Elmi Leisner

Time dependent effects on contractile properties, fibre population, myosin light chains and enzymes of energy metabolism in intermittently and continuously stimulated fast twitch muscles of the rabbit

SummaryFast-twitch tibialis anterior and extensor digitorum longus rabbit muscles were subjected to long-term intermittent (8 h daily) or continuous (24 h daily) indirect stimulation with a frequency pattern resembling that of a slow motoneuron.Increases in time to peak of isometric twitch contraction were observed without parallel changes in the pattern of myosin light chains or alterations in the distribution of slow and fast fibres as discernible by the histochemical ATPase reaction. However, changes in the fibre population and in the myosin light chain pattern were observed after intermittent stimulation periods exceeding 40 days or continuous stimulation periods longer than 20 days. Under these conditions even higher increases were found in contraction time. In one animal a complete change in fibre population was observed. In this case myosin light chains of the slow (LCS1, LCS2) and of the fast type (LCf1) were obviously synthetized simultaneously within the same fibre. Early changes in the enzyme activity pattern of energy metabolism indicated a conversion of the fibres including their mitochondrial population. These changes and the earlier reported changes in the sarcoplasmic reticulum are probably responsible for the early changes in contractile properties which occur before the transformation of the myosin.

Immunofluorescent localization of glycogenolytic and glycolytic enzyme proteins and of malate dehydrogenase isozymes in cross-striated skeletal muscle and heart of the rabbit.

SummarySpecific antisera against glycogen phosphorylase, phosphofructokinase, aldolase, glyceraldehyde-phosphate dehydrogenase, enolase, lactate dehydrogenase, cytosolic and mitochondrial malate dehydrogenase from rabbit muscle were obtained from sheep. The gammaglobulins were used for indirect immunofluorescent localization of the respective enzymes in rabbit skeletal muscle and heart. In stretched skeletal muscle a cross-striation like distribution was observed for all enzymes studied. In the case of mitochondrial malate dehydrogenase this pattern is due to the staining of I-band mitochondria. In cross-sections, an intense staining of the sarcolemma and of subsarcolemmal mitochondria was observed. Comparative analyses with polarized light revealed that the cytosolic enzymes under study are distributed in the relaxed muscle fibre predominantly within the isotropic zones. The same distribution holds also for heart. In contracting muscle a decrease in cross-striated fluorescence and a faint staining of the interfibrillar spaces suggests a location also within the interfibrillar space.

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

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

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

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

Partial fast-to-slow conversion of regenerating rat fast-twitch muscle by chronic low-frequency stimulation.

Chronic low-frequency stimulation (CLFS) of rat fast-twitch muscles induces sequential transitions in myosin heavy chain (MHC) expression from MHCIIb → MHCIId/x → MHCIIa. However, the ‘final’ step of the fast-to-slow transition, i.e., the upregulation of MHCI, has been observed only after extremely long stimulation periods. Assuming that fibre degeneration/regeneration might be involved in the upregulation of slow myosin, we investigated the effects of CLFS on extensor digitorum longus (EDL) muscles regenerating after bupivacaine-induced fibre necrosis. Normal, non-regenerating muscles responded to both 30- and 60-day CLFS with fast MHC isoform transitions (MHCIIb → MHCIId → MHCIIa) and only slight increases in MHCI. CLFS of regenerating EDL muscles caused similar transitions among the fast isoforms but, in addition, caused significant increases in MHCI (to ∼30% relative concentration). Stimulation periods of 30 and 60 days induced similar changes in the regenerating bupivacaine-treated muscles, indicating that the upregulation of slow myosin was restricted to regenerating fibres, but only during an early stage of regeneration. These results suggest that satellite cells and/or regenerating fast rat muscle fibres are capable of switching directly to a slow program under the influence of CLFS and, therefore, appear to be more malleable than adult fibres.

Specific impulse patterns regulate acetylcholinesterase activity in skeletal muscles of rats and rabbits.

In rats, acetylcholinesterase (AChE) activity in the fast muscles is several times higher than in the slow soleus muscle. The hypothesis that specific neural impulse patterns in fast or slow muscles are responsible for different AChE activities was tested by altering the neural activation pattern in the fast extensor digitorum longus (EDL) muscle by chronic low‐frequency stimulation of its nerve. In addition, the soleus muscle was examined after hind limb immobilization, which changed its neural activation pattern from tonic to phasic. Myosin heavy‐chain (MHC) isoforms were analyzed by gel electrophoresis. Activity of the molecular forms of AChE was determined by velocity sedimentation. Low‐frequency stimulation of the rat EDL for 35 days shifted the profile of MHC II isoforms toward a slower MHCIIa isoform. Activity of the globular G1 and G4 molecular forms of AChE decreased by a factor of 4 and 10, respectively, and became comparable with those in the soleus muscle. After hind limb immobilization, the fast MHCIId isoform, which is not normally present, appeared in the soleus muscle. Activity of the globular G1 form of AChE increased approximately three times and approached the levels in the fast EDL muscle. In the rabbit, on the contrary to the rat, activity of the globular forms of AChE in a fast muscle increased after low‐frequency stimulation. The results demonstrate that specific neural activation patterns regulate AChE activity in muscles. Great differences, however, exist among different mammalian species in regard to muscle AChE regulation. J. Neurosci. Res. 47:49–57, 1997.