Multiple Modes of Calcium-Induced Calcium Release in Sympathetic Neurons II: A [Ca²+]_i- and Location-Dependent Transition from Endoplasmic Reticulum Ca Accumulation to Net Ca Release

CICR from an intracellular store, here directly characterized as the ER, usually refers to net Ca²+ release that amplifies evoked elevations in cytosolic free calcium ([Ca²+]_i). However, the companion paper (Albrecht, M.A., S.L. Colegrove, J. Hongpaisan, N.B. Pivovarova, S.B. Andrews, and D.D. Friel. 2001. J. Gen. Physiol. 118:83–100) shows that in sympathetic neurons, small [Ca²+]_i elevations evoked by weak depolarization stimulate ER Ca accumulation, but at a rate attenuated by activation of a ryanodine-sensitive CICR pathway. Here, we have measured depolarization-evoked changes in total ER Ca concentration ([Ca]_ER) as a function of [Ca²+]_i, and found that progressively larger [Ca²+]_i elevations cause a graded transition from ER Ca accumulation to net release, consistent with the expression of multiple modes of CICR. [Ca]_ER is relatively high at rest (12.8 ± 0.9 mmol/kg dry weight, mean ± SEM) and is reduced by thapsigargin or ryanodine (5.5 ± 0.7 and 4.7 ± 1.1 mmol/kg, respectively). [Ca]_ER rises during weak depolarization (to 17.0 ± 1.6 mmol/kg over 120s, [Ca²+]_i less than ∼350 nM), changes little in response to stronger depolarization (12.1 ± 1.1 mmol/kg, [Ca²+]_i ∼700 nM), and declines (to 6.5 ± 1.0 mmol/kg) with larger [Ca²+]_i elevations (>1 μM) evoked by the same depolarization when mitochondrial Ca²+ uptake is inhibited (FCCP). Thus, net ER Ca²+ transport exhibits a biphasic dependence on [Ca²+]_i. With mitochondrial Ca²+ uptake enabled, [Ca]_ER rises after repolarization (to 16.6 ± 1.8 mmol/kg at 15 min) as [Ca²+]_i falls within the permissive range for ER Ca accumulation over a period lengthened by mitochondrial Ca²+ release. Finally, although spatially averaged [Ca]_ER is unchanged during strong depolarization, net ER Ca²+ release still occurs, but only in the outermost ∼5-μm cytoplasmic shell where [Ca²+]_i should reach its highest levels. Since mitochondrial Ca accumulation occurs preferentially in peripheral cytoplasm, as demonstrated here by electron energy loss Ca maps, the Ca content of ER and mitochondria exhibit reciprocal dependencies on proximity to sites of Ca²+ entry, possibly reflecting indirect mitochondrial regulation of ER Ca²+ transport.