Abstract

Reductions in work output during repeated contractions of rat medial gastrocnemius muscles (37 degrees C) were compared with changes in muscle metabolite concentrations. Three different exercise protocols were used in which the total number of stimuli and the length excursion were the same. The muscles performed a series of either 10, 25 or 40 repeated contractions at velocities of 20, 50 and 80 mm/s for groups A, B and C, respectively. In group A work output decreased steadily to 66 of the output in the first contraction. In groups B and C work output decreased to less than 10 of the first contraction. Changes in phosphocreatine and lactate concentrations were similar for all groups. However, very low ATP concentrations (about 35 of the resting value) were observed in groups B and C, compared with about 65 in group A. Inosine 5-monophosphate (IMP) production was 9.9 micro-mol/g dry wt in group A and about 18 micro-mol/g dry wt in groups B and C. The results suggest fatigue does not depend on changes in intracellular inorganic phosphate and pH but possibly on changes in nucleotide metabolism. Verf.-Referat