Agonist-mediated activation of PLA2 initiates Ca2+ mobilization in intestinal longitudinal smooth muscle.

Recent studies have shown that Ca2+ mobilization in longitudinal muscle is initiated by inositol 1,4,5-trisphosphate (IP3)-independent Ca2+ influx that acts as a trigger for Ca(2+)-induced Ca2R release. The present study examined whether arachidonic acid (AA) acts as mediator of the initial Ca2+ influx. Cholecystokinin octapeptide caused transient concentration-dependent increase in AA release in dispersed intestinal longitudinal but not circular muscle cells followed by sustained increase in both muscle cell types. The initial increase in AA release coincided with the initial Ca2+ transient and muscle contraction: all three events were abolished by guanosine 5'-O-(2-thiodiphosphate), pertussis toxin (PTX), and the phospholipase A2 (PLA2) inhibitor, dimethyleicosadienoic acid, but were not affected by calphostin C or neomycin. Exogenous AA caused concentration-dependent contraction and increase in cytosolic free Ca2+ ([Ca2+]i) in longitudinal but not circular muscle cells; both events were abolished by Ca2+ channel blockers. Depletion of Ca2+ stores with thapsigargin attenuated with thapsigargin attenuated agonist- and AA-mediated increase in [Ca2+]i and contraction in longitudinal muscle cells: the residual [Ca2+]i increase (35%) and contraction (25%) reflected the component of Ca2+ influx. We conclude that AA released by agonist-mediated G protein-dependent PTX-sensitive activation of PLA2 mediates Ca2+ influx, which then triggers Ca(2+)-induced Ca2+ release. The process is independent of phosphatidylinositol hydrolysis and occurs exclusively in longitudinal smooth muscle, in which Ca2+ release channels are highly sensitive to Ca2+, ryanodine, and cyclic ADP-ribose and insensitive to IP3.