Biol. Pharm. Bull. 29(9) 1897—1899 (2006)

levels, leading to activation of myosin light chain kinase (MLCK) in smooth muscle and, as a result, MLC phosphorylation. This action is followed immediately by a decrease in MLC phosphatase (MLCP) activity and a resultant increase in MLC20 phosphorylation, which induces further contraction, that is Ca sensitization. Multiple second messengers/signaling pathways including the RhoA/Rho-associated coiled-coil forming protein kinase (ROCK) and protein kinase C (PKC) pathways, have reportedly been linked to Ca sensitization. An inhibitor protein specific for MLCP was isolated from pig aorta smooth muscle extracts and named CPI-17 for PKC-potentiated inhibitory protein for heterotrimeric myosin light chain phosphatase of 17 kDa. Phosphorylation of Thr 38 in CPI-17 converts it to a potent MLCP inhibitor with an IC50 of 5 nM. 10,11) Phospho-CPI-17 enhances myosin phosphorylation and contraction of permeabilized arterial smooth muscle. Airway hyperresponsiveness (AHR) associated with heightened airway resistance and inflammation is the asthmatic characteristic feature. The importance of AHR in the cause of bronchial asthma was suggested by the correlation with the severity of the illness. Our previous studies found the occurrence of both in vivo and in vitro hyperresponsiveness to ACh in rats that were actively sensitized and repeatedly challenged with aerosolized antigen. In this animal model of AHR, no significant difference in the ACh-induced increase in cytosolic Ca concentration of the main bronchial smooth muscle was observed between the control and AHR rats. Indeed, Ca sensitization in bronchial preparation of repeatedly antigen challenged rats was significantly enhanced as compared with that of control animals. Moreover, an augmentation of CPI-17 expression and AChinduced phosphorylation of CPI-17 was induced in bronchial smooth muscle of repeatedly antigen challenged rats. ET-1 is a potent contracting substance for various smooth muscles including airways. In addition to the classical Ca mediated contraction, ET-1 also induced Ca sensitization of contraction. ET-1 stimulation induces phosphorylation of CPI-17 in rabbit femoral artery and vas deferens. However, it is not clear whether CPI-17 is involved in ET-1induced Ca sensitization in airway smooth muscle of AHR rats. Endothelin-1 is an elevated level of endothelin-1 has been reported in the bronchoalveolar lavage fluids from asthmatic patients. Thus, it might be important for asthma therapy to understand the detailed mechanism of endothelin-1-induced airway smooth muscle contraction. In the present study, phosphorylation of CPI-17 and MLC induced by ET-1 was determined in bronchial smooth muscle of rats. Furthermore, the levels of CPI-17 phosphorylation induced by ET-1 in bronchial smooth muscles of the AHR rats were compared with those of control animals.