Contraction of rabbit skinned skeletal muscle fibers at low levels of magnesium adenosine triphosphate

Clusters of bound Ca(2+) initiate contraction in fast skeletal muscle

Ca(2+)-binding to troponin C ultimately controls force in muscle leading to the expectation that the two curves, pCa/force and pCa/Ca(2+) binding, will coincide. Using an improved fluorescence apparatus to measure Ca(2+)-binding, we confirm a displacement between the position and shape of the pCa/Ca(2+)-binding and pCa/force curves. This displacement may be part of a mechanism that reduces the noise inherent in the control process. There must always be some Ca(2+)-binding events even at 10 or 100nM, well below threshold for muscle contraction. To minimize the response to such random binding events we suggest that clusters of adjacent Ca(2+)-binding sites must be filled before contraction is initiated. Clusters promote the reconfiguration of the thin filament to the "On" state; this simultaneously increases thin filaments' affinity for myosin heads and of troponin C for Ca(2+) producing the highly cooperative pCa/force curve. The cluster requirement displaces the Ca(2+)-binding from the force curve as observed. The thin filament conformational changes and the accompanying affinity increases introduce a discontinuity in the pCa/Ca(2+)-binding curve. The curve, therefore, is most appropriately fit by two separate Hill equations, a simple non-cooperative one (midpoint, pK1, n1∼1) for the foot and a second cooperative one (pK2, n2∼2.5) for the upper part. With this fit pK2 is larger than pK1 as our argument requires, in contrast to fitting to the sum of two Hill equations. It also expresses the idea that there may be three states of the thin filament.

Thin-filament linked regulation of smooth muscle myosin

Phosphorylation of the regulatory light chain subunit of smooth muscle myosin is sufficient, but not necessary for muscle contraction. It has been suggested that thin-filament regulation may also contribute to the regulation of contraction. A hallmark feature of regulated thin filaments, previously described for vertebrate skeletal muscle, is the capacity of strong-binding or rigor-like cross bridges to "turn-on" the actin filament. Turned-on thin filaments stimulate cross-bridge attachment even in the absence of calcium. The present study utilized an in vitro sliding-filament motility assay to test for thin-filament regulation of both unphosphorylated and phosphorylated smooth muscle myosins. Regulated thin-filaments were reconstituted from skeletal muscle actin and chicken gizzard smooth muscle tropomyosin (TmCG), and then turned-on either (1) by rigor cross bridges at low concentrations of MgATP, or (2) by adding N-ethyl-maleimide-modified skeletal subfragment S1(NEM-S1), which forms rigor-like bonds in the presence of MgATP. For control actin.TmCG filaments, force production by unphosphorylated myosin was 0.5% of that produced by thiophosphorylated myosin. The force exerted on actin.Tm filaments by both unphosphorylated and phosphorylated myosins was increased by reducing the [MgATP] to 10-100 microM MgATP (rigor-dependent activation). Force was also increased by actin.TmCG filaments that had been turned-on by NEM-S1 binding, with force production by unphosphorylated myosin increased 80-fold vs. 2.3-fold for thiophosphorylated myosin. TmCG was required for increased force production with both low MgATP and NEM-S1. Unloaded filament velocity for NEM-S1-activated thin filaments was 0.72 micron/sec with unphosphorylated myosin compared to 1.24 microns/sec with thiophosphorylated myosin. Taken together, these results suggest that thin-filament regulation may play a role in the activation of both unphosphorylated and phosphorylated smooth muscle myosins and suggest a possible mechanism for activation of slowly cycling unphosphorylated cross bridges (i.e. latch-state) during tonic contractions of smooth muscle.

Effects of ADP and low ATP on the Ca(2+)-sensitive transient contraction upon photolysis of caged ATP in rat muscle fibres: a study on the Bremel-Weber type cooperation

Caged ATP was photolysed in rat psoas fibres under various conditions to examine whether ADP plays a special role in the 'Bremel-Weber type cooperation', viz. the Ca(2+)-like action of the rigor cross-bridges. Various concentrations of ATP (0.25-1.6 mM) were photoreleased in the presence of various concentrations of ADP (0-0.4 mM) at approximately 8 or 20 degrees C. The Ca2+ concentration was set at around 0.2 microM in order that the ATP-induced contraction was significant but short. Both lower [ATP] and higher [ADP] resulted in a slower detachment of the rigor cross-bridges and a larger contraction after the detachment. ADP did not seem to affect the relationship between the rate of detachment and the size of the Ca(2+)-sensitive component of the subsequent contraction. It is concluded that there is little evidence that the ADP-bound rigor cross-bridges are more potent than the nucleotide-free ones in the Bremel-Weber type cooperation that is seen in the ATP-induced transient contraction. The mechanism by which the ADP-specific, Ca(2+)-independent contraction occurs immediately after the release of ATP remains to be clarified.

Probing the coupling of Ca2+ and rigor activation of rabbit psoas myofibrillar ATPase with ethylene glycol

We have exploited solvent perturbation to probe the coupling of Ca2+ and rigor activation of the ATPase of myofibrils from rabbit psoas. Three techniques were used: overall myofibrillar ATPases by the rapid-flow quench method; kinetics of the interaction of ATP with myofibrils by fluorescence stopped-flow; and myofibrillar shortening by optical microscopy. Because of its extensive use with muscle systems, ranging from myosin subfragment-1 to muscle fibres, we chose 40% ethylene glycol as the relaxing agent. At 4 degrees C, the glycol had little effect on the myofibrillar ATPase at low [Ca2+], but at high [Ca2+] the activity was reduced 50-fold, close to the level found under relaxing conditions, and there was no shortening. However, the ATPase of chemically cross-linked myofibrils (permanently activated even without Ca2+) was reduced only 3-4-fold. The lesser reduction of the ATPase of permanently activated myofibrils was also observed in single turnover experiments in which activation occurs by a few heads in the rigor state activating the remaining heads. The addition of ADP, which also promotes strong head-thin filament interactions, also activated the ATPase but only in the presence of Ca2+. Further experiments revealed that in 40% ethylene glycol, Ca2+ does initiate shortening but only with the aid of strong interactions and at temperatures above 15 degrees C. This confirms that in the organized and intact myofibril, Ca2+ and rigor activation are coupled, as proposed previously for regulated actomyosin subfragment-1.

Calcium regulation of tension redevelopment kinetics with 2-deoxy-ATP or low [ATP] in rabbit skeletal muscle

The correlation of acto-myosin ATPase rate with tension redevelopment kinetics (k(tr)) was determined during Ca(+2)-activated contractions of demembranated rabbit psoas muscle fibers; the ATPase rate was either increased or decreased relative to control by substitution of ATP (5.0 mM) with 2-deoxy-ATP (dATP) (5.0 mM) or by lowering [ATP] to 0.5 mM, respectively. The activation dependence of k(tr) and unloaded shortening velocity (Vu) was measured with each substrate. With 5.0 mM ATP, Vu depended linearly on tension (P), whereas k(tr) exhibited a nonlinear dependence on P, being relatively independent of P at submaximum levels and rising steeply at P > 0.6-0.7 of maximum tension (Po). With dATP, Vu was 25% greater than control at Po and was elevated at all P > 0.15Po, whereas Po was unchanged. Furthermore, the Ca(+2) sensitivity of both k(tr) and P increased, such that the dependence of k(tr) on P was not significantly different from control, despite an elevation of Vu and maximal k(tr). In contrast, lowering [ATP] caused a slight (8%) elevation of Po, no change in the Ca(+2) sensitivity of P, and a decrease in Vu at all P. Moreover, k(tr) was decreased relative to control at P > 0.75Po, but was elevated at P < 0.75Po. These data demonstrate that the cross-bridge cycling rate dominates k(tr) at maximum but not submaximum levels of Ca(2+) activation.

Troponin C modulates the activation of thin filaments by rigor cross-bridges

Extraction of troponin C (TnC) from skinned muscle fibers reduces maximum Ca2+ and rigor cross-bridge (RXB)-activated tensions and reduces cooperativity between neighboring regulatory units (one troponin-tropomyosin complex and the seven associated actins) of thin filaments. This suggests that TnC has a determining role in RXB, as well as in Ca(2+)-dependent activation processes. To investigate this possibility further, we replaced fast TnC (fTnC) of rabbit psoas fibers with either CaM[3,4TnC] or cardiac TnC (cTnC) and compared the effects of these substitutions on Ca2+ and RXB activation of tension. CaM[3,4TnC] substitution has the same effect on Ca(2+)- and RXB-activated tensions; they are reduced 50%, and cooperativity between regulatory units is reduced 40%. cTnC substitution also reduces the maximum Ca(2+)-activated tension and cooperativity. But with RXB activation the effects on tension and cooperativity are opposite; cTnC substitution potentiates tension but reduces cooperativity. We considered whether tension potentiation could be explained by increased activation by cycling cross-bridges (CXBs), but the concerted transition formalism predicts fibers will fail to relax in high substrate and high pCa when CXBs are activator ligands. It predicts resting tension, which is not observed in either control or cTnC-substituted fibers. Rather, it appears that cTnC facilitates RXB activation of fast fibers more effectively than fTnC. The order of RXB-activated tension facilitation is cTnC > fTnC > CaM[3,4TnC] > empty TnC-binding sites. Comparison of the structures of fTnC, CaM[3,4TnC], and cTnC indicates that the critical region for this property lies in the central helix or N-terminal domain, including EF hand motifs 1 and 2.

Myosin binding-induced cooperative activation of the thin filament in cardiac myocytes and skeletal muscle fibers

Myosin binding-induced activation of the thin filament was examined in isolated cardiac myocytes and single slow and fast skeletal muscle fibers. The number of cross-bridge attachments was increased by stepwise lowering of the [MgATP] in the Ca(2+)-free solution bathing the preparations. The extent of thin filament activation was determined by monitoring steadystate isometric tension at each MgATP concentration. As pMgATP (where pMgATP is -log [MgATP]) was increased from 3.0 to 8.0, isometric tension increased to a peak value in the pMgATP range of 5.0-5.4. The steepness of the tension-pMgATP relationship, between the region of the curve where tension was zero and the peak tension, is hypothesized to be due to myosin-induced cooperative activation of the thin filament. Results showed that the steepness of the tension-pMgATP relationship was markedly greater in cardiac as compared with either slow or fast skeletal muscle fibers. The steeper slope in cardiac myocytes provides evidence of greater myosin binding-induced cooperative activation of the thin filament in cardiac as compared with skeletal muscle, at least under these experimental conditions of nominal free Ca2+. Cooperative activation is also evident in the tension-pCa relation, and is dependent upon thin filament molecular interactions, which require the presence of troponin C. Thus, it was determined whether myosin-based cooperative activation of the thin filament also requires troponin C. Partial extraction of troponin C reduced the steepness of the tension-pMgATP relationship, with the effect being significantly greater in cardiac than in skeletal muscle. After partial extraction of troponin C, muscle type differences in the steepness of the tension-pMgATP relationship were no longer apparent, and reconstitution with purified troponin C restored the muscle lineage differences. These results suggest that, in the absence of Ca2+, myosin-mediated activation of the thin filament is greater in cardiac than in skeletal muscle, and this apparent cooperativity requires the presence of troponin C on thin filament regulatory strands.

How muscle may contract

A new molecular model is proposed for muscle contraction, that involves the electrical charging of the long (C-terminal) alpha-helical part of the head of the myosin molecule (S1) while the head is attached to actin; as it charges the alpha-helical part moves in the radial electric field between the filaments. The alpha-helical part snaps back when the myosin molecule is discharged electrically, at the moment that ATP binds to the active enzymatic site. This snap-back model explains several puzzling phenomena in contractility, as well as providing a physical explanation for the origin of an impulsive force that drives muscle contraction.

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.

Early steps of the Mg(2+)-ATPase of relaxed myofibrils. A comparison with Ca(2+)-activated myofibrils and myosin subfragment 1

The early steps of the Mg(2+)-ATPase activity of relaxed rabbit psoas myofibrils were studied in a buffer of near-physiological ionic strength at 4 degrees C by the rapid flow quench technique. The initial ATP binding steps were studied by the ATP chase, and the cleavage and release of product steps by the Pi burst method. The data obtained were interpreted by [formula: see text] where M represents the myosin heads with or without actin interaction. This work is a continuation of our study on Ca(2+)-activated myofibrils [Houadjeto, M., Travers, F., & Barman, T. (1992) Biochemistry 31, 1564-1569]. Here the constants obtained with relaxed myofibrils were compared with those with activated myofibrils and myosin subfragment 1 (S1). We find that whereas Ca2+ increases 80X the release of products (k4), it has little effect upon the kinetics of the initial binding and cleavage steps. As with activated myofibrils and S1, the second-order binding constant for ATP (k2/K1) was about 1 microM-1 s-1 and the ATP was bound very tightly. With activated myofibrils, it was difficult to obtain an estimate for the koff for ATP(k-2) but it is much less than kcat. Here with relaxed myofibrils we estimate k-2 less than 8 x 10(-4) s-1, which is considerably smaller than kcat (0.019 s-1) and also previous estimates for this constant. The overall Kd for ATP to relaxed myofibrils is less than 8 x 10(-10) M. With S1 this Kd is about 10(-11) M.(ABSTRACT TRUNCATED AT 250 WORDS)

Contraction of glycerinated rabbit slow-twitch muscle fibers as a function of MgATP concentration

We have measured the isometric tension and force-velocity relationships of glycerinated rabbit slow-twitch semimembranosus muscle as a function of MgATP concentration ([MgATP]) and have compared the results with those obtained previously from fast-twitch psoas muscle. We find that isometric tension decreases as [MgATP] increases. The magnitude of the decrease is not as great as observed in psoas. Maximum shortening velocity (Vmax) exhibits classical Michaelian saturation behavior with respect to [MgATP] with a Michaelis constant (Km) for half-maximal velocity of 18 microM and a value at saturating [MgATP] of 0.6 muscle lengths/s. Similar values were observed in fibers from soleus, another slow-twitch muscle. The corresponding values in rabbit psoas muscle are 150 microM and 1.6 lengths/s. Compared with psoas, in semimembranosus muscle Km decreases by a factor of approximately 10, whereas Vmax decreases by about a factor of 3. Thus, although in a nonphysiological regime, at low [MgATP], a "fast" muscle actually has a lower shortening velocity than a "slow" muscle.

Regulation of binding of subfragment 1 in isolated rigor myofibrils

A steric-hindrance model has been used to explain the regulation of muscle contraction by tropomyosin-troponin complex. The regulation of binding was studied by microscopic observation of mixtures of fluorescent subfragment 1 (S1) with rigor myofibrils at different actin-to-S1 ratios and in the presence and absence of calcium. Procedures were adapted to protect the critical thiols of S1 before conjugation to thiol-specific fluorochromes, this giving fluorescent S1 with unaltered enzyme activity. S1 binding was greatest in the I band (except at the Z-lines) in the presence of calcium regardless of the [S1]. The patterns in the absence of calcium depended on the actin-to-S1 ratios: low [S1], binding in the myosin-actin overlap region; intermediate [S1], highest binding at the A-I junction; high [S1], greatest binding in the I-band. The two distinct binding patterns observed at low [S1] were demonstrated by dual-channel fluorescence microscopy when myofibrils were sequentially incubated with fluorescent S1 without calcium followed by a different fluorescent S1 with calcium. These observations support the concept of rigor activation of actin sites. The change in the pattern upon increasing [S1] without calcium demonstrate cooperative interactions along the thin filament. However, these interactions (under the conditions used without calcium) do not appear to extend over greater than 2-3 tropomyosin-troponin-7 actin functional units.

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.

Co-operative activation of skeletal muscle thin filaments by rigor crossbridges. The effect of troponin C extraction

When Ca2+ binds to troponin C (TnC), all 26 troponin-tropomyosin (Tn-Tm) complexes of a regulatory strand change in concert from the inactive to the active configuration. To see if the complexes respond similarly when they are activated by rigor crossbridges in the absence of Ca2+, we determined the slope (ns) of the bell-shaped pS/tension (pS = -log [MgATP], where S = MgATP2-) relationship between pS 5, where the tension is maximal, and pS 2.3, where fibers are fully relaxed. In control skinned rabbit psoas fibers the ns value is greater than 4; it progressively decreases with TnC extraction. This decrease in ns with TnC extraction is analogous to the decrease in the slope (Hill coefficient) of the pCa/tension (pCa = -log [Ca2+]) relationship with extraction. Complete TnC extraction reduces the maximum substrate-induced tension by only 25%; in contrast, it reduces the maximum Ca2+ induced tension to zero. The effects of TnC extraction on the slope of the pS/tension curve are explained by the assumptions that (1) extracted Tn-Tm complexes no longer change in concert with their neighbors but change independently of them, and (2) co-operative signals cannot cross extracted Tn-Tm complexes. The ns value, therefore, like the nH, is a direct function of the number of contiguous, intact, Tn-Tm complexes in a stretch of a regulatory strand. To describe qualitatively the bi-phasic pS/tension relationship, the mono-phasic pCa/tension relationship, and the effects of TnC extraction on them, we introduce a version of the concerted-transition formalism which includes two activating ligands, Ca2+ and rigor crossbridges.

Myocyte deenergization and intracellular free calcium dynamics

Intracellular free calcium in adult rat heart ventricular myocytes was monitored by single cell fura-2 fluorescence microscopy. The average resting free calcium in rod-shaped quiescent cells was 125 nM (range 70-200 nM). When cells were deenergized with an inhibitor (amytal) and an uncoupler (carbonyl-cyanide m-chlorophenylhydrazone) of oxidative phosphorylation, there was a small but significant increase (125-380 nM) in intracellular free calcium during the transition to a highly contracted (square) rigor form. After the onset of contracture, which occurred 5-15 min after addition of the above compounds, the increase in free calcium was slow for the first 20 min, reaching a value of only 750 nM. Thereafter, the rate of increase accelerated and 50 min after contracture, free calcium was approximately 3 microM. The increase in free calcium was absolutely dependent on extracellular calcium but was not inhibited by high concentrations of verapamil (2-7 microM), suggesting influx via the Na+-Ca2+ exchange transporter as the cause of calcium increase. However, in calcium repletion protocols the rate of increase in sodium-loaded myocytes was greatly accelerated if cells were not depleted of ATP, confirming suggestions that ATP loss partially inhibits Na+-Ca2+ exchange.

Dependency of the force-velocity relationships on Mg ATP in different types of muscle fibers from Xenopus laevis

MgATP binding to the actomyosin complex is followed by the dissociation of actin and myosin. The rate of this dissociation process was determined from the relationship between the maximum velocity of shortening and the MgATP concentration. It is shown here that the overall dissociation rate is rather similar in different types of muscle fibers. The relation between MgATP concentration and the maximum shortening velocity was investigated in fast and slow fibers and bundles of myofibrils of the iliofibularis muscle of Xenopus laevis at 4 degrees C from which the sarcolemma was either removed mechanically or made permeable by means of a detergent. A small segment of each fiber was used for a histochemical determination of fiber type. At 5 mM MgATP, the fast fibers had a maximum shortening velocity (Vmax) of 1.74 +/- 0.12 Lo/s (mean +/- SEM) (Lo: segment length at a sarcomere length of 2.2 microns). For the slow fibers Vmax was 0.41 +/- 0.15 Lo/s. In both cases, the relationship between Vmax and the ATP concentration followed the hyperbolic Michaelis-Menten relation. A Km of 0.56 +/- 0.06 mM (mean +/- SD) was found for the fast fibers and of 0.16 +/- 0.03 mM for the slow fibers. Assuming that Vmax is mainly determined by the crossbridge detachment rate, the apparent second order dissociation rate for the actomyosin complex in vivo would be 3.8.10(5) M-1s-1 for the fast fibers and 2.9.10(5) M-1 s-1 for the slow fibers. Maximum power output as a function of the MgATP concentration was derived from the force-velocity relationships. At 5 mM MgATP, the maximum power output in fast fibers was (73 +/- 8) mW.g-1 dry weight and (15 +/- 5) mW.g-1 in slow fibers. The Km for MgATP for the maximum power output for the fast fibers was (0.15 +/- 0.03) mM, which is about a factor of 4 lower than the Km for Vmax. The implications of these results are discussed in terms of a kinetic scheme for crossbridge action.

Co-operative interactions between troponin-tropomyosin units extend the length of the thin filament in skeletal muscle

Ca2+ binding to troponin C (TnC), a subunit of the thin filament regulatory strand, activates vertebrate skeletal muscle contraction. Tension, however, increases with Ca2+ too abruptly to be the result of binding to sites on individual TnCs. Because extraction of one TnC on average per regulatory strand dramatically reduces the slope of the tension/Ca2+ relationship, we proposed that all 26 troponin-tropomyosin complexes of the regulatory strand form a co-operative system. This study of permeabilized (chemically skinned) rabbit psoas fibers analyzes the extraction time-course, the distribution of extraction sites on regulatory strands and the effects of extraction on the co-operativity of the tension/Ca2+ relationship. Two components of TnC are resolved in the time-course of extraction: a "rapidly extracting" component that can be selectively removed without affecting tension or co-operativity, and a "slow extracting" component whose loss reduces tension and co-operativity. Extraction of [14C]TnC shows that the slowly extracting component is lost randomly, so that, after removal of 5% of the TnC, most extracted strands have lost one TnC. Extraction interrupts the transmission of co-operativity by dividing the regulatory strand into smaller, independent co-operative systems; it reduces tension by preventing Ca2+ activation of TnC-depleted regulatory units. Co-operativity of the tension/Ca2+ relationship is modeled with the concerted-transition formalism for intact systems of 26 regulatory units, and for the smaller systems in extracted fibers.

Sliding movement of single actin filaments on one-headed myosin filaments

The myosin molecule consists of two heads, each of which contains an enzymatic active site and an actin-binding site. The fundamental problem of whether the two heads function independently or cooperatively during muscle contraction has been studied by methods using an actomyosin thread, superprecipitation and chemical modification of muscle fibres. No clear conclusion has yet been reached. We have approached this question using an assay system in which sliding movements of fluorescently labelled single actin filaments along myosin filaments can be observed directly. Here, we report direct measurement of the sliding of single actin filaments along one-headed myosin filaments in which the density of heads was varied over a wide range. Our results show that cooperative interaction between the two heads of myosin is not essential for inducing the sliding movement of actin filaments.

Inhibition of calcium influx in isolated adult rat heart cells by ATP depletion

Using 45Ca, indo1, and quin2, calcium uptake was measured in isolated quiescent adult rat heart cells under different metabolic conditions. Exposure of cells in a medium containing 1 mM CaCl2 to rotenone and uncoupler resulted in adenosine triphosphate (ATP) depletion from 17.08 +/- 2.26 to 0.63 +/- 0.11 nmol/mg within 8 minutes, and the cells went into contracture. In this time, the cells lost 1.65 +/- 0.1 nmol Ca/mg of total rapidly exchangeable cellular calcium, and the level of free cytosolic calcium as measured by indo1 rose from 47.4 +/- 16.3 nM to 79.8 +/- 27.6 nM. The subsequent rate of rise of intracellular free calcium concentration was just 4 nM/min for at least 40 minutes. Therefore, we investigated the effect of ATP depletion on the rate of calcium entry. In cells loaded with sodium by ouabain treatment without calcium, the initial rate of calcium influx on calcium addition was inhibited by 82-84% when cellular ATP was depleted, as measured by 45Ca or indo1. Quin2 also showed a strong inhibition of calcium influx by ATP depletion, but itself also caused a strong inhibition of calcium influx. The rate of calcium influx declined even further in ATP-depleted cells after the initial influx: Between 1 and 12 minutes after calcium addition, the residual 45Ca uptake rate of the first minute was inhibited by an additional 90%. We conclude that ATP depletion per se does not quickly elevate cytoplasmic free calcium and that such an elevation is prevented by a very strong inhibition of the rate of calcium entry.

Effects of partial extraction of troponin complex upon the tension-pCa relation in rabbit skeletal muscle. Further evidence that tension development involves cooperative effects within the thin filament

Partial extraction of troponin C (TnC) decreases the Ca2+ sensitivity of tension development in mammalian skinned muscle fibers (Moss, R. L., G. G. Giulian, and M. L. Greaser. 1985. Journal of General Physiology. 86:585), which suggests that Ca2+-activated tension development involves molecular cooperativity within the thin filament. This idea has been investigated further in the present study, in which Ca2+-insensitive activation of skinned fibers from rabbit psoas muscles was achieved by removing a small proportion of total troponin (Tn) complexes. Ca2+-activated isometric tension was measured at pCa values (i.e., -log[Ca2+]) between 6.7 and 4.5: (a) in control fiber segments, (b) in the same fibers after partial removal of Tn, and (c) after recombination of Tn. Tn removal was accomplished using contaminant protease activity found in preparations of LC2 from rabbit soleus muscle, and was quantitated using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and scanning densitometry. Partial Tn removal resulted in the development of a Ca2+-insensitive active tension, which varied in amount depending on the duration of the extraction, and concomitant decreases in maximal Ca2+-activated tensions. In addition, the tension-pCa relation was shifted to higher pCa values by as much as 0.3 pCa unit after Tn extraction. Readdition of Tn to the fiber segments resulted in the reduction of tension in the relaxing solution to control values and in the return of the tension-pCa relation to its original position. Thus, continuous Ca2+-insensitive activation of randomly spaced functional groups increased the Ca2+ sensitivity of tension development in the remaining functional groups along the thin filament. In addition, the variation in Ca2+-insensitive active tension as a function of Tn content after extraction suggests that only one-third to one-half of the functional groups within a thin filament need to be activated for complete disinhibition of that filament to be achieved.

Potassium and ionic strength effects on the isometric force of skinned twitch muscle fibres of the rat and toad

A study was carried out to investigate the effects of ionic strength and monovalent cations on isometric, Ca2+-activated force and rigor responses in mechanically skinned muscle fibres. Three types of skeletal muscle fibres were used: rat fast- and slow-twitch fibres and toad twitch fibres. The contractile apparatus of rat slow-twitch fibres was affected differently from that of rat fast-twitch and amphibian twitch fibres when changing the ionic strength (expressed either in terms of ionic equivalents as I or formally as gamma/2) and [K+]. Thus, the apparent sensitivity to Ca2+ decreased substantially more in slow-twitch fibres (by a factor of 20) than in the other fibre types (by a factor of 12) when I and [K+] were increased from 94 to 354 mM and from 56 to 316 mM respectively. Maximum Ca2+-activated force, however, declined only by a factor of 2.2 in slow-twitch fibres compared with 4.2 in the other fibre types, when I was increased from 154 to 354 mM. In slow-twitch fibres the force oscillations of myofibrillar origin were found to increase substantially in amplitude, duration and frequency at low values of I and almost disappeared at high ionic strength. At low values of I, it was also discovered that ca. 50% of the fast-twitch fibres responded with myofibrillar force oscillations when submaximally activated. The characteristics of these oscillations were different from those of slow-twitch fibres. Rigor force levels were found to decline markedly with increasing iota and [K+] in all fibre types. Unexpectedly, once rigor force was established in a certain ionic environment, the level of force was stable regardless of further changes in ionic strength and monovalent cation concentration. These results indicate that the rigor cross-bridges can be formed in different stable positions and that the probability of attachment in certain positions (rather than the total number of cross-bridges that can be formed) is influenced by the ionic conditions. Further experimental evidence provided in this study shows that the increase in [K+] is mainly responsible for the decrease of the Ca2+-sensitivity of the contractile apparatus and that ionic strength (expressed as I rather than gamma/2) influences markedly the maximal Ca2+-activated force, the maximum steepness of the pCa-force relations and the oscillatory processes of myofibrillar origin.

The effects of partial extraction of TnC upon the tension-pCa relationship in rabbit skinned skeletal muscle fibers

The activation of contraction in vertebrate skeletal muscle involves the binding of Ca2+ to low-affinity binding sites on the troponin C (TnC) subunit of the regulatory protein troponin. The present study is an investigation of possible cooperative interactions between adjacent functional groups, composed of seven actin monomers, one tropomyosin, and one troponin, along the same thin filament. Single skinned fibers were obtained from rabbit psoas muscles and were then placed in an experimental chamber containing relaxing solution maintained at 15 degrees C. Isometric tension was measured in solutions containing maximally and submaximally activating levels of free Ca2+ (a) in control fiber segments, (b) in the same segments after partial extraction of TnC, and finally (c) after recombination of TnC into the segments. The extraction was done at 11-13 degrees C in 20 mM Tris, 5 mM EDTA, pH 7.85 or 8.3, a procedure derived from that of Cox et al. (1981. Biochem. J. 195:205). Extraction of TnC was quantitated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the control and experimental samples. Partial extraction of TnC resulted in reductions in tension during maximal Ca activation and in a shift of the relative tension-pCa (i.e., -log[Ca2+]) relationship to lower pCa's. The readdition of TnC to the extracted fiber segments resulted in a recovery of tension to near-control levels and in the return of the tension-pCa relation to its original position. On the basis of these findings, we conclude that the sensitivity to Ca2+ of a functional group within the thin filament may vary depending upon the state of activation of immediately adjacent groups.

The inhibition of muscle contraction by adenosine 5' (beta, gamma-imido) triphosphate and by pyrophosphate

We have studied the inhibition of the contraction of glycerinated rabbit psoas muscle caused by ligands that bind to the ATPase site of myosin. Two ligands, adenosine 5' (beta, gamma-imido) triphosphate (AMPPNP) and pyrophosphate (PPi), decreased the force and stiffness developed in isometric contractions and the velocity of shortening of isotonic contractions. The force exerted by isometric fibers was measured as a function of MgATP in the presence and absence of a constant concentration of the ligands. As the MgATP concentration decreased, the inhibition of tension caused by the ligand increased, reaching approximately 50% at 25 microM MgATP and either 2 mM MgPPi or 2 mM MgAMPPNP. The maximum velocity of shortening was also measured as a function of MgATP concentration in the presence of 1 and 2 mM MgPPi and 2.5 and 5 mM MgAMPPNP. Both ligands acted as pure competitive inhibitors with Ki = 3.0 mM for PPi and 5.1 mM for MgAMPPNP. These data show that both ligands are weak inhibitors of the contraction of fibers. The results provided information on the energetics of actin-myosin-ligand states that occur in the portion of the cross-bridge cycle where MgATP binds to myosin. A simple analysis of the inhibition of velocity suggests that MgAMPPNP binds to the actomyosin complex at this step of the cycle with an effective affinity constant of approximately 2 X 10(2) M-1.

ATP-dependent movement of myosin in vitro: characterization of a quantitative assay

Sheetz and Spudich (1983, Nature (Lond.), 303:31-35) showed that ATP-dependent movement of myosin along actin filaments can be measured in vitro using myosin-coated beads and oriented actin cables from Nitella. To establish this in vitro movement as a quantitative assay and to understand better the basis for the movement, we have defined the factors that affect the myosin-bead velocity. Beads coated with skeletal muscle myosin move at a rate of 2-6 micron/s, depending on the myosin preparation. This velocity is independent of myosin concentration on the bead surface for concentrations above a critical value (approximately 20 micrograms myosin/2.5 X 10(9) beads of 1 micron in diameter). Movement is optimal between pH 6.8 and 7.5, at KCl concentrations less than 70 mM, at ATP concentrations greater than 0.1 mM, and at Mg2+ concentrations between 2 and 6 mM. From the temperature dependence of bead velocity, we calculate activation energies of 90 kJ/mol below 22 degrees C and 40 kJ/mol above 22 degrees C. Different myosin species move at their own characteristic velocities, and these velocities are proportional to their actin-activated ATPase activities. Further, the velocities of beads coated with smooth or skeletal muscle myosin correlate well with the known in vivo rates of myosin movement along actin filaments in these muscles. This in vitro assay, therefore, provides a rapid, reproducible method for quantitating the ATP-dependent movement of myosin molecules on actin.