Regulation of the smooth muscle contractile phenotype by nonmuscle myosin

The contractile phenotype of a smooth muscle can broadly be classified as phasic or tonic. Following activation, phasic smooth muscle exhibits an initial period of rapid force activation, following which force falls to a lower steady state level. In contrast, force generated by tonic smooth muscle rises slowly to a sustained steady state. The differences in contractile patterns cannot be explained by the time course of either the Ca²⁺ transient or phosphorylation of the 20-kDa regulatory myosin light chain (MLC₂₀). Therefore, a molecular marker that defines tonic and phasic smooth muscle contractile properties remains elusive. Further, smooth muscle can maintain force at low levels of MLC₂₀ phosphorylation; often referred to as the latch state. The mechanism for the latch state is unknown and has been hypothesized to be due to a number of mechanisms including the formation of slowly cycling dephosphorylated or latch cross-bridges (Hai and Murphy, Am J Physiol 253:H1365-H1371, 1988). This review will focus evidence suggesting that nonmuscle myosin IIB (NMIIB) are the latch cross-bridges in smooth muscle and NMIIB content could define the tonic contractile phenotype.