Effects of Lithium and Selective Inhibitors of Sodium-Calcium Exchanger on Its Transport Currents in Neurons and HEK293 Cells

The sodium-calcium exchanger (NCX) is essential in maintaining the intracellular calcium balance in neurons and is a regulator of glutamatergic synaptic transmission. NCX dysfunction is implicated in various neurodegenerative and mental illnesses, while pharmacological agents modulating NCX (specific antagonists, allosteric modulators, lithium ions, etc.) are considered potential drugs. However, the effects of the exchanger selective blockers or lithium ions on the electrogenic transport of NCX in neurons have not been previously studied. In this work, using a patch-clamp recording of transmembrane currents in neocortical neurons in primary culture and HEK293 cells, we investigated the effects of the selective NCX antagonists KB-R7943 and SN6 on the current-voltage characterisrics (I-V) of the cell membranes. Also, we examined the influence of lithium ions, a non-transportable NCX substrate, on the NCX transport currents. We show that in cortical neurons, transport current generated by NCX contributes to neuronal I-V, which can be detected when NCX is inhibited by KB-R7943 or SN6. When using a Ca2+-free pipette solution ([Ca2+](i) < 100 nM), the exchanger transport current can be recorded only in its reverse mode of operation, when neurons are depolarized, but not in the forward mode of operation at resting potential. Pipette solution with high content of free Ca2+ ([Ca2+](i) > 1000 nM) creates the conditions that allow forward operation of the NCX at negative membrane potentials mode of NCX at negative membrane potentials. However, in neurons, unlike HEK293, replacing more than 50% Na+ by Li+ in the extracellular solution inhibits NCX and other sodium-dependent electrogenic transport mechanisms, which is accompanied by a drop in the input resistance and considerably complicates the detection of transport currents generated by NCX. Thus, we demonstrate the possibility of a direct study of NCX transport features and pharmacological characteristics in neurons and HEK293 cells.