Group I secreted PLA2 and arachidonic acid metabolites in the maintenance of cat LES tone.

Spontaneous tone of in vitro lower esophageal sphincter (LES) circular muscle is associated with elevated levels of arachidonic acid (AA), PGF2α, and increased [35S]guanosine 5'- O-(3-thiotriphosphate) (GTPγS) binding to Gq-, Gi3-, and Gi1/i2-like G proteins. Tone and AA levels were reduced by inhibitors of a pancreatic-like (group I) secreted phospholipase A2(sPLA2), by the cyclooxygenase inhibitor indomethacin, and by the thromboxane A2 antagonist SQ-29548. In addition, pertussis toxin (PTX) reduced LES tone, confirming a role of PTX-sensitive G proteins in maintenance of LES tone. PGF2α contracted LES smooth muscle (strips and cells) and increased [35S]GTPγS binding to Gq and Gi3 in solubilized LES circular muscle membranes. PGF2α-induced contraction of LES permeable muscle cells was inhibited by Gq and Gi3 but not by Gi1/i2 and Go antibodies. The thromboxane A2 analog U-46619 contracted LES smooth muscle and increased Gqbinding. U-46619-induced contraction was inhibited by Gq but not by Gi3, Gi1/i2, and Go antibodies. LES tone and [35S]GTPγS binding were significantly reduced by indomethacin. We conclude that group I sPLA2 may mediate "spontaneous" LES tone by producing AA, which is metabolized to PGF2α and thromboxane A2. These AA metabolites activate receptors linked to Gi3 and Gq to maintain LES contraction.