The pH dependence of the cardiac sarcolemmal Ca2(+)-transporting ATPase: evidence that the Ca2+ translocator bears a doubly negative charge

H⁺-activated Na⁺ influx in the ventricular myocyte couples Ca²⁺-signalling to intracellular pH

Acid extrusion on Na(+)-coupled pH-regulatory proteins (pH-transporters), Na(+)/H(+) exchange (NHE1) and Na(+)-HCO3(-) co-transport (NBC), drives Na(+) influx into the ventricular myocyte. This H(+)-activated Na(+)-influx is acutely up-regulated at pHi<7.2, greatly exceeding Na(+)-efflux on the Na(+)/K(+) ATPase. It is spatially heterogeneous, due to the co-localisation of NHE1 protein (the dominant pH-transporter) with gap-junctions at intercalated discs. Overall Na(+)-influx via NBC is considerably lower, but much is co-localised with L-type Ca(2+)-channels in transverse-tubules. Through a functional coupling with Na(+)/Ca(2+) exchange (NCX), H(+)-activated Na(+)-influx increases sarcoplasmic-reticular Ca(2+)-loading and release during intracellular acidosis. This raises Ca(2+)-transient amplitude, rescuing it from direct H(+)-inhibition. Functional coupling is biochemically regulated and linked to membrane receptors, through effects on NHE1 and NBC. It requires adequate cytoplasmic Na(+)-mobility, as NHE1 and NCX are spatially separated (up to 60μm). The relevant functional NCX activity must be close to dyads, as it exerts no effect on bulk diastolic Ca(2+). H(+)-activated Na(+)-influx is up-regulated during ischaemia-reperfusion and some forms of maladaptive hypertrophy and heart failure. It is thus an attractive system for therapeutic manipulation. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

Simultaneous analysis of intracellular pH and Ca²⁺ from cell populations

Although changes in both pH(in) and [Ca(2+)](i) have been observed in response to a variety of agonists, it is not clear whether these ionic events work independently or are coordinated to lead to a specific physiological response. One of the fundamental problems in studying these ionic events is that changes in pH(in) modify Ca(2+) regulatory mechanisms and changes in Ca(2+) may modify pH regulation. It is desirable to use a technique that allows concomitant monitoring of these two ions in cell populations with high time resolution. Furthermore, like many Ca(2+) binding proteins, all Ca(2+)-sensitive fluoroprobes are inherently sensitive to pH owing to competition of H(+) for the Ca(2+)-binding sites. This chapter describes experimental paradigms that provide optimum conditions for simultaneous measurement of pH from the fluorescence emission of snarf-1, and Ca(2+) using fura-2. The fluorescence spectra of these compounds are sufficiently different to allow simultaneous measurement of pH and Ca(2+) both in vitro and in vivo. Moreover, the ratio of the H(+)-sensitive wavelengths of snarf-1 is unaffected by Ca(2+), or the concomitant presence of fura-2 in cells. Although the fluorescence ratio of fura-2 is insensitive to the presence of snarf-1, it is affected by pH, as indicated above. We describe procedures to correct for this effect and to obtain calibration parameters for fura-2 and snarf-1 required to facilitate analysis of pH and Ca(2+) concentrations within cell populations.

Paradoxical block of the Na+-Ca2+ exchanger by extracellular protons in guinea-pig ventricular myocytes

1. The Na+-Ca2+ exchange is a major pathway for removal of cytosolic Ca2+ in cardiac myocytes. It is known to be inhibited by changes of intracellular pH that may occur, for example, during ischaemia. In the present study, we examined whether extracellular protons (pHo) can also affect the cardiac exchange. 2. Na+-Ca2+ exchange currents (INa-Ca) were recorded from single adult guinea-pig ventricular myocytes in the whole-cell voltage-clamp configuration while [Ca2+]i was simultaneously imaged with fluo-3 and a laser-scanning confocal microscope. To activate INa-Ca, intracellular Ca2+ concentration jumps were generated by laser flash photolysis of caged Ca2+ (DM-nitrophen). 3. Exposure of the cell to moderately and extremely acidic conditions (pHo 6 and 4) was accompanied by a decrease of the peak INa-Ca to 70 % and less than 10 %, respectively. The peak INa-Ca was also inhibited to about 45 % of its initial value by increasing pHo to 10. The largest INa-Ca was found at pHo approximately 7.6. 4. Simultaneous measurements of [Ca2+]i and INa-Ca during partial proton block of the Na+-Ca2+ exchanger revealed that the exchange current was more inhibited by acidic pHo than the rate of Ca2+ transport. This observation is consistent with a change in the electrogenicity of the Na+-Ca2+ exchange cycle after protonation of the transporter. 5. We conclude that both extracellular alkalinization and acidification affect the Na+-Ca2+ exchanger during changes of pHo that may be present under pathophysiological conditions. During both extreme acidification or alkalinization the Na+-Ca2+ exchanger is strongly inhibited, suggesting that extracellular protons may interact with the Na+-Ca2+ exchanger at multiple sites. In addition, the electrogenicity and stoichiometry of the Na+-Ca2+ exchange may be modified by extracellular protons.

Distinct regulation of pHin and [Ca2+]in in human melanoma cells with different metastatic potential

We investigated whether alterations in the mechanisms involved in intracellular pH (pHin) and intracellular calcium ([Ca2+]in) homeostasis are associated with the metastatic potential of poorly (A375P) and highly (C8161) metastatic human melanoma cells. We monitored pHin and [Ca2+]in simultaneously, using the fluorescence of SNARF-1 and Fura-2, respectively. Our results indicated that steady-state pHin and [Ca2+]in between these cell types were not significantly different. Treatment of cells with NH4Cl resulted in larger pHin increases in highly than in poorly metastatic cells, suggesting that C8161 cells have a lower H+ buffering capacity than A375P. NH4Cl treatment also increased [Ca2+]in only in C8161 cells. To determine if the changes in [Ca2+]in triggered by NH4Cl treatment were due to alterations in either H+- or Ca2+-buffering capacity, cells were treated with the Ca2+-ionophore 4Br-A23187, to alter [Ca2+]in. The magnitude of the ionophore-induced [Ca2+]in increase was slightly greater in C8161 cells than in A375P. Moreover, A375P cells recover from the ionophore-induced [Ca2+]in load, whereas C8161 cells did not, suggesting that A375P may exhibit distinct [Ca2+]in regulatory mechanisms than C8161 cells, to recover from Ca2+ loads. Removal of extracellular Ca2+ ([Ca2+]ex) decreased [Ca2+]in in both cell types at the same extent. Ionophore treatment in the absence of [Ca2+]ex transiently increased [Ca2+]in in C8161, but not in A375P cells. Endoplasmic reticulum (ER) Ca2+-ATPase inhibitors such as cyclopiazonic acid (CPA) and thapsigargin (TG) increased steady-state [Ca2+]in only in C8161 cells. Together, these data suggest that the contribution of intracellular Ca2+ stores for [Ca2+]in homeostasis is greater in highly than in poorly metastatic cells. Bafilomycin treatment, to inhibit V-type H+-ATPases, corroborated our previous results that V-H+-ATPases are functionally expressed at the plasma membranes of highly metastatic, but not in poorly metastatic cells (Martínez-Zaguilán et al., 1993). Collectively, these data suggest that distinct pHin and [Ca2+]in regulatory mechanisms are present in poorly and highly metastatic human melanoma cells.

Simultaneous measurement of intracellular pH and Ca2+ in insulin-secreting cells by spectral imaging microscopy

Described is a microscopic spectral imaging approach to monitor pH and Ca2+ simultaneously from combined spectra of multiple ion indicators. Emitted light from a cell is focused onto a grating spectrograph and spectra are imaged with a cooled charge-coupled device camera. The combined spectral output of fura 2 and SNARF-1 was analyzed to follow changes in intracellular Ca2+ concentration ([Ca2+]i) and intracellular pH (pHi) simultaneously and to correct the Ca2+ signal for concurrent changes in pHi. Responses of individual hamster insulinoma (HIT-T15) cells to effectors of ion homeostasis were heterogeneous. Treatment with NH4Cl increased pHi and transiently increased [Ca2+]i. Removal of NH4Cl induced cytosolic acidification concomitant with either no change or sustained increases in [Ca2-]i. Glucose treatment generally resulted in rapid and sustained increases in both [Ca2+]i and pHi but also heterogeneous pHi and [Ca2+]i responses. Corrections of the fura 2 signal for pH were important for following Ca2+ transitions elicited by NH4Cl but were less important for glucose-induced responses. The spectral imaging microscope provides a sensitive method for simultaneous measurements of pHi and [Ca2+]i in single cells.

Selection of fluorescent ion indicators for simultaneous measurements of pH and Ca2+

The advent of fluorescent ion sensitive indicators has improved our understanding of the mechanisms involved in regulating pHi and [Ca2+]i homeostasis in living cells. However, changes in [Ca2+]i can alter pHi regulatory mechanisms and vice versa, making assignment of either ion to a particular physiological response complex. A further complication is that all fluorescent Ca2+ indicators are sensitive to protons. Therefore, techniques to simultaneously measure these two ions have been developed. Although several combinations of pH and Ca2+ probes have been used, few systematic studies have been performed to assess the validity of such measurements. In vitro analysis (i.e. free acid forms of dyes) indicated that significant quenching effects occurred when using specific dye combinations. Fura-2/SNARF-1 and MagFura-2/SNARF-1 probe combinations were found to provide the most accurate pH and [Ca2+] measurements relative to Fluo-3/SNARF-1, Ca2+-Green-1/SNARF-1, or BCECF/SNARF-1. Similar conclusions were reached when probes were calibrated after loading into cells. The magnitude of interactions between pH and Ca2+ probes could be a factor which may limit the use of certain specific combinations. Loading of probes that exhibit interactions into distinct intracellular compartments (i.e. separated by a biological membrane) abolished the quenching effects. These data indicate that interactions between the probes used to simultaneously monitor pH and Ca2+ must be considered whenever probe combinations are used.

Plasma membrane Ca(2+)-pump functional specialization in the brain. Complex of isoform expression and regulation by effectors

The plasma membrane Ca(2+)-pump (PMCA) is a key element in the removal of intracellular Ca2+. A number of PMCA pumps, encoded by a multigenic family and differing in their regulatory domains, also exist in the neuronal cells. We discuss here an idea regarding a new, higher level of specialization of PMCA protein isoforms with different sensitivities toward phospholipids and calmodulin. The idea is based on the kinetic data from PMCA stimulation by acidic phospholipids, with a combination of results describing an alternative RNA splicing at site A and C coding of regulatory domains of protein. The resulting complex modulation of the Ca(2+)-pump underlies the specific cellular requirements for Ca2+ homeostasis in a tissue-selective manner and is regulated by the level and spatial distribution of enzyme isoforms as well as by the level of their regulatory factors. The possible role of PMCA protein in the neuronal injury is also discussed.

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.

Effect of glucose on pHin and [Ca2+]in in NIH-3T3 cells transfected with the yeast P-type H(+)-ATPase

NIH-3T3 cells transfected with yeast H(+)-ATPases (RN1a cells) are tumorigenic (Perona and Serrano, 1988, Nature, 334:438). We have previously shown that RN1a cells maintain a chronically high intracellular pH (pHin) under physiological conditions. We have also shown that RN1a cells are serum-independent for growth, maintain a higher intracellular Ca2+ ([Ca2+]in), and glycolyze more rapidly than their non-transformed counterparts (Gillies et al., Proc. Natl. Acad. Sci., 1990, 87:7414; Gillies et al., Cell. Physiol. Biochem., 1992, 2:159). The present study was aimed to understand the interrelationships between glycolysis, pHin, and [Ca2+]in in RN1a cells and their non-transformed counterparts, NIH-3T3 cells. Our data show that the higher rate of glycolysis observed in RN1a cells is due to the presence of low affinity glucose transporters. Consequently, the higher rate of glycolysis is exacerbated at high glucose concentration in RN1a cells. Moreover, the maximal velocity (Vmax) for glucose utilization is up to sixfold higher in RN1a cells than in the NIH-3T3 cells, suggesting that the number of glucose transporters is higher in RN1a than NIH-3T3 cells. Glucose addition to NIH-3T3 cells results in modest decreases in both pHin and [Ca2+]in. In contrast, RN1a cells respond to glucose with a large decrease in pHin, followed by a large decrease in [Ca2+]in. The decrease in [Ca2+]in observed upon glucose addition is likely due to activation of Ca(2+)-ATPase by glycolysis, since the Ca2+ decrease is abolished by the Ca2+ ATPase inhibitors thapsigargin and cyclopiazonic acid. Glucose addition to ATP-depleted cells results in a decrease in [Ca2+]in, suggesting that ATP furnished by glycolysis is utilized by this pump.

Regulation of the erythrocyte Ca(2+)-ATPase at high pH

The activation of the Ca(2+)-ATPase from erythrocyte membranes at high pH has been investigated. Following alkalinization and in the absence of regulators, the enzyme exhibits a very high affinity for Ca2+ and a decreased maximal velocity. Either addition of calmodulin, addition of acidic phospholipids, or controlled trypsinization decreases the concentration of effector required to elicit half-maximal activation of the enzyme for calcium to similar values. The increase in affinity for Ca2+, however, is smaller than that observed at neutral pH. The maximal velocity at high pH becomes insensitive to both calmodulin and controlled proteolysis, although calmodulin binds to the protein with similar affinities at pH 7.0 and 8.0, as indicated by similarity in binding to a calmodulin-Sepharose resin and in dependence on calmodulin concentrations when the pH is increased. In contrast to the attenuated effects of calmodulin and proteolysis, at pH 8.0 the enzyme is susceptible to stimulation by phospholipids, indicating that the pathway for transduction of the signal from phospholipids is distinct from that pathway engaged by calmodulin and/or trypsinization. At pH 8.0, phosphatidylinositol induces the modulatory effect of ATP at the regulatory site but calmodulin does not. We suggest that the intraenzymic connection between the calmodulin-binding, autoinhibitory peptide and the nucleotide domain of the enzyme is impaired upon alkalinization, which would account for the differing abilities of the activators to modulate the ATP effects.

The cardiac Na(+)-Ca2+ exchanger: relative rates of calcium and sodium movements and their modulation by protonation-deprotonation of the carrier

The exchange cycle of the cardiac Na(+)-Ca2+ exchanger can be described as separate steps of Ca2+ and Na+ transport [Khananshvili, D. (1990) Biochemistry 29, 2437-2442]. In order to determine the relative rates of Na+ and Ca2+ movement during the Na(+)-Ca2+ and Ca(2+)-Ca2+ exchange modes, the ratios (R) of Na(+)-Ca2+/Ca(2+)-Ca2+ exchanges were estimated with saturating concentrations of ions at both sides of the membrane. The effect of extravesicular pH and voltage (potassium valinomycin) on the initial rates (t = 1 s) of Na(+)-Ca2+ and Ca(2+)-Ca2+ exchange were investigated by assuming that, under the conditions tested, the intravesicular pH (pH 7.4) is not affected. Na(+)- or Ca(2+)-preloaded sarcolemma vesicles were diluted rapidly in assay medium containing 45Ca and buffer (pH 5.0-10.9), and the reaction of 45Ca uptake was quenched by using a semi-rapid-mixing device. Under conditions in which [45Ca]o = [Ca]i = 250 microM, the pH-dependent curve of Ca(2+)-Ca2+ exchange shows a bell shape in the acidic range (pKa1 = 5.1 +/- 0.1 and pKa2 = 6.5 +/- 0.2) followed by activation of the exchange in the alkaline range (pKa3 = 10.0 +/- 0.2). With [45Ca]o = 250 microM and [Na]i = 160 mM, the Na(+)-Ca2+ exchange increases monotonically from pH 5.0 to 9.5 (pKa1 = 5.1 +/- 0.1, pKa2 = 7.2 +/- 0.2, and pKa3 = 9.1 +/- 0.2). At pH < 6.1, the ratio of Na(+)-Ca2+/Ca(2+)-Ca2+ exchange is close to unity (R approximately 1), while it increases to R = 3-4 in the range of pH 7.1-9.3.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium ion homeostasis in smooth muscle

Ca2+ plays an important role in the regulation of smooth-muscle contraction. In this review, we will focus on the various Ca(2+)-transport processes that contribute to the cytosolic Ca2+ concentration. Mainly the functional aspects will be covered. The smooth-muscle inositol 1,4,5-trisphosphate receptor and ryanodine receptor will be extensively discussed. Smooth-muscle contraction also depends on extracellular Ca2+ and both voltage- and Ca(2+)-release-activated plasma-membrane Ca2+ channels will be reviewed. We will finally discuss some functional properties of the Ca2+ pumps that remove Ca2+ from the cytoplasm and of the Ca2+ regulation of the nucleus.

Ion homeostasis in rat brain in vivo: intra- and extracellular [Ca2+] and [H+] in the hippocampus during recovery from short-term, transient ischemia

Changes in intra- and extracellular [Ca2+] and [H+], together with alterations in tissue PO2 and local blood flow, were measured in areas CA1 and CA3 of the hippocampus during recovery (up to 8 h) after an 8-min period of low-flow ischemia. Restoration of blood supply was followed by an immediate rise in flow and tissue PO2 above normal, with large fluctuations in both persisting for up to 4 h. In area CA1, [Ca2+]i decreased rapidly from an ischemic mean value of 30 microM to a control mean level of 73.1 nM in 20-30 min, whereas normalization of [Ca2+]e took approximately 1 h. Recovery of [Ca2+]i was accelerated by preischemic administration of a calcium antagonist, nifedipine, and a free radical scavenger, N-tert-butyl-alpha-phenylnitrone (PBN), but not by MK-801, a blocker of N-methyl-D-aspartate receptors. There was a secondary rise in [Ca2+]i in many cells beginning approximately 2 h after reperfusion. This was attenuated somewhat by PBN but not clearly influenced by either nifedipine or MK-801. Changes of [Ca2+]i in area CA3 were much smaller and slightly slower than in area CA1 and were not affected by the drugs mentioned above. In both areas CA1 and CA3, pHe and pHi fell during ischemia to an average value of 6.2, from which there was a rapid initial recovery in the first 5-10 min when blood flow was restored. Thereafter tissue pH rose slowly and did not reach control levels for approximately 1 h, and in some microareas not at all. It is concluded that (a) effective mechanisms for restoring normal [Ca2+]i remain intact after 8 min of low-flow ischemia; (b) in neurons of area CA1, some insidious change in the homeostasis of calcium triggers a secondary rise in its free cytosolic concentration, which may be causally related to activation of irreversible cell damage; and (c) the changes in [Ca2+]i and [Ca2+]e during and following 8 min of ischemia can be adequately accounted for by movements of a fixed pool of Ca between intra- and extracellular compartments, and possible mechanisms are discussed.

The Ca(2+)-transport ATPases from the plasma membrane

The initial studies on the plasma membrane (PM) Ca(2+)-transport ATPases were made in the erythrocyte, a structure that can not be taken as representing a typical eukaryotic cell. In other cell types however, the study of the PM Ca(2+)-transport ATPase is complicated by the simultaneous expression of related Ca(2+)-pumps in intracellular stores. Whereas there are as yet no known specific inhibitors for the PM Ca(2+)-transport ATPase, a number of selective inhibitors for the endo(sarco)plasmic reticulum Ca2+ pumps have been described: thapsigargin, cyclopiazonic acid and 2,5-di-(tert-butyl)-1,4-benzohydroquinone. With the recent introduction of the molecular biological approach, it became quickly obvious that a family of at least 5 different PM Ca(2+)-transport ATPase genes govern the tissue-dependent expression of PM Ca2+ pumps. Moreover alternative splicing of the primary gene transcripts was found to further enhance the number of pump variants. The PM Ca(2+)-transport ATPase are subject to modulatory control by calmodulin, by acidic phospholipids, and by the known families of protein kinases. Each of the ensuing effects are mutually related and interdependent. The wide variety PM Ca2+ pump isoforms and their regulation by such an intricate modulatory network allows the distinct tissues to adapt most adequately to the prevailing tissue and stimulus specific requirements.

Stimulation of dense tubular Ca2+ uptake in human platelets by cAMP

Elevation of intracellular cAMP is shown to increase the rate (V) and maximal extent of Ca2+ uptake by the dense tubules in intact human platelets. Elevation of [cAMP] was accomplished by preincubation with the adenylate cyclase activator forskolin or with dibutyryl-cAMP (Bt2-cAMP). The free concentration of Ca2+ in the dense tubular lumen ([Ca2+]dt) was monitored using the fluorescence of chlorotetracycline (CTC) according to protocols developed in this laboratory. The free cytoplasmic Ca2+ concentration ([Ca2+]cyt) was monitored in parallel experiments with quin2. Both [Ca2+]cyt and [Ca2+]dt were analyzed in terms of competition between pump and leak mechanisms in the plasma membrane (PM) and dense tubular membrane (DT). When platelets are incubated in media with approx. 1 microM external Ca2+, [Ca2+]cyt is approx. 50 nM and [Ca2+]dt is very low. When 2 mM external Ca2+ is added, [Ca2+]cyt rises to approx. 100 nM and the process of dense tubular Ca2+ uptake can be resolved. Forskolin (10 microM) and Bt2-cAMP increase the rate of dense tubular Ca2+ uptake (V) to 2.1 +/- 0.60 and 1.70 +/- 40 times control values (respectively). The agents also increase the final [Ca2+]dt to 1.70 +/- 0.21 and 1.72 +/- 0.60 times control values (respectively). Titrations with ionomycin (Iono) showed that the increase was due to an increase in the Vm of the dense tubular Ca2+ pump. With [Iono] = 500 nM, [Ca2+]cyt was raised to greater than or equal to 1.0 microM and Vm of the dense tubular pump was elicited. (At [Iono] = 1.0 microM, the final [Ca2+]dt values were degraded 15% due to shunting of Ca2+ uptake.) Analysis showed that forskolin (10 microM) and Bt2-cAMP (1 mM) increase the Vm by a factors of 1.56 +/- 40 and 1.56 +/- 40, respectively. Analysis showed that neither agent changed the Km of the pump significantly from its control value of 180 nM. Neither agent changed the rate constant for passive leakage of Ca2+ across the DT membrane (1.7 min-1).

Ca2+ extrusion across plasma membrane and Ca2+ uptake by intracellular stores

The aim of this review is to summarize the various systems that remove Ca2+ from the cytoplasm. We will initially focus on the Ca2+ pump and the Na(+)-Ca2+ exchanger of the plasma membrane. We will review the functional regulation of these systems and the recent progress obtained with molecular-biology techniques, which pointed to the existence of different isoforms of the Ca2+ pump. The Ca2+ pumps of the sarco(endo)plasmic reticulum will be discussed next, by summarizing the discoveries obtained with molecular-biology techniques, and by reviewing the physiological regulation of these proteins. We will finally briefly review the mitochondrial Ca(2+)-uptake mechanism.