Functional availability of sodium channels modulated by cytosolic free Ca2+ in cultured mammalian neurons (N1E‐115).

1. Whole‐cell sodium currents (INa) were measured in mouse neuroblastoma cells (N1E‐115) at different [Ca2+]i values using appropriate Ca‐EGTA buffers in the pipettes. 2. INa 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 INa 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 (Cm) 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 (iNa) and the size of INa. At pCa values of 7 and 9, iNa was identical, i.e. 0.59 and 0.58 pA, while INa/Cm 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.