抄録
Most smooth muscles including intestinal muscles and systemic vessels except pulmonary artery relax to hypoxia. We showed that at high stimulus conditions with high potassium guinea pig taenia caeci and porcine coronary artery were relaxed by hypoxia without a change in [Ca2+]i. The level of myosin regulatory light chain phosphorylation (p-MRLC) was not reduced by hypoxia in the taenia (Obara et al. 1997), but significantly reduced in the coronary (Gu et al. 2005). Thus, the mechanisms of relaxation to hypoxia are different as the p-MRLC-dissociating hypoxic relaxation in the taenia and the Ca2+-desensitizing hypoxic relaxation in the coronary. This Ca2+-desensitizing hypoxic relaxation was observed even in the skinned coronary muscle under the fixed [Ca2+]. Hypoxia relaxed the skinned coronary muscle pretreated with GTP-γS, but not with ATP-γS. The protection protocol for myosin light chain kinase and Rho kinase using ML7 and Y27632 revealed that hypoxia directly inhibits Rho kinase-dependent phosphorylation of myosin phosphatase (MYPT1), thus leading to the Ca2+-desensitizing hypoxic relaxation of the skinned coronary via decreases in both p-MYPT1 and p-MRLC. The different mechanisms between intestinal and arterial muscles may be dependent on the different property of contractile apparatus; rapid contractile activity in the intestine and slow activity in the artery. [J Physiol Sci. 2008;58 Suppl:S55]
