Characterization of the Ca2+- or Mg2+-ATPase of transverse tubule membranes isolated from rabbit skeletal muscle

Meijer and Vloedman's histochemical demonstration of mitochondrial coupling obeys Lambert-Beer's law in the myocardium

Uncoupling of mitochondrial proton pumping and adenosine triphosphate (ATP) production lowers mitochondrial efficiency. Current methods to determine mitochondrial efficiency require substantial amounts of tissue and permeabilization or isolation procedures. A simple histochemical method has been described by Meijer and Vloedman (Histochemistry 69:217–232, 1980, 10.1007/BF00489769), but this was not quantitative. We found linear correlations between (1) absorbance and sections thickness and (2) absorbance and incubation time. Because the method obeys Lambert–Beer’s law, we can estimate ATP/O₂ ratios for healthy and overloaded right-sided rat myocardium. We related mitochondrial efficiency to the ratio between cardiolipin and its precursor phosphatidylglycerol. We found a non-linear relationship between mitochondrial efficiency and this ratio, indicating that lower mitochondrial efficiency as found in experimental pulmonary hypertension may be due to altered composition of the mitochondrial inner membrane. We conclude that the histochemical method of Meijer and Vloedman can be applied to quantify mitochondrial efficiency.

Human skeletal muscle plasmalemma alters its structure to change its Ca2+-handling following heavy-load resistance exercise

High-force eccentric exercise results in sustained increases in cytoplasmic Ca2+ levels ([Ca2+]cyto), which can cause damage to the muscle. Here we report that a heavy-load strength training bout greatly alters the structure of the membrane network inside the fibres, the tubular (t-) system, causing the loss of its predominantly transverse organization and an increase in vacuolation of its longitudinal tubules across adjacent sarcomeres. The transverse tubules and vacuoles displayed distinct Ca2+-handling properties. Both t-system components could take up Ca2+ from the cytoplasm but only transverse tubules supported store-operated Ca2+ entry. The retention of significant amounts of Ca2+ within vacuoles provides an effective mechanism to reduce the total content of Ca2+ within the fibre cytoplasm. We propose this ability can reduce or limit resistance exercise-induced, Ca2+-dependent damage to the fibre by the reduction of [Ca2+]cyto to help maintain fibre viability during the period associated with delayed onset muscle soreness.

A quantitative description of tubular system Ca(2+) handling in fast- and slow-twitch muscle fibres

KEY POINTS

Current methods do not allow a quantitative description of Ca(2+) movements across the tubular (t-) system membrane without isolating the membranes from their native skeletal muscle fibre. Here we present a fluorescence-based method that allows determination of the t-system [Ca(2+) ] transients and derivation of t-system Ca(2+) fluxes in mechanically skinned skeletal muscle fibres. Differences in t-system Ca(2+) -handling properties between fast- and slow-twitch fibres from rat muscle are resolved for the first time using this new technique. The method can be used to study Ca(2+) handling of the t-system and allows direct comparisons of t-system Ca(2+) transients and Ca(2+) fluxes between groups of fibres and fibres from different strains of animals.

ABSTRACT

The tubular (t-) system of skeletal muscle is an internalization of the plasma membrane that maintains a large Ca(2+) gradient and exchanges Ca(2+) between the extracellular and intracellular environments. Little is known of the Ca(2+) -handling properties of the t-system as the small Ca(2+) fluxes conducted are difficult to resolve with conventional methods. To advance knowledge in this area we calibrated t-system-trapped rhod-5N inside skinned fibres from rat and [Ca(2+) ]t-sys , allowing confocal measurements of Ca(2+) -dependent changes in rhod-5N fluorescence during rapid changes in the intracellular ionic environment to be converted to [Ca(2+) ] transients in the t-system ([Ca(2+) ]t-sys (t)). Furthermore, t-system Ca(2+) -buffering power was determined so that t-system Ca(2+) fluxes could be derived from [Ca(2+) ]t-sys (t). With this new approach, we show that rapid depletion of sarcoplasmic reticulum (SR) Ca(2+) induced a robust store-operated Ca(2+) entry (SOCE) in fast- and slow-twitch fibres, reducing [Ca(2+) ]t-sys to < 0.1 mm. The rapid activation of SOCE upon Ca(2+) release was consistent with the presence of STIM1L in both fibre types. Abruptly introducing internal solutions with 1 mm Mg(2+) and [Ca(2+) ]cyto (28 nm-1.3 μm) to Ca(2+) -depleted fibres generated t-system Ca(2+) uptake rates dependent on [Ca(2+) ]cyto with [Ca(2+) ]t-sys reaching final plateaus in the millimolar range. For the same [Ca(2+) ]cyto , t-system Ca(2+) fluxes of fast-twitch fibres were greater than that in slow-twitch fibres. In addition, simultaneous imaging of t-system and SR Ca(2+) signals indicated that both membrane compartments accumulated Ca(2+) at similar rates and that SOCE was activated early during SR Ca(2+) depletion.

Experimental impact of aspirin exposure on rat intestinal bacteria, epithelial cells and cell line

Aspirin, a commonly used therapeutic non-steroidal anti-inflammatory drug (NSAID) is known to cause gastric mucosal damage. Intestinal bacteria having a regulatory effect on intestinal homeostasis play significant role in NSAID-induced intestinal injury. Bacteria and specific cell lines are considered to be suitable for toxicity screening and testing of chemicals. Therefore, to evaluate and compare in vitro toxicity, cultures of rat intestinal epithelial cells (IEC), isolated bacteria and IEC-6 cell line were assessed for viability, morphometric analysis, membrane transport enzymes and structural constituents for membrane damage, dehydrogenase activity test for respiratory and energy producing processes and esterase activity test for intra- and extra-cellular degradation, following the post exposure to aspirin (0—50 µg mL- 1). Similar pattern of dose-dependent changes in these parameters were observed in three types of cells. Similar in situ effects on IEC validated the in vitro findings. These findings indicate that higher aspirin concentrations may alter cellular functions of IEC and gut bacteria. Furthermore, results suggest that gut bacteria and IEC-6 cell line can be used for the initial screening of gastrointestinal cellular toxicity caused by NSAIDs.

Insulin induced translocation of Na+/K+ -ATPase is decreased in the heart of streptozotocin diabetic rats

AIM

To investigate the effect of acute insulin administration on the subcellular localization of Na(+)/K(+)-ATPase isoforms in cardiac muscle of healthy and streptozotocin-induced diabetic rats.

METHODS

Membrane fractions were isolated with subcellular fractionation and with cell surface biotinylation technique. Na(+)/K(+)-ATPase subunit isoforms were analysed with ouabain binding assay and Western blotting. Enzyme activity was measured using 3-O-methylfluorescein-phosphatase activity.

RESULTS

In control rat heart muscle alpha1 isoform of Na(+)/K(+) ATPase resides mainly in the plasma membrane fraction, while alpha2 isoform in the intracellular membrane pool. Diabetes decreased the abundance of alpha1 isoform (25 %, P<0.05) in plasma membrane and alpha2 isoform (50%, P<0.01) in the intracellular membrane fraction. When plasma membrane fractions were isolated by discontinuous sucrose gradients, insulin-stimulated translocation of alpha2- but not alpha1-subunits was detected. Alpha1-subunit translocation was only detectable by cell surface biotinylation technique. After insulin administration protein level of alpha2 increased by 3.3-fold, alpha1 by 1.37-fold and beta1 by 1.51-fold (P<0.02) in the plasma membrane of control, and less than 1.92-fold (P<0.02), 1.19-fold (not significant) and 1.34-fold (P<0.02) in diabetes. The insulin-induced translocation was wortmannin sensitive.

CONCLUSION

This study demonstrates that insulin influences the plasma membrane localization of Na(+)/K(+)-ATPase isoforms in the heart. alpha2 isoform translocation is the most vulnerable to the reduced insulin response in diabetes. alpha1 isoform also translocates in response to insulin treatment in healthy rat. Insulin mediates Na(+)/K(+)-ATPase alpha1- and alpha2-subunit translocation to the cardiac muscle plasma membrane via a PI3-kinase-dependent mechanism.

Removal from the membrane affects the interaction of rat osseous plate ecto-nucleosidetriphosphate diphosphohydrolase-1 with substrates and ions

We have characterized the kinetic properties of ectonucleoside triphosphate diphosphohydrolase 1 (E-NTPDase1) from rat osseous plate membranes. A novel finding of the present study is that the solubilized enzyme shows high- and low-affinity sites for the substrate in contrast with a single substrate site for the membrane-bound enzyme. In addition, contrary to the Michaelian chraracteristics of the membrane-bound enzyme, the site-site interactions after solubilization with 0.5% digitonin plus 0.1% lysolecithin resulted in a less active ectonucleoside triphosphate diphosphohydrolase, showing activity of about 398.3 nmol Pi min(-1) mg(-1). The solubilized enzyme has M (r) of 66-72 kDa, and its catalytic efficiency was significantly increased by magnesium and calcium ions; but the ATP/ADP activity ratio was always <2.0. Partial purification and kinetic characterization of the rat osseous plate E-NTPDase1 in a solubilized form may lead to a better understanding of a possible function of the enzyme as a modulator of nucleotidase activity or purinergic signaling in matrix vesicle membranes. The simple procedure to obtain the enzyme in a solubilized form may also be attractive for comparative studies of particular features of the active sites from this and other ATPases.

Isolation of T-tubules from skeletal muscle

The transverse tubules (T-tubules) of mammalian cardiac and skeletal muscles are invaginations of the sarcolemma. They play a crucial role in excitation-contraction coupling as well as in intracellular signaling and in regulation of glucose transport. The biochemical purification of T-tubule membranes is a difficult task, and membrane fractions enriched in transverse tubules are usually contaminated with other cell-surface and intracellular membranes. This unit includes methods that permit the isolation and purification of T-tubules from skeletal muscle.

Influence of distillery effluent on germination and growth of mung bean (Vigna radiata) seeds

Distillery effluent or spent wash discharged as waste water contains various toxic chemicals that can contaminate water and soil and may affect the common crops if used for agricultural irrigation. Toxic nature of distillery effluent is due to the presence of high amounts of organic and inorganic chemical loads and its high-acidic pH. Experimental effects of untreated (Raw) distillery effluent, discharged from a distillery unit (based on fermentation of alcohol from sugarcane molasses), and the post-treatment effluent from the outlet of conventional anaerobic treatment plant (Treated effluent) of the distillery unit were studied in mung bean (Vigna radiata, L.R. Wilczek). Mung bean is a commonly used legume crop in India and its neighboring countries. Mung bean seeds were presoaked for 6h and 30 h, respectively, in different concentrations (5-20%, v/v) of each effluent and germination, growth characters, and seedling membrane enzymes and constituents were investigated. Results revealed that the leaching of carbohydrates and proteins (solute efflux) were much higher in case of untreated effluent and were also dependent to the presoaking time. Other germination characters including percentage of germination, speed of germination index, vigor index and length of root and embryonic axis revealed significant concentration-dependent decline in untreated effluent. Evaluation of seedlings membrane transport enzymes and structural constituents (hexose, sialic acid and phospholipids) following 6 h presoaking of seeds revealed concentration-dependent decline, which were much less in treated effluent as compared to the untreated effluent. Treated effluent up to 10% (v/v) concentration reflected low-observed adverse effect levels.

Carbonic anhydrases IV and IX: subcellular localization and functional role in mouse skeletal muscle

The subcellular localization of carbonic anhydrase (CA) IV and CA IX in mouse skeletal muscle fibers has been studied immunohistochemically by confocal laser scanning microscopy. CA IV has been found to be located on the plasma membrane as well as on the sarcoplasmic reticulum (SR) membrane. CA IX is not localized in the plasma membrane but in the region of the t-tubular (TT)/terminal SR membrane. CA IV contributes 20% and CA IX 60% to the total CA activity of SR membrane vesicles isolated from mouse skeletal muscles. Our aim was to examine whether SR CA IV and TT/SR CA IX affect muscle contraction. Isolated fiber bundles of fast-twitch extensor digitorum longus and slow-twitch soleus muscle from mouse were investigated for isometric twitch and tetanic contractions and by a fatigue test. The muscle functions of CA IV knockout (KO) fibers and of CA IX KO fibers do not differ from the function of wild-type (WT) fibers. Muscle function of CA IV/XIV double KO mice unexpectedly shows a decrease in rise and relaxation time and in force of single twitches. In contrast, the CA inhibitor dorzolamide, whether applied to WT or to double KO muscle fibers, leads to a significant increase in rise time and force of twitches. It is concluded that the function of mouse skeletal muscle fibers expressing three membrane-associated CAs, IV, IX, and XIV, is not affected by the lack of one isoform but is possibly affected by the lack of all three CAs, as indicated by the inhibition studies.

Skeletal Muscle Fatigue: Cellular Mechanisms

Repeated, intense use of muscles leads to a decline in performance known as muscle fatigue. Many muscle properties change during fatigue including the action potential, extracellular and intracellular ions, and many intracellular metabolites. A range of mechanisms have been identified that contribute to the decline of performance. The traditional explanation, accumulation of intracellular lactate and hydrogen ions causing impaired function of the contractile proteins, is probably of limited importance in mammals. Alternative explanations that will be considered are the effects of ionic changes on the action potential, failure of SR Ca2+release by various mechanisms, and the effects of reactive oxygen species. Many different activities lead to fatigue, and an important challenge is to identify the various mechanisms that contribute under different circumstances. Most of the mechanistic studies of fatigue are on isolated animal tissues, and another major challenge is to use the knowledge generated in these studies to identify the mechanisms of fatigue in intact animals and particularly in human diseases.

Experimental exposure of arsenic in cultured rat intestinal epithelial cells and cell line: Toxicological consequences

Arsenic is a naturally occurring metalloid and the drinking water contamination by inorganic arsenic remains a major public health problem. The trivalent arsenic (arsenite) is more toxic than the pentavalent form (arsenate), and is known to cause gastrointestinal toxicity. Specific immortal cell lines are considered to be suitable for toxicity screening and testing of chemicals as they are easy to handle and possess most of the biochemical pathways present in the corresponding cells present in vivo. The present study was designed to evaluate and compare the in vitro toxicity of arsenite on rat intestinal epithelial cell line (IEC-6) and primary cultures of rat intestinal epithelial cells (IEC). To evaluate in vitro toxicity, cultures of IEC and IEC-6 cells were assessed for viability, morphometric analysis, membrane transport enzymes and structural constituents for membrane damage, dehydrogenase activity test for respiratory and energy producing processes and esterase activity test for intra and extra cellular degradation, following the post exposures to arsenite (0-20 ppm). Significantly similar concentration-dependent changes in these toxicity-screening parameters in IEC and IEC-6 were observed. Highest tested concentration of arsenite (20 ppm) was found to be detrimental in both IEC and IEC-6. Furthermore, to evaluate arsenite toxicity in epithelial cells of rat intestine, intestinal loops were filled with arsenite solutions and incubated for 30 min in situ. In situ studies also showed a significant arsenite concentration-dependent decline in epithelial cell membrane transport enzyme activities and total hexose and sialic acid contents. Concomitant release of membrane enzymes, hexose and sialic acid in the intestinal luminal fluid following higher arsenite exposures further indicated partial membrane damage. Similar morphological changes in IEC and IEC-6 were also evident. These findings also suggest that IEC-6 cell lines are suitable for initial screening of gastrointestinal cellular toxicity caused by arsenite.

A transverse tubule NADPH oxidase activity stimulates calcium release from isolated triads via ryanodine receptor type 1 S -glutathionylation

We report here the presence of an NADPH oxidase (NOX) activity both in intact and in isolated transverse tubules and in triads isolated from mammalian skeletal muscle, as established by immunochemical, enzymatic, and pharmacological criteria. Immunohistochemical determinations with NOX antibodies showed that the gp91(phox) membrane subunit and the cytoplasmic regulatory p47(phox) subunit co-localized in transverse tubules of adult mice fibers with the alpha1s subunit of dihydropyridine receptors. Western blot analysis revealed that isolated triads contained the integral membrane subunits gp91(phox) and p22(phox), which were markedly enriched in isolated transverse tubules but absent from junctional sarcoplasmic reticulum vesicles. Isolated triads and transverse tubules, but not junctional sarcoplasmic reticulum, also contained varying amounts of the cytoplasmic NOX regulatory subunits p47(phox) and p67(phox). NADPH or NADH elicited superoxide anion and hydrogen peroxide generation by isolated triads; both activities were inhibited by NOX inhibitors but not by rotenone. NADH diminished the total thiol content of triads by one-third; catalase or apocynin, a NOX inhibitor, prevented this effect. NADPH enhanced the activity of ryanodine receptor type 1 (RyR1) in triads, measured through [3H]ryanodine binding and calcium release kinetics, and increased significantly RyR1 S-glutathionylation over basal levels. Preincubation with reducing agents or NOX inhibitors abolished the enhancement of RyR1 activity produced by NADPH and prevented NADPH-induced RyR1 S-glutathionylation. We propose that reactive oxygen species generated by the transverse tubule NOX activate via redox modification the neighboring RyR1 Ca2+ release channels. Possible implications of this putative mechanism for skeletal muscle function are discussed.

Characterization of an ectonucleoside triphosphate diphosphohydrolase 1 activity in alkaline phosphatase-depleted rat osseous plate membranes: possible functional involvement in the calcification process

An ectonucleoside triphosphate diphosphohydrolase 1 (NTPDase1) activity present in alkaline phosphatase-depleted rat osseous plate membranes, obtained 14 days after implantation of demineralized bone particles in the subcutaneous tissue of Wistar rats, was characterized. At pH 7.5, NTPDase1 hydrolyzed nucleotide triphosphates at rates 2.4-fold higher than those of nucleotide diphosphates, while the hydrolysis of nucleotide monophosphates and non-nucleotide phosphates was negligible. NTPDase 1 hydrolyzed ATP and ADP following Michaelis-Menten kinetics with V=1278.7+/-38.4 nmol Pi/min/mg and K(M)=83.3+/-2.5 microM and V=473.9+/-18.9 nmol Pi/min/mg and K(M)=150.6+/-6.0 microM, respectively, but in the absence of magnesium and calcium ions, ATP or ADP hydrolysis was negligible. The stimulation of the NTPDase1 by calcium (V=1084.7+/-32.5 nmol Pi/min/mg; and K(M)=377.8+/-11.3 microM) and magnesium (V=1367.2+/-41.0 nmol Pi/min/mg and K(M)=595.3+/-17.8 microM) ions suggested that each ion could replace the other during the catalytic cycle of the enzyme. Oligomycin, ouabain, bafilomycin A(1), theophylline, thapsigargin, ethacrynic acid, P(1),P(5)-(adenosine-5')-pentaphosphate and omeprazole had negligible effects on the hydrolysis of ATP and ADP by NTPDase1. However, suramin and sodium azide were effective inhibitors of ATP and ADP hydrolysis. To our knowledge this is the first report suggesting the presence of NTPDase1 in rat osseous plate membranes. Considering that the ectonucleoside triphosphate diphosphohydrolase family of enzymes participates in many regulatory functions, such as response to hormones, growth control, and cell differentiation, the present observations raise interesting questions about the participation of this activity in the calcification process.

Inhibition of muscle carbonic anhydrase slows the Ca(2+) transient in rat skeletal muscle fibers

A countertransport of H(+) is coupled to Ca(2+) transport across the sarcoplasmic reticulum (SR) membrane. We propose that SR carbonic anhydrase (CA) accelerates the CO(2)-HCO reaction so that H(+) ions, which are exchanged for Ca(2+) ions, are produced or buffered in the SR at sufficient rates. Inhibition of this SR-CA is expected to reduce the rate of H(+) fluxes, which then will retard the kinetics of Ca(2+) transport. Fura 2 signals and isometric force were simultaneously recorded in fiber bundles of the soleus (SOL) and extensor digitorum longus (EDL) from rats in the absence and presence of the lipophilic CA inhibitors L-645151, chlorzolamide (CLZ), and ethoxzolamide (ETZ), as well as the hydrophilic inhibitor acetazolamide (ACTZ). Fura 2 and force signals were analyzed for time to peak (TTP), 50% decay time (t(50)), and their amplitudes. L-645151, CLZ, and ETZ significantly increased TTP of fura 2 by 10-25 ms in SOL and by 5-7 ms in EDL and TTP of force by 6-30 ms in both muscles. L-645151 and ETZ significantly prolonged t(50) of fura 2 and force by 20-55 and 40-160 ms, respectively, in SOL and EDL. L-645151, CLZ, and ETZ also increased peak force of single twitches and amplitudes of fura fluorescence ratio (R(340/380)) at an excitation wavelength of 340 to 380 nm. All effects of CA inhibitors on fura 2 and force signals could be reversed. ACTZ did not affect TTP, t(50), and amplitudes of fura 2 signals or force. L-645151, CLZ, and ETZ had no effects on myosin-, Ca(2+)-, and Na(+)-K(+)-ATPase activities, nor did they affect the amplitude and half-width of action potentials. We conclude that inhibition of SR-CA by impairing H(+) countertransport is responsible for deceleration of intracellular Ca(2+) transients and contraction times.

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.

Calcium additional to that bound to the transport sites is required for full activation of the sarcoplasmic reticulum Ca-ATPase from skeletal muscle

The sarcoplasmic reticulum Ca-ATPase is fully activated when approximately 1 microM [Ca2+] saturates the two transport sites; higher [Ca] inhibits the ATPase by competition of Ca-ATP with Mg-ATP as substrates. Here we describe a novel effect of EGTA and other chelators, raising the possibility of an additional activating effect of Ca in the sub- or low microM range. Sarcoplasmic reticulum membranes were isolated from rabbit skeletal muscles. The ATPase activity was measured after incubation at 37 degreesC in 3 mM ATP, 3 mM MgCl2, 50 mM MOPS-Tris (pH 7.2), 100 mM KCl, and variable CaCl2, EGTA and calcimycin. In the absence of added EGTA and Ca the ATPase activity is high due to contaminant Ca. The determination of the ATPase activity in the presence of increasing amounts of EGTA, without added Ca, yields a decreasing sigmoidal function. Ki ranged between 20 and 100 microM, depending on the enzyme concentration. Pi production is linear with time for several [EGTA] yielding suboptimal ATPase activities, which are inhibited by thapsigargin. These suboptimal Ca-ATPase activities are inhibited by preincubation of the enzyme in EGTA, at pH 7.2. This effect increases upon increasing EGTA concentration and preincubation time. The inhibitory effect of the previous exposure of the enzyme to EGTA is partially but significantly reverted by increasing [Ca2+] during incubations. Calcimycin and EDTA have similar effects as EGTA when added in preincubations. The effect of calcimycin is fully reverted by optimal [Ca2+] in incubations. The effects of EGTA, EDTA and calcimycin in preincubation are not additive. The results suggest that an additional calcium, lost during preincubations from a site with affinity near 1 microM, is necessary for full activation of the ATPase.

Inhibition and kinetic properties of membrane-bound carbonic anhydrases in rabbit skeletal muscles

It was the aim of this study to investigate whether the carbonic anhydrases associated with the sarcoplasmic reticulum (SR) and sarcolemmal membranes differ in their kinetic and inhibitory properties. To this end, sarcolemmal and SR membrane vesicle fractions were prepared from rabbit white and red skeletal muscles, the white muscle sarcolemmal fraction (WSL), the red muscle sarcolemmal fraction (RSL), the white muscle SR fraction (WSR), and the red muscle SR fraction (RSR). WSL displayed a specific carbonic anhydrase activity of 22.1 U . ml/mg and RSL of 7.5 U . ml/mg, whereas the SR fractions showed a much lower activity of 0.5 U . ml/mg for WSR and of 2.4 U . ml/mg for RSR. In both SR fractions phase separation experiments with Triton X-114 demonstrated that the carbonic anhydrase activity is due to a membrane-bound enzyme and not due to a cytosolic isozyme. The kinetic properties of carbonic anhydrase from the four distinct membane fractions were evaluated by determination of the Michaelis constant, Km, and of the catalytic centre activity kcat. Km appears to be somewhat lower for SR than for SL. Inhibition constants of SR and SL carbonic anhydrases were determined applying six carbonic anhydrase inhibitors: chlorzolamide, ethoxzolamide, methazolamide, benzolamide, and acetazolamide, and also cyanate. The inhibition constants of the SR fractions were significantly different from those of the corresponding sarcolemmal fractions, indicating that the carbonic anhydrase measured in the SR fractions does not originate from contaminating sarcolemmal membrane vesicles, but appears to represent a distinct carbonic anhydrase associated with the SR membrane.

In vivo protective role of Koflet (an ayurvedic preparation) against cellular toxicity caused by CCl4 and flyash

Swiss albino rats were treated in groups with CCl4, and flyash to induce cellular toxicity in the lungs and trachea. Animal groups received treatment of Koflet (K) with CCl4 (7 days) and with flyash (30 days); their general health and biochemical parameters were studied and used as an indication of cellular injuries. A significant loss was observed in body weight and food consumption in animals given only CCl4 or flyash, while simultaneous treatment with K resulted in a non significant alteration from normal control groups. Enzyme (alkaline phosphatase, Ca2+ -Mg2+ -ATPase, glutamic-oxaloacetic transaminase (GOT) and glutamic pyruvic transaminase (GPT)) activities were estimated in tissue homogenate preparation of lung, trachea and serum, which showed no significant change except for GPT activity as compared to control animals which received CCl4 or flyash with K. Similarly lung, trachea and serum contents of carbohydrate, protein, sialic acid, serum protein, serum cholesterol were estimated and it was found that alteration caused by CCl4, or flyash becomes almost non-significant compared to that of the control after the treatment of K, except for carbohydrate and serum cholesterol values. The animal group which was only treated with K did not show any significant alteration in their biochemical markers or injuries, except for cholesterol.

Characterization of two distinct intracellular GLUT4 membrane populations in muscle fiber. Differential protein composition and sensitivity to insulin

A major objective for the understanding of muscle glucose disposal is the elucidation of the intracellular trafficking pathway of GLUT4 glucose carriers in the muscle fiber. In this report, we provide functional and biochemical characterization of two distinct intracellular GLUT4 vesicle pools obtained from rat skeletal muscle. The two pools showed a differential response to insulin; thus, one showed a marked decrease in GLUT4 levels but the other did not. They also showed a markedly different protein composition as detected by quantitative vesicle immunoisolation analysis. The GLUT4 pool showing no response to insulin contained SCAMP proteins and the vSNARE proteins VAMP2 and cellubrevin, whereas only VAMP2 was found in the insulin-recruitable GLUT4 pool. SDS-PAGE and further silver staining of the immunoprecipitates revealed discrete polypeptide bands associated to the insulin-sensitive pool, and all these polypeptide bands were found in the insulin-insensitive population. Furthermore, some polypeptide bands were exclusive to the insulin-insensitive population. The presence of cellubrevin and SCAMP proteins, endosomal markers, suggest that the insulin-insensitive GLUT4 membrane population belongs to an endosomal compartment. In addition, we favor the view that the insulin-sensitive GLUT4 membrane pool is segregated from the endosomal GLUT4 population and is undergoes exocytosis to the cell surface in response to insulin. Intracellular GLUT4 membranes obtained from skeletal muscle contain cellubrevin, and VAMP2 and GLUT4-vesicles from cardiomyocytes also contain cellubrevin. This suggests that vSNARE proteins are key constituents of GLUT4 vesicles. The presence of the tSNARE protein SNAP25 in skeletal muscle membranes and SNAP25 and syntaxin 1A and syntaxin 1B in cardiomyocyte plasma membranes further suggest a role of the SNAREs in GLUT4 trafficking in muscle.

Caveolin-3 associates with developing T-tubules during muscle differentiation

Caveolae, flask-shaped invaginations of the plasma membrane, are particularly abundant in muscle cells. We have recently cloned a muscle-specific caveolin, termed caveolin-3, which is expressed in differentiated muscle cells. Specific antibodies to caveolin-3 were generated and used to characterize the distribution of caveolin-3 in adult and differentiating muscle. In fully differentiated skeletal muscle, caveolin-3 was shown to be associated exclusively with sarcolemmal caveolae. Localization of caveolin-3 during differentiation of primary cultured muscle cells and development of mouse skeletal muscle in vivo suggested that caveolin-3 is transiently associated with an internal membrane system. These elements were identified as developing transverse-(T)-tubules by double-labeling with antibodies to the alpha 1 subunit of the dihydropyridine receptor in C2C12 cells. Ultrastructural analysis of the caveolin-3-labeled elements showed an association of caveolin-3 with elaborate networks of interconnected caveolae, which penetrated the depths of the muscle fibers. These elements, which formed regular reticular structures, were shown to be surface-connected by labeling with cholera toxin conjugates. The results suggest that caveolin-3 transiently associates with T-tubules during development and may be involved in the early development of the T-tubule system in muscle.

Effect of haloperidol on the sarcoplasmic reticulum Ca-dependent adenosine triphosphatase

Several effects of the neuroleptic agent haloperidol on the sarcoplasmic reticulum (SR) Ca-dependent adenosine triphosphatase (Ca-ATPase) and Ca transport are described. Haloperidol inhibits the Ca-ATPase activity in the presence of calcimycin. The effect depends on the conditions of preexposure of the membranes to the drug: the inhibition increases with the preincubation time; Ca and Mg protect the enzyme against the effect of the drug. The inhibitory effect of haloperidol decreases upon increasing [Ca2+], at constant [Mg], and disappears at 20 mM [Mg] for any [Ca2+], and at 0.5 mM [Ca2+] for any [Mg2+]. Haloperidol also inhibits phosphorylation of the enzyme by Pi, and ATP-dependent Ca2+ uptake, in both cases with apparent Ki = 0.10-0.15 mM, and increases the rate of Ca efflux from preloaded vesicles in this concentration range. The results suggest that haloperidol interacts with the catalytic site, interfering with the effect of the divalent catalytic cation, but not at other steps of the enzymatic cycle, where Mg2+ and Ca2+ are also activators. They are consistent with a reaction model where haloperidol interacts with the E2 conformers of the enzyme, with lower affinity for the phosphoenzyme than for the dephospho species. The inhibition of Ca uptake by SR vesicles is ascribed to an increased Ca2+ permeability rather than to the inhibition of the Ca-ATPase, which requires higher concentrations of the drug.

Inactivation of ecto-ATPase activity of rat brain synaptosomes

The ecto-ATPase activity of synaptosomes plasma membrane decays exponentially as a function of time from 0.35 +/- 0.05 to 0.08 +/- 0.02 mumol ATP hydrolyzed per min per mg synaptosome protein. The first-order rate constant of inactivation is dependent on the Mg-ATP concentration varying from 0.042 +/- 0.001 min-1 with 30 microM ATP up to 0.216 +/- 0.003 min-1 with 2 mM ATP. The non-hydrolyzable ATP analogue, beta-gamma-methyleneadenosine 5'-triphosphate, did not produce inactivation of the ecto-ATPase activity. Thus, the inactivation of the ecto-ATPase activity requires hydrolysis of ATP. Product inhibition can be excluded because ADP, AMP, adenosine and inorganic phosphate up to 1 mM had no effect on the inactivation of the ecto-ATPase. Concanavalin A partially protected against the ATP-dependent inactivation. The ecto-ATPase inactivation produced by Mg-ATP is partially reverted by centrifugation, removal of the supernatant and resuspension of synaptosomes in a fresh medium. This partial reversion occurs in parallel to the release to the supernatant of phophorylated protein(s) of 90-95 kDa. Alkaline phosphatase treatment fully reverts the ecto-ATPase inactivation. We conclude that the ATP-induced inactivation is mediated, at least partially, by phosphorylation of membrane proteins.

Control of cell membrane ecto-ATPase by oligomerization state: intermolecular cross-linking modulates ATPase activity

The extracellular ATPase (ecto-ATPase) is a divalent cation-dependent nucleoside triphosphatase with an unusually high specific activity. Monoclonal antibodies, described previously [Stout, J. G., Strobel, R. S., & Kirley, T. L. (1995) J. Biol. Chem. 270, 11845-11850], and newly generated polyclonal antibodies, both raised against the chicken gizzard ecto-ATPase, were evaluated for their ability to cross-react with mammalian ecto-ATPases and were used as specific immunochemical probes to identify non-cross-linked and cross-linked ecto-ATPase. Unlike previous results obtained with the rabbit skeletal muscle ecto-ATPase enzyme, cross-linking the chicken gizzard smooth muscle ecto-ATPase with 3,3'-dithiobis(sulfosuccinimidylpropionate) (DTSSP) and dithiobis(succinimidylpropionate) (DSP) increased the activity of the enzyme which corresponded to an increase in a approximately 130 kDa immunoreactive band, proposed to be a ecto-ATPase homodimer, and a concomitant decrease in a approximately 66 kDa immunoreactive band, the ecto-ATPase monomer. Ecto-ATPase was immunochemically identified in chicken, rat, mouse, rabbit, and pig. Interestingly, under nonreducing conditions, the ecto-ATPase activity in rat and pig (unlike chicken and rabbit) was evident on Western blots as an immunoreactive band at approximately 200 kDa, proposed to be an intermolecularly disulfide-linked ecto-ATPase homotrimer. Nonreducing Western blot analysis of various rat tissues with three different monoclonal antibodies that recognize the 66 kDa chicken gizzard ecto-ATPase monomer strengthened the hypothesis that this 200 kDa band indeed represents the trimeric ecto-ATPase. After reduction, ecto-ATPase monomers were found to be approximately 66 kDa in all species examined. The differences in ecto-ATPase quaternary structure stability may account for the observed species differences in ecto-ATPase enzymatic properties. Intermolecular disulfide bonds appear to be one of the species-specific ways to stabilize the native, active ecto-ATPase quaternary structure (the homotrimer). Based on the data obtained, as well as previous data from this and other laboratories, a hypothesis was developed to explain the modulation of ecto-ATPase activity by a variety of agents, including detergents, chemical cross-linkers, lectins, antibodies, and small molecule inhibitors. It is proposed that agents and conditions stabilizing ecto-ATPase oligomers stimulate enzyme activity, whereas agents and conditions destabilizing ecto-ATPase homooligomers would inhibit the ecto-ATPase.

Expression and insulin-regulated distribution of caveolin in skeletal muscle. Caveolin does not colocalize with GLUT4 in intracellular membranes

Caveolin is believed to play an important role in sorting processes, vesicular trafficking, transmembrane signaling, and molecular transport across membranes. In this study we have evaluated the expression and distribution of caveolin in skeletal muscle and its interaction with GLUT4 glucose carriers. Caveolin was expressed to substantial levels in muscle and its expression was regulated in muscle; aging and high fat diet enhanced caveolin expression in skeletal muscle and inversely, myogenesis down-regulated caveolin in L6E9 cells. Under fasting conditions, most of caveolin was found in intracellular membranes and the caveolin present in the cell surface was found in both sarcolemma and T-tubules. Insulin administration led to a redistribution of caveolin from intracellular high density membrane fractions to intracellular lighter density fractions and to the cell surface; this pattern of insulin-induced redistribution was different to what was shown by GLUT4. These results suggests that caveolin is a component of an insulin-regulated machinery of vesicular transport in muscle. Quantitative immunoisolation of GLUT4 vesicles obtained from different intracellular GLUT4 populations revealed the absence of caveolin which substantiates the lack of colocalization of intracellular GLUT4 and caveolin. This indicates that caveolin is not involved in intracellular GLUT4 trafficking in skeletal muscle.

Ca2+/Mg(2+)-dependent ATPase activity in Hymenolepis diminuta mitochondria

Ca2+ and Mg2+ caused a concentration-dependent activation of ATP hydrolysis by mitochondrial membranes of Hymenolepis diminuta, a rat intestinal cestode. Ca2+ was the more potent, but Mg2+ the more effective. The Lineweaver-Burk plot yielded Km and Vmax values of 1.15 nM and 217.4 nmol Pi min-1 mg-1 protein for Ca(2+)-dependent activity, and 1.86 mM and 333.3 nmol Pi min-1 mg-1 protein for Mg(2+)-dependent activity, respectively. Neither Na+ nor K+, nor a combination of the two cations, induced the hydrolysis of ATP. Ouabain, a specific inhibitor of Na+/K+ ATPase, did not affect the rate of ATP hydrolysis induced by Mg2+ alone or in combination with Na+ or K+. The membrane-bound enzyme was not affected by neuraminidase and concanavalin A. Ca2+ and Mg2+ also induced appreciable hydrolysis of other nucleoside triphosphates by the membranes. Some known anthelmintics, e.g. niclosamide, praziquantel and mebendazole, had no effect on ATPase activities. In addition to other compounds including respiratory inhibitors and uncouplers of phosphorylation, ruthenium red, which blocks Ca2+ influx into the cestode mitochondria, had no influence on the rate of ATP hydrolysis induced by the cations. Triton X-100 was found most suitable for solubilization of both activities. The differences between cestode ATPase and its mammalian counterpart have been discussed.

Divalent cation-activated ecto-ATPase activity of rat glomerular mesangial cells

Evidence is presented of a Mg2+ and Ca(2+)-activated ATPase at the surface of cultured rat glomerular mesangial cells. Substrate specificity was very low when different nucleoside-5'-triphosphates were examined. The apparent Km values were 0.46 and 0.32 mM for Mg(2+)-ATPase and Ca(2+)-ATPase, respectively. Inhibition studies have revealed that this enzyme is different from the (Na+ + K+)-ATPase and from the divalent cation activated mitochondrial ATPases. Culture in the serum-free medium markedly decreased ecto-ATPase activity. Con A treatment reduced at 50 micrograms/ml by 15% the Ca(2+)-ATPase activity. Endocytosis of serum-treated zymosan (STZ) did not significantly affect ecto-ATPase activity. In contrast, endocytosis of STZ by macrophages was accompanied by a biphasic response, an increase in Mg(2+)-ATPase and Ca(2+)-ATPase activities after ingestion of smaller amounts of STZ and a marked decrease after loading doses of STZ.

Identification and partial purification of (Ca2+ or Mg2+)-ATPase in renal brush-border membranes

The protein responsible for the (Ca2+ or Mg2+)-ATPase activity in brush-border membranes from pig kidney tubular cells was characterized to distinguish this enzyme from the N-ethylmaleimide-sensitive Mg(2+)-ATPase, also present in renal brush borders. Both enzymes are clearly different in their pH optimum and their sensitivity to divalent cations, nucleoside 5'-triphosphates and inhibitors. Solubilization of the (Ca2+ or Mg2+)-ATPase from brush-border membrane vesicles was accomplished with Nonidet P-40 or dodecylmaltoside. However, simultaneous inactivation of the enzyme was inevitable. A tenfold enrichment of the ATPase activity was obtained by chromatofocusing of Nonidet-P-40-solubilized brush borders. A similar degree of purification was achieved by ion-exchange chromatography of dodecylmaltoside-solubilized preparations. From the SDS/polyacrylamide gels of partially purified (Ca2+ or Mg2+)-ATPase, a few protein bands could still be tentatively identified as responsible for the enzyme activity. Labeling of solubilized brush-border preparations with several radioactive ATP analogues also revealed that a protein band of molecular mass 90 kDa is the most probable candidate for the catalytic peptide of the (Ca2+ or Mg2+)-ATPase. Finally, immunoprecipitation as well as semi-dry blotting with antibodies generated against partially purified enzyme preparations, confirmed that a 90-kDa component is a reasonable candidate for the (Ca2+ or Mg2+)-ATPase in renal brush-border membranes.

Effect of hexachlorocyclohexane (HCH) on testicular plasma membrane of rat

The study deals with the analysis of residue of hexachlorocyclohexane (HCH) and its possible damaging potential on testicular plasma membrane of rats. In vitro studies were conducted by exposing plasma membrane of testis with 1.46 x 10(-5) M, 2.92 x 10(-5) M, 4.38 x 10(-5) M, 7.30 x 10(-5) M, 1.46 x 10(-4) M and 2.92 x 10(-4) M HCH at 37 degrees C for 15 min. The results indicate inhibition in the activity of the Ca(2+)-ATPase, Na(+)+K(+)+Mg(2+)-ATPase and 5'Nucleotidase. In vivo studies were carried out following repeated dermal exposure to HCH at a dose level of 50 or 100 mg/kg/day for 60 days to male rats. The results show significant decrease in the activities of 5'-Nucleotidase, Ca(2+)-ATPase, Na(+) + K(+)-ATPase and Mg(2+)-ATPase in the plasma membrane of testis following exposure to HCH. The analysis of the residues of HCH reveals the presence of significant quantities of its different isomers viz., alpha, beta, gamma and delta in the testicular plasma membrane of rats given in vivo dermal exposure of this pesticide. These results suggest that the presence of HCH residue may be a factor in inhibiting the marker enzymes of the plasma membrane of testis.

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.

Subcellular distribution and immunocytochemical localization of Na,K-ATPase subunit isoforms in human skeletal muscle

The expression of Na,K-ATPase isoforms was investigated in human skeletal muscle membranes isolated by subcellular fractionation. The alpha 1, alpha 2, alpha 3 and beta 1 subunits were detectable in membranes prepared from the human soleus muscle. The alpha 1 subunit was largely detected in a fraction enriched with plasma membranes (PM), its abundance in an intracellular membrane fraction (IM) accounted for only 4% of that in the PM fraction. No alpha 1 subunits were detected in membranes of sarcoplasmic reticulum (SR) origin. The PM and IM fractions were enriched with alpha 2 subunits which were less abundant in the SR-enriched fraction. The abundance of alpha 2 molecules within the IM fraction was about 75% of that in the PM fraction when the total protein content for the two fractions was taken into account. Immunocytochemical studies confirmed the localization of the alpha 1 subunit to the muscle cell surface. The alpha 2 subunit was also found to be present in the cell surface but the observation that alpha 2 immunofluorescence was diffusely dispersed throughout the muscle fibre indicated that it was also present intracellularly, consistent with its biochemical localization in the PM and IM membrane fractions. The alpha 3 subunit was detected largely in the PM fraction but the lack of good antibodies to this isoform precluded an analysis of its immunocytochemical localization. The beta 1 subunit was enriched in the PM fraction but was also detected to a modest extent in the IM.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Localization of an arginine-specific mono-ADP-ribosyltransferase in skeletal muscle sarcolemma and transverse tubules

The precise localization of a membrane-bound, arginine-specific mono-ADP-ribosyltransferase (mADP-RT) was assessed in rabbit skeletal muscle by studying membrane fractions isolated by successive sucrose density gradient centrifugations. mADP-RT activity was 10-fold enriched in sarcolemmal and T-tubular membranes. The catalytic activity, determined in preparations with mainly right-side-out vesicles, was found to be on the cytoplasmic face. As revealed by SDS-PAGE and autoradiography endogenous mADP-RT activity labeled several proteins in the range between 15 kDa and 250 kDa. T-tubules contained the highest number of [32P]ADP-ribose-labeled proteins.

Photoaffinity labelling of cardiac membrane GTP-binding proteins in response to insulin

Plasma membranes from rat cardiac ventricular tissue and insulin receptors partially purified by wheat-germ-agglutinin chromatography were subjected to direct photoaffinity labelling with [alpha-32P]GTP in order to elucidate the presence of insulin-receptor-coupled GTP-binding proteins. In plasma membranes three proteins have been identified that exhibit an enhanced photolabelling with the nucleotide in response to insulin. The apparent molecular masses of these proteins were found to be 56, 60 and 74 kDa. Photolabelling of partially purified insulin receptors showed the copurification of the 60-kDa species, whereas the 56-kDa and 74-kDa proteins could not be detected. Furthermore, the 60-kDa G-protein was found to be specifically co-immunoprecipitated with the insulin receptor. Incubation of insulin receptors with insulin increased the labelling of the 60-kDa band to 205 +/- 27% (n = 5) of control. Immuno- and ligand-blotting experiments revealed the additional presence of a 39-kDa G(o)-like protein and two G-proteins with molecular masses of 24 and 26 kDa in the receptor preparation. Under basal conditions the insulin receptor and the 60-kDa G-protein exhibited an apparent inverse distribution between plasma and microsomal membranes with the G-protein being extensively labelled in the microsomal fraction. In conclusion, our data show that, in its native environment, the cardiac insulin receptor couples to at least three GTP-binding proteins. Out of these, a 60-kDa species of microsomal origin, copurifies with the insulin receptor. It is suggested that this G-protein is associated with the insulin receptor and may be involved in insulin receptor signalling in target cells.

Insulin-induced translocation of GLUT 4 in skeletal muscle of insulin-resistant Zucker rats

The genetically obese Zucker rat (fa/fa) is an animal model with severe insulin resistance of the skeletal muscle. We investigated whether a defect of insulin-dependent glucose transporter (GLUT 4) translocation might contribute to the pathogenesis of the insulin-resistant state. fa/fa rats, lean controls (Fa/Fa) as well as normal Wistar rats were injected intraperitoneally with insulin and were killed after 2 or 20 min, respectively. Subcellular fractions were prepared from hind-limb skeletal muscle and were characterized by determination of marker-enzyme activities and immunoblotting applying antibodies against alpha 1 Na+/K+ ATPase. The relative amounts of GLUT 1 and GLUT 4 were determined in the fractions by immunoblotting with the respective antibodies. Insulin induced an approximately two-fold increase of GLUT 4 in a plasma membrane and transverse tubule enriched fraction and a decrease in the low density enriched membrane fraction in all three groups of rats. There was a high individual variation in GLUT 4 translocation efficiency within the groups. However, no statistically significant difference was noted between the groups. No effect of insulin was detectable on the distribution of GLUT 1 or alpha 1 Na+K+ ATPase. The data suggest that skeletal muscle insulin resistance of obese Zucker rats is not associated with a lack of GLUT 4 translocation.

Isolation of a Ca2+ or Mg(2+)-activated ATPase (ecto-ATPase) from bovine brain synaptic membranes

An ATPase was isolated from synaptosomal plasma membranes derived from bovine cerebral cortex. The protein has an apparent molecular mass of 50 kDa and a pI of 5.3 to 5.9. It can be labelled by incubation of intact synaptosomes with azido-GTP or azido-ATP. The isolated ATPase can be activated to a similar extent in the presence of millimolar concentrations of Mg2+ or Ca2+. It does not hydrolyze ADP. Maximal activity is obtained between pH 7.5 and 8.5. Typical inhibitors of cytoplasmic ATPases do not affect enzyme activity. The enzyme is specifically inhibited after previous incubation of intact synaptosomes in the presence of the slowly membrane-permeable enzyme inhibitor diazotized sulfanilic acid. Incubation of intact synaptosomes with diazotized sulfanilic acid results in a small increase in the apparent molecular mass of the enzyme. Our results suggest that the active site of the membrane bound enzyme faces the extracellular medium. It thus would represent an ecto-ATPase.

The MgATPase activity of rat cardiac sarcoplasmic reticulum is a function of the calcium ATPase protein

Magnesium-dependent ATPase (MgATPase) activity is associated with many E1-E2 or P-type transport ATPases including the sarcoplasmic reticulum (SR) calcium ATPase. The SR isolated from rat heart has a MgATPase activity which is 6-12 times faster than the MgATPase activity of the SR isolated from dog heart. To determine the origin of the high MgATPase activity of rat heart SR, we compared and contrasted cardiac SR isolated from both species. The preparations were similar in the following ways: (i) contamination by other organelles; (ii) the comigration of MgATPase activity with calcium-dependent ATPase (CaATPase) activity through a sucrose gradient; (iii) a similar ATPase activity sensitivity to pH and ATP concentration; (iv) the high and similar of sensitivity of ATPase activity to detergent; and (v) a similar protein profile. In both preparations, a single protein in the 105,000-Da region of polyacrylamide gels was phosphorylated by ATP, and the phosphorylated species was an acylphosphate formed in the presence and absence of calcium. Dimethyl sulfoxide, which slows acylphosphoenzyme breakdown, markedly inhibited both CaATPase and MgATPase activities of both preparations but not other enzyme activities. Importantly, the specific inhibitor of the SR calcium pump, thapsigargin, completely inhibited the CaATPase activity with an I50 of 6-7 nM; however, a higher concentration (I50 of 2 microM) was required to inhibit the MgATPase activity of the rat cardiac SR. These results provide evidence that the MgATPase activity of rat cardiac SR is part of the enzyme cycle of the calcium ATPase protein.

Rat skeletal muscle membrane associated carbonic anhydrase is 39-kDa, glycosylated, GPI-anchored CA IV

Sarcolemmal membrane vesicle preparations from white and red muscles of rat were found to contain a carbonic anhydrase which was indistinguishable from carbonic anhydrase IV from rat lung. This isozyme appears to account for all of the carbonic anhydrase activity in the sarcolemmal vesicle preparations. Digestion of 39-kDa CA IV with endoglycosidase F reduced the Mr to 36 kDa, suggesting that it contains one N-linked oligosaccharide. Treatment of sarcolemmal vesicles with phosphatidylinositol-specific phospholipase C released all of the activity, indicating that the enzyme is anchored to membranes by a phosphatidylinositol-glycan linkage. White muscle sarcoplasmic reticulum vesicles also contain a small amount of 39-kDa CA IV-type enzyme. A 52-kDa polypeptide in sarcoplasmic reticulum membranes cross-reacts with anti-human CA II and anti-rat CA II antisera, but does not bind to the sulfonamide affinity column. This cross-reacting polypeptide has no detectable CA activity.

Biogenesis of transverse tubules in skeletal muscle in vitro

The transverse (T) tubules of skeletal muscle are membrane tubules that are continuous with the plasma membrane and penetrate the mature muscle fiber radially to carry surface membrane depolarization to the sites of excitation-contraction coupling. We have studied the development of the T-tubule system in cultured amphibian and mammalian muscle cells using a fluorescent lipid probe and antibodies against T-tubules and plasma membranes. Both the lipid probe and the T-tubule antibody recognized an extensive tubular membrane system which subsequently differentiated into the T-system. At all developmental stages, the molecular composition of the T-system was distinct from that of the plasma membrane, suggesting that during myogenesis T-tubules and the plasma membrane form independently from each other and that exchange of membrane proteins between the two continuous compartments is restricted. In rat muscle cultures, T-tubule-specific antigens were first expressed in terminally differentiated myoblasts. Prior to myoblast fusion the antigens appeared as punctate label throughout the cytoplasm. Shortly after fusion the T-tubule-specific antibody labeled a tubular membrane system that extended from the perinuclear region and penetrated most parts of the cells. In contrast, the lipid probe, which labels the T-tubules by virtue of their direct continuity with the plasma membrane, only labeled short tubules extending from the plasma membrane into the periphery of the myotubes at the early stage in development. Thus, the assembly of the T-tubules appears to begin before their connections with the plasma membrane are established.

The denaturing action of lysophosphatidylcholine as studied by calorimetric and rheological techniques

The potency of lysophosphatidylcholine to perturb protein structure was investigated by differential scanning calorimetry and rheological measurements using myosin as a model protein. At physiological ionic strength (0.15 M NaCl) 5mM lysophosphatidylcholine produced a detectable reduction in the protein's enthalpy of denaturation, while concentrations less than or equal to 2 mM had no effect. At higher salt concentrations (0.6 M NaCl) lower concentrations of lysophosphatidylcholine were needed in order to reduce the enthalpy of denaturation. Also, the changes in myosin conformation, as judged from calorimetric measurements, became more extensive as the incubation temperature for myosin-lysophosphatidylcholine systems was increased from 10 degrees to 30 degrees C. Rheological techniques allowed detection of changes in the structure of filaments of myosin (in 0.15 M) upon addition of 0.2 mM lysophosphatidylcholine. The denaturing action of lysophosphatidylcholine is compared to the more familiar detergent sodium dodecyl sulphate.

Effect of carbodiimides on the activity of Mg(2+)-ATPase of slow-twitch muscle microsomal membranes

1. The hydrophobic N,N'-dicyclohexylcarbodiimide (DCCD) inhibits the activity of Mg(2+)-ATPase of slow-twitch muscle microsomal fraction. 2. The inhibition is dependent on time and concentration, with half-maximal inhibition occurring at 0.4 mM concentration of carbodiimide after a 0.5 hr incubation at room temperature. 3. ATP does not protect against the inhibition. 4. Two water-soluble carbodiimides, 1-cyclohexyl-3-(2-morpholinoethyl)-carbodiimide (CMCD) and 1-ethyl-3(3-dimethylaminopropyl)-carbodiimide (EDCD), are not inhibitory. 5. Inhibition of Mg(2+)-ATPase activity by DCCD is accompanied by covalent incorporation of the radioactive agent into the partially purified enzyme preparation.

Isolation and characterization of the Mg2(+)-ATPase from rabbit skeletal muscle sarcoplasmic reticulum membrane preparations

Preparations of sarcoplasmic reticulum vesicles, obtained according to the method of Eletr and Inesi (Biochim. Biophys. Acta (1972) 282, 174), contained both Mg2(+)-ATPase and Ca2+, Mg2(+)-ATPase activity. The two enzymes were solubilized by a mixture of digitonin and lysophosphatidylcholine and separated on a DEAE-cellulose column eluted with a discontinuous gradient of NaCl. The Mg2(+)-ATPase activity was eluted with 0.43 M NaCl. The Ca2+,Mg2(+)-ATPase was obtained by increasing the NaCl concentration of the elution medium to 0.40 M. The fraction eluted with 0.043 M NaCl was insensitive to micromolar concentrations of calcium, resistant to oligomycin, ouabain, orthovanadate and thiocyanate, and was inhibited by low concentrations of Triton X-100. The enzyme showed a single apparent Km for MgATP in the range of 0.2 mM and a Vm of 2.9 mumol Pi.min-1.mg-1 protein. Activity was maximal over a broad peak between pH 6.0-8.0. Hydrolysis of ATP was unaffected by dimethylsulfoxide concentrations up to 20% (v/v) and was inhibited at higher concentrations. The enzyme was not phosphorylated by either 32Pi or [gamma-32P]ATP at significant levels when compared with the Ca2+,Mg2(+)-ATPase in an EGTA-containing medium. The kinetic pattern of the Mg2(+)-ATPase was distinctly different from that of the Ca2+,Mg2(+)-ATPase under the same conditions. The fraction eluted from the DEAE-cellulose column was subjected to electrophoresis under non-denaturing conditions. Only one band with Mg2(+)-ATPase activity was detected. The Mg2(+)-ATPase migrated much slower than the Ca2+,Mg2(+)-ATPase under non-denaturing conditions, whereas both enzymes had a molecular mass of 105 kDa on SDS gel electrophoresis.