UNITARY CONDUCTANCE OF NA-CELLS( CHANNEL ISOFORMS IN CARDIAC AND NB2ANEUROBLASTOMA)

Unitary conductances of native Na+ channel isoforms (YNa) have been de termined under a variety of conditions, making comparisons of gamma(Na ) difficult. To allow direct comparison, we measured gamma(Na) in cell -attached patches on NB2a neuroblastoma cells and rabbit ventricular m yocytes under identical conditions [pipette solution (in mM): 280 Naand 2 Ca2+, pH 7.4; 10 degrees C]. gamma(Na) of NB2a channels, 22.9 +/ - 0.9 pS, was 21% greater than that of cardiac channels, 18.9 +/- 0.9 pS. In contrast, respective extrapolated reversal potentials, +62.4 +/ - 4.6 and +57.9 +/- 5.1 mV, were not significantly different. Several kinetic differences between the channel types were also noted. Negativ e to -20 mV, mean open time (MOT) of the NB2a isoform was significantl y less than that of cardiac channels, and, near threshold, latency to channel opening decayed more rapidly in NB2a. On the basis of analysis of MOT between -60 and 0 mV, the rate constants at 0 mV for the open- to-closed (O --> C) and open-to-inactivated (O --> I) transitions were 0.42 +/- 0.11 and 0.41 +/- 0.11 ms(-1) in NB2a and 0.10 +/- 0.06 and 1.19 +/- 0.07 ms(-1) in myocytes. The slope factors were -38.9 +/- 8.7 and +10.1 +/- 6.1 mV in NB2a and -21.3 +/- 7.1 and +23.7 +/- 4.9 mV i n myocytes. Transition rate constants were significantly different in NB2a and cardiac cells, but voltage dependence was not.