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.
Bulatko
A K
AK
Physiologisches Institut, Universität Zürich-Irchel, Switzerland.
Greeff
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NG
D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002478:Cells, Cultured; D008564:Membrane Potentials / Q000187:drug effects; D051379:Mice; D009447:Neuroblastoma; D009474:Neurons / Q000187:drug effects*; D015222:Sodium Channels / Q000187:drug effects / Q000502:physiology*; D013997:Time Factors
D015222:Sodium Channels; D002118:Calcium