Abstract
It is known for many years that arachidonic acid (AA) enhances the contraction of smooth muscle by elevating the phosphorylation level of regulatory light chain (RLC) of smooth muscle myosin (SmM). The elevation was reported to be due to enhancement of be myosin light chain (MLC) phosphatase and Rho kinase activities. In spite of the reports, we found that AA stimulated the ATPase activity even if these phosphatase and kinase were absent (J.Pharm.Sci. 94 Suppl. I, 140P, 2004). This stimulation holed true of SmMy that had been fully phosphorylated; AA stimulated the ATPase activity as well as myosin motor activity of the phosphorylated SmMy. We will show that the binding of AA to SmMy caused the stimulation.To relate of such an enhanced motor activity to the hyper-contraction such as the cerebrovascular spasm, the rabbit femoral artery was allowed to contract by K +-induced depolarization in the presence of phosphatase inhibitor of calyculin A and was observed a long-lasting, tonic contraction of the muscle. The addition of 250 μM AA caused further contraction. The western-blot of the muscle showed that RLC remained fully phosphorylated throughout the period of this tonic contraction. We speculate that AA binds to the SmMy, causing further contraction of vascular muscle. [Jpn J Physiol 55 Suppl:S15 (2005)]
