PHYSIOLOGICAL SIGNIFICANCE OF MYOSIN PHOSPHORYLATION IN SKELETAL-MUSCLE

Each S-1 or head portion of the myosin molecule in skeletal muscle con tains a subunit known as the regulatory or phosphorylatable light chai n (P-LC). Phosphorylation of the P-LC is mediated by the second messen ger Ca2+ and takes place when the muscle fibre is activated. In smooth muscle, phosphorylation of the P-LC is the principal mechanism that i nitiates contraction, but in skeletal muscle myosin P-LC phosphorylati on is not required for contraction and a definitive role has not been established. It has been proposed that P-LC phosphorylation modulates the intrinsic nature of actin-myosin interactions, leading to force po tentiation under suboptimal activation conditions. An example of this is posttetanic potentiation. This paper describes a P-LC phosphorylati on induced mechanism for force enhancement during isometric contractio n. In addition, it summarizes recent data revealing that P-LC phosphor ylation is associated with enhanced work output of fast-twitch muscle during shortening and lengthening contractions.