A crosstalk between Na⁺ channels, Na⁺/K⁺ pump and mitochondrial Na⁺ transporters controls glucose-dependent cytosolic and mitochondrial Na⁺ signals

Glucose-dependent cytosolic Na⁺ influx in pancreatic islet β cells is mediated by TTX-sensitive Na⁺ channels and is propagated into the mitochondria through the mitochondrial Na⁺/Ca²⁺ exchanger, NCLX. Mitochondrial Na⁺ transients are also controlled by the mitochondrial Na⁺/H⁺ exchanger, NHE, while cytosolic Na⁺ changes are governed by Na⁺/K⁺ ATPase pump. The functional interaction between the Na⁺ channels, Na⁺/K⁺ ATPase pump and mitochondrial Na⁺ transporters, NCLX and NHE, in mediating Na⁺ signaling is poorly understood. Here, we combine fluorescent Na⁺ imaging, pharmacological inhibition by TTX, ouabain and EIPA, with molecular control of NCLX expression, so as to investigate the crosstalk between Na⁺ transporters on both the plasma membrane and the mitochondria. According to our results, glucose-dependent cytosolic Na⁺ response was enhanced by ouabain and was followed by a rise in mitochondrial Na⁺ signal. Silencing of NCLX expression using siNCLX, did not affect the glucose- or ouabain-dependent cytosolic rise in Na⁺. In contrast, the ouabain-dependent rise in mitochondrial Na⁺ was strongly suppressed by siNCLX. Furthermore, mitochondrial Na⁺ influx rates were accelerated in cells treated with the Na⁺/H⁺ exchanger inhibitor, EIPA or by combination of EIPA and ouabain. Similarly, TTX blocked the cytosolic and mitochondrial Na⁺ responses, which were enhanced by ouabain or EIPA, respectively. Our results suggest that Na⁺/K⁺ ATPase pump controls cytosolic glucose-dependent Na⁺ rise, in a manner that is mediated by TTX-sensitive Na⁺ channels and subsequent mitochondrial Na⁺ uptake via NCLX. Furthermore, these results indicate that mitochondrial Na⁺ influx via NCLX is antagonized by Na⁺ efflux, which is mediated by the mitochondrial NHE; thus, the duration of mitochondrial Na⁺ transients is set by the interplay between these pivotal transporters.