Myofibrillar myosin ATPase activity in hindlimb muscles from young and aged rats

We tested the hypothesis that Ca ²⁺-activated myosin ATPase activity is lower in muscles of aged rats relative to muscles of young rats, independent of changes in myosin isoform expression. Myofibrils were prepared from permeabilized fibers of soleus, plantaris, and semimembranosus muscles of young (8-12 months) and aged (32-38 months) F344 × BN rats and assayed for resting myosin ATPase, Ca ²⁺-activated myosin ATPase, and myosin heavy chain (MHC) and myosin light chain (MLC) isoform compositions. Resting myosin ATPases were not affected by age in any muscle (P ≥ 0.42). Ca ²⁺-activated myosin ATPases of soleus and plantaris myofibrils were not affected by age (P ≥ 0.31) but were 16% lower in semimembranosus myofibrils from aged rats (0.448 ± 0.019 μmol P _i/min/mg) compared to young rats (0.533 ± 0.031 μmol P _i/min/mg; P = 0.03). Correspondingly, maximal unloaded shortening velocity of single semimembranosus fibers from aged rats was slow (4.6 ± 0.2 fiber lengths/s) compared with fibers from young rats (5.8 ± 0.3 fiber lengths/s; P < 0.01). No age-related changes in MHC or regulatory MLC isoforms were detected in any muscle (P ≥ 0.08) but changes in the essential MLC occurred in plantaris and semimembranosus muscles. The data indicate that Ca ²⁺-activated myosin ATPase activity is reduced with age in semimembranosus muscle, independent of age-related changes in MHC isoform expression, and is one mechanism contributing to age-related slowing of contraction in that muscle.