Myosin regulatory light chain phosphorylation inhibits shortening velocities of skeletal muscle fibers in the presence of the myosin inhibitor blebbistatin

Phosphorylation of skeletal myosin regulatory light chain (RLC) occurs in fatigue and may play a role in the inhibition of shortening velocities observed in vivo. Forces and shortening velocities were measured in permeabilized rabbit psoas fibers with either phosphorylated or dephosphorylated RLCs and in the presence or absence of the myosin inhibitor blebbistatin. Addition of 20 μM blebbistatin decreased tensions by ~80% in fibers, independent of phosphorylation. In blebbistatin maximal shortening velocities (Vmax) at 30°C, were decreased by 45% (3.2 ± 0.34 vs. 5.8 ± 0.18 lengths/s) in phosphorylated fibers but were not inhibited in dephosphorylated fibers (6.0 ± 0.30 vs. 5.4 ± 0.30). In the presence of 20 μM blebbistatin, Km for Vmax as a function of [ATP] was lower for phosphorylated fibers than for dephosphorylated fibers (50 ± 20 vs. 330 ± 84 μM) indicating that the apparent binding of ATP is stronger in these fibers. Phosphorylation of RLC in situ during fiber preparation or by addition of myosin light chain kinase yielded similar data. RLC phosphorylation inhibited velocity in blebbistatin at both 30 and 10°C, unlike previous reports where RLC phosphorylation only affected shortening velocities at higher temperatures.