Intrasynaptosomal compartmentation of calcium during depolarization-induced calcium uptake across the plasma membrane

Brain mitochondrial calcium transport: Origins of the set-point concept and its application to physiology and pathology

The transport of calcium across the inner mitochondrial membrane plays a key role in neuronal physiology and pathology. The kinetic responses of the uniporter and efflux pathways are such that a cytosolic free calcium 'set-point' can be established - above which there is net calcium accumulation into the matrix that is reversed when plasma membrane transport lowers cytosolic calcium. Pathological activation of N-methyl-d-aspartate receptor mediated sodium and calcium entry into the neuron, as occurs in stroke and spreading depression, places severe demands on both the ATP-generating and calcium loading capacities of the neuronal mitochondria as the set-point is exceeded. Experiments that led to the concept of the set-point are reviewed.

No consistent bioenergetic defects in presynaptic nerve terminals isolated from mouse models of Alzheimer's disease

Depressed cortical energy supply and impaired synaptic function are predominant associations of Alzheimer's disease (AD). To test the hypothesis that presynaptic bioenergetic deficits are associated with the progression of AD pathogenesis, we compared bioenergetic variables of cortical and hippocampal presynaptic nerve terminals (synaptosomes) from commonly used mouse models with AD-like phenotypes (J20 age 6 months, Tg2576 age 16 months, and APP/PS age 9 and 14 months) to age-matched controls. No consistent bioenergetic deficiencies were detected in synaptosomes from the three models; only APP/PS cortical synaptosomes from 14-month-old mice showed an increase in respiration associated with proton leak. J20 mice were chosen for a highly stringent investigation of mitochondrial function and content. There were no significant differences in the quality of the synaptosomal preparations or the mitochondrial volume fraction. Furthermore, respiratory variables, calcium handling, and membrane potentials of synaptosomes from symptomatic J20 mice under calcium-imposed stress were not consistently impaired. The recovery of marker proteins during synaptosome preparation was the same, ruling out the possibility that the lack of functional bioenergetic defects in synaptosomes from J20 mice was due to the selective loss of damaged synaptosomes during sample preparation. Our results support the conclusion that the intrinsic bioenergetic capacities of presynaptic nerve terminals are maintained in these symptomatic AD mouse models.

Pharmacological Characterization of the Mechanisms Involved in Delayed Calcium Deregulation in SH-SY5Y Cells Challenged with Methadone

Previously, we have shown that SH-SY5Y cells exposed to high concentrations of methadone died due to a necrotic-like cell death mechanism related to delayed calcium deregulation (DCD). In this study, we show that, in terms of their Ca²⁺ responses to 0.5 mM methadone, SH-SY5Y cells can be pooled into four different groups. In a broad pharmacological survey, the relevance of different Ca²⁺-related mechanisms on methadone-induced DCD was investigated including extracellular calcium, L-type Ca²⁺ channels, μ-opioid receptor, mitochondrial inner membrane potential, mitochondrial ATP synthesis, mitochondrial Ca²⁺/2Na⁺-exchanger, reactive oxygen species, and mitochondrial permeability transition. Only those compounds targeting mitochondria such as oligomycin, FCCP, CGP 37157, and cyclosporine A were able to amend methadone-induced Ca²⁺ dyshomeostasis suggesting that methadone induces DCD by modulating the ability of mitochondria to handle Ca²⁺. Consistently, mitochondria became dramatically shorter and rounder in the presence of methadone. Furthermore, analysis of oxygen uptake by isolated rat liver mitochondria suggested that methadone affected mitochondrial Ca²⁺ uptake in a respiratory substrate-dependent way. We conclude that methadone causes failure of intracellular Ca²⁺ homeostasis, and this effect is associated with morphological and functional changes of mitochondria. Likely, this mechanism contributes to degenerative side effects associated with methadone treatment.

Bioenergetics and transmitter release in the isolated nerve terminal

The isolated nerve terminal (or synaptosome) is the simplest preparation that allows mitochondrial bioenergetics to be studied in a physiological milieu, as well as facilitating investigation of the protein chemistry and regulation of synaptic vesicle exocytosis and recovery and providing a target for the study of the mechanism of action of numerous neurotoxins. This brief review discusses studies from our laboratory that may have provided some insight into these aspects of nerve terminal function.

Mitochondrial clearance of cytosolic Ca(2+) in stimulated lizard motor nerve terminals proceeds without progressive elevation of mitochondrial matrix [Ca(2+)]

This study used fluorescent indicator dyes to measure changes in cytosolic and mitochondrial [Ca(2+)] produced by physiological stimulation of lizard motor nerve terminals. During repetitive action potential discharge at 10-50 Hz, the increase in average cytosolic [Ca(2+)] reached plateau at levels that increased with increasing stimulus frequency. This stabilization of cytosolic [Ca(2+)] was caused mainly by mitochondrial Ca(2+) uptake, because drugs that depolarize mitochondria greatly increased the stimulation-induced elevation of cytosolic [Ca(2+)], whereas blockers of other Ca(2+) clearance routes had little effect. Surprisingly, during this sustained Ca(2+) uptake the free [Ca(2+)] in the mitochondrial matrix never exceeded a plateau level of approximately 1 microM, regardless of stimulation frequency or pattern. When stimulation ceased, matrix [Ca(2+)] decreased over a slow ( approximately 10 min) time course consisting of an initial plateau followed by a return to baseline. These measurements demonstrate that sustained mitochondrial Ca(2+) uptake is not invariably accompanied by progressive elevation of matrix free [Ca(2+)]. Both the plateau of matrix free [Ca(2+)] during stimulation and its complex decay after stimulation could be accounted for by a model incorporating reversible formation of an insoluble Ca salt. This mechanism allows mitochondria to sequester large amounts of Ca(2+) while maintaining matrix free [Ca(2+)] at levels sufficient to activate Ca(2+)-dependent mitochondrial dehydrogenases, but below levels that activate the permeability transition pore.

Roles of Na(+)-Ca2+ exchange and of mitochondria in the regulation of presynaptic Ca2+ and spontaneous glutamate release

The release of neurotransmitter from presynaptic terminals depends on an increase in the intracellular Ca2+ concentration ([Ca2+]i). In addition to the opening of presynaptic Ca2+ channels during excitation, other Ca2+ transport systems may be involved in changes in [Ca2+]i. We have studied the regulation of [Ca2+]i in nerve terminals of hippocampal cells in culture by the Na(+)-Ca2+ exchanger and by mitochondria. In addition, we have measured changes in the frequency of spontaneous excitatory postsynaptic currents (sEPSC) before and after the inhibition of the exchanger and of mitochondrial metabolism. We found rather heterogeneous [Ca2+]i responses of individual presynaptic terminals after inhibition of Na(+)-Ca2+ exchange. The increase in [Ca2+]i became more uniform and much larger after additional treatment of the cells with mitochondrial inhibitors. Correspondingly, sEPSC frequencies changed very little when only Na(+)-Ca2+ exchange was inhibited, but increased dramatically after additional inhibition of mitochondria. Our results provide evidence for prominent roles of Na(+)-Ca2+ exchange and mitochondria in presynaptic Ca2+ regulation and spontaneous glutamate release.

Mechanisms of neuronal damage in brain hypoxia/ischemia: focus on the role of mitochondrial calcium accumulation

Following a hypoxic-ischemic insult, the collapse of ion gradients results in the inappropriate release of excitatory neurotransmitters. Although excitatory amino acids such as glutamate are the likely extracellular mediators of the ensuing neuronal cell death, the intracellular events occurring downstream of glutamate receptor activation are much less clear. The present review attempts to summarize how Ca2+ overload of neurons following a hypoxic-ischemic insult is neurotoxic. In particular, the interlocked relation between mitochondrial Ca2+ accumulation and subsequent neuronal cell death is examined.

Mitochondria and neuronal glutamate excitotoxicity

The role of mitochondria in the control of glutamate excitotoxicity is investigated. The response of cultured cerebellar granule cells to continuous glutamate exposure is characterised by a transient elevation in cytoplasmic free calcium concentration followed by decay to a plateau as NMDA receptors partially inactivate. After a variable latent period, a secondary, irreversible increase in calcium occurs (delayed calcium deregulation, DCD) which precedes and predicts subsequent cell death. DCD is not controlled by mitochondrial ATP synthesis since it is unchanged in the presence of the ATP synthase inhibitor oligomycin in cells with active glycolysis. However, mitochondrial depolarisation (and hence inhibition of mitochondrial calcium accumulation) without parallel ATP depletion (oligomycin plus either rotenone or antimycin A) strongly protects the cells against DCD. Glutamate exposure is associated with an increase in the generation of superoxide anion by the cells, but superoxide generation in the absence of mitochondrial calcium accumulation is not neurotoxic. While it is concluded that mitochondrial calcium accumulation plays a critical role in the induction of DCD we can find no evidence for the involvement of the mitochondrial permeability transition.

Plasma membrane depolarization and disturbed Na+ homeostasis induced by the protonophore carbonyl cyanide-p-trifluoromethoxyphenyl-hydrazon in isolated nerve terminals

The effect of the protonophore carbonyl cyanide-p-trifluoromethoxyphenyl-hydrazon (FCCP) was studied on the intracellular [Na+], pH, and plasma membrane potential in isolated nerve terminals. FCCP induced a rise of [Na+]i at, and even below, the concentrations (0.025-1 microM) in which it is usually used in intact cells to eliminate Ca2+ uptake by mitochondria. The FCCP-induced increase of [Na+]i correlates with a fall in both the ATP level and the ATP/ADP ratio. In addition, a sudden rise of the intracellular proton concentration ([H+]i) from 83 +/- 0.4 to 124 +/- 0.7 nM was observed on the addition of FCCP (1 microM). Parallel with the rise in [H+]i, an abrupt depolarization was detected, followed by a slower decrease in the plasma membrane potential. Both the extent of the pHi change and the fast depolarization of the plasma membrane were proportional to the proton electrochemical gradient across the plasma membrane; when this gradient was increased, greater depolarization was detected. The slower decrease of the membrane potential after the fast initial depolarization was abolished when the medium contained no Na+. It is concluded that FCCP (1) gives rise to a depolarization by setting the plasma membrane potential close to the proton equilibrium potential and (2) enhances the intracellular [Na+] as a consequence of an insufficient ATP level and ATP/ADP ratio to fuel the Na+,K+/ATPase. Because both disturbed Na+ homeostasis and plasma membrane depolarization could profoundly interfere with Ca2+ homeostasis in the presence of protonophores, consideration given to these alterations may help to clarify the cellular Ca2+ sequestration processes.

Neuronal excitotoxicity: the role of mitochondria

Chronic activation of NMDA receptors by glutamate is toxic to cultured neurons. The extensive Ca2+ entry accompanying receptor activation is largely accumulated by the intracellular mitochondria, with resultant effects on mitochondrial membrane potential, ATP synthesis, glycolysis, reactive oxygen species generation and ultimately failure of cytoplasmic Ca2+ homeostasis and cell death. Each of these parameters is inter-related and in this review we describe attempts to separate out each factor to establish the sequence of events following NMDA-receptor activation. The conclusion is that mitochondrial Ca2+ accumulation is a key event in glutamate excitotoxicity, and that cells maintained by glycolysis in the absence of a mitochondrial membrane potential are highly resistant to glutamate excitotoxicity.

Action of New World scorpion venom and its neurotoxins in secretion

New World scorpion venom contains protein toxins specific for ion channels in the plasmalemma of excitable cells. The effects were examined of whole venoms from Tityus serrulatus, T. bahiensis and T. stigmurus, and some purified toxins in isolated nerve endings (synaptosomes) and pancreatic acinar cells. Both systems initiated exocytosis in a dose-dependent response to the venom or its bioactive protein toxins. Actions differed, however, such that pancreatic acinar cells required Ca2+ while cerebrocortical synaptosomes responded by a Ca(2+)-dependent mechanism, except in the case of one toxin, IV-5, that elicited a Ca(2+)-independent response. Membrane depolarization caused by scorpion venom toxins was measured via radioisotopic discharge of tetra[3H]phenylphosphonium bromide. The role of protein kinase C in second-messenger coupling in pancreatic acinar cells is favored over ion-exclusive routes characteristic of synaptosomes.

Contribution of plasma membrane and endoplasmic reticulum Ca(2+)-ATPases to the synaptosomal [Ca2+]i increase during oxidative stress

In the present study we analyzed the effect of ascorbate (0.8 mM)/Fe2+ (2.5 microM)-induced membrane lipid peroxidation on the levels of intracellular free calcium,[Ca2+]i and on the possible mechanisms involved in the perturbation of intracellular calcium homeostasis during oxidative stress. For this purpose, the influence of the ascorbate/iron oxidant system on the plasma membrane and endoplasmic reticulum Ca(2+)-dependent ATPases of brain cortical synaptosomes was studied. In addition, the influence of the peroxidative process on the uptake of calcium (45Ca2+) and on the Na+/Ca2+ exchange activity at the plasma membrane was evaluated. After ascorbate/Fe(2+)-induced membrane lipid peroxidation of the order of 18.05 +/- 4.20 nmol TBARS/mg protein, an increase in [Ca2+]i occurred, under basal or depolarizing conditions (30 mM KCl), which was dependent on the extracellular calcium concentration. Thus, for 1 and 3 mM extracellular calcium concentration, an increase of the resting [Ca2+]i values of 19.8% and 33.7% was observed, while after the K(+)-depolarization the enhancement of the [Ca2+]i was 18.4% and 29.5%, respectively. The Na+/Ca2+ exchange activity and the time-dependent influx of 45Ca2+ observed in basal conditions and after the 30 mM K(+)-depolarization, were not affected under the peroxidative conditions. The Ca(2+)-dependent ATPase activity of the synaptosomal plasma membrane was significantly depressed following peroxidation of membrane lipids, decreasing the V(max) by 48.1%, without significant changes in the affinity of the enzyme for calcium (K(m) for Ca2+ was 0.54 +/- 0.04 microM in control conditions and 0.56 +/- 0.034 microM in peroxidized conditions). The Ca(2+)-ATPase activity of the endoplasmic reticulum was also affected during ascorbate/iron-induced oxidative stress; thus, an inhibition of 45.2% was observed 5 min after adding ATP. These data suggest that the increase in synaptosomal [Ca2+]i due to oxidative stress may result from the inhibition of the plasma membrane and the endoplasmic reticulum membrane Ca(2+)-ATPase activities, probably as a result of the alteration of the lipid environment required for the maximal activity of these membrane enzymes. The consequent increase in [Ca2+]i may be responsible for the injury of the nervous tissue observed during several pathological conditions in which free radical generation seems to be involved.

Lack of involvement of [Ca2+]i in the external Ca(2+)-independent release of acetylcholine evoked by veratridine, ouabain and alpha-latrotoxin: possible role of [Na+]i

Synaptosomes were challenged by veratridine, ouabain and alpha-latrotoxin, and the release of 14C-acetylcholine (ACh) was measured in the absence of external Ca2+. We wished to test whether Ca2+ mobilized from internal stores triggered the ACh release that was independent of external Ca2+. We found that none of the agents altered the [Ca2+]i in a Ca(2+)-free medium. Buffering the intracellular Ca2+ concentration with BAPTA did not prevent the increase in release of 14C-ACh by veratridine or ouabain in the absence of Ca2+, however, it greatly reduced the release evoked in a Ca(2+)-containing medium. In parallel samples the release of ACh and the change in the internal Na+ concentration ([Na+]i) were measured. It was found that veratridine, ouabain and alpha-latrotoxin all enhanced [Na+]i in a concentration-dependent manner and a good quantitative relationship existed between the increase in [Na+]i and the release of ACh.

Synaptosomal plasma and mitochondrial membrane potentials during anoxia

The precise mechanism by which altered oxidative metabolism impairs neuronal function is unknown. Previous indirect studies suggest that anoxia's effects on the mitochondrial membrane potentials may underlie anoxia's actions. Twenty minutes of anoxia reduced the mitochondrial membrane potential of intact synaptosomes by 38-59 mV, but diminished the plasma membrane potential by only 4-10 mV. Anoxia did not alter the response of the plasma or mitochondrial membrane potentials to K+, nor did anoxia affect the reaction of the plasma membrane potential to valinomycin. However, anoxia diminished the response of the mitochondrial membrane potential to valinomycin by 50%. Thus, partial collapse of the mitochondrial membrane potential may be an important mediator of hypoxia-or anoxia-induced changes in neuronal function.

Effects of monensin and veratridine on acetylcholine release and cytosolic free Ca2+ levels in cerebrocortical synaptosomes of rats

Monensin (10(-8)-10(-4) M) caused a dose-dependent increase in the release of [3H]acetylcholine ([3H]ACh) from purified rat cerebrocortical synaptosomes, with an EC50 of approximately 1.6 x 10(-6) M. Extracellular Na+, but not Ca2+, was required for a monensin-induced increase in the release of [3H]ACh. Monensin also increased the cytosolic free Ca2+ concentration ([Ca2+]i) and uptake of 22Na+ in a dose-dependent manner. Monensin continued to cause a dose-dependent increase in [Ca2+]i in the absence of extracellular Ca2+, although an approximately 50% reduction was noted at concentrations of greater than 10(-5) M. The EC50 for the monensin-induced increase in [Ca2+]i was similar to that noted in the release of [3H]ACh. Veratridine exhibited effects similar to those of monensin, but a large portion of the increase in [Ca2+]i and [3H]ACh release was dependent on extracellular Ca2+. Measurements of rhodamine 6G fluorescence indicated that monensin and veratridine caused synaptosomal hyperpolarization and depolarization, respectively. Tetrodotoxin (10(-6) M) completely blocked all the effects of veratridine but had no effect on the activity of monensin. These results suggest that monensin increases the release of ACh at least in part by increasing [Ca2+]i, resulting from the increase in the Na+ influx through tetrodotoxin-insensitive mechanisms in rat cerebrocortical synaptosomes.

Synaptosomal [Ca2+]i as influenced by Na+/Ca2+ exchange and K+ depolarization

The modulation of the intrasynaptosomal concentration of Ca2+, [Ca2+]i, by Na+/Ca2+ exchange was studied using Indo-1 fluorescence. The electrochemical gradient of Na+ was manipulated by substituting Li+ or choline for Na+ in the external medium and, then, the influx of 45Ca2+ and the [Ca2+]i were measured. It was found that the increase in [Ca2+]i induced by K+ depolarization is lower if the value of [Ca2+]i has been previously raised by Na+/Ca2+ exchange, suggesting that Ca2+ entering by Na+/Ca2+ exchange reduces the Ca2+ entering by voltage-dependent calcium channels. Our results show that a value of [Ca2+]i of about 650 nM induced by Na+/Ca2+ exchange reduces by 50% the Ca2+ entering due to K+ depolarization and no Ca2+ enters through the channels if the [Ca2+]i is previously raised above about 800 nM. Furthermore, predepolarization of the synaptosomes in a Ca-free medium also inhibits by at least 40% the [Ca2+]i rise through Ca2+ channels. Thus, the results suggest that both predepolarization and [Ca2+]i rise due to Na+/Ca2+ exchange decrease the Ca2+ entering by voltage-sensitive Ca2+ channels. The Ca2+ entering by Na+/Ca2+ exchange might contribute to the regulation of neurotransmitter release. Our results also show that the presence of Li+ in the external medium decreases the buffering capacity of synaptosomes, probably by releasing Ca2+ from mitochondria by Li+/Ca2+ exchange.

Cytosolic free calcium concentrations in synaptosomes during histotoxic hypoxia

Altered cytosolic free calcium concentrations ([Ca2+]i) accompany impaired brain metabolism and may mediate subsequent effects on brain function and cell death. The current experiments examined whether hypoxia-induced elevations in [Ca2+]i are from external or internal sources. In the absence of external calcium, neither KCl depolarization, histotoxic hypoxia (KCN), nor the combination changed [Ca2+]i. However, with external CaCl2 concentrations as small as 13 microM, KCl depolarization increased [Ca2+]i instantaneously while hypoxia gradually raised [Ca2+]i. The combination of KCN and KCl was additive. Increasing external calcium concentrations up to 2.6 mM exaggerated the effects of K+ and KCN on [Ca2+]i, but raising medium calcium to 5.2 mM did not further augment the rise. Diminishing the sodium in the media, which alters the activity and perhaps the direction of the Na/Ca exchanger, reduced the increase in [Ca2+]i due to hypoxia, but enhanced the KCl response. The changes in ATP following K+ depolarization, KCN or their combination in the presence of physiological calcium concentrations did not parallel alterations in [Ca2+]i, which suggests that diminished activity of the calcium dependent ATPase does not underlie the elevation in [Ca2+]i. Valinomycin, an ionophore which reduces the mitochondrial membrane potential, elevated [Ca2+]i and the effects were additive with K+ depolarization in a calcium dependent manner that paralleled the effects of hypoxia. Together these results suggest that hypoxia-induced elevations of synaptosomal [Ca2+]i are due to an inability of the synaptosome to buffer entering calcium.

Na+/Ca2+ exchange activity is increased in Alzheimer's disease brain tissues

These studies were performed to determine the changes that occur in Na+/Ca2+ exchange activity in Alzheimer's disease (AD) brain tissues. Cerebral plasma membrane vesicles were purified by sucrose density gradient centrifugation from frozen postmortem hippocampal/temporal cortex tissue slices derived from age matched brains of normal, AD and non-Alzheimer dementia (NAD) origin (autopsy confirmed). Membrane marker assays (Na/K ATPase, muscarinic receptor, cytochrome c oxidase) revealed no change in membrane purity across different preparations. Thin-section electron microscopy revealed predominantly intact unilamellar vesicles. Vesicles were preincubated for 15 min (37 degrees C) in buffer containing 132 mM NaCl, 5 mM KCl, 1.3 mM MgCl2, 10 mM glucose and 10 mM HEPES (pH 7.4). Ca2+ uptake was initiated by diluting vesicles 20-fold with buffer containing either 132 mM NaCl or 132 mM choline chloride and 45CaCl2 then terminated by addition of 200 microM LaCl3 and rapid filtration. Ca2+ content increased rapidly at first and then maintained a steady plateau for up to 5 min. When the Ca2+ ionophore A23187 (10 microM) with 100 microM EGTA was added after 4 min, Ca2+ content was reduced to 10% of its original value. Ruthenium red (10 microM) had no effect on Ca2+ content. Na(+)-dependent Ca2+ uptake (Ca2+ content measured in choline chloride minus that measured in NaCl) was increased in AD brains as evidenced by both an increase in the initial rise in Ca2+ content and in elevated values of peak plateau Ca2+ content.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytosolic free calcium and ATP in synaptosomes after ischemia

Elevations in cytosolic free calcium ([Ca2+]i) precede electrophysiological alterations due to ischemia in vivo. An in vitro model of these changes would help to elucidate their molecular basis. A model of postdecapitative ischemia was used to study these interactions. Nerve endings (i.e. synaptosomes) were isolated either immediately after decapitation or at various time periods after decapitation. Synaptosomal [Ca2+]i and ATP concentrations were determined during a basal period and following depolarization. K(+)-depolarization produced an initial spike of [Ca2+]i that was followed by a new equilibrium value. Ischemia elevated the basal [Ca2+]i and the new equilibrium [Ca2+]i after KCl but suppressed the [Ca2+]i spike. However, the difference between the basal [Ca2+]i and the new equilibrium [Ca2+]i after K(+)-depolarization did not vary with ischemia. Although ischemia reduced ATP, K(+)-depolarization did not alter ATP concentrations in either the controls or the ischemia group, which suggests that synaptosomal mitochondria can meet an energy demand after ischemia. ATP was inversely related to the basal or the new equilibrium [Ca2+]i following depolarization. These changes in [Ca2+]i may underlie the alterations in neurotransmitter release and cell death following ischemia. This appears to be a useful model in which to study the molecular basis of ischemia induced changes in [Ca2+]i.

Presynaptic modulation of glutamate and dynorphin release by excitatory amino acids in the guinea-pig hippocampus

Excitatory amino acid agonists and antagonists were evaluated for their ability to affect the concomitant release of endogenous L-glutamate and dynorphin A(1-8)-like immunoreactivity from guinea-pig hippocampal mossy fiber synaptosomes. Previous work in this laboratory demonstrated that L(+)2-amino-4-phosphonobutyrate inhibits the potassium-evoked release of these endogenous neurotransmitters from guinea-pig but not rat hippocampal mossy fiber synaptosomes. Therefore, the present study was conducted to evaluate excitatory amino acid agonists as indices to the functional properties of this L(+)2-amino-4-phosphonobutyrate-sensitive glutamatergic autoreceptor on mossy fiber terminals. Low micromolar concentrations of quisqualate, but not kainate, N-methyl-D-aspartate, nor RS-alpha-amino-3-hydroxy-5-methyl-4-isoazole-propionic acid, significantly inhibited the potassium-evoked release of both L-glutamate and dynorphin A(1-8)-like immunoreactivity. Quisqualate-induced inhibition of L-glutamate release from mossy fiber terminals was antagonized by the non-N-methyl-D-aspartate antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. In contrast, high concentrations of kainate enhanced the potassium-evoked release of L-glutamate and dynorphin A(1-8)-like immunoreactivity, and this potentiation was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione. Kainate (1 mM) was the only agonist which significantly enhanced the basal release of L-glutamate, whereas the spontaneous efflux of dynorphin A(1-8)-like immunoreactivity was not affected by any of the agonists tested. The results presented in this paper suggest the existence of inhibitory and excitatory presynaptic glutamatergic autoreceptors that act to modulate the release of endogenous L-glutamate- and prodynorphin-derived peptides from guinea-pig hippocampal mossy fiber terminals. These inhibitory and excitatory autoreceptors, which are sensitive to quisqualate/L(+)2-amino-4-phosphonobutyrate or kainate, respectively, may play an important role in regulating synaptic activity at glutamatergic synapses throughout the central nervous system.

The role of Na+ in muscarinic receptor-mediated catecholamine secretion in the absence of extracellular Ca2+ in cat perfused adrenal glands

1. The role of Na+ in muscarinic receptor-mediated catecholamine secretion, which is independent of extracellular Ca2+, was investigated by observing the effect of veratridine and ouabain in perfused adrenal glands of the cat. 2. Veratridine (10(-4) M) markedly enhanced catecholamine secretion evoked by acetylcholine (ACh, 10(-4) M) during perfusion with Ca2(+)-free Locke solution containing hexamethonium (10(-3) M). The enhancement tended to be larger for noradrenaline secretion than for adrenaline secretion. Qualitatively the same result was obtained in the response to pilocarpine (5 x 10(-4) M). 3. Ouabain (10(-4) M) also enhanced ACh- and pilocarpine-induced catecholamine secretions, especially noradrenaline secretion in the absence of extracellular Ca2+. 4. Tetrodotoxin (10(-6) M) blocked the enhancing effect of veratridine on ACh-induced catecholamine secretion, but not that of ouabain in the absence of extracellular Ca2+. 5. When NaCl was replaced with sucrose, there was no secretory response to ACh regardless of the presence or absence of veratridine or ouabain. However, when ouabain, but not veratridine, was infused with Na+ before the replacement of NaCl, the response to ACh was substantially augmented. 6. These results indicate that Na+ is essential in the initiation of muscarinic receptor-mediated catecholamine secretion and its enhancement by veratridine and ouabain in the absence of extracellular Ca2+. Both drugs seem to increase the intracellular concentration of Na+ through different mechanisms and result in increases in the efficiency of Ca2+ mobilization from intracellular Ca2+ pools linked to muscarinic receptors.

12-O-tetradecanoylphorbol 13-acetate and forskolin modify muscarinic receptor-linked Ca2+ mobilization in SH-SY5Y neuroblastoma cells through different mechanisms

The phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA), which causes differentiation of SH-SY5Y neuroblastoma cells, reduces carbachol binding and carbachol-stimulated Ca2+ mobilization in these cells. The decrease in responsiveness to carbachol is due partially to a reduction in the amount of Ca2+ released by the cells and partially to a decrease in the sensitivity of the cells to carbachol. These effects probably can be attributed to a reduction in muscarinic receptor number and a decrease in receptor affinity, respectively. Forskolin, an alkaloid known to cause an increase in cellular cyclic AMP, enhances Ca2+ influx into the cells without affecting the cytosolic free Ca2+ concentration. The alkaloid causes an apparent restoration of the reduced Ca2+ release, caused by TPA, but does not affect the sensitivity of the cells to carbachol. Forskolin increases the decay of carbachol-induced increase in cytosolic Ca2+. The effects of TPA appear to be linked directly to receptor function, whereas those of forskolin are due to the effect of cyclic AMP on cellular Ca2+ metabolism.

Depolarization of human neuroblastoma cells as a result of muscarinic receptor-induced rise in cytosolic Ca2+

The role of intracellular free Ca2+ in muscarinic-receptor linked depolarization of SH-SY5Y neuroblastoma cells has been determined by using the bisoxonol membrane potential probe DiBaC4-(3) and intracellular Ca2+ indicator fura-2 respectively. Carbachol and the Ca2+ ionophore, ionomycin, at concentrations which caused similar rises in intracellular Ca2+ increased the bisoxonol fluorescence (depolarization) to the same extent. The membrane potential responses, but not the changes in intracellular Ca2+, were dependent on extracellular Na+. Ionomycin depletion of intracellular Ca2+ with EGTA and ionomycin or loading the cells with a Ca2+ buffer, BAPTA, reduced the carbachol-induced depolarization. The results suggest that a rise in intracellular Ca2+ may cause depolarization through an increase in the Na+ permeability.

Evaluation of the Ca2+ concentration in purified nerve terminals: relationship between Ca2+ homeostasis and synaptosomal preparation

The presynaptic Ca2+ concentration ([Ca]i) was evaluated by studying intracellular free Ca2+ with quin-2 and fura-2 in synaptosomal preparations. The synaptosomal preparations were purified with hyperosmotic (sucrose) and isoosmotic (Percoll) density gradient centrifugation. Synaptosomes are most viable in the heavier fractions of the density gradients. These synaptosomal fractions exhibit the lowest [Ca]i, [204 +/- 2 nM for Percoll (C-band) synaptosomes, loaded at 30 degrees C with the acetoxymethyl ester of fura-2 (fura-2-AM)], a high stability during prolonged incubations at 37 degrees C, and a more potent response to membrane depolarization by elevated extracellular [K+]. [Ca]i measurement was critically dependent on dye loading, calibration, type of dye used, synaptosomal preparation, and incubation temperature (30 degrees or 37 degrees C). Loading quin-2 in synaptosomes inserts a considerable buffer component in the synaptosomal [Ca]i regulation, and consequently there is a quin-2 dependency of [Ca]i, independent of endogenous heavy metal ions. Use of fura-2 is preferable in synaptosomes, although above a critical fura-2-AM/protein ratio during loading ester hydrolysis is not complete, giving rise to errors in [Ca]i determination. Ionomycin is a selective tool to detect the presence of partially hydrolyzed esters and saturate indicators in the cytosol with Ca2+ for calibration. Parallel studies on lactate dehydrogenase and fura-2 fluorescence indicate that synaptosomal viability is very sensitive to prolonged incubations at 37 degrees C. This study shows the applicability of measuring steady-state [Ca]i and dynamic [Ca]i changes quantitatively in fura-2-loaded synaptosomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca2+-dependent and Ca2+-independent glutamate release, energy status and cytosolic free Ca2+ concentration in isolated nerve terminals following metabolic inhibition: possible relevance to hypoglycaemia and anoxia

Hypoglycaemia and anoxia both cause massive release of glutamate from the brain in vivo, and the nature of this release was investigated using guinea-pig cerebral-cortical synaptosomes and iodoacetate and rotenone to simulate the energetic consequences of these conditions. Glutamate release (by continuous fluorimetry), cytoplasmic free Ca2+ (by fura-2), membrane potentials, ATP, ADP and creatine phosphate were determined in parallel, following the addition of iodoacetate or rotenone, alone or in combination. Ca2+-dependent glutamate release had a high energy requirement which could only be satisfied by aerobic glycolysis. Respiration using endogenous substrates, or anaerobic glycolysis following rotenone, caused a progressive inhibition of Ca2+-dependent release, correlating with a decline in the total ATP/ADP ratio and creatine phosphate. With rotenone, an increase in Ca2+-independent glutamate release was observed, correlating with a decline in plasma membrane potential. Only a slight increase in free Ca2+ was seen. Rotenone plus iodoacetate caused an almost immediate collapse of ATP/ADP ratio and a parallel loss of Ca2+-dependent glutamate release before free Ca2+ had risen to a level sufficient for exocytosis. In contrast, Ca2+-independent glutamate release increased. The Ca2+-dependent release of L-glutamate had the characteristics of an exocytotic transmitter release mechanism, being energy-dependent and triggered by elevated cytoplasmic free Ca2+ concentration. A distinct Ca2+-independent release of cytoplasmic glutamate occurred by reversal of the Na+-coupled uptake carrier, which was accelerated by a decline in the Na+ gradient. It is concluded that the Ca2+-independent release of cytoplasmic glutamate may make the major contribution to the excitotoxic release of glutamate in hypoglycaemic and anoxic conditions.

Ultrastructure of mouse foliate taste buds: synaptic and nonsynaptic interactions between taste cells and nerve fibers

High voltage electron microscopy and conventional transmission electron microscopy were used to examine the ultrastructure of foliate taste buds of mice. Computer-assisted, three-dimensional reconstructions from serial sections were used to visualize regions of interaction between taste cells and nerve fibers. Based on criteria previously established for murine vallate taste buds (Kinnamon et al., '85), foliate taste cells were classified as dark, light, or intermediate depending on their cytoplasmic content and the characteristics of their nuclei. Cells of foliate taste buds display a continuous range of morphologies, from "typical" dark cells to "typical" light cells. Cells of dark, intermediate, and light morphologies all make afferent synapses onto nerve processes, suggesting that cells of all 3 types are sensory in function. Synapses between taste cells and nerve processes may be either macular or fingerlike in shape. No efferent synapses were found. In addition to conventional synapses, taste cells exhibit 2 other types of specializations at sites of apposition with nerve fibers: subsurface cisternae and atypical mitochondria. Subsurface cisternae are narrow sacs of endoplasmic reticulum that are closely apposed to the inner leaflet of the taste cell membrane. Possible functions of subsurface cisternae include synthesis of synaptic membrane components, modification of the electrical or adhesive properties of the taste cell membrane, and exchange of trophic factors with nerve processes. Atypical mitochondria are usually much larger than typical taste cell mitochondria, and their cristae often display a swollen, twisted configuration. These mitochondria are closely apposed to the inside of the taste cell membrane adjacent to nerve fibers. Atypical mitochondria may be providing unusual amounts of energy for metabolic reactions in their vicinities or participating in calcium buffering in the taste cell. Within taste cells, presynaptic specializations, subsurface cisternae, and mitochondria are often clustered together to form "synaptic ensembles." We hypothesize that the functions served by the subsurface cisternae and mitochondria, as well as synaptic transmission, may be important in interactions between taste cells and nerve fibers.

Zinc uptake into synaptosomes

Zinc uptake was studied in synaptosomes, isolated by the Ficoll flotation technique, using the radiotracer 65Zn. True uptake of zinc could be discriminated from binding to the outside of the synaptosomes by the absence of accumulation at 0 degree C and the dependency of the rate of uptake on the medium osmolarity. The zinc uptake, studied in the presence of various zinc-complexing agents, showed saturation kinetics when analyzed in terms of [Zn]free, yielding Km = 0.25 microM. The zinc uptake was independent of both ATP and the Na+ gradient. No efflux of zinc could be demonstrated from preloaded synaptosomes due to the formation of insoluble zinc complexes inside the synaptosomes. The results are discussed in terms of the modulation of diverse neurochemical processes by zinc.

Effects of carbonyl cyanide m-chlorophenylhydrazone on quantal transmitter release and ultrastructure of frog motor nerve terminals

The quantal acetylcholine release and the ultrastructural effects of the metabolic inhibitor carbonyl cyanide m-chlorophenylhydrazone have been examined at frog neuromuscular junctions. Carbonyl cyanide m-chlorophenylhydrazone (2 microM) caused a temperature-dependent block of evoked quantal transmitter release accompanied by an increase in the rate of spontaneous quantal release. The carbonyl cyanide m-chlorophenylhydrazone-induced increase in miniature endplate potential frequency was neither antagonized nor prevented by tetrodotoxin. It also occurred in a Ca2+-free medium and after replacement of Ca2+ by Sr2+, indicating that it does not depend upon a Na+ or Ca2+ influx from the external medium but may act by releasing Ca2+ from intraterminal stores. Spontaneous quantal transmitter release was exhausted irreversibly within 4 h of carbonyl cyanide m-chlorophenylhydrazone (2 microM) action, during which time an average of 4.7 x 10(5) acetylcholine quanta were released per junction. The morphologic analysis revealed a significant temperature and time-dependent reduction in the number of synaptic vesicles with swelling and dispersion of mitochondria within the motor nerve terminals. Changes in synaptic vesicle number appear to be directly related to the intensity of transmitter release. The good correlation observed between the number of quanta secreted and the number of vesicles lost by nerve terminals in the absence of vesicle recycling provides an estimate of the initial store of transmitter quanta.

Characterization of the exocytotic release of glutamate from guinea-pig cerebral cortical synaptosomes

A continuous enzyme-linked fluorometric assay was used for determining the characteristics for glutamate exocytosis from guinea-pig cerebrocortical synaptosomes. Ca2+-dependent release can be induced not only by K+, but also by the Na+ channel activator veratridine and the Ca2+ ionophore ionomycin. K+-induced release can be inhibited by the Ca2+ channel inhibitor verapamil. Sr2+ and Ba2+ substitute for Ca2+ in promoting K+-induced release. Agents that would be predicted to transform the transvesicular pH gradient into a membrane potential are without effect on glutamate release. However, the protonophore carbonylcyanide p-trifluoromethoxyphenylhydrazone causes a time-dependent loss of exocytosis that is oligomycin insensitive and may be due to depletion of vesicular glutamate. The Ca2+-independent release of glutamate from the cytosol on depolarization is unchanged or promoted by metabolic inhibitors that lower the ATP/ADP ratio. In contrast. Ca2+-dependent release is ATP dependent and is blocked by the combined inhibition of oxidative phosphorylation and glycolysis.

Calcium and the aging nervous system

Many aspects of calcium homeostasis change with aging. Numerous calcium compartments complicate studies of altered calcium regulation. However, age-related decreases in calcium permeation across membranes and mobilization from organelles may be a common fundamental change. Deficits in ion movements appear to lead to altered coupling of calcium-dependent biochemical and neurophysiological processes and may lead to pathological and behavioral changes. The calcium-associated changes during aging probably do not occur with equal intensity in all cell types or in different parts of the same cell. Thus, cells or compartments with a high proportion of calcium activated processes would be more sensitive to diminished calcium availability. These age-related changes may predispose the brain to the development of age-related neurological disorders. The effects of decreased ion movement may be further aggravated by an age-related decline in other calcium-dependent processes. Depression of some of these calcium-dependent functions appears physiologically significant, since increasing calcium availability ameliorates age-related deficits in neurotransmission and behavior. A better understanding of the interactions between calcium homeostasis and calcium-dependent processes during aging will likely help in the design of more effective therapeutic strategies.

Role of sodium-calcium exchange in regulation of intracellular calcium in nerve terminals

Ca efflux from rat brain presynaptic nerve terminals (synaptosomes) was examined after loading the terminals with 45Ca during a brief depolarization, usually in media containing 20 microM Ca labeled with 45Ca, to assure a small (physiological) load. Efflux of 45Ca was very slow in the absence of external Na and Ca (approximately 0.5% of the load/s) and was greatly accelerated by Na and/or Ca (presumably Na+-Ca2+ and Ca2+-Ca2+ exchange, respectively). The dependence of 45Ca efflux on external Na was sigmoid, with a Hill coefficient of approximately 2.5; this implies that more than two external Na ions are required to activate the efflux of one Ca ion. The external Na (Nao)-dependent Ca efflux was inhibited by 1 mM external La, by low temperature (Q10 congruent to 2.3), and by raising external K (to depolarize the synaptosomes). With small Ca loads, the mitochondrial uncoupler, carbonyl cyanide-p-trifluoromethoxyphenylhydrazone (FCCP), had negligible effect on either Ca uptake or efflux; with large loads (greater than or equal to 5 nmol/mg protein), however, FCCP reduced the depolarization-stimulated Ca uptake and increased the Nao-dependent Ca efflux. These effects may be attributed to reduction of mitochondrial Ca sequestration. Mitochondria do not appear to sequester much Ca when the loads are smaller (and more physiological). Estimations of Ca efflux indicate that approximately 20% of a small 45Ca load (approximately 0.75 nmol Ca/mg protein) may be extruded via Na+-Ca2+ exchange within 1 s; this corresponds to a net Ca efflux of approximately 110 pmol Ca X mg protein-1 X s-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased free intrasynaptosomal Ca2+ by neurotoxic organometals: distinctive mechanisms

Effects of several alkylmetals on free intrasynaptosomal Ca2+ concentration, [Ca2+]i, were studied in vitro using the fluorescent Ca2+ indicator fura-2. Neurotoxic alkylmetals methylmercury (Met-Hg), triethyllead (TEL), triethyltin (TET), and trimethyltin (TMT) (at 2.5-30 microM) increased [Ca2+]i to different degrees. Met-Hg was the most potent, elevating [Ca2+]i 100-800 nM, dose dependently and significantly more than high K+ (150 nM) or veratridine (350 nM). The effect of Met-Hg could not be inhibited with a Ca2+ channel blocker, verapamil, nor with a Na+ channel blocker, tetrodotoxin. Inhibition of the mitochondrial Ca2+ uptake in situ with rotenone + oligomycin decreased the potency of Met-Hg to elevate [Ca2+]i but did not change the resting [Ca2+]i. Met-Hg also slightly decreased synaptosomal ATP. TEL and TET elevated [Ca2+]i by 100-200 nM. The effect of TEL, but not that of TET, could be blocked with verapamil (36%) and veratridine (67%). TEL was less efficient in the presence of ouabain. Neither TEL nor TET had significant mitochondrial effects in situ contributing to [Ca2+]i. TMT increased [Ca2+]i less than TET while dimethyltin and methyltin were inactive. These results indicate that neurotoxic derivatives of alkylmetals studied increase [Ca2+]i. This occurs mainly either by nonspecific increase (Met-Hg, TET) of Ca2+ leakage through the plasma membrane and/or specific interference with the mechanisms regulating Ca2+ fluxes through the plasma membrane (TEL).

Opiates inhibit calmodulin activation of a high-affinity Ca2+-stimulated Mg2+-dependent ATPase in synaptic membranes

A high affinity Ca2+/Mg2+ ATPase has been identified and localized in synaptic membrane subfractions. This enzyme is stimulated by low concentrations of Ca2+ (less than or equal to microM) believed to approximate the range of Ca2+ in the synaptosomal cytosol (0.1 to 5.0 microM). The opiate agonist levorphanol, in a concentration-dependent fashion, inhibited Ca2+-stimulated ATP hydrolysis in lysed synaptic membranes. This inhibition was reversed by naloxone, while dextrorphan, the inactive opiate isomer, was without effect. Inhibition by levorphanol was most pronounced in a subfraction of synaptic membranes (SPM-1). The inhibition of Ca2+-stimulated ATP hydrolysis was characterized by a reduction in Vmax for Ca2+. Levorphanol pretreatment reduced the Hill coefficient (HN) of 1.5 to 0.7, suggesting cooperative interaction between the opiate receptor and the enzyme protein. Levorphanol, but not dextrorphan, also inhibited (28%) ATP-dependent Ca2+ uptake by synaptic membranes. Opiate ligand stereoisomers were tested for their effects on calmodulin stimulating of high affinity Ca2+/Mg2+ ATPase in synaptic membranes. Levorphanol (10 microM), but not the inactive stereoisomer (+)dextrorphan, significantly inhibited (35%) the calmodulin-activated Ca2+-dependent ATP hydrolysis activity in a preparation of lysed synaptic membranes. Both Ca2+-dependent and calmodulin-dependent stimulation of the enzyme in the presence of optimal concentrations of the other co-substrate were inhibited by levorphanol (35-40%) but not dextrorphan. Inhibition of ATP hydrolysis was characterized by a reduction in Vmax for both Ca2+ and calmodulin stimulation of the enzyme. Calmodulin stimulation of enzyme activity was most pronounced in SPM-1, the membrane fraction which also exhibits the maximal opiate inhibition (40%) of the Ca2+-ATPase.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of mitochondria in the age-dependent decrease in calcium uptake of rat brain synaptosomes

Calcium uptake in rat brain synaptosomes decreases during ageing. The possible involvement of mitochondria in altered calcium homeostasis has been investigated. Mitochondria isolated from old rat brain showed decreased calcium uptake rates. Since neither the mitochondrial membrane potential nor the delta pCa decreases with age, it was concluded that variations in the driving force for calcium uptake were not the cause for impaired calcium transport in mitochondria from aged rat brain. The steady state calcium distribution in isolated aged rat brain mitochondria was achieved at higher extramitochondrial calcium concentrations than that of adults. Studying the effects of the selective release of calcium from the mitochondrial pool by the addition of an uncoupler to 45Ca loaded synaptosomes incubated in high-potassium media, it was found that the intrasynaptic mitochondrial pool and the intra/extramitochondrial 45Ca distribution also decreased considerably in 24-month-old rats. Steady state fluorescence anisotropy (rs) of diphenylhexatriene-labelled mitoplasts from 'free' brain mitochondria increased with ageing. However, since no changes in rs from synaptosomal mitochondria were found in 24-month-old rats, it is suggested that alterations in lipid dynamics are not involved in the impaired calcium uptake observed in brain mitochondria from aged rats. The implications of these findings in the calcium homeostasis of brain endings are discussed.

Quantitative measurements of the cytosolic Ca2+ activity within isolated guinea pig nerve-endings using entrapped arsenazo III and quin2

The absorbance changes of intrasynaptosomally entrapped arsenazo III have been converted into values of free Ca2+ concentration by correcting for the nonlinear response of arsenazo III at different concentrations of the dye as well as for changes in internal pH. An average resting value for free Ca2+ concentration around 0.4 microM is obtained. Depolarization with veratridine or gramicidin increases this value to around 3 microM. Measurements of cytosolic free Ca2+ with the quin2 method gives much lower values in similar conditions. The release of prelabelled [14C]noradrenaline from the nerve-endings is maximally activated when the internal free Ca2+ concentration rises as measured with arsenazo III to about 4 microM when titrated with increasing concentrations of ionophore A23187.

Restricted diffusion of tyrosine hydroxylase and phenylethanolamine N-methyltransferase from digitonin-permeabilized adrenal chromaffin cells

Tyrosine hydroxylase [TyrOHase; tyrosine 3-monooxygenase; L-tyrosine,tetrahydropteridine:oxygen oxidoreductase (3-hydroxylating), EC 1.14.16.2] and phenylethanolamine N-methyltransferase, EC 2.1.1.28) are involved in catecholamine biosynthesis and are considered soluble proteins. However, they may actually be localized on the surface of the chromaffin granule. We have used the detergent digitonin to permeabilize the plasma membrane of cultured adrenal chromaffin cells to investigate the subcellular localization of TyrOHase and PMTase. A digitonin titration of the release of proteins and catecholamines revealed the existence of at least three subcellular compartments that are distinguished by their digitonin sensitivity: (i) soluble proteins, which were released upon treatment of the cells with low digitonin concentrations (5 microM), (ii) a "digitonin-sensitive" cytoplasmic protein pool, which required higher concentrations of digitonin for release (10 microM) and included TyrOHase and PMTase, and (iii) the chromaffin granule, which was insensitive to digitonin. Analysis of the rates of release of all of these proteins revealed that the rate of TyrOHase and PMTase release was slower at 10 microM than at 40 microM digitonin, while the rates of release of the other proteins were similar at both concentrations and varied in proportion to their respective sizes. Treatment with cytoskeletal disrupting agents had no effect on TyrOHase or PMTase efflux. These data suggest that TyrOHase and PMTase are in a detergent-labile association in the cell. This is consistent with the concept that TyrOHase and PMTase may be localized on the surface of the chromaffin granule.

Calcium uptake of rat brain synaptosomes as a function of membrane potential under different depolarizing conditions

The uptake of 45Ca2+ was measured in brain synaptosomes under conditions designed to depolarize the membranes. Membrane potential was estimated from the distribution of 86Rb+ between the intra- and extracellular compartments. K+ depolarization (8-60 mM) only increased "a2+ uptake beyond a threshold depolarization of about 10 mV, whereas veratridine (5-40 microM) induced an increased Ca2+ uptake at a concentration which depolarized the membrane less than this threshold. Ouabain did not enhance Ca2+ uptake, but the depolarization it produced did not reach threshold. Ca2+ influx already stimulated by K+ depolarization can be further enhanced by veratridine without any parallel change in membrane potential. Only the pathway mediating Ca2+ uptake during K+ depolarization can be inactivated: Ca2+ uptake evoked by K+ depolarization is decreased in synaptosomes pre-depolarized in the presence of a high concentration of K+. In contrast, pre-depolarization does not change Ca2+ uptake evoked by veratridine. Ca2+ channel blockers, such as verapamil and diltiazem but not nifedipine, in concentrations of 10-100 microM, decrease the stimulation of Ca2+ uptake by high K+ concentration without influencing depolarization, whereas the effect of veratridine on Ca2+ uptake is only inhibited when its effect on Na+ channels is also prevented. It is concluded that Ca2+ uptake during K+ depolarization proceeds through voltage-dependent Ca2+ channels similar to those of squid axon, whereas veratridine activates an additional Ca2+ entry, possibly via influx through open Na+ channels. Different quantitative relationships are found between acetylcholine release of synaptosomes and the amount of Ca2+ taken up by different mechanisms: the same amount of Ca2+ uptake is accompanied by a greater increase of acetylcholine release if the uptake is induced by K+ depolarization rather than veratridine.

Subsynaptosomal calcium distribution during hypoxia and 3,4-diaminopyridine treatment

Previous results demonstrate that hypoxia (low oxygen) diminishes calcium uptake by synaptosomes. The present studies examined the effects of low oxygen on calcium homeostasis in the digitonin-resistant (mitochondrial) and the digitonin-labile (nonmitochondrial) compartments of intact synaptosomes and their relation to altered membrane potentials. A 10-min hypoxic incubation in low-potassium media reduced total (-38.3%), mitochondrial (-43.3%), and nonmitochondrial (-27.8%) calcium uptake. In high-potassium media, low oxygen reduced mitochondrial (-41.2%) and total (-34.4%) uptake whereas nonmitochondrial (+ 6%) calcium uptake was essentially unaffected. A temporal analysis of nonmitochondrial calcium uptake revealed an initial depression (0-5 min) followed by a stimulation (5-10 min). Hypoxic-induced alterations in the subsynaptosomal distribution of calcium resembled those produced by uncouplers [FCCP (carbonylcyanide-p-trifluoromethoxyphenylhydrazone) or rotenone plus oligomycin]. 3,4-Diaminopyridine partially ameliorated the hypoxic- and FCCP-induced decreases in synaptosomal calcium uptake. Low oxygen reduced the total synaptosomal membrane potential (i.e., plasma plus mitochondrial membrane potential) as measured by an increased efflux of tetraphenylphosphonium ion. This hypoxic-induced efflux of tetraphenylphosphonium was slowed by pretreatment with 3,4-diaminopyridine. Thus, both drug and membrane potential studies suggest that hypoxic-induced alterations in the subcellular distribution of calcium may be due to an uncoupling mechanism and a collapse of the synaptosomal mitochondrial membrane potential.

Human lymphocyte glucocorticoid receptors reside mainly in the cytoplasm

Karyoplasts prepared from human lymphocytes by treatment with cytochalasin B or lymphocytes permeabilized with digitonin contain only 25-40% of the total cellular glucocorticoid binding activity. The presented results thus support the original concept that unfilled steroid hormone receptors are mainly cytoplasmic.

Regulation of cytosolic calcium concentration in presynaptic nerve endings isolated from rat brain

The regulation of cytosolic Ca concentration ([Ca]i) was studied with the fluorescent Ca indicator, quin2, in pinched-off presynaptic nerve endings (synaptosomes) isolated from rat brain. The resting [Ca]i is 0.1-0.2 microM, in solutions containing 1-2 mM-Ca. [Ca]i increases by only 100-150 nM when the external Ca concentration is increased from 0.02 to 2 mM. The mitochondrial inhibitors valinomycin and fluoro-carbonyl cyanide phenylhydrazone (FCCP) increase [Ca]i by 100-200 nM. This increase is not correlated with the resting level of [Ca]i prior to the addition of inhibitors, but it is dependent on the presence of external Ca. It seems likely that the effect of these inhibitors on [Ca]i is a secondary consequence of metabolic inhibition. [Ca]i increases by about 2-fold when the external Na concentration is lowered from 145 to 5 mM, and returns to its initial level when external Na is restored. This recovery occurs also in the presence of FCCP. These results suggest that Na/Ca exchange, but not mitochondrial Ca uptake, plays a role in regulating [Ca]i and in allowing the nerve terminals to recover from Ca loading.

Dependence of cytoplasmic calcium transients on the membrane potential in isolated nerve endings of the guinea pig

The relation of changes in internal, free Ca2+, measured with arsenazo III, to the membrane potential, measured with the cyanine dye di-S-C2(5) or 86Rb+ distribution ratio, was studied in isolated guinea pig cortical nerve endings. Depolarization of the plasma membrane with veratridine or gramicidin as well as addition of ionophore A23187 led to an increase in cytosolic Ca2+. Only the response to veratridine was inhibited by tetrodotoxin. The dependence of the depolarization-induced increase in intraterminal, free Ca2+ on the membrane potential between about -50 to 0 mV was sigmoidal. A maximal increase in cytosolic Ca2+ was reached when the membrane potential was depolarized from the resting level, about -64 mV, to about -40 mV. These results show that in isolated nerve endings the activation of voltage-sensitive Ca2+ channels concomitantly leads to an increase in cytosolic, free Ca2+. Comparison of the results of the present study with the previous electrophysiological observations indicate that Ca2+ channels in synaptosomes, presynaptic nerve terminals of the squid giant synapse and cardiac cells have essentially similar voltage dependency.

Metabolic disturbances of synaptosomes isolated from ischemic gerbil brain

The effects of cerebral ischemia, induced for 10 min by bilateral common carotid ligation in the Mongolian gerbil, on the brain and synaptosomal content of phospholipids and free fatty acids were measured. Moreover, the incorporation of arachidonic acid and oleoyl-CoA into phospholipids, as well as the respiration and the accumulation of 45Ca, norepinephrine, dopamine, choline, glutamate, and gamma-aminobutyrate in the ischemic brain synaptosomal fraction were studied. Analyses of lipids showed a drop in phospholipids content with concomitant increase of lysocompounds and free fatty acids in ischemic cerebral cortex. Disturbances in lipid metabolism including rapid phospholipids hydrolysis and changes in the incorporation of arachidonic acid into inositol and choline phosphoglycerides were also shown in the synaptosomal fraction of ischemic brain. The uptake of neurotransmitter substances, expressed as a percent of control value, was reduced 21% for norepinephrine, 40% for dopamine, 20% for choline, 24% for glutamate and 13% for gamma-aminobutyrate in ischemic synaptosomes. There was no significant effect of ischemia on synaptosomal respiration and 45Ca uptake in both control and high potassium media. The inhibition of neurotransmitter uptake in ischemic brain synaptosomes may be caused by the disturbance of fatty acid metabolism.

Role of Ca2+ in pyruvate dehydrogenase interconversion in brain mitochondria and synaptosomes

The steady-state content of active (dephospho) pyruvate dehydrogenase (PDHA) of suspensions of coupled rat brain mitochondria oxidizing succinate was found to be markedly increased with increasing free Ca2+ ion concentration of the medium, with a half-maximal effect at 10(-6.43) M Ca2+. Other ions were present in these studies at concentrations appropriate for the cytosol. Depolarization of the plasma membrane of synaptosomes caused an increase in the steady-state content of PDHA, with veratridine giving a larger increase than depolarization by 33 mM-KCl. Values were 68 +/- 1% (n = 13) and 81 +/- 1% (n = 19) of maximal activity, for control incubations and incubations in the presence of 30 microM-veratridine, respectively. Measurements of cytosolic free Ca2+ concentrations ([Ca2+]cyt.) in these suspensions of synaptosomes, with the use of the fluorescent Ca2+-indicator Quin-2, indicated an increase on depolarization, with the change due to 30 microM-veratridine being larger in extent than that due to 33 mM-KCl. Values were 217 +/- 21 nM (n = 15), 544 +/- 48 nM (n = 15) and 783 +/- 75 nM (n = 14) for control, KCl-depolarized and veratridine-depolarized synaptosomes respectively. Experiments in which synaptosomes were treated with Ruthenium Red, an inhibitor of mitochondrial Ca2+ uptake, gave much lower resting contents of PDHA (42 +/- 2% of maximal), but failed to prevent totally an increase on depolarization. Addition of an excess of EGTA to the synaptosomal suspension just before the addition of veratridine resulted in a partial diminution in the response of PDHA content. Parallel studies with Quin-2 indicated no increase in [Ca2+]cyt. on addition of veratridine, under these conditions. Thus an increase in [Ca2+]cyt. forms only a part of the mechanism whereby pyruvate dehydrogenase interconversion responds to depolarization. A decrease in the ATP/ADP ratio may also be important, as inferred from the results of experiments with ouabain, which inhibits the Na+ + K+-dependent ATPase.