Contrasting Contributions of Endoplasmic Reticulum and Mitochondria to Ca2+ Handling in Neurons

loads in bullfrog sympathetic ganglion neurons. The articles include measurements of ER total [Ca] made within individual cisternae identified in cryosections from rapidly frozen cells, using energy-dispersive X-ray microanalysis. Use of the perforated patch recording technique (which avoids the cytosolic dialysis produced by whole-cell recording) allowed an impressive degree of stability in the voltage-clamped currents and fura-2– based measurements of cytosolic [Ca 2 ] (see Fig. 1 of Albrecht et al. [2001]). Measurements of ER total [Ca] are more direct than the conventional approach of inferring changes in ER [Ca] by measuring how various ER-modifying drugs influence cytosolic [Ca 2 ] transients. In fact, one of the fundamental contributions of these articles is verification that these pharmacological manipulations actually do (in most cases) produce the predicted changes in basal and stimulated total [Ca] within morphologically identified ER. For example, the relatively high resting value of ER [Ca] (12.8 mmol/kg dry weight, or 3.6 mmol/liter hydrated) is shown to be reduced to 5.5 mmol/kg dry weight when ER Ca 2 uptake is inhibited with thapsigargin, and to 4.7 mmol/kg dry weight when ER Ca 2 release channels are opened with a combination of 1 M ryanodine and 10 mM caffeine. The articles make another fundamental contribution, namely the documentation of changes in ER total [Ca] in response to the size of the Ca 2 load. Albrecht et al. (2001) present evidence that small Ca 2 loads (produced, e.g., by 2 min in 30 mm K , average cytosolic [Ca 2 ] 350 nM) are accompanied by both uptake and release of Ca 2 from ER. Since average total ER [Ca] increases from 12.8 to 17.0 mmol/kg dry weight, it is clear that uptake predominates, and that under this condition ER acts as a Ca 2 sink. The authors call this “Mode 1” behavior (Fig. 1). Hongpaisan et al. (2001) present evidence that moderate Ca 2 loads (e.g., 2 min in 50 mM K , average cytosolic [Ca 2 ] 600–800 nM) are also accompanied by both uptake and release; but release does not become regenerative, and there is no significant change in average ER [Ca]. In this situation, termed “Mode 2,” the ER might appear to be just “spinning its wheels”; but, the articles provide evidence for spatial gradients in ER behavior, as discussed below. With higher Ca 2 loads (e.g., 2 min in 50 mM K with mitochondrial Ca 2 uptake inhibited, producing peak cytosolic [Ca 2 ] exceeding 1,000 nM), there was a net loss of Ca from the ER, with total ER [Ca] falling from 12.8 to 6.5 mmol/kg dry weight. In this situation, termed “Mode 3,” ER acts as a Ca 2