Alkalinization stimulates the purified plasma-membrane Ca2+ pump by increasing its Ca2+ affinity

Involvement of non-selective Ca2+ channels in the contraction induced by alkalinization of rat anococcygeus muscle cells

Intracellular pH is a modulator of cellular functions such as smooth muscle contraction. Changes in cytosolic Ca(2+) concentration ([Ca(2+)](c)) associated with contraction are brought about by Ca(2+) influx and release from the sarcoplasmic reticulum, and alterations in the intracellular pH can affect both processes. In this work, therefore, we have investigated the Ca(2+) influx pathway that contributes to the contraction induced by the alkalinizing agent NH(4)Cl in the rat anococcygeus smooth muscle. For this purpose, we measured the isometric tension in muscle preparations, and [Ca(2+)](c) was measured on isolated cells loaded with 5 micromol/l FURA2/AM by using the ratio 340/380 nm. NH(4)Cl (10 mmol/l) induced a larger increase in [Ca(2+)](c) (100%) when compared with the [Ca(2+)](c) increase induced by 0.1 micromol/l phenylephrine (57.0+/-12.3% n=4). Incubation of the muscle preparations for 1 min in Ca(2+)-free medium reduced the contractions induced by 10 mmol/l NH(4)Cl to 11.5+/-5.1% (n=5), when compared with the contractions induced in 2.5 mmol/l Ca(2+) solution (100%). After 3 min in Ca(2+) free medium, contractions stimulated with NH(4)Cl were almost abolished (0.6+/-0.4%, n=5). In the same way, incubation with 10 micromol/l 1-[beta-[3[(4-methoxyphenyl)propoxyl]-4-methoxy-phenetyl]-1H-imidazole hydrochloride (SKF96365), a non-selective Ca(2+) channels, reduced the contractions stimulated with NH(4)Cl to 47.6+/-6.7% (n=7). On the other hand, 1 micromol/l verapamil, a voltage-operated Ca(2+) channel blocker and 0.05 micromol/l calphostin C, a protein kinase-C inhibitor, did not alter the contractions induced by NH(4)Cl. On isolated cells, [Ca(2+)](c) was reduced to 72.2+/-1.7% (n=4) by 10 micromol/l SKF96365. Taken together, our results suggest that NH(4)Cl induces contraction of rat anococcygeus smooth muscle cells, as well as [Ca(2+)](c) increase due to Ca(2+) influx through non-selective Ca(2+) channels.

Regulation of Intracellular Calcium Levels and Urokinase Activity in MDA MB 231 Cells by Quercetin

BACKGROUND

The common plant bioflavonoid, quercetin, is cytotoxic to various tumor cell lines, particularly breast cancer, by affecting the protein-kinase-C-dependent signal pathways and by cell cycle regulation. However, its role in breast cancer metastasis has not been studied so far. Increased uPA activity is evident in highly metastatic breast cancer, which is calcium dependent.

METHODS

MDA MB 231 cells were treated with various concentrations of quercetin (15-45 microg/ml). The cytotoxic effect of quercetin was assessed by MTT assay and DNA fragmentation analysis. Intracellular calcium levels were measured using Fura-2, a specific Ca2+ fluorescence indicator. Calcium uptake and release in cells treated with quercetin were measured using radioactive 45Ca2+. Urokinase enzyme activity was assayed by a casein zymogram.

RESULTS

Quercetin elicited dose- and time-dependent cytotoxicity as evidenced by the MTT assay. The maximum effect was observed at 48 h with a quercetin concentration of (45 microg/ml). DNA agarose gel electrophoresis showed dose-dependent DNA fragmentation on quercetin treatment. Quercetin caused significant depletion of cytosolic calcium levels and decreased calcium uptake from the intracellular stores. Casein zymogram showed a marked reduction of urokinase activity as evident by clear lysis bands on a dark background on treatment with quercetin.

CONCLUSION

Quercetin was found to exhibit cytotoxicity in the highly invasive breast cancer cell line MDA MB 231 in a dose- and time-dependent manner. Quercetin inhibited calcium dependent urokinase activity and hence may prove to be an effective antimetastatic agent.

Interactions between Ca(2+) and H(+) and functional consequences in vascular smooth muscle

Ca(2+) and H(+) ions can profoundly alter vascular tone. In many physiological and pathological processes, changes in the concentration of both ions occur. Thus, to understand the processes and mechanisms that modify force, it is necessary to understand what changes occur in these ions and, importantly, how they interact with each other. In this minireview, we highlight the quantitatively important mechanisms involved in the contractile responses of vascular tissues to pH change and discuss the cellular and molecular reasons underlying these responses.

A novel role of 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid as an activator of the phosphatase activity catalyzed by plasma membrane Ca2+-ATPase

The hydrolysis of p-nitrophenyl phosphate catalyzed by the erythrocyte membrane Ca2+-ATPase is stimulated by low concentrations of the compound 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), a classic inhibitor of anion transport. Enhancement of the phosphatase activity varies from 2- to 6-fold, depending on the Ca2+ and calmodulin concentrations used. Maximum stimulation of the pNPPase activity in ghosts is reached at 4-5 microM DIDS. Under the same conditions, but with ATP rather than pNPP as the substrate, the Ca2+-ATPase activity is strongly inhibited. Activation of pNPP hydrolysis by DIDS is equally effective for both ghosts and purified enzyme, and therefore is independent of its effect as an anion transport inhibitor. Binding of the activator does not change the Ca2+ dependence of the pNPPase activity. Stimulation is partially additive to the activation of the pNPPase activity elicited by calmodulin and appears to involve a strong affinity binding or covalent binding to sulfhydryl groups of the enzyme, since activation is reversed by addition of dithiothreitol but not by washing. The degree of activation of pNPP hydrolysis is greater at alkaline pH values. DIDS decreases the apparent affinity of the enzyme for pNPP whether in the presence of Ca2+ alone or Ca2+ and calmodulin or in the absence of Ca2+ (with 5 microM DIDS the observed Km shifts from 4.8 +/- 1.4 to 10.1 +/- 2.6, from 3.8 +/- 0.4 to 7.0 +/- 0.8, and from 9.3 +/- 0.7 to 15.5 +/- 1.1 mM, respectively). However, the pNPPase rate is always increased (as above, from 3.6 +/- 0.6 to 11.2 +/- 1.7, from 4.4 +/- 0.5 to 11.4 +/- 0.9, and from 2.6 +/- 0.6 to 18.6 +/- 3.9 nmol mg-1 min-1, in the presence of Ca2+ alone or Ca2+ and calmodulin or in the absence of Ca2+, respectively). ATP inhibits the pNPPase activity in the absence of Ca2+, both in the presence and in the absence of DIDS. Therefore, kinetic evidence indicates that DIDS does more than shift the enzyme to the E2 conformation. We propose that the transition from E2 to E1 is decreased and a new enzyme conformer, denoted E2*, is accumulated in the presence of DIDS.

Pseudosubstrate hydrolysis by the erythrocyte plasma membrane Ca(2+)-ATPase: kinetic evidence for a modified E1 conformation in dimethylsulfoxide

The purified Ca(2+)-ATPase of pig red cells displays a phosphatase activity towards p-nitrophenylphosphate which is inhibited by Ca2+ in the absence of solvents, and activated by calmodulin. This activity has been attributed to the E2 conformation of the enzyme. Here we show that the pNPPase activity in the absence of Ca2+ is stimulated 10-25-fold by the presence of the organic solvent dimethylsulfoxide (Me2SO). This is an activation that surpasses by severalfold that induced by calmodulin in the absence of the solvent. At 30% Me2SO, activation by calmodulin disappears. In the absence of calmodulin and at pH 7.2, the Ca2+ concentration needed for half-maximal inhibition of the pNPPase activity (K1) increases from 130 microM in the absence of Me2SO to 860 microM at 30% Me2SO. This effect of Me2SO is enhanced at pH 8.0: the K for Ca2+ increases from 2.7 microM in the absence of the solvent to 2.0 mM in its presence. However, the K0.5 for Ca2+ activation of the ATPase activity decreases from 8.3 to 2.6 microM following addition of the same Me2SO concentration. This indicates that, even in the presence of Me2SO, microM Ca2+ concentrations shift the equilibrium towards E1 but the decrease in activity that would be expected if pNPP hydrolysis were catalysed exclusively by the E2 conformation is not observed. The affinity for pNPP as a substrate increases from 2.6 mM in the absence of Me2SO to 1.6 mM in the presence of 20% Me2SO. These results suggest that Me2SO induces multiple effects in the Ca(2+)-ATPase that (i) increase the reactivity of E2 towards substrate: (ii) surpass the activation by calmodulin and, (iii) allow the enzyme to hydrolyze pNPP even when Ca2+ is bound to the high-affinity sites of the enzyme. The change in reactivity is attributed to an increase on substrate catalysis rather than on pNPP binding.

The effects of ketamine on Ca2+ movements in A7r5 vascular smooth muscle cells

To investigate the effects of ketamine on Ca2+ movement to and from intracellular Ca2+ stores and across plasma membranes, 45Ca2+ fluxes were studied in permeabilized and intact A7r5 smooth muscle cells, an established cell line derived from embryonic rat aorta. Monolayers of A7r5 cells were loaded with 45Ca2+, and the radioactivity in the collected medium and the residual activity were measured by liquid scintillation counting. Ketamine had no effect on 45Ca2+ uptake and passive leak of the nonmitochondrial Ca2+ pool in permeabilized A7r5 cells. Ketamine 1 mM had no inhibitory effect on the inositol 1,4,5-trisphosphate (InsP3, 1 microM)-induced Ca2+ release from the intracellular stores. In intact A7r5 cells, ketamine did not alter the Ca2+ extrusion from these cells under resting conditions. Addition of 10 nM vasopressin resulted in a transient Ca2+ release from the intracellular stores. Ketamine inhibited this vasopressin-induced Ca2+ release, but did not enhance Ca2+ extrusion through the plasma membrane in the period after the vasopressin effect. These results indicate that ketamine inhibits agonist-induced Ca2+ release from intracellular stores, but has no effect on Ca(2+)-uptake into intracellular stores or on Ca2+ extrusion through the plasma membrane in A7r5 smooth muscle cells.

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.

Regulation of Ca2+ transport by platelet-derived growth factor-BB in rat vascular smooth muscle cells

The present study investigates the effects of platelet-derived growth factor (PDGF) isoform BB (PDGF-BB) on cytosolic Ca2+ concentration ([Ca2+]i), Ca2+ transport, and Ca2+ pools in rat vascular smooth muscle (VSM) cells. VSM cells from thoracic aorta of Milan normotensive rats were enzymatically dispersed, cultured in 10% serum medium, and made quiescent by 72 hours in 0.3% serum medium. [Ca2+]i, Ca2+ influx, Ca2+ efflux, and exchangeable cell Ca2+ pool were evaluated by ratiometric fluorescent and radioisotope techniques. Ca2+ transport showed time-dependent changes during stimulation with PDGF-BB. The initial early responses to this peptide were transient rise in [Ca2+]i, a 30% decrease in Ca2+ influx, and a 3.6-fold increase in the rate constant for active Ca2+ efflux. Stimulation of Ca2+ efflux and inhibition of Ca2+ influx were associated with a substantial 30% reduction in the cell Ca2+ pool. This initial stimulation of Ca2+ efflux is concomitant with Ca2+ mobilization into the cytosol and is due to activation of Na(+)-independent Ca2+ efflux via the Ca2+ pump. After a 10-minute stimulation, Ca2+ influx returned to the basal value, whereas Ca2+ efflux remained 2.2-fold above control values, leading to a decline in [Ca2+]i below basal levels and a further decrease in the cell Ca2+ pool. Nearly half of this late Ca2+ efflux appears to be driven by Na(+)-Ca2+ exchange, as evidenced by its external Na+ dependence. After a 120-minute stimulation with PDGF-BB, nifedipine-sensitive Ca2+ influx is increased 37% above basal levels, and Ca2+ efflux remains elevated. During prolonged stimulation by PDGF-BB, both Ca2+ influx and efflux are stimulated, resulting in a new intracellular Ca2+ homeostasis marked by the recovery of the cell Ca2+ pool but a lowered [Ca2+]i. These final events coincide with the initiation of cell proliferation in VSM cells by PDGF-BB.

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.

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.

Conditions restricting depolarization-dependent calcium influx in synaptosomes reveal a graded response of P96 dephosphorylation and a transient dephosphorylation of P65

Temporal changes in the phosphorylation level of synaptosomal phosphoproteins following depolarization of synaptosomes were investigated under conditions restricting calcium influx. High-K+ depolarization in media of low [Na+]o (32 mM during preincubation and depolarization) at pH 6.5 resulted in a pronounced fall in the cytosolic free calcium concentration transient, and in a reduction in the initial K(+)-stimulated 45Ca2+ uptake and endogenous acetylcholine release relative to the values obtained with control synaptosomes (preincubated and depolarized in Na(+)-based media). This reduction was paralleled by a decrease in the rate of dephosphorylation of the synaptosomal protein P96. A slower dephosphorylation of P96 also was observed on exposure to 20 microM veratridine at 0.5 mM external calcium. Our results indicate that, similar to synapsin I phosphorylation, P96 dephosphorylation shows a graded response to the amount of calcium entering the presynaptic terminal. Depolarization of synaptosomes under conditions restricting the influx of calcium revealed a transient dephosphorylation (reversed within 10 s) of the phosphoprotein P65. The possible significance of this finding to the process of neurotransmitter release is discussed.

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.

Ruthenium red and compound 48/80 inhibit the smooth-muscle plasma-membrane Ca2+ pump via interaction with associated polyphosphoinositides

We will demonstrate the compound 48/80 and ruthenium red inhibit the smooth-muscle plasma-membrane Ca2+ pump by counteracting the stimulant effect of negatively charged phospholipids. Both substances did not affect the purified enzyme re-activated by pure phosphatidylcholine or phosphatidylinositol and measured in the absence of calmodulin, indicating that under these conditions they did not have a direct effect on the ATPase protein. Ruthenium red and compound 48/80 however inhibited the (Ca2(+) + Mg2+)-ATPase in the presence of phosphatidylinositol 4-phosphate and especially phosphatidylinositol 4,5-bisphosphate. The K0.5 for inhibition was 25 microM ruthenium red and 9 micrograms/ml of compound 48/80. The inhibition by ruthenium red developed slowly with half maximal inhibition occurring after about 75 s while that by compound 48/80 developed immediately within the time required for mixing. The efficacy of ruthenium red increased as the concentration of the acidic phospholipid increased, while no such cooperativity was observed for compound 48/80. Ruthenium red reduced the Vmax for Ca2+ without affecting the affinity for Ca2+, while compound 48/80 decreased both parameters. In conclusion, although ruthenium red and compound 48/80 affect the ATPase differently, both substances most likely inhibit the plasma-membrane Ca2+ pumping by counteracting the stimulation by negatively charged phospholipids.