Nitric oxide, prostanoid and non-NO, non-prostanoid involvement in acetylcholine relaxation of isolated human small arteries

1 The main purpose of the study was Co clarify to which extent nitric oxide (NO) contributes to acetylcholine (ACh) induced relaxation of human subcutaneous small arteries. 2 Arterial segments were mounted in myographs for recording of isometric tension, NO concentration and smooth muscle membrane potential. 3 In noradrenaline-contracted arteries, ACh induced endothelium-dependent relaxations. The NO synthase inhibitor, N-G-nitro-L-arginine (L-NOARG) had a small significant effect on the concentration-response curves for ACh, and in the presence of L-NOARG indomethacin only caused a small additional rightward shift in the ACh relaxation. 4 The NO scavenger, oxyhaemoglobin attenuated relaxations for ACh and for the NO donor S-nitroso-N-acetylpenicillamine (SNAP). Inhibition of guanylyl cyclase with H-1-[1,2,4]oxadiazolo[4,3a]quinoxaline-1-one (ODQ), and inhibition of protein kinase G with beta-Ethenyl-1, N2-etheno-8-bromoguanosine- 3', 5'- cyclic monophosphorothioate, Rp-isomer, slightly attenuated ACh relaxation, but abolished SNAP induced relaxation. 5 ACh induced relaxation without increases in the free NO concentration. In contrast, for equivalent relaxation, SNAP increased the NO concentration 32+/-8 nM. 6 ACh hyperpolarized the arterial smooth muscle cells with 11.4+/-1.3 mV and 10.5 +/- 1.3 mV in the absence and presence of L-NOARG, respectively. SNAP only elicited a hyperpolarization of 1.6 +/- 0.9 mV. 7 In the presence of indomethacin and L-NOARG, ACh relaxation was almost unaffected by lipoxygenase inhibition with nordihydroguaiaretic acid, or cytochrome P450 inhibition with 17-octadecynoic acid or econazole. ACh relaxation was strongly reduced by the combination of charybdotoxin and apamin, but small increments in the extracellular potassium concentration induced no relaxations. 8 The study demonstrates that the NO/L-arginine pathway is present in human subcutaneous small arteries and to a limited extent is involved in ACh induced relaxation. The study also suggests a small contribution of arachidonic acid metabolites. However, ACh relaxation is mainly dependent on a non-NO, non-prostanoid endothelium dependent hyperpolarization.