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

CR front all intracellular store, he re directly characterized as the ER, u sually refers to net Ca2+ release that amplifies evoked elevations in cytos olic 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. Fr iel. 2001. J. Gen. Physiol. 118:83-100) shows that in sympathetic neurons, small elevations evoked by weak depolarization stimulate ER Ca accumulation , but at a rate attenuated by activation of a ryanodine-sensitive CICR path way. Here, we have measured depolarization-evoked changes in total ER Ca co ncentration ([Ca](ER)) as a function of [Ca2+](i), and found that progressi vely larger [Ca2+](i) elevations cause a graded transition from ER Ca accum ulation 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 an d 4.7 +/- 1.1 mmol/kg, respectively). [Ca](ER) rises during weak depolariza tion (to 17.0 +/- 1.6 mmol/kg over 120s, [Ca2+](i) less than similar to 350 nM), changes little in response to stronger depolarization (12.1 +/- 1.1 m mol/kg, [Ca2+], similar to 700 nM), and declines (to 6.5 +/- 1.0 mmol/kg) w ith larger [Ca2+](i) elevations (>1 muM) evoked by the same depolarization when mitochondrial Ca2+ uptake is inhibited (FCCP). Thus, net ER Ca2+ trans port exhibits a biphasic dependence on [Ca2+](i). With mitochondrial Ca2+ u ptake 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 accumu lation over a period lengthened by mitochondrial Ca2+ release. Finally, alt hough spatially averaged [Ca](ER) is unchanged during strong depolarization , net ER Ca2+ release still occurs, but only in the outermost similar to5-m um cytoplasmic shell where [Ca2+], should reach its highest levels. Since m itochondrial 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.