Pathophysiology of ischemic cell death: II. Changes in plasma membrane permeability and cell volume

Intraventricular haemorrhage in premature infants: the role of immature neuronal salt and water transport

Intraventricular haemorrhage is a common complication of premature birth. Survivors are often left with cerebral palsy, intellectual disability and/or hydrocephalus. Animal models suggest that brain tissue shrinkage, with subsequent vascular stretch and tear, is an important step in the pathophysiology, but the cause of this shrinkage is unknown. Clinical risk factors for intraventricular haemorrhage are biomarkers of hypoxic-ischaemic stress, which causes mature neurons to swell. However, immature neuronal volume might shift in the opposite direction in these conditions. This is because immature neurons express the chloride, salt and water transporter NKCC1, which subserves regulatory volume increases in non-neural cells, whereas mature neurons express KCC2, which subserves regulatory volume decreases. When hypoxic-ischaemic conditions reduce active ion transport and increase the cytoplasmic membrane permeability, the effects of these transporters are diminished. Consequentially, mature neurons swell (cytotoxic oedema), whereas immature neurons might shrink. After hypoxic-ischaemic stress, in vivo and in vitro multi-photon imaging of perinatal transgenic mice demonstrated shrinkage of viable immature neurons, bulk tissue shrinkage and blood vessel displacement. Neuronal shrinkage was correlated with age-dependent membrane salt and water transporter expression using immunohistochemistry. Shrinkage of immature neurons was prevented by prior genetic or pharmacological inhibition of NKCC1 transport. These findings open new avenues of investigation for the detection of acute brain injury by neuroimaging, in addition to prevention of neuronal shrinkage and the ensuing intraventricular haemorrhage, in premature infants.

Attempts at the Characterization of In-Cell Biophysical Processes Non-Invasively-Quantitative NMR Diffusometry of a Model Cellular System

In the literature, diffusion studies of cell systems are usually limited to two water pools that are associated with the extracellular space and the entire interior of the cell. Therefore, the time-dependent diffusion coefficient contains information about the geometry of these two water regions and the water exchange through their boundary. This approach is due to the fact that most of these studies use pulse techniques and relatively low gradients, which prevents the achievement of high b-values. As a consequence, it is not possible to register the signal coming from proton populations with a very low bulk or apparent self-diffusion coefficient, such as cell organelles. The purpose of this work was to obtain information on the geometry and dynamics of water at a level lower than the cell size, i.e., in cellular structures, using the time-dependent diffusion coefficient method. The model of the cell system was made of baker’s yeast (Saccharomyces cerevisiae) since that is commonly available and well-characterized. We measured characteristic fresh yeast properties with the application of a compact Nuclear Magnetic Resonance (NMR)-Magritek Mobile Universal Surface Explorer (MoUSE) device with a very high, constant gradient (~24 T/m), which enabled us to obtain a sufficient stimulated echo attenuation even for very short diffusion times (0.2–40 ms) and to apply very short diffusion encoding times. In this work, due to a very large diffusion weighting (b-values), splitting the signal into three components was possible, among which one was associated only with cellular structures. Time-dependent diffusion coefficient analysis allowed us to determine the self-diffusion coefficients of extracellular fluid, cytoplasm and cellular organelles, as well as compartment sizes. Cellular organelles contributing to each compartment were identified based on the random walk simulations and approximate volumes of water pools calculated using theoretical sizes or molar fractions. Information about different cell structures is contained in different compartments depending on the diffusion regime, which is inherent in studies applying extremely high gradients.

Mitochondrial abnormality associates with type-specific neuronal loss and cell morphology changes in the pedunculopontine nucleus in Parkinson disease

Cholinergic neuronal loss in the pedunculopontine nucleus (PPN) associates with abnormal functions, including certain motor and nonmotor symptoms. This realization has led to low-frequency stimulation of the PPN for treating patients with Parkinson disease (PD) who are refractory to other treatment modalities. However, the molecular mechanisms underlying PPN neuronal loss and the therapeutic substrate for the clinical benefits following PPN stimulation remain poorly characterized, hampering progress toward designing more efficient therapies aimed at restoring the PPN's normal functions during progressive parkinsonism. Here, we investigated postmortem pathological changes in the PPN of PD cases. Our study detected a loss of neurons producing gamma-aminobutyric acid (GABA) as their output and glycinergic neurons, along with the pronounced loss of cholinergic neurons. These losses were accompanied by altered somatic cell size that affected the remaining neurons of all neuronal subtypes studied here. Because studies showed that mitochondrial dysfunction exists in sporadic PD and in PD animal models, we investigated whether altered mitochondrial composition exists in the PPN. A significant up-regulation of several mitochondrial proteins was seen in GABAergic and glycinergic neurons; however, cholinergic neurons indicated down-regulation of the same proteins. Our findings suggest an imbalance in the activity of key neuronal subgroups of the PPN in PD, potentially because of abnormal inhibitory activity and altered cholinergic outflow.

Extracellular diffusion parameters in the rat somatosensory cortex during recovery from transient global ischemia/hypoxia

Changes in the extracellular space diffusion parameters during ischemia are well known, but information about changes during the postischemic period is lacking. Extracellular volume fraction (alpha) and tortuosity (lambda) were determined in the rat somatosensory cortex using the real-time iontophoretic method; diffusion-weighted magnetic resonance imaging was used to determine the apparent diffusion coefficient of water. Transient ischemia was induced by bilateral common carotid artery clamping for 10 or 15 mins and concomitant ventilation with 6% O(2) in N(2). In both ischemia groups, a negative DC shift accompanied by increased potassium levels occurred after 1 to 2 mins of ischemia and recovered to preischemic values within 3 to 5 mins of reperfusion. During ischemia of 10 mins duration, alpha typically decreased to 0.07+/-0.01, whereas lambda increased to 1.80+/-0.02. In this group, normal values of alpha=0.20+/-0.01 and lambda=1.55+/-0.01 were registered within 5 to 10 mins of reperfusion. After 15 mins of ischemia, alpha increased within 40 to 50 mins of reperfusion to 0.29+/-0.03 and remained at this level. Tortuosity (lambda) increased to 1.81+/-0.02 during ischemia, recovered within 5 to 10 mins of reperfusion, and was increased to 1.62+/-0.01 at the end of the experiment. The observed changes can affect the diffusion of ions, neurotransmitters, metabolic substances, and drugs in the nervous system.

NMR spectroscopic characterization of sarcolemmal permeability during myocardial ischemia and reperfusion

This study aims to characterize the pattern of membrane disintegration during myocardial ischemia and reperfusion. Intracellular volumes were measured by 1H and 59Co NMR in isolated rat hearts during 10, 30 and 60 min of total ischemia and 30 min of reperfusion at normothermia. Perfusion with hypo-osmotic medium (210 mosm/l) increased intracellular water from 2.50+/-0.06 to 3.07+/-0.07 ml/g dry weight (P<0.001) during pre-ischemia. Hypo-osmotic swelling decreased by 16+/-3, 32+/-6 and 44+/-11% of the pre-ischemic value after 10, 30 and 60 min of ischemia (n.s., P<0.005, P<0.001) respectively, indicating that membrane permeabilization facilitated efflux of osmolytes and counterbalanced the osmotic driving force for water influx. Hypo-osmotic swelling decreased during 30 min of reperfusion by 18+/-5% in all groups (P<0.0.005 v post-ischemia). The volume of distribution of the extracellular marker cobalticyanide increased by more than 3.2+/-0.4 and 5.8+/-0.5% of the intracellular space after 30 and 60 min of ischemia respectively (P<0.001), and by an additional 2% after reperfusion. During 30 min of reperfusion, hearts released 1.6+/-0.2 and 3.2+/-0.4% of the intracellular creatine kinase contents after 30 and 60 min of ischemia, respectively (P<0.001). In addition to the correlation between ischemia duration and membrane permeability, evident from the analysis of each probe, the data showed a progressive increase in severity of membrane injury over time and permeabilization to larger molecules. 23Na NMR spectroscopy in conjunction with an extracellular shift reagent (SR) showed formation of a resonance at an intermediate chemical shift in between the intra and extracellular Na+ peaks, suggesting penetration of SR into cells with disrupted membranes. The constant chemical shift and narrow line shape of this resonance, characteristic of a homogeneous chemical environment, suggested that the distribution of SR was contained within the cytosol of cardiomyocytes. We propose that sarcolemmal membranes are gradually permeabilized to larger molecules by ischemia, and the evolving chemical instability is spatially contained within the myocyte.

Effect of lactacidosis on cell volume and intracellular pH of astrocytes

Acute traumatic or ischemic cerebral lesions are associated with tissue acidosis leading to cytotoxic brain edema, predominantly affecting astrocytes. Glial swelling from acidosis is believed to be the attempt of cells to maintain a physiological intracellular pH (pHi). However, this concept, potentially important for the development of new treatment strategies for cytotoxic brain edema, has not been validated experimentally. In the present study, cell volume and pHi of astrocytes were measured simultaneously in vitro. Exposure of suspended astrocytes to levels of acidosis found in vivo during ischemia and trauma (pH 6.8-6.2) led to a maximal increase in cell volume of 121.2% after 60 min (n = 5, p < 0.05) and to immediate intracellular acidification close to extracellular levels (pH 6.2, n = 5, p < 0.05). Inhibition of membrane transporters responsible for pHi regulation (0.1 mM amiloride for the Na+/H+ antiporter or 1 mM SITS for HCO3- -dependent transporters) inhibited cell swelling from acidosis but did not affect the profound intracellular acidification. In addition, acidosis-induced cell swelling and intracellular acidification were partly prevented by the addition of ZnCl2 (0.1 mM), an inhibitor of selective proton channels not yet described in astrocytes (n = 5, p < 0.05). In conclusion, these data demonstrate that glial swelling from acidosis is not a cellular response to defend the normal pHi, as had been thought. If these results obtained in vitro are transferable to in vivo conditions, the development of blood-brain barrier-permeable agents for the inhibition of acidosis-induced cytotoxic edema might be therapeutically useful, since they do not enhance intracellular acidosis and thus cell damage.

An in vitro rabbit retina model to study electrophysiologic and metabolic function during and following ischemia

Most in vitro studies involving neuronal ischemia use biochemical measures and/or cell counting to assess cellular death. We describe an in vitro rabbit retina model in which we measured glucose utilization, lactate production, and light-evoked compound action potentials (CAPs) to assess metabolic and functional recovery following ischemia. Under control conditions, retinal glucose utilization and lactate production (n = 7), as well as CAPs (n = 8) remained quite constant for 6-8 h. During ischemia (glucose reduced from 6 to 1 mM and oxygen from 95 to 15%), glucose utilization and lactate production fell to 50%. CAPs fell to 50% in 3-4 min, and to 0% in 8-10 min. Recovery during 3-4 h of 'return-to-control' was dependent upon the length of ischemia. Glucose utilization recovered to 63% after 1 h (n = 4) and to 18% after 2 h of ischemia (n = 6, P < 0.001). Lactate production recovered to 77% after 1 h (n = 4) and to 54% after 2 h of ischemia (n = 6, P < 0.001). CAPs returned to 51, 15, and 0.13% of the control responses after 0.5 h (n = 7), 1 h (n = 8), and 2 h (n = 5) of ischemia, respectively (P < 0.001). This avascular, blood-brain barrier-free preparation provides an opportunity to use both metabolic and functional criteria to test protection against neuronal ischemia.

Disruption of mitochondrial respiration inhibits volume-regulated anion channels and provokes neuronal cell swelling

Hypoxia and inhibitors of mitochondrial respiration impair the regulatory volume decrease (RVD) of cerebellar granule neurons after hypotonic swelling. RVD is linked to the opening of volume-regulated anion channels (VRACs). VRACs are outwardly rectifying, inactivate slowly during maintained depolarization, and are permeable to the cellular organic osmolyte taurine. Channel activation requires nonhydrolytic ATP binding and is not modulated by intracellular ADP. VRAC opening is reversibly depressed by hypoxia and by mitochondrial inhibitors such as oligomycin, rotenone, and antimycin A. These results demonstrate that neuronal VRAC activation and swelling are both tightly linked to cellular energy. Moreover, the findings reported in this work may have a particular significance for inherited mitochondrial human diseases, such as mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS), which cause brain swelling and edema.

The role of L-type voltage dependent calcium channels in stimulated [3H]norepinephrine release from canine hippocampal slices following global cerebral ischemia and reperfusion

The hippocampus is among those brain regions which are selectively vulnerable to ischemic damage. Hippocampal damage due to transient cerebral ischemia is mainly of the delayed, non-necrotic type which may arise after disruption or activation of specific cellular systems, including transmitter release through excitatory amino acid receptors. We investigated the contribution of L-type voltage dependent calcium channels (VDCCs) to glycine (GLY) potentiated N-methyl-D-aspartate (NMDA) receptor- and potassium-stimulated [3H]norepinephrine (NE) release in a canine model of global cerebral ischemia and reperfusion. Tissue was collected from four experimental groups: non-arrested controls (NA), global cerebral ischemia induced by 10 minute cardiac arrest (CA), and CA followed by 30 min or 24 hours reperfusion after restoration of spontaneous circulation. Brain slices prepared from all groups accumulated approximately equivalent amounts of [3H]NE. The sensitivity of [3H]NE release to stimulation by NMDA/GLY or elevated potassium was unchanged after ischemia and reperfusion. About 30% of release stimulated by the addition of 20 mM potassium was inhibited by the NMDA receptor-operated channel antagonist MK801 in all groups except CA in which only 4% of release was inhibited by MK801. The ability of 1 microM nitrendipine (NTP) to block stimulated release indicated that the contribution of the L-type VDCC to potassium or NMDA/GLY-stimulated release was significant only in NA and 24 hour reperfused animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Channel shutdown: a response of hippocampal neurons to adverse environments

Stretch-activated ion channels have been discovered in the membrane of many types of cells, but their presence in neurons is uncertain. We used freshly dissociated rat hippocampal neurons to study the effect of hypotonic swelling but, surprisingly, the isolated neurons did not swell. Voltage-dependent whole-cell membrane currents mediated by K+, Na+ and Ca2+ were rapidly and reversibly suppressed during sudden exposure to strongly hypo-osmotic, hyper-osmotic or glucose deficient solutions. The amplitudes of the sustained components of K+ and Ca2+ currents were more depressed than transient currents, but the rate of decay of transient K+ current greatly accelerated. The voltage dependence of activation and of steady state inactivation of residual K+ and Ca2+ currents were not shifted. The current holding membrane potential at -70 mV and therefore the conductance at that voltage were unchanged or somewhat decreased. Capacitive (charging) membrane current was not affected. Changes in tail current suggested moderate loss of cytosolic K+ in some but not in all cells. We conclude that channel shutdown is a uniform response of neuron somata and proximal dendrites to various adverse environments. Hypothetically we propose that swelling was prevented in anisosmotic conditions because membrane water permeability decreased.

Early cellular swelling during cerebral ischemia in vivo is mediated by excitatory amino acids released from nerve terminals

This study demonstrates ischemic cellular swelling in vivo detected as changes in the concentration of 14C-sucrose pre-perfused into the extracellular space (ECS) as an ECS marker. Microdialysis was utilized as a means of perfusion and measurement of the extracellular concentration of 14C-sucrose ([14C-sucrose]e). Concomitant with an abrupt increase in [K+]e at 1-3 min following the ischemia induction, [14C-sucrose]e was also rapidly elevated. Since sucrose is not taken up by either cells or capillaries, the absolute amount of 14C-sucrose in the ECS must be unchanged. The increase therefore appears to represent a relative decrease in water volume in the ECS resulting from a movement of water into the cells, i.e. cellular swelling. Ca(2+)-free perfusate containing Co2+, which has been shown to block excitatory amino acid release during cerebral ischemia, significantly delayed the increase in [14C-sucrose]e and [K+]e. Kynurenic acid, a broad-spectrum antagonist of excitatory amino acids, administered in situ through the dialysis probe also significantly delayed the increase in [14C-sucrose]e and [K+]e. These findings indicate that the early cellular swelling occurring during cerebral ischemia is a result of massive ionic fluxes mediated by excitatory amino acids which are released by a Ca(2+)-dependent exocytotic process from the nerve terminals.

Early postischemic 45Ca accumulation in rat dentate hilus

Several studies have found postischemic regional accumulation of calcium to be time-dependent and coincident with the progression of ischemic cell change. In the most vulnerable cells in the hippocampus one would therefore expect to find a primary and specific early uptake of calcium after ischemia. Autoradiograms of 45Ca and 3H-inulin distribution were investigated before and 1 h after 20 min ischemia in the rat hippocampus. Two different methodological approaches were used for administration of 45Ca: (a) administration via microdialysis probes, (b) intraventricular injection. During control conditions the 45Ca autoradiograms showed variations in distribution volume in accordance with 3H-inulin determination of extracellular space size. One hour after ischemia a massive accumulation of 45Ca was found in the dentate hilus. No change in the distribution pattern of 3H-inulin could be demonstrated 1 h after ischemia. We suggest that 45Ca accumulation in dentate hilus 1 h after ischemia is a result of increased Ca2+ uptake before irreversible cell damage occurs and is not due to passive influx of calcium across a leaky plasma membrane.

Molecular mechanisms of glial swelling in vitro

The pathophysiological chain of events occurring during cerebral ischemia is still poorly understood on a molecular level. Therefore, an in vitro model to study glial swelling mechanisms, using C6 glial cells under controlled extracellular conditions, has been established. Flow cytometry serves to determine even small cell volume changes. In this report, the effects of anoxia and acidosis on glial swelling are summarized. Anoxia alone, or in combination with iodoacetate to inhibit anaerobic glycolysis, did not cause an increase of glial volume for up to 2 h. Acidification of the incubation medium below pH 6.8, on the other hand, was immediately followed by cell swelling to 115% of normal. Amiloride or the absence of bicarbonate and Na+ in the medium significantly reduced glial swelling. The data support the contention that swelling results from an activation of the Na+/H+-antiporter to control intracellular pH. It is suggested that swelling in an ischemic penumbra is promoted by this mechanism. Therapeutic approaches to control cerebral pH might be useful to protect brain tissue in cerebral ischemia.

Acetylcholine-synthesizing amacrine cells: identification and selective staining by using radioautography and fluorescent markers

The fluorescent DNA stain 4,6,diamidino-2-phenylindole (DAPI) was applied to the cut axons of the rabbit optic tract, from which it was retrogradely transported to the retinal ganglion cell bodies. The labelled retinas were isolated from the eye and maintained in vitro in the presence of [3H]choline. They were then quick-frozen, freeze-dried, vacuum-embedded, and radioautographed on dry emulsion for identification of the acetylcholine-synthesizing cells. Inspection of the radioautographs by fluorescence microscopy showed the two labels not to co-exist: the cells that contained the transported fluorescence did not contain radioactive acetylcholine. In other animals the optic nerve was sectioned, causing retrograde degeneration of a large fraction of the ganglion cells. A population of small, round neurons in the ganglion cell layer was spared. These retinas synthesized [3H]acetylcholine at the same rate as control tissues; and radioautography showed an identical distribution of the acetylcholine-synthesizing cells. We conclude that the acetylcholine-synthesizing neurons of the ganglion cell layer are displaced amacrine cells. When DAPI was injected intraocularly instead of being applied to the optic tract, a regular mosaic of neurons in the ganglion cell layer was selectively stained, and two bands of fluorescence were observed in the inner plexiform layer, at the level where two bands of radioactive acetylcholine were observed in radioautographs. Quantitative analysis showed that the DAPI-stained cells were the same size as those that survive optic nerve section. Like the acetylcholine-synthesizing cells, they appear to be displaced amacrines; when wheatgerm agglutinin labelled by Evans blue was applied to the optic tract and DAPI was injected intraocularly, the red fluorescence of Evans blue and the blue fluorescence of DAPI accumulated in different cells. When DAPI was injected intraocularly and radioautography for acetylcholine was carried out, the cells brightly labelled by DAPI were found to have synthesized acetylcholine. We conclude that topically applied DAPI selectively labels the acetylcholine-synthesizing neurons of the ganglion cell layer. The distribution of the acetylcholine-synthesizing cells was established by counting the DAPI-labelled cells in whole-mounts.(ABSTRACT TRUNCATED AT 400 WORDS)

Ion and membrane changes in the brain during anoxia

Anoxia has two main effects on the brain, a rapid, reversible loss of function and permanent damage when the period of anoxia exceeds a critical length of time. The initial loss of function is related to a K+-conductance increase of the nerve membrane, leading to reduction of membrane resistance and hyperpolarization. After a few minutes, a non-selective increase of membrane permeability mediates rapid transfer of ions between the intra- and extracellular spaces. The subsequent rise of intracellular Ca2+ concentration may be responsible for the nerve cell death.

Earliest irreversible changes during ischemia

The ability to synthesize new protein was used as a marker of irreversible neuronal injury in experiments with isolated rabbit retinas exposed to various types of ischemic insult. The retinal neurons were able to fully recover their protein synthetic capacity after 20 min of complete ischemic anoxia, but not after 30 min. There was better toleration to either isolated substrate deprivation or complete anoxia than to both together. Increasing extracellular Mg2+ prolonged toleration to complete ischemic-anoxia. Removing Ca2+ completely from the extracellular fluid exacerbated injury. Moreover, increasing extracellular volume improved toleration to the combined insult. This experiment suggests that injured neurons may elaborate cytotoxic compounds into the extracellular fluid. This suggestion was confirmed by further experiments demonstrating exacerbation of injury following minimum insults when the retina was incubated with other already extensively damaged tissue.

Pathophysiology of ischemic cell death: I. Time of onset of irreversible damage; importance of the different components of the ischemic insult

Rabbit retina was used as an example of organized central nervous tissue in in vitro experiments designed to characterize the onset of cell death from ischemia. Retinas were subjected to progressively longer periods of different types of ischemic insult and then given an opportunity to recover before being tested for irreversible damage, using failure to reinstitute protein synthesis as the principal criterion. Anoxia was more damaging than substrate deprivation, but they were synergistic in combination. Restricting the volume of extracellular fluid during the combined deprivation, to simulate complete circulatory arrest in vivo, caused irreversible damage to occur even sooner. The cells were able to recover from 20 min of the complete ischemia, but it took them more than 2 h to do so. After 30 min, there was extensive irreversible damage. Loss of viability was usually associated with failure to reinstitute energy metabolism, as assessed by 2-deoxyglucose uptake. Under some circumstances loss of viability may have been the consequence of the failed energy metabolism. Increasing medium Mg++, prior to ischemia, to levels that greatly reduce energy requirements caused a significant improvement in the recovery of 2-deoxyglucose uptake.