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

Abstract

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.

Modulation of Endoplasmic Reticulum Ca2+ Store Filling by Cyclic ADP-ribose Promotes Inositol Trisphosphate (IP3)-evoked Ca2+ Signals

“…Specifically, acceleration of SERCA activity by cADPR promotes filling of ER stores when cytosolic Ca 2ϩ is elevated prior to activation of the IP 3 pathway, providing an anticipatory signal that will likely persist for seconds or longer depending upon rates of cADPR catabolism and ER Ca 2ϩ leakage. Indeed, the state of ER Ca 2ϩ filling has been shown to powerfully affect intracellular Ca 2ϩ signaling (46). Our results indicate that this mechanism is not necessarily constrained by a passive balance between rates of sequestration and release, and strengthen the notion that the versatility of Ca 2ϩ signaling may be enhanced not only by modulation of release channels properties from the cytosolic side, but also from luminal side of the ER through messenger pathways that modulate Ca 2ϩ sequestration mechanisms such as SERCA.…”

Anandamide Reduces Intracellular Ca2+ Concentration through Suppression of Na+/Ca2+ Exchanger Current in Rat Cardiac Myocytes

“…In addition to being found in the plasma membranes, NCX is expressed in the mitochondria and endoplasmic reticulum of excitable cells [14], [15].The NCX is an membrane protein that export Ca 2+ from cells and import Na + into the cells. The NCX removes a Ca 2+ ion in exchange for the import of three Na + ions and is considered one of the most important mechanisms for Ca 2+ removal in cardiac muscle [16], [17].…”

Tissue-Specific Expression of the Calcium Transporter Genes TRPV5, TRPV6, NCX1, and PMCA1b in the Duodenum, Kidney and Heart of Equus caballus

“…Sodium-calcium exchanger 1 (NCX1) is a counter-transporter membrane protein found in the plasma membrane, mitochondria, and endoplasmic reticulum of excitable cells [28,38], where it functions to excrete intracellular calcium. NCX1 is also involved in controlling neurosecretion, cardiac muscle relaxation, excitation-contraction coupling, and maintaining low calcium concentrations in the mitochondria [13].…”