The effects of adenosine triphosphate on the fibre volume of a muscle homogenate

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.

A period of convergence in the studies on muscle contraction and relaxation: the Ebashi's contribution

The object of this paper is to trace the growth of a fundamental problem that for a decade hindered the development of several lines of muscle research: the molecular mechanism that allows and controls contraction and relaxation of muscle fiber. Emphasis is placed on the difficulties to be overcome; thus the paper records not only the achievements and successes, but also the unavoidable failure and disappointments. The account highlights the essential contribution of Setsuro Ebashi to find the solution of the problem.

A highly sensitive method for measurement of myosin ATPase activity by reversed-phase high-performance liquid chromatography

A new method for measurement of myosin ATPase activity has been developed utilizing reversed-phase high-performance liquid chromatography (HPLC), which detects as low as 0.05 nmol of ADP hydrolyzed from ATP. After termination of the ATPase reaction by addition of perchloric acid, the hydrolysate ADP and substrate ATP were separated by reversed-phase HPLC. The absorbance of ADP was monitored at 259 nm, and the amount of ADP was quantified from its peak area on the chromatogram by use of the NIH Image computer software. Our method showed linearity over a wide range from 0.05 to 10 nmol of ADP per 20 microl with a coefficient of determination (r(2)) of 0.99. Myosin ATPase activities determined by the HPLC method were almost identical to those determined by the malachite green method, a widely used spectrophotometric method with range of detection from 1 to 8 nmol of phosphate. Because our method requires only a small volume of reaction solution, it will be a powerful tool for measuring ATPase activity of motor proteins, which are difficult to obtain in large amount.

Water loss during contracture of muscle

The relationship of contracture and exudation of water in frozenthawed frog muscle was studied. With maximum shortening, there was a water loss of 35 per cent of the weight of muscle. By restricting the contraction, it was demonstrated that the amount of water loss was proportional to the degree of shortening, there being no significant loss with isometric contraction. Muscle already shortened by tetanic stimulation also exuded water on subsequent freezing and thawing. The force of contraction could be reduced by depleting the muscle of calcium and it was shown that the amount of water exuded was also proportional to the tensile ability of the muscle. In a smooth muscle (anterior byssus retractor of Mytilus) which did not contract vigorously only a little water exuded. Contracture produced by caffeine was similarly associated with a loss of water. Microscopic studies revealed a disruption of the sarcomeres of the frozen-thawed muscle which contracted; glycerol-extracted and calcium-depleted muscles, which did not contract on freeze-thawing, did not show such disruption. Freezing and thawing of actomyosin caused a reversible syneresis of the protein. It is concluded that the exudation of the water is not merely due to the freezing and thawing but is also dependent on the contractile events.

The nature of the muscle-relaxing factor. I. An improved assay system

Analysis of the effects of deoxycholate and benzalkonium chloride on granule relaxing activity, myofibril and glycerinated fiber responses to ATP, and the sensitivity of these actomyosin-containing systems to relaxing factors have established that it is possible to differentially inactivate intrinsic granule relaxing factor. It is therefore possible to use these agents to treat myofibrils and glycerinated muscle fibers so that they may be used for the assay of muscle-relaxing factor preparations. The destruction of intrinsic relaxing factor can be produced by treating actomyosin with 1 mM deoxycholate or 0.005 per cent benzalkonium chloride for 5 minutes at room temperature.

On the mechanism of the relaxing effect of fragmented sarcoplasmic reticulum

The vesicles of fragmented sarcoplasmic reticulum, i.e. the physiological relaxing factor, remove most of the exchangeable Ca bound to actomyosin and myofibrils. The extent to which they remove Ca and the extent to which they inhibit myofibrillar activity are closely correlated. Previous work has shown that those in vitro reactions of actomyosin with ATP which are equivalent to its contraction, i.e. superprecipitation and a high ATPase activity, require the formation of a Ca-actomyosin complex, and that actomyosin after the removal of most of its bound Ca is inhibited by physiological concentrations of ATP. The evidence now suggests that the factor achieves its relaxing effect through the dissociation of the Ca-actomyosin complex and that it has no other direct influence on the biological activity of actomyosin. Experiments, showing that under similar conditions the vesicles of the factor are capable of reducing the concentration of ionized Ca in the surrounding medium to about 0.02 µM and less, suggest that the factor competes successfully with actomyosin for the available Ca through its mechanism of Ca accumulation.

On the structural continuities of the transverse tubular system of rabbit and human myocardial cells

An electron microscopic study of rabbit and human myocardium provides further evidence of the existence of two distinct components of the sarcoplasmic reticulum. A thin-walled tubular system (termed longitudinal system) is arranged in anastomosing channels sur-surrounding each sarcomere and has transverse and possibly also longitudinal connections with the tubules of adjacent sarcomeres. A thick-walled tubular system traverses the myofiber transversely at the level of the Z lines of the myofibrils. The structure of these tubules very closely resembles that of deep sarcolemmal invaginations. Indeed, the membranes of the tubules appear to be continuous with the sarcolemma in favorable sections so that there seems to be an extension of the cell membrane and extracellular fluid to all depths of the myocardial fiber. Certain physiologic data which support this concept are discussed. The calculations of A. V. Hill comparing the kinetics of diffusion and the time-distance relationships between excitation and activation in frog sartorius muscle are reconsidered for cardiac muscle.

ISOLATION OF RELAXING PARTICLES FROM RAT SKELETAL MUSCLES IN ZONAL CENTRIFUGES

Supernatants of rat skeletal muscle homogenates were fractionated by differential centrifugation and by zonal centrifugation in sucrose density gradients. Cytochrome oxidase was employed as an enzymatic marker for locating mitochondria. The subcellular fractions were also assayed for their ability to prevent the ATP-induced contraction of myofibrils. Both the mitochondrial and microsomal fractions obtained by differential fractionation were found to be rich in such relaxing activity, and the microsomal fraction was appreciably contaminated by mitochondria. In contrast to this, when fractionation was carried out by means of zonal centrifugation (4200 RPM x 205 min. to 40,000 RPM x 60 min.), relaxing activity was found to be associated only with particles having the sedimentation characteristics of microsomes (s_20,w estimated to be between 370 and 1880S). Relaxing activity was not detected in the regions of the gradient containing either the starting sample zone (soluble phase) or the mitochondrial peak. The microsomal relaxing particles showed negligible cytochrome oxidase activity.

A microassay for ATPase

A newly developed microtechnique for quantitating activity of myosin ATPase (EC 3.6.1.32) is more sensitive and less time-consuming than existing spectrophotometric methods. Measurement of ATPase activity using the new method can be accomplished in a final volume of 0.25 ml, allowing the assay to be conducted in individual wells of 96-well microplates commonly used for the enzyme-linked immunosorbent assay (ELISA). The microassay is performed by adding purified myosin to microplate wells followed by addition of ATP to initiate the enzymatic reaction. The reaction is subsequently terminated by addition of an acidic solution containing malachite green and ammonium molybdate. The level of inorganic phosphate produced by enzymatic hydrolysis of ATP is measured by scanning the microplates using a microELISA plate reader. An entire 96-well microplate can be scanned in less than 2 min, and data from the microassay can be transferred directly to a microprocessor for statistical analysis. The microassay is capable of detecting between 0.2 and 3 nmol of inorganic phosphate in a reaction volume of 50 microliter, and the ATPase activity of as little as 10 ng of rat cardiac myosin can be measured. The increased sensitivity compared with that of other spectrophotometric assays and ease of performing the microassay enable a detailed analysis of the enzymatic properties of cardiac myosin to be conducted on large numbers of small tissue specimens. Several kinetic properties of rat cardiac myosin were determined using this technique.

The origin of concepts regarding contractility, its mechanism, and its control

The function of the heart is to contract and relax rhythmically so as to propel blood, and the requirements for that function are variable, dependent on the activity of the body. Skeletal muscle also performs variable output tasks but does so by mobilizing a smaller or larger fraction of its motor units. The heart responds in all-or-none manner and must therefore have other means of achieving variable contractile strength. In the first part of this chapter, this will be discussed in a general way, considering mechanisms that heart and muscle have in common. In the second part, we shall then examine properties that are peculiar to the myocardium. The main message of the first part is primarily that much is now known about molecular processes of contraction and of excitation--contraction coupling and that these mechanisms, although subject to specialized modifications, are common to muscle and heart. Of the second part, the main feature is that the variability of excitation--contraction coupling is a major and essential characteristic of the myocardium.

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.

Calcium uptake and associated adenosine triphosphatase activity of isolated platelet membranes

A platelet subcellular fraction, sedimenting between 14,000 and 40,000 g and consisting primarily of membrane vesicles, accumulates up to 200-400 nmoles calcium/mg protein in the presence of ATP and oxalate. Steady-state levels of calcium accumulation are attained in 40-60 min. Calcium uptake requires adenosine triphosphate (ATP), is enhanced by oxalate, and is accompanied by the release of inorganic phosphate. Calcium accumulation and phosphate release require magnesium and are inhibited by Salyrgan (10 microM) and adenosine diphosphate (ADP) (1 mM), but not by ouabain (0.1 mM). The ATPase activity is stimulated by low concentrations of calcium (5-10 microM) and is inhibited by 2 mM EGTA. Electron microscopic histochemistry using lead nitrate to precipitate released phosphate results in lead precipitates localized primarily at the inner surface of membrane vesicles. These results provide evidence for a membrane ATPase that is stimulated by low concentrations of calcium and may be involved in the transport of calcium across the membrane. It is postulated that the observed calcium uptake activity is an in vitro manifestation of a calcium extrusion pump in the intact platelet.