Figure 2 - Signal transduction for LES tone.

LES tone depends on activity of a group I secreted phospholipase A₂ (sPLA₂) producing arachidonic acid (AA), which is metabolized to prostaglandin F₂ (PGF₂) and thromboxane A₂ (TXA₂). PGF₂ and TXA₂ may cross the cytoplasm and activate membrane receptors linked to G_q and G_i3. G_q activates phosphatidylinositol-specific phospholipase C (PI-PLC) to produce diacylglycerol (DAG) and IP₃-induced Ca²⁺ release. G_i3 activates phosphatidylcholine-specific phospholipase C (PC-PLC) to produce DAG. Ca²⁺ and DAG activate PKC resulting in phosphorylation of myosin light chain and contraction. (Source: Harnett et al.,³ with permission from the American Physiological Society.)