Mechanisms involved in the cellular calcium homeostasis in vascular smooth muscle: calcium pumps

The regulation of cytosolic Ca2+ homeostasis is essential for cells, and particularly for vascular smooth muscle cells. In this regulation, there is a participation of different factors and mechanisms situated at different levels in the cell, among them Ca2+ pumps play an important role. Thus, Ca2+ pump, to extrude Ca2+; Na+/Ca2+ exchanger; and different Ca2+ channels for Ca2+ entry are placed in the plasma membrane. In addition, the inner and outer surfaces of the plasmalemma possess the ability to bind Ca2+ that can be released by different agonists. The sarcoplasmic reticulum has an active role in this Ca2+ regulation; its membrane has a Ca2+ pump that facilitates luminal Ca2+ accumulation, thus reducing the cytosolic free Ca2+ concentration. This pump can be inhibited by different agents. Physiologically, its activity is regulated by the protein phospholamban; thus, when it is in its unphosphorylated state such a Ca2+ pump is inhibited. The sarcoplasmic reticulum membrane also possesses receptors for 1,4,5-inositol trisphosphate and ryanodine, which upon activation facilitates Ca2+ release from this store. The sarcoplasmic reticulum and the plasmalemma form the superficial buffer barrier that is considered as an effective barrier for Ca2+ influx. The cytosol possesses different proteins and several inorganic compounds with a Ca2+ buffering capacity. The hypothesis of capacitative Ca2+ entry into smooth muscle across the plasma membrane after intracellular store depletion and its mechanisms of inhibition and activation is also commented.     

Cellular calcium signals: nature, registration, and quantitative estimation

Calcium ions play a central role in the regulation of cellular activity. Calcium influx across the plasma membrane occurs through ion channels (voltage- and receptor-operated channels). Two intracellular channels responsible for releasing Ca2+ from the internal stores are ryanodine and IP3 receptors. Two mechanisms for Ca2+ extrusion have been identified in the sarcolemma (Ca2+ pump and Na+/Ca2+ exchanger) and one in the sarcoplasmic membrane (Ca2+ pump). Hierarchical organization of intracellular calcium signalling is presented. It is considered of opening of the single channels or of groups channels to give quarks and sparks. The methods for the determination of the intracellular Ca2+ concentration are discussed. The equation connecting [Ca2+]i with double wavelengths parameter R was obtained proceeding from three fluorescent forms of indo-1 (L, LM and LP). Using this equation permits improving calculation of [Ca2+]i.     

Mechanisms of regulation of Ca2+ levels in myometrium cells

The ways and mechanisms of the Ca2+ concentration regulation in myometrium cells are analyzed. The plasma membrane is thoroughly studied for its role in the calcium control provision for the contractile activity of the uterus. The systems of Mg2+-ATP-dependent transport of Ca2+, sodium-calcium metabolism as well as regularities of the Ca2+ passive transfer in the sarcolemma vesicles are considered. The systems of the Mg2+-ATP- and N+-dependent transport of calcium are discussed for their contribution into regulation of calcium concentration in the myoplasm. Oxytocin and ions of bivalent metals (stimulators of the contractile activity of the uterus) are studied for their effect on the activity of the sarcolemma calcium pump.     

Hormonal regulation of the Ca+ transport in blood and vascular cells

The main pathways of regulation of cytoplasm Ca2+ level with hormones and growth factors, as well as mechanisms of regulation of G-proteins, phospholipase C, Ca-channels, adenylate cyclase, guanylate cyclase, and protein kinase C, are discussed. Regulation of cytoplasm Ca2+ in vascular and blood cells with inositol phosphates, cAMP and cGMP, is stressed. The review summarises data on membrane receptors, G-proteins, protein kinases and their targets involved in regulation of Ca2+ turnover in platelets, endothelial and smooth muscle cells.     

Regulation of Na+-K+-ATPase: effect of Mg and Ca ions

The participation of Mg2+ and Ca2+ in complicated mechanisms of Na+, K(+)-ATPase regulation is discussed in the survey. The regulatory actions of Mg2+ on Na+, K(+)-ATPase such as its participation in phosphorylation and dephosphorylation of the enzyme, ADP/ATP-exchange inhibition, cardiac glycosides and vanadate binding with the enzyme, conformational changes induction during ATPase cycle are reviewed in detail. Some current views of mechanisms of above mentioned Mg2+ regulatory effects are discussed. The experimental evidence of Ca2+ immediate influence on the functional activity of Na+, K(+)-ATPase (catalytic, transport and glycoside-binding) are given. It's noted that these effects are based on the conformational changes in the enzyme and also on the phase transition in membrane induced by Ca2+. Unimmediate action of Ca2+ on Na+, K(+)-ATPase is also discussed, especially due to its effect on other membrane systems functionally linked with Na(+)-pump (for instance, due to Na+/Ca(+)-exchanger activation). It's concluded that Mg2+ and Ca2+ as "universal regulators" of the cell effectively influence the functional activity and conformational states of Na+, K(+)-ATPase.     

Molecular mechanisms of endolysosomal Ca2+ signalling in health and disease

Endosomes, lysosomes and lysosome-related organelles are emerging as important Ca2+ storage cellular compartments with a central role in intracellular Ca2+ signalling. Endocytosis at the plasma membrane forms endosomal vesicles which mature to late endosomes and culminate in lysosomal biogenesis. During this process, acquisition of different ion channels and transporters progressively changes the endolysosomal luminal ionic environment (e.g. pH and Ca2+) to regulate enzyme activities, membrane fusion/fission and organellar ion fluxes, and defects in these can result in disease. In the present review we focus on the physiology of the inter-related transport mechanisms of Ca2+ and H+ across endolysosomal membranes. In particular, we discuss the role of the Ca2+-mobilizing messenger NAADP (nicotinic acid adenine dinucleotide phosphate) as a major regulator of Ca2+ release from endolysosomes, and the recent discovery of an endolysosomal channel family, the TPCs (two-pore channels), as its principal intracellular targets. Recent molecular studies of endolysosomal Ca2+ physiology and its regulation by NAADP-gated TPCs are providing exciting new insights into the mechanisms of Ca2+-signal initiation that control a wide range of cellular processes and play a role in disease. These developments underscore a new central role for the endolysosomal system in cellular Ca2+ regulation and signalling.     

Intraluminal calcium as a primary regulator of endoplasmic reticulum function

The concentration of Ca2+ inside the lumen of endoplasmic reticulum (ER) regulates a vast array of spatiotemporally distinct cellular processes, from intracellular Ca2+ signals to intra-ER protein processing and cell death. This review summarises recent data on the mechanisms of luminal Ca2+-dependent regulation of Ca2+ release and uptake as well as ER regulation of cellular adaptive processes. In addition we discuss general biophysical properties of the ER membrane, as trans-endomembrane Ca2+ fluxes are subject to basic electrical forces, determined by factors such as the membrane potential of the ER and the ease with which Ca2+ fluxes are able to change this potential (i.e. the resistance of the ER membrane). Although these electrical forces undoubtedly play a fundamental role in shaping [Ca2+](ER) dynamics, at present there is very little direct experimental information about the biophysical properties of the ER membrane. Further studies of how intraluminal [Ca2+] is regulated, best carried out with direct measurements, are vital for understanding how Ca2+ orchestrates cell function. Direct monitoring of [Ca2+](ER) under conditions where the cytosolic [Ca2+] is known may also help to capture elusive biophysical information about the ER, such as the potential difference across the ER membrane.     

Calcium signalling in smooth muscle

Calcium signalling in smooth muscles is complex, but our understanding of it has increased markedly in recent years. Thus, progress has been made in relating global Ca2+ signals to changes in force in smooth muscles and understanding the biochemical and molecular mechanisms involved in Ca2+ sensitization, i.e. altering the relation between Ca2+ and force. Attention is now focussed more on the role of the internal Ca2+ store, the sarcoplasmic reticulum (SR), global Ca2+ signals and control of excitability. Modern imaging techniques have shown the elaborate SR network in smooth muscles, along with the expression of IP3 and ryanodine receptors. The role and cross-talk between these two Ca(2+) release mechanisms, as well as possible compartmentalization of the SR Ca2+ store are discussed. The close proximity between SR and surface membrane has long been known but the details of this special region to Ca2+ signalling and the role of local sub-membrane Ca2+ concentrations and membrane microdomains are only now emerging. The activation of K+ and Cl- channels by local Ca2+ signals, can have profound effects on excitability and hence contraction. We examine the evidence for both Ca2+ sparks and puffs in controlling ion channel activity, as well as a fundamental role for Ca2+ sparks in governing the period of inexcitability in smooth muscle, i.e. the refractory period. Finally, the relation between different Ca2+ signals, e.g. sparks, waves and transients, to smooth muscle activity in health and disease is becoming clearer and will be discussed.     

Calcium-transporting systems and calcium regulation in cardiomyocytes

The review systematizes and analyzes recent data about the role of different Ca(2+)-transport mechanisms in the regulation of Ca2+ metabolism and functional activity of the cardiomyocytes. During the cardiac action potential, Ca2+ enters the cardiomyocytes through sarcolemmal L-type calcium channels and via the Na+/Ca2+ exchange. This Ca2+ activates the release of additional Ca2+ from the sarcoplasmic reticulum. The sum of calcium from sarcolemmal influxes and sarcoplasmic release produces contractile effect. For the occurrence of relaxation, Ca2+ remove from the cytoplasm by three mechanisms, namely, sarcoplasmic Ca2+ pump, Na+/Ca2+ exchange and sarcolemmal Ca2+ pump. In this review, the structural and functional properties of the Ca2+ transport systems in the sarcolemmal membranes, sarcoplasmic reticulum and mitochondria are discussed. In addition alterations in regulation of intracellular calcium by activation of beta- and alpha-adrenergic receptors are consider.     

Roles of calcium ions in hyphal tip growth.

A role for Ca2+ in the tip growth process of fungal hyphae and other eukaryotic walled cells has been widely explored, following the earlier indications of their importance by Jaffe, Steer, and their colleagues. Analysis of the literature on fungi, with selected comparison with other tip-growing plant cells, shows that the growth rate and morphology of hyphae are sensitive to factors which influence intracellular Ca2+. These factors include variations in extracellular Ca2+ concentrations, Ca2+ ionophores, inhibitors of Ca2+ transport, and calmodulin- and Ca(2+)-binding dyes and buffers introduced into the cytoplasm. The effects of these agents appear to be mediated by a tip-high gradient of cytoplasmic free Ca2+ which is obligatorily present in all critically examined growing tips. Most recent observations agree that the gradient is very steep, declining rapidly within 10 to 20 microns of the tip. This gradient seems to be generated by the combined effects of an influx of Ca2+, via plasma membrane, possibly stretch-activated, channels localized in the hyphal tip, and subapical expulsion or sequestration of these ions. Expulsion probably involves a plasma membrane Ca(2+)-ATPase, but it is not yet possible to differentiate among mitochondria, endoplasmic reticulum, or vacuoles as the dominant sites of sequestration. It is suggested that regulation of the Ca2+ gradient in turn modulates the properties of the actin-based component of the cytoskeleton, which then controls the extensibility, and, possibly, the synthesis of the hyphal apex. Regulatory feedback mechanisms intrinsic to this model of tip growth regulation are briefly discussed, together with suggestions for future experiments which are crucial to its further elucidation and establishment.     

Ca2+ in contractile processes

The concept of Ca2+ regulation, first discovered and developed in muscle research, is historically surveyed. Ca2+ regulation mechanisms in actomyosin-dependent contractile processes are compared, emphasis being placed on the great diversity. The mode of action of Ca2+ is discussed with the examples of troponin and calmodulin, the most differentiated and conservative Ca2+-receptor proteins, respectively.     

Calcium homeostasis in trigeminal ganglion cell bodies

In primary sensory afferent neurons, Ca2+ plays a vital role in the regulation of cellular processes including receptor and synaptic plasticity, neurotransmitter and trophic factor release and gene regulation. Current understanding of the mechanisms underlying Ca2+ homeostasis of primary sensory afferent neurons is mostly derived from studies on dorsal root ganglia and nodose ganglia neuron cell bodies. Little is known about Ca2+ homeostasis in trigeminal ganglion neurons (TGNs). To determine what cellular processes contribute to electrically-evoked Ca2+ transients in TGNs, we probed Ca2+ regulatory mechanisms in TGN cell bodies from the ophthalmic division with a panel of pharmacological reagents. Ca2+ transients were evoked in fura-2 loaded TGNs by depolarizing the plasma membrane with brief (500 ms) puffs of 50 mM KCl. Cyclopiazonic acid (CPA; 5 microM), an inhibitor of the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA), significantly decreased the peak amplitude, and slowed the decay, of the KCl-evoked Ca2+ transients in TGNs. The mitochondrial protonophore, carbonyl cyanide 3-chloro-phenylhydrazone (CCCP; 5 microM) significantly increased the peak amplitude of KCl-evoked Ca2+ transients. These data demonstrate that Ca2+ stores do play a major role in Ca2+ homeostasis in TGN cell bodies. To determine the role of the sodium-calcium exchanger (NCX) in KCl-evoked Ca2+ transients in TGNs, we inhibited the exchanger with KB-R7943 (10 microM), or by replacing Na+ with Li+. NCX inhibition did not affect either the peak amplitude or the decay kinetics of the KCl-evoked Ca2+ transients. Therefore, the NCX does not play a significant role in removing cytosolic Ca2+ from TGNs. To test whether the plasma membrane calcium-ATPase (PMCA) contributes to Ca2+ extrusion, we inhibited its activity by a shift to alkaline pH (9.0). At pH 9.0, both the peak amplitude and decay time of the KCl-evoked Ca2+ transient were increased significantly. These data suggest that, in TGNs, the PMCA is the major mechanism for removing cytosolic Ca2+ following electrical activity.     

Membrane mechanisms of regulating Ca ion concentration in smooth muscle cells

The aim of this review is to summarize some current concepts on the membrane mechanisms of energy-dependent Ca2+ transport in the smooth muscles. The emphasis is placed on the properties and mechanisms of regulation of plasma membrane and endoplasmic reticulum calcium pumps, sarcolemmal sodium/calcium exchanger and mitochondrial Ca2+ transport.     

质膜钙离子ATP酶

钙离子(Ca2+)是重要的第二信使,通过与效应蛋白的结合和解离,以及在不同细胞器之间的穿梭运动而精确调控细胞活动,参与多种重要生命过程。细胞内具有精确调节Ca2+时空分布的调控系统。在静息状态下,细胞内的游离Ca2+浓度约为100 nmol/L;而当细胞受到信号刺激后,胞内的Ca2+浓度可上升至1000 nmol/L甚至更高。细胞中存在多种跨膜运送Ca2+的膜蛋白,以精确调节Ca2+浓度的时空动态变化,其中,细胞质膜上的多种Ca2+通道(包括电压门控通道、受体门控通道、储存控制通道等),以及内质网/肌质网和线粒体等胞内"钙库"膜上的雷诺丁受体、三磷酸肌醇受体等膜蛋白复合物,均可提升胞内Ca2+浓度,而细胞质膜上的钠钙交换体、质膜Ca2+-ATP酶、"钙库"膜上的内质网Ca2+-ATP酶、线粒体Ca2+单向转运体等,可将Ca2+浓度降低至静息态水平。质膜钙ATP酶是向细胞外运送Ca2+的关键膜蛋白,本文将对其结构、功能及其酶活性的调控机制做一简要综述。     

The role of sphingosine and its derivatives in the regulation of Ca2+ homeostasis

The role of sphingosine and sphingosine 1-phosphate in the regulation of Ca2+ homeostasis is reviewed. Available data suggest that there are at least three main pathways by which sphingosine and its metabolites affect Ca2+ fluxes in different cell types: (1) indirect action on Ca2+ stores, mediated by other transduction pathways; (2) direct action on the receptors of Ca2+ channels and Ca2+ pumps which are localized at the membranes of Ca2+ stores; (3) indirect action mediated by the regulation of expression of the channel-forming protein Bcl-2, which incorporates into membranes of Ca2+ stores. The effects of sphingosine and its metabolites on Ca2+ homeostasis via mechanisms (1)-(3) are considered. The combination of the mechanisms by which sphingosine affects Ca2+-signalling pathways is tissue-specific. Sphingosine and its metabolites can be physiological modulators of the intracellular Ca2+ signalling pathways.     

Potentiation of a sodium-calcium exchanger in the nuclear envelope by nuclear GM1 ganglioside

Calcium is recognized as an important intracellular messenger with a pivotal role in the regulation of many cytosolic and nuclear processes. Gangliosides of various types, especially GM1, are known to have a role in some aspects of Ca2+ regulation, operating through a variety of mechanisms that are gradually coming to light. The present study provides evidence for a sodium-calcium exchanger in the nuclear envelope of NG108-15 neuroblastoma cells that is potently and specifically activated by GM1. Immunoblot analysis revealed an unusually tight association of GM1 with the exchanger in the nuclear envelope but not with that in the plasma membrane. Exchanger and associated GM1 were located in the inner membrane of the nuclear envelope, suggesting this system could function to transfer Ca2+ between nucleoplasm and the envelope lumen. The GM1-enhanced exchange was blocked by cholera toxin B subunit while C2-ceramide, a recently discovered inhibitor of the exchanger, blocked all transfer. Exchanger activity was significantly elevated in nuclei isolated from cells that were induced to differentiate by KCl + dibutyryl-cAMP, a treatment previously shown to promote up-regulation of nuclear GM1 in conjunction with axonogenesis. Similar enhancement was achieved by addition of exogenous GM1 to nuclei from undifferentiated cells. These results suggest a prominent role for nuclear GM1 in regulation of nuclear Ca2+ homeostasis.     

Inositol 1,4,5-trisphosphate and its co-players in the concert of Ca2+ signalling--new faces in the line up

Release of Ca2+ from intracellular stores can occur by different intracellular messengers such as InsP3, cADPR and NAADP. Although in some cells messengers may operate on different stores, there are also Ca2+ stores with sensitivities for all three of these messengers. It is well documented, that InsP3- and cADPR-sensitive Ca2+ stores are involved in the activation of "store-operated Ca2+ channels" (SOCC). It has not yet been unequivocally shown, however, if Ca2+ release from stores, which respond to NAADP but not to InsP3 or cADPR, also generate signals which lead to "store-operated Ca2+ entry". Neither localization nor the mechanism of coupling to the plasma membrane of those InsP3- and cADPR-sensitive Ca2+ stores which activate SOCCs is yet clear. In this review localization and properties of InsP3-, cADPR- and NAADP-sensitive Ca2+ pools and their mutual interactions are discussed. Differential sensitivities of Ca2+ release mechanisms to InsP3, cADPR and NAADP have consequences on Ca2+ release, Ca2+ oscillations, propagation of Ca2+ waves and on activation of SOCC. Possible interaction of InsP3R and cADPR with candidates of SOCCs (TRP channels) and mechanisms involved in the regulation of SOCCs (activation-deactivation) will be elaborated.     

The role of calcium in excitation--contraction coupling in crustacean muscle fibers

The evidence that calcium (Ca) plays an important role in electrical activity and an essential role in excitation--contraction (E--C) coupling in crustacean muscles is reviewed. These muscles produce graded electrical and mechanical responses to applied depolarizations. Removal of Ca from the bath solution eliminates both responses. Addition of Ba2+ or Sr2+ to Ca-free saline restores membrane electrogenesis, and all-or-none action potentials can be induced. With Sr2+ vigorous contractions are produced, whereas Ba action potentials evoke minimal or no tension, showing that rapid depolarization of the membrane potential is not sufficient per se for E--C coupling in crab and barnacle muscle. Several inorganic (e.g., multivalent cations) and organic (e.g., aminoglycoside antibiotics) which block membrane Ca channels block electrogenesis and contraction. However, the "Ca antagonists" verapamil and D600 also block Ca uptake at intracellular storage sites, resulting in spontaneous contractions and the delayed relaxation of small contractions associated with residual Ca currents. The evidence that the Ca which enters the fibres needs to release Ca from intracellular storage sites to produce contractions is detailed and discussed. Finally, a model for E--C coupling is discussed. This model includes the sites and mechanisms of action for several chemicals which modify E--C coupling in crustacean muscle fibres.     

The effect of membrane-bound calcium on the activity of adenosine triphosphatase from erythrocytes and erythrocyte permeability for monovalent cations]

The activities of Ca2+, Mg2+-ATPase and Na+, K+-ATPase and the permeability of reconstituted human erythrocytes for Na and K ions were measured, using Ca2+-EGTA, Ca2+ATP and Ca2+-sodium citrate buffers. It was found that the increase in the Ca2+/chelate ratio caused stimulation of Ca2+, Mg2+- and Na+, K+-Atpases and an increase in the rate constants of ouabain--dependent 42K+ influx and 22Na+ efflux from the erythrocytes. The use of the Ca2+-sodium citrate system as a calcium buffer did not change the parameters of the functional state of erythrocyte membranes. The data obtained are discussed in terms of a possible role of calcium ions, which are bound to the inner surface of the erythrocyte membrane, in the regulation of the systems of active and passive transport of cations.     

Second messengers in heart cells and smooth muscle vessels

Advances in regulation by secondary messengers of Ca2+ level in cardiomyocyte and vascular smooth muscle cell cytosols with special reference to the major differences in regulatory effects in cells of the both types are reviewed. The effects of cAMP, cGMP, Ca2+, calmodulin, diacylglycerol and polyphosphoinositides on the Ca(2+)-channel, Ca(2+)-ATPase, plasmalemma, sarcoplasmic reticulum and outer membrane Na+/Ca2+ uniporter function are considered. Compartmentation of secondary messengers and protein kinase in cardiac and vascular smooth muscle cells should be taken into consideration during extrapolation of in vitro data to an in situ situation. The feasible role of impaired phosphorylation of membrane-bound proteins of cardiac and vascular smooth muscle cells in cardiac insufficiency and atherosclerosis is discussed.