Stable 5,6-Epoxyeicosatrienoic Acid Analog Relaxes Coronary Arteries Through Potassium Channel Activation

Abstract

5,6-Epoxyeicosatrienoic acid (5,6-EET) is a cytochrome P450 epoxygenase metabolite of arachidonic acid that causes vasorelaxation. However, investigations of its role in biological systems have been limited by its chemical instability. We developed a stable agonist of 5,6-EET, 5-(pentadeca-3(Z),6(Z),9(Z)-trienyloxy)pentanoic acid (PTPA), in which the 5,6-epoxide was replaced with a 5-ether. PTPA obviates chemical and enzymatic hydrolysis. In bovine coronary artery rings precontracted with U46619, PTPA (1 nmol/L to 10 μmol/L) induced concentration-dependent relaxations, with maximal relaxation of 86±5% and EC ₅₀ of 1 μmol/L. The relaxations were inhibited by the cyclooxygenase inhibitor indomethacin (10 μmol/L; max relaxation 43±9%); the ATP-sensitive K ⁺ channel inhibitor glybenclamide (10 μmol/L; max relaxation 49±6%); and the large conductance calcium-activated K ⁺ channel inhibitor iberiotoxin (100 nmol/L; max relaxation 38±6%) and abolished by the combination of iberiotoxin with indomethacin or glybenclamide or increasing extracellular K ⁺ to 20 mmol/L. Whole-cell outward K ⁺ current was increased nearly 6-fold by PTPA (10 μmol/L), which was also blocked by iberiotoxin. Additionally, we synthesized 5-(pentadeca-6(Z),9(Z)-dienyloxy)pentanoic acid and 5-(pentadeca-3(Z),9(Z)-dienyloxy)pentanoic acid (PDPA), PTPA analogs that lack the 8,9 or 11,12 double bonds of arachidonic acid and therefore are not substrates for cyclooxygenase. The PDPAs caused concentration-dependent relaxations (max relaxations 46±13% and 52±7%, respectively; EC ₅₀ 1μmol/L), which were not altered by glybenclamide but blocked by iberiotoxin. These studies suggested that PTPA induces relaxation through 2 mechanisms: (1) cyclooxygenase-dependent metabolism to 5-ether–containing prostaglandins that activate ATP-sensitive K ⁺ channels and (2) activation of smooth muscle large conductance calcium-activated K ⁺ channels. PDPAs only activate large conductance calcium-activated K ⁺ channels.