Role of phospholipase A 2 and myoendothelial gap junctions in melittin-induced arterial relaxation

Abstract

We have used preconstricted rings of rabbit superior mesenteric artery to investigate the contribution of phospholipase A 2 and gap junctional communication to endothelium-derived hyperpolarizing factor (EDHF)-type relaxations evoked by melittin, a polypeptide toxin known to mobilize arachidonic acid from the cell membrane. Arachidonyl trifluoromethyl ketone (30 μM), an inhibitor of the Ca 2+-dependent phospholipase A 2, and Gap 27 (300 μM), a connexin-mimetic peptide which attenuates intercellular communication via gap junctions, both abolished the endothelium-dependent component of EDHF-type responses evoked by melittin in the presence of the NO synthase inhibitor N G-nitro- l-arginine methyl ester ( l-NAME, 300 μM) and the cyclooxygenase inhibitor indomethacin (10 μM). By contrast, the sulfhydryl agent thimerosal (300 nM), which amplifies EDHF activity, potentiated nitric oxide (NO)/prostanoid-independent relaxations induced by melittin. Neither arachidonyl trifluoromethyl ketone nor thimerosal modulated relaxations evoked by the peptide toxin in the absence of l-NAME and indomethacin. We conclude that melittin evokes EDHF-type relaxations through activation of the endothelial Ca 2+-dependent phospholipase A 2 followed by the transmission of a chemical and/or electrical signal via myoendothelial gap junctions. This mechanism of vasorelaxation may be negatively regulated by NO.