Muscle metabolism and performance in perfused rat hindquarter during heavy exercise

An isolated perfused rat hindquarter model was used to examine muscle metabolism and performance during heavy muscular contraction. A one-pass system was used to perfuse the hindquarter for 30 min at rest and 20 min while electrically stimulated at 0.5 Hz with tetanic stimuli (100 Hz). The isometric tension generated by the gastrocnemius-plantaris-soleus muscle group was recorded continuously, and muscle biopsies were taken pre- and postperfusion. Peak tension was 2,648 +/- 55 g, decreasing to 69.3 and 51.6% of peak following 5 and 20 min of stimulation, respectively. Hindquarter O2 uptake increased from 0.56 mumol X min-1 X g perfused muscle-1 to 2.60 mumol X min-1 X g working muscle-1 after 3 min of stimulation and declined slightly thereafter. Lactate release by the hindquarter increased at the onset of stimulation, peaked at 2-4 min (23.4 +/- 1.4 mumol X min-1), and decreased to a constant amount in the final 10 min. Five minutes of stimulation produced large decreases in glycogen, creatine phosphate (CP), and ATP levels and increased lactate concentrations in all muscles sampled except the soleus. An additional 15 min of stimulation further decreased glycogen concentrations while lactate concentrations decreased in all muscles. Calculations based on the measured amounts of O2 and CP consumed and the amount of lactate produced revealed a heavy glycolytic contribution to energy production during the initial 5 min of stimulation (23%) and an aerobic domination during the final 15 min (90%). With this model heavy muscular contraction can be sustained to permit the measurement of uptake and release of substrates and products of the main energy-yielding pathways.