Effect of short time denervation on intracellular elemental content and fibre atrophy pattern of slow and fast twitch rat muscle.

Fast twitch extensor digitorum longus (EDL) and slow twitch soleus (S) muscles were studied after 5 and 16 days of denervation. Elemental contents of single muscle fibres were obtained with energy dispersive X-ray microanalyses (XRMA) applied on cryosections visualised in the scanning or scanning-transmission mode of electron microscopy. Cross sections in series with those produced for analytical electron microscopy were stained for enzyme histochemical fibre typing. A similar increase of Na and Cl was observed in EDL and S fibres after 16 days of denervation and a decrease of K was observed as well in EDL. This time after denervation is known to correspond to the maximal acetylcholine receptor synthesis extrajunctionally. There were no changes of elemental content after 5 days and thus the early membrane changes as i.a. fall in resting membrane potential and increased permeability for Na as well as increased endocytosis did not correspond to changes detectable by XRMA. Time course of atrophy differed between histochemical fibre types in EDL. In S all the muscle fibres atrophied earlier than in EDL and without any difference between fibre types. Then changes in elemental composition of muscle fibres after denervation do not seem to be related to degree and time course of muscle fibre atrophy.