Cyclical passive stretch influences the mechanical properties of the inactive cat soleus

The effects of cyclical, passive manipulation (PM, 30 min day(-1), 5 days week(-1) for 6 months) mimicking the length excursions observed during stepping on the mechanical and associated biochemical properties of the inactive cat soleus muscle were determined in five cats. Inactivity was produced via spinal cord isolation (SI), i.e. complete spinal cord transections at low thoracic and high sacral levels and bilateral dorsal rhizotomy between the transection sites. Passive manipulation was administered to one leg of each SI cat. Compared with normal controls, SI resulted in approximately 70% decrease in weight, an 80% decrease in maximum tetanic tension (Po) and an approximately 100% increase in maximum rate of shortening (Vmax) and myosin adenosine triphosphatase (mATPase) activity of the soleus. The passive manipulation regime partially ameliorated these effects. When compared with the control SI soleus, the SI-PM soleus weight and maximum tetanic tension were 12 and 21% higher, respectively, and the Vmax and mATPase activity 21 and 12% (p > 0.05) lower, respectively. Thus, inactivity resulted in a smaller and faster muscle, whereas passive manipulation for only 30 min a day tended to maintain these properties closer to normal control values. The results suggest a potential therapeutic effect of short bouts of cyclical, passive manipulation on otherwise inactive skeletal muscles.

Skeletal muscle afferent regulation of bioassayable growth hormone in the rat pituitary

There are forms of growth hormone (GH) in the plasma and pituitary of the rat and in the plasma of humans that are undetected by presently available immunoassays (iGH) but can be measured by bioassay (bGH). Although the regulation of iGH release is well documented, the mechanism(s) of bGH release is unclear. On the basis of changes in bGH and iGH secretion in rats that had been exposed to microgravity conditions, we hypothesized that neural afferents play a role in regulating the release of these hormones. To examine whether bGH secretion can be modulated by afferent input from skeletal muscle, the proximal or distal ends of severed hindlimb fast muscle nerves were stimulated ( approximately 2 times threshold) in anesthetized rats. Plasma bGH increased approximately 250%, and pituitary bGH decreased approximately 60% after proximal nerve trunk stimulation. The bGH response was independent of muscle mass or whether the muscles were flexors or extensors. Distal nerve stimulation had little or no effect on plasma or pituitary bGH. Plasma iGH concentrations were unchanged after proximal nerve stimulation. Although there may be multiple regulatory mechanisms of bGH, the present results demonstrate that the activation of low-threshold afferents from fast skeletal muscles can play a regulatory role in the release of bGH, but not iGH, from the pituitary in anesthetized rats.

Maintenance of myonuclear domain size in rat soleus after overload and growth hormone/IGF-I treatment

The purpose of this study was to determine the effects of functional overload (FO) combined with growth hormone/insulin-like growth factor I (GH/IGF-I) administration on myonuclear number and domain size in rat soleus muscle fibers. Adult female rats underwent bilateral ablation of the plantaris and gastrocnemius muscles and, after 7 days of recovery, were injected three times daily for 14 days with GH/IGF-I (1 mg/kg each; FO + GH/IGF-I group) or saline vehicle (FO group). Intact rats receiving saline vehicle served as controls (Con group). Muscle wet weight was 32% greater in the FO than in the Con group: 162 +/- 8 vs. 123 +/- 16 mg. Muscle weight in the FO + GH/IGF-I group (196 +/- 14 mg) was 59 and 21% larger than in the Con and FO groups, respectively. Mean soleus fiber cross-sectional area of the FO + GH/IGF-I group (2,826 +/- 445 microm2) was increased compared with the Con (2,044 +/- 108 microm2) and FO (2,267 +/- 301 microm2) groups. The difference in fiber size between the FO and Con groups was not significant. Mean myonuclear number increased in FO (187 +/- 15 myonuclei/mm) and FO + GH/IGF-I (217 +/- 23 myonuclei/mm) rats compared with Con (155 +/- 12 myonuclei/mm) rats, although the difference between FO and FO + GH/IGF-I animals was not significant. The mean cytoplasmic volume per myonucleus (myonuclear domain) was similar across groups. These results demonstrate that the larger mean muscle weight and fiber cross-sectional area occurred when FO was combined with GH/IGF-I administration and that myonuclear number increased concomitantly with fiber volume. Thus there appears to be some mechanism(s) that maintains the myonuclear domain when a fiber hypertrophies.

Locomotor capacity attributable to step training versus spontaneous recovery after spinalization in adult cats

Locomotor performance, hindlimb muscle activity and gait patterns during stepping were studied in step-trained and non-trained female, adult spinal cats. Changes in locomotor characteristics relative to prespinalization bipedal and quadrupedal stepping patterns were used to evaluate the effects of step training on the capacity to execute full weight-bearing stepping after spinalization. Step training consisted of full weight-bearing stepping of the hindlimbs at the greatest range of treadmill speeds possible at any given stage of locomotor recovery. In the initial stages of training the limbs were assisted as needed to execute successful steps. On the basis of two behavioral criteria, the maximum speed of treadmill stepping and the number of successful steps per unit time, the ability to step was at least 3 times greater in animals trained to step versus those allowed to recover spontaneously, i.e., the non-trained. The greater success in stepping was reflected in several physiological and kinematic properties. For example, the amplitude of electromyograph (EMG) bursts in the tibialis anterior (an ankle dorsiflexor), the amount of extension at the end of both the stance (E3) and swing (E1) phases of the step cycle, and the amount of lift of the hindlimb during swing were greater in step-trained than in non-trained spinal cats. The changes that occurred in response to training reflected functional adaptations at specific phases of the step cycle, e.g., enhanced flexor and extensor function. The improved stepping capacity attributable to step training is interpreted as a change in the probability of the appropriate neurons being activated in a temporally appropriate manner. This interpretation, in turn, suggests that step training facilitated or reinforced the function of extant sensorimotor pathways rather than promoting the generation of additional pathways. These results show that the capacity of the adult lumbar spinal cord to generate full weight-bearing stepping over a range of speeds is defined, in large part, by the functional experience of the spinal cord after supraspinal connectivity has been eliminated. These results have obvious implications with regards to 1) the possibility of motor learning occurring in the spinal cord; 2) the importance of considering "motor experience" in assessing the effect of any postspinalization intervention; and 3) the utilization of use-dependent interventions in facilitating and enhancing motor recovery.

Effects of inactivity on myosin heavy chain composition and size of rat soleus fibers

Myosin heavy chain (MHC) and fiber size properties of the adult rat soleus were determined after 4-60 days of complete inactivity, i.e., lumbar spinal cord isolation. Soleus atrophy was rapid and progressive, i.e., 25% and 64% decrease in weight and 33% and 75% decrease in fiber size after 4 and 60 days of inactivity, respectively. Changes in MHC occurred at a slower rate than the atrophic response. After 15 days there was de novo expression of type IIx MHC (approximately 10%). By 60 days, type IIx MHC accounted for 33% of the total MHC content, and 7% of the fibers contained only type IIx MHC. The relative amount of type I MHC was reduced from 93% in control to 49% after 60 days of inactivity. Therefore, the effects of 60 days of inactivity suggest that during this time period at least 75% of fiber size and approximately 40% of type I MHC composition of the adult rat soleus can be attributed to activation-related events.

Training effects on soleus of cats spinal cord transected (T12-13) as adults

Adult spinal cord transected (T12-13) cats were trained for 30 min/day, 5 days/week to either step on a treadmill (Stp-T) or stand (Std-T) for approximately 5 months. Training ameliorated soleus atrophy and enhanced maximum force capability compared to nontrained (N-T) spinal cats, with Stp-T being significantly different from N-T. Isometric twitch speed and maximum rate of shortening were unaffected by training; the soleus of all spinal groups was significantly faster than control. There was an elevation in myosin adenosine triphosphatase activity and a shift toward faster myosin heavy chain and fiber type compositions in N-T and Std-T, but not Stp-T cats. Thus, rhythmical activity involving muscle length and force changes (stepping) was more effective than a similar amount of a more static activity (standing). This specificity related to the type of training should be considered when developing rehabilitative strategies following spinal cord injury.

Growth hormone/IGF-I and/or resistive exercise maintains myonuclear number in hindlimb unweighted muscles

In the present study of rats, we examined the role, during 2 wk of hindlimb suspension, of growth hormone/insulin-like growth factor I (GH/IGF-I) administration and/or brief bouts of resistance exercise in ameliorating the loss of myonuclei in fibers of the soleus muscle that express type I myosin heavy chain. Hindlimb suspension resulted in a significant decrease in mean soleus wet weight that was attenuated either by exercise alone or by exercise plus GH/IGF-I treatment but was not attenuated by hormonal treatment alone. Both mean myonuclear number and mean fiber cross-sectional area (CSA) of fibers expressing type I myosin heavy chain decreased after 2 wk of suspension compared with control (134 vs. 162 myonuclei/mm and 917 vs. 2,076 micron2, respectively). Neither GH/IGF-I treatment nor exercise alone affected myonuclear number or fiber CSA, but the combination of exercise and growth-factor treatment attenuated the decrease in both variables. A significant correlation was found between mean myonuclear number and mean CSA across all groups. Thus GH/IGF-I administration and brief bouts of muscle loading had an interactive effect in attenuating the loss of myonuclei induced by chronic unloading.

Bed rest suppresses bioassayable growth hormone release in response to muscle activity

Hormonal responses to muscle activity were studied in eight men before (-13 or -12 and -8 or -7 days), during (2 or 3, 8 or 9, and 13 or 14 days) and after (+2 or +3 and +10 or +11 days) 17 days of bed rest. Muscle activity consisted of a series of unilateral isometric plantar flexions, including 4 maximal voluntary contractions (MVCs), 48 contractions at 30% MVC, and 12 contractions at 80% MVC, all performed at a 4:1-s work-to-rest ratio. Blood was collected before and immediately after muscle activity to measure plasma growth hormone by radioimmunoassay (IGH) and by bioassay (BGH) of tibia epiphyseal cartilage growth in hypophysectomized rats. Plasma IGH was unchanged by muscle activity before, during, or after bed rest. Before bed rest, muscle activity increased (P < 0.05) BGH by 66% at -13 or -12 days (2,146 +/- 192 to 3,565 +/- 197 microg/l) and by 92% at -8 or -7 days (2,162 +/- 159 to 4,161 +/- 204 microg/l). After 2 or 3 days of bed rest, there was no response of BGH to the muscle activity, a pattern that persisted through 8 or 9 days of bed rest. However, after 13 or 14 days of bed rest, plasma concentration of BGH was significantly lower after than before muscle activity (2,594 +/- 211 to 2,085 +/- 109 microg/l). After completion of bed rest, muscle activity increased BGH by 31% at 2 or 3 days (1,807 +/- 117 to 2,379 +/- 473 microg/l; P < 0.05), and by 10 or 11 days the BGH response was similar to that before bed rest (1,881 +/- 75 to 4,160 +/- 315 microg/l; P < 0.05). These data demonstrate that the ambulatory state of an individual can have a major impact on the release of BGH, but not IGH, in response to a single bout of muscle activity.

Effects of 14 days of spaceflight and nine days of recovery on cell body size and succinate dehydrogenase activity of rat dorsal root ganglion neurons

The cross-sectional areas and succinate dehydrogenase activities of L5 dorsal root ganglion neurons in rats were determined after 14 days of spaceflight and after nine days of recovery. The mean and distribution of the cross-sectional areas were similar to age-matched, ground-based controls for both the spaceflight and for the spaceflight plus recovery groups. The mean succinate dehydrogenase activity was significantly lower in spaceflight compared to aged-matched control rats, whereas the mean succinate dehydrogenase activity was similar in age-matched control and spaceflight plus recovery rats. The mean succinate dehydrogenase activity of neurons with cross-sectional areas between 1000 and 2000 microns2 was lower (between 7 and 10%) in both the spaceflight and the spaceflight plus recovery groups compared to the appropriate control groups. The reduction in the oxidative capacity of a subpopulation of sensory neurons having relatively large cross-sectional areas immediately following spaceflight and the sustained depression for nine days after returning to 1 g suggest that the 0 g environment induced significant alterations in proprioceptive function.

Growth hormone, IGF-I, and exercise effects on non-weight-bearing fast muscles of hypophysectomized rats

The effects of growth hormone (GH) or insulin-like growth factor I (IGF-I) with or without exercise (ladder climbing) in countering the effects of unweighting on fast muscles of hypophysectomized rats during 10 days of hindlimb suspension were determined. Compared with untreated suspended rats, muscle weights were 16-29% larger in GH-treated and 5-15% larger in IGF-I-treated suspended rats. Exercise alone had no effect on muscle weights. Compared with ambulatory control, the medial gastrocnemius weight in suspended, exercised rats was larger after GH treatment and maintained with IGF-I treatment. The combination of GH or IGF-I plus exercise in suspended rats resulted in an increase in size of each predominant fiber type, i.e., types I, I + IIa and IIa + IIx, in the medial gastrocnemius compared with untreated suspended rats. Normal ambulation or exercise during suspension increased the proportion of fibers expressing embryonic myosin heavy chain in hypophysectomized rats. The phenotype of the medial gastrocnemius was minimally affected by GH, IGF-I, and/or exercise. These results show that there is an IGF-I, as well as a GH, and exercise interactive effect in maintaining medial gastrocnemius fiber size in suspended hypophysectomized rats.

A sensitive electrophoretic method for the quantification of myosin heavy chain isoforms in horse skeletal muscle: histochemical and immunocytochemical verifications

In adult horses, three myosin heavy chain (MyHC) isoforms can be identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunohistochemistry using specific anti-MyHC monoclonal antibodies. This report studies the suitability of a consistent SDS-PAGE technique for quantifying MyHC profiles in homogenized cryostate sections of equine gluteus medius muscle biopsies (n = 18). The method used (previously described by R. J. Talmadge and R. R. Roy; J. Appl. Physiol. 1993, 75, 2337-2340) resolved MyHCs in three bands: I, IIB or IIX, and IIA from the fastest to the slowest migration band. The success rate of the protocol for yielding three well-differentiated MyHC bands was 100% and a subsequent quantification by densitometry for each MyHC isoform was obtained in all 18 muscle biopsies. The results obtained with this electrophoretic method were compared with routine myofibrillar adenosine triphosphatase histochemistry and immunohistochemistry using specific anti-MyHC monoclonal antibodies. The percent composition of the three electrophoretically separated MyHC isoforms (I, IIA and IIB or IIX) showed strong positive correlation with percentages of the area occupied in the biopsies by the three major fiber types (I, IIA, and IIB) identified histochemically (r = 0.96, P < 0.001) and immunohistochemically (r = 0.94, P < 0.01). It can be concluded that the electrophoretic method used here for measuring MyHC content is a valid alternative for muscle fiber typing in horses. As it is less costly and time-consuming than both qualitative histochemistry and immunohistochemistry, the method offers new prospects for application in equine experimental studies and veterinary medicine.

Apoptosis: a mechanism contributing to remodeling of skeletal muscle in response to hindlimb unweighting

The role of apoptosis in the elimination of myonuclei during hindlimb unloading-induced atrophy and the inhibition of apoptosis in the prevention of muscle atrophy were examined. The number of nuclei demonstrating double-stranded DNA fragmentation seen by terminal deoxynucleotidyl transferase (TDT) histochemical staining, an indicator of apoptosis, was significantly increased after 14 days of suspension. Double staining with TDT and antilaminin immunohistochemistry revealed that some TDT-positive nuclei were within the fiber lamina and were most likely myonuclei. The number of fibers containing morphologically abnormal nuclei was also significantly greater in suspended compared with control rats. Combined treatment with growth hormone and insulin-like growth factor I (GH/ IGF-I) and resistance exercise attenuated the increase in TDT-positive nuclei (approximately 26%, P > 0.05) and significantly decreased the number of fibers with morphologically abnormal nuclei. The data suggest that 1) "programmed nuclear death" contributes to the elimination of myonuclei and/or satellite cells from atrophying fibers, and 2) GH/IGF-I administration plus muscle loading ameliorates the apoptosis associated with hindlimb unloading.

Modulation of MHC isoforms in functionally overloaded and exercised rat plantaris fibers

The effects of 1 and 10 wk of functional overload (FO) of the rat plantaris with (FOTr) and without daily endurance treadmill training on its myosin heavy chain (MHC) composition were studied. After 1 and 10 wk of FO, plantaris mass was 22 and 56% greater in FO and 37 and 94% greater, respectively, in FOTr rats compared with age-matched controls. At 1 wk, pure type I and pure type IIa MHC fibers were hypertrophied in FO (39 and 44%) and FOTr (70 and 87%) rats. By 10 wk all fiber types comprising >5% of the fibers sampled showed a hypertrophic response in both FO groups. One week of FO increased the percentage of hybrid (containing both type I and type IIa MHC) fibers and of fibers containing embryonic MHC. By 10 wk, the percentage of pure type I MHC fibers was approximately 40% in both FO groups compared with 15% in controls, and the percentage of fibers containing embryonic MHC was similar to that in controls. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses showed an increase in type I MHC and a decrease in type IIb MHC in both FO groups at 10 wk, whereas little change was observed at 1 wk. These data are consistent with hypertrophy and transformation from faster to slower MHC isoforms in chronically overloaded muscles. The additional overload imposed by daily endurance treadmill training employed in this study (1.6 km/day; 10% incline) results in a larger hypertrophic response but appears to have a minimal effect on the MHC adaptations.

Influence of two weeks of non-weight bearing on rat soleus motoneurons and muscle fibers

BACKGROUND

Chronic unloading of the hindlimbs of rats results in atrophy and a shift in the metabolic and mechanical properties of the soleus muscle a predominantly slow postural muscle, towards those observed in faster muscles.

HYPOTHESIS

We hypothesized that the spinal motoneurons innervating the soleus muscle would also atrophy and show adaptations in their metabolic properties following a period of unloading.

METHODS

For 2 weeks, 10-week-old male Wistar rats were hindlimb suspended and the mean soma size and succinate dehydrogenase activity of motoneurons innervating the soleus muscle were examined. Retrograde labeling with nuclear yellow injected in the soleus muscle was used to identify the soleus motor pool. The mean fiber size and fiber type distribution from a central region of each soleus muscle was determined.

RESULTS

The soma size distribution, mean soma cross-sectional area and mean succinate dehydrogenase activity of motoneurons in the soleus motor pool were similar in control and hindlimb suspended rats. In contrast, compared to control the mean fiber size of each fiber type and the percentages of Type I fibers decreased and of Type IIa fibers increased in the soleus muscle following hindlimb suspension.

CONCLUSIONS

The results indicate that significant alterations in the phenotype and size of muscle fibers can occur without concomitant adaptations in the innervating motoneurons usually associated with motor unit types (i.e., soma size and oxidative capacity).

Human lumbosacral spinal cord interprets loading during stepping

Studies suggest that the human lumbosacral spinal cord can generate steplike oscillating electromyographic (EMG) patterns, but it remains unclear to what degree these efferent patterns depend on the phasic peripheral sensory information associated with bilateral limb movements and loading. We examined the role of sensory information related to lower-extremity weight bearing in modulating the efferent motor patterns of spinal-cord-injured (SCI) subjects during manually assisted stepping on a treadmill. Four nonambulatory subjects, each with a chronic thoracic spinal cord injury, and two nondisabled subjects were studied. The level of loading, EMG patterns, and kinematics of the lower limbs were studied during manually assisted or unassisted stepping on a treadmill with body weight support. The relationships among lumbosacral motor pool activity [soleus (SOL), medial gastrocnemius (MG), and tibialis anterior (TA)], limb load, muscle-tendon length, and velocity of muscle-tendon length change were examined. The EMG mean amplitude of the SOL, MG, and TA was directly related to the peak load per step on the lower limb during locomotion. The effects on the EMG amplitude were qualitatively similar in subjects with normal, partial, or no detectable supraspinal input. Responses were most consistent in the SOL and MG at load levels of < 50% of a subject's body weight. The modulation of the EMG amplitude from the SOL and MG, both across steps and within a step, was more closely associated with limb peak load than muscle-tendon stretch or the velocity of muscle-tendon stretch. Thus stretch reflexes were not the sole source of the phasic EMG activity in flexors and extensors during manually assisted stepping in SCI subjects. The EMG amplitude within a step was highly dependent on the phase of the step cycle regardless of level of load. These data suggest that level of loading on the lower limbs provides cues that enable the human lumbosacral spinal cord to modulate efferent output in a manner that may facilitate the generation of stepping. These data provide a rationale for gait rehabilitation strategies that utilize the level of load-bearing stepping to enhance the locomotor capability of SCI subjects.

Architectural and mechanical properties of the rat adductor longus: response to weight-lifting training

BACKGROUND

The primary objective of this study was to determine the effects of an 8 week weight-lifting program on the mechanical, histochemical, and architectural properties of the rat adductor longus muscle, a predominantly slow adductor muscle.

METHODS

The weight-lifting program was progressive such that the rats were performing three bouts of ten lifts with 300% body weight load every other day during the last 3 weeks of training. The in situ mechanical properties, fiber type composition, and architectural characteristics of the muscle were determined in control and weight-trained rats. Intramuscular electromyographic recordings were used to verify the recruitment of the adductor longus during the lifting task.

RESULTS

The adductor longus was composed predominantly of slow fibers (approximately 80% slow oxidative) and had a relatively simple architectural design, i.e., one motor end-plate band near the center of the muscle, virtually no angle of pinnation of the fibers from the line of pull, and a fiber length:muscle length ratio of 0.72. The mean fiber type composition and fiber size, the total fiber number, and the mean physiological cross-sectional area of the adductor longus were similar in the two groups of rats. The mean body weight of weight-lifting rats was significantly less than control. The weight of the adductor longus relative to body weight and its fatigue resistance were higher and the maximum rate of shortening was slower in weight-lifting than in control rats. No other mechanical property was significantly affected by the training program.

CONCLUSIONS

The results indicate that approximately 1 minute of over-load every other day by physiological recruitment of motor units can induce remodeling of the adductor longus of growing rats; i.e., the trained muscles were slower and less fatigable than control. Given that the effects on the architectural or force-generating properties of the muscles were small, the marked improvement in the ability to lift heavier loads as the training progressed appears to be more attributable to neurally related than to muscle-related phenomena.

Distribution of myosin heavy chain isoforms in non-weight-bearing rat soleus muscle fibers

The effects of 14 days of spaceflight (SF) or hindlimb suspension (HS) (Cosmos 2044) on myosin heavy chain (MHC) isoform content of the rat soleus muscle and single muscle fibers were determined. On the basis of electrophoretic analyses, there was a de novo synthesis of type IIx MHC but no change in either type I or IIa MHC isoform proportions after either SF or HS compared with controls. The percentage of fibers containing only type I MHC decreased by 26 and 23%, and the percentage of fibers with multiple MHCs increased from 6% in controls to 32% in HS and 34% in SF rats. Type IIx MHC was always found in combination with another MHC or combination of MHCs; i.e., no fibers contained type IIx MHC exclusively. These data suggest that the expression of the normal complement of MHC isoforms in the adult rat soleus muscle is dependent, in part, on normal weight bearing and that the absence of weight bearing induces a shift toward type IIx MHC protein expression in the preexisting type I and IIa fibers of the soleus.

Musculoskeletal adaptations to weightlessness and development of effective countermeasures

A Research Roundtable, organized by the American College of Sports Medicine with sponsorship from the National Aeronautics and Space Administration, met in November 1995 to define research strategies for effective exercise countermeasures to weightlessness. Exercise was considered both independently of, and in conjunction with, other therapeutic modalities (e.g., pharmacological nutritional, hormonal, and growth-related factors) that could prevent or minimize the structural and functional deficits involving skeletal muscle and bone in response to chronic exposure to weightlessness, as well as return to Earth baseline function if a degree of loss is inevitable. Musculoskeletal deficits and countermeasures are described with respect to: 1) muscle and connective tissue atrophy and localized bone loss, 2) reductions in motor performance, 3) potential proneness to injury of hard and soft tissues, and 4) probable interaction between muscle atrophy and cardiovascular alterations that contribute to the postural hypotension observed immediately upon return from space flight. In spite of a variety of countermeasure protocols utilized previously involving largely endurance types of exercise, there is presently no activity-specific countermeasure(s) that adequately prevent or reduce musculoskeletal deficiencies. It seems apparent that countermeasure exercises that have a greater resistance element, as compared to endurance activities, may prove beneficial to the musculoskeletal system. Many questions remain for scientific investigation to identify efficacious countermeasure protocols, which will be imperative with the emerging era of long-term space flight.

Correlation between myofibrillar ATPase activity and myosin heavy chain composition in equine skeletal muscle and the influence of training

BACKGROUND

The histochemical myofibrillar ATPase (mATPase) method is used routinely for identification of equine skeletal muscle fiber types, but important problems have been observed with the subdivision of fast fiber population when using this method. To verify the use of this qualitative method, a number of equine muscle biopsies were analyzed with a combination of histochemical, immunohistochemical, electrophoretic, and morphometric techniques. The influence of training on these interrelations was also evaluated.

METHODS

Five young (2-3 years old) thoroughbred horses were intensively trained for 8 months on a high-speed treadmill. Biopsies were taken from the gluteus medius muscle at the beginning, after 4 months, and at the end of the training program. Serial sections of the samples were stained by mATPase histochemistry and immunohistochemistry by using a number of monoclonal antibodies specific to selected myosin heavy chain (MyHC) isoforms. The histochemical and immunohistochemical categorization of a large number of fibers (N = 2,078) was compared fiber by fiber. The MyHC content of homogenates of the same biopsies were quantified by densitometry of a sensitive gel electrophoretic technique and compared with histochemical and immunohistochemical fiber types.

RESULTS

A large proportion of fibers examined (approximately 20%) were misclassified by traditional mATPase histochemistry. Many fibers histochemically identified as type IIB displayed both type IIa and type IIb MyHC isoforms, and nearly all type IIAB fibers in mATPase contained only the type IIa MyHC isoform by immunohistochemistry. Correlation analyses suggested a weak relation between the histochemically assessed relative cross-sectional area occupied by the three major fiber types (I, IIA, and IIB) and the electrophoretically assessed MyHC content, whereas a stronger relation was found between immunohistochemically defined fiber types and electrophoretic data. The four fiber type populations delineated according to MyHC content (I, IIA, IIAB, and IIB) had sizes and oxidative capacities significantly different from each other. No adaptation of any parameter measured to training was found. Training had no significant effect on the number of fibers misclassified by mATPase histochemistry.

CONCLUSIONS

These data demonstrate a significant limitation in mATPase histochemistry for assessing fibers containing fast MyHC isoforms. The use of monoclonal antibodies against specific MyHC isoforms seems to be a more sensitive and less subjective method.

Limited fiber type grouping in self-reinnervation cat tibialis anterior muscles

The percent and distribution patterns of three immunohistochemically identified fiber types within the anterior compartment of the cat tibialis anterior were determined 6 months after denervation and self-reinnervation. After self-reinnervation, mean frequencies of slow (9%) and fast (91%) fibers were similar to those in control (12% and 88%, respectively) muscles. However, a lower proportion of fast-1 (26%) and a higher proportion of fast-2 (65%) fibers were observed in self-reinnervated than control (32% and 56%) muscles. Quantitation of adjacencies between fibers of similar myosin heavy chain (MHC) phenotype, a measure of type grouping, revealed that the frequencies of two slow or two fast-1 fibers being adjacent in self-reinnervated muscles were similar to control. In contrast, the frequency of fast-2/fast-2 fiber adjacencies found in self-reinnervated muscles (45%) was significantly higher than in control muscles (37%). In both groups, the frequency of adjacencies between slow, fast-1, or fast-2 fibers was largely attributable to the number of each fiber type present. These data show that the incidence of grouping within each fiber type present was not altered after 6 months of self-reinnervation. Minimal changes in the spatial distribution of fiber types following self-reinnervation in adults suggests a limited degree of conversion of muscle fibers to a MHC phenotype matching the motoneuron characteristics.

Myosin heavy chain isoforms in adult equine skeletal muscle: an immunohistochemical and electrophoretic study

BACKGROUND

The aim of this study was to characterize the myosin heavy chain (MyHC) isoforms present in equine skeletal muscle.

METHODS

Muscle biopsies were removed from the superficial region of the gluteus medius muscle of five mature horses and analyzed by immunohistochemistry (using a battery of monoclonal antibodies specific for rat MyHC isoforms) and sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

RESULTS

Immunohistochemistry allowed subdivision of three different muscle fiber populations containing a single MyHC, one slow and two fast, and two hybrid populations, one containing slow and fast MyHCs and another with both fast-MyHC isoforms. Electrophoresis of MyHC confirmed the existence of three resolvable bands, with an electrophoretic mobility parallel to type I, IIa, and IIx rat MyHCs. The identities of two of these MyHCs were easily comparable with slow type I and fast type IIa MyHCs from rat skeletal muscle. However, a precise identification of the second fast MyHC was not made.

CONCLUSIONS

These results show the presence of three different MyHC isoforms in mature equine skeletal muscle, whose differential distribution defines three fiber types containing a single MyHC and two hybrid fiber populations containing either both slow and fast type IIa MyHCs or both fast MyHC isoforms.

Myosin heavy chain profile of cat soleus following chronic reduced activity or inactivity

To determine the role that normal neuromuscular activity plays in maintaining the myosin heavy chain (MHC) profile of adult cat soleus muscles, the spinal cords of 4 cats were transected (ST) and 8 cats were spinal isolated (SI) for 6 months. Nine nonoperated cats served as controls. Electrophoresis demonstrated that the soleus from control cats contained 98% type I, and 2% IIa MHCs. Both ST and SI resulted in decreased type I and increased IIa MHC, as well as de novo expression of IIb MHC. Immunohistochemistry with MHC-specific antibodies demonstrated that the soleus from control cats contained 99% type I, 1% IIa, and < 1% hybrid fibers (containing both type I and II MHCs). Following ST there were 67% type I, 17% IIa, 3% IIb, and 13% hybrid fibers. After SI, 48% of the fibers were type I, 11% were IIa, 1% were IIb, 25% were hybrid, and 15% contained embryonic MHC. Thus, normal levels of neuromuscular activity appear to be necessary for maintenance of the normal adult MHC profile in some fibers. Complete inactivation results in developmental MHC isoform expression in some fibers. Therefore, the dependence of a fiber on activity as a source of MHC modulation differs substantially among fibers even in a relatively homogeneous muscle.

Myonuclear number and myosin heavy chain expression in rat soleus single muscle fibers after spaceflight

The effects of 14 days of spaceflight on myonuclear number, fiber size, and myosin heavy chain (MHC) expression in isolated rat soleus muscle fiber segments were studied. Single soleus muscle fibers from rats flown on the Spacelab Life Sciences-2 14-day mission were compared with those from age-matched ground-based control rats by using confocal microscopy and gel electrophoresis. Spaceflight resulted in a significant reduction in the number of fibers expressing type I MHC and an increase in the number of fibers expressing type IIx or IIa MHC. Space-flight also resulted in an increase in the percentage of fibers coexpressing more than one MHC and in the reexpression of the neonatal isoform of MHC in some fibers. Fiber cross-sectional area was significantly reduced in pure type I MHC-expressing fibers and in fibers coexpressing type I+II MHC but not in fibers expressing one or more type II MHC in the flight rats. The number of myonuclei per millimeter was significantly reduced in type I MHC-expressing fibers from the flight rats but was not significantly different in type I+II and type II MHC-coexpressing fibers. Fibers expressing neonatal MHC were similar in size to control fibers but had significantly fewer myonuclei per millimeter than flight fibers not expressing neonatal MHC. In type I MHC-expressing fibers, the reduction in fiber cross-sectional area was greater than the reduction in myonuclear number; thus the average cytoplasmic volume per myonucleus was significantly lower in flight than in control fibers. The reduction in both myonuclear number and fiber size of fibers expressing type I MHC after 14 days of spaceflight supports the hypothesis that changes in the number of myonuclei may be a contributing factor to the reduction in fiber size associated with chronic unloading of the musculature.

IGF-I, growth hormone, and/or exercise effects on non-weight-bearing soleus of hypophysectomized rats

The effects of insulin-like growth factor (IGF-I) or growth hormone (GH) with and without exercise on predominantly slow muscles of hypophysectomized hindlimb-suspended (HS) rats were determined. HS resulted in a 21, 23, and 30% decrease in soleus, adductor longus, and vastus intermedius masses, respectively, compared with ambulatory rats. Compared with values in HS rats, IGF-I increased the vastus intermedius mass and GH or exercise alone increased both the soleus and vastus intermedius masses. There was a strong interactive effect between GH, but not IGF-I, and exercise in all three muscles of HS rats. The soleus fiber type distribution of HS rats was not affected by any treatment. HS resulted in a 24, 18 (P > 0.05), 32, and 20% (P > 0.05) decrease in the size of soleus fibers containing type I, IIa, I + IIa, and IIa + IIx myosin heavy chains, respectively, compared with ambulatory hypophysectomized rats. Hormone or exercise alone had no effect on fiber size in HS rats. However, all fiber sizes (except for type IIa + IIx in IGF-I with exercise rats) were larger in HS rats treated with GH or IGF-I and exercise than those in HS rats. These data indicate an interactive effect of both GH and IGF-I with exercise in maintaining fiber size of chronically non-weight-bearing predominantly slow muscles. Furthermore, the results suggest that the myosin heavy-chain phenotype in rats deficient in all pituitary factors is unresponsive to short-term administration of either GH or IGF-I or to exercise or HS.