[Ultrastructural characteristics of rat neuromuscular junctions after physical load]

It is interesting to ascertain the adaptive reaction of rat neuromuscular junctions (NMJ) of muscle fibers of different types to a chronic physical load. We examined ultrastructural changes in NMJ following both static load (pre- and postnatal ontogenesis of Wistar rats till a 2 month age took place under a constant rotation on the centrifuge at hypergravity conditions 2G), and after three kinds of dynamic loads (1/run on treadmill with a speed 35 m/min for 6 wks, 10-60 min/day; 2/swimmings, each 10 hrs/day for 10 days; 3/strength exercises on a vertical treadmill with load for 6 wks). Differences in NMJ reaction of muscle fibers of the same type to various loads were established. A low secretory activity of axonal terminals of type I muscle fibers of m. soleus was shown after the static load. The dynamic load (run) is accompanied with a high secretory activity of axonal terminals in m. soleus type I muscle fibers and of some axonal terminals of m. quadriceps femoris IIB type muscle fibers after strength exercises; the secretory activity of axonal terminals of m. quadriceps femoris IIA and IIB types muscle fibers is expressed in a lesser degree after swimming. The NMJ ultrastructure remodelling (terminal renewal) of type I muscle fibers of the 2 month old control rats increases after static and dynamic (run) loads. Some correlations between different kinds of physical load, muscle fiber type and the degree of NMJ ultrastructure transformation have been shown.