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A M Gordon

Publications and source records attributed to A M Gordon.

At least 55 records · Page 3Linked to original sources
PubMed1997

Characterization of troponin-C interactions in skinned barnacle muscle: comparison with troponin-C from rabbit striated muscle.

Previously it was shown that when troponin-C (TnC) is extracted from barnacle myofibrillar bundles they lose their Ca2+ sensitivity, which can be restored by adding back barnacle TnC (either isoform, BTnC1 or BTnC2). Thus barnacle muscle shows thin filament regulation, as does rabbit psoas skeletal muscle. In this paper we compare the interactions of barnacle and rabbit fast muscle TnC in their respective muscles. We demonstrate that muscle fibres from the giant barnacle, Balanus nubilus, contain about 186 microM kg-1 muscle tissue of BTnC1 plus BTnC2 compared to about 91 microM kg-1 of TnC in rabbit psoas muscle fibres. Extraction of BTnC is achieved using similar low ionic strength, low divalent ion Ca(2+)-low Mg2+ conditions which are required for TnC extraction in rabbit psoas skinned muscle fibres; extraction was prevented by 1 mM Mg2+. Full reconstitution of Ca(2+)-sensitivity was achieved by adding back BTnC (1 + 2, or 2). Reconstitution of barnacle muscle with rabbit fast skeletal TnC (RTnC) was more complex, with partial recovery of Ca(2+)-sensitivity with reconstitution in the presence of 3 mM Mg2+ and more fully with reconstitution in the presence of activating Ca2+ (pCa 4.0). This suggests that the barnacle TnC-TnI (troponin I) recognition sites may be more complex than in rabbit because the barnacle sites appear to have at least two different conformations or types, in which one recognizes RTnC in the presence of Mg2+ and the other only in the presence of Ca2+ and Mg2+. This is consistent with the presence of several TnI isoforms in barnacle striated myofibrils. RTnC has two C-terminal Ca(2+)-Mg2+ binding sites that are thought to be involved in the Mg(2+)-sensitive binding of RTnC in rabbit muscle, yet it has been suggested that this site in barnacle muscle does not bind Mg2+, even though Mg2+ stabilizes BTnC binding in barnacle muscle. Consistent with this stabilizing action of Mg2+, using fluorescent probes IAANS or IAE on isolated BTnC2 we demonstrate that BTnC2 binds both Ca2+ and Mg2+, but the data do not suggest direct competition. Consistent with the C-terminal sites on BTnC being Ca(2+)-specific, BTnC1 + 2 could only reconstitute low levels of force (about 1/3) in TnC-extracted rabbit skinned muscle fibers in the presence of pCa 4.0 (not just Mg2+) and only at low ionic strengths (0.09 M). Ca(2+)-activation of contraction was further examined using fluorescently labelled BTnC2 (labelled with IANBD) incorporated into skinned barnacle myofibrillar bundles. Maximal Ca2+ binding produced structural changes in BTnC which resulted in a 45% decrease in the fluorescence compared to the value at pCa 9.2. The magnitude of the fluorescence decrease paralleled the increase in force with increasing Ca2+. The Hill fits to the data gave pCa1/2 and n of 5.61 +/- 0.02 and 2.06 +/- 0.12 for force, and 5.52 +/- 0.02 and 1.88 +/- 0.10 for fluorescence. Removing MgATP to induce rigor in the fibre decreased BTnC2-NBD fluorescence only about 11%, but the addition of Ca2+ in rigor further decreased the fluorescence to a slightly larger extent than under maximal Ca2+ activating conditions. These fluorescence changes are qualitatively similar to the fluorescence enhancement seen with Ca(2+)-activation and rigor with RTnCDanz exchanged into rabbit psoas skinned muscle fibres. The data support a similar model for Ca(2+)-activation of force in barnacle muscle and in rabbit psoas skeletal muscle fibres.

Animals↗
PubMed1997

Models of calcium activation account for differences between skeletal and cardiac force redevelopment kinetics.

To explain observed differences in the activation dependence of force redevelopment kinetics between cardiac and skeletal muscle, two numerical models of contractile regulation by Ca2+ were investigated. Ca2+ binding and force production were each modelled as two-state processes with forward and reverse rate constants taken from the literature. The first model incorporates four possible thin-filament states. In the second model Ca2+ is assumed not to dissociate from a thin-filament unit in the force-generating state, resulting in three states. The four-state model can account for the activation dependence of the rate constant of tension redevelopment (ktr) seen in skeletal muscle, without requiring that Ca2+ directly modulates the kinetics of any step in the cross-bridge cycle. Using identical kinetic parameters, the three-state model shows no activation dependence of ktr, consistent with our results in cardiac muscle. Following a step increase in [Ca2+], the rate of rise in tension (as described by the rate constant kCa) varies with the final [Ca2+] for both models, consistent with experimental results from skeletal and cardiac muscle. These numerical models demonstrate that experimental measurements thought to reveal changes in kinetic parameters may simply reflect coupling between the two kinetic processes of Ca2+ binding and force generation. Furthermore, the models present possible differences in the Ca2+ activation scheme between cardiac and skeletal muscle which can account for the contrasting activation dependencies of force redevelopment kinetics.

Animals↗
PubMed1996

Kinetic studies of calcium binding to the regulatory site of troponin C from cardiac muscle.

We have studied the kinetics of the structural transitions induced by calcium binding to the single, regulatory site of cardiac troponin C by measuring the rates of calcium-mediated fluorescence changes with a monocysteine mutant of the protein (C35S) specifically labeled at Cys-84 with the fluorescent probe 2(-)[4'-(iodoacetamido)anilino]naphthalene-6-sulfonic acid. At 4 degrees C, the binding kinetics determined in the presence of Mg2+ was resolved into two phases with positive amplitude, which were completed in less than 100 ms. The rate of the fast phase increased linearly with [Ca2+] reaching a maximum of approximately 590 s-1, and that of the slow phase was approximately 100 s-1 and did not depend on Ca2+ concentration. Dissociation of bound Ca2+ from the regulatory site occurred with a rate of 102 s-1, whereas the dissociation from the two high affinity sites was about two orders of magnitude slower. These results are consistent with the following scheme for the binding of Ca2+ to the regulatory site: [formula: see text] where the asterisks denote states with enhanced fluorescence. The apparent second-order rate constant for calcium binding is Kok1 = 1.4 x 10(8) M 1 s-1. The two first-order transitions occur with observed rates of k1 + kappa-1 approximately 590 s-1 and kappa 2 + kappa-2 approximately 100 s-1, and the binding of Ca2+ to the regulatory site is not a simple diffusion-controlled reaction. These transitions provide the first information on the rates of Ca(2+)-induced conformational changes involving helix movements in the regulatory domain.

Animals↗
PubMed1996

Influence of Ca2+ on force redevelopment kinetics in skinned rat myocardium.

The influence of Ca2+ on isometric force kinetics was studied in skinned rat ventricular trabeculae by measuring the kinetics of force redevelopment after a transient decrease in force. Two protocols were employed to rapidly detach cycling myosin cross-bridges: a large-amplitude muscle length ramp followed by a restretch back to the original length or a 4% segment length step. During the recovery of force, the length of the central region of the muscle was controlled by using a segment marker technique and software feedback control. Tension redevelopment was fit by a rising exponential governed by the rate constant ktr for the ramp/restretch protocol and kstep for the step protocol. ktr and kstep averaged 7.06 s-1 and 15.7 s-1, respectively, at 15 degrees C; neither ktr nor kstep increased with the level of Ca2+ activation. Similar results were found at submaximum Ca2+ levels when sarcomere length control by laser diffraction was used. The lack of activation dependence of ktr contrasts with results from fast skeletal fibers, in which ktr varies 10-fold from low to high activation levels, and suggests that Ca2+ does not modulate the kinetics of cross-bridge attachment or detachment in mammalian cardiac muscle.

Adenosine Triphosphate↗
PubMed1995

Determination of resting free calcium in barnacle muscle using modified aequorins, buffered calcium injections, and simultaneous image-intensified video microscopy.

Knowing the resting free calcium is important in understanding the role of calcium as an intracellular second messenger. We used a bracketing (null) technique with a luminescent calcium indicator, aequorin, microinjection and image-intensification to measure free calcium in single muscle fibres from the barnacle, Balanus nubilus. We injected modified aequorins (recombinant, and hch-) which after a 30 min diffusion gave reasonable resting glows. Subsequent injection of calcium (strongly buffered with either EGTA or BAPTA, 10 mM) increased or decreased the resting glow depending on the free calcium level in the injected buffer solution. This bracketing (null) method is inherently accurate, but mechanical artifacts on calcium injection reduce the accuracy when total light emission is measured. We therefore used image-intensified video-microscopy of the injected region and video processing (Image-1) of artifact-free regions, to greatly improve the consistency. The luminescence in a pre-selected region of the muscle fibre was measured as a function of time during the injection. Solution calciums were chosen so that if the first injection decreased the resting glow, the second increased it, or vice versa, thus bracketing the true resting value. We used two methods to determine the true value bracketed by our injections: (1) a linear interpolation using the fractional changes in luminescence or (2) a power law interpolation assuming a 2.2 or 2.5 power relationship between luminescence and free calcium. Using these methods, we estimated the free calcium level in the lateral depressor fibres of freshly dredged barnacles to be 279 +/- 36 nM (+/- SD), 339 +/- 42 nM, or 352 +/- 45 nM for the linear, 2.2 and 2.5 powers respectively under the conditions of hch-aequorin and BAPTA buffers (using a K'Ca for BAPTA of 3.0 x 10(6) M-1 for our conditions). Recombinant-aequorin gave essentially the same result while EGTA buffers yielded a somewhat higher value but because of influences of pH on the K'Ca for EGTA (taken as 6.7 x 10(6) M-1 for our conditions) was considered less reliable. Minor changes in [Mg2+] upon buffer injection can lead to underestimates of the true resting [Ca2+] by at most 10%. Thus, we estimate the resting free calcium in barnacle muscle fibres to be 300-380 nM.

Aequorin↗
PubMed1995

Use of tactile afferent information in sequential finger movements.

We have investigated how tactile afferent information contributes to the generation of sequences of skilled finger movements by anesthetizing the right index fingers of experienced typists. Subjects were asked to type phrases in which the right index finger was used only once every seven to 12 keypresses. The time at which each key was depressed was recorded with a digital timer, and the translational and rotational motion of the fingers and wrist of the right hand were recorded optoelectronically from the location of reflective markers placed on the fingers. Midway through the experiment, a local anesthetic was injected at the base of the distal phalange of the right index finger. Following digital anesthesia, error rates increased considerably, mainly due to the diminished accuracy of movements of the anesthetized finger. The typing intervals following keypresses with the anesthetized fingertip were unaffected by the removal of tactile information. When errors occurred during control trials, the intervals immediately following the errors were greatly prolonged. However, errors produced with the anesthetized right index finger did not influence the timing of subsequent keypresses, implying that lack of tactile cues affected error recognition. The movement patterns during keypresses were similar before and after digital anesthesia for some subjects, while a less pronounced flexion-extension movement was seen in other subjects. The results suggest that tactile afferent information is not essential for initiating movement segments in a sequence. Rather, they emphasize the importance of this information for ensuring movement accuracy and for detecting errors.

Adult↗
PubMed1995

Asymmetric control of bilateral isometric finger forces.

We examined the ability to match the voluntary isometric finger flexion forces of the dominant and nondominant hand in humans, as well as the influence of unilateral visual feedback during this task. Right- and left-handed subjects were trained to produce a "low" force level (50 +/- 25 g) and a "high" force level (200 +/- 50 g) with the right and left index finger, separately. Following the training session, subjects were instructed to match the isometric forces of both fingers simultaneously within the required range (either low or high) so that they were perceived to be identical. The results showed an asymmetry, whereby greater forces were exerted with the index finger of the dominant hand. The asymmetry was independent of the subjects' maximum finger flexion strength. When unilateral visual feedback represented the force output of the dominant hand, the asymmetry was no longer present. In contrast, when it represented the force output of the nondominant hand, the asymmetry was not compensated. We hypothesize that these findings are the result of anatomical or physiological asymmetries inherent in the motor system controlling the production of force.

Adult↗
PubMed1995

Contribution of tactile afferent information to the control of isometric finger forces.

The ability to match the voluntary isometric force output of the right and left index fingers when the contact surfaces differ in shape was examined. Before the experiment, subjects were trained to produce both a "low" force level (50 +/- 25 g) and a "high" force level (200 +/- 50 g) with the right and left index finger, separately. Following the training session, subjects were instructed to match the forces of both fingers simultaneously within the required range (either low or high) so that the forces were perceived to be identical. One of the index fingers pushed against a conical contact pad, while the other pushed against a flat contact pad. Midway through the experiment, the two contact pads were reversed. Subjects consistently produced less force with the finger pressing against the conical pad. This asymmetry could already be seen during the beginning of the ramp increase in force and continued throughout the trial, independent of the target force levels (low or high). These findings suggest tactile afferent information at the finger-tip is important for determining the voluntary force exerted by the finger. It must be properly integrated with other peripheral information as well as with the central motor command, otherwise the perception of force is distorted. Furthermore, the perception of the force produced seemingly is dependent on the extent to which the skin of the fingertip is indented rather than the local pressure exerted at the skin.

Adult↗
PubMed1995

Tactile control of isometric fingertip forces during grasping in children with cerebral palsy.

Sensory mechanisms and sensorimotor integration have often been thought to be disturbed in children with cerebral palsy (CP). The authors investigated the influence of tactile information from the fingertips on the isometric force regulation during the lift of a small object with the precision grip in children with CP and controls. Children with CP were able to modify their grip force according to friction between the fingertips and the object and could use tactile information for anticipatory control of the force-scaling of the precision grip, but they seemed to need predictable conditions and successive lifts to build up memory representation of an object's friction.

Case-Control Studies↗
PubMed1994

Formation and lateralization of internal representations underlying motor commands during precision grip.

The capability to store and retrieve weight-related information from a lift to scale the force output during a subsequent lift was examined in 10 healthy adults and 50 children (age 2-10 years), as well as a 22-year-old patient with corpus callosum agenesis. Subjects lifted a test object between the thumb and index finger while the isometric fingertip forces were measured. The results suggest that both healthy children and adults can transfer weight-related information between the right and left hand, although a lateralization was found. Also, the storage and retrieval of weight-related information appears to be a dynamic process dependent on both previous sensory information and knowledge of future movements. Late maturation of interhemispheric connections and asymmetric loss of some information during the transfer between hemispheres suggest a lateralization of the internal representation. The patient with a corpus callosum agenesis supported this hypothesis.

Adolescent↗
PubMed1994

Unloaded shortening of skinned muscle fibers from rabbit activated with and without Ca2+.

Unloaded shortening velocity (VUS) was determined by the slack method and measured at both maximal and submaximal levels of activation in glycerinated fibers from rabbit psoas muscle. Graded activation was achieved by two methods. First, [Ca2+] was varied in fibers with endogenous skeletal troponin C (sTnC) and after replacement of endogenous TnC with either purified cardiac troponin C (cTnC) or sTnC. Alternatively, fibers were either partially or fully reconstituted with a modified form of cTnC (aTnC) that enables force generation and shortening in the absence of Ca2+. Uniformity of the distribution of reconstituted TnC across the fiber radius was evaluated using fluorescently labeled sTnC and laser scanning fluorescence confocal microscopy. Fiber shortening was nonlinear under all conditions tested and was characterized by an early rapid phase (VE) followed by a slower late phase (VL). In fibers with endogenous sTnC, both VE and VL varied with [Ca2+], but VE was less affected than VL. Similar results were obtained after extraction of TnC and reconstitution with either sTnC or cTnC, except for a small increase in the apparent activation dependence of VE. Partial activation with aTnC was obtained by fully extracting endogenous sTnC followed by reconstitution with a mixture of aTnC and cTnC (aTnC:cTnC molar ratio 1:8.5). At pCa 9.2, VE and VL were similar to those obtained in fibers reconstituted with sTnC or cTnC at equivalent force levels. In these fibers, which contained aTnC and cTnC, VE and VL increased with isometric force when [Ca2+] was increased from pCa 9.2 to 4.0. Fibers that contained a mixture of a TnC and cTnC were then extracted a second time to selectively remove cTnC. In fibers containing aTnC only, VE and VL were proportional to the resulting submaximal isometric force compared with maximum Ca(2+)-activated control. With aTnC alone, force, VE, and VL were not affected by changes in [Ca2+]. The similarity of activation dependence of VUS whether fibers were activated in a Ca(2+)-sensitive or -insensitive manners implies that VUS is determined by the average level of thin filament activation and that, with sTnC or cTnC, VUS is affected by Ca2+ binding to TnC only.

Animals↗
PubMed1994

The learning of novel finger movement sequences.

1. Experienced typists typed phrases containing words in which one isolated letter was typed with one hand, while the remaining letters were typed with the contralateral hand. 2. The translational and rotational motion of the fingers and wrist of the right hand were obtained optoelectronically from the location of reflective markers placed on the fingers. 3. Midway through the experiment, the key corresponding to the isolated letter was physically switched with another key on the keyboard, and subjects typed the letter in its new location (for 140 trials). The letter "n," typed with the right index finger, was either switched with letters normally typed with the same finger (u), with a different finger but same hand (o), with the same finger of the left hand (v), or with a different finger of the left hand (w). 4. When the words were typed normally, the interkey intervals were relatively short, and the onset of movement of the right hand began before the preceding keypress with the left hand. Thus the movement of the two hands overlapped. Furthermore, the movement to the isolated key was highly stereotypical, with little trial-to-trial variability. 5. After the transposition of keys, there were prolongations in the interkey intervals, with the largest delay occurring directly before the typing of the transposed key. Switches between homologous fingers (involving mirror movements) delayed the onset of keypresses to a lesser extent than did other switches. With practice, these delays were reduced but never reached the control level. 6. After the keyswitch, the onset of movement to the isolated key did not occur on average until after the last keypress with the contralateral hand, except when the switch involved the use of homologous fingers. In the latter case, overlapping movement of the two hands was maintained. Thus the learning of a series of discrete movements does not necessarily require that each movement segment be performed sequentially. 7. After the transposition of keys, the movement pattern and time course to a given key were similar to the movement patterns for that key observed during control trials in all conditions. Thus the learning of a series of movements may involve the use of previously learned movements under new conditions. 8. The results suggest that typing movements may be organized at several levels, including the individual keystroke and word level.

Adult↗
PubMed1993

Cross-bridges affect both TnC structure and calcium affinity in muscle fibers.

In vertebrate striated muscle, calcium binding to troponin initiates contraction, a strong interaction of actin and myosin. In isolated proteins and skinned fibers, the strong interaction of myosin with actin also affects troponin. Fluorescent labels attached to troponin C show structural changes in the TnC environment with cross-bridge attachment and also with calcium binding. Evidence that this effect of crossbridges also occurs in intact striated muscle comes from studies in partially activated cardiac or skeletal muscle by others and in barnacle muscle by us. Length changes which detach myosin cross-bridges produce a brief burst of extra calcium that can be detected by aequorin in activated, voltage clamped single barnacle muscle fibers. That this calcium is coming from calcium bound to the activating site (troponin-C) is supported by several pieces of evidence. Studies on the dependence of the extra calcium on force and the time of the length change are consistent with the amplitude of the extra calcium being proportional to the bound calcium (CaTnC) and with increased cross-bridge attachment and force increasing calcium binding to troponin-C by up to a factor of 10. Importantly, stretch of active muscle (which first detaches cross-bridges and then enhances steady force) gives a biphasic response: first extra calcium (presumably due to cross-bridge detachment) and then, decreased calcium (presumably due to enhanced calcium binding to TnC). The enhanced calcium binding we see with elevated force (via strained cross-bridges) implies that calcium binding to TnC is enhanced not only be cross-bridge attachment but also by crossbridge (or thin filament) strain. This effect of cross-bridge attachment/force on calcium binding is consistent with a dual mechanism of calcium activation of contraction. First, calcium binds to troponin in the thin filament activating strong myosin binding to the thin filament. Then, strong myosin binding in turn provides additional activation either by increasing calcium binding or by changing the thin filament structure directly allowing additional cross-bridge attachment.

Animals↗
PubMed1993

Filament overlap affects TnC extraction from skinned muscle fibres.

Recent studies on calcium regulation of muscle contraction selectively extract troponin C (TnC) from skinned skeletal muscle fibres with a low ionic strength rigor solution containing a Ca2+/Mg2+ chelator. As previous results from this laboratory and others demonstrate a crossbridge effect, especially rigor, on many of the properties of TnC, the effects of filament overlap on TnC extraction from skinned rabbit psoas muscle fibres were investigated. Tension-pCa relationships at a sarcomere length of 2.7 microns were determined before and after a 5 min TnC extraction at sarcomere lengths of 2.3, 2.5, 2.7, 3.1, 3.3 or 3.5 microns with 20 mM Tris, pH 7.8, 5 mM EDTA. The decrease in the post-extraction maximum Ca2+ activated tension, an indicator of the amount of TnC extracted, was linearly related to the overlap of the thick and thin filaments with decreases in tension being associated with a decrease in filament overlap. The smaller fibre diameter at the longer sarcomere length could facilitate diffusion of TnC from fibre segments. However, the wide range of measured diameters, 40-120 microns, accounted for only 14% of the observed tension decrement and shrinking the fibre with polyvinylpyrrolidone did not increase the tension decrement. Increasing the sarcomere length before extraction was also found to decrease the TnC content of fibre segments along with the post-extraction maximum tension. Thus, TnC appears to be preferentially extracted from non-overlap than overlap regions of the sarcomere. These results further indicate that rigor crossbridges affect TnC other than through increased Ca2+ binding and that under the conditions used here, they retard its extraction.

Animals↗
PubMed1993

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

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

Animals↗
PubMed1993

Subsarcomeric distribution of calcium in demembranated fibers of rabbit psoas muscle.

Direct measurements were made of the Ca distribution within sarcomeres of glycerinated rabbit psoas muscle fibers in rigor using electron probe x-ray microanalysis. Both analogue raster analysis and digital x-ray imaging were used to quantitate the Ca distribution along thick and thin filaments as a function of the concentration of free Ca2+. Even when corrected for the estimated contribution of Ca bound to thick filaments, the Ca measured in the region of overlap between thick and thin filaments significantly exceeded the Ca in the I-band at subsaturating concentrations of free Ca2+. At saturating levels of free Ca2+, the excess Ca in the overlap region was diminished but still statistically significant. The data thus suggest that the formation of rigor linkages exerts multiple effects on the binding of Ca2+ to thin filaments in the overlap region by increasing the affinity of troponin C for Ca2+ and possibly by unmasking additional Ca2+ binding sites. The data also show that the cooperativity invested in the thin filaments is insufficient to permit the effects of rigor cross-bridge formation on Ca2+ binding to propagate far along the thin filaments into the I-band.

Animals↗
PubMed1993

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.

Animals↗