Non‐muscle (NM) and Smooth Muscle (SM) Myosin II Activation are Regulated by Different Mechanisms During Contractile Stimulation of Airway Smooth Muscle (ASM)

Myosin II is the major motor protein in SM, and is expressed as both SM and NM isoforms. However, the function of non‐muscle myosin II in SM is not understood. We hypothesized that NM and SM myosin II may serve different functions during ASM contraction and be regulated by different mechanisms. We detected both NM myosin IIA and IIB heavy chain isoforms in canine ASM tissues. The 20 kD myosin light chain (MLC) is expressed in both NM and SM isoforms, but these isoforms cannot be distinguished immunologically. We used 2‐D gel electrophoresis to distinguish NM and SM MLC isoforms and analyze their phosphorylation in response to acetylcholine (ACh). NM MLC represented a relatively small proportion of the total cellular MLC (20–30%). Both SM and NM MLCs were phosphorylated in similar proportions in response to ACh stimulation (about 50%). When RhoA was inactivated by expressing the inactive RhoA mutant, RhoA T19N, in muscle tissues, NM MLC phosphorylation was substantially inhibited but there was little effect on SM MLC phosphorylation. The MLC kinase inhibitor, ML‐7, suppressed ACh induced SM myosin II LC phosphorylation, but had little or no effect on NM myosin II MLC phosphorylation. We conclude that MLC kinase primarily regulates the phosphorylation of SM MLC, whereas NM MLC phosphorylation depends on RhoA activation. This suggests SM and NM myosin II play distinct roles in the regulation of SM contraction. HL29289, HL074099.