The effects of calmodulin antagonist drugs on isolated sarcoplasmic reticulum from malignant hyperpyrexia susceptible swine

1. Because calcium antagonist drugs increase contracture in both control and malignant hyperpyrexia susceptible (MHS) skeletal muscle, the effect of these drugs on the sarcoplasmic reticulum (SR) was investigated. 2. The calmodulin antagonist drugs inhibited the Ca2+ dependent ATPase activity and the ATP-dependent Ca2+ uptake, and accelerated the efflux of Ca2+ from isolated SR preparations from both control and MHS skeletal muscle. These effects of calmodulin antagonist drugs on SR Ca2+ transport functions were consistent with their in vitro pharmacological effects on control and MHS muscle.     

Enhanced Ca2+ uptake and ATPase activity of sarcoplasmic reticulum in the presence of diethyl ether

The presence of diethyl ether enhances the rates of both Ca2+ uptake and ATPase activity in sarcoplasmic reticulum vesicles (SR) isolated from rabbit skeletal muscle. Stopped-flow measurements of Ca2+ transport in SR show that, in the absence of oxalate and other calcium-complexing anions, the initial velocity of the ATP-dependent Ca2+ uptake increases from 60 to 107 nmol of Ca2+/s/mg of protein when 5% (v/v) diethyl ether is present. Similar concentrations of diethyl ether increase steady state levels of Ca2+ accumulation by over 80%. Parallel to the enhancement of the rate of Ca2+ transport, diethyl ether induces an increased rate of Ca2+-dependent ATPase activity. Among four other ether compounds tested, three enhanced the rate of Ca2+ uptake, but none as effectively as diethyl ether, and a fourth reduced the rate of Ca2+ transport by the SR. These results contrast with previous observations concerning the effect of diethyl ether on ATP-dependent Ca2+ transport by SR and are now consistent with a direct pharmacological action of ether as a muscle relaxant at the level of SR Ca2+ transport.     

Modification of calcium fluxes by dimethyl sulfoxide and 2-butoxyethanol in sarcoplasmic reticulum vesicles: a possible mechanism for skeletal muscle relaxation induced by dimethyl sulfoxide

In order to investigate the mechanism of skeletal muscle relaxation induced by dimethyl sulfoxide, 2-butoxyethanol and dimethyl sulfoxide were examined for their effects on 1) Ca2+ uptake into and efflux from sarcoplasmic reticulum vesicles prepared from rabbit fast skeletal muscle and crayfish tail muscle by the murexide method, 2) ATPase activities of rabbit reticulum vesicles, 3) the isolated phrenic nerve-diaphragm preparation of the rat and 4) crayfish opener muscle preparation. Ca2+ efflux rate from rabbit reticulum vesicles was markedly decreased with increasing concentrations (5-20% v/v) of dimethyl sulfoxide without affecting the maximum Ca2+ uptake by the reticulum. 2-Butoxyethanol showed quite contrary effects. Dimethyl sulfoxide strongly inhibited the activity of basal ATPase rather than of Ca2+-dependent ATPase. 2-Butoxyethanol did not significantly inhibit the activity of basal ATPase, but markedly increased Ca2+-dependent ATPase activity. Antagonisms between dimethyl sulfoxide and caffeine were demonstrated either in contractions of crayfish opener muscles or in the Ca2+ release from crayfish sarcoplasmic reticulum vesicles. These results indicate a possibility that dimethyl sulfoxide reversibly induces skeletal muscle relaxation mainly in the sarcoplasmic reticulum by means of decreasing the rate and the amount of Ca2+ release from the reticulum.     

Inactivation of a Ca2+-induced Ca2+ release channel from skeletal muscle sarcoplasmic reticulum during active Ca2+ transport

ATP-dependent Ca2+ uptake by subfractions of skeletal muscle sarcoplasmic reticulum (SR) was studied with the Ca2+ indicator dye, antipyrylazo III. Ca2+ uptake by heavy SR showed two phases, a slow uptake phase and a fast uptake phase. By contrast, Ca2+ uptake by light SR exhibited a monophasic time course. In both fractions a steady state of Ca2+ uptake was observed when the concentration of free Ca2+ outside the vesicles was reduced to less than 0.1 microM. In the steady state, the addition of 5 microM Ca2+ to the external medium triggered rapid Ca2+ release from heavy SR but not from light SR, indicating that the heavy fraction contains a Ca2+-induced Ca2+ release channel. During Ca2+ uptake, heavy SR showed a constant Ca2+-dependent ATPase activity (1 mumol/mg protein X min) which was about 150 times higher than the rate of Ca2+ uptake in the slow uptake phase. Ruthenium red, an inhibitor of Ca2+-induced Ca2+ release, enhanced the rate of Ca2+ uptake during the slow phase without affecting Ca2+-dependent ATPase activity. Adenine nucleotides, activators of Ca2+ release, reduced the Ca2+ uptake rate. These results suggest that the rate of Ca2+ accumulation by heavy SR is not proportional to ATPase activity during the slow uptake phase due to the activation of the channel for Ca2+-induced Ca2+ release. In addition, they suggest that the release channel is inactivated during the fast Ca2+ uptake phase.     

Isolation of highly active preparations of sarcoplasmic reticulum and Ca2-dependent ATPase from cardiac muscle]

A microsomal preparation with a high ability for Ca2+ uptake has been isolated from pigeon heart. A method of further purification of Ca2+-accumulating system of heart, based on the ability of sarcoplasmic reticulum for the energy-dependent Ca2+ accumulation in the presence of oxalate, has been developed. Upon centrifugation in the gradient of sucrose and KCl concentration the fragments of sarcoplasmic reticulum, rendered "heavy" by calcium oxalate, can be separated from foreign cell membranes. The main component of heart "calcium pump" is Ca2+-dependent ATPase (making up to about 50% of all proteins of the purified reticulum), having a molecular weight of 100.000--105.000. Specific activity of heart Ca2+-ATPase as well as the ability of purified heart sarcoplasmic reticulum for Ca2+ uptake are only slightly less than those of the skeletal muscle reticulum. The data obtained suggest that heart sarcoplasmic reticulum may be efficient for providing heart muscle relaxation.     

Regulation of intracellular calcium in chick embryo fibroblast: calcium uptake by the microsomal fraction.

The total membrane fraction of a chick embryo fibroblast (CEF) homogenate accumulates calcium in an energy-dependent manner. This activity can be dissociated into azide-sensitive and azide-insensitive components. The azide-sensitive component of calcium uptake is believed to represent mitochondrial calcium uptake. The azide-insensitive component of calcium uptake is enhanced by the presence of a calcium trapping agent such as oxalate, and cannot utilize, ADP, inorganic phosphate and a Krebs cycle substrate to support uptake. The distribution of the azide-insensitive calcium uptake in subcellular fractions suggests that this uptake occurs in other than mitochondrial membranes. The membranes most likely to contribute to the azide-insensitive component of calcium uptake are the endoplasmic reticulum and plasma membrane. A microsomal preparation from CEF cells is essentially devoid of the azide-sensitive calcium uptake activity. This microsomal activity is similar in characteristics to the sarcoplasmic reticulum of skeletal muscle. However the specific activity of CEF microsomal calcium uptake system is much less than that found in the skeletal muscle system. The transport of calcium by these membranes provide a mechanism for the regulation of cytosol calcium levels and may play a role in the control of movement and growth of cultured cells.     

Ca2+ uptake, Ca2+-ATPase activity, phosphoprotein formation and phosphate turnover in a microsomal fraction of smooth muscle

Vesicles capable of phosphate-stimulated calcium uptake were isolated from the microsomal fraction of the smooth muscle of the pig stomach according to a previously described procedure which consists in increasing the density of the vesicles by loading them with calcium phosphate and isolating them by centrifugation [Raeymaekers, L., Agostini, B., and Hasselbach, W. (1981) Histochemistry, 70, 139--150]. These vesicles, which contain calcium phosphate deposits, are able to accumulate an additional amount of calcium. This calcium uptake is accompanied by calcium-stimulated ATPase activity and by the formation of an acid-stable phosphoprotein. The acid-denatured phosphoprotein is dephosphorylated by hydroxylamine, which indicates that an acylphosphate is formed. This phosphoprotein probably represents a phosphorylated transport intermediate similar to that seen with the Ca2+-ATPase of sarcoplasmic reticulum of skeletal muscle. As with the Ca2+-ATPase of sarcoplasmic reticulum vesicles, this vesicular fraction catalyses an exchange between inorganic phosphate and the gamma-phosphate of ATP (ATP-Pi exchange) which is dependent on the presence of intravesicular calcium, and an exchange of phosphate between ATP and ADP (ATP-ADP exchange). The results further indicate that the turnover rate of the calcium pump, calculated from the ratio of calcium-stimulated ATPase activity to the steady-state level of phosphoprotein, is similar to that of Ca2+-ATPase of sarcoplasmic reticulum of skeletal muscle.     

Inhibition of dicarboxylic anion transport by fluorescein isothiocyanate in skeletal sarcoplasmic reticulum.

It has been demonstrated previously that dicarboxylic anions are cotransported during ATP-dependent Ca2+ transport by skeletal muscle sarcoplasmic reticulum (SR) membranes, and that anion cotransport stimulates Ca2+ transport. In the current study, we present evidence that dicarboxylic anion cotransport and Ca2+ transport are kinetically distinct in SR, but both functions are mediated by the CaATPase protein. Preincubation of SR with 40 microM fluorescein isothiocyanate (FITC) (pH 7.0) inhibited essentially all of the Ca2+ ATPase activity, as well as active oxalate-supported and oxalate-independent 45Ca2+ accumulation. The addition of 1 mM beta, gamma-methyleneadenosine 5'-triphosphate (AMP-PCP) to the preincubation media fully protected the dicarboxylic anion-independent Ca2+ ATPase activity and the oxalate-independent active 45Ca2+ accumulation from the inhibitory effects of FITC; however, the ATP-associated [14C]oxalate accumulation, the oxalate-dependent 45Ca2+ accumulation, and the oxalate- and maleate-dependent stimulation of Ca2+ ATPase activity were not protected by AMP-PCP. Thus, the dicarboxylic anion accumulation and the stimulation of Ca2+ uptake by dicarboxylic anions could be functionally separated from the ATP-dependent, anion-independent Ca2+ translocation. FITC bound exclusively to the 100-kDa (CaATPase) and 92-kDa (phosphorylase) proteins in the SR membranes and to purified CaATPase in sodium dodecyl sulfate-polyacrylamide gel electrophoresis; 1 mM AMP-PCP inhibited 50-55% of the FITC fluorescence on the 100-kDa protein, but did not significantly alter fluorescence on the 92-kDa protein. Two-dimensional gel analysis demonstrated a single 100-kDa protein in longitudinal SR membranes. FITC appears to inhibit ATP-dependent Ca2+ transport, and dicarboxylic anion translocation through interaction at separate domains of the CaATPase protein.     

The effect of trifluoroperazine on the sarcoplasmic reticulum membrane

The inhibitory effect of trifluoroperazine (25-200 microM) on the sarcoplasmic reticulum calcium pump was studied in sarcoplasmic reticulum vesicles isolated from skeletal muscle. It was found that the lowest effective concentrations of trifluoroperazine (10 microM) displaces the Ca2+ dependence of sarcoplasmic reticulum ATPase to higher Ca2+ concentrations. Higher trifluoroperazine concentrations (100 microM) inhibit the enzyme even at saturating Ca2+. If trifluoroperazine is added to vesicles filled with calcium in the presence of ATP, inhibition of the catalytic cycle is accompanied by rapid release of accumulated calcium. ATPase inhibition and calcium release are produced by identical concentrations of trifluoroperazine and, most likely, by the same enzyme perturbation. These effects are related to partition of trifluoroperazine ino the sarcoplasmic reticulum membrane, and consequent alteration of the enzyme assembly within the membrane structure, and of the bilayer surface properties. The effect of trifluoroperazine was also studied on dissociated ('chemically skinned') cardiac cells undergoing phasic contractile activity which is totally dependent on calcium uptake and release by sarcoplasmic reticulum, and is not influenced by inhibitors of slow calcium channels. It was found that trifluoroperazine interferes with calcium transport by sarcoplasmic reticulum in situ, as well as with the role of sarcoplasmic reticulum in contractile activation.     

Reactivity of sarcoplasmic reticulum from rabbit cardiac muscle with 1-fluoro- and 1,5-difluoro- 2,4-dinitrobenzene

Sarcoplasmic reticulum preparations from rabbit cardiac and fast skeletal muscle react differentially with low concentrations of 1-fluoro- and 1,5-difluoro-2,4-dinitrobenzene. Dinitrophenylation of cardiac sarcoplasmic reticulum by 1-fluoro-2,4-dinitrobenzene is not affected by Ca2+ and is limited to the lipoprotein-lipid region. This contrasts sharply with the predominant Ca2+-dependent dinitrophenylation of the ATPase protein of rabbit skeletal sarcoplasmic reticulum by this reagent. Formation of non-serial high mol. wt. oligomers by 1,5-difluoro-2,4-dinitrobenzene is significantly greater in cardiac than in skeletal vesicles. Substrate MgATP2- does not protect rabbit cardiac sarcoplasmic reticulum ATPase activity or Ca2+ uptake from dinitrophenylation when monofunctional and bifunctional reagents are used. Chemical differences in the overall structure of the two kinds of membrane preparations can be ascertained from a comparison of the effects of Ca2+ and MgATP2- on the reactivity of these reagents.     

A possible role of protein phosphorylation in the inactivation of a Ca2+-induced Ca2+ release channel from skeletal muscle sarcoplasmic reticulum

The Ca2+-induced Ca2+ release channel in the heavy fraction of the sarcoplasmic reticulum (SR) from rabbit skeletal muscle is inactivated during ATP-dependent Ca2+ uptake (Morii, H., Takisawa, H., & Yamamoto, T. (1985) J. Biol. Chem. 260, 11536-11541). AMP, one of the adenine nucleotides which activate the Ca2+ release, delayed the onset of the channel inactivation when added early during the course of the Ca2+ uptake. However, AMP could no longer activate the channel but accelerated the inactivation when added during the later phase of the Ca2+ uptake. In SR passively loaded with Ca2+, the Ca2+ channel which had been activated by AMP and Ca2+ was not spontaneously inactivated. Similarly, during GTP-dependent Ca2+ uptake, the channel activated by AMP was not inactivated. In addition acid phosphatase markedly delayed the onset of the inactivation during ATP-dependent Ca2+ uptake, without affecting Ca2+ ATPase activity or GTP-dependent Ca2+ uptake by heavy SR. The effect of the phosphatase was completely blocked by ruthenium red, a potent inhibitor of the channel. These results suggest that the channel is inactivated through an ATP-dependent process, presumably phosphorylation of proteins in the SR membrane. This was supported by the findings that the reactivation of the inactivated channel by added Ca2+ was markedly accelerated by the addition of acid phosphatase and that several proteins of heavy SR were phosphorylated during ATP-dependent Ca2+ uptake.     

ADP-ribosylation of Ca2+-dependent ATPase in vitro suppresses the enzyme activity

We investigated the effect on the Ca2+-dependent ATPase activity of ADP-ribosylation of the enzyme from the rabbit skeletal muscle sarcoplasmic reticulum. A reconstituted ADP-ribosylation system of Ca2+-dependent ATPase in which the enzyme and ADP-ribosyltransferase, both were partially purified from the vesicles, and poly L-lysine were contained, was preincubated with 1 mM NAD, and the Ca2+-dependent ATPase activity was assayed. The NAD-dependent suppression of the enzyme activity depended on both the concentration of NAD and preincubation-time for the ADP-ribosylation, and was reversed by adding 20 mM arginine during the preincubation. These results taken together with the findings that Ca2+-dependent ATPase is a major acceptor protein for the modification in rabbit skeletal muscle sarcoplasmic reticulum [Hara et al. (1987) Biochem. Biophys. Res. Commun. 144; 856-862] suggest that Ca2+-transport in the sarcoplasmic reticulum may be regulated through changes in the rate of ADP-ribosylation of Ca2+-dependent ATPase.     

Inhibition of Na+/Ca2+ exchanger activity in cardiac and skeletal muscle sarcolemmal vesicles by monoclonal antibody 44D7

Monoclonal antibodies 44D7 and 4F2 inhibited specifically the Na+-dependent Ca2+ fluxes characteristic of the Na+/Ca2+ exchanger in cardiac and skeletal muscle sarcolemmal vesicles. Preincubation of membrane vesicles with monoclonal antibody 44D7 inhibited 90% of the Na+-dependent Ca2+ uptake measured in the first 10 s of the reaction and 50% of that measured after 60 s. Ca2+/calmodulin-dependent ATPase activity and ATP-dependent Ca2+ uptake by sarcolemmal vesicles were not affected by monoclonal antibody 44D7 whereas the Na+-dependent release of accumulated Ca2+ was inhibited. In the presence of the 44D7 antigen isolated from human kidney, monoclonal antibody 44D7 could no longer inhibit Na+-dependent Ca2+ fluxes. The distribution of 4F2 antigenic activity in the isolated muscle membrane fractions correlated with that of Na+/Ca2+ exchanger activity; cardiac and skeletal muscle sarcolemmal vesicles expressed higher levels of the antigen than skeletal muscle transverse tubule membrane, while no antigen could be detected in sarcoplasmic reticulum membranes. Our results suggest that monoclonal antibodies 44D7 and 4F2 interact either directly with the Na+/Ca2+ exchanger molecules or with some other protein(s) responsible for the regulation of this activity in the heart and skeletal muscle.     

Cause of increase in the efficiency of Ca2+ transport by fragments of sarcoplasmic reticulum from fast skeletal muscles induced by protein kinase

The effect of cAMP-dependent protein kinases from rabbit skeletal muscles on Ca2+ uptake by fragments of skeletal muscle sarcoplasmic reticulum was studied. It was shown that incubation of the sarcoplasmic reticulum fragments with protein kinase increases the rate of Ca2+ uptake without changing the activity of Ca2+-dependent ATPase. This phenomenon is not accompanied by phosphorus incorporation into the protein components of the reticulum membranes. The protein kinase preparation subjected to "self-phosphorylation" is also capable to increase the rate of Ca2+ uptake. Using (14C) -oleic acid, it was shown that the increase of the rate of Ca2+ transport under effects of the "self-phosphorylated" protein kinase occurs due to the binding of free fatty acids present in the sarcoplasmic reticulum membranes. It was found that the effect observed is due to phosphofructokinase (ATP : D-fructose-6-phosphate-1-phosphotransferase) present in the protein kinase preparation.     

Biosynthesis of the Ca2+- and Mg2+-dependent adenosine triphosphatase of sarcoplasmic reticulum in cell cultures of embryonic chick heart.

The biosynthesis of the Ca2+- and Mg2+-dependent adenosine triphosphatase of sarcoplasmic reticulum was studied in cell cultures of embryonic chick heart. Rates of synthesis were estimated from the incorporation of tritium-labeled leucine into the ATPase. Newly synthesized ATPase was isolated from cells by immunoprecipitation. Radioactive leucine incorporation into the ATPase was determined by gel electrophoresis of the immunoprecipitates and counting of gel slices containing the ATPase band. Accumulation of the ATPase was estimated from the concentration of Ca2+ and Mg2+-dependent, hydroxylamine-sensitive phosphoprotein in the whole cell membrane fraction of cultured cells. Embryonic heart cells cultured in a medium which permitted cell proliferation showed approximately linearly increasing rates of ATPase synthesis and accumulation/culture plate as the cells proliferated. When cells were cultured in a serum-free medium, cell proliferation was inhibited and there was no sustained increase in the rate of ATPase synthesis or accumulation. Inclusion of isoproterenol or dibutyryl cyclic AMP at concentrations of 10 microM up to 1 mM in serum-free culture medium failed to stimulate significantly ATPase synthesis.     

Characterization of sarcoplasmic reticulum in skinned muscle cultures

The plasma membranes of chick or rat skeletal muscles, grown in cell culture, were made permeable with saponin in a solution lacking calcium. The cells were then supplied with a medium resembling the cytosol and the ATP-dependent Ca2+ sequestration was performed. Based on the low concentration of free Ca2+ in the medium (below 5 microM), the presence of mitochondrial inhibitors and the effect of drugs that interfere with sarcoplasmic reticulum (SR) function, we assume that the measured Ca2+ accumulation expresses SR function on the saponin-treated myotubes. The development of the SR in muscle cultures is augmented as myogenesis proceeds and depends on its occurrence. Whereas creatine kinase activity is elevated immediately following cell fusion, there is a delay of at least 1 day between myoblast fusion and the increase in Ca2+ accumulation in the SR. Thyroxine or triiodothyronine caused an inhibition of Ca2+ accumulation in rat or chick muscle cultures. This inhibition could explain some of the muscle abnormalities caused by excess of thyroid hormones. A comparison was made between a white-type (fast) and heterogeneous muscle, differentiated in cell culture. There was no significant difference in SR function, indicating the important role of innervation in specifying the properties of muscle fiber types.     

The blocking effect of aliphatic hydrocarbons on Ca2+ leakage channels formed by aggregates of Ca2+-ATPase of the sarcoplasmic reticulum

The effects of aliphatic hydrocarbons within the liposomes on the Ca2+ transport function of isolated sarcoplasmic reticulum (SR) membranes of rabbit skeletal muscle, vesiculate preparation of Ca2+ dependent ATPase and proteoliposomes reconstituted from Ca2+-ATPase and egg phosphatidylcholine, were studied. It was shown that liposomes prepared from dipalmitoyl phosphatidylcholine containing aliphatic hydrocarbons increase 2 to 3 times Ca2+ accumulation by Ca2+-dependent ATPase from rabbit skeletal muscle SR. Ca2+ transport by SR vesicles increases in the presence of hydrocarbons by 15--20%. The activating effect of hydrocarbons on Ca2+ transport by proteoliposomes depends on the lipid/protein ratio. The proteoliposomes with a high lipid/protein ratio are practically insensitive to the effects of hydrocarbons. It was suggested that activation of Ca2+ transport by hydrocarbons is due to blocking of Ca2+ leakage channels formed during the aggregation of Ca2+-ATPase molecules. Treatment of membranes by formaldehyde results in the oligomerization of Ca2+-ATPase and decreases 2--4-fold the ATP-dependent accumulation of Ca2+. Subsequent addition of decane restores Ca2+ transport practically completely.     

The effect of ionomycin on calcium fluxes in sarcoplasmic reticulum vesicles and liposomes.

Ionomycin, a recently discovered calcium ionophore, inhibits the ATP-dependent active Ca2+ transport of rabbit sarcoplasmic reticulum vesicles at concentrations as low as 10(-8) to 10(-6) M. The effect is due to an increase in the Ca2+ permeability of the membrane which is also observed on liposomes. The inhibition of Ca2+ uptake is accompanied by an increase in the Ca2+-sensitive ATPase activity of sarcoplasmic reticulum vesicles.     

Studies on the heterogeneity of sarcoplasmic reticulum vesicles.

Isolated sarcoplasmic reticulum vesicles from rabbit white muscle were separated into a light (15--20% of total microsomes) and a heavy (80--85%) fraction by density gradient centifugation. The ultrastructure, chemical composition, enzymic activities and localization of membrane components in the vesicles of both fractions were investigated. From the following results it was concluded that both fractions are derived from the membranes of the sarcoplasmic reticulum system of the muscle: (i) The protein pattern of both fractions is essentially the same, except for different ratios of acidic, Ca2+-binding proteins. (ii) The 105000 dalton protein of the light fraction cross-reacts immunologically with the Ca2+-dependent ATPase of the heavy fraction. (iii) Ca2+-dependent ATPase, although of different specific activity, is found in both fractions. After rendering the vesicles leaky, specific activities in both fractions reach the same value. The light fraction was found to consist of "inside-out" vesicles by the following criteria: (i) No Ca2+ accumulation can be measured and the Ca2+-dependent ATPase activity is low and variable. (ii) The rate of trypsin digestion is lower and, compared to the heavy microsomes, a different ratio of degradation products is obtained. (iii) The sarcoplasmic reticulum membrane has a highly asymmetrical lipid distribution. This distribution of aminophospholipids is opposite to that in vesicles of heavy fraction. The light sarcoplasmic reticulum fraction has a higher phospholipid to protein ratio than the heavy one. This is consistent with the possibility that the two fractions derive from different parts of the sarcoplasmic reticulum system.     

(Ca2+ + Mg2+)-ATPase activity associated with the maintenance of a Ca2+ gradient by sarcoplasmic reticulum at submicromolar external [Ca2+]. The effect of hypothyroidism

The formation and maintenance of Ca2+-filling levels by sarcoplasmic reticulum vesicles from euthyroid (control) and hypothyroid skeletal muscle were investigated using the Ca2+-indicator quin-2, at [Ca2+] in the medium [( Cao2+]) of 0.05-0.3 microM. Rapid ATP-dependent Ca2+ uptake resulted in a steady-state Ca2+-filling level, Cai2+, within one minute. This Ca2+ gradient was maintained for at least three minutes, during which less than 20% of the ATP was consumed. Cai2+ was maximal (120 nmol/mg) for [Cao2+] greater than 0.3 microM and decreased to 40 nmol/mg at [Cao2+] of 0.05 microM. Preparations from both experimental groups showed qualitatively and quantitatively the same relationship between Cai2+ and [Cao2+] at steady state, despite a significantly lower Ca2+-pump content of hypothyroid sarcoplasmic reticulum, which resulted in a 25% lower maximal (Ca2+ + Mg2+)-ATPase activity. Maintenance of the steady state, at all levels of Cai2+, was associated with net ATP consumption by the Ca2+ pump and cycling of Ca2+, which processes were 30% slower in the hypothyroid group as compared to the control group. Determination of the passive efflux of Ca2+, as well as the fraction of leaky or unsealed sarcoplasmic reticulum fragments, excluded either of these possibilities as an explanation for the relatively high (Ca2+ + Mg2+)-ATPase rates at steady state. On the basis of these and previously reported results, it is concluded that the maintenance of a Ca2+ gradient by sarcoplasmic reticulum under physiological conditions with respect to external [Ca2+] and the concentrations of ATP, ADP and Pi, is associated with the cycling of Ca2+ coupled to net ATP hydrolysis. Using the obtained data it is calculated that the sarcoplasmic reticulum may account for 20% of the resting metabolic rate in skeletal muscle. Consequently, together with the previously reported lower sarcoplasmic reticulum content of skeletal muscle in hypothyroidism, we calculate that about one third of the decrease in basal metabolic rate in this thyroid state can be related to the alterations of the sarcoplasmic reticulum.