Fig. 5. Pathway by which smooth muscle contractility can be stimulated at constant Ca²⁺ levels. Agonists activating excitatory serpentine receptors coupled to heterotrimeric G-proteins, GTPS or arachidonic acid convert GDP-Rho-A to its GTP-bound form, which activates Rho-associated kinase. Phosphorylation of M₁₁₀ on Thr697 by Rho-associated kinase leads to inhibition of M₂₀-M₁₁₀-PP1c activity, thereby increasing the phosphorylation of Ser19 on the regulatory myosin light chain in the absence of alterations in the Ca²⁺-dependent myosin light chain kinase activity. The pathway is blocked by Rho-associated kinase inhibitor Y27632. Contractility in non-muscle cells can be activated by lysophosphatidic acid through a similar pathway.