Indo-1 measurements of intracellular free calcium in the hippocampal slice: complications of labile NADH fluorescence

Imaging synaptic zinc: promises and perils

It is well established that some excitatory nerve terminals have high concentrations of Zn(2+) in their synaptic vesicles. For some time, it has been believed that synaptic Zn(2+) is released during neurotransmission and acts as a neuromodulator. Fluorescent Zn(2+) indicators that do not penetrate membranes offer the prospect of rendering the release of Zn(2+) visible. Here, I take a critical look at fluorimetric imaging experiments devised to determine whether Zn(2+) is released and show that they are particularly susceptible to artifacts. Moreover, I will argue that recent experiments suggest that, rather than being released, Zn(2+) is presented to the extracellular space firmly coordinated to presynaptic macromolecules.

Theoretical analysis on ratiometric fluorescent indicators caused biased estimates of intracellular free calcium concentrations

Ratiometric fluorescent calcium indicator dyes have been widely used for the study of the role of Ca2+ in cell physiopathology. Although these ratiometric dyes offer several advantages over others, they suffer some drawbacks which cause serious errors in measurement of intracellular Ca2+ concentration, [Ca2+]i. The present study systematically analyzes theoretical reasons and technical sources of discrepancies occurring in the measurement of the characteristics of the agonists-induced cells [Ca2+]i. In order to avoid the errors and achieve the accurate determination of [Ca2+]i, this study proposes solutions and suggests some critical measures in both theoretical and technical aspects. Therefore, this analysis can be a valuable tool in clarifying proper usages of fluorescent dyes for [Ca2+]i measurements.

Evidence for chelatable zinc in the extracellular space of the hippocampus, but little evidence for synaptic release of Zn

Zinc colocalizes with glutamate in the synaptic vesicles of certain glutamatergic vesicles in the mammalian brain. Here, I introduce a method for detecting Zn in the extracellular space of brain slices and another method for detecting the passage of Zn out of the slice. In both cases, the fluorimetric Zn probe FluoZin-3 is used in conjunction with a slow Zn chelator, Ca-EDTA, to reduce background fluorescence. In addition, a new Zn chelator, ethylenediiminodi-2-pentanedioic acid, with little affinity for Ca or Mg is introduced. These tools are then used to show that little Zn (approximately 2 nm) is released during the course of synaptic transmission into the extracellular space. However, when hippocampal slices are subjected to a high potassium stimulus (50 mM) combined with an increase in osmolarity, Zn is externalized in the Timm's-stained areas (approximately 6 nm). This stimulus also leads to even greater Zn elevations in area CA1 that is only weakly stained by the Timm's method. Nevertheless, even under these conditions, little if any Zn makes its way out of the slices. I present evidence for a layer of Zn in the extracellular space that maps onto the Timm's stained region of the hippocampus. This Zn veneer appears to be loosely associated with molecules in the extracellular space and may be the raison d'être for vesicular Zn.

Extrusion of intracellular calcium ion after in vitro ischemia in the rat hippocampal CA1 region

Simultaneous recordings of intracellular Ca(2+) ([Ca(2+)](i)) signal and extracellular DC potential were obtained from the CA1 region in 1-[6-amino-2-(5-carboxy-2-oxazolyl)-5-benzofuranyloxy]-2-(2-amino-5-methylphenoxy)-ethane-N,N,N',N'-tetraacetic acid penta-acetoxymethyl ester (Fura-2/AM)-loaded rat hippocampal slices. Superfusion with oxygen- and glucose-deprived medium (in vitro ischemia) for 5-6 min produced a rapid rise of the [Ca(2+)](i) level in the stratum radiatum (rising phase of the [Ca(2+)](i) signal), which occurred simultaneously with a rapid negative DC potential (rapid negative potential). When oxygen and glucose were reintroduced, the increased [Ca(2+)](i) signal diminished rapidly (falling phase of the [Ca(2+)](i) signal) during the generation of a slow negative DC potential (slow negative potential), which occurred within 1 min from the onset of the reintroduction. Thereafter, the [Ca(2+)](i) signal partially and the slow negative potential completely returned to the preexposure level approximately 6 min after the reintroduction. The changes in [Ca(2+)](i) signal during and after in vitro ischemia were very similar to the changes in the membrane potential of glial cells. The rising and falling phases of [Ca(2+)](i) signal corresponded to the rapid depolarization and a depolarizing hump, respectively, in the repolarizing phase of glial cells. A prolonged application of in vitro ischemia or a reintroduction of either glucose or oxygen suppressed the falling phase after ischemic exposure. The application of ouabain (30 microM) generated both a rapid negative potential and a rapid elevation of [Ca(2+)](i), but no slow negative potential or rapid reduction in [Ca(2+)](i) were observed. When oxygen and glucose were reintroduced to slices in the Na(+)-free or ouabain- or Ni(2+)-containing medium, the falling phase was suppressed. The falling phase was significantly accelerated in Ca(2+)- and Mg(2+)-free with EGTA-containing medium. In contrast, the falling phase was significantly slower in the Ca(2+)-free with high Mg(2+)- and EGTA-containing medium. The falling phase of the [Ca(2+)](i) signal after ischemic exposure is thus considered to be primarily dependent on the reactivation of Na(+), K(+)-ATPases, while the extrusion of cytosolic Ca(2+) via the forward-mode operation of Na(+)/Ca(2+) exchangers in glial cells is thought to be directly involved in the rapid reduction of [Ca(2+)](i) after ischemic exposure.

Responses to reversible anoxia of intracellular free and bound Ca(2+) in rat cortical slices

Severe anoxia induces destabilisation of intracellular calcium homeostasis in neurones. The mechanism of this effect, and particularly the interrelationship between changes in intracellular concentration of free Ca(2+) ions and the content of the intracellular Ca(2+) stores, during and after anoxia, is not clear. We used a superfusion system of rat olfactory cortical slices for the fluorimetric estimation of changes in the intracellular concentration of free Ca(2+) ions and in the level of bound Ca(2+), utilising the fluorescent indicators Fura-2 and chlortetracycline, respectively. It was found that 10-min normoglycaemic anoxia results in simultaneous decrease in bound and increase in free Ca(2+) levels, whereas during 60-min reoxygenation, we detected an increase in both indices. The NMDA receptor antagonists MK-801 and APV attenuated changes in free Ca(2+) level during anoxia and reoxygenation and intensified anoxia-evoked decrease in bound Ca(2+) content, whereas a late post-anoxic increase in bound Ca(2+) was abolished. These data suggest that the influx of extracellular Ca(2+) to neurones via NMDA receptors, plays a critical role in the rise of intracellular free Ca(2+) concentration during and after anoxia. Biphasic changes in bound Ca(2+) content during anoxia and reoxygenation may reflect an anoxia-induced release of Ca(2+) from intracellular stores, followed later by a neuronal calcium overload and refilling of intracellular Ca(2+) binding sites.

Measurement of intracellular calcium

To a certain extent, all cellular, physiological, and pathological phenomena that occur in cells are accompanied by ionic changes. The development of techniques allowing the measurement of such ion activities has contributed substantially to our understanding of normal and abnormal cellular function. Digital video microscopy, confocal laser scanning microscopy, and more recently multiphoton microscopy have allowed the precise spatial analysis of intracellular ion activity at the subcellular level in addition to measurement of its concentration. It is well known that Ca2+ regulates numerous physiological cellular phenomena as a second messenger as well as triggering pathological events such as cell injury and death. A number of methods have been developed to measure intracellular Ca2+. In this review, we summarize the advantages and pitfalls of a variety of Ca2+ indicators used in both optical and nonoptical techniques employed for measuring intracellular Ca2+ concentration.

Involvement of the glutamate transporter and the sodium-calcium exchanger in the hypoxia-induced increase in intracellular Ca2+ in rat hippocampal slices

Hippocampal slices prepared from adult rats were loaded with fura-2 and the intracellular free Ca2+ concentration ([Ca2+]i) in the CA1 pyramidal cell layer was measured. Hypoxia (oxygen-glucose deprivation) elicited a gradual increase in [Ca2+]i in normal Krebs solution. At high extracellular sodium concentrations ([Na+]o), the hypoxia-induced response was attenuated. In contrast, hypoxia in low [Na+]o elicited a significantly enhanced response. This exaggerated response to hypoxia at a low [Na+]o was reversed by pre-incubation of the slice at a low [Na+]o prior to the hypoxic insult. The attenuation of the response to hypoxia by high [Na+]o was no longer observed in the presence of antagonist to glutamate transporter. However, antagonist to Na+-Ca2+ exchanger only slightly influenced the effects of high [Na+]o. These observations suggest that disturbance of the transmembrane gradient of Na+ concentrations is an important factor in hypoxia-induced neuronal damage and corroborates the participation of the glutamate transporter in hypoxia-induced neuronal injury. In addition, the excess release of glutamate during hypoxia is due to a reversal of Na+-dependent glutamate transporter rather than an exocytotic process.

Extracellular sodium concentration has diverse effects on the hypoxia-induced increase in intracellular Ca2+ in rat hippocampal slices

The intracellular free Ca2+ concentration ([Ca2+]i) in the CA1 pyramidal cell layer was measured using fura-2-loaded hippocampal slices prepared from adult rats. Hypoxia (oxygen-glucose deprivation) elicited a gradual increase in [Ca2+]i in normal Krebs solution. With a high extracellular sodium concentration ([Na+]o), the hypoxia-induced response was attenuated, its onset-latency was longer and the time constant of its decay phase was shorter than in controls. In contrast, hypoxia in low [Na+]o elicited a significantly enhanced response with a short onset-latency and delayed decay phase. This exaggerated response to hypoxia in low [Na+]o was reversed by pre-incubation of the slice in low [Na+]o prior to the hypoxic insult. Some possible mechanisms and the functional significance of the observed effects of [Na+]o on the hypoxia-induced increase in [Ca2+]i are discussed, with particular emphasis on the putative participation of the glutamate transporter and the sodium-calcium exchanger in hypoxia-induced neuronal injury.

Imaging free zinc in synaptic terminals in live hippocampal slices

Some glutamatergic synapses in the mammalian central nervous system exhibit high levels of free ionic zinc in their synaptic vesicles. The precise role of this vesicular zinc remains obscure, despite suggestive evidence for zinc as a neuromodulator. As a step towards elucidating the role of free zinc in the brain we have developed a method for imaging zinc release in live brain slices. A newly synthesized zinc-sensitive fluorescent probe, N-(6-methoxy-8-quinolyl)-p-carboxybenzoylsulphonamide (TFLZn), was used to monitor intracellular zinc in live rat hippocampal slices. The dye loaded into the zinc-rich synaptic vesicles of the mossy fibre terminals in the hippocampal formation. Direct electrical stimulation of the mossy fibre pathway diminished the fluorescence in the mossy fibre terminals, consistent with a stimulus-dependent zinc release. The synaptic release of zinc was followed by the rapid replenishment of the zinc levels in vesicles from an as yet unidentified intracellular zinc source. Furthermore, we present evidence that zinc may play a role in a form of long-term potentiation exhibited by the mossy fibre pathway.

Can the Indo-1 fluorescence approach measure brain intracellular calcium in vivo? A multiparametric study of cerebrocortical anoxia and ischemia

Indo-1 fluorescence was used to monitor intracellular calcium levels in the cat brain in vivo, using the approach proposed by Uematsu et al. [Uematsu D., Greenberg J. H., Reivich M., Karp A. In vivo measurement of cytosolic free calcium during cerebral ischemia and reperfusion. Ann Neurol 1988; 24: 420-428]. In addition, extracellular calcium and potassium levels, NADH redox state, electrocorticogram (ECoG), DC potential and relative cerebral blood flow were monitored simultaneously. Changes in the Indo-1 fluorescence ratio F400/F506 were monitored during anoxia, reversible ischemia and irreversible ischemia. Although these perturbations resulted in the expected changes in extracellular calcium and potassium levels, NADH redox state, ECoG and other physiological parameters, they did not result in significant increases in the F400/F506 ratio. The apparent insensitivity of the in vivo Indo-1 approach is due to the difficulty in obtaining accurate fluorescence signals from Indo-1 in the brain. Two reasons for this difficulty appear to be problems in loading Indo-1 into the brain, and problems in correcting Indo-1 fluorescence signals for changes in NADH fluorescence and changes in absorption of intrinsic chromophores. Under the conditions of our in vivo cat experiments, Indo-1 fluorescence is not a viable approach for measuring changes in cerebral intracellular calcium levels.

In vivo visualization of hippocampal cells and dynamics of Ca2+ concentration during anoxia: feasibility of a fiber-optic plate microscope system for in vivo experiments

The feasibility of a fiber-optic plate (FOP) microscope system employing a bundle of optical fibers and videomicroscopy for in vivo experiments was investigated. The FOP used here consisted of optical fibers 3 microns in diameter. By inserting the FOP into an animal, optical signals from the deep-lying tissue invisible from the surface could be obtained as two-dimensional images. Using this system, hippocampal cells stained with a fluorescent dye in an anesthetized rat were visualized. Elevation of intracellular free calcium concentration ([Ca2+]i) in the hippocampus of the rat during anoxic exposure was also detected with a fluorescent indicator dye. These results showed that the FOP microscope system was sufficiently applicable to in vivo experiments for studying tissue structure and physiological activity even in the deep regions with fluorometric techniques.

Autofluorescence as a confound in the determination of calcium levels in hippocampal slices using fura-2AM dye

Recent publications have reported calcium level determinations in slices of brain using imaging techniques and the dye fura-2AM. In general these studies ignore or deal only perfunctorily with the problem of autofluorescence in slices. This confound, which is a result of the pyridine nucleotides that are normally present in tissue, has been previously reported to interfere with Ca2+ measurements in slices. Because these pyridine compounds are involved in cell metabolism, the fluorescence intensity is labile over time following experimental manipulations. We were studying Ca2+ levels in hippocampal slices using standard imaging techniques. We found significant and variable autofluorescence at the wavelengths used for calcium determination which interfered with data interpretation in fura-treated slices. The intensity of this autofluorescence is an additive effect and is not large enough to be observed when imaging monolayers. In this paper we present a method for conducting experiments and analyzing data that decreases interference from autofluorescence. Experiments were carried out on both slices bulk loaded with fura-2AM and slices loaded with control buffer. A point to point subtraction of the control slice values gave representative calcium fluorescence values. Hippocampal slices were challenged with sodium cyanide or kainic acid. The metabolic response, seen in the fura-free slices, and the calcium response varied within and between these two treatments. Regional differences in the hippocampal sub fields were also demonstrated in response to the two treatments. These corresponded to known regional vulnerabilities to cyanide and kainate. We conclude that autofluorescence in slices need be considered when determining calcium concentrations using fura-2AM.

Causes of calcium accumulation in rat cortical brain slices during hypoxia and ischemia: role of ion channels and membrane damage

To better understand why neurons accumulate calcium during cerebral ischemia, the influence of specific ion channel inhibitors on the rise in cytosolic free calcium ([Ca2+]c) during hypoxia or ischemia was evaluated in rat cerebrocortical brain slices. [Ca2+]c was measured fluorometrically with the dye fura-2 during hypoxia (95% N2/5% CO2 or 100 microM NaCN), simulated ischemia (100 microM NaCN plus 3.5 mM iodoacetate), or 0.5-1.0 mM glutamate. Hypoxia or ischemia increased [Ca+2]c from 100-250 nM to 1,000-2,500 nM within 3-5 min. Greater than 85% of the calcium accumulation was influx from the extracellular medium. The non-competitive N-methyl-D-aspartate (NMDA) inhibitor MK-801 reduced [Ca2+]c accumulation during hypoxia, but antagonism of alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA) receptors or voltage-gated sodium or calcium channels or Na+/Ca2+ exchangers had no effect. During ischemia, combined antagonism of NMDA, AMPA and voltage-gated sodium channels slowed the rate of calcium accumulation, but not concentration at 5 min. Membrane damage, as indicated by leakage of lactate dehydrogenase into superfusate, occurred coincidentally with calcium influx and ATP loss during both hypoxia and ischemia. We conclude that cytosolic calcium changes during hypoxia or ischemia in cortical brain slices are due to multiple mechanisms, are incompletely inhibited by combined ion channel blockade, and are associated with disruption of cell membrane integrity.

Mild acidosis inhibits the rise in intracellular Ca2+ concentration in response to oxygen-glucose deprivation in rat hippocampal slices

Changes in the intracellular Ca2+ concentration ([Ca2+]i) induced by ischemia in vitro (oxygen-glucose deprivation) were continuously recorded using fura-2-loaded hippocampal slices under normal (pH 7.4), acidotic (pH 6.8) and alkalotic (pH 7.8) conditions. Oxygen-glucose deprivation induced an initial slow and a subsequent characteristic rapid increase in [Ca2+]i in most of the normal and alkalotic preparations regardless of whether or not Ca2+ was present in the bathing solutions. This characteristic rapid increase in [Ca2+]i was observed in a minority of the acidotic preparations and its latency was significantly longer in acidotic preparations than in normal and alkalotic preparations. The rise in [Ca2+]i at 10 min of oxygen-glucose deprivation was significantly smaller in the acidotic preparations than in the normal and alkalotic preparations, regardless of whether or not Ca2+ was present. At 15 min, the differences in the increase in [Ca2+]i between normal and acidotic preparations in Ca(2+)-containing solutions (2.5 mM) were insignificant. However, significant differences were still observed between the acidotic preparations and either the normal or alkalotic preparations under Ca(2+)-free conditions. These results suggest that acidosis inhibits the ischemia-induced rise in [Ca2+]i by attenuating both Ca2+ influx from the extracellular space and Ca2+ release from intracellular sites.

Fluorescence imaging of intracellular Ca2+

The measurement of intracellular Ca2+ concentrations ([Ca2+]i) is of critical importance, because many cellular functions are tightly regulated by [Ca2+]i. The fluorescent indicator, fura-2, has been used frequently to measure [Ca2+]i because of its sensitivity and specificity, and because it can be loaded into living cells with little disruption of function. Most importantly, the peak excitation wavelength of fura-2 changes when it binds Ca2+. As a consequence, measurements of fluorescence at two excitation wavelengths can be used to obtain an estimate of [Ca2+]i that is independent of dye concentration and cell thickness. Fura-2 acetoxymethyl ester (AM) is a lipid-soluble derivative that is often used because of its ability to pass through cell membranes. There are, however, several problems with the use of fura-2 AM such as intracellular compartmentation and incomplete deesterification. The availability of low-light-level cameras and computer hardware for the digitization of fluorescent images has made quantitative fluorescence microscopy possible. This technique has shown a striking spatial heterogeneity of [Ca2+]i in a variety of cell types, and has revealed substantial new information on dynamic intracellular biochemistry and signal transduction. However, the current imaging technology is not fully developed because of dye and instrumentation limitations. Further development of techniques and new probes are required to improve temporal and spatial resolution.

Influence of pH on calcium influx during hypoxia in rat cortical brain slices

BACKGROUND AND PURPOSE

Acidity of brain intracellular and extracellular fluids appears to increase brain injury from stroke, but low extracellular pH decreases the activity of N-methyl-D-aspartate receptor ion channels and decreases calcium influx into isolated neurons. To further investigate the role of acid-base balance in hypoxic brain injury, we studied the influences of intracellular and extracellular pH on calcium influx in cortical brain slices during hypoxia.

METHODS

Intracellular calcium ([Ca2+]i) and pH (pHi) were measured fluorometrically with the dyes fura-2 and biscarboxyethyl carboxyfluorescein, respectively, during two types of hypoxia: (1) slice perfusate equilibrated with N2/CO2 at pH 6.6 or 6.2 ("gaseous hypoxia") or (2) perfusate equilibrated with 95% O2/5% CO2 plus 100 mumol/L NaCN at pH 7.3, 6.6, or 6.2 ("chemical hypoxia").

RESULTS

Changes in perfusate pH under aerobic conditions did not change [Ca2+]i. However, influx of calcium caused by gaseous or chemical hypoxia increased significantly with decreasing perfusate pH. During chemical hypoxia, the elevation in [Ca2+]i at perfusate pH 6.2 was twice that at perfusate pH 7.3. Change in [Ca2+]i was correlated with perfusate pH but not pHi.

CONCLUSIONS

These results, which differ from previous studies showing acid inhibition of calcium influx in isolated neurons, suggest that low extracellular pH may exacerbate cellular injury during severe hypoxia or ischemia in the intact brain.

Investigation of factors affecting fluorometric quantitation of cytosolic [Ca2+] in perfused hearts

The goal of these studies was to examine the effects of several factors that may artifactually influence quantitation of cytosolic [Ca2+], [Ca2+]c, while using the fluorescent calcium indicator Indo-1. The following factors were investigated: 1) a possible fluorescence contribution from unhydrolized Indo-1/AM (by Mn2+ quenching), 2) Ca2+ buffering by Indo-1 (by varying [Indo-1]), 3) endothelial and mitochondrial Indo-1 loading (by bradykinin stimulation and calculations), and 4) effects of changing tissue fluorescence (predominantly NAD(P)H) on calculated [Ca2+]c during hypoxia (by a new method which allowed simultaneous determination of [Ca2+]c and changes in [NAD(P)H]). No significant contribution of Indo-1/AM was found. With increasing [Indo-1], calculated systolic [Ca2+]c fell significantly. Indo-1 incorporation (< 18%) into endothelial cells, caused a slight underestimation of systolic [Ca2+]c, while mitochondrial Indo-1 loading may cause overestimation of [Ca2+]c. With increased tissue fluorescence, during hypoxia, systolic [Ca2+]c may be underestimated by approximately 27% (for Indo-1 loading factors three to five times original tissue fluorescence). These studies suggest conditions in which experimental artifacts could be minimized to allow reliable quantitation of [Ca2+]c in intact perfused hearts using Indo-1 fluorometry. The major problem of obtaining reliable results depended on the ability to correct for changing NAD(P)H fluorescence while keeping [Indo-1] low.

Developmental changes in intracellular calcium regulation in rat cerebral cortex during hypoxia

During the first weeks of life, injury to the central nervous system caused by brief periods of oxygen deprivation greatly increases. To investigate possible causes for this change, the effects of hypoxia or application of the excitatory neurotransmitter glutamate on intracellular calcium ([Ca2+]i) and ATP were studied in rat cerebrocortical brain slices. [Ca2+]i was measured fluorometrically with the indicator Fura-2. Hypoxia (95% N2/5% CO2) or 100 microM sodium cyanide produced gradual elevations in [Ca2+]i and ATP depletion in slices from rats < 2 weeks old, but rapid changes in older rats. After 20 min, [Ca2+]i in adult slices exposed to cyanide was 1,980 +/- 310 nM; in day 1-14 animals, it was 796 +/- 181 nM (p < 0.05). Combination of cyanide and a glycolytic inhibitor (iodoacetate) rapidly elevated [Ca2+]i and depleted ATP in all age groups. Energy utilization during anoxia, assessed by measuring ATP fall in cyanide/iodoacetate-treated brain slices, increased with age. Elevations in [Ca2+]i caused by application of 500 microM glutamate increased 240% from days 1-2 to day 28, but ATP loss caused by glutamate did not change with age. The N-methyl-D-aspartate antagonist MK-801 delayed calcium entry during the initial 5-7 min of hypoxia or cyanide in rats < 2 weeks old. We conclude that anaerobic ATP production, conservation of energy by reduced ATP consumption, and reduced sensitivity to glutamate contribute to delaying elevation in [Ca2+]i in neonatal rat brain during hypoxia.

Laminar difference in tetanus-induced increase of intracellular Ca2+ in visual cortex of young rats

Changes in intracellular Ca2+ evoked by electrical stimulation of the white matter were observed by means of microfluorometry with a Ca2+ indicator, rhod-2, in slice preparations of the visual cortex obtained from young rats. Tetanic stimulation at 5 Hz for 1 min induced a marked fluorescence increase, while single-shock stimulation did not induce a sizable increase in normal perfusate. The tetanus-induced increase took place in a column-like manner from layer VI near the stimulation site to layer II/III of the cortex, although it spread horizontally in layer II/III. The magnitude of fluorescence rise was largest in layer II/III of the cortex. Since N-methyl-D-aspartate (NMDA) receptors are known to exist only on neurons, the following results are taken to indicate that the fluorescent signal is derived mostly from postsynaptic neurons: Application of NMDA in the presence of tetrodotoxin induced a marked fluorescence increase with the same laminar bias as tetanic stimulation did, and the fluorescence increase by single-shock stimulation in Mg(2+)-free medium was almost completely blocked by an antagonist for NMDA receptors. These results support the hypothesis that input-associated entry of Ca2+ into postsynaptic neurons triggers processes for induction of long-term potentiation of synaptic efficacy.

Monitoring of intracellular free calcium in perfused rat liver

Fluorescent calcium indicators have been widely used to assess cytoplasmic calcium concentration in cells. To examine the role of calcium ions on different physiological functions (e.g. in case of liver; bile secretion, glucose metabolism, etc.) there is a need for whole organ studies. We have developed a technique to estimate intracellular free calcium changes in perfused rat liver. Krebs-Henseleit perfused livers were loaded with 7 microM or 35 microM Indo-1/AM. An area 3 mm in diameter and approximately 300 microns in depth was illuminated at 340 nm. Fluorescence was monitored with photomultiplier tubes at 3 wavelengths (400 nm for Ca-bound dye, 504 nm for free dye and 464 nm for NADH). The viability of liver preparations was assessed by measurement of the concentrations of lactate dehydrogenase and alanine aminotransferase in the effluent. Loading of the livers with 7 microM Indo-1/AM via the portal vein resulted in a 5-fold increase of fluorescence at 400 nm. However the dye 'leaked' out of the liver with a half-time of 18 min. Probenecid (a specific anion carrier blocker) inhibited loss of dye in a dose dependent fashion (2.5-10 mM). Transient calcium elevations were observed in response to vasopressin (5-50 nM) at physiological levels, ethanol (0.3-0.8 M) and the calcium ionophore, ionomycin. Certain limitations were apparent with this approach: (1) it was necessary to use an anion carrier blocker to maintain a relatively steady dye concentration; (2) endogenous NADH fluorescence interfered with the calcium signal; and (3) absolute values of calcium concentration could not be determined.

Role of NMDA receptors and voltage-activated calcium channels in an in vitro model of cerebral ischemia

In an in vitro model of cerebral ischemia we investigated the functional consequences of repeated hypoxias and the potential protective effect of the N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonovaleric acid (D-APV) and the calcium channel blocker verapamil in preventing the expression of pathophysiological activity. Rat neocortical slices were exposed to nitrogen for 2-13 min and the hypoxia-induced functional modifications were monitored in layer II/III by recording the extracellular DC potential, the extracellular calcium concentration ([Ca2+]o) and the stimulus-evoked synaptic responses. Hypoxia caused a reversible 2.4-24.6 mV negative shift in the extracellular DC potential associated with a [Ca2+]o decrease from 1.2 to 0.2 mM and a complete loss of synaptic responsiveness. Repeating hypoxias induced an increase in the amplitude of this anoxic depolarization (AD) and a significant decrease in the AD onset latency. Synaptic responses partially recovered at 20 and 60 min intervals between subsequent hypoxic periods, indicating that the initial AD did not induce any short-term irreparable functional deficits. Verapamil (50 microM) caused an increase in the AD onset latency. However, in comparison to untreated controls, verapamil induced a reduction of excitatory and inhibitory responses during hypoxia probably by blocking voltage-activated calcium conductances. In addition, verapamil did not have any significant effect on the hypoxia-induced reduction of [Ca2+]o. Bath application of D-APV (30 microM) prevented the significant reduction in the AD onset latency to the second hypoxia, but had no significant effect on the AD amplitude and duration. The hypoxia-induced decrease in [Ca2+]o was not altered after addition of D-APV to the bathing medium. These data indicate that the influx of calcium through voltage-activated calcium channels and the NMDA receptor-gated ionophore does not significantly contribute to the massive depolarization observed under hypoxic conditions.

Ca2+ imaging in single living cells: theoretical and practical issues

The measurement of intracellular calcium ion concentrations [( Ca2+]i) in single living cells using quantitative fluorescence microscopy draws from a diverse set of disciplines, including cellular biology, optical physics, statistics and computer science. Over the last few years, we have devised and built a number of systems for measuring [Ca2+]i with Fura-2, and have applied them in the exploration of a wide range of biological processes controlled by Ca2+. In this report we discuss these systems and their advantages and limitations. We also describe the theoretical and practical problems associated with using Fura-2 to measure [Ca2+]i, and the solutions that we, and others, have developed to overcome them. The approaches described should provide useful guidance for others interested in imaging [Ca2+] distribution in living cells. The factors that limit current methods are discussed, and areas for future development are highlighted.