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.     

Ultrastructure of neuromuscular synapses during administration of dexamethasone

Dexamethasone administration to rats at a dose of 100 micrograms/100 g body weight for 10 days resulted in the appearance of large synaptic vesicles in axon terminals, migration of synaptic vesicles to synaptic slits, local broadening of synaptic slit, proliferation of mitochondria in pre- and postsynaptic zones of the axomuscular synapses, destruction of myofibrils and other organelles in the postsynaptic area and the presence of lysosomes in this region.     

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.     

Concentration of caveolin-3 at the neuromuscular junction in young and old rat skeletal muscle fibers.

Caveolin-3, a muscle-specific member of the caveolin family, is strongly localized to the neuromuscular junction (NMJ) in adult rat muscle fibers, where it co-localizes with alpha-bungarotoxin staining. In 24-month-old rats, less distinct staining corresponds with the normal aging changes in the NMJ. After denervation, the pattern and intensity of staining begin to break up as early as 3 days, and by 10 days little staining remains. The functional implications of this concentration of caveolin-3 at the NMJ remain obscure, but it is possible that its absence could account for some of the phenotypic characteristics of individuals with caveolin-3 mutations.     

Effects of hypothyroidism on maximum specific force in rat diaphragm muscle fibers.

We hypothesized that 1) hypothyroidism (Hyp) decreases myosin heavy chain (MHC) content per half-sarcomere in diaphragm muscle (Dia(m)) fibers, 2) Hyp decreases the maximum specific force (F(max)) of Dia(m) fibers because of the reduction in MHC content per half-sarcomere, and 3) Hyp affects MHC content per half-sarcomere and F(max) to a greater extent in fibers expressing MHC type 2X (MHC(2X)) and/or MHC type 2B (MHC(2B)). Studies were performed on single Triton X-permeabilized fibers activated at pCa 4.0. MHC content per half-sarcomere was determined by densitometric analysis of SDS-polyacrylamide gels and comparison with a standard curve of known MHC concentrations. After 3 wk of Hyp, MHC content per half-sarcomere was reduced in fibers expressing MHC(2X) and/or MHC(2B). On the basis of electron-microscopic analysis, this reduction in MHC content was also reflected by a decrease in myofibrillar volume density and thick filament density. Hyp decreased F(max) across all MHC isoforms; however, the greatest decrease occurred in fibers expressing fast MHC isoforms (approximately 40 vs. approximately 20% for fibers expressing slow MHC isoforms). When normalized for MHC content per half-sarcomere, force generated by Hyp fibers expressing MHC(2A) was reduced compared with control fibers, whereas force per half-sarcomere MHC content was higher for fibers expressing MHC(2X) and/or MHC(2B) in the Hyp Dia(m) than for controls. These results indicate that the effect of Hyp is more pronounced on fibers expressing MHC(2X) and/or MHC(2B) and that the reduction of F(max) with Hyp may be at least partially attributed to a decrease in MHC content per half-sarcomere but not to changes in force per cross bridge.     

Developmental changes in neuromuscular transmission in the rat diaphragm.

Neuromuscular transmission was studied in diaphragms from rats of three ages, 4-7 days old, 11-12 days old, and adults with the use of an in vitro phrenic nerve-hemidiaphragm preparation. Each hemidiaphragm was stimulated via either muscle or nerve with 1-s stimulus trains at frequencies from 10 to 100 Hz. The patterns of force development obtained in response to the two routes of stimulation were compared for each group. Diaphragms from adults developed maximum force in response to stimulation of approximately 40 Hz with no significant decrease in force at higher frequencies. Within each stimulus train, once peak force was achieved, it was maintained for the remainder of the stimulus and responses to nerve and muscle stimulation were almost identical. In contrast, diaphragms from 4- to 7-day-old rats developed maximum force at approximately 20 Hz; stimulation at greater than or equal to 60 Hz induced significantly less peak force. This decrease in peak force at higher frequencies was significantly larger for nerve than for muscle stimulation. In addition, during each nerve stimulus train diaphragms from 4- to 7-day-old rats were unable to maintain peak force, which decreased at frequencies greater than 20 Hz. The decrease in force reached approximately 50% of peak at stimulation frequencies greater than or equal to 60 Hz. Diaphragms from 11- to 12-day-old rats showed intermediate responses. Based on the responses to phrenic nerve stimulation, we conclude that the neonatal rat diaphragm shows marked neuromuscular transmission failure that is not seen in the adult.(ABSTRACT TRUNCATED AT 250 WORDS)     

Growth of different types of muscle fibers of the soleus muscle during postnatal ontogeny in the rat

In order to study the development of the m. soleus muscle fibers during postnatal ontogenesis in the rat, methods for revealing ATPase activity of myosin at preincubation in acidic and alcaline medium and lactate dehydrogenase and succinate dehydrogenase activity have been used. The m. soleus undergoes three stages of development. The first stage--from birth of the animal up to the 7th day. During this time the muscle is homogenous. The second stage is characterized by appearance of certain histochemical differences in the muscle fibers. The muscle becomes mixed. During the whole period (in males from the 7th up to the 175th, and in females from the 7th up to the 60th-70th day) transferring of glycolytic fibers into oxidative-glycolytic ones with their successive transformation into oxidative fibers is observed. During the third stage (in males older than 175, and in females older than 60-70 days) the m. soleus converts from the mixed into the homogenous one consisting of oxidative fibers.     

Effect of unilateral denervation on maximum specific force in rat diaphragm muscle fibers.

We hypothesize that 1) the effect of denervation (DNV) is more pronounced in fibers expressing fast myosin heavy chain (MHC) isoforms and 2) the effect of DNV on maximum specific force reflects a reduction in MHC content per half sarcomere or the number of cross bridges in parallel. Studies were performed on single Triton X-100-permeabilized fibers activated at a pCa (-log Ca2+ concentration) of 4.0. MHC content per half sarcomere was determined by densitometric analysis of SDS-PAGE gels and comparison to a standard curve of known MHC concentrations. After 2 of wk DNV, the maximum specific force of fibers expressing MHC2X was reduced by approximately 40% (MHC(2B) expression was absent), whereas the maximum specific force of fibers expressing MHC2A and MHC(slow) decreased by only approximately 20%. DNV also reduced the MHC content in fibers expressing MHC2X, with no effect on fibers expressing MHC2A and MHC(slow). When normalized for MHC content per half sarcomere, force generated by DNV fibers expressing MHC2X and MHC2A was decreased compared with control fibers. These results suggest the force per cross bridge is also affected by DNV.     

Miniature endplate currents of muscle fibers from rat diaphragm during galantamine-induced acetylcholinesterase inhibition

Miniature endplate currents (MEPC) were recorded in muscle fibers of rat diaphragm using voltage clamp technique during acetylcholinesterase (AChE) inhibition induced by various concentrations of galantamine. Their amplitude and time course began to increase at a galantamine concentration of 3.16·10^–8 g/ml. Increased concentrations of galantamine produced a greater effect. Maximum amplitude and time course were reached at a concentration of 10^–6 g/ml. The input resistance of muscle fibers increased under the effects of galantamine. In all cases MEPC fell exponentially. At a concentration of 10^–5 g/ml galantamine produced a curarelike effect; amplitude and time course of decay increased to a lesser extent than at a concentration of 10^–6 and the decay in MEPC became biphasic. Following washout of galantamine (10^–5 g/ml) the time course of MEPC first rose, then fell, returning to the initial level in 3 h, and decay again became exponential. Changes in MEPC parameters under the effects of different concentrations of galantamine and washout were closely correlated. A positive correlation was found between the time course of decay and MEPC amplitude both in the presence and absence of AChE inhibition. It is postulated that the functional importance of synaptic AChE in repressing the postsynaptic action of acetylcholine is limited and that parameters of postsynaptic response may therefore be used to evaluate its action.A. A. Ukhtomskii Institute of Physiology, A. A. Zhdanov State University, Leningrad. Translated from Neirofiziologiya, Vol. 17, No. 5, pp. 607–614, September–October, 1985.