Myosin light chain phosphorylation and phosphorylase A activity in rat extensor digitorum longus muscle.

Phosphorylation of the 18,500 dalton light chain of myosin and conversion of phosphorylase $\text{b}$ to $\text{a}$ were examined in relation to isometric tension development. Following a l sec tetanic contraction, light chain phosphate content increased from a pre-tetanic value of 0.10 to 0.75 mol phosphate/mol at 7 sec; phosphorylase $\text{a}$ activity (ratio of activity $\text{?5′AMP}\text{+5′AMP}$) increased from 0.03 to 0.42 at 4 sec and decreased to control values within 20 sec. Light chain phosphate content, however, declined much more slowly and correlated to post-tetanic potentiation of peak twitch tension. Our results suggest light chain phosphorylation is not obligatory for contraction but may play a role in post-tetanic potentiation.