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 prostaglandi n synthesis in the guinea-pig coronary artery (GPCA). This response ha s been attributed to a factor termed endothelial-derived hyperpolarizi ng factor (EDHF). Recently it has been suggested that EDHF may be a cy tochrome P450 product of arachidonic acid (AA) i.e., an epoxyeicosatri enoic acid (EET). The present study investigated whether this pathway could account for the response to ACh observed in the GPCA in the pres ence of 100 mu M N-omega-nitro-L-arginine and 10 mu M indomethacin. 2 ACh, AA and 11,12-EET each produced concentration-dependent relaxation s in arteries contracted with the H-1-receptor agonist AEP (2,2-aminoe thylpyridine). The AA-induced relaxation was significantly enhanced in the presence of the cyclo-oxygenase/lipoxygenase inhibitor, eicosatet ranynoic acid (30 mu M). 3 The cytochrome P450 inhibitors proadifen (1 0 mu M) and clotrimazole (10 mu M) inhibited ACh, lemakalim (LEM) and AA-induced relaxation, whereas 17-octadecynoic acid (100 mu M) and 7-e thoxyresorufin (10 mu M) were without effect on all three vasodilators . Proadifen and clotrimazole also inhibited ACh (1 mu M) and LEM (1 mu M)-induced hyperpolarization. 4 The ability of various potassium chan nel 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 bot h AA and 11,12-EET. In contrast, 4-aminopyridine (4-AP; 5 mM) signific antly reduced response to ACh but not responses to AA and 11,12-EET. C ombined IBTX plus (4-AP) inhibited the ACh-induced relaxation to a gre ater extent than 4-AP alone. Apamin (1 mu M), glibenclamide (10 mu M) and BaCl2 (50 mu M) had no significant effect on responses to ACh, AA and 11,12-EET. 5 IBTX (100 nM) significantly reduced both 11,12-EET (3 3 mu M) and AA (30 mu M) hyperpolarization without affecting the ACh ( 1 mu M)-induced hyperpolarization. In contrast, 4-AP significantly red uced the ACh-induced hyperpolarization without affecting either AA or 11,12-EET-induced hyperpolarizations. 6 In summary, our results sugges t that the coronary endothelium releases a factor upon application of AA which hyperpolarizes the smooth muscle. The similarity of pharmacol ogy between AA and 11,12-EET suggests that this factor is an EET. Howe ver, the disparity of pharmacology between responses to ACh versus res ponses to 11,12-EET do not support the hypothesis that EETs represent the predominant factor which ACh releases from the endothelium that le ads to NO-and prostaglandin-independent hyperpolarization and relaxati on in the GPCA.