title: Role of potassium channels in endothelium-dependent relaxation resistant to nitroarginine in the rat hepatic artery.
abstract: 1. In the presence of indomethacin (IM, 10 microM) and N omega-nitro-L- arginine (L-NOARG, 0.3 mM), acetylcholine (ACh) induces an endothelium-dependent smooth muscle hyperpolarization and relaxation in the rat isolated hepatic artery. The potassium (K) channel inhibitors, tetrabutylammonium (TBA, 1 mM) and to a lesser extent 4-aminopyridine (4-AP, 1 mM) inhibited the L-NOARG/IM-resistant relaxation induced by ACh, whereas apamin (0.1-0.3 microM), charybdotoxin (0.1-0.3 microM), iberiotoxin (0.1 microM) and dendrotoxin (0.1 microM) each had no effect. TBA also inhibited the relaxation induced by the receptor-independent endothelial cell activator, A23187. 2. When combined, apamin (0.1 microM) + charybdotoxin (0.1 microM), but not apamin (0.1 microM) + iberiotoxin (0.1 microM) or a triple combination of 4-AP (1 mM) + apamin (0.1 microM) + iberiotoxin (0.1 microM), inhibited the L-NOARG/IM-resistant relaxation induced by ACh. At a concentration of 0.3 microM, apamin + charybdotoxin completely inhibited the relaxation. This toxin combination also abolished the L-NOARG/ IM-resistant relaxation induced by A23187. 3. In the absence of L-NOARG, TBA (1 mM) inhibited the ACh-induced relaxation, whereas charybdotoxin (0.3 microM) + apamin (0.3 microM) had no effect, indicating that the toxin combination did not interfere with the L-arginine/NO pathway. 4. The gap junction inhibitors halothane (2 mM) and 1-heptanol (2 mM), or replacement of NaCl with sodium propionate did not affect the L-NOARG/IM-resistant relaxation induced by ACh. 5. Inhibition of Na+/K(+)-ATPase by ouabain (1 mM) had no effect on the L-NOARG/IM-resistant relaxation induced by ACh. Exposure to a K(+)-free Krebs solution, however, reduced the maximal relaxation by 13% without affecting the sensitivity to ACh. 6. The results suggest that the L-NOARG/IM-resistant relaxation induced by ACh in the rat hepatic artery is mediated by activation of K-channels sensitive to TBA and a combination of apamin + charybdotoxin. Chloride channels, Na+/K(+)-ATPase and gap junctions are probably not involved in the response. It is proposed that endothelial cell activation induces secretion of an endothelium-derived hyperpolarizing factor(s) (EDHF), distinct from NO and cyclo-oxygenase products, which activates more than one type of K-channel on the smooth muscle cells. Alternatively, a single type of K-channel, to which both apamin and charybdotoxin must bind for inhibition to occur, may be the target for EDHF.
authors.affiliation: Department of Clinical Pharmacology, Institute of Laboratory Medicine, Lund University Hospital, Sweden.
authors.forename: P M
authors.initials: PM
authors.lastname: Zygmunt
authors.forename: E D
authors.initials: ED
authors.lastname: Högestätt
mesh_terms: D000109:Acetylcholine / Q000494:pharmacology*; D000818:Animals; D001120:Arginine / Q000031:analogs & derivatives* / Q000494:pharmacology; D001685:Biological Factors / Q000378:metabolism; D018118:Chloride Channels / Q000037:antagonists & inhibitors; D004730:Endothelium, Vascular / Q000187:drug effects* / Q000502:physiology; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D005260:Female; D017629:Gap Junctions / Q000187:drug effects; D006499:Hepatic Artery / Q000187:drug effects; D066298:In Vitro Techniques; D007213:Indomethacin / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D009126:Muscle Relaxation / Q000187:drug effects / Q000502:physiology; D009131:Muscle, Smooth, Vascular / Q000187:drug effects; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors*; D019335:Nitroarginine; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000502:physiology*; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors
chemical_list: D001685:Biological Factors; D018118:Chloride Channels; D004791:Enzyme Inhibitors; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D000644:Quaternary Ammonium Compounds; C067410:endothelium-dependent hyperpolarization factor; D019335:Nitroarginine; D001120:Arginine; C009405:tetrabutylammonium; D019001:Nitric Oxide Synthase; D000254:Sodium-Potassium-Exchanging ATPase; D000109:Acetylcholine; D007213:Indomethacin