Differences in ammonia and adenylate metabolism in contracting fast and slow muscle

The time course of AMP deamination and IMP reamination was studied during in situ stimulation and subsequent recovery in fast-twitch (gastrocnemius) and slow-twitch (soleus) muscles of pentobarbital-anesthetized rats. Muscles were stimulated tetanically at rates that initially produced comparable decreases (40%) in tension development. In fast muscle, progressive decreases in total adenine nucleotide (TAN) contents of up to 50% were balanced by equivalent increases in IMP contents. Ammonia concentration initially increased in a 1:1 stoichiometry with changes in IMP and TAN. During recovery following stimulation, IMP removal matched, but NH3 removal exceeded the rate of TAN resynthesis. In contrast, TAN content in slow muscle was only slightly decreased (10%) during stimulation and there were no increases in IMP or NH3. Stimulation of the soleus following ligation of the blood supply did not increase TAN depletion. In both fast and slow muscle, changes in glutamate, aspartate, and alanine could be accounted for by transamination. These results illustrate a fundamental difference in adenylate metabolism during intense muscle stimulation between fast- and slow-twitch mammalian muscle.