Depolarization-induced Calcium Responses in Sympathetic Neurons: Relative Contributions from Ca²⁺ Entry, Extrusion, ER/Mitochondrial Ca²⁺ Uptake and Release, and Ca²⁺ Buffering

Many models have been developed to account for stimulus-evoked [Ca²⁺] responses, but few address how responses elicited in specific cell types are defined by the Ca²⁺ transport and buffering systems that operate in the same cells. In this study, we extend previous modeling studies by linking the time course of stimulus-evoked [Ca²⁺] responses to the underlying Ca²⁺ transport and buffering systems. Depolarization-evoked [Ca²⁺]_i responses were studied in sympathetic neurons under voltage clamp, asking how response kinetics are defined by the Ca²⁺ handling systems expressed in these cells. We investigated five cases of increasing complexity, comparing observed and calculated responses deduced from measured Ca²⁺ handling properties. In Case 1, [Ca²⁺]_i responses were elicited by small Ca²⁺ currents while Ca²⁺ transport by internal stores was inhibited, leaving plasma membrane Ca²⁺ extrusion intact. In Case 2, responses to the same stimuli were measured while mitochondrial Ca²⁺ uptake was active. In Case 3, responses were elicited as in Case 2 but with larger Ca²⁺ currents that produce larger and faster [Ca²⁺]_i elevations. Case 4 included the mitochondrial Na/Ca exchanger. Finally, Case 5 included ER Ca²⁺ uptake and release pathways. We found that [Ca²⁺]_i responses elicited by weak stimuli (Cases 1 and 2) could be quantitatively reconstructed using a spatially uniform model incorporating the measured properties of Ca²⁺ entry, removal, and buffering. Responses to strong depolarization (Case 3) could not be described by this model, but were consistent with a diffusion model incorporating the same Ca²⁺ transport and buffering descriptions, as long as endogenous buffers have low mobility, leading to steep radial [Ca²⁺]_i gradients and spatially nonuniform Ca²⁺ loading by mitochondria. When extended to include mitochondrial Ca²⁺ release (Case 4) and ER Ca²⁺ transport (Case 5), the diffusion model could also account for previous measurements of stimulus-evoked changes in total mitochondrial and ER Ca concentration.