Motor endplates in fast and slow muscles of the rat: what determines their difference?

Motor endplates in fast and slow skeletal muscles have different functional and morphological characteristics, and for brevity, are termed fast and slow respectively. We have examined the terminal arborization patterns of fast fibular and slow soleus axons 3-4 and 6 months after they reinnervated old preformed endplates or formed new ectopic endplates with denervated rat soleus muscles. Ectopic endplates formed by transplanted fibular and soleus nerves were fast and slow in appearance respectively. Both the fibular and the soleus nerves formed endplates of slow appearance when they reinnervated the original endplates. The fast appearance of ectopic fibular nerve endplates was unaffected by reinnervation of the original endplates by the slow soleus nerve. Dually innervated fibres had intermediate contraction speed compared to the fast fibres reinnervated only by the fibular nerve and the slow fibres reinnervated only by the soleus nerve. Continuous stimulation of the transplanted fibular nerve at 10 Hz for 3-4 months, starting just before the onset of ectopic endplate formation, prevented the increase in contraction speed seen without stimulation. The ectopic endplates of the stimulated axons were much smaller than usual and showed some signs of fast to slow transformation, but the transformation was incomplete and varied in degree between preparations. Transplanted soleus axons were less prone to growing along foreign pathways and to forming ectopic endplates than fibular axons.(ABSTRACT TRUNCATED AT 250 WORDS)