Effect of indomethacin on force responses and sarcoplasmic reticulum function in skinned skeletal muscle fibers and cytosolic [Ca2+] in myotubes

In this study, the effects of phospholipase A(2) ( PLA(2)) inhibitors on excitation-contraction coupling (ECC) and sarcoplasmic reticulum (SR) function were examined in skinned extensor digitorum longus (EDL) muscle fibers of the rat. The nonspecific PLA2 inhibitor indomethacin ( 200 muM) significantly increased the peak (similar to2-fold, P = 0.02) and the width (similar to6-fold, P = 0.008) of depolarization-induced force responses (DIFRs) elicited in the fibers (n = 4). Exposure of the skinned EDL fibers to indomethacin (200 muM) (n = 7) and another PLA(2) inhibitor quinacrine (200 mu M) (n = 5) resulted in the return of large DIFRs after use-dependent rundown. However, aristolochic acid (100 muM), an inhibitor of secretory PLA(2), failed to return DIFRs after rundown. Indomethacin did not protect against the loss of DIFRs induced by exposure to elevated myofibrilar [Ca2+]. Indomethacin (200 muM) produced a small but significant increase in the Ca2+ sensitivity of the contractile apparatus of skinned EDL fibers and the maximum force production. Indomethacin (200 muM) also had significant effects on SR function, increasing SR Ca2+ loading in the skinned fibers (117.2 +/- 3.0% of controls, P = 0.0008, n = 8) and inducing intracellular Ca2+ release in isolated intact flexor digitorum brevis (FDB) fibers (n = 7) and C2C12 myotubes (n = 6). These data suggest that intracellular PLA(2) may be an important modulator of ECC in skeletal muscle.