Conformational changes of the Ca2+-ATPase as early events of Ca2+ release from sarcoplasmic reticulum

Kinetics on Demand Is a Simple Mathematical Solution that Fits Recorded Caffeine-Induced Luminal SR Ca2+ Changes in Smooth Muscle Cells

The process of Ca²⁺ release from sarcoplasmic reticulum (SR) comprises 4 phases in smooth muscle cells. Phase 1 is characterized by a large increase of the intracellular Ca²⁺ concentration ([Ca²⁺]_i) with a minimal reduction of the free luminal SR [Ca²⁺] ([Ca²⁺]_FSR). Importantly, active SR Ca²⁺ ATPases (SERCA pumps) are necessary for phase 1 to occur. This situation cannot be explained by the standard kinetics that involves a fixed amount of luminal Ca²⁺ binding sites. A new mathematical model was developed that assumes an increasing SR Ca²⁺ buffering capacity in response to an increase of the luminal SR [Ca²⁺] that is called Kinetics-on-Demand (KonD) model. This approach can explain both phase 1 and the refractory period associated with a recovered [Ca²⁺]_FSR. Additionally, our data suggest that active SERCA pumps are a requisite for KonD to be functional; otherwise luminal SR Ca²⁺ binding proteins switch to standard kinetics. The importance of KonD Ca²⁺ binding properties is twofold: a more efficient Ca²⁺ release process and that [Ca²⁺]_FSR and Ca²⁺-bound to SR proteins ([Ca²⁺]_BSR) can be regulated separately allowing for Ca²⁺ release to occur (provided by Ca²⁺-bound to luminal Ca²⁺ binding proteins) without an initial reduction of the [Ca²⁺]_FSR.

An intelligent sarco-endoplasmic reticulum Ca2+ store: release and leak channels have differential access to a concealed Ca2+ pool

Simultaneous recording of cytosolic and sarco-endoplasmic reticulum (SR/ER) luminal free calcium concentrations ([Ca(2+)](i) and [Ca(2+)](L), respectively) supports the notion that release channels (RyRs and IP(3)Rs) use a concealed Ca(2+) source, likely to be associated with intra-SR/ER Ca(2+) binding proteins, whereas SR/ER Ca(2+) leak channels can only access free luminal Ca(2+). We hypothesize that Ca(2+) is trapped by oligomers of luminal Ca(2+)-binding proteins and that the opening of release channels induces the rapid liberation of this "concealed" Ca(2+) source associated with intra-ER Ca(2+) buffers. Our hypothesis may also clarify why SERCA pumps potentiate Ca(2+) release and explain quantal characteristics and refractory states of Ca(2+) release process.

Inhibition of SERCA pumps induces desynchronized RyR activation in overloaded internal Ca2+ stores in smooth muscle cells

We have previously shown that rapid inhibition of sarcoplasmic reticulum (SR) ATPase (SERCA pumps) decreases the amplitude and rate of rise (synchronization) of caffeine induced-Ca2+ release without producing a reduction of free luminal SR Ca2+ level in smooth muscle cells (Gómez-Viquez L, Guerrero-Serna G, García U, Guerrero-Hernández A. Biophys J 85: 370–380, 2003). Our aim was to investigate the role of luminal SR Ca2+ content in the communication between ryanodine receptors (RyRs) and SERCA pumps. To this end, we studied the effect of SERCA pump inhibition on RyR-mediated Ca2+ release in smooth muscle cells with overloaded SR Ca2+ stores. Under this condition, the amplitude of RyR-mediated Ca2+ release was not affected but the rate of rise was still decreased. In addition, the caffeine-induced Ca2+-dependent K+ outward currents revealed individual events, suggesting that SERCA pump inhibition reduces the coordinated activation of RyRs. Collectively, our results indicate that SERCA pumps facilitate the activation of RyRs by a mechanism that does not involve the regulation of SR Ca2+ content. Importantly, SERCA pumps and RyRs colocalize in smooth muscle cells, suggesting a possible local communication between these two proteins.

SERCA pump optimizes Ca2+ release by a mechanism independent of store filling in smooth muscle cells

Thapsigargin-sensitive sarco/endoplasmic reticulum Ca(2+) pumps (SERCAs) are involved in maintaining and replenishing agonist-sensitive internal stores. Although it has been assumed that release channels act independently of SERCA pumps, there are data suggesting the opposite. Our aim was to study the relationship between SERCA pumps and the release channels in smooth muscle cells. To this end, we have rapidly blocked SERCA pumps with thapsigargin, to avoid depletion of the internal Ca(2+) stores, and induced Ca(2+) release with either caffeine, to open ryanodine receptors, or acetylcholine, to open inositol 1,4,5-trisphosphate receptors. Blocking SERCA pumps produced smaller and slower agonist-induced [Ca(2+)](i) responses. We determined the Ca(2+) level of the internal stores both indirectly, measuring the frequency of spontaneous transient outward currents, and directly, using Mag-Fura-2, and demonstrated that the inhibition of SERCA pumps did not produce a reduction of the sarco/endoplasmic reticulum Ca(2+) levels to explain the decrease in the agonist-induced Ca(2+) responses. It appears that SERCA pumps are involved in sustaining agonist-induced Ca(2+) release by a mechanism that involves the modulation of Ca(2+) availability in the lumen of the internal stores.

RyR1/SERCA1 cross-talk regulation of calcium transport in heavy sarcoplasmic reticulum vesicles

We investigated the functional interdependence of sarco-endoplasmic reticulum Ca2+ ATPase isoform 1 and ryanodine receptor isoform 1 in heavy sarcoplasmic reticulum membranes by synchronous fluorescence determination of extravesicular Ca2+ transients and catalytic activity. Under conditions of dynamic Ca2+ exchange ATPase catalytic activity was well coordinated to ryanodine receptor activation/inactivation states. Ryanodine-induced activation of Ca2+ release channel leaks also produced marked ATPase activation in the absence of measurable increases in bulk free extravesicular Ca2+. This suggested that Ca2+ pumps are highly sensitive to Ca2+ release channel leak status and potently buffer Ca2+ ions exiting cytoplasmic openings of ryanodine receptors. Conversely, ryanodine receptor activation was dependent on Ca2+-ATPase pump activity. Ryanodine receptor activation by cytosolic Ca2+ was (i) inversely proportional to luminal Ca2+ load and (ii) dependent upon the rate of presentation of cytosolic Ca2+. Progressive Ca2+ filling coincided with progressive loss of Ca2+ sequestration rates and at a threshold loading, ryanodine-induced Ca2+ release produced small transient reversals of catalytic activity. These data indicate that attainment of threshold luminal Ca2+ loads coordinates sensitization of Ca2+ release channels with autogenic inhibition of Ca2+ pumping. This suggests that Ca2+-dependent control of Ca2+ release in intact heavy sarcoplasmic reticulum membranes involves a Ca2+-mediated "cross-talk" between sarco-endoplasmic reticulum Ca2+ ATPase isoform 1 and ryanodine receptor isoform 1.

The luminal Ca2+ transient controls Ca2+ release/re-uptake of sarcoplasmic reticulum

Our recent study (Saiki, Y., and Ikemoto, N., Biochemistry 38, 3112-3119, 1999) suggests that Ca2+ release and re-uptake of the released Ca2+ are coordinated. The following results suggest that the coordination is mediated by the luminal Ca2+ ([Ca2+]lum) transient. Upon inducing the release of the passively loaded Ca2+ from the SR with polylysine, the luminal Ca2+ ([Ca2+]lum) first increased then decreased ([Ca2+]lum transient). The activity of the SR Ca2+ ATPase was monitored at different times after inducing Ca2+ release. The phosphoenzyme (EP) formation as determined by the MANT-fluorescence increased concurrently with the initial rapid increase in the [Ca2+]lum. EP decay (pumping turnover) was accelerated concurrently with a decrease of the [Ca2+]lum. The results suggest that the [Ca2+]lum transient serves as a mediator for the acceleration of the Ca2+ re-uptake occurring soon after the induction of Ca2+ release.

Kinetic basis of quantal calcium release from intracellular calcium stores

The kinetics of Ca2+ release from canine cerebellum and rabbit skeletal muscle microsomes, mediated by the inositol 1,4,5-trisphosphate (IP3) receptor (IRC) and the ryanodine receptor (RyRC), respectively, were analyzed by a model, which considers that Ca2+ release channels undergo spontaneous inactivation. We found that: (i) both the initial rate of release (Vo) and the rate of inactivation (Vi) were saturable functions of the activating ligand concentration (CL); and (ii) the ratio of Vi/Vo, termed the relative tendency for inactivation, decreased with increasing CL. Equilibrium [3H]-IP3 binding studies, on the other hand, revealed the presence of one single class of non-co-operative IP3 sites in cerebellum membranes (Kdeq = 47 nM and Hill coefficient = 1.1). Based on the above Vi-Vo relationship and the IP3-binding data, we propose that quantal Ca2+ release through IRCs might be a result of spontaneous channel inactivation, whose rate is controlled by the ratio of IP3-occupied/free monomers in the tetrameric release channel units. Furthermore, because of the kinetic similarities between the IRC- and RyRC-mediated Ca2+ release processes, as well as between quantal Ca2+ release and channel adaptation, the same mechanism is also proposed to apply to the RyRC-mediated Ca2+ release as well as to constitute the basis of release channel adaptation.

Inactivation of the cardiac ryanodine receptor calcium release channel by nitric oxide

We have recently reported [Mészáros L.G., Minarovic I., Zahradníková A. Inhibition of the skeletal muscle ryanodine receptor calcium release channel by nitric oxide. FEBS Lett 1996; 380: 49-52] that nitric oxide (NO) reduces the activity of the skeletal muscle ryanodine receptor Ca2+ release channel (RyRC), a principal component of the excitation-contraction coupling machinery in striated muscles. Since (i) as shown here, we have obtained evidence which indicates that the NO synthase (eNOS) of cardiac muscle origin co-purified with RyRC-containing sarcoplasmic reticulum (SR) fractions; and (ii) the effects of NO donors on the release channel, as well as on cardiac function, appear somewhat contradictory, we have made an attempt to investigate the response of the cardiac RyRC to NO that is generated in situ from L-arginine in the NOS reaction. We found that L-arginine-derived NO inactivates Ca2+ release from cardiac SR and reduces the steady-state activity (i.e. open probability) of single RyRCs fused into a planar lipid bilayer. This reduction was prevented by NOS inhibitors and the NO quencher hemoglobin and was reversed by 2-mercaptoethanol. We thus conclude that: (i) in isolated SR preparations, it is possible to assess the effects of NO that is generated from L-arginine in the NOS reaction; and (ii) cardiac RyRc responds to NO in a manner which is identical to that we have previously found with the skeletal channel. These findings suggest that the direct modulation of the RyRC by NO is a signaling mechanism which likely participates in earlier demonstrated NO-induced myocardial contractility changes.

Inhibition of the skeletal muscle ryanodine receptor calcium release channel by nitric oxide

NO donors were found to reduce the rate of Ca2+ release from isolated skeletal muscle sarcoplasmic reticulum (SR) and the open probability of single ryanodine receptor Ca2+ release channels (RyRCs) in planar lipid bilayers, and these effects were prevented by the NO quencher hemoglobin and reversed by 2-mercaptoethanol. Ca2+ release assessed in skeletal muscle homogenates was also reduced by NO that was generated in situ from L-arginine by endogenous, nitro-L-arginine methylester-sensitive NO-synthase. The effect of NO on the RyRC might explain NO-induced depression of contractile force in striated muscles and, since both RyRC isoforms and NOS isoenzymes aer ubiquitous, may represent a wide-spread feedback mechanism in Ca2+ signaling; i.e. Ca-dependent activation of NO production and NO-evoked reduction of Ca2+ release from intracellular Ca2+ stores.

Coexistence of high- and low-affinity Ca2+ binding sites of the sarcoplasmic reticulum calcium pump

We have recently shown [Mészáros, L. G., & Bak, J. (1992) Biochemistry 31, 1195-1200] that, during the rapid phase of Ca2+ uptake into sarcoplasmic reticulum (SR), internalization and binding of Ca2+ to the cytoplasmic high-affinity binding sites of the Ca2+ ATPase occur simultaneously, resulting in a transient supernumerary Ca/ATP stoichiometry. Here we address the question of whether the cytoplasmic high-affinity and the luminal low-affinity Ca2+ binding sites of the SR Ca2+ ATPase also coexist. SR vesicles were loaded with Ca2+ (0-10 mM), and then the kinetics of EP formation and decomposition as well as the maximum level of EP formed from radiolabeled ATP were determined at conditions which only allow single-cycle reactions to occur: empty or Ca-loaded SR vesicles (in micromolar extravesicular Ca2+) were either mixed with ATP plus millimolar EGTA or added in amounts that set a Ca2+ ATPase/ATP ratio of 80-85 at the initiation of the reaction. The rates of EP formation and decomposition were both significantly reduced in Ca-loaded, compared to empty (ionomycin-treated), vesicles. However, the value of EPmax was unaltered by Ca-loading, suggesting the existence of the enzyme intermediate, E.Ca2(cyt).Ca2(lum), i.e., the coexistence of the cytoplasmic and the luminal Ca2+ binding sites of the Ca-pump. These results suggest that the uphill transport of Ca2+ might not be based on an alternating relocation and conversion of the Ca2+ binding sites of the Ca2+ ATPase.

Cyclic ADP-ribose as an endogenous regulator of the non-skeletal type ryanodine receptor Ca2+ channel

The skeletal and cardiac isoforms of the ryanodine receptor Ca2+ channel (RyRC) constitute the Ca2+ release pathway in sarcoplasmic reticulum of skeletal and cardiac muscles, respectively. A direct mechanical and a Ca(2+)-triggered mechanism (Ca(2+)-induced Ca2+ release) have been respectively proposed to explain the in situ activation of Ca2+ release in skeletal and cardiac muscle. In non-muscle cells, however, where the RyRC also participates in Ca2+ signalling, the mechanism of RyRC activation is unknown. Cyclic adenosine 5'-diphosphoribose (cADPR), which is present in many mammalian tissues, has been reported to induce Ca2+ release from ryanodine-sensitive intracellular Ca2+ stores in sea urchin eggs. Here we provide evidence that cADPR directly activates the cardiac but not the skeletal isoform of the RyRC. This, together with results on sea urchin eggs, suggests that cADPR is an endogenous activator of the non-skeletal type of RyRC and may thus have a role similar to inositol 1,4,5-trisphosphate in Ca2+ signalling.

Caffeine- and ryanodine-sensitive Ca2+ stores of canine cerebrum and cerebellum neurons

[3H]ryanodine binding to and Ca2+ release from microsomal fractions derived from canine cerebrum (CBR) and cerebellum (CBL) were investigated. High-affinity ryanodine binding sites were detected in both cerebrum and cerebellum microsomes [CBR: maximal binding capacity (Bmax) = 446 fmol/mg protein, dissociation constant (Kd) = 9 nM, Hill coefficient (n) = 0.95; CBL: Bmax = 650, Kd = 12, n = 1.8]. Ryanodine binding in both fractions was increased by millimolar concentrations of ATP [or its nonhydrolyzable analogue beta, gamma-methyleneadenosine 5'-triphosphate (AMP-PCP)] and micromolar concentrations of Ca2+ but was decreased by micromolar concentrations of ruthenium red, similar to that found in sarcoplasmic reticulum (SR) of striated muscle. The addition of caffeine or the sudden elevation of extravesicular Ca2+ induced a rapid La(3+)-sensitive Ca2+ release from both CBR and CBL microsomal fractions with rate constants of approximately 100 s-1, as determined by stopped-flow photometry of the Ca2+ indicator arsenazo III. The release of Ca2+ was activated by either millimolar ATP or AMP-PCP, blocked by micromolar concentrations of La3+, and significantly inhibited by 50 microM ryanodine. Mg2+ and ruthenium red in millimolar and micromolar concentrations, respectively, caused only a slight inhibition of Ca2+ release. These results indicate that rapid Ca2+ release occurs from caffeine-, Ca2+- and ryanodine-sensitive Ca2+ stores in both CBR and CBL microsomal fractions.

Non-identical behavior of the Ca2(+)-ATPase in the terminal cisternae and the longitudinal tubules fractions of sarcoplasmic reticulum

The kinetics of Ca2+ dissociation from and binding to the Ca2(+)-ATPase and the coupled tryptophan fluorescence changes were compared in the heavy and the light sarcoplasmic reticulum (SR) fractions. It was found that in the light SR both the dissociation of Ca2+ from the Ca2(+)-ATPase and the coupled tryptophan fluorescence change took place in a biphasic fashion, but they were monophasic in the heavy SR. On the other hand, the time courses of both the Ca2+ binding and the coupled tryptophan fluorescence increase were biphasic, and virtually indistinguishable between the heavy and the light SR fractions. Submicromolar ruthenium red altered the kinetics of both the Ca2+ dissociation and the coupled fluorescence change in heavy, but not in light SR; the monophasic time course characteristic of the heavy fraction became biphasic, similar to that found for light SR in the absence of ruthenium red. Extraction of non-ATPase proteins from the heavy SR vesicles also changed the kinetics of the Ca2+ dissociation-coupled fluorescence decrease in the heavy SR from monophasic to biphasic. These results suggest that the difference between the heavy and light SR Ca2(+)-ATPase in the Ca2+ dissociation kinetics is most probably produced by a ruthenium-red-sensitive interaction of the Ca2(+)-ATPase with additional protein(s), which is/are present exclusively in the heavy SR, rather than being due to a difference inherent to the Ca2(+)-ATPase polypeptide itself.

Kinetic analysis of excitation-contraction coupling

Recent studies of isolated muscle membrane have enabled induction and monitoring of rapid Ca2+ release from sarcoplasmic reticulum (SR)5 in vitro by a variety of methods. On the other hand, various proteins that may be directly or indirectly involved in the Ca2+ release mechanism have begun to be unveiled. In this mini-review, we attempt to deduce the molecular mechanism by which Ca2+ release is induced, regulated, and performed, by combining the updated information of the Ca2+ release kinetics with the accumulated knowledge about the key molecular components.

Oxidative stress and muscular dystrophy

Oxidative stress may be the fundamental basis of many of the structural, functional and biochemical changes characteristic of the inherited muscular dystrophies in animals and humans. The presence of by-products of oxidative damage, and the compensatory increases in cellular antioxidants, both indicate oxidative stress may be occurring in dystrophic muscle. Changes in the proportions and metabolism of cellular lipids, abnormal functions of cellular membranes, altered activity of membrane-bound enzymes such as the SR Ca2+-ATPase, disturbances in cellular protein turnover and energy production and a variety of other changes all indicate that these inherited muscular dystrophies appear more like the results of oxidative stress to muscle than any other type of underlying muscle disturbance. Particular details of these altered characteristics of dystrophic muscle, in combination with current knowledge on the processes of oxidative damage to cells, may provide some insight into the underlying biochemical defect responsible for the disease, as well as direct research towards the ultimate goal of an effective treatment.

Caffeine interaction with the Ca-release channels of heavy sarcoplasmic reticulum. Evidence that 170 kD Ca-binding protein is a caffeine receptor of the Ca-channels

The study of Ca2+- and caffeine-induced Ca2+ release from heavy sarcoplasmic reticulum vesicles under the different conditions suggests that Ca2+ and caffeine can interact with the common receptor of the Ca-release channels. The reticulum membranes were solubilized using nonionic detergent polyoxyethylene 9-lauryl ether, and affinity chromatography on reactive red 120-agarose was carried out. The 170 kD Ca-binding protein which is eluted by caffeine is the most probable candidate for the caffeine receptor of the Ca-channels.

The interaction of calcium and ryanodine with cardiac sarcoplasmic reticulum

The binding of [3H]ryanodine with cardiac sarcoplasmic reticulum vesicles depends on the calcium concentration. Binding in the absence of calcium appears to be non-specific because it shows no saturation up to 20 microM ryanodine. The apparent Km value for calcium varied between 2 and 0.8 microM when the ryanodine concentration varied between 10 and 265 nM. The Hill coefficient for the calcium dependence of [3H]ryanodine binding was near two. Scatchard analysis of ryanodine binding indicated a high-affinity site with a Bmax of 5.2 +/- 0.4 pmol/mg with a Kd of 6.8 +/- 0.1 nM. Preincubation under conditions in which the high-affinity sites were saturated did not result in stimulation of the calcium uptake rate indicative of closure of the calcium channel. Stimulation of calcium uptake rate occurred only at higher concentrations of ryanodine (apparent Km = 17 microM). This stimulation of the calcium uptake rate also required calcium in the submicromolar range. The data obtained support the hypothesis that ryanodine binding to the low-affinity site (Km about 17 microM) is responsible for closure of the calcium release channel and the subsequent increase in the calcium uptake rate of the sarcoplasmic reticulum. Because the number of ryanodine-binding sites is much less than the number of calcium transport pumps the channel is probably distinct from the pump.

Solubilization and separation of Ca2+-ATPase from the Ca2+-ryanodine receptor complex

Heavy sarcoplasmic reticulum (SR) preparations of rabbit skeletal muscle, which are enriched in Ca2+-release vesicles from the terminal cisternae (TC) and [3H]ryanodine receptor density, exhibit 60% of the Ca2+-ATPase activity, 58% of the EP level, and 30% of the steady state Ca2+ loading compared to membrane vesicles from the longitudinal SR. The Ca2+-ATPase of TC SR is solubilized and separated from the Ca2+-ryanodine receptor complex in the insoluble fraction on treatment with the detergent C12E9. However, a 50% decrease in receptor density is observed upon removal of the Ca2+-ATPase, suggesting a significant contribution of this protein to maintaining optimal receptor complex density.