Arachidonic acid inhibits myosin light chain phosphatase and sensitizes smooth muscle to calcium

Arachidonic acid (AA) increased, at constant Ca²⁺, the levels of force and 20-kDa myosin light chain (MLC₂₀) phosphorylation in permeabilized smooth muscle, and slowed relaxation and MLC₂₀ dephosphorylation. The Ca²⁺-sensitizing effect of AA was not inhibited by inhibitors of AA metabolism (indomethacin, nordihydroguaiaretic acid, or propyl gallate), of protein kinase C (pseudopeptide) or by guanosine-5'-O-(β-thiodiphosphate) and was abolished by oxidation of AA in air. A non-metabolizable AA analog, 5,8,11,14-eicosatetraynoic acid) also had Ca²⁺-sensitizing effects. Extensive treatment with saponin abolished the Ca²⁺-sensitizing effects of phorbol 12,13-dibutyrate and guanosine-5'-O-(γ-thiotriphosphate), but not that of AA. A purified, oligomeric MLC₂₀ phosphatase isolated from gizzard smooth muscle was dissociated into subunits by AA, and its activity was inhibited toward heavy meromyosin but not phosphorylase. We conclude that AA may act as a messenger-promoting protein phosphorylation through direct inhibition of the form of protein phosphatase(s) that dephosphorylate MLC₂₀ in vivo.