Dynamics of intracellular oxygen in PC12 cells upon stimulation of neurotransmission

Neurotransmission, synaptic plasticity, and maintenance of membrane excitability require high mitochondrial activity in neurosecretory cells. Using a fluorescence-based intracellular O-2 sensing technique, we investigated the respiration of differentiated PC12 cells upon depolarization with 100 mM K+. Single cell confocal analysis identified a significant depolarization of the plasma membrane potential and a relatively minor depolarization of the mitochondrial membrane potential following K+ exposure. We observed a two-phase respiratory response: a first intense spike lasting similar to 10 min, during which average intracellular O-2 was reduced from 85 - 90% of air saturation to 55 - 65%, followed by a second wave of smaller amplitude and longer duration. The fast rise in O-2 consumption coincided with a transient increase in cellular ATP by similar to 60%, which was provided largely by oxidative phosphorylation and by glycolysis. The increase of respiration was orchestrated mainly by Ca2+ release from the endoplasmic reticulum, whereas the influx of extracellular Ca2+ contributed similar to 20%. Depletion of Ca2+ stores by ryanodine, thapsigargin, and 4-chloro- m-cresol reduced the amplitude of respiratory spike by 45, 63, and 71%, respectively, whereas chelation of intracellular Ca2+ abolished the response. Uncoupling of the mitochondria with the protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone amplified the responses to K+; elevated respiration induced a profound deoxygenation without increasing the cellular ATP levels reduced by carbonyl cyanide p-trifluoromethoxyphenylhydrazone. Cleavage of synaptobrevin 2 by tetanus toxin, known to reduce neurotransmission, did not affect the respiratory response to K+, whereas the general excitability of (d)PC12 cells increased.