Electromyographic and electron microscopic findings in the extraocular muscle of the myotonic rat

Motor endplate involvement in the extraocular muscles of the myotonic rat

The motor endplates from the extraocular muscles of rats administered 20,25-diazacholesterol were examined by electron microscopy. Many anomalies were found at various levels of the junctional complex. Electron microscopy showed disrupted sole-plate nuclei, disruptions of the junctional sarcoplasm, atypical sarcoplasmic extensions, simplified postsynaptic areas, and interposition of Schwann cell cytoplasm between axonal terminal and muscle fiber with a reduplication of basement membrane, as well as atrophic and disrupted axonal terminals. These data indicate that the motor endplate is affected as well as the muscle fiber in myotonic dystrophy.

Dependence of membrane potential on extracellular ionic concentrations in myotonic goats and rats

Intracellular potassium concentration([K]i) and the resting sodium-to-potassium permeability ratio (alpha = PNa/PK) were determined by recording resting potentials at 25 degrees C in six solutions with variable potassium concentrations, constant [K+] + [Na+], and constant [K+] X [Cl-] product and fitting the results by nonlinear least squares to the Goldman-Hodgkin-Katz equation. Fits to models including chloride permeability terms suggest that chloride is in Donnan equilibrium in the muscles studied. External intercostal muscle was biopsied from anesthetized normal goats and goats with hereditary myotonia. In normal goat muscle, alpha was 0.012 +/- 0.001 (mean +/- SE, n = 11), and [K]i was 133 +/- 6 mM (n = 9); myotonic goat muscle did not differ from this. Extensor digitorum longus (EDL) muscle was removed from anesthetized male Wistar rats and from like rats pretreated with a single large dose of 20,25-diazacholesterol (DAC). In normal rat EDL, alpha was 0.019 +/- 0.002 (n = 12), and [K]i was 142 +/- 4 mM (n = 12). In the 16-day DAC-treated rats, alpha was significantly reduced (P < 0.002) to 0.010 +/- 0.002 (n = 8), and [K]i was slightly reduced by DAC. It is concluded that DAC reduced PNa in rat fibers. Tetrodotoxin at 5 microM did not affect either parameter in the rats.

A histological and histochemical study of changes of fiber types in experimental myotonia

In both fast and slow muscles of rats treated with 20--25 diazacholesterol there were qualitative alterations, such as changes of fiber outlines, numerous moth-eaten fibers and rare ring fibers. In addition there were generally larger groups of Type I and intermediate fibers than in normal controls ("type-grouping" tendency) in the preparations for oxidative enzymes in the extensor digitorum longus (EDL) of myotonic animals. Quantitative evaluations of EDL and soleus of myotonic rats revealed moderate hypothrophy of Type I and Type II fibers with an increase in the numbers of Type I and of Type III fibers in the EDL and a significant decrease of the nondominant fibers in the soleus muscle. The data are discussed in the light of a neurally mediated and/or direct action of the drug on the muscle fiber.