Electron microscopic studies of the T-system--the effects of hyperosmolar glycerol and dantrolene sodium on mammalian muscles

Electron microscopic studies of the T-systems in mammalian muscles were performed to investigate the effects of hyperosmolar glycerol solution and dantrolene sodium. The results indicate that in mammalian muscles the terminal cisterns of the sarcoplasmic reticulum seem to be the site of morphological changes which occur when the muscle specimens were transferred from the hypertonic glycerol solution to isotonic washing solution. When the muscle specimen was further exposed to dantrolene solution, the above changes were intensified. The changes of the terminal cisterns of SR were characterized by prominent vacuolar changes and the T-tubules were pressed and narrowed by those vacuoles secondarily. The glycerol and dantrolene technique did not cause any electron microscopic changes in the sarcolemma, myofibrils, nor Z-discs, and selective changes in the T-system, i.e. at the terminal cisterns of SR were demonstrated. This technique, excitation-contraction uncoupling, abolishes isometric twitch tension up to 99.7% and serves for the purpose of prolonged intracellular recordings of mammalian muscles without movement induced artifacts.

The mechanism of neuromuscular fatigue. A study of mammalian muscle using excitation-contraction uncoupling

Neuromuscular transmission failure was measured during rapid repetitive stimulation of the rat hemidiaphragm in vitro after excitation-contraction uncoupling by hyperosmolar glycerol, washing, and dantrolene sodium. Observations of resting membrane, miniature endplate, action, and endplate potentials (epps) showed that neuromuscular transmission failure occurred during stimulation because of a decrement of the epp. This decrement was due both to a reduced number of quanta released and to a variable reduction of quantal size. The time course of these events was studied, and limited observations were made on recovery. Observations of the difference between the epp decrement of cut muscle and of partially curarized muscle suggest a prejunctional action of curare.

Excitation-contraction uncoupling. The effect of hyperosomolar glycerol solution and antrolene sodium on mammalian muscle in vitro

In the rat hemidiaphragm, in vitro dantrolene sodium irreversibly reduced twitch tension up to 20% of the original value in 20 minutes of exposure. The phenomenon was dose related. Dantrolene sodium did not affect (1) resting membrane potential (IMP); (2) miniature endplate potential frequency of amplitude; or (3) endplate potential amplitude decrement of repetitively stimulated partially curarized muscle. A combination of hyperosmolar solutions of glycerol and dantrolene sodium abolished contraction of mammalian muscle with no demonstrable effect on neuromuscular transmission or muscle fiber RMP. With this technique, prolonged recording from repetitively activated muscle fibers is possible.