Decreasing Extracellular Na+ Concentration Triggers Inositol Polyphosphate Production and Ca2+ Mobilization

Regulation and identity of intracellular calcium stores involved in membrane cross talk in the early distal tubule of the frog kidney

The early distal tubule (EDT) of the frog nephron, similar to the thick ascending limb in mammals, mediates the transepithelial absorption of NaCl. The continued absorption of NaCl in the face of varying Na(+) load is maintained by coordination of the activity of ion-transporting proteins in the apical and basolateral membranes, so-called pump-leak coupling. Previous studies identified intracellular Ca(2+), originating from an intracellular Ca(2+) store, as playing a key role in pump-leak coupling in the EDT (Cooper GJ, Fowler MR, and Hunter M. Pflügers Arch 442: 243-247, 2001). The purpose of the experiments described in this paper was to identify the intracellular Ca(2+) storage pools in the renal diluting segment. Store Ca(2+) movements were monitored by the fluorescence of mag-fura 2 in permeabilized segments of frog EDTs. The presence of both ATP and Ca(2+) was required to maintain store Ca(2+) content. Removal of either of these substrates resulted in a passive leak of Ca(2+) from the stores. The uptake of Ca(2+) into the store was sensitive to the SERCA inhibitor 2,5-di(tert-butyl) hydroquinone, whereas Ca(2+) release from the store was stimulated by IP(3) but not cADPR. Store Ca(2+) was insensitive to the mitochondrial ATP synthase inhibitor oligomycin, and, under conditions that energized Deltapsi(m), the complex 1 inhibitor rotenone and the protonophore FCCP. Ionomycin was able to mobilize store Ca(2+) following exposure to IP(3). These results suggest that the endoplasmic reticulum is a dominant Ca(2+) store in the frog EDT. A second pool, sensitive to ionomycin but not IP(3), may overlap with the IP(3)-sensitve pool. The data also rule out any contribution by mitochondria to EDT Ca(2+) cycling.

Effect of hypotonic shock on cultured pavement cells from freshwater or seawater rainbow trout gills

The effect of hypotonic shock on cultured pavement gill cells from freshwater (FW)- and seawater (SW)-adapted trout was investigated. Exposure to 2/3rd strength Ringer solution produced an increase in cell volume followed by a slow regulatory volume decrease (RVD). The hypotonic challenge also induced a biphasic increase in cytosolic Ca(2+) with an initial peak followed by a sustained plateau. Absence of external Ca(2+) did not modify cell volume under isotonic conditions, but inhibited RVD after hypotonic shock. [Ca(2+)](i) response to hypotonicity was also partially inhibited in Ca-free bathing solutions. Similar results were obtained whether using cultured gill cells prepared from FW or SW fishes. When comparing freshly isolated cells with cultured gill cells, a similar Ca(2+) signalling response to hypotonic shock was observed regardless of the presence or absence of Ca(2+) in the solution. In conclusion, gill pavement cells in primary culture are able to regulate cell volume after a cell swelling and express a RVD response associated with an intracellular calcium increase. A similar response to a hypotonic shock was recorded for cultured gill cells collected from FW and SW trout. Finally, we showed that calcium responses were physiologically relevant as comparable results were observed with freshly isolated cells exposed to hypoosmotic shock.

A zinc-sensing receptor triggers the release of intracellular Ca2+ and regulates ion transport

Changes in extracellular zinc concentration participate in modulating fundamental cellular processes such as proliferation, secretion, and ion transport in a mechanism that is not well understood. Here, we show that a micromolar concentration of extracellular zinc triggers a massive release of calcium from thapsigargin-sensitive intracellular pools in the colonocytic cell line HT29. Calcium release was blocked by a phospholipase-C inhibitor, indicating that formation of inositol 1,4,5-triphosphate is required for zinc-dependent calcium release. Zinc influx was not observed, indicating that extracellular zinc triggered the release. The Ca(i)2+ release was zinc specific and could not be triggered by other heavy metals. Furthermore, zinc failed to activate the Ca(2+)-sensing receptor heterologously expressed in HEK293 cells. The zinc-induced Ca(i)2+ rise stimulated the activity of the Na(+)/H(+) exchanger in HT29 cells. Our results indicate that a previously uncharacterized extracellular, G protein-coupled, Zn(2+)-sensing receptor is functional in colonocytes. Because Ca(i)2+ rise is known to regulate key cellular and signal-transduction processes, the zinc-sensing receptor may provide the missing link between extracellular zinc concentration changes and the regulation of cellular processes.

Cell surface, Ca2+(cation)-sensing receptor(s): one or many?

In mammals Ca2+ concentration in the extracellular fluids ([Ca2+]o) is essential for a number of vital processes varying from bone mineralization to blood coagulation, regulation of enzymatic processes, modulation of permeability and excitability of plasma membranes. For this reason [Ca2+]o is under strict control of a complex homeostatic system that includes parathyroid glands, kidneys, bones and intestine. The extracellular Ca(2+)-sensing receptor (CaR) is an essential component of this system, regulating parathyroid hormone secretion, calcium (and magnesium) excretion by the kidney, bone remodeling and Ca2+ reabsorption by the gastrointestinal tract. Structurally, the CaR is a novel member of a growing G protein-coupled receptor superfamily, which includes metabotropic glutamate receptors (mGluRs) [1], [gamma]-aminoisobutyric acid (GABA-B) receptors [2] and vomeronasal organ receptors [3]. Initially identified from bovine parathyroid glands [4], within the 5 years following its identification CaR presence has rapidly been identified as extending to organs where the link with mineral ion metabolism has not been elucidated (i.e. brain, stomach, eye, skin and many other epithelial cells) (see [5] for review). The role of the receptor in these regions is largely unknown, but it appears to be somewhat related to phenomena such as chemotaxis, cell proliferation and programmed cell death. This review will describe the discovery of a novel class of ion-sensing receptor(s), receptor-effector coupling and the roles of the CaR inside and outside the Ca2+o homeostatic system.

Protein kinase C-mediated up-regulation of Na+/Ca2+-exchanger in rat hepatocytes determined by a new Na+/Ca2+-exchanger inhibitor, KB-R7943

The regulatory mechanism of the plasma membrane Na+/Ca2+-exchanger in isolated rat hepatocytes was studied using microspectrofluorometry and 45Ca2+ uptake methods. Exposure of single hepatocytes to low-Na+ solutions induced an increase in the intracellular Ca2+ concentration ([Ca2+]i) which depended on the presence of extracellular Ca2+. 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate (KB-R7943), a novel selective inhibitor of Na+/Ca2+-exchangers, inhibited the initial rate of [Ca2+]i increase induced by exposure to the low-Na+ solution (IC50 = 2 microM). KB-R7943 also reduced the initial rate of 45Ca2+ uptake (IC50 = 4 microM). The increase in [Ca2+]i induced by exposure to the low-Na+ solution was inhibited by pre-incubation with 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7, 50 microM), but not with N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide (H-8, 60 microM) or a tyrosine kinase inhibitor, genistein (100 microM). Furthermore, taurocholate and phorbol-12,13-dibutyrate, both of which activate protein kinase C, promoted the increase in [Ca2+]i. These [Ca2+]i increases were sensitive to KB-R7943. Our results indicate that the Na+/Ca2+-exchanger is up-regulated via protein kinase C. The activity of Na+/Ca2+-exchangers is not evident under normal physiological conditions, suggesting that the exchanger may be activated under pathophysiological conditions.

Cadmium perturbs calcium homeostasis in rat osteosarcoma (ROS 17/2.8) cells; a possible role for protein kinase C

The mechanism of the toxic effects of Cd2+ on bone cell function is not completely understood at this time. This study was designed to characterize the effect of Cd2+ on Ca2+ metabolism in ROS 17/2.8 cells. Cells were labeled with (45)Ca (1.87 mM Ca) for 20 h in the presence of 0.01, 0.1, or 1.0 microM Cd2+ and kinetic parameters were determined from (45)Ca efflux curves. Three kinetic compartments described the intracellular metabolism of (45)Ca. Cd2+ (0.01 microM) caused an approximate 9 x increase in Ca2+ flux across the plasma membrane and a decrease in the most rapidly exchanging intracellular Ca2+ compartment (S1). However, there was no change in total cell Ca2+, indicating an increased cycling of Ca2+ across the plasma membrane. Flux between S1 and the intermediate Ca2+ compartment (S2) was also increased and S2 increased significantly. All Cd2+ induced changes in Ca2+ homeostasis were obliterated by concurrent treatment with 0.1 microM calphostin C (CC), a potent protein kinase C (PKC) inhibitor. This data suggests that Cd2+ perturbs Ca2+ metabolism via a PKC dependent process.

Altered sodium-calcium exchange in afferent arterioles of the spontaneously hypertensive rat

Studies were performed to determine if there is a derangement in Na-Ca exchange activity in afferent (AA) and efferent (EA) arterioles from 3- and 9-week-old spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats. Cytosolic calcium concentration ([Ca2+]i) was assessed using microscope-based photometry in fura-2 loaded arterioles bathed in a Ringer's solution. Baseline [Ca2+]i was similar in the AA of 3- and 9-week-old WKY and SHR. In AA from 3-week-old rats, [Ca2+]i increased by 89 +/- 15 nM in WKY and by 73 +/- 13 nM in SHR during decreases in bath sodium concentration ([Na+]e) from 150 to 2 mM (Na+ replaced with n-methyl-D-glucamine). In 9-week-old hypertensive SHR (SBP = 150 mm Hg), increases in [Ca2+]i were attenuated (24 +/- 3 nM) relative to 3-week-old WKY and SHR, and 9-week-old WKY (90 +/- 9 nM; P < 0.05). Likewise, the rate of removal of Ca2+ in the continued presence of 2 mM Nae (Ca2+ sequestration and/or extrusion) was markedly reduced in AA of 9-week-old SHR (-0.15 +/- 0.03 nM/second) versus 3-week-old SHR (-0.72 +/- 0.12 nM/second) and 3- and 9-week-old WKY (-0.49 +/- 0.10 and -0.67 +/- 0.14 nM/second). In other experiments, AAs were preincubated in 1 mM ouabain to increase intracellular [Na+]. This maneuver augmented the increase in [Ca2+]i obtained with removal of Na+e; however, the responses obtained in 9-week-old SHR arterioles were still attenuated compared to those obtained in arterioles for 3- and 9-week-old WKY and 3-week-old SHR. These results suggest that exchanger number and/or sensitivity to the transmembrane Na gradient was reduced in the SHR AA. In EA, baseline [Ca2+]i was similar in 3- and 9-week-old WKY and SHR. In contrast to AA, the magnitude of Na-dependent and Na-independent changes in [Ca2+]i was not different in the EA of 3- and 9-week-old WKY and SHR. These results indicate that regulation of Na-Ca exchange activity may differ between AA and EA segments. Furthermore, diminished Na-Ca exchange and Na-independent Ca2+ sequestering/extrusion mechanisms could contribute to altered AA [Ca2+]i in the SHR.

Na/Ca exchange in the basolateral membrane of the A6 cell monolayer: role in Cai homeostasis

The presence of a Na/Ca exchanger in A6 cells was investigated by measuring intracellular calcium (Cai) fluctuations and the 45Ca fluxes through the basolateral membranes (blm) of the cell monolayer. Removal of Na+ from the medium produced a transient increase in Cai followed by a regulatory phase returning Cai to control levels in 3-4 min, this phase being greatly accelerated (< 60 s) by NaCl addition (apparent Km of approximately 5 mM Na+). The Cai increase was only found with the Na(+)-free medium on the basolateral side of the cell monolayer. A twofold increase in the 45Ca influx was observed under these conditions. In Ca(2+)- depleted cells, the initial Cai increase after Ca2+ addition to the medium was greater when the putative Na/Ca exchanger was not functioning (i.e. in a Na(+)-free medium). 45Ca effluxes through the blm of the monolayer were greatly and transiently increased by a Na(+)-free medium on the serosal side and blocked by orthovanadate (1 mM). The Cai increased induced by a hypo-osmotic shock was greater in cells bathed in a Na(+)-medium, conditions expected to block the activity of the Na/Ca exchanger. These findings support the hypothesis that a Na/Ca exchanger is present on the blm of A6 cells and affirm its role in Cai homeostasis in steady-state conditions and following osmotic shock. In addition, a Ca2+ pump also located on the blm and Ca2+ stores sensitive to inositol 1,4,5-trisphosphate were found to be implicated in Cai homeostasis.

Muscarinic stimulation of gallbladder epithelium. II. Fluid transport, cell volume, and ion permeabilities

Activation of muscarinic receptors in the fluid-absorptive epithelium of the Necturus gallbladder elevates cytosolic Ca2+ concentration, transiently hyperpolarizes the cell membrane voltages, and decreases the apparent fractional resistance of the apical membrane [G. A. Altenberg, M. Subramanyam, J. S. Bergmann, K. M. Johnson, and L. Reuss. Am. J. Physiol. 265 (Cell Physiol. 34): C1604-C1612, 1993]. In these studies, we show that at the peak of the hyperpolarization both apical and basolateral membrane resistances (Ra and Rb, respectively) decreased, but in 2-3 min Ra returned to control values while Rb rose to a level approximately 60% higher than control. The acetylcholine (ACh)-induced decrease in Ra is caused by activation of apical membrane maxi K+ channels secondary to elevation of cytosolic Ca2+ concentration. The increase in Rb is due to decreases in K+ and Cl- conductances. ACh had no effects on cell KCl content or water volume, although K+ conductance transiently increased. These results can be explained by the changes in basolateral membrane conductances. ACh did not alter fluid absorption. In conclusion, ACh has complex time-dependent effects on K+ and Cl- electrodiffusive permeabilities without measurable changes in cell volume or in the rate of transepithelial fluid transport.

Receptor-operated Ca2+ signaling and crosstalk in stimulus secretion coupling

In the cells of higher eukaryotic organisms, there are several messenger pathways of intracellular signal transduction, such as the inositol 1,4,5-trisphosphate/Ca2+ signal, voltage-dependent and -independent Ca2+ channels, adenylate cyclase/cyclic adenosine 3',5'-monophosphate, guanylate cyclase/cyclic guanosine 3',5'-monophosphate, diacylglycerol/protein kinase C, and growth factors/tyrosine kinase/tyrosine phosphatase. These pathways are present in different cell types and impinge on each other for the modulation of the cell function. Ca2+ is one of the most ubiquitous intracellular messengers mediating transcellular communication in a wide variety of cell types. Over the last decades it has become clear that the activation of many types of cells is accompanied by an increase in cytosolic free Ca2+ concentration ([Ca2+]i) that is thought to play an important part in the sequence of events occurring during cell activation. The Ca2+ signal can be divided into two categories: receptor- and voltage-operated Ca2+ signal. This review describes and integrates some recent views of receptor-operated Ca2+ signaling and crosstalk in the context of stimulus-secretion coupling.

Ca2+ oscillations in the rabbit renal cortical collecting system induced by Na+ free solutions

The presence of a Na(+)-Ca2+ exchange system has been previously demonstrated at the basolateral side of the cortical collecting system. The role of such an exchanger in maintaining low intracellular [Ca2+] ([Ca2+]i) in this nephron segment is now investigated. Cells from the connecting tubule and cortical collecting duct of rabbit kidneys were isolated by immunodissection with mAb R2G9 and subsequently cultured on glass coverslips. [Ca2+]i was measured in single cells using quantitative fluorescence microscopy. Surprisingly, isoosmotic substitution of extracellular Na+ ([Na+]o) for N-methylglucamine generated [Ca2+]i oscillations in individual cells instead of an anticipated sustained increase in [Ca2+]i. The amplitude of these oscillations ranged between 150 to 600 nM (average 308 +/- 19 nM) and occurred at a frequency of 0.63 +/- 0.03 min-1, with a duration of 44 +/- 2 seconds per spike. Oscillations were only observed in response to [Na+]o less than 5 mM and lasted until Na+o was re-introduced. The compound U73122 (10 microM), a new phospholipase C inhibitor, inhibited [Ca2+]i oscillations, which strongly suggests that IP3 generation initiates [Ca2+]i oscillations. [Ca2+]i oscillations were independent of extracellular Ca2+ and could not be inhibited by lanthanum ions, indicative for an intracellular source for the generation of Ca2+ spikes. Addition of thapsigargin, a specific inhibitor of endoplasmic reticulum Ca(2+)-ATPase activity, induced a considerable intracellular Ca2+ release, after which [Ca2+]i oscillations could no longer be provoked. Caffeine (20 mM) reversibly inhibited the Ca2+ oscillations, which implies that Ca(2+)-induced Ca2+ release is involved in generating these oscillations.(ABSTRACT TRUNCATED AT 250 WORDS)

Stable expression of the cardiac sodium-calcium exchanger in CHO cells

A line of Chinese hamster ovary (CHO) cells called CK1.4 was produced by transfection with the gene for the bovine cardiac Na(+)-Ca2+ exchanger. CK1.4 cells stably expressed substantial exchange activity and exchanger protein as shown by immunoprecipitation. Exchange activity was quantified as 45Ca2+ influx that depended on both increasing intracellular Na+ and lowering the concentration of external Na+. Replacing external Na+ with K+ slightly increased 45Ca2+ uptake by CK1.4 cells with basal Na+ and greatly increased 45Ca2+ uptake by Na(+)-loaded cells. Neither exchange activity nor exchanger protein was detected in the nontransfected parental line. By contrast to CK1.4 cells, replacing external Na+ with K+ decreased 45Ca2+ uptake in the nontransfected cells whether or not they were Na+ loaded. Changes in cytosolic free Ca2+ determined with fura-2 were consistent with the 45Ca2+ uptake data. Analysis of poly(A)(+)-RNA by Northern blot confirmed that CK1.4 cells, but not the parental line, expressed the exchanger. Expression of the exchanger was also observed in aortic myocytes and a renal epithelial cell line (LLC-MK2) but not in other lines of renal epithelial cells (MDCK, LLC-PK1) or human dermal fibroblasts. The cardiac exchanger produced substantial 45Ca2+ efflux from CK1.4 cells in response to hormone-evoked release of stored Ca2+. CK1.4 cells are an attractive model for studies of the regulation of the cardiac exchanger because they stably express sufficient exchanger for biochemical and immunological analysis.

Genistein inhibits calcium release by platelet-derived growth factor but not bradykinin or cadmium in human fibroblasts

Cd2+ provokes inositol trisphosphate production and releases stored Ca2+, apparently by binding to a zinc site in the external domain of an orphan receptor. One microM Cd2+ evokes an immediate spike in cytosolic free Ca2+, which is similar to that evoked by bradykinin. Platelet-derived growth factor (PDGF) also increases free Ca2+ in human dermal fibroblasts, but there is a distinct lag before free Ca2+ rises in response to PDGF. Genistein, which selectively inhibits tyrosine kinases, markedly inhibited Ca2+ mobilization evoked by PDGF. Calcium mobilization triggered by cadmium or bradykinin was relatively insensitive to genistein. The PDGF receptor is known to be a tyrosine kinase, which phosphorylates and thereby activates phospholipase C gamma, whereas a G protein couples the bradykinin receptor to another phospholipase C isoform. These findings support the hypothesis that the orphan receptor triggered by cadmium is coupled to phospholipase C via a G protein.

Cytosolic free calcium regulation in response to acute changes in intracellular pH in vascular smooth muscle

This study examined the mechanisms whereby alterations of intracellular pH (pHi) impact on free cytosolic calcium (Cai2+) in cultured rat aortic vascular smooth muscle cells (VSMC) assayed in the presence of HCO3/CO2. Rapid cell alkalinization, effected by the exposure to NH4Cl or removal of CO2 from the superfusate, produced a rapid increase in Cai2+. The rise in Cai2+ was markedly diminished when sarcoplasmic reticulum (SR) Ca2+ stores had been depleted by prior exposure to arginine vasopressin (AVP) in Ca(2+)-free media or when SR release and reuptake of Ca2+ were blocked by the addition of 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester (TMB-8), but was unaffected by the removal of external Ca2+ or inhibition of Ca2+ entry using NiCl2. Cell acidification also resulted in a rapid increase in Cai2+. This Cai2+ increase was most apparent when pHi was very low (< 6.6) and was unaffected by removal of external Ca2+ or NiCl2 addition. Unlike the effect of cell alkalinization, the increase in Cai2+ associated with cell acidification was not prevented by pretreatment with AVP or TMB-8. We conclude that, in cultured VSMC, acute intracellular alkalinization and, to a lesser extent, acidification result in release of Ca2+ from internal stores. Alkalinization increases Cai2+ by promoting its release from a store which is AVP and TMB-8 sensitive, most likely the SR. Cell acidification increases Cai2+ from an intracellular store(s) that is neither AVP nor TMB-8 sensitive. The increase in Cai2+ produced by cell acidification may be explained on the basis of cell buffering such that, as cytosolic H+ increases, it displaces Cai2+ from internal buffers with similar affinities for Ca2+ and H+.

Intracellular calcium, currents, and stimulus-response coupling in endothelial cells

Vascular endothelium appears to be a unique organ. It not only responds to numerous hormonal and chemical signals but also senses changes in physical parameters such as shear stress, producing mediators that modulate the responses of numerous cells, including vascular smooth muscle, platelets, and leukocytes. In many cases, the initial response of endothelial cells to these diverse signals involves elevation of cytosolic Ca2+ and activation of Ca(2+)-dependent enzymes, including nitric oxide synthase and phospholipase A2. Both the release of Ca2+ from intracellular stores, most likely the endoplasmic reticulum, and the influx of Ca2+ from the extracellular space contribute to the [Ca2+]i increase. The most important trigger for Ca2+ release is inositol 1,4,5-trisphosphate, which is generated by the action of phospholipase C, a plasmalemmal enzyme activated in many cases by the receptor-G protein cascade. Ca2+ influx appears to be related to the activity of receptor-G protein-enzyme complex and to the degree of fullness of the endoplasmic reticulum but does not involve voltage-gated Ca2+ channels. The magnitude of the Ca2+ influx depends on the electrochemical gradient, which is modulated by the membrane potential, Vm. Under basal conditions, Vm is dominated by a large inward rectifier K+ current. Some stimuli, e.g., acetylcholine, have been shown to hyperpolarize Vm, thus increasing the electrochemical gradient for Ca2+, which appears to be modulated by activation of Ca(2+)-dependent K+ and Cl- currents. However, the lack of potent and specific blockers for many of the described or postulated channels (e.g., nonselective cation channel, Ca(2+)-activated Cl- channel) makes an estimation of their effect on endothelial cell function rather difficult. Possible future directions of research and clinical implications are discussed.

A putative lectin-binding receptor mediates cadmium-evoked calcium release

Nanomolar concentrations of cadmium (Cd2+) produce an immediate rise in free Ca2+ in human dermal fibroblasts, which is mostly caused by the release of stored Ca2+ via inositol trisphosphate. Here we have used lectins to evaluate the hypothesis that a cell surface glycoprotein mediates the response to Cd2+. A prior incubation with wheat germ agglutinin (WGA) or certain other lectins inhibited calcium release evoked by Cd2+. WGA reversibly inhibited Cd(2+)-evoked calcium release as indicated by measurements of cytosolic free Ca2+ and 45Ca2+ efflux. WGA half-maximally inhibited Ca2+ release at 1.2 x 10(-7) M. The Kd for the binding of fluoresceinylated WGA was 2.8 x 10(-7) M. Chitotriose dissociated fluoresceinylated WGA from the cells and restored cadmium responsiveness. WGA inhibited Cd(2+)-evoked 45Ca2+ efflux similarly at 18 and 37 degrees C. A brief incubation with chitotriose at 18 or 10 degrees C reversed the inhibition by WGA. WGA neither bound 109Cd2+ nor affected 109Cd2+ uptake by the cells. Succinylated WGA, which binds N-acetylglucosamine but not N-acetylneuraminic acid, failed to inhibit Ca2+ release evoked by Cd2+. WGA probably inhibits Ca2+ release produced by Cd2+ by binding to N-acetylneuraminic acid in the external domain of a plasma membrane receptor.

Reversible desensitization of fibroblasts to cadmium receptor stimuli: evidence that growth in high zinc represses a xenobiotic receptor

The xenobiotic Cd2+ triggers the production of inositol trisphosphate and releases stored Ca2+ in certain cell types, apparently by binding to a zinc site in the external domain of an "orphan" receptor (no known endogenous stimulus). Cd2+ and bradykinin evoke similar spikes in cytosolic free Ca2+. Growth in high Zn2+ (100-200 microM) abolished the free Ca2+ spike evoked by Cd2+ without affecting the spike produced by bradykinin. Growth in high Zn2+ almost abolished Cd(2+)-evoked production of [3H]inositol mono-, bis-, and trisphosphate. Bradykinin-evoked [3H]inositol phosphate production was not affected by growth in high Zn2+. Growth in high Zn2+ nearly prevented the stimulation of 45Ca2+ efflux by Cd2+ without affecting the stimulation of 45Ca2+ efflux by bradykinin or histamine. Removing Zn2+ from the culture medium and incubating the cells for several hours fully restored responsiveness to Cd2+. Cycloheximide, actinomycin D, or tunicamycin prevented the restoration of Cd2+ responsiveness, indicating that resensitization requires macromolecular synthesis. Growth in high Zn2+ reversibly abolished Ca2+ mobilization evoked by two additional stimuli: a decrease in extracellular pH or Na+ concentration. These findings support the hypothesis that the three stimuli (Cd2+ or a decrease in external pH or Na+ concentration) activate the same orphan receptor. Growth in high Zn2+ apparently desensitizes the cells to the Cd2+ receptor stimuli by repressing receptor synthesis.

Effect of sodium deprivation on contraction and charge movement in frog skeletal muscle fibres

Measurements of isometric tension were performed in single twitch skeletal muscle fibres and the effect of extracellular Na+ removal on contraction was investigated. Na+ withdrawal brought about an increase in the amplitude of K+ contractures and their time course became faster. The potentiation of K+ contractures depended strongly on extracellular Ca2+ and developed slowly following an exponential time course with a time constant of approximately 8 min. Removal of extracellular Na+ greatly increased the amplitude of caffeine contractures and lowered its threshold: caffeine (0.5 mM) had no effect on resting tension in Ringer's but produced contractures in Na(+)-free solutions. Intramembrane charge movement (charge 1) was monitored in contracting voltage-clamped segments of frog skeletal muscle fibres using the triple-Vaseline-gap technique. Movement of charge 1 did not depend on the presence of extracellular Na+. However, the mechanical threshold decreased by approximately 10 mV at several pulse durations and the charge which produced just detectable contractions decreased by approximately 5 nC microF-1 in the absence of extracellular Na+. Intracellular heparin (40 mg ml-1) increased the mechanical threshold by approximately 20 mV without affecting the movement of charge 1. The effect of Na(+)-free solutions on the mechanical threshold was additive to that of heparin. It is concluded that the effects of Na(+)-withdrawal on contraction take place at a location beyond the voltage sensor of excitation-contraction coupling.

Immunocytochemical study of dystrophin in cultured mouse muscle cells by the quick-freezing and deep-etching method

Dystrophin, the protein product of the Duchenne muscular dystrophy (DMD) gene, is deficient in patients with DMD and in mdx mice. It is immunocytochemically localized in skeletal muscle sarcolemma. However, little is known about the three-dimensional ultrastructural localization of dystrophin and its relationship with other cytoskeletal proteins. We found that dystrophin is localized irregularly, just underneath the plasma membrane in normal cultured mouse myotubes, by using the quick-freezing and deep-etching (QF-DE) method; it was found to be closely linked to actin-like filaments (8-10 nm in diameter), most of which were decorated with myosin subfragment 1, and was attached to the cytoplasmic side of the plasma membrane. These results suggest that dystrophin might play an important role in the preservation of cell membrane stability by connecting actin cytoskeletons with the cytoplasmic side of the plasma membrane.

Presence of Na+/Ca2+ exchange activity and its role in regulation of intracellular calcium concentration in bovine adrenal chromaffin cells

The presence of a Na+/Ca2+ exchanger in bovine adrenal chromaffin cells was demonstrated by measuring the efflux of 45Ca2+ which had been preloaded into cells by a brief depolarization. The efflux of 45Ca2+ was dependent on extracellular Na+ (Na+o); 45Ca2+ efflux was significantly decreased by replacing Na+o with N-methylglucamine (NMG), or Li+. Replacement of Na+o by NMG increased the resting intracellular Ca2+ concentration ([Ca2+]i) of freshly isolated chromaffin cells. This could be reversed by adding Na+, suggesting that Na+/Ca2+ exchanger activity was involved in maintaining [Ca2+]i at its resting level. The initial rate of Na(+)-dependent [Ca2+]i recovery after Ca2+ loading by depolarization was dependent on the level of [Ca2+]i. There was an apparent linear relationship between the activity of the Na+/Ca2+ exchanger and [Ca2+]i both in the presence and absence of Na+o. When cells were treated with other stimuli, including 10 microM DMPP or 40 mM caffeine, the ability of the stimulated cells to decrease [Ca2+]i was significantly reduced upon replacing Na+o with NMG. Our data show that the Na+/Ca2+ exchanger is one of the major pathways for regulating [Ca2+]i in chromaffin cells in both resting and stimulated states.

Calcium signals in growth factor signal transduction

There is a substantial amount of information which has been obtained concerning the effects of growth factors on [Ca2+]i in proliferating cells. A number of different mitogens are known to induce elevations in [Ca2+]i and some characterization of the Ca2+ response to different classes of mitogens has been obtained. In addition, much is known about whether the Ca2+ response to a particular growth factor occurs as the result of an influx of external Ca2+ or a mobilization of internal Ca2+ stores. In addition, a considerable amount of information is available on the mechanism by which the Ins(1,4,5)P3-sensitive internal Ca2+ store takes up and releases Ca2+. However, there is still a large deficiency in our information concerning other Ca2+ stores in proliferating cells as well as in our knowledge of the mechanisms for regulating Ca2+ entry pathways. Much more data addressing these issues exists for other types of agonist-stimulated cells, and we have discussed much of it in this review article. While the wealth of data in nonproliferating cells provides some indications of what mechanisms might be involved in the growth factor-induced changes in [Ca2+]i, it is clear that much work must be done in proliferating cells to fully understand how external factors such as growth factors control [Ca2+]i. In addition, much work remains to be done in identifying the mechanisms for the internal control of [Ca2+]i as cells move through the cell cycle and in identifying the role that these changes in [Ca2+]i may play throughout the cell cycle.

Extracellular Na+ dependency of free cytosolic Ca2+ regulation in aortic vascular smooth muscle cells

This study examined contribution of Na(+)-dependent processes to the regulation of free cytosolic calcium (Ca2+i) in cultured vascular smooth muscle cells (VSMC) using fura-2. Removal of Na+ from superfusate (replacement with choline) resulted in an increment of Ca2+i that was greatly augmented by pretreatment with ouabain. Under both conditions, Ca2+i increase was followed by partial recovery to a new steady state that was still significantly higher than that seen before removal of external Na+ (Na+o). In ouabain-pretreated cells lowering of Na+o caused progressive increases in Ca2+i. Addition of NiCl2, a Na(+)-Ca2+ exchange inhibitor, completely blocked the increase in Ca2+i produced by removal of Na+o, indicating that the Na(+)-Ca2+ antiporter was responsible for observed Ca2+i changes. Ca2+i increase produced by reduction of Na+o was also seen after depletion of inositol trisphosphate-sensitive Ca2+ stores with repeated pulses of angiotensin II or after blockade of sarcoplasmatic reticulum Ca2+ release with TMB-8 but was not observed in the absence of external Ca2+. These observations indicate that the source of Ca2+i increase in response to changes in the transmembrane Na+ gradient is largely external, and potentiation of the Ca2+i surge by ouabain suggests Ca2+ influx via the Na(+)-Ca2+ exchanger operating in the reverse mode. The relative contribution of a Na(+)-dependent and -independent component of Ca2+i recovery was investigated by superfusing cells with ionomycin in a Na(+)-free medium and later adding Na+ to the medium. This Ca2+ ionophore increased Ca2+i to a peak, and this was followed by a rapid but partial recovery to a new steady state. Readdition of varying amounts of Na+ to the superfusate, in the continued presence of ionomycin, resulted in concentration-related decline in Ca2+i, thereby uncovering a substantial contribution of a Na(+)-dependent mechanism of Ca2+i regulation. Decline of Ca2+i produced by readdition of Na+ was blocked by addition of NiCl2 to the superfusate. Our findings thereby provide evidence for Ca2+i regulation in VSMC via a Na(+)-dependent mechanism, consistent with a Na(+)-Ca2+ exchanger, which acts as a Ca2+ efflux mechanism when Ca2+i is elevated. Na(+)-Ca2+ exchanger acts as a Ca2+ influx mechanism when intracellular Na+ is elevated by prior exposure to ouabain.

Sodium-calcium exchange in renal epithelial cells: dependence on cell sodium and competitive inhibition by magnesium

Kinetic properties of Na(+)-Ca2+ exchange in a renal epithelial cell line (LLC-MK2) were assessed by measuring cytosolic free Ca2+ with fura-2 and 45Ca2+ influx. Replacing external Na+ with K+ produced relatively small increases in free Ca2+ and 45Ca2+ uptake unless the cells were incubated with ouabain. Ouabain markedly increased cell Na+ and strongly potentiated the effect of replacing external Na+ with K+ on free Ca2+ and 45Ca2+ uptake. 45Ca2+ influx in 140 mM K+ or N-methyl-D-glucamine minus influx in 140 mM Na+ was used to quantify Na(+)-Ca2+ exchange activity of Na(+)-loaded cells. The dependence of exchange on cell Na+ was sigmoidal; the K0.5 was 26 +/- 3 mmol/liter cell water space, and the Hill coefficient was 3.1 +/- 0.2. The kinetic features of the dependence of exchange on cell Na+ partly account for the small increase in Ca2+ influx when all external Na+ is replaced by K+. Besides raising cell Na+ ouabain appears to activate the exchanger. Magnesium competitively inhibited exchange activity. The potency of Mg2+ was 8.2-fold lower with potassium instead of N-methyl-D-glucamine or choline as the replacement for external Na+. Potassium also increased the Vmax of exchange by 86% and had no effect on the Km for Ca2+. The exchanger does not cause detectable 22Na(+)-Mg2+ exchange and does not appear to require K+ or transport 86Rb+. Although exchange activity was plentiful in the epithelial cells from monkey kidney, others from amphibian, canine, opossum, and porcine kidney had no detectable exchange activity. All of the measured kinetic properties of Na(+)-Ca2+ exchange in the renal epithelial cells are very similar to those of the exchanger in rat aortic myocytes.

Modulation of Na+/K+ exchange potentiates lipopolysaccharide-induced gene expression in murine peritoneal macrophages

The role of Na+/K+ exchange in regulating lipopolysaccharide (LPS)-mediated induction of cytokine gene expression has been examined in murine peritoneal macrophages. Depletion of K+ from the culture medium resulted in a three- to five-fold potentiation of tumor necrosis factor-alpha (TNF alpha), KC (gro), and IP-10 mRNA expression in LPS-treated macrophages. The potentiating effect was apparently the result of inhibition of Na+/K+ exchange through the Na+/K(+)-adenosine triphosphatase (ATPase) because ouabain-mediated inhibition of Na+/K(+)-ATPase was also able to potentiate cytokine mRNA expression as much or more than did K+ depletion. The effects of K+ depletion or ouabain treatment were not caused by depolarization of the macrophage membrane because depolarization mediated by elevating extracellular K+ levels was inhibitory to cytokine mRNA expression. Depletion of Na+ by substitution with choline in the culture medium also markedly potentiated LPS-induced gene expression. The Na+/H+ antiporter was not, however, involved in potentiating cytokine expression because treatment of macrophages with amiloride either had no effect on or was inhibitory to the LPS-induced changes in mRNA levels. The potentiation of gene expression was selective and was at least partially the result of increased transcriptional activity of each gene. Whereas Na+ depletion and ouabain both inhibited 86Rb+ uptake by macrophages, treatment with LPS had no effect either on Rb+ uptake or on efflux. Thus altered Na+/K+ exchange is not a component of the primary signalling pathway(s) mediating response to LPS. Nevertheless, modulation of macrophage Na+/K+ exchange by agents encountered during an inflammatory response may be an important determinant of the magnitude and quality of specific gene expression.

Cytosolic acidification leads to Ca2+ mobilization from intracellular stores in single and populational parietal cells and platelets

Regulatory relationship and gain control between cytosolic free Ca2+ concentration (Cai) and cytosolic pH (pHi) were evaluated by two different cell types, gastric parietal cells, and blood platelets. Studies were carried out in both single cells and populations of cells, using Ca2(+)-indicative probe fura-2 (1-(2-(5'-carboxyoxazol-2'-yl)-6-aminobenzofuran-5-oxy)-2-(2 '-amino-5'- methylphenoxy)ethane-N,N,N',N'-tetraacetic acid) and pH-indicative probe BCECF (2',7'-bis(carboxyethyl)carboxyfluorescein). Stimulation of single and populational parietal cells and platelets with gastrin and thrombin, respectively, resulted in an increase in Cai. In both populational cell types, an initial change in pHi during agonist stimulation occurred almost simultaneously with the mobilization of Ca2+; an initial transient decrease in pHi was followed by a slower increase in pHi above the prestimulation level. When populational platelets were preloaded with the Ca2+ chelator BAPTA (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid), the thrombin-induced initial large increase in Cai was apparently inhibited, whereas the pHi decrease induced by thrombin was not altered. This suggests that the initial Cai change is not a prerequisite for the pHi change. The effect of pHi on Cai was examined next. In both single and populational cell types, application of the K(+)-H+ ionophore nigericin, which induced a transient decrease in pHi, led to the release of Ca2+ from intracellular stores. In single parietal cells double-labeled with fura-2 and BCECF, a temporal decrease in pHi preceded the rise in Cai after stimulation with nigericin. A decrease in pHi and an increase in Cai occurred at 1.5 and 4 s, respectively. In single parietal cells, replacement of medium Na+ with N-methyl-D-glucamine (NMG+), which also induced a decrease in pHi, resulted in repetitive Ca2+ spike oscillations. The source of Ca2+ utilized for the Ca2+ oscillation that was induced by NMG+ originated from the agonist-sensitive pool. Thus, several maneuvers, which were capable of decreasing pHi, led to an increase in Cai. Cytosolic acidification may be a part of the trigger for Ca2+ mobilization from intracellular stores in both parietal cells and platelets.

Effects of extracellular sodium on cytosolic calcium, PGE2 and cAMP in papillary collecting tubule cells

An increase in cytosolic calcium (Cai2+), induced by an increase in extracellular calcium concentration or addition of the calcium ionophore A23187, has been shown to suppress basal and AVP responsive cAMP and inhibit water flow in the collecting tubule. In the present study, the relationships between extracellular Na+ concentration, Cai2+, PGE2 and AVP-responsive cAMP production were examined in cultured rat papillary collecting tubule (RPCT) cells. Reducing extracellular Na+ concentration from 144 to 24 mM increased Cai2+ and PGE2 production approximately below 144 mM on Cai2+ or PGE2 was not due to a change in media osmolality or chloride concentration, since these parameters were maintained at constant levels by addition of tetramethylammonium chloride (TMA) or choline chloride. Exposure of RPCT cells to media containing 24 mM Na+ significantly suppressed basal and AVP responsive cAMP compared to that observed at 144 mM Na+. This suppression was mimicked by the calcium ionophore A23187 which also increased Cai2+ and PGE2. The increase in Cai2+ and PGE2 and the suppression of basal and AVP responsive cAMP, which were observed at 24 versus 144 mM Na+, were abolished in calcium free media and were likely due to influx of extracellular calcium. Indomethacin did not prevent the suppressive effects of reducing extracellular Na+ concentration below 144 mM on basal or AVP responsive cAMP, suggesting that the enhanced production of PGE2 did not mediate the reduction in AVP responsiveness. In contrast to cAMP, reductions in extracellular Na+ concentration from 144 to 24 mM did not influence basal cGMP or the cGMP responses to atrial natriuretic peptide or nitroprusside.(ABSTRACT TRUNCATED AT 250 WORDS)

Volume regulation in rat pheochromocytoma cultured cells submitted to hypoosmotic conditions

The mechanisms at work in cell volume regulation have been studied in PC12 cultured cells. Results show, for the first time to our knowledge, that the volume readjustment process occurring after application of a hypoosmotic saline is sensitive to amiloride, IBMX and forskoline. The process is also inhibited by quinine hydrochloride and trifluoperazine. Volume readjustment is concomtant with a decrease in K+ and Cl- intracellular levels. The decrease in K+ level can be related to an assymetrical change in the fluxes in and out of the ion as shown by flux kinetics studies using Rb86. These results are interpreted considering that the control of the activity of the ion channel pathways associated with volume readjustment in PC12 cells may implicate the Ca(2+)-calmodulin - cAMP system.

Identification and characteristics of a Na+/Ca2+ exchanger in cultured human mesangial cells

Excitable cells express Na+/Ca2+ exchange activity among other mechanisms modulating rapid fluctuations of cytosolic free Ca2+ ([Ca2+]i). We studied functions and regulation of a Na+/Ca2+ exchanger in cultured human glomerular mesangial cells. Fura-2-loaded confluent monolayers reacted to removal of ambient Na+ with an immediate, transient elevation of [Ca2+]i, assessed by single/dual wavelength fluorometry. Peak [Ca2+]i was inversely correlated with the extracellular Na+ concentration. Ca2+ influx was the sole mechanism implicated, as the [Ca2+]i rise was prevented by EGTA. The process was inhibited by 1 mM amiloride, but not by blockers of voltage-operated Ca2+ channels. Re-addition of Na+ resulted in a rapid decrease of [Ca2+]i, indicating bimodal operation of the exchanger. Na(+)-loading the cells with monensin and ouabain enhanced Ca2+ uptake. Prior stimulation of [Ca2+]i with the thromboxane A2 mimetic, U-46619, or angiotensin II also increased Ca2+ uptake upon subsequent Na+ removal, suggesting induction of the exchanger by vasoconstrictors. Moreover, the magnitude of agonist-induced [Ca2+]i transients was amplified by Na+ removal, indicating that the exchanger modulates the effects of vasoconstrictors. These results demonstrate that an inducible Na+/Ca2+ antiporter is operative in resting and stimulated human mesangial cells, further confirming their smooth muscle origin and potential regulatory role on glomerular hemodynamics.

Calcium dependence of ouabain-induced contraction in aortas from spontaneously hypertensive rats

The calcium sensitivity of ouabain-induced contractions of aortic strips from spontaneously hypertensive rat (SHR) was examined using several drugs which affect Na+ and Ca2+ movements across the cell membrane, and the results were compared with those obtained with age-matched Wistar-Kyoto rat (WKY). The Ca2+ concentration-response curves (10(-3) M ouabain-treated preparations) made with aortic strips from SHR lay to the left of those made with aortic strips from WKY (Ca EC50 values: SHR, 0.51 +/- 0.16 mM, n = 6; WKY, 1.23 +/- 0.41 mM, n = 7; P less than 0.05). Amiloride (a Na+ entry blocker) and nifedipine (a Ca2+ entry blocker) attenuated the sensitivity to Ca2+ of SHR and WKY aortic strips. With 2 x 10(-4) M amiloride, WKY vessels showed a 1.3-fold increase in the Ca EC50 value and SHR a 2.1-fold increase. With 10(-6) M nifedipine. WKY vessels showed a 1.1-fold increase in the Ca EC50 value and SHR a 1.5-fold increase. Addition of monensin (Na ionophore) produced a dose-dependent potentiation in ouabain-treated aorta from WKY, but not in ouabain-treated aorta from SHR. Addition of 1.5 x 10(-5) M A23187 (Ca ionophore) eliminated the difference between the Ca2(+)-induced contractions in aortas from SHR and WKY. These results suggest that enhancement of Ca2+ influx by Na(+)-Ca2+ exchange and/or voltage-dependent Ca2+ channels in vascular smooth muscle cell membranes may be an important factor in the difference between ouabain-induced contractions in aorta from SHR and WKY.

Characterization of ouabain-induced phosphoinositide hydrolysis in brain slices of the neonatal rat

The effect of the Na/K-ATPase inhibitor ouabain on phosphoinositide (Ptdlns) hydrolysis was studied in rat brain cortical slices. Ouabain induced a dose-dependent accumulation of inositol phosphates (InsPs) which was much higher in neonatal rats (1570 +/- 40% of basal) than in adult animals (287 +/- 18% of basal). For this reason, all experiments were conducted with 7 day-old rats. Strophantidin caused a similar stimulation of Ptdlns hydrolysis, although it was less potent than ouabain. The order of potency for ouabain-stimulated InsPs accumulation in brain areas was hippocampus greater than cortex greater than brainstem greater than cerebellum. The effect of ouabain was not blocked by antagonists for the muscarinic, alpha1 -adrenergic and glutamate receptors. Also ineffective were the K+ channel blockers 4-aminopyridine and tetraethylammonium, the sodium channel blocker tetrodotoxin, and the calcium channel blocker verapamil, whereas the Na/Ca exchanger blocker amiloride partially antagonized the effect of ouabain. The accumulation of InsPs induced by ouabain was additive to that of carbachol and norepinephrine, as well as to that induced by high K+ and veratrine, but not to that of glutamate. Removal of Na+ ions from the incubation buffer completely prevented the accumulation of InsPs induced by ouabain. The effect of ouabain was also dependent upon extracellular calcium and was under negative feedback control of protein kinase C. Despite the higher effect of ouabain on Ptdlns hydrolysis of immature rats, the density of [3H]ouabain binding sites, as well as the activity of Na/K-ATPase were higher in adult animals. Furthermore, a poor correlation was found between ouabain-stimulated Ptdlns hydrolysis and [3H]ouabain binding in brain regions.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of human platelets by collagen occurs by a Na+/H+ exchanger independent mechanism

In stimulated human platelets dense-granule secretion in response to the 'weak agonists' ADP, adrenaline, platelet activating factor and low concentrations of thrombin as well as Ca2+ mobilisation in response to thrombin are enhanced by a Na+/H+ exchanger. In the present study the role of this antiport in collagen stimulated human platelets was examined. While stimulation of platelets loaded with the fluorescent intracellular pH-sensitive dye, bis-carboxyethyl-5-(6)-carboxyfluorescein (BCECF) with thrombin resulted in the activation of the Na+/H+ exchanger, activation of this antiport did not occur in collagen-stimulated platelets. The lack of antiport activity in response to collagen using BCECF-loaded platelets correlated with the lack of any functional role of the antiport in collagen stimulated platelets. In the presence of a Na+/H+ exchange inhibitor, ethylisopropylamiloride, neither collagen-induced platelet aggregation or dense-granule secretion was affected. Furthermore, while the removal of extracellular Na+ (Na+ext), a condition that also prevents activation of the antiport, inhibited dense-granule secretion in response to a low concentration of thrombin, collagen-induced secretion was potentiated. This potentiatory effect could not be attributed to changes in either the membrane potential or in collagen-induced phospholipase C or protein kinase C activity. The present results indicate that in contrast to the 'weak agonists' (1) collagen-induced platelet activation does not require activation of the Na+/H+ exchanger and (2) Na+ext per se is an inhibitor of collagen-induced secretion.

Intracellular free calcium concentration/force relationship in rabbit inferior vena cava activated by norepinephrine and high K+

The changes in isometric force and the underlying fluctuations in intracellular free calcium concentration ([Ca2+]i) were monitored simultaneously in thin sheets of rabbit inferior vena cava loaded with the fluorescent Ca2+ indicator fura-2. In resting tissues bathed in physiological saline solution, the estimated [Ca2+]i was approximately 105 nM. The alpha-adrenergic agonist norepinephrine (10 microM) caused an initial rise in [Ca2+]i to 264 nM during force development, which dropped to 216 nM during force maintenance. The maintained norepinephrine-induced increase in force and [Ca2+]i was reversed in Ca2(+)-free (2 mM EGTA) solution. Membrane depolarization by high K+ (80 mM) significantly increased [Ca2+]i to 234 nM. Compared to norepinephrine, high K+ caused about the same steady-state increase in [Ca2+]i, but a smaller increase in force. [Ca2+]i/force curves were constructed at different concentrations of extracellular Ca2+, with either norepinephrine or high K+ as a stimulant. The curve generated with norepinephrine was located to the left of that generated with high K+.

Lowering extracellular sodium or pH raises intracellular calcium in gastric cells

The dependence of cytoplasmic free [Ca] (Cai) on [Na] and pH was assessed in individual parietal cells of intact rabbit gastric glands by microfluorimetry of fura-2. Lowering extracellular [Na] (Nao) to 20 mM or below caused a biphasic Cai increase which consisted of both release of intracellular Ca stores and Ca entry across the plasma membrane. The Ca increase was not blocked by antagonists of Ca-mobilizing receptors (atropine or cimetidine) and was independent of the replacement cation. Experiments in Ca-free media and in Na-depleted cells indicated that neither phase was due to reversal of Na/Ca exchange. The steep dependence of the Cai increase on Nao suggested that the response was not due to lowering intracellular [Na] (Nai). The effects of low Nao on Cai were also completely independent of changes in intracellular pH (pHi). Cai was remarkably stable during changes of pHi of up to 2 pH units, indicating that H and Ca do not share a cytoplasmic buffer system. Such large pH excursions required determination of the pH dependence of fura-2. Because fura-2 was found to decrease its affinity for Ca as pH decreased below 6.7, corrections were applied to experiments in which large pHi changes were observed. In contrast to the relative insensitivity of Cai to changes in pHi, decreasing extracellular pH (pHo) to 6.0 or below was found to stimulate release of intracellular Ca stores. Increased Ca entry was not observed in this case. The ability of decreases in Nao and pHo to stimulate release of intracellular Ca stores suggest interactions between Na and H with extracellular receptors.