Influence of intermittent long-term stimulation on contractile, histochemical and metabolic properties of fibre populations in fast and slow rabbit muscles

Metabolic anatomy of brain: a comparison of regional capillary density, glucose metabolism, and enzyme activities

Regional variations in capillary density, glucose utilization rate, and activities of the glycolytic enzyme lactate dehydrogenase and the mitochondrial enzyme cytochrome oxidase were compared in the rat brain. The distributions of capillaries and enzymes were studied by means of histochemical staining techniques, and glucose metabolism was measured by means of [14C]2-deoxyglucose autoradiography. Analysis of 18 gray and five white matter regions revealed a positive correlation between capillary density and glucose utilization rate. A negative correlation was found between capillary density and lactate dehydrogenase among gray matter structures. Analysis of capillaries and enzymes was also performed within laminated histological fields: hippocampus, olfactory bulb, and olfactory cortex. In general, this revealed reciprocal patterns of staining for lactate dehydrogenase and cytochrome oxidase. Capillary density paralleled cytochrome oxidase activity. The zones of intense staining for lactate dehydrogenase and cytochrome oxidase corresponded to the synaptic terminal fields of different input pathways. These findings demonstrate distinct distributions of a glycolytic and an oxidative enzyme within the brain which are at least partly associated with pathway specificity.

Actomyosin adenosine triphosphatase activities of the cat infrahyoid muscles

By use of actomyosin ATPase histochemistry, it was found that there were large differences among the three cat infrahyoid muscles (sternohyoid, sternothyroid, and thyrohyoid) with respect to their percentages of different muscle fiber types. It has been established that the individual activity patterns of the component motor units in each muscle drive the biochemical and physiologic differentiation of the muscle fibers associated with each motor unit. Therefore, the data obtained in the present investigation provide an indication of the characteristics of long-term use of each of the various types of motor units, as well as the associated differences in the physiologic capacities of the different motor unit types composing each of these infrahyoid muscles.

Species-specific effects of chronic nerve stimulation upon tibialis anterior muscle in mouse, rat, guinea pig, and rabbit

Tibialis anterior (TA) muscle of mouse, rat, guinea pig, and rabbit was indirectly stimulated for 10 h/day at 10 Hz up to 28 days. Changes in the activity levels of hexokinase (HK), phosphofructokinase (PFK), glyceraldehydephosphate dehydrogenase (GAPDH), lactate dehydrogenase (LDH), creatine kinase (CK), citrate synthase (CS), malate dehydrogenase (MDH), 3-hydroxyacyl-CoA dehydrogenase (HADH), and beta-hydroxybutyrate dehydrogenase (HBDH) were compared. Although the direction of changes in the enzyme activity pattern was in accordance with previous findings on rabbit TA, the magnitude of the responses varied markedly between the mammals under study. Mouse TA was almost unaffected. A major effect of chronic stimulation in rat, guinea pig and rabbit was an increase in enzyme activities of aerobic-oxidative metabolism. According to intrinsic differences of the muscles under study, the increases varied among the species and appeared to be inversely related to the basal levels of these enzymes in the unstimulated muscles. Conversely, glycolytic enzyme activities (PFK, GAPDH, LDH) markedly decreased in rat, guinea pig, and rabbit, and were only slightly reduced in mouse. Changes in HK and HBDH activities displayed the largest variations in the induced change between species. These results indicate species-specific patterns of metabolic adaptation to increased contractile activity.

Relationship between parvalbumin content and the speed of relaxation in chronically stimulated rabbit fast-twitch muscle

The time courses of changes in parvalbumin (PA) content, isometric twitch tension, and half-relaxation time (1/2 RT) were studied in rabbit tibialis anterior muscle following chronic 10 Hz nerve stimulation of 1-21 days. Up to 5 days stimulation had no effect on PA content, but it induced a slight (10-15%) increase in the 1/2 RT. This change occurred together with the previously observed 50% decrease in Ca2+-uptake by the SR (Leberer et al. 1987). While prolonged stimulation produced no further decrease in the Ca2+-uptake by the SR, PA content declined after 5 days of stimulation. The reduction in PA content was accompanied by a progressive lengthening of the 1/2 RT. However, the increase in 1/2 RT was particularly pronounced after PA had fallen below 50% of its normal value. A 90% reduction in PA coincided with a 60% increase in the 1/2 RT. By this time the staircase phenomenon, normally observed in fast-twitch muscle, was completely abolished. Although the changes in PA content and 1/2 RT were not linearly related, these results suggest that PA plays an important role in the relaxation process of mammalian fast-twitch muscle.

Effects of intermittent electrical stimulations on muscle catabolism in intensive care patients

To determine whether muscular contractions obtained by electrical stimulation in immobilized patients are able to reduce muscle catabolism, we studied 10 patients (65-79 yr old) hospitalized in the intensive care unit for postoperative ventilatory failure or cerebral infarction. Artificial nutrition was the same for each patient during the 9-day study period. Two periods of 4 days were defined and randomized for each patient, separated by one day. During the muscular stimulation (MS) period, intermittent electrical stimulation of the muscles of the legs (external electrodes), were performed daily during 2 X 30 mn. During the other period, muscular stimulations were not performed. Urinary excretion of nitrogen (micro-Kjeldhal digestion and Nessler procedure), creatinine (Jaffe reaction), and 3-methyl histidine (3-MH) (gas phase chromatography) was measured every day. (table; see text) We conclude that a significant decrease in 3-MH and creatinine excretion is observed during the MS period. In intensive care unit patients, muscle protein breakdown may be influenced by intermittent muscular electrical stimulation.

Effects of low and high frequency patterns of stimulation on contractile properties, enzyme activities and myosin light chain accumulation in slow and fast denervated muscles of the chicken

The effects of denervation and direct stimulation in fast and slow latissimus dorsii muscles were investigated in chicken. In slow ALD muscle, denervation resulted in an incompleteness of the relaxation, a decrease in MDH and CPK activities and an increase in fast myosin light chains (MLC) accumulation. Direct stimulation at either fast or slow rhythm prevented the effects of denervation on relaxation and CPK activity but was ineffective on MDH activity and fast MLC accumulation. Moreover, direct stimulation of denervated ALD caused rhythm-dependent change in tetanic contraction. In fast PLD muscle, the main changes in muscle properties following denervation were a slowing down of the time course of the twitch and an incompleteness of the relaxation, a decrease in LDH and CPK activities and in LC3F accumulation. Stimulation at a high frequency partly prevented the effects of denervation and resulted in a large accumulation of LC3F, while a low frequency stimulation did not restore the twitch time to peak, increased MDH activity and induced synthesis of slow MLC. This study emphasizes the role of muscle activity and its pattern in some properties of slow and fast chicken muscles following denervation.

Myosin polymorphism in single fibers of chronically stimulated rabbit fast-twitch muscle

Rabbit tibialis anterior (TA) muscles were indirectly stimulated (10 Hz, 24 h/d) for 30 d and 60 d and single fibers were analysed using a combined histochemical and biochemical technique (Staron and Pette 1986, 1987a, b). After 30 d of chronic stimulation there was a pronounced increase in the normally rare (0.5%) C fiber population (i.e., fibers containing slow- and fast-myosins in varying ratios). At this time, C fibers amounted to almost 60% of the total population. In the 60 d stimulated muscles, the major population (98%) consisted of an atypical type It fiber. This fiber type which was not detectable in normal TA muscle, differed histochemically and biochemically from type I fibers. It contained the slow-myosin light chains LC1s and LC2s, the heavy chain HCI, and, in addition, high concentrations of the fast-myosin alkali light chain LC1f and possibly traces of a heavy chain with an electrophoretic mobility comparable with that of the fast-myosin heavy chain HCIIa. These It fibers were occasionally observed in the unstimulated, contralateral TA muscles which also contained an increased population of C fibers (1.3-6.3%). Although the transformation even after 60 d of chronic stimulation was incomplete, these changes demonstrate the ability of muscle fibers to adapt in a specific manner to altered functional demands brought about by an altered stimulus pattern. In addition, the pronounced heterogeneity of the fiber population undergoing transformation indicates a nonuniform response to a uniform stimulus pattern.

The effect of isometric short-term electrical stimulation on denervated muscle

Electrical stimulation was applied daily for 20 minutes to denervated rabbit extensor digitorum longus muscle. One group was stimulated with short tetani, another with 1-Hz frequency, using isometric contractions for both. Tetanic stimulation induced severe fibrosis and is harmful to denervated muscle. One Hertz stimulation retarded denervation-induced fatigue and atrophy, as well as slowing of contraction time.

Fibre sizes and histochemical staining characteristics in normal and chronically stimulated fast muscle of cat

1. Normal and chronically stimulated peroneus longus muscles of the cat's hind limb were studied with respect to fibre size and staining properties for myofibrillar (myosin) adenosine triphosphatase (ATPase) and succinate dehydrogenase (SDH) activity. The intensity of staining for SDH activity was measured by microphotometry from the central portions of the muscle fibres ('core-SDH staining'). For comparison, histochemical properties were also studied in non-stimulated soleus muscles. 2. On account of the pH sensitivity of their myofibrillar ATPase, about 18% of the fibres in normal peroneus longus muscles were classified as type I, and about half of the remainder as II A and II B respectively. 3. In the normal peroneus longus muscles, the mean diameter of single muscle fibres generally varied between about 25 and 75 micron, whereby the average size of type I less than type II. 4. In the normal peroneus longus muscles the staining intensity for core SDH varied over a wide range. The average heaviness of staining was clearly ranked in the order type I greater than type II A greater than type II B. 5. Chronic stimulation was given to the deafferented common peroneal nerve by aid of a portable and remotely controlled mini-stimulator. The stimulation was delivered in 'tonic' patterns (greater than or equal to 50% of total time taken up by activity) of 'fast' (20 or 40 Hz) or 'slow' (5 or 10 Hz) rates. 6. Prior to the period of long-term stimulation, the cats had been subjected to a dorsal rhizotomy and hemispinalization on the ipsilateral (left) side. In the absence of chronic stimulation, these operations had no evident effects on the sizes or staining properties of peroneus longus fibres. 7. After 8 weeks of treatment with tonic patterns of stimulation, the fibres of peroneus longus muscles clearly became more similar to each other with respect to their diameter as well as their staining for ATPase and SDH activity. With respect to ATPase staining, however, the chronically stimulated peroneus longus fibres had become more similar to non-stimulated soleus fibres than to non-stimulated type I fibres of peroneus longus. With respect to the staining for core SDH, the chronically stimulated fibres all became similar to normal II A fibres of peroneus longus. The 'fast' and 'slow' patterns of chronic stimulation had the same effects on the staining properties. 8. Chronically stimulated peroneus longus muscles showed a decrease in fibre diameter which corresponded, roughly, to the concomitant decrease in muscle weight.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of spinal cord stimulation on the metabolism of developing latissimus dorsii muscles in chick embryo

Influence of chronic spinal cord stimulation upon some characteristic enzyme activities of energy metabolism was investigated in slow anterior (ALD) and fast posterior (PLD) latissimus dorsii muscles of the chick embryo. During embryonic life, oxidative metabolism (as evaluated by the activity of malate dehydrogenase (MDH] represents the main energetic pathway in both slow and fast muscles. At the end of embryonic life, an increase in anaerobic (as evaluated by the activity of lactate dehydrogenase (LDH] and creatine phosphokinase (CPK) activities occurs in PLD muscle. Chronic spinal cord stimulation at a low frequency was performed from the 10th day to the 16th day of embryonic development. In ALD, the enzyme activities were unaffected, while in PLD a concomitant decrease in LDH and CPK activities was observed.

Low frequency chronic electrical stimulation of normal and dystrophic chicken muscle

The fast-twitch posterior latissimus dorsi muscle of normal and genetically dystrophic chickens was subjected to continuous indirect electrical stimulation at 10 Hz for periods of 4-8 weeks. To sustain this in vivo nerve stimulation an internally implantable miniature stimulator device was designed. This regime of stimulation caused complete fatigue of the normal muscle within 5 min of its initiation. The dystrophic muscles maintained a very small degree of contractile activity during this initial phase. Tangible twitching of the muscle returned in 5 week birds between 3 and 5 days and in 10 week birds between 11 and 16 days after implantation. After 4 weeks of stimulation, no significant change was measured in the time-to-peak of the isometric twitch response, nor in the half-relaxation time. The resistance to fatigue was significantly increased in the stimulated muscles when tested with a series of tetani at 40 Hz. The mean fibre area was decreased, in all muscles stimulated for longer than 3 weeks, in comparison to their contralateral controls, except where fibre splitting in dystrophic birds abnormally reduced the control value. The majority fibre type of the muscle was changed from type IIB to IIA. The histochemical reactions for both NADH-linked oxidation and phosphorylase were distinctly increased in the stimulated muscles. In normal muscle, stimulation increased somewhat the number of nuclei per unit area and changed their intracellular distribution, so that a greater proportion was found adjacent to the sarcolemma. The normal posterior latissimus dorsi muscle responded to chronic stimulation with increases of 3-6-fold in its acetylcholinesterase (AChE) activity. The maximum change in AChE occurred after 2 weeks stimulation; a steady level, 3 times that of the control unstimulated muscle, persisted at later times. Chronic stimulation suppressed the over-production of AChE that is characteristic of dystrophic chicken fast-twitch muscle, to attain a level comparable to the AChE activity in a stimulated normal muscle. Stimulation exerted a strong normalizing influence on dystrophic muscle, as assessed morphologically. The characteristic fibre rounding, fibre hypertrophy and myonuclear proliferation were reduced. This influence was most marked where the stimulation was initiated before the major pathological changes had occurred, but was also significant when commenced in strongly affected birds of 10-11 weeks.

An investigation of arterial insufficiency in rat hindlimb. A combined 31P-n.m.r. and bloodflow study

A small animal model of arterial insufficiency is presented which involves unilateral femoral artery ligation and section. Invoked alterations in metabolism and perfusion of the affected muscle mass have been investigated 12 h, 4, 7 and 14 days post-ligation by 31P-n.m.r. and microsphere infusion, both at rest and during isometric muscle contraction at 1 Hz. At rest, the concentration of phosphocreatine was similar to the mean control value (36.0 +/- 1.0 mM) from 4 days post-ligation, but was significantly lower at 12 h (28.5 +/- 3.6 mM). Inorganic phosphate concentrations were significantly elevated for 7 days post-ligation. No significant differences were noted in intramuscular pH. Upon stimulation of the affected muscle mass, a time-dependent improvement in phosphocreatine utilization was observed such that 14 days post-ligation phosphocreatine utilization was not significantly different from mean control values. A similar amelioration was noted for the contraction-induced fall in intramuscular pH. At rest, no significant differences in bloodflow to the muscles of the ligated limb compared with the unaffected contralateral limb were observed. However, isometric contraction of the affected muscle mass resulted in a markedly reduced hyperaemic response 12 h post-ligation. Thereafter, a time-dependent improvement in tissue perfusion during stimulation was observed which paralleled the improvements in phosphocreatine utilization and intramuscular pH changes. The results presented are discussed with respect to the interrelationship between oxygen delivery, high energy phosphate utilization and force maintenance.

Effect of exercise on the motor unit

The motoneuron part of this review deals with the changes in recruitment and firing rates of the motor unit types upon changes from a physically inactive life to endurance or strength training. The muscle fibers react to prolonged exercise by adaptation to a higher level of performance. A matter of discussion is the prerequisites for a transformation between the basic muscle fiber types, slow twitch and fast twitch, during voluntary (transsynaptic) activity, which is demonstrated after artificial nerve stimulation. The review includes current knowledge of muscle fiber transformation as an adaptive response to increased usage either by electrical stimulation or by transsynaptic neuronal activity. The metabolic adaptation related to increased endurance is reviewed with special reference to effects on muscle fibers. The increase in strength as a result of high resistance training is mainly the result of increased muscle cross-section. Whether this is solely the result of an increase in size of individual fibers or an increased fiber number is a controversial matter.

[Effects of intermittent muscle stimulation on muscle catabolism in patients immobilized in the ICU]

Are muscular contractions obtained by electrical stimulation able to reduce muscle catabolism in immobilized patients? Ten patients (65 to 79 yr old), hospitalized in an intensive care unit for postoperative failure or cerebral infarction, were studied during nine days. Artificial nutrition was the same for each patient during the study. Two periods of four days where defined and randomized for each patient, separated by one day; during the stimulation period (S), intermittent electrical stimulation of the muscles of the legs (external electrodes) was performed daily 2 X 30 min; during the non-stimulation period (NS), muscular stimulation was not performed. Urinary excretion of nitrogen (micro-Kjeldhal digestion and Nessler procedure), creatinine (Jaffé reaction) and 3-methylhistidine were measured every day. Results (X +/- SD) are as follows: the nitrogen balance (g/d) was -1.29 +/- 1.26 during the NS period and 1.43 +/- 1.10 during the S period (NS); 3-methylhistidine (mumol/kg/d) was 3.78 +/- 0.37 during the NS period and 3.15 +/- 0.32 during the S period (p less than 0.01); creatinine (mumol/kg/d) was 92.9 +/- 6.8 during the NS period and 72.9 +/- 25 during the S period (p less than 0.01). It is concluded that a significant decrease in 3-methylhistidine and creatinine excretions is observed during the S period. In intensive care unit patients, muscle protein breakdown may be influenced by intermittent electrical muscle stimulation.

Changes of energy metabolism, myosin light chain composition, lactate dehydrogenase isozyme pattern and fibre type distribution of denervated fast-twitch muscle from rabbit after low frequency stimulation

The influence of low frequency (8-10 Hz) electrical stimulation on denervated fast-twitch muscle from rabbit was investigated. Prolonged direct stimulation of denervated muscle resulted in higher oxidative enzyme activities. Furthermore, single fibre analyses for succinate dehydrogenase showed a more uniform distribution of activity in stimulated-denervated muscle when compared to normal muscle. As was also the case following stimulation of innervated muscle, glycolytic enzymes were decreased in activity and the LDH-isozyme pattern was also shifted towards heart type. No change of the myosin light chain pattern could be observed after 56 days of stimulation.

Effects of percutaneous electrical stimulation on functional ability, plasma creatine kinase, and pectoralis musculature of normal and genetically dystrophic chickens

The breast musculature of genetically dystrophic Line 413 and genetically related normal Line 412 chickens were treated in three separate trials with high-frequency electrical stimulation (ES). Beginning on days 7 or 14 ex ovo, each bird received three ES treatments per week. Each stimulation cycle repeated five times per day consisted of 15 s "on" followed by 50 s "off". In the third trial only, the birds were additionally treated beginning day 3 ex ovo with either leucine (100 mg/kg) or the proteinase inhibitor Ep475 (10 mg/kg). ES significantly delayed the onset of righting disability in the dystrophic chickens. However, this improvement was temporary and could be masked by single treatments of either leucine or Ep475. Plasma creatine kinase activities were increased generally in both the stimulated normal and dystrophic birds. In two trials ES increased the relative muscle mass, and in one trial increased protein. ES had little effect on normal muscle mass or protein. However, ES treatment together with either leucine or Ep475 appeared to improve both normal and dystrophic muscle mass and protein. Furthermore ES decreased dystrophic muscle calcium but not acetylcholinesterase activity. On the other hand, ES had no effect on the total normal muscle calcium but increased normal acetylcholinesterase values. In both normal and dystrophic muscle samples, ES treatment in combination with leucine appeared to increase the mean muscle fiber diameters and number of myonuclei, and in the case of the dystrophic muscle, appeared to decrease the relative proportion of vacuolated, degenerating, and intensely oxidative histochemical fibers. In general, stimulation (especially in combination with leucine) appears to alter in varying degrees the phenotypic expression of the muscle disease exhibited in the dystrophic chicken.

Neural control of phenotypic expression in mammalian muscle fibers

In this review, the present knowledge about the mechanisms involved in the control of the phenotypic expression of mammalian muscle fibers is summarized. There is a discussion as to how the activity imposed on the muscle fibers by the motoneuron finally induces in the muscle cells the expression of those genes that define its particular phenotype. The functional and molecular heterogeneity of skeletal muscle is thus defined by the existence of motor units with varied function, while the homogeneity of muscle fibers belonging to the same motor unit is yet another indication of the importance of activity in the control of gene expression of the mammalian muscle fiber.

Influence of direct low frequency stimulation on contractile properties of denervated fast-twitch rabbit muscle

A continuous electrical 8 Hz impulse pattern imposed directly via implanted electrodes on denervated fast twitch muscle induced changes in its contractile characteristics. Compared with non-stimulated denervated muscle, stimulated muscle showed slowing of contraction time and improved fatigue resistance. The reaction for succinic dehydrogenase was more intense in the denervated stimulated muscle, indicating an increased capacity of oxidative enzymes. The rate of atrophy was not influenced by stimulation. The 8 Hz frequency pattern is the mediator for these changes in the characteristics of denervated muscles. It demonstrates a comparable effect on innervated muscle. The contralateral normal innervated muscle was also influenced by the electrical stimulation. Contraction time as well as twitch tension were increased. This finding is important when using the normal muscle as intraindividual control.

Effects of different patterns of long-term stimulation on blood flow, fuel uptake and enzyme activities in rabbit fast skeletal muscles

Long-term electrical stimulation (14-28 days) of rabbit fast muscles (tibialis anterior, TA and extensor digitorum longus, EDL) using intermittent high frequency (3 trains per min of 5 s duration at 40 Hz, for 8 h per day) produced changes in enzyme activities similar to those found with continuous stimulation at a frequency occurring in nerves to slow muscles (10 Hz). The activity of citrate synthetase, 3-hydroxyacyl-CoA dehydrogenase and succinate dehydrogenase increased two to 3-fold within 28 days. There was a 4-fold increase in hexokinase whereas phosphofructokinase, pyruvate kinase, lactate dehydrogenase and fructose-1,6-diphosphatase decreased to about 60% of the activity levels in the contralateral unstimulated muscles. Blood flow and oxygen consumption at rest were not changed even after 28 days of stimulation, but were increased during contractions in muscles stimulated at either frequency, the level being twice as high as in control muscles. Glucose uptake was similar to that in control muscles both at rest and during contractions and the output of lactate was similar to that found in control muscles in muscles stimulated at 40 Hz. Muscles stimulated at 10 Hz had smaller lactate output. Thus intermittent stimulation at high frequency (40 Hz) and continuous low frequency (10 Hz) produced similar changes in aerobic metabolism and fuel uptake provided that the total number of stimuli was comparable and that the stimulation was carried out for sufficiently long period.

Effects of fast and slow patterns of tonic long-term stimulation on contractile properties of fast muscle in the cat

Different physiological rates of 'tonic' long-term electrical stimulation (rates 5-40 Hz; activity greater than or equal to 50% total time) were delivered to the left-side common peroneal nerve of the cat hind limb. The duration of treatment was 8 weeks, and the animals had previously been subjected to a left-side hemispinalization and dorsal rhizotomy. In the absence of stimulation, these operations had no slowing or weakening effects on peroneal muscle contraction. The minimum two-pulse interval that gave a summation of tension (neuromuscular refractory period) was longer for stimulated than for non-stimulated muscles. Twitches of chronically stimulated muscles had become prolonged by more than 100%. Corresponding changes were found in the tension-frequency relation and in the 'sag'-behaviour of the stimulated muscles. There were no differences between the 'fast' (20 or 40 Hz pulse rates) and the 'slow' (5 or 10 Hz pulse rates) patterns of tonic stimulation with respect to their effects on speed-related muscle properties. Furthermore, during the period of chronic stimulation, the prolongation of twitch contraction time occurred along the same time course for the fast and slow patterns of tonic treatment. All chronically stimulated muscles had become weaker than normal. In comparison to the slow patterns, the present fast patterns of long-term activation caused (1) a smaller amount of decline in maximum muscle force, (2) a smaller twitch: tetanus ratio, and (3) the retention of a normal amount of post-tetanic potentiation of twitch size (decreased by the slow patterns). When tested by a series of 40 Hz bursts, force was better maintained in chronically stimulated muscles than in normal ones. These effects on fatigue resistance were the same for the fast and slow patterns of long-term activation. In peroneus longus muscles contralateral to the side of chronic activation, an evident impairment had commonly occurred in the capability to maintain force during tetani at the high rates needed for a maximum tetanic contraction. The results are discussed in relation to problems concerning the long-term effects of motoneuronal activity patterns on the contractile properties of their muscle units.

Functional electrical exercise: a comprehensive approach for physical conditioning of the spinal cord injured patient

The purpose of the present study was to define a comprehensive approach to the rehabilitation of the paralyzed individual. Consequently, we have examined the cardiopulmonary effects of active physical therapy (APT) (as measured by blood pressure [BP], heart rate [HR], equivalent ventilatory rate [ VE], oxygen consumption rate [VO2], blood lactate [LA], and selected blood gases [pO2, pCO2 and pH]), and the skeletal effects of APT (bone density changes). The equipment utilized in this study were an isokinetic leg trainer, an exercise bicycle ergometer, and a single axis vibration platform. The procedures involved standard exercise protocols followed by the eight SCI subjects (four paraplegics and four quadriplegics) utilizing the above equipment, an evaluation of the cardiopulmonary response to the leg trainer and bicycle ergometer, and a computed tomography study of bone density changes in response to lower leg vibration. The following conclusions were reached: 1) APT results in an increase in cardiovascular stability; over a four-week period mean resting BP increased by an average 29 mmHg in four quadriplegic subjects, while mean exercising BP was reduced by an average 10 mmHg. 2) BP and HR responses to exercise are variable as a function of the level of the injury and type of location of exercise. 3) VO2 correctly showed level of the exercise and LA showed the stress; however, ventilation was variable with hyperventilation in some subjects. 4) Arterial pO2, pCO2, pH are variable during exercise; with complete quadriplegics typical pO2 at rest was 82 mmHg, showing venous admixture as with Pickwickian syndrome with CO2 retention and low arterial pO2 during exercise. 5) APT results in an increase in bone mineral density through impact loading of the paralyzed extremity.

Effects of chronic stimulation on the metabolic heterogeneity of the fibre population in rabbit tibialis anterior muscle

Chronic indirect stimulation (10 Hz) was performed on rabbit tibialis anterior muscle. Long-term stimulation (52-140 days) produced a transformation of the fast tibialis anterior into a slow red muscle as judged from the histochemistry of myofibrillar actomyosin ATPase, the pattern of myosin light chains and the thorough rearrangement of the enzyme activity pattern of energy metabolism. Activity levels of citrate synthetase (CS), malate dehydrogenase (MDH), succinate dehydrogenase (SDH), 3-hydroxy-acyl-CoA dehydrogenase (HAD), and lactate dehydrogenase (LDH) were determined quantitatively by either microbiochemical assays (CS, MDH, HAD and LDH) on microdissected, single fibres or by kinetic microphotometry on cross-sectioned fibres (SDH). The activity profiles of these enzymes displayed pronounced scattering in the fibre population of the unstimulated muscle. Despite a several fold increase in the activities of CS, MDH, SDH and HAD and a pronounced decrease in LDH, chronic stimulation failed to abolish the metabolic heterogeneity of the fibre population. It is possible that chronic indirect stimulation cannot produce uniformity of fibres because of continuing diverse natural activity of the motor units.

Relationships between early alterations in parvalbumins, sarcoplasmic reticulum and metabolic enzymes in chronically stimulated fast twitch muscle

The present study compares the time courses of the early changes in parvalbumin content, in the properties of the sarcoplasmic reticulum (SR) and in activity and isozyme patterns of metabolic enzymes in chronically (12 h/day) stimulated fast twitch tibialis anterior (TA) muscle of the rabbit. Under the chosen conditions of stimulation, the first significant changes appeared after 6 days. Except for the delayed reduction in pyruvate kinase, the time course of the changes were the same. After 14 days of stimulation, parvalbumin decreased to 37% and Ca2+-ATPase activity of the SR to 29% of normal values. The transformation of the SR was also reflected by a 64% decrease of the 115000-Mr Ca2+-pumping peptide and a 5-fold increase in a 30000-Mr peptide. Following an identical time course, the mitochondrial activities of citrate synthase, 3-hydroxyacyl-CoA dehydrogenase and ketoacid-CoA transferase increased 2.9, 3.0 and 3.7-fold respectively. A similar time course was observed in the M to H-type transition of the lactate dehydrogenase isozymes. The cause of these changes is discussed as it relates to altered transcriptional and/or translational activities. It is suggested that an increase in free intracellular Ca2+ caused by increased contractile activity, which is then perpetuated by the decrease in Ca2+-binding and sequestering capacities, might be the signal for such altered synthetic activities.

Fibre type specific transformations in the enzyme activity pattern of rat vastus lateralis muscle by prolonged endurance training

The alterations in activity patterns of representative enzymes in energy metabolism were investigated in the superficial (white) and deep (red) portions of the fast vastus lateralis muscle of the adult rat in response to prolonged endurance training. It was found that following 15 weeks of extreme training (final running duration: 210 min per day, 27 m/min at 15 degree grade), increases in the activities of marker enzymes of the citric acid cycle (citrate synthase), beta-oxidation (3-hydroxyacyl CoA dehydrogenase), and ketone body utilization (3-ketoacid CoA transferase) as well as of glutamate pyruvate transaminase occurred in both regions of the muscle, with the greatest increase being observed in the superficial portion (2.6-4.2-fold). Pronounced increases were also seen for hexokinase which showed highest activities after 7 weeks of training. Conversely, decreases were noted for various glycogenolytic, glycolytic and gluconeogenic enzymes (phosphorylase, glyceraldehydephosphate dehydrogenase, pyruvate kinase, lactate dehydrogenase and fructose-1,6-diphosphatase). Reduction in the activities of these enzymes was most pronounced in the deep portion of the muscle. These results demonstrate a fundamental rearrangement of the energy metabolism of the muscle in response to prolonged, high intensity training. In the case of the deep portion of the vastus lateralis muscle, which has been shown to be composed of a large percentage of fast oxidative-glycolytic fibres (FOG), the enzyme profile becomes similar to the slow oxidative (SO) fibre. In the superficial portion which contains predominantly fast glycolytic fibres (FG), the enzyme profile becomes similar to FOG fibres.(ABSTRACT TRUNCATED AT 250 WORDS)

Two telestimulation systems for chronic indirect muscle stimulation in caged rabbits and mice

Telestimulation systems are described for chronic indirect muscle stimulation in caged rabbits and mice. Both systems use a 5 MHz carrier frequency transmission and consist of a transmitter and a receiver. The latter is fixed to the back of the animal. The system for rabbits uses pulse width modulation for transmitting stimulation frequency and amplitude. Duration of the stimulation impulse is generated in the receiver. Clock batteries in the receiver generate impulse energy. The impulse amplitude varies by only 1%. In the system used for mice, impulse energy is transmitted together with the stimulation frequency. This is achieved by a receiver containing two separate coils which are opposed to each other in an angle of 80 degrees C. In contrast to the rabbit system, the duration of the stimulation impulse is generated by the impulse width of the 5 MHz carrier. The amplitude of the stimulation impulse depends on the amplitude of the carrier. Due to the geometry of induction coil and receiver, impulse intensity varies at maximum by only 10%.

Effect of chronic electrostimulation of the sacral roots on the striated urethral sphincter

Chronic electrostimulation of the sacral roots induces an increase in urethral resistance due to activation of muscles of the striated urethral sphincter. Histochemical study of the urethral striated musculature (nonstimulated) revealed 3 main varieties of fibers: 1) Slow twitch fibers (35 per cent of the whole muscle); 2) Fast twitch fatiguable fibers (52 per cent); and 3) An intermediate type, fast twitch fatigue resistant (13 per cent). Ten dogs with chronically implanted electrodes at the 2nd sacral root were subjected to a program of prolonged stimulation under anesthesia. After prolonged electrostimulation, we detected hypertrophy of the striated muscle fibers of the urethra, anal sphincter and stimulated side of the tail. The stimulated muscle fibers showed a higher overall oxidative activity than the controls. This occurred both in intact and spinalized animals. We thus expect the urethral striated musculature to be more resistant to fatigue because of the increased oxidative activity. Stimulated fibers also had increased glycolytic activity as shown by the enhanced intermyofibrillar deposition, especially in the fast twitch fibers. The increased glycolytic activity may also increase fatigue resistance by producing energy during periods of low oxygen supply at the peak of muscular contraction. As a consequence of increase in oxidative and glycolytic capacities and muscular hypertrophy, we expect that the striated musculature of the urethra will be not only more resistant to fatigue but also capable of generating higher tension. Both are important in achieving continence via electrostimulation of sacral nerve roots. Approximately 3 months after conclusion of the stimulation program these changes had gradually reverted to the normal prestimulation level.

Histochemical and physiological properties of cat motor units after self-and cross-reinnervation

1. This report describes selected histochemical and physiological properties of the motor units of adult cat soleus muscle approximately one year after self- and cross-reinnervation with the nerve of the heterogenous flexor hallucis longus (f.h.l.). Self-reinnervated f.h.l. motor units are also considered. Whole muscles were tested for fibre reaction to alkaline pre-incubated ATPase, alpha-glycerophosphate dehydrogenase (alpha-GPD) and reduced nicotinamide adenine dinucleotide diaphorase (NADH-D). Motor units were isolated and studied by splitting the ventral root in acute preparations.2. The histochemical fibre type profile in the self-reinnervated muscle was comparable to normal muscle as was mean twitch contraction time, twitch-tetanus ratio and fatigue index. The mean tetanic tension of the soleus self- and cross-reinnervated motor units appeared close to a normal soleus whereas the mean tetanic tension of the f.h.l. self-reinnervated units was significantly less than a normal f.h.l.3. An average of 14% of the fibres of the soleus cross-reinnervated muscles had high ATPase and a alpha-GPD staining intensity in contrast to normal and self-reinnervated soleus in which such fibres are absent. Thus alkaline lability of myofibrillar ATPase increased in some fibres of what was originally a homogeneous population. The small increase in the number of densely staining fibres for ATPase at an alkaline pH (14%) was associated with a 73% decrease in (mean) contraction time (41 +/- 11 ms) of the thirty-three cross-reinnervated muscle units studied, with no unit's contraction time greater than 60 ms. Mean contraction times for the self-reinnervated soleus and f.h.l. muscles were 78 +/- 31 ms and 27 +/- 8 ms respectively.4. All fibres of the soleus cross-reinnervated muscles showed intense reaction to NADH-D, as was true of self-reinnervated soleus. This staining pattern is typical of normal soleus. In concordance, these motor units consistently demonstrated a high resistance to fatigue when stimulated for a four-minute period.5. These results suggest that in the adult self-and cross-reinnervated soleus muscle, there is some active mechanism which regulates the eventual size of motor units as reflected by tetanic tension.6. Change in contraction time from that typical for a soleus unit to that similar to an f.h.l. unit remains incomplete one year after cross-reinnervation. Within this time this partial change in single motor units reflects incomplete neural control of this property rather than a mixture of self- and foreign-innervation.7. A greater degree of independence from neural control to conversion of the histochemically demonstrated myofibrillar ATPase activity exists than is the case for contraction time.

Increase of muscle mitochondrial content with age in murine muscular dystrophy

Comparison of morphological features of gastrocnemius muscle fibers in normal and dystrophic (dy2J) mice during development was undertaken to determine the time course of increased oxidative capacity in dystrophic fibers. Measurements of mitochondrial volume percent and of Z-line width were made in superficial fast-twitch fibers using electron microscopy and stereological techniques. Dystrophic fibers develop a progressively higher mitochondrial volume percent than normal fibers after 1 month of age. Z-line width is positively correlated with mitochondrial volume percent. The results support the hypothesis that progressive changes in muscle fiber properties result from abnormal neural activity (pseudomyotonia) in dystrophic animals.

Role of the motor nerve in activity-induced enzymatic adaptation in skeletal muscle

The sciatic nerve was cut on one side in 11 male cats, and a piece of the nerve was removed. The cats were then divided at random into two groups, a stimulation group (S) of five cats and a control group (C) of six cats. Bilateral electrical stimulation (2 Hz) of the gastrocnemius muscle (directly or via the motor nerve) was carried out in the S cats 4 h/day, 3 days/wk for 4 wk. The voltage delivered was adjusted in each cat so that both gastrocnemius muscles lifted identical loads the same distance. The activity of the tricarboxylic acid cycle marker enzyme succinate dehydrogenase (SDH) per unit of muscle weight more than doubled in response to stimulation both in the intact and the denervated gastrocnemius muscle. Stimulation did not affect the activity of the glycolytic marker enzyme 6-phosphofructokinase (PFK) or muscle capillarization. Denervation resulted in pronounced (approx 50%) fiber atrophy, which was not prevented by the stimulation. It is concluded that the presence of the motor nerve per se is not necessary for an activity-induced adaptation of the oxidative capacity of skeletal muscle.

The effect of different patterns of long-term stimulation on contractile properties and myosin light chains in rabbit fast muscles

Fast rabbit skeletal muscles (tibialis anterior and extensor digitorum longus) were stimulated for 2-28 days by electrodes implanted in the vicinity of the peroneal nerve to produce maximal contractions at two different frequency patterns: that occurring naturally in nerves to slow muscles (10 Hz continuously) or three bursts of tetani (40 Hz) per minute, each 5s in duration. Both types of frequency produced muscles more resistant to fatigue during isometric twitch contractions, and led to a prolongation of contraction time greater and more consistent with 10 Hz than with 40 Hz. The twitch/tetanus ration was significantly higher in muscles stimulated at 10 Hz for 3-4 weeks but was not different from controls in muscles stimulated at 40 Hz. Both types of stimulation led to the appearance of myosin light chains characteristic of slow muscles. Muscles stimulated for 4 weeks at 40 Hz developed greater twitch tension per gram, and had significantly smaller cross-sectional area of myofibrils than control muscles. It is concluded that long-term electrical stimulation of fast muscles can affect some muscle contractile properties to resemble those of slow muscles irrespective of frequency of stimulation, provided the total number of stimuli is comparable, the duration of stimulation is long enough (minimum 2 weeks) and all motor units are activated.

Electrical stimulation of denervated muscle prevents decreases in oxidative enzymes

The influence of muscular contraction on the oxidative enzymes and the diameters of muscle fibers was investigated. Soleus muscles of guinea pigs were denervated for four weeks. The denervated fibers showed a reduction in the intensity of staining for beta-hydroxybutyrate dehydrogenase, cytochrome oxidase, succinate dehydrogenase, and NADH-dependent tetrazolium reductase. Denervation also resulted in a decrease in fiber diameter. Denervated soleus muscles were electrically stimulated to contract over a four-week period at a frequency normally received by slow contracting muscles. Electrical stimulation caused the stain intensity of histochemical reactions for oxidative enzymes to appear to be normal or greater than normal in 90% of the denervated fibers. Stimulation also caused 69% of the denervated fibers to be of normal or greater than normal size. The results demonstrate that contraction of denervated muscle by electrical stimulation prevents the loss of oxidative enzymes and the atrophy associated with denervation.

The reorganization of subcellular structure in muscle undergoing fast-to-slow type transformation. A stereological study

Transformation of fast-twitch into slow-twitch skeletal muscle was induced in adult rabbits by chronic low-frequency stimulation and studied at the ultrastructural level. With the use of stereological techniques, a time course was established for changes in mitochondrial volume, sarcotubular system, and Z-band thickness for periods of stimulation ranging from 6 h to 24 weeks. T-tubules, terminal cisternae, and sarcoplasmic reticulum decreased at an early stage and reached levels typical of slow muscle after only 2 weeks of stimulation. Transformation of Z-band structure took place between 1 1/2 and 3 weeks after the onset of stimulation. Mitochondrial volume increased several fold over the first 3 weeks of stimulation, and fell rapidly after 7 weeks, although it still remained above the levels typical of slow muscle. Although there was no sign of degradation and regeneration of the muscle fibers themselves, considerable structural reorganization was evident at the subcellular level after 1 week of stimulation. The fibers passed through a less well organized transitional stage in which fibers could not be assigned to a normal ultrastructural category. After 3 weeks all of the stimulated fibers could be assigned to the normal slow-twitch category although some subcellular irregularities persisted even after 24 weeks. The ultrastructural alterations are discussed in relation to functional and biochemical changes in the whole muscle.

Histochemical characteristics of muscle fiber types in the posterior cricoarytenoid muscle

The muscle fiber type composition of the human posterior cricoarytenoid muscle (PCA) was examined using a large battery of histochemical techniques. Staining for myosin ATPase (pH 9.9) indicated that the muscles were composed of 52% ± 11.8 SD type 1 (slowly contracting) fibers and 48% ± 11.9 SD type 2 (rapidly contracting) fibers. In order to obtain information concerning the probable fatigue resistance of the type 2 fibers, serial sections were processed to determine the relative extent of ATPase inactivation at various pH levels in the acid range and to obtain data concerning the relative activities of oxidative and glycolytic enzymes and their substrates. The great majority of the type 2 fibers were of the 2A (fatigue resistant) fiber type. This indication of a capacity for prolonged activity was substantiated by the presence of high activities of succinic dehydrogenase, a mitochondrial enzyme which is involved in oxidative metabolism. Type 2C fibers (generally considered to be an undifferentiated fiber type) were also present but relatively rare. The overall enzyme profiles of many of the muscle fibers in the human PCA differed from those typical of fibers having the same alkaline ATPase and acid ATPase characteristics in most other mammalian muscles. Since muscle fiber biochemistry reflects the activity pattern of the motor unit, these unusual enzyme profiles may be the result of activity patterns that are associated with the inspiratory cycle and/or patterns of activity that are relatively specific to the PCA. Four of the ten muscles examined had unequivocal evidence of muscle fiber type grouping, a manifestation of partial denervation followed by reinnervation. This is interesting since most of the cases were in the fifth decade. Muscle fiber type grouping has been shown to occur selectively in certain other human muscles and to increase with age, eventually resulting in muscle atrophy. This suggests the possibility that the human PCA is for some reason selectively vulnerable to partial denervation and indicates the need for more extensive data concerning the relationship of muscle fiber type grouping in the PCA and other laryngeal muscles to age.

The adaptive response of skeletal muscle to increased use

Skeletal muscle undergoes profound changes in morphological, physiological, and biochemical character when subjected to prolonged periods of increased use. Although increased use may be brought about in a variety of ways, the results show consistent features. In particular, endurance exercise and chronic stimulation differ only in degree: the properties which change in response to exercise are also those which change at an early stage of stimulation; the properties which are resistant to change under exercise conditions change only after prolonged stimulation. There is therefore a hierarchy of stability in the properties of skeletal muscle which is revealed in its response to changing functional demands. The adaptive potential of muscle provides a logical framework for understanding neural influences on the emergence of fiber types during muscle development. It is also relevant to the study of pathological conditions which may involve a sustained departure from normal postural and locomotor patterns of activity.

Succinate dehydrogenase activity in fibres classified by myosin ATPase in three hind limb muscles of rat

1. Succinate dehydrogenase (SDH) activity was assessed in situ in single fibres of cross-sectioned extensor hallucis longus, extensor digitorum longus, and soleus muscles of rat by means of microphotometric recordings of initial maximum reaction rates. 2. Each fibre assessed for SDH activity was subjectively classified into myosin subgroups by its histochemical reaction for myofibrillar actomyosin ATPase (myosin ATPase) following preincubation at pH 4.6 according to Brooke & Kaiser (1970). 3. The majority of fibres classified into myosin types I and IIa were highly reactive for SDH, such that those myosin groups could be interchangeable with the metabolic subgroups of Peter, Barnard, Edgerton, Gillespie & Stempel (1972); myosin I = slow-twitch oxidative, myosin IIa = fast-twitch oxidative glycolytic. 4. The myosin type IIb fibres, however, demonstrated marked variability in activity levels of SDH. Over 40% of those fibres had high SDH activity, and thus could not be equated with the metabolic subgroup fast-twitch glycolytic. 5. The histochemical reaction for myosin ATPase in muscle fibres therefore cannot be used as a reliable means to predict the fibres' metabolic characteristics.

A histochemical and electron microscopic study of a fast- and a slow-twitch muscle in genetically spastic mice

Fast and slow muscle fibers were studied in the flexor digitorum longus (FDL) and soleus (SOL) muscles, respectively, in control and spastic mice. HIstochemical and electron microscopic studies indicated an increased number of mitochondria, a decreased deposition of glycogen and a vesiculation and distension of the sarcoplasmic reticulum in many fast-twitch fibers of the spastic FDL. Similar findings were not evident in the slow-twitch fibers of the spastic SOL. Since the spastic condition causes increased muscular activity as a result of more rapid and prolonged nerve impulse firing, these findings reinforce the idea that a muscle fiber's oxidative capabilities are a function of its activity.

Effects of denervation and simple disuse on rates of oxidation and on activities of four mitochondrial enzymes in type I muscle

To differentiate the effect of muscle contractile activity from that of motor nerve on oxidative processes in type I muscle, oxidative processes were studied in muscle after immobilization and after denervation. The two processes led to similar atrophy of muscle weight and of the mean diameter of muscle fibers. Disuse of soleus muscle (type I) did not affect rates of oxidation of 14C-labeled substrates although these were reduced by disuse of the vastus lateralis (type II). Disuse of the soleus did not affect activities of several mitochondrial enzymes assayed by histochemical or biochemical methods. However, denervation of the soleus did lead to a fall in metabolic rates and enzyme activities. The activity of 3-hydroxybutyrate dehydrogenase fell more than did the activities of succinic dehydrogenase, lipoamide dehydrogenase, or cytochrome-c oxidase in both homogenates and in mitochondrial fractions. These results suggest nerve may regulate mitochondrial enzymes in type I muscle. The mechanism appears to be different from that which regulates oxidative processes in type II muscle.

Is resistance of a muscle to fatigue controlled by its motoneurones?

The original experiment of Buller et al. and the many subsequent confirmatory reports clearly show that the time-to-peak tension and many other speed-related parameters of slow and fast muscle fibres are dictated by the motoneurone. It has been concluded that the motoneurone exerts this control of the physiological and associated biochemical properties by the frequency at which it excites the muscle fibre. However, no studies have been reported on the fatigue properties and the associated biochemical characteristics after cross-reinnervation. Based on the 'size principle' of motoneurones, it would be reasonable to assume that a muscle fibre reinnervated by a small motoneurone would be active often and that this would be manifested biochemically as an elevated oxidative capacity. Also, it has been shown repeatedly that the mitochondrial content of a muscle fibre can be modified by daily endurance type exercise. Thus, it would seem that the motoneurone at least indirectly also controls the mitochondrial content of a muscle fibre by controlling the degree of activity. We have now tested this hypothesis using self- and cross-reinnervated muscles in cats. We found that fast- and slow-twitch muscles retained their characteristic fatigue resistance properties regardless of whether the nerve to which they had become connected had originally innervated a fatigue-resistant or relatively fatiguable muscle.

Associated changes in Ca2+ and Sr2+ activation properties and fiber proteins in cross-reinnervated rabbit soleus

Changes in the Ca2+ and Sr2+ activation properties of functionally skinned slow-twitch soleus fibers were measured and compared with those of normal fast-twitch extensor digitorum longus (EDL) following cross-reinnervation of soleus with the nerve to EDL. Most of the fibers showed either complete transformation of activtion properties (66%) or remained unchanged (34%). The change in sensitivity to divalent cations was correlated with changes in the proteins present in fibers pooled on the basis of their activation properties. The banding patterns of the 35,000- and 37,500-dalton proteins (tropomyosin and troponin T) in cross-reinnervated soleus were correspondingly transformed to those of normal EDL. Slow and fast myosin light chains were present in the pooled cross-reinnervated fibers. Fiber distributions based on activation properties were confirmed by histochemical features. For the first time it has been demonstrated that cross-reinnervation produced changes in the activation properties of soleus fibers and associated changes in the regulatory proteins measured.

Muscle fibrillation caused by cytochalasin-B applied to the motor nerve

Cytochalasin-B, a drug known to interfere with axoplasmic transport, evoked fibrillary potentials in the geniohyoid muscle when applied to its motor nerve. Despite this denervation-like effect, neuromuscular transmission remained normal. Some contractile characteristics of the muscle were studied. It was found that contraction time, isometric twitch tension, and half-relaxation time were not altered by the drug treatment. The present findings show that neurogenic molecular factors conveyed by axoplasmic transport to the nerve terminal are involved in the regulation of some muscle membrane characteristics but do not modify the muscle contractile features.

Comparison of isometric muscle training and electrical stimulation supplementing isometric muscle training in the recovery after major knee ligament surgery. A preliminary report

Eight patients undergoing reconstruction of the anterior cruciate ligament were randomly allocated into two groups. The control group received a standard plaster cast and isometric muscle training. The stimulated group received a standard plaster cast, isometric training, and percutaneous electrical stimulation during the recovery period. The patients were examined clinically and with repeated muscle biopsies before surgery, 1 week after surgery, and 5 weeks after surgery at the time of removal of the cast. The electrically stimulated group had better muscle function from a clinical point of view and their succinate dehydrogenase activities were significantly higher than those in the control group. Electrical stimulation thus could prevent the fall in oxidative enzyme activity which was noted in the control group. The results suggest that percutaneous electrical stimulation may be a way of preventing muscle atrophy after major knee ligament surgery in athletes.

Molecular transformations in sarcoplasmic reticulum of fast-twitch muscle by electro-stimulation

Chronic electro-stimulation of fast-twitch rabbit muscle with the frequency pattern received by a slow-twitch muscle induces a progressive transformation of the sarcoplasmic reticulum. After 2 days stimulation activities of Ca2+-dependent ATPase and of Ca2+ transport begin to decrease, and are paralleled by a progressive decrease in Ca2+-dependent and Ca2+, Mg2+-dependent phosphoprotein formation, reduced rate of dephosphorylation and a rearrangement of the electrophoretic polypeptide and phosphoprotein patterns. These findings suggest a transformation of the sarcoplasmic reticulum to resemble that of a slow-twitch muscle. This transformation is paralleled by increase in time-to-peak of twitch contraction and half relaxation time and occurs before conversion of the myosin light chain pattern is observed. The parallel time course of changes in contractile properties of stimulated muscle and the molecular and functional properties of the sarcoplasmic reticulum emphasizes the definitive role of the latter in determining the twitch characteristics of fast and slow twitch muscles.