[Fast kinetics of adenosine triphosphate dependent Ca 2+ uptake by fragmented sarcoplasmic reticulum]

Thermodynamics of Cation Binding to the Sarcoendoplasmic Reticulum Calcium ATPase Pump and Impacts on Enzyme Function

Sarcoendoplasmic reticulum Ca²⁺-ATPase (SERCA) is a transmembrane pump that plays an important role in transporting calcium into the sarcoplasmic reticulum (SR). While calcium (Ca²⁺) binds SERCA with micromolar affinity, magnesium (Mg²⁺) and potassium (K⁺) also compete with Ca²⁺ binding. However, the molecular bases for these competing ions' influence on the SERCA function and the selectivity of the pump for Ca²⁺ are not well-established. We therefore used in silico methods to resolve molecular determinants of cation binding in the canonical site I and II Ca²⁺ binding sites via (1) triplicate molecular dynamics (MD) simulations of Mg²⁺, Ca²⁺, and K⁺-bound SERCA, (2) mean spherical approximation (MSA) theory to score the affinity and selectivity of cation binding to the MD-resolved structures, and (3) state models of SERCA turnover informed from MSA-derived affinity data. Our key findings are that (a) coordination at sites I and II is optimized for Ca²⁺ and to a lesser extent for Mg²⁺ and K⁺, as determined by MD-derived cation-amino acid oxygen and bound water configurations, (b) the impaired coordination and high desolvation cost for Mg²⁺ precludes favorable Mg²⁺ binding relative to Ca²⁺, while K⁺ has limited capacity to bind site I, and (c) Mg²⁺ most likely acts as inhibitor and K⁺ as intermediate in SERCA's reaction cycle, based on a best-fit state model of SERCA turnover. These findings provide a quantitative basis for SERCA function that leverages molecular-scale thermodynamic data and rationalizes enzyme activity across broad ranges of K⁺, Ca²⁺, and Mg²⁺ concentrations.

Cross-bridge blocker BTS permits direct measurement of SR Ca2+ pump ATP utilization in toadfish swimbladder muscle fibers

Because the major processes involved in muscle contraction require rapid utilization of ATP, measurement of ATP utilization can provide important insights into the mechanisms of contraction. It is necessary, however, to differentiate between the contribution made by cross-bridges and that of the sarcoplasmic reticulum (SR) Ca2+ pumps. Specific and potent SR Ca2+ pump blockers have been used in skinned fibers to permit direct measurement of cross-bridge ATP utilization. Up to now, there was no analogous cross-bridge blocker. Recently, N-benzyl-p-toluene sulfonamide (BTS) was found to suppress force generation at micromolar concentrations. We tested whether BTS could be used to block cross-bridge ATP utilization, thereby permitting direct measurement of SR Ca2+ pump ATP utilization in saponin-skinned fibers. At 25 microM, BTS virtually eliminates force and cross-bridge ATP utilization (both <4% of control value). By taking advantage of the toadfish swimbladder muscle's unique right shift in its force-Ca2+ concentration ([Ca2+]) relationship, we measured SR Ca2+ pump ATP utilization in the presence and absence of BTS. At 25 microM, BTS had no effect on SR pump ATP utilization. Hence, we used BTS to make some of the first direct measurements of ATP utilization of intact SR over a physiological range of [Ca2+]at 15 degrees C. Curve fits to SR Ca2+ pump ATP utilization vs. pCa indicate that they have much lower Hill coefficients (1.49) than that describing cross-bridge force generation vs. pCa (approximately 5). Furthermore, we found that BTS also effectively eliminates force generation in bundles of intact swimbladder muscle, suggesting that it will be an important tool for studying integrated SR function during normal motor behavior.

ATP utilization for calcium uptake and force production in different types of human skeletal muscle fibres

The contractile properties and ATPase activity of skinned human skeletal muscle fibres from vastus lateralis were examined. Fibre types were resolved from single fibre segments by SDS-polyacrylamide gel electrophoresis. ATPase activity was determined by enzymatic coupling of ATP resynthesis to the oxidation of NADH. The partitioning of ATPase activity into (a) calcium-activated activity due to actomyosin (AM) interaction, (b) calcium-activated activity of the sarcoplasmic reticular (SR) calcium pump, and (c) basal (calcium independent) activity was investigated by comparing ATP utilization before and after exposure of the preparations for 30 min to a solution containing 0.5 % Triton X-100, which effectively abolished the SR ATPase activity. Partitioning of ATPase activity was also determined by measuring ATP utilization and force at different concentrations of butanedione monoxime (BDM), which inhibits AM interaction. The results obtained with Triton X-100 and BDM were similar. At saturating Ca2+ concentrations and 20 degrees C, the AM, SR and basal ATPase activities per litre cell volume (+/- S.E.M.) amounted to 46 +/- 4, 51 +/- 4 and 19 +/- 2 muM s-1 in type I fibres (n = 21), 139 +/- 14, 69 +/- 8 and 30 +/- 3 muM s-1 in type IIA fibres (n = 25), 137 +/- 22, 175 +/- 28 and 26 +/- 8 muM s-1 in type IIA/B fibres (n = 4) and 108 +/- 13, 169 +/- 42 and 32 +/- 8 muM s-1 in type IIB fibres (n = 2). These results indicate that ATP utilization for SR Ca2+ pumping in fast fibres is considerably larger than in slow fibres. The SR ATPase activity in human muscle represents a considerable fraction of the total (AM + SR + basal) ATPase activity.

Superfast contractions without superfast energetics: ATP usage by SR-Ca2+ pumps and crossbridges in toadfish swimbladder muscle

1. The rate at which an isometrically contracting muscle uses energy is thought to be proportional to its twitch speed. In both slow and fast muscles, however, a constant proportion (25-40 %) of the total energy has been found to be used by SR-Ca2+ pumps and the remainder by crossbridges. We examined whether SR-Ca2+ pumps account for a larger proportion of the energy in the fastest vertebrate muscle known (the toadfish swimbladder), and whether the swimbladder muscle utilizes energy at the superfast rate one would predict from its mechanics. 2. The ATP utilization rates of the SR-Ca2+ pumps and crossbridges were measured using a coupled assay system on fibres skinned with saponin. Surprisingly, despite its superfast twitch speed, the ATP utilization rate of swimbladder was no higher than that of much slower fast-twitch amphibian muscles. 3. The swimbladder achieves tremendous twitch speeds with a modest steady-state ATP utilization rate by employing two mechanisms: having a small number of attached crossbridges and probably utilizing intracellular Ca2+ buffers (parvalbumin) to spread out the time over which Ca2+ pumping can occur. 4. Finally, although the total ATP utilization rate was not as rapid as expected, the relative proportions used by SR-Ca2+ pumps and the crossbridges were similar to other muscles.

ATP utilization for calcium uptake and force production in skinned muscle fibres of Xenopus laevis

1. A method has been developed to discriminate between the rate of ATP hydrolysis associated with calcium uptake into the sarcoplasmic reticulum (SR) and force development of the contractile apparatus in mechanically or saponin-skinned skeletal muscle fibres. The rate of ATP hydrolysis was determined in fibres of different types from the iliofibularis muscle of Xenopus laevis by enzymatic coupling of ATP re-synthesis to the oxidation of NADH. 2. The ATPase activity was determined before and after exposure of the preparations for 30 min to a solution containing 0.5% Triton X-100, which effectively abolishes the SR ATPase activity. The fibres were activated in a solution containing 5 mM caffeine to ensure that calcium uptake into the SR was maximal. 3. At saturating Ca2+ concentrations the actomyosin (AM) and SR ATPase activities in fast-twitch fibres, at 4.3 degrees C, amounted to 1.52 +/- 0.07 and 0.58 +/- 0.10 mumol s-1 (g dry wt)-1, respectively (means +/- S.E.M.; n = 25). The SR ATPase activity was 25% of the total ATPase activity. At submaximal calcium concentrations the AM ATPase activity varied in proportion to the isometric force. 4. The calcium sensitivity of the SR ATPase was larger than that of the AM ATPase and its dependence on [Ca2+] was less steep. The AM ATPase activity was half-maximal at a pCa of 6.11 (pCa = -log [Ca2+]) whereas the SR ATPase activity was half-maximal at a pCa of 6.62. 5. In Triton X-100-treated fibres, at different 2,3-butanedione monoxime (BDM) concentrations, the AM ATPase activity and isometric force varied proportionally. The SR ATPase activity determined by extrapolation of the total ATPase activity in mechanically skinned or saponin-treated fibres to zero force, was independent of the BDM concentration in the range studied (0-20 mM). The values obtained for the SR ATPase activity in this way were similar to those obtained with Triton X-100 treatment. 6. The AM ATPase activity in slow-twitch fibres amounted to 0.74 +/- 0.13 mumol s-1 (g dry wt)-1, i.e. about a factor of two smaller than in fast-twitch fibres. The SR ATPase activity amounted to 0.47 +/- 0.07 mumol s-1 (g dry wt)-1, i.e. rather similar to the value in fast-twitch fibres. The proportion of the total ATPase activity that was due to SR ATPase (40%) was larger than in fast-twitch fibres. 7. The temperature dependence of the AM and SR ATPase activities in fast-twitch fibres differed. In the temperature range 5-10 degrees C, the relative changes in AM and SR ATPase activities for a 10 degrees C temperature change (Q10) were 3.9 +/- 0.3 and 7.2 +/- 1.5, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Interdependence of H+ and K+ fluxes during the Ca(2+)-pumping activity of sarcoplasmic reticulum vesicles

The release of H+ during the oxalate-supported Ca2+ uptake in sarcoplasmic reticulum vesicles is kinetically coincident with the initial phase of Ca2+ accumulation. The Ca2+ uptake is increased and the H+ release is decreased in the presence of KCl and other monovalent chloride salts as expected for a H(+)-monovalent cation exchange. The functioning of the Ca(2+)-pump is disturbed by the presence of potassium gluconate and to a lesser extent, of choline chloride. These salts do not inhibit the ATPase activity of Ca(2+)-permeable vesicles, suggesting a charge imbalance inhibition which is specially relevant in the case of gluconate. Therefore, K+, and also Cl-, appear to be involved in secondary fluxes during the active accumulation of Ca2+. The microsomal preparation seems homogeneous with respect to the K(+)-channel, showing an apparent rate constant for K+ release of approximately 25 s-1 measured with the aid of 86Rb+ tracer under equilibrium conditions. A Rb+ efflux, sensitive to Ca(2+)-ionophore, can be also detected during the active accumulation of Ca2+. The experimental data suggest that both monovalent cations and anions are involved in a charge compensation during the Ca2+ uptake and H+ release. Fluxes of these highly permeable ions would contribute to cancel the formation of a resting membrane potential through the sarcoplasmic reticulum membrane.

H+ countertransport and electrogenicity of the sarcoplasmic reticulum Ca2+ pump in reconstituted proteoliposomes

The Ca2+ transport adenosine triphosphatase of sarcoplasmic reticulum was reconstituted in unilamellar liposomes prepared by reverse-phase evaporation. The size of the resulting proteoliposomes was similar to that of native sarcoplasmic reticulum vesicles, but their protein content was much lower, with a protein/lipid ratio (wt/wt) of 1:40-160, as compared with 1:1 in the native membrane. The proteoliposomes sustained adenosine triphosphate-dependent Ca2+ uptake at rates proportional to the protein content (1-2 mumol Ca2+/mg protein/min), reaching asymptotic levels corresponding to a lumenal calcium concentration of 10-20 mM. The low permeability of the proteoliposomes permitted direct demonstration of Ca2+/H+ countertransport and electrogenicity by parallel measurements in the same experimental system. Countertransport of one H+ per one Ca2+ was demonstrated, and inhibition of the Ca2+ pump by lumenal alkalinization was relieved by the H+ ionophore carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone. Consistent with the countertransport stoichiometry, net positive charge displacement was produced by Ca2+ transport, as revealed by a rapid oxonol VI absorption rise. The initial rise and the following steady-state level of oxonol absorption were highest when SO4(2-) was the prevalent anion and lowest in the presence of the lipophilic anion SCN-. The influence of anions was attributed to potential driven counterion compensation. The absorption rise was rapidly collapsed by addition of valinomycin in the presence of K+. Experimentation with Ca2+ and H+ ionophores was consistent with a primary role of Ca2+ and H+ in net charge displacement. The estimated value of the steady-state electrical potential observed under optimal conditions was approximately 50 mV and was accounted for by the estimated charge transfer associated with Ca2+ and H+ countertransport under the same conditions.

Calcium transients and calcium release in rat fast-twitch skeletal muscle fibres

1. Calcium transients were recorded from cut segments of fast-twitch rat skeletal muscle fibres stretched to 3.7-4.0 microns per sarcomere and voltage clamped at a holding potential of -80 mV using the double Vaseline-gap technique. Calcium transients were monitored simultaneously with the two calcium indicators antipyrylazo III (AP III) and fura-2. AP III was used to record the calcium changes in response to 10-200 ms depolarizing pulses to different membrane potentials while fura-2 monitored the slow decay of the transient (during 16-20 s) and the resting calcium concentration. Experiments were performed at 14-17 degrees C. 2. For 50-100 ms depolarizing pulses calcium transients were first detected between -30 and -20 mV in a total of twenty-one fibres. The transients recorded with AP III showed a plateau for small pulses (-20 mV) and a steady increase during stronger pulses (-10 mV and more positive). Upon repolarization the transients decayed towards the baseline. The signal recorded simultaneously with fura-2 showed a continuous increase of the transient during the pulses at all membrane potentials. The amplitude of the calcium transients for the large pulses could not be followed with fura-2 due to saturation of the dye. 3. The signals obtained with both dyes were used to determine the kinetics of the calcium-fura-2 reaction inside the fibres. The mean values of the kinetic parameters were: the on rate constant (kon) = 5.1 x 10(8) M-1s-1, the off rate constant (koff) = 26 s-1, and koff/kon (KD) = 69.7 nM. 4. The fast phase of decay of the calcium transients after the pulses was studied from the records obtained with AP III. For depolarizing pulses of the same duration, the rate of decay of the transients after the pulse was slower the stronger the depolarization. For pulses to the same membrane potential, the rate of decay was slower the longer the pulse duration. Both stimulating patterns indicated saturation of the removal system in the muscle fibres due to occupancy of slowly equilibrating myoplasmic calcium binding sites by released calcium. 5. The fast phase of decay of the signals obtained with AP III was well fitted with a model of the system for removing calcium from the myofilament space. 6. The rate of calcium release (Rrel) from the sarcoplasmic reticulum was calculated once the removal system was characterized in the same fibre.(ABSTRACT TRUNCATED AT 400 WORDS)

Improved resolution of the initial fast phase of heavy sarcoplasmic reticulum Ca2+ uptake by Ca2+:antipyrylazo III dual-wavelength spectroscopy

The effect of ATP upon difference absorbance due to Ca2+ and Mg2+ complexation with the metallochromic dye, Antipyrylazo III (AP III), was investigated. At divalent cation concentrations appropriate for Sarcoplasmic Reticulum Ca2+ transport, wavelengths (greater than 670 nm) were found whereupon the addition of up to 1mM nucleotide did not alter divalent cation:AP III difference absorbance. At these sample wavelengths an initial rapid uptake of Ca2+ by Heavy SR (HSR) was clearly resolved by dual wavelength spectroscopy of Ca2+:dye difference absorbance. Elimination of ATP interference of Ca2+:AP III absorbance by Mg2+ elevation (3-10mM) was shown to be an inappropriate general strategy for AP III spectroscopic studies of HSR Ca2+ transport due to Mg2+ inhibition of ryanodine receptor mediated Ca2+ release.

Factors affecting aerobic recovery heat production and recovery ratio of frog sartorius

1. Sartorius muscles of Rana temporaria, equilibrated at 20 degrees C in Ringer solution buffered with phosphates, were stimulated isometrically for 0.2 up to 0.75 s at lengths varying from 1.03 to 1.48 times rest length, L0. The aerobic recovery heat was measured for 10.5 min after contraction. 2. The recovery heat production had a complex time course, showing a variable delay to maximum, declining thereafter. In most cases, the rate of heat production did not decrease monotonically; attention was focused on the slow exponential decay which only persisted from 1.5-5 min after contraction. This latter part of the time course was considered as strictly aerobic and characterized by the time constant tau s. 3. Increasing the tetanus duration from 0.2 to 0.75 s increased initial heat Qi and recovery heat Qr in proportion, so that the recovery ratio R (Qr/Qi) did not change; it was equal to 1.29 +/- 0.03 (S.E.M.; n = 44) for muscles at about L0. The kinetics of heat production were modified with longer tetani; in particular, tau s was increased from 2.2 to 5.2 min. 4. When muscles were stretched beyond L0, as long as there was no increase of the resting heat rate (stretch response or 'Feng effect'), recovery heat production had a similar evolution to that in muscles at about L0; R was constant and equal to 1.21 +/- 0.03 (n = 46). 5. When muscles were sufficiently stretched to develop a stretch response, R increased proportionally to the stretch response. The effect seemed independent of the contractile machinery, as it vanished concomitantly with the stretch response, while force and Qi remained unchanged for the length considered. The kinetics were also modified--the delay to maximum was no longer detected and tau s most likely increased. 6. Substitution of 60% of the NaCl of the Ringer solution by NaI (mol/mol) produced a significant increase of R, mainly due to the increase of Qr. 7. The results show that neither the time course nor the amount of aerobic recovery heat Qr are strictly determined by the amount of initial heat Qi. The hypothesis is discussed that Qr might include a variable fraction due to processes which are not directly implicated in the actin-myosin interactions, possibly those involving the cytosolic Ca2+ concentration and the rate of resting metabolism.

Lanthanum as a calcium-substituting ion for binding to sarcoplasmic reticulum ATPase

Ca2+ binding and internalization in sarcoplasmic reticulum ATPase can be investigated by the use of La3+ as a Ca2+ analog. Displacement kinetics of Ca2+ bound by La3+ in native vesicles is a slow biphasic process (k1 = 0.55 s-1 and k2 = 0.05 s-1) that is consistent with the existence of two Ca2+ binding populations whereas in leaky vesicles there appears to be a single population (k = 0.57 s-1). Rapid quench experiments demonstrate that Ca2+ internalization occurs with an initial burst (approximately 8 nmol/mg protein) associated with the presence of a phosphate-donor substrate in the reaction medium. While acid quenching for measurements of phosphoenzyme is instantaneous, La3+ quenching allows completion of one catalytic and transport cycle due to the slow La3+ exchange with Ca2+. This explains the apparent inconsistencies in the kinetics and stoichiometry of phosphoenzyme formation and Ca2+ internalization that are observed under certain experimental conditions.

Effects of halothane, caffeine, dantrolene and tetracaine on the calcium permeability of skeletal sarcoplasmic reticulum of malignant hyperthermic pigs

Preparing skeletal sarcoplasmic reticulum from both normal and malignant hyperthermia susceptible pigs, the effects of various drugs on the passive calcium permeability of these sarcoplasmic reticulum preparations were studied. It was found that, in the absence of halothane, the permeability of heavy sarcoplasmic reticulum prepared from malignant hyperthermia susceptible pigs was much higher than that of normal pigs. It was observed that halothane, at concentrations above 10 microM (well below anesthetic concentrations, which are on the order of 1 mM), increased the permeability of sarcoplasmic reticulum. The Hill coefficient of the effect of halothane ranged from 1.96 to 2.25, suggesting that some kind of cooperativity was involved in this reaction. The effects of caffeine were similar to those of halothane. Inhibitors, such as tetracaine and ruthenium red inhibited both the calcium permeability and the halothane-induced increment. The Hill coefficient of the effect of tetracaine was 1.75. The mode of inhibition suggests that tetracaine directly binds with the calcium channel to inhibit the calcium efflux. On the contrary, dantrolene did not affect the calcium permeability of the sarcoplasmic reticulum. However, it inhibited the halothane-induced and caffeine-induced increments of the permeability. The Hill coefficient of inhibition by dantrolene ranged from 2.3 to 3.9, suggesting that several molecules of dantrolene may interact cooperatively with one calcium release channel to inhibit the effect of halothane. These results suggest that dantrolene has a unique inhibitory action, which may be related to its efficacy in ameliorating the syndrome of malignant hyperthermia.

In vitro lipid oxidation modifies proteins and functional properties of sarcoplasmic reticulum

Changes in protein and fatty acid compositions of flounder sarcoplasmic reticulum during NADH plus ascorbate-dependent lipid peroxidation in vitro were related to the ability of the sarcoplasmic reticulum to sequester Ca+2. Progressive accumulation of high-molecular-weight protein components occurred concomitantly with loss of Ca+2-sequestering activity. Part of this polymerized protein may be the dimer or trimer of Ca+2, Mg+2-ATPase. Loss in Ca+2, Mg+2-ATPase protein could account for over 60% of the polymerized protein. Rate of loss of polyunsaturated fatty acids was C22:6 greater than C20:4 greater than C20:5 greater than C22:5. Loss of polyunsaturated fatty acids and accumulation of thiobarbituric acid-reactive substances occurred concomitantly with protein polymerization.

Effect of age of winter flounder on some properties of the sarcoplasmic reticulum

Sarcoplasmic reticulum (SR) from winter flounder (Pseudopleuronectes americanus) of various ages was investigated with regard to fluorescent age pigment (FAP), potential of enzymic lipid peroxidation, fatty acid composition, protein pattern and Ca2+-sequestering ability. FAP accumulation, protein pattern and Ca2+-sequestering ability were also evaluated after the SR were incubated in vitro with the peroxidizing cofactors: NADH, ADP and Fe3+. No difference in FAP content was observed in SR isolated from fish of different ages. However, a higher accumulation of FAP in muscle tissue from old fish was observed. A slow but significant increase in FAP in SR was observed when the SR from both young and old fish were incubated with peroxidizing cofactors. It was demonstrated that 4-year-old fish had higher levels of NADH-dependent lipid peroxidation in their SR than 2-year-old fish. No differences in fatty acid composition were observed in the SR of fish aged from 2 to 8 years old. Two species of high-molecular-weight protein were found to increase in SR isolated from aging fish. These species resembled the polymerized proteins derived after incubation of the SR with the peroxidizing cofactors. Ca2+-uptake decreased when the SR was incubated with peroxidizing cofactors, but the results are difficult to interpret since Ca2+-uptake of SR was affected by captive stress.

Fatigue in isometric contraction in a single muscle fibre: a compartmental calcium ion flow model

Fatigue in muscle is a complex biological phenomenon which has so far eluded a definite explanation. Many biochemical and physiological models have been suggested in the literature to account for the decrement in the ability of muscle to sustain a given level of force for a long time. Some of these models have been critically analysed in this paper and are shown to be not able to explain all the experimental observations. A new compartmental model based on the intracellular calcium ion movement in muscle is proposed to study the mechanical responses of a muscle fibre. Computer simulation is performed to obtain model responses in isometric contraction to an impulse and a train of stimuli of long duration. The simulated curves have been compared with experimentally observed mechanical responses of the semitendinosus muscle fibre of Rana pipiens. The comparison of computed and observed responses indicates that the proposed calcium ion model indeed accounts very well for the muscle fatigue.

Transmembrane calcium movement in 20,25-diazacholesterol myotonia

An abnormality in myoplasmic Ca2+ regulation has frequently been proposed in 20,25-diazacholesterol (20,25-D) myotonia. We report here the results of several studies of transmembrane Ca2+ movement in this animal model. (i) Physiologic Ca2+ release by intact sarcoplasmic reticulum (SR) was examined in chemically skinned single muscle fibers preloaded in EGTA-buffered Ca2+ solutions (pCa2+7.0 to 6.4). Isometric tension development and Ca2+ release thresholds in response to Cl- or caffeine showed no differences between control and 20,25-D fibers at any pCa2+. (ii) The kinetics of energy-dependent Ca2+ accumulation in purified SR vesicles were followed spectrophotometrically using Ca2+-sensitive dyes. The apparent rate for ATP-dependent Ca2+ uptake and Ca2+ sequestering capacity were unchanged in SR from 20,25-D animals vs. controls. (iii) Surface membrane Ca2+ATPase activity was measured in red blood cell ghosts and sarcolemma. Enzyme Vmax was decreased by 25 to 50% in both membranes in the 20,25-D-treated animals with a compensatory increase in the number of Ca2+ATPase molecules. In general, the SR handling of Ca2+ appears normal in 20,25-D myotonia, although the activity of Ca2+ATPase in membranes with high sterol content may be altered in response to changes in the lipid environment in this model.

Voltage-dependence of Ca2+ uptake and ATP hydrolysis of reconstituted Ca2+-ATPase vesicles

Ca2+-ATPase from sarcoplasmic reticulum was reconstituted into phospholipid/cholesterol (9:1) vesicles (RO). Sucrose density gradient centrifugation of the RO vesicles separated a light layer (RL) with a high lipid/protein ratio and a heavy layer (RH). RH vesicles exhibited a high rate of Ca2+-dependent ATP hydrolysis but did not accumulate Ca2+. RL vesicles, on the other hand, showed an initial molar ratio of Ca2+ uptake to ATP hydrolysis of approximately 1.0. Internal trapping of transported Ca2+ facilitated studies over periods of several minutes. Ca2+ transport and ATP hydrolysis declined concomitantly, reaching levels near 0 with external Ca2+ concentrations less than or equal to 2 microM. Ca2+ uptake was inhibited by the Ca2+ ionophore A23187, the detergent Triton X-100, and the metabolic inhibitor quercetin. Ca2+ transport generated a transient electrical potential difference, inside positive. This finding is consistent with the hypothesis that the Ca2+ pump is electrogenic. Steady state electrical potentials across the membrane were clamped by using potassium gradients and valinomycin, and monitored with voltage-sensitive dyes. Over a range of +50 to -100 mV, there was an inverse relationship between the initial rate of Ca2+ uptake and voltage, but the rate of ATP hydrolysis was nearly constant. In contrast, lowering the external Ca2+ concentration depressed both transport and ATP hydrolysis. These findings suggest that the membrane voltage influences the coupling between Ca2+ transport and ATP hydrolysis.

Sarcoplasmic reticulum from flounder muscle having improved lipid peroxidative activity

A procedure for the isolation of sarcoplasmic reticulum from winter flounder (Pseudopleuronectes americanus) resulted in a fraction with a specific activity of lipid peroxidation two to three times that of previous preparations. In addition, good stability of the NADH-dependent lipid peroxidative activity was achieved. There appeared to be minimal contamination of the preparation with lysosomes and mitochondria. The flounder sarcoplasmic reticulum was highly active with respect to ATPase and calcium uptake. The membrane fraction contained 43% lipid and 57% protein; 60% of the lipids were phospholipids. Phosphatidylcholine was the major phospholipid present.

Action of mercurials on the active and passive transport properties of sarcoplasmic reticulum

The effect of Hg2+ and CH3-Hg+ on the passive and active transport properties of the Ca2+-Mg2+-ATPase-rich fraction of skeletal sarcoplasmic reticulum (SR) is reported. The agents abolish active transport, at 10(-5) and 10(-4) M concentrations, respectively. Addition of the mercurials was also shown to release actively accumulated Ca2+. The mercurials increase the passive Ca2+ and Mg2+ permeability in the absence of ATP at the same concentrations at which they inhibit transport. It is proposed that both effects are the result of direct binding of the mercurials to the SH groups of the Ca2+-Mg2+-ATPase pump, altering the conformational equilibria of the pump. The agents were also shown to increase the passive KCl permeability. The SR preparation consists of two vesicle populations with respect to K+ permeability, one with rapid KCl equilibration faciliated by a monovalent cation channel function and one with slow KCl equilibration. The mercurials increase the rates of KCl equilibration in both fractions, but produce higher rates in the fraction containing the channel function. The results are discussed in terms of pump and channel function and are compared with results for the electrical behavior of the CA2+-Mg2+-ATPase and other SR proteins in black lipid membranes, as presented by others.

Parvalbumins and muscle relaxation: a computer simulation study

The distribution of Ca2+ and Mg2+ among the 'regulatory' cation binding sites of troponin (T-sites) and the strong, Ca2+-Mg2+ binding sites of troponin and parvalbumins (P-sites) in the sarcoplasm of a muscle was calculated. At rest, 60% of the T-sites were metal free, while 92% of the P-sites were loaded with Mg2+. In response to a Ca2+ pulse, troponin-calcium (T-Ca) complexes were rapidly formed, while the binding of Ca2+ to P-sites was limited by the slow rate of dissociation of the parvalbumin-magnesium (P-Mg) complexes. Muscle activation was not prevented by a high content of parvalbumins. Parvalbumin and the sarcoplasmic reticulum (SR) pump were complementary relaxing factors that removed Ca2+ from the cytosol and from the T-sites. Parvalbumins dominated the first part of relaxation, while the action of the SR was essential to ensure the return to a very low level of free Ca2+ ion and of T-Ca. After relaxation, a large fraction of the Ca2+ pulse was still bound to parvalbumins and returned slowly to the SR during the recovery. When the SR activity was reduced, the presence of parvalbumins preserved a fast rate of relaxation, at least for a few contractions. This may have a high adaptive value in cold-blooded animals.

Calcium ion regulation in barnacle muscle fibers and its relation to force development

It is possible to use the photoproteins aequorin and obelin within the slowly responding striated muscle films to provide both qualitative and quantitive information as to the nature of the calcium-dependent reactions for force development. It seems that these reactions are not in equilibrium with the Ca2+ during a transient response. In addition, these experiments suggest that kinetically at least two calciums are required per functional unit for force development and that this active site is competed for by Mg2+, H+, and K+ ions.

Functional characteristics of reconstituted sarcoplasmic reticulum membranes as a function of the lipid-to-protein ratio

The ATP-induced Ca2+ accumulation efficiency and rates of Ca2+ uptake of the reconstituted sarcoplasmic reticulum (RSR) model membrane system were measured over an extended range of lipid-to-protein (L/P) molar ratios and were compared to those of isolated light sarcoplasmic reticulum (LSR). Highly purified sarcoplasmic reticulum (SR), dissociated in the presence of deoxycholate, was reconstituted for several L/P ratios, according to the same procedure, forming closed membranes vesicles composed of greater than 95% Ca2+ pump protein and SR lipids which were capable of ATP-induced Ca2+ accumulation in the absence of oxalate or other Ca2+ precipitating agents. This suggests that dissociation of SR and reconstitution to form RSR does not significantly affect the ability of the Ca2+ pump protein incorporated into the SR lipid bilayer to establish Ca2+ gradients. Electron micrographs of fixed and stained dispersions of RSR revealed a structural organization of the membrane that was dependent upon the L/P molar ratio. RSR with L/P greater than 88 were composed of closed vesicles whose membranes stained asymmetrically, similar to that observed for LSR. Closed vesicles of RSR with L/P less than 88 were composed of membrane that stained symmetrically. In addition, reconstituted SR preparations with well-defined L/P molar ratios greater than 88 possess a functional behavior similar to that of LSR (in the absence of oxalate, energy efficiencies are 60-70% and apparent initial uptake rates are 80% that of isolated LSR controls); RSR preparations with L/P less than 88 are characterized by significantly depressed values of the energy efficiencies and apparent initial uptake rates especially at low L/P ratios. Thus, we are the first to report a reconstituted SR model membrane system capable of attaining rates of Ca2+ uptake comparable to isolated LSR controls at comparable L/P ratios in the absence of oxalate or other Ca2+ precipitating agents.

Calcium release and ionic changes in the sarcoplasmic reticulum of tetanized muscle: an electron-probe study

Approximately 60-70% of the total fiber calcium was localized in the terminal cisternae (TC) in resting frog muscle as determined by electron-probe analysis of ultrathin cryosections. During a 1.2 s tetanus, 59% (69 mmol/kg dry TC) of the calcium content of the TC was released, enough to raise total cytoplasmic calcium concentration by approximately 1 mM. This is equivalent to the concentration of binding sites on the calcium-binding proteins (troponin and parvalbumin) in frog muscle. Calcium release was associated with a significant uptake of magnesium and potassium into the TC, but the amount of calcium released exceeded the total measured cation accumulation by 62 mEq/kg dry weight. It is suggested that most of the charge deficit is apparent, and charge compensation is achieved by movement of protons into the sarcoplasmic reticulum (SR) and/or by the movement of organic co- or counterions not measured by energy dispersive electron-probe analysis. There was no significant change in the sodium or chlorine content of the TC during tetanus. The unchanged distribution of a permeant anion, chloride, argues against the existence of a large and sustained transSR potential during tetanus, if the chloride permeability of the in situ SR is as high as suggested by measurements on fractionated SR. The calcium content of the longitudinal SR (LSR) during tetanus did not show the LSR to be a major site of calcium storage and delayed return to the TC. The potassium concentration in the LSR was not significantly different from the adjacent cytoplasmic concentration. Analysis of small areas of I-band and large areas, including several sarcomeres, suggested that chloride is anisotropically distributed, with some of it probably bound to myosin. In contrast, the distribution of potassium in the fiber cytoplasm followed the water distribution. The mitochondrial concentration of calcium was low and did not change significantly during a tetanus. The TC of both tetanized and resting freeze-substituted muscles contained electron-lucent circular areas. The appearance of the TC showed no evidence of major volume changes during tetanus, in agreement with the estimates of unchanged (approximately 72%) water content of the TC obtained with electron-probe analysis.

Structural and functional lability induced by diethyl ether on the sarcoplasmic reticulum membrane

Structural and functional changes occurring in sarcoplasmic reticulum vesicles following exposure to low concentrations (5--7%, v/v) of diethyl ether in aqueous media, were studied by electron microscopy and by kinetic measurements of Ca2+ transport and ATPase activity. Electron microscopy of thin sectioned and freeze-fractured sarcoplasmic reticulum vesicles provided detailed resolution of Ca-ATPase amphiphilic molecules displaying 'lollipop' portions on the outer surface of the vesicle, and non-polar moieties penetrating the membrane's hydrophobic interior. This asymmetric disposition of ATPase molecules was disrupted in vesicles exposed to ether and then centrifuged and/or resuspended in aqueous media. Such vesicles had a tendency to undergo fragmentation, and the distribution of ATPase molecules was markedly altered. The continuous fuzzy layer of lollipops became discontinuous, and the intramembranous particles became randomly distributed over both the concave and the convex freeze-fracture membrane faces. Functionally, the vesicles lost their ability to accumulate calcium in the presence of ATP, although high rates of ATPase activity were maintained. Vesicles which were simply exposed to ether, without being subjected to centrifugation and/or homogenization, did not appear altered ultrastructurally, and retained their ability to accumulate calcium. In fact, the enzyme turnover and the maximal levels of calcium uptake were increased. It is concluded that diethyl ether interferes with lipid-lipid and protein-lipid interactions in the sarcoplasmic reticulum vesicle membrane, thereby facilitating molecular motions which may be a limiting factor in the transport mechanism. On the other hand, these weakened interactions permit structural denaturation and loss of the ability to maintain a transmembrane Ca2+ gradient when the vesicles are subjected to mechanical perturbations which are harmless in the absence of ether.

Calcium buffering in presynaptic nerve terminals. Free calcium levels measured with arsenazo III

The particulate fraction from osmotically shocked synaptosomes ('synaptosomal membrances') sequesters Ca when incubated with ATP]containing solutions. This net accumulation of Ca can reduce the free [Ca2+] of the bathing medium to sub-micromolar levels (measured with arsenazo III). Two distinct types of Ca sequestration site are responsible for the Ca2+ buffering. One site, presumed to be smooth endoplasmic reticulum, operates at low [Ca2+] (less than 1 microM), and has a relatively small capacity. Ca sequestration at this site is prevented by the Ca2+ ionophore, A-23187, but not by mitochondrial poisons. The secone (mitochondrial) site, in contrast, is blocked by the mitochondrial uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone, and oligomycin. Since the intraterminal organelles can buffer [Ca2+] to about 0.3-0.5 microM, this may be an upper limit to the normal resting level of [Ca2+]i in nerve terminals. In the steady state, total cell Ca and [Ca2+]i will be governed principally be Ca transport mechanisms in the plasmalemma; the intracellular organelle transport systems then operate in equilibrium with this [Ca2+]. During activity, however, Ca rapidly enters the terminals and [Ca2+]i rises. The intracellular buffering mechanisms then come into play and help to return [Ca2+]i toward the resting level; the non-mitochondrial Ca sequestration mechanism probably plays the major role in this Ca buffering.

Effect of length and caffeine on isometric tetanus relaxation of frog sartorius muscles

In an isometric tetanus of frog sartorius muscle the total relaxation time increased linearly with change in length from 0.7 to 1.4 times rest length. Maximal rate of relaxation, measured from the time derivative (dp/dt) of tension decay, decreased with both decrease and increase from rest length in correlation with the generated tetanus tension. Stretching the muscle did not significantly affect the times to maximal rate, positive and negative inflexion points but greatly increased the time to total relaxation from the negative inflexion point. Caffeine at 2 mM, acting on muscles at rest length, also slowed the relaxation and decreased the maximal rate of tension decay. However, caffeine increased the times to maximal rate, positive and negative inflexion points without significantly affecting time to total relaxation from the negative inflexion point. These results suggest that caffeine slows an earlier step in relaxation, while stretch slows a later step. It is proposed that muscle relaxation is a two step process: an initial step that is regulated by the rate of Ca2+ uptake by sarcoplasmic reticulum, and a later step that is mostly controlled by the speed of dissociation of remaining cross-bridges.

Functional heterogeneity of the sarcoplasmic reticulum within sarcomeres of skinned muscle fibers

Precipitation of Ca oxalate in the sarcoplasmic reticulum of chemically skinned rabbit psoas fibers caused an increase in light scattering which was proportional to the amount of Ca accumulated per unit fiber volume. The increase in scattering was used to measure net accumulation rates and steady-state Ca capacities of the sarcoplasmic reticulum in single fibers. The data obtained were qualitatively and quantitatively similar to those reported for isolated vesicle preparations. Under conditions in which Ca was not depleted from the medium, Ca accumulation was linear with time over much of its course. Steady-state capacities were independent of the Ca concentration; uptake rates were half-maximal at 0.5 microM Ca++ and saturated above about 1.0 microM. Both rate and capacity varied with the oxalate concentration, being maximal at oxalate concentrations greater than or equal to mM and decreasing in proportion to one another at lower concentrations, with a threshold near 0.25 mM. At the lower loads, electron micrographs showed many sarcoplasmic reticulum elements empty of precipitate alongside others that were full, whereas virtually all were filled in maximally loaded fibers. These data indicate that the Ca oxalate capacity of each fiber varies with the number and volume of elements in which Ca oxalate crystals can form at a given oxalate concentration, and that individual regions of the sarcoplasmic reticulum within each sarcomere differ in their ability to support Ca oxalate precipitation. Our working hypothesis is that this range in ability to form Ca oxalate crystals involves differences in ability to accumulate and retain ionized Ca inside the sarcoplasmic reticulum.

A method for studying the depolarization-induced calcium ion release from fragmented sarcoplasmic reticulum

An optical method of studying the 'depolarizatoin'-induced Ca2+ release from isolated sarcoplasmic reticulum was presented. The method, which involves the use of metallochromic indicators, has the advantage of being able to perform the rapid kinetic measurement of the release. It was suggested that the velocity of the 'depolarization'-induced Ca2+ release was rapid enough to account for the velocity of muscle contraction if the phenomenon is involved in the contraction mechanism. The change of membrane potential was also measured optically using a potential-sensitive dye. The possibility that this type of release is caused by osmotic effects is discussed.