A comparison of vesicles derived from terminal cisternae and longitudinal tubules of sarcoplasmic reticulum isolated from rabbit skeletal muscle

Thyroid hormones differentially regulate the distribution of rabbit skeletal muscle Ca2 + -ATPase (SERCA) isoforms in light and heavy sarcoplasmic reticulum

The sarcoplasmic reticulum (SR) is composed of two fractions, the heavy fraction that contains proteins involved in Ca2+ release, and the light fraction enriched in Ca(2+)-ATPase (SERCA), an enzyme responsible for Ca2+ transport from the cytosol to the lumen of SR. It is known that in red muscle thyroid hormones regulate the expression of SERCA 1 and SERCA 2 isoforms. Here we show the effects of thyroid hormone on SERCA expression and distribution in light and heavy SR fractions from rabbit white and red muscles. In hyperthyroid red muscle there is an increase of SERCA 1 and a decrease of SERCA 2 expression. This is far more pronounced in the heavy than in the light SR fraction. As a result, the rates of Ca(2+)- ATPase activity and Ca(2+)-uptake by the heavy vesicles are increased. In hypothyroidism we observed a decrease in SERCA 1 and no changes in the amount of SERCA 2 expressed. This promoted a decrease of both Ca(2+)-uptake and Ca(2+)-ATPase activity. While the major differences in hyperthyroidism were found in the heavy SR fraction, the effects of hypothyroidism were restricted to light SR fraction. In white muscle we did not observe any significant changes in either hypo- or hyperthyroidism in both SR fractions. Thus, the regulation of SERCA isoforms by thyroid hormones is not only muscle specific but also varies depending on the subcellular compartment analyzed. These changes might correspond to the molecular basis of the altered contraction and relaxation rates detected in thyroid dysfunction.

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

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

An assay for sarcoplasmic reticulum Ca2(+)-ATPase activity in muscle homogenates

A spectrophotometric method is described for the determination of sarcoplasmic reticulum (SR) Ca2(+)-ATPase activity (EC 3.1.6.38) in unfractionated muscle homogenates. Conditions were established that give maximal SR Ca2(+)-ATPase activity, while eliminating Ca2(+)-dependent myofibrillar ATPase activity and reducing Ca2(+)-independent or background ATPase activity. High [Ca2+] (20 mM) could be used to selectively inhibit the SR Ca2+ ATPase. Identification of the Ca2(+)-dependent ATPase activity in muscle homogenates as being SR Ca2+ ATPase was based on a comparison of several parameters using homogenate material and purified SR. The following parameters were compared and found to be the same in homogenate and SR: activation and inactivation between 0 and 20 mM Ca2+, temperature dependence, sensitivity toward Triton X-100, and the maximal level of inhibition of ATPase activity achieved by an antibody specific for SR Ca2+ ATPase. The method is illustrated with the analysis of homogenates prepared from freeze-dried muscle fibers and thin sections of muscles typically used in microscope analyses as well as an analysis of freshly prepared homogenates from various types of muscle, which shows a good correlation over a wide range between SR specific Ca2(+)-uptake and -ATPase activities. In addition, a simple, easily constructed cuvette is described which allows the analysis of less than 5 micrograms of tissue (wet weight) in a volume of 25 microliters.

Effects of compound 48/80 on the Ca2+ release by reversal of the Ca2+ pump and by the Ca2+ channel of sarcoplasmic reticulum membranes

The effect of the calmodulin antagonist, compound 48/80, on the Ca2+ release from skeletal muscle sarcoplasmic reticulum was investigated. Both the Ca2+ release by reversal of the Ca2+ pump and the Ca2+ release by the Mg2(+)-controlled Ca2+ channel were studied. It was observed that, when reversal of the pump is inoperative and Mg2+ is not present in the reaction medium, 48/80 stimulates Ca2+ release from the vesicles. In contrast, in the presence of Mg2+, which blocks the Ca2+ channel, 48/80 inhibits Ca2+ release induced by ADP and Pi. This effect is strong at low concentrations of Pi (approximately 1 mM), whereas high concentrations (approximately 15 mM) protect the system against the drug. Furthermore, it was observed that 48/80 has a maximum effect on the channel-mediated Ca2+ release at concentrations of about 20 micrograms/ml, whereas maximal inhibition of the pump-mediated Ca2+ release occurs at concentrations of about 60-80 micrograms/ml. The results indicate that both the Ca2+ channel complex and the Ca2(+)-ATPase may be target systems for the effects of 48/80 on the Ca2+ transport activity of sarcoplasmic reticulum. However, the Ca2+ channel is more sensitive to the drug, suggesting an involvement of calmodulin on this mechanism of Ca2+ release.

Transverse tubule calcium regulation in malignant hyperthermia

Transverse tubule (TT) calcium transport and permeability were examined in the inherited skeletal muscle disorder malignant hyperthermia (MH). ATP-dependent calcium uptake by TT vesicles isolated from normal and MH-susceptible (MHS) pig muscle had a similar dependence on ionized Ca2+ concentration (K1/2 for Ca2+ of 0.21 +/- 0.04 and 0.25 +/- 0.05 microM for MHS and normal TT, respectively), as well as a similar Vmax (20.9 +/- 2.0 and 23.7 +/- 4.5 nmol Ca/mg protein/min for MHS and normal TT, respectively). Furthermore, the stimulation of calcium uptake by either calmodulin or cAMP-dependent protein kinase was similar in normal and MHS TT. Halothane concentrations greater than 2 mM inhibited calcium uptake by either normal or MHS TT to a similar extent (IC50 = 8 mM). Dantrolene (10 microM), nitrendipine (1 microM), and Bay K 8644 (1 microM) had no significant effect on either the initial rates of calcium uptake or maximal calcium accumulation of either MHS or normal TT vesicles. However, in the absence of any added agents, maximum calcium accumulation by MHS TT was significantly less than by normal TT (90 +/- 10 versus 130 +/- 9 nmol Ca/mg protein after 15 min of uptake). This difference was not due to an increased permeability of MHS TT to calcium, nor was it due to a difference in the sarcoplasmic reticulum contamination (less than 5%) of the MHS and normal preparations. Although our results indicate there is no significant defect in MHS TT calcium regulation, the diminished maximum calcium accumulation by MHS TT may contribute to the abnormal sarcoplasmic calcium homeostasis in skeletal muscle during an MH crisis.

Inositol 1,4,5-trisphosphate-induced calcium release from canine aortic sarcoplasmic reticulum vesicles

Inositol 1,4,5-trisphosphate-induced calcium release from canine aortic smooth muscle sarcoplasmic reticulum vesicles was examined using the calcium indicator antipyrylazo III. Calcium release was initiated by addition of inositol 1,4,5-trisphosphate (IP3) to aortic vesicles 7 min after initiation of ATP-supported calcium uptake. Half-maximal calcium release occurred at 1 microM IP3, with maximal calcium release amounting to 25 +/- 2% of the intravesicular calcium (n = 12, 9 preparations). Ruthenium red (10-20 microM), which has been reported to block IP3-induced calcium release from skeletal muscle sarcoplasmic reticulum, did not inhibit aortic IP3-induced calcium release. Elevation of Mg2+ concentration from 0.06 to 7.8 mM inhibited aortic IP3-induced calcium release 75%, which contrasts with the Mg2+-insensitive IP3-induced calcium release from platelet reticular membranes. The IP3-dependence of aortic calcium release suggested that Mg2+ acted as a noncompetitive inhibitor. Thus, aortic sarcoplasmic reticulum vesicles contain an IP3-sensitive calcium pathway which is inhibited by millimolar concentrations of Mg2+, but which is not inhibited by Ruthenium red and so differs from the previously described IP3-sensitive calcium pathways in skeletal muscle and platelet reticular membranes.

Phospholamban stoichiometry in canine cardiac muscle sarcoplasmic reticulum

Treatment of cardiac sarcoplasmic reticulum with the crosslinking reagent dithiobis (succinimidyl propionate) in the presence of 125I-calmodulin, resulted in the formation of a 40,000-dalton affinity labeled component, consisting of a 1:1, phospholamban: 125I-calmodulin complex. In parallel experiments, sarcoplasmic reticulum was phosphorylated in the presence of calmodulin and [gamma-32P]ATP, and then treated with the crosslinking reagent to produce an affinity labeled component consisting of a 1:1, calmodulin: 32P-phospholamban complex. These experiments permitted determination of the amount of 125I and 32P incorporated into the 40,000-dalton complexes, as well as the amount of 32P incorporated into the 23,000-dalton form of phospholamban. If 1 mol of Ca2+-dependent ATPase phosphoprotein represents 1 mol of 100,000-dalton Ca2+-dependent ATPase monomer, then there are 4.88 +/- 1.33 mol Ca2+-dependent ATPase/mol of phospholamban. If there are 2 mol of Ca2+-dependent ATPase phosphoprotein/mol of 100,000-dalton Ca2+-dependent ATPase monomer, then there are 9.76 +/- 2.66 mol Ca2+-dependent ATPase/mol phospholamban.

Thiophosphorylation of skeletal muscle sarcoplasmic reticulum in porcine malignant hyperpyrexia

1. The basal, Ca2+-dependent and Mg2+-dependent thiophosphorylation of malignant hyperpyrexia-susceptible (MHS) porcine skeletal muscle was investigated. 2. Seven major proteins of Mr 100,000-11,000 were substrates for thiophosphorylation. 3. Sodium molybdate significantly elevated all levels of thiophosphorylation in control sarcoplasmic reticulum, but did not effect the Ca2+-dependent thiophosphorylation of MHS samples. 4. These results suggest that MHS sarcoplasmic reticulum may have altered sensitivity to protein phosphatase inhibition.

Effect of halothane on Ca2+-induced Ca2+ release from sarcoplasmic reticulum vesicles isolated from rat skeletal muscle

Halothane induces the release of Ca2+ from a subpopulation of sarcoplasmic reticulum vesicles that are derived from the terminal cisternae of rat skeletal muscle. Halothane-induced Ca2+ release appears to be an enhancement of Ca2+-induced Ca2+ release. The low-density sarcoplasmic reticulum vesicles which are believed to be derived from nonjunctional sarcoplasmic reticulum lack the capability of both Ca2+-induced and halothane-induced Ca2+ release. Ca2+ release from terminal cisternae vesicles induced by halothane is inhibited by Ruthenium red and Mg2+, and require ATP (or an ATP analogue), KCl (or similar salt) and extravesicular Ca2+. Ca2+-induced Ca2+ release has similar characteristics.

The effect of hypothyroidism on sarcoplasmic reticulum in fast-twitch muscle of the rat

The effects of hypothyroidism on the Ca2+-transport capabilities of fast-twitch muscle (m. gastrocnemius) of the rat were studied in whole-muscle homogenate and isolated sarcoplasmic reticulum. Hypothyroidism did not affect the percentage recovery and the vesicle composition of the sarcoplasmic reticulum fraction, the total lipid and phospholipid-to-protein ratios and the protein composition (both qualitative and quantitative). Also the Ca2+-loading capacity of purified sarcoplasmic reticulum, in the presence of oxalate, and the Ca2+ and pH dependence of both the uptake reaction and the coupled ATPase activity were unchanged. However, the homogenate Ca2+-loading capacity and the Ca2+-uptake activity were depressed, as was the yield of purified sarcoplasmic reticulum. The results indicate a 31% reduction of the entire sarcoplasmic reticulum membrane system per volume of muscle. Ca2+/ATP coupling ratios, determined in purified sarcoplasmic reticulum vesicles by measurement of initial rates of net Ca2+ uptake and Ca2+-Mg2+-dependent hydrolysis of ATP, were found to be 1.48 +/- 0.06 and 2.08 +/- 0.05 in the euthyroid and hypothyroid groups, respectively. Identical values were obtained with a recently described Ca2+-pulse method (Meltzer, S. and Berman, M.C. (1984) Anal. Biochem. 138, 458-464), i.e., 1.53 +/- 0.06 and 2.01 +/- 0.03 in the euthyroid and hypothyroid groups, respectively. Passive Ca2+ efflux from sarcoplasmic reticulum was the same in both groups (30 nmol/mg per min), as was the fraction of vesicles that did not show net uptake of Ca2+ (less than 10%), which makes it unlikely that these parameters provide an explanation for the differences in the coupling ratio. The energy of activation of the (Ca2+ + Mg2+)-ATPase was increased in hypothyroidism, which may point to changes in the phospholipid environment of the enzyme. Physiological concentrations of T3 and T4 had no effect on the (Ca2+ + Mg2+)-ATPase in vitro, but all observed changes in the hypothyroid state could be reversed within 14 days by administration of T3 to hypothyroid animals. Approximate calculations indicate that the observed changes in the sarcoplasmic reticulum as a result of thyroid-hormone depletion may contribute significantly to the decrease in relaxation rate and the decrease in energy consumption during contraction.

Effects of Mg2+ on calcium accumulation by two fractions of sarcoplasmic reticulum from rabbit skeletal muscle

Calcium accumulation by two fractions of sarcoplasmic reticulum presumably derived from longitudinal tubules (light vesicles) and terminal cisternae (heavy vesicles) was examined radiochemically in the presence of various free Mg2+ concentrations. Both fractions of sarcoplasmic reticulum exhibited a Mg2+-dependent increase in phosphate-supported calcium uptake velocity, though half-maximal velocity in heavy vesicles occurred at a much higher free Mg2+ concentration than that in light vesicles (i.e., approx. 0.90 mM vs. approx. 0.02 mM Mg2+). Calcium uptake velocity in light vesicles correlated with Ca2+-dependent ATPase activity, suggesting that Mg2+ stimulated the calcium pump. Calcium uptake velocity in heavy vesicles did not correlate with Ca2+-dependent ATPase activity, although a Mg2+-dependent increase in calcium influx was observed. Thus, Mg2+ may increase the coupling of ATP hydrolysis to calcium transport in heavy vesicles. Analyses of calcium sequestration (in the absence of phosphate) showed a similar trend in that elevation of Mg2+ from 0.07 to 5 mM stimulated calcium sequestration in heavy vesicles much more than in light vesicles. This difference between the two fractions of sarcoplasmic reticulum was not explained by phosphoenzyme (EP) level or distribution. Analyses of calcium uptake, Ca2+-dependent ATPase activity, and unidirectional calcium flux in the presence of approx. 0.4 mM Mg2+ suggested that ruthenium red (0.5 microM) can also increase the coupling of ATP hydrolysis to calcium transport in heavy vesicles, with no effect in light vesicles. These functional differences between light and heavy vesicles suggest that calcium transport in terminal cisternae is regulated differently from that in longitudinal tubules.

Calcium release from two fractions of sarcoplasmic reticulum from rabbit skeletal muscle

Calcium release from sarcoplasmic reticulum vesicles presumably derived from longitudinal tubules (LSR) and terminal cisternae (HSR) of rabbit skeletal muscle was investigated by dual wavelength spectrophotometry using the calcium-indicator antipyrylazo III. In 120 mM KCl, 5 mM MgCl2, 30 microM, CaCl2, 50 microM MgATP, 100 microM antipyrylazo III, 40 mM histidine (pH 6.8, 25 degrees C), LSR and HSR sequestered approx. 115 nmol calcium/mg, and then spontaneously released calcium. Analysis of ATP hydrolysis and phosphoenzyme level during LSR and HSR calcium sequestration indicated that this calcium release process was passive, occurring in the virtual absence of ATP and phosphoenzyme. Moreover, subsequent addition of ATP reinitiated the calcium sequestration-release sequence. Calcium release by HSR was more than 4-times faster than that by LSR. Analysis of the calcium release phase demonstrated a biexponential decay for both LSR (0.10 and 0.63 min-1) and HSR (0.26 and 1.65 min-1), suggestive of heterogeneity within each fraction. Replacement of 120 mM KCl with either 120 mM choline chloride, 240 mM sucrose, or H2O reduced maximal calcium sequestration by LSR, but had less effect on LSR calcium release rate constants. In the case of HSR, these changes in the ionic composition of the medium drastically reduced calcium release rate constants with little effect on calcium content. These marked differences between LSR and HSR are consistent with the hypothesis that the calcium permeability of the terminal cisternae is greater and more sensitive to the ionic environment than is that of the longitudinal tubules of sarcoplasmic reticulum.

Mechanism of calcium release from skeletal sarcoplasmic reticulum

Ca2+ -induced Ca2+ release at the terminal cisternae of skeletal sarcoplasmic reticulum was demonstrated using heavy sarcoplasmic reticulum vesicles. Ca2+ release was observed at 10 mum Ca2+ in the presence of 1.25 mm free Mg2+ and was sensitive to low concentrations of ruthenium red and was partially inhibited by valinomycin. These results suggest that the Ca2+ -induced Ca2+ release is electrogenic and that an inside negative membrane potential created by the Ca2+ flux opens a second channel that releases Ca2+. Results in support of this formulation were obtained by applying a Cl- gradient or K+ gradient to sarcoplasmic reticulum vesicles to initiate Ca2+ release. Based on experiments the following hypothesis for the excitation-contraction coupling of skeletal muscle was formulated. On excitation, small amounts of Ca2+ enter from the transverse tubule and interact with a Ca2+ enter from the transverse tubule and interact with a Ca2+ receptor at the terminal cisternae and cause Ca2+ release (Ca2+ -induced Ca2+ release). This Ca2+ flux generates an inside negative membrane potential which opens voltage-gated Ca2+ channels (membrane potential-dependent Ca2+ release) in amounts sufficient for contraction.

Acetylcholinesterase from sarcoplasmic reticulum of white muscle

1. Two membrane fractions were separated from rabbit white muscle SR by discontinuous sucrose gradient. 2. Both crude and membrane fractions were shown to contain AChE, Ca2+-stimulated and Ca2+- independent ATPase activities. 3. 1% W/V Triton X-100 solubilized most of the AChE and Ca2+-stimulated ATPase but the Ca2+- independent ATPase was poorly solubilized. 4. AChE was sensitive to BW284c51, non-sensitive to ethopropazine and presented inhibition by excess of the substrate, ATCh. 5. Polyacrylamide gel electrophoresis from Triton-treated crude SR revealed several bands of AChE and ATPase activities. 6. SDS-gel electrophoresis from crude SR showed two polypeptides specifically labelled with [3H]DFP.