Cathepsin D and MMP-9 activity increase following a high intensity exercise in hind limb muscles of young rats

The influence of an intensive exercise regime on cathepsin D and MMP-9 activity in hind limb muscles was investigated. We hypothesized that high-intensity exercise would increase the number of these proteins, indicating their involvement in the pathogenesis of exercise-induced muscle injury. Muscle fibers from the gastrocnemius and soleus were used from young (6-mo-old) female rats (n = 6) who completed 10 consecutive days of treadmill running at high intensity (34 m min(-1) gradually up to 40 min per day), compared with nonrunning, age and sex-matched rats (n = 6). After a high-intensity exercise regime, cathepsin D activity significantly increased in the gastrocnemius (from 6.6 x 10(-3) to 10.7 x 10(-3) or 61% nM tyrosine x mg-1 protein x min-1) and the soleus (from 5.9 x 10(-3) to 8.9 x 10(-3) or 66%). The activity level of mRNA MMP-9, expressed as ng mg(-1) protein, increased in both muscles subjected to intensity running. The results of this study suggest that high-intensity running results in an elevation in the activity of lysosomal enzymes involved in matrix protein degradation.