Use of 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid (5FBAPTA) in the measurement of free intracellular calcium in the brain by 19F-nuclear magnetic resonance spectroscopy

Is there a path beyond BOLD? Molecular imaging of brain function

The dependence of BOLD on neuro-vascular coupling leaves it many biological steps removed from direct monitoring of neural function. MRI based approaches have been developed aimed at reporting more directly on brain function. These include: manganese enhanced MRI as a surrogate for calcium ion influx; agents responsive to calcium concentrations; approaches to measure membrane potential; agents to measure neurotransmitters; and strategies to measure gene expression. This work has led to clever design of molecular imaging tools and many contributions to studies of brain function in animal models. However, a robust approach that has potential to get MRI closer to neurons in the human brain has not yet emerged.

The Phenomenon of “Pe-ischaemic Conditioning” in the Brain only Partly involves the NMDA Receptor: A Magnetic Resonance Study

We have investigated in more detail our previous observations on a form of ischaemic pre-conditioning "metabolic adaptation", i.e.--that sequential metabolic insults (hypoxia followed 40 min later by combined hypoxia + hypoglycaemia, or vice versa) are less injurious (monitored by increased [Ca2+]i and decreased PCr) than the immediate combined insult. We have now observed that the "adaptation" occurs between 10 and 20 min. Pre-treatment of the tissues with 10 microM-MK801 showed that it had no effect on the increase in [Ca2+]i caused by the sequential insult and only partially blocked the increase observed by exposure to the immediate combined insult. Exposure to both the delayed and immediate combined insults with low extracellular Ca2+ resulted in a two-fold increase in [Ca2+]i, similar to the increase observed with normal extracellular Ca2+ in the presence of MK801. The results are discussed in terms of the possible origins of the increases in [Ca2+]i.

Hypo-osmotic swelling-activated release of organic osmolytes in brain slices: implications for brain oedema in vivo

A decrease in the intracellular levels of osmotically active species has invariably been seen after swelling of mammalian brain tissue preparations. The exact identity of the species, and the manner of their decrease, remain to be described. We investigated the swelling-activated decrease of organic osmolytes in rat cortical brain slices using (1)H- and (31)P-magnetic resonance spectroscopy. We found that acute hypo-osmotic shock causes decreases in the levels of a range of intracellular amino acids and amino acid derivatives, N-acetyl-aspartate, creatine, GABA, glutamate, hypotaurine, and also in the levels of the methylamines glycerol-phosphorylcholine, phosphorylcholine and choline. Incubation of cortical slices with the anion channel blockers niflumic acid and tamoxifen caused inhibition of organic osmolyte efflux, suggesting that such osmolyte efflux occurs through anion channels. Intracellular phosphocreatine was also seen to decrease during acute hypo-osmotic superfusion, although intracellular ATP remained constant. In addition, the acidification of an intracellular compartment was observed during hypo-osmotic superfusion. Our results suggest a link between brain energy reserve and brain osmoregulation.

Sepsis increases intracellular free calcium in brain

Using magnetic resonance methods and a clinically relevant rodent model of sepsis, we have made in vivo measurements of increased intracellular calcium in a pathologic state in the CNS. The intracellular calcium concentration was increased nearly twofold in septic rat brain compared with controls (p < 0.0001). This result, in a fully intact functioning mammalian system, ties together a previous spectrum of indirect evidence from numerous laboratories suggesting an important role for elevated intracellular calcium in sepsis. In addition, levels of the proinflammatory cytokine tumor necrosis factor-a were elevated threefold in septic rat brain (p < 0.02), and electron microscopic examination revealed scattered injury in approximately 0.25% of glial cells. These findings are discussed in light of the current understanding of the pathophysiology of sepsis.

Magnetic resonance spectroscopy studies on changes in cerebral calcium and zinc and the energy state caused by excitotoxic amino acids

Under control conditions, superfused hippocampal slices exhibited a significantly higher phosphocreatine (PCr)/ATP ratio than cortical slices; the evidence suggests that this is due to lower concentrations of ATP, rather than higher concentrations of PCr. Glutamate caused relatively rapid decreases in PCr and ATP levels to approximately 45%, accompanied or immediately followed by an increased free intracellular calcium concentration ([Ca2+]i) and the release of Zn2+ in the cortex. In the hippocampus PCr and ATP decreased further to approximately 20% of control values, but the changes in [Ca2+]i and Zn2+ content were slower. This is in contrast to the effects of depolarisation, which produced the same rapid changes in the energy state and [Ca2+]i, with no detectable Zn2+, in both tissues. NMDA causes effects similar to those of glutamate in the cortex (decreases in the energy state, increased [Ca2+]i, and release of Zn2+). Pretreatment of the cortex for 1 h with the NMDA blocker MK-801 prevented all of the observed effects of NMDA. In contrast, pretreatment with MK-801 had no detectable effect on the increase in [Ca2+]i or the decreases in PCr and ATP caused by glutamate, although it prevented the release of zinc. The results are discussed in relation to the function of the NMDA subtype of glutamate receptor in excitotoxicity.

Intracellular chelation of calcium prevents cell damage following severe hypoxia in the rat cerebral cortex as studied by NMR spectroscopy ex vivo

Nuclear magnetic resonance (NMR) spectroscopy was used to quantify metabolic recovery (by 31P NMR) and neuronal damage (by 1H NMR) following aglycaemic hypoxia in superfused cortical brain slices. Slices were incubated either in the absence or presence of a cell-permeant Ca2+ chelator, 1,2-bis-(2-amino-phenoxy)ethane-N,N,N',N'-tetra-acetic acid acetoxy ester (BAPTA-AM) before exposure to hypoxia in the presence or absence of 1.2 mM Ca2+. Hypoxia in the presence of Ca2+ resulted in metabolic damage as well as time-dependent reduction of a neuronal metabolite, N-acetyl aspartate. The recovery was improved only temporarily by BAPTA under these conditions. Hypoxia in the absence of external Ca2+ did not cause any detectable signs of damage in BAPTA-loaded slices. These data show that combined inhibition of influx and intracellular chelation of Ca2+ render the brain cortex tolerable to severe energy failure.

Excitotoxic amino acids cause appearance of magnetic resonance spectroscopy-observable zinc in superfused cortical slices

(1) The effects of glutamate and NMDA on the free intracellular calcium concentration ([Ca2+]i) have been followed in superfused cortical slices using the 19F-magnetic resonance indicator 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid (5FBAPTA). (2) Glutamate (0.5 or 1 mM) caused a 75-100% increase in [Ca2+]i, and a new resonance was attributed to zinc-5FBAPTA, which was confirmed from its disappearance in the presence of a high-affinity chelator of heavy metals, N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine. The appearance of zinc occurred with or just after the rise in [Ca2+]i and was independent of Mg2+. (3) NMDA, N-methyl-DL-aspartate, or N-methyl-L-aspartate (10-200 microM) caused a slower increase in [Ca2+]i, and zinc was observed in some but not all experiments. When present, zinc appeared later than the increase in [Ca2+]i. These changes were also independent of Mg2+. (4) Decreases in both phosphocreatine and ATP were observed in all of these studies. (5) The results are discussed in terms of the proposed role of zinc as a modulator of excitotoxicity. Observations of zinc after exposure to glutamate or more slowly to NMDA, but not after depolarisation or deprivation of glucose and O2 (where increases also occur in [Ca2+]i), suggest that the cellular damage caused by the latter insults (depolarisation and fuel deprivation as in ischaemia) involves mechanisms not solely attributable to release of excitotoxins.

19F NMR calcium changes, edema and histology in neonatal rat brain slices during glutamate toxicity

Respiring neonatal cerebrocortical slices (350 microns thick), loaded with the free calcium indicator 5F-BAPTA, were perfused in a 20-mm-diameter glass NMR tube with oxygenated artificial CSF, exposed to extracellular glutamate and studied at 4.7 Tesla with 19F NMR spectroscopy. 31P/1H NMR spectra, obtained concurrently, were used to assess slice integrity from determinations of intracellular pH, ATP, PCr, lactate and N-acetylaspartate. 60-min periods were induced of recoverable and nonrecoverable glutamate toxicity-defined from changes in NMR metabolites. In other NMR studies, where 5F-BAPTA was not used, metabolic toxicity was modulated by three glutamate receptor antagonists: dizocilpine, NBQX and kynurenic acid. Outcome measurements were made of edema, determined invasively in isolated slices from % swelling and water content and from histological changes in Nissl stains of slice sections. Edema was (1) detectable in all slices within minutes after onset of glutamate exposure, though never in untreated control slices, and (2) modulated differently by dizocilpine, NBQX and kynurenate. Correlations were observed between edema and NMR decreases in PCr and ATP. Nissl stains of sections from slices treated with the most protective agent, dizocilpine, showed preservation of neuronal processes. As was expected in 7-day-old rats with immature NMDA receptors, 19F NMR spectroscopy revealed only small increases in free intracellular calcium ([Ca2+]i). These occurred late during glutamate exposure and reversed early during glutamate washout. The studies demonstrate that it is possible to study correlations between repeated noninvasive NMR spectra in ensembles of brain slices and invasive measures of early cellular responses.

19F nuclear magnetic resonance studies of free calcium in heart cells

19F nuclear magnetic resonance is used in conjunction with 5,5'-difluoro-1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (5FBapta), a fluorinated calcium chelator, to report steady-state intracellular free calcium levels ([Ca2+]i) in populations of resting, quiescent, isolated adult heart cells. 31P nuclear magnetic resonance shows that 5FBapta-loaded cells maintain normal intracellular high-energy phosphates, pH, and free Mg2+. The intracellular free calcium concentration of well perfused, isolated heart cells is 61 +/- 5 nM, measured with 5FBapta, which has a dissociation constant (Kd) for calcium chelation of 500 nM. A similar value is obtained with Quin-MF, another fluorinated calcium chelator with Kd and maximum calcium sensitivity at 80 nM. We find that the steady-state level of intracellular free calcium is increased by decreased extra-cellular sodium concentration, omission of extracellular magnesium, decreased extracellular pH, hyperglycemia, and upon treatment with lead acetate. Further, extracellular ATP caused a large transient increase in [Ca2+]i. Thus, while heart cells maintain a very low level of intracellular free Ca2+, acute alterations in extracellular environment can cause derangement of calcium homeostasis, resulting in measurable increases in [Ca2+]i.

Neither moderate hypoxia nor mild hypoglycaemia alone causes any significant increase in cerebral [Ca2+]i: only a combination of the two insults has this effect. A 31P and 19F NMR study

(1) The energy state and free intracellular calcium concentration ([Ca2+]i) of superfused cortical slices were measured in moderate hypoxia (approximately 65 microM O2), in mild hypoglycaemia (0.5 mM glucose), and in combinations of the two insults using 19F and 31P NMR spectroscopy. (2) Neither hypoxia nor hypoglycaemia alone caused any significant change in [Ca2+]i. Hypoxia caused a 40% fall in phosphocreatine (PCr) content but not in ATP level, and hypoglycaemia produced a slight fall in both (as expected from previous studies). These changes in the energy state recovered on return to control conditions. (3) A combined sequential insult (hypoxia, followed by hypoxia plus hypoglycaemia) produced a 100% increase in [Ca2+]i and a decrease in PCr level to approximately 25% of control. The reverse combined sequential insult (hypoglycaemia, followed by hypoglycaemia plus hypoxia) had the same effect. On return to control conditions there was some decrease in [Ca2+]i and a small increase in PCr content, but neither recovered to control levels. (4) Exposure of the tissue to the combined simultaneous insult (hypoxia plus hypoglycaemia) immediately after the control spectra had been recorded resulted in a fivefold increase in [Ca2+]i and a similar decrease in PCr level to 20-25% of control. There was little if any change of [Ca2+]i or PCr level on return to control conditions. (5) These results are discussed in terms of metabolic adaptation of some but not all of the cortical cells to the single type of insult, which renders the tissues less vulnerable to the combined insult.

Tolerance of low intracellular pH during hypercapnia by rat cortical brain slices: A 31P/1H NMR study

Metabolic tolerance of low intracellular pH (pH(i)) was studied in well-oxygenated, perfused, neonatal, rat cerebrocortical brain slices (350 microns thick) by inducing severe hypercapnia. In each of 17 separate experiments 80 brain slices (approximately 3.2 g wet weight) were suspended in an NMR tube, perfused with artificial CSF (ACSF), and studied at 4.7 T with 31P and 1H NMR spectroscopy. Spectra obtained every 5 min monitored relative concentrations of lactate or high-energy phosphate metabolites, from which pH(i) and extracellular pH were determined. Unperturbed slice preparations were metabolically stable for > 10 h, with no significant changes occurring in pHi, ATP, phosphocreatine (PCr), inorganic phosphate, or lactate. Different levels of hypercapnia were produced by sequentially perfusing slices with the following different ACSF batches, each having previously been equilibrated with a specific mixture of CO2 in oxygen: (a) 10% CO2, 15 min of perfusion; (b) 30% CO2, 15 min of perfusion; (c) 50% CO2, 15 min of perfusion; (d) 70% CO2, 30 min of perfusion; (e) 50% CO2, 15 min of perfusion; (f) 30% CO2, 15 min of perfusion; and (g) 10% CO2, 15 min of perfusion. At the completion of this protocol slices were again perfused with fresh ACSF that was equilibrated with a 95% O2/5% CO2 gas mixture. In each of five separate 1H and 31P experiments, brain slices were recovered within 2 h after termination of exposure to high CO2. The pHi was determined from measurements of the chemical shift difference between phosphoethanolamine and PCr, using a calibration curve obtained for our preparation.(ABSTRACT TRUNCATED AT 250 WORDS)

Nuclear magnetic resonance studies on the effects of decreased external sodium on guinea pig cerebral cortex slices and the permeabilities of various sodium substitutes

Decreasing the external sodium concentration ([Na+]e) to 10 mM in the presence of 280 mM sucrose had no significant effect on phosphocreatine (PCr) or on intracellular pH (pHi) as assessed using 31P nuclear magnetic resonance spectroscopy. Zero [Na+]e in the presence of 300 mM sucrose caused a fall in PCr levels to 50% of control values, and the pHi fell to 6.85 from a control value of 7.30. 1H nuclear magnetic resonance spectroscopy confirmed that the sucrose had not entered the tissue. The decreases in PCr content and in pHi, known to occur on depolarization using 40 mM external potassium concentration ([K+]e), were further decreased in the presence of 10 mM [Na+]e), to 51.4 +/- 4.0 and 6.80 +/- 0.10% of control values, respectively. The free intracellular magnesium concentration was significantly increased from a control value of 0.37 +/- 0.10 mM to 0.66 +/- 0.13 mM (p less than 0.001), when [Na+]e was decreased to 10 mM, but was not further affected by high [K+]e or zero Na+. Membrane permeabilities of the sodium substitutes N-methyl-D-glucamine (NMG), tris(hydroxymethyl)aminomethane (Tris), tetramethylammonium (TMA), and choline were assessed using 1H nuclear magnetic resonance spectroscopy. In the presence of 10 mM [Na+]e, NMG, TMA, and choline (all at 140 mM) were taken up and remained within the tissue for at least 2 h, but no uptake of Tris (140 mM) or sucrose (above) could be detected. Tissue lactate levels (from the lactate/N-acetyl aspartate ratio) increased in the presence of the substitutes that were taken up, although no change in pH was detected.

Nuclear magnetic resonance spectroscopy study on energy metabolism, intracellular pH, and free Mg2+ concentration in the brain of transgenic mice overexpressing human ornithine decarboxylase gene

We have generated a transgenic mouse line strikingly overexpressing the human ornithine decarboxylase (ODC) gene in their brain. Brain ODC activity was increased in the transgenic animals by a factor of 70 in comparison with their nontransgenic littermates. The content of brain putrescine, the product of ODC, was greater than 60 mumol/g of tissue in the transgenic mice, whereas in the normal animals it was below the level that could be detected by an HPLC method. The concentrations of the higher polyamines (spermidine and spermine) were not significantly different from control values. 31P nuclear magnetic resonance (31P NMR) spectroscopy analyses revealed a significantly reduced (40%) free Mg2+ concentration as calculated from the chemical shift differences of the nucleoside triphosphate alpha and beta peaks in the brains of the transgenic animals. The lower free Mg2+ concentration in the brains of ODC transgenic mice was not a consequence of altered intracellular pH or changes in cellular high-energy metabolites. 1H NMR showed no differences in brain choline/N-acetylaspartate and total creatine/N-acetylaspartate ratios between the two animal groups. These ODC transgenic animals may serve as models in vivo for studies on cerebral postischemic events and on epilepsy, as polyamines are supposed to be involved in these processes.

Recovery of intracellular pH in cortical brain slices following anoxia studied by nuclear magnetic resonance spectroscopy: role of lactate removal, extracellular sodium and sodium/hydrogen exchange

[31P]- and [1H]nuclear magnetic resonances recorded in an interleaved fashion were used in order to quantify high-energy phosphates, intracellular pH and lactate in cortical brain slices of the guinea-pig superfused in a CO2/HCO3(-)-buffered medium during and after anoxic insults. The volume-averaged intracellular pH and energy status of the preparation following anoxia were determined. In the presence of external Na+, intracellular pH normalized in 3 min and was significantly more alkaline from 10 to 12 min of recovery, but lactate remained elevated for 12 min of reoxygenation following anoxia. The amount of lactate removed was only 40% of the quantity of acid extruded showing operation of H+ neutralizing transmembrane mechanisms other than transport of lactic acid. Amiloride (1 or 2 mM) did not prevent the recovery of intracellular pH, but it blocked the "overshoot" of the alkalinization at 10-12 min of recovery. In a medium containing 70 mM K+, 60 mM Na+ and 0.1 mM Ca2+, the recovery of pH, but not lactate washout, was significantly delayed. Removal of external Na+ caused severe energetic failure, decreases both in oxygen uptake and in N-acetyl aspartate concentration, indicating loss of viable tissue. In Na(+)-free superfusion, lactic acidosis caused a more severe drop in intracellular pH than in the presence of Na+. Complexing of extracellular Ca2+ in the Na(+)-free medium inhibited the acidification by 0.38 pH units during anoxia which is as much as the acidification caused by lactate accumulation in the absence of Na+. In Na(+)-free medium intracellular pH recovered, however, from an anoxic level to a normoxic value in 6 min. Metabolic damage of the slice preparation induced by anoxia in the absence of Na+ was as profound in the presence as in the absence of Ca2+ showing that accumulation of Ca2+ is not the only reason for the damage. It is concluded that recovery of intracellular pH from lactic-acidosis can occur independently of energetic recovery and involves acid extrusion mechanism(s) that is(are) dependent on external Na+ and sensitive to high K+.

Effects of N-methyl-D-aspartate on [Ca2+]i and the energy state in the brain by 19F- and 31P-nuclear magnetic resonance spectroscopy

The effects of N-methyl-D-aspartate (NMDA) on the free intracellular Ca2+ concentration [( Ca2+]i) and the energy state in superfused cerebral cortical slices have been studied using 19F- and 31P-nuclear magnetic resonance spectroscopy. [Ca2+]i was measured using the calcium indicator 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid (5FBAPTA). NMDA (10 microM) in the absence of extracellular Mg2+ caused the expected rise in [Ca2+]i but produced an impairment of the energy state: the phosphocreatine (PCr) content was decreased by 42%, and the Pi/PCr ratio was increased by 55%. There was no detectable change in ATP or free intracellular Mg2+ concentration. Increasing the NMDA concentration in the superfusing medium to 100 or 400 microM caused no further increase in [Ca2+]i or further decrease in PCr content, but the Pi/PCr ratio continued to rise. The impairment of the energy state preceded the effect on [Ca2+]i, and these changes were irreversible on return to control conditions. Repeating the experiments in the presence of 1.2 mM extracellular Mg2+ resulted in similar changes in the energy state, with no change in [Ca2+]i. The possibilities that the effects were due to membrane depolarisation or to the presence of 5FBAPTA within the tissues were eliminated. The results suggest that low concentrations (10 microM) of NMDA produce an impaired energy state independent of the presence of extracellular Mg2+ and that the decreased energy state is not due to the changes in [Ca2+]i, which are seen only in the absence of extracellular Mg2+.