Single Na channels in mouse neuroblastoma cell membrane. Indications for two open states

Single sodium channel currents were studied in mouse neuroblastoma cells. Channel conductance (gamma) was 11.6 +/- 3.4 pS at 6-8 degrees C. Unitary current amplitudes and channel activity increased with increasing temperature. The Arrhenius plot of the conductance was linear between 5 and 35 degrees C with an activation enthalpy of 27.1 kJ/mole (Q10 = 1.28). Amplitude distributions were fitted by the sum of two Gaussian functions indicating the presence of two different single channel amplitudes: smaller i1 and larger i2. The relative probability of appearance of i1, which has a shorter mean open time, was higher during the early phase of depolarization (t less than 16 ms). The open time histograms, inactivating phase of macroscopic currents and delay time histograms were fitted by the sum of two exponentials. The distinct kinetic and steady-state parameters reflect two open states of sodium channels.