Glycolytic Muscle Fibers Research Articles

Altered Ca2+ sensitivity of tension in single skeletal muscle fibres from MyoD gene-inactivated mice.

1. Single, fast glycolytic skeletal muscle fibres were isolated from wild-type (MyoD+/+) and MyoD mutant mice (MyoD-/-), which lack a functional copy of the MyoD gene. Fibres were chemically permeabilized to permit manipulation and control of the ionic environment of the otherwise intact myofilament apparatus. 2. Results show a fivefold greater variability in the [Ca2+] required for half-maximum tension generation among individual MyoD-/- fibres in comparison with controls (p < 0.05). 3. Consistent with this finding, Western blot analysis showed a sevenfold greater variability in the isoform expression pattern of the thin filament regulatory protein troponin T in Myod-/- compared with control fibres (p < 0.05). 4. Electrophoretic analysis of single-fibre segments indicated no apparent alteration in the isoform expression pattern of other regulatory and contractile proteins. In addition, other parameters of contractile function, including velocity of unloaded shortening, and maximum force production, were not significantly different between MyoD-/- and MyoD ø fibres. 5. These findings indicate that the thin filament structure- function relationship is altered due to the MyoD mutation and suggest that MyoD plays a role in establishing and/or maintaining the differentiated phenotype of adult fast skeletal muscle fibres.

A gene with homology to myogenin is expressed in developing myotomal musculature of the rainbow trout and in vitro during the conversion of myosatellite cells to myotubes

We report the cloning of a new trout myogenic cDNA which encodes helix-loop-helix protein homologous to the myogenic factor myogenin. Northern analyses indicate that trout myogenin (Tmyogenin) transcripts accumulate in large amounts in the myotomal masculature of embryos and frys. In adults, transcripts concentrate within the thin lateral layer of red (slow oxydative) muscle fibres. They are present only in low amounts in white (fast glycolytic) muscle fibres which constitute the major part of the trunk musculature. Using an in vitro myogenesis system, we observed that the trout myogenin encoding gene is not activated until myosatellite cells fuse to generate multinucleated myotubes, indicating that Tmyogenin lies downstream of muscle determination factors. All these observations show that in a major taxinomic group like teleosts, a gene with homology to myogenin exists. Its activation during myogenesis suggests that it acts as a major developmental regulator of muscle differentiation.

Skeletal muscle glycolytic and oxidative enzyme capacities are determinants of insulin sensitivity and muscle composition in obese women

Regional fat distribution is an important determinant of insulin resistance in obesity. In the current study, the relationship between skeletal muscle insulin sensitivity, mid-thigh muscle composition, and the metabolic profile of muscle was investigated. Muscle composition was assessed by computed tomography of the mid-thigh, and by activities of marker enzymes of aerobic-oxidative and glycolytic pathways and muscle fiber typing using biopsies of the vastus lateralis muscle. Muscle with reduced Hounsfield attenuation on computed tomography scans was increased in proportion to obesity, and was strongly related to insulin resistance, reduced muscle oxidative capacity, and increased anaerobic and glycolytic capacities by muscle. These findings suggest that as part of its expression of insulin resistance, skeletal muscle of obese individuals is also poorly equipped for substrate oxidation and manifests increased storage of fat.

Effect of alcohol and electrical stimulation on leakage of creatine kinase from isolated fast and slow muscles of rat.

Binge drinking of alcohol may lead to acute alcoholic myopathy with rhabdomyolysis, which is characterized by skeletal muscle damage, elevated serum creatine kinase (CK), and myoglobinuria. This study was undertaken to test whether alcohol acts directly on the skeletal muscles to enhance the leakage of CK, and to assess the influence of fiber-type composition and repetitive contractions of the muscle on the effect of alcohol. After 4 hr of incubation in normal physiological solution at 37 degrees C, mean leakage of CK was 0.7 units/mg from isolated rat extensor digitorum longus (EDL), which has more fast-twitch glycolytic muscle fibers, and 1.2 units/mg from the soleus, which has more slow-twitch oxidative muscle fibers. Ethanol at 0.1, 0.2, and 0.5% concentrations caused significantly greater increase in leakage of CK from soleus than from EDL. In normal physiological solution, electrical stimulation at 1 Hz for 4 hr increased the leakage of CK by about the same degree in both EDL and soleus. In the presence of 0.1 and 0.2% ethanol, electrical stimulation markedly potentiated the alcohol-induced leakage of CK from both soleus and EDL. These results indicate that alcohol increases the leakage of CK by acting directly on skeletal muscle fibers, especially of the slow-twitch oxidative type, and that repeated muscle contractions potentiate the alcohol effect. These studies suggest that exercise may increase the chances of rhabdomyolysis in the alcoholics.

Organization of single motor units in feline sartorius.

1. We depleted single motor units in feline sartorius muscles of glycogen by stimulating their motoneurons intracellularly. We mapped the intramuscular distribution of depleted fibers by inspecting histological cross-sections throughout the length of sartorius. 2. We selected ten depleted motor units for detailed study and quantitative analysis. Nine motor units were located in the anterior head of sartorius. One was located in a muscle whose distal half appeared to have been damaged some time before the acute experiment. A single motor unit was located in the medial head of sartorius. 3. Five motor units were composed of fast-twitch glycolytic (FG) muscle fibers, two of fast-twitch oxidative glycolytic (FOG) muscle fibers, and three of slow-twitch oxidative (SO) muscle fibers. Estimates of the numbers of depleted fibers in motor units of anterior sartorius indicated that FG motor units were larger (mean 566 fibers) than FOG and SO motor units (SO mean 190, FOG mean 156 fibers). The SO motor unit in the damaged muscle had 550 fibers. One motor unit depleted in the medial head of sartorius had 270 fibers with FG profiles. 4. Muscle fibers belonging to each anterior motor unit were never distributed throughout the whole cross-section of anterior sartorius at any proximodistal level. Furthermore, fibers were distributed nonuniformly along the proximodistal axis of the muscle. In most muscles at least a few depleted fibers were found at all proximodistal levels. However, in one normal muscle and the damaged muscle, depleted fibers were confined to the proximal end. 5. The fibers in the medial motor unit were confined to a strip that did not extend across the whole cross-section of the muscle head. Fibers within this strip were scattered quite evenly from origin to insertion. This medial FG motor unit occupied a smaller territory and contained fewer fibers than anterior motor units of the same histochemical type. 6. These results show that sartorius motor units are not distributed uniformly in the mediolateral plane; those in anterior sartorius were distributed asymmetrically in the proximodistal axis as well. This finding has important functional implications for the way in which we model force development and transmission in sartorius and other long muscles.

Endurance and resistance exercise induce muscle fiber type specific responses in androgen binding capacity

This study examined the effects of different exercise training programs on androgen receptor content and receptor affinity to dihydrotestosterone in fast glycolytic (FG) and slow oxidative (SO) skeletal muscle fibers in rats. Twenty-four male Sprague-Dawley rats were equally divided into three groups: control, endurance exercise trained and resistance exercise trained. After the exercise programs were completed, the extensor digitorum longus (EDL), predominantly a FG muscle, and the soleus, predominantly a SO muscle, were isolated, weighed and both androgen receptor content and affinity to dihydrotestosterone were determined. Resistance training evoked a significant ( P < 0.05) hypertrophic response in the soleus but not the EDL. Endurance training was not associated with any significant hypertrophy in either the soleus or the EDL. Neither the endurance nor the resistance training program resulted in changes in androgen receptor affinity to dihydrotestosterone. However, alterations in androgen receptor content were noted. The endurance training program resulted in a significant increase in androgen receptor content in the soleus, but no significant difference in the EDL. The resistance training program elicited a significant decrease in androgen receptor content in the soleus, and a significant increase in the EDL. These results indicate that different exercise stimuli induce changes in androgen receptor content that are specific to skeletal muscle fiber type.

Effect of soft diet and aging on rat masseter muscle and its motoneuron

To determine the effect of a soft diet and aging on the masticatory motor unit, we investigated the morphologic and metabolic properties of the superficial masseter muscle and its motoneurons in rats. Twenty rats were divided into four groups of five rats: rats fed a hard diet until 4 months after birth (hard, young), rats fed a soft diet until 4 months after birth (soft, young), rats fed a hard diet until 22 months after birth (hard, old), and rats fed a soft diet until 22 months after birth (soft, old). The diameter of the fast-twitch oxidative glycolytic muscle fiber was significantly smaller in the soft than the hard, and in the old than the young groups. The glycolytic enzyme (phosphofructokinase) activity of the muscle was significantly weaker in the old than the young group. There was no significant difference in soma diameter of the motoneurons between the soft and hard group, while the diameter was significantly larger in the old than in the young group. There was no significant difference in NADH-diaphorase activity of the motoneurons between the soft and hard group, while significantly less activity was demonstrated in the old than in the young group. The reduction in motor unit activity caused by the soft diet is considered to influence the morphologic and metabolic properties in the superficial masseter muscle but not in its motoneurons. The reduction in the oxidative enzyme activity of motoneurons with aging may occur regardless of the reduction in motor unit activity.

Ultrastructural characteristics of zebrafish spinal motoneurons innervating glycolytic white, and oxidative red and intermediate muscle fibers

Spinal motoneurons in the zebrafish were classified using morphological criteria. Dorsomedial white motoneurons which innervate the fast, glycolytic white muscle fiber compartment were distinguished from ventrolateral red and intermediate motoneurons which innervate the slow, oxidative, red and intermediate muscle fiber compartments. Synapses on cell somata and cell organelles were studied in detail. The motoneurons which innervate white muscle fibers (W motoneurons) are considerably larger than those which innervate red and intermediate muscle fibers (RI motoneurons; W > RI). Significant differences were also found in the size of the nucleus (W > RI) and in the ratio size nucleus/size soma (W < RI); small differences were found regarding endoplasmic reticulum (W > RI) and mitochondria (W < RI). There were no differences in synaptic apposition length or percentage of terminals with flat vesicles. Small differences were discerned with regard to covering percentages (W < RI) and percentage of terminals with round vesicles (W > RI). Terminals with dense cored vesicles appeared on W motoneuron somata only. Within the motoneuron population, there was a positive correlation between the coverage of terminals containing flat vesicles and the perimeter of the cell soma. In RI motoneurons, there was a positive correlation between the perimeter of the cell and the amount of endoplasmic reticulum. A negative correlation was found between the RI cell perimeter and mitochondria, which is in line with a high succinate dehydrogenase activity in small cells.

Selective accumulation of MyoD and myogenin mRNAs in fast and slow adult skeletal muscle is controlled by innervation and hormones.

Each of the myogenic helix-loop-helix transcription factors (MyoD, Myogenin, Myf-5, and MRF4) is capable of activating muscle-specific gene expression, yet distinct functions have not been ascribed to the individual proteins. We report here that MyoD and Myogenin mRNAs selectively accumulate in hindlimb muscles of the adult rat that differ in contractile properties: MyoD is prevalent in fast twitch and Myogenin in slow twitch muscles. The distribution of MyoD and Myogenin transcripts also differ within a single muscle and correlate with the proportions of fast glycolytic and slow oxidative muscle fibres, respectively. Furthermore, the expression of a transgene consisting of a muscle-specific cis-regulatory region from the myoD gene controlling lacZ was primarily associated with the fast glycolytic fibres. Alteration of the fast/slow fibre type distribution by thyroid hormone treatment or by cross-reinnervation resulted in a corresponding alteration in the MyoD/Myogenin mRNA expression pattern. These findings show that the expression of specific myogenic helix-loop-helix regulators is under the control of innervation and humoral factors and may mediate differential control of contractile protein gene expression in adult muscle.

Decreased muscle speed, strength and fatigability following two hours of tourniquet-induced ischaemia.

Contractile and morphological properties of the rabbit tibialis anterior muscle were measured 48 hours following a two-hour ischaemic episode. Ischaemia was induced using a specially-designed pneumatic tourniquet placed on the rabbit thigh. Maximum tetanic tension of muscle subjected to ischaemia (381 +/- 77 g) was only about 30% of the tension generated by control muscles (1,212 +/- 67 g). The rate of rise of tetanic tension of muscles subjected to ischaemia (15.9 +/- 3 g/ms) was only 33% of control values (44.5 +/- 5.9 g/ms). Muscle fatigue index increased significantly from 0.22 +/- 0.7 in control muscles to 0.55 +/- 0.09 in ischaemic muscles suggesting that muscles subjected to ischaemia had a greater endurance capacity than control muscles. Morphologically, focal necrotic regions and inflammatory cells were observed in ischaemic muscle fibers. Taken together, these data are consistent with selective damage to the fast glycolytic muscle fibers within the ischaemic tibialis anterior muscles. Thus, ischaemia results in overall decreased muscle speed, strength and fatigability.

The Effect of Increased Functional Load on the Activation of Satellite Cells in the Skeletal Muscle of Adult Rats

The comparison of the number of satellite cells in the fast oxidative glycolytic muscle fibers of m. quadriceps femoris in males of normal Wistar rats aged 16-17 weeks (5 animals) and rats trained for endurance (5 animals) was made. After six weeks of treadmill running at a speed of 35 m/min in an increasing regimen, the number of satellite cells in trained animals (9.59 +/- 1.09%) increased over 2.5-fold in comparison to controls (3.41 +/- 0.26%) within 24 hours. At the ultrastructural level complete and focal injuries of some muscle fibers and the partial denervation of individual muscle fibers were recorded in trained rats. It was proposed that the focal functional denervation of muscle fibers could be one of the factors for the activation of satellite cells in endurance exercise. Some trained rats demonstrated small groups of extra- and perisynaptic satellite cells under the basal membrane of muscle fibres, partial detachment of satellite cells from the surface of muscle fiber and presence in the interstitial space cell containing filaments.

Enzymes of transmitter and energy metabolism in cat middle ear muscles

Activities of the enzymes choline acetyltransferase (ChAT) and acetylcholinesterase (AChE), which metabolize the neuromuscular transmitter acetylcholine, and malate and lactate dehydrogenase (MDH and LDH), enzymes of oxidative and glycolytic energy metabolism, respectively, were measured in the middle ear muscles of the cat. For comparison, the same enzyme activities were measured in extraocular muscle tissue and in three hindlimb muscles rich in either slow oxidative (soleus), fast glycolytic (white part of vastus lateralis), or fast oxidative glycolytic (plantaris) muscle fibers. ChAT and AChE activities were much higher in middle ear muscles than in hindlimb muscles, consistent with a denser neuromuscular innervation, as in extraocular muscles. By contrast, MDH and LDH activities were remarkably low in the middle ear muscles, lower than in any of the hindlimb muscles or the extraocular muscles. Denervation of the stapedius muscle by peripheral transection of the facial nerve resulted in decreases in all four enzyme activities without associated changes in the tensor tympani. Surgical ablation of the peripheral facial nerve supply to the stapedius muscle appears to be a feasible option for producing its denervation. The results suggest some rather specialized chemical characteristics for the middle ear muscles.

Enzyme histochemical profiles of fish spinal motoneurons after cordotomy and axotomy of motor nerves

Histochemical profiles were made of identified spinal motoneurons from normal adult zebrafish and from animals subjected to cordotomy or unilateral axotomy of the motor nerves. The lesions caused an increase of the myotomal area with oxidative muscle fibers. We studied the question: do changes in the myotomal muscle configuration concur with changes in the enzyme histochemical profiles of innervating motoneurons? Based on the location and size of cell somata, two categories of motoneurons are distinguished: large white (W) motoneurons that innervate the deep fast, glycolytic muscle fibers, and smaller red and intermediate (RI) motoneurons that innervate the superficial slow oxidative and intermediate muscle fibers. In normal animals, glucose-6-phosphate dehydrogenase activity is high in the large W motoneurons and relatively low in the small RI motoneurons. The reverse holds for succinate dehydrogenase activity. W and RI motoneurons show similar nicotinamide adenine dinucleotide diaphorase activity. Short- (2 weeks) and long- (8 weeks) term effects of lesions were studied. The results show that: (1) the 3 types of lesions lead to prolonged changes in the enzyme histochemical profiles of spinal motoneurons. The type of change depends on the type of lesion and on the type of motoneuron; (2) unilateral axotomy of the motor nerves affects the histochemical characteristics of spinal motoneurons and the myotomal muscle fiber type configuration on the ipsi- and contralateral side. The contralateral effects are conceived as adaptations to maintain a left-right symmetry in the motor output.

Impaired control of respiration in iron-deficient muscle mitochondria

Dietary iron deficiency (ID) decreases iron-containing proteins and hence respiratory capacity of skeletal muscle mitochondria (SMM), but noniron components are much less affected. Using a hexokinase plus glucose ATP-utilizing system, we studied control of respiration in isolated SMM from rats of variable iron status: ID, ID 3 days after intraperitoneal treatment with iron dextran, and control. We found that sensitivity of respiratory control (e.g., ATP/ADP at a given oxygen consumption) was positively related to state 3 respiratory capacity. Titration studies with carboxyatractyloside, a noncompetitive inhibitor of adenine nucleotide translocase (AdNT), revealed that AdNT concentration was unaffected by iron status. However, the turnover number of AdNT was markedly reduced by ID and improved with iron treatment. We conclude that in ID SMM, decreased maximal respiratory capacity is paralleled by impaired sensitivity to putative controllers of oxidative phosphorylation at any respiratory rate, despite normal levels of AdNT. A second study was designed to determine possible consequences of impaired sensitivity of respiratory control on motor unit recruitment during exercise. ID and normal rats were subjected to a program of walking treadmill exercise. Although exercise failed to induce any changes in oxidative enzyme levels in control rat, ID animals and exhibited substantial mitochondrial enzyme adaptation in hindlimb skeletal muscle. Furthermore, the most consistent enzymatic changes were observed to occur in fast glycolytic muscle fibers. These results suggest marked alterations in the pattern of muscle fiber recruitment during mild exercise in ID rodents and support the hypothesis that sensitivity of respiratory control in SMM is an important determinant of motor unit recruitment during aerobic exercise.

HISTOCHEMICAL PROPERTIES OF MUSCLE ALLOGRAFTS ENHANCED VIA CYCLOSPORINE

Until recently, the transplantation of skeletal muscle across a major histocompatibility barrier has proved difficult. However, with the advent of cyclosporine (CsA), it has become possible to achieve extended survival across such histocompatibility barriers. To date, very little is known about the histochemical, biochemical, immunological or contractile properties of long-term-surviving muscle allografts. Consequently, it was the focus of this study to histochemically examine muscle allografts prolonged with CsA and determine the cross-sectional area of fast glycolytic muscle fibers. Measurements of cross-sectional area were made because they are an important correlate to the amount of tension a muscle can generate. Animals were assigned randomly to one of three groups: control (normal) (n = 5), syngeneic (n = 4), and allogeneic (n = 4). Muscle allografts were performed by transplanting the gastrocnemius of an ACI rat (RT1a) hindlimb into the hindlimb of a Lewis rat (LEW;RT1(1]. The syngeneic model consisted of an ACI-to-ACI transplant. Animals in the allograft group were given CsA (8 mg/kg/day) until the time of sacrifice. At approximately 100 days following transplantation, both syngeneic and allogeneic muscles were removed from the recipient, and quickly frozen in isopentane cooled by liquid nitrogen. Muscle fibers were classified as slow-oxidative (SO), fast-oxidative-glycolytic (FOG), or fast-glycolytic (FG) based upon their staining for myofibrillar ATPase and NADH-dehydrogenase. From each muscle, the cross-sectional area of approximately 175 FG muscle fibers was determined. The fast-glycolytic muscle fibers of both the syngeneic and allogeneic grafts demonstrated a substantial decrease in cross-sectional area. The mean value (+/- SD) for the fibers of the allografted muscle was 1165 +/- 533 microns 2. The mean (+/- SD) fiber cross-sectional area for the fibers of the syngeneic muscle was 973 +/- 421 microns 2. These values contrast with a mean (+/- SD) value of 3552 +/- 601 microns 2 for fibers from age-matched control animals. The differences between the syngeneic and allogeneic muscles were not significant (P greater than 0.05). However, both exhibited significant (P less than 0.01) atrophy compared with the control muscle.

Electromyography of the fin musculature of the cuttlefish Sepia officinalis.

The musculature of the fins of the cuttlefish Sepia officinalis (Mollusca, Cephalopoda) was studied with electromyography to test predictions of the functional role of the various muscle masses. Previous research had shown the fins to consist of a tightly packed, three-dimensional array of muscle with distinct zones of anaerobic glycolytic and oxidative muscle fibres. In addition, a network of crossed oblique connective tissue fibres was observed within the musculature. In a previous paper a model of the function of the muscle and connective tissue was presented. In the present paper, we present recordings of electrical activity from the various muscle bundles in the fin, in conjunction with the output from an electronic movement-monitoring device, and correlate muscle activity with both the phase and the intensity of the fin-beat cycle. The results obtained here support the hypothesis that the oxidative muscle fibres produce gentle fin movements and are consistent with the hypothesis that the network of crossed oblique connective tissue fibres provides skeletal support. The results also support predictions that the anaerobic glycolytic muscle fibres both produce vigorous fin movements and provide support for that movement. This study provides a critical test of models of the role of the tightly packed, three-dimensional array of muscle found in muscular hydrostats such as the arms and tentacles of cephalopods and tongues of mammals and lizards.

Thermal acclimation induces adaptive changes in subcellular structure of fish skeletal muscle.

Stereological analyses of electron micrographs were used to quantify physiologically important ultrastructures of slow-twitch oxidative (red) and fast-twitch glycolytic (white) muscle fibers from striped bass (Morone saxatilis) acclimated to 25 and 5 degrees C. The fraction of cell volume occupied by the mitochondria [volume density, Vv (mit,f)] of red fibers increases from 0.286 +/- 0.018 to 0.448 +/- 0.024 between 25 and 5 degrees C; Vv (mit,f) of white fibers increased from 0.027 +/- 0.003 and 0.040 +/- 0.004 at 25 and 5 degrees C, respectively. Because of a concomitant increase in the mass of oxidative muscle, acclimation from 25 to 5 degrees C results in an increase in total mitochondrial volume per 100 g body wt from 2.58 to 6.73 cm3 in oxidative muscle and from 2.46 to 3.40 cm3 in fast glycolytic muscle. Mitochondria of red fibers are in more clustered arrays after cold acclimation. Size and cristae surface densities of individual mitochondria are not affected markedly by acclimation, suggesting true cold-induced proliferation rather than enlargement of organelles. Harmonic means of intermitochondrial spacing in red fibers decreases from 2.64 to 1.43 micron between 25 and 5 degrees C. This reduces diffusion path lengths between sarcoplasmic and mitochondrial compartments proportionately, compensating for decreases in diffusivity of aqueous solutes. Intracellular lipid droplets of red fibers markedly increase in volume density from 0.006 +/- 0.003 at 25 degrees C to 0.079 +/- 0.014 at 5 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective damage of fast glycolytic muscle fibres with eccentric contraction of the rabbit tibialis anterior.

Acta Physiologica ScandinavicaVolume 133, Issue 4 p. 587-588 Selective damage of fast glycolytic muscle fibres with eccentric contraction of the rabbit tibialis anterior R. L. LIEBER, R. L. LIEBER Division of Orthopaedics and Rehabilitation, Veterans Administration Medical Center and University of California, San Diego, USASearch for more papers by this authorJ. FRIDÉN, J. FRIDÉN Department of Hand and Reconstructive Surgery, Umeå University Hospital, S-901 85 Umeå, SwedenSearch for more papers by this author R. L. LIEBER, R. L. LIEBER Division of Orthopaedics and Rehabilitation, Veterans Administration Medical Center and University of California, San Diego, USASearch for more papers by this authorJ. FRIDÉN, J. FRIDÉN Department of Hand and Reconstructive Surgery, Umeå University Hospital, S-901 85 Umeå, SwedenSearch for more papers by this author First published: August 1988 https://doi.org/10.1111/j.1748-1716.1988.tb08446.xCitations: 141AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article.Citing Literature Volume133, Issue4August 1988Pages 587-588 RelatedInformation

Regulation of binding of phosphofructokinase to myofibrils in the red and white muscle of the barred sand bass, Paralabrax nebulifer (Serranidae).

The binding of phosphofructokinase (PFK) to myofibrils from the white muscle of the fish Paralabrax nebulifer (Girard, 1854) is sensitive to factors known to be allosteric regulators of PFK activity. PFK in Triton-X-100-extracted muscle remains bound to myofibrils at pH 7.0 and is fully solubilized by increasing the pH to 8.0. The curve describing the pH-dependence of PFK binding to myofibrils is similar in its steepness to pH versus activity curves of PFK at low temperature. Nucleotides are also potent modulators, preventing the association of PFK with myofibrils at concentrations between 20 and 60 mumol l-1 of ATP, ADP, MgATP or GTP, listed in order of effectiveness. PFKs in the red and white muscle extracts differ in their pH-dependence of binding to myofibrils, and their kinetic and regulatory properties (response to citrate, pH and fructose-2,6-bisphosphate). Reversible binding of PFK to myofibrils may be important in the control of glycolysis, especially in the highly glycolytic white muscle fibres.

Changes in the metabolic profile of the equine gluteus medius as a function of sampling depth

1. 1. Cross sections from the middle of the gluteus medius were removed from 10 adult horses and used to evaluate changes in histochemically determined muscle fiber type and biochemically determined metabolic enzyme activities as a function of sample depth. 2. 2. Muscle fiber types determined using histochemical methods for myosin ATPase (pH 9.4) and succinic dehydrogenase (SDH) activity indicated percent fast-twitch glycolytic (FG) muscle fibers decreased and slow-twitch oxidative (SO) fibers increased as a function of increasing sampling depth. 3. 3. Percent histochemically determined fast-twitch oxidative glycolytic (FOG) fibers decresed slightly only in the deepest region of the gluteus medius. 4. 4. Citrate synthase (CS) enzymatic activity, used as a marker for mitchondrial oxidative potential, increased 2.5-fold activity per g of muscle protein from 1 to 8 cm sampling depth. 5. 5. 3-hydroxyacyl-CoA dehydrogenase (HAD) enzymatic activity, used as a marker for lipid oxidation potential, increased 3-fold in activity per g of muscle protein when the depth increased from 1 to 8 cm. 6. 6. Phosphorylase (PS) enzymatic activity, used as a marker for potential glycogen utilization, decreased 50% in activity per g of muscle protein when going from 1 to 8 cm. 7. 7. Lactate dehydrogenase (LDH) enzymatic activity, used as a marker for anaerobic glycolytic potential, decreased about 50% in activity as the sampling depth increased from 1 to 8 cm. 8. 8. In summary, the superficial portion of the equine gluteus medius was found to be more glycolytic and less aerobic in its metabolic profile than deeper regions. The muscle became progressively more aerobic and less glycolytic with increasing sampling depth.