Biomedical subjects

J A Faulkner

Publications and source records attributed to J A Faulkner.

At least 19 recordsLinked to original sources

PubMed1998

Muscle regeneration in young and old rats: effects of motor nerve transection with and without marcaine treatment.

We tested the hypothesis that after skeletal muscle regeneration in old compared with young rats damage to the motor nerve rather than damage to muscle fibers determines the magnitude of the deficits in muscle mass and maximum force (Po). The mass and Po of extensor digitorum longus (EDL) muscles of young (4 months) and old (24 months) male rats were compared two months following (i) Marcaine treatment plus simultaneous motor nerve transection, (ii) motor nerve transection alone, and (iii) Marcaine treatment alone (from data compiled previously). In both the nerve transection-only and Marcaine with nerve transection groups the recovery of mass and Po was significantly greater in young than in old rats. This is in contrast to our previous data showing that in the absence of nerve damage Marcaine-treated muscle in old rats regenerates as well as that in young rats. Our hypothesis was supported, and we conclude that impaired axonal regeneration, re-establishment of nerve-muscle contact, or both, is the critical component in the impaired regeneration of muscle grafts in old as compared with young rats.

Aging↗

PubMed1997

Sarcomere dynamics and contraction-induced injury to maximally activated single muscle fibres from soleus muscles of rats.

1. The focal nature of contraction-induced injury to skeletal muscle fibres may arise from heterogeneities in sarcomere length that develop during contractions. We tested the hypothesis that when a maximally activated single permeabilized fibre segment is stretched and a deficit in maximum isometric force (force deficit) is produced, the regions of sarcomeres with the longest lengths of prior to the stretch contain the majority of the damaged sarcomeres when the fibre is returned to optimum length (Lo) after the stretch. 2. Single fibre segments (n = 16) were obtained from soleus muscles of rats. Average sarcomere length at five discrete positions along the length of each fibre was determined by lateral deflection of a diode laser spot. Diffraction patterns were obtained while fibres were relaxed and immediately before, during and after a single stretch of 40% strain relative to Lo. Following the stretch, the regions of each fibre that potentially contained damaged sarcomeres were identified by an increased scatter of the first-order diffraction patterns. The damage was confirmed by light and electron microscopy. 3. While single fibre segments were in relaxing solution, the mean value for all of the average sarcomere lengths sampled (n = 80) was 2.53 +/- 0.01 microns (range, 2.40-2.68 microns). During the maximum isometric contraction before each stretch, the mean sarcomere length decreased to 2.42 +/- 0.02 microns and the range increased to 2.12-3.01 microns. 4. During the stretch of 40% strain, all regions of sarcomeres were stretched onto the descending limb of the length-force curve, but sarcomere lengthening was non-uniform. After the stretch, when the maximally activated fibres were returned to Lo, the force deficit was 10 +/- 1%. Microscopic evaluation confirmed that the regions with the longest sarcomere lengths before the stretch contained the majority of the damaged sarcomeres after the stretch. We conclude that when heterogeneities in sarcomere length develop in single permeabilized fibre segments during a maximum isometric contraction, the sarcomeres in the regions with the longest lengths are the most susceptible to contraction-induced injury.

Animals↗

PubMed1997

Contractile properties of diaphragm muscle segments from old mdx and old transgenic mdx mice.

Diaphragm muscles of young (4- to 6-mo-old) mdx mice show severe fiber necrosis and have normalized forces and powers 60 and 46% of the values for control C57BL/10 mice. In contrast, microinjection of mdx mouse embryos with a truncated dystrophin minigene has produced young transgenic mdx (tg-mdx) mice with a level of dystrophin expression and structural and functional properties of diaphragm muscle strips measured in vitro not different from those of control mice. Whether dystrophin expression and functional corrections persist for the life span of these animals is not know. We tested the null hypothesis that, in old (24 mo) tg-mdx mice, dystrophin expression is adequate and diaphragm muscle strips have forces and powers not different from values for diaphragm muscle strips from young tg-mdx mice or control mice. Compared with control values, diaphragm muscle strips from old mdx mice had normalized forces and powers of 48 and 31%, respectively. Expression of dystrophin persisted in diaphragm muscles of old tg-mdx mice, and functional properties were not different from diaphragm muscles of young tg-mdx or young or old control mice. These results suggest that, with a transgenic animal approach, dystrophin expression and functional corrections persist for the life span of the animals.

Aging↗

PubMed1997

Pliometric contraction-induced injury of mouse skeletal muscle: effect of initial length.

For single pliometric (lengthening) contractions initiated from optimal fiber length (Lf), the most important factor determining the subsequent force deficit is the work input during the stretch. We tested the hypothesis that regardless of the initial length, the force deficit is primarily a function of the work input. Extensor digitorum longus muscles of mice were maximally activated in situ and lengthened at 2 Lf/s from one of three initial fiber lengths (90, 100, or 120% of Lf) to one of three final fiber lengths (150, 160, or 170% of Lf). Maximal isometric force production was assessed before and after the pliometric contraction. No single mechanical factor, including the work input (r2 = 0.34), was sufficient to explain the differences in force deficits observed among groups. Therefore, the force deficit appears to arise from a complex interaction of mechanical events. With the data grouped by initial fiber length, the correlation between the average work and the average force deficit was high (r2 = 0.97-0.99). Consequently, differences in force deficits among groups were best explained on the basis of the initial fiber length and the work input during the stretch.

Animals↗

PubMed1997

Functional deficits in medial gastrocnemius grafts in rats: relation to muscle metabolism and beta-AR regulation.

This study tested the hypothesis that alterations in the metabolic integrity of grafted muscle contribute to its diminished ability to sustain power. Compared with control muscles, muscles studied 120 days after the grafting procedure had lower specific force and sustained power. The sustained power protocol resulted in a depletion of muscle glycogen in control (83%) and grafted (85%) animals. Grafts had lower pre- and poststimulation glycogen, diminished citrate synthase activity, and greater hexokinase activity. No differences were observed in phosphofructokinase activity, glucose transporter GLUT-4 content, fiber type, beta-adrenergic-receptor (beta-AR) density, or binding affinity. Isoproterenol-stimulated adenylyl cyclase activity was lower in grafted vs. control muscle, suggesting an uncoupling of the beta-AR-effector complex. Thus the diminished ability of the grafted muscle to sustain power may be explained, in part, by a decrease in energy available from glycogen stores and/or a decrease in oxidative capacity.

Adenylyl Cyclases↗

PubMed1997

Contraction-induced injury to the extensor digitorum longus muscles of rats: the role of vitamin E.

Three days after a protocol of 225 pliometric (lengthening) contractions was administered to in situ extensor digitorum longus muscles of rats, the force deficit was 64 +/- 7% and the percentage of damaged muscle fibers was 38 +/- 5% of the control values. We then tested the hypothesis that at 3 h and 3 days after the protocol an elevation in the muscle vitamin E content would decrease the force deficit, the percentage of damaged muscle fibers, and the serum activities of creatine kinase and pyruvate kinase. The 5-8 days of intravenous injections of alpha-tocopherol increased muscle vitamin E content threefold compared with vehicle (ethanol)-treated rats. Despite the difference in vitamin E content, the force deficit and number of damaged fibers were not different. After the contraction protocol, the serum creatine kinase and pyruvate kinase activities of the vehicle-treated rats increased fourfold at 3 h and twofold at 3 days, whereas the vitamin E-treated rats showed no change. We conclude that vitamin E treatment did not ameliorate either the induction of the injury or the more severe secondary injury at 3 days. Despite the absence of evidence for an antioxidant function, the lack of any increase in serum enzyme activities for vitamin E-treated rats at 3 h and 3 days supported a role for vitamin E in the prevention of enzyme loss after muscle damage.

Animals↗

PubMed1996

The magnitude of the initial injury induced by stretches of maximally activated muscle fibres of mice and rats increases in old age.

1. Our purpose was to compare the susceptibilities of muscles in animals of different ages to the injuries induced by stretching the contracting muscle. Single stretches provide an effective method for studying the factors that contribute to the initiation of contraction-induced injury. We hypothesized that, for maximally activated muscles in old compared with young or adult mice, the work input during a single stretch of any given strain is not different, but for a given work input the magnitude of the injury is greater. 2. The force deficit resulting from each single stretch was calculated as the decrease in the maximum isometric force expressed as a percentage of the maximum force prior to the stretch. Force deficits were compared 1 min after single stretches of in situ extensor digitorum longus (EDL) muscles of young, adult and old mice. In addition, measurements of force deficits immediately following single stretches of single permeabilized fibre segments from EDL muscles of young and old rats permitted investigation of the initial injury at the level of the contractile apparatus. 3. For maximally activated EDL muscles in young, adult and old mice, no differences were observed for the work input during stretches of any given strain. Furthermore, the relationships between the work and the resultant force deficit were not different for muscles in young and adult mice. In contrast, compared with the work-force deficit relationships for muscles in either young or adult mice, the relationship was significantly steeper for muscles in old mice. For single permeabilized fibres from muscles of old rats, the force deficits immediately after single stretches were greater than those observed for fibres from muscles of young rats. We conclude that the increased susceptibility of muscles in old animals to contraction-induced injury resides at least in part within the myofibrils.

Aging↗

PubMed1996

Properties of single motor units in medial gastrocnemius muscles of adult and old rats.

1. The purpose of this study was to determine the role of motor unit remodelling in the deficit that develops in the maximum isometric tetanic force (Fo) of whole medial gastrocnemius (MGN) muscles in old compared with adult rats. The Fo values and morphological data were determined for MGN muscles and eighty-two single motor units in muscles of adult (10-12 months) and sixty-two units in those of old (24-26 months) F344 rats. During an unfused tetanus, fast and slow (S) motor units were identified by the presence and absence of sag, respectively. Fast-fatigable (FF) and fast-fatigue-resistant (FR) units were classified by fatigue indices less than or greater than 0.50, respectively. 2. For old rats, whole MGN muscle Fo was 29% less than the value of 11.2 N measured for adult rats. The deficit in whole muscle Fo of old rats resulted from equivalent decreases in the number of motor units, 16% smaller than the adult value of ninety-seven, and in the mean motor unit Fo value, 14% less than the adult value of 117 mN. 3. With ageing, little motor unit remodelling occurred in FR units, whereas the S and FF motor units demonstrated dramatic, but opposing, changes. For S units, the number of units remained constant, but the number of fibres per motor unit increased 3-fold from 57 to 165. In contrast, the number of FF units decreased by 34% and the number of fibres per motor unit of the remaining units decreased to 86% of the adult value of 333. The age-related remodelling of motor units appeared to involve denervation of fast muscle fibres with reinnervation of denervated fibres by axonal sprouting from slow fibres.

Aging↗

PubMed1996

Functional evaluation at the medial gastrocnemius donor site in rats.

The transfer of a skeletal muscle from a donor to a recipient site creates an initial deficit in the structure and function of the muscle group from which it originates. Removal of the donor muscle induces hypertrophy of the remaining synergistic muscles, which compensate for part of the deficit at the donor site. The medial gastrocnemius (MGN) muscle is a frequently utilized donor muscle. Compared with the mass and force production of the control four-muscle plantar flexor group in rats, removal of the MGN muscle creates an initial deficit of approximately 36 percent. At 60, 90, and 120 days after removal of the MGN muscle, the degree of compensation of the remaining three-muscle plantar flexor group (lateral gastrocnemius, soleus, and plantaris muscles) was evaluated. The mass of the three-muscle group increased 13 percent over the time course studied, but was still 28 percent less than the mass of the control four-muscle group. Similarly, the maximum force of the three-muscle group increased 27 percent, but was 21 percent lower than the control four-muscle group. The authors propose a model that illustrates the function restored at a donor site in terms of the percentage of the total muscle group comprised by the donor muscle and the ability of the remaining muscle group to compensate for its removal.

Adaptation, Physiological↗

PubMed1996

Transgenic mdx mice expressing dystrophin with a deletion in the actin-binding domain display a "mild Becker" phenotype.

The functional significance of the actin-binding domain of dystrophin, the protein lacking in patients with Duchenne muscular dystrophy, has remained elusive. Patients with deletions of this domain (domain I) typically express low levels of the truncated protein. Whether the moderate to severe phenotypes associated with such deletions result from loss of an essential function, or from reduced levels of a functional protein, is unclear. To address this question, we have generated transgenic mice that express wild-type levels of a dystrophin deleted for the majority of the actin-binding domain. The transgene derived protein lacks amino acids 45-273, removing 2 of 3 in vitro identified actin interacting sites and part of hinge 1. Examination of the effect of this deletion in mice lacking wild-type dystrophin (mdx) suggests that a functional domain I is not essential for prevention of a dystrophic phenotype. However, in contrast to deletions in the central rod domain and to full-length dystrophin, both of which are functional at only 20% of wild-type levels, proteins with a deletion in domain I must be expressed at high levels to prevent a severe dystrophy. These results are also in contrast to the severe dystrophy resulting from truncation of the COOH-terminal domain that links dystrophin to the extracellular matrix. The mild phenotype observed in mice with domain I-deletions indicates that an intact actin-binding domain is not essential, although it does contribute to an important function of dystrophin. These studies also suggest the link between dystrophin and the subsarcolemmal cytoskeleton involves more than a simple attachment of domain I to actin filaments.

Actinin↗

PubMed1996

The regeneration of noninnervated muscle grafts and marcaine-treated muscles in young and old rats.

Free grafts of the extensor digitorum longus (EDL) muscle in 4-month-old rats regenerate 2-3 times better than in 24-month-old rats. Based on these data, we formulated the working hypothesis that deficient reinnervation is one of the most important age-related environmental factors within the host that might account for the poor regeneration. In the present experiments, we compared the regeneration of EDL muscles in two groups of young and old rats: (a) 21-day grafts, with fibers regenerating in the absence of nerves, and (b) Marcaine-treated muscle with fibers regenerating in the presence of uninterrupted innervation. The specific hypothesis was that, under each of these circumstances, reinnervation was not involved and age-related differences in regeneration would not be seen. Differences were assessed by measurements of mass and maximum isometric force normalized to values for age-matched control muscles. In the absence of nerves, the degree of regeneration in 21-day noninnervated EDL grafts was not significantly different between young and old rats. Similarly, when EDL muscles were damaged by Marcaine and regenerated in the presence of uninterrupted innervation, no differences were noted between young and old rats. These data support the working hypothesis that a deficiency in reinnervation with increasing age accounts, at least in part, for the poorer success of muscle regeneration in grafts in old compared with young rats.

Aging↗

PubMed1996

Contraction-induced injury to single fiber segments from fast and slow muscles of rats by single stretches.

Susceptibility to contraction-induced injury was investigated in single permeabilized muscle fiber segments from fast extensor digitorum longus and slow soleus muscles of rats. We tested the hypotheses that, after single stretches of varying strains and under three conditions of Ca2+ activation (none, submaximum, and maximum), 1) the magnitude of the deficit in maximum isometric force is dependent on the work done to stretch the fiber, and 2) for each condition of activation and strain, fast fibers incur greater force deficits than slow fibers. When all data on force deficits were analyzed together, the best predictors of the overall force deficits for both fast and slow muscle fibers were linear regression models that introduced the simultaneous but independent effects of strain and average force (r2 = 0.52 and 0.63, respectively). Under comparable conditions, greater force deficits were produced in fast than slows fibers. Despite differences in the strain required to produce injury in fast and slow muscle fibers, for a given force deficit, the ultrastructural damage was strikingly similar.

Animals↗

PubMed1995

Injury to muscle fibres after single stretches of passive and maximally stimulated muscles in mice.

1. Our purpose was to investigate the initial mechanisms responsible for contraction-induced injury. Most studies of mechanisms of contraction-induced injury have been based on observations made either shortly after many repeated contractions at the peak of fatigue, or days after, at the peak of delayed onset injury. As a result, conclusions based on these studies are complicated by interactions of mechanical and biochemical events, as well as the passage of time. We studied the initial mechanical events associated with contraction-induced injury immediately following single stretches of whole skeletal muscles of mice in situ. 2. We tested the hypothesis that immediately following a single stretch, the severity of contraction-induced injury is a function of both strain and average force. Consequently, the work done to stretch the muscle would be the best predictor of the magnitude of injury. Extensor digitorum longus muscles were adjusted to optimum length for force (L(o)). Passive (not stimulated) and maximally activated muscles were exposed to single stretches of 10, 20, 30, 50 or 60% strain, relative to muscle fibre length (Lf), at a rate of 2 Lf s-1. 3. The magnitude of injury was represented by the force deficit 1 min after the stretch expressed as a percentage of the maximum force prior to the stretch. The occurrence of injury was confirmed directly by electron microscopic analysis of the ultrastructure of muscle fibres that were fixed immediately following single stretches. 4. For active muscles, a single stretch of only 30% strain produced a significant force deficit, whereas for passive muscles, a larger strain was required. Stretches of greater than 50% strain resulted in greater force deficits for passive than for maximally activated muscles. For either condition, the work done to stretch the muscle was the best predictor of the magnitude of injury, accounting for 76% of the variability in the force deficit for maximally activated muscles, and 85% for passive muscles.

Animals↗

PubMed1995

Muscle atrophy and weakness with aging: contraction-induced injury as an underlying mechanism.

In old compared with young animals, muscle mass is decreased by 30% to 40%, and maximum force and power are decreased to an even greater extent. The age-related declines in muscle mass and muscle function are similar to those that occur with decreased physical activity. Despite the similarities, we conclude that the losses in muscle mass, force, and power are not due solely to old animals being less active, but rather accrue from intrinsic age-related changes in muscles and in muscle fibers that appear to be immutable and irreversible. The intrinsic changes are associated with denervation of fast fatigable fibers and motor units and motor unit remodeling, which may be initiated by contraction-induced injury. The mechanisms remain unresolved for the weakness, the fatigability, the high susceptibility to contraction-induced injury, and the impaired recovery from injury demonstrated by the skeletal muscles of old animals.

Adaptation, Physiological↗

PubMed1995

Expression of full-length and truncated dystrophin mini-genes in transgenic mdx mice.

Duchenne and Becker muscular dystrophy are caused by defects in the dystrophin gene, and are candidates for treatment by gene therapy. We have shown previously that overexpression of a full-length dystrophin cDNA prevents the development of dystrophic symptoms in mdx mice. We show here that this functional correction can be achieved by expressing the full-length muscle isoform at a lower level than is present in control animals. Gene therapy for DMD may necessitate the use of truncated dystrophin mini-genes to accommodate the limited cloning capacity of current-generation viral delivery vectors. We have constructed both murine and human mini-genes deleted for exons 17-48, and have demonstrated that expression of either mini-gene can almost completely prevent the development of dystrophic symptoms in transgenic mdx mice. These results suggest that viral-mediated expression of moderate levels of a truncated dystrophin could be an effective treatment for DMD.

Animals↗

PubMed1995

Muscle fatigue in old animals. Unique aspects of fatigue in elderly humans.

Muscle atrophy, weakness, injury, and fatigue are inevitable and immutable concomitants of old age. Atrophy results from a gradual process of fiber denervation with loss of some fibers and atrophy of others. Fast fibers show more denervation and atrophy than slow fibers. Some fast fibers are reinnervated by axonal sprouting from slow fibers resulting in remodeling of motor units. With aging, the decreases in strength and power are greater than expected from the loss in muscle mass. Contraction-induced injury is proposed as a mechanism of the fast fiber denervation. With atrophy and weakness, human beings show a dramatic decrease in endurance and increase in fatigability with aging, but strength and endurance training slows the process.

Aging↗

PubMed1994

Tubular nerve guide and epineurial repair: comparison of techniques for neurorrhaphy.

Experiments were conducted on 36 male, Sprague-Dawley rats. In 10 animals, neurorrhaphy was performed on the peroneal nerve with epineurial repair and, in 11 animals, with a tubular polyethylene nerve guide. The authors tested the hypothesis that, following transient denervation of a skeletal muscle by transection of a peroneal nerve, the restoration of maximum force and of maximum specific force developed after insertion of a tubular nerve guide, will not be different from that developed after microsurgical epineurial neurorrhaphy. The contractile properties of the extensor digitorum longus (EDL) muscle, innervated by the peroneal nerve, were evaluated after an average of 116 days. The maximum tetanic force of EDL muscles with epineurial repair and nerve guide were 84 percent and 75 percent, respectively, of the value for control EDL muscles. The specific forces of the muscles in both groups were not different from the control values. The conclusion is that, following stabilization after transection and repair, each of the two methods was equally effective in restoring the ability of the muscle to develop force.

Anastomosis, Surgical↗

PubMed1994

Prevention of dystrophic pathology in mdx mice by a truncated dystrophin isoform.

The C-terminal domain of dystrophin is alternatively spliced to produce a variety of tissue and developmental stage-specific isoforms. Recent studies suggest that the C-terminal domain binds to the dystrophin-associated glycoprotein complex (DGC) in muscle, but little is known about the functional significance of the alternative splicing or what role individual isoforms may play in specific tissues. The major dystrophin transcript in brain lacks exons 71-74, and encodes an isoform not observed in skeletal muscle. To explore the capacity of this truncated isoform to function in muscle, we have generated transgenic mice expressing a murine dystrophin mini-gene missing exons 71-74. Uniform expression of this construct on a mutant mdx mouse background results in normal muscle morphology and physiology, and prevents the development of muscular dystrophy. These mice also display normal expression and localization of the DGC, suggesting that the alternatively spliced exons are not required for dystrophin function in skeletal muscle. An additional line of mice was analyzed that had a mosaic pattern of expression. These mice display a markedly milder phenotype than mdx mice, despite the expression of dystrophin in only half the muscle fibers. These results indicate that viral delivery of dystrophin to a simple majority of fibers in a muscle group would greatly reduce the dystrophic pathology associated with Duchenne muscular dystrophy.

Alternative Splicing↗