STUDIES ON ADENOSINETRIPHOSPHATASE OF MUSCLE

A century of exercise physiology: effects of muscle contraction and exercise on skeletal muscle Na+,K+-ATPase, Na+ and K+ ions, and on plasma K+ concentration-historical developments

This historical review traces key discoveries regarding K+ and Na+ ions in skeletal muscle at rest and with exercise, including contents and concentrations, Na+,K+-ATPase (NKA) and exercise effects on plasma [K+] in humans. Following initial measures in 1896 of muscle contents in various species, including humans, electrical stimulation of animal muscle showed K+ loss and gains in Na+, Cl- and H20, then subsequently bidirectional muscle K+ and Na+ fluxes. After NKA discovery in 1957, methods were developed to quantify muscle NKA activity via rates of ATP hydrolysis, Na+/K+ radioisotope fluxes, [3H]-ouabain binding and phosphatase activity. Since then, it became clear that NKA plays a central role in Na+/K+ homeostasis and that NKA content and activity are regulated by muscle contractions and numerous hormones. During intense exercise in humans, muscle intracellular [K+] falls by 21 mM (range - 13 to - 39 mM), interstitial [K+] increases to 12-13 mM, and plasma [K+] rises to 6-8 mM, whilst post-exercise plasma [K+] falls rapidly, reflecting increased muscle NKA activity. Contractions were shown to increase NKA activity in proportion to activation frequency in animal intact muscle preparations. In human muscle, [3H]-ouabain-binding content fully quantifies NKA content, whilst the method mainly detects α2 isoforms in rats. Acute or chronic exercise affects human muscle K+, NKA content, activity, isoforms and phospholemman (FXYD1). Numerous hormones, pharmacological and dietary interventions, altered acid-base or redox states, exercise training and physical inactivity modulate plasma [K+] during exercise. Finally, historical research approaches largely excluded female participants and typically used very small sample sizes.

The real reason why ATP hydrolysis is spontaneous at pH > 7: It's (mostly) the proton concentration!

Common wisdom holds that ATP hydrolysis is spontaneous because of the weakness of its phosphoanhydride bonds, electrostatic repulsion within the polyanionic ATP4- molecule, and resonance stabilization of the inorganic phosphate and ADP products. By examining the pH-dependence of the hydrolysis Gibbs free energy, we show that in fact, above pH 7, ATP hydrolysis is spontaneous due mainly to the low concentration of the H+ that is released as product. Hence, ATP is essentially just an electrophilic target whose attack by H2 O causes the acidity of the water nucleophile to increase dramatically; the spontaneity of the resulting acid ionization supplies much of the released Gibbs free energy. We also find that fermentation lowers pH not due to its organic acid products (e.g., lactic, acetic, formic, or succinic acids), but again, due to the H+ product of ATP hydrolysis.

Excitation-contraction coupling in mammalian skeletal muscle: Blending old and last-decade research

The excitation–contraction coupling (ECC) in skeletal muscle refers to the Ca²⁺-mediated link between the membrane excitation and the mechanical contraction. The initiation and propagation of an action potential through the membranous system of the sarcolemma and the tubular network lead to the activation of the Ca²⁺-release units (CRU): tightly coupled dihydropyridine and ryanodine (RyR) receptors. The RyR gating allows a rapid, massive, and highly regulated release of Ca²⁺ from the sarcoplasmic reticulum (SR). The release from triadic places generates a sarcomeric gradient of Ca²⁺ concentrations ([Ca²⁺]) depending on the distance of a subcellular region from the CRU. Upon release, the diffusing Ca²⁺ has multiple fates: binds to troponin C thus activating the contractile machinery, binds to classical sarcoplasmic Ca²⁺ buffers such as parvalbumin, adenosine triphosphate and, experimentally, fluorescent dyes, enters the mitochondria and the SR, or is recycled through the Na⁺/Ca²⁺ exchanger and store-operated Ca²⁺ entry (SOCE) mechanisms. To commemorate the 7^th decade after being coined, we comprehensively and critically reviewed “old”, historical landmarks and well-established concepts, and blended them with recent advances to have a complete, quantitative-focused landscape of the ECC. We discuss the: 1) elucidation of the CRU structures at near-atomic resolution and its implications for functional coupling; 2) reliable quantification of peak sarcoplasmic [Ca²⁺] using fast, low affinity Ca²⁺ dyes and the relative contributions of the Ca²⁺-binding mechanisms to the whole concert of Ca²⁺ fluxes inside the fibre; 3) articulation of this novel quantitative information with the unveiled structural details of the molecular machinery involved in mitochondrial Ca²⁺ handing to understand how and how much Ca²⁺ enters the mitochondria; 4) presence of the SOCE machinery and its different modes of activation, which awaits understanding of its magnitude and relevance in situ; 5) pharmacology of the ECC, and 6) emerging topics such as the use and potential applications of super-resolution and induced pluripotent stem cells (iPSC) in ECC. Blending the old with the new works better!

Discovery of the regulatory role of calcium ion in muscle contraction and relaxation: Setsuro Ebashi and the international emergence of Japanese muscle research

In the early 1950s Setsuro Ebashi was a graduate student at Tokyo University studying the biochemical models of muscle contraction. The muscle components in these models contracted in the presence of ATP, but what caught his attention was that the components did not relax when ATP was exhausted. Ebashi decided in 1952 to attempt to elucidate the mechanism of muscle relaxation using these models. This decision started a journey that would lead him to be the first to propose the calcium concept of muscle contraction and relaxation in 1961. It was an unpopular theory with biochemists who refused to accept that anything as simple as an inorganic ion, Ca²⁺, could control anything as important as muscle contraction. Ebashi was convinced that he was correct. He proceeded to show that micromolar concentrations of Ca²⁺ activated contraction. In 1961 he discovered the particulate nature of the ATP-dependent relaxing factor (the sarcoplasmic reticulum) and determined that it acted by binding Ca²⁺. Most notably, in 1966 he discovered troponin, the Ca²⁺ receptor in muscle, which mediated Ca²⁺ control of contraction. Ebashi's discoveries were considered the most important in the muscle field since the 1950s. Ebashi had to overcome the doubt of the scientific community. This story is one of great scientific achievement against great odds that marked the emergence of Japanese muscle research onto the international scientific stage.NEW & NOTEWORTHY Setsuro Ebashi proposed the calcium concept of muscle contraction and relaxation in 1961. It was a very unpopular theory. He showed that Ca²⁺ activated contraction and that the sarcoplasmic reticulum caused relaxation by binding Ca²⁺ in an ATP-dependent manner. Most notably, he discovered the receptor that mediated Ca²⁺ control of contraction and named it "troponin." Ebashi's discoveries are considered to be the most important in the muscle field since the 1950s.

Historical perspective on heart function: the Frank-Starling Law

More than a century of research on the Frank-Starling Law has significantly advanced our knowledge about the working heart. The Frank-Starling Law mandates that the heart is able to match cardiac ejection to the dynamic changes occurring in ventricular filling and thereby regulates ventricular contraction and ejection. Significant efforts have been attempted to identify a common fundamental basis for the Frank-Starling heart and, although a unifying idea has still to come forth, there is mounting evidence of a direct relationship between length changes in individual constituents (cardiomyocytes) and their sensitivity to Ca²⁺ ions. As the Frank-Starling Law is a vital event for the healthy heart, it is of utmost importance to understand its mechanical basis in order to optimize and organize therapeutic strategies to rescue the failing human heart. The present review is a historic perspective on cardiac muscle function. We "revive" a century of scientific research on the heart's fundamental protein constituents (contractile proteins), to their assemblies in the muscle (the sarcomeres), culminating in a thorough overview of the several synergistically events that compose the Frank-Starling mechanism. It is the authors' personal beliefs that much can be gained by understanding the Frank-Starling relationship at the cellular and whole organ level, so that we can finally, in this century, tackle the pathophysiologic mechanisms underlying heart failure.

Reiji Natori, Setsuro Ebashi, and excitation-contraction coupling

The achievements of Natori and Ebashi, which greatly contributed to the progress in studies of excitation-contraction coupling, were reviewed. Natori succeeded in removing the cell membrane of an isolated fiber of skeletal muscle to prepare a skinned fiber, which still responded to an electrical stimulation with propagated contraction. Skinned fibers showed elastic extensibility beyond the elastic limit of intact muscle fibers. Based on this elasticity Natori predicted the presence of an elastic components, later found as connectin. Skinned fibers, an excellent experimental system, contributed greatly to the progress in subsequent studies. Ebashi showed that the essential principle of the relaxing factor was not the ATP-regenerating enzymes as generally thought, but a particulate fraction with MgATPase. Then he clearly showed that a minute amount of Ca(2+) is necessary for the contractile reaction of actomyosin, and that the relaxing factor strongly accumulates Ca(2+) in the presence of ATP and causes relaxation by the removal of Ca(2+). He further discovered that the Ca(2+)-induced regulation of the contractile reaction of the myosin-actin system requires the presence of tropomyosin and a new protein, troponin. Troponin binds to a specific site on tropomyosin, which in turn binds to actin in the thin filament. Troponin is the Ca(2+)-receptive protein, and changes in troponin molecules upon Ca(2+) binding is transmitted to actin through tropomyosin to regulate the actin-myosin interaction. Through these findings, the excitation was connected by Ca(2+) with the contraction.

Calcium signalling: Past, present and future

Ca2+ is a universal second messenger controlling a wide variety of cellular reactions and adaptive responses. The initial appreciation of Ca2+ as a universal signalling molecule was based on the work of Sydney Ringer and Lewis Heilbrunn. More recent developments in this field were critically influenced by the invention of the patch clamp technique and the generation of fluorescent Ca2+ indicators. Currently the molecular Ca2+ signalling mechanisms are being worked out and we are beginning to assemble a reasonably complete picture of overall Ca2+ homeostasis. Furthermore, investigations of organellar Ca2+ homeostasis have added complexity to our understanding of Ca2+ signalling. The future of the Ca2+ signalling field lies with detailed investigations of the integrative function in vivo and clarification of the pathology associated with malfunctions of Ca2+ signalling cascades.

Slippage and uncoupling in P-type cation pumps; implications for energy transduction mechanisms and regulation of metabolism

P-type ATPases couple scalar and vectorial events under optimized states. A number of procedures and conditions lead to uncoupling or slippage. A key branching point in the catalytic cycle is at the cation-bound form of E(1)-P, where isomerization to E(2)-P leads to coupled transport, and hydrolysis leads to uncoupled release of cations to the cis membrane surface. The phenomenon of slippage supports a channel model for active transport. Ability to occlude cations within the channel is essential for coupling. Uncoupling and slippage appear to be inherent properties of P-type cation pumps, and are significant contributors to standard metabolic rate. Heat production is favored in the uncoupled state. A number of disease conditions, include ageing, ischemia and cardiac failure, result in uncoupling of either the Ca(2+)-ATPase or Na(+)/K(+)-ATPase.

The Ca(2+)-ATPase of the sarcoplasmic reticulum in skeletal and cardiac muscle. An overview from the very beginning to more recent prospects

The discovery of the ATP-driven calcium pump in the sarcoplasmic reticulum membranes reaches back to the postwar (World War II) years and would not be possible without the generous support by the American scientific community. It was this community that in pre- and postwar years gave shelter to many European scientists, which in return stimulated scientific development in the United States. These pre- and postwar relations helped to establish the calcium pump as a physiologically relevant mechanism in all kinds of cells. The pump and its counterpart, the calcium release channel, proved to be controlled by various intrinsic mechanisms. Rising hydrogen concentrations as occurring in ischemic muscles switch off pump activity and counteract allosterically caffeine-induced calcium release (CICR). Rising phosphate or the presence of other calcium-precipitating anions, on the other hand, prevents pump inhibition by intraluminal calcium precipitation, which, simultaneously, can increase the quantity of releasable calcium. The inactivation of CICR by removing medium chloride must be considered as a hint of additional mechanisms by which calcium-dependent activity regulation can be modified.

Ultra-violet and infra-red investigations on muscle

Infra-red absorption spectroscopy of muscle has already been carried out, using the Burch reflecting microscope (Barer, Cole & Thompson 1949: Barer, Thompson & Williams unpublished). There are considerable difficulties involved in this type of work. In the first place it is rather doubtful whether such measurements will ever be possible on living muscle owing to the presence of water, which possesses intense absorption bands in some of the most useful regions of the infra-red spectrum. It may be possible to overcome this difficulty to some extent by using heavy water which has a different absorption spectrum. It is in principle possible to obtain information similar to that given by infra-red spectroscopy, even in the presence of water, by means of Raman spectroscopy, but the technical difficulties involved, particularly fight scattering by colloids, would seem to preclude this method of attack so far as muscle is concerned. Our infra-red measurements have hitherto been confined to dried material. The results indicate that there is little prospect of working with whole muscles, as even single isolated striated fibres of the frog, rabbit and crab were usually too thick. However, it was possible to obtain good spectra in the chemically important region from 3 to 14/µon exceptionally thin single fibres or on artificially compressed fibres. An attempt was made to detect dichroism by means of polarized infra-red radiation, but to our surprise none was observed throughout the 3 to 14µrange, even though the material used showed strong birefringence in the visible region. Hr Stocken and I have recently examined certain molecular models of muscle, in the fight of the work of Ambrose, Elliott & Temple (1949) on myosin, and it now appears possible that infra-red dichroism of muscle might be expected to manifest itself only under rather special conditions. We hope to put these theoretical deductions to experimental test. As regards measurements on muscle in the ultra-violet region, the position is much more promising. It is quite possible to determine the absorption spectrum of theAorIband in living single fibres. The entire spectrum from about 230 mµin the ultra-violet to over 600 mµ, in the visible can be recorded simultaneously, using the reflecting microscope. This technique can also be used with polarized ultra-violet fight, in order to detect variation of dichroism in crystals at different wave-lengths (Barer, Jope & Perutz unpublished), and I intend to apply it to the study of dichroism in muscle fibres. Another new possibility is the observation of birefringence, as well as dichroism, in the ultra-violet. I have recently carried out experiments with a view to developing a new type of ultra-violet polarizer and it should now be possible to use the reflecting microscope as an ultra-violet polarizing microscope.

The relationship between phospholipid content and Ca2+-ATPase activity in the sarcoplasmic reticulum

The relationship between the phospholipid composition of sarcoplasmic reticulum and the activity of the Ca2+, Mg2+-stimulated ATPase was analyzed by digestion of membrane phospholipids with phospholipase C and A2 enzymes of diverse specificity and by detergent extraction. Phospholipase C of Clostridium perfringens and Clostridium welchii, that hydrolyze preferentially phosphatidylcholine (PC), inhibited the Ca2+-ATPase activity parallel with the depletion of phosphatidylcholine from the membrane. Phospholipase C of Bacillus cereus hydrolyzed in addition to PC, phosphatidylethanolamine (PE) and phosphatidylserine (PS), causing complete inhibition of Ca2+-stimulated ATPase activity. Digestion of sarcoplasmic reticulum with the phospholipase A2 of snake or bee venom produced similar effects. The phosphatidylinositol (PI)-specific phospholipases of B. cereus and Bacillus thuringiensis caused less than 10% inhibition of the Ca2+-ATPase, accompanied by the hydrolysis of more than 70% of the phosphatidylinositol content of the membrane, without significant change in PC, PE and PS content. The inhibition of ATPase activity by the C type phospholipases was nearly completely reversed by octaethyleneglycol dodecyl ether (C12E8). These experiments suggest that the full phospholipid content of native sarcoplasmic reticulum (congruent to 100 mol phospholipid per mol Ca2+-ATPase), is required for ATPase activity and there is no indication that PE, PS, and PI play a specific role in ATP hydrolysis. Extraction of sarcoplasmic reticulum phospholipids by detergents such as deoxycholate, cholate and C12E8 also caused proportional inhibition of ATPase activity with the decrease in phospholipid content; the parallel extraction of PC, PE and PI left the phospholipid composition largely unchanged during delipidation. These observations do not support the requirement for a 'lipid annulus' of congruent to 30 phospholipid molecules/Ca2+-ATPase as proposed by Hesketh et al. ((1976) Biochemistry 15, 4145-4151) or the specific interaction of phosphatidylethanolamine with the ATPase molecule proposed by Bick et al. ((1991) Arch. Biochem. Biophys. 286, 346-352).

Adenosine triphosphatase activity associated with purified cholinergic synaptic vesicles of Torpedo marmorata

A rapid method for purifying Torpedo electric organ vesicles is described, which employs an isoosmotic continuous sucrose-glycine gradient followed by chromagography on CPG-10-3000 porous glass beads. The synaptic vesicles have a buoyant density of 1.057 g/ml. The purified vesicles are free of cholinesterase, lactate dehydrogenase and Na+, K+-stimulated ATPase activity. They contain a ouabaininsensitive, Na+, K+-inhibited, Mg2+, Ca2+-stimulated ATPase activity. This is further stimulated by acetylcholine but not by choline.

Cholesterol in muscle membranes

The composition of skeletal muscle microsomes is reviewed. Evidence for the involvement of cholesterol in the transport of calcium by vesicles derived from the sarcoplasmic reticulum is considered. Results obtained by non aqueous extractions of skeletal muscle microsomes, and by use of the cholesterol analogue 20, 25 diazacholesterol indicate that cholesterol is not involved in calcium transport by vesicles of sarcoplasmic reticulum origin. Use of density perturbation procedures indicating that cholesterol is present in muscle membranes other than those of the sarcoplasmic reticulum involved in calcium transport is discussed. The distribution of membranal cholesterol in muscle is compared to that in other tissues.

Distribution of adenosine triphosphatase and thamine pyrophosphatase in the digestive system of Heteropneustes fossils

Histochemical localization of adenosine triphosphatase and thiamine pyrophosphatase in the digestive system of the teleost fish, Heteropneustes fossilis has been studied. In the stomach, ATPase activity is observed in the mucosa, gastric glands and muscularis. The activity is stronger in the muscularis. Very weak TPPase activity is localized only in the mucosa and gastric glands. In the intestinal mucosa ATPase activity is stronger especially, along the brush border. Mild activity is also found in the connective tissue network and their nuclei, muscularis and serosa. In the posterior portion of the intestine and rectum, the localization pattern is similar to that of intestine but the activity is weaker. TPPase activity in the intestine and rectum is restricted only to the goblet shaped mucus secreting cells. In the liver, strong activity of ATPase and moderate activity of TPPase are found in the cytoplasm as well as the nuclei of the hepatic cells.