Membrane potential difference and intracellular cation concentrations in human placental trophoblast cells in culture

1. The electrochemical gradients for Na+ and K+ were assessed in a cell culture model of trophoblast differentiation. 2. Membrane potential difference (E(m)), intracellular water and Na+ and K+ contents were measured in choriocarcinoma cells (JAr cell line; 96% of which are undifferentiated trophoblast cells) and in mononucleate and multinucleate (differentiated) cytotrophoblast cells isolated from the human placenta at term. 3. There was a significant fall in E(m) from -57 mV in JAr cells, to -48 and -40 mV in mono- and multinucleate cytotrophoblast cells, respectively Treatment with ouabain (1 mM for 15 min) depolarized the JAr cell membrane by 15 mV but did not affect cytotrophoblast cell membrane potential. 4. Intracellular K+ concentration was similar in JAr, mono- and multinucleate cytotrophoblast cells but Na+ concentration was higher in mononucleate cytotrophoblast cells compared with JAr cells. 5. Ouabain treatment (3 mM for 15 min) caused a small increase (4.5%) in cell water in mononucleate cytotrophoblast cells but lowered K+ (similar to 30%) and increased Na+ concentration (similar to 125 %) in all the trophoblast cells studied. 6. The K+ equilibrium potential (E(K)) was more negative than E(m) in all cells and the difference between E(K) and E(m) was smaller in JAr cells (-25 mV) than in mono- and multinucleate cytotrophoblast cells (-33 and -43 mV, respectively). 7. The Na+ equilibrium potential (E(Na)) was positive in the trophoblast cells and the difference between E(Na), and E(m) was 122, 100 and 100 mV in JAr, mono- and multinucleate cytotrophoblast cells, respectively. 8. These results suggest that the electrochemical gradient for K+ is affected by the stage of trophoblast cell differentiation. In contrast, the electrochemical gradient for Naf is similar in mono- and multinucleate cytotrophoblast cells.