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 [Ca(2+)] responses, but few address how responses elicited in specific cell types are defined by the Ca(2+) 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 [Ca(2+)] responses to the underlying Ca(2+) transport and buffering systems. Depolarization-evoked [Ca(2+)](i) responses were studied in sympathetic neurons under voltage clamp, asking how response kinetics are defined by the Ca(2+) handling systems expressed in these cells. We investigated five cases of increasing complexity, comparing observed and calculated responses deduced from measured Ca(2+) handling properties. In Case 1, [Ca(2+)](i) responses were elicited by small Ca(2+) currents while Ca(2+) transport by internal stores was inhibited, leaving plasma membrane Ca(2+) extrusion intact. In Case 2, responses to the same stimuli were measured while mitochondrial Ca(2+) uptake was active. In Case 3, responses were elicited as in Case 2 but with larger Ca(2+) currents that produce larger and faster [Ca(2+)](i) elevations. Case 4 included the mitochondrial Na/Ca exchanger. Finally, Case 5 included ER Ca(2+) uptake and release pathways. We found that [Ca(2+)](i) responses elicited by weak stimuli (Cases 1 and 2) could be quantitatively reconstructed using a spatially uniform model incorporating the measured properties of Ca(2+) 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 Ca(2+) transport and buffering descriptions, as long as endogenous buffers have low mobility, leading to steep radial [Ca(2+)](i) gradients and spatially nonuniform Ca(2+) loading by mitochondria. When extended to include mitochondrial Ca(2+) release (Case 4) and ER Ca(2+) transport (Case 5), the diffusion model could also account for previous measurements of stimulus-evoked changes in total mitochondrial and ER Ca concentration.
Patterson
Michael
M
Department of Neurosciences, Case Western Reserve University, Cleveland, OH 44106, USA.
Sneyd
James
J
Friel
David D
DD
D000818:Animals; D002021:Buffers; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000502:physiology; D002478:Cells, Cultured; D003593:Cytoplasm / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D008928:Mitochondria / Q000378:metabolism*; D008962:Models, Theoretical; D009431:Neural Conduction / Q000502:physiology; D009474:Neurons / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D011892:Rana catesbeiana; D019831:Sodium-Calcium Exchanger / Q000502:physiology; D013564:Sympathetic Nervous System / Q000378:metabolism*; D013997:Time Factors
D002021:Buffers; D019831:Sodium-Calcium Exchanger; D002118:Calcium