Motor units and histochemistry in rat lateral gastrocnemius and soleus muscles: evidence for dissociation of physiological and histochemical properties after reinnervation.

A reexamination of the question of specificity of reinnervation of fast and slow muscle was undertaken using the original "self" nerve supply to the fast lateral gastrocnemius (LG) and slow soleus muscles in the rat hindlimb. This paradigm takes advantage of the unusual situation of a common nerve branch, which supplies both a fast and slow muscle, and of the opportunity to keep the reinnervating nerve in its normal position. In addition it provides a test of the effects of cross-reinnervation among muscles of the same functional group. The properties of soleus and LG muscles and of individual muscle units were characterized in normal rats and in rats 4-14 mo after cutting the lateral gastrocnemius-soleus (LGS) nerve and suture of the proximal stump to the dorsal surface of the LG muscle. Individual muscle units were functionally isolated by stimulation of single motor axons to LG or soleus muscle contained in teased filaments in the L4 and L5 ventral roots. Motor units were classified as fast contracting fatiguable (FF), fast contracting fatigue resistant (FR), and slow (S) on the basis of criteria described in the cat by Burke et al. and applied to rat muscle units by Gillespie et al. Muscle fibers were classified as fast glycolytic (FG), fast oxidative glycolytic (FOG), and slow oxidative (SO) on the basis of histochemical staining for myosin ATPase, nicotinamide-adenine dinucleotide diaphorase (NADH-D), and alpha-glycerophosphate (alpha-GPD). Reinnervated muscles developed less force and weighed less in accordance with having fewer than normal motor units and having lost denervated muscle fibers. Normal LG contained a small proportion of S-type motor units (9%), whereas the majority (80%) of control soleus units were S type. After reinnervation, each muscle contained similar proportions of fast and slow motor units with S-type units constituting 30% of units in both muscles. When compared with the normal motor-unit sample, there was no significant change in average twitch and tetanic force in reinnervated muscles for each type of motor unit. However, the range within each type was greater, and there was considerable overlap between types. Twitch contraction time was inversely correlated with force in normal and reinnervated muscles as shown previously in self- and cross-reinnervated LGS in the cat. Changes in proportions of motor units in reinnervated LG were accompanied by corresponding changes in histochemical muscle types. This contrasted with reinnervated soleus in which the proportion of muscle fiber types was not significantly changed from normal despite significant change in motor-unit proportions.(ABSTRACT TRUNCATED AT 400 WORDS)