Unitary conductance of Na⁺ channel isoforms in cardiac and NB2a neuroblastoma cells

Unitary conductances of native Na⁺ channel isoforms (γ(Na)) have been determined under a variety of conditions, making comparisons of γ(Na) difficult. To allow direct comparison, we measured γ(Na) in cell-attached patches on NB2a neuroblastoma cells and rabbit ventricular myocytes under identical conditions [pipette solution (in mM): 280 Na⁺ and 2 Ca²⁺, pH 7.4; 10°C]. γ(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. Negative to -20 mV, mean open time (MOT) of the NB2a isoform was significantly 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 0.47 ± 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 + 10.7 ± 6.1 mV in NB2a and -27.3 ± 7.1 and +23.7 ± 4.9 mV in myocytes. Transition rate constants were significantly different in NB2a and cardiac cells, but voltage dependence was not.