Factors influencing the ascending limb of the sarcomere length-tension relationship in rabbit skinned muscle fibres

Myosin cross-bridge kinetics slow at longer muscle lengths during isometric contractions in intact soleus from mice

Muscle contraction results from force-generating cross-bridge interactions between myosin and actin. Cross-bridge cycling kinetics underlie fundamental contractile properties, such as active force production and energy utilization. Factors that influence cross-bridge kinetics at the molecular level propagate through the sarcomeres, cells and tissue to modulate whole-muscle function. Conversely, movement and changes in the muscle length can influence cross-bridge kinetics on the molecular level. Reduced, single-molecule and single-fibre experiments have shown that increasing the strain on cross-bridges may slow their cycling rate and prolong their attachment duration. However, whether these strain-dependent cycling mechanisms persist in the intact muscle tissue, which encompasses more complex organization and passive elements, remains unclear. To investigate this multi-scale relationship, we adapted traditional step-stretch protocols for use with mouse soleus muscle during isometric tetanic contractions, enabling novel estimates of length-dependent cross-bridge kinetics in the intact skeletal muscle. Compared to rates at the optimal muscle length (L_o), we found that cross-bridge detachment rates increased by approximately 20% at 90% of L_o (shorter) and decreased by approximately 20% at 110% of L_o (longer). These data indicate that cross-bridge kinetics vary with whole-muscle length during intact, isometric contraction, which could intrinsically modulate force generation and energetics, and suggests a multi-scale feedback pathway between whole-muscle function and cross-bridge activity.

The spatial distribution of thin filament activation influences force development and myosin activity in computational models of muscle contraction

Striated muscle contraction is initiated by Ca²⁺ binding to, and activating, thin filament regulatory units (RU) within the sarcomere, which then allows myosin cross-bridges from the opposing thick filament to bind actin and generate force. The amount of overlap between the filaments dictates how many potential cross-bridges are capable of binding, and thus how force is generated by the sarcomere. Myopathies and atrophy can impair muscle function by limiting cross-bridge interactions between the filaments, which can occur when the length of the thin filament is reduced or when RU function is disrupted. To investigate how variations in thin filament length and RU density affect ensemble cross-bridge behavior and force production, we simulated muscle contraction using a spatially explicit computational model of the half-sarcomere. Thin filament RUs were disabled either uniformly from the pointed end of the filament (to model shorter thin filament length) or randomly throughout the length of the half-sarcomere. Both uniform and random RU 'knockout' schemes decreased overall force generation during maximal and submaximal activation. The random knockout scheme also led to decreased calcium sensitivity and cooperativity of the force-pCa relationship. We also found that the rate of force development slowed with the random RU knockout, compared to the uniform RU knockout or conditions of normal RU activation. These findings imply that the relationship between RU density and force production within the sarcomere involves more complex coordination than simply the raw number of RUs available for myosin cross-bridge binding, and that the spatial pattern in which activatable RU are distributed throughout the sarcomere influences the dynamics of force production.

Myosin filament sliding through the Z-disc relates striated muscle fibre structure to function

Striated muscle contraction requires intricate interactions of microstructures. The classic textbook assumption that myosin filaments are compressed at the meshed Z-disc during striated muscle fibre contraction conflicts with experimental evidence. For example, myosin filaments are too stiff to be compressed sufficiently by the muscular force, and, unlike compressed springs, the muscle fibres do not restore their resting length after contractions to short lengths. Further, the dependence of a fibre's maximum contraction velocity on sarcomere length is unexplained to date. In this paper, we present a structurally consistent model of sarcomere contraction that reconciles these findings with the well-accepted sliding filament and crossbridge theories. The few required model parameters are taken from the literature or obtained from reasoning based on structural arguments. In our model, the transition from hexagonal to tetragonal actin filament arrangement near the Z-disc together with a thoughtful titin arrangement enables myosin filament sliding through the Z-disc. This sliding leads to swivelled crossbridges in the adjacent half-sarcomere that dampen contraction. With no fitting of parameters required, the model predicts straightforwardly the fibre's entire force-length behaviour and the dependence of the maximum contraction velocity on sarcomere length. Our model enables a structurally and functionally consistent view of the contractile machinery of the striated fibre with possible implications for muscle diseases and evolution.

Length-dependent Ca2+ activation in skeletal muscle fibers from mammalians

We tested the hypotheses that 1) a decrease in activation of skeletal muscles at short sarcomere lengths (SLs) is caused by an inhibition of Ca(2+) release from the sarcoplasmic reticulum (SR), and 2) the decrease in Ca(2+) would be caused by an inhibition of action potential conduction from the periphery to the core of the fibers. Intact, single fibers dissected from the flexor digitorum brevis from mice were activated at different SLs, and intracellular Ca(2+) was imaged with confocal microscopy. Force decreased at SLs shorter than 2.1 μm, while Ca(2+) concentration decreased at SLs below 1.9 μm. The concentration of Ca(2+) at short SL was lower at the core than at the peripheries of the fiber. When the external concentration of Na(+) was decreased in the experimental media, impairing action potential conduction, Ca(2+) gradients were observed in all SLs. When caffeine was used in the experimental media, the gradients of Ca(2+) were abolished. We concluded that there is an inhibition of Ca(2+) release from the sarcoplasmic reticulum (SR) at short SLs, which results from a decreased conduction of action potential from the periphery to the core of the fibers.

Titin stiffness modifies the force-generating region of muscle sarcomeres

The contractile units of striated muscle, the sarcomeres, comprise the thick (myosin) and thin (actin) filaments mediating active contraction and the titin filaments determining "passive" elasticity. We hypothesized that titin may be more active in muscle contraction by directly modulating thick-filament properties. We used single-myofibril mechanical measurements and atomic force microscopy of individual sarcomeres to quantify the effects of sarcomere strain and titin spring length on both the inter-filament lattice spacing and the lateral stiffness of the actin-myosin overlap zone (A-band). We found that strain reduced the lattice spacing similarly in sarcomeres with stiff (rabbit psoas) or compliant titin (rabbit diaphragm), but increased A-band lateral stiffness much more in psoas than in diaphragm. The strain-induced alterations in A-band stiffness that occur independently of lattice spacing effects may be due to titin stiffness-sensing by A-band proteins. This mechanosensitivity could play a role in the physiologically important phenomenon of length-dependent activation of striated muscle.

Length dependence of striated muscle force generation is controlled by phosphorylation of cTnI at serines 23/24

  According to the Frank-Starling relationship, greater end-diastolic volume increases ventricular output. The Frank-Starling relationship is based, in part, on the length-tension relationship in cardiac myocytes. Recently, we identified a dichotomy in the steepness of length-tension relationships in mammalian cardiac myocytes that was dependent upon protein kinase A (PKA)-induced myofibrillar phosphorylation. Because PKA has multiple myofibrillar substrates including titin, myosin-binding protein-C and cardiac troponin I (cTnI), we sought to define if phosphorylation of one of these molecules could control length-tension relationships. We focused on cTnI as troponin can be exchanged in permeabilized striated muscle cell preparations, and tested the hypothesis that phosphorylation of cTnI modulates length dependence of force generation. For these experiments, we exchanged unphosphorylated recombinant cTn into either a rat cardiac myocyte preparation or a skinned slow-twitch skeletal muscle fibre. In all cases unphosphorylated cTn yielded a shallow length-tension relationship, which was shifted to a steep relationship after PKA treatment. Furthermore, exchange with cTn having cTnI serines 23/24 mutated to aspartic acids to mimic phosphorylation always shifted a shallow length-tension relationship to a steep relationship. Overall, these results indicate that phosphorylation of cTnI serines 23/24 is a key regulator of length dependence of force generation in striated muscle.

Catchlike property in human adductor pollicis muscle

The "catchlike" property is defined as the dramatic force increase in skeletal muscles when a single pulse is added at the onset of a sub-tetanic low-frequency stimulation train. This property has been observed in single motor units, whole animal and human muscles. It is an inherent property of muscle fibres and is not related to an increase in motor unit recruitment. Despite an abundance of observations, its origin remains unclear. The aim of this study was to induce the catchlike property in human adductor pollicis and identify its possible origin. Thumb adduction forces were measured using ulnar nerve electrical stimulation at 10Hz for reference trains (RTs) with one extra pulse 8ms after the first stimulation pulse for the experimental trains (ETs). Tests were performed at two muscle length and three stimulation levels and muscle stiffness and potentiation were quantified for all test conditions. The ETs showed higher forces and greater rates of force increase than the RTs. In addition, force increase was more pronounced at short compared to long muscle length, but no differences were found in force increase for the three stimulation levels. Furthermore, potentiation and stiffness were similar across all experimental conditions. Together, these results suggest that the increase in force associated with the catchlike property is neither caused by an increased proportion of attached cross-bridges nor potentiation of the muscle, but appears to be muscle length dependent and present in both slow and fast motor units.

Sarcomere length-dependent Ca2+ activation in skinned rabbit psoas muscle fibers: coordinated regulation of thin filament cooperative activation and passive force

In skeletal muscle, active force production varies as a function of sarcomere length (SL). It has been considered that this SL dependence results simply from a change in the overlap length between the thick and thin filaments. The purpose of this study was to provide a systematic understanding of the SL-dependent increase in Ca²⁺ sensitivity in skeletal muscle, by investigating how thin filament “on–off” switching and passive force are involved in the regulation. Rabbit psoas muscles were skinned, and active force measurements were taken at various Ca²⁺ concentrations with single fibers, in the short (2.0 and 2.4 μm) and long (2.4 and 2.8 μm) SL ranges. Despite the same magnitude of SL elongation, the SL-dependent increase in Ca²⁺ sensitivity was more pronounced in the long SL range. MgADP (3 mM) increased the rate of rise of active force and attenuated SL-dependent Ca²⁺ activation in both SL ranges. Conversely, inorganic phosphate (Pi, 20 mM) decreased the rate of rise of active force and enhanced SL-dependent Ca²⁺ activation in both SL ranges. Our analyses revealed that, in the absence and presence of MgADP or Pi, the magnitude of SL-dependent Ca²⁺ activation was (1) inversely correlated with the rate of rise of active force, and (2) in proportion to passive force. These findings suggest that the SL dependence of active force in skeletal muscle is regulated via thin filament “on–off” switching and titin (connectin)-based interfilament lattice spacing modulation in a coordinated fashion, in addition to the regulation via the filament overlap.

Length dependence of force generation exhibit similarities between rat cardiac myocytes and skeletal muscle fibres

According to the Frank-Starling relationship, increased ventricular volume increases cardiac output, which helps match cardiac output to peripheral circulatory demand. The cellular basis for this relationship is in large part the myofilament length-tension relationship. Length-tension relationships in maximally calcium activated preparations are relatively shallow and similar between cardiac myocytes and skeletal muscle fibres. During twitch activations length-tension relationships become steeper in both cardiac and skeletal muscle; however, it remains unclear whether length dependence of tension differs between striated muscle cell types during submaximal activations. The purpose of this study was to compare sarcomere length-tension relationships and the sarcomere length dependence of force development between rat skinned left ventricular cardiac myocytes and fast-twitch and slow-twitch skeletal muscle fibres. Muscle cell preparations were calcium activated to yield 50% maximal force, after which isometric force and rate constants (k(tr)) of force development were measured over a range of sarcomere lengths. Myofilament length-tension relationships were considerably steeper in fast-twitch fibres compared to slow-twitch fibres. Interestingly, cardiac myocyte preparations exhibited two populations of length-tension relationships, one steeper than fast-twitch fibres and the other similar to slow-twitch fibres. Moreover, myocytes with shallow length-tension relationships were converted to steeper length-tension relationships by protein kinase A (PKA)-induced myofilament phosphorylation. Sarcomere length-k(tr) relationships were distinct between all three cell types and exhibited patterns markedly different from Ca(2+) activation-dependent k(tr) relationships. Overall, these findings indicate cardiac myocytes exhibit varied length-tension relationships and sarcomere length appears a dominant modulator of force development rates. Importantly, cardiac myocyte length-tension relationships appear able to switch between slow-twitch-like and fast-twitch-like by PKA-mediated myofibrillar phosphorylation, which implicates a novel means for controlling Frank-Starling relationships.

Impact of osmotic compression on sarcomere structure and myofilament calcium sensitivity of isolated rat myocardium

Changes in interfilament lattice spacing have been proposed as the mechanism underlying myofilament length-dependent activation. Much of the evidence to support this theory has come from experiments in which high-molecular-weight compounds, such as dextran, were used to osmotically shrink the myofilament lattice. However, whether interfilament spacing directly affects myofilament calcium sensitivity (EC50) has not been established. In this study, skinned isolated rat myocardium was osmotically compressed over a wide range (Dextran T500; 0–6%), and EC50 was correlated to both interfilament spacing and I1,1/ I1,0 intensity ratio. The latter two parameters were determined by X-ray diffraction in a separate group of skinned muscles. Osmotic compression induced a marked reduction in myofilament lattice spacing, concomitant with increases in both EC50 and I1,1/ I1,0 intensity ratio. However, interfilament spacing was not well correlated with EC50 ( r2 = 0.78). A much better and deterministic relationship was observed between EC50 and the I1,1/ I1,0 intensity ratio ( r2 = 0.99), albeit with a marked discontinuity at low levels of dextran compression; that is, a small amount of external osmotic compression (0.38 kPa, corresponding to 1% Dextran T500) produced a stepwise increase in the I1,1/ I1,0 ratio concomitant with a stepwise decrease in EC50. These parameters then remained stable over a wide range of further applied osmotic compression (up to 6% dextran). These findings provide support for a “switch-like” activation mechanism within the cardiac sarcomere that is highly sensitive to changes in external osmotic pressure.

Relationship between isometric force and myofibrillar MgATPase at short sarcomere length in skeletal and cardiac muscle and its relevance to the concept of activation heat

1. This paper has been written in recognition of the seminal contributions to cardiac and skeletal muscle energetics made by Professor Colin Gibbs during his distinguished academic career. 2. The paper focuses on what is now known about the relationship between Ca2+-activated isometric force production and myofibrillar MgATPase in intact and skinned (surface membrane rendered permeable) skeletal and cardiac muscle preparations at short sarcomere lengths. 3. The relevance of this relationship to understanding the interactions between the actin and myosin filaments at the cross-bridge level in the region of double actin filament overlap and the cellular basis of 'activation heat' measurements in intact striated muscles is discussed.

Length-dependent activation in three striated muscle types of the rat

The process whereby sarcomere length modulates the sensitivity of the myofilaments to Ca(2+) is termed length-dependent activation. Length-dependent activation is a property of all striated muscles, yet the relative extent of length-dependent activation between skeletal muscle and cardiac muscle is unclear. Although length-dependent activation may be greater in fast skeletal muscle (FSM) than in slow skeletal muscle (SSM), there has not been a well controlled comparison of length-dependent activation between skeletal muscle and cardiac muscle (CM). Accordingly, we measured sarcomere length-dependent properties in skinned soleus (SSM), psoas (FSM) and ventricular trabeculae (CM) of the rat under carefully controlled conditions. The free Ca(2+)-force relationship was determined at sarcomere lengths (SL) of 1.95 microm, 2.10 microm and 2.25 microm and fitted to a modified Hill equation. FSM and SSM were more sensitive to Ca(2+) than CM. Length-dependent activation was ordered as CM > FSM > SSM. Cooperativity as measured by the Hill coefficient of the Ca(2+)-force relationship was not significantly different between CM and FSM, both of which exhibited greater cooperativity than SSM. SL did not significantly alter this parameter in each muscle type. To establish whether the observed differences can be explained by alterations in interfilament spacing, we measured myofilament lattice spacing (LS) by synchrotron X-ray diffraction in relaxed, skinned muscle preparations. LS was inversely proportional to SL for each muscle type. The slope of the SL-LS relationship, however, was not significantly different between striated muscle types. We conclude that (1) length-dependent activation differs among the three types of striated muscle and (2) these differences in the length-dependent properties among the striated muscle types may not solely be explained by the differences in the response of interfilament spacing to changes in muscle length in relaxed, skinned isolated muscle preparations.

Effects of guandinoethane sulfonate on contraction of skeletal muscle

Guanidinoethane sulfonic acid (GES), a chemical and biological analog of taurine, decreases rat muscle taurine content when added to drinking water. Over the same period, GES appears in muscle. GES supplementation is often used to study the effect of taurine depletion on physiological mechanisms, without taking into account the possible actions of GES. The purpose of the present study was to investigate the specific actions of GES on contraction of skeletal muscle. In mice EDL muscle, the time delay needed to observe a 20% force decrease after the end of a tetanic stimulation was higher in GES-supplemented than in control muscle. This observation in GES-supplemented muscle could be explained by the action of taurine or GES on several targets, beside others the rate of Ca2+ uptake by sarcoplasmic reticulum (SR) and the Ca2+ sensitivity of myofilaments. SR of rat EDL was isolated by successive centrifugations. The effect of 20 mM taurine or GES on the rate of Ca2+ uptake by SR was measured with the fluorescent Ca2+ indicator fura-2. The results show that the rate of Ca2+ uptake by SR is not modified in the presence of taurine or GES. The Ca2+ sensitivity of myofilaments was studied in chemically skinned fibers in the presence of 20 mM taurine or GES. Both taurine and GES increased the myofilament sensitivity to Ca2+. Thus, the prolonged relaxation time of GES-supplemented muscle can be attributed to an increase in myofilament sensitivity to Ca2+. This higher sensitivity is not due to a decrease in muscle taurine content but rather to an increased GES concentration.

Myofilament calcium sensitivity in skinned rat cardiac trabeculae: role of interfilament spacing

The increase in myofilament Ca(2+) responsiveness on an increase in sarcomere length (SL) is, in part, the cellular basis for Frank-Starling's law of the heart. It has been suggested that a decrease in myofilament lattice spacing (LS) in response to an increase in SL underlies this phenomenon. This hypothesis is supported by previous studies in which reduced muscle width induced by osmotic compression was associated with an increase in Ca(2+) sensitivity, mimicking those changes observed with an increase in SL. To evaluate this hypothesis, we directly measured LS by synchrotron x-ray diffraction as function of SL in skinned rat cardiac trabeculae bathed in 0% to 6% dextran solutions (MW 413 000). We found that EC(50), [Ca(2+)] at which force is half-maximal, at SL between 1.95 and 2.25 microm did not vary in proportion to LS when 3% or 6% dextran solutions were applied. We also found that moderate compression (1% dextran) of skinned trabeculae at SL=2.02 microm reduced LS (LS=42.29+/-0.14 nm) to match that of uncompressed fibers at a long SL (SL=2.19 microm; LS=42.28+/-0.15 nm). Whereas increasing SL from 2.02 to 2.19 microm significantly increased Ca(2+) sensitivity as indexed by the EC(50) parameter (2.87+/-0.11 micromol/L to 2.52+/-0.12 micromol/L), similar reduction in myofilament lattice spacing achieved by compression with 1% dextran did not alter Ca(2+) sensitivity (2.87+/-0.10 micromol/L) at the short SL. We conclude that alterations in myofilament lattice spacing may not be the mechanism that underlies the sarcomere length-induced alteration of calcium sensitivity in skinned myocardium.

Strong binding of myosin increases shortening velocity of rabbit skinned skeletal muscle fibres at low levels of Ca(2+)

1. At low levels of activation, unloaded shortening of skinned skeletal muscle fibres takes place in two phases: an initial phase of high-velocity shortening followed by a phase of low-velocity shortening. The basis for Ca(2+) dependence of unloaded shortening velocity (V(o)) in the low-velocity phase was investigated by varying the level of thin filament activation with Ca(2+) and N-ethyl-maleimide myosin subfragment-1 (NEM-S1), a non-tension-generating, strong binding derivative of subfragment-1. V(o) was measured with the slack-test method. 2. Treatment of skinned fibres with 5 microM NEM-S1 eliminated the low-velocity phase of shortening but had no effect on the high-velocity phase of shortening during submaximal activation with Ca(2+), or on V(o) during maximal activation with Ca(2+). 3. Extensive washout of NEM-S1 from the treated fibres restored the low-velocity phase of shortening and returned low-velocity V(o) to pre-treatment values. 4. The effect of NEM-S1 to increase low-velocity V(o) can be explained in terms of a model in which strong binding myosin cross-bridges activate the thin filament to a state in which the rate of ADP release from the actin-myosin-ADP complex and the rate of cross-bridge detachment from actin are accelerated during unloaded shortening.

Length-dependent effects of osmotic compression on skinned rabbit psoas muscle fibers

The goal of this study was to characterize the interrelationship between sarcomere length and interfilament spacing in the control of Ca2+ sensitivity in skinned rabbit psoas muscle fibers. Measurements were made at sarcomere lengths 2.0, 2.7 and 3.4 microm. At 2.7 microm the fiber width was reduced by 17% relative to that at 2.0 microm and the pCa50 for force development was increased by approximately 0.3 pCa units. In the presence of 5% Dextran T-500 the fiber width at sarcomere length 2.0 microm was also decreased by 17% and the Ca2+ sensitivity was increased to the same value as at 2.7 microm. In contrast, at sarcomere length 2.7 microm the addition of as much as 10% Dextran T-500 had no effect on Ca2+ sensitivity. At sarcomere length 3.4 microm there was an additional 7% compression and the Ca2+ sensitivity was increased slightly (approximately 0.1 pCa units) relative to that at 2.7 microm. However at 3.4 microm the addition of 5% Dextran T-500 caused the Ca2+ sensitivity to decrease to the level seen at 2.0 microm. Given that the skinning process causes a swelling of the filament lattice it is evident that the relationship between sarcomere length and Ca2+ sensitivity observed in skinned fibers may not always be applicable to intact fibers. These data are consistent with measurements of Ca2+ in intact fibers which indicate that there might be a decline in Ca2+ sensitivity at long sarcomere lengths.

Effects of MgADP on length dependence of tension generation in skinned rat cardiac muscle

The effect of MgADP on the sarcomere length (SL) dependence of tension generation was investigated using skinned rat ventricular trabeculae. Increasing SL from 1.9 to 2.3 microm decreased the muscle width by approximately 11% and shifted the midpoint of the pCa-tension relationship (pCa(50)) leftward by about 0.2 pCa units. MgADP (0.1, 1, and 5 mmol/L) augmented maximal and submaximal Ca(2+)-activated tension and concomitantly diminished the SL-dependent shift of pCa(50) in a concentration-dependent manner. In contrast, pimobendan, a Ca(2+) sensitizer, which promotes Ca(2+) binding to troponin C (TnC), exhibited no effect on the SL-dependent shift of pCa(50), suggesting that TnC does not participate in the modulation of SL-dependent tension generation by MgADP. At a SL of 1. 9 microm, osmotic compression, produced by 5% wt/vol dextran (molecular weight approximately 464 000), reduced the muscle width by approximately 13% and shifted pCa(50) leftward to a similar degree as that observed when increasing SL to 2.3 microm. This favors the idea that a decrease in the interfilament lattice spacing is the primary mechanism for SL-dependent tension generation. MgADP (5 mmol/L) markedly attenuated the dextran-induced shift of pCa(50), and the degree of attenuation was similar to that observed in a study of varying SL. The actomyosin-ADP complex (AM.ADP) induced by exogenous MgADP has been reported to cooperatively promote myosin attachment to the thin filament. We hereby conclude that the increase in the number of force-generating crossbridges on a decrease in the lattice spacing is masked by the cooperative effect of AM.ADP, resulting in depressed SL-dependent tension generation.

Relaxation of diaphragm muscle

Relaxation is the process by which, after contraction, the muscle actively returns to its initial conditions of length and load. In rhythmically active muscles such as diaphragm, relaxation is of physiological importance because diaphragm must return to a relatively constant resting position at the end of each contraction-relaxation cycle. Rapid and complete relaxation of the diaphragm is likely to play an important role in adaptation to changes in respiratory load and breathing frequency. Regulation of diaphragm relaxation at the molecular and cellular levels involves Ca(2+) removal from the myofilaments, active Ca(2+) pumping by the sarcoplasmic reticulum (SR), and decrease in the number of working cross bridges. The relative contribution of these mechanisms mainly depends on sarcomere length, muscle tension, and the intrinsic contractile function. Increased capacity of SR to take up Ca(2+) can arise from increased density of active SR pumping sites or in slow-twitch fibers from phosphorylation of phospholamban, whereas impaired coupling between ATP hydrolysis and Ca(2+) transport into the SR or intracellular acidosis reduces SR Ca(2+) pump activity. In experimental conditions of decreased contractile performance, slowed, enhanced, or unchanged relaxation rates have been reported in vitro. In vivo, a slowing in the rate of decline of the respiratory pressure is generally considered an early reliable index of respiratory muscle fatigue. Impaired relaxation rate may, in turn, favor mismatch between blood flow and metabolic demand, especially at high breathing frequencies.

The effects of dantrolene on the contraction, relaxation, and energetics of the diaphragm muscle

Dantrolene is used in patients with muscle spasticity and is the only known effective treatment for malignant hyperthermia. However, its effects on muscle relaxation and energetics are unknown and may have important consequences in diaphragmatic function. We studied the effects of dantrolene (10(-8) to 10(-4) M) on diaphragm muscle strips (n = 12) in the hamster in vitro (Krebs-Henseleit solution, 29 degrees C, 95% oxygen/5% carbon dioxide) in response to tetanic stimulation (50 Hz). We studied contraction and relaxation under isotonic and isometric conditions, as well as energetics. Data are mean +/- SD. Dantrolene induced a negative inotropic effect in the hamster diaphragm (active force at 10(-4) M: 34% +/- 7% of baseline; P < 0.05) but did not significantly modify the curvature of the force-velocity relationship. Dantrolene did not significantly modify isotonic relaxation. Dantrolene, up to 10(-5) M, did not significantly impair isometric relaxation. In conclusion, dantrolene induced a marked negative inotropic effect on diaphragm muscle without affecting myothermal efficiency and relaxation.

IMPLICATIONS

Dantrolene induced a significant and concentration-dependent negative inotropic effect on diaphragm muscle but did not modify isotonic relaxation, which suggests no alteration of the calcium reuptake by the sarcoplasmic reticulum; up to 10(-5) M dantrolene did not alter isometric relaxation, i.e., myofilament calcium sensitivity. Dantrolene did not modify the energetics of diaphragm muscle.

Comparison of putative cooperative mechanisms in cardiac muscle: length dependence and dynamic responses

Length-dependent steady-state and dynamic responses of five models of isometric force generation in cardiac myofilaments were compared with similar experimental data from the literature. The models were constructed by assuming different subsets of three putative cooperative mechanisms. Cooperative mechanism 1 holds that cross-bridge binding increases the affinity of troponin for Ca2+. In the models, cooperative mechanism 1 can produce steep force-Ca2+ (F-Ca) relations, but apparent cooperativity is highest at midlevel Ca2+ concentrations. During twitches, cooperative mechanism 1 has the effect of increasing latency to peak as the magnitude of force increases, an effect not seen experimentally. Cooperative mechanism 2 holds that the binding of a cross bridge increases the rate of formation of neighboring cross bridges and that multiple cross bridges can maintain activation of the thin filament in the absence of Ca2+. Only cooperative mechanism 2 can produce sarcomere length (SL)-dependent prolongation of twitches, but this mechanism has little effect on steady-state F-Ca relations. Cooperativity mechanism 3 is designed to simulate end-to-end interactions between adjacent troponin and tropomyosin. This mechanism can produce steep F-Ca relations with appropriate SL-dependent changes in Ca2+ sensitivity. With the assumption that tropomyosin shifting is faster than cross-bridge cycling, cooperative mechanism 3 produces twitches where latency to peak is independent of the magnitude of force, as seen experimentally.

Changes in interfilament spacing mimic the effects of myosin regulatory light chain phosphorylation in rabbit psoas fibers

The modulatory effect of myosin regulatory light chain phosphorylation in mammalian skeletal muscle, first documented as posttetanic potentiation of twitch tension, was subsequently shown to enhance the expression and development of tension at submaximal levels of activating calcium. Structural analyses demonstrated that thick filaments with phosphorylated myosin regulatory light chains appeared disordered: they lost the near-helical, periodic arrangement of myosin head characteristic of the relaxed state. We suggested that disordered heads may be more mobile than ordered heads and are likely to spend more time close to their binding sites on thin filaments. In this study we determined that the physiological effects of phosphorylation could be mimicked by decreasing the lattice spacing between the thick and the thin filaments, either by osmotic compression with dextran or by increasing the sarcomere length of permeabilized rabbit psoas fibers. Phosphorylation of regulatory light chains by incubation of permeabilized fibers with myosin light chain kinase and calmodulin, followed by low levels of activating calcium, potentiated tension development at resting or lower sarcomere lengths in the absence of dextran but had no additional effect on tension potentiation or development in fibers with decreased lattice spacing due to either osmotic compression or increased sarcomere length.

Effects of myofibrillar bundle diameter on the unloaded shortening velocity of skinned skeletal muscle fibres

Using both slack tests and force clamp experiments, the velocity of unloaded shortening (Vu; Vu(st), slack test; Vu(fc), force clamp) was determined for maximally Ca(2+)-activated myofibrillar bundles. These were obtained by mechanically splitting single muscle fibres of rat, rabbit, crab and lobster skeletal muscles. A comparison was made between the Vu of thick (mammalian: 45-70 microns mean diameter; crustacean: 90-175 microns) and thin (mammalian: 25-40 microns; crustacean: 35-85 microns) preparations of the same muscle fibre. The bundle diameter had opposite effects on Vu in mammalian and crustacean muscle fibres. The Vu of thin mammalian bundles was about 0.6 times that of the thick ones, whereas in crustacean preparations this ratio was about 1.5. The kinetics of stretch-induced delayed force increase of maximally Ca(2+)-activated fibres (stretch activation) appeared not to differ between the thick and thin bundles from any animal preparation. Control experiments showed that the observed diameter effects on Vu are not due to differences in the chemical environment of the myofilaments. One possible explanation is that the intrinsic physical factors of the myofibrils modify Vu differently during progressive shortening in mammalian and crustacean preparations.

Sarcomere length dependence of the rate of tension redevelopment and submaximal tension in rat and rabbit skinned skeletal muscle fibres

1. We examined the hypothesis that in skeletal muscle the steep relationship between twitch tension and sarcomere length (SL) within the range 2.30 to 1.85 microns involves SL-dependent alterations in the rate of tension development. 2. In skinned preparations of both rat slow-twitch and rabbit fast-twitch skeletal muscle fibres the rate of tension redevelopment (ktr) at 15 degrees C was reduced at short SL (approximately 2.00 microns) compared with a longer SL (approximately 2.30 microns). In submaximally activated fibres, the decrease in ktr over this range of lengths was greater in fast-twitch fibres (38% reduction) than in slow-twitch fibres (14% reduction). 3. Ca2+ sensitivity of tension, as assessed as the pCa (-log[Ca2+]) for half-maximal activation, or pCa50, decreased to a greater extent in rabbit fast-twitch skeletal muscle fibres than in slow-twitch fibres from both rabbit and rat when SL was reduced from approximately 2.30 to approximately 1.85 microns. The delta pCa50 over this SL range was 0.24 +/- 0.07 pCa units in fast-twitch fibres from rabbit psoas muscle. The delta pCa50 for slow-twitch fibres from rabbit and rat soleus muscle was 0.08 +/- 0.02 and 0.10 +/- 0.04 pCa units, respectively. 4. Osmotic compression of both slow-twitch and fast-twitch fibres at a SL of 2.00 microns increased ktr to values similar to those obtained at a SL of 2.30 microns in the absence of dextran. This result indicates that the slower rate of tension redevelopment at short SL is due in large part to the increase in interfilament lattice spacing associated with shorter SL. 5. Taken together, these results suggest that length dependence of twitch tension is, in part, due to length dependence of isometric cross-bridge interaction kinetics, an effect that is mediated by length-dependent changes in interfilament lattice spacing.

Ca2+ binding to troponin C in skinned skeletal muscle fibers assessed with caged Ca2+ and a Ca2+ fluorophore. Invariance of Ca2+ binding as a function of sarcomere length

Ca2+ sensitivity of tension varies with sarcomere length in both skeletal and cardiac muscles. One possible explanation for this effect is that the Ca2+ affinity of the regulatory protein troponin C decreases when sarcomere length is reduced. To examine length dependence of Ca2+ binding to troponin C in skeletal muscle, we developed a protocol to simultaneously monitor changes in sarcomere length, tension, and Ca2+ concentration following flash photolysis of caged Ca2+. In this protocol, [Ca2+] was rapidly increased by flash photolysis of caged Ca2+, and changes in [Ca2+] due to photolysis and the subsequent binding to troponin C were assessed using a Ca2+ fluorophore. Small bundles of fibers from rabbit skinned psoas muscles were loaded with Ca2+ fluorophore (Fluo-3) and caged Ca2+ (dimethoxynitrophenamine or o-nitrophenyl-EGTA). The bundles were then transferred to silicone oil, where [Ca2+]free, tension, and sarcomere length were monitored before and after photolysis of caged Ca2+. Upon photolysis of caged Ca2+, fluorescence increased and then decayed to a new steady-state level within approximately 1 s, while tension increased to a new steady-state level within approximately 1.5 s. After extracting troponin C, fibers did not generate tension following the flash, but steady-state post-flash fluorescence was significantly greater than when troponin C was present. The difference in [Ca2+]free represents the amount of Ca2+ bound to troponin C. In fibers that were troponin C-replete, Ca2+ binding to troponin C did not differ at short (approximately 1.97 microm) and long (approximately 2.51 microm) sarcomere length, yet tension was approximately 50% greater at the long sarcomere length. These results show that the affinity of troponin C for Ca2+ is not altered by changes in sarcomere length, indicating that length-dependent changes in Ca2+ sensitivity of tension in skeletal muscle are not related to length-dependent changes in Ca2+ binding affinity of troponin C.

Mechanics of feline soleus: II. Design and validation of a mathematical model

We have developed a mathematical model to describe force production in cat soleus during steady-state activation over a range of fascicle lengths and velocities. The model was based primarily upon a three element design by Zajac but also considered the many different features present in other previously described models. We compared quantitatively the usefulness of these features and putative relationships to account for a set of force and length data from cat soleus wholemuscle described in a companion paper. Among the novel features that proved useful were the inclusion of a short-length passive force resisting compression, a new normalisation constant for connective-tissue lengths to replace the potentially troublesome slack length, and a new length dependent term for lengthening velocities in the force-velocity relationship. Each feature of this model was chosen to provide the most accurate description of the data possible without adding unneeded complexity. Previously described functions were compared with novel functions to determine the best description of the experimental data for each of the elements in the model.

Regional involvement of an endothelium-derived contractile factor in the vasoactive actions of neuropeptide Y in bovine isolated retinal arteries

1. In vitro experiments in a microvascular myograph were designed in order to investigate the effects of human neuropeptide Y (NPY), its receptor subtype and the mechanisms underlying NPY actions in bovine isolated retinal proximal (PRA) and distal (DRA) arteries. 2. A single concentration of NPY (10 nM) induced a prompt and reproducible contraction which reached a plateau within 1-4 min, after which the response returned to baseline over the next 2-10 min. Cumulative addition of NPY induced concentration-dependent contractions of bovine retinal arteries, with an EC50[M] of 1.7 nM and a maximal response equal to 54 +/- 8% of Emax (absolute maximal contractile levels of vessels) and not different from that obtained by a single addition of the peptide. There were no significant differences in either sensitivity or maximal response to NPY between PRA and DRA. 3. Porcine NPY and the selective Y1-receptor agonist, [Pro34]NPY, also induced concentration-dependent contractions of the retinal arteries with a potency and maximal response not significantly different from those of human NPY; in contrast, the selective Y2-receptor agonist, NPY(13-36), caused only a 5% contraction at the highest concentration used. 4. Removal of extracellular Ca2+ or pretreatment with the 1,4-dihydropyridine Ca(2+)-channel blocker, nifedipine (1 microM), reduced the contractile response of 10 nM NPY to 18.4 +/- 3.3% (n = 6) and 18.6 +/- 3.9% (n = 6); respectively, of the controls. 5. Mechanical removal of the endothelium depressed the maximal contraction elicited by NPY in PRA but did not affect either sensitivity or maximal response to the peptide in DRA. In endothelium-intact arteries, blockade of the cyclo-oxygenase pathway with 3 microM indomethacin increased resting tension in both PRA and DRA and significantly inhibited sensitivity and maximal contraction to NPY of PRA and DRA, respectively. The thromboxane A2 (TXA2)/prostaglandin H2 (PGH2) receptor antagonist, SQ30741, reduced both sensitivity and maximal contraction to NPY in PRA but not in DRA. 6. In endothelium-denuded PRA, indomethacin but not SQ30741 significantly reduced NPY maximal response and induced a marked increase in resting tension suggesting a basal release of a vasodilator prostanoid from smooth muscle cells. 7. Superoxide dismutase (SOD) (150 u ml-1) reduced the maximal contraction to NPY in PRA. Inhibition of the nitric oxide (NO) synthase with NG-nitro-L-arginine (L-NOARG) (30 microM), enhanced sensitivity and maximal contraction to NPY in both PRA and DRA. In the presence of L-NOARG, SOD did not further inhibit NPY responses in PRA. 8. NPY (10 nM) induced a 2.9 fold leftwards shift of the noradrenaline concentration-response curves in PRA and increased maximal response by 50 +/- 16%. Neither 1 nor 10 nM NPY affected noradrenaline responses in DRA. [Pro34]NPY (10 nM), but not NPY(13-36), mimicked the potentiating effect of NPY on noradrenaline responses in PRA. 9. TXA2 analogue, U46619, at 10 nM elicited 3.6 fold leftwards shift of the noradrenaline concentration-responses curves in PRA and increased the maximal contraction by 32 +/- 3%, whereas in the presence of 1 microM SQ30741, 10 nM NPY did not potentiate noradrenaline responses. 10. The present results indicate that NPY may play a role in the regulation of retinal blood flow through both a direct contractile action, independent of the vessel size and a potentiation of the responses induced by noradrenaline in the proximal part of the retinal circulation, both effects being mediated by Y1 receptors. NPY promotes Ca2+ influx through voltage-dependent Ca2+ channels and stimulates the synthesis of contractile prostanoids in PRA and DRA, although only in PRA does the peptide trigger the release of an endothelium-derived contractile factor which facilitates the contraction and also seems to account for the potentiating effect of NPY.

Osmotic compression of single cardiac myocytes eliminates the reduction in Ca2+ sensitivity of tension at short sarcomere length

According to the Frank-Starling relation, cardiac output varies as a function of end-diastolic volume of the ventricle. The cellular basis of the relation is thought to involve length-dependent variations in Ca2+ sensitivity of tension; ie, as sarcomere length is increased in cardiac muscle, Ca2+ sensitivity of tension also increases. One possible explanation for this effect is that the decrease in myocyte diameter as muscle length is increased reduces the lateral spacing between thick and thin filaments, thereby increasing the likelihood of cross-bridge interaction with actin. To examine this idea, we measured the effects of osmotic compression of single skinned cardiac myocytes on Ca2+ sensitivity of tension. Single myocytes from rat enzymatically digested ventricles were attached to a force transducer and piezoelectric translator, and tension-pCa relations were subsequently characterized at short sarcomere length (SL), at the same short SL in the presence of 2.5% dextran, and at long SL. The pCa (-log[Ca2+]) for half-maximal tension (ie, pCa50) increased from 5.54 +/- 0.09 to 5.65 +/- 0.10 (n = 7, mean +/- SD, P < .001) as SL was increased from approximately 1.85 to approximately 2.25 microns. Osmotic compression of myocytes at short length also increased Ca2+ sensitivity of tension, shifting tension-pCa relations to [Ca2+] levels similar to those observed at long length (pCa50, 5.68 +/- 0.11). These results support the idea that the length dependence of Ca2+ sensitivity of tension in cardiac muscle arises in large part from the changes in interfilament lattice spacing that accompany changes in SL.

Length dependence of Ca2+ sensitivity of tension in mouse cardiac myocytes expressing skeletal troponin C

1. Beat-to-beat performance of myocardium is highly dependent on sarcomere length. The physiological basis for this effect is not well understood but presumably includes alterations in the extent of overlap between thick and thin filaments. Sarcomere length dependence of activation also appears to be involved since length-tension relationships in cardiac muscle are usually steeper than those in skeletal muscle. 2. An explanation recently proposed to account for the difference between length-tension relationships is that the cardiac isoform of troponin C (cTnC) has intrinsic properties that confer greater length-dependent changes in the Ca2+ sensitivity of tension than does skeletal troponin C (sTnC), presumably due to greater length-dependent changes in the Ca(2+)-binding affinity of cTnC. To test this hypothesis, transgenic mice were developed in which fast sTnC was expressed ectopically in the heart. This allowed a comparison of the length dependence of the Ca2+ sensitivity of tension between myocytes having thin filaments that contained either endogenous cTnC or primarily sTnC. 3. In myocytes from both transgenic and normal mice, the Ca2+ sensitivity of tension increased similarly when mean sarcomere length was increased from approximately 1.83 to 2.23 microns. In both cases, the mid-point (pCa50) of the tension-pCa (i.e. -log[Ca2+]) relationship shifted 0.12 +/- 0.01 pCa units (mean +/- S.E.M.) in the direction of lower Ca2+. 4. We conclude that the Ca2+ sensitivity of tension in myocytes changes as a function of sarcomere length but is independent of the isoform of troponin C present in the thin filaments.

Mean sarcomere length-force relationship of rat muscle fibre bundles

To study how sarcomere length inhomogeneities and the duration of activation affect sarcomere length-force characteristics of muscle, the mean sarcomere length-force relationship was determined for twitches and at 100 and 300 ms during tetanic activation for rat extensor digitorum longus and gastrocnemius medialis muscle fibre bundles. Mean sarcomere length is the mean length of all sarcomeres within the fibre, calculated by dividing fibre length by the number of sarcomeres in series in the fibre. The twitch mean sarcomere length-force relationship is shifted to larger sarcomere lengths (optimum mean sarcomere length = 2.69 microns) compared to the relationships determined at 100 or 300 ms of tetanic activation (optimum mean sarcomere length = 2.38 microns), which were the same. It is shown that the normalized Gordon et al. rationale results in a large overestimate of force (at most 68% of force at a sarcomere length of 1.60 microns) for mean sarcomere lengths between 1.4 and 2.0 microns, and in an underestimate of force between 2.3 and 3.0 microns. It is concluded that modelling skeletal mammalian muscle length-force relationships can be improved by using mean sarcomere length-force relations of mammalian fibres instead of the normalized rationale of Gordon et al. derived from a selected homogeneous part of frog fibre.

Determinants of loaded shortening velocity in single cardiac myocytes permeabilized with alpha-hemolysin

Force-velocity relations were obtained from single cardiac myocytes isolated by enzymatic digestion of rat myocardium and permeabilized with the pore-forming staphylococcal toxin alpha-hemolysin. Single cardiac myocytes were attached to a force transducer and piezoelectric translator and viewed with an inverted microscope to allow periodic monitoring of sarcomere length during experiments. Permeabilized cells were activated by immersion in a bath of known [Ca2+]. We report that the Ca2+ sensitivity of cells obtained by enzymatic digestion and permeabilized using alpha-hemolysin is similar to that reported previously for mechanically disrupted ventricular myocardium; however, the tension-pCa relation is less steep in the new preparation. During isotonic measurements, force was clamped to various loads using a rapid-response servo system. All recordings of shortening under load were distinctly curvilinear, and analysis of data involved fitting each shortening recording with a single exponential curve and calculating the value of the slope at the initial time of the load clamp. In addition, the presence of significant resting force at initial sarcomere lengths in these cells required that the possibility of alteration of velocity due to the presence of resting force be addressed. The maximum shortening velocity in fully Ca(2+)-activated single ventricular myocytes studied by this method was 2.83 muscle lengths per second on average. The basis for curvilinear shortening is postulated to be multifactorial in cardiac muscle, involving a combination of shortening inactivation and one or more passive elasticities that resist stretch or compression depending on sarcomere length. Shortening velocity shows a dependence on myosin isoform content when cells from a single heart are compared; however, this relation does not hold when cells from different hearts are compared. The behavior of single alpha-hemolysin-permeabilized myocyte shortening under loaded conditions at lower levels of Ca2+ is also described. During submaximal Ca2+ activation, initial shortening velocities are faster than those observed in maximally activated cells. This may be due to contributions of high passive force to increase shortening velocity under conditions of low active force generation, when passive force in the cell is a greater proportion of the total force and there are fewer bound crossbridges.

Calcium-independent activation of skeletal muscle fibers by a modified form of cardiac troponin C

A conformational change accompanying Ca2+ binding to troponin C (TnC) constitutes the initial event in contractile regulation of vertebrate striated muscle. We replaced endogenous TnC in single skinned fibers from rabbit psoas muscle with a modified form of cardiac TnC (cTnC) which, unlike native cTnC, probably contains an intramolecular disulfide bond. We found that such activating TnC (aTnC) enables force generation and shortening in the absence of calcium. With aTnC, both force and shortening velocity were the same at pCa 9.2 and pCa 4.0. aTnc could not be extracted under conditions which resulted in extraction of endogenous TnC. Thus, aTnC provides a stable model for structural studies of a calcium binding protein in the active conformation as well as a useful tool for physiological studies on the primary and secondary effects of Ca2+ on the molecular kinetics of muscle contraction.

Force-calcium relations in skinned twitch and slow-tonic frog muscle fibres have similar sarcomere length dependencies

The sarcomere length (SL) dependence of the calcium sensitivity of force was measured in skinned single twitch and slow-tonic muscle fibres from frog and toad. Twitch and slow-tonic fibres were characterized by location, appearance, physiological response to calcium and by protein band patterns from sodium-dodecyl-sulphate polyacrylamide gel electrophoresis (SDS-PAGE). Force-calcium relations were determined for each fibre type at two sarcomere lengths, 2.4 and 3.1 microns. Bathing solution ionic strength (IS) was 200 mM and solution pH was 7.0, 6.0 or 5.5; experiments were also done at IS = 120 mM and pH 7.0. At all pHs and ionic strengths tested, slow-tonic fibres exhibited a slower time course of force development and were more sensitive to calcium than were twitch fibres. Lowering IS increased calcium sensitivity and lowering pH decreased calcium sensitivity in both fibre types. Increasing SL increased the calcium sensitivity of force in both twitch and slow-tonic fibres at pH 7.0 and at both 200 and 120 mM IS. Lowering pH caused a decrease in the length dependence of calcium sensitivity of both fibre types; at pH 5.5 the calcium sensitivity of force in slow-tonic fibres exhibited a slight decrease with increasing SL.

Ca2+ regulation of mechanical properties of striated muscle. Mechanistic studies using extraction and replacement of regulatory proteins

Extraction of regulatory proteins from thick and thin filaments of vertebrate striated muscle has proven to be an important approach in elucidating roles of these proteins in regulating contraction and in probing specific mechanisms of activation. For some proteins, such as LC2 and C protein, extraction has been fundamental in demonstrating the importance of these proteins in modulating contraction and the kinetics of cross-bridge interaction. For other proteins, such as TnC and troponin, extraction has provided significant insight into the importance of thin-filament intermolecular cooperativity in modulating Ca2+ sensitivity of the contractile process. A combination of extraction and readdition has provided a means of introducing mutated or derivatized proteins into fibers to accomplish a variety of experimental objectives. The use of this approach is likely to grow with the need to test the functional consequences of site-specific mutations as part of studies directed to mechanisms of regulation or altered regulation in heart and skeletal muscles under normal and pathophysiological conditions. Such studies are likely to include extraction in combination with other probes of function such as flash photolysis of reaction substrates or products within the cross-bridge interaction cycle. Although extraction is a powerful approach and is likely to be extended to proteins not discussed in this review, an essential element of experimental design in studies such as these is that appropriate control experiments be done to verify that observed effects of the extraction protocol are specifically attributable to the protein that is removed.

Developmental changes in troponin T isoform expression and tension production in chicken single skeletal muscle fibres

1. The Ca2+ sensitivity of tension development was characterized in single skinned fibres from the slow anterior latissimus dorsi (ALD), fast posterior latissimus dorsi (PLD), and fast pectoralis major (PM) muscles of the chicken at adult and neonatal (2 weeks post-hatch) stages of development. In the adult, the PM was most sensitive, the ALD intermediate, and the PLD least sensitive to Ca2+. 2. PM and PLD fibres were less sensitive to Ca2+ at the neonatal stage of development than in the adult. However, ALD fibres exhibited no age-dependent changes in Ca2+ sensitivity. 3. Characterization of regulatory protein composition indicated that the PM and PLD fibres had identical fast isoforms of troponin C and troponin I at each developmental stage examined, but there were muscle-specific and age-dependent expressions of troponin T isoforms in these fibres. 4. In the ALD fibres, identical slow isoforms of troponin C, troponin I and tropomyosin were found at each stage. In addition, the troponin T isoform that was present did not change with age. 5. The results suggest a relationship between the specific troponin T isoform composition of individual muscle fibres and their calcium sensitivities of tension development.

Force, length, and Ca(2+)-troponin C affinity in skeletal muscle

On the basis of isotopic methods it has been found that force generation promotes increased Ca2+ binding to troponin C in cardiac muscle [P. Hofmann and F. Fuchs. Am. J. Physiol. 253 (Cell Physiol. 22): C541-C546, 1987] but not in skeletal muscle (J. Muscle Res. Cell Motil. 6: 477, 1985). However, studies with skinned rabbit psoas muscle fibers containing substituted fluorescent troponin C analogues indicate that force-generating cross bridges do promote increased Ca2+ binding in skeletal muscle (K. Güth and J. D. Potter. J. Biol. Chem. 262: 13627-13635, 1987). We have reexamined this question using a modified contraction-relaxation protocol in which Ca2+ binding to detergent-treated rabbit psoas fibers was measured either during steady-state force development or after relaxation was induced by one of two myosin ATPase inhibitors, vanadate or 2,3-butanedione monoxime. A standard double-isotope technique was used to measure Ca2+ binding. Another set of experiments was done in which force was reduced by releasing muscle fibers from sarcomere lengths of 2.4-2.6 microns to 1.5-1.7 microns, and bound Ca2+ was determined either before or after the release. No statistically significant effect of force generation or sarcomere length on Ca(2+)-troponin C affinity was observed. Thus the discrepancy remains between results obtained with isotopic and fluorescence methods. It is possible that in skinned fibers emission from fluorescence probes is more closely related to protein-protein interactions than to the amount of Ca2+ bound to troponin C.

Kinetics of adenosine triphosphate hydrolysis by shortening myofibrils from rabbit psoas muscle

1. Using a solenoid-operated mixing device, time-resolved measurements were made of shortening and accompanying ATP hydrolysis at 20 degrees C by myofibrils prepared from rabbit psoas muscle. 2. The extent of ATP hydrolysis was determined by an improved Malachite Green method for determination of inorganic phosphate (Pi) in the presence of a large excess of ATP. For the measurement of the change in sarcomere length by phase contrast microscopy, shortening was terminated without delay and artifact by a mixture of 0.2 M-acetate (pH 4.6) and 1.25% (v/v) glutaraldehyde. 3. The shortening velocity per half-sarcomere was 10 microns s-1 in 25 mM-KCl for sarcomere lengths above 1.4 microns, and at least 12 microns s-1 in 150 mM-KCl for sarcomere lengths above 1.7 microns. During this rapid shortening, there was no significant ATP turnover by cross-bridges (upper 95% confidence limit: 0.14 mol (mol of myosin head)-1 in 25 mM-KCl; 0.12 mol mol-1 in KCl solutions greater than or equal to 100 mM). 4. When the sarcomeres shortened below 1.7 microns in KCl concentrations greater than 100 mM or below 1.4 microns in 25 mM-KCl, there was a transient acceleration of ATP hydrolysis (delayed ATP hydrolysis), which was then followed by a steady slow hydrolysis. 5. The magnitudes (+/- estimated standard deviation) of delayed ATP hydrolysis by myofibrils of initial sarcomere length 2.4 microns were 0.42 +/- 0.19, 0.31 +/- 0.10 and 0.17 +/- 0.09 mol (mol myosin head)-1 in 25 mM, 100 mM and 150 mM-KCl, respectively. For myofibrils of sarcomere length 2.0 microns, however, it decreased to 0.24 +/- 0.10 mol mol-1 in 25 mM-KCl or to an insignificant level in 150 mM-KCl. 6. These results indicate that most of the ATP hydrolysis products remain bound to cross-bridges during rapid shortening, and that when the force opposing shortening increases, a proportion of cross-bridges rapidly dissociate the products and enter the next ATP cycle, which diminishes with the decrease in shortening distance as well as the increase in ionic strength. Such behaviour of the cross-bridge is probably a manifestation of its energetic and kinetic properties in the state with bound ADP and Pi interacting with actin filaments at zero load and at a transition from zero to non-zero loads.

The role of troponin C in the length dependence of Ca(2+)-sensitive force of mammalian skeletal and cardiac muscles

1. Skinned fibre preparations of right ventricular trabeculae, psoas and soleus muscles from hamster and rabbit were activated by Ca2+ and the length dependencies of their pCa (-log [Ca2+])-force relationships were compared. 2. Ca2+ sensitivity of the myocardium was higher at 2.2-2.4 microns than that at 1.7-1.9 microns. The length dependence was at least twofold greater in cardiac muscle than in fast skeletal fibres at identical temperatures and salt concentrations. Slow-twitch fibres gave a response similar to that in the myocardium. 3. The effect of the troponin C (TnC) phenotype on the length dependence of Ca2+ sensitivity was measured on both fast skeletal fibres and cardiac muscle with TnC exchange in situ. The length-induced increase in Ca2+ sensitivity was found to be greater in the presence of cardiac TnC than with fast skeletal TnC. Thus the results indicate that a certain domain of TnC is specialized in this length function, and that this domain is different in the two phenotypes. 4. The possibility that the enhanced length dependence of Ca2+ sensitivity after cardiac TnC reconstitution was attributable to reduced TnC binding was excluded when the length dependence of partially extracted fast fibres was reduced to one-half the normal value after a 50% deletion of the native TnC. 5. Two recombinant forms of cardiac TnC (kindly provided by Dr John Putkey, Houston, TX, USA) were used next, to investigate the roles of two specific domains in TnC in the control of length dependence of Ca2+ sensitivity and in the contraction-relaxation switching of cardiac muscle: 6. Using mutant CBM1 [corrected], in which site 1 was modified such as to bind the 4th Ca2+ ion, as in skeletal TnC, the length-induced Ca2+ sensitivity in cardiac muscle was suppressed. The effect was intermediate between cardiac and skeletal TnCs under the same conditions. The pSr (-log [Sr2+])-force relationship of cardiac muscle was also measured. In the presence of the mutant, skinned trabeculae manifest pSr-activation curves identical to those of fast fibres. This indicates that the metal ion binding properties of site 1 in TnC modulate the regulatory action of site 2. 7. Using mutant CBM2A, in which site 2 was inactivated, the activation of cardiac muscle by both Ca2+ and Sr2+ ions was completely blocked. This is the expected result, since both regulatory sites were now inactive, regulatory site 1 being normally inactive in cardiac muscle.(ABSTRACT TRUNCATED AT 400 WORDS)

C-protein limits shortening velocity of rabbit skeletal muscle fibres at low levels of Ca2+ activation

1. Effects on maximum shortening velocity (Vmax) due to partial extraction of C-protein were investigated in skinned fibres from rabbit psoas muscles. Up to 80% of endogenous C-protein was extracted, as assessed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) of fibre segments obtained before and after the extraction protocol. Vmax was obtained at 15 degrees C by measuring the times required to take up various amounts of slack imposed at one end of the fibre. 2. During maximal activation with Ca2+, Vmax in control fibres was 4.26 +/- 0.16 (mean +/- S.E.M., n = 7) muscle lengths per second (ML/s). Following extraction of approximately 40% of endogenous C-protein, Vmax in these same fibres was 4.41 +/- 0.24 ML/s. 3. At sufficiently low levels of submaximal activation, high- and low-velocity phases of unloaded shortening were observed. Partial extraction of C-protein significantly increased Vmax in the low-velocity phase but had no effect on the high-velocity phase. The effect on low-velocity Vmax was fully reversed by re-addition of purified C-protein. 4. At low levels of activation, the amount of shortening to the break-point between the high- and low-velocity phases was not significantly affected by C-protein extraction. Under control conditions the average break-point was 85.6 +/- 3.1 nm/half-sarcomere, while 84.1 +/- 3.1 nm/half-sarcomere was obtained following partial extraction of C-protein. 5. These results are considered in terms of a model in which an internal load slows Vmax at low levels of activation once a given amount of active shortening has occurred. C-protein may contribute to this internal load either by binding to actin and myosin or by influencing mechanical properties of myosin cross-bridges.

Length vs. active force relationship in single isolated smooth muscle cells

The length vs. active force relationship (L-F) may provide information about changes in smooth muscle contractile protein interactions as muscle length changes. To characterize the L-F in single toad stomach smooth muscle cells, cells were attached to a force measurement system, electrically stimulated, and isometric force and elastic modulus (an estimate of the number of attached cross bridges) determined at different cell lengths. Cells generated maximum stress (Pmax = 152.5 mN/mm2) and elastic modulus (Eact = 0.68 x 10(4) mN/mm2) at their rest length (Lcell = 78.0 microns; distance between cell attachments). At shorter lengths, active force and elastic modulus declined proportionally with active force eliminated at 0.4 Lcell. Stretching the relaxed cells up to 1.4 Lcell shifted the subsequent L-F along the length axis by the amount of the stretch but did not change Pmax or the shape of the L-F. In activated cells, force was a function of cell length rather than of shortening history. We interpret these findings as evidence that 1) Lcell is close to the optimum length for force generation, 2) the decline in force at lengths less than Lcell results from a reduced number of attached cross bridges, and 3) stretching relaxed smooth muscle cells may not move the contractile units to new positions on their L-F.

Substitution of cardiac troponin C into rabbit muscle does not alter the length dependence of Ca2+ sensitivity of tension

1. The isometric length-tension relationship for cardiac muscle is generally steeper than for skeletal muscle in the physiological range of sarcomere lengths. Recent studies suggest that cardiac troponin C (cTnC) may have intrinsic properties that confer greater length-dependent changes in Ca2+ sensitivity of tension than for skeletal troponin C (sTnC). We tested this hypothesis by characterizing tension-pCa (pCa is -log[Ca2+]) relationships in rabbit skinned psoas muscle fibres at mean sarcomere lengths of 2.32 and 1.87 microns both before and after partial replacement of endogenous sTnC with cTnC. 2. In untreated control fibres, the mid-point (pCa50) of the tension-pCa relationship shifted to lower pCa by 0.15 +/- 0.02 pCa units, i.e. became less sensitive to Ca2+, when sarcomere length was reduced, and the relationship became steeper. 3. Partial extraction of endogenous sTnC and reconstitution with cTnC resulted in no change in the length-dependent shift of pCa50 when reconstitution with cTnC was more than 95% complete; however, when reconstitution was less than 95% complete, there were significant increases in the length-dependent shift in pCa50. 4. An increase in the length-dependent shift of pCa50 was also observed in fibres from which sTnC was partially extracted, but no cTnC was subsequently re-added. 5. We conclude that differences in type of TnC alone are not sufficient to explain differences between skeletal and cardiac muscles in the length dependence of Ca2+ sensitivity of tension.

The determinants of skeletal muscle force and power: their adaptability with changes in activity pattern

A kinetic model of the cross-bridge cycle in skeletal muscle is presented with special reference to the rate limiting steps regulating the peak rate of force development (dP/dt), peak force (P0), and the maximal shortening speed (Vmax). Force production in skeletal muscle is dependent on the number of cross-bridges in the strongly bound, high-force state (AM'-ADP), and during a peak isometric contraction this state is the dominant cross-bridge form. The peak force and power output of a muscle depends upon numerous factors to include: (1) muscle and fiber size and length; (2) architecture, such as the angle and physical properties of the fiber-tendon attachment, and the fiber to muscle length ratio; (3) fiber type; (4) number of cross-bridges in parallel; (5) force per cross-bridge; (6) peak dP/dt; (7) force-velocity relationship; (8) fiber Vmax; (9) force-pCa2+ relationship: and (10) the force-frequency (action potential Hz) relationship. In this paper, we discuss these determinants of force and power output, and consider how they adapt to both muscle unloading (induced by hindlimb suspension) and programs of regular endurance exercise. Slow- and fast-twitch fibers have similar capacities to generate specific tension (kg cm-2). However, fast fibers show a considerably higher peak dP/dt, Vmax, and power output. The high Vmax of the fast-twitch fiber is likely due to the high myofibrillar ATPase activity of the fast myosin isozyme. Both hindlimb suspension and regular endurance exercise have been shown to induce fiber type specific changes in single fiber function. For example, fiber size and the peak tetanic tension of the slow oxidative (SO), fast oxidative glycolytic (FOG), and fast glycolytic (FG) fiber types were generally unaltered by endurance exercise-training. In contrast, hindlimb suspension produced cell atrophy in all fiber types and a reduced specific tension in the SO but not the FOG or FG fiber types. Both exercise-training and HS shifted the force-pCa curve to the right, and increased the Vmax of the SO fiber type. From the standpoint of work capacity or the ability to move a load, the important functional property is power output. Peak power is obtained at loads considerably below 50% of PO, and it is correlated with the percentage of fast-twitch fibers. Peak power can be increased by both dynamic and isometric programs of exercise-training.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of tension and stiffness due to reduced pH in mammalian fast- and slow-twitch skinned skeletal muscle fibres

1. The pH dependence of the Ca2+ sensitivities of isometric tension and stiffness was investigated at 10 and 15 degrees C in skinned single fibres from rat and rabbit fast- and slow-twitch skeletal muscles. Stiffness was determined by recording the tension responses to sinusoidal length changes (3.3 kHz); peak-to-peak amplitude of the length change was monitored by laser diffraction and averaged approximately 1.3 nm (half-sarcomere)-1. We have assumed that stiffness provides a measure of the number of cross-bridge attachments to actin. 2. At maximal Ca2+ activation, stiffness was depressed by 22 +/- 2% (mean +/- S.E.M.) in fast-twitch fibres but was unchanged in slow-twitch fibres when pH was lowered from 7.00 to 6.20. As reported previously, maximum tension was depressed by 20-45% at low pH, with the effect being greater in fast-twitch compared to slow-twitch fibres. 3. In fast-twitch fibres at 10 and 15 degrees C the Ca2+ concentrations for half-maximal activation of tension and stiffness were increased at low pH. In slow-twitch fibres, similar effects were observed at 15 degrees C, but at 10 degrees C there were no changes in the Ca2+ sensitivities of tension and stiffness when pH was lowered. 4. Linear relationships between relative tension and relative stiffness were obtained at all temperatures, with slopes of 1.04 +/- 0.01 at pH 7.00 and 0.76 +/- 0.01 at pH 6.20 in fast- and slow-twitch fibres, indicating that force per cross-bridge attachment is similarly reduced at low pH in both types of fibres. 5. In both fast- and slow-twitch fibres, rigor tension (no added ATP or creatine phosphate; pCa 9.0) was depressed by 35 +/- 7% and stiffness by 12 +/- 4% when pH was reduced from 7.00 to 6.20. Since cross-bridge cycling is inhibited in rigor the effect of low pH to depress rigor tension suggests that pH directly modulates the strength of the bond formed between actin and myosin. 6. The effect of low pH to reduce the apparent number of cross-bridge attachments during maximal Ca2+ activation in fast- but not slow-twitch fibres could account for the greater H(+)-induced depression of maximum force in fast-twitch fibres. In both fibre types, the decrease in the number of cross-bridge attachments at submaximal concentrations of Ca2+ may in part account for the loss in Ca2+ sensitivity of tension at low pH, due perhaps to a reduction in co-operative activation of the thin filament by bound cross-bridges.

Effects of partial extraction of light chain 2 on the Ca2+ sensitivities of isometric tension, stiffness, and velocity of shortening in skinned skeletal muscle fibers

Various functional roles for myosin light chain 2 (LC2) have been suggested on the basis of numerous and predominantly in vitro biochemical studies. Using skinned fibers from rabbit psoas muscle, the present study examines the influence of partial removal of LC2 on isometric tension, stiffness, and maximum velocity of shortening at various levels of activation by Ca2+. Isometric tension, stiffness, and velocity of shortening were measured at pCa values between 6.6 and 4.5 (a) in a control fiber segment, (b) in the same fiber segment after partial removal of LC2, and (c) after recombination with LC2. The extraction solution contained 20 mM EDTA, 20 or 50 mM KCl, and either imidazole or PO4(2-) as a pH buffer (pH 7.0). The amount of LC2 extracted varied with the temperature, duration of extraction, and whether or not troponin C (0.5 mg/ml) was added to the extraction solution. Extraction of 20-40% LC2 resulted in increased active tensions in the range of pCa's between 6.6 and 5.7, but had no effect upon maximum tension. The tension-pCa relationship was left-shifted to lower [Ca2+] by as much as 0.2 pCa units after LC2 extraction. At low concentrations of Ca2+, an increase in stiffness proportional to the increase in tension was observed. Readdition of LC2 to these fiber segments resulted in a return of tension and stiffness to near control values. Stiffness during maximal activation was unaffected by partial extraction of LC2. LC2 extraction was shown to uniformly decrease (by 25-30%), the velocity of shortening during the high velocity phase but it did not significantly affect the low velocity phase of shortening. This effect was reversed by readdition of purified LC2 to the fiber segments. On the basis of these findings we conclude that LC2 may modulate the number of cross-bridges formed during Ca2+ activation and also the rate of cross-bridge detachment during shortening. These results are consistent with the idea that LC2 may modulate contraction via an influence upon the conformation of the S1-S2 hinge region of myosin.

Inhibition of neurotensin-induced miosis by blockade of ocular dopamine pathways

In previous work we have determined that intracameral (IC) administration of neurotensin (NT) produces strong miosis in rabbits. However, the pharmacological mechanism of this response remains undetermined. Blockade of alpha and beta-adrenoceptor subtypes with phenoxybenzamine and propranolol, blockade of M1 muscarinic receptors with atropine or blockade of mu opioid receptors with naloxone did not affect NT-induced miosis. Of interest however was the observation that destruction of ocular dopamine (DA) nerve endings with 6-hydroxydopamine (6-OHDA) + desmethylimipramine (DMI), or blockade of D-2 DA receptors with haloperidol significantly inhibited the miotic response to IC NT. These findings indicate that an intact iridic DA pathway is required for the expression of NT-induced miosis.

Dissociation of force from myofibrillar MgATPase and stiffness at short sarcomere lengths in rat and toad skeletal muscle

1. Single fast-twitch fibres from the extensor digitorum longus muscle of the rat, Rattus norvegicus, and single twitch fibres from the iliofibularis muscle of the cane toad, Bufo marinus, were mechanically skinned and then used to measure maximally Ca2+-activated [( Ca2+] greater than 0.03 mmol l-1) isometric force production, myofibrillar MgATPase activity and fibre stiffness at different sarcomere lengths. MgATP hydrolysis was linked by an enzyme cascade to the oxidation of NADH (nicotinamide adenine dinucleotide, reduced form) and was monitored by a microfluorimetric system. Fibre stiffness was measured from the amplitude of force oscillations generated by small sinusoidal length changes. 2. At sarcomere lengths which were optimal for isometric force production (around 2.7 microns for rat and 2.2 microns for toad fibres) the myofibrillar MgATPase activity (mean +/- S.E.M.) at 21-22 degrees C was found to be 3.80 +/- 0.53 molecules MgATP hydrolysed s-1 per myosin head for eight rat fibres and 6.35 +/- 0.77 s-1 per myosin head for four toad fibres. 3. At sarcomere lengths shorter than 2.7 microns in rat fibres and 2.2 microns in toad fibres, MgATPase and stiffness remained elevated and close to their respective values at 2.7 microns in rat fibres and 2.2 microns in toad fibres even when the isometric force decreased to near zero levels. 4. The dissociation at short sarcomere lengths of myofibrillar MgATPase activity and fibre stiffness from isometric force suggests that the cross-bridge cycle is not greatly affected by double actin filament overlap with the myosin filaments at short sarcomere lengths. Moreover, the results suggest that cross-bridges can be formed by myosin with actin filaments projecting from the nearest Z-line and from the Z-line in the other half of the sarcomere. 5. These results help to reconcile energetic and mechanical data obtained by others at short sarcomere lengths and can be explained within the framework of the sliding filament theory.