Relationship of arachidonic acid release to porcine coronary artery relaxation

In porcine coronary artery endothelium-dependent relaxation to bradykinin is in part attributed to a chemically unidentified factor, termed endothelium-derived hyperpolarizing factor (EDHF). We hypothesize that arachidonic acid, acting through a cyclooxygenase-independent mechanism, is responsible for EDHF production. To define the relationship between EDHF production and arachidonic acid release we investigated the role of phospholipase C in bradykinin-induced relaxation and prostaglandin I₂ production (an index of arachidonic acid release) in porcine coronary artery. The phospholipase C inhibitor U73122 (1 μmol L) abolished bradykinin- induced, nitric oxide-mediated relaxation but did not inhibit either bradykinin-induced, EDHF-mediated relaxation or prostaglandin I₂ production. However, when given at a larger dose (20 μmol) U73122 abolished both bradykinin-induced. EDHF-mediated relaxation and prostaglandin I₂ production. Similarly, the calcium-ATPase inhibitor thapsigargin, given at a dose (1 μmol/L) that abolished bradykinin-induced increase in intracellular calcium concentration in cultured porcine coronary artery endothelial cells, eliminated both bradykinin-induced. EDHF-mediated relaxation and prostaglandin I₂ production. Although thapsigargin abolished bradykinin- induced prostaglandin I₂ production, the basal production of prostaglandin I₂ was enhanced, and contraction of endothelium-intact rings was attenuated. These latter responses are most likely related to enhanced basal arachidonic acid release and associated EDHF production. These observations suggest that phospholipase C activation and increased intracellular calcium concentration are required for both bradykinin-induced arachidonic acid release and EDHF production in porcine coronary artery. Moreover, EDHF production in porcine coronary artery appears to the closely associated with arachidonic acid release, thus supporting the hypothesis that arachidonic acid, acting through a cyclooxygenase-independent mechanism, is responsible for EDHF production in porcine coronary artery.