A major role for the Rho‐associated coiled coil forming protein kinase in G‐protein‐mediated Ca 2+ sensitization through inhibition of myosin phosphatase in rabbit trachea

G protein‐mediated Ca 2+ sensitization of airway smooth muscle contraction was investigated with respect to the relative importance of Rho‐associated coiled coil forming protein kinase (ROCK) and protein kinase C (PKC). We examined the effects of Y‐27632, a ROCK inhibitor, and GF 109203X, a PKC inhibitor, on guanosine 5′‐ O ‐(3‐thiotriphosphate) (GTPγS)‐induced contraction in α‐toxin‐ or β‐escin‐permeabilized rabbit trachea. Although pre‐treatment with Y‐27632 dose‐dependently inhibited GTPγS (10 μ M )‐induced Ca 2+ sensitization of α‐toxin‐permeabilized trachea, a Y‐27632‐insensitive component (approximately 16% of the maximum contraction) was retained during the early phase of the GTPγS response in the presence of Y‐27632 (100 μ M ). GF 109203X (5 μ M ) abolished 1 μ M 4β‐phorbol 12, 13‐dibutyrate (PDBu)‐induced, but only partially inhibited the GTPγS‐induced Ca 2+ sensitization. A combination of Y‐27632 (100 μ M ) and GF 109203X (5 μ M ) totally abolished the GTPγS response. GTPγS caused only a small contraction in the absence of Ca 2+ . Wortmannin (30 μ M ), a myosin light chain kinase (MLCK) inhibitor, completely inhibited Ca 2+ ‐induced contraction. ATP‐triggered contraction of the strip which had been treated with calyculin A (1 μ M ), a phosphatase inhibitor, in rigor solutions was markedly slowed by worthmannin (30 μ M ), but not by Y‐27632 (100 μ M ), in the presence of GTPγS and Ca 2+ . GTPγS, but not PDBu, contracted the β‐escin‐permeabilized trachea in the absence of Ca 2+ , but the presence of Ca 2+ ‐independent MLCK. We conclude that ROCK plays a primary role in G‐protein‐mediated Ca 2+ sensitization, which requires MLCK activity, with minor contribution of PKC to the early phase of contraction, and PDBu utilizes conventional PKC(s) in airway smooth muscle.British Journal of Pharmacology (1999) 128 , 925–933; doi: 10.1038/sj.bjp.0702864