Spatial heterogeneity in the mechanisms contributing to acetylcholine-induced dilatation in the rabbit isolated ear

1 Using an X-ray microangiographic technique in rabbit isolated perfused ears preconstricted with 5-HT (300 nM) and histamine (300 nM), we investigated the combined actions of N-omega-nitro-L-arginine methyl ester (L-NAME) and indomethacin on acetylcholine-induced depressor responses. 2 Under control conditions, acetylcholine (10 nM-30 mu M) induced a concentration-dependent reversal of the presser response, reaching a maximum of 66.0 +/- 13.6% (n = 6). In the presence of L-NAME (300 mu M) and indomethacin (10 mu M), this depressor action was reduced, reaching a maximum of 38.6 +/- 5.9% (n = 6). 3 The control response was associated with substantial vasodilatation in the central ear artery (G(0)), a smaller dilatory action on first generation branch arteries (GI) and no effect on second generation branch arteries (G(2)). In the presence of L-NAME and indomethacin, vasodilatation occurred in G(2) with no effect in G(0) or G(1). 4 Two calcium-activated K+ channels blockers, charybdotoxin (ChTX; 10 nM) and penitrem A (100 nM), further inhibited, but did not abolish, the L-NAME- and indomethacin-resistant response to acetylcholine (10 nM-300 mu M). Both agents abolished the vasodilatory action of acetylcholine in G(2). 5 In conclusion, L-NAME and indomethacin induced a shift in acetylcholine-induced vasodilatation from G(0) and G(1) to G(2). This is consistent with the suggestion that nitric oxide dominates in larger vessels whilst other mechanisms dominate in smaller vessels. The L-NAME- and indomethacin-resistant component was inhibited by ChTX and penitrem A, suggesting it is mediated, at least in part, by activation of K-Ca channels and could therefore involve a hyperpolarising mediator such as endothelium-derived hyperpolarising factor.