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 Ca$^{2+}$-and Sr$^{2+}$-dependent changes in phosphorylation. Phosphorylation closely corresponded with the Ca$^{2+}$- and Sr$^{2+}$-activated tensions. Adenosine 5′-O (3′-thiotriphosphate) only in the presence of Ca$^{2+}$ induced irreversible Ca$^{2+}$-insensitive activation of tension and thiophosphorylation of the 20,000-dalton light chains, and blocked incorporation of $^{32}$P from [γ-$^{32}$P]adenosine triphosphate into the myosin light chains.