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

1 Non-adrenergic 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 mu m 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 mu M) and atropine (1 mu M) produced frequency-dependent relaxation of small arteries. Pretreatment with tetrodotoxin (1 mu M) abolished the relaxation and desensitization with capsaicin (10 mu M) strongly inhibited the relaxation. 4 Pretreatment with a P2Y-purinoceptor antagonist, basilen blue (3 mu M) or a human calcitonin gene-related peptide (hCGRP) receptor antagonist, hCGRP(8-37) (1 mu M) 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 mu M) and the NO scavenger oxyhaemoglobin (OxyHb; 20 mu M) but not by NO synthase inhibitors N(G)-nitro-L-arginine methyl ester (L-NAME; 300 mu M) or N(G)-nitro-L-arginine (L-NOARG; 300 mu M). 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 mu M), 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 mu M). 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.