H+-dependent calcium uptake into an IP3-sensitive calcium pool from rat parotid gland

Mechanisms of Cadmium Neurotoxicity

Cadmium is a heavy metal that increasingly contaminates food and drink products. Once ingested, cadmium exerts toxic effects that pose a significant threat to human health. The nervous system is particularly vulnerable to prolonged, low-dose cadmium exposure. This review article provides an overview of cadmium's primary mechanisms of neurotoxicity. Cadmium gains entry into the nervous system via zinc and calcium transporters, altering the homeostasis for these metal ions. Once within the nervous system, cadmium disrupts mitochondrial respiration by decreasing ATP synthesis and increasing the production of reactive oxygen species. Cadmium also impairs normal neurotransmission by increasing neurotransmitter release asynchronicity and disrupting neurotransmitter signaling proteins. Cadmium furthermore impairs the blood-brain barrier and alters the regulation of glycogen metabolism. Together, these mechanisms represent multiple sites of biochemical perturbation that result in cumulative nervous system damage which can increase the risk for neurological and neurodegenerative disorders. Understanding the way by which cadmium exerts its effects is critical for developing effective treatment and prevention strategies against cadmium-induced neurotoxic insult.

Factors influencing riverine utilization patterns in two sympatric macaques

Many species of terrestrial animals, including primates, live in varied association with the aquatic (e.g., riverine or coastal) environment. However, the benefits that each species receive from the aquatic environment are thought to vary depending on their social and ecological characteristics, and thus, elucidating those benefits to each species is important for understanding the principles of wild animal behaviour. In the present study, to gain a more complete picture of aquatic environment use, including social and ecological factors in primates, factors affecting riverine habitat utilization of two macaque species (Macaca nemestrina and M. fascicularis) were identified and qualitative comparisons were made with sympatric proboscis monkeys (Nasalis larvatus), which have different social and ecological characteristics. Temporal variation in sighting frequency of macaques at the riverbanks was positively related to the fruit availability of a dominant riparian plant species and negatively related to the river water level which affects the extent of predation pressure. Riverine utilization of macaques was greatly influenced by distribution and abundance of food (especially fruit) resources, possibly in association with predation pressure. Additionally, qualitative ecological comparisons with sympatric proboscis monkeys suggest that the drivers of riverine utilization depend on the feeding niches of the species, and different anti-predator strategies resulting from their differing social structures.

Lipocalin-2 (24p3/neutrophil gelatinase-associated lipocalin (NGAL)) receptor is expressed in distal nephron and mediates protein endocytosis

In the kidney, bulk reabsorption of filtered proteins occurs in the proximal tubule via receptor-mediated endocytosis (RME) through the multiligand receptor complex megalin-cubilin. Other mechanisms and nephron sites for RME of proteins are unclear. Recently, the secreted protein 24p3 (lipocalin-2, neutrophil gelatinase-associated lipocalin (NGAL)), which is expressed in the distal nephron, has been identified as a sensitive biomarker of kidney damage. A high-affinity receptor for 24p3 (24p3R) that is involved in endocytotic iron delivery has also been cloned. We investigated the localization of 24p3R in rodent kidney and its role in RME of protein-metal complexes and albumin. Immunostaining of kidney tissue showed expression of 24p3R in apical membranes of distal tubules and collecting ducts, but not of proximal tubule. The differential expression of 24p3R in these nephron segments was confirmed in the respective cell lines. CHO cells transiently transfected with 24p3R or distal tubule cells internalized submicromolar concentrations of fluorescence-coupled proteins transferrin, albumin, or metallothionein (MT) as well as the toxic cadmium-MT (Cd2+(7)-MT) complex, which caused cell death. Uptake of MT or transferrin and Cd2+(7)-MT toxicity were prevented by picomolar concentrations of 24p3. An EC50 of 123±50 nM was determined for binding of MT to 24p3R by microscale thermophoresis. Hence, 24p3R binds proteins filtered by the kidney with high affinity and may contribute to RME of proteins, including 24p3, and to Cd2+(7)-MT toxicity in distal nephron segments.

Adaptive redistribution of NBCe1-A and NBCe1-B in rat kidney proximal tubule and striated ducts of salivary glands during acid-base disturbances

The cellular distribution of the NH2-terminal electrogenic Na+-HCO3(-) cotransporter (NBCe1) variants NBCe1-A and NBCe1-B has been investigated in rat kidney and submandibular gland (SMG) under physiological conditions and after systemic acid-base perturbations. Moreover, the in vivo data were complemented in vitro by using an immortalized cell line derived from the S1 segment of the proximal tubule (PT) of normotensive Wistar-Kyoto rats (WKPT-0293 Cl.2). NBCe1-A was basolaterally localized in PT cells, whereas NBCe1-B exhibited intracellular and basolateral distribution. SMG showed transcript and protein expression for NBCe1-A and NBCe1-B. NBCe1-B was basolaterally localized in duct cells; NBCe1-A was found intracellularly in salivary striated ducts and apically in main duct cells. Acute metabolic acidosis significantly increased cells that showed basolateral NBCe1-A in the PT, indicating increased HCO3(-) reabsorption, and significantly decreased cells that exhibited basolateral NBCe1-B in the salivary ducts, suggesting decreased HCO3(-) secretion. Chronic acidosis had no effect on NBCe1 distribution in PT but significantly increased the percentage of cells with basolateral NBCe1-A in salivary striated duct cells, suggesting increased HCO3(-) reabsorption. In contrast, chronic alkalosis caused adaptive redistribution of NBCe1-A and NBCe1-B in renal PT, favoring decreased HCO3(-) reabsorption. In vitro, WKPT-0293 Cl.2 cells expressed key acid-base transporters. Extracellular alkalosis downregulated NBCe1-A protein. WKPT-0293 Cl.2 cells are therefore a useful model to study renal acid-base regulation in vitro. The results propose redistribution of the transporters as a potential posttranslational regulation modus during acid-base disturbances. Moreover, the data demonstrate that renal PT and salivary duct epithelia respond to acid-base disturbances by an opposite redistribution pattern for NBCe1-A and NBCe1-B, reflecting specialized functions as the HCO3(-)-reabsorbing and HCO3(-)-secreting epithelium, respectively.

Na+/H+ exchanger isoforms are differentially regulated in rat submandibular gland during acid/base disturbances in vivo

Acute metabolic acidosis and alkalosis cause a series of homeostatic adaptive responses in the kidney and other epithelia. We hypothesized that acid/base disturbances might affect the expression of Na(+)/H(+) exchanger (NHE) isoforms in salivary glands and determined the expression and cellular distribution of NHE3 and NHE4 in rat submandibular glands of controls and after imposed acute or chronic metabolic acidosis or alkalosis in vivo. Reverse transcription/polymerase chain reaction, in situ hybridization, and immunohistochemistry were applied by using specific primers, antisense probes, and antibodies, respectively. The results showed NHE3 and NHE4 transcript expression and protein abundance in rat submandibular gland. NHE3 was apically localized in duct cells, whereas NHE4 was found basolaterally distributed in acinar and duct cells. Acute acidosis and alkalosis and chronic acidosis had no effect on NHE3 and NHE4 expression and localization. In contrast, chronic metabolic alkalosis significantly decreased the number of apically stained NHE3 duct cells but had no effect on NHE3 mRNA expression. The results demonstrate, for the first time, the presence of NHE4 protein in salivary glands. The data also indicate the distinct regulation and adaptive changes of different isoforms of the same transporter in rat submandibular gland as a response to acid/base disturbances.

Ca2+ storage in Trypanosoma brucei: the influence of cytoplasmic pH and importance of vacuolar acidity

The hypothesis that changes in cytosolic pH effect the release from intracellular compartments of stored calcium in Trypanosoma brucei was addressed by the use of procyclic and bloodstream trypomastigotes of T. brucei loaded with the fluorescent reagents 2',7'-bis-(2-carboxyethyl)-5(and 6)-carboxyfluorescein (BCECF) to measure intracellular pH (pHi), or fura 2 to measure intracellular free calcium ([Ca2+]i). Experiments were performed in EGTA-containing buffers, so increases in [Ca2+]i reflected release of stored calcium rather than Ca2+ entry. Nigericin reduced pHi and increased [Ca2+]i in loaded cells, whilst propionate reduced pHi, but did not affect [Ca2+]i, and NH4Cl increased both variables, so there appears to be no correlation between pHi and [Ca2+]i. Treatment of the cells with the calcium ionophore ionomycin under similar conditions (nominal absence of extracellular Ca2+) resulted in an increase of [Ca2+]i which was greatly increased by addition of either NH4Cl, nigericin or the vacuolar H(+)-ATPase inhibitor bafilomycin A1. Similar results were obtained when the order of additions was reversed or when digitonin-permeabilized cells were used with the Ca2+ indicator arsenazo III. The results suggest that more Ca2+ is stored in this acidic compartment in procyclic than in bloodstream forms. Taking into account the relative importance of the ionomycin-releasable and the ionomycin-plus-NH4Cl-releasable Ca2+ pools, it is apparent that a significant amount of the Ca2+ stored in T. brucei trypomastigotes is present in the acidic compartment thus identified.

Intracellular Ca2+ storage in acidocalcisomes of Trypanosoma cruzi

The use of digitonin to permeabilize the plasma membrane of Trypanosoma cruzi allowed the identification of a non-mitochondrial nigericin- or bafilomycin A1-sensitive Ca(2+)-uptake mechanism. Proton uptake, as detected by ATP-dependent Acridine Orange accumulation, was also demonstrated in these permeabilized cells. Under these conditions Acridine Orange was concentrated in abundant cytoplasmic round vacuoles. This latter process was inhibited (and reversed) by bafilomycin A1, nigericin and NH4Cl in different stages of T. cruzi. Ca2+ released Acridine Orange from permeabilized cells, suggesting that the dye and Ca2+ were being accumulated in the same acidic compartment and that Ca2+ was taken up in exchange for protons. Addition of bafilomycin A1 (5 microM), nigericin (1 microM) or carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP; 1 microM) to fura 2-loaded epimastigotes increased their intracellular Ca2+ concentration ([Ca2+]i). Although this effect was more noticeable in the presence of extracellular Ca2+, it was also observed in its absence. Addition of NH4Cl (10-40 mM) to different stages of T. cruzi, in the nominal absence of extracellular Ca2+ to preclude Ca2+ entry, increased both [Ca2+]i in fura 2-loaded cells, and intracellular pH (pHi) in 2',7'-bis-(2-carboxyethyl)-5-(and -6)-carboxyfluorescein acetoxymethyl ester (BCECF)-loaded cells. Treatment of the cells with the Ca2+ ionophore ionomycin under similar conditions (nominal absence of extracellular Ca2+) resulted in an increase in [Ca2+]i and a significantly higher increase in [Ca2+]i after addition of NH4Cl, nigericin or bafilomycin A1, all agents which increase the pH of acidic compartments and make ionomycin more effective as a Ca(2+)-releasing ionophore. Similar results were obtained when the order of additions was reversed. Taking into account the relative importance of the ionomycin-releasable and the ionomycin plus NH4Cl-releasable Ca2+ pools, it is apparent that most of the Ca2+ stored in different stages of T. cruzi is present in the acidic compartment thus identified. Taken together, these results are consistent with the presence of a Ca2+/H+ exchange system in an acidic vacuole, which we have named the 'acidocalcisome' and which appears to be a unique organelle present in trypanosomatids.

Ca2+/H+ exchange in acidic vacuoles of Trypanosoma brucei

The use of digitonin to permeabilize the plasma membrane of Trypanosoma brucei procyclic and bloodstream trypomastigotes allowed the identification of a non-mitochondrial nigericin-sensitive Ca2+ compartment. The proton ionophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) was able to cause Ca2+ release from this compartment, which was also sensitive to sodium orthovanadate. Preincubation of the cells with the vacuolar H(+)-ATPase inhibitor bafilomycin A1 greatly reduced the nigericin-sensitive Ca2+ compartment. Bafilomycin A1 inhibited the initial rate of ATP-dependent non-mitochondrial Ca2+ uptake and stimulated the initial rate of nigericin-induced Ca2+ release by permeabilized procyclic trypomastigotes. ATP-dependent and bafilomycin A1- and 7-chloro-4-nitrobenz-2-oxa-1,3-diazole (NBD-Cl)-sensitive Acridine Orange uptake was demonstrated in permeabilized cells. Under these conditions Acridine Orange was concentrated in abundant cytoplasmic round vacuoles by a process inhibited by bafilomycin A1, NBD-Cl, nigericin, and Ca2+. Vanadate or EGTA significantly increased Acridine Orange uptake, while Ca2+ released Acridine Orange from these preparations, thus suggesting that the dye and Ca2+ were being accumulated in the same acidic vacuole. Acridine Orange uptake was reversed by nigericin, bafilomycin A1 and NH4Cl. The results are consistent with the presence of a Ca2+/H(+)-ATPase system pumping Ca2+ into an acidic vacuole, that we tentatively named the acidocalcisome.

Calcium accumulation by organelles within Myxicola axoplasm

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

ATP-driven Ca2+/H+ antiport in acid vesicles from Dictyostelium

Amoebae of the cellular slime mold Dictyostelium discoideum possess an extensive and dynamic endomembrane system that includes many types of acidic vacuoles. A light membrane fraction from Dictyostelium, rich in vacuolar-type H(+)-ATPase, has been described [Padh, H., Lavasa, M. & Steck, T.L. (1989) J. Cell Biol. 108, 865-874]. Here, we show that this "acidosomal" fraction also contains a high-affinity vanadate-sensitive Ca2+ uptake activity that is stimulated by the pH gradient formed by the H(+)-ATPase. We attribute this Ca2+ uptake to the presence of a H(+)-countertransporting Ca(2+)-ATPase, pumping Ca2+ into an acidic compartment.

Spatial distribution of intracellular, free Ca2+ in isolated rat parotid acini

The spatial distribution of intracellular, free Ca2+ ([Ca2+]i) in rat parotid acini was measured by imaging fura-2 fluorescence from individual acinar cells by means of a digital imaging microscope. Upon cholinergic stimulation in a Krebs-Ringer bicarbonate buffer at (37 degrees C), [Ca2+]i increased synchronously at both the basolateral and luminal membranes as well as in all cells of the secretory endpiece, reaching peak [Ca2+]i levels 1 s after stimulation. Atropine addition caused a rapid down-regulation of [Ca2+]i, which, however, never reached prestimulatory levels. When acini were stimulated in a medium containing 5 nM Ca2+, the Ca2+ mobilization arising from internal pools caused an increase in [Ca2+]i predominantly near the basolateral area, where the endoplasmic reticulum is located, and standing Ca2+ gradients were observed for up to 10 s. A mathematical model is developed to simulate the time courses of the Ca2+ profiles through the cytoplasm using estimated values of the Ca2+ diffusion coefficients and the cytosolic Ca2+ buffering capacity. It is concluded that under physiological conditions, the Ca2+ release from the endoplasmic reticulum is responsible for the activation of the basolaterally located K+ channels. Furthermore, Ca2+ influx from the interstitium is responsible for much of the rise in [Ca2+]i near the luminal membranes, where the Cl- channels are supposed to be located.

Single-channel and Fura-2 analysis of internal Ca2+ oscillations in HeLa cells: contribution of the receptor-evoked Ca2+ influx and effect of internal pH

Patch-clamp and Fura-2 experiments were performed in order to investigate the calcium oscillations due to H1 receptor stimulation in HeLa cells. The cytosolic calcium fluctuations occurring directly at the plasma membrane inner face were detected by measuring the activity of calcium-dependent potassium channels. This method also allowed measurement of changes in intracellular potential using as indicator the amplitude of the channel current jump. The average internal calcium concentration was obtained from Fura-2 experiments carried out at either the single-cell level or from a small population of cells in monolayer. The results indicate that the internal calcium oscillations in HeLa cells arise from a biphasic process with an initial phase independent of the presence of external calcium. External calcium was found, however, to become essential once the regular oscillatory process has been established. Removing external calcium after this initial phase produced a rapid decay in the burst frequency and eventually a complete abolition of the oscillations. In addition, the calcium oscillations occurring during the external-calcium-dependent phase could be blocked by calcium entry blockers such as Co2+ or La3+, or abolished by perfusing the external medium with a high-K+ solution. Experiments were also performed in which the cell internal pH (pHi) was changed by removing the external bicarbonate or by adding NH4Cl to the bathing solution. The results obtained under these conditions indicate that an increase in internal pH abolishes selectively the appearance of calcium spikes without increasing the basal calcium level, while a cellular acidification maintains or stimulates the calcium oscillatory process. It was also observed that the inhibitory effect of alkaline pH was independent of external calcium, and that calcium oscillations could always be seen at alkaline pH during the initial phase of histamine stimulation. On the basis of these results, it is proposed that the internal calcium oscillations in HeLa cells depend on the release of calcium from internal pools, which are reloaded via a pH-dependent mechanism. Part of the calcium sequestration occurring during the oscillatory process would be carried out, however, by pH-insensitive calcium compartments.

Single chloride channels in endosomal vesicle preparations from rat kidney cortex

Endocytotic vesicles from rat kidney cortex, isolated by differential centrifugation and enriched on a Percoll gradient, contain both an electrogenic H+ translocation system and a conductive chloride pathway. Using the dehydration/rehydration method, we fused vesicles of enriched endosomal vesicle preparations and thereby made them accessible to the patch-clamp technique. In the fused vesicles, we observed Cl- channels with a single-channel conductance of 73 +/- 2 pS in symmetrical 140 mM KCl solution (n = 25). The current-voltage relationship was linear in the range of -60 to +80 mV, but channel kinetic properties depended on the clamp potential. At positive potentials, two sublevels of conductance were discernible and the mean open time of the channel was 10-15 msec. At negative voltages, only one substate could be resolved and the mean open time decreased to 2-6 msec. Clamp voltages more negative than -50 mV caused reversible channel inactivation. The channel was selective for anions over cations. Ion substitution experiments revealed an anion permeability sequence of Cl- = Br- = I- greater than SO4(2-) approximately F-. Gluconate, methanesulfonate and cyclamate were impermeable. The anion channel blockers 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS, 1.0 mM) and 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB, 0.1 mM) totally inhibited channel activity. Comparisons with data obtained from radiolabeled Cl(-)-flux measurements and studies on the H+ pump activity in endocytotic vesicle suspensions suggest that the channel described here is involved in maintenance of electroneutrality during ATP-driven H+ uptake into the endosomes.

Characterization of inositol 1,4,5-trisphosphate-sensitive (IsCaP) and -insensitive (IisCaP) nonmitochondrial Ca2+ pools in rat pancreatic acinar cells

We have measured Ca2+ uptake and Ca2+ release in isolated permeabilized pancreatic acinar cells and in isolated membrane vesicles of endoplasmic reticulum prepared from these cells. Ca2+ uptake into cells was monitored with a Ca2+ electrode, whereas Ca2+ uptake into membrane vesicles was measured with 45Ca2+. Using inhibitors of known action, such as the H+ ATPase inhibitors NBD-Cl and NEM, the Ca2+ ATPase inhibitor vanadate as well as the second messenger inositol 1,4,5-trisphosphate (IP3) and its analog inositol 1,4,5-trisphosphorothioate (IPS3), we could functionally differentiate two nonmitochondrial Ca2+ pools. Ca2+ uptake into the IP3-sensitive Ca2+ pool (IsCaP) occurs by a MgATP-dependent Ca2+ uptake mechanism that exchanges Ca2+ for H+ ions. In the absence of ATP Ca2+ uptake can occur to some extent at the expense of an H+ gradient that is established by a vacuolar-type MgATP-dependent H+ pump present in the same organelle. The other Ca2+ pool takes up Ca2+ by a vanadate-sensitive Ca2+ ATPase and is insensitive to IP3 (IisCaP). The IsCaP is filled at "higher" Ca2+ concentrations (approximately 10(-6) mol/liter) which may occur during stimulation. The low steady-state [Ca2+] of approximately 10(-7) mol/liter is adjusted by the IisCaP. It is speculated that both Ca2+ pools can communicate with each other, the possible mechanism of which, however, is at present unknown.