Mitochondria and other calcium buffers of squid axon studied in situ

A controversial issue: Can mitochondria modulate cytosolic calcium and contraction of skeletal muscle fibers?

Mitochondria are characterized by a high capacity to accumulate calcium thanks to the electrochemical gradient created by the extrusion of protons in the respiratory chain. Thereby calcium can enter crossing the inner mitochondrial membrane via MCU complex, a high-capacity, low-affinity transport mechanism. Calcium uptake serves numerous purposes, among them the regulation of three dehydrogenases of the citric cycle, apoptosis via permeability transition, and, in some cell types, modulation of cytosolic calcium transients. This Review is focused on mitochondrial calcium uptake in skeletal muscle fibers and aims to reanalyze its functional impact. In particular, we ask whether mitochondrial calcium uptake is relevant for the control of cytosolic calcium transients and therefore of contractile performance. Recent data suggest that this may be the case, at least in particular conditions, as modified expression of MCU complex subunits or of proteins involved in mitochondrial dynamics and ablation of the main cytosolic calcium buffer, parvalbumin.

Carriers, exchangers, and cotransporters in the first 100 years of the Journal of General Physiology

Jennings reviews the many contributions of JGP articles to our current understanding of solute transporter mechanisms.

Transporters, pumps, and channels are proteins that catalyze the movement of solutes across membranes. The single-solute carriers, coupled exchangers, and coupled cotransporters that are collectively known as transporters are distinct from conductive ion channels, water channels, and ATP-hydrolyzing pumps. The main conceptual framework for studying transporter mechanisms is the alternating access model, which comprises substrate binding and release events on each side of the permeability barrier and translocation events involving conformational changes between inward-facing and outward-facing conformational states. In 1948, the Journal of General Physiology began to publish work that focused on the erythrocyte glucose transporter—the first transporter to be characterized kinetically—followed by articles on the rates, stoichiometries, asymmetries, voltage dependences, and regulation of coupled exchangers and cotransporters beginning in the 1960s. After the dawn of cDNA cloning and sequencing in the 1980s, heterologous expression systems and site-directed mutagenesis allowed identification of the functional roles of specific amino acid residues. In the past two decades, structures of transport proteins have made it possible to propose specific models for transporter function at the molecular level. Here, we review the contribution of JGP articles to our current understanding of solute transporter mechanisms. Whether the topic has been kinetics, energetics, regulation, mutagenesis, or structure-based modeling, a common feature of these articles has been a quantitative, mechanistic approach, leading to lasting insights into the functions of transporters.

Mitochondrial dynamics in peripheral neuropathies

SIGNIFICANCE

Mitochondrial dynamics describes the continuous change in the position, size, and shape of mitochondria within cells. The morphological and functional complexity of neurons, the remarkable length of their processes, and the rapid changes in metabolic requirements arising from their intrinsic excitability render these cells particularly dependent on effective mitochondrial function and positioning. The rules that govern these changes and their functional significance are not fully understood, yet the dysfunction of mitochondrial dynamics has been implicated as a pathogenetic factor in a number of diseases, including disorders of the central and peripheral nervous systems.

RECENT ADVANCES

In recent years, a number of mutations of genes encoding proteins that play important roles in mitochondrial dynamics and function have been discovered in patients with Charcot-Marie-Tooth (CMT) disease, a hereditary peripheral neuropathy. These findings have directly linked mitochondrial pathology to the pathology of peripheral nerve and have identified certain aspects of mitochondrial dynamics as potential early events in the pathogenesis of CMT. In addition, mitochondrial dysfunction has now been implicated in the pathogenesis of noninherited neuropathies, including diabetic and inflammatory neuropathies.

CRITICAL ISSUES

The role of mitochondria in peripheral nerve diseases has been mostly examined in vitro, and less so in animal models.

FUTURE DIRECTIONS

This review examines available evidence for the role of mitochondrial dynamics in the pathogenesis of peripheral neuropathies, their relevance in human diseases, and future challenges for research in this field.

Sexual hormones: effects on cardiac and mitochondrial activity after ischemia-reperfusion in adult rats. Gender difference

In this work we studied the influence of sex hormones on heart and mitochondrial functions, from adult castrated female and male, and intact rats. Castration was performed at their third week of life and on the fourth month animals were subjected to heart ischemia and reperfusion. Electrocardiogram and blood pressure recordings were made, cytokines levels were measured, histopathological studies were performed and thiobarbituric acid reactive species were determined. At the mitochondrial level respiratory control, transmembranal potential and calcium management were determined; Western blot of some mitochondrial components was also performed. Alterations in cardiac function were worst in intact males and castrated females as compared with those found in intact females and castrated males, cytokine levels were modulated also by hormonal status. Regarding mitochondria, in those obtained from hearts from castrated females without ischemia-reperfusion, all evaluated parameters were similar to those observed in mitochondria after ischemia-reperfusion. The results show hormonal influences on the heart at functional and mitochondrial levels.

Role of the mitochondrial sodium/calcium exchanger in neuronal physiology and in the pathogenesis of neurological diseases

In neurons, as in other excitable cells, mitochondria extrude Ca(2+) ions from their matrix in exchange with cytosolic Na(+) ions. This exchange is mediated by a specific transporter located in the inner mitochondrial membrane, the mitochondrial Na(+)/Ca(2+) exchanger (NCX(mito)). The stoichiometry of NCX(mito)-operated Na(+)/Ca(2+) exchange has been the subject of a long controversy, but evidence of an electrogenic 3 Na(+)/1 Ca(2+) exchange is increasing. Although the molecular identity of NCX(mito) is still undetermined, data obtained in our laboratory suggest that besides the long-sought and as yet unfound mitochondrial-specific NCX, the three isoforms of plasmamembrane NCX can contribute to NCX(mito) in neurons and astrocytes. NCX(mito) has a role in controlling neuronal Ca(2+) homeostasis and neuronal bioenergetics. Indeed, by cycling the Ca(2+) ions captured by mitochondria back to the cytosol, NCX(mito) determines a shoulder in neuronal [Ca(2+)](c) responses to neurotransmitters and depolarizing stimuli which may then outlast stimulus duration. This persistent NCX(mito)-dependent Ca(2+) release has a role in post-tetanic potentiation, a form of short-term synaptic plasticity. By controlling [Ca(2+)](m) NCX(mito) regulates the activity of the Ca(2+)-sensitive enzymes pyruvate-, alpha-ketoglutarate- and isocitrate-dehydrogenases and affects the activity of the respiratory chain. Convincing experimental evidence suggests that supraphysiological activation of NCX(mito) contributes to neuronal cell death in the ischemic brain and, in epileptic neurons coping with seizure-induced ion overload, reduces the ability to reestablish normal ionic homeostasis. These data suggest that NCX(mito) could represent an important target for the development of new neurological drugs.

Correlation between oxidative Stress and Alteration of Intracellular Calcium Handling in Isoproterenol-Induced Myocardial Infarction

Myocardial Ca(2+) overload and oxidative stress are well documented effects associated to isoproterenol (ISO)-induced myocardial necrosis, but information correlating these two issues is scarce. Using an ISO-induced myocardial infarction model, 3 stages of myocardial damage were defined: pre-infarction (0-12 h), infarction (12-24 h) and post-infarction (24-96 h). Alterations in Ca(2+) homeostasis and oxidative stress were studied in mitochondria, sarcoplasmic reticulum and plasmalemma by measuring the Ca(2+) content, the activity of Ca(2+) handling proteins, and by quantifying TBARs, nitric oxide (NO) and oxidative protein damage (changes in carbonyl and thiol groups). Free radicals generated system, antioxidant enzymes and oxidative stress (GSH/GSSG ratio) were also monitored at different times of ISO-induced cardiotoxicity. The Ca(2+) overload induced by ISO was counterbalanced by a diminution in the ryanodine receptor activity and the Na(+)-Ca(+2) exchanger as well as by the increase in both calcium ATPases activities (vanadate- and thapsigargine-sensitive) and mitochondrial Ca(2+) uptake during pre-infarction and infarction stages. Pro-oxidative reactions and antioxidant defences during the 3 stages of cardiotoxicity were observed, with maximal oxidative stress during the infarction. Significant correlations were found among pro-oxidative reactions with plasmalemma and sarcoplasmic reticulum Ca(2+) ATPases, and ryanodine receptor activities at the onset and development of ISO-induced infarction. These findings could be helpful in the design of antioxidant therapies in this pathology.

Physiology and Pathophysiology of the Calcium Store in the Endoplasmic Reticulum of Neurons

The endoplasmic reticulum (ER) is the largest single intracellular organelle, which is present in all types of nerve cells. The ER is an interconnected, internally continuous system of tubules and cisterns, which extends from the nuclear envelope to axons and presynaptic terminals, as well as to dendrites and dendritic spines. Ca2+release channels and Ca2+pumps residing in the ER membrane provide for its excitability. Regulated ER Ca2+release controls many neuronal functions, from plasmalemmal excitability to synaptic plasticity. Enzymatic cascades dependent on the Ca2+concentration in the ER lumen integrate rapid Ca2+signaling with long-lasting adaptive responses through modifications in protein synthesis and processing. Disruptions of ER Ca2+homeostasis are critically involved in various forms of neuropathology.

Dissection of mitochondrial Ca2+ uptake and release fluxes in situ after depolarization-evoked [Ca2+](i) elevations in sympathetic neurons

We studied how mitochondrial Ca2+ transport influences [Ca2+](i) dynamics in sympathetic neurons. Cells were treated with thapsigargin to inhibit Ca2+ accumulation by SERCA pumps and depolarized to elevate [Ca2+(i); the recovery that followed repolarization was then examined. The total Ca2+ flux responsible for the [Ca2+](i) recovery was separated into mitochondrial and nonmitochondrial components based on sensitivity to the proton ionophore FCCP, a selective inhibitor of mitochondrial Ca2+ transport in these cells. The nonmitochondrial flux, representing net Ca2+ extrusion across the plasma membrane, has a simple dependence on [Ca2+](i), while the net mitochondrial flux (J(mito)) is biphasic, indicative of Ca+) accumulation during the initial phase of recovery when [Ca2+](i) is high, and net Ca2+ release during later phases of recovery. During each phase, mitochondrial Ca2+ transport has distinct effects on recovery kinetics. J(mito) was separated into components representing mitochondrial Ca2+ uptake and release based on sensitivity to the specific mitochondrial Na(+)/Ca2+ exchange inhibitor, CGP 37157 (CGP). The CGP-resistant (uptake) component of J(mito) increases steeply with [Ca2+](i), as expected for transport by the mitochondrial uniporter. The CGP-sensitive (release) component is inhibited by lowering the intracellular Na(+) concentration and depends on both intra- and extramitochondrial Ca2+ concentration, as expected for the Na(+)/Ca2+ exchanger. Above approximately 400 nM [Ca2+](i), net mitochondrial Ca2+ transport is dominated by uptake and is largely insensitive to CGP. When [Ca2+](i) is approximately 200-300 nM, the net mitochondrial flux is small but represents the sum of much larger uptake and release fluxes that largely cancel. Thus, mitochondrial Ca2+ transport occurs in situ at much lower concentrations than previously thought, and may provide a mechanism for quantitative control of ATP production after brief or low frequency stimuli that raise [Ca(2+)](i) to levels below approximately 500 nM.

Dynamic properties of an inositol 1,4,5-trisphosphate- and thapsigargin-insensitive calcium pool in mammalian cell lines

The functional characteristics of a nonacidic, inositol 1,4,5-trisphosphate- and thapsigargin-insensitive Ca2+ pool have been characterized in mammalian cells derived from the rat pituitary gland (GH3, GC, and GH3B6), the adrenal tissue (PC12), and mast cells (RBL-1). This Ca2+ pool is released into the cytoplasm by the Ca2+ ionophores ionomycin or A23187 after the discharge of the inositol 1,4,5-trisphosphate-sensitive store with an agonist coupled to phospholipase C activation and/or thapsigargin. The amount of Ca2+ trapped within this pool increased significantly after a prolonged elevation of intracellular Ca2+ concentration elicited by activation of Ca2+ influx. This pool was affected neither by caffeine-ryanodine nor by mitochondrial uncouplers. Probing mitochondrial Ca2+ with recombinant aequorin confirmed that this pool did not coincide with mitochondria, whereas its homogeneous distribution across the cytosol, as revealed by confocal microscopy, and its insensitivity to brefeldin A make localization within the Golgi complex unlikely. A proton gradient as the driving mechanism for Ca2+ uptake was excluded since ionomycin is inefficient in releasing Ca2+ from acidic pools and Ca2+ accumulation/release in/from this store was unaffected by monensin or NH4Cl, drugs known to collapse organelle acidic pH gradients. Ca2+ sequestration inside this pool, thus, may occur through a low-affinity, high-capacity Ca2+-ATPase system, which is, however, distinct from classical endosarcoplasmic reticulum Ca2+-ATPases. The cytological nature and functional role of this Ca2+ storage compartment are discussed.

Mitochondria contribute to Ca2+ removal in smooth muscle cells

Recent evidence, from a variety of cell types, suggests that mitochondria play an important role in shaping the change in intracellular calcium concentration ([Ca2+]i) that occurs during physiological stimulation. In the present study, using a range of inhibitors of mitochondrial Ca2+ uptake, we have examined the contribution of mitochondria to Ca2+ removal from the cytosol of smooth muscle cells following stimulation. In voltage-clamped single smooth muscle cells, we found that following a 8-s train depolarizing pulses, the rate of Ca2+ extrusion from the cytosol was reduced by more than 50% by inhibitors of cytochrome oxidase or exposure of cells to the protonophore carbonyl cyanide P-trifluoromethoxy-phenylhydrazone. Using the potential-sensitive indicator-tetramethyl rhodamine ethyl ester, we confirmed that the effect of these agents was associated with depolarization of the mitochondrial membrane. Since, the primary function of the mitochondria is to provide the cell's ATP, it could be argued that it is the ATP supply to the ion pumps which is limiting the rate of Ca2+ removal. However, experiments carried out with the mitochondrial Ca2+ uniporter inhibitor ruthenium red produced similar results, while the ATP synthetase inhibitor oligomycin had no effect, suggesting that the effect was not due to ATP insufficiency. These results establish that mitochondria in smooth muscle cells play a significant role in removing Ca2+ from the cytosol following stimulation. The uptake of Ca2+ into mitochondria is proposed to stimulate mitochondrial ATP production, thereby providing a means for matching increased energy demand, following the cell's rise in [Ca2+]i, with increased cellular ATP production.

Antagonists of excitatory amino acids and endogenous opioid peptides in the treatment of experimental central nervous system injury

Trauma to the central nervous system can lead to primary injuries occurring at the time of impact as well as secondary or delayed injury processes that can result from cellular hypoxia, oligemia/ischemia, edema and swelling, and intracranial hypertension that are manifested over a period of hours to weeks after the initial event. Although the mechanisms underlying delayed tissue injury are poorly understood, they appear to be associated with endogenous neurochemical changes resulting from traumatic nervous system injury. These neurochemical changes may include excessive neurotransmitter release, deregulation of ion homeostasis, and the synthesis, release, or activation of various "autodestructive" neurochemical factors. Experimental studies over the past decade indicate that these alterations mediate important components of the neurochemical cascade leading to central nervous system injury. Furthermore, pharmacologic manipulations of these neurochemical changes have been reported to attenuate secondary central nervous system damage, ameliorate neuronal death, and promote functional recovery after central nervous system injury. This article focuses on the role of excitatory amino acid neurotransmitters, endogenous opioid peptides, and magnesium in the pathophysiology of central nervous system injury and on the therapeutic manipulation of these systems to improve functional outcome after central nervous system injury.

Calcium accumulation by organelles within Myxicola axoplasm

1. 45Ca2+ accumulation into inulin-inaccessible compartments within cytoplasm from the giant axon of Myxicola infundibulum was measured as a function of free calcium, pH, and time. Accumulation reached a maximum after 1 h and remained stable for at least 3 h. 2. At 0.5, 5, and 50 microM [Ca2+], in the presence of 1 mM ATP or 5 mM succinate, steady-state calcium uptake had a bell-shaped dependence on pH with a maximum near pH 7. Uptake was abolished by the proton uncoupling reagent carbonyl cyanide p-trifluoromethoxy-phenylhydrazone (FCCP, 4 micrograms ml-1). 3. Uptake of the membrane permeant cation, [14C]-tetraphenylphosphonium (TPP+), also had a bell-shaped dependence on pH with a maximum pH approximately 7, suggesting a pH dependence of the electrical potential of a membrane enclosed cytoplasmic compartment. Cyanide (2 mM) inhibited TPP+ uptake. 4. Inositol 1,4,5-trisphosphate (IP3, 10 microM), reduced steady-state calcium accumulation by 20-22% at 0.5 microM free calcium, pH 7 (P < 0.01, n = 16) and at 5 microM free calcium, pH 8 (P < 0.0005, n = 35). No effects of IP3 were found at other pH or calcium concentrations. 5. Neither guanosine 5'-triphosphate (GTP) nor inositol 1,3,4,5-tetrakisphosphate (IP4) had an effect on calcium uptake (5 microM [Ca2+], pH 8). 6. At 0.5 microM free calcium; vanadate (10 microM) inhibited 20-30%, of the 45Ca2+ accumulation, thapsigargin (33 nM) inhibited 20-30%, and cyanide (2 mM) plus oligomycin B (2 micrograms ml-1), or valinomycin (1 microM), inhibited 70-80%. The fraction of uptake sensitive to thapsigargin fell as the free calcium increased; however, the sensitivity of uptake to cyanide plus oligomycin B was approximately 80% for 0.5, 5.0, and 50 microM [Ca2+]. 7. Thapsigargin had no additional inhibiting effect in the presence of cyanide plus oligomycin B. IP3 had no effect in the presence of cyanide plus oligomycin B or other mitochondrial inhibitors. 8. Results suggest the presence of both mitochondrial (70-80%) and non-mitochondrial (20-30%) calcium pools in this system (at 0.5-5.0 microM Ca2+). The apparent non-mitochondrial uptake (sensitive to thapsigargin, or IP3) is not detectable in the presence of mitochondrial inhibitors. We interpret these results as evidence of functional communication between mitochondrial and non-mitochondrial calcium stores.

Cytosolic calcium homeostasis in fungi: roles of plasma membrane transport and intracellular sequestration of calcium

Cytosolic free calcium ([Ca2+]c) has been measured in the mycelial fungus Neurospora crassa with Ca2(+)-selective microelectrodes. The mean value of [Ca2+]c is 92 +/- 15 nM and it is insensitive to external pH values between 5.8 and 8.4. Simultaneous measurement of membrane potential enables the electrochemical potential difference for Ca2+ across the plasma membrane to be estimated as about -60 kJ.mol-1-a value that cannot be sustained either by a simple Ca2(+)-ATPase, or, in alkaline conditions, by straightforward H+/Ca2+ exchange with a stoichiometric ratio of less than 5 H+/Ca2+. We propose that the most likely alternative mechanism of Ca2+ efflux is ATP-driven H+/Ca2+. In accord with this proposal, depletion of the ATP level from 2.5 to 0.5 mM by CN- elicits an increase in [Ca2+]c, but only in alkaline conditions in which the putative H+/Ca2(+)-ATPase would be selectively stalled. The insensitivity of Ca2+ homeostasis to CN- in more acid conditions implies that the Km (ATP) of the transport system is 100 microM or less. The increase in [Ca2+]c in the presence of CN- at pH 8.4 (50 nM.min-1) is compared with 45Ca2+ influx (0.62 mM.min-1) under the same conditions. The proportion of entering Ca2+ remaining free in the cytosol is only 8 x 10(-5), and since the concentration of available chelation sites on Ca2(+)-binding proteins is unlikely to exceed 100 microM, a major role for the fungal vacuole in short-term Ca2+ homeostasis is indicated. This notion is supported by the observation that cytosolic Ca2+ homeostasis is disrupted by a protonophore, which rapidly abolishes the driving force (a transmembrane pH difference) for Ca2+ uptake into fungal vacuoles.

Effects of metabolic blockade on the regulation of intracellular calcium in dissociated mouse sensory neurones

1. Impaired intracellular Ca2+ concentration ([Ca2+]i) regulation may underlie alterations in neuronal function during hypoxia or hypoglycaemia and may initiate cell damage. We have used the Ca2(+)-sensitive fluorophore, Fura-2, to study the regulation of [Ca2+]i in neurones isolated from mouse dorsal root ganglia. Mean resting [Ca2+]i was 163 +/- 11 nM (mean +/- S.E.M., n = 38). 2. Depolarization by exposure to 20 or 30 mM-K+ caused a rapid Co2(+)- and Cd2(+)-sensitive rise in [Ca2+]i, which subsequently declined with a time course usually fitted by the sum of two exponential functions. 3. Interference with mitochondrial function (by CN- or FCPP) or with glycolysis (by glucose removal) all raised [Ca2+]i by up to 220%. Addition of FCCP in the presence of CN- further increased [Ca2+]i. The response to CN- was still seen in the absence of extracellular Ca2+, although it attenuated rapidly, indicating release from an intracellular store. 4. Either CN- or glucose removal increased the rise in [Ca2+]i induced by K+ 2- to 3-fold and slowed recovery, suggesting interference with sequestration or extrusion of [Ca2+]i. 5. Resting [Ca2+]i rose when external Na+ was replaced by Li+ or N-methyl-D-glucamine, demonstrating the presence of a Na(+)-Ca2+ exchange process. However, Na+ replacement had only a slight effect on the handling of a Ca2+ load. 6. We conclude that (i) Ca2+ is released into the cytoplasm from intracellular organelles when energy supplies are reduced: (ii) that the extrusion or sequestration of Ca2+ entering the cell during electrical activity is rapidly impaired by interference with mitochondrial metabolism: and (iii) Na(+)-Ca2+ exchange makes only a small contribution to intracellular Ca2+ homeostasis. 7. [Ca2+]i would thus be expected to rise in vivo during hypoxia or hypoglycaemia and may initiate alterations in neuronal function. However, if a rise in Ca2+ is an important cause of cell damage in cerebral hypoxaemia, the combination of excitation and hypoxia will lead to the largest increases in [Ca2+]i, while hypoxia alone appears to cause only a small increase in [Ca2+]i in quiescent cells.

Interaction of intracellular ion buffering with transmembrane-coupled ion transport

The role of the Na/Ca exchanger in the control of cellular excitability and tension development is a subject of current interest in cardiac physiology. It has been suggested that this coupled transporter is responsible for rapid changes in intracellular calcium activity during single beats, generation of plateau currents, which control action potential duration, and control of intracellular sodium during Na/K pump suppression, which may occur during terminal states of ischemia. The actual behavior of this exchanger is likely to be complex for several reasons. First, the exchanger transports two ionic species and thus its instantaneous flux rate depends on both intracellular sodium and calcium activity. Secondly, the alteration in intracellular calcium activity, which is caused by a given transmembrane calcium flux, and which controls the subsequent exchanger rate, is a complex function of available intracellular calcium buffering. The buffers convert the ongoing transmembrane calcium fluxes into changes in activity that are a small and variable fraction of the change in total calcium concentration. Using a number of simple assumptions, we model changes in intracellular calcium and sodium concentration under the influence of Na/Ca exchange, Na/K ATPase and Ca-ATPase pumps, and passive sodium and calcium currents during periods of suppression and reactivation of the Na/K ATPase pump. The goal is to see whether and to what extent general notions of the role of the Na/Ca exchanger used in planning and interpreting experimental studies are consistent with its function as derived from current mechanistic assumptions about the exchanger. We find, for example, that based on even very high estimates of intracellular calcium buffering, it is unlikely that Na/Ca exchange alone can control intracellular sodium during prolonged Na/K pump blockade. It is also shown that Na/Ca exchange can contaminate measurements of Na/K pump currents under a variety of experimental conditions. The way in which these and other functions are affected by the dissociation constants and total capacity of the intracellular calcium buffers are also explored in detail.

Aging increases calcium influx at motor nerve terminal

To determine whether increased transmitter release from soleus nerve terminals of old C57BL/6J mice is caused by an altered Ca2+ regulation, the time course of post-tetanic potentiation of miniature endplate potential (MEPP) frequency was used as an indicator of the kinetics of Ca2+ metabolism in young (10 months) and old (24 months) mice. Post-tetanic potentiation properties were studied in either (1) 0.2 mM Ca2+, 5.0 mM Mg2+ Krebs; or (2) Ca2(+)-free/EGTA Krebs to eliminate Ca2+ influx, and thereby isolated Ca2+ buffering. In the 0.2 mM Ca2+ Krebs, the time constants of decay of augmentation (TA) and potentiation (TP) were longer in old (TA = 10.3 +/- 1.0 sec, TP = 195.3 +/- 5.4 sec) than in young (TA = 7.0 +/- 0.7 sec, TP = 78.8 +/- 6.6 sec) nerve terminals. Evoked transmitter release was measured in 0.4 mM Ca2+, 2.75 mM Mg2+ Krebs. Quantal content of the endplate potential was positively correlated with TA (r = 0.95) and with TP (r = 0.98). In the Ca2(+)-free/EGTA Krebs, there was no difference in post-tetanic potentiation properties between young and old terminals. These results suggest that Ca2+ influx into the soleus nerve terminal increases with aging. This may explain, at least in part, the increased quantal content observed at old terminals.

Measurements of intracellular Ca2+ in dissociated type I cells of the rabbit carotid body

1. The carotid body chemoreceptors are stimulated in situ by cyanide (CN-), which mimics the effect of hypoxia. We have shown that CN- increases a calcium-dependent potassium conductance (gK(Ca)) in single type I cells dissociated from the carotid body of the rabbit. We have now used the Ca2(+)-sensitive fluorophore, Fura-2, to measure intracellular Ca2+ directly in single type I cells. 2. CN- reversibly increased [Ca2+]i from approximately 90 nM to a mean of approximately 200 nM. Some of this Ca2+ originated from an intracellular store, which was depleted by exposure to Ca2(+)-free solutions. Prolonged application of CN- caused a sustained increase in [Ca2+]i, suggesting that CN- impairs the removal or sequestration of Ca2+. 3. pHi measured with the dye BCECF (2,7-bis(2-carboxyethyl)-5(and-6)-carboxyfluorescein) did not change consistently in response to CN-, although pHi changed predictably in response to both ammonium chloride and to acidification of the superfusate with CO2. 4. Potassium-induced depolarization (35 mM-K+) caused a large, cadmium-sensitive rise in [Ca2+]i. The K(+)-induced Ca2+ load was used to study the regulation of [Ca2+]i. 5. The clearance of a Ca2+ load was slowed either by removal of [Na+]o or by application of CN-. This shows that both a Na+-Ca2+ exchange and an energy-dependent process or processes contribute to the regulation of [Ca2+]i. 6. Carbachol (CCh, 10-100 microM), which also hyperpolarizes type I cells, caused a small transient rise in [Ca2+]i, indicating release from an exhaustible intracellular pool. The response to CN- was unaffected by prior or continued exposure to CCh, suggesting that the two stimuli operate by distinct mechanisms. 7. The increased gK(Ca) seen in type I cells in response to CN- thus reflects a change in cellular Ca2+ homeostasis. The rise in [Ca2+]i presumably underlies the documented increase in transmitter release from the carotid body in response to CN-. If chemotransduction is a consequence of the release of transmitters from the type I cell, the response of the carotid body to CN-, and possibly also to hypoxia, is thus a direct consequence of the energy dependence of Ca2+ homeostasis in the type I cell.

Calcium efflux and intracellular exchangeable calcium in mammalian nonmyelinated nerve fibers

Calcium efflux was measured in desheathed rabbit vagus nerves loaded with 45Ca2+. The effects of extracellular calcium, sodium, phosphate, potassium and lanthanum ions on the calcium efflux were investigated and the distribution of intracellular calcium determined by kinetic analysis of 45Ca2+ efflux profiles. The 45Ca2+ desaturation curve can be adequately described by three exponential terms. The rate constant of the first component (0.2 min-1) corresponds to an efflux from an extracellular compartment. The two slow components had rate constants of 0.03 and 0.08 min-1 and represent the efflux from two intracellular pools. The amounts of exchangeable calcium in these two pools, after a loading period of 150 min, were 0.170 and 0.102 mmol/kg wet weight, respectively. The total calcium efflux in physiological conditions amounted to about 24 fmol cm-2 sec-1. The magnitude of the two intracellular compartments as well as the total calcium efflux were markedly affected by extracellular phosphate, sodium and lanthanum, whereas the corresponding rate constants remained almost unchanged. Phosphate reversed the effect of sodium withdrawal on the calcium efflux: in the absence of phosphate, sodium withdrawal increased the calcium efflux to 224%, but in the presence of phosphate, sodium withdrawal decreased calcium efflux to 44%. Phosphate also affected the increase in calcium efflux produced by inhibitors of mitochondrial calcium uptake, suggesting that two different mitochondrial pools contribute to the control and regulation of intracellular calcium and of the transmembrane calcium transport.

Calcium regulation by and buffer capacity of molluscan neurons during calcium transients

The properties of Ca-regulation and -buffering of physiological levels of Ca-transients were examined in the soma of Archidoris monteryensis neurons. The rate of recovery from a Ca-transient was examined with two experimental protocols; in one the pulse duration was kept constant and its amplitude was varied, and in the other the duration was varied while the amplitude was kept constant. These experiments revealed that the recovery from a Ca-transient was approximately a first order process and the apparent first order rate constant was dependent on the duration of Ca-influx. The calcium buffer capacity of the cytoplasm was determined by an indirect method which utilised measured amounts of intracellular EGTA to reduce transient changes in free calcium. An equation for the cytoplasmic buffer capacity was derived on the assumption that the capacities of exogenous and endogenous Ca buffers summate linearly. The resting cytoplasmic Ca buffer capacity was 45.2 microM/delta pCa, when it was assumed that the incoming Ca diffuses a distance of 10 microns into the cytoplasm. For a diffusion distance of 5 microns it was 34.5 microM/delta pCa. In both cases, the buffer capacity increased with an increase in the size of Ca transient.

Electrically coupled but chemically isolated synapses: dendritic spines and calcium in a rule for synaptic modification

An influential model of learning assumes synaptic enhancement occurs when there is pre- and post-synaptic conjunction of neuronal activity, as proposed by Hebb (1949) and studied in the form of long-term potentiation (LTP). There is evidence that LTP has a post-synaptic locus of control and is triggered by an elevation of intracellular calcium ion concentration, [Ca2+]i. Since synapses which undergo LTP are usually situated on dendritic spines, three effects of spine morphology on this system should be considered: (i) synapses on spines are chemically isolated by the barrier to Ca2+ diffusion due to the spine neck dimensions; (ii) the resistance of the spine neck permits a given synaptic current to bring about greater depolarization (of the spine head membrane) than the same current into a dendrite; while (iii) the spine neck resistance does not significantly attenuate current flow (in the dendrite to spine direction) because of the relatively high impedance of the spine head, and this permits electrical coupling via the dendritic tree. The specificity of LTP to activated synapses on depolarized cells has recently been attributed to special properties of the receptor-linked channel specifically activated by N-methyl-D-aspartate (NMDA). This admits calcium and other ions only when there is both depolarization and receptor activation. However, consideration of point (ii) suggests that, for spines with high resistance necks, the current through a synapse on the spine head will cause sufficient depolarization to unblock the NMDA channel. Thus, the properties of the NMDA channel do not account for the requirement for conjunction of pre- and post-synaptic activity, if these channels are located on the spine head. This suggests that additional mechanisms are required to explain why it is necessary to depolarize the post-synaptic cell in order to induce LTP. As an alternative, it is postulated that there exist voltage-sensitive calcium channels (VSCCs) on the spine head membrane, of a type which require greater membrane depolarization for activation. To generate the greater depolarization required, both pre- and post-synaptic activation would be necessary. If so, the role of dendritic or somatically located NMDA channels may be to "prime" neurons for LTP by enchancing voltage-dependent responses. A corollary is that spine resistance may regulate the threshold number of synapses required to produce LTP. It is predicted that, on spines with very high neck resistance (say, greater than 600 M omega), synaptic current alone may produce sufficient depolarization to activate VSCCs.(ABSTRACT TRUNCATED AT 400 WORDS)

ATP-dependent and ATP-independent calcium buffers determined in isolated squid axoplasm by axoplasmic dilution using calcium-selective electrodes

Ca2+ buffering in axoplasm extruded from the squid giant axon was studied by axoplasmic dilution and Ca2+-selective electrodes. ATP-dependent Ca2+ buffering was lost when metabolism and endogenous ATP production were inhibited by axoplasmic dilution. Under these conditions ('low ATP' axoplasmic suspensions) Ca2+ activity (aCa) increased spontaneously. This increase was prevented by ATP addition. Addition of a Ca2+ load to axoplasmic suspensions following ATP addition ('normal ATP' axoplasmic suspensions) did not increase aCa. When a Ca2+ load was added to low ATP axoplasmic suspensions the value of aCa was only 5% of that expected from the Ca2+ load in a non-Ca2+ buffered system. Addition of the Ca2+ ionophore A23187 to Ca2+-loaded normal ATP axoplasmic suspensions increased aCa to a value 5% of that expected in a non-Ca2+ buffered system. Addition of A23187 or KCN to low ATP axoplasmic suspensions did not increase aCa above that produced by axoplasmic dilution. These data indicate the presence of ATP-dependent and ATP-independent Ca2+ buffering mechanisms in the squid axoplasm and that the ATP-dependent Ca2+ buffering was mediated by membrane-limited organelles presumably the mitochondria and endoplasmic reticulum.

Relation of acetylcholine release to Ca2+ uptake and intraterminal Ca2+ concentration in guinea-pig cortex synaptosomes

[14C]Acetylcholine (ACh) release and parallel alterations in 45Ca2+ uptake and intrasynaptosomal free CA2+ concentration ([Ca2+]i) were measured in guinea-pig brain cortex synaptosomes. Depolarization by high K+ concentrations caused a rapid transient increase in Ca2+ uptake, terminating within 60 s (rate constant = 0.060 s-1; t1/2 = 11.6 s). This resulted in a rapid increase (within 1 s) in [Ca2+1]i, which then fell to a maintained but still-elevated plateau level (t1/2 for the decline was 15 s). Peaks of [Ca2+]i showed a sigmoidal dependence on depolarization, contrasting with the simple linear dependence of plateau levels of [Ca2+]i. The K+-evoked ACh release also had two phases: a fast initial increase (t1/2 = 11.3 s), which terminated within 60 s, was followed by a slow additional increase during sustained depolarizations of up to 10 min. Depolarization by veratridine led to a slow gradual increase in Ca2+ uptake (t1/2 = 130 s) over a 10-min incubation period, whereas an elevated plateau level of [Ca2+]i was achieved within 2 min (without a rapid peak elevation). The Ca2+-dependent fraction of the veratridine-evoked ACh release correlated with the increase in [Ca2+]i rather than with Ca2+ uptake. Using two different methods of depolarization partially circumvented the time limitations imposed by a buffering Ca2+ indicator and we suggest that, in the main, ACh is released in bursts associated with [Ca2+]i transients.

Problems in assessing altered calcium homeostasis in the aging nervous system

Defining a role for altered calcium homeostasis on functional deficits in the aging brain is not only hindered by the complexity and abundance of the regulatory systems involved, but also by the inability to assess calcium regulation in vivo.

Calcium buffering in axons and axoplasm of Loligo

1. Ca-selective micro-electrodes were used to measure free Ca concentration in axons and extruded axoplasm. 2. Free Ca in axons immersed in artificial sea water containing 3 mM-Ca averaged 77 nM in freshly dissected axons and 4.9 microM in cyanide- or azide-poisoned axons. 3. Extruded axoplasm maintained a free Ca only a little higher than that of the axons from which it was obtained. 4. Axoplasmic buffering was investigated by titrating isolated axoplasm with CaCl2 or K-EGTA and monitoring the change in free Ca. Energy-dependent and energy-independent components of Ca binding could be recognized in fresh axoplasm. The energy-dependent fraction could be further subdivided into Ruthenium Red-sensitive and Ruthenium Red-insensitive components and the energy-independent fraction into a component of high affinity and rather low capacity and another component of low affinity and large capacity. 5. The Ruthenium Red-sensitive process could accumulate many millimoles Ca per kilogram axoplasm while still maintaining a free Ca close to 100 nM. After injection of Ruthenium Red into fresh axoplasm, binding is dramatically altered so that it closely resembles that in a metabolically poisoned preparation. 6. The Ruthenium Red-insensitive process has a small capacity and appears to be capable of lowering free Ca to about 200 nM. It can, however, lower free Ca to 50-150 nM if oxalate is also present. 7. Simultaneous measurement of pH and free Ca showed that axoplasmic pH only begins to fall appreciably in response to added Ca when mitochondrial Ca buffering becomes impaired. 8. Raising axoplasmic levels of Na or Li, but not K, tends to bring about a concomitant rise in free Ca.

The post-injury responses in trauma and ischemia: secondary injury or protective mechanisms?

Transient injuries to the central nervous system, whether due to trauma or ischemia, often produce long lasting metabolic derangements, lipid peroxidation, edema, and falls in blood flow at the lesion site. Because these post-injury responses are believed to be causes of secondary injury, much research effort has been devoted to developing therapies that prevent them. Recent studies suggest that excessive Ca entry into injured cells instigates these post-injury responses. A new theory is proposed to explain these post-injury responses. This theory posits that Ca ions entering dying cells activate phospholipases that break down membranes to release phosphates. The phosphates then bind and precipitate Ca ions, producing the profound and prolonged decreases in extracellular Ca activity that have been observed in traumatized spinal cords and ischemic brains. The phospholipase activity also facilitates release of lipid peroxides which enhance edema and reduce blood flow. Both of these in turn decrease Ca diffusion to the lesion site and slow the recovery of extracellular Ca activity, giving the tissue time to recover and avoiding the consequences of rapid restoration of extracellular Ca activity. The theory suggests that central nervous tissues evolved these Ca-activated responses as a general mechanism to protect neurons against excessive Ca entry. Brain and spinal cord tissues contain very high concentrations of phosphates, many times greater than is necessary to bind all the Ca ions in the tissues. This excessive Ca buffering capacity enables the tissue to sacrifice a small proportion of severely injured cells to reduce Ca entry into less severely injured neurons. This process will also rapidly eliminate moribund cells that may otherwise linger and consume oxygen and metabolic substrates better utilized by the remaining cells. If confirmed, this theory raises serious questions concerning the current experimental therapeutic approaches to CNS trauma and stroke. Therapy should perhaps be designed to optimize rather than to abort the post-injury responses.

Characterization of the ATP-promoted aspect of Na+-Ca2+ exchange present in squid retinal nerve axolemma

Using an in vitro system which consists of an axolemma-rich vesicle fraction prepared from squid retinal nerve fibers, an Na+-Ca2+ exchange process has been characterized and appears identical with that reported in squid giant axon. This exchange is absolutely dependent on the establishment of an Na+ gradient, shows monovalent and divalent cation specificity and is highly sensitive to monensin, A23187 and valinomycin but not to ouabain, digitoxigenin, vanadate, pentylenetetrazole, tetrodotoxin or tetraethylammonium. Furthermore, it was found that the exchange process is enhanced by the addition of ATP. This ATP-promoted aspects of Na+-Ca2+ exchange shares many similar characteristics with Na+-Ca2+ ATP hydrolysis and may indicate a common mechanism for both activities via a protein phosphorylation-dephosphorylation event.

Hippocampal long-term potentiation increases mitochondrial calcium pump activity in rat

45Ca2+ pump activity was measured in hippocampal homogenates after the induction of long-term potentiation (LTP) of perforant path-dentate gyrus synapses in vivo. The mitochondrial poison, sodium azide, was used to separate mitochondrial from non-mitochondrial 45Ca2+ uptake. High-frequency stimulation of the perforant path input produced a selectively increased mitochondrial uptake of calcium. This increased pump activity may permit granule cells to more effectively buffer higher intracellular calcium levels associated with LTP.

Continuous measurement of calcium influx in mammalian nonmyelinated nerve fibers: effects of Nao, Cao, and electrical activity

A new technique for continuous monitoring of the cellular calcium was developed and used for studying the effects of external and internal Na (Nao and Nai), external Ca (Cao), Ca ionophore A23187, and electrical activity on membrane-bound and intracellular Ca in mammalian nonmyelinated nerve fibers. Increasing Cao increased both the membrane-bound and the intracellular Ca. Lowering Nao increased the membrane-bound fraction of Ca indicating that lack of Nao enhanced the capacity of the plasma membrane to bind Ca, and produced an increase of the internal Ca pool. Increasing Nai by treatment with ouabain enhanced the Ca inflow in both, the presence and absence of Nao, presumably by stimulating the Cao/Nai exchange. The Ca ionophore A23187 produced a large and irreversible increase in the intracellular Ca without affecting the membrane-bound fraction. On the other hand, electrical activity, which is known to produce a large increase of the total Ca in squid axon, had no measurable effect on the total calcium content in our preparation. It is concluded that in mammalian nerve fibers a Ca load by exposition to Na-free solution or to A23187 produces an accumulation of Ca into the intracellular Ca stores, whereas during electrical activity the membrane-associated extrusion mechanisms are able to maintain the intracellular Ca2+ below the threshold for intracellular sequestration. Furthermore, the results indicate that the intracellular sequestration mechanisms are dependent on the internal concentration of Na.

Calcium and proton buffering and diffusion in isolated cytoplasm from Myxicola axons

Ion-selective electrodes recorded the pH (7.49 +/- 0.05, n = 8) and pCa (6.72 +/- 0.03, n = 40) in samples (approximately 1 microliter) of isolated Myxicola axoplasm mounted within 760-micron diameter plastic tubes. We determined the interactions between Ca2+ and H+ on axoplasmic buffers by microinjecting CaCl2 or HCl into the axoplasmic samples at a distance 75-125 micron from the tips of the electrodes (distance = r). When axoplasmic pH was lowered 0.97 +/- 0.095 from its resting value (measured at r = 125 micron) by injecting 4 nmol HCl, pCa dropped 0.30 +/- 0.05 (n = 6). When expressed in units of concentration, these data show that a HCl injection of approximately 4 mmol/l axoplasm increased H+ and Ca2+ activity by approximately 0.3 microM. Lowering axoplasmic pCa 2.20 +/- 0.43 (r = 75 micron) (n = 3) by injecting 40 pmol CaCl2 had only a small effect on pH. In other experiments, two Ca2+ electrodes measured the Ca2+ activity 125 and 375 micron from the site of CaCl2 injection. Evidence of Ca2+ buffering was obtained when the Ca2+ activity at these two locations was below that expected for simple Ca2+ diffusion away from the injection site. Centrifuged axoplasm (100,000 g) taken from the bottom of the centrifuge tube had a somewhat greater Ca2+ buffering capacity than that taken from the top of the tube. Electron microscopic studies of the centrifuged axoplasm showed a greater concentration of mitochondria and other axoplasmic vesicles in the bottom of the centrifuge tube. Ruthenium red (20-40 micrograms/ml) greatly reduced Ca2+ buffering. The mitochondrial inhibitors CN (2 mM) and oligomycin (a mixture of oligomycin A, B, and C, 5 micrograms/ml) also reduced Ca2+ buffering but were not as effective as ruthenium red. Axoplasm in which ATP and mitochondrial substrates were removed by dialysis was unable to lower free Ca2+ when the concentration of this ion was elevated to approximately 10 microM. In the presence of oligomycin to block mitochondrial ATPase, and with Mg2+ -ATP as the only source of energy, axoplasm lowered Ca2+ activity slowly; with succinate as the only metabolic substrate, axoplasm rapidly lowered the Ca2+ activity from approximately 10 microM to below 1 microM.

Dependence of ionized and total Ca in squid axons on Nao-free or high-Ko conditions

The level of intracellular Ca in squid axons (both ionized and total Ca) was studied as a function of the experimental variables [Na]i, [Na]o, pHi, cyanide, and depolarization. Ionized Ca was measured by following the light emission of aequorin while total Ca was measured by the atomic absorption analysis of samples of axoplasm. Aequorin glow is known to be increased either by the application of Nao-free solutions or by depolarization produced by external solutions containing greater than normal K concentrations. The present results show that if [Na]i is low, the depolarization that is brought about by solutions with elevated [K] leads to a resting light emission that is decreased rather than increased, as is the case when [Na]i is high. In axons where [Na]i is varied, a comparison of the increments in light emission produced by the application first of Na-free and then of high-K solutions shows that they have an identical dependence on [Na]i, with a half-activation of Ca entry produced by an [Na]i of 25-30 mM. Changes in pHi affect the aequorin signal produced by depolarization, with acidification reducing and alkanization increasing the response. Cyanide did not greatly affect the size of the signal resulting from either Nao removal or that from depolarization.

Presynaptic calcium diffusion from various arrays of single channels. Implications for transmitter release and synaptic facilitation

A one-dimensional model of presynaptic calcium diffusion away from the membrane, with cytoplasmic binding, extrusion by a surface pump, and influx during action potentials, can account for the rapid decay of phasic transmitter release and the slower decay of synaptic facilitation following one spike, as well as the very slow decline in total free calcium observed experimentally. However, simulations using this model, and alternative versions in which calcium uptake into organelles and saturable binding are included, fail to preserve phasic transmitter release to spikes in a long tetanus. A three-dimensional diffusion model was developed, in which calcium enters through discrete membrane channels and acts to release transmitter within 50 nm of entry points. Analytic solutions of the equations of this model, in which calcium channels were distributed in active zone patches based on ultrastructural observations, were successful in predicting synaptic facilitation, phasic release to tetanic spikes, and the accumulation of total free calcium. The effects of varying calcium buffering, pump rate, and channel number and distribution were explored. Versions appropriate to squid giant synapses and frog neuromuscular junctions were simulated. Limitations of key assumptions, particularly rapid nonsaturable binding, are discussed.

ATP-dependent calcium accumulation by non-mitochondrial organelles of axoplasm isolated from Myxicola giant axons

Axoplasm from freshly isolated Myxicola giant axons was mixed with small volumes of 'artificial axoplasm' containing 45Ca and either CaEGTA/EGTA or CaDTPA/DTPA buffers giving various nominal values of [Ca2+]. The axoplasm samples were centrifuged at 100 000 X g for 30 min to form a pellet and the percentage of 45Ca bound to the pellet was determined. The fraction of bound calcium rose with increasing values of [Ca2+] along an S-shaped curve. Carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) was used to reveal the presence of mitochondrial Ca uptake. At physiological values of [Ca2+], around 100 nM, Ca uptake was insensitive to FCCP. As [Ca2+] was elevated, increasing sensitivity to FCCP was noted above [Ca2+] = 0.5 microM. At low values of [Ca2+], including the physiological range, Ca binding was significantly reduced by vanadate and quercetin, agents known to inhibit Ca uptake mediated by Ca2+-activated ATPase reactions. Inhibition of Ca binding by these agents was approximately 50% at physiological values of [Ca2+]. ATP depletion decreased the percentage of Ca binding by the pellet at physiological [Ca2+]. The results suggest that about 50% of the Ca buffering by particulate matter in axoplasm is via organelles requiring intact Ca2+-ATPase reaction at physiological values of [Ca2+].

Uptake of calcium by the endoplasmic reticulum of the frog photoreceptor

We studied retinal photoreceptors of Rana pipiens by using techniques designed to investigate calcium localization. Particularly useful were methods in which intracellular sites of calcium uptake were detected by incubation of saponin-treated isolated retinas in calcium-containing media, with oxalate present as a trapping agent. With these procedures, cell compartments accumulate deposits, which can be shown to contain calcium by x-ray microanalysis. Calcium accumulation was prominent in the rough endoplasmic reticulum in the myoid region. In addition, deposits were observed in agranular reticulum and in certain Golgi-associated compartments of the myoid region, in mitochondria, in axonal reticulum, and in agranular reticulum of presynaptic terminals. Calcium was also detected in the endoplasmic reticulum of retinas fixed directly upon isolation, by a freeze-substitution method. The factors influencing accumulation of calcium in the endoplasmic reticulum were evaluated by a semiquantitative approach based on determining the relative frequency of calcium oxalate crystals under varying conditions. Calcium accumulation was markedly enhanced by ATP. Studies with a nonhydrolyzable ATP analogue (adenylyl- imidodiphosphate ) and with inhibitors of the sarcoplasmic reticulum Ca2+-Mg2+ ATPase (mersalyl and tetracaine) indicated that this ATP-dependent calcium uptake reflects an energy-dependent process roughly comparable to that in the sarcoplasmic reticulum.

Spatially nonuniform changes in intracellular calcium ion concentrations

The spatial variation of changes in intracellular calcium ions were studied with a one-dimensional scanning microphotometer. Changes in intracellular calcium were measured with a metallochromic dye, arsenazo III. Both the magnitude and the kinetics of changes in calcium were dramatically different in different regions of a cell. In Limulus ventral photoreceptors the maximum change was probably restricted to the rhabdomeric lobe.

Effects of internal sodium and hydrogen ions and of external calcium ions and membrane potential on calcium entry in squid axons

Squid giant axons were impaled with electrodes to measure pNai, pHi, Em, and were injected with either aequorin or arsenazo III to measure [Ca]i or with phenol red to measure [H]i. Depolarization of such axons with elevated [K] in sea water leads to a Ca entry that is a function of [Ca]o, [Na]i, and [H]i. With saturating [Na]i half-maximal Ca entry is produced by a [Ca]o of 0.58 mM. With saturating [Ca]o, depolarization produced by 450 mM-K+ leads to half-maximal Ca entry when [Na]i is 25 mM; entry is virtually undetectable if [Na]i is 18 mM. If [Ca]o is 50 mM, Ca entry upon depolarization as measured with aequorin is phasic with a rapid phase of light emission and a plateau; Ca entry as measured with arsenazo III shows no such phasic behaviour, absorbance vs. time is a square wave that closely follows the depolarization vs. time trace. Both detectors of [Ca]i show a square-wave response if [Ca]o is 3 mM. The introduction of 2 mM-CN into the sea water bathing the axon does not affect the response to depolarization nor does the destruction of most of the ATP in the axon following the injection of apyrase. If axons are microinjected with phenol red rather than arsenazo, the entry of Ca produces an acidification in the peripheral parts of the axoplasm. Other experiments measuring [Ca]i show that Ca entry is strongly inhibited by a decrease in pHi. Making sea water alkaline with pH buffers scarcely affects the Ca entry induced by depolarization; making axoplasm alkaline by adding NH4+ to sea water greatly enhances Ca entry by Na/Ca exchange and also enhances the ability of axoplasmic buffers to absorb Ca.

Measurements of intracellular ionized calcium in squid giant axons using calcium-selective electrodes

Ca2+-selective electrodes have been used to measure free intracellular Ca2+ concentrations in squid giant axons. Electrodes made of glass cannulas of about 20 microns in diameter, plugged with a poly(vinyl chloride) gelled sensor were used to impale the axons axially. They showed a Nernstian response to Ca2+ down to about 3 microM in solutions containing 0.3 M K+ and 0.025 M Na+. Sub-Nernstian but useful responses were obtained up to pCa 8. The electrodes showed adequate selectivity to Ca2+ over Mg2+, H+, K+ and Na+. To calibrate them properly, a set of standard solutions were prepared using different Ca2+ buffers (EGTA, HEEDTA, nitrilotriacetic acid) after carefully characterizing their apparent Ca2+ association constants under conditions resembling the axoplasmic environment. In fresh axons incubated in artificial seawater containing 4 mM Ca2+, the mean resting intracellular ionized calcium concentration was 0.106 microM (n = 15). The Ca2+-electrodes were used to investigate effects of different experimental procedures on the [Ca2+]i. The main conclusions are: (i) intact axons can extrude calcium ions at low [Ca2+]i levels by a process independent of external Na+; (ii) poisoned axons can extrude calcium ions at high levels of [Ca2+]i by an external Na+-dependent process. The level of free intracellular Ca attained at these latter conditions is about an order to magnitude greater than the resting physiological value.

Intramitochondrial and extramitochondrial free calcium ion concentrations of suspensions of heart mitochondria with very low, plausibly physiological, contents of total calcium

The 2-oxoglutarate dehydrogenase of intact rat heart mitochondria is activated by Ca2+, with 50% activation at approximately 0.5 nmol of total Ca/mg of mitochondrial protein, in the presence of Pi and Mg2+. Mitochondrial Ca contents in excess of 2 nmol/mg of protein result in 100% activation of the enzyme. Investigation of Ca2+ release from the mitochondria using the metallochromic indicator Arsenazo III defines a S0.5 of 5.4 +/- 0.4 nmol of Ca/mg of protein, when the endogenous Ca content of the mitochondria is progressively depleted with EGTA, prior to the initiation of the release process being studied. The subsequent determination of matrix free Ca2+ concentration by the "null-point" technique has allowed expression of these results in terms of free concentration rather than Ca content, with an activity coefficient of approximately 0.001 for matrix Ca2+. From the above, Ca2+ efflux from heart mitochondria is not saturated at the mitochondrial Ca contents or Ca2+ concentrations which give effective regulation of dehydrogenase activity. A consequence is that heart mitochondria do not buffer the pCa of the extramitochondrial medium at these Ca contents (less than 2 nmol/mg of protein), and this is shown in direct measurements of extramitochondrial pCa. This is taken to question the physiological significance of mitochondrial buffering of cytosolic free Ca2+ in normal heart.

A study of calcium compartments in rat brain cortex thin slices: effects of veratridine, lithium and of a mitochondrial uncoupler

The efflux kinetics of 45Ca from rat brain cortex thin slices previously equilibrated with it, was studied in a superfusion system. Two first order kinetic components of efflux from the tissue were found: k2 = 0.0667 min-1, that was unchanged by lowering the temperature from 37 degrees C to 15 degrees, and k3 = 0.0167 min-1 at 37 degrees C, that was reduced to 0.0897 min-1 at 15 degrees C. This suggests that k2 represents efflux from the extracellular space, and k3 that from the cellular compartment. Addition of the mitochondrial uncoupler carbonyl cyanide, m-chlorophenylhydrazone (CCCP) (10(-5)M) increased the efflux fractional rate constant of 45Ca by 35%, while no change in efflux was induced by 10 mM caffeine. Veratridine (10(-5)M) drastically reduced 45Ca efflux if superfusion was with physiological salt solution (150 mM sodium present), but not if 50 mM lithium replaced an equivalent amount of sodium in the superfusion fluid. This lithium-containing solution did not affect 45Ca efflux in the absence of veratridine. These results indicate that mitochondria accumulate only a minor fraction of intracellular 45Ca; that 45Ca possibly turns over very rapidly in the endoplasmic reticulum, and that most of 45Ca is present in a different, non-mitochondrial, non endoplasmic reticular compartment, the nature of which can be only conjectured.

Anoxia increases potassium conductance in hippocampal nerve cells

The effect of anoxia on nerve cell function was studied by intra- and extracellular microelectrode recordings from the CA1 and CA3 region in guinea pig hippocampal slices. Hyperpolarization and concomitant reduction of the nerve cell input resistance was observed early during anoxia. During this period the spontaneous activity first disappeared, then the evoked activity gradually disappeared. The hyperpolarization was followed by depolarization and an absence of a measurable input resistance. All the induced changes were reversed when the slice was reoxygenated. Reversal of the electro-chemical gradient for Cl- across the nerve cell membrane did not affect the course of events during anoxia. Aminopyridines blocked the anoxic hyperpolarization and attenuated the decrease of membrane resistance, but had no effect on the later depolarization. Blockers of synaptic transmission. Mn++, Mg++ and of Na+-channels (TTX) were without effect on the nerve cell changes during anoxia. It is suggested that the reduction of nerve cell excitability in anoxia is primarily due to increased K+-conductance. Thus, the nerve cells are hyperpolarized and the input resistance reduced, causing higher threshold and reduction of synaptic potentials. The mechanism of the K+-conductance activation is unknown at present.