Myopathic changes in indirectly stimulated mouse diaphragm after ecothiopate in vitro.

Mouse phrenic nerve-hemidiaphragms were stimulated in vitro in the presence of the anticholinesterase ecothiopate iodide and prepared for light and electron microscopy at different times during and after the appearance of prolonged contractions localized at the endplate. The earliest changes were at the subsynapse, without damage to the plasma membrane, and comprised hypercontraction of the sarcomeres, dilatation and vesiculation of the sarcoplasmic reticulum and the mitochondria, and dissolution of the Z-lines. Later there was damage to the plasma membrane. Also appearing later in the junctional region, but separated from the subsynapse by apparently normal muscle, were extrasynaptic hypercontractions, with a plasma membrane initially undamaged, but which became permeable after the contractile material divided into contraction clots. A hypothesis is proposed for the formation of such hypercontractions by abnormal mechanical factors arising from different contractile states along the length of the fibre, and is discussed with the role of prolonged transmitter action in the aetiology of myopathy.