Intracellular EDTA mimics parvalbumin in the promotion of skeletal muscle relaxation.

Parvalbumin (PA) is an intracellular Ca2+-binding protein found in some muscle and nerves. Its ability to bind Ca2+ and facilitate skeletal muscle relaxation is limited by its Mg2+ off-rate. EDTA serves as an "artificial" PA in that it exhibited similar rate constants for Mg2+ (3 s-1) and Ca2+ (0.7 s-1) dissociation at 10 degrees C. When introduced into frog skeletal muscle, EDTA increased the relaxation rate by approximately 2.7-fold, and with increasing tetanus duration, EDTA lost its ability to contribute to relaxation (and Ca2+ sequestration) at its Mg2+ off-rate. Intracellular EDTA recovered its ability to contribute to muscle relaxation and Ca2+ sequestration at its Ca2+ off-rate. Like PA, EDTA's contribution to muscle relaxation and Ca2+ sequestration was more clearly observed when the SR Ca-ATPase was inhibited. Introduction of EDTA into rat soleus muscle, which has low [PA], increased the relaxation rate in a manner that was analogous to the way in which PA facilitates relaxation of frog skeletal muscle. Thus intracellular EDTA serves as an effective mimic of PA, and its use should aid in our understanding of PA's function in muscle and nerve.

Determinants of relaxation rate in skinned frog skeletal muscle fibers.

The influences of sarcomere uniformity and Ca2+ concentration on the kinetics of relaxation were examined in skinned frog skeletal muscle fibers induced to relax by rapid sequestration of Ca2+ by the photolysis of the Ca2+ chelator, diazo-2, at 10 degreesC. Compared with an intact fiber, diazo-2-induced relaxation exhibited a faster and shorter initial slow phase and a fast phase with a longer tail. Stabilization of the sarcomeres by repeated releases and restretches during force development increased the duration of the slow phase and slowed its kinetics. When force of contraction was decreased by lowering the Ca2+ concentration, the overall kinetics of relaxation was accelerated, with the slow phase being the most sensitive to Ca2+ concentration. Twitchlike contractions were induced by photorelease of Ca2+ from a caged Ca2+ (DM-Nitrophen), with subsequent Ca2+ sequestration by intact sarcoplasmic reticulum or Ca2+ rebinding to caged Ca2+. These twitchlike responses exhibited relaxation kinetics that were about twofold slower than those observed in intact fibers. Results suggest that the slow phase of relaxation is influenced by the degree of sarcomere homogeneity and rate of Ca2+ dissociation from thin filaments. The fast phase of relaxation is in part determined by the level of Ca2+ activation.

Role of calcium and crossbridges in modulation of rates of force development and relaxation in skinned muscle fibers.

The influence of Ca2+ and force generating crossbridges on the kinetics of force development and relaxation was examined in skinned muscle fibers activated by photolytic release of Ca2+ from a caged calcium or inactivated by photolytic uptake of Ca2+ by a caged Ca2+ chelator. In frog fibers at 10 degrees C, decreasing the Ca2+ released from caged calcium to an extent that resulted in 50% of maximum force development produced an approximate seven-fold decrease in the rate of contraction. In contrast decreasing the number of force generating crossbridges by partial extraction of troponin C (TnC) or addition of vanadate caused only minor changes in contraction rate. Thus the rate of force development decreases dramatically with decreases in the Ca2+ level which suggests that a step in the crossbridge cycle may be Ca2+ dependent. The kinetics of relaxation induced by photolysis of diazo-2 was: a) slowed by stabilization of the sarcomeres by repeated releases and re-stretches during contraction and b) accelerated when the amplitude of force development was decreased by decreasing the [Ca2+] which induced a steady contraction. The half time of relaxation decreased by approximately two- to three-fold, when 50% of maximum force was developed. One interpretation of these results is that decreasing the number of force generating crossbridges may speed relaxation by inducing a decreased affinity of TnC for Ca2+ and thus accelerating the Ca2+ dissociation rate from TnC and thereby increasing relaxation rate.

Role of calcium and cross bridges in determining rate of force development in frog muscle fibers.

The influence of Ca2+ and force-generating cross bridges on the kinetics of force development was examined in skinned frog muscle fibers activated by photolytic release of Ca2+ from caged Ca2+ at 10 degrees C. Isometric force development was fit by a double exponential equation with rates of 44.4 s-1 (kc1) and 6.1 s-1 (kc2); kc1 was not significantly different from the rate of force development observed in intact fibers. Maximum activation by caged Ca2+ from preexisting submaximal force produced rates of contraction similar to those observed with maximum activation from zero force. Decreasing the Ca2+ level to an extent that resulted in 50% of maximum force development produced an approximately sevenfold decrease in kc1 and no change in kc2. Partial extraction of troponin C reduced kc1 only slightly (by 16%), whereas decreasing the number of force-generating cross bridges by vanadate did not decrease kc1. Neither treatment altered kc2. Thus the rate of force development increases dramatically with increases in Ca2+ level.

Parvalbumin relaxes frog skeletal muscle when sarcoplasmic reticulum Ca(2+)-ATPase is inhibited.

Inhibition of sarcoplasmic reticulum (SR) Ca(2+)-adenosinetriphosphatase (ATPase) with 2,5-di-(tert-butyl)-1,4-benzohydroquinone (TBQ) in frog skeletal muscle fibers at 10 degrees C prolonged the half time of the fall of the Ca2+ transient by 62% and twitch force by 100% and increased peak force by 120% without increasing the amplitude of the Ca2+ signal. In the presence of TBQ the rate of relaxation and the rate of fall of Ca2+ became progressively slower in a series of twitches until relaxation failed. Relaxation rate decreased with a time course (approximately 2 s-1) similar to the Mg2+ off rate from purified parvalbumin (PA; 3.6 s-1). TBQ slowed the rate of fall of Ca2+ (5-fold) and force (8-fold) in a 0.3-s tetanus so that the rate of fall of Ca2+ (approximately 2.5 s-1) was similar to the Mg2+ off rate from PA. TBQ caused a near total failure of both Ca2+ sequestration and relaxation in a 1.1-s tetanus, during which PA would be saturated with Ca2+ and could not contribute to relaxation. Thus, when the SR Ca(2+)-ATPase is inhibited, Mg(2+)-PA can sequester Ca2+ and produce relaxation at a rate that is defined by the Mg2+ off rate from PA.

Role of parvalbumin in relaxation of frog skeletal muscle.

Experiments were done to test the hypothesis that parvalbumin (PA) promotes relaxation in frog skeletal muscle. Single fibers and purified PA from R. temporaria skeletal muscle were used to determine the relationship between Ca2+ and Mg2+ dissociation rates from PA and changes in relaxation rate as a function of isometric tetanus duration at 0 degrees C. Relaxation rate slows as a function of tetanus duration with a rate constant of 1.18 s-1. Recovery of relaxation rate after a prolonged tetanus exhibits a rate constant of 0.12 s-1. Dissociation rate constants for Mg2+ and Ca2+ from purified PA are 0.93 s-1 and 0.19 s-1, respectively. Thus rates of slowing and recovery of relaxation rate may be controlled by Mg2+ and Ca2+ dissociation from PA, respectively. The influence of temperature on relaxation rate and on Ca2+ and Mg2+ dissociation rates from purified PA also was examined. The magnitude of slowing of relaxation rate with increasing tetanus duration, relative to the final, steady value of relaxation rate, is greater at 0 than at 10 degrees C. In the 0 to 10 degrees C range, the Q10 for relaxation rate increases with increasing tetanus duration. Both of these observations can be explained if the Q10 for Ca2+ uptake by the sarcoplasmic reticulum is greater than the Q10 for Ca2+ sequestration by PA during relaxation. When Ca2+ and Mg2+ dissociation rates from PA at various temperatures are compared to other proposed indicators of PA function, it is concluded that PA facilitates relaxation of frog skeletal muscle in the 0 to 20 degrees C range.

Effect of temperature on relaxation rate and Ca2+, Mg2+ dissociation rates from parvalbumin of frog muscle fibres.

1. Influence of temperature on relaxation rate as a function of isometric tetanus duration and on Ca2+ and Mg2+ dissociation rates from purified parvalbumin (PA) was examined to test the hypothesis that PA promotes relaxation in frog skeletal muscle. Single fibres and PA IVB from Rana temporaria skeletal muscle were utilized. 2. The magnitude of slowing of relaxation rate with increasing tetanus duration, relative to the final, steady value of relaxation rate, is 3-fold greater at O than at 10 degrees C. 3. In the 0-10 degrees C range, the Q10 for relaxation rate increases from 2.3 to 3.7 with increasing tetanus duration. 4. Dissociation of Ca2+ and Mg2+ from PA exhibited: (i) rate constants of 1.03 +/- 0.03 s-1 (mean +/- S.D., n = 5) and 3.42 +/- 0.14 s-1 (n = 5) at 20 degrees C and (ii) Q10 values of 2.3 and 1.9 in the 0-20 degrees C range, respectively. 5. Time courses of slowing of relaxation rate with increasing tetanus duration and recovery of relaxation rate with rest after a prolonged tetanus at 10 degrees C are similar to rates of dissociation of Mg2+ and Ca2+ from PA, respectively, as previously reported at 0 degree C. 6. Both the temperature dependence of the relative magnitude of slowing of relaxation rate and the increased Q10 of relaxation rate with increased tetanus duration can be explained if the Q10 for Ca2+ uptake by the sarcoplasmic reticulum is greater than the Q10 for Ca2+ sequestration by PA during relaxation. When Ca2+ and Mg2+ dissociation rates from PA at various temperatures are compared to other proposed indicators of PA function, it is concluded that PA facilitates relaxation of frog skeletal muscle in the 0-20 degrees C range.

Parvalbumin content and Ca2+ and Mg2+ dissociation rates correlated with changes in relaxation rate of frog muscle fibres.

1. Experiments were done to test the hypothesis that parvalbumin (PA) promotes relaxation in frog skeletal muscle. Single fibres and purified PA from Rana temporaria skeletal muscle were used to determine the relationship between PA concentration ( [PA] ), Ca2+ and Mg2+ dissociation rates from PA and changes in rate of relaxation as a function of tetanus duration at 0 degrees C. 2. Total [PA] in fibres from tibialis anterior muscles is 0.76 +/- 0.20 mmol PA l-1 myoplasmic water (mean +/- S.D., n = 25) with 65% PA IVa and 35% PA IVb, where PA IVa and PA IVb are PA isoforms. 3. Relaxation rate from an isometric tetanus shows progressively as a function of tetanus duration with an exponential time course and a rate constant of 1.18 +/- 0.35 s-1 (n = 17). Time course of recovery of relaxation rate after a prolonged tetanus is exponential with a rate constant of 0.12 +/- 0.02 s-1 (n = 14). 4. The extent of recovery of relaxation rate after a prolonged tetanus was correlated with total [PA] in fibres (correlation coefficient (r) = 0.80, n = 7; P less than 0.05). 5. Dissociation rate constants for Mg2+ and Ca2+ from purified PA are 0.93 +/- 0.02 s-1 (n = 5) and 0.19 +/- 0.01 s-1 (n = 5), respectively. Dissociation rate constants were not significantly different for PA isoforms IVa and IVb. These rate constants are similar to the rate constants determined for the time courses of slowing and recovery of relaxation rate, respectively. 6. Results suggest that the time courses of slowing and recovery of relaxation rate may be controlled, to a large extent, by Mg2+ and Ca2+ dissociation from PA, respectively. This evidence supports a role for PA in facilitating relaxation during a tetanus in frog skeletal muscle at 0 degrees C.

Influence of temperature on mechanics and energetics of muscle contraction.

Results gleaned from use of temperature as a probe to study skeletal muscle performance and mechanisms of activation and contraction are reviewed. Steady-state and non-steady-state responses to changes in temperature are considered. Temperature sensitivities, Q10 values, of mechanical and energetic parameters range from nearly 1 to greater than 5 in frog skeletal muscle. Factors that are less temperature sensitive (Q10 less than or equal to 1.5) are peak tetanic force, instantaneous stiffness, curvature of force-velocity relation, magnitude of labile heat, and mechanical efficiency. Rates with intermediate temperature sensitivities (Q10 greater than 2 but less than 3) include rate of isometric force development, maximum shortening velocity, and relaxation from a brief tetanus. Rates with high temperature sensitivities (Q10 greater than 3) include cross-bridge turnover during an isometric tetanus, isometric economy, maximum power output, Ca2+ sequestration by sarcoplasmic reticulum, relaxation from a prolonged tetanus, and recovery metabolism. The observation that the Q10 for relaxation rate depends on tetanic duration can be explained in terms of the possible role of parvalbumin as a soluble relaxing factor.

Variation in the normalized tetanic force of single frog muscle fibres.

1. The forces produced in maximal fixed-end tetani of single fibres isolated from the anterior tibialis muscle of the frog Rana temporaria have been measured at sarcomere lengths of 2.2 microns and temperatures near 0 and 10 degrees C. 2. When normalized by either cross-sectional area or dry weight per unit length at a sarcomere length of 2.2 microns, the forces vary over a twofold range. 3. The normalized force is not significantly correlated with the velocity of unloaded shortening or the twitch characteristics of the fibres. Lack of variability of these two quantities (together with histochemical evidence) suggest that only one fibre type is present in the experimental sample. 4. The steady rate of energy liberation (stable, heart rate) of the fibres during isometric tetani is positively correlated with the normalized force, indicating that extra ATP splitting is required to produce higher forces. 5. Fibres with a higher ratio of dry weight per unit length to cross-sectional area ('dry density') show a higher force when normalized by area, but not when normalized by dry weight per unit length. 6. Fibres with a more circular cross-sectional profile produce more force when normalized by either cross-sectional area or dry weight per unit length. The significance of this correlation is unclear. 7. The contribution of various sources to the total overall variation in normalized force is assessed. It is suggested that a diffusible substance or substances may be involved in modulating fibre force.

Phase-front measurements of an injection-locked AlGaAs laser-diode array.

The phase-front quality of the primary spatial lobe emitted from an injection-locked gain-guided AlGaAs laser diode array is measured by using an equal-path, phase-shifting Mach-Zehnder interferometer. Root-mean-square phase errors of 0.037 +/- 0.003 wave (lambda/27) are measured for the single spatial lobe, which contained 240-mW cw output power in a single longitudinal mode. This phase-front quality corresponds to a Strehl ratio of S = 0.947, which results in a 0.23-dB power loss from the single lobe's ideal diffraction-limited power. These values are comparable with those measured for single-stripe index-guided AlGaAs lasers.

Temperature dependence of the crossbridge cycle during unloaded shortening and maximum isometric tetanus in frog skeletal muscle.

The primary objective of this study was to determine if the rate limiting step in the crossbridge cycle was the same during maximum rate of shortening and during maintenance of maximum tension in an isometric contraction. To this end the temperature dependence, Q10, of the crossbridge cycle was estimated during unloaded shortening and maximum isometric tetanus. Isolated semitendinosus muscles from the frog were studied at 0 and 10 degrees C. Crossbridge cycling during unloaded shortening was determined from the velocity of unloaded shortening estimated by the slack step technique. Crossbridge cycling during maintained isometric tetanus was determined from the steady rate of energy liberation during the tetanus after allowance for energy liberation due to Ca2+ cycling. The Q10 of the velocity of unloaded shortening was 2.5 and that of the steady rate of energy liberation was 4.6. After correction for the temperature dependence of energy liberation associated with Ca2+ cycling (5.7), the estimated Q10 of the steady rate of energy liberation became 3.9. These estimates of the Q10 of the crossbridge cycle are significantly different. These results support the conclusion that the rate limiting steps during unloaded shortening and maximum isometric force maintenance occur at different steps in the crossbridge cycle. Further the high Q10 of the energy liberation due to Ca2+ cycling may relate to the high concentration of parvalbumin in frog muscle. A second objective of this study was to document in the same muscle the variation of Q10s of mechanical and energetic properties of contraction. Over this temperature range the Q10s ranged from 1.1 to 5.7.

Absolute values of myothermic measurements on single muscle fibres from frog.

The heat and force produced in tetanic contraction of single fibres from anterior tibialis muscle of the frog Rana temporaria have been observed at measured temperatures close to 1 and 10 degrees C. Heat was measured using a Hill-Downing type thermopile. In control experiments with a resistor of known heat capacity comparable to a single muscle fibre, it was found that Peltier and Joule heating produced identical thermopile outputs. The Peltier method was used to introduce a known amount of heat into the system in each experiment with a muscle fibre. From the response to this heating the heat capacity of each preparation was obtained and used to calculate the absolute amount of heat production from the thermopile output. The heat produced during tetanic contraction (H) could be described by Aubert's equation [H = Ha (1 - e-t/tau) + hbt]. In some fibres there was no labile heat (Ha), whereas in others it was clearly present. The stable heat rate (hb) was strongly temperature dependent (Q10 = 4.06). At 0 degree C the stable heat rate (normalized by dry weight) in the single fibres was significantly greater than that in whole anterior tibialis muscle.

Energetics and mechanics of frog skeletal muscle in hypotonic solution.

Hypotonic solutions are known to potentiate muscle force production and increase actomyosin ATPase activity in solution. As such, both the rate and amount of ATP hydrolysis should increase during contraction. This was tested indirectly by measuring force and energy liberation in Rana pipiens semitendinosus muscles at 0 degrees C in hypotonic solution. Force and the amount and rate of energy liberation increased. This increase is consistent with the interpretation that the rate of ATP hydrolysis is increased in hypotonic solution. Muscles, stretched to beyond myofilament overlap, liberate a substantial fraction of the energy liberated at maximal myofilament overlap. This energy liberation, the activation heat, is thought to reflect the energy utilized to cycle Ca2+. Hypotonic solution decreased the amplitude and the rate of the activation heat, suggesting that the amount and rate of Ca2+ cycled by the sarcoplasmic reticulum is reduced. Thus in hypotonic solution, force production and the rate of ATP hydrolysis by the cross bridges are potentiated despite an apparent decrease in Ca2+ cycling.

Energetics of activation in frog skeletal muscle at sarcomere lengths beyond myofilament overlap.

Experiments were designed to gain information about the effects of extremely long sarcomere lengths (greater than 3.8 microns) on muscle activation. The amount of energy liberated in an isometric twitch by muscles stretched to sarcomere lengths where myofilament overlap is vanishingly small (greater than 3.6 microns) is thought to be an indirect measure of the Ca2+ cycled during contraction. The effects of altering sarcomere length from 3.8 to 4.3 microns on the amount of Ca2+ cycled was measured using twitch energy liberation as an indicator of the Ca2+ cycled. Twitch energy liberation decreased by approximately 20% over this sarcomere length region, suggesting that the amount of Ca2+ released by a single action potential is not altered dramatically when a muscle is stretched to extreme lengths.

Energetic aspects of skeletal muscle contraction: implications of fiber types.

In this chapter fundamental energetic properties of skeletal muscles as elucidated from isolated muscle preparations are described. Implications of these intrinsic properties for the energetic characterization of different fiber types and for the understanding of locomotion have been considered. Emphasis was placed on the myriad of physical and chemical techniques that can be employed to understand muscle energetics and on the interrelationship of results from different techniques. The anaerobic initial processes which liberate energy during contraction and relaxation are discussed in detail. The high-energy phosphate (approximately P) utilized during contraction and relaxation can be distributed between actomyosin ATPase or cross-bridge cycling (70%) and the Ca2+ ATPase of the sacroplasmic reticulum (30%). Muscle shortening increases the rate of approximately P hydrolysis, and stretching a muscle during contraction suppresses the rate of approximately P hydrolysis. The economy of an isometric contraction is defined as the ratio of isometric mechanical response to energetic cost and is shown to be a fundamental intrinsic parameter describing muscle energetics. Economy of contraction varies across the animal kingdom by over three orders of magnitude and is different in different mammalian fiber types. In mammalian skeletal muscles differences in economy of contraction can be attributed mainly to differences in the specific actomyosin and Ca2+ ATPase of muscles. Furthermore, there is an inverse relationship between economy of contraction and maximum velocity of muscle shortening (Vmax) and maximum power output. This is a fundamental relationship. Muscles cannot be economical at developing and maintaining force and also exhibit rapid shortening. Interestingly, there appears to be a subtle system of unknown nature that modulates the Vmax and economy of contraction. Efficiency of a work-producing contraction is defined and contrasted to the economy of contraction. Unlike economy, maximum efficiency of work production varies little across the animal kingdom. There are difficulties associated with the measurement of maximum efficiency of contraction, and it has yet to be determined unequivocally if the maximum efficiency of contraction varies in different fiber types. The intrinsic properties of force per cross-sectional area, economy, and Vmax determine the basic energetic properties of skeletal muscles. Nonetheless, the mechanics and energetics of skeletal muscles in the body are profoundly influenced by muscle architecture, attachment of muscles to the skeleton, and motor unit organization.(ABSTRACT TRUNCATED AT 400 WORDS)