Abstract
CICR from an intracellular store, here directly characterized as the ER, usually refers to net Ca(2)+ 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 approximately 350 nM), changes little in response to stronger depolarization (12.1 +/- 1.1 mmol/kg, [Ca2+](i) approximately 700 nM), and declines (to 6.5 +/- 1.0 mmol/kg) with larger [Ca2+](i) elevations (>1 microM) 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 approximately 5-microm 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 Ca(2)+ transport.