Endothelium-dependent sensory NANC vasodilatation: involvement of ATP,CGRP and a possible NO store

1. Non-adrenergic non-cholinergic (NANC) vasodilator nerves regulate tone in certain vascular beds. We have investigated the mechanisms of the NANC dilator response in the isolated small mesenteric artery of the rabbit by use of the tension myograph.
2. Small second or third order (150–300 μm in diameter) arteries of the rabbit mesenteric bed were mounted in a Mulvany tension myograph. Responses to electrical field stimulation (EFS) and exogenous vasodilators were investigated.
3. EFS (0.5–16 Hz, 10 V, 0.3 ms for 5 s), in the presence of guanethidine (5 μM) and atropine (1 μM) produced frequency-dependent relaxation of small arteries. Pretreatment with tetrodotoxin (1 μM) abolished the relaxation and desensitization with capsaicin (10 μM) strongly inhibited the relaxation.
4. Pretreatment with a P2Y-purinoceptor antagonist, basilen blue (3 μM) or a human calcitonin gene-related peptide (hCGRP) receptor antagonist, hCGRP_8–37 (1 μM) suppressed the NANC relaxation by approximately 40–60 % in each case and combined pretreatment almost abolished the relaxation.
5. The EFS-induced relaxation was suppressed by endothelium-removal, pretreatment with the soluble guanylyl cyclase inhibitor ODQ (1 μM) and the NO scavenger oxyhaemoglobin (OxyHb; 20 μM) but not by NO synthase inhibitors N^G-nitro-L-arginine methyl ester (L-NAME; 300 μM) or N^G-nitro-L-arginine (L-NOARG; 300 μM). Combined pretreatment with ODQ and CGRP_8–37 almost abolished the relaxation.
6. A P2Y-purinoceptor agonist, 2-methylthio ATP, produced endothelium-dependent relaxation which was inhibited by L-NAME and ODQ (1 μM), whilst hCGRP produced endothelium-independent and ODQ-insensitive relaxation.
7. Ultraviolet light (320 nm, 5 shots over 20 s) produced relaxation that was blocked by both OxyHb and ODQ but not by N^G-monomethyl-L-arginine (L-NMMA, 300 μM).
8. The present study suggests that EFS-induced NANC relaxation of the mesenteric small artery of the rabbit is mediated mainly by capsaicin-sensitive sensory C-fibres and that both ATP and CGRP are involved. The action of ATP released by EFS appears to be endothelium-dependent and involve activation of soluble guanylyl cyclase, but is resistant to inhibitors of NO synthase. The response to CGRP is endothelium-independent. These results show that ATP and CGRP account fully for the NANC relaxation of this vessel type and that the endothelium is involved in NANC-induced relaxation. The endothelium-dependent part of the response is consistent with the release of NO, either from NO synthase, incompletely inhibited by the NO synthase inhibitors, or by some preformed stores.