Alterations in lipid and calcium metabolism associated with seizure activity in the postischemic brain

Transient ischemia is known to lead to a long-lasting depression of cerebral metabolic rate and blood flow and to an attenuated metabolic and circulatory response to physiological stimuli. However, the corresponding responses to induced seizures are retained, demonstrating preserved metabolic and circulatory capacity. The objective of the present study was to explore how a preceding period of ischemia (15 min) alters the release of free fatty acids (FFAs) and diacylglycerides (DAGs), the formation of cyclic nucleotides, and the influx/efflux of Ca(2+), following intense neuronal stimulation. For that purpose, seizure activity was induced with bicuculline for 30 s or 5 min at 6 h after the ischemia. Extracellular Ca(2+) concentration (Ca(2+)(e)) was recorded, and the tissue was frozen in situ for measurements of levels of FFAs, DAGs, and cyclic nucleotides. Six hours after ischemia, the FFA concentrations were normalized, but there was a lowering of the content of 20:4 in the DAG fraction. Cyclic AMP levels returned to normal values, but cyclic GMP content was reduced. Seizures induced in postischemic animals showed similar changes in Ca(2+)(e), as well as in levels of FFAs, DAGs, and cyclic nucleotides, as did seizures induced in nonischemic control animals, with the exception of an attenuated rise in 20:4 content in the DAG fraction. We conclude that, at least in the neocortex, seizure-induced phospholipid hydrolysis and cyclic cAMP/cyclic GMP formation are not altered by a preceding period of ischemia, nor is there a change in the influx/efflux of Ca(2+) during seizure discharge or in associated spreading depression.

Cyclosporin A enhances survival, ameliorates brain damage, and prevents secondary mitochondrial dysfunction after a 30-minute period of transient cerebral ischemia

Cyclosporin A (CsA) has been shown to be efficacious in protecting against ischemic injury after short periods (5 to 10 min) of forebrain ischemia. The present experiments were undertaken to study if a long period of forebrain ischemia (30 min), induced at a brain temperature of 37 degrees C, is compatible with survival and if the brain damage incurred can be ameliorated by CsA. The results showed that animals subjected to 30 min of forebrain ischemia at a brain temperature of 37 degrees C failed to survive after the first 24 h of recovery and showed extensive neuronal necrosis in all selectively vulnerable regions after 1 day of survival. CsA, when injected in combination with an intracerebral lesion to open the blood-brain barrier, markedly prolonged the survival time. CsA-injected animals also showed amelioration of histological lesions, an effect that was sustained for at least 4 days. Experiments with mitochondria isolated from the neocortex and hippocampus showed that state 3 respiratory rates decreased during ischemia, recovered after 1 and 3 h of recirculation, and then showed a secondary decline at 6 h. Administration of CsA prevented this secondary decline. Measurements of neocortical cerebral blood flow showed that there was no secondary hypoperfusion prior to secondary mitochondrial dysfunction, implying that changes in blood flow may not be responsible for the rapidly developing, secondary brain damage. The results thus demonstrate that if brain temperature is upheld at 37 degrees C, a 30-min period of ischemia is not compatible with survival after the first day of recovery, and gross histopathological damage develops within that period. CsA was efficacious in prolonging animal survival, ameliorating brain damage, and preventing the secondary mitochondrial dysfunction. Since CsA blocks the mitochondrial permeability transition pore its action may, at least in part, be on mitochondrial integrity and function.

Characteristics of the calcium-triggered mitochondrial permeability transition in nonsynaptic brain mitochondria: effect of cyclosporin A and ubiquinone O

The objective of the present study was to assess the capacity of nonsynaptic brain mitochondria to accumulate Ca2+ when subjected to repeated Ca2+ loads, and to explore under what conditions a mitochondrial permeability transition (MPT) pore is assembled. The effects of cyclosporin A (CsA) on Ca2+ accumulation and MPT pore assembly were compared with those obtained with ubiquinone 0 (Ubo), a quinone that is a stronger MPT blocker than CsA, when tested on muscle and liver mitochondria. When suspended in a solution containing phosphate (2 mM) and Mg2+ (1 mM), but no ATP or ADP, the brain mitochondria had a limited capacity to accumulate Ca2+ (210 nmol/mg of mitochondrial protein). Furthermore, when repeated Ca2+ pulses (40 nmol/mg of protein each) saturated the uptake system, the mitochondria failed to release the Ca2+ accumulated. However, in each instance, the first Ca2+ pulse was accompanied by a moderate release of Ca2+, a release that was not observed during the subsequent pulses. The initial release was accompanied by a relatively marked depolarization, and by swelling, as assessed by light-scattering measurements. However, as the swelling was <50% of that observed following addition of alamethicin, it is concluded that the first Ca2+ pulse gives rise to an MPT in a subfraction of the mitochondrial population. CsA, an avid blocker of the MPT pore, only marginally increased the Ca(2+)-sequestrating capacity of the mitochondria. However, CsA eliminated the Ca2+ release accompanying the first Ca2+ pulse. The effects of CsA were shared by Ubo, but when the concentration of Ubo exceeded 20 microM, it proved toxic. The results thus suggest that brain mitochondria are different from those derived from a variety of other sources. The major difference is that a fraction of the brain mitochondria, studied presently, depolarized and showed signs of an MPT. This fraction, but not the remaining ones, contributed to the chemically and electron microscopically verified mitochondrial swelling.

Role and mechanisms of secondary mitochondrial failure

Ischemia is accompanied by mitochondrial dysfunction, as assessed by measurements of mitochondrial respiratory activities in vitro. Following brief periods of ischemia, mitochondrial function is usually normalized during reperfusion. However, particularly after ischemia of longer duration, reperfusion may be accompanied by secondary mitochondrial failure. After short periods of ischemia this is observed in selectively vulnerable areas and, after intermediate to long periods of ischemia, in other areas as well. However, it has remained unsettled if the mitochondrial dysfunction is the result or the cause of cell death. Although it has been commonly assumed that such failure is secondary to cell injury by other mechanisms, recent results suggest that mitochondrial dysfunction may be the cause of cell death. Indirect evidence for this postulate is provided by experiments showing that cyclosporin A (CsA), when allowed to cross the blood-brain barrier, is a potent neuroprotectant. CsA is a virtually specific blocker of the mitochondrial permeability transition (MPT) pore, a voltage-gated channel allowing molecules and ions with a mass < 1500 Daltons to pass the inner mitochondrial membrane. Experiments on isolated cells in vitro demonstrate that cell calcium accumulation or oxidative stress triggers the assembly of an MPT pore, which leads to collapse of the mitochondrial membrane potential, to ATP hydrolysis, to enhanced production of reactive oxygen species (ROS), and to cell death. The beneficial effect of CsA could thus be related to its ability to block the MPT pore. Longer periods of ischemia, such as occurs after transient middle cerebral artery (MCA) occlusion, lead to pan-necrotic lesions (infarction). In the rat, recirculation following 2 h of MCA occlusion leads to partial normalization of the bioenergetic state but this is followed within 4-6 h by secondary bioenergetic failure. The latter seems unrelated to blockade of the microcirculation, but correlates to secondary mitochondrial failure. The brain damage incurred is ameliorated by the spin trap alpha-phenyl-N-butyl nitrone (PBN) and by the immunosuppressant FK506 even when given 1-3 h after the start of recirculation. The two drugs also prevent the secondary mitochondrial failure during early recirculation, suggesting that such failure is pathogenetically important. Probably, though, the mitochondrial dysfunction involves not only the assembly of an MPT pore but also other mechanisms. Since recirculation is associated with release of mitochondrial proteins it is not unlikely that such proteins, e.g. cytochrome c, trigger cascades of events leading to cell death.6.

Calcium metabolism of focal and penumbral tissues in rats subjected to transient middle cerebral artery occlusion

The present experiments were undertaken to define changes in tissue calcium metabolism in focal and perifocal ("penumbral") tissues following 2 h of transient middle cerebral artery occlusion (MCAO) in rats, induced with an intraluminal filament occlusion technique. The extracellular calcium concentration ([Ca2+]e) was measured with ion-selective microelectrodes in neocortical focus and penumbra. For measurement of total tissue calcium content, tissue samples from these areas were collected and analyzed with atomic absorption spectrometry. During MCAO, [Ca2+]e in a neocortical focal area fell from a normal value of about 1.2 mM to values around 0.1 mM, suggesting translocation of virtually all extracellular calcium to intracellular fluids. Recirculation was accompanied by re-extrusion of calcium within 5-7 min; however, [Ca2+]e never returned to normal but stabilized at about 50% of the control value for the first 6 h, and decreased further after 24 h. In penumbral areas, [Ca2+]e showed the expected transient decreases associated with spreading depression-like (or ischemic) depolarization waves. Recirculation was followed by return of [Ca2+]e towards normal values. In the focus, water content increased from about 79% to about 80.4% at the end of the 2-h period of ischemia. After 2 h and 4 h of recirculation, the edema was aggravated (mean values 81.9% and 81.2%, respectively). After 6 h and 24 h, the edema was more pronounced (83.6% and 83.8%, respectively). In the penumbra, no significant edema was observed until 6 h and 24 h of recirculation. The total tissue calcium content in the focus (expressed by unit dry weight) increased at the end of the ischemia period demonstrating calcium translocation from blood to tissue. After 6 h and 24 h, the content increased two- to threefold, compared with control. Changes in the penumbra were qualitatively similar but less pronounced, and a significant increase was not observed until after 6 h of recirculation. The results suggest that 2 h of MCAO leads to a profound perturbation of cell calcium metabolism. In focal areas, cells fail to extrude the calcium that is gradually accumulated during reperfusion and show massive calcium overload after the first 4-6 h of recirculation. Penumbral tissues show a similar increase in calcium concentration after 6 h of recirculation.

Calcium in ischemic cell death

BACKGROUND

This review article deals with the role of calcium in ischemic cell death. A calcium-related mechanism was proposed more than two decades ago to explain cell necrosis incurred in cardiac ischemia and muscular dystrophy. In fact, an excitotoxic hypothesis was advanced to explain the acetylcholine-related death of muscle end plates. A similar hypothesis was proposed to explain selective neuronal damage in the brain in ischemia, hypoglycemic coma, and status epilepticus.

SUMMARY OF REVIEW

The original concepts encompass the hypothesis that cell damage in ischemia-reperfusion is due to enhanced activity of phospholipases and proteases, leading to release of free fatty acids and their breakdown products and to degradation of cytoskeletal proteins. It is equally clear that a coupling exists between influx of calcium into cells and their production of reactive oxygen species, such as .O2, H2O2, and .OH. Recent results have underscored the role of calcium in ischemic cell death. A coupling has been demonstrated among glutamate release, calcium influx, and enhanced production of reactive metabolites such as .O2-, .OH, and nitric oxide. It has become equally clear that the combination of .O2- and nitric oxide can yield peroxynitrate, a metabolite with potentially devastating effects. The mitochondria have again come into the focus of interest. This is because certain conditions, notably mitochondrial calcium accumulation and oxidative stress, can trigger the assembly (opening) of a high-conductance pore in the inner mitochondrial membrane. The mitochondrial permeability transition (MPT) pore leads to a collapse of the electrochemical potential for H+, thereby arresting ATP production and triggering production of reactive oxygen species. The occurrence of an MPT in vivo is suggested by the dramatic anti-ischemic effect of cyclosporin A, a virtually specific blocker of the MPT in vitro in transient forebrain ischemia. However, cyclosporin A has limited effect on the cell damage incurred as a result of 2 hours of focal cerebral ischemia, suggesting that factors other than MPT play a role. It is discussed whether this could reflect the operation of phospholipase A2 activity and degradation of the lipid skeleton of the inner mitochondrial membrane.

CONCLUSIONS

Calcium is one of the triggers involved in ischemic cell death, whatever the mechanism.

The immunosuppressant drug FK506 ameliorates secondary mitochondrial dysfunction following transient focal cerebral ischemia in the rat

Recirculation following 2 h of focal ischemia due to transient middle cerebral artery (MCA) occlusion has previously been found to be accompanied by an initial, partial recovery of the cellular bioenergetic state and of mitochondrial respiratory functions, with secondary deterioration during the first 2-4 h of reflow. Both the free radical spin trap alpha-phenyl-N-tert-butyl nitrone (PBN) and the immunosuppressant drug FK506 ameliorate the damage incurred by the 2-h period of focal ischemia, even when given 1-3 h after the start of the recirculation. The primary objective of this study was to find out if FK506, like PBN, prevents the secondary deterioration of mitochondrial function, as this can be studied in vitro. Since this proved to be the case, we addressed the question of whether the secondary mitochondrial dysfunction and bioenergetic failure were related to a secondary compromise of microcirculation and cellular oxygen delivery. Six groups of male Wistar rats were studied for measurement of mitochondrial respiratory activity (total, n = 36). One group was used as control (n = 6). In the other groups of animals, MCA occlusion of 2 h duration was induced by an intraluminal filament technique, Neocortical focal and perifocal ("penumbra") tissues were sampled after 2 h of ischemia (n = 6) and after 1 h (n = 6), 2 h (n = 6 with vehicle), and 4 h (n = 6 with vehicle; n = 6 with FK506) of recirculation. The vehicle or 1.0 mg.kg-1 of FK506 was injected intravenously after 1 h of recirculation. Homogenates were prepared, and stimulated (+ADP), nonstimulated (-ADP), and uncoupled respiratory rates were measured polarographically. The uncoupling agent used was carbonyl cyanide m-chlorophenylhydrazone. Local CBF and tissue oxygen tension were evaluated by laser-Doppler flowmetry and PO2 microelectrodes, respectively, throughout the whole periods of 2 h of ischemia and 4 h of recirculation, using a remote MCA occlusion technique. After 2 h of ischemia, the penumbra showed a moderate decrease and the focus a marked decrease in ADP-stimulated and uncoupled respiratory rates, with a marked fall in the respiratory control ratio, defined as ADP-stimulated divided by nonstimulated respiration. Recirculation (1 h) brought about partial recovery, but continued reflow (2 and 4 h) was associated with a secondary deterioration of respiratory functions. The secondary deterioration was prevented by FK506. The results thus confirm previous findings showing that secondary mitochondrial dysfunction occurs following transient focal cerebral ischemia and demonstrate that FK506, like PBN, improves the in vitro performance of mitochondria in focal and penumbral areas. Following MCA occlusion, local CBF in a penumbral area and tissue PO2 in a focal area decreased to about 30 and 5% of control, respectively. However, recirculation brought about rapid recovery of blood flow and oxygen delivery. During the whole 4-h period of recirculation, local CBF and tissue PO2 were maintained close to 100% and at about 160% of the preischemic level, respectively. The results make it highly unlikely that the secondary bioenergetic failure during recirculation is due to a compromised microcirculation. It follows that oxygen delivery is not rate-limiting for recovery events. Very likely, FK506 (and PBN) acts at the cellular level to improve mitochondrial energy functions.

Extracellular potassium in a neocortical core area after transient focal ischemia

BACKGROUND AND PURPOSE

Occlusion of the middle cerebral artery (MCAO) results in bioenergetic failure in the densely ischemic core areas. During reperfusion, transient recovery of the bioenergetic state is followed by secondary deterioration. In this study, we recorded the extracellular potassium concentrations in the cortical core during 2 hours of MCAO, as well as during recovery. One group of animals with recirculation periods of 6 to 8 hours was given the free radical spin trap alpha-phenyl-N-tert-butyl nitrone (PBN).

METHODS

The experiments were performed on adult male Wistar rats (305 to 335 g). The right MCA was occluded by an intraluminal filament technique. For [K+]e measurements a craniotomy was made over the right cortex, and an ion-sensitive microelectrode was lowered into the ischemic focus. Recording of [K+]e was continued for 2 hours. After 48 hours of reperfusion, infarction size was estimated with 2,3,5-triphenyltetrazolium chloride.

RESULTS

During MCA occlusion, [K+]e rose to approximately 60 mmol/L. However, several animals showed transient (and partial) periods of repolarization accompanied by a decrease in [K+]e. Immediately on reperfusion, the [K+]e started to recover and reached baseline levels (2.5 mmol/L) within 3 to 5 minutes. During the first 6 hours of recovery, [K+]e was stable at about 2.5 mmol/L, but after this period a moderate increase in the [K+]e was observed. This was not observed in animals injected with PBN 1 hour after reperfusion.

CONCLUSIONS

The data suggest that delayed cell membrane dysfunction, as reflected in a rise in [K+]e, occurs after about 6 hours of reperfusion and that treatment with PBN in a single dose ameliorates or delays such deterioration of plasma membrane function.

Effects of preischemic hyperglycemia on brain damage incurred by rats subjected to 2.5 or 5 minutes of forebrain ischemia

BACKGROUND AND PURPOSE

The objective of this study was to explore whether preischemic hyperglycemia, which is known to aggravate brain damage due to transient global or forebrain ischemia of intermediate duration (10 to 20 minutes), increases the density of selective neuronal necrosis, as observed primarily in the CA1 sector of the hippocampus after brief periods of forebrain ischemia in rats (2.5 and 5 minutes).

METHODS

Anesthetized rats were subjected to two-vessel forebrain ischemia of 2.5- or 5-minute duration. Normoglycemic or hyperglycemic rats were either allowed a recovery period of 7 days for histopathological evaluation of neuronal necrosis in the hippocampus, isocortex, thalamus, and substantia nigra or were used for recording of extracellular concentrations of Ca2+ ([Ca2+]c), K+, or H+, together with the direct current (DC) potential.

RESULTS

Ischemia of 2.5- or 5-minute duration gave rise to similar damage in the CA1 sector of the hippocampus in normoglycemic and hyperglycemic groups (10% to 15% and 20% to 30% of the total population, respectively). However, in hyperglycemic animals subjected to 2.5 minutes of ischemia, CA1 neurons never depolarized and [Ca2+]c did not decrease. In the 5-minute groups, the total period of depolarization was 2 to 3 minutes shorter in hyperglycemic than in normoglycemic groups. This fact and results showing neocortical, thalamic, and substantia nigra damage in hyperglycemic animals after 5 minutes of ischemia demonstrate that although hyperglycemia delays the onset of ischemic depolarization and hastens repolarization and extrusion of Ca2+, it aggravates neuronal damage due to ischemia.

CONCLUSIONS

These results reinforce the concept that hyperglycemia exaggerates brain damage due to transient ischemia and prove that this exaggeration is observed at the neuronal level. The results also suggest that the concept of the duration of an ischemic transient should be qualified, particularly if ischemia is brief, ie. < 10 minutes in duration.

Calcium-related damage in ischemia

The objective of this hypothesis article is to review evidence supporting a role for calcium in mediating ischemic brain damage, and to present data which puts mitochondrial dysfunction in the center of interest. The assumptions/postulates put forward, relating to global/forebrain and to focal ischemia, are as follows. (1) In brief ischemia of the global/forebrain type neuronal necrosis, particularly in the CA1 sector of the hippocampus, is conspicuously delayed. It is postulated that the initial events during ischemia, and in the immediate recirculation period, lead to a perturbation of cell calcium homeostasis, with a gradual postischemic rise in the free cytosolic calcium concentration (Ca2+i). When the latter reaches a certain limiting value mitochondria start accumulating calcium. It is hypothesized that intramitochondrial calcium accumulation triggers a permeability transition of the inner mitochondrial membrane (MPT), leading to production of reactive oxygen species, release of calcium, and an increase in the cytosol calcium concentration of a potentially adverse nature. (2) If ischemia of this "cardiac arrest" type is prolonged, or complicated by preischemic hyperglycemia, neuronal necrosis is enhanced and pan-necrotic lesions appear. Such insults are known to cause rapidly developing mitochondrial failure, but the involvement of calcium has not yet been demonstrated. (3) In focal ischemia, core tissues probably suffer a metabolic insult similar to that affecting brain tissues in global/forebrain ischemia. Thus, calcium influx and calcium overload of mitochondria are predictable, but available data only demonstrate rapidly developing, secondary energy failure, mitochondrial dysfunction, and enhanced influx of 45Ca. Thus, although secondary mitochondrial failure has been proved, a causative link between calcium influx and bioenergetic failure remains to be proved. Perifocal, penumbral tissues are exposed to spontaneously occurring depolarisation waves, leading to cellular efflux of K+ and influx of Ca2+. The latter may lead to gradual mitochondrial calcium overload triggering a MPT, and cell death. Although conclusive evidence has not yet been presented available results suggest a link between calcium influx, mitochondrial overload, and cell death.

Molecular mechanisms of acidosis-mediated damage

The present article is concerned with mechanisms which are responsible for the exaggerated brain damage observed in hyperglycemic animals subjected to transient global or forebrain ischemia. Since hyperglycemia enchances the production of lactate plus H+ during ischemia, it seems likely that aggravation of damage is due to exaggerated intra- and extracellular acidosis. This contention is supported by results showing a detrimental effect of extreme hypercapnia in normoglycemic rats subjected to transient ischemia or to hypoglycemic coma. Enhanced acidosis may exaggerate ischemic damage by one of three mechanisms: (i) accelerating free radical production via H(+)-dependent reactions, some of which are catalyzed by iron released from protein bindings by a lowering of pH, (ii) by perturbing the intracellular signal transduction pathway, leading to changes in gene expression or protein synthesis, or (iii) by activating endonucleases which cause DNA fragmentation. While activation of endonucleases must affect the nucleus, the targets of free radical attack are not known. Microvessels are considered likely targets of such attack in sustained ischemia and in trauma; however, enhanced acidosis is not known to aggravate microvascular dysfunction, or to induce inflammatory responses at the endothelial-blood interface. A more likely target is the mitochondrion. Thus, if the ischemia is of long duration (30 min) hyperglycemia triggers rapidly developing mitochondrial failure. It is speculated that this is because free radicals damage components of the respiratory chain, leading to a secondary deterioration of oxidative phosphorylation.

Influence of hyperglycemia and of hypercapnia on cellular calcium transients during reversible brain ischemia

The object of the study was to find out how preischemic hyperglycemia (in normocapnic animals) or excessive hypercapnia (in normoglycemic animals) affect the calcium transient during ischemia, as this can be assessed by measurements of the extracellular calcium concentration ([Ca2+]e). To that extent, normocapnic-normoglycemic control animals were compared with animals with induced hyperglycemia or hypercapnia, all being subjected to 10 min of forebrain ischemia, the [Ca2+]e and d.c. potential being measured with ion-sensitive glass microelectrodes. Hyperglycemia and hypercapnia delayed the loss of ion homeostasis following induction of ischemia. Furthermore, both hyperglycemia and hypercapnia reduced the delay of Ca2+ extrusion upon recirculation. As a result, both hyperglycemia and hypercapnia significantly reduced the ischemic calcium transient, as this was assessed by calculating the duration of maximal calcium load of cells. The results make it less likely that aggravation of brain damage by hyperglycemia or excessive hypercapnia is related to a further derangement of cell calcium homeostasis.

The influence of calcium transients on intracellular pH in cortical neurons in primary culture

The objective of this study was to assess the influence of Ca2+ influx on intracellular pH (pHi) of neocortical neurons in primary culture. Neurons were exposed to glutamate (100-500 microM) or KCl (50 mM), and pHi was recorded with microspectrofluorometric techniques. Additional experiments were carried out in which calcium influx was triggered by ionomycin (2 microM) or the calcium ionophore 4-Br-A23187 (2 microM). Glutamate exposure either caused no, or only a small decrease in pHi (delta pH approximately 0.06 units). When a decrease was observed, a rebound rise in pHi above control was observed upon termination of glutamate exposure. In about 20% of the cells, the acidification was more pronounced (delta pH approximately 0.20 units), but all these cells had high control pHi values, and showed gradual acidification. Exposure of cells to 50 mM KCl consistently increased pHi. Since this increase was similar in the presence and nominal absence of HCO3-, it probably did not reflect influx of HCO3- via a Na(+)-HCO3- symporter. Furthermore, since it occurred in the absence of external Ca2+ (or a measurable rise in Cai2+) it seemed independent of Ca2+ influx. It is tentatively concluded that the rise in pHi was due to reduced passive influx of H+ along the electrochemical gradient, which is reduced by depolarization. In Ca(2+)-containing solutions, depolarization led to a rebound increase in pHi above control. This, and the rebound found after glutamate transients, may reflect Ca(2+)-triggered phosphorylation and upregulation of the Na+/H+ antiporter which extrudes H+ from the cell.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of acidosis on hypoglycemic brain damage

The objective of the study was to explore whether hypoglycemic brain damage is affected by super-imposed acidosis. To that end, animals with insulin-induced hypoglycemic coma, defined in terms of a negative DC potential shift, massive release of K+, or cellular uptake of Ca2+, were exposed to excessive hypercapnia (PaCO2 approximately 200 or approximately 300 mm Hg) during the last 25 min of the 30-min coma period. Animals were allowed to survive for 7 days before their brains were fixed by perfusion, and the cell damage was assessed by light microscopy. Other animals were analyzed with respect to changes in extracellular pH (pHe) or extracellular K+ or Ca2+ concentrations (K+e and Ca2+e, respectively). The total CO2 content (TCO2) was also measured to allow derivation of intracellular pH (pHi). The increase in PaCO2 to 190 +/- 15 and 312 +/- 23 mm Hg (means +/- SD) reduced the pHe from a predepolarization value of approximately 7.4 and a postdepolarization value (after the first 5 min of coma) of approximately 7.3 to 6.8 and 6.7, respectively. The corresponding mean pHi values were 6.7 and 6.5. The hypercapnia did not alter the K+e, which rose to 50-60 mM at the onset of hypoglycemic coma, but it increased the Ca2+e from approximately 0.05 to 0.10-0.16 mM. Normocapnic animals with induced hypoglycemic coma of 30-min duration showed the expected neuronal lesions in the neocortex, hippocampus, and caudoputamen. Hypercapnia clearly aggravated this damage, particularly in the caudoputamen, subiculum, and CA1 region of the hippocampus, and caused additional damage to cells in the CA3 region and piriform cortex. A rise in CO2 tension from approximately 200 to 300 mm Hg did not further aggravate the damage. The results thus demonstrate that relative moderate acidosis aggravates damage that is believed to be mostly neuronal, sparing glia cells and vascular tissue.

Induced spreading depressions in energy-compromised neocortical tissue: calcium transients and histopathological correlates

Mechanisms causing gradual recruitment of damaged cells in the penumbra zone around the core of a focal ischaemic lesion may encompass irregularly occurring depolarization waves of the spreading depression (SD) type, each leading to transient loading of cells with calcium. It has been speculated that, when elicited in an underperfused or otherwise energy-compromised tissue, such depolarization waves lead to cell damage. We assessed under what conditions the calcium transients during KCl-induced SDs are prolonged, and explored if marked prolongation of the transients leads to brain damage. Cerebral blood flow (CBF) was reduced by marked hypocapnia. Tissue oxygenation was reduced by arterial hypoxia, without or with unilateral carotid artery occlusion, or by occlusion of the carotid arteries in normoxic, anaesthetized rats. In all animals the DC potential and extracellular calcium concentration (Ca2+e) were measured before and during a series of SDs. The animals were recovered for histopathological assessment. Hypoxia alone (Pao2, 32.5 +/- 3.8 mmHg) increased mean and total depolarization times, but repeated SDs elicited over 1.7 (+/-0.4) h failed to induce cell damage. Unilateral carotid artery occlusion further prolonged the SD waves but, in spite of total depolarization times of up to 40 min during 2 h, only two out of seven animals showed damage, localized to caudoputamen and parietal cortex, as well as to the subiculum, CA1 and CA3 sectors of the hippocampus. Bilateral carotid artery occlusion was associated with the most pronounced prolongation of the DC potential shifts and Ca2+ transients, with total depolarization times of up to 70 min. In spite of this, only four out of 13 animals showed brain damage and in two of these the damage was contralateral. The results justify modification of the hypothesis stating that SD-like depolarizations in the perifocal penumbra zone per se is what leads to gradual recruitment of such tissues in the infarction process. It is suggested that additional factors are required, such as a larger reduction in CBF, or the proximity of cells at risk to necrotic tissue.

Influence of acid-base changes on the intracellular calcium concentration of neurons in primary culture

The influence of changes in intra- and extracellular pH (pHi and pHe, respectively) on the cytosolic, free calcium concentration ([Ca2+]i) of neocortical neurons was studied by microspectrofluorometric techniques and the fluorophore fura-2. When, at constant pHe, pHi was lowered with the NH4Cl prepulse technique, or by a transient increase in CO2 tension, [Ca2+]i invariably increased, the magnitude of the rise being proportional to delta pHi. Since similar results were obtained in Ca(2+)-free solutions, the results suggest that the rise in [Ca2+]i was due to calcium release from intracellular stores. The initial alkaline transient during NH4Cl exposure was associated with a rise in [Ca2+]i. However, this rise seemed to reflect influx of Ca2+ from the external solution. Thus, in Ca(2+)-free solution NH4Cl exposure led to a decrease in [Ca2+]i. This result and others suggest that, at constant pHe, intracellular alkalosis reduces [Ca2+]i, probably by enhancing sequestration of calcium. When cells were exposed to a CO2 transient at reduced pHe, Ca2+ rose initially but then fell, often below basal values. Similar results were obtained when extracellular HCO3- concentration was reduced at constant CO2 tension. Unexpectedly, such results were obtained only in Ca(2+)-containing solutions. In Ca(2+)-free solutions, acidosis always raised [Ca2+]i. It is suggested that a lowering of pHe stimulates extrusion of Ca2+ by ATP-driven Ca2+/2H+ antiport.

Regulation of intra- and extracellular pH in the rat brain in acute hypercapnia: a re-appraisal

Recent results have demonstrated that intracellular pH (pHi) in nerve and glial cells is not regulated back to normal during CO2 exposure if extracellular pH (pHe) is reduced. This raises the question about regulation of pHi and pHe in vivo. In order to successively reduce pHe we exposed animals to incremental increases in CO2 tension (11, 27.5, 42.5%) and studied regulation of pHi during the first 90 min of hypercapnia. Extracellular pH, as well as Na+, K+, and Cl- concentrations, were also measured, as were whole tissue contents of Na+, K+, and Cl-. At all CO2 tensions studied, pHe slowly increased during CO2 exposure. In animals breathing 11% CO2 (delta pHe approximately 0.2 units), pHi increased slowly. However, in animals exposed to 27.5% CO2 or 42.5% CO2 (delta pHe > 0.4 units), no regulation of pHi was observed. Extracellular HCO3- concentrations increased substantially already during the first 15 min of hypercapnia (not significant in animals breathing 42.5% CO2), and then gradually rose. These increases were accompanied by a decrease in Cl- and an increase in Na+ concentration, K+ concentration remaining constant. The total tissue content of these ions remained constant, suggesting that extracellular HCO3- concentration increases by Cl-/HCO3- antiport and/or by Na+.2HCO3- symport, the HCO3- emanating from intracellular sources. The results challenge the dogma of the supremacy of mechanisms regulating pHi, and suggest that brain cells, possibly astrocytes, regulate pHe at the expense of their own pH homeostasis. By inference, we further conclude that regulation of pHi normally occurs only if pHe is first regulated back close to normal value.

The influence of pH on glutamate- and depolarization-induced increases of intracellular calcium concentration in cortical neurons in primary culture

The present experiments, carried out on neocortical neurons in primary culture with measurements of cytosolic calcium concentrations ([Ca2+]i) by microspectrofluorometric techniques, were designed to study how changes in extra- and intracellular pH (pHe and pHi, respectively) modulate the rise in [Ca2+]i due to glutamate exposure or potassium (K+)-induced depolarization. Although a reduction in pHe/pHi per se increased [Ca2+]i, the acidosis attenuated both the peak rise in [Ca2+]i following exposure to glutamate, and the plateau level observed during prolonged exposure. As a result, cells exposed to solutions with low pH consistently had lower [Ca2+]i values upon glutamate exposure than cells studied at normal pH. Alkalosis, i.e., an increase in pHe/pHi, had the opposite effect, accentuating the glutamate-induced [Ca2+]i transients. Experiments designed to separate changes due to extra- and intracellular pH suggested that the decisive event was the change in pHe. These results are consistent with the known effect of pHe on calcium flux through NMDA-gated ion channels. However, lowering of pHe had an equivalent effect on the rise in [Ca2+]i triggered by exposure of the cells to a K+ concentration of 50 mM. Thus, acidosis reduces influx of calcium through both agonist-operated and voltage-sensitive channels to such an extent that efflux/sequestration mechanisms suffice to maintain a lower [Ca2+]i.

The influence of pH on cellular calcium influx during ischemia

The objective of this study was to explore how alterations in tissue pH during ischemia influence cell calcium uptake, as this is reflected in the extracellular calcium concentration (Ca2+e). Variations in pH were achieved by making animals hypo-, normo- or hyperglycemic prior to cardiac arrest ischemia or by increasing preischemic PCO2 in normoglycemic animals. For comparison, the N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine maleate (MK-801) was given prior to induction of ischemia. In some experiments the effect of acidosis on K+ efflux and Na+ influx were studied as well. In hypoglycemic subjects, the reduction of Ca2+e during ischemia was very rapid, 90% of the reduction occurring within 4.7 s. Normoglycemic animals showed a slower rate of reduction of Ca2+e. Hyperglycemic animals displayed an even slower rate of reduction and a biphasic response in which the initial, faster influx of Ca2+ was followed by a conspicuously slow one. This second phase led to a very gradual decrease in Ca2+e, a stable level being reached first after 6-7 min. This marked delay in calcium influx during ischemia was very similar in hypercapnic animals, who showed an extracellular pH during ischemia as low as hyperglycemic subjects. The effect of acidosis was duplicated by MK-801, suggesting that low pH reduces calcium influx by blocking NMDA-gated ion channels.

Acidosis induced by hypercapnia exaggerates ischemic brain damage

Although preischemic hyperglycemia is known to aggravate damage due to transient ischemia, it is a matter of controversy whether or not this is a result of the exaggerated acidosis. It has recently been reported that although tissue acidosis of a comparable severity could be induced in normoglycemic dogs by an excessive rise in arterial CO2 tension, short-term functional recovery was improved, rather than compromised. In the present experiments we induced excessive hypercapnia (PaCO2, approximately 300 mm Hg) in normoglycemic rats before inducing forebrain ischemia of 10-min duration. This reduced the brain extracellular pH to values normally encountered in hyperglycemic rats subjected to ischemia. The events induced by hypercapnia clearly enhanced ischemic brain damage, as assessed histologically after 7 days of recovery. We hypothesize that the decisive event was an exaggerated decrease in extra- and intracellular pH and that the results thus demonstrate an adverse effect of acidosis. However, since postischemic seizures did not occur in the hypercapnic ischemic rats, the results also demonstrate that changes in intra-extracellular pH and bicarbonate concentrations modulated ischemic damage in an unexpected way.

The influence of repeated spreading depression-induced calcium transients on neuronal viability in moderately hypoglycemic rats

The calcium transients which are associated with spreading depression (SD) do not lead to neuronal necrosis, even if the SDs are repeated over hours. We have previously shown that a restriction of energy production by moderate hypoglycemia prolongs the calcium transients during SD. In the present experiments, we explored whether such prolonged transients lead to neuronal necrosis. To that end, SDs were elicited for 2 h by topical application of KCl in anesthetized rats at plasma glucose concentrations of 6, 3, and 2 mM. The animals were then allowed to recover, and they were studied histopathologically after 7 days. In two other groups, hypoglycemic coma of 5 min duration (defined in terms of the d.c. potential shift) was induced either without or with a preceding train of SDs. These animals were also evaluated with respect to histopathological alterations. SDs elicited for 2 h did not give rise to neuronal damage when elicited at plasma glucose concentration of 6 mM, and, of the animals maintained at 3 and 2 mM, only a few animals showed (mild) damage. In general, therefore, repeated SDs with calcium transients of normal or increased duration fail to induce neuronal damage. The results suggest that, if calcium transients are responsible for a gradual extension of the infarct into the penumbra zone of a focal ischemic lesion some additional pathophysiological factors must be present, such as overt energy failure, acidosis, or microvascular damage. A hypoglycemia-induced calcium transient of 5 min duration gave no or only moderate neuronal damage. However, if a series of SDS were elicited in the precoma period, the damage was exaggerated.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain calcium metabolism in hypoglycemic coma

The present experiments were designed to provide information on brain calcium metabolism during hypoglycemic coma. We specifically wished to evaluate changes in extracellular calcium concentration (Ca2+e) during prolonged hypoglycemic coma and recovery and to assess whether Ca2+e falls to similar values during hypoglycemia and ischemia. To that end, Ca2+e and K+e in neocortical tissue were recorded by ion-sensitive microelectrodes during hypoglycemic coma of 30 min duration and during 15 min of recovery. Cardiac arrest ischemia was induced either at the end of the period of hypoglycemia or after 15 min of recovery. Hypoglycemic coma, as reflected by a DC potential shift and by cellular release of K+, was accompanied by a sustained decrease in Ca2+e from approximately 1.2 to approximately 0.02 mM, i.e., to approximately 1% of control. Infusion of glucose was followed by a biphasic recovery of Ca2+e, starting within 2 min of infusion. During the first phase, completed within the initial 3-4 min, Ca2+e rose to about 25% of control. During the second phase, Ca2+e slowly increased toward normal within 25-30 min. Ischemia, when induced at the end of the period of hypoglycemia, was accompanied by a rise in Ca2+e to about 0.1 mM, i.e., about 10% of control. A similar value was recorded when ischemia was induced after 15 min of recovery following a 30-min hypoglycemic coma. Although the present results do not give information on Ca2+i during hypoglycemic coma, it is tempting to conclude that partial preservation of the nucleoside triphosphate stores, and absence of acidosis, allow some binding and sequestration of the calcium entering the cell.(ABSTRACT TRUNCATED AT 250 WORDS)

Recovery of mitochondrial and plasma membrane function following hypoglycemic coma: coupling of ATP synthesis, K+ transport, and changes in extra- and intracellular pH

The primary objective of the present study was to evaluate the recovery of plasma and mitochondrial membrane functions after 30 min of hypoglycemic coma and to establish whether a lingering accumulation of free fatty acids (FFAs) delays the recovery. A secondary objective was to study whether production of metabolic acids following glucose infusion leads to a fall in intracellular pH (pHi). Phosphocreatine, creatine, ATP, ADP, and AMP, as well as glycogen, glucose, lactate, pyruvate, and FFAs of rat brain cortex and caudoputamen were measured, and "free" ADP was calculated from the creatine kinase equilibrium. Extracellular pH (pHe) and K+ concentration (K+e) were measured with ion-sensitive microelectrodes, and pHi was derived by the CO2 method. Glucose injection was followed by resumption of oxidative phosphorylation within approximately 2 min and by an equally rapid restoration of normal K+e levels. These functions recovered although tissue FFAs remained elevated for at least 7-8 min. Tissue lactate content increased only moderately and production of metabolic acids did not lead to intracellular acidosis. After 15 min of recovery, pHi was moderately increased, although pHe fell toward 7.0. It is speculated that the dissociation between intra- and extra-cellular pH is compatible with an up-regulation of an Na+/H+ antiporter, e.g., by phosphorylation.

Coupling of cellular energy state and ion homeostasis during recovery following brain ischemia

The present experiments were undertaken to explore the relationship between recovery of cerebral energy state following transient ischemia, and resumption of Na+/K+ transport, as this is reflected in changes in extracellular K+ concentration ([K+]c). Cerebral energy state was evaluated by measurements of cerebral cortical concentrations of phosphocreatine (PCr), ATP, ADP, and AMP at the end of 15 min of severe, incomplete ischemia, as well as after 2 and 5 min of recirculation. Derivation of intracellular pH (pHi) allowed calculation of 'free' ADP (ADPf) and AMP (AMPf) concentrations. Changes in [K+]e were measured by an ion-sensitive microelectrode. The results showed that tissue ATP concentration, which was close to zero after 15 min of ischemia, rose to 30% of control after 2 min, and to 60% of control after 5 min of recirculation. However, since the adenine nucleotide pool was reduced by the ischemia the latter value represents extensive or complete phosphorylation of that pool, as reflected in a normalized ATP/ADPf ratio. During recirculation, the concentration of pyruvate rose, but the lactate content remained unchanged, suggesting that the substrate for oxidative metabolism was exogenous glucose. Resumption of Na+/K+ transport, as reflected in the [K+]e began after 2-3 min, and a normal [K+]e was attained within 5 min. The results demonstrate that transport of Na+ and K+ is resumed at tissue ATP concentrations which are only 30-40% of control. It is discussed whether this reflects relatively extensive rephosphorylation of the remaining adenine nucleotide pool, or if compartmentation of adenine nucleotides exists during recirculation.

The influence of insulin-induced hypoglycemia on the calcium transients accompanying reversible forebrain ischemia in the rat

The primary objective of this study was to explore why preischemic hypoglycemia, which restricts tissue acidosis during the ischemic insult, does not ameliorate cell damage incurred as a result of transient ischemia. The question arose whether hypoglycemia (plasma glucose concentration 2-3 mM) delays resumption of extrusion of Ca2+ from cells during recirculation. Measurements of extracellular Ca2+ concentration during forebrain ischemia of 15 min duration proved that this was the case. Thus, normoglycemic animals resumed Ca2+ extrusion upon recirculation after a delay of 1.5-2.0 min, and hypoglycemic ones after an additional delay which could amount to 3-4 min. We attempted to explore the cause of this delay. At first sight, the results suggested that resumption of oxidative phosphorylation upon recirculation was substrate limited. However, glucose infusion during ischemia or just after recirculation failed to accelerate Ca2+ extrusion from the cells. A comparison between non-injected and insulin-injected animals at equal plasma glucose concentrations suggested that insulin was responsible for the delay. On analysis, the delay proved to be related to a sluggish recovery of cerebral blood flow. The results suggest that when cell damage is evaluated after transient ischemia in hypo- and normoglycemic subjects, attention should be directed to the period of cell calcium 'overload'. Unobserved differences in the duration of the calcium transient may also confound interpretation of data on the effects of insulin.