Multiple Modes of Calcium-induced Calcium Release In Sympathetic Neurons II: A [Ca super(2+)] sub(i) and Location-dependent Transition from Endoplasmic Reticulum Ca Accumulation to Net Ca Release

• Published in: The Journal of general physiology Vol. 118; no. 1; pp. 101 - 112
• Main Authors: Hongpaisan, J, Pivovarova, N B, Colegrove, S L, Leapman, R D, Friel, D D, Andrews, S B
• Format: Journal Article
• Language: English
• Published: 01-07-2001
• ISSN: 0022-1295

CICR from an intracellular store, here directly characterized as the ER, usually refers to net Ca super(2+) release that amplifies evoked elevations in cytosolic free calcium ([Ca super(2+)] sub(i)). However, the companion paper shows that in sympathetic neurons, small [Ca super(2+)] sub(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] sub(ER)) as a function of [Ca super(2+)] sub(i), and found that progressively larger [Ca super(2+)] sub(i) elevations cause a graded transition from ER Ca accumulation to net release, consistent with the expression of multiple modes of CICR. [Ca] sub(ER) is relatively high at rest (12.8 plus or minus 0.9 mmol/kg dry weight, mean plus or minus SEM) and is reduced by thapsigargin or ryanodine (5.5 plus or minus 0.7 and 4.7 plus or minus 1.1 mmol/kg, respectively). [Ca] sub(ER) rises during weak depolarization (to 17.0 plus or minus 1.6 mmol/kg over 120s, [Ca super(2+)] sub(i) less than similar to 350 nM), changes little in response to stronger depolarization (12.1 plus or minus 1.1 mmol/kg, [Ca super(2+)] sub(i) similar to 700 nM), and declines (to 6.5 plus or minus 1.0 mmol/kg) with larger [Ca super(2+)] sub(i) elevations (>1 mu M) evoked by the same depolarization when mitochondrial Ca super(2+) uptake is inhibited (FCCP). Thus, net ER Ca super(2+) transport exhibits a biphasic dependence on [Ca super(2+)] sub(i). With mitochondrial Ca super(2+) uptake enabled, [Ca] sub(ER) rises after repolarization (to 16.6 plus or minus 1.8 mmol/kg at 15 min) as [Ca super(2+)] sub(i) falls within the permissive range for ER Ca accumulation over a period lengthened by mitochondrial Ca super(2+) release. Finally, although spatially averaged [Ca] sub(ER) is unchanged during strong depolarization, net ER Ca super(2+) release still occurs, but only in the outermost similar to 5- mu m cytoplasmic shell where [Ca super(2+)] sub(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 Ca super(2+) entry, possibly reflecting indirect mitochondrial regulation of ER Ca super(2+) transport.