FUNCTIONAL AVAILABILITY OF SODIUM-CHANNELS MODULATED BY CYTOSOLIC-FREE CA2+ IN CULTURED MAMMALIAN NEURONS (N1E-115)

Whole-cell sodium currents (I-Na) were measured in mouse neuroblastoma cells (N1E-115) at different [Ca2+](i) values using appropriate Ca-EGTA buffers in the pipettes. 2. I-Na was found to be larger at pCa 7 than at pCa 8 or 9 with a ratio of 1:0.65 or 0.55, respectively. The steady-state inactivation (h(infinity) curve) was independent of [Ca2+](i), thus excluding surface charge effects as a cause of the Ca2+ effect. 3. Recovery of I-Na from slow inactivation after changing from resting (-30 to -40 mV) to holding potential (-70 mV) occurred in a similar way at all pCa values. The Ca2+ effect appears to be independent of slow inactivation and to occur within the first 2 min of pipette buffer-cytoplasm equilibration. 4. The cell membrane capacitance (C-m) was independent of [Ca2+](i), thus excluding exo- or endocytosis of sodium channel-containing membrane as a cause of the Ca2+ effect. 5. Non-stationary fluctuation analysis was used to determine simultaneously the single channel current (i(Na)) and the size of I-Na. At pCa values of 7 and 9, i(Na) was identical, i.e. 0.59 and 0.58 pA, while I-Na/C-m differed, i.e. 41.1 and 22.2 pA pF(-1), respectively. The peak open probability at 0 mV was about 0.5 for both pCa values indicating that [Ca2+](i) controls the fraction of channels available for activation. 6. Since [Ca2+](i) in other neurons varies between 30 and 100 nM in the resting and active state, respectively, the present data suggest a modulatory role for [Ca2+](i) in neuronal excitability.