Extracellular calcium depletion transiently elevates oxygen consumption in neurosecretory PC12 cells through activation of mitochondrial Na ⁺ /Ca ²⁺ exchange

Fluctuating extracellular Ca ²⁺ regulates many aspects of neuronal (patho)physiology including cell metabolism and respiration. Using fluorescence-based intracellular oxygen sensing technique, we demonstrate that depletion of extracellular Ca ²⁺ from 1.8 to ≤0.6mM by chelation with EGTA induces a marked spike in O ₂ consumption in differentiated PC12 cells. This respiratory response is associated with the reduction in cytosolic and mitochondrial Ca ²⁺ , minor depolarization on the mitochondrial membrane, moderate depolarization of plasma membrane, and no changes in NAD(P)H and ATP. The response is linked to the influx of extracellular Na ⁺ and the subsequent activation of mitochondrial Na ⁺ /Ca ²⁺ and Na ⁺ /H ⁺ exchange. The mitochondrial Na ⁺ /Ca ²⁺ exchanger ( _m NCX) activated by Na ⁺ influx reduces Ca ²⁺ and increases Na ⁺ levels in the mitochondrial matrix. The excess of Na ⁺ activates the mitochondrial Na ⁺ /H ⁺ exchanger (NHE) increasing the outward pumping of protons, electron transport and O ₂ consumption. Reduction in extracellular Na ⁺ and inhibition of Na ⁺ influx through the receptor operated calcium channels and plasmalemmal NHE reduce the respiratory response. Inhibition of the _m NCX, L-type voltage gated Ca ²⁺ channels or the release of Ca ²⁺ from the endoplasmic reticulum also reduces the respiratory spike, indicating that unimpaired intercompartmental Ca ²⁺ exchange is critical for response development.