Figure 2.

Putative model of force generation and maintenance in ASM involving MLCK, PKC and calponin. When a Ca²⁺-mobilizing agonist, acting through a G-protein-coupled receptor (GPCR), is applied to an ASM cell of the contractile phenotype, there occurs a transient increase in the [Ca²⁺]_i of sufficient magnitude to occupy all four Ca²⁺-binding sites on calmodulin (CM). In this form Ca²⁺-bound CM is able to active myosin light chain kinase (MLCK), which phosphorylates the 20 kDa light chains of myosin (LC₂₀) permitting actin activation of myosin ATPase, cross-bridge cycling and, in turn, force generation. When the [Ca²⁺]_i returns towards the resting levels, Ca²⁺ dissociate from CM and MLCK is dephosphorylated by myosin light chain phosphatase (MLCP). However, in many cases, force is maintained. One explanation for Ca²⁺-independent force generation is the activation of a novel protein kinase C (PKC) isoform such as PKCε by diacylglycerol (DAG), which is also generated by the contractile agonist and phosphorylates an actin-linked protein, calponin (Cal). Under resting conditions Cal inhibits the ability of actin to activate myosin ATPase and so keeps the muscle relaxed. However, in the phosphorylated state, Cal dissociates from actin thereby relieving its inhibitory effect on myosin ATPase. Ca²⁺-independent contraction then ensues due to the phosphorylation of LC₂₀ by the resting activity of MLCK. Cal is thought to be regulated by reversible phosphorylation and protein phosphatase 2A (PP2A) has been shown to dephosphorylate Cal to a form that prevents actin-activated myosin ATPase activity (see text for further details).