Chicken gizzard: Relation between calcium-activated phosphorylation and contraction

Of the proteins in mechanically disrupted chicken gizzard fibers (no functional sarcolemma) only the 20,000-dalton light chains of myosin underwent large (a²⁺- and Sr²⁺-dependent changes in phosphorylation. Phosphorylation closely corresponded with the Ca²⁺- and Sr²⁺-activated tensions. Adnosine 5'-O (3'- thiotriphosphate) only in the presence of Ca²⁺ induced irreversible Ca²⁺-insensitive activation of tension and thiophosphorylation of the 20,000-dalton light chains, and blocked incorporation of ³²P from [γ-³²P]adenosine triphosphate into the myosin light chains.