Contribution of Rho kinase to the early phase of the calcium-contraction coupling in airway smooth muscle

We investigated theoretically and experimentally the role of Rho kinase (RhoK) in Ca2+-contraction coupling in rat airways. Isometric contraction was measured on tracheal, extrapulmonary and intrapulmonary bronchial rings. Intracellular [Ca2+] was recorded in freshly isolated tracheal myocytes. Stimulation by carbachol (0.3 and 10 mu m) and 50 mm external KCl induced a short-time, Hill-shaped contraction obtained within 90 s, followed by a sustained or an additional delayed contraction. Responses of [Ca2+](i) to acetylcholine consisted in a fast peak followed by a plateau and, in 42% of the cells, superimposed Ca2+ oscillations. The RhoK inhibitor Y27632 (10 mu m) did not alter the [Ca2+](i) response. Whatever the agonist, Y27632 did not modify the basal tension but decreased the amplitude of the short-duration response, without altering the additional delayed contraction. The Myosin Light Chain Phosphatase (MLCP) inhibitor calyculin A increased the basal tension and abolished the effect of RhoK. KN93 (Ca2+-calmodulin-dependent protein kinase II inhibitor) and DIDS (inhibitor of Ca2+-activated Cl- channels) had no influence on the RhoK effect. We built a theoretical model of Ca2+-dependent active/inactive RhoK ratio and subsequent RhoK-dependent MLCP inactivation, which was further coupled with a four-state model of the contractile apparatus and Ca2+-dependent MLCK activation. The model explains the time course of the short-duration contraction and the role of RhoK by Ca2+-dependent activation of MLCK and RhoK, which inactivates MLCP. Oscillatory and non-oscillatory [Ca2+](i) responses result in a non-oscillatory contraction, the amplitude of which is encoded by the plateau value and oscillation frequency. In conclusion, Ca2+-dependent but CaMK II-independent RhoK activation contributes to the early phase of the contractile response via MLCP inhibition.