Effect of tibial bone resection on the development of fast- and slow-twitch skeletal muscles in foetal sheep.

To determine if longitudinal bone growth affects the differentiation of fast- and slow-twitch muscles, the tibial bone was sectioned at 90 days gestation in foetal sheep so that the lower leg was permanently without structural support. At 140 days (term is approximately 147 days) the contractile properties of whole muscles, activation profiles of single fibres and ultrastructure of fast- and slow-twitch muscles from the hindlimbs were studied. The contractile and activation profiles of the slow-twitch soleus muscles were significantly affected by tibial bone resection (TIBX). The soleus muscles from the TIBX hindlimbs showed: (1) a decrease in the time to peak of the twitch responses from 106.2 +/- 10.7 ms (control, n = 4) to 65.1 +/- 2.48 ms (TIBX, n = 5); (2) fatigue profiles more characteristic of those observed in the fast-twitch muscles: and (3) Ca2+ - and Sr2+ -activation profiles of skinned fibres similar to those from intact hindlimbs at earlier stages of gestation. In the FDL, TIBX did not significantly change whole muscle twitch contraction time, the fatigue profile or the Ca2+ - and Sr2+ -activation profiles of skinned fibres. Electron microscopy showed an increased deposition of glycogen in both soleus and FDL muscles. This study shows that the development of the slow-twitch phenotype is impeded in the absence of the physical support normally provided by the tibial bone. We suggest that longitudinal stretch is an important factor in allowing full expression of the slow-twitch phenotype.

Developmental changes in the activation properties and ultrastructure of fast- and slow-twitch muscles from fetal sheep.

At early stages of muscle development, skeletal muscles contract and relax slowly, regardless of whether they are destined to become fast- or slow-twitch. In this study, we have characterised the activation profiles of developing fast- and slow-twitch muscles from a precocial species, the sheep, to determine if the activation profiles of the muscles are characteristically slow when both the fast- and slow-twitch muscles have slow isometric contraction profiles. Single skinned muscle fibres from the fast-twitch flexor digitorum longus (FDL) and slow-twitch soleus muscles from fetal (gestational ages 70, 90, 120 and 140 days; term 147 days) and neonatal (8 weeks old) sheep were used to determine the isometric force-pCa (pCa = -log10[Ca2+]) and force-pSr relations during development. Fast-twitch mammalian muscles generally have a greatly different sensitivity to Ca2+ and Sr2+ whereas slow-twitch muscles have a similar sensitivity to these divalent cations. At all ages studied, the force-pCa and force-pSr relations of the FDL muscle were widely separated. The mean separation of the mid-point of the curves (pCa50-pSr50) was approximately 1.1. This is typical of adult fast-twitch muscle. The force-pCa and force-pSr curves for soleus muscle were also widely separated at 70 and 90 days gestation (pCa50-pSr50 approximately 0.75); between 90 days and 140 days this separation decreased significantly to approximately 0.2. This leads to a paradoxical situation whereby at early stages of muscle development the fast muscles have contraction dynamics of slow muscles but the slow muscles have activation profiles more characteristic of fast muscles. The time course for development of the FDL and soleus is different, based on sarcomere structure with the soleus muscle developing clearly defined sarcomere structure earlier in gestation than the FDL. At 70 days gestation the FDL muscle had no clearly defined sarcomeres. Force (N cm-2) increased almost linearly between 70 and 140 days gestation in both muscle types and there was no difference between the Ca(2+)- and Sr(2+)-activated force throughout development.

Mechanical sensitivity of muscle afferents in a nerve treated with colchicine.

The experiments reported here demonstrate that the mechanical sensitivity of peripheral nerve fibres typically seen after injury can be induced without overtly injuring the nerve, but by simply applying colchicine topically to the nerve. In cats anaesthetised with pentobarbitone sodium, the medial gastrocnemius nerve was exposed and 10 mM colchicine applied topically for 15 min. The animals recovered from the operation normally and showed no subsequent motor deficit. Six days later animals were re-anaesthetised, a laminectomy carried out and responses recorded in single afferents at the level of the dorsal root. It was found that many afferents, particularly those with conduction velocities in the group II-III range, had become sensitive to local mechanical stimulation of the nerve in the region treated with colchicine and showed slowly adapting responses to stretch of the nerve. Many of the smaller fibres exhibited spontaneous activity. Mechanically sensitive afferents exhibited impulse conduction blocks at the colchicine-treated site. Some afferents, which appeared to conduct impulses normally through the treated region, were associated with muscle receptors having normal response properties. However, other muscle receptors were clearly abnormal and were insensitive to muscle stretch or contraction or exhibited only phasic responses. When the nerve was cut proximal to the colchicine-treated site, some, but not all, spontaneous activity was abolished. It was subsequently shown using a collision technique that the activity in some axons had its origin in the cell body in the dorsal root ganglion. In one experiment, it was shown that after nerve section proximal to the colchicine-treated region three of five axons switched their activity from a peripheral to a central origin. It is postulated that colchicine disrupts fast axonal transport of mechanically sensitive or voltage-sensitive ion channels, from the cell body to the peripheral terminals of the axons, leading to an accumulation of these channels at the treated site. This induces mechanical sensitivity and spontaneous activity. It is postulated that interruption of a retrogradely transported signal induces the spontaneous activity in the cell body. These experiments suggest that an important influence is exerted by the cell body on the peripheral terminals of mechanoreceptors to confer on them their normal response properties.

Age-associated changes in the innervation of muscle fibers and changes in the mechanical properties of motor units.

In both humans and animals there is a progressive loss of muscle strength with age. Tests of handgrip and knee extension in men show that some decline in strength is evident by the age of 55 years and is pronounced by the age of 65, compared with the 25- to 35-year period when strength is at a maximum. A comparable age-related decline in peak force development has also been shown in hind-limb muscles of aged rats. Motoneurons and consequently motor units are lost with age, and this is apparent in man after the age of 60. Again, a comparable decline has been demonstrated in the motoneuron population of hindlimb muscles of rats aged 20-24 months. Loss of motoneurons in young adults (through either injury or disease) results in the remaining intact motoneurons sprouting to innervate the denervated fibers. This capacity for sprouting has been shown to be seriously impaired in the hindlimb muscles of aged rats. Furthermore, the well-established relationship between motor unit size and fatigability (smaller units tend to be more fatigue resistant) also tends to break down, with large units just as likely to be fatigable as fatigue resistant. The normally large, fatigable motor units also appear to be reduced in size in the aged muscles. The age-related loss of motoneurons and associated loss of muscle fibers accounts in part for the reduced functional capacity of muscle with age. The reason for the impairment of the aged motoneuron remains to be investigated, but it may relate to the integrity of the oxidative metabolic pathways within the cell, given that mitochondrial respiratory chain function is known to be reduced with age.

Growth and differentiation of fast and slow muscles in fetal sheep, and the effects of hypophysectomy.

1. Isometric contractile characteristics of fast-twitch (flexor digitorum longus, FDL; medial gastrocnemius, MG) and slow-twitch (soleus) muscles were determined in pentobarbitone-anaesthetized fetal sheep between 90 and 140 days gestation. Five fetuses were hypophysectomized (HPX) at 90-95 days gestation and then studied at 138-140 days. 2. At 90-95 days gestation the time to peak of single twitch contractions for the soleus, MG and FDL were not significantly different from each other; the mean value (+/-S.E.M.) for all the muscles at this age was 77.6 +/- 9.0 ms. At 120-125 days gestation the MG and FDL contracted significantly faster (44.0 +/- 0.9 and 40.8 +/- 1.8 ms, respectively) than at 90-95 days, and did not change significantly thereafter. In contrast, the soleus muscle contracted more slowly (111.9 +/- 6.6 ms) at 138-140 days than at 90-95 days and 120-125 days gestation. 3. Soleus muscle consisted of type I fibres at all gestational ages. There was no significant change with gestational age in the relative numbers of type I and II fibres in the MG and FDL, but in the diaphragm the number of type I fibres increased and the number of type II fibres decreased between 125 and 138 days gestation. 4. HPX abolished the normal increase of soleus weight relative to body weight between 125 and 138 days but did not alter the change of twitch contraction time with age. HPX significantly prolonged twitch time to peak and time to half-relaxation of MG and time to half-relaxation of FDL at 138 days. 5. The maximum rate of rise of the isometric tetanic contraction was unchanged by HPX in all three hindlimb muscles, but fatigue of MG and FDL was increased. 6. The relative proportions of different fibre types in the hindlimb muscles and the diaphragm were unchanged by HPX, but there was a significant decrease in mean areas of type I and II fibres in the FDL and MG of the HPX fetuses.

Shortening heat in slow- and fast-twitch muscles of the rat.

Shortening heat has been reported in several amphibian skeletal muscles. In this investigation, shortening heat has been investigated in both soleus and extensor digitorum longus (EDL) muscles of young rats. The procedure involved shortening the muscles through two different distances, at near maximum velocity and at the onset of a summated twitch from different initial lengths. At the end of the shortening period, the muscle contracted isometrically, and the stress and associated heat production were recorded. These heat-stress data were compared with heat-stress data of isometric twitches at different initial lengths. There was a parallel upward shift in energy output when shortening occurred, indicating the presence of a shortening heat. Shortening heat increased with the distance shortened in soleus, but this was not the case for EDL. The values for the shortening heat coefficient for both muscle types are slightly higher than those reported for amphibian skeletal muscle and suggest that shortening heat is a significant component of the energy output of mammalian skeletal muscle.

The universality of bioenergetic disease and amelioration with redox therapy.

Overt mitochondrial diseases associated with mitochondrial DNA mutations are characterized by a decline in mitochondrial respiratory function. Similarly, a progressive decline in mitochondrial respiratory function associated with mitochondrial DNA mutations is clearly evidenced in aged human subjects. This communication is concerned with the development of a rat model for the study of bioenergy decline associated with the ageing process and overt mitochondrial diseases. The model involves the treatment of young rats with AZT to induce skeletal and cardiac myopathies. It has shown that there is a decline in soleus muscle function in vivo and that this decline is mirrored in the capacity of heart sub-mitochondrial particles to maintain bioenergy function. Coenzyme Q10 and several analogs were administered with AZT as potential therapeutics for the re-energization of affected tissues. Coenzyme Q10 and especially decyl Q were found to be therapeutically beneficial by both in vivo improvement in soleus muscle function and in vitro cardiac mitochondrial membrane potential capacity. Sub-mitochondrial particles were also prepared from heart mitochondria of young and aged rats. The particles prepared from the aged rats were found to have a decreased ability to maintain membrane potential as compared to those derived from the young rats.

Mechanical sensitivity of regenerating myelinated skin and muscle afferents in the cat.

These experiments describe the responses of myelinated skin and muscle afferent nerve fibres at a neuroma to stretch, local pressure and vibration in the anaesthetised cat. The sural nerve and the nerve supplying the medial gastrocnemius were studied. Neuroma formation was encouraged by placing the cut end of the nerve in a cuff made of synthetic material (Gore-tex). By 6 days after nerve section, the two nerves contained mechanically sensitive afferents. No motor fibres appeared to be mechanically sensitive. Mechanically sensitive sural afferents responded to ramp stretch of the nerve, applied at the cuff, with a single impulse or brief burst of impulses. The majority of gastrocnemius afferents responded to stretch with slowly adapting trains of impulses. Many muscle group II afferents exhibited a steady resting discharge, while group I afferents had an intermittent or bursting resting discharge or were silent. Those group I axons which showed resting activity had a low stretch threshold and were probably Ia fibres. Many of the silent units were also stretch sensitive. It is proposed that the spontaneously active units and silent units with low stretch thresholds were Ia fibres, while silent units with high stretch thresholds were Ib fibres. Both sural and gastrocnemius afferents responded to locally applied vibration. The mean peak response frequency for sural units was 170 Hz (+/- 70 Hz SD). For gastrocnemius units it was 325 Hz (+/- 86 Hz SD). Group I muscle afferents responded to higher frequencies of vibration than group II afferents. In four experiments the nerve was treated at a site a few millimetres proximal to the point of section with the axonal transport blocker colchicine. Twenty-five millimolar colchicine blocked impulse conduction at its point of application. Nevertheless, mechanically sensitive areas developed in the nerve just proximal to the treated region. Ten millimolar colchicine did not block impulse conduction, but led to dispersion of mechanosensitive areas to more proximal regions of the mechanosensitive areas to more proximal regions of the nerve. This result suggests that the disruption of orthograde axonal transport by colchicine leads to development of mechanically sensitive areas in axons further back from their cut ends. Local application of the drugs succinyl choline, tetra-ethyl ammonium and gadolinium had no effect on levels of resting activity or on mechanical sensitivity of afferents in the cuff. The potassium channel blocker 4-aminopyridine, on the other hand, produced an increase in the levels of resting activity and in the stretch responses of afferents.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional development of fetal limb muscles: a review of the roles of activity, nerves and hormones.

Animals that are immature at birth with respect to postural and locomotor control (e.g. cats, rats) possess incompletely differentiated 'fast-twitch' and 'slow-twitch' muscles at birth; full development proceeds slowly in the postnatal period and involves myogenic, hormonal, neural and behavioural factors. The gradual emergence of specific motor patterns and the exercise of individual muscle groups is thought to play a major role in the final development of each muscle and the fibre types which comprise them. In contrast, precocial species such as the sheep are born with skeletal muscles, especially those of the limbs, which are fully differentiated at birth. The relative importance of neural and hormonal factors in allowing this functional specialization to occur in the presumed absence of significant load-bearing exercise in the intrauterine environment is unclear. In this brief review, the changes which occur in contractile function and fibre type differentiation during the last one-third of gestation in fetal sheep are described, and some of the factors which influence this development are considered.

Activity and motor unit size in partially denervated rat medial gastrocnemius.

The aim of the study was to determine whether increased motoneuron activity induced by treadmill walking would alter the extent of motoneuron sprouting in the partially denervated rat medial gastrocnemius muscle. An extensive partial denervation was effected by unilateral section of the L5 ventral root, and it is very likely that all units remaining in the medial gastrocnemius were used in treadmill walking. Rats were trained for 1.5 h/day and after 14 days were walking at least 1 km/day. Motor unit characteristics were determined 24 days after the partial denervation and were compared with units from partially denervated control (PDC) animals and with units from normal (control) animals. In PDC rats, force developed by slow, fast fatigue-resistant, and fast intermediate-fatigable motor units increased substantially compared with control animals; that of fast-fatigable units did not increase. In partially denervated exercised animals, force developed by slow and fast-fatigue-resistant units showed no further increase, but fast-intermediate- and fast-fatigable units showed significant increases compared with those in PDC animals. The changes in force were closely paralleled by changes in innervation ratios. We concluded that neuronal activity is an important factor in determining the rate of motoneuron sprouting.

Recovery of muscle after different periods of denervation and treatments.

Three aspects of reinnervation and recovery of skeletal muscle following various periods of denervation were investigated: (1) the effect of duration of denervation; (2) the effect of hyperthyroidism on recovery; and (3) whether the muscle or the nerve limits recovery. The rat medial gastrocnemius (MG) nerve was cut and then resutured after 0, 3, 7, 21, or 56 days. In a second group of animals, the MG muscle was denervated and, in addition, the animal received triiodothyronine (T3) supplementation during reinnervation. The third group of animals had the denervated MG muscle reinnervated by a larger number of newly transected foreign axons. The force produced by the reinnervated muscle depends on the period that the muscle was denervated. Recovery was impaired when the period of denervation exceeded 7 days. T3 treatment did not benefit the return of force production, nor did providing the muscle with a larger number of newly transected axons.

Motor units within the normal rat medial gastrocnemius.

The contractile properties, fatiguability and axonal conduction velocity were determined for 118 motor units in the medial gastrocnemius of eighteen rats. Fast-twitch and slow-twitch units could be categorized on the basis of the 'sag' test. For the purposes of statistical comparison the fast-twitch units were classified on the basis of their fatiguability as fast-fatiguable (FF), fast intermediate (FI) and fast fatigue-resistant (FR). As such, FR units tended to have a longer isometric twitch time course than other fast-twitch units. On the basis of peak tetanic force FF units were largest (mean 341 +/- 120 mN) followed by FI (145 +/- 85 mN), FR (87.3 +/- 38) and slow units (40.4 +/- 11.0 mN). There were no differences in motor axonal conduction velocity. Although units were categorized, it is clear that for all characteristics investigated the fast-twitch units exist as a continuum. Some of the largest FF units (peak force 350-500 mN) failed to maintain force with high-frequency stimulation (300 Hz) and this was associated with a failure of the EMG signal.

Developmental changes in hindlimb muscles and diaphragm of sheep.

In this study, plasma thyroxine, contractile and histochemical (adenosinetriphosphatase and NADH) characteristics of soleus (SOL), medial gastrocnemius (MG), and extensor digitorum longus (EDL) were examined in 140-day-gestation fetal sheep and in 2-, 5-, and 30-day-old lambs and adult ewes. Electrophoretic separation of myosin heavy chains was also done on all muscles and the diaphragm. There were no differences in the twitch contraction and relaxation times of MG and EDL at the different ages; in contrast SOL contraction times were significantly shorter in the fetus and newborn than in the adult. Fast glycolytic fibers first appeared in EDL, MG, and diaphragm at 5, 30, and 5 days after birth, respectively. The proportion of slow oxidative fibers decreased after birth and with postnatal development in EDL, whereas they increased in MG and diaphragm. Plasma thyroxine concentrations were higher in the fetus and day-old lambs than in 2-, 5-, and 30-day-old lambs or adult sheep. It is suggested that contractile specialization of the fast-twitch diaphragm, MG, and EDL is largely achieved in utero and is probably mediated by thyroid hormone. In contrast, SOL changed postnatally, probably influenced by the altered neural drive.

Alterations in the contractile properties of motor units within the ageing rat medial gastrocnemius.

The decline in muscle force accompanying senescence is associated with a loss of motor nerves which provides some opportunity for collateral reinnervation and alterations in the mechanical properties of surviving motor units. The contribution of the different motor unit types to this neuromuscular reorganization was investigated by determining the isometric contractile properties of single motor units within the medial gastrocnemius of rats aged 791 +/- 39 days. These were compared with a group of similarly isolated and classified units within young adult rats aged between four and five months. The estimated number of units decreased significantly from 93 +/- 28 in young adults to 66 +/- 19 in senescent animals. There was a preferential loss of fast motor units and a significant loss of myelinated axons within the muscle nerve. The maximum tetanic force developed by FF motor units decreased with age while that of the more fatigue resistant FI and FR motor units increased. Other contractile properties were unaltered and there was no change in any contractile parameter for S unit types. Alterations in the cross-sectional areas and proportions of histochemically classified muscle fibres reflected these changes. The proportion of FG fibres declined with age and there was evidence of denervation, particularly within the peripheral rim of fast type fibres. The number of muscle fibres within the medial gastrocnemius decreased with age. These results indicate that fast motor units are most severely affected during the early stages of senescence. Within this population the FF motor units which have the largest innervation ratios in the young adult may suffer preferential degeneration.

Effect of partial denervation on motor units in the ageing rat medial gastrocnemius.

The ageing neuromuscular system is thought to undergo a continual process of reorganization as motoneurones are lost and surviving motor nerves reinnervate neighbouring denervated muscle fibres. However, the extent to which collateral reinnervation is able to compensate for neural deficits in the ageing individual is unknown. The ability of the senescent motoneurone to increase the size of its peripheral field was therefore investigated following transection of the right L5 ventral root in male Sprague Dawley rats aged 775 +/- 50 days. This procedure resulted in an extensive partial denervation of the right medial gastrocnemius muscle. After a recovery period of between 28 and 31 days the isometric contractile properties of surviving motor units were compared to control motor units from both the contralateral muscle and a group of unoperated control animals aged 791 +/- 39 days. Motor unit force was found to be unchanged after partial denervation and the absence of any alteration in motor unit size was confirmed by histological analysis. However, the time course of the isometric twitch was significantly longer for both fast and slow motor unit types and the conduction velocity of motoneurones innervating fast units was decreased following partial denervation. These results demonstrate that senescent motor nerves are unable to substantially increase the size of their peripheral fields by extensive collateral reinnervation.

Immunity to nerve growth factor and the effect on motor unit reinnervation in the rabbit.

The trophic effects of nerve growth factor (NGF) on sympathetic, peripheral afferent, and other neural crest-derived cells have been intensively investigated. More recently, NGF has been shown to have an influence on motoneurons. This study was undertaken to investigate whether NGF had any influence on the mechanical or histological properties of reinnervated motor units. Three groups of rabbits were used: normal rabbits, rabbits in which the nerve to medial gastrocnemius (MG) was cut and allowed to reinnervate for 56 days, and rabbits in which the MG nerve reinnervated in the presence of immunity to NGF. Immunity to NGF did not affect the ability of motor axons to reinnervate a muscle, nor were the contractile characteristics of the motor units altered. The size of horseradish peroxidase-labeled motoneurons was not influenced by immunization against NGF; however, the distribution of afferent neuron sizes was altered. Conduction velocity of motor axons proximal to the neuroma was significantly faster after immunization against NGF. Transection and subsequent reinnervation by a peripheral nerve normally causes an increase in myelin thickness proximal to the neuroma. However, immunization against NGF appeared to decrease the magnitude of myelin thickening. It was concluded that immunization against NGF affects motor axonal conduction velocity via an influence on the neural crest-derived Schwann cells.

Pathological aspects of Australian Stringhalt.

Nine horses with clinical signs of Australian Stringhalt were killed and tissues collected for a detailed pathological study. Lesions were limited to peripheral nerves and muscles. The most severely affected nerves were the superficial and deep peroneal, distal tibial, plantar digital, volar and recurrent laryngeal nerve with changes characterised by a selective loss of large diameter myelinated fibres with various degrees of demyelination, fibrosis, Schwann cell proliferation and onion-bulb formation. A routine evaluation of the brain and spinal cord by light microscopy failed to reveal any consistent abnormalities. Morphometric analysis of deep peroneal and recurrent laryngeal nerves confirmed the reduced number of large diameter myelinated axons. Teased fibre preparations of these nerves did not show any abnormalities in internodal distance. The most severe muscle lesions were in the long and lateral digital extensors, cranial tibial, dorsal cricoarytenoid, gracilis and lateral deep digital flexor with extensive atrophy of fibres and diffuse fibrosis. Histochemical evaluation of the long digital extensor from 3 affected horses showed an abnormally wide distribution in fibre size and a reduction in type II fibres compared with controls. These lesions are consistent with a distal axonopathy leading to neurogenic muscle atrophy. The distribution of neuromuscular lesions in Australian Stringhalt may be explained by the susceptibility of longer, larger myelinated nerve fibres to injury, but the cause for this distal axonopathy remains unknown.

Sprouting of fusimotor neurones after partial denervation of the cat soleus muscle.

Normally, gamma motoneurones innervate only the intrafusal fibres of muscle spindles. This is a report of sprouting of gamma motoneurones to innervate extrafusal muscle fibres following partial denervation of the soleus muscle of kittens. In eight newborn animals, the L7 ventral root was cut on one side under anaesthesia and the animals were then allowed to recover. At approximately 100 days of age animals were reanaesthetised and a study made of mechanical properties of motor units whose axons ran in the S1 ventral root and supplied the partially denervated soleus muscle. Evidence was obtained for sprouting of all surviving alpha motoneurones. In addition, in four experiments axons conducting within the gamma range, on stimulation, produced measurable tension. In one experiment, stimulation of one such gamma axon also produced specific fusimotor effects on four afferents identified as coming from primary endings of muscle spindles. The gamma axon was therefore a fusimotor axon. The effect observed on stimulation of the gamma axon suggested a largely dynamic action. Other examples of gamma axons were encountered that on stimulation produced tension, but which could not be specifically associated with spindles. In addition, a number of gamma axons that did not develop tension were shown, on stimulation, to have fusimotor effects that were static in action. It is concluded that in extensively denervated muscles gamma motoneurones may sometimes sprout to innervate extrafusal fibres. The mechanical properties of the extrafusal fibres innervated by such gamma axons were similar to those of ordinary alpha motor units.

Effects of thyroidectomy on development of skeletal muscle in fetal sheep.

The influence of the thyroid gland on the functional and histochemical development of fast- and slow-twitch skeletal muscle of fetal sheep has been studied in euthyroid fetal sheep (n = 6) and athyroid fetuses (n = 4) surgically thyroid-ectomized at 70-75 days of gestation. Two fast-twitch muscles, the medial gastrocnemius and extensor digitorum longus, and the slow-twitch soleus muscle were studied at the fetal age of 140 days gestation. The athyroid fetuses had significantly slower twitch contraction and relaxation times in both the medial gastrocnemius and extensor digitorum longus muscles compared with the euthyroid fetuses. Twitch contraction and relaxation times of the soleus were not different in the two groups. Thyroidectomy resulted in an increase in the proportion of fast (type II) muscle fibers and myosin, as shown histochemically and by gel electrophoresis of heavy-chain myosins. These results indicate that the functional maturation of the fast-twitch muscles of sheep is influenced by the presence of an intact thyroid gland from at least 70 days of gestation. In contrast, the slow-twitch soleus muscle fiber diameter and twitch contraction and relaxation times were not different in the two groups.

Use of phenytoin to treat horses with Australian stringhalt.

Five horses with Australian stringhalt were treated with 15 mg/kg phenytoin orally for 2 weeks. During the second week of the trial, 3 of the horses were given an additional dose of 10 mg/kg phenytoin. The response to treatment was clinically assessed by grading the severity of the gait abnormality at the walk, trot, turning and backing twice daily. There was a significant (P less than 0.05) improvement in the gait abnormality when pre-treatment values were compared with the mean of the last 3 assessments before treatment stopped. When reassessed 2 weeks after treatment ceased, there remained a significant (P less than 0.05) improvement compared with pre-treatment values at the trot and on backing, but not at the walk or turning. Surface electromyographic recordings were made weekly from the long digital extensor muscle, and there was a change to a near normal recording by the end of treatment. Plasma phenytoin concentrations were monitored during the trial, and the dose rates used achieved a steady state with a mean plasma level of 37 +/- 7 mumol/l. There was wide variability between plasma concentrations in different horses, although there was no difference in absorption between administration of the phenytoin as a paste, or when it was mixed in the feed. Although mild tranquilization was seen after treatment, there were no clinical, haematological or biochemical signs of toxicity from the phenytoin therapy.