Ionomycin releases calcium from the sarcoplasmic reticulum and activates Na+/Ca2+ exchange in vascular smooth muscle cells

Calcium tunneling through the ER and transfer to other organelles for optimal signaling in Toxoplasma gondii

Ca 2+ signaling in cells begins with the opening of Ca 2+ channels in either the plasma membrane (PM) or the endoplasmic reticulum (ER) and results in a dramatic increase in the physiologically low (<100 nM) cytosolic Ca 2+ level. The temporal and spatial Ca 2+ levels are well regulated to enable precise and specific activation of critical biological processes. Ca 2+ signaling regulates pathogenic features of apicomplexan parasites like Toxoplasma gondii which infects approximately one-third of the world's population. T. gondii relies on Ca 2+ signals to stimulate traits of its infection cycle and several Ca 2+ signaling elements play essential roles in its parasitic cycle. Active egress, an essential step for the infection cycle of T. gondii is preceded by a large increase in cytosolic Ca 2+ most likely by release from intracellular stores. Intracellular parasites take up Ca 2+ from the host cell during host Ca 2+ signaling events to replenish intracellular stores. In this work, we investigated the mechanism by which intracellular stores are replenished with Ca 2+ and demonstrated a central role for the SERCA-Ca 2+ -ATPase to keep not only the ER filled with Ca 2+ but also acidic stores. We also show mitochondrial Ca 2+ uptake, by transfer of Ca 2+ from the ER most likely through membrane contact sites. We propose a central role for the ER in tunneling of calcium from the extracellular milieu through the ER to other organelles.

HIGHLIGHTS

The T. gondii ER efficiently takes up Ca 2+ that enters the cytosol from the extracellular milieu. Filling of acidic stores in T. gondii appears to be dependent on the filling of the ER The mitochondrion of T. gondii has no direct access to extracellular calcium but is able to take up Ca 2+ by transfer from the ER and/or acidic stores.

A plastid two-pore channel essential for inter-organelle communication and growth of Toxoplasma gondii

Two-pore channels (TPCs) are a ubiquitous family of cation channels that localize to acidic organelles in animals and plants to regulate numerous Ca²⁺-dependent events. Little is known about TPCs in unicellular organisms despite their ancient origins. Here, we characterize a TPC from Toxoplasma gondii, the causative agent of toxoplasmosis. TgTPC is a member of a novel clad of TPCs in Apicomplexa, distinct from previously identified TPCs and only present in coccidians. We show that TgTPC localizes not to acidic organelles but to the apicoplast, a non-photosynthetic plastid found in most apicomplexan parasites. Conditional silencing of TgTPC resulted in progressive loss of apicoplast integrity, severely affecting growth and the lytic cycle. Isolation of TPC null mutants revealed a selective role for TPCs in replication independent of apicoplast loss that required conserved residues within the pore-lining region. Using a genetically-encoded Ca²⁺ indicator targeted to the apicoplast, we show that Ca²⁺ signals deriving from the ER but not from the extracellular space are selectively transmitted to the lumen. Deletion of the TgTPC gene caused reduced apicoplast Ca²⁺ uptake and membrane contact site formation between the apicoplast and the ER. Fundamental roles for TPCs in maintaining organelle integrity, inter-organelle communication and growth emerge.

BRAF and NRAS mutated melanoma: Different Ca2+ responses, Na+/Ca2+ exchanger expression, and sensitivity to inhibitors

Calcium is a ubiquitous intracellular second messenger, playing central roles in the regulation of several biological processes. Alterations in Ca²⁺ homeostasis and signaling are an important feature of tumor cells to acquire proliferative and survival advantages, which include structural and functional changes in storage capacity, channels, and pumps. Here, we investigated the differences in Ca²⁺ homeostasis in vemurafenib-responsive and non-responsive melanoma cells. Also, the expression of the Na⁺/Ca²⁺ exchanger (NCX) and the impact of its inhibition were studied. For this, it was used B-RAF^V600E and NRAS^Q61R-mutated human melanoma cells. The intracellular Ca²⁺ chelator BAPTA-AM decreased the viability of SK-MEL-147 but not of SK-MEL-19 and EGTA sensitized NRAS^Q61R-mutated cells to vemurafenib. These cells also presented a smaller response to thapsargin and ionomycin regarding the cytosolic Ca²⁺ levels in relation to SK-MEL-19, which was associated to an increased expression of NCX1, NO basal levels, and sensitivity to NCX inhibitors. These data highlight the differences between B-RAF^V600E and NRAS^Q61R-mutated melanoma cells in response to Ca²⁺ stimuli and point to the potential combination of clinically used chemotherapeutic drugs, including vemurafenib, with NCX inhibitors as a new therapeutic strategy to the treatment of melanoma.

Inducible nitric oxide synthase-mediated injury in a mouse model of acute salivary gland dysfunction

AIM

Inducible nitric oxide synthase (iNOS) is a key regulator of the innate immune system. The aim of the current study was to explore whether innate immune-mediated iNOS and reactive nitrogen species acutely perturb acinar cell physiology and calcium homeostasis of exocrine salivary tissues.

METHODS

Innate immunity in the submandibular gland of C57BL/6 mice was locally activated via intraductal retrograde infusion of polyinosinic:polycytidylic acid (poly (I:C). Expressions of iNOS and the activity of the reactive nitrogen species peroxynitrite, were evaluated by immunohistochemistry. Mice were pre-treated with the selective iNOS inhibitor aminoguanidine in order to substantiate the injurious effect of the nitrosative signal on the key calcium regulator sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA2b) and calcium signalling.

RESULTS

Challenging salivary gland innate immunity with poly (I:C) prompted upregulated expression of iNOS and the generation of peroxynitrite. Inhibition of iNOS/peroxynitrite revealed the role played by upregulated nitrosative signalling in: dysregulated expression of SERCA2b, perturbed calcium homeostasis and loss of saliva secretion.

CONCLUSION

iNOS mediates disruption of exocrine calcium signalling causing secretory dysfunction following activation of innate immunity in a novel salivary gland injury model.

Toll-Like Receptors Promote Mitochondrial Translocation of Nuclear Transcription Factor Nuclear Factor of Activated T-Cells in Prolonged Microglial Activation

Microglia are resident macrophages in the CNS that scavenge pathogens, dying cells, and molecules using pattern recognition Toll-like receptors (TLRs). Nuclear factor of activated T-cells (NFAT) family transcription factors also regulate inflammatory responses in microglia. However, whether there exists cross talk between TLR and NFAT signaling is unclear. Here we show that chronic activation of murine microglia by prolonged stimulation of Toll-like receptor 4 (TLR4) ligand lipopolysaccharides (LPSs) leads to unexpected translocation of NFAT1 into mitochondria. This mitochondrial import of NFAT1 is independent of calcium/calcineurin signaling. Instead, inhibition of Toll/interleukin 1 receptor domain-containing adapter-inducing interferon-β (TRIF) pathway blocks the mitochondrial translocation of NFAT1. Functionally, inhibition of NFAT1 reduces the TRIF-mediated expression of interferon-β and compromises the production of ATP and reactive oxygen species in LPS-treated microglia. Therefore, our findings reveal a new inflammatory signaling pathway that links TLR with NFAT in regulating cytokine production and mitochondrial activity during chronic microglial activation.

SIGNIFICANCE STATEMENT

Nuclear factor of activated T-cells (NFAT) family transcription factors are known to undergo nuclear translocation in response to inflammatory stimulation. In this study, we uncovered a surprise transportation of NFATs into mitochondria in microglia after a prolonged treatment with bacteria endotoxin lipopolysaccharides (LPSs). LPSs activated Toll-like receptor 4 and its downstream Toll/interleukin 1 receptor-domain-containing adapter-inducing interferon-β (TRIF) to regulate the mitochondrial translocation of NFAT in microglia, whereas genetic inhibition of NFAT1 compromised TRIF-mediated cytokine production and reduced ATP and reactive oxygen species generation. These findings reveal a previously undescribed mitochondrial translocation of NFAT in microglia responding to extended activation of Toll-like receptor-mediated signaling transduction pathways.

Unconventional neuroprotection against Ca2+ -dependent insults by metalloporphyrin catalytic antioxidants

We evaluated whether both inert and catalytically active metalloporphyrin antioxidants, meso-substituted with either phenyl-based or N-alkylpyridinium-based groups, suppress Ca(2+)-dependent neurotoxicity in cell culture models of relevance to cerebral ischemia. Representatives from both metalloporphyrin classes, regardless of antioxidant strength, protected cultured cortical neurons or PC-12 cultures against the Ca(2+) ionophores ionomycin or A23187, by suppressing neurotoxic Ca(2+) influx. Some metalloporphyrins suppressed excitotoxic Ca(2+) influx indirectly induced by the Ca(2+) ionophores in cortical neurons. Metalloporphyrins did not quench intracellular fluorescence, suggesting localization to the plasma membrane interface and/or interference with Ca(2+) ionophores. Metalloporphyrins suppressed ionomycin-induced Mn(2+) influx, but did not protect cortical neurons against pyrithione, a Zn(2+) ionophore. In other Ca(2+)-dependent paradigms, Ca(2+) influx via plasma membrane depolarization, but not through reversal of plasmalemmal Na(+)/Ca(2+) exchangers, was modestly suppressed by Mn(III)meso-tetrakis(4-benzoic acid)porphyrin (Mn(III)TBAP) or by an inert analog, Zn(II)TBAP. Mn(III)TBAP and Zn(II)TBAP potently protected cortical neurons against long-duration oxygen-glucose deprivation (OGD), performed in the presence of antagonists of NMDA, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate and L-type voltage-gated Ca(2+) channels, raising the possibility of an unconventional mode of blockade of transient receptor protein melastatin 7 channels by a metalloTBAP family of metalloporphyrins. The present study extends the range of Ca(2+)-dependent insults for which metalloporphyrins demonstrate unconventional neuroprotection. MetalloTBAPs appear capable of targeting an OGD temporal continuum.

The effect of nimodipine on calcium homeostasis and pain sensitivity in diabetic rats

1. The pathogenesis of diabetic neuropathy is a complex phenomenon, the mechanisms of which are not fully understood. Our previous studies have shown that the intracellular calcium signaling is impaired in primary and secondary nociceptive neurons in rats with streptozotocin (STZ)-induced diabetes. Here, we investigated the effect of prolonged treatment with the L-type calcium channel blocker nimodipine on diabetes-induced changes in neuronal calcium signaling and pain sensitivity. 2. Diabetes was induced in young rats (21 p.d.) by a streptozotocin injection. After 3 weeks of diabetes development, the rats were treated with nimodipine for another 3 weeks. The effect of nimodipine treatment on calcium homeostasis in nociceptive dorsal root ganglion neurons (DRG) and substantia gelatinosa (SG) neurons of the spinal cord slices was examined with fluorescent imaging technique. 3. Nimodipine treatment was not able to normalize elevated resting intracellular calcium ([Ca(2+)]( i )) levels in small DRG neurons. However, it was able to restore impaired Ca(2+) release from the ER, induced by either activation of ryanodine receptors or by receptor-independent mechanism in both DRG and SG neurons. 4. The beneficiary effects of nimodipine treatment on [Ca(2+)]( i ) signaling were paralleled with the reversal of diabetes-induced thermal hypoalgesia and normalization of the acute phase of the response to formalin injection. Nimodipine treatment was also able to shorten the duration of the tonic phase of formalin response to the control values. 5. To separate vasodilating effect of nimodipine Biessels et al., (Brain Res. 1035:86-93) from its effect on neuronal Ca(2+) channels, a group of STZ-diabetic rats was treated with vasodilator - enalapril. Enalapril treatment also have some beneficial effect on normalizing Ca(2+) release from the ER, however, it was far less explicit than the normalizing effect of nimodipine. Effect of enalapril treatment on nociceptive behavioral responses was also much less pronounced. It partially reversed diabetes-induced thermal hypoalgesia, but did not change the characteristics of the response to formalin injection. 6. The results of this study suggest that chronic nimodipine treatment may be effective in restoring diabetes-impaired neuronal calcium homeostasis as well as reduction of diabetes-induced thermal hypoalgesia and noxious stimuli responses. The nimodipine effect is mediated through a direct neuronal action combined with some vascular mechanism.

Modulation of agonist-induced Ca2+ release by SR Ca2+ load: direct SR and cytosolic Ca2+ measurements in rat uterine myocytes

Release of Ca2+ from sarcoplasmic reticulum (SR) is one of the most important mechanisms of smooth muscle stimulation by a variety of physiologically active substances. Agonist-induced Ca2+ release is considered to be dependent on the Ca2+ content of the SR, although the mechanism underlying this dependence is unclear. In the present study, the effect of SR Ca2+ load on the amplitude of [Ca2+]i transients elicited by application of the purinergic agonist ATP was examined in uterine smooth muscle cells isolated from pregnant rats. Measurement of intraluminal Ca2+ level ([Ca2+]L) using a low affinity Ca indicator, mag-fluo-4, revealed that incubation of cells in a high-Ca2+ (10 mM) extracellular solution leads to a substantial increase in [Ca2+]L (SR overload). However, despite increased SR Ca2+ content this did not potentiate ATP-induced [Ca2+]i transients. Repetitive applications of ATP in the absence of extracellular Ca2+, as well as prolonged incubation in Ca2+-free solution without agonist, depleted the [Ca2+]L (SR overload). In contrast to overload, partial depletion of the SR substantially reduced the amplitude of Ca2+ release. ATP-induced [Ca2+]i transients were completely abolished when SR Ca2+ content was decreased below 80% of its normal value indicating a steep dependence of the IP3-mediated Ca2+ release on the Ca2+ load of the store. Our results suggest that in uterine smooth muscle cells decrease in the SR Ca2+ load below its normal resting level substantially reduces the IP3-mediated Ca2+ release, while Ca2+ overload of the SR has no impact on such release.

Intracellular calcium and outer hair cell electromotility

The influence of increased intracellular calcium level on outer hair cell (OHC) electromotility was examined by means of transcellular electrical stimulation in a partitioning microchamber. Electromotile activity was measured before and after application of the calcium ionophore ionomycin, which promotes the inflow of extracellular calcium, as well as its release from intracellular calcium stores. The ionomycin solvent, dimethyl sulphoxide (DMSO), by itself elicited a significant decrease in the magnitude of OHC electromotility. The DMSO effect was counteracted by 10 microM ionomycin and was reversed by 50 microM ionomycin. The increase in electromotility is partially mediated by a calmodulin-dependent mechanism, since W7, a calmodulin antagonist, attenuated the 50 microM ionomycin-induced motility increase. Our results suggest that the electromotility magnitude increase in isolated OHCs due to ionomycin is a calcium/calmodulin-dependent phenomenon.

Intracellular and extracellular calcium utilization during hypoxic vasoconstriction of cyclostome aortas

Hypoxic vasoconstriction (HV) is an intrinsic response of mammalian pulmonary and cyclostome aortic vascular smooth muscle. The present study examined the utilization of calcium during HV in dorsal aortas (DA) from sea lamprey and New Zealand hagfish. HV was temporally correlated with increased free cytosolic calcium (Ca2+c) in lamprey DA. Extracellular calcium (Ca2+o) did not contribute significantly to HV in lamprey DA, but it accounted for 38.1 +/- 5.3% of HV in hagfish DA. Treatment of lamprey DA with ionomycin, ryanodine, or caffeine added to thapsigargin-reduced HV, whereas HV was augmented by BAY K 8644. Methoxyverapamil (D600) in zero Ca2+o did not affect HV in lamprey DA, nor did it prevent further constriction when Ca2+o was restored during hypoxia in hagfish DA. Removal of extracellular sodium (Na+o) caused a constriction in both species. Lamprey DA relaxed to prehypoxic tension following return to normoxia in zero Na+o, whereas relaxation was inhibited in hagfish DA. Relaxation following HV was inhibited in lamprey DA when Na+o and Ca2+o were removed. These results show that HV is correlated with [Ca2+]c in lamprey DA and that Na+/Ca2+ exchange is used during HV in hagfish but not lamprey DA. Multiple receptor types appear to mediate stored intracellular calcium release in lamprey DA, and L-type calcium channels do not contribute significantly to constriction in either cyclostome.

Two distinct Ca(2+)-dependent signaling pathways regulate the motor output of cochlear outer hair cells

The outer hair cells (OHCs) of the cochlea have an electromotility mechanism, based on conformational changes of voltage-sensitive "motor" proteins in the lateral plasma membrane. The translocation of electrical charges across the membrane that accompanies electromotility imparts a voltage dependency to the membrane capacitance. We used capacitance measurements to investigate whether electromotility may be influenced by different manipulations known to affect intracellular Ca(2+) or Ca(2+)-dependent protein phosphorylation. Application of acetylcholine (ACh) to the synaptic pole of isolated OHCs evoked a Ca(2+)-activated apamin-sensitive outward K(+) current. It also enhanced electromotility, probably because of a phosphorylation-dependent decrease of the cell's axial stiffness. However, ACh did not change the voltage-dependent capacitance either in conventional whole-cell experiments or under perforated-patch conditions. The effects produced by the Ca(2+) ionophore ionomycin mimicked those produced by ACh. Hyperpolarizing shifts of the voltage dependence of capacitance and electromotility were induced by okadaic acid, a promoter of protein phosphorylation, whereas trifluoperazine and W-7, antagonists of calmodulin, caused opposite depolarizing shifts. Components of the protein phosphorylation cascade-IP(3) receptors and calmodulin-dependent protein kinase type IV-were immunolocalized to the lateral wall of the OHC. Our results suggest that two different Ca(2+)-dependent pathways may control the OHC motor output. The first pathway modulates cytoskeletal stiffness and can be activated by ACh. The second pathway shifts the voltage sensitivity of the OHC electromotile mechanism and may be activated by the release of Ca(2+) from intracellular stores located in the proximity of the lateral plasma membrane.

A new microsystem for automated electrorotation measurements using laser tweezers

We have developed a new microsystem for fast, automated studies of reactions and kinetics of single cells with biochemical or pharmacological agents. A cell spins in an external rotating electric field and the frequency dependence characterises the passive dielectric properties of membrane and cytoplasm. We use a planar microelectrode chip with microchannel (easily covered with a removable slip) for the application of frequencies exceeding 250 MHz to determine cytoplasmic properties in low and high conductivity electrolyte solutions. The laser tweezers serve as a bearing system, rotation is induced by microelectrodes and rotation speed is recorded automatically. This opens up new possibilities in biotechnology, e.g. for drug screening as demonstrated by measuring the influence of ionomycin on the passive dielectric properties of T-lymphoma cells. Additionally, a possible infrared-induced long-term cell damage could be observed by electrorotation and is discussed.

Interactions between Ca2+ and cAMP in ecdysteroid secretion from the prothoracic glands of Bombyx mori

The interaction between Ca2+ and cAMP in the mediation of ecdysteroid secretion from prothoracic glands (PGs) of Bombyx mori was investigated in vitro. Omission of Ca2+ from the PGs' incubation medium decreased basal ecdysteroid secretion from day 3 until day 6. On day 6, the ability of forskolin or 3-isobutyl-1-methylxanthine (IBMX) to stimulate ecdysteroid secretion was affected by the omission of Ca2+ from the medium. The cAMP agonist Sp-adenosine 3',5'-cyclic monophosphothioate (Sp-cAMPS) and the cAMP analogue dibutyryl cyclic AMP (dbcAMP) stimulated ecdysteroid secretion even in the absence of Ca2+ from the medium. The Sp-cAMPS-stimulated ecdysteroid secretion was inhibited by the cAMP antagonist Rp-adenosine 3',5'-cyclic monophosphothioate (Rp-cAMPS) and the L-type Ca2+ channel blocker verapamil. Both the Ca2+ ionophore A23187 and the L-type Ca2+ channel agonist S(-) Bay K 8644 could stimulate ecdysteroid secretion. The A23187-induced ecdysteroid secretion was partially inhibited by Rp-cAMPS. The combined results indicate that Ca2+ and cAMP signaling pathways can cooperatively, as well as independently, stimulate ecdysteroid secretion from the PGs.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Evidence against a major role for Ca2+ in hypoxia-induced gene expression in human hepatoma cells (Hep3B)

1. The human hepatoma cell line Hep3B is a widely used model for studies of hypoxia-related gene expression. Cytosolic free calcium concentration ([Ca2+]i) has been implicated in cellular oxygen-sensing processes. We investigated whether calcium ions have a significant impact on the production of erythropoietin (EPO) and vascular endothelial growth factor (VEGF). 2. We found that the calcium ionophore ionomycin induced a rapid and sustained increase of [Ca2+]i while thapsigargin, an inhibitor of endoplasmic reticulum calcium ATPase, only caused a 20 % elevation of [Ca2+]i within 10 min after application. However, the calcium content of intracellular stores was considerably reduced by thapsigargin after an incubation period of 24 h. 3. Variations in [Ca2+]o did not result in altered EPO or VEGF secretion rates. Ionomycin decreased EPO production while the lowering of VEGF production was not statistically significant. In the presence of extracellular Ca2+ the membrane permeant calcium chelator BAPTA-AM stimulated the production of EPO (P < 0.05) but not of VEGF while EGTA-AM, a closely related agent, affected neither EPO nor VEGF formation under these conditions. Incubation with thapsigargin resulted in decreased EPO synthesis (P < 0.05) but stimulated VEGF secretion (P < 0.05). 4. In the absence of extracellular calcium, EGTA-AM led to an accumulation of hypoxia-inducible factor-1alpha (HIF-1alpha). This treatment significantly stimulated VEGF synthesis but also decreased EPO secretion (P < 0.05). 5. Our data suggest that the calcium transient and the cytosolic Ca2+ concentration do not play a key role in hypoxia-induced EPO and VEGF production in Hep3B cells.

In situ characterization of the Ca2+ sensitivity of large conductance Ca2+-activated K+ channels: implications for their use as near-membrane Ca2+ indicators in smooth muscle cells

The Ca2+ sensitivity of large conductance Ca2+- and voltage-activated K+ channels (BKV,Ca) has been determined in situ in freshly isolated myocytes from the guinea pig urinary bladder. In this study, in situ denotes that BKV,Ca channel activity was recorded without removing the channels from the cell. By combining patch clamp recording in the cell-attached configuration and microfluorometry of fura-2, we were able to correlate BKV,Ca channel activity with changes in cytoplasmic intracellular [Ca2+] ([Ca2+]i). The latter were induced by ionomycin, an electroneutral Ca2+ ionophore. At 0 mV, the Hill coefficient (nH) and the [Ca2+]i to attain half of the maximal BKV,Ca channel activity (Ca50) were 8 and 1 microM, respectively. The data suggest that this large Hill number was not a consequence of the difference between the near-membrane [Ca2+] ([Ca2+]s) and the bulk [Ca2+]i, indicated by fura-2. High Hill numbers in the activation by Ca2+ of BKV,Ca channels have been seen by different groups (e.g., filled squares in Fig. 4 of Silberberg, S. D., A. Lagrutta, J. P. Adelman, and K. L. Magleby. 1996. Biophys. J. 70:2640-2651). However, such high nH has always been considered a peculiarity rather than the rule. This work shows that a high Ca2+ cooperativity is the normal situation for BKV,Ca channels in myocytes from guinea pig urinary bladder. Furthermore, the Ca50 did not display any significant variation among different channels or cells. It was also evident that BKV,Ca channel activity could decrease in elevated [Ca2+]i, either partially or completely. This work implies that the complete activation of BKV,Ca channels occurs with a smaller increment in [Ca2+]s than previously expected from in vitro characterization of the Ca2+ sensitivity of these channels. Additionally, it appears that the activity of BKV,Ca channels in situ does not strictly follow changes in near-membrane [Ca2+].

Characterization of sodium-calcium exchange in rabbit renal arterioles

Experiments were performed to test the hypothesis that renal arterioles exhibit Na-Ca exchange capability and that this process is regulated by protein kinase C (PKC). Glomeruli with attached arterioles were dissected from rabbit kidney and loaded with fura-2 for measurement of intracellular calcium concentration ([Ca2+]i) using microscope-based photometry. In tissue bathed in Ringer's solution containing 150 mM Na+ and 1.5 mM Ca2+, afferent and efferent arteriolar [Ca2+]i averaged 136 +/- 6 and 154 +/- 7 nM, respectively. Removal of extracellular Na+ increased afferent arteriolar [Ca2+]i by 70 +/- 7 mM, while efferent arteriolar [Ca2+]i only increased by 39 +/- 5 nM (P < 0.01 vs. afferent arteriole). These responses were inhibited by 6 nM Ni2+ and required extracellular Ca2+, but were unaffected by 10 microM diltiazem. After incubation in 500 microM ouabain, 5 microM monensin, and 5 microM nigericin, [Ca2+]i responses to removal of extracellular Na+ were exaggerated significantly, averaging 174 +/- 50 nM in afferent arterioles and 222 +/- 82 nM in efferent arterioles (NS vs. afferent arterioles). Moreover, responses to removal of extracellular Na+ were enhanced by 100 nM phorbol 12-myristate 13-acetate, an affect which was blocked by PKC inhibition (25 nM K252b). These data indicate that both afferent and efferent arterioles express the Na-Ca exchanger, and that PKC activity impacts on exchange capacity in these vessels.

Intracellular acidification associated with changes in free cytosolic calcium. Evidence for Ca2+/H+ exchange via a plasma membrane Ca(2+)-ATPase in vascular smooth muscle cells

The purpose of this study was to define the mechanism whereby agonists that increase free cytosolic calcium (Cai2+) affect intracellular pH (pHi) in smooth muscle. Rat aortic vascular smooth muscle cells grown on coverslips were loaded with BCECF/AM or fura-2/AM for continuous monitoring of pHi or Cai2+, respectively, in a HCO3-/CO2- containing medium. Recovery from rapid increases in Cai2+ produced by 1 microM angiotensin (Ang) II (delta Cai2+ -229 +/- 43 nM) or 1 microM ionomycin (delta Cai2+ -148 +/- 19 nM) was accompanied by a fall in pHi (delta pHi, -0.064 +/- 0.0085 P < 0.01, and -0.05 +/- 0.012 pH units, P < 0.01, respectively). Neither the fall in pHi nor the rise in Cai2+ elicited by Ang II was prevented by pretreatment with agents which block the action of this agonist on pHi via the stimulation of the Cl/HCo3 exchangers (DIDS, 50 microM) or the Na+/H+ antiporter (EIPA, 50 microM). In the presence of DIDS and EIPA, Ang II produced a fall in pHi (delta pHi, -0.050 +/- 0.014, P < 0.01) and a rise in Cai2+ (delta Ca2+ 252 +/- 157 nM, P < 0.01). That the change in pHi was secondary to changes in Cai2+ was inferred from the finding that, when the rise in Cai2+ elicited by Ang II was prevented by preincubation with a Ca2+ buffer, BAPTA (60 microM), the fall in pHi was abolished as well (delta pHi, 0.0014 +/- 0.0046). The pHi fall produced by Ang II and ionomycin was prevented by cadmium at a very low concentration (20 nM) which is known to inhibit plasma membrane Ca(2+)-ATPase activity (delta pHi -0.002 +/- 0.0006 and -0.0016 pH units, respectively). Cadmium also blunted Cai2+ recovery after Ang II and ionomycin. These findings suggest that the fall in pHi produced by these agents is due to H+ entry coupled to Ca2+ extrusion via the plasma membrane Ca(2+)-ATPase. Our results indicate that agonists that increase Cai2+ cause intracellular acidification as a result of Ca2+/H+ exchange across the plasma membrane. This process appears to be mediated by a plasma membrane Ca(2+)-ATPase which, in the process of extruding Ca2+ from the cell, brings in [H+] and thus acidifies the cell.

The effects of N-ras oncogene expression on PDGF-BB stimulated responses in cultured mouse myoblasts

The role of the ras oncogene in the signalling pathway triggered by platelet-derived growth factor BB (PDGF-BB) has been investigated in a cell line which normally differentiates into myotubes. Following the activation of the N-ras oncogene, however, the cells proliferate and form foci. PDGF-BB stimulated the phosphorylation of tyrosine in several cellular proteins of molecular weight 185, 160, 94, 54, 44, 42 kDa and furthermore Ca2+ was released from internal stores. Activation of the N-ras gene by treatment of cells with dexamethasone (DEX) inhibited these responses to PDGF-BB. On the other hand, both ras-induced and -non induced cells responded to bradykinin (BK), foetal calf serum (FCS) and ionomycin (ION) by releasing Ca2+ from intracellular stores. The inhibition of the response to PDGF-BB in ras-activated cells has been further investigated. The binding of [125I]-PDGF-BB to its receptors was low and western blotting showed a low level of PDGF-BB receptor protein. This was in marked contrast to the receptor number seen in cells grown in growth medium or fusion promoting medium. These results indicate that cells transformed with the N-ras oncogene fail to respond to platelet-derived growth factor and exhibit a very low level of PDGF receptors. This suggests a role for the ras oncogene in the earliest steps of the signalling pathway.

Differential effects of maitotoxin on calcium entry and ciliary beating in the rabbit ciliated tracheal epithelium

The marine toxin maitotoxin (MTX) induces stimulation of ciliary beating in primary cultures of rabbit tracheal epithelial cells. The response is time- and concentration-dependent. External calcium is an absolute requirement, although at a very low concentration (50 microM for maximal effect). Pretreatment of the cells with MTX induces an early (5 min) and sustained ( > or = 24 h) homologous desensitization. The response to MTX is strongly inhibited by trifluoperazin (an inhibitor of Ca-calmodulin-dependent enzymes) and by chelation of [Ca]i with BAPTA. However, the magnitude and kinetics of [Ca]i rise elicited by MTX do not correlate with those of the ciliary beat frequency (CBF) increase: the CBF increase is transient (with a peak at 5-10 min) while the [Ca]i rise is sustained; the CBF increase occurs at concentrations of MTX which are without an effect on [Ca]i; the CBF increase is not inhibited by 200 microM verapamil, genistein or okadaic acid, which inhibit the MTX-induced [Ca]i rise. The CBF increase is strongly inhibited by antagonists of arachidonic acid metabolism, mepacrine and nordiguaiaretic acid. However, MTX does not stimulate cAMP synthesis. These results suggest that calcium is not the only factor involved in the biological effects of MTX and even suggest that MTX may primarily stimulate phospholipid breakdown in the cell membrane.

Intra- and extracellular calcium modulates stereocilia stiffness on chick cochlear hair cells

Segments of the chick basilar papilla were isolated and maintained in culture medium. The sensory hair bundle of individual hair cells was observed with light microscopy and stimulated with a water microjet at 600 Hz. Hair bundle motion was slowed by illuminating the microscope with stroboscopic light, and water jet intensity was systematically varied in decibel (dB) steps until a visual detection level (VDL) threshold of hair bundle motion was achieved. The VDL threshold of many hair cells was measured in each isolated papilla. However, only one of eight extracellular calcium concentrations (0.0, 0.0001, 0.001, 0.01, 0.1, 1.25, 6.0, and 12.0 mM) was used with each papilla. In a second series, a calcium ionophore (ionomycin) was added to the culture medium, and VDL thresholds were again measured at seven of these extracellular calcium concentrations. With extracellular calcium alone, the stimulus level needed to achieve threshold was reduced by 2.73 dB between 0.1 and 0.01 mM. This change in threshold represented a 1.37-fold decrease in hair bundle stiffness. When ionomycin was added to the culture medium, a progressively greater stimulus intensity was needed to achieve threshold as calcium concentration increased. The 11.7-dB increase in threshold, with the addition of ionomycin, between 0.0001 and 6.0 mM extracellular calcium was equivalent to a 3.85-fold increase in bundle stiffness. These large changes in hair-bundle stiffness, as a function of the extra- or intracellular calcium environment, may play an important role in the micromechanical behavior of the hair cell during sound simulation.

Carbachol-induced potassium release in rat parotid acini: comparison of the roles of cytosolic Ca2+ and protein kinase C

Carbachol (CCh) stimulated K+ release from rat parotid acini. Treatment with the intracellular Ca2+ antagonist 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) or the intracellular Ca2+ chelator 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) strongly suppressed the CCh-induced K+ release. Combined addition of the Ca2+ ionophore ionomycin and the microsomal Ca(2+)-ATPase inhibitor thapsigargin caused a rapid increase in cytosolic Ca2+ concentration ([Ca2+]i) and resulted in a marked release of K+. In the absence of extracellular Ca2+, CCh or a combination of ionomycin and thapsigargin caused a transient release of K+ which correlated well with the transient change in [Ca2+]i. On the other hand, phorbol 12-myristate 13-acetate (PMA) did not potentiate the CCh-induced K+ release, although the CCh-induced amylase release was significantly enhanced in the presence of PMA. Staurosporine, a protein kinase C-inhibitor, did not inhibit the CCh-induced K+ release, which was in contrast with its inhibitory effect on amylase release. These results suggest that the K+ release from rat parotid acini induced by CCh stimulation is mediated by a rapid increase in [Ca2+]i but is not associated with activation of protein kinase C. This signal pathway is different from that for amylase release where activation of protein kinase C plays an important role.

Characterization of Ca2+ signals generated by extracellular nucleotides in supporting cells of the organ of Corti

ATP has been demonstrated to act as a co-transmitter or neuromodulator in various physiological processes. There is recent evidence that ATP receptors, characterized as P2 purinergic receptors, are expressed in the sensory hair cells of the auditory organ. The aim of the present study was to know whether other cell types of the organ of Corti, the supporting cells, were also sensitive to external ATP. In both types of supporting cells considered in this study, Deiters' cells (DCs) and Hensen's cells (HEs), extracellular ATP at sub-micromolar concentrations evoked a transient increase in [Ca2+]i as monitored with fluorescence microscopy using the calcium probe Indo-1. An apparent Kd of 0.5 and 0.9 microM was determined for DCs and HEs, respectively. Furthermore, we demonstrated that ATP stimulated Ca2+ release from internal stores in DCs, but not in HEs. Dynamic calcium imaging by confocal laser scanning microscopy of ATP induced Ca2+ mobilization demonstrated a calcium wave propagation in the cell body of DCs which originated in the phalangeal processes, suggesting a functional organization of Ca2+ sequestering stores in DCs.

Ca2+ influx via Na(+)-Ca2+ exchange in immortalized aortic myocytes. II. Feedback inhibition by [Ca2+]i

Depolarization with 50 mM K+ evoked a spike in cytosolic free Ca2+ ([Ca2+]i) and increased 45Ca2+ uptake in immortalized aortic myocytes. The following evidence indicates that the electrogenic Na(+)-Ca2+ exchanger caused the Ca2+ influx that was evoked by K+ depolarization. First, K+ depolarization had no effect on [Ca2+]i and 45Ca2+ uptake in cells with basal Na+ but strikingly increased both in Na(+)-loaded cells. Second, the [Ca2+]i increases produced by K+ depolarization depended hyperbolically on external Ca2+ (50% maximum concentration = 1.5 mM). Third, the increases in [Ca2+]i and 45Ca2+ uptake were greater when external Na+ was replaced with K+ rather than with N-methyl-D-glucamine or choline. A series of K+ depolarizations elicited a sequence of [Ca2+]i spikes, provided there was a short incubation at 5 mM K+ between the depolarizations. A prior K+ depolarization almost abolished the 45Ca2+ uptake response to K+ depolarization. The inhibition of exchange activity by a prior K+ depolarization required external Ca2+ and was completely reversible. A prior incubation with angiotensin II, platelet-derived growth factor, or ionomycin also inhibited exchange activity. Moderate [Ca2+]i increases probably feedback inhibit Ca2+ influx via the exchanger by a kinetic mechanism. Inactivation of the exchanger, together with Ca2+ extrusion or sequestration, causes the rapid decrease in [Ca2+]i from the peak evoked by depolarization.

Role of sarcolemmal membrane sodium-calcium exchange in vascular smooth muscle tension

A body of information obtained by experiments with intact tissues, isolated cells, and sarcolemmal vesicles indicates, beyond a reasonable doubt, that a specific Na(+)-Ca2+ exchange system exists in vascular smooth muscle. However, its role in the regulation of cytosolic free-Ca2+ concentration and cell tension under physiological conditions remains unclear. Under pharmacological conditions in which the Na(+)-K+ pump is inhibited either by digitalis glycosides or K(+)-free medium, Na(+)-Ca2+ exchange may be modulated by increases in cytosolic free Na+ to increase the cytosolic free-Ca2+ concentration and cell tension. Under pathological conditions in which the cytosolic Na+ concentration is increased as a result of inhibition of the Na(+)-K+ pump by endogenous ouabain or a digitalis-like factor, or activation of the Na(+)-H+ exchange or passive permeability of Na+, the Na(+)-Ca2+ exchange activity of vascular smooth muscle and the nerve terminal may play an important role in the development and/or maintenance of hypertension. These and other premises remain to be confirmed or discounted.

A comparison of calcium homeostasis in isolated and attached growth cones of the snail Helisoma

This study examines the capability of growth cones from identified neurons of the snail Helisoma trivolvis to perform calcium homeostasis. Calcium influx into the cytoplasm was eliminated or increased experimentally to alter [Ca]i, and the compensatory response of the growth cone was measured with the fluorescent calcium indicator Fura-2. Growth cones compensated for both increases and decreases in calcium influx by restoring [Ca]i towards basal levels under both types of challenges. The intrinsic ability of growth cones to control [Ca]i was examined in physically isolated growth cones. Isolated growth cones demonstrated essentially identical calcium homeostatic properties to their intact counterparts, indicating that mechanisms governing calcium homeostasis exist intrinsically in the growth cone. Such independence may add significantly to the growth cone's potential to locally interpret and respond to stimuli encountered en route to its appropriate target.

Inhibition of sodium-calcium and sodium-proton exchangers by amiloride congeners in arterial muscle cells

The inhibitory potencies of several amiloride congeners towards Na(+)-Ca2+ and Na(+)-H+ exchange were compared in rat aortic myocytes. N-(2,4-Dimethylbenzyl)amiloride (DMB) was 10 times more potent towards Na(+)-Ca2+ than Na(+)-H+ exchange. Amiloride and ethylisopropylamiloride were about 5,000 and 10,000 times more potent toward Na(+)-H+ than Na(+)-Ca2+ exchange respectively. N-(3,4-Dichlorobenzyl)amiloride was almost equipotent towards both exchangers. About 40 nM ethylisopropylamiloride inhibited Na(+)-H+ exchange by 50%. Ethylisopropylamiloride (10 microM) had no effect on basal or angiotensin-evoked 45Ca2+ efflux or net Ca2+ efflux. In contrast to ethylisopropylamiloride, 25-50 microM DMB, which strongly inhibits Na(+)-Ca2+ exchange, markedly decreased both 45Ca2+ efflux and net Ca2+ efflux produced by angiotensin. Replacing extracellular Na+ with N-methyl-D-glucamine inhibited angiotensin-evoked 45Ca2+ efflux similarly to DMB. Neither DMB nor Na+ placement had any effect on basal or angiotensin-evoked production of [3H]inositol phosphates. These findings suggest that Na(+)-H+ exchange has no major influence on short-term Ca2+ regulation and provide evidence that Na(+)-Ca2+ exchange is a major pathway of rapid Ca2+ efflux in stimulated arterial muscle cells.

Energy dependence of sodium-calcium exchange in vascular smooth muscle cells

Three different types of mitochondrial poisons (oligomycin, antimycin A, and dinitrophenol) strongly inhibited Na(+)-Ca2+ exchange in aortic myocytes. Exchange activity was assayed as 45Ca2+ uptake that depended on inverting the Na+ gradient and was inhibited by 25 microM dimethylbenzamil. Glucose markedly decreased the inhibition of exchange activity by these three poisons. Glucose also prevented rotenone from inhibiting exchange and depleting cellular ATP. In the absence of glucose, rotenone decreased ATP and exchange activity with half-times of 0.8 and 0.9 min, respectively. Almost eliminating cellular ATP with rotenone maximally inhibited exchange by 80%. Repletion of ATP with glucose substantially restored Na(+)-Ca2+ exchange activity. Ca2+ uptake by organelles, subsequent to entry via exchange for Na+, does not appear to contribute significantly to exchange activity as assayed in intact myocytes. The specific activity of Na(+)-Ca2+ exchange was approximately 30 nmol.min-1.mg protein-1. These findings suggest that ATP modulates exchange activity and that there are approximately 150,000 Na(+)-Ca2+ exchangers per cell, assuming that the turnover number is 1,000 s-1.