Arachidonic acid mediates non-capacitative calcium entry evoked by CB1-cannabinoid receptor activation in DDT1 MF-2 smooth muscle cells

Cannabinoid CB1‐receptor stimulation in DDT1 MF‐2 smooth muscle cells induces a rise in [Ca2+]i, which is dependent on extracellular Ca2+ and modulated by thapsigargin‐sensitive stores, suggesting capacitative Ca2+ entry (CCE), and by MAP kinase. Non‐capacitative Ca2+ entry (NCCE) stimulated by arachidonic acid (AA) partly mediates histamine H1‐receptor‐evoked increases in [Ca2+]i in DDT1 MF‐2 cells. In the current study, both Ca2+ entry mechanisms and a possible link between MAP kinase activation and increasing [Ca2+]i were investigated. In the whole‐cell patch clamp configuration, the CB‐receptor agonist CP 55, 940 evoked a transient, Ca2+‐dependent K+ current, which was not blocked by the inhibitors of CCE, 2‐APB, and SKF 96365. AA, but not its metabolites, evoked a transient outward current and inhibited the response to CP 55,940 in a concentration‐dependent manner. CP 55,940 induced a concentration‐dependent release of AA, which was inhibited by the CB1 antagonist SR 141716. The non‐selective Ca2+ channel blockers La3+ and Gd3+ inhibited the CP 55,940‐induced current at concentrations that had no effect on thapsigargin‐evoked CCE. La3+ also inhibited the AA‐induced current. CP 55,940‐induced AA release was abolished by Gd3+ and by phospholipase A2 inhibition using quinacrine; this compound also inhibited the outward current. The CP 55,940‐induced AA release was strongly reduced by the MAP kinase inhibitor PD 98059. The data suggest that in DDT1 MF‐2 cells, AA is an integral component of the CB1 receptor signaling pathway, upstream of NCCE and, via PLA2, downstream of MAP kinase. © 2005 Wiley‐Liss, Inc.