The site of calcium binding in relation to the activation of myofibrillar contraction

A short history of the development of mathematical models of cardiac mechanics

Cardiac mechanics plays a crucial role in atrial and ventricular function, in the regulation of growth and remodelling, in the progression of disease, and the response to treatment. The spatial scale of the critical mechanisms ranges from nm (molecules) to cm (hearts) with the fastest events occurring in milliseconds (molecular events) and the slowest requiring months (growth and remodelling). Due to its complexity and importance, cardiac mechanics has been studied extensively both experimentally and through mathematical models and simulation. Models of cardiac mechanics evolved from seminal studies in skeletal muscle, and developed into cardiac specific, species specific, human specific and finally patient specific calculations. These models provide a formal framework to link multiple experimental assays recorded over nearly 100 years into a single unified representation of cardiac function. This review first provides a summary of the proteins, physiology and anatomy involved in the generation of cardiac pump function. We then describe the evolution of models of cardiac mechanics starting with the early theoretical frameworks describing the link between sarcomeres and muscle contraction, transitioning through myosin-level models to calcium-driven systems, and ending with whole heart patient-specific models.

Effect of ethanol on troponin calcium binding

The Chelex resin method was found to be suitable for studying drug effects on Ca2+ binding of proteins. In comparison to conventional dialysis techniques, the Chelex method has the following advantages: Ca2+-EGTA buffer is not necessary, free Ca2+ concentration as low as 10(-9) M can be determined directly, and the reaction is complete in 30 min, thus creating fewer problems with protein denaturation at elevated temperatures. Methods to cope with problems inherent to this assay, such as the excluded volume effect of the resin and protein adsorption by the resin are described. The validity of the method was confirmed by the measurements of Ca2+ binding of troponin in the presence and absence of Mg2+. Using this method, it was demonstrated that ethanol concentration as high as 25% does not influence the Ca2+ binding of troponin.

The binding of calcium to detergent-extracted rabbit psoas muscle fibres during relaxation and force generation

Rigor complexes between actin and myosin have been shown to cause increased binding of Ca2+ to troponin C. A similar effect of force-generating crossbridges has been suggested as an explanation for the coupling between load and activation which has been observed in skeletal and cardiac muscle. The goal of this study was to test the hypothesis that Ca2+-troponin affinity during crossbridge cycling is load-dependent. Ca2+-binding to detergent-extracted rabbit psoas fibres was measured during ATP-induced force generation and in the relaxed state. To compare Ca2+ binding in the latter two states it was necessary to establish conditions in which ATP-induced force could be regulated independently of free Ca2+ concentration. Such conditions were obtained by the use of either the ATPase inhibitor sodium vanadate or the substitution of MgITP for MgATP as an energy source. This study showed that in the presence of MgATP (or MgITP) the amount of Ca2+ bound to the myofilaments at a given free Ca2+ concentration was independent of the force generated. Thus force per se is not a determinant of Ca2+-troponin affinity.

Action of ADP and ethanol on muscle proteins in vitro

The effects of ADP and ethanol on Ca2+ binding of troponin and the superprecipitation of actomyosin were studied. Ca2+ binding of troponin-tropomyosin complex bound to polystyrene particles (Lytron) was increased by ADP, and this increase was inhibited by ethanol. However, Ca2+ binding of the complex as measured by equilibrium dialysis and by the chelex resin method was not influenced by either ADP or ADP plus ethanol. Ca2+ binding of the thin filament, myosin-ghost myofibrils and myofibrils was also not inhibited. Superprecipitation of actomyosin was augmented in the presence of ADP, and the enhancement was inhibited by ethanol. However, this effect of ADP or ethanol was not observed in the presence of an inhibitor of myokinase, p1, p5-di (adenosine-5') pentaphosphate (Ap5A). In the presence of Ap5A, superprecipitation of actomyosin was enhanced when small amounts of ATP (10 microM) and ADP (100 microM) were added 5 min prior to the addition of 2 mM ATP to start the reaction. The enhancement of superprecipitation of actomyosin by ADP may be caused by nuclei produced by a low concentration of ATP which is produced from ADP by contaminating myokinase activity. These data suggest that ADP and ethanol influence Ca2+ binding of the troponin-tropomyosin complex bound to a solid phase, but their effects on superprecipitation may not necessarily reflect muscle contraction in vivo.

Thin filament proteins and thin filament-linked regulation of vertebrate muscle contraction

Recent developments in the field of myofibrillar proteins will be reviewed. Consideration will be given to the proteins that participate in the contractile process itself as well as to those involved in Ca-dependent regulation of striated (skeletal and cardiac) and smooth muscle. The relation of protein structure to function will be emphasized and the relation of various physiologically and histochemically defined fiber types to the proteins found in them will be discussed.

A reexamination of the chelex competitive calcium binding assay

The chelex competitive calcium binding assay has been examined as a tool for the analysis of the kinetic parameters of calcium binding substances. Scatchard analysis of calcium binding demonstrates that chelex binds calcium with apparent negative cooperativity or with more than one class of calcium binding sites, and therefore, cannot be used to provide accurate estimations of the dissociation constant or total number of binding sites on an unknown ligand. Results presented in this study indicate that the chelex assay can be effectively used for the qualitative detection of calcium binding substances in tissue extracts or biological fluids.

Calcium and magnesium binding to thin and thick filaments in skinned muscle fibres: electron probe analysis

Electron probe analysis of ultrathin cryosections with high spatial resolution was used to determine in situ the concentrations of Ca2+ and Mg2+ bound in the absence of ATP to myofilaments in the I and A-bands of skinned frog skeletal muscle. At 2.2 x 10(-11) M Ca2+ and 2.7 x 10(-9) M Mg2+, the inexchangeably bound Mg2+ in the I-band was equivalent to the amount of divalent cations known to be inexchangeably bound to F-actin, while the Ca2+ bound to the I-band was not significantly above zero. The bound Mg2+ in the I-band was not exchangeable with Ca2+ even when the skinned fibres were exposed to 10 mM Ca2+ solution. These results clearly indicate that Mg2+, rather than Ca2+, is the divalent cation bound to F-actin in the thin filaments in situ. In the presence of 1 mM Mg2+, the exchangeable Ca2+ bound to the I-band was increased as a function of the free Ca2+, while that in the A-band was not significantly changed with [Ca2+] up to 2 x 10(-5) M, and increased to approximately 0.8 mol Ca2+ per mol myosin at 10(-4) M Ca2+. At a saturating free Ca2+ in Tris-Cl solution, the bound Ca2+ content (2-3 mol Ca2+ per mol troponin) of the nonoverlapping I-band was unexpectedly low; the replacement of Tris with Na+ enhanced Ca2+ binding to the level equivalent to 3-4 mol Ca2+ per mol troponin. The depressant effect of Tris on Ca2+ binding was greater in the absence of Mg2+. High concentrations of Tris also reduced the maximum tension induced by 10(-4) M Ca2+ buffered with 10 mM EGTA. At 1.3 x 10(-7) M Ca2+, thought to be close to the cytoplasmic free Ca2+ in resting muscle, the I-band bound a significant amount of Ca2+: equivalent to about 1 mol Ca2+ per mol troponin. In rabbit myofibrils there was a significant amount (approximately 1.5 mol/mol myosin) of Ca2+ bound by the A-band at a free Ca2+ of 10(-4) M.

Calcium release and ionic changes in the sarcoplasmic reticulum of tetanized muscle: an electron-probe study

Approximately 60-70% of the total fiber calcium was localized in the terminal cisternae (TC) in resting frog muscle as determined by electron-probe analysis of ultrathin cryosections. During a 1.2 s tetanus, 59% (69 mmol/kg dry TC) of the calcium content of the TC was released, enough to raise total cytoplasmic calcium concentration by approximately 1 mM. This is equivalent to the concentration of binding sites on the calcium-binding proteins (troponin and parvalbumin) in frog muscle. Calcium release was associated with a significant uptake of magnesium and potassium into the TC, but the amount of calcium released exceeded the total measured cation accumulation by 62 mEq/kg dry weight. It is suggested that most of the charge deficit is apparent, and charge compensation is achieved by movement of protons into the sarcoplasmic reticulum (SR) and/or by the movement of organic co- or counterions not measured by energy dispersive electron-probe analysis. There was no significant change in the sodium or chlorine content of the TC during tetanus. The unchanged distribution of a permeant anion, chloride, argues against the existence of a large and sustained transSR potential during tetanus, if the chloride permeability of the in situ SR is as high as suggested by measurements on fractionated SR. The calcium content of the longitudinal SR (LSR) during tetanus did not show the LSR to be a major site of calcium storage and delayed return to the TC. The potassium concentration in the LSR was not significantly different from the adjacent cytoplasmic concentration. Analysis of small areas of I-band and large areas, including several sarcomeres, suggested that chloride is anisotropically distributed, with some of it probably bound to myosin. In contrast, the distribution of potassium in the fiber cytoplasm followed the water distribution. The mitochondrial concentration of calcium was low and did not change significantly during a tetanus. The TC of both tetanized and resting freeze-substituted muscles contained electron-lucent circular areas. The appearance of the TC showed no evidence of major volume changes during tetanus, in agreement with the estimates of unchanged (approximately 72%) water content of the TC obtained with electron-probe analysis.

The time-course of Ca2+ exchange with calmodulin, troponin, parvalbumin, and myosin in response to transient increases in Ca2+

We have modeled the time-course of Ca2+ binding to calmodulin, troponin, parvalbumin, and myosin in response to trains of transient increases in the free myoplasmic calcium ion concentration (pCa). A simple mathematical expression was used to describe each pCa transient, the shape and duration of which is qualitatively similar to those thought to occur in vivo. These calculations assumed that all individual metal binding sites are noninteracting and that Ca2+ bind competitively to the Ca2+-Mg2+ sites of troponin, parvalbumin, and myosin. All the on-and-off rate constants for both Ca2+ and Mg2+ were obtained either from the literature or from our own research. The percent saturation of the Ca2+-Mg2+ sites with Ca2+ was found to change very little in response to each pCa transient in the presence of 2.5 X 10(-3)M Mg2+. Our analysis suggests that the Ca2+ content of these sites is a measure of the intensity and frequency of recent muscle activity because large changes in the Ca2+ occupancy of these sites can occur with repeated stimulation. In contrast, large rapid changes in the amount of Ca2+ bound to the Ca2+-specific sites of troponin and calmodulin are induced by each pCa transient. Thus, only sites of the "Ca2+-specific" type can act as rapid Ca2+-regulatory sites in muscle. Fluctuation in the total amount of Ca2+ bound to these sites in response to various types of pCa transients further suggests that in vivo only about one-half to one-third of the total steady-state myofibrillar Ca2+-binding capacity exchanges Ca2+ during any single transient.

The effect of magnesium ions on the binding of calcium ions to glycerinated rabbit psoas muscle fibers

The effect of Mg2+ on Ca2+ bound to glycerinated rabbit psoas muscle was studied by means of a double-isotope technique. The troponin-C content of the fibers was analyzed by quantitative sodium dodecyl sulfate polyacrylamide gel electrophoresis and found to be 0.5 mumol/g fiber protein. In the absence of Mg2+ the fibers bound a maximum of 3.5 mumol Ca2+/g protein. This value could be readily accounted for in terms of the four Ca2+-binding sites of troponin-C and the two divalent cation-binding sites of myosin. In the presence of 1 mM Mg2+ the entire Ca2+ titration curve was shifted downward with a maximum bound Ca2+ of slightly more than 2 mumol/g, or 4 mol Ca2+/mol troponin-C. Further increase in Mg2+ concentration to 10 mM had little effect on Ca2+ binding when the free Ca2+ concentration was in the upper part of the physiological range (5 . 10(-7)-5 . 10(-6) M) but caused a marked reduction when the Ca2+ concentration was low. The results are consistent with biochemical data showing two groups of (Mg2+/Ca2+)-binding sites. One group, located in myosin, has a high affinity for Mg2+ while the other group, located on troponin-C, has a low affinity for Mg2+. If the free Mg2+ concentration in muscle is in the range of 2--5 mM, as suggested by recent data, it can be inferred that the binding sites on myosin will never be occupied by Ca2+ under physiological conditions.

Phosphorylation of an actin.tropomyosin.troponin complex from human skeletal muscle

A human skeletal actin.tropomyosin.troponin complex was phosphorylated in the presence of [gamma-32 P]ATP, Mg2+, adenosine 3':5'-monophosphate (cyclic AMP) and cyclic AMP-dependent protein kinase (protein kinase). Phosphorylation was not observed when the actin complex was incubated in the absence of protein kinase or 1 microM cyclic AMP. In the presence of 10(-7) M Ca2+ and protein kinase 0.1 mole of [32P]phosphate per 196 000 g of protein was incorporated. This was two-fold higher than the [32P]phosphate content of a rabbit skeletal actin complex but two-fold lower than that of a bovine cardiac actin complex. At high Ca2+, 5.10(-5) M, little change in the phosphorylation of a human skeletal actin complex occurred. Phosphoserine and phosphothreonine were identified in the [32P]phosphorylated actin complex. Polyacrylamide gel electrophoresis in sodium dodecyl sulfate showed that 60% of the label was associated with the tropomyosin binding component of troponin. The inhibitory component of troponin contained 16% of the bound [32P]phosphate. Increasing the Ca2+ concentration did not significantly decrease the [32P]phosphate content of the phosphorylated proteins in the actin complex. No change in the distribution of phosphoserine or phosphothreonine was observed. Half maximal calcium activation of the ATPase activity of reconstitute human skeletal actomyosin made with the [32P] phosphorylated human skeletal actin complex was the same as a reconstituted actomyosin made with an actin complex incubated in the absence of protein kinase at low or high Ca2+.

Cooperative interactions between calcium-binding sites on glycerinated muscle fibers. The influence of cross-bridge attachment

A double isotope technique and EGTA buffers were used to measure the binding of Ca2+ to rabbit psoas muscle fibers extracted with detergent and glycerol. These experiments were designed to test the effect of rigor complex formation, determined by the degree of filament overlap, on the properties of the Ca2+-binding sites in the intact filament lattice. In the presence of 5 mM MgCl2 (no ATP), reduction of filament overlap was associated with a reduced binding of Ca2+ over the entire range of free Ca2+ concentrations (5.10(-8)-2.10(-5) M). With maximum filament overlap (sarcomere length 2.1-2.2 micrometer) the maximum bound Ca2+ was equivalent to 4 mol Ca2+/mol troponin and there was significant positive interaction between binding sites, as shown by Scatchard and Hill plots. With no filament overlap (sarcomere length 3.8-4.4 micrometer) the maximum bound Ca2+ was equivalent to 3 mumole Ca2+/mol troponin and graphical analysis indicated a single class of non-interacting sites. The data provide evidence that when cross-bridge attachments between actin and myosin filaments are formed not only does an additional Ca2+ binding site appear, but cooperative properties are imposed upon the binding sites.

Regulation of tension in skinned muscle fibers: effect of high concentrations of Mg-ATP

The control of tension in skinned fibers by Mg-ATP and Ca described in previous publications has been studied at high substrate concentrations over a wide range of temperature and salt concentration. Curves of tension versus pCa shift systemically to the right as [Mg-ATP] increases. The maximum Ca-activated tension of a skinned fiber declines at sufficiently high substrate concentrations. This behavior is described by a generalization of the scheme given in the earlier reports.

The binding of calcium to glycerinated muscle fibers in rigor. The effect of filament overlap

The binding of Ca2+ to glycerinated rabbit psoas fibers of varying sarcomere length was measured with a double isotope technique and ethyleneglycol-bis-(beta-aminoethylether)-N,N'-tetraacetic acid buffers. Experiments were carried out under rigor conditions with fiber bundles pre-set at different lengths prior to extraction with detergent and glycerol. These experiments were designed to test whether rigor complex formation, determined by the degree of filament overlap, enhances Ca2+-receptor affinity in the intact filament lattice, as it does in reconstituted actomyosin systems. The Ca2+-receptor affinity, as indicated by the free Ca2+ concentration at half-saturation and by the slopes of Scatchard plots, was found to be relatively unaffected by variations in filament overlap. However, the maximum bound Ca2+ was significantly reduced in stretched fibers. With maximum filament overlap the bound Ca2+ was equivalent to 4 mol per mol troponin. When stretched to zero overlap the fibers bound a maximum of 3 mol Ca2+ per mol troponin. When fibers with maximum overlap were incubated in the presence of 5 mM MgATP there was a reduction in the number of Ca2+-binding sites equivalent to that caused by stretching the fibers. These findings, taken together with other data in the literature, suggest that in the intact filament lattice at least one of the Ca2+-binding sites is present only when cross-bridge attachments are formed.

Cooperative interactions between the contractile proteins of cardiac and skeletal muscle

The calcium activation of the ATPase (ATP phosphohydrolase, EC 3.6.1.3) activity of cardiac actomyosin reconstituted from bovine cardiac myosin and a complex of actin-tropomyosin-troponin extracted from bovine cardiac muscle at 37 degrees C was studied and compared with similar proteins from rabbit fast skeletal muscle. The proteins of the actin complex were identified by polyacrylamide gel electrophoresis in sodium dodecyl sulfate. Half-maximal activation of the cardiac actomyosin was seen at a calcium concentration of 1.2 +/- 0.002 (S.E. of mean) muM. A hybridized reconstituted actomyosin made with cardiac myosin and the actin-tropomyosin-troponin complex extracted from rabbit skeletal muscle was also activated by calcium but the half-maximal value was shifted to 0.65 +/- 0.02 (S.E. of mean) muM Ca2+. Homologous rabbit skeletal actomyosin showed half-maximal activation at 0.90 +/- 0.01 (S.E. of mean) muM Ca2+ and the value for a hybridized actomyosin made with rabbit skeletal myosin and the actin-complex from cardiac muscle was found at 1.4 +/- 0.03 (S.E. of mean) muM Ca2+ concentration. Kinetic analysis of the Ca2+ activated ATPase activity of reconstituted bovine cardiac actomyosin indicated some degree of cooperativity with respect to calcium. Double reciprocal plots of reconstituted actomyosins made with bovine cardiac actin complex were curvilinear and significantly different than those of reconstituted actomyosins made with the rabbit fast skeletal actin complex. The Ca2+-dependent cooperativity was of a mixed type as determined from Hill plots for homologous reconstituted bovine cardiac and rabbit fast skeletal actomyosin. The results show that cooperative interactions in reconstituted actomyosins were greater when the actin-tropomyosin-troponin complex was derived from cardiac than skeletal muscle.

Tension in mechanically disrupted mammalian cardiac cells: effects of magnesium adenosine triphosphate

1. Maximum and submaximum Ca-activated tension in mechanically disrupted rat ventricular fibres was examined in solutions containing 30 micron, 100 micron and 4 mM-MgATP and either 50 micron or 1 mM ionized Mg. 2. In the absence of added Ca, significant amounts of base-line tension (up to 50% of maximum) develop in solutions containing less than 30 micron-MgATP. This effect is Mg-dependent; more tension is produced with 50 micron-Mg than with 1 mM. 3. Increasing the MgATP concentration shifts the pCa-% maximum tension relationship in the direction of increasing Ca required for activation. At 50 micron-Mg the pCa which produces 50% maximum tension is 5-8, 5-3 and 5-5 for the 30 micron, 100 micron and 4 mM-MgATP solutions. The effect of MgATP on position is relatively independent of the Mg concentration. 4. The steepness of the pCa-% maximum tension curve increases as MgATP is elevated to the millimolar range. The Hill coefficients for the different MgATP curves at 50 micron-Mg are 1-1, 1-3 and 3-0. This change in steepness accounts for the slightly lower Ca concentration needed for half-maximum tension as the MgATP concentration is increased to millimolar levels. Raising the Mg concentration to 1 mM greatly diminishes the effect of MgATP on the slope of the pCa-tension relationship. 5. The maximum tnesion a fibre bundle can produce decreases as the amount of MgATP is raised from micromolar to millimolar levels. For 50 muM-Mg, maximum tension drops about 35% as MgATP is raised from 30 micronM to 4 mM. For any concentraiton of MgATP, maximum tension is higher at 1 mM-Mg than at 50 micron-Mg. 6. Existing theories of interaction between myosin heads and the thin filament are sufficient to account for the effects of MgATP on the position of the pCa-tension curves and on maximum tension. The effects on slope are less satisfactorily explained.

Cooperative binding of calcium to glycerinated skeletal muscle fibers

The binding of 45Ca2+ to glycerinated rabbit psoas fibers was measured by means of a double isotope technique. With 5 mM Mg2+ (no ATP) binding was half-maximal at 1.4-10(-6) M Ca2+ and the maximal amount bound was 1.6 mumol/g protein. At less than 50% saturation, the Scatchard plot had a positive slope and the Hill coefficient was 2.2. At greater than 50% saturation, the Scatchard plot was linear with a negative slope (K' = 0.8 - (10(6) M-1) and the Hill coefficient was 1.0. In the absence of Mg2+, binding was half-maximal at 3 - 10(-7) M Ca2+ and the maximal amount bound was 2.9 mumol/g protein. The Scatchard plot indicated two classes of sites with K' values of about 2 - 10(7) and 2 - 10(6) M-1. The Hill coefficient in the mid-saturation range was approx. 0.6. The data indicate that in the presence of the Mg2+ binding to about half of the total Ca2+ binding sites is suppressed and there is a strong positive cooperativity involving half of the ramaining sites.

Ca2+ dependence of tension and ADP production in segments of chemically skinned muscle fibers

Both ADP production and tension have been measured in segments of chemically skinned fibers contracting at different Ca2+ concentrations. Full mechanical activation occurred between pCa 7.00 and pCa 6.50. The total ATPase was due to both actomyosin and non-actomyosin ATPase. Actomyosin ATPase was observed at pCa 7.09 without accompanying tension. The Ca2+ dependence of tension was steeper than actomyosin ATPase. This finding implies some rate constants of the mechano-chemical cycle are Ca2+ dependent. Non-actomyosin ATPase was measured in fibers stretched beyond overlap of the thick and thin filaments. Sarcoplasmic reticulum was isolated and sarcoplasmic reticulum activity was measured in vitro under the same conditions as the single-fiber experiments. Non-actomyosin ATPase in the single fibers was found to be small compared to maximally activated actomyosin ATPase but larger than the ATPase that could be attributed to sarcoplasmic reticulum activity.

Calcium content and exchange in frog skeletal muscle

Calcium content and exchange in frog ELD IV muscle were examined employing the efflux technique. 2. Muscle calcium was found to exchange with four time constants, 21-5 sec 2-7, 32 and 1244 min. 3. All calcium was found to be exchangeable with more than half the total amount residing in an extracellular compartment. 4. Results obtained from ELD IV muscles and single fibres were identical. 5. Muscle calcium content was found to remain constant up to 20 hr in vitro. 6. Extra exchange of calcium occurs upon contraction. This extra exchange appears to occur in the most slowly exchanging component. 7. The data are discussed in relation to morphological and autoradiographic findings and a model of calcium exchange in skeletal muscle is presented.

Calcium-accumulating properties of subcellular fractions of bovine vascular smooth muscle

This study examined the calcium transport properties of two subcellular fractions of bovine aorta obtained by differential centrifugation. The vesicular fraction had a high-affinity (Km = 1.05 X 10(-6)M) calcium transport mechanism which could be potentiated by using the calcium-precipitating anion, oxalate. The mitochondria-enriched fraction's calcium transport system had a lower affinity for calcium than did that of the vesicular fraction. The calcium capacity of the vesicular fraction was determined by comparing the steady-state calcium uptake in the presence of oxalate in the whole homogenate with the same uptake by the vesicular fraction alone. A calcium capacity of about 100 mumoles Ca2+/kg aorta was obtained. It is concluded that the vesicular fraction has all of the properties of a major site of subcellular calcium regulation in vascular smooth muscle.

Inhibition of the vasodilator effect of hypercapnic acidosis by hypercalcemia in dogs and rats

The vasodilator effect of local hypercapnic acidosis, produced by intra-arterial infusion of an acid buffer solution consisting of a mixture of glycine and hydrochloric acid, in the perfused hind limb and in the perfused gastrocnemius muscle of anesthetized dogs was examined at normal plasma calcium concentration and during elevation of the plasma calcium concentration produced by intra-arterial infusion of calcium chloride. Hypercalcemia inhibited the vasodilator action of hypercapnic acidosis so that the effect was not apparent until venous blood pH decreased below 7. The dilator effect of local application of acidified Ringer's solution on arterioles of the cremaster muscle of the rat was also inhibited by elevation of the plasma calcium concentration induced by intravenous administration of calcium chloride. The available evidence suggests that this inhibition of the dilator effect of hypercapnic acidosis by calcium ions is related to competition between calcium and hydrogen ions for binding at one or more cellular sites such as the sarcolemma, the sarcoplasmic reticulum, or the contractile proteins of vascular smooth muscle.

The effects of caffeine on the contraction of the frog heart

1. The force of contraction and the membrane potentials have been measured from preparations of frog heart, with methods that also allow the rapid exchange of the extracellular fluid.2. In regularly beating preparations, caffeine induces only weak contractures at temperatures above 15 degrees C, but it does cause a marked potentiation of the twitch responses; longer exposure results in a depression of contraction. The build up and decline of the twitch strength, on addition and on removal of caffeine approximates to a single exponential, time constant 26-45 sec, and this time constant is not altered by variation of the [Ca](o), the stimulus rate or the caffeine concentration. This time course of the change of twitch strength is less complex than the changes seen when either [Ca](o) or the stimulus rate is altered, suggesting a more direct action of caffeine on excitation-contraction coupling.3. Caffeine increases the strength of the contractures initiated by potassium-rich or sodium-depleted solutions in isolated atrial trabeculae.4. After the spontaneous relaxation of the contracture, evoked by either sodium-free or potassium-rich fluids, the application of caffeine initiates a redevelopment of tension. This caffeine contracture is transient and its strength is dependent on the caffeine concentration. The response in sodium-free solution can be elicited in the virtual absence of extracellular calcium.5. Local anaesthetics antagonize the caffeine contracture.6. The results suggest that the sarcoplasmic reticulum of frog heart muscle plays an important role in the initiation and control of contraction and relaxation.

Calcium requirements for cardiac myofibrillar activation

The amounts of calcium required to achieve various levels of myofibrillar activation in the dog heart were determined by measuring the dependence of myofibrillar calcium binding, myofibrillar adenosinetriphosphatase (ATPase), and isometric tension on free calcium concentration. Myofibrillar ATPase was half-maximal at 2.4 x 10 -6 M free calcium, and tension development was half-maximal at 2.0 x 10 -6 M free calcium. No simple relation between calcium binding and activation was found. For example, between 10 -8 M and 10 -6 M free calcium, an appreciable amount of calcium was bound to the myofibrils, but there was little activation of isometric tension. On the other hand, myofibrillar calcium binding was not saturated at levels of free calcium at which both tension and ATPase were maximal; therefore, it appears that only a portion of the total myofibrillar calcium binding sites control ATPase and tension. Using the information derived from the binding and activation studies together with our determination of the myofibrillar content of the dog heart, 47.5 mg myofibrillar protein/g wet heart, we calculated the calcium required to achieve various levels of myofibrillar activation in the intact ventricle. By this calculation method, development of half-maximal tension required 22.4 µmoles calcium/kg wet heart, and development of maximal isometric tension required 92.8 µmoles/kg wet heart.

Ionic strength and the contraction kinetics of skinned muscle fibers

The influence of KCl concentration on the contraction kinetics of skinned frog muscle fibers at 5-7 degrees C was studied at various calcium levels. The magnitude of the calcium-activated force decreased continuously as the KCl concentration of the bathing solution was increased from 0 to 280 mM. The shortening velocity at a given relative load was unaffected by the level of calcium activation at 140 mM KCl, as has been previously reported by Podolsky and Teichholz (1970. J. Physiol. [Lond.]. 211: 19), and was independent of ionic strength when the KCl concentration was increased from 140 to 280 mM. In contrast, the shortening velocity decreased as the KCl concentration was reduced below 140 mM; the decrease in velocity was enhanced when the fibers were only partially activated. In the low KCl range, the resting tension of the fibers increased after the first contraction cycle. The results suggest that in fibers activated at low ionic strength some of the cross bridges that are formed are abnormal in the sense that they retard shortening and persist in relaxing solution.

The onic dependence of the strength and spontaneous relations of the potassium contracture induced in the heart of the frog Rana pipiens

1. The tension generated by isolated frog atrial trabecules, during exposure to solutions containing a high potassium concentration, is not maintained but spontaneously relaxes. The final part of this relaxation can be fitted by a single exponential function.2. The recovery of the tension generating mechanisms following the spontaneous relaxation of a potassium contracture depends on the preceding membrane potential and the time since the last contracture.3. The rate of the exponential phase of the spontaneous relaxation is independent of the [K](o) and hence the membrane potential, the [Ca](o); and when the [Ca](o)/[Na](o) (2) ratio is maintained it is also independent of the [Na](o). This relaxation is not influenced by atropine or pronethalol.4. When sodium is totally excluded from the bathing medium the rate of relaxation of a later potassium contracture is much increased. It is argued that this change is due to a fall in the intracellular sodium concentration.5. The consequences of these results are discussed, and the hypothesis that is favoured would require that contraction is induced by a transient release of calcium into the sarcoplasm, probably triggered by a potential dependent, and probably also transient, influx of calcium through the cell membrane. Relaxation is supposed to occur when this activator-calcium is then removed by an intracellular relaxing system that resembles the sarcoplasmic reticulum of other muscles. What this intracellular structure might be, is also discussed.

Membrane current and contraction in frog atrial fibres

1. Membrane current and mechanical activity were recorded from short segments of frog atrial muscle strips using a double sucrose gap voltage clamp arrangement. Experiments were performed at 4-7 degrees C. Two types of contraction were observed dependent upon the duration of the clamp.2. Short-lasting depolarizations caused a flow of Ca inward current, I(Ca), and development of a phasic contraction. Time to peak tension approximated 400 msec. Both I(Ca) and contraction, as functions of membrane potential, had a threshold of about - 40 mV and were maximal at inside positive potentials in normal Ringer fluid. Peak tension decreased at strong depolarizations.3. The minimum time of depolarization required for initiation of a phasic contraction was 40-70 msec. The time necessary for full activation of contraction was 200-300 msec and comparable to the period of time covered by the flow of I(Ca).4. There was no marked change in peak tension upon repetitive depolarization to the same membrane potential.5. Restoration of (phasic) contractility after a preceding contraction was strongly dependent on the level of membrane potential between conditioning and test pulse. Restoration was half complete at potentials around - 45 mV.6. Long-lasting depolarizations generated tonic (sustained) contractions superimposed on the phasic (transient) ones. Threshold potential for initiation of tonic contractions was usually positive to the threshold of phasic contractions. The time taken to attain the final level of tension ranged between 0.7 and 3 sec. Plateau tension, as a function of membrane potential, increased with increasing depolarization and reached a flat maximum at about + 50 mV in normal Ringer fluid.7. At membrane potentials near zero level, plateau tension developed by the tonic mechanism was about twice peak tension due to phasic contraction.8. Removal of Ca ions from the external medium resulted in an almost complete abolition of phasic contraction within 1-2 min and a gradual decrease of tonic contraction during the first 10 min. Application of a ;Ca inhibitor' to normal Ringer fluid caused a strong reduction of both I(Ca) and phasic contraction without affecting tonic contractions.9. It is concluded that phasic contractions are directly activated by the flow of I(Ca). Generation of tonic contractions may be attributed to a Ca transfer mechanism different from I(Ca) or a release of Ca from intracellular stores.