Biomedical subjects

L Missiaen

Publications and source records attributed to L Missiaen.

At least 127 records · Page 7Linked to original sources

PubMed1990

Uterine norepinephrine levels are correlated with contraction force, but not with the occurrence of preeclampsia.

The aim of the present experiments was to investigate a possible correlation between the occurrence of preeclampsia and the levels of norepinephrine in human myometrial biopsies obtained from the lower uterine segment at the time of cesarean section. Norepinephrine was measured directly by the technique of muscle CATS, and indirectly by measuring the effect of their release on the K(+)-induced force development of the myometrial strip. Two distinct types of force development depending on the level of endogenous norepinephrine were observed, but they were not correlated with the development of preeclampsia. This was confirmed by the absence of correlation between uterine norepinephrine levels and the occurrence of preeclampsia. Therefore, no correlation between myometrial catecholamine content and the development of preeclampsia could be demonstrated.

Biopsy↗

PubMed1989

Role of arginine residues in the stimulation of the smooth-muscle plasma-membrane Ca2+ pump by negatively charged phospholipids.

Negatively charged phospholipids strongly stimulate the purified plasma membrane Ca2+ pump of erythrocytes [Enyedi, Flura, Sarkadi, Gardos & Carafoli (1987) J. Biol. Chem. 262, 6425-6430] and of smooth muscle [Missiaen, Raeymaekers, Wuytack, Vrolix, De Smedt & Casteels, (1989) Biochem. J. 263, 687-694]. We have investigated the role of arginine residues in the interaction of these acidic phospholipids with the smooth-muscle Ca2+ transport ATPase. The arginine-modifying reagent phenylglyoxal inhiibited the ATPase activity in a time-dependent fashion by decreasing the Vmax. of the Ca2(+)-activation curve. Low concentrations of PtdIns, PtdIns4P, PtdIns(4,5) P2, phosphatidylserine and phosphatidic acid partially prevented this inactivation. This protective effect was however not apparent at higher concentrations of PtdIns4P, PtdIns(4,5) P2 and phosphatidic acid, which may be related to the previously observed inhibition of the enzyme at higher concentrations of these phospholipids. These findings indicate that the functionally important interaction of the acidic lipids with the protein occurs at least partially via arginine residue(s).

Aldehydes↗

PubMed1989

Phospholipid-protein interactions of the plasma-membrane Ca2+-transporting ATPase. Evidence for a tissue-dependent functional difference.

The aim of the present work was to investigate the stimulation of the plasma-membrane Ca2+-transporting ATPase by negatively charged phospholipids. The Ca2+-transporting ATPase was purified from pig stomach smooth muscle and from pig erythrocytes, and was reactivated with phosphatidylcholine (PC) in the presence and absence of negatively charged phospholipids. The substitution of phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PIP), phosphatidylinositol 4,5-bisphosphate (PIP2), phosphatidic acid (PA) or phosphatidylserine (PS) for PC induced profound changes in the Vmax, the K0.5 and the Hill coefficient of the Ca2+-activation curves for both ATPases. Low concentrations of each of the negatively charged phospholipids increased the Vmax., but high ratios of PIP, PIP2 or PA to PC decreased this parameter. PI, PA and PS increased the Vmax. of the erythrocyte enzyme to a larger extent than that of the smooth-muscle enzyme. This difference was less pronounced for PIP and absent for PIP2. PI (greater than 20% PC substituted), PIP, PIP2, PA and PS all increased the affinity of the two Ca2+-transporting ATPases for Ca2+ in the following order of potency: PIP2 greater than PIP greater than PI approximately PS approximately PA. PI, PA and PS increased the Ca2+ affinity of the smooth-muscle enzyme more than that of the erythrocyte enzyme; this difference was less pronounced for PIP and absent for PIP2. Even in the presence of calmodulin, all of the negatively charged phospholipids were still able to increase the Vmax. of the erythrocyte enzyme, whereas only PIP and PIP2 increased the affinity for Ca2+. The effect of PI at low concentrations (less than 20%) on the erythrocyte enzyme was peculiar in that it caused a decrease in the Ca2+ affinity instead of an increase. This effect was not observed for the smooth-muscle enzyme. All of the negatively charged phospholipids slightly increased the Hill coefficient for Ca2+ of both ATPases, and this effect was additive to that of calmodulin. The stimulation of the erythrocyte enzyme exhibited positive co-operativity towards PI and PIP, whereas that of the smooth-muscle enzyme did not. It is concluded (1) that there is a correlation between the number of negative charges on the phospholipids (PIP2 greater than PIP greater than PA approximately PI approximately PS) and the magnitude of their effect on the Vmax. and the K0.5 for Ca2+, and (2) that the action of the lipids on the smooth-muscle enzyme differs from that on the erythrocyte enzyme, indicating that these two Ca2+-transporting ATPases are not the same.

Animals↗

PubMed1989

cDNA cloning and sequencing of phospholamban from pig stomach smooth muscle.

Phospholamban cDNA from pig stomach smooth muscle was cloned and sequenced. The 737-nucleotide-residue cDNA contained an open reading frame of 156 nucleotide residues encoding a peptide of 52 amino acid residues (Mr 6080). This peptide shares 100% sequence identity with dog cardiac-muscle phospholamban. It differs from rabbit cardiac-muscle and slow-twitch skeletal-muscle phospholamban only at position 2, which is a glutamic acid residue in rabbit phospholamban, but an aspartic acid residue in the pig smooth-muscle protein. Northern-blot analysis reveals the presence of several phospholamban mRNAs in smooth muscle, but a 900-nucleotide-residue and a 2800-nucleotide-residue transcript predominate.

Adenosine Triphosphatases↗

PubMed1989

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

The finding that negatively charged phospholipids activate the plasma-membrane (Ca2+ + Mg2+)-ATPase and that polycations counteract this stimulation suggest that negative charges in the environment of the ATPase protein could be important for its function. The aim of the present work was to investigate whether changing the charges on the ATPase protein itself by modifying the pH within the physiological range affects the activity of the purified plasma-membrane Ca2+ pump from stomach smooth muscle. Increasing the pH from 6.9 to 7.4 and using 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid (BAPTA) as a Ca2+ buffer, doubled the ATPase activity at 0.3 microM-Ca2+ in the presence of 100% phosphatidylcholine (PC) or after substituting 20% of the PC by negatively charged phospholipids PtdIns, PtdIns4P, phosphatidylserine and phosphatidic acid. This stimulatory effect was due to an increased affinity of the enzyme for Ca2+, while the Vmax. remained unaffected. In the case of PtdIns(4,5)P2, a stimulatory effect upon alkalinization was only observed at a PtdIns(4,5)P2 concentration of 10%. When a concentration of 20% was used, alkalinization decreased the Vmax. and no stimulatory effect on the ATPase at 0.3 microM-Ca2+ could be observed. Alkalinization not only stimulated the purified Ca2+ pump, but it also increased the activity of the enzyme in a plasma-membrane-enriched fraction from stomach smooth muscle by a factor of 2.06. The ionophore A23187-induced Ca2+ uptake in closed inside-out vesicles also increased by a factor of 2.54 if the pH was changed from 6.9 to 7.4. This finding indicates that the effect of pH is most likely to be exerted at the cytoplasmic site of the Ca2+ pump protein.

Animals↗

PubMed1989

Polyamines and neomycin inhibit the purified plasma-membrane Ca2+ pump by interacting with associated polyphosphoinositides.

We investigated the effect of spermine, spermidine, putrescine and neomycin on the activity of the plasma-membrane Ca2+ pump and on its stimulation by negatively charged phospholipids and calmodulin. Millimolar concentrations of spermine and to a lesser extent of spermidine decreased the ATPase activity in the presence of phosphatidylinositol 4,5-bisphosphate (PIP2), without affecting the stimulation by phosphatidylinositol 4-phosphate (PIP). Sub-millimolar concentrations of neomycin inhibited the stimulation of the ATPase by PIP and by PIP2. Neomycin was more effective at the higher concentrations of PIP and PIP2. We discuss that these findings are compatible with the hypothesis that PIP and PIP2 bind to the ATPase and that several of these molecules have to be available to stimulate the ATPase.

Animals↗

PubMed1989

AIF4-induced inhibition of the ATPase activity, the Ca2+-transport activity and the phosphoprotein-intermediate formation of plasma-membrane and endo(sarco)plasmic-reticulum Ca2+-transport ATPases in different tissues. Evidence for a tissue-dependent functional difference.

AIF4- inhibits the (Ca2+ + Mg2+)-ATPase activity of the plasma-membrane and the sarcoplasmic-reticulum Ca2+-transport ATPase [Missiaen, Wuytack, De Smedt, Vrolix & Casteels (1988) Biochem. J. 253, 827-833]. The aim of the present work was to investigate this inhibition further. We now report that AIF4- inhibits not only the (Ca2+ + Mg2+)-ATPase activity, but also the ATP-dependent 45Ca2+ transport, and the formation of the phosphoprotein intermediate by these pumps. Mg2+ potentiated the effect of AIF4-, whereas K+ had no such effect. The plasma-membrane Ca2+-transport ATPase from erythrocytes was 20 times less sensitive to inhibition by AIF4- as compared with the Ca2+-transport ATPase from smooth muscle. The endoplasmic-reticulum Ca2+-transport ATPase from smooth muscle was inhibited to a greater extent than the sarcoplasmic-reticulum Ca2+-transport ATPase of slow and fast skeletal muscle.

Adenosine Triphosphatases↗

PubMed1989

Measurement of microsomal ATPase activities: a comparison between the inorganic phosphate-release assay and the NADH-coupled enzyme assay.

The specific activity of the Mg2+-ATPase and the (Ca2+ + Mg2+)-ATPase has been measured in a microsomal fraction from pig antral smooth muscle with the phosphate-release assay and the NADH-coupled enzyme assay, and the release of inorganic phosphate as a function of time is compared with the concomitant production of ADP. Both assays are found to overestimate the true Mg2+-ATPase activity. The adenylate kinase inhibitor P1,P5-di(adenosine-5'-)pentaphosphate (Ap5A) reduces the specific activity of the Mg2+-ATPase measured in the NADH-coupled enzyme assay to about half of its original value; however, it does not affect the specific activity of the Mg2+-ATPase in the Pi-release assay. The considerable overestimation of the Mg2+-ATPase activity in the NADH-coupled enzyme assay results from a combined action of an ATP pyrophosphatase (ATP in equilibrium AMP + PPi) and adenylate kinase activity contaminating the microsomes. The adenylate kinase activity in the microsomes catalyses the conversion of AMP formed by the ATP pyrophosphatase together with ATP into two ADP's. Also the phosphate-release assay is prone to an overestimation artefact because an inorganic pyrophosphatase will degrade the pyrophosphate and thus lead to additional Pi-production. Measurements of AMP and NAD+ production by HPLC confirmed our proposed reaction scheme. The same (Ca2+ + Mg2+)-ATPase activity is found in both assays, because the (Ca2+ + Mg2+)-ATPase activity is calculated from the difference in ATPase activity in the presence and absence of Ca2+, so that as a consequence the interfering activities are automatically subtracted.

Adenosine Diphosphate↗

PubMed1989

TPA- and agonist-induced force development in myometrium from pregnant and non-pregnant rats.

In myometrium from pregnant rats, 100 nM-TPA elevated resting tension and initially slightly enhanced the contraction induced by 138 mM-KCl. After 20 min this force development significantly declined. In saponin-treated skinned myometrial cells from pregnant rats, 100 nM-TPA enhanced the contraction induced by 0.3 microM-Ca2+, but reduced that induced by 1 microM-Ca2+. These findings suggest that the excitatory and inhibitory actions of TPA on the myometrium are probably due to its action on the contractile proteins. In myometrium from non-pregnant rats, TPA affected neither the resting tension, nor the amplitude of the evoked contractions, nor the Ca2+-induced contractions in skinned myometrium. While TPA only affected tension development in pregnant rats, both 1 mM-carbachol and 90 nM-oxytocin induced a tonic contraction in Ca-free solution independently of the hormonal status of the rats. The latter finding makes it unlikely that activation of protein kinase C is involved in the agonist-induced tonic force development in Ca-free solution.

Animals↗

PubMed1989

AlF-4 inhibits the accumulation of Ca in the endoplasmic reticulum in intact myometrial strips, but not in the rabbit ear artery.

AlF-4, known to be a potent modulator of G-proteins, also inhibits purified "P"-type cation-transport ATPases (Missiaen et al. 1988). The aim of the present work is to find out whether AlF-4 also inhibits these ATPases in intact cells. We therefore have studied the effect of AlF-4 on the force development and on 45Ca2+ -fluxes of muscle strips from rat myometrium and rabbit ear artery. 1 mM-NaF plus 10 microM-AlCl3 induces in both tissues a tonic contraction that is completely blocked by 0.5 mM-deferoxamine. The contractile response in myometrium exceeds that of the ear artery. These contractions are independent of an activation of G-proteins but their amplitude depends on [Ca2+]o. Application of AlF-4 during the loading of the endoplasmic reticulum of myometrium with Ca2+ dramatically reduces the amount of stored Ca2+ as estimated from its release induced by 1 mM-carbachol during incubation in Ca-free solution. This effect could be due to a predominant inhibitory effect of AlF-4 on the (Ca2+ + Mg2+)-ATPase of the endoplasmic reticulum. Such effect could not be established in rabbit ear artery. The increase of the fractional loss of 45Ca2+ induced by 10 microM-norepinephrine in rabbit ear artery is not inhibited by AlF-4. It is concluded that the inhibition of the endoplasmic-reticulum (Ca2+ + Mg2+)-ATPase in intact myometrial cells warns us against explaining all effects of AlF-4 on intact cells only by an activation of G-proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Aluminum↗

PubMed1988

AlF4- reversibly inhibits 'P'-type cation-transport ATPases, possibly by interacting with the phosphate-binding site of the ATPase.

The only known cellular action of AlF4- is to stimulate the G-proteins. The aim of the present work is to demonstrate that AlF4- also inhibits 'P'-type cation-transport ATPases. NaF plus AlCl3 completely and reversibly inhibits the activity of the purified (Na+ + K+)-ATPase (Na+- and K+-activated ATPase) and of the purified plasmalemmal (Ca2+ + Mg2+)-ATPase (Ca2+-stimulated and Mg2+-dependent ATPase). It partially inhibits the activity of the sarcoplasmic-reticulum (Ca2+ + Mg2+)-ATPase, whereas it does not affect the mitochondrial H+-transporting ATPase. The inhibitory substances are neither F- nor Al3+ but rather fluoroaluminate complexes. Because AlF4- still inhibits the ATPase in the presence of guanosine 5'-[beta-thio]diphosphate, and because guanosine 5'-[beta gamma-imido]triphosphate does not inhibit the ATPase, it is unlikely that the inhibition could be due to the activation of an unknown G-protein. The time course of inhibition and the concentrations of NaF and AlCl3 required for this inhibition differ for the different ATPases. AlF4- inhibits the (Na+ + K+)-ATPase and the plasmalemmal (Ca2+ + Mg2+)-ATPase noncompetitively with respect to ATP and to their respective cationic substrates, Na+ and Ca2+. AlF4- probably binds to the phosphate-binding site of the ATPase, as the Ki for inhibition of the (Na+ + K+)-ATPase and of the plasmalemmal (Ca2+ + Mg2+)-ATPase is shifted in the presence of respectively 5 and 50 mM-Pi to higher concentrations of NaF. Moreover, AlF4- inhibits the K+-activated p-nitrophenylphosphatase of the (Na+ + K+)-ATPase competitively with respect to p-nitrophenyl phosphate. This AlF4- -induced inhibition of 'P'-type cation-transport ATPases warns us against explaining all the effects of AlF4- on intact cells by an activation of G-proteins.

Adenosine Triphosphatases↗

PubMed1988

Effect of ovarian steroids on membrane ATPase activities in microsomes (microsomal fractions) from rat myometrium. Inhibition of a component of the Mg2+-activated ATPase by Ca2+-calmodulin and by oxytocin.

The activities of Mg2+-ATPase (Mg2+-activated ATPase), (Ca2+ + Mg2+)-activated ATPase and (Na+ + K+)-activated ATPase have been determined in microsomes (microsomal fractions) obtained from rat myometrium under different hormonal conditions. Animals were either ovariectomized and treated for a prolonged period of time with 17 beta-oestradiol or progesterone, or myometria were obtained at day 21 of pregnancy. In each case the endometrium was carefully removed. The Mg2+-ATPase consists of two components: an inactivating labile component and a second constant component. The rate of ATP hydrolysis by the labile component of the Mg2+-ATPase declines exponentially as a function of time after adding the membranes to the assay medium; this inactivation is caused by the presence of ATP in the medium. This ATPase activity inhibited by ATP is catalysed by a labile enzyme and hence it gradually diminishes within a few hours, even when the microsomes are kept on ice. This labile component has the highest activity in microsomes from pregnant rats, a lower activity in progesterone-treated rats, and the lowest in 17 beta-oestradiol-treated rats. This component of the Mg2+-ATPase is not affected by 90 nM-oxytocin. The constant component of the Mg2+-ATPase must be ascribed to a different enzyme, which, in contrast with the labile component, is very stable and not affected by the hormonal status of the animal. This constant component of the Mg2+-ATPase is inhibited both by Ca2+-calmodulin, and by oxytocin in microsomes from pregnant and from progesterone-treated animals, whereas such inhibition does not occur in microsomes from 17 beta-oestradiol-treated animals. The activity of the (Na+ + K+)-activated ATPase is not dependent on the hormonal status of the animal. Myometrial microsomes present an ATP-dependent Ca2+ transport, irrespective of the hormonal condition, but only in microsomes obtained from rats treated with 17 beta-oestradiol, can a (Ca2+ + Mg2+)-activated ATPase activity be demonstrated. This activity can be stimulated by calmodulin.

Adenosine Triphosphate↗

PubMed1988

Characterization of the Mg2+-activated ATPase activity in smooth-muscle membranes. NADH oxidase and adenylate kinase interfere with the NADH-coupled enzyme assay.

The apparent Mg2+-activated ATPase activity measured by the continuous NADH-coupled enzyme assay was studied in a number of microsomal preparations obtained from smooth muscle of the myometrium from pregnant or 17 beta-oestradiol-pretreated rats, the bovine aorta, the guinea-pig taenia coli, the rabbit ear artery and pig antrum. It was shown that this ATPase assay is prone to the effects of a number of artefacts that are tissue-dependent. The apparent Mg2+-ATPase activity in microsomes (microsomal fractions) from myometrium, aorta and taenia coli declines non-linearly during the assay. Its initial high rate gradually diminishes over 15-60 min, depending on the type of smooth muscle, to a constant value. This decline depends on the presence of ATP and can be partially prevented by concanavalin A. The non-linearity is limited in microsomes from rabbit ear artery. In microsomes from antrum the apparent Mg2+-ATPase activity actually increases with time, albeit gradually. Storage on ice of the microsomes of the aorta, and especially of myometrium of pregnant rats and of taenia coli, is accompanied over a few hours after their preparation by a gradual suppression of the component of the Mg2+-ATPase activity that is inhibited by ATP. The Mg2+-ATPase activity in microsomes from antrum remains constant. NADH oxidase activity accounts for 10% of the Mg2+-ATPase activity in microsomes from stomach smooth muscle. The apparent initial non-linearity of the Mg2+-ATPase activity in that tissue is due to a time-dependent decrease of a rotenone-sensitive NADH oxidase activity. The adenylate kinase activity, as deduced from the effect of the adenylate kinase inhibitor P1,P5-di(adenosine-5') pentaphosphate, could account for 45.0, 35.0 and 31.0% respectively of the Mg2+-ATPase activity in microsomes from stomach, myometrium and aorta. No adenylate kinase activity could be detected in microsomes from ear artery and taenia coli. When microsomes from stomach smooth muscle were separated on a sucrose gradient, the contribution of adenylate kinase and NADH oxidase to the Mg2+-ATPase activity was most pronounced in the higher-density fractions. Part of the NADH oxidase activity and of the Mg2+-ATPase activity, and most of the adenylate kinase activity, are not sedimented at 224000 gmax. for 30 min and may therefore be present as soluble enzymes.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenine Nucleotides↗

PubMed1988

Carbachol partially inhibits the plasma-membrane Ca2+-pump in microsomes from pig stomach smooth muscle.

A plasmalemmal enriched membrane fraction, prepared from pig stomach smooth-muscle, contains a calmodulin-stimulated (Ca2+ + Mg2+)-ATPase and presents an ATP-dependent 45Ca-uptake. If these smooth-muscle strips are preincubated with 10(-3) M-carbachol, this Ca2+ + Mg2+)-ATPase and the 45Ca-uptake are reduced by 21.4% and 13.5%, respectively, as compared to controls. This inhibitory effect of carbachol can be completely blocked by atropine. Carbachol does neither affect the passive permeability of the microsomes to 45Ca, nor the passive 45Ca-binding to the vesicles. Neither does it exert an effect on the proportion of closed inside-out plasma-membrane vesicles. Likewise, preincubation of rat myometrium with 90 nM-oxytocin induces a 20.4% inhibition of the ATP-dependent 45Ca-uptake, without having an effect on the passive 45Ca-binding, the permeability to 45Ca or the sideness of the vesicles. From these results, it is concluded that some agonists as carbachol and oxytocin induce a decrease in the activity of the plasmalemmal Ca2+-pump.

Animals↗

PubMed1988

Ryanodine reduces the amount of calcium in intracellular stores of smooth-muscle cells of the rabbit ear artery.

We have investigated the mechanism of action of ryanodine on intact and skinned smooth-muscle cells of the rabbit ear artery. The amplitude of the phasic response induced by low noradrenaline (NA) concentrations (less than 30 nM) was inhibited by 10 microM ryanodine, while that elicited by high NA concentrations (greater than 100 nM) was not affected. The phasic contractions induced by both low and high NA concentrations in Ca2+-free solution containing 2 mM EGTA were suppressed by 10 microM ryanodine. The rate of 45Ca efflux in Krebs solution was enhanced by 10 microM ryanodine, while the increased 45Ca efflux induced by 10 microM NA was inhibited by ryanodine. 10 microM ryanodine did not affect the contractile proteins in saponin-treated smooth-muscle cells. The intracellular Ca2+ stores of these skinned cells could be filled by exposing these cells to a solution containing 0.6 microM Ca2+. After a wash in a Ca2+-free solution, a contraction due to a release of the accumulated Ca2+ could be induced by either 25 mM caffeine or 20 microM inositol 1,4,5-trisphosphate (InsP3) or 10 microM A23187. These contractions did not occur if 10 microM ryanodine was present during Ca2+ loading. The addition of ryanodine during the Ca2+-free wash did not affect the subsequent force development. These observations indicate that ryanodine, in the presence of Ca2+, depletes the intracellular Ca2+ stores, and that this depletion is responsible for the inhibition of the component of the NA-induced contraction which depends on the release of Ca2+ from intracellular stores.

Alkaloids↗

PubMed1988

Properties of intracellular calcium stores in pregnant rat myometrium.

1. The properties of the Ca2+ stores in myometrium of 21-day pregnant rats were studied by recording the contractile responses of saponin-treated skinned muscles. 2. After accumulation of Ca2+ into the stores in the presence of 5 mM NaN3, inositol 1,4,5-trisphosphate (InsP3) at concentrations exceeding 3 microM produced a contraction. The amplitude of this contraction was maximal at about 20 microM. A second application of 20 microM InsP3 produced a smaller contraction than the first one. However after reloading the stores with Ca2+, 20 microM InsP3 produced a contraction of the same amplitude as the initial one. 3. After application of 20 microM InsP3, 1 microM A23187 still evoked a large contraction. If A23187 was applied first, the subsequent application of InsP3 or A23187 no longer induced a contraction, even after Ca2+ loading. 4. Guanosine triphosphate (GTP) or arachidonic acid, both 100 microM neither evoked a contraction nor enhanced the subsequent contraction elicited by 20 microM InsP3. 5. Caffeine 25 mM did not induce a contraction nor did it affect the contraction elicited by 20 microM InsP3. 6. The results indicate that in pregnant rat myometrium InsP3 releases Ca2+ from intracellular stores as has been proposed in vascular smooth muscles.

Animals↗

PubMedDate not supplied

Molecular physiology of the SERCA and SPCA pumps.

Intracellular Ca(2+)-transport ATPases exert a pivotal role in the endoplasmic reticulum and in the compartments of the cellular secretory pathway by maintaining a sufficiently high lumenal Ca(2+) (and Mn(2+)) concentration in these compartments required for an impressive number of vastly different cell functions. At the same time this lumenal Ca(2+) represents a store of releasable activator Ca(2+) controlling an equally impressive number of cytosolic functions. This review mainly focuses on the different Ca(2+)-transport ATPases found in the intracellular compartments of mainly animal non-muscle cells: the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) pumps. Although it is not our intention to treat the ATPases of the specialized sarcoplasmic reticulum in depth, we can hardly ignore the SERCA1 pump of fast-twitch skeletal muscle since its structure and function is by far the best understood and it can serve as a guide to understand the other members of the family. In a second part of this review we describe the relatively novel family of secretory pathway Ca(2+)/Mn(2+) ATPases (SPCA), which in eukaryotic cells are primarily found in the Golgi compartment.

Amino Acid Sequence↗

PubMedDate not supplied

Xestospongin C is an equally potent inhibitor of the inositol 1,4,5-trisphosphate receptor and the endoplasmic-reticulum Ca(2+) pumps.

Xestospongins, a group of macrocyclic bis-1-oxaquinolizidines isolated from the Australian sponge, Xestospongia species, are potent blockers of the inositol 1,4,5-trisphosphate (IP(3))-induced Ca2+ release in bi-directional Ca2+-flux conditions. We have now studied the effects of xestospongin C on the (45)Ca2+ uptake and the uni-directional (45)Ca2+ efflux in permeabilized A7r5 smooth-muscle cells. Xestospongin C not only inhibits the IP(3)-induced Ca2+ release, but is also an equally potent blocker of the endoplasmic-reticulum Ca2+ pump, while it has no effect on the passive Ca2+ leak. The inhibition of the IP(3) receptor did not depend on the IP(3), Ca2+ or ATP concentration. Xestospongin C can, therefore, not be considered as a selective blocker of IP(3) receptors.

Animals↗