Characterization of endothelium- dependent relaxation in guinea pig basilar artery - effect of hypoxia and role of cytochrome P450 mono-oxygenase.
In the guinea pig basilar artery, acetylcholine and the calcium ionophore A23187 induced endothelium-dependent relaxations, which were not significantly affected by the nitric oxide (NO) synthase inhibitor Nomega-nitro-L-arginine (L-NOARG; 0.3 mM) or the guanylate cyclase inhibitor ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one; 1-10 microM), or by these inhibitors combined. However, acetylcholine (10 microM) and A23187 (3 microM) each significantly increased the tissue level of cGMP in the absence but not in the presence of L-NOARG, suggesting that NO is released from the vascular endothelium in this blood vessel. Treatment with the potassium (K) channel inhibitors charybdotoxin (0.1 microM) plus apamin (0.1 microM), a toxin mixture previously shown to inhibit relaxations mediated by endothelium-derived hyperpolarizing factor (EDHF) in this artery, had no effect on the A23187-induced relaxation but slightly inhibited the response to acetylcholine (Emax was reduced by 24%). When the action of EDHF was prevented by these K channel inhibitors, the remaining relaxation was abolished by either ODQ (1 microM) or L-NOARG (0.3 mM), indicating that NO, apart from EDHF, contributes to the endothelium-dependent relaxations. Furthermore, ODQ (10 microM) abolished the relaxation induced by the NO donor S-nitroso-N-acetylpenicillamine. Thus, activation of soluble guanylate cyclase seems to be the only mechanism through which NO causes relaxation in this artery. When vessels were exposed to grave hypoxia (pO2 = 6 mm Hg), the NO-mediated relaxation (induced by acetylcholine in the presence of charybdotoxin plus apamin) disappeared. In contrast, EDHF-mediated responses (elicited by acetylcholine in the presence of L-NOARG) were only marginally affected by hypoxia (Emax was reduced by 16%). 17-Octadecynoic acid (50 microM) and 5,8,11,14-eicosatetraynoic acid (10 microM), inhibitors of cytochrome P450-dependent oxidation of arachidonic acid, failed to inhibit the acetylcholine-induced relaxation in the presence of L-NOARG. The cytochrome P450-dependent arachidonic acid metabolite 11,12-epoxyecosatrienoic acid (0.3-3.0 microM) had no relaxant effect per se. In conclusion, EDHF and NO are both mediators of endothelium-dependent relaxations in the guinea pig basilar artery. However, during grave hypoxia, EDHF alone mediates acetylcholine-induced relaxation. The results further suggest that EDHF is not a metabolite of arachidonic acid formed by cytochrome P450 mono-oxygenase or generated by another oxygen-dependent enzyme in this artery.
Petersson
J
J
Department of Clinical Pharmacology, Institute of Laboratory Medicine, Lund University Hospital, Lund, Sweden.
Zygmunt
P M
PM
Jönsson
P
P
Högestätt
E D
ED
D015126:8,11,14-Eicosatrienoic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D001030:Apamin / Q000494:pharmacology; D016718:Arachidonic Acid / Q000037:antagonists & inhibitors; D001488:Basilar Artery / Q000503:physiopathology*; D001685:Biological Factors / Q000502:physiology; D018999:Charybdotoxin / Q000494:pharmacology; D003577:Cytochrome P-450 Enzyme System / Q000502:physiology*; D004338:Drug Combinations; D004730:Endothelium, Vascular / Q000503:physiopathology*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006162:Guanylate Cyclase / Q000037:antagonists & inhibitors; D006168:Guinea Pigs; D000860:Hypoxia / Q000503:physiopathology*; D008297:Male; D009569:Nitric Oxide / Q000502:physiology; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors; D011188:Potassium / Q000494:pharmacology; D012016:Reference Values; D014664:Vasodilation / Q000502:physiology*
D001685:Biological Factors; D004338:Drug Combinations; D004791:Enzyme Inhibitors; C067410:endothelium-dependent hyperpolarization factor; D018999:Charybdotoxin; D001030:Apamin; D016718:Arachidonic Acid; D009569:Nitric Oxide; C046783:11,12-epoxy-5,8,14-eicosatrienoic acid; D003577:Cytochrome P-450 Enzyme System; D019001:Nitric Oxide Synthase; D006162:Guanylate Cyclase; D015126:8,11,14-Eicosatrienoic Acid; D011188:Potassium