Calcium oxalate and calcium phosphate capacities of cardiac sarcoplasmic reticulum

Application of Calcium Kinetics Characterization in Cardiac Disease Modeling and Drug Discovery

Calcium regulation is essential in virtually any cell due to its critical role as a second messenger in multiple signaling pathways [...].

Calcium Uptake in Crude Tissue Preparation

The various isoforms of the sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA) are responsible for the Ca(2+) uptake from the cytosol into the endoplasmic or sarcoplasmic reticulum (ER/SR). In some tissues, the activity of SERCA can be modulated by binding partners, such as phospholamban and sarcolipin. The activity of SERCA can be characterized by its apparent affinity for Ca(2+) as well as maximal enzymatic velocity. Both parameters can be effectively determined by the protocol described here. Specifically, we describe the measurement of the rate of oxalate-facilitated (45)Ca uptake into the SR of crude mouse ventricular homogenates. This protocol can easily be adapted for different tissues and animal models as well as cultured cells.

Cellular mechanism of the voltage-dependent change in slow potentials generated in circular smooth muscle of the guinea-pig gastric corpus

The cellular mechanism of the voltage-dependent properties of slow potentials were investigated in single bundles of circular smooth muscle isolated from the gastric corpus of guinea-pig using conventional microelectrode recordings. Hyperpolarization of the membrane by current injection decreased the frequency and increased the amplitude of slow potentials linearly. At potentials negative of -80 mV, slow potential generation was abolished and a periodic generation of clustered unitary potentials was evident. Application of cyclopiazonic acid (CPA, 20 microM) or thapsigargin (1 microM; inhibitors of Ca(2+)-ATPase), carbonyl cyanide m-chlorophenyl hydrazone (CCCP, 0.1 microM; mitochondrial protonophore) or 2-aminoethoxydiphenyl borate (2-APB, 20 microM; inhibitor of IP(3) receptor-mediated Ca(2+) release) depolarized the membrane and reduced or inhibited the amplitude and frequency of slow potentials: repolarization of the membrane to the resting level by current injection resulted in a recovery of the amplitude of slow potentials in the presence of CPA or CCCP, but not 2-APB. The slow potentials abolished by thapsigargin did not recover upon membrane repolarization. The altered frequency of slow potentials by 2-APB, CPA or CCCP was not reversed by membrane repolarization to control potentials. Depolarization of the membrane by about 10 mV with high-potassium solution also reduced the amplitude and increased the frequency of slow potentials in a manner restored by repolarization to control potentials upon current injection, suggesting that membrane depolarization did not affect the voltage dependency of pacemaker activity. The results indicate that in corpus circular muscles the voltage dependency of the frequency and amplitude of slow potentials requires a functional Ca(2+) store and mitochondria.

Metabolic factors contributing to altered Ca2+ regulation in skeletal muscle fatigue

AIM

Skeletal muscle fatigue is characterized by a failure to maintain force production or power output during intense exercise. Many recent studies on isolated fibres have used brief repetitive tetanic contractions to mimic fatigue resulting from intensive exercise and to investigate the underlying cellular mechanisms. Such studies have shown that characteristic changes in Ca2+ regulation occur during fatiguing stimulation. This includes prolongation of the 'Ca2+-tails' which follow each period of tetanic stimulation and a progressive rise in resting [Ca2+]. More importantly, the final stage of fatigue is associated with a rapid decrease in tetanic [Ca2+]i and force. These fatigue-induced changes in sarcoplasmic reticulum (SR) Ca2+ regulation are temporally associated with alterations in the intracellular levels of phosphate metabolites and a causal relationship has often been proposed. The aim of this review is to evaluate the evidence linking changes in the levels of phosphate metabolites and altered Ca2+ regulation during fatigue.

RESULTS

The following current hypotheses will be discussed: (1) the early changes in Ca2+ regulation reflect alterations in the intracellular levels of phosphate metabolites, (2) inhibition of the SR Ca2+ release mechanism (e.g. caused by ATP depletion and increased [Mg2+]) contributes to the decrease in tetanic [Ca2+]i during the final stages of fatigue and (iii) delayed entry of inorganic phosphate ions (Pi) into the SR, followed by precipitation of calcium phosphate (Ca-Pi), can explain the fatigue-induced decrease in tetanic [Ca2+]i.

CONCLUSION

There is strong evidence that changes in phosphate metabolite levels contribute to early changes in SR Ca2+ regulation during fatigue and that inhibition of the SR Ca2+ release mechanism can partially explain the rapid decrease in tetanic [Ca2+]i during the final stages of fatigue. While precipitation of Ca-Pi may occur within the SR during fatigue, there is currently insufficient evidence to establish whether this contributes to the late decline in tetanic [Ca2+]i.

Physiological characterization of the Na(+)/Ca(2+) exchanger (NCX) in hepatopancreatic and antennal gland basolateral membrane vesicles isolated from the freshwater crayfish Procambarus clarkii

The purpose of this study was to physiologically characterize the basolateral Na(+)/Ca(2+) exchanger (NCX) in basolateral membrane vesicles (BLMVs) of hepatopancreas and antennal gland of intermolt crayfish. Conditions were optimized to measure Na(+)-dependent Ca(2+) uptake and retention in the BLMV including use of intravesicular (IV) oxalate and measuring initial uptake rates at 20 s. Na(+)-dependent Ca(2+) uptake rate into BLMV was temperature insensitive. Na(+)-dependent Ca(2+) uptake rate was dependent upon free Ca(2+) with saturable Michaelis-Menten kinetics determined as follows: hepatopancreas, maximal uptake rate (J(max))=2.45 nmol/mg per min, concentration at which carrier operates at half-maximal uptake rate (K(m))=0.69 microM Ca(2+); antennal gland, J(max)=13.2 nmol/mg per min, K(m)=0.59 microM Ca(2+). The two vesicle populations exhibited different sensitivity to putative NCX inhibitors. Benzamil had no effect on Na(+)-dependent Ca(2+) uptake rate in hepatopancreas; in antennal gland it was inhibitory at concentrations up to 30 microM and was stimulatory at higher concentrations. Conversely the inhibitor quinacrine was inhibitory at 10 microM in hepatopancreas and was stimulatory at 1000 microM; meanwhile it was ineffective in antennal gland BLMV. Short circuiting the BLMV had no effect on Na(+)-dependent Ca(2+) uptake rate suggesting that the process may be electroneutral. Compared with another prominent basolateral transporter in hepatopancreas the plasma membrane Ca(2+) ATPase (PMCA), the NCX has 70-fold greater J(max) (at comparable temperature) and a lower affinity. In antennal gland the NCX has 40-fold greater J(max) and a lower affinity. In hepatopancreas and antennal gland BLMV NCX appears to determine the rate of basolateral Ca(2+) efflux in intermolt.

Interdependent effects of inorganic phosphate and creatine phosphate on sarcoplasmic reticulum Ca2+ regulation in mechanically skinned rat skeletal muscle

1. The effects of creatine phosphate (CP) and inorganic phosphate (Pi) on sarcoplasmic reticulum (SR) Ca2+ regulation were investigated in mechanically skinned muscle fibres from rat extensor digitorum longus (EDL) muscles. Changes in [Ca2+] were detected using fura-2 fluorescence, during continuous perfusion or when the solution surrounding the preparation was restricted to approximately 6 microl by stopping perfusion. 2. In solutions with 5 mM ATP and 10 mM CP, stopping the flow for 2-3 min had no effect on [Ca2+] within the bath. This suggests that SR Ca2+ uptake is balanced by an efflux under these conditions. 3. In solutions with CP, the introduction of Pi induced a small transient rise in [Ca2+], due to Ca2+ loss from the SR. Following equilibration with solutions containing Pi (> or = 5 mM), a maintained decrease in [Ca2+] occurred when the flow was stopped. This is consistent with calcium phosphate (Ca-Pi) precipitation within the SR, resulting in maintained Ca2+ uptake. 4. In the absence of CP, the [Ca2+] within the bath increased progressively when the flow was stopped. This rise in [Ca2+] was inhibited by an alternative ATP regenerating system comprising phosphoenolpyruvate (PEP) and pyruvate kinase (PK). Therefore, the loss of Ca2+ from the SR may result from local ADP accumulation and the consequent reversal of the SR Ca2+ pump. 5. In the absence of CP, the initial Ca2+ release associated with the introduction of Pi increased markedly. Following prolonged equilibration with solutions containing Pi, a rise in [Ca2+] occurred within the bath when the flow was stopped. Maintained Ca2+ uptake associated with Ca-Pi precipitation was not apparent at any level of Pi tested (1-60 mM), when CP was absent. 6. These results suggest that withdrawal of CP is associated with activation of a SR Ca2+ efflux pathway. This may involve reversal of the SR Ca2+ pump, due to local ADP accumulation. In the absence of CP, the dominant influence of Pi appears to involve further Ca2+ efflux via the SR Ca2+ pump. The possible relevance of these effects to skeletal muscle fatigue is considered.

Regulation of Ca2+ transport by sarcoplasmic reticulum Ca2+-ATPase at limiting [Ca2+]

The factors regulating Ca2+ transport by isolated sarcoplasmic reticulum (SR) vesicles have been studied using the fluorescent indicator Fluo-3 to monitor extravesicular free [Ca2+]. ATP, in the presence of 5 mM oxalate, which clamps intravesicular [Ca2+] at approximately 10 microM, induced a rapid decline in Fluo-3 fluorescence to reach a limiting steady state level. This corresponds to a residual medium [Ca2+] of 100 to 200 nM, and has been defined as [Ca2+]lim, whilst thermodynamic considerations predict a level of less than 1 nM. This value is similar to that measured in intact muscle with Ca2+ fluophores, where it is presumed that sarcoplasmic free [Ca2+] is a balance between pump and leaks. Fluorescence of Fluo-3 at [Ca2+]lim was decreased 70% to 80% by histidine, imidazole and cysteine. The K0.5 value for histidine was 3 mM, suggesting that residual [Ca2+]lim fluorescence is due to Zn2+. The level of Zn2+ in preparations of SR vesicles, measured by atomic absorption, was 0.47+/-0.04 nmol/mg, corresponding to 0.1 mol per mol Ca-ATPase. This is in agreement with findings of Papp et al. (Arch. Biochem. Biophys., 243 (1985) 254-263). Histidine, 20 mM, included in the buffer, gave a corrected value for [Ca2+]lim of 49+/-1.8 nM, which is still higher than predicted on thermodynamic grounds. A possible 'pump/leak' mechanism was tested by the effects of varying active Ca2+ transport 1 to 2 orders with temperature and pH. [Ca2+]lim remained relatively constant under these conditions. Alternate substrates acetyl phosphate and p-NPP gave similar [Ca2+]lim levels even though the latter substrate supported transport 500-fold slower than with ATP. In fact, [Ca2+]lim was lower with 10 mM p-NPP than with 5 mM ATP. The magnitude of passive efflux from Ca-oxalate loaded SR during the steady state of [Ca2+]lim was estimated by the unidirectional flux of 45Ca2+, and directly, following depletion of ATP, by measuring release of 40Ca2+, and was 0.02% of Vmax. Constant infusion of CaCl2 at [Ca2+]lim resulted in a new steady state, in which active transport into SR vesicles balances the infusion rate. Varying infusion rates allows determination of [Ca2+]-dependence of transport in the absence of chelating agents. Parameters of non-linear regression were Vmax=853 nmol/min per mg, K0.5(Ca)=279 nM, and nH(Ca)=1.89. Since conditions employed in this study are similar to those in the sarcoplasm of relaxed muscle, it is suggested that histidine, added to media in studies of intracellular Ca2+ transients, and in the relaxed state, will minimise contribution of Zn2+ to fluophore fluorescence, since it occurs at levels predicted in this study to cause significant overestimation of cytoplasmic free [Ca2+] in the relaxed state. Similar precautions may apply to non-muscle cells as well. This study also suggests that [Ca2+]lim in the resting state is a characteristic feature of Ca2+ pump function, rather than a balance between active transport and passive leakage pathways.

Mechanisms underlying phosphate-induced failure of Ca2+ release in single skinned skeletal muscle fibres of the rat

1. Single mechanically skinned fibres from rat extensor digitorum longus (EDL) muscles were used to investigate the mechanisms underlying inorganic phosphate (Pi) movements between the myoplasm and the sarcoplasmic reticulum (SR). Force transients elicited by caffeine/low Mg2+ application were used to assess the rate of Pi-induced inhibition of SR Ca2+ release and the subsequent recovery of Ca2+ release following removal of myoplasmic Pi. 2. Myoplasmic Pi reduced SR Ca2+ release in a concentration- and time-dependent manner. A 10 s exposure to 10, 20 and 50 mM myoplasmic Pi reduced SR Ca2+ release by 12 +/- 9, 29 +/- 5 and 82 +/- 5 %, respectively. 3. Removal of myoplasmic ATP at the time of Pi exposure significantly increased the rate and extent of SR Ca2+ release inhibition. For example, Ca2+ release was reduced by 86 +/- 6 % (n = 6) after 20 s exposure to 20 mM Pi in the absence of ATP compared with only 47 +/- 5 % (n = 5) in the presence of ATP. 4. The half and full recovery times for SR Ca2+ release following washout of myoplasmic Pi were 35 s and approximately 7 min, respectively. Recovery of Ca2+ release was unaffected by the absence of ATP during washout of Pi but was prevented when fibres were washed in the presence of high myoplasmic Pi (30 mM). Neither the Pi transporter blocker phenylphosphonic acid (PHPA) nor the anion channel blockers anthracene-9-carboxylic acid (9-AC) and 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) affected the rate of recovery of SR Ca2+ release. 5. These results show that Pi entry and exit from the SR occur primarily through a passive pathway that is insensitive to well-known anion channel blockers. Pi inhibition of SR Ca2+ release appears to be a complicated phenomenon influenced by the rate of Pi movement across the SR as well as by the rate, extent and species of Ca2+-Pi precipitate formation in the SR lumen. The more rapid inhibitory effect of Pi in the absence of myoplasmic ATP suggests that Pi may inhibit SR Ca2+ release more efficiently during the later stages of fatigue.

Comparison of sarcoplasmic reticulum capabilities in toadfish (Opsanus tau) sonic muscle and rat fast twitch muscle

The sonic muscle of the oyster toadfish, Opsanus tau, can produce unfused contractions at 300 Hz. Electron microscopy shows a great abundance of the Sarcoplasmic reticulum (SR) in this muscle, but no functional characterization of the capabilities of the SR has been reported. We measured the oxalate-supported Ca2+ uptake rate and capacities of homogenates of toadfish sonic muscle and rat extensor digitorum longus (EDL) muscle, and estimated the number of pump units by titration with thapsigargin, a high-affinity, specific inhibitor of the SR Ca-ATPase. The Ca2+ uptake rate averaged 70.9 +/- 9.5 mumol min -1 per g tissue for the toad fish sonic muscle, and 73.5 +/- 3.7 mumol min -1 g-1 for rat EDL. The capacity for Ca2+ -oxalate uptake was 161 +/- 20 mumol g -1 and 33 +/- 2 mumol g -1 for toadfish sonic muscle and rat EDL, respectively. Thus, the rates of Ca2+ uptake were similar in the two muscles, but the toadfish sonic muscle had about five times the capacity of the rat EDL. The number of pumps as estimated by thapsigargin titration was 68 +/- 4 nmol of Ca-ATPase per g tissue in the toadfish, and 42 +/- 5 nmol Ca-ATPase per g tissue in the rat EDL. The turnover number, defined as the Ca2+ uptake divided by the number of pumps, was 1065 +/- 150 min -1 for toadfish and 1786 +/- 230 min -1 for rat EDL (p < 0.05) at 37 degrees C. The Ca2+ uptake rate of toadfish sonic muscle at 22 degree C, a typical temperature for calling toadfish, averaged 42 +/- 1% of its rate at 37 degree C. At these operating temperatures, the toadfish SR is likely to be slower than the rat fast-twitch SR, yet the toadfish sonic muscle supports more rapid contractions. One explanation for this is that the voluminous SR provides activator Ca2+ for contraction, but the abundant parvalbumin plays a major role in relaxation.

[Calcium transport in microsomes isolated from rat parotid gland. Effects of ADP and an ATP-regenerating system]

Calcium loading of a rat parotid microsomal fraction is greatly increased by an ATP-regenerating system (phosphocreatine and creatine phosphokinase). This effect is neither a consequence of a rise in the ATP concentration nor of an increased formation of inorganic phosphate originating from hydrolysis of ATP or phosphocreatine. Addition of ADP to the incubation medium provokes an inhibition of Ca2+ influx and a stimulation of Ca2+ efflux by the microsomal fraction. These results suggest that the stimulation of Ca2+ uptake by the ATP-regenerating system is due, at least in part, to an increase of Ca2+ influx and a slowing down of Ca2+ efflux as consequence of a decrease of ADP availability. It is proposed that the effect of ADP on Ca2+ movements could account for the action of certain agonists on intracellular Ca2+ concentration. Moreover, the InsP3 responsive Ca2+ pool was also shown to be enlarged by the ATP-regenerating system without modification of InsP3 sensitivity.

Comparative effects of inorganic phosphate and oxalate on uptake and release of Ca2+ by the sarcoplasmic reticulum in saponin skinned rat cardiac trabeculae

1. Ventricular trabeculae from the right ventricle of rat heart were suspended in a 6 microliters bath and "skinned' with saponin (50 mg ml-1). Preparations were perfused with solutions mimicking the intracellular milieu and the [Ca2+] within the bath was monitored continuously using fura-2. 2. Application of 20 mM caffeine released Ca2+ from the sarcoplasmic reticulum (SR), resulting in a transient increase in the fura-2 fluorescence ratio. Caffeine-induced Ca2+ transients were smaller in the presence of 30 or 60 mM inorganic phosphate (Pi). This depressive effect of Pi on SR function was reversed by 10 mM creatine phosphate (CP). Caffeine-induced Ca2+ transients were also reduced in the presence of 10 mM oxalate, although this effect was not reversed by CP. 3. When perfusion was stopped in the presence of 30 or 60 mM Pi, the [Ca2+] within the bath remained constant. However, when the flow was stopped in the presence of 60 mM Pi and 10 mM CP, a prolonged decrease in [Ca2+] occurred, consistent with precipitation of calcium phosphate within the SR. A similar decrease in [Ca2+] was observed when perfusion was stopped in the presence of 2 or 20 mM oxalate, in the absence or presence of CP. 4. The SR was Ca2+ depleted by withdrawal of ATP and exposure to 20 mM caffeine. Perfusion was then stopped and ATP reapplied, resulting in a maintained decrease in [Ca2+] within the bath, due to SR Ca2+ uptake. Net Ca2+ uptake was markedly reduced in the presence of 30 mM Pi. In contrast, 20 mM oxalate increased Ca2+ uptake and the [Ca2+] within the bath continued to fall over 2-3 min. 5. Introduction of Pi released Ca2+ from the SR. Ryanodine (100 microM) abolished caffeine-induced Ca2+ release while Pi-induced Ca2+ release was unaffected. Pi-induced Ca2+ release was reduced in the constant presence of 20 mM caffeine or 10 mM CP and was abolished completely by disruption of the SR membrane with Triton X-100. Pi-induced Ca2+ release occurred after abolition of SR Ca2+ uptake by ATP withdrawal. 6. These results suggest that the Pi-induced decrease in releasable Ca2+ does not result from precipitation of calcium phosphate within the SR lumen. Pi inhibits net SR Ca2+ uptake, but this appears to result from activation of a ryanodine-insensitive Ca2+ efflux pathway rather then inhibition of Ca2+ uptake. Possible mechanisms are considered, including reversal of the SR Ca2+ pump.

Pulmonary microsomes contain a Ca(2+)-transport system sensitive to oxidative stress

A variety of events, including inhalation of atmospheric chemicals, trauma, and ischemia-reperfusion, may cause generation of reactive oxygen species in the lung and result in airways constriction. The specific metabolic mechanisms that translate oxygen radical production into airways constriction are yet to be identified. In the lung, calcium homeostasis is central to release of bronchoactive and vasoactive chemical mediators and to regulation of smooth muscle cell contractility, i.e., airway constriction. In the present work, we characterized Ca(2+)-transport in the microsomal fraction of mouse lungs, and determined how reactive oxygen species, generated by Fe2+/ascorbate and H2O2/hemoglobin, affected Ca2+ transport. The microsomal fraction of pulmonary tissue accumulated 90 +/- 5 nmol Ca2+/mg protein by an ATP-dependent process in the presence of 15 mM oxalate, and 16 +/- 2 nmol Ca2+ in its absence. In the presence of oxalate, the rate of Ca2+ uptake was 50 +/- 5 nmol Ca2+/min per mg protein at pCa 5.9 (37 degrees C). The Ca(2+)-ATPase activity was 50-60 nmol Pi/min per mg protein (pCa 5.9, 37 degrees C) in the presence of alamethicin. Inhibitors of mitochondrial H(+)-ATPase had no effect on the Ca2+ transport. Half-maximal activation of Ca2+ transport was produced by 0.4-0.5 microM Ca2+. Endoplasmic reticulum Ca(2+)-pump (SERC-ATPase) was found to be predominantly responsible for the Ca(2+)-accumulating capacity of the pulmonary microsomes. Incubation of the microsomes in the presence of either Fe2+/ascorbate or H2O2/hemoglobin resulted in a time-dependent accumulation of peroxidation products (TBARS) and in inhibition of the Ca2+ transport. The inhibitory effect of Fe2+/ascorbate on Ca2+ transport strictly correlated with the inhibition of the Ca(2+)-ATPase activity. These results are the first to indicate a highly active microsomal Ca2+ transport system in murine lungs which is sensitive to endogenous oxidation products. The importance of this system to pulmonary disorders exacerbated by oxidative chemicals remains to be studied.

Physiological concentrations of inorganic phosphate affect MgATP-dependent Ca2+ storage and inositol trisphosphate-induced Ca2+ efflux in microsomal vesicles from non-hepatic cells

1. MgATP-dependent 45Ca2+ uptake by microsomes obtained from various non-hepatic tissues, namely rat brain, rat solid Morris hepatoma 3924A and human platelets, was measured in the presence of P(i) at low, cytosol-like, concentrations. 2. Increasing P(i) concentrations (0.5-3 mM) caused a progressive enlargement of the 45Ca(2+)-storage capacity of all the microsomal fractions. 3. As a result of P(i) stimulation of Ca2+ uptake, 45Ca2+ and [32P]P(i) were co-accumulated by the three microsomal fractions. 4. The time course for 45Ca2+ and [32P]P(i) accumulation in brain microsomes revealed a biphasic 45Ca2+ uptake: a rapid phase was followed by a second, slower, phase, which depended on the presence of P(i). During the P(i)-dependent phase, the uptake of 45Ca2+ was paralleled by the uptake of [32P]Pi. 5. The passive efflux of Ca2+ was paralleled by the efflux of P(i) and vice versa. In fact, the inhibition of active Ca2+ uptake by excess EGTA, or lowering the P(i) concentration of the incubation system by dilution, caused the release of 45Ca2+ and [32P]P(i) from 45Ca2+ or [32P]P(i) pre-loaded brain microsomes. The Ca2+ ionophore A23187 also released 45Ca2+ and [32P]P(i). 6. Ca2+ efflux by A23187 was rapid (t 1/2 approx. 2 s) and independent of the extent of intravesicular Ca2+ loading, which indicates that Ca2+ and P(i) do not form intravesicular insoluble complexes. 7. The progressive increase in Ca2+ accumulation, depending on P(i) stimulation, resulted in a proportional increase in the amount of Ca2+ releasable by InsP3 in the three non-hepatic microsomal fractions and in digitonin-permeabilized platelets. 8. Concomitantly to Ca2+, microsomal P(i) was also released by InsP3.

Inorganic phosphate decreases the Ca2+ content of the sarcoplasmic reticulum in saponin-treated rat cardiac trabeculae

1. Measurements of [Ca2+] were made in saponin-permeabilized rat ventricular trabeculae using the fluorescent indicator Indo-1. Application of caffeine (20 mM) caused a transient rise in [Ca2+] within the preparation as a result of Ca2+ release from the sarcoplasmic reticulum (SR). The size of the caffeine-induced Ca2+ transient was related to the amount of Ca2+ accumulated by the SR prior to addition of caffeine. Caffeine-induced Ca2+ release was abolished by ryanodine (10 microM), an inhibitor of SR Ca2+ release. 2. At a bathing [Ca2+] of 0.2 microM, the amount of Ca2+ released from the SR on addition of caffeine was sufficient to generate a tension transient. Ca2+ and tension responses were stabilized by application of caffeine at regular intervals (2 min). Addition of 10 mM inorganic phosphate (Pi) induced a transient increase in [Ca2+] within the preparation due to a net release of Ca2+ from the SR. The amplitude of subsequent caffeine-induced Ca2+ transients were reduced to 65 +/- 7.5% (mean +/- S.D., n = 13) of control. In addition, the accompanying tension transient fell to 45 +/- 6.9% of control. Removal of Pi caused a transient decrease in the [Ca2+] within the preparation consistent with a net increase in Ca2+ uptake by the SR. Subsequent caffeine-induced Ca2+ and tension transients returned to control levels. 3. Inclusion of Pi (2-30 mM) in the perfusing solution decreased the size of caffeine-induced Ca2+ and tension transients in a dose-dependent manner. 4. Addition of 10 mM ADP caused a transient increase in [Ca2+] and depressed subsequent caffeine-induced Ca2+ transients to a greater extent than 10 mM Pi. Despite the reduction in Ca2+ release from the SR, tension responses were larger in the presence of 10 mM ADP than under control conditions. This is a consequence of an increase in Ca(2+)-activated force by ADP. 5. A decrease in the amplitude of caffeine-induced Ca2+ transients also occurred on changing from a solution containing 1 mM ADP and 10 mM Pi to a solution with 10 mM ADP and 1 mM Pi. This confirms the previous observation that ADP is more effective than Pi at reducing caffeine-induced Ca2+ released from the SR. 6. Spontaneous oscillations of [Ca2+] and tension occurred in the presence of 0.5 microM Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of perfusate [Ca2+] on cardiac sarcoplasmic reticulum Ca2+ release channel in isolated rat hearts

The effect of perfusate [Ca2+] on the function of cardiac sarcoplasmic reticulum (CSR) was assessed by the oxalate-supported Ca2+ uptake rate of ventricular homogenates of isolated rat hearts maintained in a modified Langendorff preparation. The total Ca2+ pumping activity of the CSR was determined by using 20 microM ruthenium red or 625 microM ryanodine to close the CSR Ca2+ release channel. The homogenate Ca2+ uptake rate in the absence of ruthenium red or ryanodine decreased progressively with increasing perfusate [Ca2+] (25.7 +/- 1.2, 21.4 +/- 1.5, 17.2 +/- 1.1, and 16.3 +/- 1.2 [mean +/- SEM] nmol Ca2+.min-1.mg-1 for hearts perfused for 5 minutes with 0.2, 1.4, 2.8, and 5.6 mM Ca2+, respectively; p = 0.0001; n = 8). This depression was not observed when Ca2+ uptake was assayed in the presence of ryanodine or ruthenium red. Since the Ca2+ uptake in the presence of ryanodine or ruthenium red is determined by the Ca(2+)-ATPase, this result suggests that perfusion with varying [Ca2+] did not affect the Ca(2+)-ATPase. The observed decrease in Ca2+ uptake in the absence of ryanodine or ruthenium red is caused by an increased efflux through the ryanodine-sensitive Ca2+ release channel. When hearts perfused for 5 minutes with 0.2 or 5.6 mM Ca2+ were reperfused for 10 minutes with 1.4 mM Ca2+, homogenate Ca2+ uptake rates were restored to near control levels. These effects of perfusate Ca2+ were not direct effects, because changes in the [Ca2+] of the homogenization medium did not alter the homogenate Ca2+ uptake activity in either the presence or absence of ryanodine. The homogenate Ca2+ uptake rates were unaffected by prior active loading of the CSR with Ca2+. These results suggest a regulatory role of perfusate Ca2+ in increasing the open state of the ryanodine-sensitive Ca2+ release channel that is distinct from the beat-to-beat regulation of Ca2+ release from the CSR by Ca2+ (Ca(2+)-induced Ca2+ release).

Calcium transport by sarcoplasmic reticulum of vascular smooth muscle: I. MgATP-dependent and MgATP-independent calcium uptake

The components of 45calcium (Ca) uptake were studied in saponin skinned rat caudal artery. The steady-state Ca content increased when the free Ca concentration was varied from 10(-8) to 10(-4) M but was reduced by azide when the free Ca concentration exceeded 3.1 microM. The azide sensitivity and low affinity for Ca were consistent with functional mitochondria. The azide-insensitive component consisted of a small bound and a larger releasable Ca fraction. After skinning in Triton X-100, approximately 4 mumol Ca/kg wet tissue remained, which represented a tightly bound but slowly exchangeable Ca pool. The Ca content was independent of the free Ca concentration and MgATP, and it was not released with A-23187 or Ca. The Ca content of the larger fraction was a higher order function of the free Ca concentration and was released with A-23187, indicating it resided within a membrane-bounded structure. Ca uptake by the releasable fraction was increased by oxalate, MgATP, phosphocreatine, temperature, phosphate, and ruthenium red and represents Ca sequestered by the sarcoplasmic reticulum (SR) with little contribution from other Ca binding or storage sites. It is described by the coefficients Umax = 96.94 mumol/kg wet tissue, K1/2 = 0.75 microM, and Hill coefficient = 1.70. The SR in this preparation regulates cytosolic Ca concentrations under physiological conditions and can accumulate Ca by MgATP-dependent and MgATP-independent process. The larger, MgATP-dependent Ca uptake is described by the coefficients Umax = 72.87 mumol/kg wet tissue, K1/2 = 0.8 microM, and Hill coefficient = 2.09 and is consistent with Ca sequestered by the Ca-transport ATPase of smooth muscle SR. The smaller, MgATP-independent uptake is described by the coefficients Umax = 24.14 mumol/kg wet tissue, K1/2 = 0.56 microM, and Hill coefficient = 1.01 and represents Ca sequestered by an unidentified mechanism or by a subpopulation of SR.

Seasonal variations in the rate and capacity of cardiac SR calcium accumulation in a hibernating species

The rate of calcium uptake and the level of calcium accumulation was measured in cardiac muscle SR from hibernating and nonhibernating Richardson's ground squirrels. In whole heart homogenates, the rate of calcium uptake was higher (P less than 0.05) in hibernating animals than it was in active animals. Further purification of homogenates into sacroplasmic reticulum (SR) preparations showed that the hibernating animals had the highest rate of calcium uptake and the greatest level of calcium accumulation. These results could not be explained by variations in non-SR membrane contaminants nor by changes in the maximal activity or total amount of a SR marker enzyme, the Ca(2+)-ATPase. The addition of ryanodine to the calcium uptake medium increased the level of calcium accumulation in all groups by a similar amount. It is concluded that the high rate of calcium uptake by isolated cardiac SR vesicles from hibernating ground squirrels reflects the activity of the organelle in vivo, and that the ability of the ryanodine-insensitive population of SR vesicles to accumulate calcium is affected by hibernation.

Stabilization of rat cardiac sacroplasmic reticulum Ca2+ uptake activity and isolation of vesicles with improved calcium uptake activity

The Ca2+ uptake activity of rat cardiac sacroplasmic reticulum (CSR) in ventricular homogenates is highly unstable, and this instability probably accounts for the low specific activity of Ca2+ uptake in previously reported fractions of isolated rat CSR. The instability was observed at either 0 degrees or 37 degrees, but the Ca2+ uptake activity was relatively stable at 25 degrees. The decay of Ca2+ uptake activity at 0 degrees could not be prevented by either PMSF or leupeptin, but dithiothreitol exerted some protective effects. Sodium metabisulfite prevented decay of the Ca2+ uptake activity of homogenates kept on ice but not of homogenates kept at 37 degrees. We also found that release of the CSR from the cellular debris required homogenization in high KCl. This distinguishes rat CSR from canine CSR. Isolated CSR was produced by a combination of differential centrifugation and discontinuous sucrous gradient centrifugation. The average rate of the sustained oxalate-supported calcium uptake in the resulting CSR fraction was 0.36 mumol/min-mg in the absence of CSR calcium channel blockers and 0.67 mumol/min/mg in the presence of 10 microM ruthenium red. Thus, this preparation has the advantage of containing both the releasing and non-releasing fractions of the CSR. The Ca2(+)-ATPase rates averaged 1.07 mumol/min/mg and 0.88 mumol/min-mg in the absence and presence of ruthenium red, respectively. Although these rates are higher than previously reported rates, this CSR preparation should still be considered a 'crude' preparation. A major distinction between the rat CSR and dog CSR was the lower content of Ca2(+)-ATPase in rat CSR, as judged by SDS-PAGE. Preparations of CSR isolated by this method may be useful in evaluating alterations in CSR function.

MgATP-dependent accumulation of calcium ions and inorganic phosphate in a liver reticular pool

1. MgATP-dependent Ca2+ uptake by rat liver microsomal preparations and permeabilized hepatocytes was measured in the presence or absence of Pi. 2. Monitoring of free Ca2+ in incubation systems with a Ca2+ electrode in the presence of Pi (2-7 mM) revealed a biphasic Ca2+ uptake, with the onset of a second, Pi-dependent, Ca2+ accumulation. 3. Increasing Pi concentrations (up to 10 mM) caused a progressive enlargement of 45Ca2(+)-loading capacity of microsomal fractions. 4. As a result of Pi stimulation of active Ca2+ uptake, [32P]Pi and 45Ca2+ were co-accumulated. 5. Experiments with permeabilized hepatocytes revealed that the amount of Ca2+ releasable by myo-inositol 1,4,5-trisphosphate is unaffected by Pi.

Differential effect of global ischemia on the ryanodine-sensitive and ryanodine-insensitive calcium uptake of cardiac sarcoplasmic reticulum

The effect of ischemia on the function of cardiac sarcoplasmic reticulum (SR) was assessed by the calcium uptake rate of rat whole-heart homogenates in the presence of 10 mM oxalate. Previous studies have shown that this uptake is restricted to the SR. The contribution of the ryanodine-sensitive fractions of the SR to the total homogenate uptake was assessed by using 20 microM ruthenium red and 625 microM ryanodine to close the SR calcium release channel under previously established optimal conditions. Global ischemia of 10, 15, 30, and 60 minutes depressed homogenate calcium uptake rate 19 +/- 2%, 50 +/- 6%, 65 +/- 3%, and 81 +/- 5%, respectively. This decrease was not observed when the uptake rates were measured after closure of the calcium channel with ryanodine or ruthenium red. Similar results were obtained with a Langendorff in vitro perfusion preparation, in which calcium uptake was decreased 35 +/- 5%, 37 +/- 8%, 58 +/- 7%, and 64 +/- 4% after 10, 15, 30, and 60 minutes of ischemia, but no significant decrease was observed when homogenate uptake rates were measured in the presence of ryanodine. Thus, ischemia caused a depression in the calcium uptake rate of cardiac SR only when this activity was measured in the absence of SR calcium channel blockers. Reperfusion of ischemic hearts in a Langendorff preparation resulted in recovery of homogenate calcium uptake activity that correlated well with the return to sinus rhythm of the reperfused hearts. These reperfused hearts showed no change in the calcium uptake rate measured in the presence of ryanodine. These results suggest that the decrease in homogenate calcium uptake caused by ischemia is not due to a defect in calcium pumping capabilities but is due to an increased efflux through the ryanodine-sensitive calcium release channel of cardiac SR.

The rate and capacity of calcium uptake by sarcoplasmic reticulum in fast, slow, and cardiac muscle: effects of ryanodine and ruthenium red

The rate and capacity of oxalate-supported calcium uptake was measured in homogenates of rat fast, slow, and cardiac muscle. The contribution of the releasing fraction of the sarcoplasmic reticulum (SR) to the calcium uptake abilities was estimated using ruthenium red or ryanodine to block the release channel. A relatively small fraction (12-20%) of the calcium pumping activity was associated with the release channel in skeletal muscle compared to 50% or more in cardiac muscle. The total capacity of the SR in the muscle types was in the ratio 1:0.75:1.5 for cardiac, slow, and fast muscle, respectively, while the rates of uptake were in the ratio 1:3.8:14.4. The major difference in the muscle types appears to be the density of pumping activity in the SR rather than the volume of the SR. The difference in the density of pumping activity is due to intrinsic differences in the kinetics of the calcium pump units and in their surface density.

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.