PURINE NUCLEOSIDE FORMATION IN RAT SKELETAL-MUSCLE FIBER TYPES

To determine the capacity for purine nucleotide degradation among skel etal muscle fiber types, we established energy-depleted conditions in muscles of the rat hindlimb by inducing muscle contraction during isch emia. After 5, 10, 15, or 20 min of ischemic contractions, representat ive muscle sections were freeze-clamped and analyzed for purine nucleo tides, nucleosides, and bases. Fast-twitch muscle sections accumulated about fourfold more IMP than the slow-twitch red soleus muscle. Inosi ne begins to accumulate at <0.5 mumol/g IMP in slow-witch muscle and a t approximately 2 mumol/g IMP in fast-twitch muscle. This suggests tha t inosine is formed intracellularly by 5'-nucleotidase acting on IMP a nd that the activity and/or substrate affinity of the 5'-nucleotidase present in slow-twitch muscle may be higher than in fast-twitch muscle . At similar concentrations of precursor IMP, slow-twitch muscle has a greater capacity for purine nucleoside formation and should be more d ependent on salvage and de novo synthesis of purine for the maintenanc e of muscle adenine nucleotides. Fast-twitch muscles are better able t o retain IMP for subsequent reamination due to their lower capacity to degrade IMP to inosine.