Fast efflux of Ca2+ mediated by the sarcoplasmic reticulum Ca2(+)-ATPase

Calcium as a Trojan horse in mental diseases-The role of PMCA and PMCA-interacting proteins in bipolar disorder and schizophrenia

Although first mentions about calcium disturbances in psychiatric diseases appeared more than 30 years ago, the most recent genomic and proteomic findings confirmed a significant role of Ca²⁺ and Ca²⁺-regulated pathways in development of neuropathological processes, including bipolar disorder and schizophrenia. Moreover, last decades have shown that due to multifactorial nature of both diseases, impairment in neuronal calcium homeostasis may depend not only on disturbed Ca²⁺ entry system, but also on altered extrusion system. A pivotal role in Ca²⁺ clearance mechanism is played by plasma membrane Ca²⁺-ATPase (PMCA), the enzyme responsible for returning the elevated levels of cytosolic Ca²⁺ back to the resting state. In this paper we summarize the current knowledge about the role of PMCA in bipolar disorder and schizophrenia pathologies, as well as the contribution of several proteins that by interaction with PMCA modify signal transduction mechanisms.

The effect of the anticancer drugs tamoxifen and hydroxytamoxifen on the calcium pump of isolated sarcoplasmic reticulum vesicles

The interactions of tamoxifen (TAM) and its active metabolite 4-hydroxytamoxifen (OHTAM) with the sarcoplasmic reticulum (SR) Ca(2+)-pump were investigated. The turnover of the Ca(2+)-ATPase is strongly inhibited by both drugs at low concentrations that do not significantly perturb the lipid organization of SR membranes. Moreover, TAM decreases Ca(2+) accumulation by SR Ca(2+)-ATPase and increases in parallel the ATP hydrolysis, decreasing the energetic efficiency of the Ca(2+)-pump (Ca (2+)ATP coupling ratio) by about 70% at 30 muM. This uncoupling of ATP hydrolysis from Ca(2+) accumulation is a putative consequence of structural defects induced on membranes, since the ATP hydrolysis at low residual Ca(2+) (Ca(2+) not supplemented) is also stimulated. On the other hand, OHTAM decreases the Ca(2+) uptake to a greater extent than TAM but, unlike TAM, it inhibits ATP hydrolysis. Thus, the Ca (2+)ATP ratio is decreased by about 47% at 30 muM OHTAM; this effect is not a consequence of membrane disruption, since the ATP-splitting activity decreases in parallel to Ca(2+) accumulation and no significant effect is detected for ATP hydrolysis at low residual Ca(2+). The inhibition of the Ca(2+)-pump by OHTAM is putatively related to a direct interaction with the regulatory sites of the enzyme or interactive perturbations at the lipid-protein interface. The effect may result from a decrease of efficiency in the energy transmission and transduction between the ATP use at the catalytic site and the channeling process involved in Ca(2+) translocation. Therefore, the effects of the drugs on the Ca(2+)-pump are different and rule out an unitary mechanism of action on the basis of bilayer structure perturbations.

Plasma membrane Ca2+-ATPases in the nervous system during development and ageing

Calcium signaling is used by neurons to control a variety of functions, including cellular differentiation, synaptic maturation, neurotransmitter release, intracellular signaling and cell death. This review focuses on one of the most important Ca²⁺ regulators in the cell, the plasma membrane Ca²⁺-ATPase (PMCA), which has a high affinity for Ca²⁺ and is widely expressed in brain. The ontogeny of PMCA isoforms, linked to specific requirements of Ca²⁺ during development of different brain areas, is addressed, as well as their function in the adult tissue. This is based on the high diversity of variants in the PMCA family in brain, which show particular kinetic differences possibly related to specific localizations and functions of the cell. Conversely, alterations in the activity of PMCAs could lead to changes in Ca²⁺ homeostasis and, consequently, to neural dysfunction. The involvement of PMCA isoforms in certain neuropathologies and in brain ageing is also discussed.

Toxic effects of chlorpromazine on Carassius auratus and its oxidative stress

Under laboratory conditions, ecotoxicological effects of chlorpromazine (CPZ) on freshwater goldfish (Carassius auratus) were examined using the toxic culture experiment. The results showed that the median lethal concentration (LC(50)) of CPZ toxic to Carassius auratus in 24, 48 and 96 h was 1.11, 0.43 and 0.32 mg/L, respectively. Thus, CPZ is an extreme toxicant to goldfish. Furthermore, there were significantly positive correlations between the ecotoxicological effects of CPZ and its concentrations, and the toxicity became higher as the exposure time increased. The activity of superoxide dismutase (SOD) and catalase (CAT) in goldfish livers was significantly influenced by CPZ. At the same exposure time, the activity of SOD reduced first, and increased then, whereas the activity of CAT enhanced first and decreased then. At the same exposure levels of CPZ, the activity of SOD and CAT changed similarly, decreased first, then increased and decreased at last. Within the range of exposure concentrations, the changes in the activity of CAT can more easily reflect the oxidation stress in Carassius auratus by CPZ than those of SOD.

Chlorpromazine binding to Na+, K+-ATPase and photolabeling: involvement of the ouabain site monitored by fluorescence

This work reports the results of ultraviolet irradiation on the interaction of the phototoxic antipsychotic drug chlorpromazine (CPZ) with the sodium pump Na+, K+-ATPase. The study was performed by monitoring the fluorescence modifications of CPZ itself and of the specific probe anthroylouabain (AO). CPZ association with Na+, K+-ATPase was found to modify the kinetics of CPZ-photodegradation. It was demonstrated that UV irradiation produces a stable fluorescent photoproduct of CPZ covalently bound to Na+, K+-ATPase. The fluorescent probe AO, which specifically binds to the extracellular ouabain site of the pump, was used to localize the CPZ binding site. UV-irradiation of AO-labeled Na+, K+-ATPase treated with CPZ at concentration about 20 microM produced dose-dependent modifications of the AO fluorescence, e.g. increased quantum yield and blue shift. The results demonstrated that CPZ binds near the ouabain site. The photo-induced reaction of CPZ with AO-labeled Na+, K+-ATPase protected the ouabain site from the aqueous environment. It was also found that UV irradiation of CPZ-treated enzyme obstructs the binding of AO, which suggested occlusion of the ouabain site. This effect can be evaluated for a potential use of CPZ in photochemotherapy.

Inhibition of plasma membrane Ca2+-ATPase by heparin is modulated by potassium

Heparin is related to several protein receptors that control Ca2+ homeostasis. Here, we studied the effects of heparin on the plasma membrane Ca2+-ATPase from erythrocytes. Both ATP hydrolysis and Ca2+ uptake were inhibited by heparin without modification of the steady-state level of phosphoenzyme formed by ATP. Calmodulin did neither modify the inhibition nor the binding of heparin. Inhibition by heparin was counteracted by K+ but not by Li+. This effect was extended to other sulfated polysaccharides with high number of sulfate residues. Hydrolysis of p-nitrophenylphosphate was equally inhibited by heparin. No evidence for enzyme uncoupling was observed: Ca2+ uptake and ATP hydrolysis remained tightly associated at any level of heparin, and heparin did not increase the passive Ca2+ efflux of inside-out vesicles. Vanadate blocked this efflux, indicating that the main point of Ca2+ escape from these vesicles was linked to the Ca2+ pump. It is discussed that sulfated polysaccharides may physiologically increase the steady-state level of Ca2+ in the cytosol by inhibiting the Ca2+ pumps in a K+ (and tissue) regulated way. It is suggested that heparin regulates the plasma membrane Ca2+-ATPase by binding to the E2 conformer.

Ca2+ Transport by the Synaptosomal Plasma Membrane Ca2+-ATPase and the Effect of Thioridazine

Thioridazine inhibits the activity of the synaptic plasma membrane Ca(2+)-ATPase from pig brain and slightly decreases the rate of Ca(2+) accumulation by synaptic plasma membrane vesicles in the absence of phosphate. However, in the presence of phosphate, thioridazine increases the rate of Ca(2+) accumulation into synaptic plasma membrane vesicles. Phosphate anions diffuse through the membrane and form calcium phosphate crystals, reducing the free Ca(2+) concentration inside the vesicles and the rate of Ca(2+) leak. The higher levels of Ca(2+) accumulation obtained in the presence of thioridazine could be explained by a reduction of the rate of slippage on the plasma membrane ATPase.

Analysis of polyamines as markers of (patho)physiological conditions

The aliphatic polyamines, putrescine, spermidine and spermine, are normal cell constituents that play important roles in cell proliferation and differentiation. The equilibrium between cellular uptake and release and the balanced activities of biosynthetic and catabolic enzymes of polyamines are essential for normal homeostasis in the proliferation and functions of cells and tissues. However, the intracellular polyamine content increases in hyperplastic or neoplastic growth. Although the involvement of polyamines in physiological and pathological cell proliferation and differentiation has been well established, the role they play is quite different in relation to cell systems and animal models and is dependent on inducer agents and stimuli. Also, the experimental procedures used to deplete polyamines have been shown to influence the cell responses. In this paper, the assay methods currently in use for polyamines are reviewed and compared with respect to sensitivity, reproducibility and applicability to routine analysis. The relevance of polyamine metabolism and the uptake/release process in many physiological and pathological processes is highlighted, and the cellular polyamine pathways are discussed in relation to the possible diagnostic and therapeutic significance of these mediators.

Uncoupled ATP hydrolysis and thermogenic activity of the sarcoplasmic reticulum Ca2+-ATPase: coupling effects of dimethyl sulfoxide and low temperature

The sarcoplasmic reticulum Ca(2+)-ATPase transports Ca(2+) using the energy derived from ATP hydrolysis. During catalysis, part of the energy is used to translocate Ca(2+) across the membrane, and part is dissipated as heat. At 35 degrees C the heat released during the hydrolysis of each ATP molecule varies depending on the formation of a Ca(2+) gradient across the membrane. With leaky vesicles (no gradient) the heat released varies between 9 and 12 kcal/mol of ATP cleaved, and with intact vesicles (gradient), the heat released increases to 20-24 kcal/mol of ATP. After Ca(2+) accumulation, 82% of the Ca(2+)-ATPase activity is not coupled to Ca(2+) transport, and the ratio between Ca(2+) transported and ATP cleaved is 0.3. The addition of 20% dimethyl sulfoxide (v/v) to the medium or decreasing the temperature from 35 to 20 degrees C abolishes the difference of heat produced during ATP hydrolysis in the presence and absence of a gradient. This is accompanied by a simultaneous inhibition of the uncoupled ATPase activity and an increase of the Ca(2+)/ATP ratio from 0.3 to 1.3-1.4. It is concluded that the uncoupled Ca(2+)-ATPase is responsible for both the low Ca(2+)/ATP ratio measured during transport and the difference of heat produced during ATP hydrolysis in the presence and absence of a gradient.

Curcumin, a molecule that inhibits the Ca2+-ATPase of sarcoplasmic reticulum but increases the rate of accumulation of Ca2+

Curcumin, an important inhibitor of carcinogenesis, is an inhibitor of the ATPase activity of the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum (SR). Inhibition by curcumin is structurally specific, requiring the presence of a pair of -OH groups at the 4-position of the rings. Inhibition is not competitive with ATP. Unexpectedly, addition of curcumin to SR vesicles leads to an increase in the rate of accumulation of Ca(2+), unlike other inhibitors of the Ca(2+)-ATPase that result in a reduced rate of accumulation. An increase in the rate of accumulation of Ca(2+) is seen in the presence of phosphate ion, which lowers the concentration of free Ca(2+) within the lumen of the SR, showing that the effect is not passive leak across the SR membrane. Rather, simulations suggest that the effect is to reduce the rate of slippage on the ATPase, a process in which a Ca(2+)-bound, phosphorylated intermediate releases its bound Ca(2+) on the cytoplasmic rather than on the lumenal side of the membrane. The structural specificity of the effects of curcumin on ATPase activity and on Ca(2+) accumulation is the same, and the apparent dissociation constants for the two effects are similar, suggesting that the two effects of curcumin could follow from binding to a single site on the ATPase.

Correlation between uncoupled ATP hydrolysis and heat production by the sarcoplasmic reticulum Ca2+-ATPase: coupling effect of fluoride

The sarcoplasmic reticulum Ca(2+)-ATPase transports Ca(2+) using the chemical energy derived from ATP hydrolysis. Part of the chemical energy is used to translocate Ca(2+) through the membrane (work) and part is dissipated as heat. The amount of heat produced during catalysis increases after formation of the Ca(2+) gradient across the vesicle membrane. In the absence of gradient (leaky vesicles) the amount of heat produced/mol of ATP cleaved is half of that measured in the presence of the gradient. After formation of the gradient, part of the ATPase activity is not coupled to Ca(2+) transport. We now show that NaF can impair the uncoupled ATPase activity with discrete effect on the ATPase activity coupled to Ca(2+) transport. For the control vesicles not treated with NaF, after formation of the gradient only 20% of the ATP cleaved is coupled to Ca(2+) transport, and the caloric yield of the total ATPase activity (coupled plus uncoupled) is 22.8 kcal released/mol of ATP cleaved. In contrast, the vesicles treated with NaF consume only the ATP needed to maintain the gradient, and the caloric yield of ATP hydrolysis is 3.1 kcal/mol of ATP. The slow ATPase activity measured in vesicles treated with NaF has the same Ca(2+) dependence as the control vesicles. This demonstrates unambiguously that the uncoupled activity is an actual pathway of the Ca(2+)-ATPase rather than a contaminating phosphatase. We conclude that when ATP hydrolysis occurs without coupled biological work most of the chemical energy is dissipated as heat. Thus, uncoupled ATPase activity appears to be the mechanistic feature underlying the ability of the Ca(2+)-ATPase to modulated heat production.

Slippage and uncoupling in P-type cation pumps; implications for energy transduction mechanisms and regulation of metabolism

P-type ATPases couple scalar and vectorial events under optimized states. A number of procedures and conditions lead to uncoupling or slippage. A key branching point in the catalytic cycle is at the cation-bound form of E(1)-P, where isomerization to E(2)-P leads to coupled transport, and hydrolysis leads to uncoupled release of cations to the cis membrane surface. The phenomenon of slippage supports a channel model for active transport. Ability to occlude cations within the channel is essential for coupling. Uncoupling and slippage appear to be inherent properties of P-type cation pumps, and are significant contributors to standard metabolic rate. Heat production is favored in the uncoupled state. A number of disease conditions, include ageing, ischemia and cardiac failure, result in uncoupling of either the Ca(2+)-ATPase or Na(+)/K(+)-ATPase.

Uncoupled ATPase activity and heat production by the sarcoplasmic reticulum Ca2+-ATPase. Regulation by ADP

Sarcoplasmic reticulum vesicles of rabbit skeletal muscle accumulate Ca2+ at the expense of ATP hydrolysis. The heat released during the hydrolysis of each ATP molecule varies depending on whether or not a Ca2+ gradient is formed across the vesicle membrane. After Ca2+ accumulation, a part of the Ca2+-ATPase activity is not coupled with Ca2+ transport (Yu, X., and Inesi, G. (1995) J. Biol. Chem. 270, 4361-4367). I now show that both the heat produced during substrate hydrolysis and the uncoupled ATPase activity vary depending on the ADP/ATP ratio in the medium. With a low ratio, the Ca2+ transport is exothermic, and the formation of the gradient increases the amount of heat produced during the hydrolysis of each ATP molecule cleaved. With a high ADP/ATP ratio, the Ca2+ transport is endothermic, and formation of a gradient increased the amount of heat absorbed from the medium. Heat is absorbed from the medium when the Ca2+ efflux is coupled with the synthesis of ATP (5.7 kcal/mol of ATP). When there is no ATP synthesis, the Ca2+ efflux is exothermic (14-16 kcal/Ca2+ mol). It is concluded that in the presence of a low ADP concentration the uncoupled ATPase activity is the dominant route of heat production. With a high ADP/ATP ratio, the uncoupled ATPase activity is abolished, and the Ca2+ transport is endothermic. The possible correlation of these findings with thermogenesis and anoxia is discussed.

The effects of phenothiazines and other calmodulin antagonists on the sarcoplasmic and endoplasmic reticulum Ca(2+) pumps

The effects of a number of phenothiazines and other calmodulin antagonists on the Ca(2+)-ATPase activity of sarcoplasmic reticulum (SR) and endoplasmic reticulum (ER) were investigated. The drugs used in this study were trifluoperazine, calmidazolium, fluphenazine, chlorpromazine, W-7, and calmodulin-binding peptide. Our results showed that calmidazolium and calmodulin-binding peptide were the most potent inhibitors of skeletal muscle SR Ca(2+)-ATPase activity (isoform SERCA 1) (IC(50) values of 0.5 and 7 microM, respectively), while W-7 was the least potent inhibitor (IC(50), 125 microM). All of the antagonists had little effect on the cerebellar ER Ca(2+)-ATPase activity (isoform SERCA 2b), except for trifluoperazine, which had a biphasic effect, causing stimulation at low concentrations and inhibition at higher concentrations. Our results suggest that the effects of these calmodulin antagonists are independent of calmodulin and that they inhibit the Ca(2+)-ATPase in an isoform-specific manner. It was found that these antagonists inhibit the skeletal muscle isoform of the Ca(2+) pump by altering the Ca(2+) affinity and the associated Ca(2+)-binding steps, as well as possibly stabilising the E1 conformational state of the enzyme.

ATP synthesis and heat production during Ca(2+) efflux by sarcoplasmic reticulum Ca(2+)-ATPase

Vesicles derived from the sarcoplasmic reticulum of rabbit white skeletal muscle were loaded with Ca(2+) and used to measure the rates of Ca(2+) efflux, heat production, and ATP synthesis from ADP and P(i). It was found that the Ca(2+)-ATPase can function in three different forms: (i) it absorbs heat from the medium (5 Kcal/mol Ca(2+)) when the efflux was coupled with ATP synthesis; (ii) it converts the energy derived from the gradient into heat (30 Kcal/mol Ca(2+)) when Mg(2+) is removed from the medium and the synthesis of ATP is impaired; and (iii) the ATPase becomes uncoupled when the different ligands of the enzyme are removed from the medium. As a result, there is no ATP synthesis and no heat production or absorption during Ca(2+) efflux. The Ca(2+) efflux, ATP synthesis, and heat production were inhibited by thapsigargin, a specific inhibitor of the Ca(2+)-ATPase.

Energy interconversion by the sarcoplasmic reticulum Ca2+-ATPase: ATP hydrolysis, Ca2+ transport, ATP synthesis and heat production

The sarcoplasmic reticulum of skeletal muscle retains a membrane bound Ca2+-ATPase which is able to interconvert different forms of energy. A part of the chemical energy released during ATP hydrolysis is converted into heat and in the bibliography it is assumed that the amount of heat produced during the hydrolysis of an ATP molecule is always the same, as if the energy released during ATP cleavage were divided in two non-interchangeable parts: one would be converted into heat, and the other used for Ca2+ transport. Data obtained in our laboratory during the past three years indicate that the amount of heat released during the hydrolysis of ATP may vary between 7 and 32 Kcal/mol depending on whether or not a transmembrane Ca2+ gradient is formed across the sarcoplasmic reticulum membrane. Drugs such as heparin and dimethyl sulfoxide are able to modify the fraction of the chemical energy released during ATP hydrolysis which is used for Ca2+ transport and the fraction which is dissipated in the surrounding medium as heat.

Vanadate oligomer inhibition of passive and active Ca2+ translocation by the Ca2+ pump of sarcoplasmic reticulum

'Monovanadate' containing mainly monomeric, dimeric and tetrameric vanadate species or 'decavanadate', containing mainly decameric vanadate species inhibits the passive and the active efflux of Ca2+ through the sarcoplasmic reticulum calcium pump. When the efflux of Ca2+ by sarcoplasmic reticulum vesicles is not associated with ATP synthesis both vanadate solutions inhibit the passive efflux of Ca2+. However, only 'decavanadate' exerts noticeable effects when the efflux of Ca2+ is associated with ATP synthesis being the active efflux of Ca2+ almost completely inhibited by decameric species concentration as low as 40 microM.

Insight into the uncoupling mechanism of sarcoplasmic reticulum ATPase using the phosphorylating substrate UTP

Ca(2+) transport and UTP hydrolysis catalyzed by sarcoplasmic reticulum Ca(2+)-ATPase from skeletal muscle was studied. A passive Ca(2+) load inside microsomal vesicles clearly decreased the net uptake rate and the final accumulation of Ca(2+) but not the UTP hydrolysis rate, causing energy uncoupling. In the absence of passive leak, the Ca(2+)/P(i) coupling ratio was 0.7-0.8. UTP hydrolysis did not maintain a rapid component of Ca(2+) exchange between the cytoplasmic and lumenal compartments as occurs with ATP. The uncoupling process in the presence of UTP is associated with: (i) the absence of a steady state accumulation of ADP-insensitive phosphoenzyme; (ii) the cytoplasmic dissociation of Ca(2+) bound to the ADP-sensitive phosphoenzyme; and (iii) the absence of enzyme inhibition by cyclopiazonic acid. All these characteristics confirm the lack of enzyme conformations with low Ca(2+) affinity and point to the existence of an uncoupling mechanism mediated by a phosphorylated form of the enzyme. Suboptimal coupling values can be explained in molecular terms by the proposed functional model.

Ca(2+) release and heat production by the endoplasmic reticulum Ca(2+)-ATPase of blood platelets. Effect of the platelet activating factor

Different sarco/endoplasmic reticulum Ca(2+)-ATPases isoforms are found in blood platelets and in skeletal muscle. The amount of heat produced during ATP hydrolysis by vesicles derived from the endoplasmic reticulum of blood platelets was the same in the absence and presence of a transmembrane Ca(2+) gradient. Addition of platelets activating factor (PAF) to the medium promoted both a Ca(2+) efflux that was arrested by thapsigargin and an increase of the yield of heat produced during ATP hydrolysis. The calorimetric enthalpy of ATP hydrolysis (DeltaH(cal)) measured during Ca(2+) transport varied between -10 and -12 kcal/mol without PAF and between -20 and -24 kcal/mol with 4 microM PAF. Different from platelets, in skeletal muscle vesicles a thapsigargin-sensitive Ca(2+) efflux and a high heat production during ATP hydrolysis were measured without PAF and the DeltaH(cal) varied between -10 and -12 kcal/mol in the absence of Ca(2+) and between -22 up to -32 kcal/mol after formation of a transmembrane Ca(2+) gradient. PAF did not enhance the rate of thapsigargin-sensitive Ca(2+) efflux nor increase the yield of heat produced during ATP hydrolysis. These findings indicate that the platelets of Ca(2+)-ATPase isoforms are only able to convert osmotic energy into heat in the presence of PAF.

Uridine triphosphate-sensitive pathway of Ca2+ release from the sarcoplasmic reticulum of rat skeletal muscle fibers

The pyrimidine nucleotide, uridine triphosphate (UTP), was tested with skinned skeletal muscle fibers in order to investigate the UTP-sensitive pathway of Ca2+ release from the sarcoplasmic reticulum. The presence of ryanodine (200 microM), ruthenium red (10 microM) or heparin (2.5 mg/ml) did not affect the tension elicited in the presence of UTP, demonstrating that the UTP-induced Ca2+ release involved neither ryanodine nor inositol triphosphate-sensitive channels. Drugs such as compound 48/80 or cyclopiazonic acid used to inhibit Ca2+-ATPase in its reverse function appeared to be, respectively, non-specific or without any inhibitory effect on the tension induced by UTP. Finally, the UTP-induced tension as well as the trifluoperazine-induced tension were abolished in the presence of spermidine (50 mM), supporting the hypothesis that the UTP-sensitive pathway of the SR Ca2+ release might occur through the uncoupled calcium ATPase.

Oxidative damage to sarcoplasmic reticulum Ca2+-ATPase AT submicromolar iron concentrations: evidence for metal-catalyzed oxidation

The sarcoplasmic reticulum (SR) calcium ATPase carries out active Ca2+ pumping at the expense of ATP hydrolysis. We have previously described the inhibition of SR ATPase by oxidative stress induced by the Fenton reaction (Fe2+ + H2O2 --> HO. + HO- + Fe3+). Inhibition was not related to peroxidation of the SR membrane nor to oxidation of ATPase thiols, and involved fragmentation of the ATPase polypeptide chain. The present study aims at further characterizing the mechanism of inhibition of the Ca2+-ATPase by oxygen reactive species at Fe2+ concentrations possibly found in pathological conditions of iron overload. ATP hydrolysis by SR vesicles was inhibited in a dose-dependent manner by micromolar concentrations of Fe2+, H2O2, and ascorbate. Measuring the rate constants of inactivation (k inact) at different Fe2+ concentrations in the presence of saturating concentrations of H2O2 and ascorbate (100 microM each) revealed a saturation profile with half-maximal inactivation rate at ca. 2 microM Fe2+. Inhibition was not affected by addition of 200 microM Ca2+ to the medium, indicating that it was not related to iron binding to the high affinity Ca2+ binding sites in the ATPase. Furthermore, inhibition was not prevented by the water-soluble hydroxyl radical scavengers mannitol or dimethylsulfoxide, nor by butylated hydroxytoluene (a lipid peroxidation blocker) or dithiothreitol (DTT). However, when Cu2+ was used instead of Fe2+ in the Fenton reaction, ATPase inhibition could be prevented by DTT. We propose that functional impairment of the Ca2+-pump may be related to oxidative protein fragmentation mediated by site-specific Fe2+ binding at submicromolar or low micromolar concentrations, which may occur in pathological conditions of iron overload.

Control of heat production by the Ca2+-ATPase of rabbit and trout sarcoplasmic reticulum

The sarcoplasmic reticulum Ca2+-ATPase of rabbit skeletal muscle can convert the energy derived from a Ca2+ gradient into heat (L. de Meis, M. L. Bianconi, and V. A. Suzano. FEBS Lett. 406: 201-204, 1997). In this report, it is shown that this conversion varies depending on the temperature and on whether rabbit (endotherm) or trout (poikilotherm) sarcoplasmic reticulum vesicles are used. The gradient doubled the yield of heat produced during ATP hydrolysis and the calorimetric enthalpy of ATP hydrolysis (DeltaHcal) value found with both rabbit and trout varied between -10 and -12 kcal/mol in leaky vesicles (no gradient) and between -20 and -22 kcal/mol with intact vesicles (gradient). For the rabbit, the difference of DeltaHcal measured with and without gradient was detected in the range of 30-35 degrees C and disappeared when the temperature was decreased below 30 degrees C. For the trout, the difference was detected between 20 and 25 degrees C and disappeared below 20 degrees C. The effect of the gradient on the DeltaHcal for ATP hydrolysis was modified by DMSO, trifluoperazine, and heparin sodium.

Insulin receptor substrate proteins create a link between the tyrosine phosphorylation cascade and the Ca2+-ATPases in muscle and heart

Following phosphorylation by the insulin receptor kinase, the insulin receptor substrates (IRS)-1 and IRS-2 bind to and activate several Src homology 2 (SH2) domain proteins. To identify novel proteins that interact with IRS proteins in muscle, a human skeletal muscle cDNA expression library was created in the lambdaEXlox system and probed with baculovirus-produced and tyrosine-phosphorylated human IRS-1. One clone of the 10 clones which was positive through three rounds of screening represented the C terminus of the human homologue of the adult fast twitch skeletal muscle Ca2+-ATPase (SERCA1) including the cytoplasmic tail and part of transmembrane region 10. Western blot analysis of extracts of rat muscle demonstrated co-immunoprecipitation of both IRS-1 and IRS-2 with the skeletal muscle Ca2+-ATPase (SERCA1) and the cardiac muscle isoform (SERCA2). In both cases, injection of insulin stimulated a 2- to 6-fold increase in association of which was maximal within 5 min. In primary cultures of aortic smooth muscle cells and C2C12 cells, the insulin-stimulated interaction between IRS proteins and SERCA1 and -2 was dose-dependent with a maximum induction at 100 nM insulin. This interaction was confirmed in a "pull down" experiment using a glutathione S-transferase fusion protein containing the C terminus of the human SERCA isoform and phosphorylated IRS-1 in vitro and could be blocked by a FLVRES-like domain peptide present in the human SERCA sequence. Affinity chromatography of phosphopeptide libraries using the glutathione S-transferase fusion protein of the C terminus of SERCA1 indicated a consensus sequence for binding of XpYGSS; this is identical to potential tyrosine phosphorylation sites at position 431 of human IRS-1 and at position 500 of human IRS-2. In streptozotocin diabetic rats the interaction between IRS proteins and SERCA1 in skeletal muscle and SERCA2 in cardiac muscle was significantly reduced. Taken together, these results indicate that the IRS proteins bind to the Ca2+-ATPase of the sarcoplasmic reticulum in an insulin-regulated fashion, thus creating a potential link between the tyrosine phosphorylation cascade and effects of insulin on calcium.

Activation of sodium transport and intracellular sodium lowering by the neuroleptic drug chlorpromazine

Chlorpromazine (CPZ), a commonly used antipsychotic drug, at high concentration was found to reduce significantly the sodium content of both rat (Rattus norvegicus) and toad (Bufo marinus) liver cells. This reduction in intracellular sodium was demonstrated using 22Na+ flux and measurement of cell sodium content. The results suggest that the sodium-lowering effect of CPZ stemmed from a stimulation of sodium transport rather than from an inhibition of sodium influx (i.e., sodium channels), cell damage, or Na+:Na+ exchange. CPZ was found to interfere with the binding of ouabain to the sodium pump, although a simple reduction in sodium pump inhibition did not account for the sodium-lowering effect. CPZ was able to negate the effects of monensin, a sodium ionophore, suggesting a substantial capacity to activate sodium transport. The intracellular sodium-lowering action of CPZ through the activation of sodium transport represents a new property previously undescribed for this drug.

Control of energy fluxes by the sarcoplasmic reticulum Ca2+-ATPase: ATP hydrolysis, ATP synthesis and heat production

The experiments described indicate that heat is released when Ca2+ leaks through the Ca2+-ATPase of sarcoplasmic reticulum vesicles. In the presence of a transmembrane Ca2+ concentration gradient, agents that modify the amount of ATP synthesized from ADP and Pi also modify the amount of heat produced by the hydrolysis of each ATP molecule. Thus, in the presence of heparin, less ATP is synthesized and more heat is produced. Conversely, with dimethyl sulfoxide more ATP is synthesized and less heat is produced. The data indicate that between limits (-10 to -30 kcal/mol) the Ca2+-ATPase can regulate the interconversion of energy in such a way as to vary the fraction of energy derived from ATP hydrolysis which is converted into heat and that which is converted into other forms of energy.

Anticancer drugs, ionophoric peptides, and steroids as substrates of the yeast multidrug transporter Pdr5p

Pdr5p is the yeast Saccharomyces cerevisiae ATP-binding cassette transporter conferring resistance to several unrelated drugs. Its high overproduction in Pdr1p transcription factor mutants allows us to study the molecular mechanism of multidrug transport and substrate specificity. We have developed new in vivo and in vitro assays of Pdr5p-mediated drug transport. We show that in spite of little sequence homology, and inverted topology in respect to that of mammalian P-glycoproteins, Pdr5p shares with them common substrates. Pdr5p extrudes rhodamines 6G and 123, from intact yeast cells in an energy-dependent manner. Plasma membrane preparations from a Pdr5p-overproducing strain exhibit ATP hydrolysis-dependent, osmotically sensitive rhodamine 6G fluorescence quenching. The quenching is competitively inhibited by micromolar concentrations of many anticancer drugs, such as vinblastine, vincristine, taxol, and verapamil, and of ionophoric peptides as well as steroids. In contrast, other anticancer drugs, like colchicine and some multidrug resistance modifiers, such as quinidine, exert noncompetitive inhibition. Our experimental system opens new possibilities for the analysis of structure-function relationship of multidrug transporter substrates and inhibitors.

Effect of haloperidol on the sarcoplasmic reticulum Ca-dependent adenosine triphosphatase

Several effects of the neuroleptic agent haloperidol on the sarcoplasmic reticulum (SR) Ca-dependent adenosine triphosphatase (Ca-ATPase) and Ca transport are described. Haloperidol inhibits the Ca-ATPase activity in the presence of calcimycin. The effect depends on the conditions of preexposure of the membranes to the drug: the inhibition increases with the preincubation time; Ca and Mg protect the enzyme against the effect of the drug. The inhibitory effect of haloperidol decreases upon increasing [Ca2+], at constant [Mg], and disappears at 20 mM [Mg] for any [Ca2+], and at 0.5 mM [Ca2+] for any [Mg2+]. Haloperidol also inhibits phosphorylation of the enzyme by Pi, and ATP-dependent Ca2+ uptake, in both cases with apparent Ki = 0.10-0.15 mM, and increases the rate of Ca efflux from preloaded vesicles in this concentration range. The results suggest that haloperidol interacts with the catalytic site, interfering with the effect of the divalent catalytic cation, but not at other steps of the enzymatic cycle, where Mg2+ and Ca2+ are also activators. They are consistent with a reaction model where haloperidol interacts with the E2 conformers of the enzyme, with lower affinity for the phosphoenzyme than for the dephospho species. The inhibition of Ca uptake by SR vesicles is ascribed to an increased Ca2+ permeability rather than to the inhibition of the Ca-ATPase, which requires higher concentrations of the drug.

Site-directed mutagenesis studies of energy coupling in the sarcoplasmic reticulum Ca(2+)-ATPase

Site-directed mutagenesis studies identifying residues important to energy transduction in the sarcoplasmic reticulum Ca(2+)-ATPase are reviewed. Mutations blocking the crucial E1P to E2P transition are located in the small and the large cytoplasmic domains, in the stalk segment S4 linking transmembrane segment M4 with the catalytic site, as well as in transmembrane segments M4 and M8. Mutations that block the dephosphorylation of the E2P phosphoenzyme intermediate are located in transmembrane segments M4, M5, and M6, i.e., in the same domain as the Ca(2+)-binding sites. Removal of the sidechain of Tyr763 located at the boundary between transmembrane segment M5 and the corresponding stalk segment S5 linking M5 with the catalytic site leads to uncoupling of ATP hydrolysis from Ca2+ uptake. Uncoupling may be due to efflux through the Ca(2+)-ATPase of Ca2+ that has been transported, and may thus be caused by a defective gating process in the late part of the catalytic cycle. A nearby located residue Lys758 is also involved in energy coupling, since its substitution with Ile activates dephosphorylation at high pH and slows the E2 to E1 transition.

Oxidative damage to sarcoplasmic reticulum Ca(2+)-pump induced by Fe2+/H2O2/ascorbate is not mediated by lipid peroxidation or thiol oxidation and leads to protein fragmentation

The major protein in the sarcoplasmic reticulum (SR) membrane is the Ca2+ transporting ATPase which carries out active Ca2+ pumping at the expense of ATP hydrolysis. The aim of this work was to elucidate the mechanisms by which oxidative stress induced by Fenton's reaction (Fe(2+)+H2O2-->HO.+OH-+Fe3+) alters the function of SR. ATP hydrolysis by both SR vesicles (SRV) and purified ATPase was inhibited in a dose-dependent manner in the presence of 0-1.5 mM H2O2 plus 50 microM Fe2+ and 6 mM ascorbate. Ca2+ uptake carried out by the Ca(2+)-ATPase in SRV was also inhibited in parallel. The inhibition of hydrolysis and Ca2+ uptake was not prevented by butylhydroxytoluene (BHT) at concentrations which significantly blocked formation of thiobarbituric acid-reactive substances (TBARS), suggesting that inhibition of the ATPase was not due to lipid peroxidation of the SR membrane. In addition, dithiothreitol (DTT) did not prevent inhibition of either ATPase activity or Ca2+ uptake, suggesting that inhibition was not related to oxidation of ATPase thiols. The passive efflux of 45Ca2+ from pre-loaded SR vesicles was greatly increased by oxidative stress and this effect could be only partially prevented (ca 20%) by addition of BHT or DTT. Trifluoperazine (which specifically binds to the Ca(2+)-ATPase, causing conformational changes in the enzyme) fully protected the ATPase activity against oxidative damage. These results suggest that the alterations in function observed upon oxidation of SRV are mainly due to direct effects on the Ca(2+)-ATPase. Electrophoretic analysis of oxidized Ca(2+)-ATPase revealed a decrease in intensity of the silver-stained 110 kDa Ca(2+)-ATPase band and the appearance of low molecular weight peptides (MW < 100 kDa) and high molecular weight protein aggregates. Presence of DTT during oxidation prevented the appearance of protein aggregates and caused a simultaneous increase in the amount of low molecular weight peptides. We propose that impairment of function of the Ca(2+)-pump may be related to aminoacid oxidation and fragmentation of the protein.

Short and long range functions of amino acids in the transmembrane region of the sarcoplasmic reticulum ATPase. A mutational study

Mutational analysis of several amino acids in the transmembrane region of the sarcoplasmic reticulum ATPase was performed by expressing wild type ATPase and 32 site-directed mutants in COS-1 cells followed by functional characterization of the microsomal fraction. Four different phenotype characteristics were observed in the mutants: (a) functions similar to those sustained by the wild type ATPase; (b) Ca2+ transport inhibited to a greater extent than ATPase hydrolytic activity; (c) inhibition of transport and hydrolytic activity in the presence of high levels of phosphorylated enzyme intermediate; and (d) total inhibition of ATP utilization by the enzyme while retaining the ability to form phosphoenzyme by utilization of P(i). Analysis of experimental observations and molecular models revealed short and long range functions of several amino acids within the transmembrane region. Short range functions include: (a) direct involvement of five amino acids in Ca2+ binding within a channel formed by clustered transmembrane helices M4, M5, M6, and M8; (b) roles of several amino acids in structural stabilization of the helical cluster for optimal channel function; and (c) a specific role of Lys297 in sealing the distal end of the channel, suggesting that the M4 helix rotates to allow vectorial flux of Ca2+ upon enzyme phosphorylation. Long range functions are related to the influence of several transmembrane amino acids on phosphorylation reactions with ATP or P(i), transmitted to the extramembranous region of the ATPase in the presence or in the absence of Ca2+.

Two simultaneous binding sites for nucleotide analogs are kinetically distinguishable on the sarcoplasmic reticulum Ca(2+)-ATPase

Erythrosin B and eosin Y stimulate p-nitrophenyl phosphate hydrolysis by purified sarcoplasmic reticulum Ca(2+)-ATPase by nearly 2-3 fold in the presence of Ca(2+). This stimulation is not due to the change on the apparent affinity for substrate but is indeed due to acceleration of the turnover rate of the enzyme. Stimulation reaches a maximum at approximately 5 microM erythrosin or 20 microM eosin and is strictly dependent on the presence of Ca(2+) in reaction media, while higher concentrations of dye progressively inhibit phosphatase activity. Labeling with fluorescein isothiocyanate (FITC) largely shifts the Km for p-nitrophenyl phosphate (pNPP) and completely abolishes the stimulation of phosphatase activity induced by erythrosin in the presence of Ca(2+), apparently by FITC impairing dye binding to an activator site and allowing only manifestation of an inhibitory binding site. In the absence of Ca(2+), both erythrosin and eosin inhibit pNPP hyrolysis with Ic50 values 3-4 fold higher than the maximally stimulatory enzyme with FITC, which by its turn does not affect pNPPase activity in absence of Ca(2+). It is suggested that stimulation and inhibition of phosphatase activity are related to two simultaneous and physically different nucleotide analog binding sites.

Dissection of the functional domains of the sarcoplasmic reticulum Ca(2+)-ATPase by site-directed mutagenesis

The results of site-directed mutagenesis studies of the sarcoplasmic reticulum Ca(2+)-ATPase are reviewed. More than 250 different point mutants have been expressed in cell culture and analysed by a panel of functional assays. Thereby, 40-50 important amino acid residues have been pinpointed, and the mutants have been assigned to functional classes: the Ca(2+)-affinity mutants, the phosphorylation-negative mutants, the ATP-affinity mutants, the E1P mutants, the E2P mutants, and the uncoupled mutants. Moreover, regions important to the specific inhibition by thapsigargin have been identified by analysis of Ca(2+)-ATPase/Na+,K(+)-ATPase chimeric constructs.

Ligand-gated channel of the sarcoplasmic reticulum Ca2+ transport ATPase

In resting muscle, cytoplasmic Ca2+ concentration is maintained at a low level by active Ca2+ transport mediated by the Ca2+ ATPase from sarcoplasmic reticulum. The region of the protein that contains the catalytic site faces the cytoplasmic side of the membrane, while the transmembrane helices form a channel-like structure that allows Ca2+ translocation across the membrane. When the coupling between the catalytic and transport domains is lost, the ATPase mediates Ca2+ efflux as a Ca2+ channel. The Ca2+ efflux through the ATPase channel is activated by different hydrophobic drugs and is arrested by ligands and substrates of the ATPase at physiological pH. At acid pH, the inhibitory effect of cations is no longer observed. It is concluded that the Ca2+ efflux through the ATPase may be sufficiently fast to support physiological Ca2+ oscillations in skeletal muscle, that occur mainly in conditions of intracellular acidosis.

The Ca(2+)-ATPase isoforms of platelets are located in distinct functional Ca2+ pools and are uncoupled by a mechanism different from that of skeletal muscle Ca(2+)-ATPase

Vesicles derived from the dense tubular system of platelets possess a Ca(2+)-ATPase that can use either ATP or acetyl phosphate as a substrate. In the presence of phosphate as a precipitating anion, the maximum amount of Ca2+ accumulated by the vesicles with the use of acetyl phosphate was only one-third of that accumulated with the use of ATP. Vesicles derived from the sarcoplasmic reticulum of skeletal muscle accumulated equal amounts of Ca2+ regardless of the substrate used. When acetyl phosphate was used in platelet vesicles, the transport of Ca2+ was inhibited by Na+, Li+, and K+; in sarcoplasmic reticulum vesicles, only Na+ caused inhibition. When ATP was used as substrate, the different monovalent cation had no effect on either sarcoplasmic reticulum or platelet vesicles. The catalytic cycle of the Ca(2+)-ATPase is reversed when a Ca2+ gradient is formed across the vesicle membrane. The stoichiometry between active Ca2+ efflux and ATP synthesis was one in platelet vesicles and two in sarcoplasmic reticulum vesicles. The coupling between ATP synthesis and Ca2+ efflux in sarcoplasmic reticulum vesicles was abolished by arsenate regardless of whether the vesicles were loaded with Ca2+ using acetyl phosphate or ATP. In platelets, uncoupling was observed only when the vesicles were loaded using acetyl phosphate. In both sarcoplasmic reticulum and platelet vesicles, the effect of arsenate was antagonized by thapsigargin (2 microM), micromolar Ca2+ concentrations, P(i) (5-20 mM), and MgATP (10-100 microM). Trifluoperazine also uncoupled the platelet Ca2+ pump but, different from arsenate, this drug was effective in vesicles that were loaded using either ATP or acetyl phosphate. Trifluoperazine enhanced Ca2+ efflux from both sarcoplasmic reticulum and platelet vesicles; thapsigargin, Ca2+, Mg2+, or K+ antagonized this effect in sarcoplasmic reticulum but not in platelet vesicles. The data indicate that the Ca(2+)-transport isoforms found in sarcoplasmic reticulum and in platelets have different kinetic properties.

Structure-function relationships of cation translocation by Ca(2+)- and Na+, K(+)-ATPases studied by site-directed mutagenesis

Site-directed mutagenesis studies of the sarcoplasmic reticulum Ca(2+)-ATPase have pinpointed five amino acid residues that are essential to Ca2+ occlusion, and these residues have been assigned to different parts of a Ca2+ binding pocket with channel-like structure. Three of the homologous Na+, K(+)-ATPase residues have been shown to be important for binding of cytoplasmic Na+ at transport sites. In addition, three of the above mentioned Ca(2+)-ATPase residues appear to participate in the countertransport of H+, and two of the Na+, K(+)-ATPase residues to participate in the countertransport of K+. Residues involved in energy transducing conformational changes have also been identified by mutagenesis. In the Ca(2+)-ATPase, ATP hydrolysis is uncoupled from Ca2+ transport following mutation of a tyrosine residue located at the top of transmembrane segment M5. This tyrosine, present also in the Na+, K(+)-ATPase, may play a critical role in closing the gate to a transmembrane channel.

Inhibition of human LAK-cell activity by the anti-depressant trifluoperazine

The anti-depressive drug trifluoperazine (TFP) was studied on in vitro immune responses. TFP proved to be an inhibitor of lymphokine-activated killer (LAK) cells in its generative step, as well as in its effector phase. Natural killer (NK) activity and interleukin-2 (IL-2) or mitogen-induced lymphocyte proliferation were just as sensitive to the drug effects, whereas the division of tumor cells was more resistant. The mechanism through which TFP suppresses these lymphocytic systems remains unclear. It does not, however, affect an early stage of cellular activation as the addition of the drug as late as 24 h after the start of the culture was still inhibitory for lymphocyte mitogenesis. Neither the expression of CD25, nor that of CD56 was affected by TFP, and exogenous IL-2 was unable to overcome the suppression of proliferation. In relation to cell-mediated cytotoxicity, TFP partially interfered with the effector/target binding. However, addition of lectin to the assay did not overcome the inhibition of lysis produced by the drug. Although further work remains to be done, the effect of TFP on immune responses must be taken into consideration when treating immunosuppressed patients.

Functional consequences of alterations to amino acids at the M5S5 boundary of the Ca(2+)-ATPase of sarcoplasmic reticulum. Mutation Tyr763-->Gly uncouples ATP hydrolysis from Ca2+ transport

The roles of the hydrophobic side chains of residues Phe760, Ile761, Tyr763, Leu764, and Ile765 located at the M5S5 boundary of the Ca(2+)-ATPase of sarcoplasmic reticulum were analyzed by site-directed mutagenesis. Substitution of Tyr763 with glycine resulted in a new phenotypic variant of the Ca(2+)-ATPase that catalyzed a high rate of Ca(2+)-activated ATP hydrolysis without net accumulation of Ca2+ in the microsomal vesicles. The ATPase activity of the Tyr763-->Gly mutant displayed characteristics similar to the ATPase activity of the wild-type enzyme measured in the presence of calcium ionophore, and the mutant was able to form the ADP-insensitive phosphoenzyme intermediate. Mutants Phe760-->Gly, Ile761-->Gly, Leu764-->Gly, and Ile765-->Gly were able to accumulate Ca2+. In mutants Leu764-->Gly and Ile765-->Gly, the turnover rate was low due to inhibition of dephosphorylation of the ADP-insensitive phosphoenzyme intermediate. On the other hand, mutant Leu764-->Lys dephosphorylated rapidly. Mutants Phe760-->Gly and Leu764-->Lys displayed apparent Ca2+ affinities that were reduced two and three orders of magnitude, respectively, relative to that of the wild-type.

Uncoupling of occlusion from ATP hydrolysis activity in sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase

The uncoupling of Ca2+ transport from ATP hydrolysis in the sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase by trypsin digestion was re-investigated by comparing ATPase activity with the ability of the enzyme to occlude Eu3+ (a transport parameter) after various tryptic digests. With this method, re-examination of uncoupling by tryptic digest of the ATPase revealed that TD2 cleavage (Arg-198) had no effect on either occlusion or ATPase activity. Digestion past TD2 in the presence of 5 mM Ca2+ and at 25 degrees C resulted in the loss of about 70% of the ATPase activity, but no loss of occlusion. Digestion past TD2 in the presence of 5 mM Ca2+, 3 mM ATP, and at 25 degrees C resulted in a partially uncoupled enzyme complex which retained about 50% of the ATPase activity, but completely lost the ability to occlude Eu3+. Digest past TD2 in the presence of 5 mM Ca2+ and 3 mM AMP-PNP (a non-hydrolyzable ATP analog) at 25 degrees C resulted in no loss of occlusion, thus revealing the absolute requirement of ATP during the digest to eliminate occlusion. From these findings we conclude that uncoupling of Ca2+ transport from ATPase activity is possible by tryptic digestion of the (Ca2+ + Mg2+)-ATPase. Interestingly, only after phosphorylation of the enzyme do the susceptible bond(s) which lead to the loss of occlusion become exposed to trypsin.

Refilling the inositol 1,4,5-trisphosphate-sensitive Ca2+ store in neuroblastoma x glioma hybrid NG108-15 cells

Bradykinin-induced increases in the intracellular free Ca2+ concentration ([Ca2+]i) were recorded in single NG108-15 cells with indo-1-based dual-emission microfluorimetry (50% effective concentration, 16 nM). A 1-min exposure to 30 nM bradykinin completely depleted the inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ store; refilling the store required extracellular Ca2+ (half time, 2 min). Refilling the IP3-sensitive store was completely blocked by 1 microM La3+ and 10 microM nitrendipine, but not 10 microM verapamil, 10 microM flunarizine, 1 microM nitrendipine, or 0.1 microM La3+. Thapsigargin irreversibly depleted the Ca2+ store and prevented its refilling (half-maximal inhibitory concentration, 3 nM). Influx of Ca2+ across the plasma membrane did not increase after depletion of the IP3-sensitive store by exposure to bradykinin, although maintained presence of the agonist produced significant Ca2+ influx. Similarly, Mn2+ and Ba2+ influx, as measured by indo-1 quenching and spectral shifts, did not increase following depletion of IP3-sensitive store. In contrast to depletion of the IP3-sensitive Ca2+ store by bradykinin, thapsigargin (10 nM) treatment produced Ca2+ and Ba2+ influx. We conclude that after Ca2+ mobilization, the IP3-sensitive Ca2+ store in NG108-15 cells is refilled with cytoplasmic Ca2+ via a thapsigargin-sensitive Ca(2+)-Mg(2+)-ATPase. Cytoplasmic Ca2+ is replenished by a persistent leak of Ca2+ across the plasma membrane. This leak is not modulated by the status of the intracellular Ca2+ store. In NG108-15 cells, agonist and thapsigargin-evoked Ca2+ entry are mediated by activation of plasmalemmal Ca2+ channels independent of the status of the IP3-sensitive intracellular Ca2+ store.

The enhancement of Ca2+ efflux from sarcoplasmic reticulum vesicles by urea

Urea, in nondenaturing concentrations, inhibited Ca2+ uptake by sarcoplasmic reticulum vesicles with no concomitant effect on ATP hydrolysis. This inhibition was antagonized by 5 mM oxalate and 20 mM orthophosphate. At concentrations of 0.2 to 1.0 M, urea induced an increase in the Ca2+ efflux from preloaded vesicles diluted in a medium at pH 7.0 containing 2 mM ethylene glycol bis(beta-aminoethyl ether)N,N'-tetraacetic acid, 0.1 mM orthophosphate, and 0.1 mM MgCl2. The urea-induced efflux was arrested by ligands of the (Ca(2+)-Mg2+) ATPase, namely, K+, Mg2+, Ca2+, and ADP, and by ruthenium red and the polyamines spermine, spermidine, and putrescine. In the case of polyamines a dissociation between the effect on the efflux and the net Ca2+ uptake was observed, as only the efflux could be blocked by the drugs. Glycine betaine, trimethylamine-N-oxide, and sucrose antagonized the effects of urea on both the net Ca2+ uptake and the rate of Ca2+ efflux.

Calcium transport by sarcoplasmic reticulum of vascular smooth muscle: II. Effects of calmodulin and calmodulin inhibitors

The role of calmodulin (CaM) in modulating calcium (Ca) uptake by sarcoplasmic reticulum (SR) of vascular smooth muscle was studied in saponin skinned strips of rat caudal artery. Exogenous CaM concentrations ranging from 0.3-1.8 microM did not statistically change the steady state MgATP-dependent Ca content, the MgATP-independent Ca content, or the oxalate-stimulated Ca influx. Calmidazolium (CDZ), W-7, and trifluoperazine (TFP) were used to examine the potential effect of an endogenous CaM pool on inward Ca transport. The IC50 of these antagonists for inhibition of Ca-CaM-stimulated phosphodiesterase activity and Ca-activated superprecipitation of canine aortic actomyosin was measured and found to be in the low micromolar range with a rank order of potency for inhibition of CDZ greater than TFP greater than W-7. In skinned tissues, micromolar concentrations of antagonists that inhibited CaM-mediated reactions in isolated enzyme systems did not reduce Ca content or oxalate-stimulated Ca influx. At higher concentrations of 100-200 microM, the MgATP-dependent Ca content was significantly reduced by TFP and W-7 but not by CDZ. The order of potency for inhibition of Ca uptake was TFP greater than W-7 greater than CDZ. The MgATP-independent Ca content was significantly decreased only by 200 microM TFP. Although none of these inhibitors significantly altered Ca efflux at concentrations up to 100 microM, Ca release was significantly stimulated by all three at 200 microM. The TFP-stimulated Ca release was partially inhibited by ruthenium red. The results indicate that neither exogenous CaM nor an endogenous CaM pool directly modulates inward Ca transport by the SR of saponin skinned caudal artery. The inhibition of Ca uptake produced by hundred micromolar concentrations of CaM antagonists fails to correlate with the order of and with the potency of inhibition measured in isolated enzyme systems. This suggests that the inhibition of Ca uptake produced by high concentrations of these antagonists may be independent of a specific interaction with CaM. The activation of Ca release by high concentrations of CaM antagonists may involve a nonspecific increase in membrane permeability as well as modulation of a membrane Ca channel.

Functional evidence of a transmembrane channel within the Ca2+ transport ATPase of sarcoplasmic reticulum

Ca2+ efflux can be studied conveniently following dilution of sarcoplasmic reticulum (SR) vesicles preloaded with 45Ca2+ by active transport. The rates of efflux are highly dependent on ATPase substrates and cofactors (Pi, Mg2+, Ca2+ and ADP) in the efflux medium. On the other hand, phenothiazines stimulate efflux through a passive permeability channel with no coupled catalytic events. Efflux activation by manipulation of catalytically active ATPase ligands, as well as by the catalytically inactive phenothiazines, can be prevented by thapsigargin, which is a highly specific inhibitor of the Ca(2+)-ATPase. This demonstrates that the passive channel activated by phenothiazines is an integral part of the ATPase, and can operate either uncoupled or coupled to catalytic events.