Stable potentials were recorded with microelectrodes in an in vivo preparation of the mammary gland from the anesthetized lactating mouse. Location of the microelectrode tip was determined by ionophoretic injection of the fluorescent dye Lucifer yellow CH. Fifteen dye injections were localized to mammary alveolar cells; the average recorded potential for these penetrations was -49 +/- 2 mV. Cell-to-cell dye transfer between alveolar cells was observed with all intracellular Lucifer yellow injections. Ten dye injections were localized to the alveolar lumina with an average recorded potential of -35 +/- 2 mV. With these penetrations Lucifer yellow spread rapidly to many alveolar lumina. These findings indicate that stable potentials can be obtained from both cells and lumina in the in vivo mammary gland, demonstrating the feasibility of electrophysiological studies of the mammary epithelium. The presence of a large transepithelial potential provides evidence for physiologically tight junctions between mammary alveolar cells. In addition, the distribution of Lucifer yellow shows that mammary alveolar cells are coupled and suggests that milk flows freely between alveolar lumina.
SUMMARY1. The ratio of intracellular potasium to sodium in the mammary cells of the lactating mouse was compared with the ratio of potassium to sodium in the mouse milk to determine whether the sodium and potassium concentrations in milk are governed by a Donnan equilibrium as postulated earlier (Peaker 1977(Peaker a, 1978.2. An efflux technique was.used to determine the average intracellular sodium of 23-0 + 12 ,umol/g tissue (± S.E. of mean). The intracellular potassium, determined by calculating the amount ofpotassium contained in both the interstitial and milk spaces and subtracting these values from the total tissue potassium, was 62+1 Jumol/g tissue. The mean intracellular potassium to sodium ratio, calculated from individual efflux experiments, was 2-7 + 0-2.3. The total, interstitial, and milk water spaces were measured by tissue drying, sodium efflux, and lactose efflux, respectively. The average values ( ±S.E. of mean) obtained were 0 700+0 004 ml/g tissue, 0-150 +0016 ml/g tissue and 0 064 + 0-004 ml/g tissue. Based on these values the intracellular water space was 0-49 + 002 ml/g tissue.4. Intracellular concentrations of sodium and potassium calculated from the intracellular amounts of sodium and potassium and intracellular water space were 47 + 3 mm and 129 + 5 mm, respectively.5. The concentrations of potassium and sodium in mouse milk were 47 +1 mm and 26+1 mm. The mean potassium to sodium ratio ( +S.E. of mean) calculated from individual milk samples was 1-8+0-1.6. The milk ratio of potassium to sodium is significantly different (P < 0-001) from the intracellular water ratio of potassium to sodium. This finding, in a tight epithelium such as the lactating mouse mammary gland, suggests that both sodium and potassium cannot be distributed passively across the apical membrane and an active transport process must exist for one or both of these ions in this membrane.
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