Quantitative Analysis of Mitochondrial Ca(2+) Uptake and Release Pathways in Sympathetic Neurons: Reconstruction of the Recovery after Depolarization-Evoked [Ca(2+)](i) Elevations

Rate equations for mitochondrial Ca(2+) uptake and release and plasma membrane Ca(2+) transport were determined from the measured fluxes in the preceding study and incorporated into a model of Ca(2+) dynamics. It was asked if the measured fluxes are sufficient to account for the [Ca(2+)](i) recovery kinetics after depolarization-evoked [Ca(2+)](i) elevations. Ca(2+) transport across the plasma membrane was described by a parallel extrusion/leak system, while the rates of mitochondrial Ca(2+) uptake and release were represented using equations like those describing Ca(2+) transport by isolated mitochondria. Taken together, these rate descriptions account very well for the time course of recovery after [Ca(2+)](i) elevations evoked by weak and strong depolarization and their differential sensitivity to FCCP, CGP 37157, and [Na(+)](i). The model also leads to three general conclusions about mitochondrial Ca(2+) transport in intact cells: (1) mitochondria are expected to accumulate Ca(2+) even in response to stimuli that raise [Ca(2+)](i) only slightly above resting levels; (2) there are two qualitatively different stimulus regimes that parallel the buffering and non-buffering modes of Ca(2+) transport by isolated mitochondria that have been described previously; (3) the impact of mitochondrial Ca(2+) transport on intracellular calcium dynamics is strongly influenced by nonmitochondrial Ca(2+) transport; in particular, the magnitude of the prolonged [Ca(2+)](i) elevation that occurs during the plateau phase of recovery is related to the Ca(2+) set-point described in studies of isolated mitochondria, but is a property of mitochondrial Ca(2+) transport in a cellular context. Finally, the model resolves the paradoxical finding that stimulus-induced [Ca(2+)](i) elevations as small as ∼300 nM increase intramitochondrial total Ca(2+) concentration, but the steady [Ca(2+)](i) elevations evoked by such stimuli are not influenced by FCCP.