INTERACTIONS BETWEEN ENDOTHELIUM-DERIVED RELAXING FACTORS IN THE RAT HEPATIC-ARTERY - FOCUS ON REGULATION OF EDHF

1 In rat isolated hepatic arteries contracted with phenylephrine, acet ylcholine and the calcium ionophore A23187 each elicit endothelium-dep endent relaxations, which involve both nitric oxide (NO) and endotheli um-derived hyperpolarizing factor (EDHF). However, the contribution of prostanoids to these responses, and the potential interaction between EDHF and other endothelium-derived relaxing factors have not been exa mined. 2 In the presence of the NO synthase inhibitor N-G-nitro-L-argi nine (L-NOARG, 0.3 mM) and a mixture of charybdotoxin (0.3 mu M) and a pamin (0.3 mu M), inhibitors of the target potassium (K) channel(s) fo r EDHF, acetylcholine and A23187 each induced a concentration-dependen t and almost complete relaxation, which was abolished in the additiona l presence of indomethacin (10 mu M). Thus, in addition to EDHF and NO , a relaxing factor(s) generated by cyclo-oxygenase (COX) contributes to endothelium-dependent relaxation in the rat hepatic artery. 3 The r esting membrane potentials of endothelium-intact and endothelium-denud ed vascular segments were -57 mV and -52 mV, respectively (P>0.05). In intact arteries, the resting membrane potential was not affected by L -NOARG plus indomethacin, but reduced to -47 mV in the presence of cha rybdotoxin plus apamin. Acetylcholine and A23187 (10 mu M each) elicit ed a hyperpolarization of 13 mV and 15 mV, respectively. The hyperpola rization induced by these agents was not affected by L-NOARG plus indo methacin (12mV and 14mV, respectively), but reduced in the presence of charybdotoxin plus apamin (7 mV and 10 mV, respectively), and abolish ed in the combined presence of charybdotoxin, apamin and indomethacin. 4 The NO donor 3-morpholino-sydnonimine (SIN-1) induced a concentrati on-dependent relaxation, which was unaffected by charybdotoxin plus ap amin, but abolished by the selective soluble guanylate cyclase inhibit or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one (ODQ, 10 mu M) SIN-1 ( 10 mu M) did not alter the resting membrane potential in endothelium-d enuded vascular segments. 5 The COX-dependent relaxation induced by ac etylcholine was abolished following exposure to 30 mM KCI, but unaffec ted. by glibenclamide (10 mu M). The prostacyclin analogue iloprost in duced a concentration-dependent relaxation, which was also abolished i n 30 mM KCl and unaffected by the combined treatment with glibenclamid e, charybdotoxin and apamin. Iloprost (10 mu M) induced a glibenclamid e-resistant hyperpolarization (8 mV with and 9 mV without glibenclamid e) in endothelium-denuded vascular segments. 6 Exposure to SIN-1 or il oprost did not affect the EDHF-mediated relaxation induced by acetylch oline (i.e. in the presence of L-NOARG and indomethacin). Replacement of L-NOARG with the NO scavenger oxyhaemoglobin (10 mu M) or the solub le guanylate cyclase inhibitor ODQ (10 mu M) or methylene blue (10 mu M), which all significantly inhibited responses to endothelium-derived NO, did not affect the acetylcholine-induced relaxation in the presen ce of indomethacin, indicating that endogenous NO also does not suppre ss EDHF-mediated responses. 7 These results show that, in addition to EDHF and NO, an endothelium-derived hyperpolarizing factor(s) generate d by COX contributes significantly to endothelium-dependent relaxation in the rat heptic artery. Neither this factor nor NO seems to regulat e EDHF-mediated responses. Thus, EDHF does not serve simply as a 'back -up' system for NO and prostacyclin in this artery. However, whether E DHF modulates the NO and COX pathways remains to be determined.