Transitional stages in the histochemical development of muscle fibres during post-natal growth

Adaptive conditioning of skeletal muscle in a large animal model (Sus domesticus)

Recognition of the adaptive capacity of mammalian skeletal muscle has opened the way to a number of clinical applications. For most of these, the fast, fatigue-susceptible fibres need to be transformed stably to fast, fatigue-resistant fibres that express the 2A myosin heavy chain isoform. The thresholds for activity-induced change are size-dependent, so although the requisite patterns of electrical stimulation are known for the rabbit, in humans these same patterns would produce type 1 fibre characteristics, with an undesirable loss of contractile speed and power. We have used histochemistry, immunohistochemistry and electrophoretic separations to evaluate a possible conditioning regime in a large animal model. Stimulation of the porcine latissimus dorsi muscle with a phasic 30-Hz pattern for up to 41 days converted all type 2X and 2A/2X fibres to 2A with only a small increase in the type 1 population, from 17% to 22%. Stimulation for longer periods increased the proportion of type 1 fibres to 52%. Based on this model, stimulation regimes designed to achieve a stable 2A phenotype in humans should deliver fewer stimulating impulses, possibly by a factor of 2, than the pattern assessed here. Any such pattern needs to be tested for at least 8 weeks.

A successive histochemical staining for succinate dehydrogenase and "reversed"-ATPase in a single section for the skeletal muscle fibre typing

A procedure is described which simplifies the classification of skeletal muscle fibres in that it allows a simultaneous evaluation of both the oxidative capacity and the intensity of "reversed" ATPase of the fibres, and thus enables to distinguish three fibre types - SO, FOG and FG - in one tissue section. After preincubation at pH 4.1-4.2 the cryostat section is incubated for succinate dehydrogenase (SDH) and subsequently for "reversed"-ATPase. This is followed by the fixation with neutral buffered formaldehyde. The results of typing of chicken, minipig and rabbit fibres in a single muscle section stained with this technique are identical to those obtained with the usual method based on a comparison of serial sections of which one is stained for SDH activity the other for "reversed"-ATPase activity.

Alternative method for quantitative enzyme histochemistry of muscle fibers. Application of photographic densitometry combined with atomic absorption spectrophotometry

The present study examines the use of photographic densitometry combined with atomic absorption spectrophotometry for the quantitation of enzyme activities (SDH and ATPase) in fresh frozen sections of rat tibialis anterior muscles. The technique eliminates some difficulties which are inherent in other methods. The reliability of the technique was found to be in the 98% range; the results were precise for all samples studied. The use of SDH to separate muscle fibers into "types" was found to be totally inaccurate since a full spectrum of activities was observed. ATPase activities could separate easily into two groups, but a continuum of ATPase activities was observed in the fast-twitch fibers. The simultaneous use of both enzymes was capable of separating the FG, FOG and SO fibers; however, variation within a single type was considerable and a great deal of information was lost when using any classification system. The continuum of SDH activities indicates the motor units are arranged as a spectrum of fatigue-resistant contractile units. The range of ATPase activities observed is comparable to ranges of motor unit contraction times emphasizing the importance of this enzyme in the regulation of contraction speed.

Low temperature activation of post mortem glycogenolysis in bovine skeletal muscle fibres

Trimmed strips of sternomandibularis muscles taken from freshly-slaughtered cattle were placed in an isotonic myograph and cooled to 1 degree C. Spontaneous activity due to neuromuscular irritability was minimized by keeping muscle surfaces moist and anaerobic and was monitored by electromyography. Muscle strips were removed and frozen for histochemical analysis after they had completed their initial phase of cold-induced shortening (several hours). Control strips maintained for an equal time at 24 degrees C rarely depleted the stainable glycogen in any of their muscle fibres so as to become PAS-negative. In chilled muscle strips, however, glycogenolysis was activated in some muscle fibres and they became PAS-negative. In serial sections, most of the PAS-negative fibres exhibited strong ATPase and weak succinate dehydrogenase activity. Fibres with weak ATPase and strong succinate dehydrogenase activity rarely became PAS-negative. These results are in agreement with biochemical reports of a cold-induced (less than 5 degrees C) activation of glycolysis in skeletal muscle post mortem. Investigations on untrimmed lengths of excised sternomandibularis muscle indicated that longitudinal muscle damage caused in cutting muscle strips for the myograph and/or their more rapid rate of initial cooling had facilitated the depletion of stainable glycogen.

Comparison of red and white muscles by cytophotometry of their muscle fibre populations

Samples from two red muscles (vastus intermedius and vastus medialis) and two white muscles (biceps femoris and gluteus medius) were taken from four pigs. Serial transverse sections were reacted for ATPase and NADH oxidative activity. Sections were mapped with a projection microscope so that the staining intensity of individual fibres for the two reactions could be measured with a simple microscope photometer. Transmission values at 600 nm were converted to units of 0-10 for the range from darkest to lightest staining fibres on each section to cancel variation in staining intensity between sections. The aim of the study was to use simple cytophotometry instead of subjective judgement in the categorization of different histochemical types of muscle fibres. Cytophotometry enabled clear resolution of the major fibre types (types I and II using the ATPase reaction), partial resolution of more variable characteristics (NADH oxidative activity in type I and II fibres) and no resolution of subtle subtypes (IA and IB with the NADH oxidative reaction). However, between the major fibre types, cytophotometry revealed variable numbers of fibres with transitional characteristics. There were more of these fibres in red muscles. With sections reacted for ATPase, transmission values for low magnification fields containing 100 to 200 fibres were correlated (r = -0.91) with the ratio of type I:II fibres.