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Results for “Calcium-Transporting ATPases”

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At least 883 records · Page 49Linked to original sources
PubMed1990

Ultrastructural localization of gangliosides, calcium and a high-affinity Ca(2+)-ATPase in nerve terminals: a contribution to the possible functional role of gangliosides.

By means of newly developed electron microscopical techniques (electron spectroscopic imaging, ESI; electron energy loss spectroscopy, EELS; immunogold labelling) a specific accumulation of endogenous calcium within the synaptic cleft and a distinct localization of a high-affinity Ca(2+)-ATPase at the inner sides of the pre- and postsynaptic membrane of nerve cells from fish brain have been demonstrated. Additionally, a differentiation-dependent expression of polysialoganglioside epitopes on the outer surface of nerve terminals in clustered arrangements was demonstrated using their ultracytochemical detection by means of the monoclonal antibody Q211. These results which are in agreement with parallel biochemical investigations on modulatory effects of exogenous gangliosides on a high-affinity Ca(2+)-ATPase in the CNS of vertebrates support our hypothesis that Ca(2+)-ganglioside complexes act as modulators for the processes of synaptic transmission and long-term neuronal adaptations.

Animals↗
PubMed1991

Analysis of calcium activated chloride current fluctuations in Xenopus laevis oocytes.

Fluctuations of calcium activated chloride currents were investigated in oocytes of Xenopus laevis. The method of noise analysis and the model of chloride channels activation by calcium ions were used to estimate the chloride channels lifetime and the average frequency of current fluctuations, which depends on changes of cytoplasmic calcium concentration. This current fluctuations can be evoked by activation of cholinergic receptors or inhibition by Na3VO4 of plasma membrane Ca(2+)-ATPase. The average opening lifetime of chloride channels was approximately 100 ms. The frequency of fluctuations increased with the increasing extracellular calcium concentrations and external ACh concentrations. Caffeine in 2 mmol/l concentration changed the current fluctuations into oscillations with a period of about 18-20s. Ten mmol/l caffeine fully inhibited the oscillation activity.

Action Potentials↗
PubMed1991

Further observations on pineal brain sand formation and evolution in man.

Acervuli obtained from fragments of the human pineal glands of subjects of both sexes and age ranging from 23 to 87 years were analyzed by light microscopy after histochemical stains and by EDS-microanalysis. It was found that the sub-units and acervuli are positive to P.A.S., Alcian Blue pH 2.5, Gomori-Bargmann procedures for the presence of proteins, glycoproteins, proteoglycans and of the neurosecretory material in different layers of the sub-units and acervuli. We suppose that the increase of Mg++ in the sub-unit from the core to the periphery is responsible for the inhibition of following hydroxyapatite deposition and for growth. We suggest that the presence of glycoproteins and proteoglycans can represent the aggregation factor for bindings between disulphide bonds and calcium.

Adult↗
PubMed1991

Effect of selenite on epithelium of cultured rabbit lens.

Selenite (Se) cataract in rabbit lenses was investigated in vitro to define target sites of Se that might be involved in calcium elevation and lens opacification. Experiments in which the anterior or the posterior surface of the lens was exposed to Se showed that anterior exposure led to ionic imbalances and opacification in the whole lens. Posterior exposure to Se (1 mM, 2 hr) had no effect. Se treatment (0.1 mM) of epithelial homogenates led to a 56% loss of thiol (SH) groups, and treatment of lenses cultured in Se led to a 22% loss. Experiments to assess the effects of Se on SH groups of Ca-ATPase showed that the transport enzyme was not affected by the poison. To determine whether this negative finding was due to the lack of accessibility of Se for SH sites in an ordered membrane, Ca-ATPase was also assayed in homogenate preparations treated with Se; still no inhibition of Ca-ATPase activity was observed. Therefore, an alternative explanation of calcium elevation was explored. The passive movement of labeled chloride (36Cl) was found to be twice as fast in Se-treated lenses as it was in control lenses. Measurement of the lens voltage indicated an 18-mV depolarization in Se-treated lenses, suggesting that Se increased membrane permeability. All cataractogenic changes that occurred after Se treatment were irreversible-despite intervention with external application of reduced glutathione or cysteine. This finding suggests that irreversible loss of SH groups in lens membranes is important in maintaining ion homeostasis.

Animals↗
PubMed1991

Depletion of cellular calcium accelerates protein degradation in the endoplasmic reticulum.

In this study the effects of A23187 and thapsigargin on the degradation of T-cell antigen receptor-beta (TCR-beta) and CD3-delta in the endoplasmic reticulum have been studied. Preliminary experiments showed that these drugs had different effects on the secretory pathway. Depletion of cellular calcium pools by incubation of cells with A23187 in calcium-free medium blocked transport between the endoplasmic reticulum and the Golgi apparatus whereas thapsigargin caused a modest increase in transport. When added to cells transfected with TCR-beta or CD3-delta the drugs caused an immediate stimulation of proteolysis of presynthesized protein and at maximum effective concentrations caused a 3-fold increase in the rate of degradation. They did not affect the lag period of 1 h which precedes degradation of newly synthesized proteins. Chelation of cytosolic calcium also accelerated degradation, suggesting that depletion of calcium from the endoplasmic reticulum was the main stimulus of proteolysis and that increased degradation was not caused by a transient increase in cytosolic calcium levels. The selectivity of degradation in the endoplasmic reticulum was maintained. A23187 had no effect on the stability of CD3-gamma nor co-transfected epsilon-beta dimers. Calcium depletion increased the overall rate of degradation in the endoplasmic reticulum and increased the rate of proteolysis of an "anchor minus" beta chain. The results suggested that proteolysis within the endoplasmic reticulum may be regulated by the high concentrations of Ca2+ which are stored in the organelle. Ca2+ may be required for protein folding. Calcium depletion may have caused the beta and delta chains to adopt a conformation that was more susceptible to proteolysis. Alternatively, calcium depletion may have disrupted the lumenal content of the endoplasmic reticulum and increased the access of proteases to potential substrates.

Animals↗
PubMed1991

Mechanism of inhibition of sarcoplasmic reticulum Ca2(+)-ATPase by active site cross-linking. Impairment of nucleotide binding slows nucleotide-dependent phosphoryl transfer, and loss of active site flexibility stabilizes occluded forms and blocks E2-P formation.

Investigation of the properties of Ca2(+)-ATPase of sarcoplasmic reticulum cross-linked at the active site with glutaraldehyde showed that ATP binding affinity and rate of ATP-dependent phosphorylation and Ca2+ occlusion were decreased 2-3 orders of magnitude compared with the native enzyme. Cross-linkage had little effect on or marginally increased the rate of acetyl phosphate- and p-nitrophenyl phosphate-supported Ca2+ occlusion. Ca2+ binding or Ca2(+)-induced changes in tryptophan fluorescence were unaffected. High levels of phosphoenzyme (up to 4 nmol/mg of protein) were obtained, with 2 mol of Ca2+ occluded/mol of E-P. Dephosphorylation and deocclusion occurred together at a slow rate (k = 0.01 s-1) and were stimulated in a monophasic manner up to 20-fold by ADP. Cross-linking inhibited E2-P formation from Pi in 30% (v/v) dimethyl sulfoxide by more than 95%. Induction of turnover of the native ATPase, under conditions designed to yield high steady state levels of E1 approximately P(2Ca), results in a 3-4-fold increase in reactivity of active site residues to glutaraldehyde. The results show that cross-linkage sterically impairs nucleotide binding, changing ATP and ADP into relatively poor substrates, slowing nucleotide-dependent phosphoryl transfer and Ca2+ occlusion and deocclusion. The forward reaction with smaller substrates is unaffected. Another major effect of the cross-link is to inhibit E2-P formation, causing accumulation of E1 approximately P(2Ca) during enzyme turnover and preventing phosphorylation by Pi in the reverse direction. We suggest that occlusion and deocclusion of cations at the transport site of the native enzyme are linked to a two-step cleft closure movement at the active site and that the crosslink stabilizes occluded forms of the pump because it blocks part of this tertiary structural change. The latter could normally be propagated through linking helices to the distal side of the pump to destabilize the cations and open the transport sites to the lumen.

Adenosine Triphosphate↗
PubMed1991

Interaction of potassium and magnesium with the high affinity calcium-binding sites of the sarcoplasmic reticulum calcium-ATPase.

The sarcoplasmic reticulum Ca2(+)-ATPase of skeletal muscle has two high affinity calcium sites, one of fast access ("f" site) and one of slow access ("s" site). In addition to Ca2+ these sites are able to interact with other cations like Mg2+ or K+. We have studied with a stopped-flow method the modifications produced by Mg2+ and K+ on the kinetics of the intrinsic fluorescence changes produced by Ca2+ binding to and dissociation from the Ca2(+)-ATPase of sarcoplasmic reticulum. The presence of Mg2+ ions (K1/2 = 0.5 mM at pH 7.2) leads to the appearance of a rapid phase in the Ca2+ binding, which represents half of the signal amplitude at optimal Mg2+. The presence of K+ greatly accelerates both the Ca2+ binding and the Ca2+ dissociation reactions, giving, respectively, a 4- and 8-fold increase of the rate constant of the induced fluorescence change. K+ ions also increase the rate of the 45Ca/40Ca exchange reaction at the s site measured by rapid filtration. These results lead us to build up a model for the Ca2(+)-binding mechanism of the sarcoplasmic reticulum Ca2(+)-ATPase in which Mg2+ and K+ participate at particular steps of the reaction. Moreover, we propose that, in the absence of Ca2+, this enzyme may be the pathway for monovalent ion fluxes across the sarcoplasmic reticulum membrane.

Animals↗
PubMedDate not supplied

[The immediate and late effects of ionizing radiation on the Ca2+-accumulating capacity of the plasma membranes of rat aorta smooth-muscle cells].

A study was made of Ca2(+)-accumulating ability of plasma membranes of nonstriated cells of rat aorta 3, 10, 30, 180 and 365 days after whole-body gamma irradiation with doses of 0.5, 1.0 and 4.0 Gy. The radiation doses used were shown to cause structural and functional modification of Ca2(+) pump which was a function of dose. The process under study was normalized during the first months following irradiation.

Animals↗
PubMed1991

Isoform switching of the sarco(endo)plasmic reticulum Ca2+ pump during differentiation of BC3H1 myoblasts.

We have studied the expression of the gene 2 for the sarco(endo)plasmic reticulum Ca2+ pump (SERCA2) in BC3H1 cells. Myogenic differentiation not only activated the SERCA2 expression but it also induced an isoform switch. Undifferentiated myoblasts only expressed the SERCA2b isoform (non-muscle) whereas differentiated myocytes predominantly contained the SERCA2a isoform (cardiac/slow skeletal muscle). The isoform switch was documented by immunoblot analysis with isoform-specific antibodies. This observation was confirmed at the mRNA level by using antisense RNA probes specific for class 1 (SERCA2a) or class 2 (SERCA2b) messengers. The expression of the SERCA2a isoform after differentiation was accompanied by a decreased sensitivity of the Ca2+ uptake in permeabilized cells to the Ca2+ pump inhibitor thapsigargin.

Animals↗
PubMed1977

Ca2+-activated ATPase of rat liver plasma membrane.

Rat liver plasma membranes hydrolyze ATP in the presence of Ca2+. The rate of hydrolysis is different when Mg2+ions are present in the incubation system. Several parameters differentiate Ca2+-ATPase from Mg2+-ATPase: a) the Km of ATP hydrolysis for Ca2+ (2.25 x 10(-4) M) is lower than for Mg2+ (2.14 x 10(-3) M); b) the shape of the activation curve is hyperbolic in the presence of Ca2+ and sigmoid in the presence of Mg2+; c) Mg2+-ATPase shows two different values of activation energy while Ca2+-ATPase presents only a single value; d) Ca2+-ATPase is inhibited, while Mg2+-ATPase is unaffected by cyclic AMP. Ca2+-ATPase is localized on the plasma membrane and is not inhibited by cysteine. It does not hydrolyze substrates different from nucleotides triphosphate, such as glucose-1-phosphate or alpha-glycero-phosphate. The enzyme is probably related to a mechanism of calcium transport.

Adenosine Triphosphate↗
PubMed1977

Studies on adenosine triphosphatase activity of rat cardiac myosin in isoproterenol-induced cardiac hypertrophy.

Ca2+- and K+ -activated ATPase activity of cardiac myosin from normal and hypertrophied rat hearts was investigated. Cardiac hypertrophy was induced by isoproterenol treatment. A nearly 40% increase in heart mass was seen after seven consecutive days of isoproterenol injection (5 mg/kg) as determined by either heart weight expressed as per cent of body weight or by dry heart weight and total protein content. The measurement of ATP hydrolysis revealed that cardiac myosin from isoproterenol-treated rats had a significant decrease (P less than 0.01) in Ca2+-activated ATPase activity at low ionic strength (0.05 M KCl) in the presence of 5 and 10 mM Ca2+. In contrast, in a high ionic strength medium (0.50 M KCl) the K+- and Ca2+-activated ATPase activity of myosin prepared from hypertrophied myocardium remained unchanged. Comparative analysis of protein present in the light chains of myosin showed no alteration in the proportion of LC1 to LC2 in the myosin from hypertrophied hearts, however, a decrease in the absorption of myosin in the u.v. region was observed. On the basis of our results one can hypothesize that there may be some conformational change in the myosin molecule from hypertrophied myocardium, thereby modifying both Ca2+-sensitivity and ATPase activity at a low KCl concentration.

Animals↗
PubMed1979

Toxic substances and cell membrane function.

The exposed location and functional importance of cell membranes make them particularly susceptible to the toxic effects of many chemicals. The likelihood of such effects has been appreciated for many years. However, the recent advent of new techniques has greatly increased our understanding of the complexities of membrane structure and function. These data make it quite clear that the interaction of toxic compounds with either the protein or the lipid component of cell membranes may substantially alter membrane function. This paper summarizes the current concepts of membrane structure and function and discusses the techniques currently in use to study cell membranes. Several examples are presented in which xenobiotics significantly alter membrane function. These include effects of heavy metals on passive ion permeability, impairment of osmoregulation and calcium transport by organochlorine pesticides, inhibition of the transport of neurotransmitter metabolites by phenoxyacetic acid herbicides in choroid plexus, and reduction in intestinal nutrient transport by heavy metals. Hence the study of the interactions of foreign compounds with membrane function may enhance our understanding of mechanisms both of toxicity and of basic membrane function.

2,4-Dichlorophenoxyacetic Acid↗
PubMed1979

Nerve cell calcium as a messenger for opiate and endorphin actions.

The studies outlined in this chapter provide convincing evidence for the role of calcium in pharmacological and biochemical actions of opiates on the central nervous system. The ability of the synaptosomes to accumulate excess calcium during narcotic exposure represents a singular major example of cellular adaptation at the level of the membrane. These findings together with the possibility that CDR and calcium may mediate a number of processes involving neurotransmitter activity provide many avenues for the future study of opiates and other central nervous system depressants.

Animals↗
PubMed1979

Na,K-ATPase in excitation-contraction coupling of vascular smooth muscle from cattle.

Lowering the extracellular K+ content from 6 to 0.6 mM causes a rise, and elevation from 6 to 8.5 mM a fall of 45Ca++ efflux from the vascular smooth muscle cells of the arteria carotis communis of cattle. In contrast, a level of 17 mM K+ has no influence. Removal of extracellular calcium does not block these effects. 10(-4) M ouabain also induces a rise in Ca++ efflux, additional potassium reduction then being without effect; 10(-9) M ouabain is of no influence. The 45Ca++ efflux kinetics correlates with the activity of the isolated Na,K-ATPase. Tonus increases of the vascular strips by 10(-4) M ouabain and potassium deficiency cannot be blocked by 4 mM lanthanum or removal of extracellular calcium. Unlike sodium, potassium stimulates the active Ca++ binding and the activity of the Ca-ATPase of the microsomal fraction. The ative Ca++ binding of the mitochondria is stimulated by both ions. It is postulated that the activity of the plasma membrane Na,K-pump is able to regulate the tonus of big arteries through alteration of Ca++ storage processes.

Animals↗