions added to the incubation medium reduced the rate of K+ (Rb+) accumulation. K+ (Rb+) uptake was markedly reduced by both barium ions and quinine; the inhibitory effects were not additive suggesting they are acting at a single locus. Increasing the pH of the medium stimulated the Ba2(+)-sensitive component of K+ (Rb+) influx. The results show that the transport properties of the goats' milk membrane fraction used here are similar to the apical aspect of the intact mammary epithelium. Therefore this membrane preparation may be a good source and model to study mammary secretory cell apical membrane ion transport.

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