Endothermic force generation in skinned cardiac muscle from rat

Mechanisms of myosin II force generation: insights from novel experimental techniques and approaches

Myosin II is a molecular motor that converts chemical energy derived from ATP hydrolysis into mechanical work. Myosin II isoforms are responsible for muscle contraction and a range of cell functions relying on the development of force and motion. When the motor attaches to actin, ATP is hydrolyzed and inorganic phosphate (Pi) and ADP are released from its active site. These reactions are coordinated with changes in the structure of myosin, promoting the so-called "power stroke" that causes the sliding of actin filaments. The general features of the myosin-actin interactions are well accepted, but there are critical issues that remain poorly understood, mostly due to technological limitations. In recent years, there has been a significant advance in structural, biochemical, and mechanical methods that have advanced the field considerably. New modeling approaches have also allowed researchers to understand actomyosin interactions at different levels of analysis. This paper reviews recent studies looking into the interaction between myosin II and actin filaments, which leads to power stroke and force generation. It reviews studies conducted with single myosin molecules, myosins working in filaments, muscle sarcomeres, myofibrils, and fibers. It also reviews the mathematical models that have been used to understand the mechanics of myosin II in approaches focusing on single molecules to ensembles. Finally, it includes brief sections on translational aspects, how changes in the myosin motor by mutations and/or posttranslational modifications may cause detrimental effects in diseases and aging, among other conditions, and how myosin II has become an emerging drug target.

Temperature Effects on Force and Actin⁻Myosin Interaction in Muscle: A Look Back on Some Experimental Findings

Observations made in temperature studies on mammalian muscle during force development, shortening, and lengthening, are re-examined. The isometric force in active muscle goes up substantially on warming from less than 10 °C to temperatures closer to physiological (>30 °C), and the sigmoidal temperature dependence of this force has a half-maximum at ~10 °C. During steady shortening, when force is decreased to a steady level, the sigmoidal curve is more pronounced and shifted to higher temperatures, whereas, in lengthening muscle, the curve is shifted to lower temperatures, and there is a less marked increase with temperature. Even with a small rapid temperature-jump (T-jump), force in active muscle rises in a definitive way. The rate of tension rise is slower with adenosine diphosphate (ADP) and faster with increased phosphate. Analysis showed that a T-jump enhances an early, pre-phosphate release step in the acto-myosin (crossbridge) ATPase cycle, thus inducing a force-rise. The sigmoidal dependence of steady force on temperature is due to this endothermic nature of crossbridge force generation. During shortening, the force-generating step and the ATPase cycle are accelerated, whereas during lengthening, they are inhibited. The endothermic force generation is seen in different muscle types (fast, slow, and cardiac). The underlying mechanism may involve a structural change in attached myosin heads and/or their attachments on heat absorption.

DCM-related tropomyosin mutants E40K/E54K over-inhibit the actomyosin interaction and lead to a decrease in the number of cycling cross-bridges

Two DCM mutants (E40K and E54K) of tropomyosin (Tm) were examined using the thin-filament extraction/reconstitu­tion technique. The effects of the Ca²⁺, ATP, phos­phate (Pi), and ADP concentrations on isometric tension and its transients were studied at 25°C, and the results were com­pared to those for the WT protein. Our results indicate that both E40K and E54K have a significantly lower T_HC (high Ca²⁺ ten­sion at pCa 4.66) (E40K: 1.21±0.06 T_a, ±SEM, N = 34; E54K: 1.24±0.07 T_a, N = 28), a significantly lower T_LC (low- Ca²⁺ tension at pCa 7.0) (E40K: 0.07±0.02 T_a, N = 34; E54K: 0.06±0.02 T_a, N = 28), and a significantly lower T_act (Ca²⁺ activatable tension) (T_act = T_HC–T_LC, E40K: 1.15±0.08 T_a, N = 34; E54K: 1.18±0.06 T_a, N = 28) than WT (T_HC = 1.53±0.07 T_a, T_LC = 0.12±0.01 T_a, T_act = 1.40±0.07 T_a, N = 25). All tensions were normalized to T_a ( = 13.9±0.8 kPa, N = 57), the ten­sion of actin-filament reconstituted cardiac fibers (myocardium) under the standard activating conditions. The Ca²⁺ sensitivity (pCa₅₀) of E40K (5.23±0.02, N = 34) and E54K (5.24±0.03, N = 28) was similar to that of the WT protein (5.26±0.03, N = 25). The cooper­a­tivity increased significantly in E54K (3.73±0.25, N = 28) compared to WT (2.80±0.17, N = 25). Seven kinetic constants were deduced using sinusoidal analysis at pCa 4.66. These results enabled us to calculate the cross-bridge distribution in the strongly attached states, and thereby deduce the force/cross-bridge. The results indicate that the force/cross-bridge is ∼15% less in E54K than WT, but remains similar to that of the WT protein in the case of E40K. We conclude that over-inhibition of the actomyosin interaction by E40K and E54K Tm mutants leads to a decreased force-generating ability at systole, which is the main mechanism underlying the early pathogenesis of DCM.

Thin filament-reconstituted skinned muscle fibers for the study of muscle physiology

We review the use of thin filament-reconstituted muscle fibers in the study of muscle physiology. Thin filament extraction and reconstitution protocol is a powerful technique to study the role of each component of the thin filament. It is also useful for studying the properties of genetically modified molecules such as actin and tropomyosin. We also review the combination of this protocol with sinusoidal analysis, which will provide a solid technique for determining the effect of regulatory proteins on actomyosin interaction and concomitant cross-bridge kinetics. We suggest that thin filament-reconstituted muscle fibers are an ideal system for studying muscle physiology especially when gene modifications of actin or tropomyosin are involved.

The role of thin filament cooperativity in cardiac length-dependent calcium activation

Length-dependent activation (LDA) is a prominent feature of cardiac muscle characterized by decreases in the Ca(2+) levels required to generate force (i.e., increases in Ca(2+) sensitivity) when muscle is stretched. Previous studies have concluded that LDA originates from the increased ability of (strong) cross-bridges to attach when muscle is lengthened, which in turn enhances Ca(2+) binding to the troponin C (TnC) subunit of the troponin complex. However, our results demonstrate that inhibition of strong cross-bridge attachment with blebbistatin had no effect on the length-dependent modulation of Ca(2+) sensitivity (i.e., EC(50)) or Ca(2+) cooperativity, suggesting that LDA originates upstream of cross-bridge attachment. To test whether LDA arises from length dependence of thin-filament activation, we replaced native cTnC with a mutant cTnC (DM-TnC) that is incapable of binding Ca(2+). Although progressive replacement of native cTnC with DM-TnC caused an expected monotonic decrease in the maximal force (F(max)), DM-TnC incorporation induced much larger increases in EC(50) and decreases in Ca(2+) cooperativity at short lengths than at long lengths. These findings support the conclusion that LDA arises primarily from the influence of length on the modulation of the Ca(2+) cooperativity arising from interaction between adjacent troponin-tropomyosin complexes on the thin filament.

Crossbridge mechanism(s) examined by temperature perturbation studies on muscle

An overall view of the contractile process that has emerged from -temperature-studies on active muscle is outlined. In isometric muscle, a small rapid temperature-jump (T-jump) enhances an early, pre-phosphate release, step in the acto-myosin (crossbridge) ATPase cycle and induces a characteristic rise in force indicating that crossbridge force generation is endothermic (force rises when heat is absorbed). Sigmoidal temperature dependence of steady force is largely due to the endothermic nature of force generation. During shortening, when muscle force is decreased, the T-jump force generation is enhanced; conversely, when a muscle is lengthening and its force increased, the T-jump force generation is inhibited. Taking T-jump force generation as a signature of the crossbridge - ATPase cycle, the results suggest that during lengthening the ATPase cycle is truncated before endothermic force generation, whereas during shortening this step and the ATPase cycle, are accelerated; this readily provides a molecular basis for the Fenn effect.

Force and power generating mechanism(s) in active muscle as revealed from temperature perturbation studies

The basic characteristics of the process of force and power generation in active muscle that have emerged from temperature studies are examined. This is done by reviewing complementary findings from temperature-dependence studies and rapid temperature-jump (T-jump) experiments and from intact and skinned fast mammalian muscle fibres. In isometric muscle, a small T-jump leads to a characteristic rise in force showing that crossbridge force generation is endothermic (heat absorbed) and associated with increased entropy (disorder). The sensitivity of the T-jump force generation to added inorganic phosphate (Pi) indicates that a T-jump enhances an early step in the actomyosin (crossbridge) ATPase cycle before Pi-release. During muscle lengthening when steady force is increased, the T-jump force generation is inhibited. Conversely, during shortening when steady force is decreased, the T-jump force generation is enhanced in a velocity-dependent manner, showing that T-jump force generation is strain sensitive. Within the temperature range of ∼5–35◦C, the temperature dependence of steady active force is sigmoidal both in isometric and in shortening muscle. However, in shortening muscle, the endothermic character of force generation becomes more pronounced with increased velocity and this can, at least partly, account for the marked increase with warming of the mechanical power output of active muscle.

Temperature jump induced force generation in rabbit muscle fibres gets faster with shortening and shows a biphasic dependence on velocity

We examined the tension responses to ramp shortening and rapid temperature jump (<0.2 ms, 3-4 degrees C T-jump) in maximally Ca(2+)-activated rabbit psoas muscle fibres at 8-9 degrees C (the fibre length (L(0)) was approximately 1.5 mm and sarcomere length 2.5 microm). The aim was to investigate the strain sensitivity of crossbridge force generation in muscle. The T-jump induced tension rise was examined during steady shortening over a wide range of velocities (V) approaching the V(max) (V range approximately 0.01 to approximately 1.5 L(0) s(1)). In the isometric state, a T-jump induced a biphasic tension rise consisting of a fast (approximately 50 s(1), phase 2b) and a slow (approximately 10 s(1), phase 3) component, but if treated as monophasic the rate was approximately 20 s(1). During steady shortening the T-jump tension rise was monophasic; the rate of tension rise increased linearly with shortening velocity, and near V(max) it was approximately 200 s(1), approximately 10x faster than in the isometric state. Relative to the tension reached after the T-jump, the amplitude increased with shortening velocity, and near V(max) it was 4x larger than in the isometric state. Thus, the temperature sensitivity of muscle force is markedly increased with velocity during steady shortening, as found in steady state experiments. The rate of tension decline during ramp shortening also increased markedly with increase of velocity. The absolute amplitude of T-jump tension rise was larger than that in the isometric state at the low velocities (<0.5 L(0) s(1)) but decreased to below that of the isometric state at the higher velocities. Such a biphasic velocity dependence of the absolute amplitude of T-jump tension rise implies interplay between, at least, two processes that have opposing effects on the tension output as the shortening velocity is increased, probably enhancement of crossbridge force generation and faster (post-stroke) crossbridge detachment by negative strain. Overall, our results show that T-jump force generation is strain sensitive and becomes considerably faster when exposed to negative strain. Thus the crossbridge force generation step in muscle is both temperature sensitive (endothermic) and strain sensitive.

Tropomyosin period 3 is essential for enhancement of isometric tension in thin filament-reconstituted bovine myocardium

Tropomyosin (Tm) consists of 7 quasiequivalent repeats known as "periods," and its specific function may be associated with these periods. To test the hypothesis that either period 2 or 3 promotes force generation by inducing a positive allosteric effect on actin, we reconstituted the thin filament with mutant Tm in which either period 2 (Delta2Tm) or period 3 (Delta3Tm) was deleted. We then studied: isometric tension, stiffness, 6 kinetic constants, and the pCa-tension relationship. N-terminal acetylation of Tm did not cause any differences. The isometric tension in Delta2Tm remained unchanged, and was reduced to approximately 60% in Delta3Tm. Although the kinetic constants underwent small changes, the occupancy of strongly attached cross-bridges was not much different. The Hill factor (cooperativity) did not differ significantly between Delta2Tm (1.79 +/- 0.19) and the control (1.73 +/- 0.21), or Delta3Tm (1.35 +/- 0.22) and the control. In contrast, pCa(50) decreased slightly in Delta2Tm (5.11 +/- 0.07), and increased significantly in Delta3Tm (5.57 +/- 0.09) compared to the control (5.28 +/- 0.04). These results demonstrate that, when ions are present at physiological concentrations in the muscle fiber system, period 3 (but not period 2) is essential for the positive allosteric effect that enhances the interaction between actin and myosin, and increases isometric force of each cross-bridge.

Temperature-dependence of isometric tension and cross-bridge kinetics of cardiac muscle fibers reconstituted with a tropomyosin internal deletion mutant

The effect of temperature on isometric tension and cross-bridge kinetics was studied with a tropomyosin (Tm) internal deletion mutant AS-Delta23Tm (Ala-Ser-Tm Delta(47-123)) in bovine cardiac muscle fibers by using the thin filament extraction and reconstitution technique. The results are compared with those from actin reconstituted alone, cardiac muscle-derived control acetyl-Tm, and recombinant control AS-Tm. In all four reconstituted muscle groups, isometric tension and stiffness increased linearly with temperature in the range 5-40 degrees C for fibers activated in the presence of saturating ATP and Ca(2+). The slopes of the temperature-tension plots of the two controls were very similar, whereas the slope derived from fibers with actin alone had approximately 40% the control value, and the slope from mutant Tm had approximately 36% the control value. Sinusoidal analysis was performed to study the temperature dependence of cross-bridge kinetics. All three exponential processes A, B, and C were identified in the high temperature range (30-40 degrees C); only processes B and C were identified in the mid-temperature range (15-25 degrees C), and only process C was identified in the low temperature range (5-10 degrees C). At a given temperature, similar apparent rate constants (2pia, 2pib, 2pic) were observed in all four muscle groups, whereas their magnitudes were markedly less in the order of AS-Delta23Tm < Actin < AS-Tm approximately Acetyl-Tm groups. Our observations are consistent with the hypothesis that Tm enhances hydrophobic and stereospecific interactions (positive allosteric effect) between actin and myosin, but Delta23Tm decreases these interactions (negative allosteric effect). Our observations further indicate that tension/cross-bridge is increased by Tm, but is diminished by Delta23Tm. We conclude that Tm affects the conformation of actin so as to increase the area of hydrophobic interaction between actin and myosin molecules.

Use of thin filament reconstituted muscle fibres to probe the mechanism of force generation

The technique of selective removal of the thin filament by gelsolin in bovine cardiac muscle fibres, and reconstitution of the thin filament from isolated proteins is reviewed, and papers that used reconstituted preparations are discussed. By comparing the results obtained in the absence/presence of regulatory proteins tropomyosin (Tm) and troponin (Tn), it is concluded that the role of Tm and Tn in force generation is not only to expose the binding site of actin to myosin, but also to modify actin for better stereospecific and hydrophobic interaction with myosin. This conclusion is further supported by experiments that used a truncated Tm mutant and the temperature study of reconstituted fibres. The conclusion is consistent with the hypothesis that there are three states in the thin filament: blocked state, closed state, and open state. Tm is the major player to produce these effects, with Tn playing the role of Ca2+ sensing and signal transmission mechanism. Experiments that changed the number of negative charges at the N-terminal finger of actin demonstrates that this part of actin is essential to promote the strong interaction between actin and myosin molecules, in addition to the well-known weak interaction that positions the myosin head at the active site of actin prior to force generation.

Temperature change does not affect force between regulated actin filaments and heavy meromyosin in single-molecule experiments

The temperature dependence of sliding velocity, force and the number of cross-bridges was studied on regulated actin filaments (reconstituted thin filaments) when they were placed on heavy meromyosin (HMM) attached to a glass surface. The regulated actin filaments were used because our previous study on muscle fibres demonstrated that the temperature effect was much reduced in the absence of regulatory proteins. A fluorescently labelled thin filament was attached to the gelsolin-coated surface of a polystyrene bead. The bead was trapped by optical tweezers, and HMM-thin filament interaction was performed at 20-35 degrees C to study the temperature dependence of force at the single-molecule level. Our experiments showed that there was a small increase in force with temperature (Q10 = 1.43) and sliding velocity (Q10 = 1.46). The small increase in force was correlated with the small increase in the number of cross-bridges (Q10 = 1.49), and when force was divided by the number of cross-bridges, the result did not depend on the temperature (Q(10) = 1.03). These results demonstrate that the force each cross-bridge generates is fixed and independent of temperature. Our additional experiments demonstrate that tropomyosin (Tm) in the presence of troponin (Tn) and Ca2+ enhances both force and velocity, and a truncated mutant, Delta23Tm, diminishes force and velocity. These results are consistent with the hypothesis that Tm in the presence of Tn and Ca2+ exerts a positive allosteric effect on actin to make actomyosin linkage more secure so that larger forces can be generated.

Endothermic force generation, temperature-jump experiments and effects of increased [MgADP] in rabbit psoas muscle fibres

We studied, by experiment and by kinetic modelling, the characteristics of the force increase on heating (endothermic force) in muscle. Experiments were done on maximally Ca2+-activated, permeabilized, single fibres (length approximately 2 mm; sarcomere length, 2.5 microm) from rabbit psoas muscle; [MgATP] was 4.6 mM, pH 7.1 and ionic strength was 200 mM. A small-amplitude (approximately 3 degrees C) rapid laser temperature-jump (0.2 ms T-jump) at 8-9 degrees C induced a tension rise to a new steady state and it consisted of two (fast and slow) exponential components. The T-jump-induced tension rise became slower as [MgADP] was increased, with half-maximal effect at 0.5 mM [MgADP]; the pre- and post-T-jump tension increased approximately 20% with 4 mM added [MgADP]. As determined by the tension change to small, rapid length steps (<1.4%L0 complete in <0.5 ms), the increase of force by [MgADP] was not associated with a concomitant increase of stiffness; the quick tension recovery after length steps (Huxley-Simmons phase 2) was slower with added MgADP. In steady-state experiments, the tension was larger at higher temperatures and the plot of tension versus reciprocal absolute temperature was sigmoidal, with a half-maximal tension at 10-12 degrees C; the relation with added 4 mM MgADP was shifted upwards on the tension axis and towards lower temperatures. The potentiation of tension with 4 mM added MgADP was 20-25% at low temperatures (approximately 5-10 degrees C), but approximately 10% at the physiological temperatures (approximately 30 degrees C). The shortening velocity was decreased with increased [MgADP] at low and high temperatures. The sigmoidal relation between tension and reciprocal temperature, and the basic effects of increased [MgADP] on endothermic force, can be qualitatively simulated using a five-step kinetic scheme for the crossbridge/A-MATPase cycle where the force generating conformational change occurs in a reversible step before the release of inorganic phosphate (P(i)), it is temperature sensitive (Q10 of approximately 4) and the release of MgADP occurs by a subsequent, slower, two-step mechanism. Modelling shows that the sigmoidal relation between force and reciprocal temperature arises from conversion of preforce-generating (A-M.ADP.P(i)) states to force-bearing (A-M.ADP) states as the temperature is raised. A tension response to a simulated T-jump consists of three (one fast and two slow) components, but, by combining the two slow components, they could be reduced to two; their relative amplitudes vary with temperature. The model can qualitatively simulate features of the tension responses induced by large-T-jumps from low starting temperatures, and those induced by small-T-jumps from different starting temperatures and, also, the interactive effects of P(i) and temperature on force in muscle fibres.

What do we learn by studying the temperature effect on isometric tension and tension transients in mammalian striated muscle fibres?

The significance of transient analysis of isometric tension and its temperature dependence on the molecular mechanisms of contraction is reviewed. The kinetic analysis of tension transient is essential to establish the elementary steps of the cross-bridge cycle. The temperature study is essential to deduce thermodynamic parameters of the force generation step, from which surface area changes associated with hydrophobic interaction and ionic interaction can be calculated. Experimental evidence suggests that a large scale hydrophobic and stereospecific interaction takes place at the time of force generation. This interaction is promoted by regulatory proteins troponin and tropomyosin, which is the basis for endothermic force generation. The six state cross-bridge model with two apparent rate constants is capable of explaining the temperature dependence of isometric tension and tension transients induced by temperature jump experiments. This model was previously proposed to account for the tension transients induced by sinusoidal length changes [Kawai and Halvorson (1991) Biophys J 59: 329-342]. The series compliance model is suitable for explaining the temperature effect on the stiffness data as the function of temperature, leading to the conclusion that the series compliance accounts for 40 +/- 5% of the total compliance in activated psoas fibres at 20 degrees C. These results are consistent with the hypothesis that tension per cross-bridge remains the same at different temperatures, and that it is the population shift that gives rise to the characteristic temperature effect on isometric tension.

Force generation induced by rapid temperature jumps in intact mammalian (rat) skeletal muscle fibres

We examined the tension (force) responses induced by rapid temperature jumps (T-jumps) in electrically stimulated, intact fibre bundles (5-10 fibres, fibre length approximately 2 mm) isolated from a foot muscle (flexor hallucis brevis) of the rat; the muscle contains approximately 90 % type 2 fast fibres. In steady state experiments, the temperature dependence of the twitch tension was basically similar to that previously described from other fast muscles; the tetanic tension increased 3- to 4-fold in raising the temperature from approximately 2 to 35 degrees C and the relation between the tetanic tension and the reciprocal absolute temperature was sigmoidal with half-maximal tension at 9.5 degrees C. A rapid T-jump of 3-5 degrees C was induced during a contraction by applying an infrared laser pulse (lambda = 1.32 micro, 0.2 ms) to the 50 microl trough containing the fibre bundle immersed in physiological saline. At approximately 10 degrees C, a T-jump induced a large transient tension rise when applied during the rising phase of a twitch contraction, the amplitude of which decreased when the T-jump was delayed with respect to the stimulus; a T-jump probably perturbs an early step in excitation-contraction coupling. No transient increase was seen when a T-jump was applied during twitch relaxation. When applied during the plateau of a tetanic contraction a T-jump induced a tension rise to a higher steady tension level; the tension rise after a T-jump was 2-3 times faster than the corresponding phase of the initial tension rise in a tetanus. The approach to a new steady tension level after a T-jump was biphasic with a fast (phase 2b, approximately 35 s-1 at 10 degrees C) and a slow component (phase 3, < 10 s-1). The rates of both components increased (Q10 approximately 3) but their amplitudes decreased with increase of the steady temperature. These results from tetanized intact fibres are consistent with the thesis previously proposed from studies on Ca2+-activated skinned fibres, that the elementary force generation step in muscle is enhanced by increased temperature; the findings indicate that an endothermic molecular step underlies muscle force generation.

An asymmetry in the phosphate dependence of tension transients induced by length perturbation in mammalian (rabbit psoas) muscle fibres

The effects of inorganic phosphate (P(i), a product released during ATP hydrolysis in active muscle) on tension transients induced by length perturbation (approximately 0.3 ms) were examined in chemically skinned (0.5 % Brij), maximally Ca(2+)-activated rabbit psoas muscle fibres at 10 degrees C (ionic strength 200 mM, pH 7.1). In one type of experiment, the tension transients induced by length release and stretch of a standard amplitude (0.4-0.5 % of L(o), muscle fibre length) were examined at a range of added [P(i)] (range 3-25 mM). The steady active tension was depressed approximately 45 % with 25 mM added P(i). The initial tension recovery (from T(1), extreme tension reached after length step, to T(2), tension after quick recovery) was analysed by half-time measurement and also by exponential curve fitting - extracting a fast (phase 2a) and a slow (phase 2b) component. The tension decay after a stretch became faster with increased [P(i)], whereas the quick tension rise induced by a length release was insensitive to added P(i). Consequently, the asymmetry in the speed of tension recovery from stretch and release was reduced at high [P(i)]. A plot of the phase 2b rate (or 1/half-time) of tension decay after stretch versus [P(i)] was approximately hyperbolic and showed saturation at higher [P(i)] levels. In a second type of experiment, the tension transients induced by length steps of different amplitudes were examined in control (no added P(i)) and in the presence of 25 mM added P(i). Over a range of length step amplitudes (up to 1 % L(0)), the tension decay after stretch was consistently faster in the presence of P(i) than in the control; this was particularly pronounced in phase 2b. The rate of tension rise after length release remained high but similar in the presence and absence of added P(i). These observations indicate that a stretch and release perturb different molecular steps in the crossbridge cycle. The P(i) sensitivity of tension decay (phase 2b) after stretch is similar to that seen using other perturbations (e.g. [P(i)] jumps, hydrostatic pressure jumps and temperature jumps and sinusoidal length oscillations). The results indicate that the P(i)-sensitive force generation identified in previous studies is strain sensitive (as expected), but it is seen only with respect to positive strain (stretches).

Temperature effect on isometric tension is mediated by regulatory proteins tropomyosin and troponin in bovine myocardium

The effect of temperature on isometric tension with and without the regulatory proteins tropomyosin and troponin was studied in bovine myocardium using a thin filament removal and reconstitution protocol. In control bovine myocardium, isometric tension increased linearly with temperature in the range 5-40 degrees C: isometric tension at 10 and 30 degrees C was 0.65 and 1.28 times that at 20 degrees C, respectively, with a Q10 of about 1.4. In actin filament-reconstituted myocardium without regulatory proteins, the temperature effect on isometric tension was less: isometric tension at 10 and 30 degrees C was 0.96 and 1.17 times that at 20 degrees C, respectively, with a Q10 of about 1.1. The temperature dependence of the apparent rate constants was studied using sinusoidal analysis. The temperature dependence of 2pi(b) (rate constant of delayed tension phase) did not vary significantly with the regulatory proteins under the standard activating condition (5 mM MgATP, 8 mM P(i), 200 mM ionic strength, pCa 4.66, pH 7.00). Q10 for 2pi(b) in control and actin filament-reconstituted myocardium was 3.8 and 4.0, respectively. There were two phases to the temperature dependence of 2pi(c) (rate constant of quick recovery). In control and thin filament-reconstituted myocardium, Q10 for 2pi(c) was approximately 5.5 in the low temperature range (< or = 25 degrees C) and 2.7 in the high temperature range (> or = 30 degrees C). In actin filament-reconstituted myocardium, Q10 for 2pi(c) was 8.5 in the low temperature range and 3.6 in the high temperature range. The above results demonstrate that regulatory proteins augment the temperature dependence of isometric tension, indicating that the regulatory proteins may modify the actomyosin interaction.

Effects of inorganic phosphate on endothermic force generation in muscle

Using a rapid (ca. 0.2 ms) laser temperature jump technique, the rate of endothermic force generation was examined in single-skinned (rabbit psoas) muscle fibres when they were exposed to different levels of inorganic phosphate (a product released during ATP hydrolysis in active muscle). The steady force is reduced by increased phosphate but the apparent rate constant of force generation induced by a standard temperature jump (from ca. 9 degrees C to ca. 12 degrees C) increases two- to threefold when the phosphate added is increased from zero to ca. 25 mM. The increase in the apparent rate constant also exhibits saturation at higher phosphate levels and the relation is hyperbolic. Detailed examination of the data, particularly in relation to our pressure release experiments, leads to a scheme for the molecular steps involved in phosphate release and force generation in active muscle fibres, where phosphate release from attached cross-bridges involves three reversible and sequentially faster molecular steps. Step one is a moderately slow, pre-force generation step that probably represents a transition of cross-bridges from non-specific to stereospecific attached states. Step two is moderately fast and represents endothermic cross-bridge force generation (temperature sensitive) and step three is a very rapid phosphate release. Such a scheme accommodates findings from a variety of different studies, including pressure perturbation experiments and other studies where the effect of phosphate on muscle force was studied.