Cerebral ischemia leads to a massive increase in cytoplasmic calcium activity resulting from an influx of calcium ions into cells and a release of calcium from mitochondria and endoplasmic reticulum (ER). It is widely believed that this increase in cytoplasmic calcium activity plays a major role in ischemic cell injury in neurons. Recently, this concept was modified, taking into account that disturbances occurring during ischemia are potentially reversible: it then was proposed that after reversible ischemia, calcium ions are taken up by mitochondria, leading to disturbances of oxidative phosphorylation, formation of free radicals, and deterioration of mitochondrial functions. The current review focuses on the possible role of disturbances of ER calcium homeostasis in the pathologic process culminating in ischemic cell injury. The ER is a subcellular compartment that fulfills important functions such as the folding and processing of proteins, all of which are strictly calcium dependent. ER calcium activity is therefore relatively high, lying in the lower millimolar range (i.e., close to that of the extracellular space). Depletion of ER calcium stores is a severe form of stress to which cells react with a highly conserved stress response, the most important changes being a suppression of global protein synthesis and activation of stress gene expression. The response of cells to disturbances of ER calcium homeostasis is almost identical to their response to transient ischemia, implying common underlying mechanisms. Many observations from experimental studies indicate that disturbances of ER calcium homeostasis are involved in the pathologic process leading to ischemic cell injury. Evidence also has been presented that depletion of ER calcium stores alone is sufficient to activate the process of programmed cell death. Furthermore, it has been shown that activation of the ER-resident stress response system by a sublethal form of stress affords tolerance to other, potentially lethal insults. Also, disturbances of ER function have been implicated in the development of degenerative disorders such as prion disease and Alzheimer's disease. Thus, disturbances of the functioning of the ER may be a common denominator of neuronal cell injury in a wide variety of acute and chronic pathologic states of the brain. Finally, there is evidence that ER calcium homeostasis plays a key role in maintaining cells in their physiologic state, since depletion of ER calcium stores causes growth arrest and cell death, whereas cells in which the regulatory link between ER calcium homeostasis and protein synthesis has been blocked enter a state of uncontrolled proliferation.

1. Brainstem slices were taken from mature rats. In the dorsal vagal nucleus (DVNX), membrane potentials (Em) of neurons (DVNs) and glia, as well as extracellular oxygen, K+ and pH (Po2, aKO, pHO), were analysed during metabolic disturbances.2. Postsynaptic potentials of DVNs, elicited by repetitive electrical stimulation of the solitary tract (TS), led to a secondary glial depolarization of up to 25 mV, a fall in Po. of up to 150 mmHg, a rise in extracellular aKo of up to 9 mm, and a fall in pHo of about 0-2 pH units.3. Hypoxic superfusates produced tissue anoxia, leading to an aK. increase of less than 2 mM and a pHo fall of 024 + 0 04 pH units (mean + S.D.). Glucose-free solution evoked, after a delay of more than 8 min, a slow rise in aKo of 1-9 + 0-8 mm, accompanied by a mean increase in pHo of 0-24 + 0-13 pH units. After pre-incubation in glucose-free solution, anoxia elevated aKo by up to 15 mm, whereas the anoxia-induced pHo decrease was completely blocked. 4. In 45 of 118 DVNs, anoxia elicited a persistent hyperpolarization of 15'6 + 5 0 mV. In the remaining DVNs, anoxic exposure either did not produce a change in Em (37 %) or led to a depolarization of less than 10 mV (25 %). A stable depolarization of 9 + 3 8 mV was detected in glial cells during anoxia. Similar responses were revealed in oxygenated glucose-free solution after a delay of 12-60 min.5. The metabolism-related hyperpolarizations were blocked by 100-500 /uM tolbutamide or 20-100 AM glibenclamide, leading to recovery of spontaneous (0-5-6 Hz) spike discharge. In these cells, 400-500 uM diazoxide evoked hyperpolarizations and blockade of spontaneous activity. 6. In DVNs and glial cells, a progressive depolarization of up to 40 mV in amplitude developed during anoxic exposure after pre-incubation in glucose-free solution. 7. The results show that oxygen or glucose depletion does not impair the viability of DVNX cells. The contribution of neuronal ATP-sensitive K+ (KATP) channels to this tolerance is discussed.

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We have used thapsigargin (TG), a specific, irreversible inhibitor of endoplasmic reticulum (ER) Ca2+‐ATPases, and caffeine, an agonist of the ryanodine receptor, to study the effect of emptying of ER calcium stores on protein synthesis in neuronal cells. TG at 1 µM caused a permanent inhibition of protein synthesis in hippocampal slices from 3‐week‐old rats but no inhibition in slices prepared from 2‐month‐old animals. Caffeine at 10 mM caused a reduction of protein synthesis in both 3‐week‐ and 2‐month‐old rats immediately after exposure, but complete recovery of protein synthesis occurred within 30 min after treatment. In neuronal cells, TG produced an almost complete inhibition of protein synthesis that was only partially reversed over a 24‐h recovery period. TG did not significantly affect neuronal ATP levels or energy charge. Fifty percent inhibition of protein synthesis was achieved with ∼5 nM TG. Recovery of protein synthesis after TG treatment was significantly hindered when serum was omitted from the medium after TG exposure, suggesting that serum promotes recovery of ER calcium homeostasis. It is concluded that TG is a suitable tool for the study of the mechanisms of protein synthesis inhibition after transient cerebral ischemia. The possibility that disturbances in ER calcium homeostasis may contribute to the pathological process of ischemic cell death is discussed.