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 beta cells is mediated by TTX-sensitive Na+ channels and is propagated into the mitochondria through the mitochondrial Na+/Ca2+ exchanger, NCDC. 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. (C) 2014 Elsevier Ltd. All rights reserved.