Extensor motoneurone properties are altered immediately before and during fictive locomotion in the adult decerebrate rat

This study examined motoneurone properties during fictive locomotion in the adult rat for the first time. Fictive locomotion was induced via electrical stimulation of the mesencephalic locomotor region in decerebrate adult rats under neuromuscular blockade to compare basic and rhythmic motoneurone properties in antidromically identified extensor motoneurones during: (1) quiescence, before and after fictive locomotion; (2) the 'tonic' period immediately preceding locomotor-like activity, whereby the amplitude of peripheral flexor (peroneal) and extensor (tibial) nerves are increased but alternation has not yet occurred; and (3) locomotor-like episodes. Locomotion was identified by alternating flexor-extensor nerve activity, where the motoneurone either produced membrane oscillations consistent with a locomotor drive potential (LDP) or did not display membrane oscillation during alternating nerve activity. Cells producing LDPs were referred to as such, while those that did not were referred to as 'idle' motoneurones. LDP and idle motoneurones during locomotion had hyperpolarized spike threshold (Vth ; LDP: 3.8 mV; idle: 5.8 mV), decreased rheobase and an increased discharge rate (LDP: 64%; idle: 41%) during triangular ramp current injection even though the frequency-current slope was reduced by 70% and 55%, respectively. Modulation began in the tonic period immediately preceding locomotion, with a hyperpolarized Vth and reduced rheobase. Spike frequency adaptation did not occur in spiking LDPs or firing generated from sinusoidal current injection, but occurred during a sustained current pulse during locomotion. Input conductance showed no change. Results suggest motoneurone modulation occurs across the pool and is not restricted to motoneurones engaged in locomotion.

Adaptations of motoneuron properties to chronic compensatory muscle overload

The aim of the study was to determine whether chronic muscle overload has measurable effect on electrophysiological properties of motoneurons (MNs), and whether duration of this overload influences intensity of adaptations. The compensatory overload was induced in the rat medial gastrocnemius (MG) by bilateral tenotomy of its synergists (lateral gastrocnemius, soleus, and plantaris); as a result, only the MG was able to evoke the foot plantar flexion. To assure regular activation of the MG muscle, rats were placed in wheel-equipped cages and subjected to a low-level treadmill exercise. The intracellular recordings from MG motoneurons were made after 5 or 12 wk of the overload, and in a control group of intact rats. Some of the passive and threshold membrane properties as well as rhythmic firing properties were considerably modified in fast-type MNs, while remaining unaltered in slow-type MNs. The significant changes included a shortening of the spike duration and the spike rise time, an increase of the afterhyperpolarization amplitude, an increase of the input resistance, a decrease of the rheobase, and a decrease of the minimum current necessary to evoke steady-state firing. The data suggest higher excitability of fast-type MNs innervating the overloaded muscle, and a shift towards electrophysiological properties of slow-type MNs. All of the adaptations could be observed after 5 wk of the compensatory overload with no further changes occurring after 12 wk. This indicates that the response to an increased level of chronic activation of MNs is relatively quick and stable.

Plasticity of rat motoneuron rhythmic firing properties with varying levels of afferent and descending inputs

Hindlimb motoneuron excitability was compared among exercise-trained (E), sedentary (S), and spinal cord transected (T) Sprague-Dawley rats by examining the slope of the frequency-current (F/I) relationship with standard intracellular recording techniques in rats anesthetized with ketamine-xylazine. The T group included spinal transected and spinal isolated rats; the E animals were either spontaneously active (exercise wheel) or treadmill trained; and rats in the S group were housed in pairs. An analysis of motoneuron initial [1st interspike interval (ISI)], early (mean of 1st three ISIs), and steady-state (mean of last 3 ISIs) discharge rate slopes resulting from increasing and decreasing 500-ms injected square-wave depolarizing current pulses was used to describe rhythmic motoneuron properties. The steepest slope occurred in the S group (55.3 ± 22.2 Hz/nA), followed by the T group (35.5 ± 15.3 Hz/nA), while the flattest slope was found in the E group (25.4 ± 10.9 Hz/nA). The steepest steady-state slope occurred in the S group but was found to be similar between the T and E groups. Furthermore, a spike-frequency adaptation (SFA) index revealed a slower adaptation in motoneurons of the E animals only (∼40% lower). Finally, evidence for a secondary range of firing existed more frequently in the T group (41%) compared with the S (12%) and E (31%) groups. The lower F/I slope and lower SFA index of motoneurons for E rats may be a result of an increase in Na(+) conductance at the initial segment. The results show that motoneuronal rhythmic firing behavior is plastic, depending on the volume of daily activation and on intact descending pathways.

Insight into skeletal muscle mechanotransduction: MAPK activation is quantitatively related to tension

The mechanism by which mechanical forces acting through skeletal muscle cells generate intracellular signaling, known as mechanotransduction, and the details of how gene expression and cell size are regulated by this signaling are poorly understood. Mitogen-activated protein kinases (MAPKs) are known to be involved in mechanically induced signaling in various cell types, including skeletal muscle where MAPK activation has been reported in response to contraction and passive stretch. Therefore, the investigation of MAPK activation in response to mechanical stress in skeletal muscle may yield important information about the mechanotransduction process. With the use of a rat plantaris in situ preparation, a wide range of peak tensions was generated through passive stretch and concentric, isometric, and eccentric contractile protocols, and the resulting phosphorylation of c-Jun NH(2)-terminal kinase (JNK), extracellular regulated kinase (ERK), and p38 MAPKs was assessed. Isoforms of JNK and ERK MAPKs were found to be phosphorylated in a tension-dependent manner, such that eccentric > isometric > concentric > passive stretch. Peak tension was found to be a better predictor of MAPK phosphorylation than time-tension integral or rate of tension development. Differences in maximal response amplitude and sensitivity between JNK and ERK MAPKs suggest different roles for these two kinase families in mechanically induced signaling. A strong linear relationship between p54 JNK phosphorylation and peak tension over a 15-fold range in tension (r(2) = 0.89, n = 32) was observed, supporting the fact that contraction-type differences can be explained in terms of tension and demonstrating that MAPK activation is a quantitative reflection of the magnitude of mechanical stress applied to muscle. Thus the measurement of MAPK activation, as an assay of skeletal muscle mechanotransduction, may help elucidate mechanically induced hypertrophy.

Habitual exercise enhances neuromuscular transmission efficacy of rat soleus muscle in situ

Rat motor nerve terminals and the endplates they interact with exhibit changes to varying patterns of use, as when exposed to increased activation in the form of endurance exercise training. The extent to which these changes affect neuromuscular transmission efficacy is uncertain. In this study, the effects of habitual exercise on the electrophysiological properties of neuromuscular transmission in rat soleus muscle were investigated using a novel in situ approach. Consistent with previous reports, miniature endplate potential frequency was enhanced by habitual exercise. Other passive properties, such as resting membrane potential, miniature endplate potential amplitude, and "giant" miniature endplate potential characteristics were unaltered by the training program. Full-size endplate potentials were obtained by blocking soleus muscle action potentials with mu-conotoxin GIIIb. Quantal content values were 91.5 and 119.9 for control and active groups, respectively (P < 0.01). We also measured the rate and extent of endplate potential amplitude rundown during 3-s trains of continuous stimulation at 25, 50, and 75 Hz; at 50 and 75 Hz, we found both the rate and extent of rundown to be significantly attenuated (10--20%) in a specific population of cells from active rats (P < 0.05). The results establish the degree of activity-dependent plasticity as it pertains to neuromuscular transmission in a mammalian slow-twitch muscle.

Changes in electrophysiological properties of tibial motoneurones in the rat following 4 weeks of tetrodotoxin-induced paralysis

In this study, we test the hypothesis that 4 weeks tetrodotoxin (TTX) paralysis altered the passive membrane properties of rat tibial motoneurones. Impulse activity along the sciatic nerve was blocked for 4 weeks using TTX delivered by an osmotic minipump to a Silastic cuff placed around the nerve. That portion of the sample exhibiting the 20% slowest After-hyperpolarization (AHP) decay time (AHPd), and which therefore included presumptive type S motoneurons, demonstrated responses (reduced AHPd, increased rheobase and rheobase voltage), which were not evident in the rest of the sample (presumptive fast motoneurons), in which an increased AHPd, in fact, was found. The results thus support the hypothesis that retrograde signals from inactive slow and fast muscle fibers have different effects on their innervating motoneurones.

Myosin heavy chains in fibers of TTX-paralyzed rat soleus and medial gastrocnemius muscles

The expression of five myosin heavy chain (MHC) isoforms was analyzed in the rat soleus (Sol) and the deep and superficial medial gastrocnemius (dGM, sGM) muscle after 2 and 4 wk of TTX paralysis by using immunohistochemical techniques. In Sol, after 4 wk of paralysis, fibers containing type I MHC were either pure type I (14%) or also contained developmental (D; 76%), IIa (26%), or IIx (18%) MHC. Values for corresponding fibers in dGM were 8.5, 65, 38, and 22%. Also, by 4 wk an increase was seen in the proportions of fibers expressing IIa MHC in Sol (from 16 to 38%) and dGM (from 24 to 74%). In a region of sGM in control muscles containing pure IIb fibers, a major proportion (86%) remained pure after 4 wk of paralysis, with the remainder coexpressing IIb and IIx. The results indicate that TTX-induced muscle paralysis results in an increase in fibers containing multiple MHC isoforms and that the D isoform appears in a major proportion of these hybrid fibers.

Fatigability of rat hindlimb muscles after acute irreversible acetylcholinesterase inhibition

The purpose of this study was to investigate the functional impact of acute irreversible inhibition of acetylcholinesterase (AChE) on the fatigability of medial gastrocnemius and plantaris muscles of Sprague-Dawley rats. After treatment with methanesulfonyl fluoride (a lipid-soluble anticholinesterase), which reduced their AChE activity by >90%, these muscles were subjected to an in situ indirect stimulation protocol, including a series of isolated twitch and tetanic contractions preceding a 3-min fatigue regimen (100-ms trains at 75 Hz applied every 1.5 s). During the first minute of the fatigue regimen, the effects of AChE inhibition were already near maximal, including marked reductions in peak tension and the force-time integral (area), as well as a decrement of compound muscle action potential amplitudes within a stimulus train. Neuromuscular transmission failure was the major contributor of the force decreases in the AChE-inhibited muscles. However, despite this neuromuscular transmission failure, muscles of which all AChE molecular forms were nearly completely inhibited were still able to function, although abnormally, during 3 min of intermittent high-frequency nerve stimulation.

Salbutamol effect in spinal cord injured individuals undergoing functional electrical stimulation training

OBJECTIVE

Preliminary study to investigate possible changes in skeletal muscle morphology and function, as well as hormonal and metabolic effects, after treatment with a selective beta2-adrenergic receptor agonist.

DESIGN

Double-blind, placebo-controlled trial.

PARTICIPANTS

Three individuals with spinal cord injury (SCI).

INTERVENTION

Two-week treatment with salbutamol (2mg) or placebo (ascorbic acid, 50mg) twice a day. Program of functional electronic stimulation (FES) cycling for 30 minutes twice a week.

MAIN OUTCOME MEASURES

Body weight, three measures of leg circumference (gluteal furrow, one third of subischial height up from tibial-femoral joint space, and minimum circumference above the knee), muscle fiber area, and total work output per session.

RESULTS

There were increases in body weight (2.30 +/- .70kg), leg circumferences (gluteal furrow 1.70 +/- .27cm, one third subischial height 1.53 +/- 1.65cm, minimum circumference above the knee .43 +/- .04cm), and muscle (vastus lateralis) cross-sectional area (1,374 +/- 493 to 2,446 +/- 1,177microm2) after salbutamol treatment, whereas quadriceps muscle contractile function was not modified. Total work output during FES cycling sessions was increased more during salbutamol treatment (64%) compared with training alone (27%). Salbutamol treatment was associated with a large decrease in skeletal muscle beta-adrenergic receptor density.

CONCLUSION

Although some side effects were noted, these results suggest that a short treatment with the beta2-adrenergic receptor agonist salbutamol during a training program with FES cycling could be beneficial in patients with SCI.

Nifedipine does not impede clenbuterol-stimulated muscle hypertrophy

The mechanism(s) responsible for beta2-adrenergic receptor-mediated skeletal muscle and cardiac hypertrophy remains undefined. This study examined whether calcium influx through L-type calcium channels contributed to the development of cardiac and skeletal muscle (plantaris; gastrocnemius; soleus) hypertrophy during an 8-day treatment with the beta2-adrenergic receptor agonist clenbuterol. Concurrent blockade of L-type calcium channels with nifedipine did not reverse the hypertrophic action of clenbuterol. Moreover, nifedipine treatment alone resulted in both cardiac and soleus muscle hypertrophy (6% and 7%, respectively), and this effect was additive to the clenbuterol-mediated hypertrophy in the heart and soleus muscles. The hypertrophic effects of nifedipine were not associated with increases in total beta-adrenergic receptor density, nor did nifedipine reverse clenbuterol-mediated beta-adrenergic receptor downregulation in either the left ventricle or soleus muscle. Both nifedipine and clenbuterol-induced hypertrophy increased total protein content of the soleus and left ventricle, with no change in protein concentration. In conclusion, our results support the hypothesis that beta2-adrenergic receptor agonist-induced muscle hypertrophy is mediated by mechanisms other than calcium influx through L-type calcium channels.

Endurance training increases acetylcholine receptor quantity at neuromuscular junctions of adult rat skeletal muscle

The aim of the study was to test the hypothesis that a 16 week endurance training program would alter the abundance of endplate-associated nicotinic acetylcholine receptors (nAChR) in various rat skeletal muscles. We found a 20% increase in endplate-specific [125I]alpha-bungarotoxin binding in several muscles of trained rats, accompanied by equal susceptibility of toxin binding to the inhibitory effect of D-tubocurarine in sedentary and trained muscles. We conclude that the neuromuscular junction adaptations that occur with increased chronic activation include an increase in nAChR number. Results of experiments designed to determine nAChR turnover also suggest that this effect is mediated by an alteration in the receptor's metabolic state. The potential implications and mechanisms of this adaptation are discussed.

The case for adaptability of the neuromuscular junction to endurance exercise training

Although the adaptability of the neuromuscular junction (NMJ) has been demonstrated using the models of denervation/reinnervation, electrical stimulation, development, aging, and pathological states, relatively little is known about the effects of increased chronic voluntary use on the morphology and physiological function of the NMJ. A review of findings relating to adaptations in the various pre- and postsynaptic components of the NMJ with exercise training is presented. These findings are discussed as they pertain to NMJ function during exercise. Other physiological modulators of the NMJ, such as trophic factors released by nerve terminals and muscles, and circulating substances are discussed in terms of possible roles they may play in training-induced adaptations.

The effect of diazepan and exercise training on selected biochemical and histochemical properties of rat skeletal muscle

The effects of chronic diazepam (D) treatment and exercise training on total body mass (TBM), microsomal protein yield (MPY), calcium uptake by fragmented sarcoplasmic reticulum (SR), muscle fibre cross-sectional area, and both PFK and SDH activities were investigated in the tibialis anterior (TA), soleus (Sol), and plantaris (Plt) muscles of 50 male albino Sprague-Dawley rats. Rats were assigned randomly to control (C), sprint-trained (S), or endurance-trained (E) groups. Training was of 12 weeks duration. One-half of each group received daily intraperitoneally D doses of 5 mg kg-1 of TBM. Exercise reduced TBM (p < 0.05); increased the relative BM of the TA (E = 2.02 +/- 0.02, p < 0.01) and Plt (E = 1.15 +/- 0.02, p < 0.01; S = 1.13 +/- 0.03, p < 0.01), as well as the Ca++ uptake of the Sol SR (C = 0.08 +/- 0.02, E = 0.16 +/- 01, p < 0.05). MPY was elevated in S-Sol (C = 1.12 +/- 0.6, S = 1.52 +/- 0.1, p < 0.01). D elevated Sol MPY as well as TA PFK. S-trained animals had lower mean fibre areas than the E-trained (D-treated and untreated) animals. The elevated relative masses of TA and Plt are explained by a decreased TBM with exercise. The increased Ca++ uptake of the Sol indicates that E enhances this function, and the increased MPY probably implies an increased SR. The D could be responsible for the D-elevated Sol MPY as well as the TA PFK. El D did not reduce neuromuscular activity to a level adversely affecting oxidative enzyme activity, but in the case of PFK activity in the TA muscle, such a reduction was evident.

Chronic beta-blockade increases skeletal muscle beta-adrenergic-receptor density and enhances contractile force

The effects of a chronic 14-day administration of a selective beta2-adrenergic-receptor antagonist (ICI-118551) on skeletal muscle were evaluated in female Sprague-Dawley rats. Chronic ICI-118551 treatment did not modify muscle mass, oxidative potential, or protein concentration of the medial gastrocnemius muscle, suggesting that maintenance of these skeletal muscle characteristics is not dependent on beta2-adrenergic-receptor stimulation. However, the drug treatment increased beta-adrenergic-receptor density of the lateral gastrocnemius (42%) and caused an increase in specific (g/g) isometric in situ contractile forces of the medial gastrocnemius [twitch, 56%; tetanic (200 Hz), 28%]. The elevated contractile forces observed after a chronic treatment with ICI-118551 were completely abolished when the beta2-adrenergic antagonist was also administered acutely before measurement of contractile forces, suggesting that this response is beta2-adrenergic-receptor dependent. Possible mechanisms for the increased forces were studied. Caffeine administration potentiated twitch forces but had little effect on tetanic force in control animals. Administration of dibutyryl adenosine 3',5'-cyclic monophosphate in control animals also resulted in small increases of twitch force but did not modify tetanic forces. We conclude that increases in beta-adrenergic-receptor density and the stimulation of the receptors by endogenous catecholamines appear to be responsible for increased contractile forces but that the mechanism remains to be demonstrated.

The effects of tetrodotoxin-induced muscle paralysis on the physiological properties of muscle units and their innervating motoneurons in rat

1. Although the inactivity of a slow muscle (cat soleus) induced via nerve impulse blockade has been demonstrated to have some axotomy-like effects (decreased after-hyperpolarization (AHP) duration) on its innervating motoneurons, the reported effects of inactivity on motoneurons which innervate fast muscles containing mixtures of motor unit types are equivocal. This study was designed to determine the effect of a period (2 weeks) of complete hindlimb muscle paralysis, via tetrodotoxin (TTX) blockade of sciatic nerve impulses, on the contractile (muscle units) and electrophysiological (motoneurons) properties of motor units in the rat gastrocnemius. Motoneuron properties were also compared with those of rats subjected to sciatic nerve axotomy 2 weeks earlier. 2. At the time of the terminal experiment (24 h after the removal of the TTX delivery system) in anaesthetized animals, properties of tibial motoneurons (i.e. rheobase current, input resistance, time course of after-potentials) were determined using conventional microelectrode techniques. For those tibial motoneurons innervating the gastrocnemius, muscle unit responses (i.e. twitch force and time course, maximum tetanic tension, fatigability) were also recorded in response to current injection. 3. Consistent with previously reported whole-muscle responses to TTX-induced disuse, the TTX-treated gastrocnemius muscle units showed weaker tetanic forces, prolonged twitches and elevated twitch/tetanic ratios. These effects were similar for motor units classified as small, medium and large according to their tetanic tension-generating capacities. Muscle unit fatigue resistances appeared to be unchanged. 4. The mean values, distributions and ranges of tibial motoneuron properties were similar between control and TTX-treated groups for rheobase, input resistance and AHP half-decay time. In the case of the latter, the proportion of motoneurons possessing "slow' AHP half-decay times (> 20 ms) was not significantly different in control (17%) and TTX-treated groups (11%). 5. Motoneurons axotomized 2 weeks earlier had a significantly higher (42-69%) mean input resistance and a longer (34-42%) mean AHP half-decay time when compared with the control and TTX-treated groups. 6. It appears that, for fast muscles containing several different motor unit types, TTX-induced axon blockade does not produce similar effects on motoneuron intrinsic properties to those evoked by axotomy. This lack of effect on the distribution and range of these properties of tibial motoneurons indicates that none of the motoneurons which innervate muscles of mixed fibre type are particularly susceptible to the decreased activity and the atrophy-associated muscle changes produced by this condition. Thus, the apparent 'retrograde signalling' of muscle on motoneuron properties reported previously for the cat soleus may be specific to this particular muscle or species.

Properties of sprouted rat motor units: effects of period of enlargement and activity level

The effects of short and prolonged partial denervation of lateral gastrocnemius muscles in sedentary and active rats (running) were examined. In PD muscles of sedentary animals the mean motor unit (MU) tetanic force after 30 days was not different than that measured after 90 days. Increased locomotor activity over the same period (voluntary running, approximately equal to 6 km/day) resulted in an increase in mean MU tetanic force of enlarged MUs (28%). The absence of a significant increase in mean muscle fiber area suggested an activity-related enhancement of motoneuron sprouting. However, the small magnitude of this increase, relative to the potential for further sprouting, indicates the activity effect is not strong and may be partly due to fiber area changes not evident with whole muscle analysis. Nonetheless, these data demonstrate that daily locomotor activity can enhance the tension-generating capacity of chronically enlarged MUs.

Clenbuterol has a greater influence on untrained than on previously trained skeletal muscle in rats

The effects of clenbuterol, a selective beta 2-adrenergic agonist, and of exercise training on the properties of skeletal muscle were studied in the hindlimb of sedentary and trained rats. A 2-week training programme, consisting of climbing on a grid with a load attached to the tail, did not increase the muscle mass of the soleus, the plantaris and the gastrocnemius muscles or modify the isometric in situ contractile properties of the medial gastrocnemius muscle. The only change observed in a 12-week training regimen was a significant increase in contractile forces (expressed in grams per gram of muscle) of the medial gastrocnemius muscle at sub-tetanic stimulating frequencies (twitch 42%, 25Hz 45% and 50Hz 47%). Both training programmes significantly increased fatigue resistance of the medial gastrocnemius muscle. A 2-week oral treatment with clenbuterol significantly increased the muscle mass of the soleus (19.8%), plantaris (16.9%) and gastrocnemius (15.3%) muscles in all animals treated with the agonist. However, clenbuterol had different effects in animals beginning their training programme than in animals that had been trained for the previous 10 weeks. Specifically, clenbuterol caused a significant increase in gastrocnemius muscle mass in the former group but not in the latter. These results suggest that the responses to the combination of clenbuterol and training in previously trained skeletal muscles are not as marked as those observed in untrained muscles.

Similar age-related changes in two regions of muscle with different properties

Two regions of the medial gastrocnemius (MG) muscle, with different contractile properties and innervated by different nerve branches, were investigated in male Sprague-Dawley rats at three ages: 2-3 months, 6-7 months and 24-25 months (i.e., the '50% survival age and beyond': a recommended definition of aged rodents derived from lifespan data on a given colony). At the 50% survival age, both regions of the MG showed decreased mass, slowed contraction times and a decreased number of fast-twitch, but not slow-twitch, muscle fibres. The 40% loss of fast-twitch muscle fibres was not reflected in the loss of motoneurones, suggesting that muscle degeneration precedes motoneurone loss at the 50% survival age in the rat.

Effects of fatigue of rat EDL in situ on metabolism of phosphoinositides

The study was conducted to determine the effect of persistent fatigue in situ on the inositol phosphate second messenger system proposed to constitute a step in excitation-contraction coupling. Rat EDL, after stimulation in situ for 1 hr (100 Hz for 330 ms, 1/s), showed increased incorporation of myo-[3H]inositol into membrane phosphoinositides during a subsequent 4-h incubation period. The rate of hydrolysis of this pool resulting from 10 sec of tetanic stimulation, as well as the rate of production of inositol phosphates InsP, InsP2, and InsP3, were significantly reduced in fatigued muscles. These results suggest that the metabolic changes that parallel the alteration in contractile response with fatigue reflect a disruption in E-C coupling.

Chronic exercise increases SNAP-25 abundance in fast-transported proteins of rat motoneurones

The aim of the present study was to determine whether endurance exercise training selectively modifies the relative abundance of some of the proteins that are subjected to fast axonal transport. Rats were trained on treadmill for 11-13 weeks. [35S]methionine was injected into the ventral horn of L4-L5 spinal cord segments, and transported [35S]methionine-labelled proteins were analysed on fluorograms of sodium dodecyl sulphate polyacrylamide gels. The proportion of a 28 kDa protein increased significantly after training, from 4.9% in controls of 7.7% in trained animals. Two-dimensional electrophoresis and immunoprecipitation identified it as SNAP-25/SuP. The increased availability of SNAP-25, a synaptic protein, may constitute part of the molecular basis of exercise-induced changes in nerve terminal morphology and physiology.

Acetylcholinesterase adaptation to voluntary wheel running is proportional to the volume of activity in fast, but not slow, rat hindlimb muscles

Chronic enhancement of neuromuscular activity by forced exercise training programmes results in selective adaptation of the G4 acetylcholinesterase (AChE) molecular form in hindlimb fast muscles of the rat, with only minor and non-selective AChE changes in the soleus. In order to shed further light on the physiological significance of this G4 adaptation to training, we turned to a voluntary exercise model. The impact of 5 days and 4 weeks of voluntary wheel cage running on AChE molecular forms was examined in four hindlimb fast muscles and the slow-twitch soleus from two rat strains. Inbred Fisher and Sprague-Dawley rats, placed in live-in wheel cages, exercised spontaneously for distances which progressively increased up to an average of approximately 3 and 18 km/day, respectively, by the end of week 4. Fast muscles responded to this voluntary activity by massive G4 increases (up to 420%) with almost no changes in A12, so that by week 4 the tetramer became the main AChE component of these muscles. The additional G4 was composed primarily of amphiphilic molecules, suggesting a membrane-bound state. The G4 content of fast muscles was highly correlated with the distance covered by the rats during the 5 days before they were killed (r = 0.850-0.879, P < 0.001 in three muscles). The soleus muscle, in turn, responded to wheel cage activity by a marked selective reduction of its asymmetric forms--up to 45% for A12. This A12 decline, already maximal by day 5 of wheel cage running, showed no relationship with the distance covered.(ABSTRACT TRUNCATED AT 250 WORDS)

Motoneurons innervating two regions of rat medial gastrocnemius muscle with differing contractile and histochemical properties

The medial gastrocnemius (MG) muscle, which receives its innervation by two extramuscular nerve branches, is representative of muscles which show a particular form of muscle compartmentalization (i.e. a regional specialization of muscle fibers) in which there is a 'deep' oxidative region and a 'superficial' low-oxidative region. Differential recruitment of motor units from these two regions of the MG has been reported for different functional tasks. Our goal was to determine if the organization of the MG motoneuron pool-muscle complex with its two extramuscular nerve branches could account for the phenomenon of regional specialization of muscle fibers. The two extramuscular nerve branches innervated muscle subvolumes which differed in contractile properties and fiber type percentages. The MG proximal nerve branch (NBr) innervated mostly high-oxidative and slow fibers, but with some low-oxidative fast fibers. The distal NBr innervated mostly low-oxidative fibers, but also a small proportion of high-oxidative and slow fibers. These results suggest that the two nerve branches do not strictly define a superficial/deep organization of fiber types in the MG. The number and soma size characteristics of motoneurons supplying the two extramuscular nerve branches showed that the motoneurons innervating the deep more oxidative muscle region, supplied by the proximal NBr, were smaller than those innervating the superficial, primarily low-oxidative, region supplied by the distal NBr. Our findings indicate that the MG motoneuron pool-muscle complex of the Sprague-Dawley rat will lend itself to studies of how the various motor unit types within a given spinal motor complex adapt to different conditions (e.g. aging, disease, injury, exercise).

Physiological properties of motoneurons innervating different muscle unit types in rat gastrocnemius

1. The contractile properties of gastrocnemius muscle units and the electrophysiological properties of their innervating motoneurons were examined in anesthetized adult male Sprague-Dawley rats in situ, using conventional microelectrode techniques. 2. Muscle units (n = 70) were classified as fast (F) or slow (S) on the basis of the degree of force summation during stimulation at 25 Hz, as well as fatigue resistance, in response to current injection into innervating motoneurons. S units categorized using these criteria were also found to invariably demonstrate twitch half-relaxation times (RT1/2) > 28 ms, with F units demonstrating RT1/2 < 27 ms. Some overlap was present between F and S units in twitch time-to-peck tension (TPT). 3. S muscle units were innervated by motoneurons with significantly higher afterhyperpolarization (AHP) times-to-peak and half-decay times, AHP amplitudes, and input resistances than F units. Motoneurons innervating S units also demonstrated slower mean axon conduction velocity than F units. 4. F units were further classified as fast fatiguing (FF), fast fatigue-resistant (FR), or fast intermediate (FI) on the basis of their fatigue resistance. Muscle unit forces were different among the unit types such that FF > FI > FR > S. Twitch TPT and RT1/2 differed such that FF < FI, FR < S. 5. No differences among FF, FI, and FR units were found for measures of AHP time course, AHP amplitude, rheobase, or input resistance. The only motoneuron property that differed among F unit types was axon conduction velocity, which was lower for FF than for FI and FR units. 6. The best relationships between muscle unit and motoneuron physiological properties were between expression of twitch time course, which included RT1/2 and AHP half-decay time (r = 0.73 to 0.74). S units always had AHP half-decay times > 20 ms, whereas for F units, values never exceeded 19 ms. Correlations between these variables increased when only S units were considered (r = 0.86-0.97), and were weak and not significant among F units only. 7. The only other significant correlations between muscle unit and motoneuron properties > 0.5 were between input resistance and expressions of twitch time course (r = 0.56-0.66). These relationships were nonexistent when type S units were not included in the analysis. 8. A sample of tibial motoneurons (n = 98) that innervated hindlimb muscles other than gastrocnemius was combined with the gastrocnemius motoneurons to examine interrelationships among motoneuron properties.(ABSTRACT TRUNCATED AT 400 WORDS)

Histochemical and contractile responses of rat medial gastrocnemius to 2 weeks of complete disuse

We studied the histochemical and in situ contractile changes in a rat ankle extensor, medial gastrocnemius, in which activation of muscle fibres by motoneurones was blocked for 14 days, using the sodium channel blocker tetrodotoxin applied to the sciatic nerve. Muscles were atrophied and showed slower twitch responses, greater fusion at subtetanic frequencies of stimulation, and higher twitch/tetanic ratios. Tetanic force/mm2 of fibre area and fatiguability were unchanged. Type II fibres were more atrophied and showed greater decreases in mitochondrial succinate dehydrogenase activity than type I fibres. The contractile changes resulting from complete disuse do not occur in models in which weight-bearing alone has been removed (space flight, hindlimb suspension), suggesting that the residual motoneurone activity reported in models of weightlessness is sufficient to prevent these responses. Similarly, the finding of a greater type II fibre susceptibility to complete disuse, which differs from the pattern seen in models of weightlessness, suggest that this residual motoneurone activity in the latter influences atrophic responses in a manner that is variable among motor unit types, to produce the reported preferential type I atrophy characteristic of removal of weight-bearing.

Effects of exercise training on components of the motor unit

Relatively little is known as to how the motor unit (the motoneurone and its innervated muscle fibres) adapts. Using the model of compensatory overload of the rat plantaris, we have described the complex patterns of adaptation which differ across motor units within a single muscle. These adaptations, measured using histochemical and physiological techniques in combination, emphasize the difficulty of predicting whole muscle responses from those of its constituent muscle fibres and motor units. In addition, they suggest that subtle changes may occur in the way motor units are used during training that are not evident when looking at whole muscles. Evidence of the responses of the motoneurone component of the motor unit to overload is fragmentary; nonetheless, other models of neuromuscular adaptation show the extent to which motor nerves and muscles adapt in a coordinated manner, with functional consequences that may be relevant to the exercise training model. More information on overload-induced changes in motoneurone and muscle properties, how these changes are coordinated, and their functional consequences, is necessary in order to better understand the training model.

Rostrocaudal pattern of fiber-type changes in an overloaded rat ankle extensor

Our aim was to quantify the overload-induced hypertrophy and conversion of fiber types (type II to I) occurring in the medial head of the gastrocnemius muscle (MG). Overload of MG was induced by a bilateral tenotomy/retraction of synergists, followed by 12-18 wk of regular treadmill locomotion (2 h of walking/running per day on 3 of 4 days). We counted all type I fibers and determined type I and II mean fiber areas in eight equidistant sections taken along the length of control and overloaded MG. Increase in muscle weights (31%), as well as in total muscle cross-sectional areas (37%) and fiber areas (type I, 57%; type II, 34%), attested to a significant hypertrophic response in overloaded MG. An increase in type I fiber composition of MG from 7.0 to 11.5% occurred as a result of overload, with the greatest and only statistically significant changes (approximately 70-100%) being found in sections taken from the most rostral 45% of the muscle length. Results of analysis of sections taken from the largest muscle girth showed that it significantly underestimated the extent of fiber conversion that occurred throughout the muscle as a whole. These data obtained on the MG, which possesses a compartmentalization of fiber types, support the notion that all fiber types respond to this model with a similar degree of hypertrophy. Also, they emphasize the complex nature of the adaptive changes that occur in these types of muscles as a result of overload.

Muscle acetylcholinesterase adapts to compensatory overload by a general increase in its molecular forms

We have investigated the impact of compensatory overload on the content of acetylcholinesterase (AChe) molecular forms in the rat fast-twitch medial gastrocnemius (MG). Overload was induced by way of a bilateral tenotomy of the MG's functional synergists coupled to a daily walking training program (15 m/min, 30% incline, up to 60 min per session, 12-18 wks). This latter condition ensured that the MG were used on a regular basis. In comparison to control values, overloaded MG showed 25 and 19% increases (P less than 0.05) in muscle wet weight and protein concentration, respectively. The content in AChe (activity per muscle) was also increased in these MG (28%, P less than 0.05). Sedimentation analyses revealed a general elevation in the content of AChe molecular forms, with A8, G2, and G1 displaying significant changes (35-42%, P less than 0.05). In a second group of rats, daily running training (27 m/min, 30% incline, using the same timetable) was supplemented to the compensatory overload. In this group, the additional running training led to a greater hypertrophic response as attested to by increases (P less than 0.05) in the MG wet weight (41%) and protein concentration (35%) in comparison to controls. However, total AChe content of these muscles was increased to an extent similar to that observed in the MG subjected only to compensatory overload (24%, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Swimming training increases the G4 acetylcholinesterase content of both fast ankle extensors and flexors

The effect of endurance swimming training on AChE molecular forms was examined in 2 groups of functionally antagonist rat muscles, including ankle extensors and flexors. This exercise regimen, which entails predominant dynamic activity (i.e., involving extensive shortening) of both groups of muscles, resulted in marked selective G4 increases in all fast muscles. The G4 elevation exhibited by the ankle flexors was in sharp contrast to the G4 reduction reported in these same muscles following running training, during which their action is predominantly tonic. The results strengthen the conclusion that predominantly dynamic activity increases the G4 content of mature innervated fast muscles.

TTX-induced muscle disuse alters Ca2+ activation characteristics of myofibril ATPase

1. Previous reports of the effects of disuse induced by tetrodotoxin (TTX) have demonstrated alterations in muscle function suggesting changes in the quality of contractile proteins. 2. We extended these studies to the effects of TTX-induced disuse on the Ca2+ activation characteristics of myofibrillar ATPase of the rat gastrocnemius. 3. Atrophic responses were as previously reported (St-Pierre, D.M.M. and Gardiner P.F. (1985) Effect of disuse on mammalian fast-twitch muscle: joint fixation compared with neurally applied tetrodotoxin. Exp. Neurol. 90, 635-651; St-Pierre, D.M.M. et al. (1987). Recovery of muscle from tetrodotoxin-induced disuse and the influence of daily exercise; 1. Contractile properties. Exp. Neurol. 98, 472-488.) with a significant decrease in left gastrocnemius weight compared to control (C) (1.25 +/- 0.06 for C vs 0.72 +/- 0.04 for TTX, X +/- SEM, P less than or equal to 0.01). 4. Myofibrillar protein yield (mg/g wet weight) was also depressed (92.8 +/- 4.5 for C vs 70.3 +/- 3.7 for TTX; P less than or equal to 0.01). 5. Maximum ATPase of myofibrils (nmol Pi/mg/min) was decreased (441 +/- 28 for C vs 181 +/- 30 for TTX, P less than or equal to 0.01). 6. Furthermore, the Hill n which reflects the cooperative aspects of Ca2+ activation of the myofibrillar ATPase was depressed (1.58 +/- 0.07 for C vs 1.29 +/- 0.09 for TTX; P less than or equal to 0.01). 7. The results suggest that muscle perturbations resulting from disuse are partially related to changes in the myofibril.

Ca2+ activation properties of myofibrillar ATPase from fatigued rat plantaris

1. Muscle fatigue following long-duration rhythmic activity is often characterized by reduced force following a single impulse and at low-frequencies of stimulation. 2. Although this response is generally attributed to an alteration in excitation-contraction coupling, the possibility that the responsiveness of myofibrillar proteins to a given Ca2+ signal is altered has never been ruled out. 3. In this study, rat plantaris muscles were subjected to an in situ regimen of contractions (100 Hz, lasting 100 msec, once every 750 msec, for 1 hr), and allowed to recover for 15 min. 4. Twitch, 100 Hz, and 200 Hz forces were reduced by 79%, 49% and 17% respectively, at this time. 5. In myofibrils isolated from these muscles, maximum activity of Ca2+ activated myofibrillar ATPase, Ca2+ sensitivity (pCa 50), and co-operatively (Hill n), were not different from non-fatigued muscles. 6. It appears, therefore, that the Ca2+ activation properties of myofibrillar ATPase do not contribute to this pattern of fatigue.

Diabetes affects retrograde but not anterograde transport of sciatic nerve phosphofructokinase in Sprague-Dawley rats

Phosphofructokinase activity was measured in the sciatic nerve of streptozotocin-induced diabetic and nondiabetic rats. Average steady-state phosphofructokinase activity was obtained from three consecutive segments of the mid-femoral region in the left sciatic nerve in both diabetic (4 and 24 weeks) and nondiabetic, age-matched animals. Over time, phosphofructokinase activity significantly decreased (p less than 0.05) with diabetes, with no effect demonstrated within similar age-groups. The accumulation of phosphofructokinase activity was accomplished by ligating the mid-femoral region of the right sciatic nerve for 24 h. Anterograde and retrograde axonal transport of phosphofructokinase was measured in the 3-mm segment proximal and distal to the ligature, respectively. There was a trend (p = 0.0627) towards a decline in net proximal accumulation (mean proximal minus mean background) with age. Net distal (mean distal minus mean background) activity declined by 80% (p less than 0.05) in the control group between 4 and 24 weeks of the diabetic state. However, diabetic animals did not experience the same age-related decline in retrograde transport. The findings suggest that diabetes affects the age-associated evolution of retrograde transport, presumably a reflection of the neuropathy occurring in the distal axon branches, without altering anterograde transport to any appreciable extent.

To what extent is hindlimb suspension a model of disuse?

The extent to which the remaining active or passive components of muscle mechanical stress not associated with weightbearing are involved in preserving muscle morphological and functional characteristics in the rodent hindlimb suspension model is not known. Such information would be relevant to the construction of appropriate countermeasures for the disuse atrophy associated with muscle unloading. This question was addressed by superimposing 2 weeks of hindlimb suspension and neuromuscular quiescence, achieved by the chronic neural application of the sodium channel blocker tetrodotoxin. A major portion of the muscle size characteristics of the fast anti-gravity gastrocnemius and plantaris, and the functional characteristics of the plantaris, were maintained by the full range voluntary activity remaining after suspension. Muscle mass of the slow soleus was compromised regardless of this residual activity. Indeed, for fast ankle extensors, hindlimb unloading resembles more closely a model of normal usage than of disuse, but for slow extensors this condition appears to be extremely detrimental.

The "fastness" of rat motoneurones: time-course of afterhyperpolarization in relation to axonal conduction velocity and muscle unit contractile speed

In normal adult rats, intracellular recordings were obtained from motoneurones of the medial gastrocnemius (MG) and other tibial-nerve muscle branches. Among the whole population of tibial motoneurones, there was a significant negative correlation between the duration of afterhyperpolarization (AHP) and axonal conduction velocity (CV). However, this AHP vs CV relationship differed from that previously demonstrated in cats, in that for a given axonal CV, the AHPs were briefer in the rat than in the cat. For MG cells, twitches were also recorded from their muscle units. As has previously been observed in cats, there was a significant correlation between the time-course of the AHP of a motoneurone and the time-course of its muscle unit twitch. This relationship was, in principle, similar between the two species, but the AHPs and the twitches were briefer in the rat. The present results provide the first demonstration, for the rat model, of the presence of a functionally relevant "speed-match" between the intrinsic properties of alpha-motoneurones and those of their muscle units.

Influence of weight bearing on the adaptations of rat plantaris to ablation of its synergists

The present study was designed to determine the contribution of weight bearing to the adaptations of the plantaris (PL) to synergist removal. PL from female rats were exposed to 28 days of a simultaneous condition of synergist ablation and hindlimb suspension. At 28 days, contractile responses and morphological measures were obtained and compared with muscles that either had synergists intact or were weight bearing or a combination of both. Synergist ablation prolonged PL maximum isometric twitch tension (Pt), time to peak tension (12%), and one-half relaxation time (12%); increased Pt (26%), maximum isometric tetanic tension (Po, 44%), fatigue resistance (FI, 42%), and fast fiber cross-sectional area (FT CSA, 20%); and decreased Pt/Po (13%) over nonablation counterparts. Suspension decreased PL Pt (26%), Po (26%), rest length (16%), FT CSA (31%), and slow-twitch fiber (ST) number (24%) but increased FI (75%) over weight-bearing counterparts. PL from weight-bearing animals were heavier than from suspended animals, and the extent of this response was greatest after synergist removal. Whole muscle and ST CSA and ST area contribution were greater only in weight-bearing synergist ablation muscles. Daily weight bearing (4 h) in synergist ablation hindlimb suspension groups caused PL weights and ST expressions to be halfway between 24-h suspension and 24-h weight-bearing groups. Our results suggest that weight bearing is not essential to the induction of several adaptations associated with synergist ablation but is required to cause the large muscle mass and ST expression characteristic of this model.

Influence of overload on recovery of rat plantaris from partial denervation

A functional index of neural adaptability is the capacity of motoneurons to extend and establish supernumerary connections with neighboring denervated muscle fibers. The purpose of this study was to guage this response in rat plantaris muscles subjected to increased levels of activity resulting from the surgical removal of the synergistic gastrocnemius and soleus muscles. Thirty-seven days of overload increased plantaris absolute (69%) and relative (82%) weight, whole muscle (35%) and individual fiber (37%) mean cross-sectional area, half-relaxation time (1/2RT; 25%), and maximum tetanic tension (P0; 21%). In a separate group of animals that had undergone 30 days of overload, three-quarters of the plantaris muscle fibers were denervated by sectioning radicular nerve L4. At 7 days postlesion, contractile responses were obtained from sprouting motor units remaining in radicular nerve L5, and the results compared to a nonoverloaded group that had undergone this same procedure. Twitch time to peak tension and 1/2RT were prolonged in normal partially denervated (PD) and overloaded partially denervated (OPD) muscles, and this response was significantly greater in the overloaded muscles. Both PD and OPD muscles increased twitch tension (38%) and peak tension developed at 25 Hz (34%) to a similar extent, during recovery from partial denervation. These increases, attributable to sprouting of L5 motor axon collaterals, were matched in PD muscles with a corresponding increase in P0, a response which did not occur in OPD muscles. Additionally, a more extensive decrease in P0 occurred as a result of partial denervation in OPD muscles compared with whole muscle P0 of nondenervated muscle (L4 plus L5 stimulation).(ABSTRACT TRUNCATED AT 250 WORDS)

Physiological responses of rat plantaris motor units to overload induced by surgical removal of its synergists

1. Rat plantaris muscles were subjected to chronic overload by the surgical removal of the soleus and most of the gastrocnemius muscles. Twelve to 16 wk later whole muscle and motor unit (ventral root dissection technique) contractile properties as well as histochemistry were determined. 2. Motor units were categorized as fast, fatigable (FF), fast, intermediate fatigue-resistant (FI), fast, fatigue-resistant (FR), and slow (S) based on contractile characteristics. Muscle fibers were identified as type I and type II according to myofibrillar ATPase staining. 3. Whole muscles demonstrated increases in wet weight, tetanic force, proportion of type I fibers, and mean cross-sectional areas of both type I and II fibers, as a result of chronic overload. 4. Tetanic tension increased by the same relative magnitude in all motor units whereas twitch tension remained unchanged. A significant change in the proportions of the motor unit types occurred in overloaded muscles, such that the latter contained higher proportions of FF and S units, and lower proportions of FI and FR units, than normal muscles. 5. The fatigue profile of a composite constructed from a summation of motor unit responses revealed that the overloaded plantaris displayed fatigue resistance similar to that of the normal plantaris for a given absolute force output. 6. Glycogen-depleted fibers of hypertrophied single motor units demonstrated uniform myofibrillar ATPase and SDH staining characteristics suggesting that metabolic adaptations among fibers of the same unit were similar after 12-16 wk of overload. 7. The finding that overload caused a uniform increase in the tetanic strength of all motor units, whereas alterations in fatigue resistance varied in degree and direction among unit types, demonstrate that these two properties are not controlled in parallel in this model. The smallest units maintain or even increase their fatigue resistance during the hypertrophic process, whereas high threshold units actually decrease in fatigue resistance.

Fast axonal transport of labeled proteins in motoneurons of exercise-trained rats

In this study, the fast orthograde axonal transport of radiolabeled proteins was measured to determine the effects of endurance-running training on transport velocity and amounts of transported proteins in rat sciatic motoneurons. Female rats were subjected to a progressive running-training program for 10-12 wk. Twenty-four hours after the last training session, rats underwent right L4-L5 dorsal root ganglionectomy. The next day, 20 microCi of [3H]leucine was injected bilaterally in the vicinity of the motoneuronal cell bodies supplying the sciatic nerve, to study axonal transport parameters. Results showed that peak and average transport velocities of labeled proteins were significantly (P less than 0.05) increased by 22 and 29%, respectively, in the deafferented nerves of the runners as compared with controls. Moreover, the amount of total transported protein-bound radioactivity was increased in both left (40%) and right (37%) sciatic nerves of the runners. An exhaustive exercise session reduced (P less than 0.05) peak displacement (8%) and total transported protein-bound radioactivity (36%) in the sciatic nerves of control rats, whereas no changes were noticed in trained animals. The data suggest that chronic endurance running induces significant adaptations in the fast axonal transport of labeled proteins.

Recovery of muscle from tetrodotoxin-induced disuse and the influence of daily exercise. 2. Muscle enzymes and fatigue characteristics

The fatigue characteristics and the activities of oxidative and glycolytic enzymes were determined in tetrodotoxin (TTX)-induced disuse muscles and in muscles of animals recovering from TTX-induced disuse (TTX-rec). In addition, the effects of additional daily exercise (grid-climbing and swimming) on the fatigue and metabolic profiles of muscles from TTX-rec and control animals were investigated. The activities of citrate synthase (CS), phosphofructokinase (PFK), and alpha-glycerophosphate dehydrogenase (alpha-GPD) were depressed following 28 days of inactivity produced by the chronic neural application of TTX. The response of these muscles to a pattern of stimulation that has been used to classify fast-twitch motor units according to their fatigability (6) (330 ms, 40 Hz, l/s, 4 min) was not affected to any great extent by inactivity, except for a loss in the ability to summate or maintain forces during each 330-ms burst, as fatigue developed. After 28 days of recovery, the concentration of CS had returned to normal, whereas the concentrations of PFK and alpha-GPD remained depressed. TTX-rec muscles, on the other hand, appeared more resistant to fatigue than control muscles, based on several indices of muscle fatigue. Control and TTX muscles responded similarly to daily training. Swimming but not climbing increased the activity of CS and the fatigue resistance of the muscle. Neither exercise influenced the activity of PFK and alpha-GPD. Although the activity of CS was influenced by the level of neuromuscular usage, the former did not appear to play a dominant role in determining the fatigue resistance of the muscle, emphasizing the need to consider other factors as primary determinants of muscle fatigue.

Absence of staircase following disuse in rat gastrocnemius muscle

Repetitive stimulation of mammalian fast-twitch skeletal muscles will normally result in a positive staircase response. This phenomenon was investigated in the rat gastrocnemius muscle following a 2-week period of tetrodotoxin-induced disuse. Muscle inactivity was imposed by superfusing tetrodotoxin in saline over the left sciatic nerve via an implanted osmotic pump. In situ isometric contractile responses to double pulse stimulation and repetitive stimulation at 10 Hz were determined the day after removal of the pump. Two weeks of disuse resulted in 40% muscle weight loss. A twitch contraction gave the same force when expressed per gram of wet muscle weight in control muscles, 317 +/- 24.6 (means +/- SE) g/g, as compared with tetrodotoxin-treated muscles, 328 +/- 24.2 g/g. Both contraction time and half-relaxation time were prolonged following treatment with tetrodotoxin. Repetitive stimulation at 10 Hz resulted in a positive staircase response in the control muscles, but not in muscles of the tetrodotoxin-treated rats. The observed changes in the time course of the twitch contraction with repetitive stimulation following tetrodotoxin-induced disuse are consistent with alterations in sarcoplasmic reticulum handling of calcium. It is not certain if there is a change following disuse in the mechanism normally associated with staircase or if this mechanism is merely opposed by an early fatigue.

Recovery of muscle from tetrodotoxin-induced disuse and the influence of daily exercise. 1. Contractile properties

The extent of recovery in rat gastrocnemius muscles which have undergone atrophic changes due to 4 weeks of inactivity caused by sciatic nerve superfusion of tetrodotoxin, was investigated. Control and tetrodotoxin-treated female Sprague-Dawley rats were subjected to either a daily program of grid-climbing or swim-training for 4 weeks, or benefited from cage activity only. In situ contractile properties of the gastrocnemius were then measured. Twenty-eight days of recovery was insufficient to allow a complete reversal of the changes produced by 28 days of inactivity. The gastrocnemius remained atrophied (25%) and tetanically weaker than normal in the recovered animals. The maximal rate of rise of the twitch was partially recovered whereas that developed during a maximal contraction was normal. Maximal rate of rise expressed relative to the tension output was elevated, however, for both twitch (%Pt/ms) and maximal contraction (%Po/ms). Swimming did not influence the recovery of muscle size or strength. Grid-climbing, on the other hand, produced heavier and stronger muscles and a faster recovery of %Pt/ms. This study clearly illustrates the importance of muscle load in regulating muscle size and strength.

Changes in rat plantaris motor unit profiles with advanced age

The physiological characteristics of single motor units in rat plantaris muscles were determined in situ, for young adult (3 months) and very old (30-34 months) Fischer 344 rats. Old muscles generated 43% less tetanic force (P0) per gram. Motor units classified as "slow", using criteria of fatigue resistance and "sag" during unfused tetani, had a mean P0 which was 255% of that in young muscles, while fast motor units were similar in P0 in the two groups. Estimates were made of motor unit numbers using whole muscle and mean motor unit P0 values. The typical young plantaris contained 48 units, of which 5-6 were slow, while old plantaris contained 29 units, of which 11 were slow. In spite of this large increase in slow motor unit presence (increased mean motor unit P0, plus increased number) in old muscles, a comparatively modest (72%) increase occurred in the muscle cross-section occupied by histochemically demonstrated slow fibres. During senescence, there occurs a loss in muscle tetanic force capability which is accompanied by a loss of motor units and a reorganization of the remaining motor unit profile. An increase in slow motor unit number and size with advancing age can evidently occur without concomitant histochemical changes. Motor units do not "dedifferentiate", but maintain their physiological distinctiveness into very old age.

Patterns of EMG activity of rat plantaris muscle during swimming and other locomotor activities

The purpose of the study was to examine the patterns of electromyographic (EMG) activity of the rat plantaris during loaded swimming in comparison with other locomotor activities. Five female Sprague-Dawley rats were implanted with chronic bipolar electrodes in the plantaris muscle of the left hindlimb under pentobarbital anesthesia. Characteristics of EMG bursts recorded while the conscious rat was performing treadmill walking (0.24 m/s) were stable and reproducible 10-14 days postsurgery. Following this stabilization period, records of EMG activity were obtained during walking, loaded swimming (6.5 g attached to tail), and several other locomotor tasks. Compared to walking, EMG bursts during loaded swimming were significantly higher (67%) in maximum amplitude, one-third as long in duration, and occurred at a greater rate (4.4 vs. 1.7 bursts/s, P less than 0.05). Swimming bursts were of higher amplitudes than those of all other activities examined and reached 65% of the EMG amplitude recorded following stimulation of the sciatic nerve with supramaximal voltage. The addition of a mass to the animal's tail during swimming did not increase the EMG burst amplitudes but resulted in a higher frequency of bursts. Compared with treadmill walking, loaded swimming elicited burst of high variability in amplitude. Swimming in the rat involves rapid, extensive activation of plantaris, thus providing an exercise model to study the adaptability of the neuromuscular system to prolonged activity of this type.

Fast axonal transport of acetylcholinesterase in rat sciatic motoneurons is enhanced following prolonged daily running, but not following swimming

The effects of increases in neuronal activity on fast axonal transport of acetylcholinesterase (AChE) in sciatic motoneurons were studied by subjecting rats to daily running or swimming training (8 weeks). Net accumulation of AChE activity proximal and distal to a ligature served to evaluate orthograde and retrograde transport. Results showed that runners had greater orthograde and retrograde transport of AChE as compared to control animals, while no changes were found in swimmers. These adaptations in the runners were caused by the long-term nature of the training regimen since an acute exercise session had no effect on AChE transport. The observed changes may be attributed to an increase in the mobile fraction of AChE in the motoneurons. Since swimming training had no effect on transport but entails a high level of neuronal activity, it is suggested that increased impulse activity is not the factor mediating the adaptations in axonal transport of AChE which resulted from running training.

Contractile and electromyographic characteristics of rat plantaris motor unit types during fatigue in situ

1. The ventral root dissection technique was used to obtain contractile and electromyogram (e.m.g.) characteristics of ninety-five plantaris motor units in situ in pentobarbitone-anaesthetized rats (n = 20). 2. Motor units demonstrated a wide spectrum of sizes, contractile speeds, and fatigue indices, and were categorized in the same manner as cat hind-limb motor units. Fast-fatigable (f.f.) and fast-intermediate fatigue resistant (f.i.) motor units constituted 20.2 and 25.5% of the motor unit population but together generated over 75% of the cumulative tetanic force. Fast-fatigue resistant (f.r.) and slow motor units composed 43.6 and 10.6% of the population while producing less than 25% of the aggregate tetanic force. 3. Only f.f. and a portion of f.i. motor units demonstrated extensive e.m.g. amplitude reductions during a standard fatigue test. Mean percentage e.m.g. decrease (from the first spike of the first burst to the last spike of the last burst) was 74.0 +/- 27.7% for f.f. units and 28.3 +/- 31.0% (mean +/- S.D.) for f.i. motor units. Relationships between percentage e.m.g. decline and motor unit size (tetanic force) showed significant (P less than 0.01) positive correlations in f.f. (r = 0.71) and f.i. (r = 0.69) motor units. 4. Backward extrapolation of the time course of the force-e.m.g. relationship during the fatigue test revealed that declines in e.m.g. may explain 15, 21 and 66% of the force losses in f.r., f.i. and f.f. motor units. Slow motor units were fatigue resistant and demonstrated a mean e.m.g. decline of 4.3 +/- 6.2%. 5. Indirectly stimulated whole muscle was more fatigable than a composite constructed from motor unit data because of more severe e.m.g. amplitude reductions in the former. 6. The motor unit mechanical and electrical responses during the fatigue test do not summate linearly during whole muscle contractile activity. This is most likely due to the presence, during whole muscle activity, of metabolic changes during the fatigue regimen which influence neuromuscular propagation of excitation, which are not as severe during single motor unit activity.

Posttetanic potentiation and skeletal muscle fatigue: interactions with caffeine

The purpose of this study was to determine the interaction of three factors that modify twitch contraction amplitude in the rat gastrocnemius muscle in situ: posttetanic potentiation, fatigue, and caffeine. Posttetanic (200 Hz for 1 s) twitch responses were observed before and after 15 Hz stimulation for 6 min (group FS), injection of caffeine (75 mg/kg dissolved in saline, group NC), a combination of both repetitive stimulation and caffeine injection (group FC), or no treatment (group NS). Developed tension increased significantly with posttetanic potentiation and caffeine injection and these potentiating factors were additive (group NC). Repetitive stimulation attenuated the twitch response and the fatigued muscle was still responsive to the potentiating factors. Posttetanic potentiation was accomplished primarily by a significant increase in the peak rate of force development whereas caffeine potentiation and fatigue were effected with a proportional change in contraction time. It seems likely that the mechanism of posttetanic potentiation is not the same as the mechanism of caffeine-induced potentiation. Caffeine-induced potentiation is known to be related to increased release of calcium. Because changes in contraction time with fatigue were opposite to those associated with caffeine potentiation, it is proposed that the attenuated twitch response in fatigue results from reduced release of calcium.