Prolactin effects on ion transport across cultured mouse mammary epithelium

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Abstract
Prolactin is a known osmoregulatory hormone in lower vertebrates, and recent evidence indicates that this hormone modulates ionic concentrations in milk. In an ultrastructurally and biochemically differentiated primary cell culture system in which mouse mammary epithelium is maintained on floating collagen gels, prolactin causes an increase in short-circuit current (Isc) of monolayers of cells derived from midpregnant (24.6 to 48.0 .mu.A .cntdot. cm-2) and lactating (10.4 to 16.1 .mu.A .cntdot. cm-2) glands. Transepithelial potential differences (basal side ground) average about -12 mV and are similar to those seen in vivo. Prelactating mammary epithelial cell cultures have transepithelial resistances ranging from 374 .OMEGA. .cntdot. cm2 (prolactin present) to 507 .OMEGA. .cntdot. cm2 (prolactin absent), and lactating cell cultures have resistances averaging almost 1,000 .OMEGA. .cntdot. cm2. Prolactin effects require at most 1 day of culture maintenance in prolactin-containing medium, and the effects are not due to known contamination of prolactin preparations with arginine vasopressin or growth hormone. Medium concentrations of prolactin as low as 1 ng/ml can elicit these effects. In prelactating cell cultures not treated with prolactin, the Isc is equal to the rate of Na absorption. Prolactin increases Na absorption 4-fold but increases Isc only 2-fold. Prolactin induces other active transport; neither K nor Cl movements can account for this additional transport. Resistance values, current-voltage plots, and permeability coefficients indicate that in vitro mammary epithelium is a moderately tight tissue. Comparisons with intact glands indicate that in vitro mammary epithelium closely resembles its in vivo counterpart. Floating collagen gel cultures appear suitable for elucidating transport properties in cellularly heterogeneous and structurally complex mammalian tissues.