Study on the Contraction of Smooth Muscle Myofibrils from the Chicken Gizzard

Abstract

1. Phosphorylation of the myosin light chain in chicken gizzard myofibrils and its effects on myofibrillar ATPase activity were investigated under the condition of the contracture of the myofibrils. 2. The myosin light chain was phosphorylated by endogenous light chain kinase, after incubating the myofibrils for two hours at 30°C with ATP, Mg²⁺ and Ca²⁺. After standing overnight, the phosphorylated light chain was dephosphorylated by endogenous light chain phosphatase. 3. Phosphorylated and phosphorylated-dephosphorylated myofibrils showed much lower ATPase activities than did control myofibrils and were quite similar in ATPase activity. 4. The phosphorylated myofibrils apparently showed a negative Ca²⁺ -sensitivity in Mg²⁺ -ATPase. In contrast, the phosphorylated-dephosphorylated myofibril Mg²⁺ -ATPase was not affected by the presence or absence of Ca²⁺ 5. The superprecipitation particles formed by phosphorylated-dephosphorylated myosin could not be dissolved in 0.6 M NaCl, although control actomyosin gel and the superprecipitation particles formed by phosphorylated myosin could be easily dissolved under the same conditions. 6. It is concluded that the phosphorylated-dephosphorylated process of the actomyosin system in gizzard myofibrils might result in stronger actin-myosin interaction.