Characteristics of the chloride conductance in muscle fibers of the rat diaphragm

In muscle fibers from the rat diaphragm, 85% of the resting membrane ion conductance is attributable to Cl-. At 37 degree C and pH 7.0, GCl averages 2.11 mmho/cm2 while residual conductance largely due to K+ averages 0.34 mmho/cm2. The resting GCl exhibits a biphasic temperature dependence with a Q10 of 1.6 between 6 degree C and 25 degree C and a Q10 of nearly 1 between 25 degree C and 40 degree C. Decreasing external pH reversibly reduced GCl; the apparent pK for groups mediating this decrease is 5.5. Increasing pH up to 10.0 had no effect on GCl. Anion conductance sequence and permeability sequence were both determined to be Cl-greater than Br-greater than or equal to I-greater than CH3SO4-. Lowering the pH below 5.5 reduced the magnitude of the measured conductance to all anions but did not alter the conductance sequence. The permeability sequence was likewise unchanged at low pH. Experiments with varying molar ratios of Cl- and I- indicated a marked interaction between these ions in their transmembrane movement. Similar but less striking interaction was seen between Cl- and Br-. Current-voltage relationships for GCl measured at early time-points in the presence of Rb+ were linear, but showed marked rectification with longer hyperpolarizing pulses (greater than 50ms) due to a slow time-and voltage-dependent change in membrane conductance to Cl-. This nonlinear behavior appeared to depend on the concentration of Cl- present but cannot be attributed to tubular ion accumulation. Tubular disruption with glycerol lowers apparent GCl but not GK, suggesting that the transverse tubule (T-tubule) system is permeable to Cl- in this species. Quantitative estimates indicate that up to 80% of GCl may be associated with the T tubules.     

Metabolic and phenotypic adaptations of diaphragm muscle fibers with inactivation

Zhan, Wen-Zhi, Hirofumi Miyata, Y. S. Prakash, and Gary C. Sieck. Metabolic and phenotypic adaptations of diaphragm musclefibers with inactivation. J. Appl.Physiol. 82(4):1145-1153, 1997.We hypothesizedthat metabolic adaptations to muscle inactivity are most pronouncedwhen neurotrophic influence is disrupted. In ratdiaphragm muscle(Dia_m), 2 wk ofunilateral denervation or tetrodotoxin nerve blockade resulted in areduction in succinate dehydrogenase (SDH) activity of type I, IIa, andIIx fibers (~50, 70, and 24%, respectively) and a decrease in SDHvariability among fibers (~63%). In contrast, inactivity induced byspinal cord hemisection at C₂ (ST)resulted in much less change in SDH activity of type I and IIa fibers(~27 and 24%, respectively) and only an ~30% reduction in SDHvariability among fibers. Actomyosin adenosinetriphosphatase (ATPase)activities of type I, IIx, and IIb fibers in denervated andtetrodotoxin-treated Dia_m werereduced by ~20, 45, and 60%, respectively, and actomyosin ATPasevariability among fibers was ~60% lower. In contrast, onlyactomyosin ATPase activity of type IIb fibers was reduced (~20%) inST Dia_m. These results suggestthat disruption of neurotrophic influence has a greater impact onmuscle fiber metabolic properties than inactivity per se.

     

Developmental effects on myonuclear domain size of rat diaphragm fibers.

During early postnatal development in rat diaphragm muscle (Diam), significant fiber growth and transitions in myosin heavy chain (MHC) isoform expression occur. Similar to other skeletal muscles, Diam fibers are multinucleated, and each myonucleus regulates the gene products within a finite volume: the myonuclear domain (MND). We hypothesized that postnatal changes in fiber cross-sectional area (CSA) are associated with increased number of myonuclei so that the MND size is maintained. The Diam was removed at postnatal days 14 (P-14) and 28 (P-28). MHC isoform expression was determined by SDS-PAGE. Fiber CSA, myonuclear number, and MND size were measured using confocal microscopy. By P-14, significant coexpression of MHC isoforms was present with no fiber displaying singular expression of MHCNeo. By P-28, singular expression was predominant. MND size was not different across fiber types at P-14. Significant fiber growth was evident by P-28 at all fiber types (fiber CSA increased by 61, 93, and 147% at fibers expressing MHCSlow, MHC2A, and MHC2X, respectively). The number of myonuclei per unit of fiber length was similar across fibers at P-14, but it was greater at fibers expressing MHC2X at P-28. The total number of myonuclei per fiber also increased between P-14 and P-28 at all fiber types. Accordingly, MND size increased significantly by P-28 at all fiber types, and it became larger at fibers expressing MHC2X compared with fibers expressing MHCSlow or MHC2A. These results suggest that MND size is not maintained during the considerable fiber growth associated with postnatal development of the Diam.     

Sphingomyelinase depresses force and calcium sensitivity of the contractile apparatus in mouse diaphragm muscle fibers

Diseases that result in muscle weakness, e.g., heart failure, are characterized by elevated sphingomyelinase (SMase) activity. In intact muscle, SMase increases oxidants that contribute to diminished muscle force. However, the source of oxidants, specific processes of muscle contraction that are dysfunctional, and biochemical changes underlying the weakness elicited by SMase remain unknown. We tested three hypotheses: 1) SMase-induced depression of muscle force is mediated by mitochondrial reactive oxygen species (ROS), 2) SMase depresses force and calcium sensitivity of the contractile apparatus, and 3) SMase promotes oxidation and phosphorylation of myofibrillar proteins. Our experiments included intact muscle bundles, permeabilized single fibers, and isolated myofibrillar proteins. The mitochondrial-targeted antioxidant d-Arg-2',6'-dimethyl-Tyr-Lys-Phe-NH(2), decreased cytosolic oxidants and protected intact muscle bundles from weakness stimulated by SMase. SMase depressed maximal calcium-activated force by 20% in permeabilized single fibers (in kN/m(2): control 117 ± 6; SMase 93 ± 8; P < 0.05). Calcium sensitivity of permeabilized single fibers decreased from 5.98 ± 0.03 (control) to 5.91 ± 0.02 (SMase; P < 0.05). Myofibrillar protein nitrotyrosines, carbonyls, and phosphorylation were unaltered by SMase. Our study shows that the fall in specific force of intact muscle elicited by SMase is mediated by mitochondrial ROS and can be attributed largely to dysfunction of the contractile apparatus.     

Initial ultrastructural changes in the muscle fibers of the diaphragm in rats exposed to chlorophos

Ultrastructural changes of rat diaphragm muscle fibers were studied after administration of chlorophos, i. e. organophosphorus inhibitor. Observations were made 10-180 seconds after treatment (concentrations--18 and 24 mM). The swelling of mitochondria and the increase in the length of sarcomeres were observed simultaneously. These changes were phasic. The swelling of mitochondria is probably due to the increase in energetical activity of muscle fibres.     

Effect of nutritional deprivation on diaphragm contractility and muscle fiber size

The influence of nutritional deprivation on the contractile and fatigue properties of the diaphragm was studied in adult rats. Food access was restricted to one-third of normal daily intake until the body weight of nutritionally deprived (ND) animals was approximately 50% of controls (CTL). Isometric contractile properties were studied in an in vitro nerve muscle strip preparation. Both twitch (Pt) and tetanic (Po) tensions of diaphragms from the ND animals were markedly reduced compared with CTL; however, Pt/Po was higher for the ND group. The shape of the force-frequency curve (normalized to Po) was generally similar between the two groups, except at 5 and 10 pulses/s stimulation, where greater relative tensions were produced in diaphragms from the ND animals. Diaphragm fatigue was induced by repetitive stimulation at either 20 or 100 pulses/s. Endurance time (defined as the time required for tension to fall to 50% of initial) of diaphragms from ND animals was prolonged at both 20 and 100 pulses/s. Immediately after induction of fatigue, force-frequency curves for both ND and CTL diaphragms were shifted to the right. However, this rightward shift was attenuated in the ND group compared with CTL. Nutritional deprivation had no effect on the proportions of different fiber types within the diaphragm but did result in a significant decrease in the cross-sectional area of both fast-and slow-twitch fibers. This decrease in cross-sectional area was significantly greater for fast-twitch fibers. We conclude that these changes in diaphragm contractile and fatigue properties occur as a result of the influence of malnutrition on muscle fiber cross-sectional area.     

Metabolic capacity of muscle fibers from high-altitude natives

We evaluate the effects of chronic hypoxia on the metabolic phenotype of the muscle fiber types of humans. The subjects were three Quechua natives residing in the Peruvian Andes at an altitude greater than 3300 m, and three lowlanders from below 700 m. Biopsy specimens were obtained from the vastus lateralis muscles of volunteers. Muscle fibers were identified histochemically as type 1 (oxidative), 2a (oxidativeglycolytic) or 2b (glycolytic). The relative contribution of each fiber type to the total cross-sectional area of each biopsy sample was determined. In individual fibers, the activities of malate dehydrogenase (MDH, citric acid cycle), lactate dehydrogenase (LDH, glycolysis) and adenylokinase (high-energy phosphate) were quantified. The cross-sectional area of the muscle occupied by each fiber type is comparable between Quechuas and lowlanders. Type 1 fibers are the only fiber type to demonstrate statistically significant (P 50.05) differences in enzyme activities between Quechuas and lowlanders. MDH activity is, on average, 19.6% less (P 0.0001) and LDH activity 28.1% more (P 0.0001) in the type 1 fibers of the Quechuas. Chronic hypoxia appears to produce a shift from oxidative to glycolytic metabolism in those fibers which are typically the, most aerobic in human muscle.     

Occurrence and regional distribution of striated muscle fibers in the rat pineal gland

Summary In a total of 96 rat pineals studied 31 were found to contain striated muscle fibers or their precursors. The muscle fibers were most frequently present in the stalk region and more frequently found in the left than in the right hemisphere. Size measurements revealed that the lengths of pineal muscle cell nuclei differ only slightly from those of the sphincter muscle of the iris. However, the yellowish appearance of pineal muscle cell nuclei under darkfield investigation, a phenomenon observed in all muscular tissues of mesenchymal origin and connective tissue cells, may support the hypothesis that pineal musculature is of mesenchymal rather than ectodermal origin.Supported by a grant (Vo 135/4) of the Deutsche Forschungsgemeinschaft within the Schwerpunktprogramm Neuroendokrinologie     

Influence of corticosteroids on myonuclear domain size in the rat diaphragm muscle.

Skeletal muscle fibers are multinucleated. Each myonucleus regulates gene products and protein expression in only a restricted portion of the muscle fiber, the myonuclear domain (MND). In the rat diaphragm muscle (DIAm), corticosteroid (CoS) treatment causes atrophy of fibers containing myosin heavy chain (MHC): MHC2X and/or MHC2B. We hypothesized that DIAm fiber MND size is maintained during CoS-induced atrophy. Adult male rats received methylprednisolone for 11 days at 1 (CoS-Low, n = 8) or 8 mg x kg(-1) x day(-1) (CoS-High, n = 8). Age-matched (CTL-AgeM, n = 8), sham-operated (SHAM-AgeM, n = 8), and weight-matched (CTL-WtM, n = 8) animals served as controls. In single DIAm fibers, cross-sectional area (CSA), MND size, and MHC expression were determined. Fiber CSA and MND size were similar in CTL-AgeM and SHAM-AgeM groups. Only fibers containing MHCslow or MHC2A displayed smaller CSA in CTL-WtM than in CTL-AgeM and SHAM-AgeM groups, and MND size was reduced in all fibers. Thus fibers containing MHCslow and MHC2A maintain the number of myonuclei, whereas MHC2X or MHC2B fibers show loss of myonuclei during normal muscle growth. Both CoS groups displayed smaller CSA and MND size than CTL-AgeM and SHAM-AgeM groups. However, compared with CTL-WtM DIAm fibers, only fibers containing MHC2X or MHC2B displayed reduced CSA and MND size after CoS treatment. Thus little, if any, loss of myonuclei was associated with CoS-induced atrophy of MHC2X or MHC2B DIAm fibers. In summary, MND size does not appear to be regulated during CoS-induced DIAm atrophy.