Lanthanum can be transported by the sodium-calcium exchange pathway and directly triggers catecholamine release from bovine chromaffin cells

Effective assessment of lanthanide ion delivery into live cells by paramagnetic NMR spectroscopy

We report an effective assessment of lanthanide ion (Ln3+) delivery into live cells by paramagnetic NMR spectroscopy. Free Ln3+ ions are toxic to live cells resulting in a gradual leakage of target proteins to the extracellular media. The citrate-Ln3+ complex is an efficient and mild reagent over the free Ln3+ form for live cell delivery.

The Structural Basis of Long-Term Potentiation in Hippocampal Synapses, Revealed by Electron Microscopy Imaging of Lanthanum-Induced Synaptic Vesicle Recycling

Hippocampal neurons in dissociated cell cultures were exposed to the trivalent cation lanthanum for short periods (15–30 min) and prepared for electron microscopy (EM), to evaluate the stimulatory effects of this cation on synaptic ultrastructure. Not only were characteristic ultrastructural changes of exaggerated synaptic vesicle turnover seen within the presynapses of these cultures—including synaptic vesicle depletion and proliferation of vesicle-recycling structures—but the overall architecture of a large proportion of the synapses in the cultures was dramatically altered, due to large postsynaptic “bulges” or herniations into the presynapses. Moreover, in most cases, these postsynaptic herniations or protrusions produced by lanthanum were seen by EM to distort or break or “perforate” the so-called postsynaptic densities (PSDs) that harbor receptors and recognition molecules essential for synaptic function. These dramatic EM observations lead us to postulate that such PSD breakages or “perforations” could very possibly create essential substrates or “tags” for synaptic growth, simply by creating fragmented free edges around the PSDs, into which new receptors and recognition molecules could be recruited more easily, and thus, they could represent the physical substrate for the important synaptic growth process known as “long-term potentiation” (LTP). All of this was created simply in hippocampal dissociated cell cultures, and simply by pushing synaptic vesicle recycling way beyond its normal limits with the trivalent cation lanthanum, but we argued in this report that such fundamental changes in synaptic architecture—given that they can occur at all—could also occur at the extremes of normal neuronal activity, which are presumed to lead to learning and memory.

Lanthanide-Binding Tags for 3D X-ray Imaging of Proteins in Cells at Nanoscale Resolution

We report the application of lanthanide-binding tags (LBTs) for two- and three-dimensional X-ray imaging of individual proteins in cells with a sub-15 nm beam. The method combines encoded LBTs, which are tags of minimal size (ca. 15-20 amino acids) affording high-affinity lanthanide ion binding, and X-ray fluorescence microscopy (XFM). This approach enables visualization of LBT-tagged proteins while simultaneously measuring the elemental distribution in cells at a spatial resolution necessary for visualizing cell membranes and eukaryotic subcellular organelles.

Immune dysfunction and liver damage of mice following exposure to lanthanoids

In an effort to investigate the effects of exposure to lanthanoids (Ln) on the immune response and liver function, mice were orally exposed to LaCl3 , CeCl3 , and NdCl3 at 2, 10, and 20 mg/kg doses for 30 days, respectively; lymphocyte counts, serum IgM level, hematological indices, biochemical parameters of liver functions, and histopathological changes in Ln(3+) -treated mice were assessed. Indeed, 20 mg/kg Ln(3+) significantly inhibited mice growth and reduced the counts of white blood cells, platelets, and reticulocyte in mice blood. Specifically, in these Ln(3+) -treated mice, CD3+, CD4+, CD8+, CD19+ and NK cells, and CD4+/CD8+ ratio as well as serum IgM level were decreased. Furthermore, liver function was disrupted, as evidenced by the increased alanine aminotransferase, total bilirubin, total bile acid and triglycerides, and the decreased glucose and ratio of albumin to globulin. The cytoarchitecture damage and fatty degeneration in liver caused by Ln(3+) at 20 mg/kg dose were also observed. Our findings showed that exposure to Ln affected the cell and humoral immunity and disturbed liver function in mice. In addition, Ce(3+) was found to exhibit higher toxicity than La(3+) and Nd(3+).

Ion permeation and block of the gating pore in the voltage sensor of NaV1.4 channels with hypokalemic periodic paralysis mutations

Hypokalemic periodic paralysis and normokalemic periodic paralysis are caused by mutations of the gating charge–carrying arginine residues in skeletal muscle Na_V1.4 channels, which induce gating pore current through the mutant voltage sensor domains. Inward sodium currents through the gating pore of mutant R666G are only ∼1% of central pore current, but substitution of guanidine for sodium in the extracellular solution increases their size by 13- ± 2-fold. Ethylguanidine is permeant through the R666G gating pore at physiological membrane potentials but blocks the gating pore at hyperpolarized potentials. Guanidine is also highly permeant through the proton-selective gating pore formed by the mutant R666H. Gating pore current conducted by the R666G mutant is blocked by divalent cations such as Ba²⁺ and Zn²⁺ in a voltage-dependent manner. The affinity for voltage-dependent block of gating pore current by Ba²⁺ and Zn²⁺ is increased at more negative holding potentials. The apparent dissociation constant (K_d) values for Zn²⁺ block for test pulses to −160 mV are 650 ± 150 µM, 360 ± 70 µM, and 95.6 ± 11 µM at holding potentials of 0 mV, −80 mV, and −120 mV, respectively. Gating pore current is blocked by trivalent cations, but in a nearly voltage-independent manner, with an apparent K_d for Gd³⁺ of 238 ± 14 µM at −80 mV. To test whether these periodic paralyses might be treated by blocking gating pore current, we screened several aromatic and aliphatic guanidine derivatives and found that 1-(2,4-xylyl)guanidinium can block gating pore current in the millimolar concentration range without affecting normal Na_V1.4 channel function. Together, our results demonstrate unique permeability of guanidine through Na_V1.4 gating pores, define voltage-dependent and voltage-independent block by divalent and trivalent cations, respectively, and provide initial support for the concept that guanidine-based gating pore blockers could be therapeutically useful.

Molecular substrates mediating lanthanide-evoked neurotransmitter release in central synapses

Noncanonical secretagogues such as hypertonicity or alpha-latrotoxin have been extremely informative in studying neurotransmission. Lanthanum and lanthanides can also trigger neurotransmitter release through an unknown mechanism. Here, we studied the effect of lanthanides on neurotransmission in hippocampal cultures. Application of 2 mM La3+ caused rapid and robust neurotransmitter release within seconds. In addition, transient application of La3+ uncovered a sustained facilitation of miniature neurotransmission. The response to La3+ was detectable at 2 microM and increased in a concentration-dependent manner<or=2 mM. Rapid effect of La3+ was independent of extracellular and intracellular Ca2+ and did not require La3+ entry into cells or activation of phospholipaseCbeta. Synapses deficient in synaptobrevin-2, the major synaptic vesicle soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein in the brain, did not display any rapid release in response to La3+, whereas the slow facilitation of release detected after La3+ removal remained intact. In contrast, preincubation with intracellular Ca2+ chelators selectively attenuated the delayed release triggered by La3+. Moreover, synapses deficient in synaptotagmin-1 maintained a rapid response to La3+, suggesting that La3+-triggered neurotransmitter release does not require synaptotagmin-1 as a sensor. Therefore La3+ has two separate effects on synaptic transmission. For its rapid action, La3+ interacts with a target on the surface membrane, and unlike other forms of release, it triggers strictly synaptobrevin-2-dependent fusion, implying that in central synapses synaptobrevin-2 function is secretagogue specific. For the delayed action, La3+ may act intracellularly after its entry or through intracellular Ca2+ via a mechanism that does not require synaptobrevin-2.

The co-presence of Na+/Ca2+-K+ exchanger and Na+/Ca2+ exchanger in bovine adrenal chromaffin cells

We have previously shown that there is high Na(+)/Ca(2+) exchange (NCX) activity in bovine adrenal chromaffin cells. In this study, by monitoring the [Ca(2+)](i) change in single cells and in a population of chromaffin cells, when the reverse mode of exchanger activity has been initiated, we have shown that the NCX activity is enhanced by K(+). The K(+)-enhanced activity accounted for a significant proportion of the Na(+)-dependent Ca(2+) uptake activity in the chromaffin cells. The results support the hypothesis that both NCX and Na(+)/Ca(2+)-K(+) exchanger (NCKX) are co-present in chromaffin cells. The expression of NCKX in chromaffin cells was further confirmed using PCR and northern blotting. In addition to the plasma membrane, the exchanger activity, measured by Na(+)-dependent (45)Ca(2+) uptake, was also present in membrane isolated from the chromaffin granules enriched fraction and the mitochondria enriched fraction. The results support that both NCX and NCKX are present in bovine chromaffin cells and that the regulation of [Ca(2+)](i) is probably more efficient with the participation of NCKX.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

A constitutively active nonselective cation conductance underlies resting Ca2+ influx and secretion in bovine adrenal chromaffin cells

We have combined fluorimetric measurements of the intracellular free Ca(2+) concentration ([Ca(2+)](i)) with the patch clamp technique, to investigate resting Ca(2+) entry in bovine adrenal chromaffin cells. Perfusion with nominally Ca(2+)-free medium resulted in a rapid, reversible decrease in [Ca(2+)](i), indicating a resting Ca(2+) permeability across the plasma membrane. Simultaneous whole-cell voltage-clamp showed a resting inward current that increased when extracellular Ca(2+) (Ca(2+)(o)) was lowered. This current had a reversal potential of around 0 mV and was carried by monovalent or divalent cations. In Na(+)-free extracellular medium there was a reduction in current amplitude upon removal of Ca(2+)(o), indicating the current can carry Ca(2+). The current was constitutively active and not enhanced by agents that promote Ca(2+)-store depletion such as thapsigargin. Extracellular La(3+) abolished the resting current, reduced resting [Ca(2+)](i) and inhibited basal secretion. Abolishment of resting Ca(2+) influx depleted the inositol 1,4,5-trisphosphate-sensitive Ca(2+) store without affecting the caffeine-sensitive Ca(2+) store. The results indicate the presence of a constitutively active nonselective cation conductance, permeable to both monovalent and divalent cations, that can regulate [Ca(2+)](i), the repletion state of the intracellular Ca(2+) store and the secretory response in resting cells.

Lanthanum induces extracellular signal-regulated kinase phosphorylation through different mechanisms in HeLa cells and NIH 3T3 cells

Lanthanum ion (La3+) was generally regarded as calcium antagonist and was used as calcium channel blocker. However, its potential biological effects on cells were poorly understood. In the present work, it was found that La3+ could induce rapid extracellular signal-regulated kinase (ERK) phosphorylation in both HeLa cells and NIH 3T3 cells, but different mechanisms were involved. At a concentration of 30 microM or higher, La3+ enters the cells and activates ERK through a mechanism involving calmodulin activation inside the cells, which is similar to the action of intracellular Ca2+. However, at lower concentration, free La3+ promoted ERK phosphorylation in NIH 3T3 cells outside the cells through an unknown La3+ sensing mechanism, while Ca2+ exerted much weaker effect. The present results suggested that the biological effects of La3+ on cells maybe involve mechanisms beyond calcium antagonist.

Lanthanum Effect on the Dynamics of Tight Junction Opening and Closing

We present a comparative study in frog urinary bladders (FUB) and A6 cell monolayers (A6CM) on the effect of La3+ on tight junction (TJ) dynamics. These tissues react similarly to changes of basolateral Ca2+ (Ca(2+)bl), while responding differently to the action of La3+(bl). In FUB, La(3+)bl shows a Ca(2+)-antagonistic effect that promotes TJ opening in the presence of a normal Ca(2+)bl concentration. In A6CM, in contrast, La(3+)bl always shows a clear Ca(2+)-agonistic effect. The fact that a concentration of La(3+)bl one fifth of the normal Ca(2+)bl leads in FUB to TJ opening and in A6CM to a complete recovery of the TJ seal indicates a high affinity of La3+ for the Ca(2+)-binding sites in both tissues. In FUB, apical La3+ (La(3+)ap) exhibits, differently from its basolateral effect, an evident Ca(2+)-agonistic effect, suggesting a dual effect of La3+, depending on which side of the bladder La3+ is applied. In A6CM La(3+)ap has a Ca(2+)-agonistic effect similar to La(3+)bl. The effects of La(3+)bl in FUB and in A6CM are consistent, according to our previous publications, with La3+ acting antagonistically or agonistically, respectively, on the Ca2+ binding sites of zonula adhaerens. Despite the fact that the effect of La(3+)ap is clear in both tissues, its site of action is yet to be determined. Protonation of the Ca(2+)-binding sites causes a decrease of its agonistic effect on A6CM, consistent with a negatively charged binding site. In A6CM La3+ apparently replaces Ca2+, mimicking the effect of Ca2+ triggering the cascade of events leading to TJ closure. In FUB, La3+ interacts with the binding sites, dislodging Ca2+, with a high affinity, but this interaction is inadequate to initiate or sustain the process of junction closing. Possibly, the difference between the two preparations resides in subtle conformation differences of the outer segment of E-cadherin molecules.

Mitochondrial Na+ overload is caused by oxidative stress and leads to activation of the caspase 3- dependent apoptotic machinery

Oxidative stress is one of the major causes of cell death. Using time-lapse confocal recording of live cardiomyocytes, we showed that H2O2 (OH*) caused a marked increase in Na+ and Ca2+ levels in both the cytosol ([Na]cyt, [Ca]cyt) and mitochondria ([Na]m, [Ca]m). The H2O2-induced intracellular Na+ ([Na]i) overload contributed to the H2O2-induced [Ca]cyt/[Ca]m overload via activation of the reverse mode of the Na-Ca exchanger. When myocytes were treated for 40 min with 100 microM H2O2 in normal medium, then returned to H2O2-free medium, the percentage of apoptotic cells increased from 4% at 0 h to 55 and 85% at 4.5 and 16 h, respectively. H2O2-induced apoptosis was completely prevented by using Na-free, but not Ca-free, medium. When a Na+ ionophore cocktail in Ca-free medium was used instead of H2O2 to increase the [Na]i by more than 30 mM without any change in the [Ca]i, cytochrome c release and caspase 3-dependent apoptosis occurred, showing that [Na]i overload per se induced apoptosis. We also showed that the increase in the mitochondrial, but not the cytosolic, Na+ levels resulted in the opening of the permeation transition pore, followed by cytochrome c release. Our findings therefore suggest that H2O2-induced [Na]m overload is an important upstream signal for the apoptotic machinery, and the prevention of [Na]m overload thus represents a particularly attractive target for strategies aimed at preventing oxidative stress-induced cell death.

Lanthanum is transported by the sodium/calcium exchanger and regulates its activity

La3+ uptake was measured in fura 2-loaded Chinese hamster ovary cells expressing the bovine cardiac Na+/Ca2+ exchanger (NCX1.1). La3+ was taken up by the cells after an initial lag phase of 50-60 s and achieved a steady state within 5-6 min. Neonatal cardiac myocytes accumulated La3+ in a similar manner. La3+ uptake was due to the activity of the exchanger, because no uptake was seen in nontransfected cells or in transfected cells that had been treated with gramicidin to remove cytosolic Na+. The low rate of La3+ uptake during the lag period resulted from insufficient cytosolic Ca2+ to activate the exchanger at its regulatory sites, as shown by the following observations. La3+ uptake occurred without a lag period in cells expressing a mutant of NCX1.1 that does not exhibit regulatory activation by cytosolic Ca2+. The rate of La3+ uptake by wild-type cells was increased, and the lag phase was reduced or eliminated, when the cytosolic Ca2+ concentration was increased before initiating La3+ uptake. La3+ could substitute for Ca2+ at very low concentrations to activate exchange activity. Thus preloading cells expressing NCX1.1 with a small quantity of La3+ increased the rate of exchange-mediated Ca2+ influx by 20-fold; in contrast, cytosolic La3+ partially inhibited Ca2+ uptake by the regulation-deficient mutant. With an estimated KD of 30 pM for the binding of La3+ to fura 2, we conclude that cytosolic La3+ activates exchange activity at picomolar concentrations. We speculatively suggest that endogenous trace metals might activate exchange activity under physiological conditions.

Complexation of ytterbium to human transferrin and its uptake by K562 cells

There is an increasing interest in the use of lanthanides in medicine. However, the mechanism of their accumulation in cells is not well understood. Lanthanide cations are similar to ferric ions with regard to transferrin binding, suggesting transferrin-receptor mediated transport is possible; however, this has not yet been confirmed. In order to clarify this mechanism, we investigated the binding of Yb3+ to apotransferrin by UV-Vis spectroscopy and stopped-flow spectrophotometry, and found that Yb3+ binds to apotransferrin at the specific iron sites in the presence of bicarbonate. The apparent binding constants of these sites showed that the affinity of Yb3+ is lower than that of Fe3+and binding of Yb3+ in the N-lobe is kinetically favored while the C-lobe is thermodynamically favored. The first Yb3+ bound to the C-lobe quantitatively with a Yb/apotransferrin molar ratio of < 1, whereas the binding to the other site is weaker and approaches completeness by a higher molar ratio only. As demonstrated by 1H NMR spectra, Yb3+ binding disturbed the conformation of apotransferrin in a manner similar to Fe3+. Flow cytometric studies on the uptake of fluorescein isothiocyanate labeled Yb3+-bound transferrin species by K562 cells showed that they bind to the cell receptors. Laser scanning confocal microscopic studies with fluorescein isothiocyanate labeled Yb3+-bound transferrin and propidium iodide labeled DNA and RNA in cells indicated that the Yb3+ entered the cells. The Yb3+-transferrin complex inhibited the uptake of the fluorescein labeled ferric-saturated transferrin (Fe2-transferrin) complex into K562 cells. The results demonstrate that the complex of Yb3+-transferrin complex was recognized by the transferrin receptor and that the transferrin-receptor-mediated mechanism is a possible pathway for Yb3+ accumulation in cells.

Inhibition of neuronal nicotinic acetylcholine receptors by La(3+)

A study was made of the effects of La(3+) on neuronal alpha 2 beta 4 nicotinic acetylcholine receptors expressed in Xenopus oocytes. La(3+) by itself (up to 10 microM) did not elicit significant membrane currents. However, La(3+) reversibly inhibited the ionic currents induced by acetylcholine (IC(50)=13.5+/-4.3 microM). When La(3+) and acetylcholine were simultaneously applied onto an oocyte, the level of inhibition of the acetylcholine response was the same as when the oocyte was first preincubated with La(3+) and then exposed to acetylcholine plus La(3+). In the presence of La(3+), the EC(50) decreased from 43.8+/-6.4 to 26.5+/-5.1 microM, suggesting a small increase in the affinity of acetylcholine for the receptors through a noncompetitive mechanism. The inhibition of acetylcholine response was independent of the membrane potential. From these results we conclude that La(3+) regulates nicotinic receptors, reversibly and noncompetitively, presumably by inhibiting allosterically the receptor through interactions at an external domain of the receptor complex.

Removal of Ca(2+) following depolarization-evoked cytoplasmic Ca(2+) transients in freshly dissociated pyramidal neurones of the rat dorsal cochlear nucleus

Cytoplasmic [Ca(2+)] ([Ca(2+)](i)) was measured using Fura-2 in pyramidal neurones isolated from the rat dorsal cochlear nucleus (DCN). The kinetic properties of Ca(2+) removal following K(+) depolarization-induced Ca(2+) transients were characterized by fitting exponential functions to the decay phase. The removal after small transients (<82 nM peak [Ca(2+)](i)) had monophasic time course (time constant of 6.43 +/- 0.48 s). In the cases of higher Ca(2+) transients biphasic decay was found. The early time constant decreased (from 3.09 +/- 0.26 to 1.46 +/- 0.11 s) as the peak intracellular [Ca(2+)] increased. The value of the late time constant was 18.15 +/- 1.60 s at the smallest transients, and showed less dependence on [Ca(2+)](i). Blockers of Ca(2+) uptake into intracellular stores (thapsigargin and cyclopiazonic acid) decreased the amplitude of the Ca(2+) transients and slowed their decay. La(3+) (3 mM) applied extracellularly during the declining phase dramatically changed the time course of the Ca(2+) transients as a plateau developed and persisted until the La(3+) was present. When the other Ca(2+) removal mechanisms were available, reduction of the external [Na(+)] to inhibit the Na(+)/Ca(2+) exchange resulted in a moderate increase of the time constants. It is concluded that in the isolated pyramidal neurones of the DCN the removal of Ca(2+) depends mainly on the activity of Ca(2+) pump mechanisms.

Mobilizing store Ca(2+) in the presence of La(3+) evokes exocytosis in bovine chromaffin cells

The effect on exocytosis of La(3+), a known inhibitor of plasma membrane Ca(2+)-ATPases and Na(+)/Ca(2+) exchangers, was studied using cultured bovine adrenal chromaffin cells. At high concentrations (0.3-3 mM), La(3+) substantially increased histamine-induced catecholamine secretion. This action was mimicked by other lanthanide ions (Nd(3+), Eu(3+), Gd(3+), and Tb(3+)), but not several divalent cations. In the presence of La(3+), the secretory response to histamine became independent of extracellular Ca(2+). La(3+) enhanced secretion evoked by other agents that mobilize intracellular Ca(2+) stores (angiotensin II, bradykinin, caffeine, and thapsigargin), but not that due to passive depolarization with 20 mM K(+). La(3+) still enhanced histamine-induced secretion in the presence of the nonselective inhibitors of Ca(2+)-permeant channels SKF96365 and Cd(2+), but the enhancement was abolished by prior depletion of intracellular Ca(2+) stores with thapsigargin. La(3+) inhibited (45)Ca(2+) efflux from preloaded chromaffin cells in the presence or absence of Na(+). It also enhanced and prolonged the rise in cytosolic [Ca(2+)] measured with fura-2 during mobilization of intracellular Ca(2+) stores with histamine in Ca(2+)-free buffer. The results suggest that the efficacy of intracellular Ca(2+) stores in evoking exocytosis is enhanced dramatically by inhibiting Ca(2+) efflux from the cell.

Effect of Gd3+ on bradykinin-induced catecholamine secretion from bovine adrenal chromaffin cells

1. The effects of Gd3+ on bradykinin- (BK-) induced catecholamine secretion, 45Ca2+ efflux and cytosolic [Ca2+] were studied using bovine adrenal chromaffin cells. 2. BK increased secretion in a Ca2+-dependent manner. From 1 - 100 microM, Gd3+ progressively inhibited secretion induced by 30 nM BK to near-basal levels, however from 0.3 - 3 mM Gd3+ dramatically enhanced BK-induced secretion to above control levels. Gd3+ also increased basal catecholamine secretion by 2 - 3 fold at 1 mM. These effects were mimicked by Eu3+ and La3+. 3. Gd3+ enhanced secretion induced by other agonists that mobilize intracellular Ca2+ stores, but simply blocked the response to K+. 4. Gd3+ still enhanced basal and BK-induced secretion in Ca2+-free solution or in the presence of 30 microM SKF96365, however both effects of Gd3+ were abolished after depleting intracellular Ca2+ stores. 5. Gd3+ (1 mM) reduced the rate of basal 45Ca2+ efflux by 57%. In Ca2+-free buffer, BK transiently increased cytosolic [Ca2+] measured with Fura-2. The [Ca2+] response to BK was substantially prolonged in the presence of Gd3+ (1 mM). 6. The results suggest that Gd3+ greatly enhances the efficacy of Ca2+ released from intracellular stores in evoking catecholamine secretion, by inhibiting Ca2+ extrusion from the cytosol. This suggests that intracellular Ca2+ stores are fully competent to support secretion in chromaffin cells to levels comparable to those evoked by extracellular Ca2+ entry. Drugs that modify Ca2+ extrusion from the cell, such as lanthanide ions, will be useful in investigating the mechanisms by which intracellular Ca2+-store mobilization couples to Ca2+-dependent exocytosis.

The events relating to lanthanide ions enhanced permeability of human erythrocyte membrane: binding, conformational change, phase transition, perforation and ion transport

The binding and uptake of Gd3+ ions by human erythrocytes in vitro were studied by determining the Gd contents in membrane and in cytosol by means of particle-induced X-ray emission (PIXE) spectrometry. Results obtained from varied incubation time revealed that the Gd3+ ions bind to the membrane proteins and lipids at first. Gd3+ binding to the membrane lipids and proteins lasts 0 approximately 20 and 20 approximately 100 ms respectively, as shown by the stopped-flow studies. Then a fraction of Gd3+ ions diffuses through the membrane. The kinetics of Gd3+ binding indicates that the binding to phospholipids is prior to that to the membrane proteins, but a portion of the lipid-bound Gd3+ redistributed later to the proteins. PIXE studies showed that the entry of Gd3+ increased the influx of Ca2+ and Cl-. By monitoring the changes in fluorescence of proteins and that of the Ln3+, the uptake of La3+, Eu3+, Gd3+ and Tb3+ was shown to be a process comprising a series of events. Binding to the membrane molecules induces the phase transition of lipid bilayer and conformational changes and aggregation of membrane proteins. Conformational changes of the proteins were characterized by Fourier transform IR spectroscopy (FT-IR) deconvolved spectra, i.e. alpha-helix content decreases while beta-sheet increases. ESR spectra of MSL-labeled proteins reflect the aggregation state related with the conformational change. [31P]NMR spectra of membrane lipid bilayer revealed the Ln3+ ions induced hexagonal (H(II)) phase formation. Phase transition and aggregation of membrane proteins cause the formation of domain structure and perforation in the membrane. These alterations in membrane structure are responsible for the Ln3+ enhanced membrane permeability. Thus the previous Ln3+ binding will facilitate the across-membrane transport of other Ln3+ ions through the membrane.

Lanthanides stimulate [3H]tyramine binding in the rat striatum

Low (up to 100 nM) and high (approximately 100 microM) concentrations of lanthanides and Ca2+-ions, respectively, stimulated [3H]tyramine binding ([3H]TY) to rat striatal membranes, a putative marker for the vesicular transporter of dopamine. On the other hand, lanthanides (approximately 100 microM) inhibited or stimulated TY binding in striatal and extrastriatal (cortex, cerebellum) tissues, respectively. The binding increases by lanthanum (La3+) appeared to depend on endogenous Ca2+, whereas, those induced in EDTA-pretreated membranes were Ca2+-independent. The La3+-induced, apparent increase in the Bmax for [3H]TY binding seemed to reflect a retarded rate of dissociation of the ligand from its targets, rather than a larger availability of functionally-relevant, vesicular transport-related TY sites. This indicates uncertain mechanisms of present La3+ effects.

Histamine causes Ca2+ entry via both a store-operated and a store-independent pathway in bovine adrenal chromaffin cells

The characteristics and properties of the increase in cytosolic [Ca2+] that occurs in bovine adrenal medullary chromaffin cells on exposure to histamine have been investigated. Specifically, these experiments were conducted to determine how much external Ca2+ enters the cell through a (capacitative) Ca2+ entry pathway activated as a consequence of intracellular Ca2+ store mobilization, relative to that which enters independently of store depletion via other channels activated by histamine. In Fura-2 loaded cells continued exposure to histamine (10 microM) caused a rapid but transient increase in cytosolic [Ca2+] followed by a lower plateau that was sustained as long as external Ca2+ was present. In the absence of external Ca2+, only the initial brief transient was observed. In cells previously treated with thapsigargin (100 nM) in Ca(2+)-free medium to deplete the internal Ca2+ stores, histamine caused no increase in cytosolic [Ca2+] when external Ca2+ was absent. Re-introduction of external Ca2+ to thapsigargin-treated store-depleted cells caused a sustained increase in cytosolic [Ca2+] that was further increased (P < 0.0002) upon exposure to histamine. The histamine-evoked increase was prevented by the H1-receptor antagonist, mepyramine (2 microM). A comparison was made between store-dependent Ca2+ entry consequent upon store mobilization with histamine in Ca(2+)-free medium and plateau phase Ca2+ entry resulting from stimulation with histamine in Ca(2+)-containing medium. The latter was found to be approximately 3 times greater in magnitude than the former (P << 0.0001) at the same concentration of histamine (10 microM). It is concluded that histamine causes Ca2+ entry not only via a capacitative entry pathway secondary to internal store mobilization, but also causes substantial Ca2+ entry through other pathways.

Mechanism of lanthanum inhibition of extracellular ATP-evoked calcium mobilization in MDCK cells

We have studied the effects of La3+ on ATP-evoked rises in intracellular calcium levels ([Ca2+]i) measured by fura-2 fluorimetry in Madin Darby canine kidney (MDCK) cells. ATP evoked [Ca2+]i rises dose-dependently with an EC50 of 2.5 microM. The trigger for the Ca2+ signal was a release of Ca2+ from the inositol-1,4,5-trisphosphate (IP3)-sensitive stores because the signal was completely blocked by pretreatment with the endoplasmic reticulum (ER) Ca2+ pump inhibitor thapsigargin (TG) or the phospholipase C (PLC) inhibitor U73122. Both the peak height and area under the curve of 10 microM ATP-evoked Ca2+ signal was reduced by approximately 50% by extracellular Ca2+ removal, suggesting that ATP induced capacitative Ca2+ entry. La3+ inhibited the ATP-evoked Ca2+ signal dose-dependently when added before or after ATP. Pretreatment of 0.1 mM La3+ inhibited approximately 90% of the Ca2+ signal induced by 10 microM ATP. The mechanisms underlying the La3+ inhibition appear to involve not only block of capacitative Ca2+ entry but also interference with ATP binding to the ATP receptors.

Calcium site specificity. Early Ca2+-related tight junction events

The molecular mechanisms by which Ca2+ and metal ions interact with the binding sites that modulate the tight junctions (TJs) have not been fully described. Metal ions were used as probes of these sites in the frog urinary bladder. Basolateral Ca2+ withdrawal induces the opening of the TJs, a process that is abruptly terminated when Ca2+ is readmitted, and is followed by a complete recovery of the TJ seal. Mg2+ and Ba2+ were incapable of keeping the TJ sealed or of inducing TJ recovery. In addition, Mg2+ causes a reversible concentration-dependent inhibition of the Ca2+-induced TJ recovery. The effects of extracellular Ca2+ manipulation on the TJs apparently is not mediated by changes of cytosolic Ca2+ concentration. The transition elements, Mn2+ and Cd2+, act as Ca2+ agonists. In the absence of Ca2+, they prevent TJ opening and almost immediately halt the process of TJ opening caused by Ca2+ withdrawal. In addition, Mn2+ promotes an almost complete recovery of the TJ seal. Cd2+, in spite of stabilizing the TJs in the closed state and halting TJ opening, does not promote TJ recovery, an effect that apparently results from a superimposed toxic effect that is markedly attenuated by the presence of Ca2+. The interruption of TJ opening caused by Ca2+, Cd2+, or Mn2+, and the stability they confer to the closed TJs, might result from the interaction of these ions with E-cadherin. Addition of La3+ (2 microM) to the basolateral Ca2+-containing solution causes an increase of TJ permeability that fully reverses when La3+ is removed. This effect of La3+, observed in the presence of Ca2+ (1 mM), indicates a high La3+ affinity for the Ca2+-binding sites. This ability of La3+ to open TJs in the presence of Ca2+ is a relevant aspect that must be considered when using La3+ in the evaluation of TJ permeability of epithelial and endothelial membranes, particularly when used during in vivo perfusion or in the absence of fixatives.

Opposite effects of lanthanum on different types of nicotinic acetylcholine receptors

The effects of lanthanum (La3+) were studied on muscle and neuronal nicotinic acetylcholine receptors (AChRs) expressed in Xenopus oocytes. La3+ exerts a dose-dependent positive modulation on alpha1 beta1 gamma8 muscle AChRs, whereas it modulates negatively either alpha2 beta2, alpha2 beta4 or alpha3 beta4 neuronal AChRs. Moreover, La3+ appears to accelerate the desensitization of neuronal receptors. In both muscle and neuronal AChRs, the respective potentiating or inhibiting effects of La3+ on the ACh-currents are voltage-independent, suggesting that La3+ is acting at a site located in the external domain of the receptor.

Biphasic effects of chromium compounds on catecholamine secretion from bovine adrenal medullary cells

CrO3 was found to affect norepinephrine release in a biphasic manner: at concentrations above 100 microM, it inhibited, while at concentrations below 10 microM, it enhanced DMPP- and high K+-induced [3H]norepinephrine (NE) release from bovine adrenal medullary cells. Similar effects were found for K2Cr2O7. CrO3 inhibited the 45Ca2+ uptake induced by DMPP and high K+, suggesting that the voltage-gated Ca2+ channels are possible sites of the inhibitory action of CrO3. CrCl3, possessing a trivalent state in contrast to the hexavalent states of CrO3, K2Cr2O7, inhibited DMPP-induced [3H] release and inhibited, to a lesser extent, high K+-induced [3H]-NE release, suggesting that nicotinic receptors are also possible sites of Cr3+ action. In medullary cells permeabilized with digitonin, both CrO3 and CrCl3 induced [3H]-NE release from cells preloaded with [3H]-NE. In intact cells, CrO3 but not CrCl3 enhanced secretagogue-induced [3H]-NE release and entered into the cells as demonstrated by fluorescence quenching experiments. These results suggest that chromium compounds can induce catecholamine secretion after entering the cytoplasm. The enhancement of norepinephrine release induced by chromium ions appears to be due to interference with the intracellular functions of Ca2+ in the cytoplasm.

Sodium-calcium exchange: recent advances

Na-Ca exchange proteins are involved in Ca homeostasis in a wide variety of tissues. Unique Na-Ca exchangers have been identified by molecular biological approaches and it appears that these may represent a superfamily of ion transporters, similar to that identified for ion channels. Major advances in our understanding of these transporters have occurred in the past decade by combining molecular approaches with electrophysiological analyses. The regulatory and transport properties of Na-Ca exchangers are beginning to become understood in molecular detail. It also appears that the physiological roles of Na-Ca exchange may be quite complex. This brief review highlights some recent advances in Na-Ca exchange research obtained through the combination of molecular biological and electrophysiological approaches.

Use of La3+ to distinguish activity of the plasmalemmal Ca2+ pump from Na+/Ca2+ exchange in arterial myocytes

La3+ was tested for its ability to distinguish external Na+ (Nao)-independent Ca2+ efflux via the plasma membrane (PM) Ca2+ pump from Nao-dependent Ca2+ efflux via Na+/Ca2+ exchange. Fura-2 loaded cultured rat aortic myocytes were used with digital imaging to measure the cytosolic free Ca2+ concentration ([Ca2+]cyt) and to monitor La3+ entry. At a La3+ concentration ([La3+]o) of 0.25 mM, but not at lower concentrations, La3+ entered the cells; 0.01 mM verapamil blocked this entry. Transient increases in [Ca2+]cyt were evoked by unloading the sarcoplasmic reticulum with cyclopiazonic acid (CPA)+caffeine (CAF) in Na,Ca-free medium (to inhibit Ca2+ extrusion via Na+/Ca2+ exchange and Ca2+ influx). La3+ (0.03-0.25 mM with verapamil) augmented the Ca2+ transients and slowed Nao-independent [Ca2+]cyt recovery in a dose-dependent manner (IC50 approximately 0.01 mM La3+). This La(3+)-sensitive recovery was apparently mediated by the PM Ca2+ pump. The effects of La3+ were reversible: [Ca2+]cyt returned promptly toward base line when La3+ was washed out in Na,Ca-free medium containing CPA + CAF. Reintroduction of extracellular Na+ during [Ca2+]cyt recovery ([La3+]o = 0.06-0.25 mM) significantly speeded recovery, indicating that the Na+/Ca2+ exchanger was not inhibited by [La3+]o < or = 0.25 mM. The La(3+)-sensitive (Nao-independent) and Nao-dependent [Ca2+]cyt recovery rates were comparable. In Na(+)-loaded cells, < or = 0.25 mM La3+ also did not affect Na+/Ca2+ exchange mediated Ca2+ influx. In medium containing Na+ and Ca2+, 0.125 mM La3+ abolished the serotonin (5-HT) evoked plateau responses that resulted from Ca2+ entry via Ca2+ channels. In Na,Ca-free medium, but not Ca-free medium, however, La3+ converted 5-HT evoked Ca2+ transients into sustained responses. We conclude that low [La3+]o (0.06-0.25 mM) inhibits the PM Ca2+ pump, but spares Na+/Ca2+ exchanger mediated Ca2+ influx and efflux in arterial myocytes.

Mn2+ can substitute for Ca2+ in causing catecholamine secretion but not for increasing tyrosine hydroxylase phosphorylation in bovine adrenal chromaffin cells

The ability of the divalent cation manganese (Mn2+) to substitute for calcium (Ca2+) both in triggering catecholamine release and in stimulating catecholamine synthesis, as indicated by an increase in tyrosine hydroxylase (TOH) phosphorylation, has been determined in bovine adrenal medullary chromaffin cells maintained in tissue culture. Mn2+ was found to enter chromaffin cells through pathways activated by nicotinic receptor stimulation and potassium depolarisation, and via the Na1:Ca0 exchange mechanism in Na(+)-loaded cells. Like Ca2+, entry of Mn2+ through these pathways triggered immediate catecholamine release and, like Ca2+, maintained quantitatively comparable release at least up to 40 min. Unlike Ca2+, Mn2+ did not stimulate an increase in TOH phosphorylation in intact chromaffin cells, even over a prolonged time course, but Mn2+ did stimulate increased TOH phosphorylation in lysed cell preparations showing that its lack of effect in the intact cells was not due to inhibition of the specific phosphorylation pathway. In lysed cell preparations, Mn2+ stimulated also phosphorylation of a different spectrum of proteins to Ca2+, and of the same proteins to different extents. In particular, P80 (MARCKS protein) was more intensely phosphorylated in the presence of Mn2+ than in the presence of Ca2+. Since TOH phosphorylation always occurs when intracellular Ca2+ is increased, the absence of an increase with Mn2+ indicates that none of its intracellular effects could have occurred as a consequence of Mn2+ mobilisation of intracellular Ca2+. In summary, the data show that Mn2+ is a surrogate for Ca2+ in triggering and maintaining catecholamine release, but does not substitute for Ca2+ in stimulating TOH phosphorylation.

Ca2+ clearance mechanisms in isolated rat adrenal chromaffin cells

1. Intracellular Ca2+ clearance mechanisms were studied in rat adrenal chromaffin cells, by measuring slow tail currents through small-conductance Ca(2+)-activated K+ channels and using indo-1 photometry following depolarization-induced Ca2+ loading. 2. Following several-hundred millisecond depolarizations, [Ca2+]i decayed in three phases. An initial fast decay was followed by a long-lasting, low plateau, then [Ca2+]i returned to the resting level slowly. 3. Replacement of external Na+ moderately slowed [Ca2+]i decay, indicating a contribution of plasma membrane Na(+)-Ca2+ exchange. 4. Raising external pH or application of extracellular Eosin of La3+ prolonged slow tail currents, indicating a contribution of plasma membrane Ca(2+)-ATPase to Ca2+ clearance. 5. Ca(2+)-induced Ca2+ release from caffeine-sensitive stores occurred during depolarization. 6. Inhibition of endoplasmic reticulum Ca(2+)-ATPase had little effect on Ca2+ clearance. 7. Slow tail currents and [Ca2+]i decay following 0.2 - 2 s depolarizations were much prolonged by mitochondrial inhibition with carbonyl cyanide m-chlorophenylhydrazone (CCCP) or Ruthenium Red, which abolished the initial rapid decay and plateau of [Ca2+]i. 8. In conclusion, mitochondrial Ca2+ uptake plays a major role in Ca2+ clearance by rapidly and reversibly sequestering Ca2+ during depolarization-evoked Ca2+ loads.

Mechanisms for Ca signaling in vascular smooth muscle: resolved from 45Ca uptake and efflux experiments

Established cell lines are now widely used in experiments concerning vascular smooth muscle (VSM) function; however, considerable evidence suggests that cultured VSM cells are functionally different from VSM cells in intact blood vessels. In order to test the hypothesis that calcium signaling mechanisms are comparable in these two preparations, we developed a new method for high resolution 45Ca efflux studies in A7r5 cells. Briefly, this method involves plating cells in the lumen of a tubular glass efflux chamber and, after loading the cells with 45Ca, perfusing the chamber with a physiological saline solution and collecting the effluent. Using this method we found that the plasma membrane in cultured cells is not rate limiting for calcium efflux, since the efflux curves from both permeabilized and intact cells are kinetically the same. We also found the plasma membrane is not rate limiting in whole aortic segments by using a depolarizing solution followed by dihydropyridine solution. Thus, we demonstrated that the data obtained from cells or tissues with intact membranes reveal information about the intracellular stores (sarcoplasmic reticulum and mitochondria). Combining efflux data with a detailed kinetic model of cellular Ca transport allows least-squares estimation of the rate constants for release and uptake of Ca2+ by intracellular stores with a high degree of confidence (CV < 25%) as well as the Ca2+ contents and transmembrane fluxes associated with these stores. Quantitative comparison of results obtained from A7r5 cells with those we previously obtained for rabbit aortic segments reveals marked similarities and suggests that A7r5 cells serve as excellent model experiments for VSM cell Ca2+ homeostasis.

A difference in the cellular mechanisms of secretion of adrenaline and noradrenaline revealed with lanthanum in bovine chromaffin cells

A comparison of the effectiveness of the trivalent cation, lanthanum (La3+) relative to Ca2+ in causing adrenaline and noradrenaline release from bovine adrenal medullary chromaffin cells has been made. In cells maintained in tissue culture and permeabilised with digitonin, both La3+ and Ca2+ triggered catecholamine release. La3+ was more effective than Ca2+: the EC50 for La3+ was shifted to the left of that for Ca2+ by close to one order of magnitude for both adrenaline and noradrenaline. With respect to adrenaline, the same maximal release was triggered by the two cations, but with respect to noradrenaline, La3+ triggered a significantly greater release than did Ca2+. Mixtures of experimental media containing both La3+ and Ca2+ caused release of adrenaline and noradrenaline in amounts that approximated closely the sum of the releases caused by Ca2+ and La3+ alone. The data strongly imply that either the release mechanisms for adrenaline and for noradrenaline from their respective chromaffin cells are different, or the cellular mechanisms that regulate release from the two cells are different.

Dissociation of subsarcolemmal from global cytosolic [Ca2+] in myocytes from guinea-pig coronary artery

1. Changes in cytosolic Ca2+ concentration (delta[Ca2+]c) were measured by global indo-1 fluorescence and compared with changes in subsarcolemmal Ca2+ concentration (delta[Ca2+]sl) indicated by Ca(2+)-activated K+ currents (IK(Ca)). 2. At -50 mV holding potential, 10mM caffeine increased both IK(Ca) and [Ca2+]c without measurable delay. While IK(Ca) peaked within 0.3 +/- 0.16 s (mean +/- S.D.) and decayed to 50% within 0.4 +/- 0.2 s, delta[Ca2+]c peaked within 1.5 +/- 0.5 s and decayed to 50% within 5.2 +/- 1.0 s. The different time courses support the idea that [Ca2+]sl and [Ca2+]c deviate. 3. When 10 mM caffeine was applied 20 s after an initial 2 s caffeine application, IK(Ca) was suppressed to 22 +/- 5% and delta [Ca2+]c to 40 +/- 4%. During the following 1 min caffeine-free period, IK(Ca) recovered to 61 +/- 7% while delta [Ca2+]c remained at 40 +/- 3%. The differences between IK(Ca) and delta[Ca2+]c suggest that Ca2+ deprivation and Ca2+ refilling is faster in peripheral than in central sarcoplasmic reticulum (SR). 4. During the loading period of indo-1, a spontaneous delta[Ca2+]c of 30-80 nM appeared both at -50 mV and at more positive potentials. The amplitude of spontaneous delta[Ca2+]c increased with the amplitude, the frequency or the fusion of spontaneous transient outward currents (STOCs). 5. Block of sarcolemmal Ca2+ fluxes by 1 mM La3+ increased [Ca2+]c by 250 +/- 100 nM and suppressed the spontaneous delta[Ca2+]c. However, La3+ did not significantly retard the rate of decay of STOCs which may therefore be limited by Ca2+ diffusion into the cytosol and not by Ca2+ extrusion. 6. The dissociation of IK(Ca) (or STOCs) and delta[Ca2+]c may indicate a Ca2+ concentration gradient during Ca2+ release directed from the sarcolemma towards the centre of the cell, which later reverses direction.

Dominant role of mitochondria in clearance of large Ca2+ loads from rat adrenal chromaffin cells

Cytosolic Ca2+ (Ca2+c) clearance from adrenal chromaffin cells was studied by whole-cell patch clamp and indo-1 Ca2+ photometry after influx of Ca2+ through voltage-dependent Ca2+ channels. We isolated the rates of Ca2+c clearance by several mechanisms using combinations of the following agents (with their expected targets): Li+ or TEA substituted for Na+ (Na(+)-Ca2+ exchange), 1 mM La3+ applied after the depolarization (Na(+)-Ca2+ exchange and plasma membrane Ca(2+)-ATPase), 1 microM thapsigargin (pumping into reticular stores), and 2 microM carbonyl cyanide m-chlorophenylhydrazone (uptake into mitochondria). Remarkably, whenever [Ca2+]c rose above approximately 500 nM, Ca2+c clearance by mitochondria exceeded clearance by either Na(+)-Ca2+ exchange or the Ca2+ pumps of the plasma and reticular membranes. As [Ca2+]c fell again, Ca2+ reemerged from mitochondria, prolonging the final return to basal levels.

The effect of extracellular polyvalent cations on bovine anterior pituitary cells. Evidence for a Ca(2+)-sensing receptor coupled to release of intracellular calcium stores

We have investigated the effects of extracellular cations ([ION]ex) on cytosolic free calcium levels ([Ca2+]i) in bovine anterior pituitary (bAP) cells, using single-cell microfluorimetry. Increasing the [Ca2+]ex from 1 mM to 20 mM caused [Ca2+]i to increase in 64 +/- 14% of bAP cells. The [Ca2+]ex-induced [Ca2+]i increase was observed when cells were maintained in the presence of the voltage-gated-calcium-channel antagonist nitrendipine, but not when cells were treated with thapsigargin. Addition of [La3+]ex (5-15 microM) decreased [Ca2+]i, whereas 30 microM-1 mM caused a [Ca2+]i rise in 60.9 +/- 8.8% of bAP cells. [La3+]ex-induced [Ca2+]i changes were abolished by treating bAP-cells with either thapsigargin or ionomycin, but not nitrendipine. [La3+]ex at 15 microM did not increase [Ca2+]i in any cells tested, but when cells were treated with thimerosal, [La3+]ex (15 microM) caused a [Ca2+]i increase in 62.5 +/- 12.2% of bAP cells. In the presence of 1 mM [Ca2+]ex, successive additions of La3+ caused successive [Ca2+]i rises, but in nominally [Ca2+]ex-free medium only the first addition of [La3+]ex caused a [Ca2+]i rise. Addition of thyroliberin (TRH) in the presence of 1 mM [Ca2+]ex, caused [Ca2+]i to increase in 70% of bAP cells; subsequent addition of [La3+]ex (1 mM) only caused [Ca2+]i increases in 75% of those cells which had already responded to TRH. However, all cells which responded to 1 mM [La3+]ex also responded subsequently to TRH. After treatment with TRH in medium that was nominally [Ca2+]ex free, addition of La3+ (0.5-1 mM) did not increase [Ca2+]i in any cells tested. The number of cells which showed [La3+]ex-induced [Ca2+]i increases decreased in culture: only 21.75 +/- 2.2% cells responded after 7-11 days. When cells were cultured for 7-11 days in the presence of tunicamycin, [La3+]ex failed to increase [Ca2+]i in any cells tested. [Mn2+]ex rapidly quenched the Fura-2 signal measured from all bAP cells, but at 10 mM it also triggered a [Ca2+]i rise in about 60% of bAP cells. The Mn(2+)-induced [Ca2+]i rise was specifically abolished in cells cultured in the presence of tunicamycin although quenching was still observed. From these data we suggest that bAP cells may express a polyvalent cation receptor coupled to the release of calcium from intracellular stores.