Gerhard Asmussen

Mechanical properties of the isolated inferior oblique muscle of the rabbit

The passive-mechanical and dynamic properties of the rabbit inferior oblique muscle IO were studied in vitro at 35°C. The influence of length on the resting tension and isometric contractions were determined. Maximum twitch tension and fusion tension were developed at optimum length (Lo) an extension of the muscle to about 1.15 timesLR, the resting length of the IO in situ. A linear relation was found between length and tension in the activated muscle. An increase in stimulation frequency induced a parallel shift in the curves to higher tension but the slope of the curves remained unchanged. On an average the IO had in response to direct massive stimulation a twitch contraction time of 6.4 ms and a half-relaxation time of 7.0 ms. At stimulation with 300 Hz or above the tetanus fused. Stimulus frequencies above fusion frequency increased the rate of tension rise but not the maximum tetanic tension. The maximum tetanic tension was about 6.4 N/cm2, and the twitch: tetanus ratio was 0.1. To prolonged tetanic stimulations the IO exhibited a high fatigue resistance. Cooling the muscle to 25°C was followed by an increase in the time parameters of single twitches and tetanic contractions, a decrease of the tension developed in a fused tetanus and a small potentiation of the twitch. Following a repetitive stimulation a small post-tetanic potentiaion of the twitch was observed.

The Force-velocity Relation of the Rabbit Inferior Oblique Muscle Influence of Temperature

The contractile properties of the rabbit inferior oblique muscle (IO) were studied in vitro with direct stimulation at temperatures between 20 and 35°C. Isovelocity releases were used to determine the force/ velocity relation. Cooling the muscle from 35°C to 20°C increased contraction and half-relaxation times of single twitches with a temperature coefficient (Q10) of 0.4, but did not affect significantly the twitch tension. The tetanic tension increases with increasing temperature (Q10=1.32). Cooling decreased the maximum shortening velocity of the IO with a Q10 of 1.6 and the maximum mechanical power with a Q10 of 2.3. At 35°C, the maximum speed of shortening of the muscle (19±2 muscle lengths/s, mean ± SEM) corresponded to a maximum shortening velocity of the sarcomeres of 57±6 μm/s. This value is similar to data obtained for extraocular muscles (EOM) of smaller rodents (mice and rats). In comparison with mammalian limb muscles the isometric and force-velocity properties of mammalian EOM appear to be virtually independent of the size of the animal. Thus, IO is a fast-twitch muscle endowed with a maximum velocity of shortening higher than that of fast-twitch skeletal muscle, but using a tetanic mechanical power lower than that produced by slow-twitch muscle: the combination of these properties makes it ideally suited to move an ocular globe of low mass at high velocity.

Maximal shortening velocities, isomyosins and fibre types in soleus muscle of mice, rats and guinea-pigs.

1. Guinea‐pig soleus contains only type I fibres and slow isomyosin, SM2. Rat and mouse soleus contain about 70% of type I fibres and a mixture of isomyosins: slow, SM2 and intermediate, IM. Many rat soleus muscles contain a third isomyosin of a slow type, SM1. 2. The maximal velocity of unloaded shortening, V0, is largest in mouse soleus (6.11 Lf s‐1), slowest in guinea‐pig soleus (1.67 Lf s‐1) and intermediate in rat soleus (4.16 Lf s‐1) (Lf = fibre length). 3. In guinea‐pig soleus, V0 is equal to the maximal velocity (Vmax) computed using the Hill force‐velocity relationship; V0 is approximately twice as large as Vmax in mouse and rat soleus. 4. V0 measures the unloaded shortening velocity of the fastest fibres whereas Vmax is a function of the force‐velocity characteristics of all the fibres contained in the muscle. 5. V0 increases according to the isomyosin composition of the fibres in the sequence SM2 less than SM1 + IM less than IM.

Effects on skeletal muscle fibres of diabetes andGinkgo biloba extract treatment

Combined cytophotometric and morphometric analysis of muscle fibre properties and myosin heavy chain electrophoresis were performed on extensor digitorum longus and soleus muscles from healthy rats and rats with streptozotocin-induced diabetes. Moreover, the protective effect of Ginkgo biloba extract, a potent oxygen radical scavenger, on diabetic muscles was investigated. Changes in fibre type-related enzyme activities, fibre type distribution, fibre cross areas and myosin isoforms were found. In muscles of diabetic rats, a metabolic shift was measured mainly in fibres with oxidative metabolism. Fast-oxidative glycolytic fibres showed a shift to more glycolytic metabolism and about a third transformed into fast-glycolytic fibres. Slow-oxidative fibres became more oxidative. Fibre atrophy was measured in diabetic muscles dependent on fibre type and muscle. Different fibre types atrophied to a different degree. Therefore, a decreased area percentage of slow fibres and an increased area percentage of fast fibres of the whole muscle cross section in both muscles were found. This is supported by reduced slow and increased fast myosin heavy chain isoforms. These alterations of diabetic muscle fibres could be due to less motion of diabetic rats and diabetic neuropathy. After treatment with Ginkgo biloba extract, enzyme activities were increased mainly in oxidative fibres of diabetic muscles, which was interpreted as protective effect. Generally, the soleus muscle with predominant oxidative metabolism was more vulnerable to diabetic alterations and Ginkgo biloba extract treatment than the extensor digitorum longus muscle with predominant glycolytic metabolism.

Anabolic steroids (metenolone) improve muscle performance and hemodynamic characteristics in cardiomyoplasty

The loss of force and mass in the conditioned latissimus dorsi muscle are principal reasons for the poor improvement in hemodynamic functioning attained by cardiomyoplasty. Using 24 sheep, we investigated the effect of anabolic steroids on the hemodynamic, histologic, and myophysiologic characteristics in the setting of cardiomyoplasty. In 12 of the animals (group A), the latissimus dorsi muscles were electrically conditioned with an Itrel pulse generator; in the remaining 12 animals (group B), the electrical conditioning was combined with the administration of an anabolic hormone (metenolone; 100 mg/week). The hemodynamic measurements were performed during isolated perfusion of the subclavian artery (maintenance of pressure in the muscles), while all other circulation variables were held at the exact and reproducible value of zero by inducing ventricular fibrillation. Maximum force and muscle mass showed a significant increase in group B (maximum force: group A, 4.23 +/- 0.55 kp, and group B, 6.0 +/- 3.14 kp; muscle mass: group A, +11.07% +/- 1.06%, and group B, +79.9% +/- 40.8%). The ratio of type I to type II fibers after 12 weeks was 65.2% to 34.8% in group A and 96.7% to 3.3% in group B, as opposed to 19.9% to 80.1% in the control group. No side effects of the anabolic steroids were observed during the experiment. In the hemodynamic studies, we were able to demonstrate a further significant increase in the left ventricular pressure, fractional fiber shortening value, ejection fraction, stroke volume, cardiac output, and stroke work when using conditioned latissimus dorsi muscles that were additionally treated with metenolone.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in mechanical properties of the inferior oblique muscle of the rabbit after denervation

The inferior oblique muscle (IO) of the rabbit was denervated. The mechanical properties of the muscles were determined at 35°C in vitro 2–76 days after the operation. After denervation the muscles exhibited a considerable hypertrophy. The cross-sectional area of muscles denervated 30 days or longer grew to about twice as innervated controls. The length-tension relationship of passive or activated longterm denervated IO differed from normal in a higher stiffness. After denervation the time parameters of single twitches and tetanic contractions evoked by massive stimulation were prolonged, the fusion frequency was decreased, and the fatigue resistance was increased. During the first week after cutting the motor nerve both twitch and tetanic tensions decreased drastically. A minimum was reached at the end of the second weak. In the subsequent weeks the tension development was increased again, but per unit cross-sectional area it was always smaller than in innervated muscles. After denervation the twitch: tetanus ratio was increased. Cooling ro 25°C was followed by an increase in time parameters of single twitches and tetanic contractions and by a depression of twitch and tetanic tensions. Following a repetitive stimulation denervated IO showed a posttetanic depression of the single twitch.

Contractures in normal and denervated inferior oblique muscle of the rabbit.

Isometric contracture responses of normal and denervated inferior oblique muscles (IO) of the rabbit have been investigated in vitro at 35°C. The threshold concentration for eliciting potassium contractures was about 20 mM K+. In normal IO low potassium concentrations up to about 50 mM K+ evoked only sustained contractures, higher concentrations were responded by contractures with an initial transient component. The transient tension development was maximal at about 100 mM K+ the sustained component at 80 mM K+. After denervation the characteristic time course of the contractures was not changed, but the tension output of the preparations was diminished and long-term denervated IO have a somewhat lowered threshold. In normal IO acetylcholine (ACh), succinylcholine (SCh) and choline (Ch) caused also sustained contractures, the threshold doses were about 5 μM for ACh and SCh and 500 μM for Ch. The ACh sensitivity of the preparations was increased by physostigmine and decreased or abolished byd-tubocurarine. Denervation increased the drug sensitivity but the shape of the contractures was hardly influenced. The properties of slow tonic muscle fibres in mammalian extraocular muscles (EOM) probably responsible for sustained contractures and their changes after denervation are discussed.

Neuromuskuläre Reinnervation mimischer Muskulatur Eine histomorphologische Untersuchung am M. zygomaticus des Kaninchens

ZusammenfassungZur Beurteilung der Reinnervations- und Degenerationsprozesse der mimischen Muskulatur wurde am Kaninchen die denervierte Portio auricularis des M. zygomaticus sowohl primär als auch sekundär mit einem neuromuskulären Transplantat aus dem M. sternohyoideus mit der innervierenden Ansa hypoglossi reinnerviert und 6 bzw. 12 Wochen danach histologisch untersucht. Anhand der ATPase-Reaktion wurde eine Differenzierung der Muskelfasertypen vorgenommen und histomorphologische Merkmale der Denervation und Reinnervation ausgewertet. 6 Wochen nach Denervation und primärer neuromuskulärer Transplantation waren Fasertypgruppierungen als Reinnervationszeichen in Transplantatnähe nachweisbar. Nach einer Reinnervationsphase von 12 Wochen konnte das Fortschreiten eines Reinnervationsvorgangs anhand von Fasertypgruppierungen in transplantatfernen Muskelarealen nachgewiesen werden. Gleichzeitig stellte sich eine zunehmende Differenzierung der Fasertypen in den Gruppierungen dar.In allen denervierten und reinnervierten Gruppen waren pathologische Kalibervariationen der Muskelfaser nachweisbar. Primär reinnervierte Muskeln zeigten im Laufe einer Reinnervationsphase von 12 Wochen eine Normalisierungstendenz der Kalibervariationen. Sekundär, im Intervall von 6 Wochen nach Denervation transplantierte Muskeln, zeigten im Vergleich zu primär transplantierten Muskeln eine ausgeprägtere Faserpolymorphie mit einer hochgradig pathologisch erhöhten Kalibervariation. Der interfaszikuläre Raum war deutlich verbreitert. Die Reinnervationstendenz mit Nachweis von Fasertypgruppierungen war vergleichbar zu primär transplantierten Muskeln und war ebenfalls bereits 6 Wochen nach Transplantation nachweisbar.AbstractTo evaluate occurrences of reinnervation and degeneration, the portio auricularis of the zygomatic muscle was reinnervated (either primary or secondary reinnervation) with an interval of 6 weeks, using a neuromuscular transplant of the sternohyoid muscle, which was left at the ansa hypoglossi. Histological examination was performed 6 and 12 weeks later. Enzymatic stains for myofibrillar ATPase were used for differentiation of muscle fibre types. Histomorphological features of denervation and reinnervation were analysed. Six weeks after denervation and primary neuromuscular transplantation, type grouping in regions close to the transplanted muscle were detectable as signs of reinnervation. After a period of 12 weeks, the progressing reinnervation was proved by type grouping in distal areas of the muscle far from the area of transplantation. At the same time, differentiation of fibre typing in the type grouping areas increased.All denervated and reinnervated muscles had pathological variations of fibre diameter. Primary reinnervated muscles showed tendency of normalization in variation of diameter. Compared with primary reinnervated muscles, secondary reinnervated muscles with an interval of 6 weeks showed pronounced polymorphology of fibres and a high pathological increase of the variation of fibre diameters. The interfascicular space was significantly spread. The tendency of reinnervation with incidence of fibre type grouping was comparable to primary reinnervated muscles and also occurred 6 weeks after transplantation.