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PubMed1991

Controlled expansion of peripheral nerves: comparison of nerve grafting and nerve expansion/repair for canine sciatic nerve defects.

The inherent disadvantages of nerve grafting have made it necessary to find alternative techniques for treating segmental nerve loss. This study compares the techniques of nerve grafting and nerve expansion/repair for the management of nerve injuries with segmental nerve loss in an animal model. Bilateral segmental sciatic nerve defects were created in eight dogs. On one side a 2-cm segment was excised and replaced with a nerve graft; on the other side a 2-cm defect was created with ligatures and the nerve underwent preliminary expansion and then repair. Eighteen months later nerve conduction velocity (NCV), gastrocnemius contraction force (GCF), and muscle weight (GMW) were determined for the seven surviving animals. NCV for the expanded repair was 58.66 +/- 34.18 m/sec and 47.73 +/- 7.93 for the grafted repair (p = 0.4); GCF was 619.04 +/- 353.70 gm for the expanded repair and 726.80 +/- 415.78 gm (p = 0.2) for the grafted repair; and GMW was 82.80 +/- 5.68 gm for the expanded repair and 109.55 +/- 20.63 gm (p = 0.02) for the grafted repair. The data suggest that: 1) conventional tissue expansion techniques can be used successfully to repair segmental peripheral nerve defects; 2) NCV and GCF are comparable for grafting and expansion/repair techniques although GMW is significantly higher in the grafted group; and 3) nerve expansion/repair may prove to be a useful alternative to grafting.

Action Potentials↗
PubMed1999

Ex vivo evaluation of a roller screw linear muscle actuator for an implantable ventricular assist device using trained and untrained latissimus dorsi muscles.

In this study, the power output and contraction length of trained and untrained canine latissimus dorsi (LD) muscles were measured using a roller screw linear muscle actuator (RSLMA). The RSLMA consisted of a roller screw-nut assembly and translation unit to convert the linear pull force of the muscle into an axial displacement of the roller screw. When a cable wound around a spool attached at each end of the roller screw nut was pulled in either direction, the nut was rotated which in turn advanced the roller screw in the axial direction. Under anesthesia, a Telectronics myostimulator (Model 7220) was implanted in the subcutaneous area of the canine left thoracic region with its bipolar intramuscular leads implanted around the thoracodorsal nerve and the distal muscle. A total of 6 dogs went through the myostimulator implantation, followed with 8 weeks of continuous stimulation. After completion of the training, the contraction lengths of the trained and untrained LD muscles were measured, and they were 4.25 and 5.5 cm, respectively, while the instantaneous power outputs were 4.24 and 8 W, respectively. Although untrained muscles could provide much higher instantaneous power immediately following the start of the stimulation, it diminished rapidly. On the other hand, the trained muscle showed prolonged fatigue resistance. The thoracolumbar and humeral approaches in attaching the actuator cable did not show a difference with respect to muscle power output. The trained LD muscle can provide sufficient power in the range of 3-4 W to drive a left heart assist device, but its long-term evaluation awaits development of an appropriate muscle-device interface for chronic in vivo application.

Animals↗
PubMed1994

Mechanism of inhibition of smooth muscle of guinea-pig taenia coli by chloramphenicol.

The effects of antibiotic chloramphenicol (CAP) on Ca(2+)-ATPase activity and muscle tension were examined in guinea-pig taenia coli. In general, when CAP was added to the resting tissue no inhibition was observed except when a tonus was present, caused by either ouabain, high K+ or acetylcholine. Ouabain and high K(+)-induced sustained contractions were concentration-dependently inhibited by CAP. The sustained contraction induced by high K+ was more strongly inhibited by CAP than ouabain (IC50 value: high K+ 0.29 mumol/ml; ouabain 0.34 mumol/ml). In Ca(2+)-free solution, inhibition of ouabain-induced sustained contracture by CAP was more pronounced. CAP increased the activity of Ca(2+)-ATPase in taenia coli in all experiments. In presence of cystine, CAP-induced inhibition and increase in Ca(2+)-ATPase activity could not be observed. CAP analogue thiamphenicol (TAP), devoid of p-NO2 group, showed insignificant response on smooth muscle inhibition and Ca(2+)-ATPase activity. These findings suggest that CAP inhibits smooth muscle contractility by decreasing cytosolic Ca2+ ([Ca2+]i) level through a cGMP mediated increase in Ca(2+)-ATPase activity and this action is possibly related with the p-NO2 group present in its molecule like other nitro-compounds.

Acetylcholine↗
PubMed2002

Combinatorial and cross-fiber averaging transform muscle electrical responses with a large stochastic component into deterministic contractions.

Pyloric muscles of the stomatogastric neuromuscular system of the lobster Panulirus interruptus produce highly deterministic (range, less than +/- 6% of mean amplitude) contractions in response to motor nerve stimulation with unchanging spike bursts containing physiological (5-10) spike numbers. Intracellular recordings of extrajunctional potentials (EJPs) evoked in these muscles by motor nerve stimulation revealed a large, apparently stochastic amplitude variation (range, +/- 36% of mean amplitude). These observations raised the question of how do electrical responses with a large amplitude variation give rise to deterministic muscle output? We show here that this question is likely resolved by (1) combinatorial averaging within individual muscle fibers of the multiple EJPs that occur in motor neuron bursts, and (2) averaging across muscle fibers whose electrical responses are uncorrelated. Synapses with high inherent variability are also present in vertebrate CNSs. Combinatorial averaging in multispike inputs would also reduce variation in postsynaptic response at these synapses. The data reported here provide further support that bursting presynaptic activity could make such synapses functionally deterministic as well.

Action Potentials↗
PubMed2000

Spaceflight effects on single skeletal muscle fiber function in the rhesus monkey.

The purpose of this investigation was to understand how 14 days of weightlessness alters the cellular properties of individual slow- and fast-twitch muscle fibers in the rhesus monkey. The diameter of the soleus (Sol) type I, medial gastrocnemius (MG) type I, and MG type II fibers from the vivarium controls averaged 60 +/- 1, 46 +/- 2, and 59 +/- 2 microm, respectively. Both a control 1-G capsule sit (CS) and spaceflight (SF) significantly reduced the Sol type I fiber diameter (20 and 13%, respectively) and peak force, with the latter declining from 0.48 +/- 0.01 to 0.31 +/- 0.02 (CS group) and 0.32 +/- 0.01 mN (SF group). When the peak force was expressed as kiloNewtons per square meter (kN/m(2)), only the SF group showed a significant decline. This group also showed a significant 15% drop in peak fiber stiffness that suggests that fewer cross bridges were contracting in parallel. In the MG, SF but not CS depressed the type I fiber diameter and force. Additionally, SF significantly depressed absolute (mN) and relative (kN/m(2)) force in the fast-twitch MG fibers by 30% and 28%, respectively. The Ca(2+) sensitivity of the type I fiber (Sol and MG) was significantly reduced by growth but unaltered by SF. Flight had no significant effect on the mean maximal fiber shortening velocity in any fiber type or muscle. The post-SF Sol type I fibers showed a reduced peak power and, at peak power, an elevated velocity and decreased force. In conclusion, CS and SF caused atrophy and a reduced force and power in the Sol type I fiber. However, only SF elicited atrophy and reduced force (mN) in the MG type I fiber and a decline in relative force (kN/m(2)) in the Sol type I and MG type II fibers.

Animals↗
PubMed1993

Dynamic effects and pharmacokinetics of lemakalim in the isolated guinea-pig heart.

Myocardial effects of lemakalim were studied in the isolated retrogradely perfused and spontaneously beating guinea-pig heart. Pharmacokinetic analysis of drug accumulation and disposition showed two-compartment characteristics with phasic half-times of about 0.27 and 2.1 min., respectively. Myocardial clearance was 6.8 ml min.-1, and myocardial drug accumulation was 5-fold. Dynamic parameters were studied during increasing lemakalim concentrations from 1.4 nM up to 10 microM. Dynamic steady-states developed within 3-4 min. Increase of coronary flowrate showed an Emax of 185% with an EC50 of 60 nM. Lemakalim produced biphasic inhibitory actions on contraction velocity and amplitude. Emax and EC50 for the two phases were: 48% and 47% and 100 nM and 65 nM, respectively, for the first phase and 100% in both cases and 1.7 microM and 1.6 microM for the second phase. Heart frequency increased to a maximum of 15% above baseline at 0.3 microM. Oxygen consumption increased progressively at concentrations above 7 nM and reached a maximum of 175% at 0.3 microM. The frequency-corrected QT-interval was biphasically shortened to a maximal extent of 30% at the highest concentration. Myocardial efficiency expressed as the ratio of amplitude times frequency to oxygen consumption exhibited a progressive decline to about 20% of control values. PQ- and QRS-intervals showed only minor or no changes, respectively. No arrythmogenic effects were observed. The study demonstrated a rapid and very moderate accumulation of lemakalim in the guinea-pig heart accompanied by a marked increase in coronary flowrate and a progressive negative inotropic effect followed by reduced myocardial efficiency.

Animals↗
PubMed1986

Reversibility of diabetic cardiomyopathy with insulin in rabbits.

Diabetes mellitus causes a cardiomyopathy in human subjects, independent of atherosclerotic coronary artery disease. Ventricular papillary muscle function studies in chronically diabetic rats and rabbits have shown diminished contractility and a prolonged duration of contraction. In rats there was complete reversibility of these changes with insulin therapy. However, the effects of insulin on the myocardial mechanics of diabetic rabbits have not been studied. Therefore, rabbits diabetic for 3-4 mo (after alloxan injection) were treated with PZI insulin for 3-4 mo, and the mechanical performance of their right ventricular papillary muscles was compared with that of untreated diabetic animals and age-matched controls. Insulin therapy normalized serum glucose concentration. All abnormalities in papillary muscle function were completely reversed in insulin-treated animals. Norepinephrine (NE) dose responses were also evaluated in muscles from all groups. There were no differences in the positive inotropic effects of NE between groups. However, the data suggested, in diabetic animals a blunted response of peak relaxation rate to NE; this abnormality was reversed in muscles from treated animals. These findings indicate that previous work on diabetic rats can be extended to diabetic rabbits and suggest that chronic insulin therapy completely reverses the contractile alterations in hearts from these diabetic animals.

Animals↗
PubMed1999

Development of fatigue during repeated eccentric-concentric muscle contractions of plantar flexors in patients with stroke.

OBJECTIVE: To better understand the mechanisms behind fatigue in muscles affected by a neuromuscular disease. DESIGN: Patients recruited by convenience compared to age-matched healthy subjects from a population study. SETTING: University hospital laboratory. METHODS: Repetitive eccentric-concentric plantar flexions at 60 degrees/sec were performed on a dynamometer until exhaustion. The mean power frequency and root mean square of the electromyogram were recorded, and work was calculated. SUBJECTS: Both legs of seven patients with upper motor neuron lesion from stroke and one leg of healthy men were tested. RESULTS: There were no significant (p > .05) differences in number of cycles performed or decrease of work between any of the tested legs. There was a significant (p > .05) difference in work performed by the affected leg and the reference group. Mean power frequency decreased significantly (p > .05) for the gastrocnemius muscle in the nonaffected leg and for the reference group, while no such decreases were found in the affected leg. The statistical methods used were the nonparametric tests: the Wilcoxon one-sample for differences between paired observations, and the Mann-Whitney U for differences between groups. CONCLUSION: A reduction in work in high-intensity dynamic muscle activity may not be associated with a reduction in mean power frequency, especially in patients with altered supraspinal control. There may be peripheral fatigue factors not reflected in the electromyographic activity.

Brain Infarction↗
PubMed1992

Mechanical properties of human bronchial smooth muscle in vitro.

The in vitro mechanical properties of smooth muscle strips from 10 human main stem bronchi obtained immediately after pneumonectomy were evaluated. Maximal active isometric and isotonic responses were obtained at varying lengths by use of electrical field stimulation (EFS). At the length (Lmax) producing maximal force (Pmax), resting tension was very high (60.0 +/- 8.8% Pmax). Maximal fractional muscle shortening was 25.0 +/- 9.0% at a length of 75% Lmax, whereas less shortening occurred at Lmax (12.2 +/- 2.7%). The addition of increasing elastic loads produced an exponential decrease in the shortening and velocity of shortening but increased tension generation of muscle strips stimulated by EFS. Morphometric analysis revealed that muscle accounted for 8.7 +/- 1.5% of the total cross-sectional tissue area. Evaluation of two human tracheal smooth muscle preparations revealed mechanics similar to the bronchial preparations. Passive tension at Lmax was 10-fold greater and maximal active shortening was threefold less than that previously demonstrated for porcine trachealis by us of the same apparatus. We attribute the limited shortening of human bronchial and tracheal smooth muscle to the larger load presumably provided by a connective tissue parallel elastic component within the evaluated tissues, which must be overcome for shortening to occur. We suggest that a decrease in airway wall elastance could increase smooth muscle shortening, leading to excessive responses to contractile agonists, as seen in airway hyperresponsiveness.

Aged↗
PubMed2003

Muscle mass, isokinetic torque, and functional capacity in women with osteoarthritis of the knee.

OBJECTIVES: To investigate the relations between cross-sectional area and concentric and eccentric torques in the quadriceps and hamstring muscles and to determine how functional capacity relates to pain, muscle mass, and concentric and eccentric knee torques in women who have bilateral osteoarthritis (OA) of the knee. DESIGN: Randomized, descriptive study. SETTING: A university exercise physiology laboratory in Turkey. PARTICIPANTS: Eighteen women with bilateral knee OA (grades 2 or 3) graded radiographically. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Selected functional tests included the 15-m walk, rising from a chair, descending stairs, and stair climbing. Pain during the functional tests was subjectively measured on an 11-point scale (range, 0-10). Concentric and eccentric torques of the quadriceps and hamstring muscles were measured by isokinetic dynamometry with angular velocities of 60 degrees, 120 degrees, and 180 degrees /s; cross-sectional areas of the quadriceps and hamstring muscles were measured by computed tomography. RESULTS: Eccentric torque was significantly (P range, <.05 to.001) greater than concentric torque for the quadriceps (range, 16%-100%) and hamstring (range, 50%-158%) muscles at all angular velocities. Torque-velocity curves for concentric and eccentric contractions were almost identical to those found in healthy young and elderly people. According to r(2) values, cross-sectional area of the quadriceps and hamstring muscles explained 24% to 61% (r(2) range,.24-.61) and 38% to 51% (r(2) range,.38-.51) of the variations in concentric and eccentric peak torques, respectively. Very small to moderate correlations (.01-.75) were observed among torque at any velocity and the variables of functional capacity and pain. For stair-climbing times, the best predictor variable was the eccentric hamstring to concentric quadriceps torque ratio. For stair descending, it was the concentric hamstring to eccentric quadriceps torque ratios. These torque ratios explained 81% (r(2)=.81) and 61% (r(2)=.61) of the variations, respectively. CONCLUSIONS: The findings in a patient group with bilateral OA of the knee showed that (1) eccentric torque is greater than concentric torque in knee muscles; (2) the correlation coefficients (r) between concentric and eccentric torques at different velocities (r range,.63-.86), but not between reciprocal torque ratios (r range,.02-.69), are good; (3) cross-sectional area cannot be considered as a single predictor of peak torque for either quadriceps or hamstring muscles; and (4) the variation in descending stairs and stair-climbing capacities can be explained by the reciprocal torque ratios of knee muscles.

Activities of Daily Living↗
PubMed1994

The effects of toluene diisocyanate and of capsaicin on human bronchial smooth muscle in vitro.

Toluene diisocyanate contracts guinea-pig bronchial smooth muscle through a mechanism involving capsaicin-sensitive sensory nerves. In the present study, we investigated the effects of toluene diisocyanate, capsaicin and tachykinins on isolated human bronchi. In 44 rings, toluene diisocyanate (0.3 mM) produced a relaxation which averaged 16.9 +/- 1.1%, in ten rings it produced a shortening that was 15.1 +/- 3.3% and in ten preparations it gave no response. A second administration of toluene diisocyanate (0.3 mM) always produced a relaxation (n = 13, 18.1 +/- 3.9%). Capsaicin (0.03 mM) produced shortening in 15 (35 +/- 6.6%) and relaxation in 11 preparations (41 +/- 6.8%), whereas a second administration caused shortening in nine (25.1 +/- 6.1%) and relaxation in 16 rings (36.4 +/- 4.9%). When toluene diisocyanate was given after two consecutive capsaicin administrations, we observed shortening in two rings (10.0 +/- 3.6%), relaxation in ten rings (15.9 +/- 3.6%), and no response in four preparations. To test the role of NK1 and NK2 receptors in these conflicting responses, we performed concentration-response curves to different tachykinins. Substance P, neurokinin A and neurokinin A-(4-10), a specific NK2 receptor agonist, gave a concentration-dependent shortening, with neurokinin A being the most effective and neurokinin A-(4-10) the least. The specific NK1 receptor agonist, [Sar9, Met(O2)11]substance P, produced both shortening and relaxation. We conclude that toluene diisocyanate and capsaicin may produce both shortening and relaxation in isolated human bronchi through NK1 receptors.

Acetylcholine↗
PubMed2002

Complexities in ETS-domain transcription factor function and regulation: lessons from the TCF (ternary complex factor) subfamily. The Colworth Medal Lecture.

The ETS-domain transcription factor family can be divided into a series of subfamilies. Elk-1 represents the founding member of the ternary complex factor (TCF) subfamily. By focusing on the TCF subfamily, we can demonstrate the complexities that exist in the function and regulation of ETS-domain transcription factors. This article focuses on Elk-1 in detail and summarizes the functions of other TCFs. The key themes covered include the domain structure of the TCFs, the mechanisms of complex formation with serum response factor, regulation of TCFs by mitogen-activated protein kinase cascades, and transcriptional regulatory properties of the TCFs. Finally, the emerging role of the TCFs in vivo is discussed. A picture is developing indicating that, while these proteins exhibit significant sequence and functional conservation, key differences in their structure and regulation are being identified which may relate to unique functions of these proteins in vivo.

Amino Acid Sequence↗
PubMed2001

Hemodynamic effects of supervised calf muscle exercise in patients with venous leg ulceration: a prospective controlled study.

HYPOTHESIS: Because more than two thirds of patients with venous ulcer have an impaired calf muscle pump, enhancement of its ejecting ability with physical training may generate an improved hemodynamic milieu sufficient to promoting ulcer healing. This study evaluated the effects of short-term supervised calf exercise on calf muscle pump function and venous hemodynamics in limbs with venous ulceration. DESIGN: Prospective controlled study. SETTINGS: University-associated tertiary care hospital. PATIENTS: The study consisted of 2 groups. An exercise group comprised 10 patients (median age, 72 years) receiving supervised isotonic calf muscle exercise for 7 consecutive days. A control group comprised 11 patients matched with those in the exercise group for age, sex, ulcer size, and ulcer duration (all, P>.09). Patients in both groups had perimalleolar venous leg ulcers, impaired calf muscle function (ejection fraction, <60%), and full ankle joint movement. INTERVENTIONS: After providing a complete clinical history, both groups underwent a physical examination, venous duplex scanning, and air plethysmography. The venous filling index, venous volume, residual venous volume, and residual volume fraction of the calf on standing were measured plethysmographically at baseline and on day 8, in addition to calf muscle endurance as determined by the maximal number of plantar flexions performed against a fixed 4-kg resistance during 6 minutes (1 flexion/s). Operators were blinded to the subject's group. Exercise in the first group entailed consecutive active plantar flexions using a standardized 4-kg resistance pedal ergometer. Subjects daily completed 3 sets of flexions of 6 minutes each. All patients had short-stretched compression bandaging. MAIN OUTCOME MEASURES: The ejected venous volume and ejection fraction were evaluated in both groups at baseline and on day 8. RESULTS: Both groups had a similar hemodynamic performance at baseline for all the variables evaluated (P>.10). After 7 days of exercise, patients in the exercise group improved their ejected venous volume by 67.5%, ejection fraction by 62.5%, residual venous volume by 25% (all 3, P =.006), and their residual volume fraction by 28.6% (P =.008). Changes in the control group within the same period were small (all, P>.10). By day 8, the exercise group had a significantly better ejected venous volume (P<.001) and ejection fraction (P<.001) than the control group. The venous filling index and the venous volume did not change (P>.50) in either study group. Calf muscular endurance in the exercise group increased 135%, from a median 153 plantar flexions at baseline to 360 on day 7 (P<.001). CONCLUSIONS: By increasing the muscular endurance, efficacy, and power of the calf muscle, isotonic exercise improves its ejecting ability and the global hemodynamic status in limbs with venous ulceration. Prospective evaluations of the clinical effects of calf muscle pump strengthening for the treatment of venous leg ulceration are indicated by the results of this study.

Aged↗
PubMed1996

Neural influence on slow muscle properties: inactivity with and without cross-reinnervation.

The effects of altering (cross-reinnervation, CR) and/or eliminating (spinal isolation, SI) the activation-dependent neural input to the cat soleus for 8 months on its mechanical and biochemical properties were determined. The percent fast fibers was 0, 27, 38, and 54% for normal control (NC), NC-CR, SI, and SI-CR cats. Calcium-activated whole muscle myofibrillar adenosine triphosphatase activity was higher in SI-CR and NC cats. Isometric twitch speed properties and frequency of stimulation:tension response were faster in all experimental groups compared to NC. Maximum twitch and tetanic (Po) force, physiological cross-sectional area (PCSA), and specific tension (Po/PCSA) were unaffected in NC-CR cats, but reduced in SI and SI-CR cats. The soleus was more fatigable in SI-CR compared to NC cats. Alpha-glycerophosphate dehydrogenase activity was higher in all experimental groups compared to NC. All eight parameters reflecting the type of muscle were shifted toward those observed in a "faster" muscle in SI-CR vs. SI cats. These data suggest that electrically silent motoneurons can influence type-related skeletal muscle properties.

Animals↗
PubMed1996

The effect of progressive resistance training in rheumatoid arthritis. Increased strength without changes in energy balance or body composition.

OBJECTIVE: To demonstrate the feasibility of high-intensity progressive resistance training in rheumatoid arthritis (RA) patients compared with healthy control subjects. METHODS: Eight subjects with RA, 8 healthy young subjects, and 8 healthy elderly subjects underwent 12 weeks of high-intensity progressive resistance training, while 6 elderly subjects performed warm-up exercises only. Fitness, body composition, energy expenditure, function, disease activity, pain, and fatigue were measured at baseline and followup. RESULTS: All 3 training groups demonstrated similar improvements in strength compared with the change among control subjects (RA group 57% [P < 0.0005], young exercise group 44% [P < 0.01], elderly exercise group 36% [P < 0.05]). Subjects with RA had no change in the number of painful or swollen joints but had significant reductions in self-reported pain score (21% [P < 0.05]) and fatigue score (38% [P = 0.06]), improved 50-foot walking times (mean +/- SD 10.4 +/- 2.2 seconds versus 8.3 +/- 1.5 seconds [P < 0.005]), and improved balance and gait scores (48.9 +/- 3.8 versus 50.4+/- 2.0 [P = 0.07]). CONCLUSION: High-intensity strength training is feasible and safe in selected patients with well-controlled RA and leads to significant improvements in strength, pain, and fatigue without exacerbating disease activity or joint pain.

Adult↗
PubMed1985

Euler buckling in a wheelbarrow obstacle course: a catastrophe with complex lag.

This article studies human physical work capacity under increasing load from a general systems theory perspective. There are several points of generalization between the structure of nonliving (building materials) and living systems (humans) with respect to stress, strain, and fatigue. A catastrophe model for Euler buckling was transposed and tested for human performance in a wheelbarrow obstacle course under varying loads. Subjects were 129 employees of a Midwest manufacturing plant. A cusp model was hypothesized and verified (R2 = .68, control R2 = .11) where vertical load was the asymmetry factor, and body balance, height, and sex-related differences all contributed to bifurcation. A catastrophe model in codimension 10 was also invoked to explain memory in the system. Principal control variables were exercise habits, weight, balance, and sex-related differences (R2 = .75). The core model of human load-to-failure was concluded to be similar to that for Euler buckling: additional complexities were discovered which were attributed in part to systemic memory. Discussion points included the use of large dimension catastrophe models for problems involving complex lag effects, and the transposability of the model to the organizational systemic level.

Adipose Tissue↗
PubMedDate not supplied

Experimental myotonia induced in denervated muscles by 2,4-D.

Rats were denervated in one hind limb and injected with 2,4-dichlorophenoxyacetic acid (2,4-D). Isotonic tetanic contractions of the muscles treated with 2,4-D after more than 10 days of denervation revealed prolonged relaxation times similar to those of the intact side and characteristic of clinical myotonia. No myotonic discharges were observed in the muscles denervated for more than 10 days and treated with 2,4-D. The increase in threshold for action potential generation secondary to denervation is suggested as the factor limiting the initiation of the repetitive discharges.

2,4-Dichlorophenoxyacetic Acid↗
PubMed1990

Twitch relaxation of the cat soleus muscle at different lengths and temperatures.

We recorded isometric and isotonic twitches, in situ, from the cat soleus at various muscle lengths and temperatures. At a given temperature the duration of isometric twitches increased approximately 60% for each 10% increase in muscle length, which was primarily owing to decreases in the rate of relaxation. For the relaxation of isometric twitches recorded at different muscle lengths, the equivalent activation energies determined were the same (13.2 +/- 0.3 kcal/M). The duration of isotonic twitch contractions increased only 20% for each 10% increase in muscle length. Even a small amount of shortening (3%) diminished the dependence of twitch duration on muscle length. In this case, twitch duration increased approximately 30% for every 10% increase in muscle length. Hence, even small changes in internal and/or external compliance (eg, changes in the tendon-fiber continuity) can greatly influence twitch duration. Our findings are consistent with the hypotheses that in the cat soleus, Ca2+ sequestration is primarily governed by a single energy dependent process and that the Ca2+ sensitivity of the contractile apparatus increases with increasing sarcomere length.

Animals↗