|Abstract or Summary
- Skeletal muscle damage induced by lengthening ("eccentric" or
"pliometric") contractions cause an immediate loss in maximal tetanic force (P₀) and an increase in protein degradation by unidentified endogenous mechanisms.
We hypothesized that increased proteolysis following active lengthening injury is
mediated by the Ca²⁺ dependent protease calpain. To test this hypothesis we
constructed an apparatus capable of inducing lengthening-contraction injury in rat
extensor digitorum longus (EDL) and measured the calpain-specific proteolysis of
a-fodrin in muscles subjected to either 90 in-situ active lengthening contractions
(17.4 + 0.3% of fiber length), 90 active isometric contractions, 90 passive
lengthening contractions (18.4 ± 0.1% of fiber length) or no contractile treatment.
Sixty minutes after the exercise treatments, isometric force had declined 4 ± 2% of P₀ in the isometric-contraction-treated muscles (n = 6), 4 ± 1% in the passivelengthening
treatment (n = 3), and 5 ±3% in muscles that received no treatment
other than isometric test contractures (n = 3). In contrast, force declined 53 ± 3% of P₀ (P <0.01 vs. all other treatments) in muscles subjected to active lengthening
contractions (n = 6). Calpain-mediated fodrinolysis produces 145 kDa and 150 kDa
peptides that retain immunoreactivity to the intact ct-fodrin antibody. Densitometric
analysis of a-fodrin Western blots showed that levels of these peptides were not
different between the active isometric, passive lengthening, no treatment or
contralateral muscle groups. In contrast, levels of the 145 kDa and 150 kDa
peptides in muscles subjected to lengthening contractions were 5.75-fold greater (P
<0.01) than the combined mean of the non-injury treatments and 8.5-fold greater
than contralateral muscles (P <0.01). These data indicate that calpain-mediated
proteolysis is increased following in situ lengthening contraction-induced injury in
rat EDL muscles.
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