NA+ CHANNELS IN CARDIAC AND NEURONAL CELLS DERIVED FROM A MOUSE EMBRYONAL CARCINOMA CELL-LINE

1. Cells from a pluripotent murine embryonal carcinoma cell line (P19) were differentiated in vitro into cells with neurone- and cardiac-like phenotypes. Cells treated with 0.5 mum retinoic acid developed into neurone-like cells possessing extensive neurites. Dimethyl sulphoxide treatment (0.5%) produced large, spontaneously contracting cell aggregates with many properties of cardiac cells. 2. The neurone- and cardiac-like cells contained voltage-sensitive Na+ channels with properties similar to those of native neuronal and cardiac cells. 3. We used whole-cell patch clamp techniques to measure inward currents from the neurone- and cardiac-like cells. Undifferentiated (untreated) cells had only small inward currents (peak of -0.15 nA in 150 mm external Na+). The peak inward current in the neurone-like and cardiac-like cells was -1.2 nA (in 154 mm external Na+) and -2.8 nA (in only 46 mm Na+), respectively. These large currents were absent when the external solution contained no Na+. 4. Tetrodotoxin (TTX) blocked the Na+ currents in the neurone-and cardiac-like cells in a dose-dependent manner. The K(d) for TTX block of the Na+ current in the neurone-like cells was 6.7 nm. The Na+ current in the cardiac-like cells was much more resistant to TTX; the half-blocking concentration was two orders of magnitude higher, 710 nm. 5. The kinetic properties of the Na+ channel currents in the neurone- and cardiac-like cells were similar but developed over somewhat different voltage ranges. The voltage sensitivity of activation was similar in both cell t pes but the activation midpoint voltage was different: -12 mV in the neuronal cells and -34 mV for cardiac cells. Inactivation of the neuronal Na+ channels had a mid-point near -47 mV and was more sensitive to the membrane voltage than inactivation of the cardiac channels. The mid-point of inactivation for the cardiac Na+ channels was -80 mV.