Endothelium-dependent relaxation and hyperpolarization in guinea-pig coronary artery: role of epoxyeicosatrienoic acid

1. Acetylcholine (ACh) elicits an endothelium-dependent relaxation and hyperpolarization in the absence of nitric oxide (NO) and prostaglandin synthesis in the guinea-pig coronary artery (GPCA). This response has been attributed to a factor termed endothelial-derived hyperpolarizing factor (EDHF). Recently it has been suggested that EDHF may be a cytochrome P450 product of arachidonic acid (AA) i.e., an epoxyeicosatrienoic acid (EET). The present study investigated whether this pathway could account for the response to ACh observed in the GPCA in the presence of 100 μM N^ω-nitro-L-arginine and 10 μM indomethacin.
2. ACh, AA and 11,12-EET each produced concentration-dependent relaxations in arteries contracted with the H₁-receptor agonist AEP (2,2-aminoethylpyridine). The AA-induced relaxation was significantly enhanced in the presence of the cyclo-oxygenase/lipoxygenase inhibitor, eicosatetranynoic acid (30 μM).
3. The cytochrome P450 inhibitors proadifen (10 μM) and clotrimazole (10 μM) inhibited ACh, lemakalim (LEM) and AA-induced relaxation, whereas 17-octadecynoic acid (100 μM) and 7-ethoxyresorufin (10 μM) were without effect on all three vasodilators. Proadifen and clotrimazole also inhibited ACh (1 μM) and LEM (1 μM)-induced hyperpolarization.
4. The ability of various potassium channel blockers to inhibit relaxation responses elicited with ACh, AA and 11,12-EET was also determined. Iberiotoxin (IBTX; 100 nM) was without effect on responses to ACh but significantly reduced responses to both AA and 11,12-EET. In contrast, 4-aminopyridine (4-AP; 5 mM) significantly reduced response to ACh but not responses to AA and 11,12-EET. Combined IBTX plus (4-AP) inhibited the ACh-induced relaxation to a greater extent than 4-AP alone. Apamin (1 μM), glibenclamide (10 μM) and BaCl₂ (50 μM) had no significant effect on responses to ACh, AA and 11,12-EET.
5. IBTX (100 nM) significantly reduced both 11,12-EET (33 μM) and AA (30 μM) hyperpolarization without affecting the ACh (1 μM)-induced hyperpolarization. In contrast, 4-AP significantly reduced the ACh-induced hyperpolarization without affecting either AA or 11,12-EET-induced hyperpolarizations.
6. In summary, our results suggest that the coronary endothelium releases a factor upon application of AA which hyperpolarizes the smooth muscle. The similarity of pharmacology between AA and 11,12-EET suggests that this factor is an EET. However, the disparity of pharmacology between responses to ACh versus responses to 11,12-EET do not support the hypothesis that EETs represent the predominant factor which ACh releases from the endothelium that leads to NO- and prostaglandin-independent hyperpolarization and relaxation in the GPCA.