Ultrastructure and metabolism of skeletal muscle fibres in the tench: Effects of long-term acclimation to hypoxia

Summary Tench (Tinca tinca) were acclimated to either aerated (P _{O 2} 17.6 KPa) or hypoxic (P _{O 2} 1.5 KPa) water for 6 weeks.Acclimation to hypoxia resulted in a decrease in mitochondrial volume fraction in both slow (22.9 to 15.0 %) and fast glycolytic (4.5 to 1.8 %) myotomal muscles fibres (P<0.01).Intermyofibrillar mitochondrial populations (4.4 to 1.2% slow; 0.6 to 0.04% fast fibres) were affected to a greater extent than those in the subsarcolemmal zone (18.5 to 13.8% slow; 3.9 to 1.8% fast fibres). After acclimation to hypoxia, cytochrome-oxidase activities decreased by 31 and 33 % in slow and fast fibres, respectively, but were maintained in the liver.Fibre size remained unchanged and actively differentiating fibres were observed in muscles from both groups of fish. Hypoxia resulted in a significant increase in myofibrillar volume fraction in both slow (43.1 to 56.1 %) and fast glycolytic fibres (73.1 to 82.7%) (P<0.05).Glycogen concentrations (mg/100g tissue) for liver (6616) slow muscle (1892) and fast muscle (334) were similar for fish acclimated to aerated or hypoxic water. Acclimation to hypoxia increased carnitine palmitoyl transferase activity (moles substrate utilised g·dry wt_-1 min^-1) in slow (0.42 to 1.1), fast glycolytic muscle (<0.01 to 0.15) and liver (1.1 to 3.7) indicating an enhanced capacity for fatty acid oxidation.Phosphofructokinase activities of fast glycolytic fibres were similar in fish acclimated to either aerated or hypoxic water, consistent with an unaltered capacity for anaerobic glycogenolysis. Hexokinase activities (moles substate utilised, g·dry wt^-1 min^-1) decreased in fast fibres (1.2 to 0.4) but were maintained in the slow muslce (2.1 to 2.5) and liver (4.5 to 4.8) of hypoxic fish. The activities of phosphofructokinase in slow muscle and phosphofructokinase, pyruvate kinase and lactate dehydrogenase in liver were two times higher in fish acclimated to hypoxia. An enhanced capacity for glycolysis in these tissues may reflect a reduced threshold for anaerobic metabolism during activity and/or an adaptation for acute exposure to anoxia in fish acclimated to hypoxia.Abbreviations/Glossary CO cytochrome oxidase activity - CPT carnitine palmitoyltransferase activity - HK hexokinase activity - LDH lactate dehydrogenase activity - PFK phosphofructokinase activity - PK pyruvate kinase activity - Vv volume fractions of cell components - normoxic fish acclimated to aerated water - hypoxic fish acclimated to reduced oxygen tensions - P _{O 2} partial pressure of oxygen tension A preliminary account of part of this work was presented at theXth European Meeting on Muscle and Cell Motility held at Galway, Ireland, in September 1981     

Comparison of the Molecular Forms of the Cholinesterases in Tissues of Normal and Dystrophic Chickens

Abstract: The levels and molecular forms of acetylcholinesterase (AChE, EC 3.1.1.7) and pseudocholinesterase (ΦChE, EC 3.1.1.8) were examined in various skeletal muscles, cardiac muscles, and neural tissues from normal and dystrophic chickens. The relative amount of the heavy (H_c) form of AChE in mixed-fibre-type twitch muscles varies in proportion to the percentage of glycolytic fast-twitch fibres. Conversely, muscles with higher levels of oxidative fibres (i.e., slow-tonic, oxidative-glycolytic fast-twitch, or oxidative slow-twitch) have higher proportions of the light (L) form of AChE. The effects of dystrophy on AChE and ΦChE are more severe in muscles richer in glycolytic fast-twitch fibres (e.g., pectoral or posterior latissimus dorsi, PLD); there is no alteration of AChE or ΦChE in a slow-tonic muscle. In the pectoral or PLD muscles from older dystrophic chickens, however, the AChE forms revert to a normal distribution while the ΦChE pattern remains abnormal. Muscle ΦChE is sensitive to collagenase in a similar way as is AChE, thus apparently having a similar tailed structure. Unlike skeletal muscle, cardiac muscle has very high levels of ΦChE, present mainly as the L form; AChE is present mainly as the medium (M) form, with smaller amounts of L and H_c. The latter pattern of AChE forms resembles that seen in several neural tissues examined. No alterations in AChE or ΦChE were found in cardiac or neural tissues from dystrophic chickens.     

Multiple Molecular Forms of Acetylcholine Receptors in Cultured Skeletal Muscle Cells: Subcellular Localization and Characterization

Abstract: Skeletal muscle cells of newborn rats, cultured in the absence of neuronal influence, were found to contain two types of cell surface acetylcholine receptors as demonstrated by isoelectric focusing. The isoelectric points of the two types of receptors were indistinguishable from those of junctional and extrajunctional types of receptors in mature animals. The cultured cells had two classes of intracellular α-bungarotoxin (αBT) binding components; one had the same sedimentation coefficient as that of surface receptors (9S), and the other had much smaller apparent molecular weights. Only a single major component was detected by isoelectric focusing analysis of the 9s intracellular aBT binding component, with a PI value close to that of the extra junctional receptor. These results suggest that the junctional and extrajunctional types of receptors may be synthesized through a common precursor.     

Hereditary motor endplate disease (med) of the mouse: Observations on dissociated myogenic cells and their development in culture

Summary Myogenic cells from mice homozygous for the lethal mutation motor endplate disease (med/med) were grown in culture. Like muscle cells taken from wild type (+/?) litter mates they fused to form myotubes which contracted, developed cross striations, and exposed acetylcholine receptors (AChR) on their surface. However, a decrease of 30% in the number of mononucleated cells per unit fresh weight of muscle was observed as early as 2–3 days postnatal, i.e., at least one week prior to the onset of physiological symptoms. Hence, in addition to influencing the functional maintenance of motor endplates, the med gene seems to control early events in muscle development.     

Enzyme histochemistry of the mesocoxal muscles of Periplaneta americana

Histochemical techniques have been employed to characterize enzymatic activity in the mesocoxal muscles of the cockroach, Periplaneta americana. Through our studies of the enzymes myosin-ATPase, NADH reductase, succinic dehydrogenase (SDH), and lactic dehydrogenase (LDH), we were able to classify fibers within these muscles according to criteria established for muscle fibers of vertebrates. Many of the mesocoxal muscles possess two different and distinct populations of fibers, whereas the remaining muscles are homogeneous with respect to their constituent fibers. The data presented here indicate biochemical heterogeneity for muscles of differing structural and functional features and possible neurotrophic influences upon oxidative enzymes and myosin-ATPase isozymes.     

Subsarcolemmal mitochondria and capillarization of soleus muscle fibers in young rats subjected to an endurance training

Summary Rats, 6 weeks old, were subjected to a program of endurance running for 3, 6 and 12 weeks. 0.5 to 0.8 m thick sections of Epon embedded soleus muscles were studied with morphometric methods.In cross-sections the area occupied by subsarcolemmal mitochondria was independent of the age, but was 53% higher after 12 weeks of training. The mean depth of the zones with subsarcolemmal mitochondria increased only 15% to about 0.9 m. Thus, the subsarcolemmal mitochondria showed a pronounced spreading at the muscle fiber surface in trained muscles. — The number of capillaries per fiber decreased slightly in controls and increased not significantly in trained muscles.It is concluded that the subsarcolemmal mitochondria supply the energy for the active transport of metabolites through the sarcolemma in oxidative muscle fibers, and that they are the limiting factor for endurance performance of the soleus muscle fibers because the changes in the capillarization were only small. It is suggested that the subsarcolemmal and the interfibrillar mitochondria have different functions and may therefore represent different types of mitochondria which can be distinguished by their morphology as well as by their biochemical properties.     

Ultrastructural alterations in skeletal muscle fibers of streptozotocin-diabetic rats

Summary The ultrastructure of fast-twitch-oxidative-glycolytic (FOG), fasttwitch-glycolytic (FG) and slow-twitch-oxidative (SO) fibers in plantaris and soleus muscles of normal and streptozotocin-diabetic rats was studied. In the diabetic animals, the mitochondria of FOG and SO fibers showed a loss of cristae and an increase in electron-dense granules. There was also an increased number of lipid droplets in close proximity to the mitochondria and the nuclei, and a separation of individual muscle nuclei to form satellite cells. Higher incidences of surface projections and sarcoplasmic splittings at the nuclear region were noticed in SO fibers. The FG fibers showed some disorientation of the T-tubular system. It is concluded that streptozotocin-diabetes has differential effects on the fine structure of the three fiber types of rat skeletal muscle.Supported by USPHS Grant AM 18280-04, Boston University Grant GRS-405-BI, and a grant-in-aid award from Sigma Xi Society     

Histochemical properties of skeletal muscle fibers in streptozotocin-diabetic rats

Summary The response of rat gastrocnemius muscle fibers to chronic streptozotocin-diabetes was studied. Transverse sections of this muscle from normal and diabetic rats were histochemically assayed for reduced diphosphopyridine nucleotide-diaphorase, myofibrillar adenosine triphosphatase, mitochondrial alpha-glycerophosphate dehydrogenase, beta-hydroxybutyrate dehydrogenase, and alkaline phosphatase activities. Cross-sectional areas of the fiber types were measured, and fiber capillarization and populations estimated. Chemically-induced diabetes appeared to have little effect on the metabolic or morphological properties of slow-twitch fibers. However, a general dedifferentiation occurred in the 2 fast-twitch fiber populations. There was a loss of oxidative potential in the fast-twitch-oxidative-glycolytic fibers, and a significant decrease in size in the fast-twitch-glycolytic fibers. No change in the proportions of slow- and fast-twitch fibers in the muscles of diabetic rats occurred. It is concluded that hypoinsulinism has differential effects on the 3 fiber types in heterogeneous rat skeletal muscle, and that slow-twitch fibers are least affected by the diabetic condition.     

Splicing of the Muscle-Specific Plasma Membrane Ca2+-ATPase Isoform PMCA1c Is Associated with Cell Fusion in C2 Myocytes

Abstract: The regulation of intracellular calcium is essential for proper muscle function. Muscle cells have several mechanisms for dealing with the rapid and large changes in cytosolic calcium level that occur during contraction. Among these is the plasma membrane Ca²⁺-ATPase (PMCA), which pumps calcium from the cytosol to the extracellular space. We have previously shown that in human fetal muscle the PMCA1 isoforms present are PMCA1a-d, with PMCA1b and c predominating. Alternative splicing of mRNAs encoding proteins involved in muscle contraction is common in developing muscle. Therefore, we examined the expression of muscle-specific PMCA mRNAs in pre- and postfusion mouse C2 myoblasts. The housekeeping form of the Ca²⁺-ATPase, PMCA1b, was found at all times and under all conditions. However, the other predominating isoform found in muscle, PMCA1c, was expressed on myotube formation. Simple cell-cell contact was not sufficient to induce PMCA1c expression, as cells plated at confluence but harvested before myotube formation did not express PMCA1c. The induction of this muscle-specific Ca²⁺-ATPase at myotube formation suggests that it may play an important role in muscle function.