ABSTRACT
Hyperpolarization and relaxation mediated by endothelium-derived hyperpolarizing factor ( E D H F ) in the porcine coronary artery involve a cytochrome P450-epoxygenase-derived metabolite. To further characterize the cellular mechanisms underlying EDHF-mediated hyperpolarization and relaxation of the porcine coronary artery, the effects of inhibitors of cytochrome P450-epoxygenase enzymes, potassium channels and gap junctional communication were compared. Sulfaphenazole, a selective cytochrome P450 2C9 inhibitor, attenuated EDHF-mediated hyperpolarizations and relaxations. In the presence of A 9-tetrahydrocannabinol, previously reported to inhibit E D H F production, both EDHF-mediated responses were also blocked to the same extent. The combination of barium plus ouabain had no effect on EDHF-mediated hyper polarization, but the combination of charybdotoxin plus apamin abolished both the EDHF-mediated hyperpolarization and relaxation. Iberiotoxin, an inhibitor of large conductance Ca 2 +-dependent K + channels, and 18a-glycyrrhetinic acid, a gap junction uncoupling agent, failed to significantly affect EDHF-mediated responses in bicarbonatebuffered physiological salt solution, but inhibited those recorded in bicarbonatefree solution. These results confirm the crucial role of products of cytochrome P450-epoxygenase in EDHF-mediated responses in the porcine coronary artery. They also indicate that the N a + / K + ATPase, inwardly rectifying potassium channels and gap junctional communication play little or no role in mediating these responses. Moreover, EDHF-mediated hyperpolarization can be rendered sensitive to iberiotoxin and 18a-glycyrrhetinic acid by the removal of extracellular bicarbonate.
