Reverse NCX Attenuates Cellular Sodium Loading in Metabolically Compromised Cortex

Abstract

In core regions of ischemic stroke, disruption of blood flow causes breakdown of ionic gradients and, ultimately, calcium overload and cell death. In the surrounding penumbra, cells may recover upon reperfusion, but recovery is hampered by additional metabolic demands imposed by peri-infarct depolarizations (PIDs). There is evidence that sodium influx drives PIDs, but no data exist on PID-related sodium accumulations in vivo. Here, we found that PIDs in mouse neocortex are associated with propagating sodium elevations in neurons and astrocytes. Similar transient sodium elevations were induced in acute tissue slices by brief chemical ischemia. Blocking NMDA-receptors dampened sodium and accompanying calcium loads of neurons in tissue slices, while inhibiting glutamate transport diminished sodium influx into astrocytes, but amplified neuronal sodium loads. In both cell types, inhibition of sodium/calcium exchange (NCX) increased sodium transients. Blocking NCX also significantly reduced calcium transients, a result confirmed in vivo. Our study provides the first quantitative data on sodium elevations in peri-infarct regions in vivo. They suggest that sodium influx drives reversal of NCX, triggering a massive secondary calcium elevation while promoting export of sodium. Reported neuroprotective effects of NCX activity in stroke models might thus be related to its dampening of ischemia-induced sodium loading.

Action Potential Firing Induces Sodium Transients in Macroglial Cells of the Mouse Corpus Callosum

“…In addition, we performed SDS-PAGE and Western blots of tissue dissected from corpus callosum of animals at different postnatal stages (P5, 10,15,25). For both GLT-1 and GLAST several distinct bands (60,130, 180 kd and 55, 130, 150, 190 kd, respectively) were detected in all age groups most likely due to monomeric and multimeric fractions of the transporters ( Figure 7A), confirming earlier studies indicating that both exist as homomultimers [45,46].…”

“…Moreover, sodium influx into astrocytes changes the driving force for sodium-dependent secondary active transporters. For example, this may result in the reversal of the plasma membrane Na + /Ca 2+ -exchanger (NCX), which then switches from a calcium exporter to an importer for calcium, contributing to intracellular calcium increases under both physiological and pathophysiological conditions [12][13][14][15].…”

Relation between activity‐induced intracellular sodium transients and ATP dynamics in mouse hippocampal neurons

“…; Gerkau et al . ). Briefly, SBFI‐AM‐loaded organotypic tissue slices were first superfused with nominally Na + ‐free calibration‐ACSF, containing ouabain (100 µ m ), an inhibitor of the Na + /K + ‐ATPase, as well as the ionophores gramicidin (3 µ m ) and monensin (10 µ m ) to equilibrate intra‐ and extracellular Na + .…”

“…; Gerkau et al . ), increasing [Na + ] caused a decrease in fluorescence emission of SBFI and traces were inverted for illustration purposes after normalization to fluorescence levels at 0 m m (Fig. A ).…”

“…Afterwards, slices were exposed to calibration-ACSFs containing different [Na + ] and resulting changes in SBFI fluorescence in neuronal somata were recorded using multiphoton excitation. As reported before (Rose et al 1999;Meier et al 2006;Gerkau et al 2018), increasing [Na + ] caused a decrease in fluorescence emission of SBFI and traces were inverted for illustration purposes after normalization to fluorescence levels at 0 mM ( Fig. 1A).…”