Variable stoichiometric efficiency of Ca2+ and Sr2+ transport by the sarcoplasmic reticulum ATPase

Fluorescence lifetime FRET assay for live-cell high-throughput screening of the cardiac SERCA pump yields multiple classes of small-molecule allosteric modulators

We have used FRET-based biosensors in live cells, in a robust high-throughput screening (HTS) platform, to identify small-molecules that alter the structure and activity of the cardiac sarco/endoplasmic reticulum calcium ATPase (SERCA2a). Our primary aim is to discover drug-like small-molecule activators that improve SERCA's function for the treatment of heart failure. We have previously demonstrated the use of an intramolecular FRET biosensor, based on human SERCA2a, by screening two different small validation libraries using novel microplate readers that detect the fluorescence lifetime or emission spectrum with high speed, precision, and resolution. Here we report results from FRET-HTS of 50,000 compounds using the same biosensor, with hit compounds functionally evaluated using assays for Ca2+-ATPase activity and Ca2+-transport. We focused on 18 hit compounds, from which we identified eight structurally unique scaffolds and four scaffold classes as SERCA modulators, approximately half of which are activators and half are inhibitors. Five of these compounds were identified as promising SERCA activators, one of which activates Ca2+-transport even more than Ca2+-ATPase activity thus improving SERCA efficiency. While both activators and inhibitors have therapeutic potential, the activators establish the basis for future testing in heart disease models and lead development, toward pharmaceutical therapy for heart failure.

FRET assay for live-cell high-throughput screening of the cardiac SERCA pump yields multiple classes of small-molecule allosteric modulators

We have used FRET-based biosensors in live cells, in a robust high-throughput screening (HTS) platform, to identify small-molecules that alter the structure and activity of the cardiac sarco/endoplasmic reticulum calcium ATPase (SERCA2a). Our primary aim is to discover drug-like small-molecule activators that improve SERCA’s function for the treatment of heart failure. We have previously demonstrated the use of an intramolecular FRET biosensor, based on human SERCA2a, by screening a small validation library using novel microplate readers that can detect the fluorescence lifetime or emission spectrum with high speed, precision, and resolution. Here we report results from a 50,000-compound screen using the same biosensor, with hit compounds functionally evaluated using Ca 2+ -ATPase and Ca 2+ -transport assays. We focused on 18 hit compounds, from which we identified eight structurally unique compounds and four compound classes as SERCA modulators, approximately half of which are activators and half are inhibitors. While both activators and inhibitors have therapeutic potential, the activators establish the basis for future testing in heart disease models and lead development, toward pharmaceutical therapy for heart failure.

FRET assay for live-cell high-throughput screening of the cardiac SERCA pump yields multiple classes of small-molecule allosteric modulators

We have used FRET-based biosensors in live cells, in a robust high-throughput screening (HTS) platform, to identify small-molecules that alter the structure and activity of the cardiac sarco/endoplasmic reticulum calcium ATPase (SERCA2a). Our primary aim is to discover drug-like small-molecule activators that improve SERCA’s function for the treatment of heart failure. We have previously demonstrated the use of an intramolecular FRET biosensor, based on human SERCA2a, by screening a small validation library using novel microplate readers that can detect the fluorescence lifetime or emission spectrum with high speed, precision, and resolution. Here we report results from a 50,000-compound screen using the same biosensor, with hit compounds functionally evaluated using Ca 2+ -ATPase and Ca 2+ -transport assays. We focused on 18 hit compounds, from which we identified eight structurally unique compounds and four compound classes as SERCA modulators, approximately half of which are activators and half are inhibitors. While both activators and inhibitors have therapeutic potential, the activators establish the basis for future testing in heart disease models and lead development, toward pharmaceutical therapy for heart failure.

A Large-Scale High-Throughput Screen for Modulators of SERCA Activity

The sarco/endoplasmic reticulum Ca-ATPase (SERCA) is a P-type ion pump that transports Ca²⁺ from the cytosol into the endoplasmic/sarcoplasmic reticulum (ER/SR) in most mammalian cells. It is critically important in muscle, facilitating relaxation and enabling subsequent contraction. Increasing SERCA expression or specific activity can alleviate muscle dysfunction, most notably in the heart, and we seek to develop small-molecule drug candidates that activate SERCA. Therefore, we adapted an NADH-coupled assay, measuring Ca-dependent ATPase activity of SERCA, to high-throughput screening (HTS) format, and screened a 46,000-compound library of diverse chemical scaffolds. This HTS platform yielded numerous hits that reproducibly alter SERCA Ca-ATPase activity, with few false positives. The top 19 activating hits were further tested for effects on both Ca-ATPase and Ca²⁺ transport, in both cardiac and skeletal SR. Nearly all hits increased Ca²⁺ uptake in both cardiac and skeletal SR, with some showing isoform specificity. Furthermore, dual analysis of both activities identified compounds with a range of effects on Ca²⁺-uptake and ATPase, which fit into distinct classifications. Further study will be needed to identify which classifications are best suited for therapeutic use. These results reinforce the need for robust secondary assays and criteria for selection of lead compounds, before undergoing HTS on a larger scale.

The relationship between form and function throughout the history of excitation-contraction coupling

Franzini-Armstrong reviews the development of the excitation–contraction coupling field over time.

The concept of excitation–contraction coupling is almost as old as Journal of General Physiology. It was understood as early as the 1940s that a series of stereotyped events is responsible for the rapid contraction response of muscle fibers to an initial electrical event at the surface. These early developments, now lost in what seems to be the far past for most young investigators, have provided an endless source of experimental approaches. In this Milestone in Physiology, I describe in detail the experiments and concepts that introduced and established the field of excitation–contraction coupling in skeletal muscle. More recent advances are presented in an abbreviated form, as readers are likely to be familiar with recent work in the field.

Coral skeletal geochemistry as a monitor of inshore water quality

Coral reefs maintain extraordinary biodiversity and provide protection from tsunamis and storm surge, but inshore coral reef health is degrading in many regions due to deteriorating water quality. Deconvolving natural and anthropogenic changes to water quality is hampered by the lack of long term, dated water quality data but such records are required for forward modelling of reef health to aid their management. Reef corals provide an excellent archive of high resolution geochemical (trace element) proxies that can span hundreds of years and potentially provide records used through the Holocene. Hence, geochemical proxies in corals hold great promise for understanding changes in ancient water quality that can inform broader oceanographic and climatic changes in a given region. This article reviews and highlights the use of coral-based trace metal archives, including metal transported from rivers to the ocean, incorporation of trace metals into coral skeletons and the current 'state of the art' in utilizing coral trace metal proxies as tools for monitoring various types of local and regional source-specific pollution (river discharge, land use changes, dredging and dumping, mining, oil spills, antifouling paints, atmospheric sources, sewage). The three most commonly used coral trace element proxies (i.e., Ba/Ca, Mn/Ca, and Y/Ca) are closely associated with river runoff in the Great Barrier Reef, but considerable uncertainty remains regarding their complex biogeochemical cycling and controlling mechanisms. However, coral-based water quality reconstructions have suffered from a lack of understanding of so-called vital effects and early marine diagenesis. The main challenge is to identify and eliminate the influence of extraneous local factors in order to allow accurate water quality reconstructions and to develop alternate proxies to monitor water pollution. Rare earth elements have great potential as they are self-referencing and reflect basic terrestrial input.

The role of skeletal-muscle-based thermogenic mechanisms in vertebrate endothermy

Thermogenesis is one of the most important homeostatic mechanisms that evolved during vertebrate evolution. Despite its importance for the survival of the organism, the mechanistic details behind various thermogenic processes remain incompletely understood. Although heat production from muscle has long been recognized as a thermogenic mechanism, whether muscle can produce heat independently of contraction remains controversial. Studies in birds and mammals suggest that skeletal muscle can be an important site of non-shivering thermogenesis (NST) and can be recruited during cold adaptation, although unequivocal evidence is lacking. Much research on thermogenesis during the last two decades has been focused on brown adipose tissue (BAT). These studies clearly implicate BAT as an important site of NST in mammals, in particular in newborns and rodents. However, BAT is either absent, as in birds and pigs, or is only a minor component, as in adult large mammals including humans, bringing into question the BAT-centric view of thermogenesis. This review focuses on the evolution and emergence of various thermogenic mechanisms in vertebrates from fish to man. A careful analysis of the existing data reveals that muscle was the earliest facultative thermogenic organ to emerge in vertebrates, long before the appearance of BAT in eutherian mammals. Additionally, these studies suggest that muscle-based thermogenesis is the dominant mechanism of heat production in many species including birds, marsupials, and certain mammals where BAT-mediated thermogenesis is absent or limited. We discuss the relevance of our recent findings showing that uncoupling of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) by sarcolipin (SLN), resulting in futile cycling and increased heat production, could be the basis for NST in skeletal muscle. The overall goal of this review is to highlight the role of skeletal muscle as a thermogenic organ and provide a balanced view of thermogenesis in vertebrates.

Inhibition mechanism of the intracellular transporter Ca2+-pump from sarco-endoplasmic reticulum by the antitumor agent dimethyl-celecoxib

Dimethyl-celecoxib is a celecoxib analog that lacks the capacity as cyclo-oxygenase-2 inhibitor and therefore the life-threatening effects but retains the antineoplastic properties. The action mechanism at the molecular level is unclear. Our in vitro assays using a sarcoplasmic reticulum preparation from rabbit skeletal muscle demonstrate that dimethyl-celecoxib inhibits Ca²⁺-ATPase activity and ATP-dependent Ca²⁺ transport in a concentration-dependent manner. Celecoxib was a more potent inhibitor of Ca²⁺-ATPase activity than dimethyl-celecoxib, as deduced from the half-maximum effect but dimethyl-celecoxib exhibited higher inhibition potency when Ca²⁺ transport was evaluated. Since Ca²⁺ transport was more sensitive to inhibition than Ca²⁺-ATPase activity the drugs under study caused Ca²⁺/P_i uncoupling. Dimethyl-celecoxib provoked greater uncoupling and the effect was dependent on drug concentration but independent of Ca²⁺-pump functioning. Dimethyl-celecoxib prevented Ca²⁺ binding by stabilizing the inactive Ca²⁺-free conformation of the pump. The effect on the kinetics of phosphoenzyme accumulation and the dependence of the phosphoenzyme level on dimethyl-celecoxib concentration were independent of whether or not the Ca²⁺–pump was exposed to the drug in the presence of Ca²⁺ before phosphorylation. This provided evidence of non-preferential interaction with the Ca²⁺-free conformation. Likewise, the decreased phosphoenzyme level in the presence of dimethyl-celecoxib that was partially relieved by increasing Ca²⁺ was consistent with the mentioned effect on Ca²⁺ binding. The kinetics of phosphoenzyme decomposition under turnover conditions was not altered by dimethyl-celecoxib. The dual effect of the drug involves Ca²⁺-pump inhibition and membrane permeabilization activity. The reported data can explain the cytotoxic and anti-proliferative effects that have been attributed to the celecoxib analog. Ligand docking simulation predicts interaction of celecoxib and dimethyl-celecoxib with the intracellular Ca²⁺ transporter at the inhibition site of hydroquinones.

Impaired left ventricular mechanical and energetic function in mice after cardiomyocyte-specific excision of Serca2

Sarco(endo)plasmic reticulum Ca2+ -ATPase (SERCA)2 transports Ca2+ from the cytosol into the sarcoplasmic reticulum of cardiomyocytes and is essential for maintaining myocardial Ca2+ handling and thus the mechanical function of the heart. SERCA2 is a major ATP consumer in excitation-contraction coupling but is regarded to contribute to energetically efficient Ca2+ handling in the cardiomyocyte. Previous studies using cardiomyocyte-specific SERCA2 knockout (KO) mice have demonstrated that decreased SERCA2 activity reduces the Ca2+ transient amplitude and induces compensatory Ca2+ transport mechanisms that may lead to more inefficient Ca2+ transport. In this study, we examined the relationship between left ventricular (LV) function and myocardial O2 consumption (MVo2) in ex vivo hearts from SERCA2 KO mice to directly measure how SERCA2 elimination influences mechanical and energetic features of the heart. Ex vivo hearts from SERCA2 KO hearts developed mechanical dysfunction at 4 wk and demonstrated virtually no working capacity at 7 wk. In accordance with the reported reduction in Ca2+ transient amplitude in cardiomyocytes from SERCA2 KO mice, work-independent MVo2 was decreased due to a reduced energy cost of excitation-contraction coupling. As these hearts also showed a marked impairment in the efficiency of chemomechanical energy transduction (contractile efficiency, i.e, work-dependent MVo2), hearts from SERCA2 KO mice were found to be mechanically inefficient. This ex vivo evaluation of mechanical and energetic function in hearts from SERCA2 KO mice brings together findings from previous experimental and mathematical modeling-based studies and demonstrates that reduced SERCA2 activity not only leads to mechanical dysfunction but also to energetic dysfunction.

Ca(2+)/H (+) exchange, lumenal Ca(2+) release and Ca (2+)/ATP coupling ratios in the sarcoplasmic reticulum ATPase

The Ca²⁺ transport ATPase (SERCA) of sarcoplasmic reticulum (SR) plays an important role in muscle cytosolic signaling, as it stores Ca²⁺ in intracellular membrane bound compartments, thereby lowering cytosolic Ca²⁺ to induce relaxation. The stored Ca²⁺ is in turn released upon membrane excitation to trigger muscle contraction. SERCA is activated by high affinity binding of cytosolic Ca²⁺, whereupon ATP is utilized by formation of a phosphoenzyme intermediate, which undergoes protein conformational transitions yielding reduced affinity and vectorial translocation of bound Ca²⁺. We review here biochemical and biophysical evidence demonstrating that release of bound Ca²⁺ into the lumen of SR requires Ca²⁺/H⁺ exchange at the low affinity Ca²⁺ sites. Rise of lumenal Ca²⁺ above its dissociation constant from low affinity sites, or reduction of the H⁺ concentration by high pH, prevent Ca²⁺/H⁺ exchange. Under these conditions Ca²⁺ release into the lumen of SR is bypassed, and hydrolytic cleavage of phosphoenzyme may yield uncoupled ATPase cycles. We clarify how such Ca²⁺pump slippage does not occur within the time length of muscle twitches, but under special conditions and in special cells may contribute to thermogenesis.

Ablation of sarcolipin decreases the energy requirements for Ca2+ transport by sarco(endo)plasmic reticulum Ca2+-ATPases in resting skeletal muscle

The purpose of this study was to examine the effects of sarcolipin (SLN) on sarco(endo) plasmic reticulum Ca(2+)-ATPase (SERCA pump) energetics in vivo and resting skeletal muscle metabolic rate. Using SLN knockout (Sln(-/-)) mice we show that SLN ablation increases the transport stoichiometry of SERCA pumps (Ca(2+) uptake/Ca(2+)-ATPase activity) and decreases the relative contribution of SERCA pumps to resting oxygen consumption (VO2) in soleus without affecting soleus or whole body VO2. These data suggest that the lower energy requirements for Ca(2+) cycling in resting skeletal muscle of Sln(-/-) mice do not impact significantly either skeletal muscle or whole body metabolic rate.

Uncoupling of sarcoplasmic reticulum Ca²⁺-ATPase by N-arachidonoyl dopamine. Members of the endocannabinoid family as thermogenic drugs

BACKGROUND AND PURPOSE

The sarcoplasmic reticulum Ca²⁺-ATPase (SERCA) plays a role in thermogenesis. The exogenous compound capsaicin increased SERCA-mediated ATP hydrolysis not coupled to Ca²⁺ transport. Here, we have sought to identify endogenous compounds that may function as SERCA uncoupling agents.

EXPERIMENTAL APPROACH

Using isolated SR vesicles from rabbits, we have screened for endogenous compounds that uncouple SERCA. We have also studied their ability to deplete cytoplasmic ATP from human skeletal muscle cells in culture.

KEY RESULTS

Studies on SR vesicles showed that the endogenous lipid metabolite N-arachidonoyl dopamine (NADA) was a potent stimulator of SERCA uncoupling. NADA stabilized an E₁-like pump conformation that had a lower dephosphorylation rate, low affinity for Ca²⁺ at the luminal sites and a specific proteinase K cleavage pattern involving protection of the C-terminal p83C fragment from further cleavage. Moreover, we found a significantly decreased cytoplasmic ATP levels following treatment of skeletal muscle cells with 100 nM NADA. This effect was dependent on the presence of glucose and abolished by pretreatment with the specific SERCA inhibitor thapsigargin, regardless of the presence of glucose.

CONCLUSIONS AND IMPLICATIONS

NADA is an endogenous molecule that may function as SERCA uncoupling agent in vivo. Members of the endocannabinoid family exert concerted actions on several Ca²⁺-handling proteins. Uncoupling of SERCA by exogenous compounds could be a novel post-mitochondrial strategy for reduction of cellular ATP levels. In addition, signalling networks leading to SERCA uncoupling can be explored to study the importance of this ion pump in pathophysiological conditions related to metabolism.

Inhibition of the intracellular Ca(2+) transporter SERCA (Sarco-Endoplasmic Reticulum Ca(2+)-ATPase) by the natural polyphenol epigallocatechin-3-gallate

The use of a microsomal preparation from skeletal muscle revealed that both Ca(2+) transport and Ca(2+)-dependent ATP hydrolysis linked to Sarco-Endoplasmic Reticulum Ca(2+)-ATPase are inhibited by epigallocatechin-3-gallate (EGCG). A half-maximal effect was achieved at approx. 12 μM. The presence of the galloyl group was essential for the inhibitory effect of the catechin. The relative inhibition of the Ca(2+)-ATPase activity decreased when the Ca(2+) concentration was raised but not when the ATP concentration was elevated. Data on the catalytic cycle indicated inhibition of maximal Ca(2+) binding and a decrease in Ca(2+) binding affinity when measured in the absence of ATP. Moreover, the addition of ATP to samples in the presence of EGCG and Ca(2+) led to an early increase in phosphoenzyme followed by a time-dependent decay that was faster when the drug concentration was raised. However, phosphorylation following the addition of ATP plus Ca(2+) led to a slow rate of phosphoenzyme accumulation that was also dependent on EGCG concentration. The results are consistent with retention of the transporter conformation in the Ca(2+)-free state, thus impeding Ca(2+) binding and therefore the subsequent steps when ATP is added to trigger the Ca(2+) transport process. Furthermore, phosphorylation by inorganic phosphate in the absence of Ca(2+) was partially inhibited by EGCG, suggesting alteration of the native Ca(2+)-free conformation at the catalytic site.

Phosphatidylserine stimulation of Drs2p·Cdc50p lipid translocase dephosphorylation is controlled by phosphatidylinositol-4-phosphate

Here, Drs2p, a yeast lipid translocase that belongs to the family of P(4)-type ATPases, was overexpressed in the yeast Saccharomyces cerevisiae together with Cdc50p, its glycosylated partner, as a result of the design of a novel co-expression vector. The resulting high yield allowed us, using crude membranes or detergent-solubilized membranes, to measure the formation from [γ-(32)P]ATP of a (32)P-labeled transient phosphoenzyme at the catalytic site of Drs2p. Formation of this phosphoenzyme could be detected only if Cdc50p was co-expressed with Drs2p but was not dependent on full glycosylation of Cdc50p. It was inhibited by orthovanadate and fluoride compounds. In crude membranes, the phosphoenzyme formed at steady state at 4 °C displayed ADP-insensitive but temperature-sensitive decay. Solubilizing concentrations of dodecyl maltoside left this decay rate almost unaltered, whereas several other detergents accelerated it. Unexpectedly, the dephosphorylation rate for the solubilized Drs2p·Cdc50p complex was inhibited by the addition of phosphatidylserine. Phosphatidylserine exerted its anticipated accelerating effect on the dephosphorylation of Drs2p·Cdc50p complex only in the additional presence of phosphatidylinositol-4-phosphate. These results explain why phosphatidylinositol-4-phosphate tightly controls Drs2p-catalyzed lipid transport and establish the functional relevance of the Drs2p·Cdc50p complex overexpressed here.

The Ca2+-ATPase (SERCA1) is inhibited by 4-aminoquinoline derivatives through interference with catalytic activation by Ca2+, whereas the ATPase E2 state remains functional

Several clotrimazole (CLT) and 4-aminoquinoline derivatives were synthesized and found to exhibit in vitro antiplasmodial activity with IC(50) ranging from nm to μm values. We report here that some of these compounds produce inhibition of rabbit sarcoplasmic reticulum Ca(2+)-ATPase (SERCA1) with IC(50) values in the μm range. The highest affinity for the Ca(2+)-ATPase was observed with NF1442 (N-((3-chlorophenyl)(4-((4-(7-chloroquinolin-4-yl)piperazin-1-yl)methyl)phenyl)methyl)-7-chloro-4-aminoquinoline) and NF1058 (N-((3-chlorophenyl)(4-(pyrrolidin-1-ylmethyl)phenyl)methyl)-7-chloro-4-aminoquinoline),yielding IC(50) values of 1.3 and 8.0 μm as demonstrated by measurements of steady state ATPase activity as well as single cycle charge transfer. Characterization of sequential reactions comprising the ATPase catalytic and transport cycle then demonstrated that NF1058, and similarly CLT, interferes with the mechanism of Ca(2+) binding and Ca(2+)-dependent enzyme activation (E(2) to E(1)·Ca(2) transition) required for formation of phosphorylated intermediate by ATP utilization. On the other hand, Ca(2+) independent phosphoenzyme formation by utilization of P(i) (i.e. reverse of the hydrolytic reaction in the absence of Ca(2+)) was not inhibited by NF1058 or CLT. Comparative experiments showed that the high affinity inhibitor thapsigargin interferes not only with Ca(2+) binding and phosphoenzyme formation with ATP but also with phosphoenzyme formation by utilization of P(i) even though this reaction does not require Ca(2+). It is concluded that NF1058 and CLT inhibit SERCA by stabilization of an E(2) state that, as opposed to that obtained with thapsigargin, retains the functional ability to form E(2)-P by reacting with P(i).

Ca2+ release to lumen from ADP-sensitive phosphoenzyme E1PCa2 without bound K+ of sarcoplasmic reticulum Ca2+-ATPase

During Ca(2+) transport by sarcoplasmic reticulum Ca(2+)-ATPase, the conformation change of ADP-sensitive phosphoenzyme (E1PCa(2)) to ADP-insensitive phosphoenzyme (E2PCa(2)) is followed by rapid Ca(2+) release into the lumen. Here, we find that in the absence of K(+), Ca(2+) release occurs considerably faster than E1PCa(2) to E2PCa(2) conformation change. Therefore, the lumenal Ca(2+) release pathway is open to some extent in the K(+)-free E1PCa(2) structure. The Ca(2+) affinity of this E1P is as high as that of the unphosphorylated ATPase (E1), indicating the Ca(2+) binding sites are not disrupted. Thus, bound K(+) stabilizes the E1PCa(2) structure with occluded Ca(2+), keeping the Ca(2+) pathway to the lumen closed. We found previously (Yamasaki, K., Wang, G., Daiho, T., Danko, S., and Suzuki, H. (2008) J. Biol. Chem. 283, 29144-29155) that the K(+) bound in E2P reduces the Ca(2+) affinity essential for achieving the high physiological Ca(2+) gradient and to fully open the lumenal Ca(2+) gate for rapid Ca(2+) release (E2PCa(2) → E2P + 2Ca(2+)). These findings show that bound K(+) is critical for stabilizing both E1PCa(2) and E2P structures, thereby contributing to the structural changes that efficiently couple phosphoenzyme processing and Ca(2+) handling.

Luminal Ca(2+) content regulates intracellular Ca(2+) release in subepicardial myocytes of intact beating mouse hearts: effect of exogenous buffers

Ca(+)-induced Ca(2+) release tightly controls the function of ventricular cardiac myocytes under normal and pathological conditions. Two major factors contributing to the regulation of Ca(2+) release are the cytosolic free Ca(2+) concentration and sarcoplasmic reticulum (SR) Ca(2+) content. We hypothesized that the amount of Ca(2+) released from the SR during each heart beat strongly defines the refractoriness of Ca(2+) release. To test this hypothesis, EGTA AM, a high-affinity, slow-association rate Ca(2+) chelator, was used as a tool to modify luminal SR Ca(2+) content. An analysis of the cytosolic and luminal SR Ca(2+) dynamics recorded from the epicardial layer of intact mouse hearts indicated that the presence of EGTA reduced the diastolic SR free Ca(2+) concentration and fraction of SR Ca(2+) depletion during each beat. In addition, this maneuver shortened the refractory period and accelerated the restitution of Ca(2+) release. As a consequence of the accelerated restitution, the frequency dependence of Ca(2+) alternans was significantly shifted toward higher heart rates, suggesting a role of luminal SR Ca(2+) in the genesis of this highly arrhythmogenic phenomenon. Thus, intra-SR Ca(2+) dynamics set the refractoriness and frequency dependence of Ca(2+) transients in subepicardial ventricular myocytes.

Modelling sarcoplasmic reticulum calcium ATPase and its regulation in cardiac myocytes

When developing large-scale mathematical models of physiology, some reduction in complexity is necessarily required to maintain computational efficiency. A prime example of such an intricate cell is the cardiac myocyte. For the predictive power of the cardiomyocyte models, it is vital to accurately describe the calcium transport mechanisms, since they essentially link the electrical activation to contractility. The removal of calcium from the cytoplasm takes place mainly by the Na(+)/Ca(2+) exchanger, and the sarcoplasmic reticulum Ca(2+) ATPase (SERCA). In the present study, we review the properties of SERCA, its frequency-dependent and beta-adrenergic regulation, and the approaches of mathematical modelling that have been used to investigate its function. Furthermore, we present novel theoretical considerations that might prove useful for the elucidation of the role of SERCA in cardiac function, achieving a reduction in model complexity, but at the same time retaining the central aspects of its function. Our results indicate that to faithfully predict the physiological properties of SERCA, we should take into account the calcium-buffering effect and reversible function of the pump. This 'uncomplicated' modelling approach could be useful to other similar transport mechanisms as well.

A thermodynamic model of the cardiac sarcoplasmic/endoplasmic Ca(2+) (SERCA) pump

We present a biophysically based kinetic model of the cardiac SERCA pump that consolidates a range of experimental data into a consistent and thermodynamically constrained framework. The SERCA model consists of a number of sub-states with partial reactions that are sensitive to Ca(2+) and pH, and to the metabolites MgATP, MgADP, and Pi. Optimization of model parameters to fit experimental data favors a fully cooperative Ca(2+)-binding mechanism and predicts a Ca(2+)/H(+) counter-transport stoichiometry of 2. Moreover, the order of binding of the partial reactions, particularly the binding of MgATP, proves to be a strong determinant of the ability of the model to fit the data. A thermodynamic investigation of the model indicates that the binding of MgATP has a large inhibitory effect on the maximal reverse rate of the pump. The model is suitable for integrating into whole-cell models of cardiac electrophysiology and Ca(2+) dynamics to simulate the effects on the cell of compromised metabolism arising in ischemia and hypoxia.

Cold tolerance in hypothyroid rabbits: role of skeletal muscle mitochondria and sarcoplasmic reticulum Ca2+ ATPase isoform 1 heat production

Brown adipose tissue (BAT) is involved in rat and mice thermoregulation, and heat produced by BAT depends on the concerted action of thyroid hormones and catecholamines. Little is known about cold-induced thermogenesis in mammals that have little or no BAT, such as rabbits. In these animals, thermogenesis primarily occurs in skeletal muscle. In this work, we have studied the effect of cold acclimation (4 C for 10 d) in normal and hypothyroid rabbits. It is known that hypothyroid rats die after a few hours of cold exposure. We now show that, different from rats, hypothyroid rabbits sustain their body temperature and survive after 10 d cold exposure. When compared with rabbits kept at room temperature, the muscles of cold-exposed rabbits showed a dark red color characteristic of oxidative muscle fibers. According to this pattern, we observed that in both normal and hypothyroid rabbits, cold exposure promotes an increase in oxygen consumption by skeletal muscle mitochondria. Moreover, in red muscle, cold acclimation induces an increase in the expression and activity of sarcoplasmic reticulum Ca(2+) ATPase isoform 1 (SERCA1), one of the muscle enzymes involved in heat production. We conclude that rabbit cold tolerance is probably related to increased muscle oxidative metabolism and heat production by SERCA1 and that these changes are not completely dependent on normal thyroid function.

Capsaicin stimulates uncoupled ATP hydrolysis by the sarcoplasmic reticulum calcium pump

In muscle cells the sarcoplasmic reticulum (SR) Ca(2+)-ATPase (SERCA) couples the free energy of ATP hydrolysis to pump Ca(2+) ions from the cytoplasm to the SR lumen. In addition, SERCA plays a key role in non-shivering thermogenesis through uncoupled reactions, where ATP hydrolysis takes place without active Ca(2+) translocation. Capsaicin (CPS) is a naturally occurring vanilloid, the consumption of which is linked with increased metabolic rate and core body temperature. Here we document the stimulation by CPS of the Ca(2+)-dependent ATP hydrolysis by SERCA without effects on Ca(2+) accumulation. The stimulation by CPS was significantly dependent on the presence of a Ca(2+) gradient across the SR membrane. ATP activation assays showed that the drug reduced the nucleotide affinity at the catalytic site, whereas the affinity at the regulatory site increased. Several biochemical analyses indicated that CPS stabilizes an ADP-insensitive E(2)P-related conformation that dephosphorylates at a higher rate than the control enzyme. Under conditions where uncoupled SERCA was specifically inhibited by the treatment with fluoride, low temperatures, or dimethyl sulfoxide, CPS had no stimulatory effect on ATP hydrolysis by SERCA. It is concluded that CPS stabilizes a SERCA sub-conformation where Ca(2+) is released from the phosphorylated intermediate to the cytoplasm instead of the SR lumen, increasing ATP hydrolysis not coupled with Ca(2+) transport. To the best of our knowledge CPS is the first natural drug that augments uncoupled SERCA, presumably resulting in thermogenesis. The role of CPS as a SERCA modulator is discussed.

Capsazepine, a synthetic vanilloid that converts the Na,K-ATPase to Na-ATPase

Capsazepine (CPZ), a synthetic capsaicin analogue, inhibits ATP hydrolysis by Na,K-ATPase in the presence but not in the absence of K(+). Studies with purified membranes revealed that CPZ reduced Na(+)-dependent phosphorylation by interference with Na(+) binding from the intracellular side of the membrane. Kinetic analyses showed that CPZ stabilized an enzyme species that constitutively occluded K(+). Low-affinity ATP interaction with the enzyme was strongly reduced after CPZ treatment; in contrast, indirectly measured interaction with ADP was much increased, which suggests that composite regulatory communication with nucleotides takes place during turnover. Studies with lipid vesicles revealed that CPZ reduced ATP-dependent digitoxigenin-sensitive (22)Na(+) influx into K(+)-loaded vesicles only at saturating ATP concentrations. The drug apparently abolishes the regulatory effect of ATP on the pump. Drawing on previous homology modeling studies of Na,K-ATPase to atomic models of sarcoplasmic reticulum Ca-ATPase and on kinetic data, we propose that CPZ uncouples an Na(+) cycle from an Na(+)/K(+) cycle in the pump. The Na(+) cycle possibly involves transport through the recently characterized Na(+)-specific site. A shift to such an uncoupled mode is believed to produce pumps mediating uncoupled Na(+) efflux by modifying the transport stoichiometry of single pump units.

The structural basis of calcium transport by the calcium pump

The sarcoplasmic reticulum Ca2+-ATPase, a P-type ATPase, has a critical role in muscle function and metabolism. Here we present functional studies and three new crystal structures of the rabbit skeletal muscle Ca2+-ATPase, representing the phosphoenzyme intermediates associated with Ca2+ binding, Ca2+ translocation and dephosphorylation, that are based on complexes with a functional ATP analogue, beryllium fluoride and aluminium fluoride, respectively. The structures complete the cycle of nucleotide binding and cation transport of Ca2+-ATPase. Phosphorylation of the enzyme triggers the onset of a conformational change that leads to the opening of a luminal exit pathway defined by the transmembrane segments M1 through M6, which represent the canonical membrane domain of P-type pumps. Ca2+ release is promoted by translocation of the M4 helix, exposing Glu 309, Glu 771 and Asn 796 to the lumen. The mechanism explains how P-type ATPases are able to form the steep electrochemical gradients required for key functions in eukaryotic cells.

Thermogenic activity of Ca2+-ATPase from skeletal muscle heavy sarcoplasmic reticulum: The role of ryanodine Ca2+ channel

The sarcoplasmic reticulum Ca(2+) ATPase 1 (SERCA 1) is able to handle the energy derived from ATP hydrolysis in such a way as to determine the parcel of energy that is used for Ca(2+) transport and the fraction that is converted into heat. In this work we measured the heat production by SERCA 1 in the two sarcoplasmic reticulum (SR) fractions: the light fraction (LSR), which is enriched in SERCA and the heavy fraction (HSR), which contains both the SERCA and the ryanodine Ca(2+) channel. We verified that although HSR cleaved ATP at faster rate than LSR, the amount of heat released during ATP hydrolysis by HSR was smaller than that measured by LSR. Consequently, the amount of heat released per mol of ATP cleaved (DeltaH(cal)) by HSR was lower compared to LSR. In HSR, the addition of 5 mM Mg(2+) or ruthenium red, conditions that close the ryanodine Ca(2+) channel, promoted a decrease in the ATPase activity, but the amount of heat released during ATP hydrolysis remained practically the same. In this condition, the DeltaH(cal) values of ATP hydrolysis increased significantly. Neither Mg(2+) nor ruthenium red had effect on LSR. Thus, we conclude that heat production by SERCA 1 depends on the region of SR in which the enzyme is inserted and that in HSR, the DeltaH(cal) of ATP hydrolysis by SERCA 1 depends on whether the ryanodine Ca(2+) channel is opened or closed.

The presence of sarcolipin results in increased heat production by Ca(2+)-ATPase

Skeletal muscle sarcoplasmic reticulum of large mammals such as rabbit contains sarcolipin (SLN), a small peptide with a single transmembrane alpha-helix. When reconstituted with the Ca(2+)-ATPase from skeletal muscle sarcoplasmic reticulum into sealed vesicles, the presence of SLN leads to a reduced level of accumulation of Ca(2+). Heats of reaction of the reconstituted Ca(2+)-ATPase with ATP were measured using isothermal calorimetry. The heat released increased linearly with time over 30 min and increased with increasing SLN content. Rates ATP hydrolysis by the reconstituted Ca(2+)-ATPase were constant over a 30-min time period and were the same when measured in the presence or absence of an ATP-regenerating system. The calculated values of heat released per mol of ATP hydrolyzed increased with increasing SLN content and fitted to a simple binding equation with a dissociation constant for the SLN.ATPase complex of 6.9 x 10(-4) +/- 2.9 x 10(-4) in units of mol fraction per monolayer. It is suggested that the interaction between Ca(2+)-ATPase and SLN in the sarcoplasmic reticulum could be important in thermogenesis by the sarcoplasmic reticulum.

Role of sarco/endoplasmic reticulum Ca(2+)-ATPase in thermogenesis

Enzymes are able to handle the energy derived from the hydrolysis of phosphate compounds in such a way as to determine the parcel that is used for work and the fraction that is converted into heat. The sarco/endoplasmic reticulum Ca(2+)-ATPases (SERCA) is a family of membrane-bound ATPases that are able to transport Ca(2+) ion across the membrane using the chemical energy derived from ATP hydrolysis. The heat released during ATP hydrolysis by SERCA may vary from 10 up to 30 kcal/mol depending on the SERCA isoform used and on whether or not a Ca(2+) gradient is formed across the membrane. Drugs such as heparin, dimethyl sulfoxide and the platelet-activating factor (PAF) are able to modify the fraction of the chemical energy released during ATP hydrolysis that is used for Ca(2+) transport and the fraction that is dissipated in the surrounding medium as heat. The thyroid hormone 3,5,3'-triiodo L: -thyronine (T(3)) regulates the expression and function of the thermogenic SERCA isoforms. Modulation of heat production by SERCA might be one of the mechanisms involved in the increased thermogenesis found in hyperthyroidism.

oscillations in a model of energy-dependent uptake by the endoplasmic reticulum

Active Ca2+ transport in living cells necessitates controlled supply of metabolic energy. Direct coupling between sarco/endoplasmic reticulum (ER) Ca2+ ATPases (SERCA) and intracellular energy-generation sites has been well established experimentally. On the basis of these experimental findings we propose a pump-driven model to investigate complex dynamic properties of a cell system. The model describes the pump process both by the Ca2+ ATPase itself and by a suitable description of the glycolysis. The associated set of differential equations shows a rich behavior, the solutions ranging from simple periodic oscillations to complex patterns such as bursting and spiking. Recent experimental results on calcium oscillations in Xenopus laevis oocytes and on dynamic patterns of intracellular Ca2+ concentrations in electrically non-excitable cells are well described by corresponding theoretical results derived within the proposed model. The simulation results are further compared to spontaneous [Ca2+] oscillations in primitive endodermal cells.

The thermogenic activity of rat brown adipose tissue and rabbit white muscle Ca2+-ATPase

The Ca2+-ATPase (SERCA) found in vesicles derived from the sarco/endoplasmic reticulum vesicles of rats brown adipose tissue and rabbit white muscle were identified by gel electrophoresis, Western blot, electron microscopy and immunolabeling with gold particles. In both tissues, the isoform found was SERCA 1. The Ca2+ affinity of the fat SERCA 1 was different from the muscle isoform. The degree of uncoupling is estimated measuring the ratio between Ca2+ transport and ATP cleaved. In brown fat vesicles the degree of uncoupling varied depending on the Ca2+ concentration of the medium. This was not observed in vesicles derived from muscle. At all Ca2+ concentrations tested, the uncoupling was not related to Ca2+ leakage from the membrane and was far more pronounced in fat than in muscle vesicle. When a Ca2+ gradient was formed across the vesicles membrane the heat released during ATP hydrolysis varied between 22 and 26 Kcal/mol in both fat and muscle vesicles but in the absence of a gradient the heat released was 17 Kcal/mol in fat and 12 Kcal/mol in muscle. The data reported indicate that the SERCA 1 of brown adipocytes is far more thermogenic than the white muscle SERCA 1, and suggest that, in addition to storing Ca2+ inside the endoplasmic reticulum, the SERCA 1 may represent a source of heat production contributing to the thermogenic function of brown adipose tissue.

Calcium pumps in the central nervous system

Two families of Ca2+ transport ATPases are involved in the maintenance of Ca2+ homeostasis in the nervous system, the plasma membrane Ca2+-ATPase that pumps Ca2+ to the extracellular medium and the intracellular sarco/endoplasmic reticulum Ca2+-ATPase that transports Ca2+ from the cytosol to the endoplasmic reticulum. Both types of calcium pumps show precise regulatory properties and they are localized in specific subcellular regions. In this review, we describe the functional and regulatory properties of both families of calcium pumps, their distribution in nerve cells, and their involvement in neurological disorders. The functional characterization of neuronal calcium pumps is very important in order to understand the biochemical processes involved in the maintenance of intracellular calcium in synaptic terminals.

Functional approach to the catalytic site of the sarcoplasmic reticulum Ca(2+)-ATPase: binding and hydrolysis of ATP in the absence of Ca(2+)

Isolated sarcoplasmic reticulum vesicles in the presence of Mg(2+) and absence of Ca(2+) retain significant ATP hydrolytic activity that can be attributed to the Ca(2+)-ATPase protein. At neutral pH and the presence of 5 mM Mg(2+), the dependence of the hydrolysis rate on a linear ATP concentration scale can be fitted by a single hyperbolic function. MgATP hydrolysis is inhibited by either free Mg(2+) or free ATP. The rate of ATP hydrolysis is not perturbed by vanadate, whereas the rate of p-nitrophenyl phosphate hydrolysis is not altered by a nonhydrolyzable ATP analog. ATP binding affinity at neutral pH and in a Ca(2+)-free medium is increased by Mg(2+) but decreased by vanadate when Mg(2+) is present. It is suggested that MgATP hydrolysis in the absence of Ca(2+) requires some optimal adjustment of the enzyme cytoplasmic domains. The Ca(2+)-independent activity is operative at basal levels of cytoplasmic Ca(2+) or when the Ca(2+) binding transition is impeded.

Functional effect of hydrogen peroxide on the sarcoplasmic reticulum membrane: uncoupling and irreversible inhibition of the Ca2+-ATPase protein

The chemical treatment of sarcoplasmic reticulum vesicles with H2O2 affects both Ca2+ transport and the hydrolytic activity supported by the Ca2+-ATPase protein. Ca2+ transport was much more sensitive to inhibition than ATPase activity and the decrease in Ca2+ transport was not the result of an increase in membrane permeability. The Ca2+/Pi uncoupling can be attributed to the own catalytic mechanism of the enzyme. Under conditions of high uncoupling, Ca2+ binding to the transport sites was barely affected and accumulation of phosphorylated species during the enzyme cycling gave almost maximal levels. These are features defining intramolecular uncoupling mediated by a phosphorylated form of the enzyme. Severe inhibition of the hydrolytic activity was observed when higher peroxide concentrations and leaky vesicles were used. These experimental conditions diminished maximal Ca2+ binding and the steady-state phosphoenzyme level. The low hydrolytic activity can be ascribed to a decrease in the rate of enzyme dephosphorylation.

Status epilepticus induced by pilocarpine and Ca2+ transport by microsome in the hippocampus of rats

An increase in intra-neuronal Ca(2+) concentration has been associated to status epilepticus (SE). Ca(2+) is stored in the endoplasmic reticulum, mediated by the Ca(2+)-ATPases (SERCAs). Here we studied the Ca(2+)-ATPase activity and the SERCA2b distribution in the hippocampus of rats submitted to 5h of SE. The Ca(2+)-uptake was measured using [45Ca]CaCl(2) and the hippocampal distribution of SERCA2b was analyzed by immunohistochemistry. A reduction in the amount of cells expressing SERCA2b in CA1, CA3 and dentate gyrus was observed. However, the surviving cells of these regions increased the SERCA2b immunoreactivity, when compared with control tissues. The Ca(2+)-ATPase activity measured in all hippocampal formation was not modified by SE. These results suggest that SE promotes a redistribution of SERCA2b in the hippocampus as a compensatory Ca(2+)-transport mechanism.

Heat of PPi Hydrolysis Varies Depending on the Enzyme Used

With yeast-soluble inorganic pyrophosphatase, the heat released during PP(i) hydrolysis was -6.3 kcal/mol regardless of the KCl concentration in the medium. With the membrane-bound pyrophosphatase of corn vacuoles, the heat released varies between -23.5 and -7.5 kcal/mol depending on the KCl concentration in the medium and whether or not a H(+) gradient is formed across the vacuole membranes. The data support the proposal that enzymes are able to handle the energy derived from phosphate compound hydrolysis in such a way as to determine the parcel that is used for work and the fraction that is converted into heat.

Time-resolved charge movements in the sarcoplasmatic reticulum Ca-ATPase

The time-resolved kinetics of the Ca(2+)-translocating partial reaction of the sarcoplasmatic reticulum Ca-ATPase was investigated by ATP-concentration jump experiments. ATP was released by an ultraviolet light flash from its inactive precursor and charge movements in the membrane domain of the ion pumps were detected by the fluorescent styryl dye 2BITC. Two oppositely directed cation movements were found, which were assigned to Ca(2+) release and H(+) binding. The faster process with a typical time constant of 30 ms reports the rate-limiting process before Ca(2+) release, probably the conformation transition E(1) --> E(2). The following, slow uptake of positive charge had a pH-dependent time constant, which was 1 s at low pH and approximately 3 s at pH > 8. This process is assigned to an electrically silent conformational relaxation of the state P-E(2) preceding H(+) binding. This interpretation is in agreement with the observation that the fast process was independent of the substrate concentrations (i.e., when [Ca(2+)] > 200 nM, and [ATP] > 20 micro M). The slow process was independent of the Ca(2+) concentration. The activation energy of the resolved processes was between 80 kJ/mol and 90 kJ/mol, which is comparable to the activation energy of the enzymatic activity (92 kJ/mol) and these high values point to conformational changes underlying rate-limiting steps of the pump cycle.

Hyperthyroidism increases the uncoupled ATPase activity and heat production by the sarcoplasmic reticulum Ca2+-ATPase

The sarcoplasmic reticulum Ca2+-ATPase is able to modulate the distribution of energy released during ATP hydrolysis, so that a portion of energy is used for Ca2+ transport (coupled ATPase activity) and a portion is converted into heat (uncoupled ATPase activity). In this report it is shown that T4 administration to rabbits promotes an increase in the rates of both the uncoupled ATPase activity and heat production in sarcoplasmic reticulum vesicles, and that the degree of activation varies depending on the muscle type used. In white muscles hyperthyroidism promotes a 0.8-fold increase of the uncoupled ATPase activity and in red muscle a 4-fold increase. The yield of vesicles from hyperthyroid muscles is 3-4-fold larger than that obtained from normal muscles; thus the rate of heat production by the Ca2+-ATPase expressed in terms of g of muscle in hyperthyroidism is increased by a factor of 3.6 in white muscles and 12.0 in red muscles. The data presented suggest that the Ca2+-ATPase uncoupled activity may represent one of the heat sources that contributes to the enhanced thermogenesis noted in hyperthyroidism.

Brown adipose tissue Ca2+-ATPase: uncoupled ATP hydrolysis and thermogenic activity

In this report a sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) was identified in rats brown adipose tissue. Electrophoretic analysis of brown fat microssomal protein yields a 110-kDa band that is reactive to SERCA 1 antibody but is not reactive to SERCA 2 antibodies. Nevertheless, the kinetics properties of the brown fat SERCA differ from the skeletal muscle SERCA 1 inasmuch they manifest a different Ca2+ affinity and a much higher degree of ATPase/Ca2+ uncoupling. A SERCA enzyme is not found in white fat. Fatty acids promoted Ca2+ leakage from brown fat vesicles. The heat released during ATP hydrolysis was -24.7 kcal/mol when a Ca2+ gradient was formed across the vesicles membrane and -14.4 kcal/mol in the absence of a gradient. The data reported suggest that in addition to storing Ca2+ inside the endoplasmic reticulum, the Ca2+-ATPase may represent a source of heat production contributing to the thermogenic function of brown adipose tissue.

The thermogenic function of the sarcoplasmic reticulum Ca2+-ATPase of normal and hyperthyroid rabbit

After formation of a Ca(2+) gradient, the amount of heat released during the hydrolysis of each mol of ATP cleaved (DeltaH(cal)) varies depending on the Ca(2+)-ATPase isoform expressed by the muscle cell. In vesicles derived from the sarcoplasmic reticulum of white muscle (SERCA 1) most of the ATP cleaved is not coupled to Ca(2+) transport, and the DeltaH(cal) varies between -20 and -22 kcal/mol. In contrast, in vesicles derived from red muscle (SERCA 2a) the hydrolysis of ATP is coupled with Ca(2+) transport, and the DeltaH(cal) varies between -12 and -14 kcal/mol. Hyperthyroidism increases the rate of heat production by the Ca(2+)-ATPase fourfold in white muscle and 40-fold in red muscle. In hyperthyroid rabbits, the amount of sarcoplasmic reticulum protein recovered from white and red muscle is four- to fivefold greater than that obtained from control rabbits. Hyperthyroid red muscle expresses SERCA 1, and the vesicles derived from these muscle hydrolyze ATP through a catalytic route that is not coupled to Ca(2+) transport, thus increasing the amount of heat released during ATP hydrolysis, the DeltaH(cal) varying between -20 and -22 kcal/mol.

Dissecting the hydrolytic activities of sarcoplasmic reticulum ATPase in the presence of acetyl phosphate

Sarcoplasmic reticulum vesicles and purified Ca(2+)-ATPase hydrolyze acetyl phosphate both in the presence and absence of Ca(2+). The Ca(2+)-independent activity was fully sensitive to vanadate, insensitive to thapsigargin, and proceeded without accumulation of phosphorylated enzyme. Acetyl phosphate hydrolysis in the absence of Ca(2+) was activated by dimethyl sulfoxide. The Ca(2+)-dependent activity was partially sensitive to vanadate, fully sensitive to thapsigargin, and associated with steady phosphoenzyme accumulation. The Ca(2+)/P(i) coupling ratio at neutral pH sustained by 10 mm acetyl phosphate was 0.57. Addition of 30% dimethyl sulfoxide completely blocked Ca(2+) transport and partially inhibited the hydrolysis rate. Uncoupling induced by dimethyl sulfoxide included the accumulation of vanadate-insensitive phosphorylated enzyme. When acetyl phosphate was the substrate, the hydrolytic pathway was dependent on experimental conditions that might or might not allow net Ca(2+) transport. The interdependence of both Ca(2+)-dependent and Ca(2+)-independent hydrolytic activities was demonstrated.

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.

The slippage of the Ca2+ pump and its control by anions and curcumin in skeletal and cardiac sarcoplasmic reticulum

Ca(2+) transport by sarcoplasmic reticulum (SR) ATPase occurs with an optimal coupling ratio of 2 Ca(2+) per ATP in pre-steady state. However, slippage of the pump and lower coupling ratios are observed in steady state. Slippage depends on the presence of high Ca(2+) in the lumen of SR vesicles and high nucleotide in the medium. Thereby, Ca(2+) and/or nucleotide-bound phosphoenzyme intermediates accumulate and undergo uncoupled cleavage, before vectorial translocation of bound Ca(2+) in the forward direction of the cycle or before productive reversal to ATP synthesis. Transport efficiency and coupling ratios are improved by reduction of nucleotide concentration in the presence of ATP regenerating systems and/or complexation of luminal Ca(2+) with phosphate or oxalate. Curcumin (1-5 microm) lowers the concentration of phosphate or oxalate required to reduce slippage of the Ca(2+) pump. Thereby, under appropriate conditions, curcumin favors kinetic flow, completion of productive cycles, and improvement of coupling ratios. The findings obtained with isolated SR vesicles suggest that slippage is an important phenomenon under prevailing conditions of muscle fibers in situ. Ca(2+) transport and its slippage can be improved by curcumin in cardiac as well as in skeletal SR, raising the possibility of pharmacological interventions to correct defective Ca(2+) homeostasis. Higher curcumin concentrations (5-30 microm), however, inhibit overall ATPase activity and Ca(2+) transport by interfering with phosphoenzyme formation with ATP or P(i).

Kinetics of the Ca(2+), H(+), and Mg(2+) interaction with the ion-binding sites of the SR Ca-ATPase

Electrochromic styryl dyes were used to investigate mutually antagonistic effects of Ca(2+) and H(+) on binding of the other ion in the E(1) and P-E(2) states of the SR Ca-ATPase. On the cytoplasmic side of the protein in the absence of Mg(2+) a strictly competitive binding sequence, H(2)E(1) <==> HE(1) <==> E(1) <==> CaE(1) <==> Ca(2)E(1), was found with two Ca(2+) ions bound cooperatively. The apparent equilibrium dissociation constants were in the order of K(1/2)(2 Ca) = 34 nM, K(1/2)(H) = 1 nM and K(1/2)(H(2)) = 1.32 microM. Up to 2 Mg(2+) ions were also able to enter the binding sites electrogenically and to compete with the transported substrate ions (K(1/2)(Mg) = 165 microM, K(1/2)(Mg(2)) = 7.4 mM). In the P-E(2) state, with binding sites facing the lumen of the sarcoplasmatic reticulum, the measured concentration dependence of Ca(2+) and H(+) binding could be described satisfactorily only with a branched reaction scheme in which a mixed state, P-E(2)CaH, exists. From numerical simulations, equilibrium dissociation constants could be determined for Ca(2+) (0.4 mM and 25 mM) and H(+) (2 microM and 10 microM). These simulations reproduced all observed antagonistic concentration dependences. The comparison of the dielectric ion binding in the E(1) and P-E(2) conformations indicates that the transition between both conformations is accompanied by a shift of their (dielectric) position.

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.

Testing the versatility of the sarcoplasmic reticulum Ca(2+)-ATPase reaction cycle when p-nitrophenyl phosphate is the substrate

A detailed characterization of p-nitrophenyl phosphate as energy-donor substrate for the sarcoplasmic reticulum Ca(2+)-ATPase was undertaken in this study. The fact that p-nitrophenyl phosphate can be hydrolyzed in the presence or absence of Ca(2+) by the purified enzyme is consistent with the observed phenomenon of intramolecular uncoupling. Under the most favorable conditions, which include neutral pH, intact microsomal vesicles, and low free Ca(2+) in the lumen, the Ca(2+)/P(i) coupling ratio was 0.6. A rise or decrease in pH, high free Ca(2+) in the lumenal space, or the addition of dimethyl sulfoxide increase the intramolecular uncoupling. Alkaline pH and/or high free Ca(2+) in the lumen potentiate the accumulation of enzyme conformations with high Ca(2+) affinity. Acidic pH and/or dimethyl sulfoxide favor the accumulation of enzyme conformations with low Ca(2+) affinity. Under standard assay conditions, two uncoupled routes, together with a coupled route, are operative during the hydrolysis of p-nitrophenyl phosphate in the presence of Ca(2+). The prevalence of any one of the uncoupled catalytic cycles is dependent on the working conditions. The proposed reaction scheme constitutes a general model for understanding the mechanism of intramolecular energy uncoupling.

Kinetic analysis of a model of the sarcoplasmic reticulum Ca-ATPase, with variable stoichiometry, which enhances the amount and the rate of Ca transport

The sarcoplasmic reticulum (SR) Ca-dependent adenosinetriphosphatase (Ca-ATPase) actively transports Ca2+ from the myoplasm to the SR lumen. Under optimal conditions a 2:1 stoichiometry of Ca transport/ATP hydrolysis has been observed, but lower stoichiometries have been reported under several circumstances. A lower stoichiometry under conditions of high Ca2+ load, although thermodynamically less efficient, could in theory increase the rate and the maximal amount of Ca uptake. We analysed, by computing simulation, the transient kinetics of a model of the SR Ca-ATPase with variable stoichiometry. The model is based on current experimental reports and includes the most relevant properties of the system. The results show an acceleration in the rate of Ca uptake, an increase in the net Ca transport, and an increase in the rate of [Ca2+] reduction in the medium, which might be physiologically useful to increase the rate of Ca pumping at high Ca load of the sarcoplasmic reticulum.

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.