Biochemical properties of isolated transverse tubular membranes

Influence of kaempferol, a flavonoid compound, on membrane-bound ATPases in streptozotocin-induced diabetic rats

CONTEXT

Kaempferol is a flavonoid found in many edible plants (e.g. tea, cabbage, beans, tomato, strawberries, and grapes) and in plants or botanical products commonly used in traditional medicine. Numerous preclinical studies have shown that kaempferol have a wide range of pharmacological activities, including antioxidant, anti-inflammatory, anticancer, cardioprotective, neuroprotective, and antidiabetic activities.

OBJECTIVE

The present study investigates the effect of kaempferol on membrane-bound ATPases in erythrocytes and in liver, kidney, and heart of streptozotocin (STZ)-induced diabetic rats.

MATERIALS AND METHODS

Diabetes was induced into adult male albino rats of the Wistar strain, by intraperitoneal administration of STZ (40 mg/kg body weight (BW)). Kaempferol (100 mg/kg BW) or glibenclamide (600 µg/kg BW) was administered orally once daily for 45 d to normal and STZ-induced diabetic rats. The effects of kaempferol on membrane-bound ATPases (total ATPase, Na(+)/K(+)-ATPase, Ca(2+)-ATPase, and Mg(2+)-ATPase) activity in erythrocytes and in liver, kidney, and heart were determined.

RESULTS

In our study, diabetic rats had significantly (p < 0.05) decreased activities of total ATPases, Na(+)/K(+)-ATPase, Ca(2+)-ATPase, and Mg(2+)-ATPase in erythrocytes and tissues. Oral administration of kaempferol (100 mg/kg BW) or glibenclamide (600 µg/kg BW) for a period of 45 d resulted in significant (p < 0.05) reversal of these enzymes' activities to near normal in erythrocytes and tissues when compared with diabetic control rats.

DISCUSSION AND CONCLUSION

Thus, obtained results indicate that administration of kaempferol has the potential to restore deranged activity of membrane-bound ATPases in STZ-induced diabetic rats. Further detailed investigation is necessary to discover kaempferol's action mechanism.

Extracellular ATP signaling during differentiation of C2C12 skeletal muscle cells: role in proliferation

Evidence shows that extracellular ATP signals influence myogenesis, regeneration and physiology of skeletal muscle. Present work was aimed at characterizing the extracellular ATP signaling system of skeletal muscle C2C12 cells during differentiation. We show that mechanical and electrical stimulation produces substantial release of ATP from differentiated myotubes, but not from proliferating myoblasts. Extracellular ATP-hydrolyzing activity is low in myoblasts and high in myotubes, consistent with the increased expression of extracellular enzymes during differentiation. Stimulation of cells with extracellular nucleotides produces substantial Ca(2+) transients, whose amplitude and shape changed during differentiation. Consistently, C2C12 cells express several P2X and P2Y receptors, whose level changes along with maturation stages. Supplementation with either ATP or UTP stimulates proliferation of C2C12 myoblasts, whereas excessive doses were cytotoxic. The data indicate that skeletal muscle development is accompanied by major functional changes in extracellular ATP signaling.

The GDA1_CD39 superfamily: NTPDases with diverse functions

The first comprehensive review of the ubiquitous "ecto-ATPases" by Plesner was published in 1995. A year later, a lymphoid cell activation antigen, CD39, that had been cloned previously, was shown to be an ecto-ATPase. A family of proteins, related to CD39 and a yeast GDPase, all containing the canonical apyrase conserved regions in their polypeptides, soon started to expand. They are now recognized as members of the GDA1_CD39 protein family. Because proteins in this family hydrolyze nucleoside triphosphates and diphosphates, a unifying nomenclature, nucleoside triphosphate diphopshohydrolases (NTPDases), was established in 2000. Membrane-bound NTPDases are either located on the cell surface or membranes of intracellular organelles. Soluble NTPDases exist in the cytosol and may be secreted. In the last 15 years, molecular cloning and functional expression have facilitated biochemical characterization of NTPDases of many organisms, culminating in the recent structural determination of the ecto-domain of a mammalian cell surface NTPDase and a bacterial NTPDase. The first goal of this review is to summarize the biochemical, mutagenesis, and structural studies of the NTPDases. Because of their ability in hydrolyzing extracellular nucleotides, the mammalian cell surface NTPDases (the ecto-NTPDases) which regulate purinergic signaling have received the most attention. Less appreciated are the functions of intracellular NTPDases and NTPDases of other organisms, e.g., bacteria, parasites, Drosophila, plants, etc. The second goal of this review is to summarize recent findings which demonstrate the involvement of the NTPDases in multiple and diverse physiological processes: pathogen-host interaction, plant growth, eukaryote cell protein and lipid glycosylation, eye development, and oncogenesis.

The T-tubule membrane ATP-operated P2X4 receptor influences contractility of skeletal muscle

Evidence indicates that extracellular ATP may have relevant functions in skeletal muscle, even though the physiological role and distribution of specific signaling pathway elements are not well known. The present work shows that P2X4 receptor, an extracellular ATP-regulated cell membrane channel permeable to Ca2+, is expressed in several tissues of the rat, including skeletal muscle. A specific antibody detected a protein band of approximately 60 kDa. Immunofluorescence demonstrated that P2X4 has an intracellular localization, and confocal analysis revealed that the receptor colocalizes with the T-tubule membrane DHP receptor. Considering that the natural agonist of P2X4 is ATP, we explored if changes of extracellular ATP levels could occur in contracting skeletal muscle to regulate the channel. In vitro experiments showed that substantial ATP is released and rapidly hydrolyzed after electrical stimulation of rat muscle fibers. Results show that the presence of ATP-degrading enzymes (hexokinase/apyrase), inhibitors of P2X receptors or Ca2+-free conditions, all abolished the progressive twitch tension potentiation produced in soleus muscle by low-frequency (0.05 Hz) stimulation. These data reveal that ATP-mediated Ca2+ entry, most likely through P2X4 receptor, may play an important role in modulating the contractility of skeletal muscle.

Enzymatic and transcriptional regulation of human ecto-ATPase/E-NTPDase 2

We have characterized the regulation of expressed human ecto-ATPase (E-NTPDase 2), a cell surface integral membrane glycoprotein. Ecto-ATPase activity is inhibited by parameters that decrease membrane protein interaction, i.e., detergents and high temperatures. These inhibitory effects are overcome when membranes are pretreated with concanavalin A or chemical cross-linking agents that increase the amounts of ecto-ATPase oligomers. Cross-linking agents also abrogate substrate inactivation of the ecto-ATPase, a unique characteristic of the enzyme. These effects indicate that the magnitude of negative substrate regulation is dependent on quaternary structures of the protein, which likely involves interaction of transmembrane domains. The importance of transmembrane domains of ecto-ATPase in activity modulation is demonstrated further by the stimulatory effect of digitonin, a steroid glycoside that preferentially interacts with cholesterol in the membranes but does not promote oligomer formation. These results indicate that ecto-ATPase activity is regulated by a multitude of mechanisms, some of which may have physiological significance. Ecto-ATPase is also susceptible to transcriptional regulation. Ecto-ATPase gene expression is increased in a human hepatoma whereas it is undetectable in the normal liver.

The metal coordination of sCD39 during ATP hydrolysis

BACKGROUND

The hydrolysis of ATP and ADP by ecto-nucleoside triphosphate diphosphohydrolase 1 (CD39) requires divalent cations, like Ca2+ and Mg2+. In spite of considerable work, it is not clear whether divalent cations bind to the enzyme in the absence of nucleotide or only as nucleotide-Me+2 complex. Here we study the protein ligands for Me+2.

RESULTS

When VO2+ was used as a substitute for Ca2+, the ATPase activity of soluble CD39 was 25% of that with Ca2+ as cofactor. Protein ligands of the VO2+-nucleotide complex bound to the catalytic site of soluble CD39 were characterized by electron paramagnetic resonance (EPR) spectroscopy. The EPR spectrum contained one species designated T with VO2+-AMPPNP as ligand. Two species D1 and D2 were observed when VO2+-AMPCP was bound to soluble CD39. The results suggest that species D1 and D2 represent the metal-ADP complexes at the catalytic site of soluble CD39 corresponding to the intermediate formed during ATP hydrolysis and the substrate for further hydrolysis, respectively.

CONCLUSIONS

VO2+ can functionally substitute for Ca2+ as a cofactor of sCD39, and it produces four different EPR features when bound in the presence of different nucleotides or in the absence of nucleotide. The metal coordination for each conformation corresponding to each EPR species is proposed, and the mechanism of sCD39 catalysis is discussed.

Regulation of chicken gizzard ecto-ATPase activity by modulators that affect its oligomerization status

The major ectonucleoside triphosphate phosphohydrolase in the chicken gizzard smooth muscle membranes is an ecto-ATPase, an integral membrane glycoprotein belonging to the E-ATPase (or E-NTPDase) family. The gizzard ecto-ATPase is distinguished by its unusual kinetic properties, temperature dependence, and response to a variety of modulators. Compounds that promote oligomerization of the enzyme protein, i.e., concanavalin A, chemical cross-linking agent, and eosin iodoacetamide, increase its activity. Compounds that inhibit some ion-motive ATPases, e.g., sulfhydryl reagents, xanthene derivatives, NBD-halides, and suramin, also inhibit the gizzard ecto-ATPase, but not another E-ATPase, the chicken liver ecto-ATP-diphosphohydrolase, which contains the same conserved regions as the ecto-ATPase. Furthermore, inhibition of the gizzard ecto-ATPase by these compounds as well as detergents is not prevented by preincubation of the membranes with the substrate, ATP, indicating that their interaction with the enzyme occurs at a locus other than the catalytic site. On the other hand, the inhibitory effect of these compounds, except suramin, is abolished or reduced if the membranes are preincubated with concanavalin A. It is concluded that these structurally unrelated modulators exert their effect by interfering with the oligomerization of the ecto-ATPase protein. Our findings suggest that, under physiological conditions, the gizzard smooth muscle ecto-ATPase may exhibit a range of activities determined by membrane events that affect the status of oligomerization of the enzyme.

Regulation of transverse tubule ecto-ATPase activity in chicken skeletal muscle

Transverse tubule (T-tubule) ecto-ATPase from chicken skeletal muscle is an integral membrane glycoprotein that seems to exist as a homodimer and exhibits unusual properties. Treatment of T-tubule membranes with concanavalin A (Con A) did not significantly affect the thermal variation of the fluorescence anisotropy of vesicles labelled with 1,6-diphenyl-1,3,5-hexatriene or trimethylammonium-1,6-diphenyl-1,3,5-hexatriene. Cross-linking of membrane components with glutaraldehyde elicited effects on ecto-ATPase activity very similar to those of Con A treatment: a severalfold increase in activity, a decrease in Triton X-100 sensitivity and a requirement to be present before ATP to exert its action. In addition, glutaraldehyde and Con A normalized the temperature dependence and the kinetic behaviour of the enzyme. Membrane-perturbing agents (detergents, alcohols and cholesterol oxidase), with the sole exception of digitonin, caused a marked decrease in ecto-ATPase activity; the prior presence of Con A prevented this inhibition, whereas when the lectin was added after the membrane perturbing agent, recovery of the activity was not always possible. The addition of nucleotides before Con A led to a suppression of ecto-ATPase stimulation; it occurred when the nucleotide was hydrolysed (ATP or UTP) and when it was not (adenosine 5'-[beta,gamma-imido]triphosphate) and even in the presence of 3 mM P(i). A model is proposed for the complex regulatory mechanisms of chicken T-tubule ecto-ATPase that involves the occurrence of two different catalytic states in an equilibrium modulated by lectins and cross-linking agents, by the structure of the membrane and by the presence of ligands for a regulatory site.

Inhibition of an ecto-ATP-diphosphohydrolase by azide

Cell surface ATPases (ecto-ATPases or E-ATPases) hydrolyze extracellular ATP and other nucleotides. Regulation of extracellular nucleotide concentration is one of their major proposed functions. Based on enzymatic characterization, the E-ATPases have been divided into two subfamilies, ecto-ATPases and ecto-ATP-diphosphohydrolases (ecto-ATPDases). In the presence of either Mg2+ or Ca2+, ecto-ATPDases, including proteins closely related to CD39, hydrolyze nucleoside diphosphates in addition to nucleoside triphosphates and are inhibited by millimolar concentrations of azide, whereas ecto-ATPases appear to lack these two properties. This report presents the first systematic kinetic study of a purified ecto-ATPDase, the chicken oviduct ecto-ATPDase (Strobel, R.S., Nagy, A.K., Knowles, A.F., Buegel, J. & Rosenberg, M.O. (1996) J. Biol. Chem. 271, 16323-16331), with respect to ATP and ADP, and azide inhibition. Km values for ATP obtained at pH 6.4 and 7.4 are 10-30 times lower than for ADP and the catalytic efficiency is greater with ATP as the substrate. The enzyme also exhibits complicated behavior toward azide. Variable inhibition by azide is observed depending on nucleotide substrate, divalent ion, and pH. Nearly complete inhibition by 5 mm azide is obtained when MgADP is the substrate and when assays are conducted at pH 6-6.4. Azide inhibition diminishes when ATP is the substrate, Ca2+ as the activating ion, and at higher pH. The greater efficacy of azide in inhibiting ADP hydrolysis compared to ATP hydrolysis may be related to the different modes of inhibition with the two nucleotide substrates. While azide decreases both Vmax and Km for ADP, it does not alter the Km for ATP. These results suggest that the apparent affinity of azide for the E.ADP complex is significantly greater than that for the free enzyme or E.ATP. The response of the enzyme to three other inhibitors, fluoride, vanadate, and pyrophosphate, is also dependent on substrate and pH. Taken together, these results are indicative of a discrimination between ADP and ATP by the enzyme. A mechanism of azide inhibition is proposed.

Ecto-ATPase activity of alpha-sarcoglycan (adhalin)

alpha-Sarcoglycan is a component of the sarcoglycan complex of dystrophin-associated proteins. Mutations of any of the sarcoglycan genes cause specific forms of muscular dystrophies, collectively termed sarcoglycanopathies. Importantly, a deficiency of any specific sarcoglycan affects the expression of the others. Thus, it appears that the lack of sarcoglycans deprives the muscle cell of an essential, yet unknown function. In the present study, we provide evidence for an ecto-ATPase activity of alpha-sarcoglycan. alpha-Sarcoglycan binds ATP in a Mg2+-dependent and Ca2+-independent manner. The binding is inhibited by 3'-O-(4-benzoyl)benzoyl ATP and ADP. Sequence analysis reveals the existence of a consensus site for nucleotide binding in the extracellular domain of the protein. An antibody against this sequence inhibits the binding of ATP. A dystrophin.dystrophin-associated protein preparation demonstrates a Mg-ATPase activity that is inhibited by the antibody but not by inhibitors of endo-ATPases. In addition, we demonstrate the presence in the sarcolemmal membrane of a P2X-type purinergic receptor. These data suggest that alpha-sarcoglycan may modulate the activity of P2X receptors by buffering the extracellular ATP concentration. The absence of alpha-sarcoglycan in sarcoglycanopathies leaves elevated the concentration of extracellular ATP and the persistent activation of P2X receptors, leading to intracellular Ca2+ overload and muscle fiber death.

Ectonucleotidases of avian gizzard smooth muscle and liver plasma membranes: a comparative study

Extracellular nucleotides, e.g., ATP, ADP, and UTP, are important signaling molecules which elicit various physiological responses in different tissues. Their degradation is catalyzed by ectonucleotidases which are located on cell surfaces. Most tissues have a mixed population of ectonucleotidases. In this report, the ATP and ADP hydrolyzing ectonucleotidases of chicken gizzard smooth muscle and liver plasma membranes were studied. The two membranes exhibited marked differences in the ratio of ATPase/ADPase activities, activation by divalent cations, thermal stability, responses to detergents and cross-linking agents, and sensitivity to several enzyme inhibitors. The ATPase activity of chicken gizzard membranes is (i) labile to heat and detergents; (ii) activated by concanavalin A and disuccinimidyl suberate, both cross-linking agents; (iii) inhibited by mercurials; and (iv) insensitive to high concentrations of azide, a known inhibitor of ecto-ATP diphosphohydrolases (ecto-ATP/Dase). In contrast, the liver membrane ATPase and ADPase activities are more stable to treatment by heat and detergents and insensitive to cross-linking agents and mercurials, but are inhibited by azide. A low ADP hydrolase activity in the gizzard membranes could be distinguished from both the gizzard ATPase and the liver ATPase/ADPase. This ADP hydrolase, which is markedly stimulated by NBD-Cl, accounts for most of the ADP hydrolysis activity in gizzard membranes. It is concluded that the major ectonucleotidase in the gizzard membranes is an ecto-ATPase whereas that in the liver membranes is an ecto-ATP/Dase. That both membranes contain a mixed population of the ecto-ATPase and ecto-ATP/Dase, but in different proportions, is further demonstrated by immunochemical characterization. The different composition of ectonucleotidases in the two membranes is expected to have an important effect on the regulation of hydrolysis of extracellular ATP as well as the concentration of extracellular adenine nucleotides in the gizzard and liver tissues.

The transmembrane domains of ectoapyrase (CD39) affect its enzymatic activity and quaternary structure

Mammalian ectoapyrase (CD39) is an integral membrane protein with two transmembrane domains and a large extracellular region. The enzymatic activity of ectoapyrase is inhibited by most detergents used for membrane protein solubilization. In contrast, the enzymatic activities of soluble E-type ATPases, including potato tuber (Solanum tuberosum) apyrase and parasite ecto-ATPase, are not affected by detergents. Here we show that ectoapyrase is a tetramer and that detergents that reduce the activity of the enzyme promote dissociation of the tetramer to monomers. We expressed a secreted form of the ectoapyrase in COS-7 cells by fusing the signal peptide of murine CD4 with the extracellular domain of the ectoapyrase. The soluble ectoapyrase is catalytically active and its activity is not affected by detergents. Mutants of the ectoapyrase with only the NH2- or the COOH-terminal transmembrane domain are membrane-bound, and their activity is no longer affected by detergents. The enzymatic activity of all of the mutant proteins is less than that of the native enzyme. These results suggest that the proper contacts between the transmembrane domains of the monomers in the tetramer are necessary for full enzymatic activity.

Golgi localization and functional expression of human uridine diphosphatase

A full-length E(ecto)-ATPase (Plesner, L. (1995) Int. Rev. Cytol. 158, 141-214) cDNA was cloned from a human brain cDNA library; it encodes a 610-amino acid protein that contains two putative transmembrane domains. Heterologous expression of this protein in COS-7 cells caused a significant increase in intracellular membrane-bound nucleoside phosphatase activity. The activity was highest with UDP as substrate and was stimulated by divalent cations in the following order: Ca2+ >> Mg2+ > Mn2+. The results of immunofluorescence staining indicate that this protein is located in the Golgi apparatus. UDP hydrolysis was increased in the presence of Triton X-100 or alamethicin, an ionophore that facilitates movement of UDP across the membrane, suggesting that the active site of this UDPase is on the luminal side of the Golgi apparatus. This is the first identification of a mammalian Golgi luminal UDPase gene. Computer-aided sequence analysis of the EATPase superfamily indicates that the human UDPase is highly similar to two hypothetical proteins of the nematode Caenorhabditis elegans and to an unidentified 71.9-kDa yeast protein and is less related to the previously identified yeast Golgi GDPase.

T-tubule membranes from chicken skeletal muscle possess an enzymic cascade for degradation of extracellular ATP

The chicken T-tubule Mg2+-ATPase is an integral membrane glycoprotein that presents properties different from those of other ATPases located in skeletal muscle cells and exhibits ATP-hydrolysing activity on the extracellular side of the transverse tubule (TT) membranes. In this study we demonstrate that TT vesicles purified from chicken skeletal muscle possess ecto-ADPase and ecto-5'-nucleotidase activities that, along with ecto-ATPase, are able to sequentially degrade extracellular ATP to ADP, AMP and adenosine. Characterization studies of these TT ectonucleotidases revealed remarkable differences between ecto-ATPase and ecto-ADPase activities with respect to thermal stability, temperature dependence of the hydrolytic activity, effect of ionic strength, kinetic behaviour, divalent cation preference and responses to azide, N-ethylmaleimide, NaSCN, Triton X-100 and concanavalin A. Ecto-ATPase, but not ecto-ADPase, was inhibited by a polyclonal antibody against the chicken TT ecto-ATPase. On the basis of these results we propose that ATP and ADP hydrolysis are accomplished by two distinct enzymes and therefore the TT ecto-ATPase is not an apyrase. 5'-Nucleotidase activity was inhibited by adenosine 5'-[alpha,beta-methylene]diphosphate and concanavalin A, followed simple Michaelis-Menten kinetics and was released from the membranes by treatment with phosphatidylinositol-specific phospholipase C, indicating that AMP hydrolysis in T-tubules is catalysed by a typical ecto-5'-nucleotidase. Results obtained from electrophoresis experiments under native conditions suggest that ecto-ATPase, ecto-ADPase and 5'-nucleotidase might be associated, forming functional complexes in the T-tubule membranes. The TT ectonucleotidases constitute an enzymic cascade for the degradation of extracellular ATP that might be involved in the regulation of purinergic signalling in the muscle fibre.

Regulatory differences among avian ecto-ATPases

Extracellular ATP is found to produce a variety of important biological responses. Ecto-ATPases are located on numerous cell types in many different species, regulate extracellular ATP levels and can be a key step in generating adenosine. Studies conducted on chicken ecto-ATPases from liver and cardiac and smooth muscle show a variety of differing properties including (1) different apparent Km's, (2) lectin sensitivity, (3) responses to detergents, (4) responses to lipid mediators, and (5) responsiveness to nucleotide-mimetic affinity labels. These results suggest that although each enzyme hydrolyzes extracellular ATP, they should each be viewed as a distinct subtype of the whole ecto-ATPase family due to their differential responses, largely linked to proposed regulatory phenomenon.

Cholesterol increases the thermal stability of the Ca2+/Mg(2+)-ATPase of cardiac microsomes

The effect of membrane cholesterol on the thermal inactivation of Ca2+/Mg(2+)-ATPase activity of bovine cardiac microsome was measured and compared to the thermal denaturation profiles of the microsomes as measured by differential scanning calorimetry (DSC). Inactivation, defined as loss of activity, and denaturation, defined as conformational unfolding, were irreversible under the conditions used. Both thermal inactivation of Ca2+/Mg(2+)-ATPase activity and thermal denaturation were shifted to higher temperatures in microsomes enriched with cholesterol (37 +/- 5 micrograms cholesterol/mg protein, cholesterol/phospholipid molar ratio 0.31) compared to control microsomes (15 +/- 3 micrograms cholesterol/mg protein, molar ratio 0.12). Thermal inactivation was measured by two methods: first, measuring activity at room temperature as a function of heating to elevated temperatures at 1 K/min, where inactivation temperatures (T1, temperature of half activity) were 58.9 +/- 0.3 degrees C for control membranes and 59.9 +/- 0.1 degrees C for cholesterol-enriched membranes, respectively. Second, measuring ATPase activity as a function of time at constant temperature, where T1 values of 57.6 +/- 0.5 degrees C and 59.2 +/- 0.5 degrees C were determined for control and cholesterol-enriched membranes, respectively. DSC profiles of microsomal membranes consisting of a number of overlapping peaks were obtained. A well resolved component (transition C) was observed with a transition temperature (T 1/2) of 58.2 degrees C. This T 1/2, which is a measure of conformational stability, correlates with the T1 for Ca2+/Mg(2+)-ATPase activity and is 1.9 +/- 0.6 K higher in cholesterol-enriched membranes. Thus, the increased resistance to inactivation appears to be due to increased conformational stability of the protein induced by cholesterol, demonstrating that a change in lipid composition can influence the stability of an integral membrane protein in a natural membrane. The increased stability is of sufficient magnitude to account for the previously observed correlation between cholesterol content and resistance to heat shock in several cell lines.

Chicken oviductal ecto-ATP-diphosphohydrolase. Purification and characterization

An ecto-ATP diphosphohydrolase (ATPDase) was purified to homogeneity from vesiculosomes shed from chicken oviduct. First, the ecto-ATPDase-enriched vesiculosomes were concentrated by filtration, differential centrifugation, and exclusion chromatography. Next, the nonionic detergent, Nonidet P-40, was used to extract the ecto-ATPDase from vesiculosomal membranes, and the solubilized enzyme was further purified by ion exchange (DEAE-Bio-Gel) and lentil-lectin-Sepharose 4B chromatography. In the final stage, immunoaffinity chromatography was utilized to obtain purified ecto-ATPDase. More than 25,000-fold purification was achieved. Specific activity of the purified enzyme was greater than 800 micronol/min/mg of protein with MgATP as the substrate, the highest ever reported for an ATPDase. The enzyme also hydrolyzed other nucleoside triphosphates in the presence of magnesium at similar rates and CaATP and MgADP at lower rates. The molecular mass of the purified glycoprotein was 80 kDa as determined by SDS-polyacrylamide gel electrophoresis and Western blot analysis. Based on its enzymatic properties, the relationship of the chicken oviduct ecto-ATPDase with other reported ATPDases and ecto-ATPases is discussed.

Activation of the sarcoplasmic reticulum Ca2+-ATPase induced by exercise

Prolonged exercise has been shown to cause disruption of intracellular calcium homeostasis in skeletal muscle, which is normally maintained by the sarcoplasmic reticulum (SR) Ca2+-ATPase. We have investigated the response of this enzyme to increased intracellular calcium levels by investigating the functional and physical characteristics of the SR Ca2+-ATPase and membrane lipids following 2 h of treadmill running and throughout a period of post-exercise recovery. The Ca2+-ATPase of SR membranes purified from exercised rats shows increases in enzymatic activity correlating with post-exercise recovery time. Corresponding increases in active Ca2+-ATPase pump units are observed, as measured by the concentration of phosphorylated enzyme intermediate formed from ATP. However, catalytic turnover rates of the Ca2+-ATPase are unchanged. Using spin-label electron paramagnetic resonance to assess both membrane fluidity and associations between individual Ca2+-ATPase polypeptide chains, we find no exercise-induced alterations in membrane dynamics which could explain the observed increases in Ca2+-ATPase activity. Nor do we find evidence for altered membrane purification as a result of exercise. We suggest that the cell responds to the challenge of increased cytosolic calcium levels by increasing the proportion of functional SR Ca2+-ATPase proteins in the membrane for the rapid restoration of calcium homeostasis.

Dihydropyridine receptors in transverse tubules from normal and dystrophic chicken skeletal muscle

Calcium overload is a fundamental pathogenic event associated with chronic muscle degeneration in muscular dystrophies. The possibility that L-type voltage-dependent calcium channels were involved in the etiology of chicken muscular dystrophy was investigated by studying the dihydropyridine receptors in transverse tubule membranes isolated from skeletal muscle of normal (line 412) and dystrophic (line 413) chickens. The yield of T-tubular protein from dystrophic muscle was considerably increased compared with that from normal muscle (2.51 +/- 0.18 vs 1.04 +/- 0.31 mg protein x 100 g muscle-1). The binding of the calcium channel antagonist (+) [3H]PN200-110 to the dihydropyridine receptor in transverse tubule preparations was relatively slow, markedly affected by temperature and required divalent cations. (+) [3H]PN200-110 equilibrium binding assays revealed a single class of high-affinity sites and showed that maximum binding capacity (Bmax) (3.17 +/- 0.47 for normal and 3.51 +/- 0.52 pmol x mg protein-1 for dystrophic transverse tubules) and dissociation constant (Kd) (0.32 +/- 0.07 and 0.26 +/- 0.09 nM, respectively) were not significantly different in normal and dystrophic membranes. Kinetic studies indicated that normal and dystrophic transverse tubules did not differ significantly in association (2.54 x 10(6) and 2.27 x 10(6) M(-1)s(-1), respectively) and dissociation (8.5 x 10(-4) and 9.3 x 10(-4)s(-1), respectively) rate constants. Since dissociation kinetics for both preparations were monoexponential under all the experimental conditions employed, no low-affinity binding sites for (+) [3H]PN200-110 could be detected in chicken transverse tubules membranes. However, immunoblot assay, using a monoclonal antibody, revealed that dystrophic transverse tubules as compared with normal membranes were enriched twofold with the alpha 1-subunit of the dihydropyridine receptor. Therefore, although dihydropyridine-binding sites were not altered in transverse tubule membranes from dystrophic chicken skeletal muscle, both the increased yield in T-tubule vesicles and the enhanced immunodetection of the alpha 1-subunit of the dihydropyridine receptor, suggest that total content in dihydropyridine receptor is higher in dystrophic than in normal muscle.

Visualization of the subsarcolemmal cytoskeleton network of mouse skeletal muscle cells by en face views and application to immunoelectron localization of dystrophin

The ultrastructural organization of the highly interconnected filamentous network underneath the sarcolemma as well as the role played by the muscle protein dystrophin within this cytoskeleton remain yet unclear. More accurate information has been obtained by using a method which provides three-dimensional en face views of large membrane areas applied to mouse cultured myotubes and isolated adult skeletal muscle fibres. Two levels have been distinguished in the cytoskeleton underlying the sarcolemma: the submembranous level, partly integrated into the membrane, and the cortical level, invading the proximal cytoplasmic space. Few differences have been found between the membrane cytoskeletons of myotubes issued from 14-day-old cultures and those of adult fibres. The comparison was done with cells where dystrophin is missing (mdx mouse muscle): surprisingly, the lack of dystrophin does not induce obvious or dramatic ultrastructural disorganization, either in the cortical cytoskeletal network or in the submembranous one. Immunogold labelling of either the central-rod or the C-terminal domain of dystrophin is not located among the cortical network. This study provides additional data on the spatial ordering of subsarcolemmal cytoskeletal elements: dystrophin does not appear as a filamentous structure entirely located among subsarcolemmal cytoskeleton but seems partly embedded in membranous material.

Adaptive changes in lipid composition of skeletal sarcoplasmic reticulum membranes associated with aging

We have undertaken a detailed examination of changes associated with aging in lipid composition and corresponding physical properties of hindlimb skeletal sarcoplasmic reticulum (SR) membranes isolated from young (5 months), middle-aged (16 months), and old (28 months) Fischer strain 344 rats. Silica gel HPLC chromatography was used to separate phospholipid headgroup species. Subsequent reversed-phase HPLC was used to resolve fatty acid chain compositions of phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol species. For all three phospholipid pools, significant age-related variations are observed in the abundance of multiple molecular species, particularly those having polyunsaturated fatty acid chains. Using mass spectrometry (fast atom bombardment and tandem techniques) to distinguish ester- from ether-linked phosphatidylethanolamine species, we demonstrate that overall plasmenylethanolamine content is substantially increased with age, from 48 mol% to 62 mol%. A substantial increase is also observed in the single molecular species 18:0-20:4 phosphatidylinositol suggesting implications for signalling pathways. In addition, associated with senescence we find a significant increase in the rigidifying lipid, cholesterol. Despite these changes in lipid composition of different aged animals, the average bilayer fluidity examined at several bilayer depths with stearic acid spin labels, is not altered. Neither do we find differences in the rotational mobility of maleimide spin-labeled Ca(2+)-ATPase, as determined from saturation-transfer electron paramagnetic resonance, which is sensitive to both the fluidity of lipids directly associated with the Ca(2+)-ATPase and to its association with proteins.

Isolation and characterization of distinct domains of sarcolemma and T-tubules from rat skeletal muscle

1. Several cell-surface domains of sarcolemma and T-tubule from skeletal-muscle fibre were isolated and characterized. 2. A protocol of subcellular fractionation was set up that involved the sequential low- and high-speed homogenization of rat skeletal muscle followed by KCl washing, Ca2+ loading and sucrose-density-gradient centrifugation. This protocol led to the separation of cell-surface membranes from membranes enriched in sarcoplasmic reticulum and intracellular GLUT4-containing vesicles. 3. Agglutination of cell-surface membranes using wheat-germ agglutinin allowed the isolation of three distinct cell-surface membrane domains: sarcolemmal fraction 1 (SM1), sarcolemmal fraction 2 (SM2) and a T-tubule fraction enriched in protein tt28 and the alpha 2-component of dihydropyridine receptor. 4. Fractions SM1 and SM2 represented distinct sarcolemmal subcompartments based on different compositions of biochemical markers: SM2 was characterized by high levels of beta 1-integrin and dystrophin, and SM1 was enriched in beta 1-integrin but lacked dystrophin. 5. The caveolae-associated molecule caveolin was very abundant in SM1, SM2 and T-tubules, suggesting the presence of caveolae or caveolin-rich domains in these cell-surface membrane domains. In contrast, clathrin heavy chain was abundant in SM1 and T-tubules, but only trace levels were detected in SM2. 6. Immunoadsorption of T-tubule vesicles with antibodies against protein tt28 and against GLUT4 revealed the presence of GLUT4 in T-tubules under basal conditions and it also allowed the identification of two distinct pools of T-tubules showing different contents of tt28 and dihydropyridine receptors. 7. Our data on distribution of clathrin and dystrophin reveal the existence of subcompartments in sarcolemma from muscle fibre, featuring selective mutually exclusive components. T-tubules contain caveolin and clathrin suggesting that they contain caveolin- and clathrin-rich domains. Furthermore, evidence for the heterogeneous distribution of membrane proteins in T-tubules is also presented.

T-tubule depolarization-induced SR Ca2+ release is controlled by dihydropyridine receptor- and Ca(2+)-dependent mechanisms in cell homogenates from rabbit skeletal muscle

In vertebrate skeletal muscle, the voltage-dependent mechanism of rapid sarcoplasmic reticulum (SR) Ca2+ release, commonly referred to as excitation-contraction (EC) coupling, is believed to be mediated by physical interaction between the transverse (T)-tubule voltage-sensing dihydropyridine receptor (DHPR) and the SR ryanodine receptor (RyR)/Ca2+ release channel. In this study, differential T-tubule and SR membrane monovalent ion permeabilities were exploited with the use of an ion-replacement protocol to study T-tubule depolarization-induced SR 45Ca2+ release from rabbit skeletal muscle whole-cell homogenates. Specificity of Ca2+ release was ascertained with the use of the DHPR antagonists D888, nifedipine and PN200-110. In the presence of the "slow" complexing Ca2+ buffer EGTA, homogenates exhibited T-tubule depolarization-induced Ca2+ release comprised of an initial rapid phase followed by a slower release phase. During the rapid phase, approximately 20% of the total sequestered Ca2+ (approximately 30 nmol 45Ca2+/mg protein), corresponding to 100% of the caffeine-sensitive Ca2+ pool, was released within 50 ms. Rapid release could be inhibited fourfold by D888. Addition to release media of the "fast" complexing Ca2+ buffer BAPTA, at concentrations > or = 4 mM, nearly abolished rapid Ca2+ release, suggesting that most was Ca2+ dependent. Addition of millimolar concentrations of either Ca2+ or Mg2+ also greatly reduced rapid Ca2+ release. These results show that T-tubule depolarization-induced SR Ca2+ release from rabbit skeletal muscle homogenates is controlled by T-tubule membrane potential- and by Ca(2+)-dependent mechanisms.

Use of thermal analysis to distinguish magnesium and calcium stimulated ATPase activity in isolated transverse tubules from skeletal muscle

The presence of calcium stimulated adenosine triphosphatase (Ca2+,Mg(2+)-ATPase) activity in isolated transverse tubule (t-tubule) membranes is distinguished from magnesium adenosine triphosphatase (Mg(2+)-ATPase) activity on the basis of differing thermal stabilities. The Mg(2+)-ATPase is the major protein component of the t-tubule membrane, and it can be difficult to discriminate between the low levels of Ca2+ stimulated ATPase activity found in isolates of t-tubules compared to the much higher Mg(2+)-ATPase activity. Thermal analysis reveals different inactivation temperatures (Ti) for the proteins responsible for ATP dependent calcium transport (Ti = 49 degrees C) and Mg(2+)-ATPase activity (Ti = 57 degrees C) in isolated t-tubule membranes. The differential scanning calorimetry profile of t-tubule membranes consists of three major components with transition temperatures (Tm) of 51 degrees C, 57 degrees C and 63 degrees C. Denaturation of the component with Tm = 57 degrees C correlates with inactivation of Mg(2+)-ATPase activity, and denaturation of the Tm = 51 degrees C component correlates with the inactivation of Ca2+,Mg(2+)-ATPase activity and calcium transport. The functions of the t-tubule membrane component or components that denature with Tm = 63 degrees C have yet to be identified. The lack of stimulation of calcium transport in isolated t-tubules by oxalate, the impermeability of isolated t-tubules to oxalate, and experiments performed on t-tubules with defined amounts of sarcoplasmic reticulum (SR) added suggest that contamination of the isolated t-tubules by SR is unlikely to account for the level of Ca2+,Mg(2+)-ATPase activity detected. The presence of a Ca2+,Mg(2+)-ATPase in the t-tubule membrane would provide a mechanism that may be involved in the partial removal of calcium that is accumulated in the junctional space during muscle relaxation or calcium that is released from the terminal cisternae of sarcoplasmic reticulum during excitation-contraction coupling.

Ecto-ATPases: identities and functions

Ecto-ATPases are ubiquitous in eukaryotic cells. They hydrolyze extracellular nucleoside tri- and/or diphosphates, and, when isolated, they exhibit E-type ATPase activity, (that is, the activity is dependent on Ca2+ or Mg2+, and it is insensitive to specific inhibitors of P-type, F-type, and V-type ATPases; in addition, several nucleotide tri- and/or diphosphates are hydrolysed, but nucleoside monophosphates and nonnucleoside phosphates are not substrates). Ecto-ATPases are glycoproteins; they do not form a phosphorylated intermediate during the catalytic cycle; they seem to have an extremely high turnover number; and they present specific experimental problems during solubilization and purification. The T-tubule Mg2+-ATPase belongs to this group of enzymes, which may serve at least two major roles: they terminate ATP/ADP-induced signal transduction and participate in adenosine recycling. Several other functions have been discussed and identity to certain cell adhesion molecules and the bile acid transport protein was suggested on the basis of cDNA clone isolation and immunological work.

Modulation of Pi transport in skeletal muscle by insulin and IGF-1

In vivo, skeletal muscle Pi uptake influences both muscle cellular [Pi] and plasma [Pi], and may mediate the hypophosphataemic effects of insulin and insulin-like growth factor 1 (IGF-1). These effects were investigated in the cultured mouse myoblast cell line G8 and the isolated incubated rat soleus. The low Km for Pi in G8 cells is consistent with in vivo evidence that muscle cell [Pi] is partially protected against changes in plasma [Pi]. Insulin and IGF-1 stimulated Na-dependent Pi influx: in G8 cells both increased Vmax, with no change in Km, but while the insulin response occurred within 15 min and rapidly reversed upon insulin withdrawal, the response to IGF-1 occurred only after 60 min and persisted at least 60 min following IGF-1 withdrawal. Furthermore, only the IGF-1 response was inhibited by cycloheximide. We suggest that IGF-1 operates through de novo protein synthesis, while insulin stimulates transporter recruitment to the cell surface.

The role of phosphoinositide metabolism in Ca2+ signalling of skeletal muscle cells

1. The mobilization of Ca2+ from intracellular stores by D-myo-inositol 1,4,5-triphosphate[Ins(1,4,5)P3] is now widely accepted as the primary link between plasma membrane receptors that stimulate phospholipase C and the subsequent increase in intracellular free Ca2+ that occurs when such receptors are activated (Berridge, 1993). Since the observations of Volpe et al. (1985) which showed that Ins(1,4,5)P3 could induce Ca2+ release from isolated terminal cisternae membranes and elicit contracture of chemically skinned muscle fibres, research has focused on the role of Ins(1,4,5)P3 in the generation of SR Ca2+ transients and in the mechanism of excitation-contraction coupling (EC-coupling). 2. The mechanism of signal transduction at the triadic junction during EC-coupling is unknown. Asymmetric charge movement and mechanical coupling between highly specialized triadic proteins has been proposed as the primary mechanism for voltage-activated generation of SR Ca2+ signals and subsequent contraction. Ins(1,4,5)P3 has also been proposed as the major signal transduction molecule for the generation of the primary Ca2+ transient produced during EC-coupling. 3. Investigations on the generation of Ca2+ transients by Ins(1,4,5)P3 have been conducted on ion channels incorporated into lipid bilayers, skinned and intact fibres and isolated membrane vesicles. Ins(1,4,5)P3 induces SR Ca2+ release and the enzymes responsible for its synthesis and degradation are present in muscle tissue. However, the sensitivity of the Ca2+ release mechanism to Ins(1,4,5)P3 is highly dependent on experimental conditions and on membrane potential. 4. While Ins(1,4,5)P3 may not be the major signal transduction molecule for the generation of the primary Ca2+ signal produced during voltage-activated contraction, this inositol polyphosphate may play a functional role as a modulator of EC-coupling and/or of the processes of myoplasmic Ca2+ regulation occurring on a time scale of seconds, during the events of contraction.

SYNTHESIS AND BIOLOGICAL ACTIVITY OF NOVEL 2-THIO DERIVATIVES OF ATP

2-Alkylthio analogues of adenosine 5'-triphosphate were synthesized and evaluated as P2y purinoceptor agonists. ATP and analogues transiently increased intracellular Ca2+ levels in C6 glioma cells and in skeletal muscle derived myotubes in culture. Most derivatives were resistant to stepwise dephosphorylation by ecto-ATPases.

Direct effects of phorbol esters and diacylglycerols on the T-tubule Mg(2+)-ATPase

T-tubule membrane vesicles isolated from skeletal muscle contain a very active Mg(2+)-ATPase (EC 3.6.1.34) which is modulated by lectins and is located in the junctional region near the sarcoplasmic reticulum membranes (1). The effects of several prominent lipophilic agents upon the ATPase have led us to evaluate the action of diacylglycerols and phorbol esters upon the enzyme. The ATPase is inhibited by submicromolar levels of the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), and the diacylglycerol, 1-oleoyl-2-acetyl-sn-glycerol (sn-OAG), with K0.5s of 0.2 and 0.5 microM, respectively. Significantly, 4-alpha-phorbol 12,13-didecanoate (4-alpha-phorbol) the TPA analogue shown to be inactive toward protein kinase C (PKC), inhibited the ATPase with a K0.5 of 0.3 microM, and 1-stearoyl-2-arachidonyl-sn-glycerol, the preferred endogenous activator of PKC, was not inhibitory toward the ATPase. 1-(5-Isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (a membrane permeant PKC inhibitor) and peptide 19-36 (the highly specific PKC pseudosubstrate inhibitor) were both without effect upon the ATPase and did not affect TPA inhibition. ATPase activity was not altered under phosphorylating conditions in experiments using exogenous rat brain PKC. ConA protected ATPase activity against inhibition by TPA, 4-alpha-phorbol, and sn-OAG. Additionally, phorbol-12,13-dibutyrate binding studies demonstrated that the ATPase was capable of significant phorbol binding with ConA protection. The data are consistent with a direct and specific effect of phorbol esters and diacylglycerols upon the ATPase, without any participation of PKC. We conclude that the transverse tubule (T-tubule) ATPase is an alternate receptor for diacylglycerol and TPA in skeletal muscle and that the mode of action of these agents upon the ATPase (inhibition) is opposite to their mode of action on PKC (activation). The data demonstrate that substantial care must be taken in ascribing either cellular or subcellular effects of phorbol esters and diacylglycerols exclusively to the activation of PKC and that alternate receptors may exist. Criteria are recommended for the demonstration of PKC-independent modulation by phorbols and diacylglycerols.

The Mg(2+)-ATPase of rabbit skeletal-muscle transverse tubule is a highly glycosylated multiple-subunit enzyme

The Mg(2+)-ATPase present in rabbit skeletal-muscle transverse tubules is an integral membrane enzyme which has been solubilized and purified previously in this laboratory [Kirley (1988) J. Biol. Chem. 263, 12682-12689]. The present study indicates that, in addition to the approx. 100 kDa protein (distinct from the sarcoplasmic-reticulum Ca(2+)-ATPase) seen previously to co-purify with the Mg(2+)-ATPase activity, there are also proteins having molecular masses of 160, 70 and 43 kDa. The 70 and 43 kDa glycosylated proteins (50 and 31 kDa after deglycosylation) are difficult to detect by SDS/PAGE before deglycosylation, owing to the broadness of the bands. Additional purification procedures, cross-linking studies and chemical and enzymic deglycosylation studies were undertaken to determine the structure and relationship of these proteins. Both the 97 and 160 kDa proteins were demonstrated to be N-glycosylated at multiple sites, the 97 kDa protein being reduced to a peptide core of 84 kDa and the 160 kDa protein to a peptide core of 131 kDa after deglycosylation. Although the Mg(2+)-ATPase activity is resistant to a number of chemical modification reagents, cross-linking inactivates the enzyme at low concentrations. This inactivation is accompanied by cross-linking of two 97 kDa molecules to one another, suggesting that the 97 kDa protein is involved in ATP hydrolysis. The existence of several proteins along with the inhibition of ATPase activity by cross-linking is consistent with the interpretation of the susceptibility of this enzyme to inactivation by most detergents as being due to the disruption of a protein complex of associated subunits by the inactivating detergents. The 160 kDa glycoprotein can be partially resolved from the Mg(2+)-ATPase activity, and is identified by its N-terminal amino acid sequence as angiotensin-converting enzyme.

Vanadate inhibition of ATP and p-nitrophenyl phosphate hydrolysis in the fragmented sarcoplasmic reticulum

The vanadate inhibition of the Ca(2+)-ATPase activity was analysed both in intact sarcoplasmic reticulum vesicles and in the presence of low concentrations of Tween 20, using ATP and p-nitrophenyl phosphate as substrates. The saturation of the internal low-affinity calcium-binding sites protects the enzyme against vanadate inhibition, because: (1) p-nitrophenyl phosphate hydrolysis is not inhibited by vanadate in intact vesicles, but inhibition developed after solubilization with detergents; (2) the vanadate inhibition of the p-nitrophenyl phosphate hydrolysis in solubilized preparations is prevented by free Ca2+ concentrations higher than 10(-3) M and vanadate competes with calcium (10(-5)-10(-3) M); and (3) the vanadate inhibition of ATP hydrolysis is decreased with an increase in vesicular Ca2+ concentration. The presence of magnesium ions is indispensable for the vanadate effect. The vanadate inhibition is non-competitive with respect to Mg-p-nitrophenyl phosphate and uncompetitive with respect to Mg-ATP. However, in the presence of dimethyl sulfoxide, which facilitates phosphorylation of the enzyme, the inhibition is converted to a competitive one with respect to a substrate. The results suggest, that in the process of enzyme operation vanadate interacts with the unliganded E form of Ca(2+)-ATPase, occupying probably an intermediate position between the E2 and E1 forms, with the formation of an E2 Van complex, that imposes the inhibition on the Ca(2+)-ATPase activity.

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

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

Co-localization of the dihydropyridine receptor and the cyclic AMP-binding subunit of an intrinsic protein kinase to the junctional membrane of the transverse tubules of skeletal muscle

Junctional transverse tubules (TT) isolated from triads of rabbit skeletal muscle by centrifugation in an ion-free sucrose gradient were compared with membrane subfractions, predominantly derived from the free portion of TT, that had been purified from sarcoplasmic reticulum membrane contaminants by three different methods. The markers used were diagnostic membrane markers and the dihydropyridine (DHP) receptor, which is a specific marker of the junctional membrane of TT. Junctional TT have a high membrane density (Bmax. 60 pmol/mg of protein) of high-affinity (Kd 0.25 nM) DHP-binding sites using [3H]PN200-110 as the specific ligand. When analysed by SDS/PAGE under reducing conditions and by Western blot techniques, the TT were found to contain a concanavalin A-binding 150 kDa glycoprotein which probably corresponds to the alpha 2-subunit of the DHP receptor. This conclusion was supported by correlative immunoblot experiments with a specific antibody. Junctional TT are further distinguished from free TT by the presence of a high number (Bmax. 20 pmol/mg of protein) of [3H]cyclic AMP receptor sites, as determined by the Millipore filtration technique of Gill & Walton [(1974) Methods Enzymol. 38, 376-381]. Use of this method means that the number of receptors may have been underestimated. The TT-bound cyclic AMP receptor was identified as a 55 kDa protein by specific photoaffinity labelling with 8-N3-[3H]cyclic AMP, and had similar phosphorylation properties and apparent molecular mass to the RII form of the regulatory subunit of cyclic AMP-dependent protein kinase. Co-localization of the intrinsic cyclic AMP-dependent protein kinase and of the DHP receptor complex to the junctional membrane of TT supports the hypothesis that the 170 kDa alpha 1-subunit of the receptor is a substrate for the kinase.

Abnormal properties of Mg2(+)-ATPase in transverse tubule membranes from dystrophic chicken

Purified transverse tubule membranes from normal and dystrophic chicken skeletal muscle were isolated by a calcium-loading procedure. Normal and dystrophic T-tubules were similar in cholesterol content and (Na+,K+)-ATPase and 5'-nucleotidase activities but a significant decrease of Mg2(+)-ATPase activity was observed in dystrophic membranes. A comparative analysis of the enzyme properties revealed that the kinetic parameters were altered in dystrophic T-tubules and the ATP-hydrolyzing activity was differently affected by the ionic strength. However, the influence of temperature and the regulatory effect of concanavalin A were the same as in normal T-tubules. Membrane fluidity was similar in both preparations as estimated by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene and trimethylammonium diphenylhexatriene. These results point to an impairment in the function of Mg2(+)-ATPase due to structural alterations of the enzyme.