Involvement of transverse tubules in induced myotonia.

A single oral dose of clofibrate (30 mg/100 gm body weight) given to 250-300 gm male rats produced myotonic discharges from both the gastrocnemius and soleus muscles 2 hours after feeding. Rats fed the same dosage for 14 consecutive days exhibited electromyographic discharges consistent with profound myotonia. Fibers removed from the superficial region of the gastrocnemius muscle served as the source of fast twitch fibers for histological studies. Slow twitch fibers were observed in the soleus muscle. After a single clofibrate feeding localized T-tubule dilation was observed in a small percentage of myofibers in both muscles. In rats fed clofibrate for 14 days, approximately 30% of the fibers of the gastrocnemius and soleus muscles had dilated T-tubules. The motochondria of the soleus muscles of these rats were characterized by dilated intercristal matrixes containing electron densities. No mitochondrial changes were observed in the fibers of the gastrocnemius muscles taken from the same rats.