NO contributes to EDHF-like responses in rat small arteries: a role for NO stores.

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Abstract
Objectives: Responses to EDHF are usually characterised in the presence of nitric oxide synthase (NOS) and cyclooxygenase (COX) inhibitors. The contribution of NO to endothelium-dependent relaxation in the presence of NOS inhibitors was assessed using NO scavengers with the objective of testing (i) whether any residual NO produces endothelium-dependent relaxation in a manner similar to EDHF and (ii) to identify the source of the residual NO. Methods: Small rat hepatic and mesenteric arteries were mounted in a tension myograph for either isometric or membrane potential measurements. Results: Relaxation to ACh was unaffected by pre-treatment with NG-nitro-l-arginine methyl ester (l-NAME, 300 μM), and indomethacin (Indo, 5 μM) in the absence or presence of 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 1 μM), nitro-l-arginine (300 μM) or l-nitro-mono-methyl-arginine (l-NMMA, 300 μM). Addition of OxyHb (20 μM) or carboxy-PTIO (300 μM) produced a significant suppression of ACh-induced relaxations (≈40%). In l-NAME+Indo treated arteries ACh-induced hyperpolarisation (Δ16.3±2.1 mV, n=8) was significantly suppressed with the addition of OxyHb (Δ10.2±1.6 mV, n=12). ACh-induced relaxation, in the presence of l-NAME+Indo+OxyHb, was abolished by raised extracellular K+, or the combination of charybdotoxin (CTX, 100 nM)+apamin (100 nM). In contrast whilst l-NAME+indo+barium+ouabain suppressed ACh-induced relaxation, the presence of OxyHb had no additional effect. Ultraviolet light induced a relaxation in arteries treated with l-NMMA+Indo (37.0±5.2%, n=9) which was sensitive to OxyHb (15.2±10.9%, n=4), and barium+ouabain (6.39±2.7%, n=4), but not CTX+apamin (37.8±2.4%, n=4). Conclusions: These findings suggest that NO contributes significantly to the “EDHF-like” response seen in rat small arteries and that the source of this NO may be preformed vascular stores.

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