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J Appl Physiol 92: 1293-1299, 2002; doi:10.1152/japplphysiol.00912.2001
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Vol. 92, Issue 3, 1293-1299, March 2002

Effects of pH on the length-dependent twitch potentiation in skeletal muscle

Dilson E. Rassier and Walter Herzog

Human Performance Laboratory, Faculty of Kinesiology, University of Calgary, Calgary, Alberta, Canada T2N 1N4


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

When muscle is elongated, there is a length dependence of twitch potentiation and an increased Ca2+ sensitivity of the myofilaments. Changes in the charge potential of myofilaments, induced by a decrease in pH, are known to abolish the length dependence of Ca2+ sensitivity. This study was aimed at testing the hypothesis that a decrease in pH, and the concomitant loss of length dependence of Ca2+ sensitivity, depresses the length dependence of staircase potentiation. In vitro, isometric twitch contractions of fiber bundles dissected from the mouse extensor digitorum longus, performed before and after 10 s of 10-Hz stimulation (i.e., the staircase potentiation protocol) were analyzed at five different lengths, ranging from optimal length for maximal force production (Lo; = 12 ± 0.7 mm) to Lo + 1.2 mm (Lo + 10%). These measurements were made at an extracellular pH of 6.6, 7.4, and 7.8 (pH changes induced by altering the CO2 concentration of the bath solution). At pH 7.4 and 7.8, the degree of potentiation after 10-Hz stimulation showed a linear decrease with increased fiber bundle length (r2 = 0.95 and r2 = 0.99, respectively). At pH 6.6, the length dependence of potentiation was abolished, and the slope of the length-potentiation relationship was not different from zero (r2 = 0.05). The results of this study indicate that length dependence of potentiation in intact skeletal muscle is abolished by lowering the pH. Because decreasing the pH decreases Ca2+ sensitivity and changes the charge potential of the filaments, the mechanism of length-dependent potentiation may be closely related to the length dependence of Ca2+ sensitivity, and changes in the charge potential of the myofilaments may be important in regulating this relationship.

force-length relation; Ca2+ sensitivity; force potentiation; twitch force


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

FORCE PRODUCTION IN SKELETAL muscle depends on many variables. Among these, length, speed of contraction, and the history of the contractile conditions have been well studied (2, 7, 8). Other variables have received less attention in the biomechanical community, and mechanistic explanations that relate these variables to force production are rare. These include force potentiation after tetanic and staircase contractions and the shift of submaximal force-length relationships to the right of the length axis compared with maximal force-length relationships. In this study, we tried to gain insight into the mechanisms of staircase potentiation as a function of muscle length.

Staircase potentiation is the increase in active twitch force that occurs for the first seconds of repetitive skeletal muscle stimulation. Staircase potentiation is important for in vivo skeletal muscle contractions, because it occurs at frequencies of stimulation that are within the physiological range (12). The degree of potentiation in intact muscles is associated with the degree of phosphorylation of the regulatory light chains of myosin (RLC) (17), which causes an increase in Ca2+ sensitivity. An increase in Ca2+ sensitivity means that there is a leftward shift in the force-pCa2+ relationship (18, 23), therefore increasing force production at a given, submaximal Ca2+ concentration.

Twitch potentiation is muscle length dependent. Potentiation is greater when the contractile response is measured at short compared with long muscle lengths (19-21). However, RLC phosphorylation is not muscle length dependent (20). Therefore the force-potentiating effects of phosphorylation must be different at different lengths, and factors other than RLC phosphorylation must produce the length-dependent effect.

Sweeney and Stull (22) and Levine et al. (10) proposed a model to explain how RLC phosphorylation might increase Ca2+ sensitivity and force production. During repetitive muscle stimulation, RLC phosphorylation is associated with a movement of the myosin cross bridge away from the thick filament because of changes in the charge potential in the RLC region of the myosin head (24). This movement of the cross bridges decreases the distance between actin binding site and cross bridge, presumably increasing the probability of cross-bridge attachment (23), which, in turn, would result in increased force (potentiation).

This model might be used to explain the length dependence of potentiation. When a muscle is elongated, the fiber diameter (13, 15) and interfilament spacing (14) decrease, thereby decreasing the distance between the myosin head and actin attachment site. This decrease in myosin-to-actin distance has been associated with an increase in Ca2+ sensitivity, as demonstrated with skinned fibers in which an increase in sarcomere length caused a leftward shift in the force-Ca2+ relation (4, 6, 13). It appears, therefore, that an increase in sarcomere length and RLC phosphorylation causes an increase in Ca2+ sensitivity because of a reduction in the distance from myosin head to actin attachment site. The fact that the effects of potentiation decrease with increasing sarcomere length might be directly associated with the decrease in lattice spacing with increasing length. This decrease in lattice spacing with increasing sarcomere length might offset, to a certain extent, the effects of RLC phosphorylation on potentiation. If this theory is correct, the length dependence of potentiation should be directly related to the length dependence of Ca2+ sensitivity, as suggested previously (19, 21).

Although it is difficult to test the theory that cross bridges approach actin on RLC phosphorylation and so cause the length dependence of potentiation in intact muscles, it is easy to formulate testable hypotheses based on this idea. If the length-dependent increase in Ca2+ sensitivity, caused by changes in interfilament spacing, is responsible for the decrease in staircase potentiation with increasing muscle lengths, then abolishing the length dependence of Ca2+ sensitivity should abolish the length dependence of potentiation. It is known that a decrease in intramuscular pH virtually eliminates Ca2+ sensitivity as a function of length (13). The purpose of this study was to test the hypothesis that a decrease in pH, which virtually eliminates Ca2+ sensitivity as a function of muscle length, also eliminates the length dependence of staircase potentiation.


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Muscle preparation. Male mice (CD1) weighing ~40 g were used in this study. Treatment of these animals was approved by the University committee for the ethical use of animals for research. Animals were deeply anesthetized with intraperitoneal injections of pentobarbital sodium (60 mg/kg). The right hindlimb was shaved, and an incision was made along its surface. The tibialis anterior was removed to expose the extensor digitorum longus (EDL). The EDL was then removed and transferred to a dissecting chamber containing alkaline solution, as described below.

By use of a pair of iris scissors and forceps under a microscope with dark-field illumination (×80 maximum magnification), a fiber bundle was dissected from the third digit of the EDL. Further dissection was performed to obtain a small fiber bundle (~10 to 30 fibers, 10-14 mm long). Care was taken to not damage the fibers. After dissection, the tendons of the dissected muscle bundle were gripped with small pieces of T-shaped aluminum foil as close to the end of the fibers as possible. Although we did not measure the compliance of the system, it is unlikely that it played a significant role in the force tracings during contractions. The muscle bundle was transferred to an experimental chamber, with continuous superfusion of a Tyrode solution. One tendon clip was attached to a force transducer, the other to a motor arm, suspending the bundle horizontally. The motor arm was controlled by computer, allowing for fine changes in muscle bundle length. Experiments were conducted at 25°C.

Solutions. During the experiments, a Tyrode solution (in mM: 121 NaCl, 5.0 KCl, 0.4 NaH2PO4, 0.5 MgCl2, 1.8 CaCl2, 24 NaHCO3, 5.5 dextrose), which was bubbled continuously with 95% O2 and 5% CO2 (pH = 7.4), was pumped through the experimental chamber. Changes in pH were obtained by bubbling the solution with 70% O2 and 30% CO2 (pH = 6.6) or by changing the original Tyrode solution to (in mM) 136.5 NaCl, 5.0 KCl, 0.4 NaH2PO4, 0.5 MgCl2, 1.8 CaCl2, 11.9 NaHCO3, 5.5 dextrose (pH = 7.8 without bubbling). Although we have not measured the intracellular pH in our preparation, this method has been tested and used by Westerblad et al. (26), and we assumed that we would have the same results.

Fiber bundle length and force measurements. The experimental group (n = 10) underwent the following procedures. During the experiments, optimal length (Lo) was defined as the length at which maximal isometric force was obtained in response to a doublet stimulation (5-ms delay). The fiber bundle was tested for a range from Lo to Lo + 1.2 mm (0.3-mm steps). The average length of the fiber bundles used in this study was 12 ± 0.7 mm. Therefore, fibers were stretched by a maximum of ~10% during the experiment. Muscle bundle force was measured with a semiconductor strain gauge transducer connected to an amplifier in a half-bridge configuration. Stimulation (Grass S88, Grass Instruments) was done with supramaximal (10-50 V) square pulses, 0.5-ms duration, through two platinum wires placed in the experimental chamber parallel to the muscle bundles.

After the muscle was set at Lo at pH 7.4, four twitch contractions (20-s intervals) and a tetanic contraction (200 Hz for 400 ms) were given to obtain reference forces. After 5 min, a double-pulse contraction was given to ensure that the fiber bundle was at Lo. This doublet was followed by four twitches (20-s intervals). After a 1-min rest period, a protocol was started to evaluate the length dependence of twitch potentiation, as described in Fig. 1. One twitch contraction was elicited at each of the five different lengths under study, with 2-s intervals between contractions (T1); then, a 10-Hz stimulation train was given for 10 s (S) to elicit the staircase potentiation. After the 10-s stimulation train, the single-twitch contractions at each of the five different lengths were repeated with 2-s intervals (T2). The order of length changes (increasing or decreasing) was randomized. The first twitch contraction after the 10-s stimulation train was ~3 s after the end of the stimulation train. The 2-s interval between twitches was sufficient to change fiber bundle length. The degree of twitch potentiation in each fiber bundle length was calculated as the force difference between twitch contractions recorded after and before the 10-s, 10-Hz stimulation trains. Forces were collected at 4,000 Hz.


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  		Fig. 1.   Schematic drawing of the stimulation protocol used during the experiments. Five twitch contractions were elicited at different fiber bundle lengths (random order) before (T1) and after (T2) a 10-s, 10-Hz stimulation train (S).

After the testing at a pH level of 7.4, a 30-min rest period was permitted, and then four twitch contractions were given at Lo to assess return to control (prestaircase) conditions. After these contractions, acidosis or alkalosis was induced. After the pH was changed to the desired level, four twitch contractions were elicited at Lo, followed by doublet stimulation and a tetanic contraction (200 Hz, 400 ms), followed by a 5-min interval. Then, after an additional four control twitches at Lo that were used to evaluate the impact of changing the pH on the twitch amplitude, the protocol described in Fig. 1 was repeated. The Lo evaluated during doublet stimulation was not changed after changes in pH were produced.

A control group (n = 12) was used to evaluate the impact of changing fiber bundle length without changing the pH. The entire experimental protocol described above and in Fig. 1 was performed at a constant pH level of 7.4. In six of these bundles, the first five twitches (T1) were recorded for increasing fiber bundle length (from Lo to Lo + 1.2 mm, in 0.3-mm steps) and the five poststaircase twitches (T2) were recorded for decreasing fiber bundle length (from Lo + 1.2 to Lo mm, in 0.3-mm steps) in the first of the three repeat protocols. In the second repeat protocol, stimulation was performed in random order. In the third repeat protocol, the first five twitches (T1) were recorded for decreasing fiber bundle length (from Lo + 1.2 mm to Lo, in 0.3-mm steps), and the five poststaircase twitches (T2) were recorded for increasing fiber bundle length (from Lo to Lo + 1.2 mm, in 0.3-mm steps). In the remaining six fiber bundles, the first and third repeat protocols were reversed; the second protocol was repeated. These experiments were also used to evaluate the possible effect of fatigue during the experiments. To check for fatigue, the first unpotentiated twitch contractions (T1) recorded at Lo before the first and the third repeat protocols were compared.

Additional control experiments (n = 6) were conducted to evaluate whether the time elapsed after the 10-s, 10-Hz stimulation train (S, Fig. 1) altered the level of potentiation. In these experiments, the entire protocol shown in Fig. 1 was performed at a pH level of 7.4 without changing the fiber bundle length after the 10-s stimulation train.

Data analysis. The active force of twitches was taken as the difference between the peak force obtained during the contraction and the passive tension, which was rarely increased in the range of lengths investigated in this study. A two-way analysis of variance with repeated measures was performed to compare the active force during the 10-s, 10-Hz stimulation among the three pH conditions. To evaluate the length-dependent twitch potentiation for each pH condition, a two-way analysis of variance with repeated measures was used, in which the twitch contractions recorded before (T1, Fig. 1) and after (T2, Fig. 1) the 10-s, 10-Hz stimulation train were compared across the five fiber bundle lengths. When significant interactions occurred, post hoc comparisons were performed with the Student-Newman-Keuls test. Linear regression was performed to compare the degree of potentiation for the three pH conditions at the different fiber bundle lengths. The control experiments were evaluated by using an analysis of variance with repeated measures. The level of significance was preset at P < 0.05 for all analyses.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Control experiments. The aim of the control experiments was to evaluate if twitch contractions were affected by 1) the mechanical maneuver of changing the fiber bundle length, 2) fatigue, or 3) the time after the staircase potentiation. It was shown that the twitch amplitude was not altered because of the way changes in fiber bundle length were made, neither before nor after repetitive stimulation (Fig. 2). The twitch amplitude was the same irrespective of whether the muscle was shortened or stretched before a contraction was elicited. Also, the twitch amplitude recorded at the beginning and end of the protocol did not differ significantly, indicating that fatigue did not influence the results (Fig. 2). Similarly, the degree of potentiation of the first and fifth (last) twitches after the 10-s, 10-Hz stimulation train was the same at all pH levels, indicating that the order of fiber bundle lengths at which twitch potentiation was measured did not influence the results.


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  		Fig. 2.   Control experiments showing the results of twitch contractions recorded before (solid symbols) and after (open symbols) a 10-s, 10-Hz stimulation train after shortening (circles) or stretching (triangles) the fiber bundle. Optimal length (Lo) is referred to as 0 mm, and changes in length are shown in the x-axis. Note that the maneuver of changing fiber bundle length did not alter the active force production during unpotentiated and potentiated twitches. Values are means ± SE.

Changes in pH and repetitive muscle stimulation at Lo. Figure 3 shows typical twitch contractions recorded at 0, 5, and 10 s of 10-Hz stimulation trains at Lo, at pH conditions of 7.4 (A) and 6.6 (B). There was a staircase potentiation response in both examples. The potentiation was greater at pH 7.4 than that at 6.6. Twitch potentiation was accompanied by an increase in the rate of force development, without a significant increase in contraction time. Contractions at pH 7.8 (not shown) were similar to those observed at pH 7.4. 


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  		Fig. 3.   Twitch contractions recorded during a typical experiment at 0 (bottom tracings), 5 (middle tracings), and 10 s (top tracings) of a 10-Hz stimulation train, at pH 7.4 (A) and 6.6 (B). A staircase potentiation is visible in both examples. In this example, potentiation was greater at pH 7.4 (52%) than that at pH 6.6 (23%).

Figure 4 shows the mean peak active forces at 0, 5, and 10 s of the 10-s, 10-Hz stimulation train at pH conditions of 6.6, 7.4, and 7.8. Active force was greater at 5 and 10 s than at the beginning of the stimulation train for all pH conditions. This increase in active force defines a positive staircase potentiation. At pH 6.6, potentiation was smaller than at pH conditions of 7.4 and 7.8. 


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  		Fig. 4.   Active force developed during 10-s, 10-Hz stimulation, measured at the 3 different pH conditions investigated in the study. The peak forces of the twitches at 0, 5, and 10 s of the 10-Hz stimulation trains are shown. For all conditions, a positive staircase response was observed. Values are means ± SE.

Force-length relation before and after repetitive stimulation. Figure 5 shows typical twitch contractions recorded before and after the 10-s, 10-Hz stimulation train, at the five lengths under investigation (from Lo to Lo + 1.2 mm, Fig. 5, A-E, respectively), at pH 7.4. Figure 6 shows the corresponding results at pH 6.6. There is an increase in active force after 10-s, 10-Hz stimulation at both pH levels shown. At a pH level of 7.4, potentiation decreases with increasing fiber bundle length (Fig. 5). The amplitude of the unpotentiated twitch contraction does not change significantly at the fiber bundle lengths between Lo and Lo + 0.6 mm, starting to decrease only at Lo + 0.9 mm. At pH 6.6, potentiation is independent of fiber bundle length (Fig. 6). Contrary to the results at pH 7.4, the amplitude of the unpotentiated twitch decreases continuously with increasing fiber bundle lengths. Contractions at pH 7.8 (not shown) produced results similar to those observed at pH 7.4. 


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  		Fig. 5.   Twitch contractions recorded during a typical experiment before (bottom tracings) and after (top tracings) a 10-s, 10-Hz stimulation train at pH 7.4, at the 5 different fiber bundle lengths tested in this study. In this example, potentiation was 61% at Lo (A), 44% at Lo + 0.3 mm (B), 38% at Lo + 0.6 mm (C), 26% at Lo + 0.9 mm (D), and 6% at Lo + 1.2 mm (E). Therefore, there was a length dependence of potentiation. Panels show only the first 0.8 s of contractions; after 1.2 s, the force returned to the resting level in all cases.



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  		Fig. 6.   Twitch contractions recorded during a typical experiment before (bottom tracings) and after (top tracings) a 10-s, 10-Hz stimulation train at pH 6.6 at the 5 different lengths tested in this study. Potentiation across lengths is similar, with values of 29% at Lo (A), 33% at Lo + 0.3 mm (B), 31% at Lo + 0.6 mm (C), 32% at Lo + 0.9 mm (D), and 27% at Lo + 1.2 mm (E). Panels show only the first 0.8 s of contractions; after 1.2 s, the force returned to the resting level in all cases.

Figure 7 shows the mean force-length relationships for twitch contractions recorded before and after the 10-s, 10-Hz stimulation, at pH conditions of 6.6 (A), 7.4 (B) and 7.8 (C). At pH 7.4, the active force of the unpotentiated twitches does not decrease until lengths beyond Lo + 0.3 mm. At Lo + 1.2 mm, twitch potentiation was virtually absent (Fig. 7B). At pH 6.6, the peak force of the unpotentiated twitches decreased continuously with increasing fiber bundle lengths (Fig. 7A). Potentiation after the 10-s, 10-Hz stimulation train remained nearly constant as a function of fiber bundle length. Both these results differ from those obtained at pH 7.4. The results obtained at pH 7.8 (Fig. 7C) were similar to those obtained at pH 7.4. Figure 8 shows that the mean potentiation after the 10-s, 10-Hz stimulation train decreased as a function of fiber bundle lengths for pH conditions of 7.4 and 7.8 but remained unaltered at pH 6.6. 


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  		Fig. 7.   Force-length relationship for twitch contractions recorded before () and after (open circle ) the 10-s, 10-Hz stimulation train, at pH 6.6 (A), 7.4 (B), and 7.8 (C). Optimal length is referred to as 0 mm, and changes in length are shown in the x-axis. At pH levels of 7.4 and 7.8, the force-length relation for unpotentiated twitches showed an active force peak that was shifted to the lengths longer than Lo, whereas the potentiated twitches show a force-length relationship that decreases with increasing fiber bundle length. At pH 6.6, force decreases linearly with increasing length before and after the 10-s, 10-Hz stimulation train, and the length dependence of staircase potentiation is abolished. Values are means ± SE.



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  		Fig. 8.   Relationship between staircase potentiation and fiber bundle length, at the three pH levels tested in the study. Optimal length is referred to as 0 mm, and changes in length are shown in the x-axis. At pH levels of 7.4 and 7.8, the degree of potentiation was inversely proportional to fiber bundle length. At pH 6.6, the slope of the potentiation-length relationship is not different from zero, showing that length no longer affects the degree of potentiation. Regression analyses yielded: AF = 59.9 - 42.3 · L (r2 = 0.95), AF = 51.9 - 37.7 · L (r2 = 0.99), and AF = 32.2 - 1.1 · L (r2 = 0.05), for pH levels of 7.4, 7.8, and 6.6, respectively (AF, active force; L, length). Values are means ± SE.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The main purpose of this paper was to test the hypothesis that decreasing the intracellular pH decreases the length dependence of twitch potentiation. This hypothesis was confirmed, because a decrease in pH from 7.4 to 6.6 abolished the length dependence of potentiation after staircase (10-s, 10-Hz) stimulation trains.

Staircase potentiation was inversely related to fiber bundle length at pH levels of 7.4 and 7.8, as previously reported in muscles stimulated in situ and at physiological pH (19-21), or in vitro at pH 7.4 (11). In previous studies, we observed that this length dependence of potentiation was directly associated with the length dependence of Ca2+ sensitivity. We suggested that the increased Ca2+ sensitivity, induced by stretching the muscle, is caused by a decrease in interfilament spacing that offsets the effects of RLC phosphorylation on potentiating the twitch contraction (19, 21).

Studies with skinned fibers provided evidence that the length-dependent increase in Ca2+ sensitivity is a function of the sarcomere lattice spacing (6, 25). The lattice spacing, measured as the relative distance between adjacent myosin filaments, decreases inversely with the square root of the length of the sarcomere (3). Taking into account that the actin-to-myosin (surface-to-surface) distance is ~14.2 nm (9), the proximity of the filaments, induced by increasing sarcomere length, may increase the probability of myosin-actin interaction, thereby increasing Ca2+ sensitivity. Lattice spacing decreases by ~1.8 nm when sarcomeres are stretched from 2.5 to 3.0 µm [for a review, see Millman (16)].

It is known that the center of mass of myosin heads is located at a radius between 13.3 and 13.5 nm from the center of the thick filament backbone in the relaxed, unphosphorylated state (10). When myosin filaments are phosphorylated, this distance increases to ~16.1 and 19.0 nm (10), and RLC phosphorylation is also associated with an increase in Ca2+ sensitivity. Therefore, with the assumption that stretching of the muscle and RLC phosphorylation do not have an additive effect, the impact of RLC phosphorylation would be diminished at increasing muscle lengths, where the myosin cross bridges are close to the actin binding sites even before the RLC are phosphorylated. This theory may be explained by changes in force-Ca2+ relationships for different experimental situations. When the RLCs are phosphorylated, there is a leftward shift in the force-Ca2+ relationship, resulting in a greater force for a given submaximal Ca2+ concentration. When the muscle is stretched, there is also a leftward shift in the force-Ca2+ relationship. Therefore, for this stretched condition, the effect of RLC phosphorylation is smaller compared with the unstretched condition.

Different mechanisms have been proposed to explain the effects of intracellular pH on Ca2+ sensitivity. First, lowering the pH is thought to influence the degree of dissociation of ion groups from the thin filaments, reducing the Ca2+ concentration near the troponin binding sites (5). Second, Ca2+ concentration between the myofilaments is influenced by the presence of fixed charges on the myofilaments, whose ionization depends on pH. The amount of Ca2+ bound to myofibrils or skinned muscle fibers decreases when pH is decreased from 7.0 to 6.5, indicating that decreasing pH is associated with a reduction of the affinity of Ca2+ to binding sites on troponin (1). In both cases, there is less activation of the muscle fibers at low pH compared with high pH, and that should decrease the active force production. We observed a systematic decrease in the twitch amplitude at pH 6.6 compared with the corresponding values at pH 7.4 and 7.8 (Fig. 7), as expected.

RLC phosphorylation is thought to increase the mobility of cross bridges by changing the charge potentials at the surface of the myosin filament (24). Therefore, it is tempting to speculate that changes in filament charge density induced by changes in sarcomere length and RLC phosphorylation regulate the length dependence of twitch potentiation in a cooperative manner. More investigation is needed to better understand the interplay between charge potentials, sarcomere length, and twitch potentiation in intact skeletal muscle.

In conclusion, we demonstrated that the length dependence of staircase potentiation is directly associated with the length dependence of Ca2+ sensitivity. This relationship is controlled, at least partially, by the changes in charge potentials on the myofilaments that help to regulate interfilament spacing.


    FOOTNOTES

Address for reprint requests and other correspondence: D. E. Rassier, Human Performance Laboratory, Faculty of Kinesiology, Univ. of Calgary, 2500 Univ. Dr., Calgary, AB, Canada T2N 1N4 (E-mail: rassier{at}kin.ucalgary.ca)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

10.1152/japplphysiol.00912.2001

Received 4 September 2001; accepted in final form 9 November 2001.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

1.   Blanchard, EM, Pan BS, and Solaro RJ. The effect of acidic pH on the ATPase activity and troponin Ca2+ binding of rabbit skeletal myofilaments. J Biol Chem 259: 3181-3186, 1984[Abstract/Free Full Text].

2.   Edman, KAP, Caputo C, and Lou F. Depression of tetanic force induced by loaded shortening of frog muscle fibres. J Physiol (Lond) 466: 535-552, 1993[Abstract/Free Full Text].

3.   Elliot, GF, Lowy J, and Worthington CR. An X-ray diffraction study of the filament lattice of striated muscle in the living state and in rigor. J Mol Biol 6: 295-305, 1963.

4.   Fuchs, F, and Wang Y. Force, length, and Ca2+-troponin C affinity in skeletal muscle. Am J Physiol Cell Physiol 261: C787-C792, 1991[Abstract/Free Full Text].

5.   Godt, RE. A simple electrostatic model can explain the effect of pH upon the force-pCa relation of skinned frog skeletal muscle fibers. Biophys J 35: 385-392, 1981[Abstract/Free Full Text].

6.   Godt, RE, and Maughan DW. Influence of osmotic compression on calcium activation and tension in skinned muscle fibers of the rabbit. Pflügers Arch 391: 334-337, 1981[ISI][Medline].

7.   Gordon, AM, Huxley AF, and Julian FJ. The variation in isometric tension with sarcomere length in vertebrate muscle fibres. J Physiol (Lond) 184: 170-192, 1966[Abstract/Free Full Text].

8.   Hill, AV. The heat of shortening and the dynamic constants of muscle. Proc R Soc Lond B Biol Sci 126: 136-195, 1938.

9.   Kawai, M, Wray JS, and Zhao Y. The effect of lattice spacing change on cross-bridge kinetics in chemically skinned rabbit psoas muscle fibers. I. Proportionality between the lattice spacing and the fiber width. Biophys J 64: 187-196, 1993[Abstract/Free Full Text].

10.   Levine, RJC, Kensler RW, Yang Z, Stull JT, and Sweeney HL. Myosin light chain phosphorylation affects the structure of rabbit skeletal muscle thick filaments. Biophys J 71: 898-907, 1996[Abstract/Free Full Text].

11.   MacIntosh, BR, and Rassier DE. Sarcomere length-dependent twitch potentiation in skeletal muscle (Abstract). Biophys J 74: 154, 1998.

12.   MacIntosh, BR, and Willis JC. Force-frequency relationship and potentiation in mammalian skeletal muscle. J Appl Physiol 88: 2088-2096, 2000[Abstract/Free Full Text].

13.   Martyn, DA, and Gordon AM. Length and myofilament spacing-dependent changes in calcium sensitivity of skeletal fibres: effects of pH and ionic strength. J Muscle Res Cell Motil 9: 428-445, 1988[ISI][Medline].

14.   Matsubara, I, and Elliot GF. X-ray diffraction studies on skinned single fibers of the frog skeletal muscle. J Mol Biol 72: 657-669, 1972[ISI][Medline].

15.   Maughan, DW, and Godt RE. Stretch and radial compression studies on relaxed skinned muscle fibers of the frog. Biophys J 28: 391-402, 1979[Abstract/Free Full Text].

16.   Millman, BM. The filament lattice of striated muscle. Physiol Rev 78: 359-391, 1998[Abstract/Free Full Text].

17.   Moore, RL, and Stull JT. Myosin light chain phosphorylation in fast and slow skeletal muscles in situ. Am J Physiol Cell Physiol 247: C462-C471, 1984[Abstract/Free Full Text].

18.   Persechini, A, Stull JT, and Cooke R. The effect of myosin phosphorylation on the contractile properties of skinned rabbit skeletal muscle fibers. J Biol Chem 260: 7951-7954, 1985[Abstract/Free Full Text].

19.   Rassier, DE, and MacIntosh BR. Length-dependence of staircase potentiation: interactions with caffeine and dantrolene sodium. Can J Physiol Pharmacol 78: 350-357, 2000[ISI][Medline].

20.   Rassier, DE, Tubman LA, and MacIntosh BR. Length-dependent potentiation and myosin light chain phosphorylation in rat gastrocnemius muscle. Am J Physiol Cell Physiol 273: C198-C204, 1997[Abstract/Free Full Text].

21.   Rassier, DE, Tubman LA, and MacIntosh BR. Caffeine and length dependence of staircase potentiation in skeletal muscle. Can J Physiol Pharmacol 76: 975-982, 1998[ISI][Medline].

22.   Sweeney, HL, and Stull JT. Phosphorylation of myosin in permeabilized mammalian cardiac and skeletal muscle cells. Am J Physiol Cell Physiol 250: C657-C660, 1986[Abstract/Free Full Text].

23.   Sweeney, HL, and Stull JT. Alteration of cross-bridge kinetics by myosin light chain phosphorylation in rabbit skeletal muscle: implications for regulation of actin-myosin interaction. Proc Natl Acad Sci USA 87: 414-418, 1990[Abstract/Free Full Text].

24.   Sweeney, HL, Yang Z, Zhi G, Stull JT, and Trybus KM. Charge replacement near the phosphorylatable serine of the myosin regulatory light chain mimics aspects of phosphorylation. Proc Natl Acad Sci USA 91: 1490-1494, 1994[Abstract/Free Full Text].

25.   Wang, Y, and Fuchs F. Osmotic compression of skinned cardiac and skeletal muscle bundles: effects on force generation, Ca2+ sensitivity and Ca2+ binding. J Mol Cell Cardiol 27: 1235-1244, 1995[ISI][Medline].

26.   Westerblad, H, Brutton JD, and Lännergren J. The effect of intracellular pH on contractile function of intact, single fibres of mouse muscle declines with increasing temperature. J Physiol (Lond) 500: 193-204, 1997[ISI][Medline].

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